Coexistence of Commensurate and Incommensurate Antiferromagnetic Groundstates in Co${}_x$NbSe${}_2$ Single Crystal

Abstract

In Co${}_x$NbSe${}_2$, crystal symmetry, and cobalt site occupation drive the formation of two distinct magnetic phases. At $x=1/4$, the centrosymmetric structure ($P$6${}_3$/$mmc$) promotes Co-Co interactions leading to the formation of an $A$-type antiferromagnetic structure phase with a transition temperature of $T_N^A$ = 169 K. At $x=1/3$, the non-centrosymmetric structure ($P$6${}_3$22) induces a lower-temperature magnetic phase with $T_N^S$ = 28 K. We report the coexistence of both substructures within a superlattice, with a nuclear propagation vector of (1/3, 1/3, 0) relative to the host lattice. Single crystals of Co${}_{0.28}$NbSe${}_2$ exhibit both magnetic transitions, with $T_N^A$ corresponding to the $x\sim 1/4$ phase and $T_N^S$ corresponding to the $x\sim 1/3$ phase. Magnetic susceptibility and specific heat measurements confirm these transitions, although only the high-temperature $T_N^A$ phase significantly affects resistivity. We successfully isolate each phase in powder samples, while single crystals with an intercalation ratio of $x=0.28$ display the coexistence of both phases in a single sample. Using single-crystal neutron diffraction, we solved the magnetic structure of the high-temperature centrosymmetric phase ($T_N^A$), and neutron powder diffraction revealed the double-$q$ magnetic structure of the low-temperature noncentrosymmetric phase ($T_N^S$)