United States

Magnetic Tunnel junctions (MTJs) with Co2FeAl Heusler alloy electrode are expected to show giant TMR ratio at room temperature (RT) because of its half-metallicity. In previous work, although Co2FeAl/MgO/CoFe-MTJ showed a TMR ratio of 330% at RT [1], it is still smaller than those for MTJs with CoFeB electrodes. In order to realize a large TMR ratio at RT, it is necessary to fabricate high-quality Heusler alloy electrodes and to control the interface at the atomic level. Therefore, I focused on molecular beam epitaxy (MBE) technique, which enables the fabrication of highly ordered Heusler alloy thin films [2] and atomic level interfacial modification [3]. In this work, we prepared epitaxial Co2FeAl thin films by MBE technique and characterized their magnetic properties to identify the half-metallicity of the Co2FeAl films. In addition, TMR effect in Co2FeAl/MgO/CoFe-MTJs fabricated by MBE were investigated.
The films were deposited on MgO(001) substrate using MBE and sputtering methods. The stacking structure of the Co2FeAl films were Cr(20)/Co2FeAl(50)/MgO(5) (unit of nm). After the fabrication of Co2FeAl layers, the films were annealed at Ta=200~700°C. Structural and magnetic properties were measured by XRD, VSM and FMR. The stacking structure of MTJs was Cr(20)/Co2FeAl(30)/MgO(2.0)/CoFe(5)/IrMn(10)/Cr(5) (unit of nm). After the microfabrication, the MTJ devices were annealed at TMTJ=275~425°C under 1T magnetic fields. TMR effects were measured by DC 4-probe method.
The Co2FeAl film with Ta=600°C showed high B2 order parameter and high magnetization. In addition, as shown in Fig.1, it showed very small damping constant α of 1.8×10-3, indicating that the film has good half-metallicity. Fig.2 shows the magnetoresistance curves of the MTJs measured at RT. A relatively large TMR ratio of 125% at TMTJ=400°C was observed and the TMR ratio can be improved by insertion of ultrathin Mg or Al into the Co2FeAl/MgO interface due to suppression of the interfacial state and oxidation.
This work was supported by NEDO, JSPS, GP-Spin Program and JST SPRING, Grant Number JPMJS2114.&lt;br&gt;&lt;br&gt;
**References** [1] W. Wang, H. Sukegawa and K. Inomata, Appl. Phys. Lett. 95, 182502 (2009)
[2] M. Oogane, A. P. McFadden and C. J. Palmstrøm, Appl. Phys. Lett. 112, 262407 (2018)
[3] K. Himi, K. Takahashi and S. Mitani, Appl. Phys. Lett. 78, 1436 (2001)
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MMM 2022

Damping constant of Co2FeAl thin film deposited by molecular beam epitaxy and tunnel magnetoresistance effect using them

Magnetic Tunnel junctions (MTJs) with Co2FeAl Heusler alloy electrode are expected to show giant TMR ratio at room temperature (RT) because of its half-metallicity. In previous work, although Co2FeAl/MgO/CoFe-MTJ showed a TMR ratio of 330% at RT [1], it is still smaller than those for MTJs with CoFeB electrodes. In order to realize a large TMR ratio at RT, it is necessary to fabricate high-quality Heusler alloy electrodes and to control the interface at the atomic level. Therefore, I focused on molecular beam epitaxy (MBE) technique, which enables the fabrication of highly ordered Heusler alloy thin films [2] and atomic level interfacial modification [3]. In this work, we prepared epitaxial Co2FeAl thin films by MBE technique and characterized their magnetic properties to identify the half-metallicity of the Co2FeAl films. In addition, TMR effect in Co2FeAl/MgO/CoFe-MTJs fabricated by MBE were investigated.
The films were deposited on MgO(001) substrate using MBE and sputtering methods. The stacking structure of the Co2FeAl films were Cr(20)/Co2FeAl(50)/MgO(5) (unit of nm). After the fabrication of Co2FeAl layers, the films were annealed at Ta=200~700°C. Structural and magnetic properties were measured by XRD, VSM and FMR. The stacking structure of MTJs was Cr(20)/Co2FeAl(30)/MgO(2.0)/CoFe(5)/IrMn(10)/Cr(5) (unit of nm). After the microfabrication, the MTJ devices were annealed at TMTJ=275~425°C under 1T magnetic fields. TMR effects were measured by DC 4-probe method.
The Co2FeAl film with Ta=600°C showed high B2 order parameter and high magnetization. In addition, as shown in Fig.1, it showed very small damping constant α of 1.8×10-3, indicating that the film has good half-metallicity. Fig.2 shows the magnetoresistance curves of the MTJs measured at RT. A relatively large TMR ratio of 125% at TMTJ=400°C was observed and the TMR ratio can be improved by insertion of ultrathin Mg or Al into the Co2FeAl/MgO interface due to suppression of the interfacial state and oxidation.
This work was supported by NEDO, JSPS, GP-Spin Program and JST SPRING, Grant Number JPMJS2114. 
**References** [1] W. Wang, H. Sukegawa and K. Inomata, Appl. Phys. Lett. 95, 182502 (2009)
[2] M. Oogane, A. P. McFadden and C. J. Palmstrøm, Appl. Phys. Lett. 112, 262407 (2018)
[3] K. Himi, K. Takahashi and S. Mitani, Appl. Phys. Lett. 78, 1436 (2001)
 
