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Magnetism in two-dimensional (2D) van der Waals (vdW) materials has emerged as a new paradigm enunciating new condensed matter phenomena, bearing potential for application in future spintronic and quantum computing devices (1,2). Among 2D vdW ferromagnets (FMs), metallic Fe3GeTe2 (FGT) is interesting owing to its high Curie temperature, uniaxial magnetic anisotropy, existence of unusual magnetic ground state. An understanding of factors responsible for unconventional magnetism and associated magnetoresistive manifestations has remained elusive. Here, we clarify the underlying physics responsible for nontrivial ground state and demonstrate tuning of emergent properties by substitution at magnetic (Fe) or nonmagnetic (Ge) sites in 2D vdW FGT.
High quality single-crystalline FGT, (Fe1-xCox)3GeTe2, Fe3(Ge1-xAsx)Te2 , (0 ≤ x ≤ 1) were grown by chemical vapor transport method. Magnetotransport measurements under H || c-axis result in sizeable anomalous Hall effect, arising from topological nodal lines in the band structure. Interestingly, transverse resistivity under H ⊥ c-axis result in unconventional magnetoresistive behavior with prominent cusp-like feature (Fig. 1) (3,4). Concomitant magneto-optical imaging indicates emergent skyrmion lattice-like structure, size varying with doping and angular magnetotransport confirms their 2D nature. Separation of magnetoresistive responses indicates dominant unconventional emergent Hall effect (3,4), tunable with magnetic or non-magnetic doping (Fig. 2), significantly larger than in other vdW, skyrmion-hosting materials (5-7). Our results deepen understanding of emergent responses from complicated spin textures prospective for topological magnetism and non-collinear spin texture-based spintronic devices.&lt;br&gt;

RRC acknowledges DST for financial support (DST/INSPIRE/04/2018/001755). We thank H. Ohno for discussions. This work is partly supported by RIEC Cooperative Research Project.&lt;br&gt;

**References:** &lt;br&gt;
(1) B. Huang et al., Nature 546, 270 (2017).&lt;br&gt;
(2) K. S. Burch et al., Nature 563, 47 (2018).&lt;br&gt;
(3) R. Roy Chowdhury et al., Sci. Rep. 11, 14121 (2021).&lt;br&gt;
(4) R. Roy Chowdhury et al., Phys. Rev. Mater. 6, 014002 (2022).&lt;br&gt;
(5) Y. Wang et al., Phys. Rev. B. 96, 134428 (pp 1-6) (2017).&lt;br&gt;
(6) S. Seki et al., Science 336, 198-201 (2012).&lt;br&gt;
(7) N. Kanazawa et al., Adv. Mater. 29, 1603227 (2017).&lt;br&gt;

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MMM 2022

Unconventional Emergent Hall Effect Phenomena and its Modification In a van der Waals Ferromagnet Fe3GeTe2

Magnetism in two-dimensional (2D) van der Waals (vdW) materials has emerged as a new paradigm enunciating new condensed matter phenomena, bearing potential for application in future spintronic and quantum computing devices (1,2). Among 2D vdW ferromagnets (FMs), metallic Fe3GeTe2 (FGT) is interesting owing to its high Curie temperature, uniaxial magnetic anisotropy, existence of unusual magnetic ground state. An understanding of factors responsible for unconventional magnetism and associated magnetoresistive manifestations has remained elusive. Here, we clarify the underlying physics responsible for nontrivial ground state and demonstrate tuning of emergent properties by substitution at magnetic (Fe) or nonmagnetic (Ge) sites in 2D vdW FGT.
High quality single-crystalline FGT, (Fe1-xCox)3GeTe2, Fe3(Ge1-xAsx)Te2 , (0 ≤ x ≤ 1) were grown by chemical vapor transport method. Magnetotransport measurements under H || c-axis result in sizeable anomalous Hall effect, arising from topological nodal lines in the band structure. Interestingly, transverse resistivity under H ⊥ c-axis result in unconventional magnetoresistive behavior with prominent cusp-like feature (Fig. 1) (3,4). Concomitant magneto-optical imaging indicates emergent skyrmion lattice-like structure, size varying with doping and angular magnetotransport confirms their 2D nature. Separation of magnetoresistive responses indicates dominant unconventional emergent Hall effect (3,4), tunable with magnetic or non-magnetic doping (Fig. 2), significantly larger than in other vdW, skyrmion-hosting materials (5-7). Our results deepen understanding of emergent responses from complicated spin textures prospective for topological magnetism and non-collinear spin texture-based spintronic devices. 

