United States

Antiferromagnets (AFM) exhibit a host of properties which make them attractive materials for applications. Although they are robust against perturbation by external
magnetic fields, magnetic phases do manifest in bipartite AFMs depending on the applied field and the material properties. The phases are surprisingly complex even in the small particle
limit where the individual lattices are uniformly magnetized. While previous work [1,2,3,4,5] has examined similar systems and determined some of the equilibrium states, this work
comprehensively identifies all critical energy points (local minima, local maxima, saddle points) under the macrospin model [6] where the energy is composed of an applied field (h&lt;sub&gt;0&lt;/sub&gt;)
aligned with a uniaxial anisotropy (h&lt;sub&gt;k&lt;/sub&gt;) mediated by a spatially homogeneous AFM exchange (h&lt;sub&gt;e&lt;/sub&gt;). Both easy axis (h&lt;sub&gt;k&lt;/sub&gt;&gt;0) and easy plane (h&lt;sub&gt;k&lt;/sub&gt;&lt;0) cases are considered.
In this analysis eleven candidate magnetization configurations are identified as critical energy states. Six of these are shown to be local energy minima (metastable) for some range of
material and field parameters, illustrated in the Fig. 1 phase diagram. Note the bi- and tristable regions in this diagram that can support hysteretic behavior. The lowest energy (ground)
states, representing long-term stable phases in the presence of thermal effects, are presented in Fig. 2. Saddle points identified by this analysis determine the energy barrier between local
minima and can be used to estimate the time scale for thermally activated switching.
The transition between phases driven by changing h&lt;sub&gt;0&lt;/sub&gt; is also studied, including in particular the well-known spin-flip and spin-flop behaviors [7]. Consideration of energy levels in this analysis shows that of the eleven candidate phases alluded to above, the five which are never local energy minima also do not occur during adiabatic phase transitions.&lt;br&gt;
**References**&lt;br&gt; 1) F. B. Anderson and H. B. Callen, Phys. Rev., 136, p. A1068 (1964).
2) N. Yamashita, J. Phys. Soc. Jpn., 32, p. 610 (1972).
3) A. N. Bogdanov and U. K. Rößler, Appl. Phys. Lett., 89, p. 163109 (2006).
4) A.-V. Plamadă, D. Cimpoesu and A. Stancu, Appl. Phys. Lett., 96, p. 122505 (2010).
5) H.-F. Li, npj Comput. Mater., 2, p. 16032 (2016).
6) V. Baltz, A. Manchon, M. Tsoi, Rev. Mod. Phys., 90, p. 015005 (2018).
7) N. Ntallis and K.G. Efthimiadis, Comput. Mater. Sci., 97, p. 42 (2015).&lt;br&gt;

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/91d94e67a526421cf301142398fc5fd1.png)&lt;br&gt;

Fig. 1: Phase diagram for energetically stable (local energy minima) configurations in an AFM. SFO (spin-flopped) is an easy-axis canted phase. All parameters and fields are scaled by h&lt;sub&gt;e&lt;/sub&gt;, denoted by a bar on top.&lt;br&gt;

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/fba2235f3ad5831dd388d3ee7e41df19.png)&lt;br&gt;

Fig. 2: Phase diagram for energetically preferred configurations (global energy minima). All parameters and fields are scaled by h&lt;sub&gt;e&lt;/sub&gt;, denoted by a bar on top.