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technical paper

#### Welcome to the 67th Annual Conference on Magnetism and Magnetic Materials, MMM 2022!

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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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Rare-earth magnets may provide useful magnetic properties for quantum technologies, including quantum transduction and quantum memories. Here we calculate the band structure of magnons in ferromagnetic Er2O3. Erbium, in the +3 state in this material, has 11 electrons in a 4f shell which are shielded from the surroundings by valence electrons. The localization of these 4f electrons around the ion core results in long coherence times that make Er2O3 a viable candidate for emerging technologies and devices such as quantum memories, single photon emitters and transduction [1].

We consider fully saturated Er2O3 as a ferromagnetic insulator that hosts magnons and approximate the interaction of the spins as magnetic dipole-dipole interaction. Similar models will describe other dipole-dipole interactions spins, such as ensembles of molecular spins. The Holstein-Primakoff representation is employed to quantize the Hamiltonian. Er2O3 has 32 erbium atoms in a non-primitive cubic unit cell. The long-range nature of magnetic dipole-dipole interaction poses a challenging issue for considering magnonic structure from dipole-dipole interactions. The dipole-dipole interaction drops as a cube power of the separation between spins, thus the strength of the coupling of the 30th nearest neighbor is 4.3% of the first nearest neighbor while the number of neighbors increases. If these long-distance neighbors are ignored the magnonic dispersion will drastically change. We will describe approaches to reach convergence in the magnon dispersion for such long-range interactions.

This work was supported as part of the Center for Molecular Quantum Transduction, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award Number DE-SC0021314. 
**References** [1] Zhong, Tian and Goldner, Philippe. "Emerging rare-earth doped material platforms for quantum nanophotonics" Nanophotonics, vol. 8, no. 11, 2019, pp. 2003-2015.