RRC acknowledges DST for financial support (DST/INSPIRE/04/2018/001755). We thank H. Ohno for discussions. This work is partly supported by RIEC Cooperative Research Project. 

**References:** 
(1) B. Huang et al., Nature 546, 270 (2017). 
(2) K. S. Burch et al., Nature 563, 47 (2018). 
(3) R. Roy Chowdhury et al., Sci. Rep. 11, 14121 (2021). 
(4) R. Roy Chowdhury et al., Phys. Rev. Mater. 6, 014002 (2022). 
(5) Y. Wang et al., Phys. Rev. B. 96, 134428 (pp 1-6) (2017). 
(6) S. Seki et al., Science 336, 198-201 (2012). 
(7) N. Kanazawa et al., Adv. Mater. 29, 1603227 (2017). 

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technical paper

#### Welcome to the 67th Annual Conference on Magnetism and Magnetic Materials, MMM 2022!

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Current-induced magnetization switching via spin-orbit torque (SOT) has been intensively investigated for conventional memory applications. With the emergence of multi-states devices that can be utilized in both memory and computing operations, research has pivoted to focus on all electrical-induced multiple resistant states to provide complementary hardware for unconventional computing [1]. SOT-based multi-state devices can mimic artificial synapses and neurons for unconventional computing due to their non-volatility, high scalability, and low power consumption. Artificial synapses are utilized for memory function, acting as junctions between pre-and post-neurons, these continuously being adjusted to tune the strength between two neurons. Artificial neurons integrate all the input stimuli and fire once the accumulated potential reaches a certain threshold. SOT manipulation of multiple magnetization states can be realized by controlling domain wall propagation and modulating domain wall nucleation to emulate synapses [2]. Domain wall propagation requires controllable pinning while domain nucleation can be effectively achieved by modifying the magnetic anisotropy energy. In this work, SOT-induced domain wall nucleation in Pt/Co system is demonstrated by engineering its interface to lower the anisotropy energy. A low magnetic anisotropy energy of 1.35×105 J/m3 has been achieved in Pt (5 nm)/Co (1.2 nm)/HfOx (2 nm)/Ti (2 nm) structure, as obtained from the hysteresis loops shown in Fig. 1(a) and (b). Electrical and Kerr microscopy measurements were carried out to determine the multiple resistance states. Figure 2(a) shows the Hall resistance Rxy change with magnetic field HOOP. Figure 2(b) shows the pulse current-induced multiple resistance; the insets show the domain images after sending a pulse current (I) from 6 to 9 mA. The results clearly indicate the multi-states in Pt/Co/HfOx/Ti system are driven by current, which is promising for synaptic memory-driven neuromorphic applications. 
**References** [1] Zhou, J., et. al., Adv. Mater. 33, 2103672 (2021).
[2] Jin, T., et. al., J. Phys. D: Appl. Phys, 52, 445001 (2019). 

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All electrical Manipulation of Multiple Resistant States via Modifying Magnetic Anisotropy for Emulating Synaptic Memory