MMM 2022

Energetics of the Spin Flop and Spin

Antiferromagnets (AFM) exhibit a host of properties which make them attractive materials for applications. Although they are robust against perturbation by external
magnetic fields, magnetic phases do manifest in bipartite AFMs depending on the applied field and the material properties. The phases are surprisingly complex even in the small particle
limit where the individual lattices are uniformly magnetized. While previous work [1,2,3,4,5] has examined similar systems and determined some of the equilibrium states, this work
comprehensively identifies all critical energy points (local minima, local maxima, saddle points) under the macrospin model [6] where the energy is composed of an applied field (h0)
aligned with a uniaxial anisotropy (hk) mediated by a spatially homogeneous AFM exchange (he). Both easy axis (hk>0) and easy plane (hk<0) cases are considered.
In this analysis eleven candidate magnetization configurations are identified as critical energy states. Six of these are shown to be local energy minima (metastable) for some range of
material and field parameters, illustrated in the Fig. 1 phase diagram. Note the bi- and tristable regions in this diagram that can support hysteretic behavior. The lowest energy (ground)
states, representing long-term stable phases in the presence of thermal effects, are presented in Fig. 2. Saddle points identified by this analysis determine the energy barrier between local
minima and can be used to estimate the time scale for thermally activated switching.
The transition between phases driven by changing h0 is also studied, including in particular the well-known spin-flip and spin-flop behaviors [7]. Consideration of energy levels in this analysis shows that of the eleven candidate phases alluded to above, the five which are never local energy minima also do not occur during adiabatic phase transitions. 
**References** 1) F. B. Anderson and H. B. Callen, Phys. Rev., 136, p. A1068 (1964).
2) N. Yamashita, J. Phys. Soc. Jpn., 32, p. 610 (1972).
3) A. N. Bogdanov and U. K. Rößler, Appl. Phys. Lett., 89, p. 163109 (2006).
4) A.-V. Plamadă, D. Cimpoesu and A. Stancu, Appl. Phys. Lett., 96, p. 122505 (2010).
5) H.-F. Li, npj Comput. Mater., 2, p. 16032 (2016).
6) V. Baltz, A. Manchon, M. Tsoi, Rev. Mod. Phys., 90, p. 015005 (2018).
7) N. Ntallis and K.G. Efthimiadis, Comput. Mater. Sci., 97, p. 42 (2015). 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/91d94e67a526421cf301142398fc5fd1.png) 

Fig. 1: Phase diagram for energetically stable (local energy minima) configurations in an AFM. SFO (spin-flopped) is an easy-axis canted phase. All parameters and fields are scaled by he, denoted by a bar on top. 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/fba2235f3ad5831dd388d3ee7e41df19.png) 

Fig. 2: Phase diagram for energetically preferred configurations (global energy minima). All parameters and fields are scaled by he, denoted by a bar on top.

technical paper

#### Welcome to the 67th Annual Conference on Magnetism and Magnetic Materials, MMM 2022!

The Conference was held from October 31 to November 4 and offers both in-person and on-demand content.

MMM 2022 is sponsored jointly by AIP Publishing and the IEEE Magnetics Society. Members of the international scientific and engineering communities interested in recent developments in fundamental and applied magnetism are invited to attend and contribute to the technical sessions. The technical program will include invited and contributed papers in oral and poster sessions, invited symposia, a tutorial, and several special sessions. This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

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Magneto-ionics (MI) provides a nonvolatile EF modulation of magnetism through the migration of ions toward/away from the magnetic interfaces that allows for low-power spintronics devices. However, the ionic mechanism involved still needs to be explored, as key properties such as reversibility have been shown to strongly depend on the chemical composition and oxidation level of the ferromagnet (FM)/oxide interface, (1–3), and on the degree of crystallinity of the system (4). We have recently shown that the gate-voltage driven degree of oxidation at the FM/oxide interface in Ta/Co20Fe60B20/HfO2 defines two MI regimes: (I) underoxidized to optimally oxidized, and (II) optimally oxidized to overoxidized, accompanied by a change in the magnetization easy axis from in-plane (IP) to out-of-plane (OOP) in regime I and from OOP to IP in regime II (3). Only regime II showed reversibility, which was linked to the different binding strength of the oxygen species in regimes I and II.
In this work, we present the MI behavior in Pt/Co60Fe20B20/HfO2, where also two MI regimes have been identified. In this system, both regime I and II are reversible, but the stability of the nonvolatile magnetic states was found to be significantly different in regime I and II. Magnetic states in regime I was found to be highly stable, with no significant loss of remanence in the hysteresis loops over several weeks after applying the gate voltage. In this regime, a significant variation of the Dzyaloshinskii-Moriya interaction was also observed. In contrast, the magnetic states in regime II (between OOP and IP) were found to evolve with time back to the OOP state within one day after applying the gate voltage. Interestingly, only the final overoxidized IP state is fully stable in regime II.
This disparity in the stability of the MI states in regimes I and II could be attributed to differences in the gate-voltage driven chemical reactions and ionic transport, where also the composition of CoFeB may play an important role. Our results show the importance of studying the link between the ionic mechanisms and the MI state stability, in view of practical applications in spintronics. 