Magnonic dispersion of Er2O3

Magnetic tunnel junctions (MTJs) are currently at the forefront of spintronics research because their large magnetoresistance (MR) and high sensitivity make them an attractive application prospect for magnetic memories and magnetic sensors [1]. MTJs with perpendicular magnetic anisotropy (PMA) are advantageous over in-plane devices due to their scalability, stronger remnant magnetization for a smaller demagnetizing field in the perpendicular direction, and higher signal-to-noise ratio due to the enhanced uniaxial orientation [1]. The perpendicular anisotropy is also essential to achieve faster magnetization switching and to minimize stray fields from a magnetoresistance junction.
Heusler compounds are one of the most promising candidates for MTJ electrodes because their half-metallic states can help achieve 100 % spin polarization at room temperature, leading to an infinite magnetoresistance ratio. Furthermore, these materials have additional advantages, including high Curie temperature and long spin diffusion length [2]. Recently, our group has discovered Weyl semi-metal behaviors [3], and PMA [4] in Heusler-based Co2MnGa ultrathin films, making it a promising material for exploring spin-polarising devices.
In this work, we used Co2MnGa electrodes to obtain perpendicularly magnetized MTJ stacks. We used thin-film sputtering and photolithography to fabricate the devices. We optimized the devices by precisely varying the thickness of each Co2MnGa and MgO layer and annealing conditions. We used the magneto-optical Kerr effect (MOKE) microscopy for domain imaging and MR measurements of the multilayer stacks. The MTJ showed strong PMA, and we could identify two distinct magnetic steps in the field sweep curve corresponding to the switching direction of the two magnetic layers. The devices also showed high MR, demonstrating that Co2MnGa-based perpendicular MTJs are a strong candidate for the next generation of spintronic devices.
**References** [1] Maciel, N.; Marques, E.; Naviner, L.; Zhou, Y.; Cai, H. Magnetic Tunnel Junction Applications. Sensors, 2019, 20 (1), 121. https://doi.org/10.3390/s20010121. 
[2] Elphick, K.; Frost, W.; Samiepour, M.; Kubota, T.; Takanashi, K.; Sukegawa, H.; Mitani, S.; Hirohata, A. Heusler alloys for spintronic devices: review on recent development and future perspectives. Sci. Technol. Adv. Mater., 2021, 22 (1), 235–271. https://doi.org/10.1080/14686996.2020.1812364.
[3] Zhang, Y.; Yin, Y.; Dubuis, G.; Butler, T.; Medhekar, N. V.; Granville, S. Berry curvature origin of the thickness-dependent anomalous Hall effect in a ferromagnetic Weyl semi-metal. npj Quantum Mater., 2021, 6 (1). https://doi.org/10.1038/s41535-021-00315-8.
[4] Ludbrook, B. M.; Ruck, B. J.; Granville, S. Perpendicular magnetic anisotropy in Co 2 MnGa and its anomalous Hall effect. Appl. Phys. Lett., 2017, 110 (6), 062408. https://doi.org/10.1063/1.4976078.
 

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Heusler Alloy Based Perpendicular Magnetic Tunnel Junctions

Metallic superlattices such as Co/Ir/Co exhibit the symmetric interlayer exchange coupling (IEC) (1). Recently, the antisymmetric interlayer exchange coupling (AIEC) was observed in a metallic superlattice when the in-plane spatial inversion symmetry was broken (2-5). However, the controllability and the applicability of AIEC have not been examined yet, and the systematic experiments exploiting well-controlled in-plane structural asymmetry are indispensable. This study systematically investigated the AIEC for the wedge-shaped Pt/Co/Ir/Co/Pt to clarify the nature of AIEC.
A double-wedged sample (DWS) of Ta (1)/Pt (2)/Co (tCo)/Ir (tIr)/Co (0.5)/Pt (2)/Ta (1) (thickness in nm) was deposited on a SiOx substrate using magnetron sputtering. The tCo and tIr were continuously varied from 0.6 to 1.6 nm and from 0 to 1.5 nm, respectively. The wedge direction of the Ir layer was orthogonal to that of the wedged bottom Co layer. DWS was patterned into Hall-bar-shaped devices, and the magnetic properties were investigated by exploiting the anomalous Hall effect (AHE). The IEC in Co/Ir/Co was confirmed from the periodic change of the saturation field (μ0Hs) against tIr. The AHE curves were largely shifted with an additional in-plane magnetic field (μ0Hip) of 50 mT (Fig. 1), indicating the existence of AIEC. The largest switching field shift (μ0ΔHsw) was 14.8 mT at tCo ~ 0.80 nm and tIr ~ 0.27 nm, which was an order of magnitude larger than the values reported previously (0.7~1.7 mT (3,5)). Similar to the tIr dependence of μ0Hs, μ0ΔHsw showed the oscillatory change against tIr with a local minimum at tIr ~ 0.87 nm, suggesting the correlation between the symmetric IEC and the AIEC. Furthermore, we developed the extended Stoner-Wohlfarth model and proposed the tunability of the AIEC effective field by the ratio of the AIEC energy and the antiferromagnetic coupling strength. Finally, we theoretically and experimentally demonstrated the perpendicular magnetization switching solely induced by μ0Hip for DWS. This study provides crucial information to manipulate the three-dimensional chiral magnetic structures (6). 

