Incommensurate antiferromagnetism with amplitude modulation in the semiconducting van der Waals magnet -UTe

Sakai, Hironori   ; Tabata, Chihiro; Kaneko, Koji   ; Tokiwa, Yoshifumi  ; Kitazawa, Takafumi  ; Kambe, Shinsaku*  ; Tokunaga, Yo  ; Haga, Yoshinori   

-UTe, a van der Waals (vdW) actinide compound with a monoclinic ZrSe-type structure, is a narrow-gap semiconductor with moments. Te NMR reveals strongly anisotropic, layer-confined spin fluctuations below 20 K, with the -axis component enhanced, and a signal wipeout at the antiferromagnetic (AFM) transition K. Single-crystal neutron diffraction finds and a longitudinal sinusoidal modulation of -axis moments with an amplitude of and AFM stacking along . A CEF singlet-singlet induced-moment framework accounts for the easy-axis anisotropy, the small heat-capacity anomaly at , the reduced ordered moment, and the exchange-driven selection of in this localized vdW magnet, establishing a constrained exchange geometry that stabilizes this in-plane incommensurate state.