**References:** 
(1) L. Herrera Diez, Y. T. Liu, M. Belmeguenai, et al., Physical Review Applied, 12, 034005 (2019); 
(2) A. Fassatoui, L. Ranno, S. Pizzini, et al., Physical Review Applied, 14, 064041 (2020), 
(3) R. Pachat, D. Ourdani, L. Herrera Diez. et al., Physical Review Applied, 15, 064055 (2021), 
(4) R. Pachat, D. Ourdani, L. Herrera Diez, et al., Adv. Mat. Inter., Accepted (2022) 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/ea145b4e40c9304eb75f27a8883b090a.png)

Stability of under/over oxidized magneto ionic states in Pt/Co60Fe20B20/HfO2

Neuromorphic computing is a particularly exciting application of spintronics, able to exploit its rich physics to achieve low-power-consumption computation inspired by the brain. Designing neuromorphic systems requires simulating the response of spintronic devices to multiple types of inputs over long time periods. Currently, the dominant approach to predicting the complex behavior of spintronic devices is micromagnetic simulation. This technique, however, requires very long simulation times, easily reaching weeks in time-dependent experiments or in micrometer-scale devices. In this work, we take an alternative road by adopting a deep learning technique, Neural Ordinary Differential Equations (ODEs), to model spintronic devices and develop spintronic systems. We show that Neural ODEs, trained on a minimal amount of data, can predict the behavior of spintronic devices with high accuracy and an extremely efficient simulation time, compared to micromagnetic simulations. For this purpose, we re-frame the formalism of Neural Ordinary Differential Equations (ODEs) [1] to the constraints of spintronics: few measured outputs, multiple inputs and internal parameters (Fig. 1). We demonstrate with Neural ODEs an acceleration factor over 200 compared to micromagnetic simulations for a neuromorphic system -- a reservoir computer made of magnetic skyrmions (20 minutes compared to three days). We also demonstrate that this state-of-the-art deep learning technique for time series modeling can be applied to model an experimental neuromorphic experiment [2], in which spin-torque nano-oscillators are used as reservoir computers to achieve a spoken digit recognition task (Fig. 2). Neural ODEs can therefore constitute a disruptive tool for developing spintronic applications in complement to micromagnetic simulations, which are time-consuming and cannot fit experiments when noise or imperfections are present [3].

Building spintronics neuromorphic systems with Neural Ordinary Differential Equations

Realization of novel topological phases in magnonic band structures represents a new opportunity for the development of low-dissipation magnonics [1-5]. The previous proposals require materials with either special crystal symmetries [2-3] or artificially modulated structures that demand advanced nanofabrication techniques [4-5], both of which bring in difficulties for experimental realization. In this work, we theoretically study the magnonic band structure of antiparallelly aligned magnetic multilayers (Fig. 1), and reveal their field-tunable topological phases [1]. We show that the long-range dipolar interaction between propagating magnons is chiral in nature, whose strength depends on the wave vector direction and therefore breaks time-reversal symmetry. Consequently, the dipolar interaction plays a role like spin-orbit coupling in electronic systems and generates bulk bands with non-zero Chern integers and ultra-localized magnonic surface states that carry chiral spin currents (Fig. 1). Through an external magnetic field, the topological phases can be switched, which therefore provides a tunable for transferring spin angular momenta in this synthetic antiparallelly aligned heterostructure. Besides analytical study, we also carry out micromagnetic simulations on the simplest multilayer system â€“ YIG/Py bilayers using MuMax3 [6]. The magnonic band structure with a degeneration point only in one half of the reciprocal space and the unidirectional propagation of spin waves (Fig. 2) both validate our analytical results. Our study provides an easy-to-implement system for realizing topologically protected magnonic surface states and low-dissipation spin transport in a tunable manner, which is expected to benefit various areas of modern spintronics and magnonics.