Magnetic skyrmions are “topologically protected” solitons, that have recently received increased attention due to their potential application as information carriers and for unconventional computing [1]. While the current-driven dynamics has been explored deeply, the theory of temperature gradient-induced dynamics - Skyrmion-Caloritronics [2] - is still at its early stages of development but it is particularly promising due to its low energy consumption.
In this work, we study the effects of thermal gradients on skyrmion motion in different systems (single-layer FM with interfacial Dzyaloshinskii-Moriya interaction (IDMI), multilayer, and synthetic antiferromagnets (SAFs)) inspired by the experimental results of Ref. [2] and from a fundamental point of view. We observe that, when driven by the entropic torque, skyrmions in single layers move from the cold to the hot region with a finite skyrmion Hall angle (Fig. 1), while in multilayers they move in the opposite direction (from hot to cold region) (Fig. 2). The latter is in qualitative agreement with the experimental observations (Fig. 3a in Ref. [2]). We demonstrate that this difference is due to the distinct scaling relations characterizing the two systems, and to the existence of a magnetostatic field gradient linked to the variation of saturation magnetization (MS) which cannot be neglected in magnetic multilayers. The numerical results are corroborated by a generalized Thiele’s equation developed for this scenario. Moreover, we show that the skyrmion Hall angle is completely suppressed in SAFs, similarly to the current-driven skyrmions [3,4].
Our results have fundamental implications in the future development of skyrmionic devices combining thermal gradients and SOTs where the proper temperature dependence of the parameters should be taken into account.
See Ref. [5] for founding statement. 
**References** [1] J. Zázvorka, et al., Nat. Nanotechnol. 14, 658 (2019).

[2] Z. Wang, et al., Nat. Electron. 3, 672 (2020).

[3] X. Zhang, et al., Nat. Commun. 7, 10293 (2016).

[4] R. Tomasello, et al., J. Phys. D. Appl. Phys. 50, 325302 (2017).

[5] This work was supported by the Project No. PRIN 2020LWPKH7 funded by the Italian Ministry of University and Research, and by the PETASPIN association (www.petaspin.com). JB acknowledges support from the Royal Society through a University Research Fellowship. E.R. acknowledges the economical support of the COST Action CA 17123 (MAGNETOFON) within the STSM program. We would like to acknowledge networking support from COST Action No. CA17123 “Ultrafast opto magneto electronics for nondissipative information technology.”

[6] W. Li, et al., Adv. Mater. 31, 1807683 (2019). 

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Skyrmion Caloritronics: a promising Strategy to manipulate Skyrmions

Spin based photovoltaic (PV) effect is novel research in energy harvesting area.Recently, spin-PV effect was demonstrated on magnetic tunnel junction-based molecular spintronics (MTJMSD).This prior work mainly focused on NiFe/AlOx/NiFe MTJ[1].To advance this field there is a critical need of investigating a wide range of magnetic materials.MgO insulator and CoFeB electrode-based MTJS are well established for showing high TMR ratio and heavily studied for computer memory operation.However,they have never been integrated into the molecular spintronics study.This study made effort to develop a fabrication method by which CoFeB/MgO/CoFeB MTJ could be produced in the cross-junction form to facilitate transport and electrical measurements.To do so we had to overcome numerous fabrication challenges.First, we investigated the air stability of CoFeB using reflectance study as a function of temperature.We observed that CoFeB was stable in air up to ~100C.To produce stable and low leakage current tunnel junctions we iteratively investigated the role of multiple bottom electrode fabrication factors through Taguchi Design of experiment.We produced bottom electrode in such a way that thin films have optimum tapered edge geometry and ~0.15 nm Rq roughness.To produce exposed side edges for molecule spin channel attachment we deposited MgO and top electrode through photolithorgraphically produced cavity perpendicular to the bottom electrode. To convert MTJ into MTJMSD we electrochemically bridged the SMM molecules across MgO and conducted current-voltage study before and after molecule spin channel creation.Interestingly, SMM produced current suppression at room temperature by ~200% at 300mV. Also, we observed PV effect. This MTJ produced 0.08 open circuit voltage and 5.19E-9 saturation current under 1sun(1000 mW/cm2) light intensity. To investigate the mechanism, we perform KPAFM. The difference between electrodes before and after SMM bridging was 33 and 150 mV, respectively. Higher difference in KPAFM voltage signifies a large difference in electrode resistivity. KPAFM also suggests the observed PV effect is mainly from the impacted ferromagnetic electrode region and not due to the interaction of light at molecular channels. 
**References** 1- P. Tyagi and C. Riso, "Molecular spintronics devices exhibiting properties of a solar cell," Nanotechnology, vol. 30, p. 495401, 2019/09/19 2019.
2- P. Tyagi and C. Riso, "Magnetic force microscopy revealing long range molecule impact on magnetic tunnel junction based molecular spintronics devices," Organic Electronics, vol. 75, p. 105421, 2019/12/01/ 2019.
3-Heersche, H. B., De Groot, Z., Folk, J. A., Van Der Zant, H. S. J., Romeike, C., Wegewijs, M. R., ... & Cornia, A. (2006). Electron transport through single Mn 12 molecular magnets. Physical review letters, 96(20), 206801.
4- Jo MH, Grose JE, Baheti K, Deshmukh MM, Sokol JJ, Rumberger EM, Hendrickson DN, Long JR, Park H, Ralph DC. Signatures of molecular magnetism in single-molecule transport spectroscopy. Nano Lett. 2006 Sep;6(9):2014-20. doi: 10.1021/nl061212i. PMID: 16968018.
5- Bogani, L., Wernsdorfer, W. Molecular spintronics using single-molecule magnets. Nature Mater 7, 179–186 (2008). https://doi.org/10.1038/nmat2133 
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Single Molecular Magnet (SMM) Producing Photovoltaic Effect(PV) on Magnetic Tunnel Junction

