Controlling the Magnetic Ground State in ${\mathrm{Cr}}_{1-x}{\mathrm{V}}_x$ Films

We demonstrate the ability to control the magnetic phase diagram of ${\mathrm{Cr}}_{1-x}{\mathrm{V}}_x(110)$ thin films grown on a W(110) substrate. Using angle-resolved photoemission, we have mapped paramagnetic and commensurate and incommensurate antiferromagnetic phases as a function of temperature, film thickness, and composition. We show that surface-localized electron states play a key role in the observed phase behaviors and suggest from this that it might be possible to control the magnetic phase by applying an external electric field.

Figure 1

(a) Schematic of the thickness-composition double wedge used in these experiments. (b) Calculated bulk chromium Fermi surface, showing the $\Gamma$-centered electron octahedron nested with the slightly larger H-centered hole octahedron. (c) 3D ARPES band map in the $\Gamma \mathrm{\text{−}}N\mathrm{\text{−}}P\mathrm{\text{−}}H$ plane of the bulk Brillouin zone, showing the dispersion of the bands that form the nested electron and hole Fermi surface octahedra.

Figure 2

2D ARPES band maps showing energy band dispersion along a line directly through the face of the electron octahedron. (a) Pure Cr film of thickness $d=6\mathrm{nm}$ at $T=30\mathrm{K}$ showing two back-folded bands indicative of the IC phase. (b) Alloy film with $x=0.06$ and $d=6\mathrm{nm}$, $T=30\mathrm{K}$, showing no back-folded band indicative of the P-phase. (c) Cr film of thickness $d=3\mathrm{nm}$ at $T=30\mathrm{K}$ showing one back-folded band indicative of the C phase. (d) Cr film of thickness $d=6\mathrm{nm}$ and $T=30\mathrm{K}$ with a monolayer of adsorbed hydrogen. The IC phase from panel (a) has transformed to the C phase, and a diffuse surface band has split from the bulk band as shown.

Figure 3

MDC maps of the regions where the back-folded bands exist. (a) Cr at $T=30\mathrm{K}$, as a function of film thickness $d$. The C phase at small $d$ transforms to the IC phase at higher $d$. (b) and (c) Films at $T=15\mathrm{K}$ as a function of $x$, at $d=2.5\mathrm{nm}$ and $d=10\mathrm{nm}$, respectively. The thin film (panel b) starts in the C phase and transforms quickly to the IC phase and then the P-phase as $x$ increases, while the thick film starts IC and becomes more IC as the P phase is approached. Zero points on the thickness or concentration axes in Fig. 3 (a–c) correspond to beginning of wedges; below these points thickness/concentration is constant and equal to zero value. (d) Cr at $d=6\mathrm{nm}$ and $T=30\mathrm{K}$ as a function of hydrogen dose. The initially split back-folded band (IC) merges to form a single back-folded band (C), and over the same coverage regime a surface band splits from the intense bulk band edge and enters a projected band gap.

Figure 4

Part of the low temperature 3D ($x$, $d$, $T$) magnetic phase diagram of clean (dashed line) and partially H-covered (solid line) ${\mathrm{Cr}}_{1-x}{\mathrm{V}}_x$ films as a function of $x$ and $d$. Filled circles are measured transition points, and the various lines are guides to the eye. $H$-adsorption shifts the phase lines to higher thickness, indicating a surface-related preference for the C phase.