It has been predicted that transverse spin current can propagate coherently (without dephasing) over a long distance in antiferromagnetically ordered metals. Here, we determine the dephasing length of transverse spin current in ferrimagnetic CoGd alloys by spin pumping measurements. A modified drift-diffusion model, which accounts for spin-current transmission through the ferrimagnet, reveals that the dephasing length is about 4-5 times longer in nearly compensated CoGd than in ferromagnetic metals. Our results confirm partial mitigation of spin dephasing in antiferromagnetically ordered metals, analogous to spin echo rephasing for nuclear and qubit spin systems.

Confirming the origin of Gilbert damping by experiment has remained a challenge for many decades, even for some of the simplest ferromagnetic metals. In this Letter, we experimentally identify Gilbert damping that increases with decreasing electronic scattering in epitaxial thin films of pure Fe. This observation of conductivity-like damping, 2 which cannot be accounted for by classical eddy current loss, is in excellent quantitative agreement with theoretical predictions of Gilbert damping due to intraband scattering. Our results resolve the longstanding question about the role of intraband scattering inGilbert damping in pure ferromagnetic metals.Damping determines how fast the magnetization relaxes towards the effective magnetic field and plays a central role in many aspects of magnetization dynamics [1,2]. The magnitude of viscous Gilbert damping governs the threshold current for spin-torque magnetic switching and auto-oscillations [3,4], mobility of magnetic domain walls [5,6], and decay lengths of diffusive spin waves and superfluid-like spin currents [7,8]. To enable spintronic technologies with low power dissipation, there is currently much interest in minimizing Gilbert damping in thin films of magnetic materials [9][10][11][12][13], especially ferromagnetic metals [14-18] that are compatible with conventional device fabrication schemes. Despite the fundamental and technological importance of Gilbert damping, its physical mechanisms in various magnetic materialseven in the simplest ferromagnetic metals, such as pure Fehave yet to be confirmed by experiment.Gilbert damping is generally attributed to spin-orbit coupling that ultimately dissipates the energy of the magnetic system to the lattice [1,2]. Kambersky's torque correlation model [19] qualitatively captures the temperature dependence of damping in some experiments [20-23] by partitioning Gilbert damping into two mechanisms due to spin-orbit coupling, namely interband and intraband scattering mechanisms, each with a distinct dependence on the electronic momentum scattering time e. For the interband scattering mechanism where magnetization dynamics can excite electron-hole pairs across different bands, the resulting Gilbert damping is "resistivity-like" as its magnitude scales with e -1 , i.e., increased electronic scattering results in 3 higher damping [24,25]. By contrast, the intraband scattering mechanism is typically understood through the breathing Fermi surface model [26], where electron-hole pairs are excited in the same band, yielding "conductivity-like" Gilbert damping that scales with e, i.e., reduced electronic scattering results in higher damping. Conductivity-like Gilbert damping was reported experimentally more than 40 years ago in bulk crystals of pure Ni and Co at low temperatures, but surprisingly not in pure Fe [20]. The apparent absence of conductivity-like damping in Fe has been at odds with many theoretical predictions that intraband scattering should dominate at low temperatures [27][28][29][30][31][32][33], although some th...

