skip to main content

Title: 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.

more » « less
Award ID(s):
Author(s) / Creator(s):
; ; ; ; ;
Publisher / Repository:
American Institute of Physics
Date Published:
Journal Name:
Applied Physics Letters
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    While induced spin polarization of a palladium (Pd) 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 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]nexchange 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.

    more » « less
    more » « less
  3. Abstract

    Strong damping‐like spin‐orbit torque (τDL) has great potential for enabling ultrafast energy‐efficient magnetic memories, oscillators, and logic. So far, the reported τDLexerted on a thin‐film magnet must result from an externally generated spin current or from an internal non‐equilibrium spin polarization in non‐centrosymmetric GaMnAs single crystals. Here, for the first time a very strong, unexpected τDLis demonstrated from current flow within ferromagnetic single layers of chemically disordered, face‐centered‐cubic CoPt. It is established here that the novel τDLis a bulk effect, with the strength per unit current density increasing monotonically with the CoPt thickness, and is insensitive to the presence or absence of spin sinks at the CoPt surfaces. This τDLmost likely arises from a net transverse spin polarization associated with a strong spin Hall effect, while there is no detectable long‐range asymmetry in the material. These results broaden the scope of spin‐orbitronics and provide a novel avenue for developing single‐layer‐based spin‐torque memory, oscillator, and logic technologies.

    more » « less
  4. Crystallographic anisotropy of the spin-dependent conductivity tensor can be exploited to generate transverse spin-polarized current in a ferromagnetic film. This ferromagnetic spin Hall effect is analogous to the spin-splitting effect in altermagnets and does not require spin-orbit coupling. First-principles screening of 41 non-cubic ferromagnets revealed that many of them, when grown as a single crystal with tilted crystallographic axes, can exhibit large spin Hall angles comparable with the best available spin-orbit-driven spin Hall sources. Macroscopic spin Hall effect is possible for uniformly magnetized ferromagnetic films grown on some low-symmetry substrates with epitaxial relations that prevent cancellation of contributions from different orientation domains. Macroscopic response is also possible for any substrate if magnetocrystalline anisotropy is strong enough to lock the magnetization to the crystallographic axes in different orientation domains. 
    more » « less
  5. Abstract

    Optically active spin defects in van der Waals materials are promising platforms for modern quantum technologies. Here we investigate the coherent dynamics of strongly interacting ensembles of negatively charged boron-vacancy ($${{{{{{{{\rm{V}}}}}}}}}_{{{{{{{{\rm{B}}}}}}}}}^{-}$$VB) centers in hexagonal boron nitride (hBN) with varying defect density. By employing advanced dynamical decoupling sequences to selectively isolate different dephasing sources, we observe more than 5-fold improvement in the measured coherence times across all hBN samples. Crucially, we identify that the many-body interaction within the$${{{{{{{{\rm{V}}}}}}}}}_{{{{{{{{\rm{B}}}}}}}}}^{-}$$VBensemble plays a substantial role in the coherent dynamics, which is then used to directly estimate the concentration of$${{{{{{{{\rm{V}}}}}}}}}_{{{{{{{{\rm{B}}}}}}}}}^{-}$$VB. We find that at high ion implantation dosage, only a small portion of the created boron vacancy defects are in the desired negatively charged state. Finally, we investigate the spin response of$${{{{{{{{\rm{V}}}}}}}}}_{{{{{{{{\rm{B}}}}}}}}}^{-}$$VBto the local charged defects induced electric field signals, and estimate its ground state transverse electric field susceptibility. Our results provide new insights on the spin and charge properties of$${{{{{{{{\rm{V}}}}}}}}}_{{{{{{{{\rm{B}}}}}}}}}^{-}$$VB, which are important for future use of defects in hBN as quantum sensors and simulators.

    more » « less