This content will become publicly available on November 29, 2023

Absorption of transverse spin current in ferromagnetic NiCu: Dominance of bulk dephasing over spin-flip scattering

In ferromagnetic metals, transverse spin currents are thought to be absorbed via dephasing—i.e., destructive interference of spins precessing about the strong exchange field. Yet, due to the ultrashort coherence length of ≈1 nm in typical ferromagnetic thin films, it is difficult to distinguish dephasing in the bulk from spin-flip scattering at the interface. Here, to assess which mechanism dominates, we examine transverse spin-current absorption in ferromagnetic NiCu alloy films with reduced exchange fields. We observe that the coherence length increases with decreasing Curie temperature, as weaker dephasing in the film bulk slows down spin absorption. Moreover, nonmagnetic Cu impurities do not diminish the efficiency of spin-transfer torque from the absorbed spin current. Our findings affirm that the transverse spin current is predominantly absorbed by dephasing inside the nanometer-thick ferromagnetic metals, even with high impurity contents.

Authors:
;  ;  ;  ;  ;
Award ID(s):
Publication Date:
NSF-PAR ID:
10382835
Journal Name:
Applied Physics Letters
Volume:
121
Issue:
22
Page Range or eLocation-ID:
Article No. 222403
ISSN:
0003-6951
Publisher:
American Institute of Physics
National Science Foundation
##### More Like this
1. Abstract

While induced polarization of a palladium overlayer on antiferromagnetic and magneto-electric Cr2O3(0001) is possible because of the boundary polarization at the Cr2O3(0001), in the single domain state, the palladium (Pd) thin film appears to be ferromagnetic on its own, likely as a result of strain. In the conduction band, we find the experimental evidence of ferromagnetic spin polarized in Pd thin films on a Cr2O3(0001) single crystal, especially in the thin limit, Pd thickness of around 1-4 nm. Indeed there is significant spin polarization in 10 Å thick Pd films on Cr2O3(0001) at 310 K, i.e. above the Néel temperature of bulk Cr2O3. While Cr2O3(0001) has surface moments that tend to align along the surface normal, for Pd on Cr2O3, the spin polarization contains an in-plane component. Strain in the Pd adlayer on Cr2O3(0001) appears correlated to the spin polarization measured in spin polarized inverse photoemission spectroscopy. Further evidence for magnetization of Pd on Cr2O3is provided by measurement of the exchange bias fields in Cr2O3/Pd(buffer)/[Co/Pd]n exchange bias systems. The magnitude of the exchange bias field is, over a wide temperature range, virtually unaffected by the Pd thickness variation between 1 and 2 nm.

2. Abstract

Van der Waals heterostructures offer great versatility to tailor unique interactions at the atomically flat interfaces between dissimilar layered materials and induce novel physical phenomena. By bringing monolayer 1 T’ WTe2, a two-dimensional quantum spin Hall insulator, and few-layer Cr2Ge2Te6, an insulating ferromagnet, into close proximity in an heterostructure, we introduce a ferromagnetic order in the former via the interfacial exchange interaction. The ferromagnetism in WTe2manifests in the anomalous Nernst effect, anomalous Hall effect as well as anisotropic magnetoresistance effect. Using local electrodes, we identify separate transport contributions from the metallic edge and insulating bulk. When driven by an AC current, the second harmonic voltage responses closely resemble the anomalous Nernst responses to AC temperature gradient generated by nonlocal heater, which appear as nonreciprocal signals with respect to the induced magnetization orientation. Our results from different electrodes reveal spin-polarized edge states in the magnetized quantum spin Hall insulator.

3. Abstract

The circumgalactic medium (CGM) plays a vital role in the formation and evolution of galaxies, acting as a lifeline between galaxies and the surrounding intergalactic medium. In this study, we leverage a unique sample of quasar pairs to investigate the properties of the CGM with absorption line tomography. We present a new sample of medium-resolution Keck/ESI, Magellan/MagE, and VLT/XSHOOTER spectra of 29 quasar pairs at redshift 2 <z< 3. We supplement the sample with additional spectra of 32 pairs from the literature, creating a catalog of 61 quasar pairs with angular separations between 1.″7 and 132.″9 and projected physical separations (r) between 14 kpc and 887 kpc. We construct a catalog of 906 metal-line absorption doublets of Civ(λλ1548, 1550) with equivalent widths ranging from 6 m Å ≤Wr,1550≤ 2053 m Å. The best-fit linear model to the log-space equivalent width frequency distribution ($logf(Wr)=mlog(Wr)+b$) of the sample yields coefficients ofm= −1.44 ± 0.16 andb= −0.43 ± 0.16. To constrain the projected extent of Civ, we calculate the transverse autocorrelation function. The flattening of the autocorrelation function at lowrprovides a lower limit for the coherence length of the metal enriched CGM—on the order ofmore »

4. Abstract

The emergence of ferromagnetism in materials where the bulk phase does not show any magnetic order demonstrates that atomically precise films can stabilize distinct ground states and expands the phase space for the discovery of materials. Here, the emergence of long-range magnetic order is reported in ultrathin (111) LaNiO3(LNO) films, where bulk LNO is paramagnetic, and the origins of this phase are explained. Transport and structural studies of LNO(111) films indicate that NiO6octahedral distortions stabilize a magnetic insulating phase at the film/substrate interface and result in a thickness-dependent metal–insulator transition att = 8 unit cells. Away from this interface, distortions relax and bulk-like conduction is regained. Synchrotron x-ray diffraction and dynamical x-ray diffraction simulations confirm a corresponding out-of-plane unit-cell expansion at the interface of all films. X-ray absorption spectroscopy reveals that distortion stabilizes an increased concentration of Ni2+ions. Evidence of long-range magnetic order is found in anomalous Hall effect and magnetoresistance measurements, likely due to ferromagnetic superexchange interactions among Ni2+–Ni3+ions. Together, these results indicate that long-range magnetic ordering and metallicity in LNO(111) films emerges from a balance among the spin, charge, lattice, and orbital degrees of freedom.

5. Abstract

Spin-orbit torque nano-oscillators based on bilayers of ferromagnetic and nonmagnetic metals are ultra-compact current-controlled microwave signal sources. They are attractive for practical applications such as microwave assisted magnetic recording, neuromorphic computing, and chip-to-chip wireless communications. However, a major drawback of these devices is low output microwave power arising from the relatively small anisotropic magnetoresistance of the ferromagnetic layer. Here we experimentally show that the output power of a spin-orbit torque nano-oscillator can be significantly enhanced without compromising its structural simplicity. Addition of a ferromagnetic reference layer to the oscillator allows us to employ current-in-plane giant magnetoresistance to boost the output power of the device. This enhancement of the output power is a result of both large magnitude of giant magnetoresistance compared to that of anisotropic magnetoresistance and their different angular dependencies. Our results hold promise for practical applications of spin-orbit torque nano-oscillators.