**References:** 
[1] M. D. Stiles, J. Magn. Magn. Mater., Vol. 200, p.322-337 (1999). 
[2] E. Y. Vedmedenko et al., Phys. Rev. Lett., Vol. 122, p.257202 (2019). 
[3] D.-S. Han et al., Nat. Mater., Vol. 18, p.703 (2019). 
[4] A. Fernández-Pacheco et al., Nat. Mater., Vol. 18, p.679 (2019). 
[5] K. Wang et al., Commun., Phys. Vol. 4, p.10 (2021). 
[6] H. Masuda et al., Phys. Rev. Appl., Vol. 17, p.054036 (2022). 

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Large antisymmetric interlayer exchange coupling enabling perpendicular magnetization switching by an in plane magnetic field

The electric-field induced spin-orbit torque (SOT) in antiferromagnetic (AFM) Weyl semimetals (WSMs) is theoretically investigated. Unlike in the ferromagnetic (FM) counterparts (1), the magnetic domain walls (DWs) in the AFM WSMs appears to lack the torsion in the magnetization and thus unable to benefit from this highly efficient mechanism that originates from the axial magnetic effect. Contrarily, our calculations illustrate that the addition of the Dzyaloshinskii-Morya interaction can introduce a twist in the magnetic moment, giving rise to the non-zero axial magnetic field and net spin current in the AFM textures subjected to an electric field. The non-zero components of the exchange fields of spin-polarized Weyl fermions are shown in Fig. 1. In the case of a Bloch DW (Fig. 1a), the direction of the driving field varies along the DW texture, significantly reducing the effect of SOT. Contrarily, the contribution of the unidirectional effective field along the Néel DW location (Fig. 1b) appears cumulative getting more efficiency. The dynamics of the DW motion is analyzed by considering the balance of energy absorption and dissipation. It reveals the need to account for the contribution of the exchange dissipation mechanism (2) beyond the Gilbert-like term to compensate the unusual superlinear rate of energy absorption by the AFM textures. The obtained DW velocity over the magnon velocity vm vs electric field shows a significant speed-up for the Néel DW in AFM WSMs (Fig.2) compared with the FM WSM as well as those in the non-topological FMs and AFMs. The results clearly indicate the significance of the axial magnetic effect in the dynamics of spin textures in AFM WSMs. 

**References:** 
(1) D. Kurebayashi and K. Nomura, "Theory for spin torque in Weyl semimetal with magnetic texture," Sci. Rep. 9, 5365 (2019). 
(2) J. H. Mentink, J. Hellsvik, D. V. Afanasiev, B. A. Ivanov, A. Kirilyuk, A. V. Kimel, O. Eriksson, M. I. Katsnelson, and Th. Rasing, "Ultrafast spin dynamics in multisublattice magnets," Phys. Rev. Lett. 108, 057202 (2012). 

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Electrical Control of Antiferromagnetic Domain Walls in Weyl Semimetals

Field-free switching of perpendicular magnetization has been observed in an epitaxial L11-ordered CuPt/CoPt bilayer [1] and attributed to spin-orbit torque (SOT) arising from the crystallographic 3m point group of the interface. Using a first-principles nonequilibrium Green’s function formalism [2] combined with the Andersen disorder model, we calculate the angular dependence of the SOT in a CuPt/CoPt bilayer and find that the magnitude of the 3m SOT is about 20% of the conventional dampinglike SOT. We study the magnetization dynamics in a perpendicularly magnetized circular nanodisk in the presence of the 3m SOT and Dzyaloshinskii-Moriya interaction using micromagnetic simulations and find that even a relatively small 3m torque can enable domain-wall-mediated field-free magnetization reversal. For further insight, we derive a collective-coordinate model describing the motion of a domain wall in a nanodisk. Numerical solutions and stability analysis of the equations of motion in this model are used to establish the roles of the dampinglike, fieldlike, and 3m SOT in the magnetization reversal process.

