United States

Arrays of magnetic nanowires (NWs) are an attractive platform with tremendous potential for developing novel magnetic applications, e.g., next-generation information technologies or sensors [1]. The synthesis of NWs by electrochemistry shows multiple degrees of freedom allowing for tailoring the composition, crystallographic structure and, consequently, magnetic properties [2,3].&lt;br/&gt;Herein, FeNi NWs were grown by electrodeposition into anodized aluminum oxide (AAO) membranes (Fig. 1a). The broad range of electrolytes and voltages used, allowed us to study the anomalous FeNi co-deposition by a compositional analysis, being explained within a modified Bocris-Drazic-Despic (BDD) model [4]. X-ray diffraction patterns showed a change from &lt;i&gt;bcc &lt;/i&gt;crystallographic structure for Fe-rich NWs to &lt;i&gt;fcc &lt;/i&gt;structure at mid and low Fe content, with biphasic &lt;i&gt;bcc&lt;/i&gt;-&lt;i&gt;fcc&lt;/i&gt; phase for equiatomic NWs.&lt;br/&gt;Tuning the synthesis conditions led to FeNi NWs with a range of magnetic hardness, observing an increase in coercivity (from 0.2 kOe to 1 kOe) for a decreased Fe content (Fig. 1b). Coercivity angular measurements showed that the magnetization reversal is dominated by transverse domain wall reversal and indicated a decrease in magnetostatic interactions among NWs as the source of the coercivity rise. First-order reversal curves (FORCs) highlighted the limited role of the magnetic interactions in the arrays, except for Fe&lt;sub&gt;0.80&lt;/sub&gt;Ni&lt;sub&gt;0.20&lt;/sub&gt; NWs which showed a FORC diagram for magnetically interacting NWs (Fig. 1c). This study shows how the magnetic properties of the NWs can be tailored through an optimized control of the synthesis conditions and adapted to the requirements of the final application.&lt;br/&gt;&lt;i&gt;Fig. 1: (a) SEM image of NWs grown inside the AAO template, (b) Hysteresis loops for FeNi NWs with different compositions, and (c) FORC diagram for Fe&lt;sub&gt;0.80&lt;/sub&gt;Ni&lt;sub&gt;0.20&lt;/sub&gt; NWs.&lt;/i&gt;&lt;br/&gt;Acknowledgements&lt;br/&gt;Authors acknowledge support from EU M-ERA.NET and MICINN through &lt;i&gt;COSMAG&lt;/i&gt; (PCI2020-112143) and &lt;i&gt;NEXUS &lt;/i&gt;(PID2020-11521RB-C21). A.J.C.-H. and E.M.P. acknowledge support from &quot;La Caixaâ€ Foundation (ID 100010434) through the Doctoral INPhINIT Incoming program (LCF/BQ/DI20/1178002) and AEI (JdC-I program, IJC2020-043011-I/MCIN/AEI/10.13039/501100011033) and EU by NextGenerationEU/PRTR, respectively.

MMM 2022

Analysis of the Magnetic Interactions in FeNi Nanowire Arrays through FORC and Angular Coercivity Measurements

