Efficient spin transport through native oxides of nickel and permalloy with platinum and gold overlayers

Abstract

We present measurements of spin pumping detected by the inverse spin Hall effect voltage and ferromagnetic resonance spectroscopy in a series of metallic ferromagnet/normal metal thin film stacks. We compare heterostructures grown in situ to those where either a magnetic or nonmagnetic oxide is introduced between the two metals. The heterostructures, either nickel with a platinum overlayer (Ni/Pt) or the nickel-iron alloy permalloy (Py) with a gold overlayer (Py/Au), were also characterized in detail using grazing-incidence X-ray reflectivity, Auger electron spectroscopy, and both SQUID and alternating-gradient magnetometry. We verify the presence of oxide layers, characterize layer thickness, composition, and roughness, and probe saturation magnetization, coercivity, and anisotropy. The results show that while the presence of a non-magnetic oxide at the interface suppresses spin transport from the ferromagnet to the non-magnetic metal, a thin magnetic oxide (here the native oxide formed on both Py and Ni) somewhat enhances the product of the spin mixing conductance and the spin Hall angle. We also observe clear evidence of an out-of-plane component of magnetic anisotropy in Ni/Pt samples that is enhanced in the presence of the native oxide, resulting in perpendicular exchange bias. Finally, the dc inverse spinHall voltages generated at ferromagnetic resonance in our Py/Au samples are large, and suggest values for the spin Hall angle in gold of 0.04 < α SH < 0.22, in line with the highest values reported for Au. This is interpreted as resulting from Fe impurities. We present indirect evidence that the Au films described here indeed have significant impurity levels.2

Thermal spin injection and interface insensitivity in permalloy/aluminum metallic nonlocal spin valves

“…While electrical spin injection in this limit invokes tunneling of spin-polarized electrons, thermal injection in the tunneling limit could proceed by incoherent spin pumping as seen in the longitudinal spin Seebeck effect [66][67][68][69][70][71][72][73][74]. In this picture, the magnetic oxide could increase the effective interfacial spin mixing conductance or allow transport of spin via (non-electronic) collective spin excitations [60,[75][76][77][78], and these effects could contribute to the SDSE signal measured here. Further experiments exploring thermal spin injection in a range of materials and with more carefully controlled and characterized interfaces are required to clarify the potential advantages of thermal spin injection for a wide range of potential spintronic applications.…”

“…As stated above, we attribute the low electrical injection signals and P I to loss of spin information as the spin polarized electrons are injected in the NM. In theory the low P I could indicate a reduced spin polarization in the bulk of the Py itself, though films made from this source in this chamber have historically not shown dramatically reduced values of M s , AMR, or of course Seebeck coefficient [29,60,61]. The most likely cause for the reduced electrical spin injection is the formation of oxidized permalloy at the FM/NM junction that was not fully removed by the RF cleaning step before Al deposition.…”

Influence of Thickness and Interface on the Low-Temperature Enhancement of the Spin Seebeck Effect in YIG Films

“…As recently observed, for metallic interfaces between a metallic ferromagnet and a nonmagnetic metal layer, interface effects can play a crucial role in determining the spin current effects and dominate the measured signals [23][24][25][26]. However, the interfaces between metals are different from insulator-metal interfaces where coupling tends to be more localized.…”

Interface-induced phenomena in magnetism

“…Fluctuations of the antiferromagnetic order provide an efficient pathway for spin current transmission [Saglam et al ., 2016]. Robust spin-transport through the antiferromagnet (insulating NiO) was ascribed to antiferromagnetic moment fluctuations [Wang et al ., 2014; Hahn et al ., 2014; Lin et al ., 2016; Zink et al ., 2016], but uncompensated spins in the antiferromagnet due to defects, grain boundaries, and interfacial roughness may also play a role. Efficient spin transmission through an antiferromagnet (NiO) was inferred from an inverse experiment on a ferromagnet/antiferromagnet/nonmagnet structure [Moriyama et al ., 2015] in which spin-current was generated by the spin Hall effect in the nonmagnetic layer and absorbed via spin-transfer torque in the ferromagnet.…”