France



Cylindrical magnetic nanowires (NWs) are promising candidates for building blocks of 3D nanoarchitectures and 3D nanotechnologies [1,2]. They present great potential for 3D applications, offering responses to many external (electrical, magnetic, mechanical, thermal, etc.) stimuli [3].&lt;br/&gt;The emerging interest toward cylindrical magnetic NWs is related to the fact that their geometry promotes topologically non-trivial 3D magnetic structures such as Bloch-point domain walls, complex configurations that connect vortex domains with opposite chirality, vortices or skyrmion tubes [4,5].&lt;br/&gt;Here, we present a study on multisegmented NWs formed by ferromagnetic (FM) segments separated either by a different FM material or a non-magnetic (NM) one. On one hand we have alternating CoNi/Ni segments (Fig. 1 (a)), where the domain structure can be tailored by alternating magnetocrystalline anisotropy and the length of each segment. On other hand, FeCo ferromagnetic layers separated by thin Cu NM layers (Fig. 1 (c)). The multisegmented NWs were prepared by electroplating filling the pores of anodic aluminum oxide membranes.&lt;br/&gt;The HRTEM analysis confirms the multisegmented morphology with a uniform diameter, and periodic and sharp interfaces. Quantitative analysis reveals that the diameter of CoNi/Ni NWs is about 140 nm with lengths between 0.6 μm and 1.4 μm for CoNi and 1 μm for Ni segments while FeCo/Cu wires with 165 nm in diameter are formed by FeCo segments with 250 nm in length separated by 50 nm Cu layers.&lt;br/&gt;XMCD-PEEM imaging of multisegmented CoNi/Ni NWs reveals that the magnetic structure in Ni segment is in a dominantly axial magnetic state while in CoNi segments it depends on their length. In NWs with the shortest CoNi segments (0.6 μm), we observed single vortex domains, in longer segments of 1.2 μm we observed one to three vortex domain states (Fig. 1 (a)), while in the NWs with the longest CoNi segments either multi-vortex or multi-transverse domains were observed.&lt;br/&gt;The micromagnetic simulations show that the remanent magnetic structure of CoNi segments depends on the direction of the previously applied field. If the direction is perpendicular to both the NW and the magnetization easy axis, the vortex domains simply expand inside the segment following the structures formed at the interfaces and resulting in one vortex domain for short segments and two or more vortex structures for longer segments. In turn, when the field is applied perpendicular to the NW but along the magnetization easy axis, transverse domains are formed. They are separated by vortex domain structures with core at the surface pointing perpendicular to it.&lt;br/&gt;Furthermore, although Ni segments are in an almost single-domain state, they are not completely saturated, and present a small magnetization curling on the NW surface. Their chirality is formed prior to formation of magnetic structures in CoNi and imprints the chirality along the next CoNi segments (Fig. 1 (b)) [5].&lt;br/&gt;The second investigated multisegmented system, FeCo/Cu (Fig. 1 (c-d)), presents a tilt at the interface between FM/NM layer. From the XMCD-PEEM images (Fig. 1 (c)) we determined that each FeCo segment presents a single vortex state which switches its magnetization under the influence of perpendicular magnetic fields. This switching is sequential and each segment has a slightly different switching field. The simulations (Fig. 1 (d)) unveil the main differences between magnetization states when the field is applied along different directions with respect to the NW axis. The tilt between adjacent segments breaks the rotational symmetry providing different magnetic potentials for vortices moving in a certain direction under the influence of applied magnetic field.&lt;br/&gt;The proper design of segments anisotropy and geometry (e.g., surface tilting) in cylindrical multisegmented nanowires opens multiple opportunities for advanced nanotechnologies in 3D magnetic nanostructures.

**References:**

[1] Fernández-Pacheco, A.; Streubel, R.; Fruchart, O.; Hertel, R.; Fischer, P.; Cowburn, R. P. , Nat. Commun. 2017, 8, 15756.&lt;br/&gt;[2] Hertel, R, Spin 2013, 03 (03), 1340009.&lt;br/&gt;[3] &lt;i&gt;“Magnetic Nano- and Microwires: Design, Synthesis, Properties and Applications”,&lt;/i&gt; Elsevier 2020, edited by M.Vazquez.&lt;br/&gt;[3] Da Col, S.; Jamet, S.; Rougemaille, N.; Locatelli, A.; Mentes, T. O.; Burgos, B. S.; Afid, R.; Darques, M.; Cagnon, L.; Toussaint, J. C.; et al. , Phys. Rev. B 2014, 89 (18), 180405.&lt;br/&gt;[4] Fernandez-Roldan, J. A.; Perez del Real, R.; Bran, C.; Vazquez, M.; Chubykalo-Fesenko, O., Nanoscale 2018, 10 (13), 5923–5927.&lt;br/&gt;[5] Bran, C.; Fernandez-Roldan, J.A.; P Del Real, R.; Asenjo, A.; Chen, Y.-S.; Zhang, J.; Zhang, X.; Fraile Rodríguez, A.; Foerster, M.; Aballe, L.; et al. , &lt;i&gt;ACS Nano&lt;/i&gt; 2020, &lt;i&gt;14&lt;/i&gt;, 12819.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/9345e275af7517d8247f940544434392.jpg)
Figure 1. Schematic illustrations of multisegmented NWs (top panels) (a) Chemical contrast and XMCD-PEEM images of a CoNi (1.2 μm)/Ni(1 μm) multisegmented NWs (b) Modelled magnetization state of the CoNi/Ni NW presented in (a), (c) XMCD-PEEM images of FeCo/Cu NWs measured in remanence after perpendicular magnetic fields were applied, (d) Simulated magnetization configurations of FeCo/Cu NW before (upper panel) and after (bottom panel) magnetic fields were applied in y-direction. Scale bar 1μm.



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 Controlled magnetization by magnetic fields in multisegmented cylindrical nanowires