Tunable Magnonic Chern Bands and Chiral Spin Currents in Magnetic Multilayers

Magnetic Tunnel Junctions (MTJ) are one of the major building blocks of spintronics, used in random access memory designs due to non-volatility, endurance and fast
switching. Regardless of the choice of writing scheme, e.g Spin-Transfer Torque (STT)[1] or Spin-Orbit Torque (SOT)[2], we need to tailor the intrinsic properties of the switchable
magnet (the Free Layer) in order to ensure a high retention time of the magnetization state, representing a trade off with respect to the switching energy, i.e the more retention, the more
energy required to operate the device.
To overcome the dilemma and enable MTJs with high retention and low writing energy, a Hybrid Free Layer (HFL) concept has been previously proposed by integrating a Synthetic
Antiferromagnet (SAF) stack into the widely used CoFeB/MgO Free Layer stack[3]. This HFL has two intrinsic interlayer couplings tailored by a SAF and SFM (synthetic ferromagnet)
spacers (Fig. 1). The SAF spacer reduces the stray field and gives an exchange coupling torque from the motion of either layer II or III, leading to faster overall motion of the stack. The
SFM spacer allows for flexibility in the choice of SAF design, as it couples the layer I directly to layer II, but also decouples the crystallization of the Tunneling Barrier interface from layer
II, preventing lattice mismatching effects and bad electronic band conditions in a wide range of materials[4].
This work explores this design focusing on compensation of ground states (where the stack adds up to no field). We demonstrate by micromagnetics that adding a SFM to a SAF stack
impacts the energy and layer trajectories in non-trivial ways. For instance, in Fig. 2 the hysteresis loop of HFL stacks with both compensated equilibrium states and Top Major (layers I, II
generate field) show that even at compensation point we have degeneracy of states, i.e regions of external field where both low/high energy states are stable. This contrasts with SAF only
stacks where outside of compensation a behavior towards saturation is observed (Fig.2 Right). Different switching speeds can also be found by tailoring the coupling strengths. 
**References** [1] S. Sakhare et al., 2018 IEEE IEDM;
[2] S. Couet et al., 2021 Symposium on VLSI Technology, 2021, pp. 1-2.
[3] E. Raymenants et al., Nature Electronics, volume 4, pgs 392–398 (2021).
[4] Liu, E. et al., IEEE Trans. Magn. 53, 1–5 (2017). 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/f4a956e9e6108a5e1fecf8f287ec16a2.png) 

Fig.1 HFL design with top MTJ. 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/5611644821e8f98ad639563066f90281.png) 

Fig. 2 HFL (left) and SAF (right) hysteresis loops.

Micromagnetic Modelling of Multi Coupled Free Layers for Magnetic Tunnel Junction Devices

The key to realizing high area recording density capability (ADC) for heat assisted magnetic recording (HAMR) is to achieve small grain size (and pitch) in the granular L10 FePt media with a sufficiently high grain aspect ratio [1]. Media such as FePt-C, FePt-SiOx, and FePt-TaOx, or multilayers made of a combination of them, appear not able to reach the desired goal because of their failure to achieve either high grain aspect ratio, small grain size, or high L10 ordering, respectively [2, 3]. In this study, using the co-sputtering technique, we demonstrate the fabrication of FePt-BN granular media with much reduced FePt grain size and high grain aspect ratio. In particular, an underlayer stack of Ta(5nm)/Cr(50)/MgO(9) is first deposited on Corning LotusTM NXT glass followed by co-sputtering of FePt and BN at an elevated substrate temperature along. A RF bias is applied to the substrate during the FePt-BN deposition with a short and optimized delay. Figure 1C shows the order parameter as a function of substrate temperature for the FePt-BN granular film media, calculated from XRD measurements. The volumetric concentration of BN is at 38%. Figure 1A on the left of the figure shows a cross-section TEM micrograph of the FePt-BN layer formed at a substrate temperature of 725 oC. Figure 1B shows a cross-section TEM picture of FePt-BN/FePt-SiOx multilayer granular film with temperature and composition graded over the grain height for producing tall grains [4] for comparison. The grain size of single layer FePt-BN film (Figure1 A) is around 4 nm with an aspect ratio of 2 while the grain size of the FePt-BN/FePt-SiOx multilayer film is around 7nm and grain height 11nm. The small grain size with a relatively high grain aspect ratio in the single layer FePt-BN film is promising for enabling high area recording density capability. 
**References** 
[1]Y. Kubota et al., “Heat-Assisted Magnetic Recording’s Extensibility to High Linear and Areal Density,” IEEE Transactions on Magnetics, vol. 54, no. 11, Nov. 2018. 
[2] A. Perumal, Y.K. Takahashi, K. Hono, “L10 FePt-C Nanogranular Perpendicular Anisotropy Films with Narrow Size Distribution”, Appl. Phys. Express 1 101301 (2008). 
[3] B. S. D. C. S. Varaprasad, M Chen, YK Takahashi, K Hono “L1-Ordered FePt-Based Perpendicular Magnetic Recording Media for Heat-Assisted Magnetic Recording” IEEE
transactions on magnetics 49 (2), 718-722 (2013) 
[4] C. Xu, B. Zhou, T. Du, B.S.D.C.S. Varaprasad, D.E. Laughlin, J.-G. Zhu, Understanding the growth of high-aspect-ratio grains in granular L10-FePt thin-film magnetic media, APL Materials. 10 (2022). 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/b46f142fcddcff5ebe95523b1fdfc98e.png) 
Figure 1 (A) Out of plane micrographs of FePt-BN (8 nm) (B) Out of plane micrographs of FePt-BN/FePt-SiO 2 (11 nm) and (C) Order parameter Vs set temperature of FePt-BN sample