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Arrays of magnetic nanowires (NWs) are an attractive platform with tremendous potential for developing novel magnetic applications, e.g., next-generation information technologies or sensors [1]. The synthesis of NWs by electrochemistry shows multiple degrees of freedom allowing for tailoring the composition, crystallographic structure and, consequently, magnetic properties [2,3]. Herein, FeNi NWs were grown by electrodeposition into anodized aluminum oxide (AAO) membranes (Fig. 1a). The broad range of electrolytes and voltages used, allowed us to study the anomalous FeNi co-deposition by a compositional analysis, being explained within a modified Bocris-Drazic-Despic (BDD) model [4]. X-ray diffraction patterns showed a change from bcc crystallographic structure for Fe-rich NWs to fcc structure at mid and low Fe content, with biphasic bcc-fcc phase for equiatomic NWs. Tuning the synthesis conditions led to FeNi NWs with a range of magnetic hardness, observing an increase in coercivity (from 0.2 kOe to 1 kOe) for a decreased Fe content (Fig. 1b). Coercivity angular measurements showed that the magnetization reversal is dominated by transverse domain wall reversal and indicated a decrease in magnetostatic interactions among NWs as the source of the coercivity rise. First-order reversal curves (FORCs) highlighted the limited role of the magnetic interactions in the arrays, except for Fe0.80Ni0.20 NWs which showed a FORC diagram for magnetically interacting NWs (Fig. 1c). This study shows how the magnetic properties of the NWs can be tailored through an optimized control of the synthesis conditions and adapted to the requirements of the final application. Fig. 1: (a) SEM image of NWs grown inside the AAO template, (b) Hysteresis loops for FeNi NWs with different compositions, and (c) FORC diagram for Fe0.80Ni0.20 NWs. Acknowledgements Authors acknowledge support from EU M-ERA.NET and MICINN through COSMAG (PCI2020-112143) and NEXUS (PID2020-11521RB-C21). A.J.C.-H. and E.M.P. acknowledge support from "La Caixaâ€ Foundation (ID 100010434) through the Doctoral INPhINIT Incoming program (LCF/BQ/DI20/1178002) and AEI (JdC-I program, IJC2020-043011-I/MCIN/AEI/10.13039/501100011033) and EU by NextGenerationEU/PRTR, respectively.

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All of the polymorphs of pure ammonium iodide NH4I have space groups that are non-polar. However, we identify that when a fraction of the hydrogen is replaced by deuterons or when small percentages of iodide are replaced by bromide or potassium, polar phases can be established. In particular, the electric polarization of the following compounds can be switched by an applied electric field: (NH4)0.67(ND4)0.33I (where D is deuteron), NH4I0.91Br0.09 and NH4I0.94K0.06 according to pyroelectric current measurements. Magnetic susceptibility measurements indicate that the polar phases may be magnetic in origin. We explain our results by taking into account the notion that every reorienting NH4+ possesses a molecular magnetic moment that can couple with that of neighboring NH4+. The magnetic moments arise from the protons (H+) having to orbit in concert about the central nitrogen atom when the ion reorients. In the room-temperature phases, enough orbits are available to avoid mutual resonance, but below a critical temperature, a subset of these orbits become energetically inaccessible so neighboring NH4+ become mutually resonant. To avoid lattice instabilities from resonant forces, the NH4+ become ordered to establish a spiral modulation of its magnetic moments. A replacement of a fraction of H with D or I with Br or K breaks the spatial-inversion symmetry of the modulating vector so an electric polarization emerges. Our findings provide the potential of adding an extra ferroelectric dimension to hydrogen-bonded materials. 

**References:** 
F. Yen, Z. Zheng and Z. Si, Angewandte Chemie, 129(44), 13863-13866 (2017) 
L. Meng, C. He and F. Yen, The Journal of Physical Chemistry C, 124, 17255−17261 (2020)

Inducing ferroelectricity in NH4I via doping of deuterons, bromide and potassium

Magnetic tunnel junctions are a key component in non-volatile memory technologies such as magnetic random access memory (MRAM). Typical two-terminal MRAM write speeds have been limited to the nanosecond regime. Currently, sub-nanosecond switching has been achieved in three-terminal structures relying on spin-orbit torque switching mechanisms. However, using this mechanism increases the footprint and number of transistors needed, ultimately limiting scalability. Here, we demonstrate that it is possible to achieve reliable sub-nanosecond switching in a two-terminal design by utilizing a Double Spin Torque magnetic tunnel junction. In this design, a second reference layer is added to the standard two terminal MTJ. Using a non-magnetic spacer, we are able to do so without sacrificing magnetoresistance like in double MTJ designs. Figure 1 shows the error rates for a single device, plotted using a normal quantile scale (using the absolute value of the standard deviation from the fifty-percent switching current density), measured at -40 C, 25 C, and 85 C, for pulse widths from 225 ps to 10 ns. We demonstrate 100% successful switching for 1e6 write attempts using 225 ps wide write pulses, over the temperature range -40 C to 85 C. Further, we show reliable sub-nanosecond switching across a few hundred devices. 1e10 write-endurance testing on a handful of devices also demonstrates that reliability can be achieved as well. Lastly, we compare our Double Spin Torque magnetic tunnel junction to three-terminal spin-orbit torque MRAM. We find that with our design, a 3-10X reduction in write power consumption can be achieved. 
**References**
 
