Multi-segmented nanowire (NW) arrays are strong candidates to improve the memory storage capacity due to their lower magnetic bit area [1]. But writing/reading information on such devices is still a challenge. Recently, micromagnetic simulations revealed that the integration of a soft magnetic segment (with lower coercivity) along the racetrack could ease the writing process by using external magnetic fields [2]. Here, we demonstrate the feasibility of using NiCu/FeCo magnetic NWs as in-line writing components in 3D racetrack memory devices. Using template-assisted DC electrodeposition [3-5], 4 samples of NW hexagonal arrays (50 nm-diameter, 100 nm of interwire distance) were produced: i) Ni90Cu10; ii) Fe20Co80; iii) bi-segmented NiCu/FeCo; and iv) multi-segmented NiCu/[FeCo/Au]3. Morphological, chemical and structural characterization revealed the formation of single, bi- and multi-segmented NW arrays with tuned dimensions and uniform compositions (Fig. 1). Magnetic hysteresis loops, ferromagnetic resonance (FMR) measurements and first-order reversal curve (FORC) diagram analysis confirmed the hard and soft magnetic properties of the FeCo and NiCu single arrays, respectively. In the interacting NiCu/FeCo bi- and multi-segmented systems, a significant reduction of the parallel coercivity occurred, while keeping the magnetization easy axis lying parallel to the NWs' axis. FORC diagrams also demonstrated a two-step reversal process in systems (Fig. 2), illustrating a FORC fingerprint typically present in soft/hard magnetic interacting bi-layered NWs [6]. Micromagnetic simulations confirmed such a two-step reversal process, revealing the presence of strong interactions at the soft/hard interface [7]. This work was supported by projects POCI-01-0145-FEDER-028676, POCI-01-0145-FEDER-031302, UIDB/04968/2020, EXPL/IF/00541/2015 and contract IF/01159/2015 from Portuguese FCT and COMPETE 2020 (FEDER); grant 02.2020/0261 from the National Research Foundation of Ukraine; and project PID2020-117024GB-C42 from the Spanish Ministerio de Ciencia e Innovacíon.

![](https://assets.underline.io/uploads/markdown_image/1/image/cd280bc8126f81abd395a71d877c76eb.jpg)

Fig. 1. Scanning electron microscopy images of the electrodeposited multi-segmented NWs.

![](https://assets.underline.io/uploads/markdown_image/1/image/22038b2a0a7bf508db8232151a5ff998.jpg)

Fig. 2. FORC diagram of the multi-segmented sample along the parallel direction.

Feasibility of Using NiCu/FeCo Coupled Bi-segmented Nanowires as In-line Writing Heads for 3D Racetrack Storage Devices

Novel magnetic nanostructures (MNS) present a unique spin arrangement in the magnetic ground state, namely spin-vortex or synthetic antiferromagnetic state. MNS showed promising results in cell separation, as a contrast enhancing agents in MRI and in magneto-mechanically induced cell annihilation. The main advantages of magneto-mechanically induced cell annihilation are the usage of weaker magnetic fields with lower frequencies, as well as the need for a lower concentration of particles [1,2]. In this work, we developed one subset of biocompatible magnetic nanostructures that exhibit a spin-vortex state. First, micromagnetic simulations, using mumax3 of sub-micron iron discs, were performed for different interdot distance and aspect-ratio (thickness/diameter). By analysing the nucleation and annihilation fields, as well as the magnetic susceptibility, it was found that the (ideal) discs could be considered as isolated for interdot distances greater than twice the radius of the disc (2R) [3]. We also found that discs with an aspect ratio between 5 and 15 should sustain the vortex state in remanence [4]. Iron nano-discs in a vortex state, with a diameter of about 500 nm, were fabricated by electron beam evaporation on a Si substrate pre-patterned by interference lithography. Then, the magnetic vortex nano-discs were released from the substrate by chemical etching of a sacrificial layer. Subsequently, cell viability and uptake assays were performed in a human leukaemia monocyte cell line (THP-1) [5]. Several concentrations of nano-discs were studied by flow cytometry. As a result, the discs were internalized by the cells and found to be innocuous to them, in the absence of an external magnetic field.

Vortex nano-discs: from micromagnetic simulations to cancer cells internalization for magneto-mechanically induced damage applications.