FePt BN Granular HAMR Media with High Grain Aspect Ratio and High L10 Ordering on Corning Lotus™ NXT Glass

The vast multi-principal element alloy (MPEA) composition space is promising for discovery of new material phases with unique combinations of properties.1 We have explored the potential to achieve high magnetic anisotropy materials in MPEA FeCoNiMnCu thin films. Thin films of FeCoNiMnCu sputtered on thermally oxidized Si/SiO2 substrate at room temperature exhibit soft magnetic properties with a coercivity on the order of 10 Oe . After post-deposition rapid thermal annealing (RTA), the films exhibit a single face-centered cubic (FCC) phase from grazing incidence X-ray diffraction (GIXRD), as shown in Fig. 1, as well as a continuous surface morphology measured by atomic force microscopy. Magnetometry measurements reveal a coercivity of 400 Oe, a nearly 40-fold increase compared to as-grown samples. First-order reversal curve (FORC) analysis2 of the RTA sample reveals a distinct feature in the high coercivity regime of the FORC distribution, as shown in Fig. 2, indicating the presence of a single magnetic phase with much enhanced coercivity. Additionally, we have explored inclusion of Pt to facilitate uniaxial structure and enhance magnetic anisotropy. We find that RTA of the FeCoNiMnCuPt films induces ordering into an L10 intermetallic phase. A significant enhancement of coercivity to over 2 kOe after RTA has been observed, compared to 5 Oe before RTA, along with the development of an out-of-plane magnetic easy-axis, reflecting the formation of a high anisotropy L10 phase. These results demonstrate the potential in achieving high magnetic anisotropy materials using the multi-principal element approach. 

This work has been supported by the NSF (ECCS-2151809). 
**References:** 
1. J.W. Yeh, S.K. Chen, S.Y. Chang et al., Adv. Eng. Mater., 6, 299-303, (2004). 
2. D.A. Gilbert, J.W. Liao, K. Liu et al., APL Mater., 2, 086106, (2014). 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/e950d258e4e85c804cb517bbdc6f1322.png) 
Fig. 1. Grazing-incidence XRD for FeCoNiMnCu films (bottom) as-grown and (top) after RTA. 
![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/ca2a4a7de03b99275018c61a1b745243.png) 
Fig. 2. (left ) Family of first-order reversal curves (FORC) and (right) FORC distribution of an FeCoNiMnCu film after RTA.

Phase Stabilization and Coercivity Enhancement in Multi principal Element FeCoNiMnCu Thin Films

