Selective Amplification of the Topological Hall Signal in Cr$_2$Te$_3$: The Role of Molecular Exchange Coupling

TL;DR

This work addresses the realization and control of topological spin textures in two-dimensional Cr2Te3 by tuning Cr intercalation during growth and by molecular adsorption at the surface. The authors demonstrate a progression from in-plane to out-of-plane magnetic anisotropy, with an intermediate noncoplanar ground state that exhibits a topological Hall effect. Complementary Monte Carlo modeling shows that interfacial exchange couplings, rather than spin-orbit effects, can generate finite spin chirality and topological Hall signals, a trend corroborated by experimental modulation of the THE via VOPc and CoPc adsorption. The findings establish exchange-coupling engineering at interfaces as a viable route to novel topological spintronic functionalities in 2D magnets, with implications for device design and control of topological transport phenomena.

Abstract

Layered magnetic transition-metal chalcogenides (TMCs) are a focal point of research, revealing a variety of intriguing magnetic and topological ground states. Within this family of TMCs, chromium telluride has garnered significant attention because of its excellent tunability in magnetic response, owing to the presence of competing magnetic exchange interactions. We here demonstrate the manipulation of magnetic anisotropy in ultra-thin Cr$_2$Te$_3$ films through growth engineering leading to a controlled transition from in-plane to out-of-plane orientation with an intermediate non-coplanar magnetic ground phase characterized by a topological Hall effect. Moreover, interfacing these films with Vanadyl phthalocyanine (VOPc) molecules prominently enhances the non-coplanar magnetic phase, attributing its presence to the competing interfacial magnetic exchange interactions over the spin-orbit-driven interfacial effects. These findings pave the way for the realization of novel topological spintronic devices through interface-modulated exchange coupling.

Figures (5)

• Figure 1: (a) Crystal structure of CrTe$_2$ (left) and Cr$_2$Te$_3$ (right) in YZ plane, showcasing three distinct types of Cr atoms (Cr$_{I,II,III}$) with the co-existence of FM and AFM exchange interactions in Cr$_2$Te$_3$. (b) Schematic of the molecular beam epitaxy co-evaporation setup for Cr$_2Te_3$ thin films, with in-situ RHEED monitoring. RHEED image of the 4 nm Cr$_2$Te$_3$ film taken along [10$\bar{1}$0] direction. (c) Coupled XRD scan ($2\theta/\omega$) of CT3, CT8 and CT12 films showing c-axis oriented growth. (d) Magnetization (M) vs Temperature in zero-field cooled (black) and field-cooled (red) procedure for CT3, CT8 and CT12 films. $M$ vs $H$ (top), AMR (in %) vs $H$ (middle) and Hall resistance (R$_{xy}$) vs $H$ (bottom) for (e) CT3, (f) CT12 and (g) CT8 devices taken at 6 K. Typical zero-field resistance values of the devices were in the range of 2 to 6 k$\Omega$ with the CT3 devices being more resistive.
• Figure 2: (a) AMR (in %) vs $H$ and (b) Hall signal ($R_{xy}$) vs $H$ of reference CT12 (4.5 nm) (black), CT12 (4.5 nm)/VOPc (4 ML) (blue) and CT12 (4.5 nm)/CoPc (4 ML) (green) devices, at 120 K (top) and 10 K(bottom). (c) Normalized Hall resistivity ($\rho$$_{xy}$(norm)) vs $H$ of reference CT8 (4.5 nm) (black) and CT8 (4.5 nm)/VoPc (4 ML) (blue) devices at 6 K. The Hall resistivity is normalized by $\rho$$_{xy}$(norm) = $\rho$$_{xy}$(H, T)/$\rho$$_{xy}$(1 T, 120 K).
• Figure 3: 3D Color map of $\rho$$_{xy}$(THE) with varying $\text{T}$ and $\text{H}$ for CT8 (4.5 nm), CT12 (4.5 nm), CT8 (4.5 nm)/VOPc (4ML), and CT12(4.5 nm)/VoPc(4 ML) devices. $\rho$$_{xy}$(THE) is obtained by removing the background anomalous Hall signal from $\rho$$_{xy}$. The values of $\rho$$_{xy}$ are in units of 1$0^{-7}$$\Omega$.cm.
• Figure 4: (a) Three-layer structure of the magnetic atoms near the considered Cr$_2$Te$_3$/V interface. Moments of Cr and V atoms were taken as 3 $\mu_B$ and 1.72 $\mu_B$, respectively. (b)-(c) Top view and side view of the system, showing different types of exchange interactions considered in the model. (d)-(e) Ground state spin configuration in the top Cr layer, obtained in Monte Carlo annealing calculation, for parameter sets (d) $J_2=0$, $J_3=0$, $J_4=0$, $J_5=0$, and (e) $J_2=-2J_1$, $J_3=0.2J_1$, $J_4=0$, $J_5=0$. In (d), we realize a ferromagnetic (FM) state. In (e), in the presence of finite $J_2$ and $J_3$, a canted state appears, for which the scalar spin chirality is non-zero. (f) Variation of the site-averaged scalar spin chirality $\chi_{\rm av}$ with $J_2$ and $J_3$, revealing that $\chi_{\rm av}$ is enhanced in a parameter regime. The values of $J_4$ and $J_5$ are kept at zero. The black dot in (f) represents the values of $J_2$ and $J_3$, close to the previously-reported parameters for this materials system bian2021covalent. The considered value of $J_1$ is 2.92 meV.
• Figure 5: (a) Variation of the site-averaged scalar spin chirality $\chi_{\rm av}$ with the exchange couplings $J_4$ and $J_5$ of the V atoms with the interfacial Cr atoms. Both $J_4$ and $J_5$ are considered ferromagnetic, revealing that $\chi_{\rm av}$ is enhanced in a parameter range. The other parameters used are $J_2=-1.2J_1$, $J_3=0.26J_1$, which are close to the previously-reported parameters for this materials system bian2021covalent. (b) Variation of $\chi_{\rm av}$ with $J_4$ and $J_5$, both considered here as antiferromagnetic, also showing that $\chi_{\rm av}$ is enhanced in a range of $J_4$ and $J_5$.