The advances in synthesis methods and the research of new magnetic effects have been the driving forces propelling the use of magnetic nanostructures in several research fields. In biomedical applications, magnetic nanoparticles, usually in the superparamagnetic state, are mainly used in magnetic fluid hyperthermia (MFH) and as contrast agents. In contrast, the novel magnetic nanostructures (MNS) studied in this work present a unique spin arrangement in the magnetic ground state, namely spin-vortex or synthetic antiferromagnetic state. MNS are mainly produced using top-down lithography techniques and physical vapour deposition methods. They are not spherical, rather disc-shaped, like coins with nanometer dimensions. MNS showed promising results in cell separation, as a contrast enhancing agents in MRI and in magneto-mechanically induced cell annihilation. Magneto-mechanically induced cell annihilation is a comparable technique with MFH for cancer therapy, where instead of superparamagnetic particles, micro/nano-discs in the spin-vortex state or synthetic antiferromagnetic nanostructures are employed. The main advantages of this novel approach are the usage of weaker magnetic fields with lower frequencies, as well as the need for a lower concentration of particles. This scenario opens new possibilities in cancer therapy, having triggered a scientific interest for MNS with unique spin configurations suitable for biomedical applications [1]. In this work, we developed one subset of biocompatible magnetic nanostructures that exhibit a spin-vortex state with interest in analysing their application in magneto-mechanically induced cell death [2]. First, micromagnetic simulations, using mumax3 of sub-micron iron discs, were performed for different interdot distance and aspect-ratio (thickness/diameter), to better understand the magnetic behaviour of these nanostructures. Two sets of samples were studied: ideal circular discs and disc-shaped nanostructures (based on images of real samples). By analysing the nucleation and annihilation fields, as well as the magnetic susceptibility, it was found that the (ideal) discs could be considered as isolated for interdot distances greater than twice the radius of the disc (2R) [3]. We also found that discs with an aspect ratio between 5 and 15 should sustain the vortex state in remanence. Iron nano-discs, with a diameter of about 500 nm, were fabricated by electron beam evaporation on a Si substrate pre-patterned by interference lithography [4]. The discs, protected by bottom and top gold layers, were fully characterized with scanning electron microscopy (SEM), X-ray diffraction (XRD), superconducting quantum interference device (SQUID), magneto-optic Kerr effect (MOKE) and ferromagnetic resonance (FMR) techniques. The synthesized nanostructures showed the desired vortex state configuration. The obtained magnetic measurements are in good agreement with the micromagnetic simulations (Fig. 1). Then, the magnetic vortex nano-discs were released from the substrate by chemical etching of a sacrificial layer. Subsequently, cell viability and uptake assays were performed in a human leukaemia monocyte cell line (THP-1). Several concentrations of nano-discs were studied by flow cytometry. As a result, the discs were internalized by the cells and found to be innocuous to them, in the absence of an external magnetic field. Work supported by Horizon 2020 MSCA grant agreement No 734801, AEI PID2019-104604RB, IT1162-19, IF/01159/2015, SFRH/BD/148563/2019, POCI-01-0145-FEDER-028676/PTDC/CTM-CTM/28676/2017 and POCI-01-0145-FEDER-031302/PTDC/FIS-MAC/31302/2017.

**References:**

[1] L. Peixoto, R. Magalhães, D. Navas, S. Moraes, C. Redondo, R. Morales, J. P. Araújo, C.T. Sousa, Magnetic nanostructures for emerging biomedical applications. Appl. Phys. Rev., 7 (2020) 011310., [2] Y. Cheng, M. E. Muroski, D.C.M.C. Petit, R. Mansell, T. Vemulkar, R. A. Morshed, Y. Hana, I. V. Balyasnikova, C.M. Horbinski, X.Huang, L. Zhangc, R. P. Cowburn, M. S. Lesniak, J. Control. Release 223 (2016) 75. [3] L. Peixoto, C. Sousa, D. Navas, and J. P. Araújo, Micromagnetic study of the vortex state in sub-micron iron discs. EPJ Web of Conferences 233 (2020) 05002. [4] B. Mora, A. Perez-Valle, C. Redondo, M.D. Boyano, R. Morales, ACS Appl. Mater. Interfaces 10 (2018) 8165.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/68db60cda0b5971db06d25b5e8aea1a8.jpg)

 Vortex nano-discs: from micromagnetic simulations to cancer cells internalization for magneto-mechanically induced damage applications.

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).