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Cylindrical magnetic nanowires (NWs) are promising candidates for building blocks of 3D nanoarchitectures and 3D nanotechnologies [1,2]. They present great potential for 3D applications, offering responses to many external (electrical, magnetic, mechanical, thermal, etc.) stimuli [3]. The emerging interest toward cylindrical magnetic NWs is related to the fact that their geometry promotes topologically non-trivial 3D magnetic structures such as Bloch-point domain walls, complex configurations that connect vortex domains with opposite chirality, vortices or skyrmion tubes [4,5]. Here, we present a study on multisegmented NWs formed by ferromagnetic (FM) segments separated either by a different FM material or a non-magnetic (NM) one. On one hand we have alternating CoNi/Ni segments (Fig. 1 (a)), where the domain structure can be tailored by alternating magnetocrystalline anisotropy and the length of each segment. On other hand, FeCo ferromagnetic layers separated by thin Cu NM layers (Fig. 1 (c)). The multisegmented NWs were prepared by electroplating filling the pores of anodic aluminum oxide membranes. The HRTEM analysis confirms the multisegmented morphology with a uniform diameter, and periodic and sharp interfaces. Quantitative analysis reveals that the diameter of CoNi/Ni NWs is about 140 nm with lengths between 0.6 μm and 1.4 μm for CoNi and 1 μm for Ni segments while FeCo/Cu wires with 165 nm in diameter are formed by FeCo segments with 250 nm in length separated by 50 nm Cu layers. XMCD-PEEM imaging of multisegmented CoNi/Ni NWs reveals that the magnetic structure in Ni segment is in a dominantly axial magnetic state while in CoNi segments it depends on their length. In NWs with the shortest CoNi segments (0.6 μm), we observed single vortex domains, in longer segments of 1.2 μm we observed one to three vortex domain states (Fig. 1 (a)), while in the NWs with the longest CoNi segments either multi-vortex or multi-transverse domains were observed. The micromagnetic simulations show that the remanent magnetic structure of CoNi segments depends on the direction of the previously applied field. If the direction is perpendicular to both the NW and the magnetization easy axis, the vortex domains simply expand inside the segment following the structures formed at the interfaces and resulting in one vortex domain for short segments and two or more vortex structures for longer segments. In turn, when the field is applied perpendicular to the NW but along the magnetization easy axis, transverse domains are formed. They are separated by vortex domain structures with core at the surface pointing perpendicular to it. Furthermore, although Ni segments are in an almost single-domain state, they are not completely saturated, and present a small magnetization curling on the NW surface. Their chirality is formed prior to formation of magnetic structures in CoNi and imprints the chirality along the next CoNi segments (Fig. 1 (b)) [5]. The second investigated multisegmented system, FeCo/Cu (Fig. 1 (c-d)), presents a tilt at the interface between FM/NM layer. From the XMCD-PEEM images (Fig. 1 (c)) we determined that each FeCo segment presents a single vortex state which switches its magnetization under the influence of perpendicular magnetic fields. This switching is sequential and each segment has a slightly different switching field. The simulations (Fig. 1 (d)) unveil the main differences between magnetization states when the field is applied along different directions with respect to the NW axis. The tilt between adjacent segments breaks the rotational symmetry providing different magnetic potentials for vortices moving in a certain direction under the influence of applied magnetic field. The proper design of segments anisotropy and geometry (e.g., surface tilting) in cylindrical multisegmented nanowires opens multiple opportunities for advanced nanotechnologies in 3D magnetic nanostructures.

**References:**

[1] Fernández-Pacheco, A.; Streubel, R.; Fruchart, O.; Hertel, R.; Fischer, P.; Cowburn, R. P. , Nat. Commun. 2017, 8, 15756. [2] Hertel, R, Spin 2013, 03 (03), 1340009. [3] “Magnetic Nano- and Microwires: Design, Synthesis, Properties and Applications”, Elsevier 2020, edited by M.Vazquez. [3] Da Col, S.; Jamet, S.; Rougemaille, N.; Locatelli, A.; Mentes, T. O.; Burgos, B. S.; Afid, R.; Darques, M.; Cagnon, L.; Toussaint, J. C.; et al. , Phys. Rev. B 2014, 89 (18), 180405. [4] Fernandez-Roldan, J. A.; Perez del Real, R.; Bran, C.; Vazquez, M.; Chubykalo-Fesenko, O., Nanoscale 2018, 10 (13), 5923–5927. [5] Bran, C.; Fernandez-Roldan, J.A.; P Del Real, R.; Asenjo, A.; Chen, Y.-S.; Zhang, J.; Zhang, X.; Fraile Rodríguez, A.; Foerster, M.; Aballe, L.; et al. , ACS Nano 2020, 14, 12819.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/9345e275af7517d8247f940544434392.jpg)
Figure 1. Schematic illustrations of multisegmented NWs (top panels) (a) Chemical contrast and XMCD-PEEM images of a CoNi (1.2 μm)/Ni(1 μm) multisegmented NWs (b) Modelled magnetization state of the CoNi/Ni NW presented in (a), (c) XMCD-PEEM images of FeCo/Cu NWs measured in remanence after perpendicular magnetic fields were applied, (d) Simulated magnetization configurations of FeCo/Cu NW before (upper panel) and after (bottom panel) magnetic fields were applied in y-direction. Scale bar 1μm.

technical paper

The transmittable torque of magnetorheological fluid (MRF) clutches can be adjusted by controlling the magnetic field inside the clutch. In order to increase their maximum transmittable torque while decreasing their volume and mass, geometry optimization is often carried out, such as in [1]. Additionally, the MRF used in them can be designed to have a higher yield stress, resulting in a higher transmittable torque. In [2], this was studied by adding micrometer-sized graphite flakes to an in-house created MRF; it was found that the inclusion of graphite flakes led to an increased yield stress. The present research aims to further study this, by investigating the effects that including graphite flakes in a commercially available MRF may have on the transmittable torque of an MRF clutch. Clutch design. The MRF clutch utilized in this research is a drum-type, permanent magnet-based clutch, made from two concentric cylinders. The external cylinder was fabricated from low carbon steel and the internal cylinder was fabricated from aluminum. A cavity for the MRF is formed between these cylinders, with a width of 0.75 mm. The external cylinder formed the main body of the output stage of the MRF clutch. The magnet assembly used to magnetize the clutch consisted of four magnets which have a hollowed-out cylinder shape when assembled together. These magnets were magnetized in the radial direction, with two of them having their south poles in the outer face, while the other two had them in the inner face. The internal cylinder formed the main body of the input stage of the clutch, driven by a DC motor. A mechanism called field blocker, presented in [3], was used to adjust the magnetization level of the MRF inside the clutch. MRF. A commercially available MRF, the LORD MRF-140CG, was used in this study. Four mixtures with 20 μm graphite flakes (282863, Sigma-Aldrich) were prepared, with a graphite content of 0.5% w/w, 1.0% w/w, 2.0% w/w, and 3.0% w/w. Experimental set-up. The MRF clutch was driven by a velocity-controlled DC motor, and its output stage was connected to one end of a 5 Nm T22 HBM torque meter. A lever was attached to the other end of this torque meter, which contacted the testing bench when rotated in one or the other direction. After contacting the bench, this lever forced the output stage of the MRF clutch to remain stationary while the input stage continued rotating. A stepper motor moved the field blocker in or out of the MRF clutch by means of a ball-screw mechanism, covering and uncovering the magnet assembly. The testing set-up is shown in Figure 1 a). Experimental procedure. The testing of the four MRF mixtures together with the MRF without graphite were carried out the following way: 1) The torque output was measured during one second without driving the clutch and with the lever in a free position. 2) The clutch was then driven at the velocity under test for a duration equal to 2 s plus the time required for the clutch to complete a rotation. 3) The clutch was driven backward at a low angular velocity for one second, in order to leave the lever in a free position. This procedure was repeated three times for each combination of five angular velocities, six field blocker positions, with and without the magnet assembly, for a total of 180 measurements per MRF mixture. The average torque value was calculated from the torque recorded during the second testing step, after the first two seconds had passed. The effects on torque due to seal friction and viscosity on the measured torque values were then removed. Results. The results of these tests are summarized in Table 1, Figure 1 b) and c). In Figure 1 b) and c), the average torque values of the tests carried out with the field blocker not covering and covering the magnets are shown (two out of the six positions tested). The torque values are higher in the case were the magnet assembly is uncovered than when it is covered. It is also seen that the difference in torque caused by the graphite content increases as the input angular velocity increases. Higher magnetization levels decrease the difference in torque (the field blocker does not block the magnetic field entirely). The mean, maximum, and minimum torque values over the six magnetization cases are shown in Table 1. It can be seen that on average, the transmittable torque increases with both graphite content and angular velocity, achieving up to 70% torque increase. Conclusion. The inclusion of graphite flakes in a commercially available MRF can be used to increase the output torque of an MRF clutch, a result which aligns to those of [2]. This effect in a clutch with aluminum cylinders is more evident at higher angular velocities.