Wuzhang Fang and Edward Schwartz contribute equally to this work. This work was supported by NSF through Grant No. DMR-1916275, and by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award No. DE-SC0021019. 
**References** [1] L. Liu et al., Nat. Nanotechnol. 16, 277 (2021).
[2] K. D. Belashchenko, A. A. Kovalev, and M. van Schilfgaarde, Phys. Rev. Materials 3, 011401(R) (2019); Phys. Rev. B 101, 020407(R) (2020).

First principles calculation of the 3m spin

Ferrimagnetic Mn4N (Fig. 1) is a promising candidate for various spintronics devices thanks to their small magnetization (Ms ~ 100 kA/m) and clear perpendicular magnetic anisotropy (Ku = 0.1 MJ/m)[1]. Ni-[2,3] and Co-doped[4] Mn4N films have magnetic compensation (MC) points. Using Ni-doped Mn4N films at the vicinity of the MC composition, our group demonstrated ultrafast domain wall motion (vDW = 3,000 m/s[5]) driven only by spin-transfer torque at RT thanks to the compensation. Additionally, In-doped Mn4N[6] films show ferromagnetism. In this work, we focus on Sn-doped Mn4N (Mn4−xSnxN) film. Although the MC in Mn4−xSnxN bulks was reported at low temperature[7], no studies have been reported on MC of Mn4−xSnxN films. Therefore, we fabricated Mn4−xSnxN epitaxial films and evaluated their properties.
25-nm-thick Mn4−xSnxN epitaxial films (x = 0.0–1.0) were grown on the MgO(001) substrates by plasma-assisted molecular beam epitaxy. Both the longitudinal and transverse resistivities (ρxx & ρxy) were measured with van der Pauw method at RT. The ordinary Hall coefficient (RH) was calculated from the slope of the ρxy-μ0H loops at high field regime. The anomalous Hall resistivities ρAH were derived by excluding the component of ordinary Hall effect from ρxy.
Fig. 2 shows the ordinary Hall coefficient (RH) and the anomalous Hall angle (θAH (= ρAH/ρxx)) of Mn4−xSnxN films as a function of Sn composition, x. xcc is the composition where the sign of RH reversed. It suggests that the dominant career-type changed from electrons (RH < 0) to holes (RH > 0) with increasing x. Moreover, the sign reversal of θAH was observed twice at the composition except xcc, indicating it was not caused by change in the career-type. Although similar results were also obtained in Ni-[2], Co-[4] and In-[6] doped Mn4N, this was the first report that the multiple sign reversals of θAH were confirmed in Mn4N-based compounds. The origin seems to be the change of magnetic structures such as MC and ferrimagnetic-to-ferromagnetic transition. 
**References** [1] T. Hirose et al., AIP Adv. 10, 025117 (2020).
[2] T. Komori et al., J. Appl. Phys. 125, 213902 (2019).
[3] T. Komori et al., J. Appl. Phys. 127, 043903 (2020).
[4] H. Mitarai et al., Phys. Rev. Mater. 4, 094401 (2020).
[5] S. Ghosh et al., Nano Lett. 21, 2580 (2021).
[6] T. Yasuda et al., J. Phys. D. Appl. Phys. 55, 115003 (2022).
[7] M. Mekata, J. Phys. Soc. Japan 17, 796 (1962). 
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Epitaxial growth of Sn doped Mn4N films and their magneto

