Short-range magnetic interaction in a monolayer $1T\text{−}{\mathrm{VSe}}_2$ film revealed by element-specific x-ray magnetic circular dichroism

We investigated the intrinsic magnetic properties of the two-dimensional ferromagnetic candidate monolayer $1T\text{−}{\mathrm{VSe}}_2$ films using x-ray magnetic circular dichroism (XMCD), an element-specific magnetic probe. By performing high-resolution measurements, we succeeded in detecting a clear XMCD signal from the atomically thin $1T\text{−}{\mathrm{VSe}}_2$ films under an external magnetic field. Through manipulation of the x-ray incidence angle, we were able to disentangle the in-plane and out-of-plane magnetic properties and found a strong magnetic anisotropy. Moreover, magnetic-field- and temperature-dependent XMCD revealed that there is no long-range ferromagnetic ordering even at 6 K, but short-range ferromagnetic and antiferromagnetic interactions between neighboring vanadium ions exist. Such low-temperature magnetic behavior signifies that the monolayer $1T\text{−}{\mathrm{VSe}}_2$ is on the verge of ferromagnetism, and this fact accounts for the reported ferromagnetism in ${\mathrm{VSe}}_2$-based heterostructures.

Figure 1

(a) Top (upper) and side (lower) views of the crystal structure of a monolayer $1T\text{−}{\mathrm{VSe}}_2$ film. (b) RHEED pattern after the sample growth. (c) Azimuthally rotated two-dimensional Brillouin zones of ${\mathrm{VSe}}_2$ (red) and HOPG (gray). (d), (e) Observed band dispersions of the HOPG (d) and the monolayer ${\mathrm{VSe}}_2$ on HOPG (e) recorded at room temperature. ARPES images are symmetrized with respect to the $\overline{\mathrm{\Gamma }}$ point. (f) Angle-integrated EDCs of the ARPES images shown in (d) and (e). (g) Calculated band dispersion of the free-standing monolayer ${\mathrm{VSe}}_2$ at the nonmagnetic state. The band dispersions along $\overline{\mathrm{\Gamma }}\text{−}\overline{M}$ and $\overline{\mathrm{\Gamma }}\text{−}\overline{K}$ directions are superimposed.

Figure 2

(a) Schematics of the experimental geometries. Upper: grazing-incidence (GI) geometry. Lower: normal-incidence (NI) geometry. (b) XAS spectra at V $L_{2,3}$ edges acquired at GI geometry. (c), (d) XMCD spectra at V $L_{2,3}$ edges with various magnetic fields at GI (c) and NI (d) geometries. (e)–(g) Same as (b)–(d) but at Se $L_{2,3}$ edges.

Figure 3

(a), (b) Magnetic-field dependence of the XMCD intensities ($M\text{−}H$ curves) at the V $L_3$ edge in the NI and GI geometries recorded at several temperatures. Due to zero remanence, the $M\text{−}H$ curves obtained for decreasing- and increasing-field sweeps were added. (c) Extracted in-plane (parallel) component of $M\text{−}H$ curves from (a) and (b). (d) Comparison of in-plane and out-of-plane $M\text{−}H$ curves at 6 K.

Figure 4

(a)–(c) Temperature dependence of the XMCD intensities ($M\text{−}T$ curves) at 10, 8, 6, 4, and 2 T in the NI, GI, and PI geometries. The XMCD intensities are symmetrized with respect to the origin. The error bars were evaluated from the difference between the $M\text{−}H$ curves and their fitting results shown in the Supplemental Material [45]. (d)–(f) Temperature dependence of the $H/\mathrm{XMCD}$ intensities in the NI, GI, and PI geometries. The inset of (d) shows the estimated Weiss temperatures, which were evaluated by linear fitting in two different temperature ranges: 6–40 K (yellow markers) and 6–20 K (gray markers), in the NI geometry. The inset of (f) shows the $H/\mathrm{XMCD}$ intensities at 10 and 2 T in the PI geometry. The gray lines indicate the linear fitting results of the $H/\mathrm{XMCD}$ intensities from 20 to 40 K.

Figure 5

(a), (b) Fitting results of nonlinear and linear components in the NI geometry. (c), (d) Same as (a) and (b) but in the PI geometry.