 Phase Diagram of Magnetization Reversal Modes in Cylindrical Ni Nanowires

Cancer is the leading cause of death in Europe after cardiovascular disease, accounting for about 20% of deaths on that continent [1]. In recent years, hyperthermia emerged as a promising treatment approach in oncology consisting in raising the temperature of cancer cells to 40-45°C to reach apoptosis i.e., programmed cell death. Differences in the vasculature of tumors and healthy tissues leads to different heat-induced damage responses. At temperatures up to 45°, the vasculature of healthy tissues allows an effective heating dissipation, while the tumor’s lumpy and disorganized ones makes them less efficient in heat dissipation. Therefore, for the same thermal dose, the healthy tissue adjacent to the tumor can remain at lower temperatures than the tumor itself [2]. One way to reach local and controlled hyperthermia is via functionalizable nanostructures that are activated by external stimuli, such as magnetic fields or electromagnetic radiation. In this context, gold nanostructures (Au-NS) have been the subject of much attention in the academic and clinical environment, due to their biocompatibility and high absorption of electromagnetic radiation in the near-infrared range caused by its surface plasmon resonance [3]. In parallel, magnetic nanostructures based on iron (Fe-NS) have also been gaining attention, since they can combine the diagnostic (as contrast agents for magnetic resonance imaging) and therapy (magnetic hyperthermia) properties [4]. Although the Fe-oxide NS are the most reported in the literature, Fe-NS are a promising alternative. The high magnetic moment of Fe-NS can improve the heat dissipation phenomena produced by the magnetic hysteresis, due to the irreversible magnetization/ demagnetization processes induced by an applied alternating magnetic field [5]. The main goal of this work is to combine the “best of both worlds” producing multifunctional Iron-Gold nanostructures (Fe-Au-NS) with high heating performance when stimulated with radiation (500-1000nm) and with alternating magnetic fields, for applications in controlled and localized hyperthermia. Fe-Au-NS were produced by two methods: 1) through the ablation of Iron and Gold targets with a femtosecond pulsed laser in liquids (such as ethanol) and 2) by electrodeposition in self-organized alumina matrices. The first technique is particularly interesting for the development of NS-Fe-Au with complex structures such as the core-shell structure [4]. In turn, the second technique allows a controlled growth of nanowires, nanotubes, and more complex morphologies such as segmented Fe-Au-Fe nanowires [6]. The Fe-Au-NS have been fully characterized with scanning electron microscopy (SEM), X-ray diffraction (XRD) and superconducting quantum interference device (SQUID) techniques. The morphological analysis showed a narrow distribution in diameter and length of the obtained structures with improved robustness and high yield, making these techniques versatile approaches strongly compatible with large scale production. Finally, we show the possibility to tune accurately the size of the nanostructures and consequently provide an easy control over the magnetic properties of these nanostructures ultimately enabling reaching the superparamagnetic regime.

**References:**

[1] World Health Organization, Regional office for Europe: http://www.euro.who.int/en/health-topics/noncommunicable-diseases/cancer/data-and-statistics [2] A. Espinosa, et al, “Magnetic (Hyper)Thermia or Photothermia? Progressive Comparison of Iron Oxide and Gold Nanoparticles Heating in Water, in Cells, and In Vivo”, Advanced Functional Materials, 28, 37 (2018) [3] Huang, et al “Cancer Cell Imaging and Photothermal Therapy in the Near-Infrared Region by Using Gold Nanorods”, Journal of American Chemical Society, 128, 2115-2120 (2004) [4] P. Wagener, et al “Solvent-surface interactions control the phase structure in laser generated iron-gold core-shell nanoparticles”, Scientific Reports, 6, 23352 (2016) [5] L. Peixoto, et al, "Magnetic nanostructures for emerging biomedical applications", Applied Physics Reviews, 7 (2020) [6] C. Sousa, et al, “Nanoporous alumina as templates for multifunctional applications” Applied Physics Reviews, 1, 031102 (2014)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/3c5f2bfac9400b165970e45f4430476c.jpg)
Fig 1. Fe-NPs synthetized via ablation of an Iron target by femtosecond laser in ethanol, in the presence of magnetic field applied post LAL

![](https://assets.underline.io/uploads/markdown_image/1/image/54f1dde19192a42c34f083352fd223ff.jpg)
Fig 2. Laser Ablation in Liquids Set-up

