While classical electronics focuses on the control of the charge current, spintronics aims at using a spin current, e.g. a flow of angular momentum for logical and memory applications. One of the main use of such a spin current is magnetization switching used for writing in memory devices. Indeed, when a spin current enters into a ferromagnet, its angular momentum (or polarization) is transmitted to the magnetization, and possibly changes its orientation. The transfer of angular momentum from the spin current to the magnetization can be done in two ways depending on the relative orientation between the polarization and the magnetization direction. If they are collinear, the polarization is absorbed by the ferromagnet through diffusive processes originating from the spin switchings during scattering on defects. However, when the magnetization and polarization are transverse, ballistic processes originating from the conduction electron’s spin rotation around the exchange field of the ferromagnet drive the polarization absorption. Understanding the mechanisms of the spin current relaxation into a ferromagnet is therefore of crucial interest for spintronics applications, and remain a seldom explored fundamental question. One of the main difficulty in studying spin current relaxation in a ferromagnetic material is the fact that a charge current generates parasitic effects through the anisotropic magnetoresistance and planar Hall effect. We consequently used lateral spin valves, allowing the measurement of the spin relaxation in a ferromagnet with different relative magnetization direction using a pure spin current, e.g. a flow of angular momentum in the absence of charge current. This measurement allowed us to observe the spin-current relaxation anisotropy in a ferromagnet and to extract the spin mixing conductance, an elusive but fundamental parameter of the spin-dependent transport [1]. Finally, these results allows us to move toward the study of magnetization dependent Spin Hall Effect in ferromagnetic materials.

Spin absorption anisotropy in lateral spin valves

The motion of domain walls by the use of the magnetic fields as well as spin polarized currents has been a major part of spintronics in the last two decades. The two mechanisms of current induced domain wall motion are Spin transfer torque(STT) and Spin orbit torques(SOT). In the recent years, compensated ferrimagnets have been in focus due to their ability to reach the magnetic and the angular momentum compensation points. At the angular mometum compensation point the precessional motion of the local magnetic moment becomes negligible which has shown to have massively increase the domain wall velocity[1,2]. Here we will focus on a rare-earth free ferrimagentic nitride called Manganese Nitride (Mn4N). We have previously shown epitaxially grown Mn4N has a very low magnetization (71 kA/m) and a very high perpedicular magnetic anisotropy (PMA) with mm sized domains[3]. This low magnetization is due to the two different Mn sub-lattices in the anti-perovskite crystal structure. With such a small magnetization and a high spin polarization led us to obtain a very high domain wall velocity of more than 900m/s at J=1.3*1012 A/m2 using pure STT[4]. In this work, we have have shown an even larger domain wall velocity approaching 3000 m/s at J=1.26*1012 A/m2 using pure STT near the magnetic and the angular compensation points in epitaxially grown Ni doped Mn4N[5]. The magnetic and angular momentum compensation points are obtained by the adjustment of Ni doping in this system at room temperature. These high domain wall velocities are comparable to the best velocities obtained by SOT. Also, in this system there is no requirement for an external in-plane field as shown in some other systems by SOT. Moreover, we also demonstrate a reversal of the domian wall motion direction after crossing the compensation points where the domain walls move in the same direction as the flow of electrons. This phenomenon will be explained with the help of ab-initio calculations.

![](https://assets.underline.io/uploads/markdown_image/1/image/c0c2ec29f2ef977b88dd4a5f908583e9.jpg)

Current density vs Velocity curves for Mn4-xNixN thin films. Filled symbols show samples before and the open symbols after the compensation point.


![](https://assets.underline.io/uploads/markdown_image/1/image/b73b2f498d533d34f95601fada6e88b3.jpg)

MS vs Velocity plot, measured for J=1*1012 A/m2(blacksquares) is compared with the best fit obtained using the q−φmodel.