No abstract

We investigate the impact of pinned antiferromagnetic order on the decoherence of spin current in polycrystalline IrMn. In NiFe/Cu/IrMn/CoFe multilayers, we coherently pump an electronic spin current from NiFe into IrMn, whose antiferromagnetic order is globally pinned by static exchangebias coupling with CoFe. We observe no anisotropic spin decoherence with respect to the orientation of the pinned antiferromagnetic order. We also observe no difference in spin decoherence for samples with and without pinned antiferromagnetic order. Moreover, although there is a pronounced resonance linewidth increase in NiFe that coincides with the switching of IrMn/CoFe, we show that this is not indicative of anisotropic spin decoherence in IrMn. Our results demonstrate that the decoherence of electron-mediated spin current is remarkably insensitive to the magnetization state of the antiferromagnetic IrMn spin sink.A spin current is said to be coherent when the spin polarization of its carriers, e.g., electrons, is locked in a uniform precessional phase. How a spin current loses its coherence, particularly as it interacts with magnetic order, is a crucial fundamental question in spintronics [1]. In a ferromagnetic metal (FM), an electronic spin current polarized transverse to the magnetization dephases quickly in the uniform ferromagnetic exchange field [2,3]. Experiments of ferromagnetic resonance (FMR) spin pumping [4][5][6], where a coherently excited spin current propagates from a FM spin source to a FM spin sink [7], show the transverse spin coherence length in FMs to be as short as ≈1 nm [8]. The dephasing of transverse spin polarization s also gives rise to a spin-transfer torque, ∝ m × s × m, acting on the magnetization m of the FM spin sink [2,3,9,10].For antiferromagnetic metals (AFMs) with staggered exchange fields, a fundamental understanding of spin transport has yet to be developed by experiment. Although the transverse spin coherence length can in principle be 1 nm [11][12][13], an electronic spin current polarized transverse to the antiferromagnetic order parameter (Néel vector l) is expected to dephase in the diffusive limit of transport [12,14]. Such spin dephasing in AFMs generates a spin-transfer torque, ∝ l × s × l [13-15], which may be crucial for emerging antiferromagnetic spintronic technologies [16][17][18][19][20].Furthermore, spin dephasing in an AFM with a uniform Néel vector may yield anisotropic decoherence, where spin absorption by the AFM is enhanced when l ⊥ s [21].By contrast, polycrystalline thin films of AFMs by themselves do not exhibit anisotropic spin decoherence on a macroscopic scale, since the grains contain a distribution of Néel vector orientations that averages out the anisotropy [22]. While polycrystalline AFMs have found commercial applications (i.e., pinning ferromagnetic layers in spin valves) [23] and been used as spin sinks [8,22,[24][25][26][27][28], their nonuniform, unpinned antiferromagnetic order poses a challenge for gaining fundamental insight into spin decoherence.To ali...

How spin-orbit torques emerge from materials with weak spin-orbit coupling (e.g., light metals) is an open question in spintronics. Here, we report on a field-like spin-orbit torque (i.e., in-plane spin-orbit field transverse to the current axis) in SiO2-sandwiched permalloy (Py), with the top Py-SiO2 interface incorporating ultrathin Ti or Cu. In both SiO2/Py/Ti/SiO2 and SiO2/Py/Cu/SiO2, this spin-orbit field opposes the classical Oersted field. While the magnitude of the spin-orbit field is at least a factor of 3 greater than the Oersted field, we do not observe evidence for a significant damping-like torque in SiO2/Py/Ti/SiO2 or SiO2/Py/Cu/SiO2. Our findings point to contributions from a Rashba-Edelstein effect or spin-orbit precession at the (Ti, Cu)-inserted interface.2 An electric current in a material with spin-orbit coupling generally gives rise to a non-equilibrium spin accumulation [1-6], which can then exert torquesi.e., spin-orbit torques (SOTs)on magnetization in an adjacent magnetic medium [7][8][9]. SOTs are often classified into two symmetries: damping-like SOT that either counters or enhances magnetic relaxation, and field-like SOT (or "spin-orbit field") that acts similarly to a magnetic field. Next generations of nanomagnetic computing devices may benefit from an improved understanding of mechanisms for SOTs and the discovery of new thin-film systems enabling large SOTs.While most efforts have focused on conductors known for strong spin-orbit coupling (e.g., 5d transition metals, topological insulators, etc.) [7,8], recent reports have shown SOTs in ferromagnets interfaced with materials that are not expected to exhibit significant spin-orbit coupling [10][11][12][13][14]. For example, a large damping-like SOT has been reported in ferromagnetic Ni80Fe20 (permalloy, Py) interfaced with partially oxidized Cu [10,11]; quantum-interference transport measurements have revealed that Cu with an oxidation gradient can, in fact, exhibit enhanced spin-orbit coupling comparable to that in heavier metals (e.g., Au) [15]. As another example of SOTs that emerge by incorporating seemingly weak spin-orbit materials, Py interfaced with a Ti seed layer and Al2O3 capping layer exhibits a sizable field-like SOT [12]. The key observed features of this spin-orbit field in Ti/Py/Al2O3 [12] are: (1) it points in-plane and transverse to the current axis, irrespective of the magnetization orientation in Py; (2) its magnitude scales inversely with the Py thickness, i.e., it is interfacial in origin; (3) it is modified significantly by the addition of an insertion layer (e.g., Cu) at the Py-Al2O3 interface. Ref. [12] claims that this spin-orbit field is governed by a Rashba-Edelstein effect (REE) [1,5,16,17] at the Py/Al2O3 and Cu/Al2O3 interfaces. However, the complicated stack structures of SiO2(substrate)/Ti/Py/(Cu/)Al2O3 with multiple dissimilar interfaces in Ref.[12] obscure the mechanisms of the spin-orbit field, particularly the roles played by the Ti and Cu layers.Here, by using simpler stack structures, we gain i...