Collinear, incommensurate antiferromagnetism in van der Waals magnet alpha-UTe3

Abstract

alpha-UTe3, a van der Waals (vdW) actinide compound with a monoclinic ZrSe3-type structure, is a narrow-gap semiconductor with 5f moments. 125Te NMR reveals strongly anisotropic, layer-confined spin fluctuations below about 20 K, with the a-axis component enhanced, and a signal wipeout at the antiferromagnetic (AFM) transition at TN = 5 K. Single-crystal neutron diffraction finds q approx. (0.17, 0.5, 0) and a longitudinal sinusoidal modulation of a-axis moments (amplitude about 0.8 muB) with AFM stacking along b. A CEF singlet-singlet induced-moment framework accounts for the easy-axis anisotropy, the small heat-capacity anomaly at TN, the reduced ordered moment, and the exchange-driven selection of q in this localized 5f vdW magnet, establishing a constrained exchange geometry stabilizing this in-plane incommensurate state.

Figures (7)

• Figure 1: (a) Crystal structure of $\alpha$-UTe$_3$, highlighting van der Waals spacings (yellow bands). (b) Projection onto the $ac$ ($c^\ast$) plane, viewed along the $b$-axis. (c) Projection onto the $ab$ plane, showing the one-dimensional U chains along $b$. The $J_i$ denote exchange interactions between U ions. (d) Local environments of the three crystallographically distinct Te sites: Te(1), Te(2), and Te(3).
• Figure 2: (a) Temperature dependence of electrical resistivity $\rho(T)$ for $\alpha$-UTe$_3$ with current in the $ab$-plane under $\mu_0H = 0$ and 9 T ($H \perp ab$). (b) Specific heat divided by temperature $C/T$ at zero field, with phonon ($C_{\rm ph}/T$), magnetic ($C_{\rm mag}/T$) contributions, and integrated entropy $S_{\rm mag}/R$ (right axis). (c) Magnetic susceptibility $\chi(T) \equiv M/H$ for $H \parallel a$, $b$, and $c^\ast$; inset: field variation of $\chi(T)$ for $H \parallel a$. (d) Magnetization $M(H)$ at $T = 5$ K for $H \parallel a$, $b$, and $c^\ast$.
• Figure 3: (a) $^{125}$Te-NMR spectra of $\alpha$-UTe$_3$ measured at $T = 20$ K and $\mu_0H_0 = 1.95$ T for magnetic fields applied along the $a$-, $b$-, and $c^\ast$-axes. A small extrinsic $^{11}$B-NMR signal from a glass slip is shaded by a gray-hatched band. (b) Temperature dependence of the nuclear spin-lattice relaxation rate $1/T_1$ for $H_0 \parallel a$, $b$, and $c^\ast$. (c) Temperature dependence of the directional spin fluctuation quantity $R_i/|A_i|^2$ for $i = a$, $b$, and $c^\ast$. The definition is given in the main text.
• Figure 4: (a) Neutron diffraction intensity map in the $(h, k, 0)$ plane for a single crystal of $\alpha$-UTe$_3$ at $T = 1.6$ K. (b) Temperature dependence of the peak profile of the magnetic reflection at $\bm{Q}=(\sim 0.83, 0.5, 0)$. Solid lines represent Gaussian fits. (c) Peak positions and integrated intensity of the magnetic reflection at $\bm{Q}=(\sim 0.83, 0.5, 0)$. The dotted lines are guides to the eye. (d) $|F_{\rm calc}|^2$ vs $|F_{\rm obs}|^2$ plot of the magnetic reflections observed at 1.6 K. The dashed line is a guide to the eye. (e) The magnetic structure model employed in the fitting analysis of the neutron data. The illustration was generated using VESTAvesta.
• Figure 5: Field-angle dependence of $^{125}$Te NMR spectra in $\alpha$-UTe$_3$ measured at $T = 20$ K and $\mu_0H = 1.95$ T. (a) Field rotated in the plane perpendicular to the $b$ axis; angle $\Theta$ is measured from the $c^\ast$ axis. (b) Field rotated in the plane perpendicular to the $a$ axis; $\Theta$ is defined similarly. (c) Field rotated in the $ab$ plane (perpendicular to the $c^\ast$ axis); angle $\Phi$ is measured from the $a$ axis. The vertical line indicates the reference frequency corresponding to $^{125}K = 0$. The hatched area represents an insensitive region due to $^{11}$B NMR signals from the glass strip used to fix the sample.