Current driven domain wall dynamics across the angular momentum compensation point in epitaxially grown ferrimagnetic Ni-doped Mn4N thin films

We show that antiperovskite Mn4N transforms from ferri- to ferromagnetism at room temperature (RT) when the Mn content is decreased by the addition of non-magnetic In atoms. Mn4N films have attracted increasing attention as candidates for the current induced domain wall motion (CIDWM) devices. They have a small magnetization (Ms ~ 100 kA/m [1]) and perpendicular magnetic anisotropy, contributing to high CIDWM mobility with low threshold current. In addition, Mn4-xNixN [2] and Mn4-xCoxN [3] epitaxial films have x-dependent magnetic compensation (MC) points at RT, probed by X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) measurements. In Mn4-xNixN nanowires at the vicinity of the MC, we demonstrated ultrafast and efficient CIDWM (3,000 m/s with j=1.2×1012 A/m2) by pure spin transfer torque at RT [4]. In this work, we focused on Mn4-xInxN epitaxial films as new candidates for such materials [5]. We grew 25nm-thick Mn4-xInxN films (x=0, 0.15 and 0.27) on MgO(001) by MBE. XAS and XMCD measurements were performed at the twin APPLE II undulator beam line BL16A of KEK-PF in Japan. Figure 1 shows XAS and XMCD spectra of Mn-L2,3 absorption edges of the samples. In the XAS spectra of all the samples, distinct shoulders marked by arrows appear at about 2 eV above the main L3 edges, which are caused by fcc-Mn(II) atoms [3]. In the XMCD spectrum of Mn4N(x=0), corner-site Mn(I) and fcc-Mn(II) atoms are attributable to peaks α and β, respectively [3]. Their different signs show that the magnetic moments of Mn(I) and Mn(II) are aligned antiparallel. When the In composition x increases up to 0.27, the sign of peak β reverses while that of peak α remains unchanged. It means that the magnetic moment of Mn(II) reverses and now aligns parallel to that of Mn(I), showing a ferri-to-ferromagnetic transition at RT. Figure 2 shows the anomalous Hall effect (AHE) curves at RT. The sign reversal of ρAHE reconfirms the reversal of the magnetic moment of Mn(II) between x = 0.15 and 0.27 in Fig. 1.

![](https://assets.underline.io/uploads/markdown_image/1/image/26c9e42c0a4ba29f78f74bc6894e54cc.jpg)

Fig. 1 XAS and XMCD spectra at Mn-L2,3edges in (a) Mn4N, (b) Mn3.85In0.15N and (c) Mn3.85In0.27N.

![](https://assets.underline.io/uploads/markdown_image/1/image/cfe5232aa05ae86551424552fbca083a.jpg)

Fig. 2 Anomalous Hall effect curves of Mn4−xInxN (x=0.15 and 0.27) films on MgO(001).

Making ferrimagnetic Mn4N ferromagnetic by non-magnetic element doping

While classical spintronics has traditionally relied on ferromagnetic metals as spin generators and spin detectors, a new approach called spin-orbitronics exploits the interplay between charge and spin currents enabled by the spin-orbit coupling in non-magnetic systems, in particular using the spin Hall effect. However, the interconversion efficiency of the direct and inverse spin Hall effect is a bulk property that rarely exceeds ten percent, and does not take advantage of interfacial and low-dimensional effects otherwise ubiquitous in spintronics. In this contribution, we report the observation of spin-to-charge current conversion in strained mercury telluride, using spin pumping experiments at room temperature. We show that a HgCdTe barrier can be used to protect the HgTe topological surface states, leading to high conversion rates, with inverse Edelstein lengths up to 2.0±0.5 nm. These measurements, associated with the temperature dependence of the resistivity, suggest that these high conversion rates are due to the spin momentum locking property of HgTe surface states [1]. We then focus on the SrTiO3 (STO)-based 2D electron system, presenting experiments performed on NiFe/Al/STO heterostructures. We investigate the nature of the spin-to-charge conversion through a combination of spin pumping, magnetotransport, spectroscopy and gating experiments, finding a very highly efficient spin-to-charge conversion, with inverse Edelstein lengths beyond 20 nm. More importantly, we demonstrate that the conversion rate can be tuned in amplitude and rate by a gate voltage. We then discuss the amplitude of the effect and its gate dependence on the basis of the electronic structure of the 2DES and highlight the importance of a long scattering time to achieve efficient spin-to-charge interconversion. Finally, we harness the electric-field-induced ferroelectric-like state of SrTiO3 to manipulate the spin–orbit properties of the two-dimensional electron gas. Using spin pumping techniques and a back-gate voltage (cf. figure), we efficiently convert spin currents into positive or negative charge currents, depending on the polarization direction [3]. This non-volatile effect opens the way to the electric-field control of spin currents and to ultralow-power spintronics, in which non-volatility would be provided by ferroelectricity rather than by ferromagnetism.