Spin waves (SW) possess specific properties that make them well-suited for microwave applications. Very often these applications rely on magnetic bodies where large populations of coherently pumped spin waves share a common space with their less coherent biproducts and with the thermal populations of spin waves. It is thus of interest to develop experimental techniques able of measuring spin waves in broad frequency intervals with a large dynamic range, ideally from the floor of the thermal population of spin waves up to the regime of large amplitudes of magnetization precession.
We have studied how a population of spin waves can be characterized from the analysis of the electrical microwave noise delivered by an inductive antenna placed in its vicinity [1]. The measurements are conducted on a synthetic antiferromagnetic thin stripe covered by a micron-sized antenna that feeds a spectrum analyzer after amplification. The antenna noise contains two contributions.
(i) The population of incoherent spin waves generates a fluctuating field that is sensed by the antenna: this is the “magnon noise.”
(ii) The antenna noise also contains the contribution of the electronic fluctuations: the Johnson-Nyquist noise. The latter depends on all impedances within the measurement circuit, which includes the antenna self-inductance. As a result, the electronic noise contains information about the magnetic susceptibility of the stripe, though it does not inform on the absolute amplitude of the magnetic fluctuations.

For micrometer-sized systems at thermal equilibrium, the electronic noise dominates and can account for the measured entire noise (see Figure 1) ; in this case the pure magnon noise cannot be determined. If in contrast the spin wave bath is not at thermal equilibrium with the measurement circuit, and if the spin wave population can be changed then one could measure a mode-resolved effective magnon temperature provided specific precautions are implemented. 
**References** [1] T. Devolder et al., Phys. Rev. B 105, 214404 (2022) 
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Measuring a Population of Spin Waves from the Electrical Noise of an Inductively Coupled Antenna