**References:**

[1] R. M. Andrade, A. B. Filho, C. B. S. Vimieiro, and M. Pinotti, “Optimal design and torque control of an active magnetorheological prosthetic knee,” Smart Mater. Struct., vol. 27, no. 10, p. 105031, 2018. [2] M. K. Thakur and C. Sarkar, “Influence of Graphite Flakes on the Strength of Magnetorheological Fluids at High Temperature and its Rheology,” IEEE Trans. Magn., vol. 56, no. 5, pp. 1–10, 2020. [3] M. A. Fernandez, J. Y. Chang, and C. Y. Huang, “Development of a passive magnetorheological fluid clutch with field-blocking mechanism,” IEEE Trans. Magn., vol. 54, no. 11, pp. 1–5, 2018.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/dd1732b1b4985924706ec3392bdc17b8.jpg)
Table 1. Percentage of the average torque values obtained using MRF with graphite above or below the average torque value obtained with the MRF without graphite.

![](https://assets.underline.io/uploads/markdown_image/1/image/53bb62aa46ec33865da9ddf7f003527d.jpg)
Figure 1. The testing set-up is presented in a). The plotted torque values when the magnet assembly was uncovered and covered are shown in b) and c), respectively.

 A Study on the Effects of Graphite Flakes on Torque Transmission of a Magnetorheological Fluid Clutch

Magnetic fluid sealing is a high-performance sealing technique compared to traditional sealing methods, but is limited to a few working conditions. Here we present a novel sealing method using nano-micron magnetic powders as sealing medium. Nano and micron-sized magnetic particles of different materials and size distribution were prepared and mixed. Packing process of magnetic particles was simulated by DEM. Leakage rate was calculated by Darcy’s Law. Finally, the static sealing experiment showed a high pressure resistance and a rather low leakage rate of this novel sealing method. 1. Introduction Sealing with magnetic fluids has been a commonly used high-performance sealing technique with its unique advantages like low friction torque, no leakage and long lifetime. However, it is not capable of resisting high pressure difference or working under extreme temperatures. Magnetic powders are mixture of nano- and micron-sized particles which can be magnetized by an external magnetic field. This research prepared nano-micron magnetic powders of great magnetic and rheological properties. Then the packing process of magnetic powders was simulated and leakage rate was calculated. A static sealing device was designed to test the sealing behavior of this novel sealing method. 2. Methods 2.1 Preparation of magnetic powders Chemical precipitation was used to prepared magnetic nanoparticles. Experimental conditions were adjusted to acquire nanoparticles of different materials, sizes and magnetic intensity. Micron particles were prepared by high-energy ball milling method. Surfactants were coated on particles to control the interparticle forces. 2.2 Simulation of packing process Interparticle forces of magnetic particles are complex and so is the packing process of magnetic powders. In this research, packing process of nano-micron magnetic powders was simulated by DEM using Hertzian-Mindlin model with JKR. Fraction, sizes, magnetization and surfactants of powder preparation, leads to changes of interparticle forces, especially cohesive and magnetic forces. Magnetic particles follow the basic equation: midvi(t)/dt = mig + Fid(t) + Fic(t) + Fim(t) vi(t) is the ith particle's velocity at time t; Fid(t), Fic(t), Fim(t) are the total drag force, net contact force and magnetic force of the ith particle, respectively. 2.3 Calculation of leakage rate by Darcy's Law Darcy's law is commonly used to describe the flow of a fluid through a porous medium. We established a microscopic model to estimate the permeability for calculation of leakage rate of the sealing device. The permeability κ is calculated by the equation: κ = μÛh / △p μ is the fluid viscosity; Û is the outlet velocity; h is the computational zone height; △p is the pressure difference. 2.4 Design and experiments of static sealing device A static sealing device was designed, where magnetic field distribution was simulated to maximize the magnetic flux density in the sealing gap. For each magnetic powder example, sealing capacity and leakage rate were measured. 2.5 Results The simulation results showed that the permeability decreased rapidly with the decrease of particle sizes, and appropriate mixture of nano- and micron-sized particles further decreased the permeability. For the powders prepared, the permeability could be as low as 5.09E-18 m-2. Considering the size of the sealing device, the calculated leakage rate was 2.22E-9 m3/s. Experiments indicated a sealing capacity ≥ 0.5 MPa and leakage rate ≤ 1E-6 Pa×m3/(s×cm). 3. Discussion By selecting magnetic materials, mixture of particles of different sizes and coating particles with surfactants, nano-micron magnetic powders can possess great magnetic and rheological properties. Proper size distribution and interparticle forces lead to a denser packing of magnetic powders. Further, a denser packing leads to a rather low leakage rate, which is verified by both simulation and sealing experiments. A novel sealing method using nano-micron magnetic powders shows distinctive property and may be a promising sealing technique.

**References:**

[1] J. Kafashan, J. Wiacek and N. Abd Rahman, Granular Matter, Vol. 21, p.80 (2019) [2] R.A. Sperling, W.J. Parak, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol. 368, p.1333-1383 (2010) [3] M. Neouze, U. Schubert, Monatshefte für Chemie - Chemical Monthly, Vol. 139, p.183-195 (2008) [4] R. Aboul Hosn, L. Sibille and N. Benahmed, Computers and Geotechnics, Vol. 95, p.30-39(2018) [5] K. Chen, Y. Tian and L. Shan, Smart Materials and Structures, Vol. 22, p.97001 (2013)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/dc98b54b36276cc5bab66afb1c8b1a3a.jpg)
Fig.1 Prepared magnetic powders

![](https://assets.underline.io/uploads/markdown_image/1/image/ab3ec5f9c5d8047ad3aa9245e09e70f4.jpg)
Fig.2 A microscopic model to calculate permeability

 A Novel Sealing Method Using Nano-micron Magnetic Powders and its Leakage Rate Analysis

