Observation of Anomalous Spin Torque Generated by a Ferromagnet

Bose, Arnab; Lam, D. D.; Bhuktare, Swapnil; Dutta, Sutapa; Singh, Harbans; Jibiki, Yuma; Goto, Minori; Miwa, Shinji; Tulapurkar, Ashwin

doi:10.1103/physrevapplied.9.064026

Cited by 54 publications

(36 citation statements)

References 52 publications

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“…Some isolate the spin Hall current whose spin direction is transverse to the magnetization [13,14]. Others measure the spin current with a magnetization-aligned spin direction [8][9][10][11]15], as would be expected for bulk spin currents in ferromagnets that have spin directions aligned with the magnetization [21]. Das et al quantify contributions from both transverse and magnetization-aligned spin directions within Py [12], providing experimental evidence supporting some of the results presented here.…”

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confidence: 74%

Intrinsic spin currents in ferromagnets

et al. 2019

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First principles calculations show that electric fields applied to ferromagnets generate spin currents flowing perpendicularly to the electric field. Reduced symmetry in these ferromagnets enables a wide variety of such spin currents. However, the total spin current is approximately the sum of a magnetization-independent spin Hall current and an anisotropic spin anomalous Hall current. Intrinsic spin currents are not subject to dephasing, enabling their spin polarizations to be misaligned with the magnetization, which enables the magnetization-independent spin Hall effect. The spin Hall conductivity and spin anomalous Hall conductivities of transition metal ferromagnets are comparable to those found in heavy metals, opening new avenues for efficient spin current generation in spintronic devices.Introduction-Over the past few decades, the study of electrical spin current generation has focused on two systems: ferromagnets without spin-orbit coupling and nonmagnets with spin-orbit coupling. In ferromagnets without spin-orbit coupling, real space and spin space are decoupled. As a result, spin currents are simple products of particle flow and spin direction, each of which independently satisfies the system's symmetries. Thus, carriers must flow parallel to the electric field and carrier spins must align with the magnetization. In nonmagnets with spin-orbit coupling, real space and spin space are coupled. Because of this coupling, the net spin current is no longer the simple product of particle flow and spin direction. Any spin current that satisfies the system's symmetries is allowed. Isotropic symmetry allows for spin currents in which the charge flow, spin flow, and spin direction are mutually orthogonal. The generation of such spin currents satisfying these constraints is known as the spin Hall effect [1][2][3][4][5][6][7].

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Intrinsic spin currents in ferromagnets

et al. 2019

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“…For the rest of this Paper, we refer to this SHE as 'conventional SHE.' To combine the virtues of transverse spin transport with magnetic recording, the conventional SHE was studied in magnetic materials with broken TRS (ferromagnets [31][32][33][34][35][36][37][38][39][40][41][42][43][44] or antiferromagnets [30,[45][46][47][48]), which revealed various phenomena associated with the interplay of SOC and magnetism. For example, ferromagnetic metals exhibit an (inverse) conventional SHE [31]; unaffected by magnetization reversal [33] it is time-reversal even.Since charge currents in ferromagnets are intrinsically spinpolarized, transverse AHE currents are spin-polarized as well and are used to generate spin torques [32,35].…”