![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/08c48e83869479112a6b8517871d5394.png)

Reliable Sub nanosecond MRAM with Double Spin

Ultrathin ferromagnetic strips, sandwiched between a heavy metal and an oxide, have been suggested as potential platforms for recording devices where the information is coded in perpendicular magnetized domains (up or down) separated domain walls (DWs). Due to the Dzyaloshinskii-Moriya interaction, such DWs depict a homochiral Néel configuration [1,2] and are displaced with high velocity (~500 m/s) by current pulses (J~1 TA/m2) [3]. However, the writing mechanism for such DW-based devices, that is, a controlled nucleation of domains and DWs, is still challenging. Nucleation is conventionally performed by injecting an electrical current pulse along an orthogonal conducting wire [1]. However, the resulting Oersted field that nucleates a new domain also annihilates an already shifted one unless an intricate device layout is used [4]. Although some ideas have been proposed to solve this issue [5,6], a simple nucleation scheme remains elusive.
Here we propose a simple geometrical design where both the nucleation and the shifting can be purely achieved by current pulses (Fig. 1(a)). A DW is initially located in the bottom-left arm of the two symmetrical arms close to its junction with the longitudinal strip (LS), where the bits will be coded and shifted. By applying a proper voltage pulse (V2 in Fig. 1(b)), the DW is pushed towards the LS, and split in two new DWs: one at the top-left arm and other entering in the LS. A second pulse along the same bottom-left arm (V2) will shift the new DW along the LS. If injected along the top-left arm (V1), another DW and domain will be nucleated whereas the former DW will be also driven along the LS. Based on this method, new domains (bits) and DWs can be simultaneously nucleated (written) and driven by alternatively applying voltage pulses between one of the two left arms and the LS of the structure. In the talk, we will show by means of realistic simulations the potential of this idea to perform the writing operation in DW-based recording devices. 
**References** [1] S. Emori et al. Nat. Mater. 12, 611 (2013).
[2] K.-S. Ryu et al. Nat. Nanotechnol. 8, 527 (2013).
[3] I. M. Miron et al. Nat. Mater. 10, 419 (2011).
[4] O. Alejos et al. Scientific Reports, 7, 11909 (2017).
[5] Z. Luo et al. Appl. Phys. Lett. 111, 162404 (2017).
[6] T. Phuong Dao et al. Nano Lett. 19, 9, 5930 (2019).
 
![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/685344f1d53b63eaa04ed5c1ea27dc65.png)

Purely current driven writing mechanism for domain wall

Efficient high power density electric drive motors require permanent magnets with high energy product, good thermal stability, and less critical materials. Nd2Fe14B-based sintered magnets are one of the primary candidates for these applications. However, Nd-Fe-B based magnets without heavy rare earth elements (HREE) have poor magnetic properties above 150°C, target operating temperature for most electric vehicles (EV). HREE, such as, Dy, have been added to the magnets to enhance the coercivity and improve their performance at elevated temperatures. The operating temperature of the magnets depends on the amount of Dy added. For the magnet to be operable at 200 oC, the amount of Dy required could exceed 10 wt.%. Unfortunately, Dy is a critical element. Therefore, the development of Dy-lean or free Nd-Fe-B magnets capable of high temperature operation becomes a great interest to the EV industry. It is well accepted that the coercivity of these magnets is controlled by the nucleation of reversal domains at locally low-anisotropy surfaces of the individual 2:14:1 grains. The coercivity is sensitive to grain boundary structure, grain boundary chemistry, and grain size. Engineering microstructure techniques to develop Dy-lean or free Nd-Fe-B magnets include grain boundary diffusion by doping with a low melting point metal or rare earth oxide (REO), and grain size reduction. In this work, PrCu eutectic alloy powder is added as a sintering aid, blending with magnet feedstock powder. The PrCu alloy forms at a grain boundary phase after the magnet was sintered. The effect of PrCu on magnetic properties of Dy-free NdFeB sintered magnets was systematically studied by varying the sintering conditions. Sintering temperature can be significantly lowered with assistance of PrCu liquid phase, which is beneficial to reduce grain growth and thus enhance the coercivity. With increasing PrCu from 0, 5, 7.5 to 10 wt.%, as shown in Fig 1, the magnets obtained Hc of 14.6, 16.6, 18.6, and 18.8 kOe, and (BH)max of 37.8, 39.5, 35.0 and 28.4 MGOe, respectively. The microstructure and PrCu roles in the magnets will be analysed and discussed. 
**References:** 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/f8031051aa11178ee2a515b84b608ca5.png) 
Fig. 1 Demagnetization curves of the magnets with different addition of PrCu

