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Register a new userSpin Hall magnetoresistance at Pt/CoFe2O4 interfaces and texture effects
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We report magnetoresistance measurements on thin Pt bars grown on epitaxial (001) and (111) CoFe2O4 (CFO) ferrimagnetic insulating films. The results can be described in terms of the recently discovered spin Hall magnetoresistance (SMR). The magnitude of the SMR depends on the interface preparation conditions, being optimal when Pt/CFO samples are prepared in situ, in a single process. The spin-mixing interface conductance, the key parameter governing SMR and other relevant spin-dependent phenomena such as spin pumping or spin Seebeck effect, is found to be different depending on the crystallographic orientation of CFO, highlighting the role of the composition and density of magnetic ions at the interface on spin mixing.
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We have studied the spin Hall magnetoresistance (SMR), the magnetoresistance within the plane transverse to the current flow, of Pt/Co bilayers. We find that the SMR increases with increasing Co thickness: the effective spin Hall angle for bilayers with thick Co exceeds the reported values of Pt when a conventional drift-diffusion model is used. An extended model including spin transport within the Co layer cannot account for the large SMR. To identify its origin, contributions from other sources are studied. For most bilayers, the SMR increases with decreasing temperature and increasing magnetic field, indicating that magnon-related effects in the Co layer play little role. Without the Pt layer, we do not observe the large SMR found for the Pt/Co bilayers with thick Co. Implementing the effect of the so-called interface magnetoresistance and the textured induced anisotropic scattering cannot account for the Co thickness dependent SMR. Since the large SMR is present for W/Co but its magnitude reduces in W/CoFeB, we infer its origin is associated with a particular property of Co.
We conducted a systematic angular dependence study of nonlinear magnetoresistance in NiFe/Pt bilayers at variable temperature and field using the Wheatstone bridge method. We successfully disentangled magnon magnetoresistance from other types of magnetoresistances based on their different temperature and field dependences. Both the spin Hall/anisotropic and magnon magnetoresistances contain sine phi and sine 3 phi components with phi the angle between current and magnetization, but they exhibit different field and temperature dependence. The competition between different types of magnetoresistances leads to a sign reversal of sine 3 phi component at a specific magnetic field, which was not reported previously. The phenomenological model developed is able to account for the experimental results for both NiFe/Pt and NiFe/Ta samples with different layer thicknesses. Our results demonstrate the importance of disentangling different types of magnetoresistances when characterizing the charge-spin interconversion process in magnetic heterostructures.
We show theoretically that both intrinsic spin Hall effect (SHE) and orbital Hall effect (OHE) can arise in centrosymmetric systems through momentum-space orbital texture, which is ubiquitous even in centrosymmetric systems unlike spin texture. OHE occurs even without spin-orbit coupling (SOC) and is converted into SHE through SOC. The resulting spin Hall conductivity is large (comparable to that of Pt) but depends on the SOC strength in a nonmonotonic way. This mechanism is stable against orbital quenching. This work suggests a path for an ongoing search for materials with stronger SHE. It also calls for experimental efforts to probe orbital degrees of freedom in OHE and SHE. Possible ways for experimental detection are briefly discussed.
Ultra-thin Pt films grown on insulating ferrimagnetic CoFe2O4 (111) epitaxial films display a magnetoresistance upon rotating the magnetization of the magnetic layer. We report here X-ray magnetic circular dichroism (XMCD) recorded at Pt-L2,3 and Pt-M3 edges. The results indicate that the Pt magnetic moment, if any, is below the detection limit (< 0.001 {mu}$_B$/Pt), thus strongly favoring the view that the presence of CoFe2O4 does not induce the formation of magnetic moments in Pt. Therefore, the observed magnetoresistance cannot be attributed to some sort of proximity-induced magnetic moments at Pt ions and subsequent magnetic-field dependent scattering. It thus follows that either bulk (spin Hall and Inverse spin Hall Effects) or interface (Rashba) spin-orbit related effects dominate the observed magnetoresistance. Furthermore, comparison of bulk magnetization and XMCD data at (Fe,Co)-L2,3 edges suggests the presence of some spin disorder in the CoFe2O4 layer which may be relevant for the observed anomalous non-saturating field-dependence of spin Hall magnetoresistance.
We investigated spin Hall magnetoresistance in FeMn/Pt bilayers, which was found to be one order of magnitude larger than that of heavy metal and insulating ferromagnet or antiferromagnet bilayer systems, and comparable to that of NiFe/Pt bilayers. The spin Hall magnetoresistance shows a non-monotonic dependence on the thicknesses of both FeMn and Pt. The former can be accounted for by the thickness dependence of net magnetization in FeMn thin films, whereas the latter is mainly due to spin accumulation and diffusion in Pt. Through analysis of the Pt thickness dependence, the spin Hall angle, spin diffusion length of Pt and the real part of spin mixing conductance were determined to be 0.2, 1.1 nm, and $5.5 * 10^{14} {Omega}^{-1} m^{-2}$, respectively. The results corroborate the spin orbit torque effect observed in this system recently.
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