Direct observation of spin accumulation in Cu induced by spin pumping

Abstract

Pure spin currents have been ubiquitous in contemporary spintronics research. Despite its profound physical and technological significance, the detection of pure spin current has largely remained indirect, which is usually achieved by probing spin-transfer torque effects or spin-to-charge conversions. By using scanning transmission x-ray microscopy, we report the direct detection and spatial mapping of spin accumulation in a nonmagnetic Cu layer without any direct charge current injection. Such a pure spin current is induced by spin pumping from a Ni 80 Fe 20 layer and is not accompanied by concomitant charge motion. The observed frequency dependence indicates that the signal is dominated by a coherent, pure spin current, but the magnitude of the spin accumulation suggests also possible additional thermal contributions. Our technique takes advantage of the x-ray magnetic circular dichroism and the synchronization of microwave with x-ray pulses, which together provide a high sensitivity for probing transient magnetic moment. From the detected x-ray signals, we observe two distinct resonance modes induced by spin pumping, which, based on micromagnetic simulations, we attribute to nonlinear microwave excitations. Our result provides a new pathway for detecting pure spin currents that originate from many spintronics phenomena, such as spin Hall and spin Seebeck effects, and which can be applied to both metal and insulator spin current sources.

Direct Imaging of the ac Component of Pumped Spin Polarization with Element Specificity

“…5. The lateral extent of the pumped ac spin polarization is different from previous findings of the dc [15] and the ac [16] components in Py/Cu heterostructures, where the polarization is confined to the area of the ferromagnet. A substantial lateral extent of the ac component of the pumped spin polarization inside the nonferromagnetic insulator can be supported by another consideration.…”

“…Unlike in ISHE experiments, a high-Z metallic material, like Pt or Pd, can be avoided and we can use Co:ZnO as a highly insulating and rather low-Z material. Therefore, the spin relaxation times and/or the spin coherence length can be expected to be much larger than, for example, in metallic Cu, where the expected decay length is of the order of 300 nm at 300 K [30] and is therefore not easily detected by the spatial resolution of the STXM [15,16]. In fact, long spin coherence lengths and times at room temperature are well known for ZnO [31] from optical measurements.…”

“…This combination allows us to utilize the known lateral resolution of a few tens of nanometers of the STXM to investigate the lateral extent of the generated spin polarization. This basic approach has already been shown to be feasible for electrical spin injection into a nonferromagnetic metal [15] and very recently to also probe the ac component in Py/Cu heterostructures [16], which was evidenced before using time-resolved measurements without spatial resolution [17]. The time-resolved detection scheme based on the internal picosecond time structure of the synchrotron allows us to sample dynamics up to the gigahertz regime, i.e., FMR frequencies [14].…”

Spin accumulation in metallic thin films induced by electronic impurity scattering

“…For Cu, the experimental observations for the normalized spin accumulation vary between 6 × 10 −9 and 3 × 10 −12 μ B cm 2 A −1 depending on the experimental situation [53,54]. These are dramatically larger than our predictions of the order of ∼10 −13 μ B cm 2 A −1 .…”

“…In contrast, by doping the full 3d series of impurities into the Pt thin film, we find that all values and profiles of normalized spin accumulation remain similar, which highlights the importance of strong intrinsic spin-orbit coupling of the host. Furthermore, our results of normalized spin accumulation for the Pt thin film are in good agreement with experimental observations [51,52], while our results of Cu thin film are at first glance incompatible with experimental findings [53,54]. This discrepancy can be attributed to the fact that in experiments ferromagnetic materials at the interface to Cu might alter the actual spin accumulation significantly, which remains an open topic for further investigations.…”