 Multifunctional Fe-Au nanostructures for biomedical applications

The study of 1D-nanostructures has been intensified in the last few decades due to the development of fabrication methods of metallic, semiconductors and insulator materials aiming applications in several areas [1]. In particular, multi-segmented magnetic nanowire (NW) arrays are prospective candidates for an extensive range of applications such as racetrack memory devices, spintronic, and drug delivery [1, 2, 3]. These possibilities are due to the number of degrees of freedom related to the intrinsic shape anisotropy combined with the potential to introduce different materials along the NW’s length. Concerning practical applications, understanding the role of layers with distinct structural and magnetic properties can be a path to avoid some issues such as system overheating and Walker breakdown in racetrack devices. For instance, micromagnetic simulations revealed that the combination of Co segments with different structures, bcc and hcp, promotes a faster way of writing information in 3D racetrack memory devices [2]. This observation results from the high magnetocrystalline anisotropy along the wire length induced by the Co-hcp structure. Although these evaluations seem promising, thorough experimental research of multi-segmented NWs with low and high anisotropy components still needs to be performed. Here, we present the FeCo layer properties’ evaluation onto multi-segmented NW arrays produced through DC electrodeposition into ordered porous alumina templates with 40 nm of pore diameter and 60 µm of length. For each layer’s deposition, a Pt electrode was used as the counter electrode, Ag/AgCl as the reference electrode, and an Au layer as the working electrode [5]. Two samples were prepared: 1) Au [200 nm] / Fe33Co67 [24 µm] / Au [10 nm] and 2) Au [200 nm] / Ni90Cu10 [1 µm] / (Fe33Co67 [13 µm] / Au [10 nm])x3, where the indicated length was obtained from scanning electron microscopy (SEM) images and the alloy’s stoichiometry from energy dispersive spectroscopy (EDS). X-ray diffraction (XRD) measurements at room temperature revealed the formation of bcc structure of FeCo with the (110) preferential orientation for both samples and an additional hcp phase with the (110) preferential orientation for FeCo tri-segmented NWs (Figure 1). The magnetic hysteresis loops acquired at room temperature for the Au/FeCo/Au arrays present the easy magnetization axis along the parallel direction, evidencing a typical strong shape anisotropy [5]. Distinctly, when three FeCo segments of 13 µm are deposited along the NW’s length, the arrays present an isotropic magnetic behavior with an increase of the coercive field when applying the magnetic field perpendicular to the nanowires’ axis (Figure 2). This change in magnetic behavior for the segmented NWs might be related to the presence of an hcp phase that induces a perpendicular magnetocrystalline anisotropy. However, further research should be performed to better comprehend the interplay between the NiCu and FeCo layers into the NW arrays. In summary, multi-segmented NWs of FeCo alloys were successfully achieved through the electrodeposition process into porous templates. The growth of FeCo segments leads to the formation of hcp structures and the observation of an isotropic magnetic behavior. This can be related not only to the structural properties but also to the interplay between FeCo segments along the NW’s length. A more complete evaluation of the magnetic properties of the produced samples should be performed to fully understand the role of the different layers in the magnetocrystalline anisotropy of the arrays. This work was financially supported by projects POCI-01-0145-FEDER-028676 and UIDB/04968/2020 from Portuguese FCT and COMPETE 2020 (FEDER) and IF/01159/2015, POCI-01-0145-FEDER-031302/PTDC/FIS-MAC/31302/2017. .

**References:**

References:: [1] C. T. Sousa et al., Appl. Phys. Rev., vol.1 , p. 031102 (2014); [2] J. Rial and M. P. Proenca, Nanomaterials, vol. 10, p. 2403 (2020); [3] D. Wang et al., Materials, vol. 12, p. 3908 (2019); [4] C. Bran et al., J. Phys. D: Appl. Phys., vol. 48, p. 145304 (2015); [5] E. M. Palmero et al., Nano Research, vol. 12, p. 1547 (2019).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/419fc130f5d8af612615b50ad0e953e0.jpg)
Figure 1: XRD patterns for Au/FeCo/Au and Au/NiCu/[FeCo/Au]3 segmented nanowires with the Bragg peak positions for bcc (green) and hcp (red) structures.

![](https://assets.underline.io/uploads/markdown_image/1/image/59bc97224bca21ea1066106778e1c21d.jpg)
Figure 2: Hysteresis loops obtained by applying the magnetic field perpendicular to the NWs’ length

Celia Sousa

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