Noncollinear spin structure in Dy-doped classical ferrimagnet

TL;DR

Noncollinear spin textures in ferrimagnetic oxide insulators offer low-damping spintronic platforms but are challenging to realize in spinel NiZAF. The authors introduce Dy-doping to create local strain gradients that break inversion symmetry and enable local DMI, employing XMCD, XRMR, EXAFS, Kerr microscopy, SQUID magnetometry, and first-principles Green's-function DFT to comprehensively characterize the system. XRMR reveals a spiral-type in-plane spin structure with depth-dependent canting and a measurable out-of-plane component; XMCD shows spin canting emerging below ~200 K, while EXAFS confirms Dy incorporation at octahedral sites with associated local strain. The work demonstrates noncollinearity in an otherwise inversion-symmetric spinel ferrite via strain-gradient–induced DMI, highlighting prospects for chiral magnetic domains and topological textures in oxide-based spintronics.

Abstract

Noncollinear spin structures have attracted tremendous attention because they offer a versatile platform for spin control and manipulation, essential in spintronics. Realizing noncollinearity in ferrimagnetic insulators is of particular interest as they can be potentially utilized in low-damping spintronics with tunable magnetic order. Within the spinel-ferrite family, Zn and Al-substituted nickel ferrite (NiZAF) has emerged as an excellent choice for low-damping spintronics. However, realizing noncollinearity in such systems remains challenging. Here, we present evidence of noncollinear spin structure in the NiZAF thin films induced by the rare earth Dy-doping, utilizing the soft x-ray spectroscopy methods such as magnetic circular dichroism and x-ray resonant magnetic reflectivity (XRMR). In particular, XRMR reveals a spiral-type spin structure, which is attributed to the Dzyaloshinskii-Moriya interaction, arising due to broken inversion symmetry by the Dy-induced local strain field as confirmed by our theoretical calculations. The realization of noncollinearity in the spinel-ferrite opens pathway to explore the possibility of chiral magnetic domains and topological spin textures exhibiting promise for oxide-based spintronics

Figures (4)

• Figure 1: Crystal structure and magnetic properties.(a) Unit cell of inverse spinel (Fd$\overline{3}$m) nickel ferrite, where green-colored arrows indicate spin directions at different crystallographic sites such as O$_{h}$ (purple regions) and T$_{d}$ (light green regions). (b) Room temperature XRD pattern collected along the q$_{z}$ direction in the vicinity of the (311) reflection for the pristine and 5% Dy-doped NiZAF thin films (curves are shifted vertically for clarity), where vertical blue arrows represent Lau oscillations. (c) Temperature-dependent magnetization measured using SQUID under the in-plane field of 10 mT during the field-cooling cycle for the pristine, 3% and 5% Dy-doped NiZAF thin films. (d) Temperature-dependent Kerr signal (MOKE) versus out-of-plane field (H$_z$) for 5% Dy-doped NiZAF, where the appearance of hysteresis loop below 200 K is indicated by the arrow.
• Figure 2: Noncollinear spin structure using XMCD.(a) XMCD spectra recorded at 300 K under 6 T using thin films with different Dy concentrations (pristine, 3% Dy-doped and 5% Dy-doped) under normal incidence at edges (a) Fe-L$_{2,3}$ and (b) Ni-L$_{2,3}$. Both octahedral (O$_{h1}$ and O$_{h2}$) and tetrahedral (T$_{d}$) sites are labelled, where spins with oxidation state of Fe$^{2+, 3+}$ cations are depicted by red arrows in (a). (c) Comparison of XMCD in the remanence state achieved from the IP and OOP fields for both Ni-L$_{2,3}$ and Fe-L$_{2,3}$ edges for the 5% Dy-doped NiZAF. (d) Temperature-dependent behavior of the spin magnetic moment (m$_{s}$) for the Ni$^{2+}$ and Fe$^{3+}$ cations, derived from the sum-rule using XMCD spectra collected at corresponding edges in remanence (0 T) and 6 T under normal incidence for the 5% Dy-doped NiZAF. Purple arrows show a cartoon view of spin-canting below 200 K.
• Figure 3: Spiral-type spin structure using XRMR.(a) Schematic of the XRMR experimental setup, where specular reflection is detected after the the left(I$^-$)/right(I$^+$) circularly polarized (CP) x-ray beam incidence under the longitudinal geometry with field direction along the sample's plane (indicated by blue arrow). (b). Magnetic asymmetry modelling at 50 K for the Fe-L$_{3}$ edge (c) in the remanence state and (d) 2 T. (e) Asymmetry modelling at 50 K for the Ni-L$_{3}$ edge at 2 T. (f) Schematic of spin structure for Fe$^{2+}$ (obtained from asymmetry modelling in remanence state) and Ni$^{2+}$ (obtained from asymmetry modelling at 2 T) occupied at the O$_{h}$-site, where the obtained value of IP angle $\gamma$ and OOP angle $\phi$ are given (represented in the coordinate axes model).
• Figure 4: Local structure and strain-gradient schematic.(a) Normalized (FY) Dy-L$_3$ edge EXAFS spectrum and $\chi(k)\times k^{2}$ interference function (inset) derived after background subtraction for the 5% Dy-doped NiZAF. (b) Magnitude of the Fourier Transformation (FT) of $\chi(k)\times k^{2}$. The two maxima labelled by A and B can be related to nearest O and Fe atoms around Dy (see Table II). The inset in (b) shows a model of the local Dy environment within the octahedrally coordinated site. (c) Schematic view of atoms in the lattice showing the strain-gradient induced by larger Dy$^{3+}$ cations, where green arrows show spin directions.