The areal recording density of hard disk drives has a potential to reach 4 Tb/in2 with the heat-assisted magnetic recording (HAMR) technology [1]. So far, the best candidate for HAMR media is FePt nanogranular thin films with the L10 ordered structure. To improve the recording density and reduce the jitter noise, the bimodal grain size distribution and nanostructural defects, that are unavoidable in actual FePt films, must be suppressed. Grains with {111} twinning variants, [200] misoriented grains, and disordered grains with negligible magnetic anisotropy constants (K = 0) can be listed among the detrimental defects. Evaluation of their volume fractions, V111, V200 and VK=0 respectively, is of a vital interest. A new micromagnetic characterization of FePt nanogranular media is proposed in this work [2]. We developed a method of constructing finite element models of FePt granular media based on high-resolution transmission electron microscopy (TEM) images taken from top and side views of the films (Fig. 1). Using the TEM image based model, an accurate micromagnetic approximation of the out-of-plane hysteresis loops was achieved for MgO(6 nm)/FePt-BN(1 nm)/FePt-(C,SiO2)(7 nm) thin films as shown in Fig. 2. It yielded parameters of uniaxial magnetic anisotropy (mean magnetic anisotropy constant K, its standard deviation, distortion of easy magnetization axes) and the volume fractions of the defects. Evaluated V111 and V200 were in excellent agreement with those determined by synchrotron XRD. After such an experimental validation, we collected a dataset of demagnetization curves by micromagnetic simulations upon different parameters of magnetic anisotropy and volume fractions of defects, then a neural network was trained to predict these parameters and use them as an initial set for the micromagnetic approximation that speeded up the approach significantly [3]. Thus, the developed TEM image based micromagnetic simulations assisted by machine learning can boost optimization of FePt HAMR media linking its nanostructural features to magnetic properties. 
**References** 
[1] K. Hono et al., MRS Bull. 43 (2018) 93. 
[2] A. Bolyachkin et al., Acta Mater. 227 (2022) 117744. 
[3] E. Dengina et al., Scripta Mater. 218 (2022) 114797. 
![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/ac606ea50887649d903f8f32221d1c31.png) 
Fig. 1. Finite element model of FePt granular media constructed based on TEM images. 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/cf38cb194d3e886d75ada05fa2dc4ba6.png) 
Fig. 2. Micromagnetic approximation of an experimental out-of-plane hysteresis loop.

Machine learning assisted micromagnetic approximation for FePt HAMR media

Sputtering technique has been the choice for fabricating L10 FePt granular thin-film media for heat-assisted magnetic recording (HAMR). A variety of amorphous grain boundary materials have been studied extensively [1,2]. Among them, SiOx has been shown to facilitate columnar growth of FePt grains; however, its limitation on deposition temperature prevents high ordering. Amorphous carbon can yield high-temperature-enduring microstructure, however, with a limitation on grain height leveling to an aspect ratio ~1. Amorphous BN was also reported to affect the ordering of FePt [3]. In this paper, we report a study for creating a granular FePt-L10 film with crystalline BN grain boundaries that facilitate small grains with high aspect ratios and high ordering. By applying RF substrate bias with a substrate temperature of 700 oC, a microstructure of columnar FePt grains separated and wrapped around by hexagonal boron nitride(h-BN) layers is produced, as shown in Fig. 1(a). The spacing of the lattice fringes in grain boundaries is ~0.34 nm, close to that of h-BN basal planes (0.331 nm). The electron energy loss spectrum (EELS) shows the π* peak at 191.8 eV in the B-K edge, confirming the formation of turbostratic h-BN. Fig.1(e) shows columnar grains with an aspect ratio of ~1.85. A similar nanostructure was observed in FePt-Carbon film deposited with RF substrate bias, in which the graphite phase was likely formed in grain boundary regions. The covalent binding within the honeycomb structured nanosheets imparts the grain boundary materials high stability at high temperatures. This study provides an extendibility of FePt-based granular media to utilize partial crystalline grain boundary materials to improve microstructure. It also presents more understanding of why carbon and BN can maintain good grain isolation at quite high temperatures. 
**References** 
[1] A. Perumal, Y.K. Takahashi, K. Hono, L10 FePt-C Nanogranular Perpendicular Anisotropy Films with Narrow Size Distribution, 1 (2008) 101301.
https://doi.org/10.1143/apex.1.101301. 
[2] 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://doi.org/10.1063/5.0089009. 
[3] B.H. Li, C. Feng, X. Gao, J. Teng, G.H. Yu, X. Xing, Z.Y. Liu, Magnetic properties and microstructure of FePt/BN nanocomposite films with perpendicular magnetic anisotropy,
Applied Physics Letters. 91 (2007). https://doi.org/10.1063/1.2798584. 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/07f5a5208e585f6f71b6c269715f7b17.png) 
Fig.1 (a) Plane-view HRTEM of FePt-hBN film, (b) FFT shows a ring pattern of h-BN (002), corresponding to a d-spacing of 0.33~ 0.35 nm; (c) and (d) grain size and center-to-center
pitch distance distributions; (e) cross-sectional TEM image of the 8-nm-thick FePt-hBN film grown on MgO seed layer. An order parameter of 0.84 was achieved with substrate
temperature around 700oC.