Spin precession and spin wave propagation with ferromagnetic resonance (FMR) rely essentially on the following points: a) the external field Hex is applied in order to set the precession axis of spins in the direction of Hex, b) the generated spin wave propagates in the direction perpendicular to the Hex, and c) the generation of spin waves owes its origin to the ferromagnetic spin exchange interaction acting between neighboring spins. Although in the case of the spin thermoelectric (STE) generation the precession of local 3d-electron spins in Fe3+ is thermally excited and its excitation is irrelevant to the resonance phenomenon,1 the three points noted above on FMR are considered to be important also for generating spin currents thermally in a ferrimagnetic insulator (FMI) due to the temperature gradient ▽T. Ref. 2 noted that the driving force for generating spin currents is due to the nonequilibrium dynamics of magnons in the FMI driven by ▽T.2 Based on our studies on STE generation1,3,4, we consider that the spin exchange interaction plays a major role as the driving force for the generation of spin currents in an FMI. The process in Fig. 1 shows the role of the ferromagnetic spin exchange interaction for generating spin currents in the FMI due to ▽T. We regard the ferromagnetic spin exchange interaction as the cause of the driving force for generating spin currents. Fig. 2 illustrates the generation of spin currents in YIG or Bi-substituted YIG by ▽T in the longitudinal spin Seebeck effect (LSSE). Each spin makes the spin precession of different magnitude due to ▽T, because each position of the spins is different in temperature. The precession of each spin then reaches a steady state, settling down to a uniform-mode-like state at tn from the initial state at t0 due to the exchange force working between spins. This idea well explains the generation of STE voltage for all the Hex and ▽T arrangements that have been reported in experiments so far. 
**References** 1M. Imamura, H. Asada and R. Nishimura, IEEE Trans. Magn., Vol. 58, p.4500111 (2022)
2K. Uchida, H. Adachi and T. Kikkawa, Proc. IEEE, Vol. 104, p.1946 (2016)
3M. Imamura, H. Asada and R. Nishimura, AIP Advances, Vol. 11, p.035143 (2021)
4M. Imamura, H. Asada and R. Nishimura, J. Magn. Magn. Mater., Vol. 550, p.169081 (2022) 
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Underlying Mechanism of the Driving Force for Generating Spin Currents Thermally in a Ferrimagnetic Insulator Due to a Temperature Gradient

Spin-orbit-torque (SOT) based magnetic devices1 have gained considerable attention due to their non-volatility, speed, and energy efficiency. SOT-induced magnetization switching studies are limited by the rise-time and duration of available pulsed current sources 2,3. Recently, ultrafast SOT switching has been demonstrated using ~ps current pulses4,obtained from an optically excited photoconductive (Auston) switch4,5, although the switching dynamics have not yet been reported. Here, we report the ultrafast magnetization dynamics measured via the time-resolved polar magneto-optical Kerr effect (MOKE)5
in a ferromagnet (inset of Fig. 1), using the ultrafast SOT
effect due to a ~ 9 ps long current pulse generated from an Auston switch. The ~ps current pulse, without any in-plane magnetic field, introduces a demagnetization of ~30% and softens the magnet at an ultrafast timescale. In the presence of a 1600 Oe symmetry-breaking in-plane field, SOT tries to rotate the magnetic moments away from the saturation state, for a negative
current pulse (blue line in Fig. 1), which when combined with the ultrafast demagnetization, shows a reduced initial demagnetization coupled with SOT-oscillations at longer timescales.
However, for positive current, it induces a coherent rotation of the softened magnetic moments toward the negative saturation. Magnetization crosses zero in ~65 ps and switches fully after
about ~200 ps (red line in Fig. 1). Sub-ns current pulse-induced SOT dynamics are governed by domain-wall propagation with an additional incubation delay due to domain nucleation 6-8.
The absence of any incubation delay combined with the fast-switching time leads us to believe that the switching is probably governed by the coherent rotation of the softened magnetic
moment due to ultrafast heating by the ~ps current pulse excitation. A micromagnetic simulation coupled with 2-temperature ultrafast heating qualitatively agrees well with the observed dynamics (dotted lines in Fig. 1). Our work sheds significant insights on the mechanism of picosecond SOT-induced magnetization dynamics. 

**References** 1. D. Polley, A. Pattabi, J. Chatterjee, et al., Appl. Phys. Lett. 120, 140501 (2022). 
2. I.M. Miron, K. Garello, G. Gaudin, et al., Nature 476, 189 (2011). 
3. K. Garello, C.O. Avci, I.M. Miron, et al., Appl. Phys. Lett. 105, 212402 (2014). 
4. K. Jhuria, J. Hohlfeld, A. Pattabi, et al., Nat. Electron. 3, 680 (2020). 
5. Y. Yang, R.B. Wilson, J. Gorchon, et al., Sci. Adv. 3, e1603117 (2017). 
6. E. Grimaldi, V. Krizakova, G. Sala, et al., Nat. Nanotechnol. 15, 111 (2020). 
7. V. Krizakova, K. Garello, E. Grimaldi, et al., Appl. Phys. Lett. 116, 232406 (2020). 
8. G. Sala, V. Krizakova, E. Grimaldi, et al., Nat. Commun. 12, 656 (2021). 