Coercivity enhancement of PrCu diffused Dy free

Curved magnetic structures often exhibit unique magnetic behavior both as a result of their curvilinear boundary conditions as well as curvature-induced energy effects, such as the curvature-induced Dzyaloshinskii-Moriya (DM) interaction and curvature induced anisotropy. These structures are also of considerable interest for technological applications. For example, magnetic systems have been proposed for a number of wearable devices, such as wearable magnetoreceptors [1] or wearable touchless sensors [2], and the development of such devices requires an understanding of the effects of curvature in magnetic systems. Such effects are best studied in 3D structures, where the curvature can have a significant spatial extent. Nanowires are relatively simple 3D curved structures capable of exhibiting unique magnetization configurations, especially during magnetization reversal [3], and are well suited for applications requiring integration with electrical contacts. Here we have simulated the magnetization reversal in cylindrical NiFe nanowires and supported the results of these simulations with in situ Lorentz TEM images. We found that the reversal of these nanowires falls into two different regimes, depending on the diameter of the nanowire. For nanowires with a diameter less than or equal to 100 nm, magnetization reversal occurred via coherent rotation of the spins. For nanowires with a diameter greater than or equal to 150 nm, reversal occurred through a winding of the magnetization at the edges that eventually resulted in a double-helix configuration at the zero-field state, shown in Figure 1. The reversal completed through the gradual unwinding of the double helix state. In both regimes, domain wall propagation was not observed to occur during reversal.

Magnetization Reversal in Cylindrical NiFe Nanowires

The need to measure high-frequency magnetic permeability has increased due to the 5th generation mobile communication system. We developed a microstrip line-type
probe and demonstrated comprehensive bandwidth permeability measurement up to 67 GHz [1]. For the permeability measurement of the thick magnetic sample, the demagnetizing effect
caused the measurement error and the shift of the ferromagnetic resonance (FMR) [2-3]. In this study, a microstrip conductor with slits has been developed to enhance the current
uniformity and reduce the demagnetizing effect in the thick magnetic material, as shown in Fig. 1 (a). This in-plane excitation decreased the demagnetizing effect. Fig. 1(b) shows the
photograph of the microstrip line type probe with slits. The probe consists of a microstrip conductor (1.3 mm in total width) on a glass substrate and a ground plane. The 15 μm wide copper
conductors (2 μm in thickness) and 15 μm wide gap were fabricated by lift-off process and rf sputtering. Also, the SMA connectors were connected with both input and output terminals.
First, S21 is calibrated by applying a strong DC field (2 T) in the vertical direction of the high-frequency field to saturate the sample. Secondly, S21 is measured without a strong DC field.
The complex permeability was optimized by the complex impedance and FEM analysis [2]. Fig. 2 shows the relative permeability of the NiZn ferrite sheet (3 mm x 1 mm, 100 μm thick).
Measured permeability agreed well with the value of the Nicolson-Ross-Weir method [4]. The measurement point was 201 from 0.1 GHz to 20 GHz. 96% of the whole real part was within
±15%, and 92% of the entire imaginary part was within ±15%. We have demonstrated that this microstrip line type probe with slits can accurately evaluate thick magnetic material because
the uniform current inside the microstrip conductor prevents the measurement error caused by the demagnetizing effect. 