α”-Fe16N2 is a promising environmentally-friendly rare-earth-free permanent magnet material with ultra-high saturation magnetization. Recent research has demonstrated experimentally through a thermally quenching treatment using γ’ phase Fe4N as a precursor to synthesize α”-Fe16N2 in bulk format. In this research, we investigated γ’-Fe4N thin film thermal decomposition process and potential localized phase transition from fcc phase to bct phase using Molecular Dynamics (MD) simulation method. As nitrogen concentration is much higher in γ’-Fe4N (5.9 wt.%) than that in α’-Fe8N or α’’-Fe16N2 (3 wt.%), Nitrogen “depletion” process must have to occur during the thermal decomposition to form possible lower-Nitrogen content bct Fe-N solid solution. A localized “Nitrogen-rich” grain boundary lattice defect model is designed to induce Nitrogen bond forming through the recombination of nitrogen pair atoms, and therefore significantly improves local reduction of Nitrogen content in the α’-Fe4N lattice sample during thermal treatment. Modified Embedded Atom Method (MEAM) interatomic potential of Fe-N system is applied. Bond forming/breaking and local Nitrogen displacement analysis (MSD) are also performed in the thermostat-controlled heating/quenching simulation process. We use virtual XRD method and formation energy calculation to assess and detect the new material phases on the thermally-treated and energy minimized lattice sample. 
**References:** [1] Jian-Ping Wang, “Environment-friendly bulk Fe16N2 permanent magnet: Review and Prospective”, Journal of Magnetism and Magnetic Materials, 497(2020) 165962 
[2] J. Zhu, G, Guo and J.-P. Wang, "Study of γ'-F4N Annealing Process Through Molecular Dynamics Modeling", TMS 2022 151st Annual Meeting, The Minerals, Metals & Materials Series, Page 109-117, https://doi.org/10.1007/978-3-030-92381-5_11 
[3] M.H. Wetzel, M.R. Schwarz, A. Leineweber, High-pressure high-temperature study of the pressure induced decomposition of the iron nitride γ′-Fe4N, Journal of Alloys and Compounds, Volume 801, 2019, Pages 438-448

Simulation of Thermal Decomposition of γ’ Fe4N using Molecular Dynamics Method

The degree of magnetic order that can be achieved in thermally artificial kagome spin ice depends on the superparamagnetic blocking temperature of the individual nanomagnets [1] and the predicted phase transition temperature of the system [2]. The interplay between these two temperature scales dictates the observable magnetic order in experiments and in this talk, I will discuss how phase transition temperatures can be independently tuned from superparamagnetic blocking temperature.

We find that by placing nanomagnetic bridges asymmetrically at the vertices of artificial kagome ice we can control the degree of frustration in the system, which allows for the independent tuning of the phase transition temperature [3]. We demonstrate that by carefully varying the widths of the bridges, we obtain various degrees of magnetic ordering and observe two magnetic phases. We then utilize the thermally active system to show that we can image the phase transition using temperature-dependent X-ray photoemission electron microscopy. 
**Reference:** 
[1] K. Hofhuis, A. Hrabec, H. Arava, N. Leo, Y.L. Huang, R.V. Chopdekar, S. Parchenko, A. Kleibert,
S. Koraltan, C. Abert, C. Vogler, D. Suess, P.M. Derlet, & L.J. Heyderman, Physical Review B 102, 180405(R) (2020) 
[2] G. Moller & R. Moessner, Physical Review B 80, 140409(R) (2009). 
[3] K. Hofhuis, S. H. Skjærvø, Z. Luo, S. Parchenko, A. Kleibert, P. M. Derlet, & L. J. Heyderman, Nature Physics 18, 699-705 (2022)

Phase Transitions in Thermally Active Artificial Kagome Spin Ice

Spin current is one of the key technologies for spintronics devices. Especially, generation of spin currents via electrical charge current has attracted a great deal of attentions from both fundamental physics and application point of views because it can manipulate magnetization vector of the ferromagnet [1]. This charge-to-spin conversion phenomena originate from spin-orbit coupling and has been widely studied in non-magnetic materials. Recently, the interest in spin conversion materials has extended to ferromagnetic materials (FMs). FMs can generate magnetization-dependent charge-to-spin conversion (MD-CSC), which is caused by the reduced mirror symmetry due to the presence of magnetization, and this MD-CSC can be used as a writing source for high-density MRAM (Fig. 1) [2]. However, the origin of this MD-CSC has not been calrified yet, for instance, whether the interfacial or bulk contribution is domoinant. In addition to this, a guideline for high spin conversion efficiency is highly required for realizing energy-efficient writing in MRAM devices. In this presentation, we systematically studied the charge-to-spin conversion in ferromagnetic material to clarify its origin [3]. Through precise measurement of spin conversion efficiency via spin-torque ferromagnetic resonance [4], we experimentally revealed that two different mechanisms, which originate from the bulk and at interface, contribute to the charge-to-spin conversion in ferromagnet. Next, we demonstrated a guideline to enhance the spin conversion efficiency which is to control the interface structure between Ni-Co alloy and non-magnetic spacer Cu spacer layer (Fig.2) [3]. Moreover, we show that even the sign of the MD-CSC can be tuned by changing the non-mangetic spacer material [5]. These findings would be a milestone toward realizing a high-density and energy-efficient spin-orbit torque MRAMs. This work was partly supported by Grant-in-Aid for Scientific Research (Nos. JP16J03105, JP25220604, JP15H05702, and JP19J01643) and CREST program (No. JPMJCR18T3) of JST. Part of this work was conducted at the AIST Nano-Processing Facility, supported by Nanotechnology Platform Program of the Ministry of Education, Culture, Sports, Science and Technologies (MEXT), Japan. 
**References** [1] A. Manchon et.al, Rev. Mod. Phys. 91, 035004 (2019). [2] S. C. Baek et.al, Nat. Mater. 17, 509-513 (2018). [3] Y. Hibino et al., Nat. Commun. 8, 15848 (2021). [4] Y. Hibino et.al, Phys. Rev. B 101, 174441 (2020). [5] Y. Hibino et al., APL Mater. 8, 041110 (2020). 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/bc101a35bb991226ebda3683a9a65441.png)