Magnetofluidic cooling (MFC) is a heat transfer technology to transfer heat using thermomagnetic convection (TMC) of a ferrofluid[1, 2]. The temperature-dependent response of the ferrofluid leads to TMC[2] under the combined effect of thermal and magnetic field gradients. Practical applications require application-specific device design and materials. Solar cells can exhibit significant performance degradation with increasing temperature[3]. Methods reported for cooling, e.g., forced water/air circulation, water spraying, floating tracking concentrating, phase-change materials, and water immersion, show several limitations[4]. Here, we report a novel MFC device for the cooling of solar cells. We first developed a simulation model to investigate the ferrofluid flow needed to cool solar cells. Studying the effect of magnetic field strength, field distribution, channel size, and hydrodynamic properties of the fluid resulted in a novel design (Figure 1). We developed a novel ferrofluid and integrated it with our MFC system. We then built a setup for quantitative measurements of the solar cell performance. As expected, the solar cells showed decreasing performance with increasing temperature without MFC. On the other hand, with MFC (yellow band regions in Figure 2), we observed significant performance improvement. Interestingly, the MFC device demonstrated a higher relative performance enhancement at higher initial solar cell temperatures due to the self-regulating behavior of the MFC device. Hence, the current MFC system offers a novel, versatile, green, energy-efficient, environmentally friendly, noise-free, and vibration-free approach for passive cooling of solar cells. Acknowledgments This research is supported by grants from the National Research Foundation, Prime Minister’s Office, Singapore, under its Campus of Research Excellence and Technological Enterprise (CREATE) programme.

**References:**

[1] V. B. Varma, M. S. Pattanaik, S. K. Cheekati, and R. V. Ramanujan, "Superior cooling performance of a single channel hybrid magnetofluidic cooling device," Energy Convers. Manage., vol. 223, p. 113465, 2020. [2] M. S. Pattanaik, V. B. Varma, S. K. Cheekati, G. Prasanna, N. M. Sudharsan, and R. V. Ramanujan, "A self-regulating multi-torus magneto-fluidic device for kilowatt level cooling," Energy Convers. Manage., vol. 198, p. 111819, 2019. [3] P. Singh and N. M. Ravindra, "Temperature dependence of solar cell performance—an analysis," Sol. Energy Mater. Sol. Cells, vol. 101, pp. 36-45, 2012. [4] J. Siecker, K. Kusakana, and B. Numbi, "A review of solar photovoltaic systems cooling technologies," Renewable Sustainable Energy Rev., vol. 79, pp. 192-203, 2017.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/64997af6a802d48d79de39536f74d9ea.jpg)
Figure 1: (a) Schematic of the application-specific MFC device for Solar cell cooling. (b) Numerical simulations of the MFC device showing temperature profile with and without MFC.

![](https://assets.underline.io/uploads/markdown_image/1/image/4793520fe2d25aa7216475af3a89d499.jpg)
Figure 2: Solar cells surface temperature and generated voltage with and without magnetofluidic cooling (MFC). Yellow bands denote the MFC of the solar cell under an applied magnetic field of 550 mT. Gray bands show the performance without MFC.

 A Novel Magnetofluidic System to Cool Solar Cells

Introduction The manipulation of cell ladened droplets in the bulk culture media is essential for precise analysis and screening of cells, drugs, or other biological objects, which can lead to various applications. With the rapid growth of microfluidic technology, the physical and chemical properties of the droplets can be controlled much more uniformly. This breakthrough leads to more targeted manipulation and handling of the cell-ladened objects. Different methods have been proposed using electrical, optical, or magnetic principles [1]. Among all these methods, magnetic force manipulation is of great interests because it is non-contact, non-thermal, and insensitive to the dielectric properties [2]. The manipulation of magnetic and non-magnetic objects with magnetic fields in various applications have been studied [3]. However, most studies focused on the handling on a single nanoparticle. Little work has been done regarding the tweezing of particle with microscale diameter. Besides, the kinematics mechanism underlying the manipulation of magnetic objects is still not fully studied. This work aimed at developing a platform that can generate and manipulate magnetic-nanoparticle-ladened hydrogel droplets with more controllable manner. The relation between the magnetic force and the behavior of hydrogel droplets was also studied. This work can provide more in-depth understanding of the coupling between magnetic force and fluidic mechanics in handling microfluidic droplets, which can be a guideline in designing magneto-microfluidic platforms for biological applications. Experiments The hydrogel droplets were generated using a microfluidic platform following standard processes [3]. The fabrication of the microfluidic device followed typical soft lithography replica molding technique, by using maskless photolithography and PDMS prepolymer curing [4]. Superparamagnetic nanoparticles (Dynabeads Myone Carboxylic Acid, Purchased from ThermoFisher) was added into the hydrogel solution and mixed completely before the generation of the droplets. The magnetic beads were used as the medium to respond to the external magnetic field; thus the hydrogel droplets can be manipulated remotely. The flow rates of the oil (HFE 7500) and hydrogel solution were carefully adjusted to generate droplets with desired diameter, about 55 um. The generation of the hydrogel droplet is illustrated in Fig. 1a. After the droplets were solidified with fixed shape, the droplets were released into cell culture media, as shown in Fig. 1b. An external magnetic field was exerted using an electromagnet to manipulate the magnetic-nanoparticle-ladened hydrogel droplets within the culture media. The magnetic field could be controlled by adjusting the current of the DC power source. The magnetic manipulation setup and the movement of hydrogel droplets in the culture media are shown in Fig. 1c, Fig. 1d and Fig. 1e. Results and discussion The microfluidic device was successfully fabricated, and the magnetic-nanoparticle-ladened hydrogel droplets were generated using the droplet microfluidic platform. The diameter of the droplet was approximately 55 um, which is suitable for cell encapsulation and other biological applications. After the hydrogel droplets were released into cell culture media, an external magnetic flied was exerted to manipulate the droplets. The results showed that the droplets could be moved in the culture media through magnetic force (Fig. 2a and Fig. 2b). By adopting superparamagnetic nanoparticles, the droplets could be moved rapidly, at a speed of 200 um/s, as shown in Fig. 2c). Since there is no remanence in the superparamagnetic nanoparticles after each manipulation, the hydrogel droplets could be manipulated for several times as desired, which is also favorable for biological applications in which multiple steps are required. In the experiments, the droplets sank to the bottom after 5 min, due to the higher density of the hydrogel droplets relative to water. The moving speed of the hydrogel droplet was much slower than when the droplets were suspended in the media. A detailed kinetics analysis of the movement of hydrogel droplets will be addressed in the full paper.