**References:** 

[1]S. Yabukami et. al., IEEE Transactions on Magnetics, Vol. 57, No. 2, 6100405 (2021). 
[2]S. Yabukami et al., IEEE Transactions on Magnetics, vol. 58, No. 2, 6100305 (2022). 
[3]K. Takagi et. al., Journal of Magnetic of Japan, vol. 46 (2022, in press). 
[4]A. M. Nicolson et. al., Transactions on Instrumentation and Measurement, vol. 19, no. 4, pp. 377-382 (1970). 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/cefd07059a335bcf9db54bfe87161218.png)

Thin Film Microstrip Line Type Probe with Slits and Permeability Measurement of Thick Magnetic Material

Current magnetic sensors have been used to detect position, angle, rotation and magnetic fields, based on three key types of technologies: Hall, anisotropic and giant
magnetoresistance effects [1]. Hall sensors are made by patterning Silicon into a cross-bar, where sensitivity can be increased by replacing Si with compound semiconductors such as InAs
and GaAs. However, these sensors suffer from large temperature dependence of their output in a finite magnetic field and have limitations in their working temperature range (between -40
and 120oC) as well as their detectable field range (between 10-2 and 102 T). We have demonstrated a linear response in magnetisation under a small magnetic field (up to +/-50 mT) in an
antidot structure consisting of ferromagnetic Fe nanoparticles dispersed in insulating MgO matrix [2]. In this study, we have adopted this system to replace ferromagnetic Fe dots with Co
and have measured magnetisation dynamics.
We co-deposited Co and MgO at the ratio of 2:1 by electron-beam evaporation on MgO(001) and (011) substrates in ultrahigh vacuum molecular beam epitaxy. The total film thickness
was 30 nm. The films were ex-situ annealed at 400oC for up to 3 hours under vacuum. All the samples show very strong magnetic anisotropy, which is different from the Fe:MgO films
previously reported [2]. The MgO[010] direction is found to be the easy axis, while the MgO[100] is the hard axis. The squareness is estimated to be 0.936 for the as-deposited film,
improving up to 0.939 for the 3-h annealed film. Along the hard axis, the magnetisation curve shows almost linear response within ± 750 Oe, which is broader than that for the Fe:MgO
films (± 500 Oe) [2]. This may indicate the Co nanoparticles dispersed in the MgO matrix are crystallised in fcc. with the epitaxial relationship of Co(001)[010]//MgO(001)[010] [3]. We
will also report the results on the Co:MgO films grown on MgO(011) and discuss the steps towards the sensor applications. 

**References:** 

[1] M. A. Khan et al., Eng. Res. Exp. 3, 022005 (2021). 
[2] M. Rummey et al., IEEE Trans. Magn. 48, 4010 (2012). 
[3] M. Hashimoto et al., J. Cryst. Growth 166, 792 (1996).

Development of a Magnetic Sensor Using Co:MgO Antidots

In the fields of magnetism and spintronics, magnetic multilayers continue to thrive as pivotal structures to functionalize magnetic interactions, including the interfacial Dzyaloshinskii-Moriya interaction (DMI), and to engineer complex non-trivial spin textures [1-3]. However, previous research has focused almost exclusively on 2D structures. The challenge in studying 3D textures is in obtaining the necessary spatial resolution and sensitivity to resolve them. Here we show that this challenge can be met by reference-aided coherent diffractive x-ray imaging combining the robust Fourier Transform Holography with phase retrieval algorithms [4,5]. Based on this amplified wide-angle scattering, we achieve 5 nm spatial resolution for spin textures in Pt/Co/Cu magnetic multilayers, which allows us to clearly resolve fine domain walls (Fig.1c). Surprisingly, while conventional low-resolution images only show the well-known stripe domain state characteristic of such multilayers, our high-resolution images additionally reveal several small, mostly circular features of much weaker contrast (Fig. 1a,b), indicating a reduced net magnetization in the out-of-plane direction. Interestingly, while these features are clearly magnetic in nature and interact with the domain walls, they do not annihilate at the largest fields available in our system (220 mT). We associate the features to a localized increased surface roughness, leading to a local loss of perpendicular magnetic anisotropy promoting in-plane spin alignment and of 3D magnetic textures.