**References:**

[1] P. Noel et al., Phys. Rev. Lett. 120, 167201 (2018) [2] D. C. Vaz et al., Nature Materials 1-7 (2019) [3] P. Noël et al., Nature, 580(7804), 483-486 (2020)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/3c004d9b544e1474370e93e56b3a4a2b.jpg)
Left: Scheme of the spin-pumping technique. A rf field is used to induce magnetization precession in the NiFe layer. This leads to the injection of a pure spin current Js into the 2D electron gaz at the surface of STO. This spin current is then converted into a charge current Jc, which can be detected using a voltmeter. The back gate is used to pole the STO. Right: For a given spin current, the produced charge currrent depends in a non-volatile way of the gate voltage.

 Spin to charge conversion in the topological insulator HgTe and in STO-based two-dimensional electron gas INVITED

The spin to charge interconversion arising from spin-orbit effects is an important tool for spin manipulation. The charge current to spin current conversion has attracted lots of attention as it can be used to write information through the spin orbit torque. The reciprocal mechanism, the spin to charge conversion, has also been found to be essential, in particular as it enables the reading process of magneto-electric spin-orbit devices [1, 2]. Whereas the efficiency of the interconversion is a crucial issue for applications, a consensus on the measured values seems to remain elusive, even for standard materials. The spin to charge interconversion can be for instance obtained by the Spin Hall Effect in platinum. The bulk contribution of platinum to the interconversion efficiency has been widely studied, and the interfacial contributions to the interconversion, such as the spin memory loss at the ferromagnet-platinum interface [3] and spin backflow [4], have been identified as key factors in the measurement of the spin to charge interconversion. These two contributions indeed decrease the amount of spin current flowing through the interface, possibly leading to a wrong estimation of the interconversion efficiency. Here, we will focus on the estimation of the interface resistance of the spin-orbit material/ferromagnetic interface, and of its exact role on spin to charge interconversion, showing that it has up to now been frequently wrongly overlooked. In this contribution, we measured electrically the inverse and direct spin to charge interconversion in ferromagnet/Pt nanostructures (Fig. 1) for different ferromagnetic materials. We find that the amplitude of the Spin Hall signal depends drastically on the magnetic material (CoFe, Co, or NiFe) used for the injection and the detection of the spin current. We show that the use of a conventional analysis, neglecting the interface resistance at the ferromagnet-Pt interface, gives effective spin Hall angle for Pt which are actually strongly dependent on the used ferromagnetic material. By using cross geometry measurement and finite element simulations, we succeeded in extract the interface resistance between Pt and the ferromagnetic materials. Using these resistance values, we show that in order to obtain a consistent spin to charge interconversion efficiency value in the different systems, an interfacial spin scattering asymmetry must be taken into account. This parameter, despite being frequently overlooked in spinorbitronics experiment, appears to be essential for a proper extraction of the spin to charge interconversion efficiency. Finally, it show that an important interface resistance can, contrarily with what could be expected, enhance the spin Hall signal in electrical measurements (Fig. 2). These results thus provide guidelines for the optimization of the optimization of the write and read processes using spin to charge interconversion in materials with strong spin orbit coupling.