**References:**

References: [1] Yi, Changqing, et al. "Microfluidics technology for manipulation and analysis of biological cells." Analytica chimica acta 560.1-2 (2006): 1-23. [2] Pamme, Nicole. "Magnetism and microfluidics." Lab on a Chip 6.1 (2006): 24-38. [3] Utech, Stefanie, et al. "Microfluidic generation of monodisperse, structurally homogeneous alginate microgels for cell encapsulation and 3D cell culture." Advanced healthcare materials 4.11 (2015): 1628-1633. [4] Nan, Lang, et al. "Oil-mediated high-throughput generation and sorting of water-in-water droplets." Microsystems & Nanoengineering 6.1 (2020): 1-10. Acknowledgements Yuan Shuai and Prof. Shum provided the idea. Yuan Shuai did the experiments in Microfluidics and Soft Matter Group, the University of Hong Kong. Yuan Shuai composed the paper. Prof. Shum and Dr. Pong directed the project and revised the paper. Information of the authors: Shuai Yuan is with the Department of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong (ysthoth@connect.hku.hk). Anderson H. C. Shum is with the Department of Mechanical Engineering, The University of Hong Kong, Hong Kong (ashum@hku.hk). Philip W. T. Pong is with the Department of Electrical and Computer Engineering, New Jersey Institute of Technology, and the Department of Electrical and Electronic Engineering, The University of Hong Kong (philip.pong@njit.edu).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/fd782eaeb1cfdd24d1ad098290524f53.jpg)
Fig. 1. a) Generation of magnetic nanoparticle ladened hydrogel droplets using microfluidic platform, b) Release of the hydrogel droplets into the cell culture media. c) Setup of the magnetic manipulation platform. d) Illustration of the movement of hydrogel droplets suspended in the culture media in response to the external magnetic field. e) Movement of hydrogel droplets that sank to the bottom.

![](https://assets.underline.io/uploads/markdown_image/1/image/7ad7f5b67300470f8ae35f503485f209.jpg)
Fig. 2. a) and b) Movement of the magnetic-nanoparticle-ladened hydrogel droplets under magnetic flied. The hydrogel droplets at the bottom of the Petri dish moved much more slowly than the floating droplets. c) Moving speed of the suspended and sank hydrogel droplets, respectively. The measurement of the suspended droplets stopped at 5 s because the droplets moved outside the scope.

 Magnetic manipulation of hydrogel droplets in culture media for biological applications

Cobalt ferrite nanoparticles (NPs) are competitive candidates in nanomedicine and biomedical applications, especially in the fields of magnetic drug delivery, magnetic resonance imaging, and separation and detection of biomolecules [1,2]. The reason behind is a suitable combination of excellent chemical and magnetic characteristics, such as a high chemical stability, surface active sites, and ease of synthesis and functionalization, together with a high anisotropy constant, a high coercivity, and a moderate saturation magnetization. However, in order to control the functional response of cobalt ferrite NPs, one of the biggest challenges is to quantitatively disentangle the dependence of the composition, structure, or surface chemistry onto the overall magnetic response. In this study we unravel the key role played by the Co2+ cations on the destabilization of the collinear ferrimagnetism in cobalt ferrite NPs, by combining an advanced synthesis approach with a broad set of world-class complementary local probes. A set of samples of monodispersed, 8 nm cobalt ferrite NPs of identical stoichiometry but with a progressive inclusion of structural defects was prepared [3,4]. SQUID magnetometry results show a rapid degradation of the collinear ferrimagnetism as the structural disorder increases within the NPs, and even samples that are almost free from crystallographic defects exhibit relatively large values of the high-field susceptibility suggesting the occurrence of canting at least for some of the cations and sites [5]. As local characterization probes, synchrotron-based, element-, valence- and site- specific X-ray spectroscopy and magnetometry on ensembles of NPs was combined with high resolution transmission electron microscopy of selected, individual NP (Fig. 1) [5]. The analysis of element-specific X-Ray Magnetic Circular Dichroism (XMCD) spectra and hysteresis loops for all cationic sites reveals that the collinear alignment of the Co2+ cations in octahedral sites is significantly more affected by the structural disorder than in any other cation. This is because structural defects cause local distortions of the crystal field acting on the orbital component of the cations, yielding effective local anisotropy axes that cause a prevalent Co2+ spin canting through the spin–orbit coupling, owing to the relatively large value of the partially unquenched moment of these cations, as found by XMCD. As the number of structural defects increases, the rest of the cations are progressively dragged off the ferrimagnetic alignment, being the Fe3+ cations in tetrahedral sites the last ones to be affected by the disorder because the canting takes place first in octahedral sites thanks to their smaller number of next-nearest neighbors in the tetrahedral sublattice [5]. Our results demonstrate the key role of the Co2+ cations on the destabilization of the collinear ferrimagnetism in Co-ferrite NPs as their crystalline quality worsens and may help clarify the often conflicting, large variability of magnetic properties in the literature of Co-ferrite NPs with slightly different structural features. Our work may provide new avenues to interpret and control the functional response of Co-ferrite NPs, of relevance in the fields of rare-earth free permanent magnets, biological separation and detection, or nanomedicine. Acknowledgements The work was supported by Spanish MCIU and AEI (MAT2015-68772-P; PGC2018-097789-B-I00) and European Union FEDER funds. M.E.T. acknowledges Spanish MINECO for the Ph.D. contract BES-2016-077527. A.F.R. and C.M. acknowledge the financial support from the EU CALIPSO Transnational Access programme. Part of this work was performed at the Swiss Light Source, Paul Scherrer Institut, Switzerland. The measurements at FEI TITAN3 of the Laboratorio de Microscopias Avanzadas (LMA), Instituto de Nanociencia de Aragon (INA), Universidad de Zaragoza, are also gratefully acknowledged. S.R.V.A. thanks the Swiss National Science Foundation for financial support under project Nr. 169467.

**References:**

[1] S. Y. Srinivasan, K.M. Paknikar, D. Bodas, et al. Applications of Cobalt Ferrite Nanoparticles in Biomedical Nanotechnology. Nanomedicine. Future Medicine Ltd. May 2018, pp 1221–1238. https://doi.org/10.2217/nnm-2017-0379. [2] S. Amiri, H. Shokrollahi, The Role of Cobalt Ferrite Magnetic Nanoparticles in Medical Science. Materials Science and Engineering C. Elsevier January 2013, pp 1–8. https://doi.org/10.1016/j.msec.2012.09.003. [3] C. Moya, M. D. P.Morales, X. Batlle, A. Labarta, Phys. Chem. Chem. Phys. 17, 13143 (2015). [4] C. Moya, G. Salas, M. D. P. Morales, et al. J. Mater. Chem. C 3, 4522 (2015). [5] C. Moya, A. Fraile Rodríguez, M. Escoda-Torroella, et al., J. Phys Chem. C (2020) (in press). https://dx.doi.org/10.1021/acs.jpcc.0c06657.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/176a07ed0dc34fb44fc96344b80c0136.jpg)
Fig. 1. Monodisperse samples of 8 nm NPs with a tunable amount of structural defects prepared by thermal decomposition of Fe(III) and Co(II) acetylacetonates with a variable concentration of 1,2-hexadecanediol. HRTEM images and element-specific XMCD loops of the Co2+ cations corresponding to highly defective (a, c) and highly crystalline nanoparticles (b, d).