Revealing 3D Magnetic Textures in [Pt/Co/Cu]15 Multilayers by Coherent X ray Imaging with 5 nm Resolution

The recent development of magnetic tomography [1] and laminography [2] techniques has allowed the quantitative characterization of 3D magnetization vector maps in thin films and nanostructures. Thus, vector analysis methods can be used to understand the experimental magnetization configurations and obtain a unified description of the different types of 3D domain walls and magnetic singularities. In this work we have studied by X-ray vector magnetic tomography (XVMT) the 3D magnetization configuration of a 140 nm thick permalloy microstructure [3] and a ferrimagnetic GdCo/NdCo/GdCoâ€™ trilayer [4]. In the Py microstructure, we observe a domain wall with non-trivial 3D configuration decorated by Bloch points such as the one in Fig. 1: composed of a linear tail-to-tail wall and a circulating vortex across the sample thickness. The corresponding emergent field Be (calculated in the right part of Fig. 1) reveals the underlying topological singularity with charge -1, that acts as a sink of Be lines. Changes in domain wall chirality are mediated by topological dipoles and triplets linked by horizontal bundles of emergent field lines. In addition, surface magnetic textures and helical vortices are found at the cross sections of vertical bundles of Be lines [3]. In the GdCo/NdCo/GdCoâ€™ trilayer, the competition between anisotropy, exchange and magnetostatic interactions creates a peculiar stripe domain pattern with an exchange spring wall across the thickness at the top GdCo/NdCo interface [4]. The role of this boundary as a preferred site for nucleation of singularities will be discussed in detail (see e.g. the emergent field dipole in Fig. 2, composed of two Bloch points at different sample depths and opposite topological charges). Work supported by Spanish MICIN and Asturias FICYT.

X ray vector magnetic tomography of Bloch points in magnetic microstructures and multilayers

Multiferroic materials have garnered wide interest for their exceptional static and dynamical magnetoelectric properties (1). In particular, type-II multiferroics exhibit an inversion-symmetry-breaking magnetic order which directly induces a ferroelectric polarization through various mechanisms, such as the spin-current or the inverse Dzyaloshinskii-Moriya effect. This intrinsic coupling between the magnetic and dipolar order parameters results in record-strength magnetoelectric effects (2). In this context, there has been a recent surge of interest in 2D materials possessing such intrinsic multiferroic properties, enabling the integration and control of magnetoelectric effects in artificial heterostructures and nanoelectronic devices (3,4).
In this talk, I will present our recent study and realization of type-II multiferroic order in a single atomic layer of the transition metal-based van der Waals material NiI2 (5). The multiferroic state of NiI2 is characterized by an inversion-symmetry-breaking helimagnetic order which induces a chirality-dependent electrical polarization. Using circular dichroic Raman measurements, we directly probed the magneto-chiral ground state and its electromagnon modes originating from dynamic magnetoelectric coupling. Using birefringence and second-harmonic generation measurements, we observed a highly anisotropic electronic state simultaneously breaking three-fold rotational and inversion symmetry to support polar order. The evolution of the optical signatures as a function of temperature and layer number surprisingly revealed an ordered magnetic, polar state that persists down to the ultrathin limit of monolayer NiI2 (6). 

**References:** 
(1) Matsukura, F., Tokura, Y. and Ohno, H. Control of magnetism by electric fields. Nat. Nanotech. 10, 209 (2015). 
(2) Spaldin, N. A. and Ramesh, R. Advances in magnetoelectric multiferroics. Nat. Mater. 18, 203 (2019). 
(3) Huang, B. et al. Electrical control of 2D magnetism in bilayer CrI3. Nat. Nanotech. 13, 544 (2018). 
(4) Jiang, S., Li, L., Wang, Z., Mak, K. F. and Shan, J. Controlling magnetism in 2D CrI3 by electrostatic doping. Nat. Nanotech. 13, 549 (2018). 
(5) Kurumaji, T. et al. Magnetoelectric responses induced by domain rearrangement and spin structural change in triangular-lattice helimagnets NiI2 and CoI2. Phys. Rev. B 87, 014429 (2013). 
(6) Song, Q. et al. Evidence for a single-layer van der Waals multiferroic. Nature 602, 601 (2022) 
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