**References:**

[1] S. Manipatruni et al. Nature Phys. 14, 338 (2018) [2] V. T. Pham, M. Cosset-Cheneau et al, arXiv:2011.12207 (2020) [3] J.-C. Rojas-Sanchez et al, Phys. Rev. Lett. 112, 106602 (2014) [4] W. Zhang et al, Nature Phys. 11, 496–502 (2015)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/6123e54e5e7c2ea0aa50c47e29d7ef35.jpg)
Sketches presenting the measurement using the ferromagnet (FM)/ spin-orbit coupling (SOC) nanostructure. The ferromagnetic electrodes allows to probe the spin accumulation and the charge-spin interconversion, and can reciprocally be used to inject a spin current in the SOC material to study the spin-charge interconversion. The studied SOC material is Pt.

![](https://assets.underline.io/uploads/markdown_image/1/image/a874514a41b1600a02d21f56caa56179.jpg)
Evolution of the interconversion signal when changing the interface resistance RA. The parameter γ represents the amplitude of the spin scattering assymetry.

 Role of the interfacial asymmetric spin scattering at ferromagnet-Pt interfaces

The spin to charge interconversion has been intensively studied in 5d materials such as platinum. In these non-magnetic systems, the relation between the charge current, the spin current and its polarization is strongly constrained by symmetries. Recently, a strong spin to charge interconversion efficiency has been obtained in 3d ferromagnets with strong spin orbit interactions such as NiCu. In ferromagnetic materials, the symmetry breaking induced by the presence of the magnetization lifts the geometrical constraints on the relation between charge and spin currents. However, the link between the ferromagnetic order and spin to charge interconversion remains unclear. By using spin pumping measurements, we injected a spin current from a CoFeB layer into the weak ferromagnets Ni60Cu40 and Ni50Cu50. The charge current generated by the spin to charge interconversion was subsequently measured across the Curie temperatures (Tc) of these alloys. A change of sign of the generated spin current was observed across the Tc when the CoFeB and NiCu layers are in direct contact. However, this sign change disappears when inserting a copper layer between the two. This indicates that two ferromagnetic materials can be coupled and influence each other at the Ferromagnetic Resonance (FMR), thus questioning the results of studies claiming the observation of spin fluctuation and spin-charge interconversion using FMR measurements. Most importantly, the fact that the generated charge current does not change across the Curie temperature of NiCu alloys indicates that the spin-charge interconversion does not depend on the magnetic phase [1]. The independence of the interconversion on the magnetic phase shows either that the band structure of NiCu is not affected, or that its change across the Tc does not influence the spin-charge interconversion. These results consequently provide an important contribution to both the experimental and theoretical understanding of the spin to charge interconversion in ferromagnetic materials.

Independence of the Inverse Spin Hall Effect with the Magnetic Phase in Thin NiCu Films

Spin-orbitronics exploits the interplay between charge and spin currents enabled by the spin-orbit coupling in non-magnetic systems. This spin-orbit coupling is indeed the phenomenon at the origin of spin-charge interconversions in bulk materials, via the spin Hall effect, and at surfaces presenting a broken inversion symmetry, via the Rashba-Edelstein effect. Recently, spin-to-charge conversion studies in Metal/SrTiO3-based two-dimensional (2D) electron gases have revealed a high conversion efficiency via the inverse Rashba-Edelstein effect [1]. The following demonstration of non-volatile electric-control of the spin-to-charge conversion further brings to light the great potential of these quantum materials for novels ultra-low power spintronics applications [2]. Achieving a metal barrier thickness study is thus desirable to gain insight on the different contributions to the charge-to-spin conversion in metal/SrTiO3-based 2D electron gases. Here, we report spin-orbit torques results obtained by spin-torque ferromagnetic resonance (ST-FMR) in Metal/SrTiO3-based 2D electron gases, for different metal barrier thicknesses. A demonstration of charge-to-spin conversion at the metal/oxide interface is firstly achieved by ST-FMR on both Ta and Mg barrier-based heterostructures. The spin-orbit torques are further investigated for different temperatures, revealing temperature-dependent symmetric and antisymmetric ST-FMR signals. We further achieve the non-volatile electric-control of the sheet resistance of the 2D electron gas in ST-FMR devices, which makes it possible to investigate the gate dependence of the charge-to-spin conversion via ST-FMR techniques. Finally, the dependence of the charge-to-spin conversion with the metallic barrier thickness is discussed, providing routes to improve the spin-orbit torques, and opening the way to optimized spintronics devices based on Metal/SrTiO3 2D electron gases.

