Using element-resolved x-ray magnetic circular dichroism (XMCD) and x-ray magnetic linear dichroism (XMLD) measurements, we determined the spin orientations of Ni, CoO and Fe films in Ni/CoO/Fe films grown on a curved MgO(0 0 1) substrate. We find that the vicinal surface of MgO(0 0 1) substrate results in a spin canting towards out-of-plane direction in the Ni and CoO films as a result of the interfacial coupling. The Ni spin canting angle increases monotonically with the vicinal angle in the studied range of 0–17° and the CoO spin canting angle increases more rapidly towards saturation only at a few degrees of the vicinal angle. The uniaxial magnetic anisotropy induced in the Ni layer by the Ni/CoO interfacial coupling is quantitatively determined and is shown to increase monotonically with the vicinal angle. Our result provides a new pathway for tailoring the spin orientation by modifying the substrate surface symmetry in combining with the ferromagnetic/antiferromagnetic interfacial interaction in thin-film based spintronic devices.

Spin pumping from ferromagnetic Fe into antiferromagnetic CoO across a Ag spacer layer was studied using ferromagnetic resonance (FMR) in Py/CoO/Ag/Fe/Ag(001). The thin Py film on top of CoO permits an alignment of the CoO Neél vector through field cooling in two otherwise equivalent [110] and [110] crystalline axes which are parallel and perpendicular to the Fe magnetization direction, respectively. Fe FMR linewidth is measured as a function of Ag thickness in 10-20-GHz frequency range and in 180-330 K temperature range. We find that there exists an anisotropy in the Fe FMR damping for parallel and perpendicular alignment of the Fe and CoO spins. However, such anisotropic damping exists only at thin Ag thickness where there exists a magnetic interlayer coupling between Fe and CoO, and vanishes at thick Ag thickness where the interlayer coupling becomes negligible but permitting spin-current transmission into CoO. Our result indicates the absence of anisotropic spin current for parallel and perpendicular alignment of the Fe and CoO spin axes.

An antiferromagnetic NiO spiral wall in Fe/NiO/Co0.5Ni0.5O/vicinal Ag(001) was created by rotating Fe magnetization and investigated using x-ray magnetic linear dichroism (XMLD). Different from the Mauri's 180° spiral wall, we find that the NiO spiral wall always switches its chirality at ∼90 rotation of the Fe magnetization, and unwinds the spiral wall back to a single domain with a further rotation of the Fe magnetization from 90° to 180°. The effect of this chirality switching on the magnetic anisotropy was studied using rotational magneto-optic Kerr effect (ROTMOKE) on Py/NiO/Co0.5Ni0.5O/vicinal Ag(001). We find that the original Mauri's model has to be corrected by an energy folding due to the chirality switching, which consequently converts the exchange bias from the Mauri's 180° spiral wall into a uniaxial anisotropy and a negative fourfold anisotropy.

We present the first theoretical and experimental evidence of time-resolved dynamic x-ray magnetic linear dichroism (XMLD) measurements of GHz magnetic precessions driven by ferromagnetic resonance in both metallic and insulating thin films. Our findings show a dynamic XMLD in both ferromagnetic Ni80Fe20 and ferrimagnetic Ni0.65Zn0.35Al0.8Fe1.2O4 for different measurement geometries and linear polarizations. A detailed analysis of the observed signals reveals the importance of separating different harmonic components in the dynamic signal in order to identify the XMLD response without the influence of competing contributions. In particular, RF magnetic resonance elicits a large dynamic XMLD response at the fundamental frequency under experimental geometries with oblique x-ray polarization. The geometric range and experimental sensitivity can be improved by isolating the 2ω Fourier component of the dynamic response. These results illustrate the potential of dynamic XMLD and represent a milestone accomplishment toward the study of GHz spin dynamics in systems beyond ferromagnetic order.

The recent discovery of spin current transmission through antiferromagnetic insulating materials opens up vast opportunities for fundamental physics and spintronics applications. The question currently surrounding this topic is: whether and how could THz antiferromagnetic magnons mediate a GHz spin current? This mismatch of frequencies becomes particularly critical for the case of coherent ac spin current, raising the fundamental question of whether a GHz ac spin current can ever keep its coherence inside an antiferromagnetic insulator and so drive the spin precession of another ferromagnet layer coherently? Utilizing element- and time-resolved x-ray pump-probe measurements on Py/Ag/CoO/Ag/Fe₇₅Co₂₅/MgO(001) heterostructures, here we demonstrate that a coherent GHz ac spin current pumped by the Py ferromagnetic resonance can transmit coherently across an antiferromagnetic CoO insulating layer to drive a coherent spin precession of the Fe₇₅Co₂₅ layer. Further measurement results favor thermal magnons rather than evanescent spin waves as the mediator of the coherent ac spin current in CoO.