 Crucial Role of the Co Cations on the Destabilization of the Ferrimagnetic Alignment in Co-Ferrite Nanoparticles with Tunable Structural Defects

Iron oxide nanoparticles (NPs) have attracted a great attention for biomedical and environmental applications due to their good biocompatibility and magnetic performance. The control over the crystal structure and oxidation state are of paramount importance to optimize their properties [1, 2]. In this work we studied the effect of the amount of both 1,2-hexadecanediol and the solvent 1-octadecene on the thermal decomposition method with iron (III) acetylacetonate. On the one hand, a higher amount of one of the reagents gives rise to single-phase, single-crystal Fe3O4 NPs with diameters below 10 nm and narrow size distributions but with moderate reaction yields. On the other hand, low amounts of either of the two reagents result in large NPs containing both Fe3O4 and FeO phases but with high values of the reaction yield. Consequently, the samples exhibited two distinct magnetic behaviors depending on the amount of these two reagents. The hysteresis loops at room temperature for the small NPs showed the typical features of superparamagnetism: values of the saturation magnetization close to the bulk one for magnetite with no coercive field. On the contrary, larger NPs showed ferrimagnetic behavior with reduced values of the saturation magnetization, as well as shifted hysteresis loops at 5 K after field cooling the sample at 1 T. Zero-field cooling-field cooling (ZFC-FC) curves below 200 K for the small NPs exhibited a peak below room temperature corresponding to the blocking temperature, while those curves for the larger particles displayed two peaks at higher temperatures which can be associated with the Verwey and Neel transitions of magnetite and wüstite phases, respectively. This shows the biphasic nature of the large NPs. With this accurate monitoring of the reaction conditions we have added an extra level of optimization to the synthesis of these NPs. In particular, we have found that, for 1 mmol of iron (III) acetylacetonate, the minimum amounts of 1,2-hexadecanediol and 1-octadecene for the preparation of monophasic, single-crystale Fe3O4 NPs are 2.5 mmol and 5 mL, respectively. This allows us to tune the properties of each sample of iron oxide NPs to its specific application. [3] Acknowledgements The work was supported by Spanish MCIU and AEI (MAT2015-68772-P; PGC2018-097789-B-I00) and European Union FEDER funds. M.E-T. acknowledge Spanish MCIU for BES-2016-077527.

**References:**

[1] A. Fraile Rodríguez, C. Moya, M. Escoda-Torroella, A. Romero, A. Labarta and X. Batlle. J. Mater. Chem. C, 6, 4, 875–882 (2018). [2] C. Moya, M. P. Morales, X. Batlle and A. Labarta. Phys. Chem. Chem. Phys., 17, 19, 13143–13149 (2015). [3] M. Escoda-Torroella, C. Moya, A. Fraile Rodríguez, X. Batlle and A. Labarta. Langmuir (2020) doi.org/10.1021/acs.langmuir.0c02221

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/790431501f371d0ebef8ec3cb20fc3ab.jpg)
Colored HRTEM images of two samples synthetized with increasing amounts of 1,2-hexadecanediol and 1-octadecence and their respective magnetization curves, showing the correlation between low amount of both reagents and a defective crystal structure and a poorer magnetic behavior.

 Selective Control of Reagents as a Tuning Knob for Iron Oxide Nanoparticles with Controlled Morphology, Oxidation State and Magnetic Response

The heating ability of magnetic nanoparticles when subjected to alternating magnetic fields is a topic of interest for biomedical applications, as in the case of magnetic hyperthermia, and more recently in the area of induction heating in heterogeneous catalysis [1-2]. In particular, AC magnetometry stands out as a powerful technique to study the dynamic hysteresis loops of the nanoparticles in the desired medium and to quantify their heating efficiency, which is proportional to the area of the cycles. When properly calibrated, this device provides more accurate and reproducible measurements than the most widespread calorimetric method, giving also information about the magnetic properties of the system such as the coercive field, magnetization saturation and susceptibility. The study of the dynamic behaviour of systems of magnetic nanoparticles under radiofrequency fields is still being investigated both experimentally and theoretically [3-5], in particular the influence of nanoparticle dipolar interactions and chain formation, due to the huge increase in the heating efficiency reported in the literature. In this work, we perform an experimental systematic study of three samples of magnetite with sizes of 12 nm, 30 nm and 50 nm as a function of the applied field, the initial temperature of the sample and of the acquisition time. Time-dependent high-frequency hysteresis loop measurements are presented, showing that, in some cases, the hysteresis strongly depends on the acquisition time of the cycle, effect that can be associated to the chain formation. In the case of the 30 nm sample, there is a strong dependence of the hysteresis cycle shape with the acquisition time, increasing the area as well as the susceptibility as the acquisition time increases; whereas the 50 nm nanoparticles show always the same hysteresis cycles independently of the acquisition time (see Fig. 1). This effect is associated to the time required for the chain formation. Similar results have been already observed by Asensio et al., in iron carbide nanoparticles [1].

**References:**

[1] J.M. Asensio, et al., Nanoscale., Vol. 11(12), p.5402-5411 (2019) [2] B. Rivas-Murias, et al., Angewandte Chemie., Vol. 132(36), p.15667-15672 (2020) [3] I. Morales, et al., Nanomaterials., Vol. 8(12), p.970 (2018) [4] I. Rodrigo, I. Castellanos-Rubio, E. Garaio, et al., International Journal of Hyperthermia., Vol. 31(1), p.976-991 (2020) [5] D.P. Valdés, E. Lima Jr, R.D. Zysler, E. De Biasi, Physical Review Applied., Vol. 14, p.014-023 (2020)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/2f6401c65514349ed6ceb4b1826c577f.jpg)
Fig. 1. Evolution of the cycles as a function of the acquisition time between 5 ms and 10 s, for an applied field of 27 mT and 50.3 kHz.

 Time-Dependent AC Magnetometry of Magnetite Nanoparticles in the Monodomain-Multidomain Limit

Over the past decade, magnetic field-induced self-assembly of magnetic inorganic nanoparticles has been studied extensively for particles having different sizes and compositions. However, relatively little attention has been devoted to how the surface chemistry of polymer nanoparticles affects their self-assembly properties. Self-assembly of nanoparticles becomes more important because novel collective and exceptional properties can be produced in the ordered array of nanoparticles [1-3]. More recently, methods using external fields such as magnetic fields have emerged as key methods to direct the assembly of inorganic nanoparticles effectively [4, 5]. In this work, we present the first direct observation of polymer nanoparticle chain formation in a magnetic field based on temperature induced surface properties of the PNIPAM layer. The colloidally stable PNIPAM coated Iron Oxide Nanoparticles were successfully synthesized by using 7-nm magnetic iron oxide nanoparticles embedded in a PNIPAM-g-PGMA shell. This temperature responsive polymer (PNIPAM-g-PGMA) was synthesized by a simple polymerization reaction at 80 oC involving the anchoring of poly(N-isopropylacrylamide) (PNIPAM) chains onto the surface of poly(glycidyl methacrylate) (PGMA). The PGMA was used because it has been explicitly shown to have the epoxide functional groups groups [6] located in the loops and tails of the core macromolecule. This work was concerned with the capability of thermoresponsive polymer nanoparticles forming chains in an external magnetic field at 25 oC and 45 oC. Chain length and width information was measured using SEM and TEM. The time-series experiment performed enabled the measurement of proton transverse relaxation rates with the magnitude of the fractional reduction in R2 being observed to decrease as the concentration of iron in polymer nanoparticles decreased for both temperatures. At 45 oC, we obtained more nanosized chain-like structures of PNIPAM-g-PGMA-NPs, which contain wider and longer chain-like substructures, owing to the hydrophobic properties of PNIPAM chain as shown in Fig. 1 (c). It is important to note that the magnetic properties of all the synthesized nanoparticles presented as superparamagnetic even after the surface was coated with PNIPAM- PGMA. The relevance of this work is that many applications of PNIPAM- PGMA coated magnetic nanoparticles are likely to involve the particles being exposed to strong magnetic fields. As such, a knowledge of the rates of chain formation and sizes of chains could be critical to predicting the behaviour of the flow of the particles in the blood stream, for example.