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confidence: 99%

Origin of the magnetic spin Hall effect: Spin current vorticity in the Fermi sea

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Neumann

Johansson

et al. 2020

Phys. Rev. Research

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The interplay of spin-orbit coupling (SOC) and magnetism gives rise to a plethora of charge-to-spin conversion phenomena that harbor great potential for spintronics applications. In addition to the spin Hall effect, magnets may exhibit a magnetic spin Hall effect (MSHE), as was recently discovered [Kimata et al., Nature 565, 627-630 (2019)]. To date, the MSHE is still awaiting its intuitive explanation. Here we relate the MSHE to the vorticity of spin currents in the Fermi sea, which explains pictorially the origin of the MSHE. For all magnetic Laue groups that allow for nonzero spin current vorticities the related tensor elements of the MSH conductivity are given. Minimal requirements for the occurrence of a MSHE are compatibility with either a magnetization or a magnetic toroidal quadrupole. This finding implies in particular that the MSHE is expected in all ferromagnets with sufficiently large SOC. To substantiate our symmetry analysis, we present various models, in particular a two-dimensional magnetized Rashba electron gas, that corroborate an interpretation by means of spin current vortices. Considering thermally induced spin transport and the magnetic spin Nernst effect in magnetic insulators, which are brought about by magnons, our findings for electron transport can be carried over to the realm of spincaloritronics, heat-to-spin conversion, and energy harvesting. I. FROM THE CONVENTIONAL TO THE MAGNETIC SPIN HALL EFFECTThe spin Hall effect (SHE) [1] and its inverse are without doubt important discoveries [2][3][4][5][6] in the field of spintronics [7,8]. They serve not only as 'working horses' for generating and detecting spin currents [9] but also as key ingredients in spin-orbit torque devices for electric magnetization switching [10][11][12]. Compared to spin-transfer torque devices [13-17], spin-orbit torque devices are faster, more robust, and consume less power upon operation [18][19][20]; for a recent review see Ref. 21. While the anomalous Hall effect (AHE) in a magnet [22] produces a transverse charge current density upon applying an electric field E, the SHE in a nonmagnet produces a transverse spin current density j γ = σ γ E (γ = x, y, z indicates the transported spin component). Mathematically, the SHE is quantified by the antisymmetric part of the spin conductivity tensor σ γ . For example, the σ z xy element comprises z-polarized spin currents in x direction as a response to an electric field in y direction.In a simple picture, the intrinsic SHE [23,24] is explained by spinning electrons that experience a spin-dependent Magnus force caused by spin-orbit coupling (SOC). It appears as if 'built-in' spin-dependent magnetic fields evoke spin-dependent Lorentz forces that result in a transverse pure spin current. The extrinsic SHE [25][26][27] is covered by Mott scattering at defects [28].Since the SHE does not rely on broken time-reversal symmetry (TRS), it is featured in nonmagnetic metals [29] or semiconductors [2]. Imposing few demands on a material's properties, a SHE can be expected in...

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“…We have earlier shown that linewidth change can be sensitively measured in planar Hall structure doing ST-FMR [Ref. 32]. We follow the same approach in this work to compare the strength of spin Hall torque (SHT) and spin Nernst torque (SNT).…”

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confidence: 99%

“…where MS is saturation magnetization, tPt, tPy are thickness of Pt and Py film respectively, H ⊥ is perpendicular magnetic anisotropy field. Now, if dc current is superimposed on the RF current then non-zero dc spin current is injected which can change the resonance linewidth of Py 2,3,32 . It also provides direct quantification of effective spin Hall angle as following:…”

Section: ( )mentioning

confidence: 99%

“…If the injected spin current density is enough, spin torque is expected on the FM causing enhanced or reduced damping depending upon the direction of spin vectors absorbed by FM (Fig 1(b-c)). We compare the change in resonance linewidth of FM due to the spin torque generated by SNE and SHE by performing spintorque ferromagnetic resonance (ST-FMR) experiment 2,4,6,30,31,32 . Basic working principle of ST-FMR is the following.…”

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Control of magnetization dynamics by spin-Nernst torque

et al. 2018

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Relativistic interaction between electron's spin and orbital angular momentum has provided efficient mechanism to control magnetization of nano-magnets. Extensive research has been done to understand and improve spin-orbit interaction driven torques generated by non-magnets while applying electric current. In this work, we show that heat current in non-magnet can also couple to its spin-orbit interaction to produce torque on adjacent ferromagnet. Hence, this work provides a platform to study spin-orbito-caloritronic effects in heavy metal/ferromagnet bi-layers.Since last few years considerable attention has been drawn to control the magnetization dynamics by pure spin current generated by spin Hall effect (SHE) 1,2,3,4 and interfacial magnetic fields 5,6 by Rashba effect. SHE and Rashba effects are relativistic phenomena which couple electron's spin and orbital motion and can be used to exert spin-orbit torques 7 , 8 . On the other hand, thermal gradient in ferromagnet can also create pure spin current 9,10,11,12,13 which can further produce thermal spin torques 14,15,16,17,18 and domain wall motion 19,20 . Conversion of heat current into spin current in a non-magnet has been shown recently via the spin Nernst effect (SNE) 21,22,23,24 . But an important question remains unanswered whether thermal gradient in non-magnet can generate spin toque owing to its spin-orbit coupling, which in turn could be used for manipulating magnetization. In this letter we demonstrate that, interplay of heat current and spin-orbit coupling in non-magnetic Platinum (Pt) can generate thermally driven spin-orbit torque (equivalent to spin Nernst torque (SNT)). SNT has been predicted recently 25,26 but it is lacking the experimental evidence. Here, we show that effective magnetic damping can be controlled by SNT while creating thermal gradient in Pt/Ni81Fe19 bilayer. This can open a new avenue to manipulate spins in magnetic nanostructures for technological applications 27,28,29 .

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Observation of Anomalous Spin Torque Generated by a Ferromagnet

Cited by 54 publications

References 52 publications

Intrinsic spin currents in ferromagnets

Intrinsic spin currents in ferromagnets

Origin of the magnetic spin Hall effect: Spin current vorticity in the Fermi sea

Control of magnetization dynamics by spin-Nernst torque

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