RF Biased sputtering induced formation of hexagonal boron nitride (h

While CoPt and FePt magnets have high magnetic anisotropy (MAE) when chemically ordered into L10 structures, their components’ high criticality calls for a search for more sustainable systems. Ab initio modelling confirms the pivotal role of the tetragonal order for this useful permanent magnetic property [1]. L10-FeNi, found in meteoritic, tetrataenite samples, also has significant uniaxial anisotropy [2,3]. We report integrated modelling of ternary FeNi-based alloys of both the propensity for L10 order and the MAE. We study Fe50Ni40X10 alloys (X = Pt, Pd, Al, Ti, Mo, Nb, Ta, W). Crucially, to guide material design, our modelling accounts for the physics that the same electrons which produce the MAE also glue the atoms into their places in the crystal structure. There are 3 steps: 1. The MAE is calculated for an ideal (prescribed) ferromagnetic (FM) L10 ordered material (top figure) - alternating Fe and Ni80X20 layers. These values, per formula unit (FU), are shown in the 5th column (col) of the table. 2. The atomic, short-range order (ASRO) in a high temperature, paramagnetic (PM), f.c.c. solid solution, Fe50Ni40X10 is calculated [4] to learn whether L10 order forms readily (col 2) and to determine the compositions of the alternating layers (col 3) if it does (lower figure). 3. The MAE of the ensuing L10 alloys are calculated (col 4). FM L10 Fe-Ni80Pt20 has a large MAE of 0.39 meV/FU but Fe-Ni80Al20’s value of 0.23meV/FU is significant and twice that of L10-FeNi. The chemical ordering, however, supported by the electrons of the materials, does not lead to these desired atomic arrangements and only the Pt, Pd, Al-doped alloys adopt any L10 order with sharply reduced MAEs (col 4). Comparison of the ASRO in PM and FM Fe50Ni50 solid solutions shows a way to circumvent this issue and explain the L10-order in meteoritic FeNi samples. Spin-polarized electronic structure is paramount to establish L10 order - PM Fe50Ni50 has no propensity whatsoever to adopt this order in sharp contrast to the alloy in a FM state which is predicted to order into the L10 structure below 670K [5]. 
[Research performed in part under the auspices of the U.S. Department of Energy, Office of Science under Award Number DE SC0022168.] 
**References:** [1] J. B. Staunton et al., https://doi.org/10.1103/PhysRevB.74.144411 
[2] R. S. Clarke and E. R. D. Scott, American Mineralogist 65, no. 7-8 (1980): 624-630. 
[3] N. Maat et al. Acta Materialia. 2020 Sep 1;196:776 
[4] C. D. Woodgate and J. B. Staunton, https://doi.org/10.1103/PhysRevB.105.115124 
[5] V. Crisan et al., https://doi.org/10.1103/PhysRevB.66.014416 

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Figure 1 
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Figure 2

Integrated ab initio computational modelling of magnetic anisotropy and L10 order in non critical transition metal magnets

Anisotropic assemblies of magnetic nanoparticles (NPs) are highly powerful for designing micro-actuators in various applications, from robotics to biomedicine. Despite great potential, the challenge remains of how to controllably fabricate them at the micro scale due to two limitations: i) the lack of high-remanence nanoscale magnetic materials, which would dictate the resolution and displacement of the actuator; and ii) the lack of robust method for the large-scale synthesis of such materials. In this work, iron cobalt (Fe-Co) magnetic nanochains are prepared with a novel strategy, where the nanochains are formed from dipole-dipole alignment of individual Fe-Co NPs in the absence of an external magnetic field. As opposed to the widely studied field-guided assembly, our spontaneous assembly reported here enables high-throughput fabrication of anisotropic Fe-Co chains. Experiments and simulations show that dipolar magnetism plays a key role in the resulting structures of the Fe-Co chains, whose morphology, width, and length can be tuned by varying the amounts of precursors and fluid rotating speed during the synthesis. The randomly-oriented Fe-Co chains show a remanent magnetization of 28 emu/g, increasing by a factor of 2 compared to the Fe-Co NPs with equal side lengths. Furthermore, this higher remanence in the nanochains is experimentally shown to be advantageous in the development of magnetic soft actuators with a much larger mechanical deformation response to an externally applied field. To demonstrate this point, anisotropic magnetic elastomer composites were fabricated and their mechanical deformation behavior tested in both bending and twisting actuation modes. Both cases showed, very large, visible deformations in low magnetic fields (&lt; 200 Oe). Thus, by developing high-remanence magnetic chains with enhanced yields, composite materials can be made to maximize their effectiveness as micro-actuators or as other device technologies.

