The Evolution of Magnetism in a Thin Film Pyrochlore Ferromagnetic Insulator

Abstract

The pyrochlore vanadates are compelling candidates for next-generation dissipationless devices. Lu2V2O7 and Y2V2O7 are ferromagnetic insulators (Tc ~ 70 K) that are believed to exhibit the magnon Hall effect and are expected to host topological magnons. Their completely dissipationless magnon edge states could be harnessed to realize low-power information transport in spintronic or magnonic devices. As a crucial step in the realization of devices, we synthesize the first thin films of pyrochlore Y2V2O7 on isostructural Y2Ti2O7 substrates and explore the evolution of their magnetic properties down to the ultrathin limit. All films are insulating ferromagnets with transition temperatures of up to the bulk value (Tc ~ 68 K) that decrease with thickness according to finite-size effects. Our films also exhibit a change in anisotropy from in-plane to out-of-plane easy axis coincident with the development of partial strain relaxation and nonzero magnetic hysteresis in an applied field. This evolution demonstrates the impact of strain on magnetic anisotropy and paves the way to tunable magnon topology.

Figures (37)

• Figure 1: Pyrochlore structure and characterization. a The cation structure of Y$_2$V$_2$O$_7$ showing the separate interpenetrating pyrochlore sublattices of corner sharing tetrahedra and an isolated 111 vanadium Kagome plane b X-ray diffraction about the 222 substrate (Y$_2$Ti$_2$O$_7$, denoted by asterisk) and film (Y$_2$V$_2$O$_7$) for the thickness series (al = atomic layers) showing thickness fringes indicative of smooth, high-quality films c HAADF-STEM micrograph of the 30 atomic layer Y$_2$V$_2$O$_7$ film along $\langle1\overline{1}0\rangle$ showing nearly indistinguishable interface between substrate and film d An EELS micrograph of the same film as (c) at approximately the same scale showing a clear interface between film (top) and substrate (bottom)
• Figure 2: Magnetic susceptibility of Y$_2$V$_2$O$_7$ thickness series. a Field-cooled magnetic susceptibility measured in a field of 500 Oe perpendicular to $\langle111\rangle$ for each sample in the thickness series b Ferromagnetic transition temperature vs film thickness ($n$, atomic layers) with a finite-size scaling fit (solid line, extrapolated along dotted line) that shows excellent agreement with both thickness series (filled circles) and superlattice samples (open circles)
• Figure 3: Magnetization vs. applied field loops. a Thickness dependence of magnetization vs. applied field loops measured at 10 K with field perpendicular to $\langle111\rangle$b Temperature dependence of magnetization vs. applied field loops with field perpendicular to $\langle111\rangle$ for 60 atomic layers of Y$_2$V$_2$O$_7$, which has T$_\text{c}$ = 63 K c Coercive field (width) of magnetization loops vs film thickness and temperature with single-phase films (filled circles) and superlattices (open circles) d Remnant magnetization of Y$_2$V$_2$O$_7$ thin films vs. thickness and temperature
• Figure 4: Magnetic anisotropy of Y$_2$V$_2$O$_7$ thin films. a-e Magnetization vs. applied field loops at 10 K for a field applied within the plane of the film (IP, perpendicular to $\langle111\rangle$) and out-of-plane (OOP, field along $\langle111\rangle$) for films with thickness (a) 250, (b) 60, (c) 45, (d) 30, and (e) 15 atomic layers f The anisotropy constant $\Delta$M$_\text{r}$ = M$_{\text{r,IP}}$ - M$_{\text{r,OOP}}$, assuming constant 1 $\mu_\text{B}$ saturation, showing a change in anisotropy between the 45 and 60 atomic layer films
• Figure S1: Oxygen coordination and crystal field levels in Y$_2$V$_2$O$_7$. a The local oxygen coordination environment of V$^{4+}$ in Y$_2$V$_2$O$_7$, which is nearly octahedral with a slight trigonal distortion (emphasized with arrows) b The crystal electric field energy levels of V$^{4+}$ with configuration 3$d^1$ where the five 3$d$ orbitals are split into two degenerate upper E$_g$ levels and three lower energy T$_{2g}$ levels by the octahedral coordination; the three T$_{2g}$ states further split into one lowest lying A$_{1g}$ state and two higher energy E$_g$' levels due to the trigonal distortion