Recently, the electrical switching of antiferromagnetic (AFM) order has been intensively investigated because of its application potential in data storage technology. Herein, we report the current switching of the AFM Néel vector in epitaxial Pd/CoO films as a function of temperature. Using combined measurements of Hall resistance (HR) and x-ray magnetic linear dichroism (XMLD) below and above the AFM Néel temperature, we unambiguously identified both magnetic and nonmagnetic contributions to the current-induced HR change. Through magnetic field-induced HR measurements, we quantitatively determined the percentage of current-induced CoO spin switching. Further, we showed that the thermal effect dominated the CoO magnetic switching more in samples with a thinner Pd layer and that samples with a thicker CoO layer required higher thermal activation for current-induced magnetic switching. These results provide a clear and comprehensive picture of current-induced AFM spin switching across the AFM Néel temperature.

Spin pumping has been studied within Ta / Ag / Ni81Fe19 (0-5 nm) / Ag (6 nm) / Co2MnGe (5 nm) / Ag / Ta large-area spin-valve structures, and the transverse spin current absorption of Ni81Fe19 sink layers of different thicknesses has been explored. In some circumstances, the spin current absorption can be inferred from the modification of the Co2MnGe source layer damping in vector network analyzer ferromagnetic resonance (VNA-FMR) experiments. However, the spin current absorption is more accurately determined from element-specific phase-resolved x-ray ferromagnetic resonance (XFMR) measurements that directly probe the spin transfer torque (STT) acting on the sink layer at the source layer resonance. Comparison with a macrospin model allows the real part of the effective spin mixing conductance to be extracted. We find that spin current absorption in the outer Ta layers has a significant impact, while sink layers with thicknesses of less than 0.6 nm are found to be discontinuous and superparamagnetic at room temperature, and lead to a noticeable increase of the source layer damping. For the thickest 5-nm sink layer, increased spin current absorption is found to coincide with a reduction of the zero frequency FMR linewidth that we attribute to improved interface quality. This study shows that the transverse spin current absorption does not follow a universal dependence upon sink layer thickness but instead the structural quality of the sink layer plays a crucial role.

Magnetic skyrmions are topological spin textures, which usually exist in noncentrosymmetric materials where the crystal inversion symmetry breaking generates the so-called Dzyaloshinskii-Moriya interaction. This requirement unfortunately excludes many important magnetic material classes, including the recently found two-dimensional van der Waals (vdW) magnetic materials, which offer unprecedented opportunities for spintronic technology. Using photoemission electron microscopy and Lorentz transmission electron microscopy, we investigated and stabilized Néel-type magnetic skyrmion in vdW ferromagnetic Fe₃GeTe₂ on top of (Co/Pd) _n in which the Fe₃GeTe₂ has a centrosymmetric crystal structure. We demonstrate that the magnetic coupling between the Fe₃GeTe₂ and the (Co/Pd) _n could create skyrmions in Fe₃GeTe₂ without the need of an external magnetic field. Our results open exciting opportunities in spintronic research and the engineering of topologically protected nanoscale features by expanding the group of skyrmion host materials to include these previously unknown vdW magnets.

Many technologically useful magnetic oxides are ferrimagnetic insulators, which consist of chemically distinct cations. Here, we examine the spin dynamics of different magnetic cations in ferrimagnetic NiZnAl-ferrite (Ni0.65Zn0.35Al0.8Fe1.2O4) under continuous microwave excitation. Specifically, we employ time-resolved x-ray ferromagnetic resonance to separately probe Fe2+/3+ and Ni2+ cations on different sublattice sites. Our results show that the precessing cation moments retain a rigid, collinear configuration to within ≈2°. Moreover, the effective spin relaxation is identical to within <10% for all magnetic cations in the ferrite. Thus, we validate the oft-assumed “ferromagnetic-like” dynamics in the resonantly driven ferrimagnetic oxide: the magnetic moments from different cations precess as a coherent, collective magnetization, despite the high contents of nonmagnetic Zn2+ and Al3+ diluting the exchange interactions.

Interfacial charge transfer in oxide heterostructures gives rise to a rich variety of electronic and magnetic phenomena. Designing heterostructures where one of the thin-film components exhibits a metal-insulator transition opens a promising avenue for controlling such phenomena both statically and dynamically. In this work, we utilize a combination of depth-resolved soft x-ray standing-wave and hard x-ray photoelectron spectroscopies in conjunction with polarization-dependent x-ray absorption spectroscopy to investigate the effects of the metal-insulator transition in LaNiO3 on the electronic and magnetic states at the LaNiO3/CaMnO3 interface. We report a direct observation of the reduced effective valence state of the interfacial Mn cations in the metallic superlattice with an above-critical LaNiO3 thickness (6 unit cells, u.c.) facilitated by the charge transfer of itinerant Ni3deg electrons into the interfacial CaMnO3 layer. Conversely, in an insulating superlattice with a below-critical LaNiO3 thickness of 2u.c., a homogeneous effective valence state of Mn is observed throughout the CaMnO3 layers due to the blockage of charge transfer across the interface. The ability to switch and tune interfacial charge transfer enables precise control of the emergent ferromagnetic state at the LaNiO3/CaMnO3 interface and, thus, has far-reaching consequences on the future strategies for the design of next-generation spintronic devices.