Experimental realization of linearly polarized x-ray detected ferromagnetic resonance

Abstract

Recently, antiferromagnetic spintronics have attracted great attention, with extensive research devoted to spin transport between ferromagnetic (FM) and antiferromagnetic (AFM) materials in FM/AFM heterostructures [1-6]. In order to enable a deeper understanding of the mechanisms behind this effect, it would be invaluable to perform time resolved studies of GHz spin dynamics in systems with antiferromagnetic order with sensitivity to each absorber site individually, however, to date experimental techniques that offer these capabilities are lacking. Here, we present the first experimental observation of dynamic X-ray magnetic linear dichroism (XMLD) from GHz spin precessions driven by ferromagnetic resonance (FMR). We explore the capabilities of this new technique in two model systems, a ferromagnetic metal, i.e., Ni80Fe20 (Py), and a ferrimagnetic insulator, i.e., Ni0.65Zn0.35Al0.8Fe1.2O4 (NZAFO). Both systems yield dynamic XMLD signals consistent with our models. Under experimental geometries with oblique x-ray polarization (i.e., when the angle between static magnetization and x-ray polarization is near 45°) the dynamic XMLD response occurs primarily at the fundamental frequency of the driving RF excitation (see Fig. 1). This effect is well suited for in-plane magnetic anisotropy studies conducted in normal incidence geometry. For studies that require complementary geometries (e.g., vertical or horizontal polarization), the dynamic signal can be extracted from the 2nd harmonic response. This also works for grazing incidence measurements that are sensitive to in- vs. out-of-plane magnetic anisotropy. The presented results are a breakthrough achievement in detecting dynamic manifestations of the XMLD effect at technologically relevant, GHz frequencies. By utilizing the sensitivity of XMLD to the spin axis rather than the spin direction, one can expand conventional x-ray detected FMR towards the direct dynamic study of antiferromagnetic order.