![](https://assets.underline.io/uploads/markdown_image/1/image/c24dfd85212876a78c60e1c793e05850.jpg)

Fig.1: ST-FMR signals in Mg/SrTiO3 heterostructure at 77K

Spin-Orbit Torques Dependence on the Metal Barrier Thickness in Metal/SrTiO3-based Two-Dimensional Electron Gases Studied by Spin-Torque Ferromagnetic Resonance

The realization of magnetization switching induced by in-plane current injection in heavy metal/ferromagnetic heterostructures has drawn increasing attention to spin-orbitronics, leading to the advent of spin-orbit torques magnetoresistive random access memories (SOT-MRAM). Compared to heavy metals, oxide 2D electron gases have emerged as alternative spin-orbitronics material systems, which benefit from an efficient spin-charge interconversion through the direct and inverse Edelstein effects. Recently, we have demonstrated an enhancement of the spin-to-charge conversion efficiency by two orders of magnitude in SrTiO3-based 2D gas compared to conventional heavy metals [1], along with a non-volatile electric-control of the spin-to-charge conversion [2]. While the sign and efficiency of the SOTs are fixed by the stack of materials in conventional SOT devices, achieving an electric-control of the mirror charge-to-spin conversion would be of great interest for developing reconfigurable SOT-MRAM and logic gates. Here we report electric-field control of spin orbit torques with electrical remanence in a perpendicular ferromagnet - SrTiO3 system. Non-volatile electric-field control of the sheet resistance is achieved with 1150% contrast, with two switchable and remanent high and low resistivity states of the 2D electron gas. Spin-orbit torques effective fields are further measured using second harmonic Hall methods. A remanent electric-field control of the SOT efficiency is demonstrated, with inversion of the sign of the SOT anti-damping-like effective field. These results are consistent with a combination of both intrinsic modulation of the SOT efficiency together with extrinsic modulation due to the non-volatile electric-control of the current injection in the 2D gas. The non-volatile control of the SOT effective field is further evidenced by reproducible inversion of the SOT torques after initializing with negative or positive voltage pulses of ±130 V, opening the way to reconfigurable spin-orbit torque memory and logic gate architectures.

![](https://assets.underline.io/uploads/markdown_image/1/image/c659e26f3f654d16f4c29d73b4605f6d.jpg)

Fig.1: (a) Structure of the sample and measurement geometry. (b) Electric-field control of spin orbit torques with electrical remanence.

Non-Volatile Electric-Field Control of Spin-Orbit Torques and Anomalous Hall Effect in Perpendicular Ferromagnet - SrTiO3 System

**Session chair:** Mingzhong Wu, Colorado State University


GOJ - Metal Spintronics

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Presentations

Spin absorption anisotropy in lateral spin valves

Current driven domain wall dynamics across the angular momentum compensation point in epitaxially grown ferrimagnetic Ni-doped Mn4N thin films

Making ferrimagnetic Mn4N ferromagnetic by non-magnetic element doping

Spin to charge conversion in the topological insulator HgTe and in STO-based two-dimensional electron gas INVITED

Role of the interfacial asymmetric spin scattering at ferromagnet-Pt interfaces

Independence of the Inverse Spin Hall Effect with the Magnetic Phase in Thin NiCu Films

Spin-Orbit Torques Dependence on the Metal Barrier Thickness in Metal/SrTiO3-based Two-Dimensional Electron Gases Studied by Spin-Torque Ferromagnetic Resonance

Non-Volatile Electric-Field Control of Spin-Orbit Torques and Anomalous Hall Effect in Perpendicular Ferromagnet - SrTiO3 System

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