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Fig. 1. Ultrafast magnetization dynamics with positive and negative ~ps current pulse direction. The theoretical analysis is shown in dotted lines.

Spin Orbit Torque Induced Ultrafast Magnetization Switching in a Ferromagnet

Co2-based full-Heusler compounds are among the most promising materials due to their exotic transport properties, high thermal stability, high Curie temperatures (Tc â‰ˆ 1100 K) for the bulk sample, high magnetic moment (~ 6 ÂµB/f.u.), low Gilbert damping constant (Î± = 0.004) and large anomalous Hall effect linked with their band structure [1-3]. The Taylor-Ulitovsky technique, known since 60-s and allows fast (up to a few hundred meters per minute) preparation quite long (up to few kilometers) and thin (typically 0.5 to 40 Âµm in diameter) metallic wires coated by flexible and insulating glass coating [4]. In addition, the existence of thin, flexible, insulating, and highly transparent glass coating, enhanced mechanical properties and fast and inexpensive fabrication method, glass coating can be useful for biomedical applications due to its biocompatibility [5]. We report on preparation and magnetic properties of Co2FeSi Heusler alloy glass-covered microwires with a metallic nucleus diameter of about 4,4 Âµm and total sample diameter of about 17,6 Î¼m. From the X-ray diffraction, XRD, analysis of the as-prepared Co2FeSi, it was shown that the structure consists of a mixture of nanocrystalline and amorphous phases. An increase in the average grain size, Dg, from 17.8 to 31.6 nm and content of nanocrystalline phase, C from 52 % to 97 % upon annealing at 873 K (1 h) is obtained from XRD analysis. Estimated from temperature dependence of magnetic moment Tc is between 1040 K and 1059 K. As illustrated in Fig. 1, the Co2-based Heusler glass-coated microwires present strong dependence of the magnetization and the structural properties on the annealing conditions. For annealing temperatures between 873 K and 973 K magnetic phase transition has been observed, as evidenced by sharp drop of the magnetic moment when we applied 50 Oe and 200 Oe external magnetic fields during FC and FH and change in hysteresis loops (see Fig.1).

Preparation and magnetic properties of Co2 based Heusler alloy glass

Switch timings of MRAM technologies, such as Spin Transfer Torque (STT) and Spin Orbit Torque (SOT), are critical in the context of SRAM replacement for last level
cache applications [1]. The operating conditions of Magnetic tunnel junction (MTJ) result from the compromise between writing current level, retention and stability during read operation.
To optimize the operating conditions for a given application and tune the physical parameters of the MTJs, we propose a new system able to extract the incubation and switching times
under writing pulses application.
The proposed system operates by acquiring and analyzing the reflected signal of the writing pulse applied on the MTJ, which could be a 2 or 3 terminal (STT[2] or SOT[3]) MRAM. The
test system includes a pulse generator, a set of RF circuits and a high-resolution scope. The test sequence operates in a fully automated manner, in a 2 steps process. Firstly, an average
signal in both P (red line) and AP (blue line) states are acquired as reference measurements. Secondly, single shots or averaged measurements of the switching events are acquired and the
reference data is subtracted, as shown in Fig.1 on STT-MRAM devices. The probability of switching and average switching time versus pulse amplitude and width can be extracted. The
validity of fast macrospin switching, without any domain wall formation can be verified. With single shot measurements, switching time of single events can be acquired, as well as the
incubation time before the switching occurs [2]. The full test flow is executed in a few seconds, enabling the possibility to test population of devices using automated probing stations.
Using the system and our 3D magnetic generator, the test protocol can also be executed under application of an external magnetic field to study the field immunity of the device and the
corresponding influence on the switching dynamics. 

**References:** 

[1] P. Barla, V. K. Joshi, and S. Bhat, J. Comput. Electron. 20, 805–837 (2021).
[2] J. Sampaio, S. Lequeux, P. Metaxas, et al., APL 103, 242415 (2013).
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