Tailoring magnetism in self-intercalated ${\mathrm{Cr}}_{1+\delta }{\mathrm{Te}}_2$ epitaxial films

Magnetic transition metal dichalcogenide (TMD) films have recently emerged as promising candidates in hosting novel magnetic phases relevant to next-generation spintronic devices. However, systematic control of the magnetization orientation, or anisotropy, and its thermal stability characterized by Curie temperature $\left(T_{\mathrm{C}}\right)$, remains to be achieved in such films. Here we present self-intercalated epitaxial ${\mathrm{Cr}}_{1+\delta }{\mathrm{Te}}_2$ films as a platform for achieving systematic/smooth magnetic tailoring in TMD films. Using a molecular-beam epitaxy based technique, we have realized epitaxial ${\mathrm{Cr}}_{1+\delta }{\mathrm{Te}}_2$ films with smoothly tunable δ over a wide range (0.33–0.82), while maintaining NiAs-type crystal structure. With increasing δ, we found monotonic enhancement of $T_{\mathrm{C}}$ from 160 to 350 K, and the rotation of magnetic anisotropy from out-of-plane to in-plane easy-axis configuration for fixed film thickness. Contributions from conventional dipolar and orbital moment terms are insufficient to explain the observed evolution of magnetic behavior with δ. Instead, ab initio calculations suggest that the emergence of antiferromagnetic interactions with δ, and its interplay with conventional ferromagnetism, may play a key role in the observed trends. This demonstration of tunable $T_{\mathrm{C}}$ and magnetic anisotropy across room temperature in TMD films paves the way for engineering different magnetic phases for spintronic applications.

Figure 1

Epitaxial growth of as-grown ${\mathrm{Cr}}_{1+\delta }{\mathrm{Te}}_2$ film. (a) Side view of crystal structure of ${\mathrm{Cr}}_{1+\delta }{\mathrm{Te}}_2$ showing intercalation of ${\mathrm{Cr}}_{\delta }$ between ${\mathrm{CrTe}}_2$ layers forming NiAs structure. (c) RHEED patterns for ${\mathrm{Al}}_2{\mathrm{O}}_3$ substrate, taken just before film deposition at 300 °C, and for as-grown film, taken at room temperature. The electron beam is injected along (100) direction of ${\mathrm{Al}}_2{\mathrm{O}}_3$ substrate. (d) STM topography (set bias: 200 mV; feedback current: 200 pA), with the root-mean-square (rms) value of surface roughness indicated. (e) XRD profile along (00L) orientation of the as-grown film. (f) XRD φ scans with respect to asymmetric peaks: (101) of ${\mathrm{Cr}}_{1+\delta }{\mathrm{Te}}_2$ (red) and (113) of ${\mathrm{Al}}_2{\mathrm{O}}_3$ (gray). (g) Relative epitaxial orientation between substrate and film in this study.

Figure 2

Structural and compositional characterization after in situ postdeposition annealing with varying temperature ($T_A$). (a) XRD intensity profiles along (00L) direction for varying TA, vertically offset for clarity. * and ** indicate diffraction peaks originating from substrate and pure Cr, respectively. Lower graph shows referential powder-diffraction patterns of pure Cr and ${\mathrm{Cr}}_5{\mathrm{Te}}_8$ ($\delta =0.25$) taken from ICDD Database. (b) Scanning electron microscope–EDS measurements of variation of δ as a function of $T_A$. The dashed parabolic trendline is a guide to the eye. (c) Variation of lattice constants $a_0$ and $c_0$ and unit-cell volume $V_0$ as a function of δ, determined from XRD measurements. Dashed lines show a fit to Vegard's law. Triangular symbols indicate corresponding parameters for bulk crystals from literature [14, 16, 19].

Figure 3

Evolution of magnetic properties for ${\mathrm{Cr}}_{1+\delta }{\mathrm{Te}}_2$ films (0.3 < δ < 0.82). Magnetization as a function of temperature, acquired by field cooling (FC) with a 1000-Oe magnetic field (H). Red and blue lines indicate data taken with H along out-of-plane (OP) and in-plane (IP) configurations, respectively. The Curie temperature $T_{\mathrm{C}}$ is indicated by an arrow. (b) Magnetization hysteresis curves at 2 K for each δ, acquired in OP (red) and IP (blue) configurations, respectively. The diamagnetic signal from substrate is subtracted in (b). The effective magnetic anisotropy $K_{\mathrm{eff}}$ was calculated from areal difference of IP and OP M(H) loops [52]. (c) The δ dependence of $T_{\mathrm{C}}$ (left axis, filled circle) and $K_{\mathrm{eff}}$ (right axis, empty square). Thick lines in (c) are guides to the eye for $T_{\mathrm{C}}$ and $K_{\mathrm{eff}}$.

Figure 4

Calculated exchange interactions for ${\mathrm{Cr}}_{1+\delta }{\mathrm{Te}}_2$ ($\delta =0$,1). (a),(b) Crystal structures of ${\mathrm{CrTe}}_2$ ($\delta =0$) and CrTe ($\delta =1$). Green and red arrows indicate direct exchange interactions for pairs of atoms in intrasublattice ($J_{11}$) and intersublattice ($J_{12}$) configurations. For simplicity, (b) does not show green arrows for the two distinct intrasublattice contributions $J_{11}$ and $J_{22}$ that are included in the calculation for CrTe [see (d)]. (c),(d) DFT-calculated exchange interactions $J_{11}$, $J_{22}$, and $J_{12}$ as a function of atomic separation for $\delta =0$ (c) and 1 (d) as per the structures in (a) and (b) respectively. Thick lines in (c) and (d) are guides to the eye for distance evolution for ${\mathrm{J}}_{\mathrm{s}}$. (e) Cartoon showing expected evolution of ground-state magnetic configuration of ${\mathrm{Cr}}_{1+\delta }{\mathrm{Te}}_2$ with varying δ, with individual arrows indicating the local orientation of magnetization.