Clarifying the Origin of Charge to

Metamagnetic Heusler compounds display a first order structural transformation associated with large changes of magnetization and/or magnetic order. Their properties can
be tuned by suitable changes in composition making them a versatile class of multifunctional materials, thanks to the strong interplay between thermal, mechanical and magnetic degrees of
freedom [1]. In addition, their hierarchically interrelated twin-with-twin microstructure, in the low temperature martensitic phase, and the strong spin-lattice coupling allow to control their
magnetic and multifunctional properties from the atomic to the micro-scale by tuning growth conditions and applying external fields [2, 3].
In my talk I will report on our recent results on nano/microscale materials obtained by different fabrication methods (e.g. epitaxial thin films, patterned and free-standing structures) [3-6].
Thin films and micro/nanostructures are of particular interest not only for the realization of new-concept devices, but also for providing insights into the magnetostructural coupling at the
different length scales. The talk will focus on microstructure engineering and microstructure related effects on the martensitic transformation, also in view of the possible exploitation of
this class of materials in energy related and smart applications.
In particular, by an accurate magnetic and structural investigation at different length-scales, I will demonstrate how growth temperature and simple post-growth treatments, i.e. postannealing
at low T, magnetic field cooling and mechanical stress, are suitable to manipulate the twin variant configuration in epitaxial thin films. X-type variants with out-of-plane
magnetic easy axis or Y-type variants with in-plane magnetic easy-axis can be selected, as well as their geometrical distribution, in films with mixed X/Y-type microstructure [3].
Taking advantage from the possibility to manipulate microstructure we have studied the role of specific martensitic configurations on the martensitic transition path. By advanced magnetic
force microscopy imaging in a wide temperature (260–350 K) and magnetic field range (up to 14 T) we have directly observed the nucleation and the self-accommodation of the martensitic
twinning configurations under zero-field, isofield and isothermal conditions. We have found that between the two possible twinning configurations, the Y-type, which nucleates first, shows
a significantly smaller thermal hysteresis as well as a sharper phase transition with respect to X-type microstructure, for all the three investigated conditions [4].
The effects of lateral size, shape and geometry on the properties of patterned epitaxially grown Ni-Mn-Ga films will also be discussed. In particular, I will present the characterization of
arrays of microstructures, obtained by means of different lithographic techniques, with lateral sizes down to 200 nm range and having different shapes and orientations with respect to the
substrate edges [5].
The last part of my talk will be devoted to free-standing nanostructures in view of their possible exploitation as micro/nano actuators exploiting microstructure-controlled actuation
mechanisms by the combined application of temperature and magnetic field [6]. 

**References:** 

[1] M. Acet, L. Mañosa, A. Planes, Handbook of Magnetic Materials (Ed: K. H. J. Buschow), Elsevier, Amsterdam 2011, pp. 231–289 
[2] P. Ranzieri et al., Adv. Mater. 2015, vol. 27, p. 4760 
[3] M. Takhsha Ghahfarokhi et al. Acta Mater. vol. 187, p. 135–145 (2020) 
[4] M. Takhsha Ghahfarokhi et al. Acta Mater. vol. 23, 117356 (2021) 
[5] M. Takhsha Ghahfarokhi, et al. Appl. Mater. Today vol. 23, p. 101058 (2021). 
[6] M. Campanini et al., Small, vol. 14, p. 1803027 (2018) 

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