Exploring Magnetic Nanochain Anisotropy for Flexible Composite Micro Actuators

The automotion of spin textures, where geometrical gradients are exploited to propagate localized magnetic states without the need of external stimuli, is a promising phenomenon for fast, low-power transmission of magnetic information, as previously demonstrated in planar nanowire systems [1]. Here, we show that automotion is an attractive mechanism in 3D nanowire circuits for the vertical interconnectivity of functional magnetic planes, alternative to the standard current-based spintronic effects normally employed to move spin textures. To showcase this new approach, we have prototyped 3D spiral nanowires by a combination of 3D-printing Focused Electron Beam Induced Deposition and physical vapor deposition methods [2, 3]. The magnetic devices (Fig. 1) are made of Permalloy and are 3 Î¼m tall, 150 nm wide, presenting curvature gradients of 0.09 Î¼m-2. Due to the high directionality of the physical deposition process, they have average thickness gradients of -5.3nm/Î¼m; this very large thickness gradient results to be the dominating mechanism responsible for 3D domain wall automotion, as micromagnetic simulations indicate. We have directly imaged the automotion of domain walls in the 3D spirals using â€œshadowâ€ X-ray photoelectron microscopy [4]. In these experiments, we nucleate a pair of domain walls at different locations within the spirals using external magnetic fields, and directly observe the evolution of the system as the field is reduced until reaching zero value (Fig. 2). The experiments show the successful automotion of the walls along the 3D interconnectors, and its competition with the intrinsic pinning present in the devices at localized regions. This work thus proposes and successfully demonstrates the automotion of spin textures as a viable effect to create 3D interconnected magnetic devices [5].

Domain Wall Automotion for Three Dimensional Magnetic Interconnectivity

Spin–orbit torques provide electrical control of magnetization dynamics in nanoscale heterostructures. Interfaces play a crucial role in these torques from creating the transfer of spin current to the magnetization or generating spin currents and spin accumulations. Spin-orbit torques tend to be localized to the interface because spin currents tend not to penetrate deep into ferromagnets unless the spins are aligned with the magnetization. However, describing a spin-orbit torque as interfacial generally implies that the spin-orbit coupling responsible for the torque is at the interface. That such torques might be important should not be a surprise given the importance of interfacial magnetocrystalline anisotropies and Dzyaloshinskii-Moriya interactions. Unfortunately, it can be difficult to differentiate between interfacial spin-orbit torques and those that originate away from the interfaces. There is no difference in the symmetries of the torques. In addition, changes in other properties can complicate the interpretation of thickness-dependent studies. Never-the-less, in looking to optimize spin-orbit torques, it is useful to understand the role of interfaces. In this talk, I describe the mechanisms that give rise to interfacial spin-orbit torques, discuss where they might be important, and speculate on the role they might play in the commercialization of devices based on spin-orbit torques. 

**References:** 

V. P. Amin, P. M. Haney, and M. D. Stiles, J. of Appl. Phys. 128, 151101 (2020). 
V. P. Amin, J. Zemen, and M. D. Stiles, Phys. Rev. Lett. 121, 136805 (2018). 
V. P. Amin and M. D. Stiles, Phys. Rev. B 94, 104420 (2016). 
V. P. Amin and M. D. Stiles, Phys. Rev. B 94, 104419 (2016).

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Unconventional Emergent Hall Effect Phenomena and its Modification In a van der Waals Ferromagnet Fe3GeTe2

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