**References:**

[1] A.T. Ngo, M.P. Pileni, Nanoparticles of cobalt ferrite: Influence of the applied field on the organization of the nanocrystals on a substrate and on their magnetic properties, Advanced Materials, 12 (2000) 276-279. [2] S.H. Sun, C.B. Murray, Synthesis of monodisperse cobalt nanocrystals and their assembly into magnetic superlattices Journal of Applied Physics, 85 (1999) 4325-4330. [3] S. Wirth, S. von Molnar, M. Field, D.D. Awschalom, Magnetism of nanometer-scale iron particles arrays Journal of Applied Physics, 85 (1999) 5249-5254. [4] L.G. Paterno, E.H.C.P. Sinnecker, M.A.G. Soler, J.P. Sinnecker, M.A. Novak, P.C. Morais, Tuning of magnetic dipolar interactions of maghemite nanoparticles embedded in polyelectrolyte layer-by-layer films, Journal of Nanoscience and Nanotechnology, 12 (2012) 6672-6678. [5] C. Hoffmann, E. Mazari, S. Lallet, R.L. Borgne, V. Marchi, C. Gosse, Z. Gueroui, Spatiotemporal control of microtubule nucleation and assembly using magnetic nanoparticles, Nature Nanotechnology, 8 (2013) 199-205. [6] N.I. Taib, Vipul Agarwal, Nicole M. Smith, Robert C. Woodward, Timothy G. St Pierre, and K. Swaminathan Iyer. , Direct correlation of PNIPAM thermal transition and magnetic resonance relaxation of iron oxide nanoparticles., Materials Chemistry Frontiers 1(2017) 2335-2340.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/eb66df8daf98544b300a29e7409203cd.jpg)
Fig. 1. TEM micrograph of the nanoparticles: (a) PNIPAM-PGMA-NPs and (d) PGMA-NPs in the absence of external magnetic field at 25 oC. Chain-like structures of PNIPAM-PGMA-NPs and PGMA-NPs at 25 oC (b and e) and 45 oC (c and f) in the presence of external magnetic field. Insets showing the details of the chain-like structures of nanoparticles. Scale bar: (a) 2 µm, (b) 5 µm, (c), (d), (e) and (f) 10 µm, (insets: (b), (e) and (f) 2 µm; (c) 5 µm).

 Chain Formation of PNIPAM coated Magnetic Nanoparticles in an External Magnetic Field and the Effect of Temperature

Bottom-up fabrication of magnetic nanostructures composed of particles with desired sizes and composition can result in significant magnetic properties desired for practical applications in high performing magnetics, spintronics, and magnetic sensors [1, 2]. Magnetic and magneto-transport properties are strongly size-dependent, and the ability to adjust nanoparticle size leads to a high control on nanostructures' behavior [3, 4]. Generation of nanoparticles by aerosol methods provides great control of particle formation, size and deposition [5, 6]. Here, we present a novel bottom-up approach for generating self-assembled magnetic nanostructures [7]. The method uses an aerosol technique based on spark ablation to produce nanoparticles (NPs) with tunable compositions guided onto any substrate of choice using combined electric and magnetic fields. Using external magnetic fields, magnetic NPs can be assembled one-by-one along any direction to form 1, 2, or 3-D structures, see Fig. 1. In these systems, both shape and magneto-crystalline contributions play a key role in the magnetic anisotropy resulting in enhanced magnetic properties such as higher coercivity [8]. The presented technique provides a new approach for self-assembling of 1, 2, and 3-D magnetic nanostructures with building blocks that can be tailored for a specific application. Fig. 1 (a-c) shows the magnetic-field-induced self-assembly of vertical and in-plane structures. The magnetic field used to control the self-assembly also aligns the easy axis of the NPs, resulting in a higher anisotropy compared to a chain with randomly oriented particles. Fig. 1 (d) shows the possibility of particle size modulation, which is similar to diameter modulation in magnetic nanowires [9]. As illustrated in Fig. 1 (e), these structures can be directly integrated on the suitable substrates for, e.g., transport measurements or practical applications, which suggests a great advantage over the common solution-based methods [10]. Furthermore, the particle generation technique can be used to generate a vast number of material systems that can be used as building blocks. We have produced nanochains (NCs) composed of Ni, Fe, Co [7, 11], and as shown in Fig. 1 (f, g), CoPd (alloy) and CoAu (segmented/1D granular) structures. By tuning the segmented structures or chemical composition of alloys, a variety of systems can be achieved with possible applications in magnetism and spintronics [5]. Fig. 2 shows the NCs transferred to a carbon-supported copper grid where they were imaged using SEM (a & d) and by Scanning Transmission X-ray Microscopy (STXM) using circular polarized X-rays. The measurements were performed at the PolLux beamline at the Swiss light source (SLS) [12]. X-ray magnetic circular dichroism (XMCD) provides magnetic contrast (b-c & e-f) with a resolution approaching that of a single particle when combined with SEM images from the same NC. Applying a strong ex-situ magnetic field produces a mostly homogeneous remanent magnetization along the NCs with only minor indications of domain formation. Applying an in-situ field of -140 mT resulted in that the XMCD contrast of several more regions along the NC turned bright. These results demonstrate that an ex-situ field can produce a strong remanent magnetization and that the weak in-situ fields can be used to switch the magnetization in some regions. It is argued that these observations are due to larger multi-domain particles that behave as nucleation centers to initiate domain formation. As shown in Fig. 2 (g), micromagnetic simulations on NCs show that the magnetization is energetically favorable along the long axis of the chains and in the same direction (single domain). However, when there is a particle with a diameter larger than the single domain size, the NC breaks up into two domains. Magneto-transport in these structures is of interest since the magnetized and demagnetized states of the chains can result in a magnetoresistance response, and also, the segmented chains composed of different materials (Such as Co/Au in Fig. 1) can act as 1-D magnetic multilayers. To investigate the magneto-transport in these systems, NCs are self-assembled directly onto the surface of a Si-chip insulated with SiO2 and pre-patterned with aligned markers. Contacts to the NCs are defined via electron beam lithography and thermal evaporation, and magneto-transport properties along with micromagnetic simulations will be discussed.

**References:**

[1] H. Wang, Y. Yu, Y. Sun, Nano, Vol. 6, p. 1-17 (2011) [2] D. Lisjak, A. J. P. i. M. S. Mertelj, Progress in Materials Science, Vol. 95, p. 286-328 (2018) [3] J. Mohapatra, M. Xing, J. Elkins, Journal of Physics D, Vol. 53, p. 504004 (2020) [4] Z. Xiang, Y Song, D Pan, Journal of Alloys and Compounds, Vol. 744, p. 432-437 (2018) [5] C. Preger, C. Bulbucan, B. O. Meuller, The Journal of Physical Chemistry C, Vol. 123, p. 16083-16090 (2019) [6] J. Fang, , L Schlag, SC Park, Advanced Materials, Vol. 28, p. 1770-1779 (2016) [7] C. Preger, M. Josefsson, R. Westerström, M. E. Messing, Nanotechnology (Submitted Article) (2021) [8] S. Singamaneni, V. N. Bliznyuk, C. Binek, Journal of Materials Chemistry, Vol. 21, p. 16819-16845 (2011) [9] L. A. Rodríguez, C. Bran, D Reyes," ACS nano, Vol. 10, p. 9669-9678 (2016) [10] X. Chen, Z. Zhou, H. E. Karahan, Small, Vol. 14, p. 1801929 (2018) [11] A. Schmidt-Ott, Spark Ablation: Building Blocks for Nanotechnology. CRC Press (2019) [12] J. Raabe, G Tzvetkov, U Flechsig, Review of scientific instruments, Vol. 79, p. 113704 (2008)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/c295a21c34ef6fdff44b2922c6d85a95.jpg)
Fig. 1. SEM image of a single vertical nanochain (NC) (a), 2-D chains along the surface of the substrate (b), and 3-D self-assembly of Co nanoparticles (c). Nanochains with particle-size modulation (d), direct integration of NCs on a Chip (e). EDS (energy-dispersive X-ray spectroscopy) mapping of a CoPd alloy (f) and CoAu segmented (g) nanochain.

![](https://assets.underline.io/uploads/markdown_image/1/image/e10ee3f10149d6228d9b0e2b9546b632.jpg)
Fig. 2. SEM image of a nanochain (a) and corresponding XMCD contrast in a field of +140 mT (b) and -140 mT (c). SEM (d) and remanent XMCD contrast after magnetizing ex-situ in +1.5 T (e). XMCD contrast after applying an in-situ field of -140 mT (f). Micromagnetic simulation of a chain composed of magnetic single-domain particles and a chain including a particle larger than the single-domain size.

 Engineered Self-assembled Magnetic Nanochains INVITED

Cylindrical magnetic nanostructures are attracting much interest in a wide range of applications (biomedical imaging, drug delivery, 3D memory storage devices, sensors, etc.). The ability to tune their magnetic properties by changing the geometry (disks, dots, wires, tubes), combined with low-cost and high-yield template fabrication methods, makes these nanostructures highly attractive for implementation in emerging technologies. However, most applications require a full understanding of the magnetization reversal mechanisms. Although it is a well-known topic, thoroughly studied for the past two decades, recent advances in characterization and micromagnetic simulation techniques renewed the attention of the scientific community towards the complete understanding of such magnetic 3D nano-elements [Bran2016, Fernández-Pacheco2017, Charilaou2018, Ruiz-Gómez2020]. In particular, Ni nanowires have been the most reported nanostructures fabricated in nanoporous alumina templates so far [Sousa2014, Proenca2012]. But, nanodisks and low-aspect ratio nanowires are still less explored, particularly regarding the full understanding of the magnetization reversal mechanisms. In this work, we have analyzed the magnetization response of cylindrical Ni nanostructures with 15 to 100 nm in diameter and 12 to 230 nm in length and compared our results with experimental data for periodic arrays. Our simulations show the presence of a critical diameter (d ≈ 30 nm) that determines the type of domain wall nucleated (transverse or vortex/skyrmion tube); and a critical length (L ≈ 100 nm) that limits the number of domain walls formed (one or two). By adjusting the diameter/length of the Ni nanocylinders, different magnetic remanent states and reversal mechanisms were observed (Figure 1). Moreover, vortex DWs were found to originate from 3D skyrmion tubes, which have been reported as one of the best configurations for magnetic storage devices. By increasing the diameter and aspect ratio (L/d) of nanowires with L &gt; 100 nm, three distinct reversal modes were observed: i) simultaneous propagation of two vortex DWs with opposite chirality; ii) propagation of only one vortex DW, or iii) spiral rotation of both DWs through the “corkscrew” mechanism (Figure 2). Only for very low aspect ratio structures (nanodisks), no skyrmion tubes were observed and reversal occurred by spiral rotation of one vortex DW. The broad range of dimensions studied in this work allowed the creation of the first complete magnetic phase diagram in cylindrical nanostructures [Proenca2021]. This universal phase diagram will be highly important for the future choice of nanoscaled dimensions in the development of novel nano-devices. This work has been supported by the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie Grant Agreement No. 734801; by the Portuguese Fundação para a Ciência e Tecnologia (FCT) through the Investigador FCT program (Contract No. IF/01159/2015); by FCT and COMPETE 2020 (FEDER) under the projects POCI-01-0145-FEDER-028676, POCI-01-0145-FEDER-031302, POCI-01-0145-FEDER-032527, POCI-01-0145-FEDER-032257 and UIDB/04968/2020; and by the Spanish Ministerio de Ciencia e Innovación under the project MAT2017-87072-C4-3-P.

**References:**

C. Bran, E. Berganza, E. M. Palmero, et al., J. Mater. Chem. C, Vol. 4, p.978 (2016) A. Fernández-Pacheco, R. Streubel, O. Fruchart, et al., Nat. Commun., Vol. 8, p15756 (2017) M. Charilaou, H. B. Braun and J. F. Löffler, Phys. Rev. Lett., Vol. 121, p.97202 (2018) S. Ruiz-Gómez, C. Fernández-González, E. Martínez, et al., Nanoscale, Vol. 12, p.17880 (2020) C. T. Sousa, D. C. Leitao, M. P. Proenca, et al., Appl. Phys. Rev., Vol. 1, p.031102 (2014) M. P. Proenca, C. T. Sousa, J. Ventura, et al., Electrochim. Acta, Vol. 72, p.215 (2012) M. P. Proenca, J. Rial, J. P. Araujo and C. T. Sousa, Magnetic reversal modes in cylindrical nanostructures: from disks to wires, submitted (2021)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/224d41da086e562fd530d5cd1dca0bff.jpg)
Figure 1: Phase diagram of the magnetization reversal modes in individual Ni nanocylinders with different lengths and diameters, when applying the magnetic field along the nanocylinder’s length (parallel direction). Insets show selected cross-sectional representations of the magnetic configuration during reversal.

![](https://assets.underline.io/uploads/markdown_image/1/image/458fea4fee1b4edbee176c880e9b95e1.png)
Figure 2: 3D representation of the magnetic configuration during reversal of the simulated Ni nanowire with 100 nm in diameter and 130 nm in length illustrating the spiral rotation of two vortex domain walls (skyrmion tubes) through the “corkscrew” mechanism when applying the magnetic field along the z-direction (parallel to the nanowires’ long axis).

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