Commensurate and incommensurate magnetic structure of the moderately frustrated antiferromagnet Li2M(WO4)2 with M=Co,Ni

Abstract

We have investigated the magnetic structure of ${\mathrm{Li}}_{2}\mathrm{Co}{({\mathrm{WO}}_{4})}_{2}$ via magnetic susceptibility, and neutron diffraction measurements. Two magnetic transitions are observed in magnetic susceptibility at ${T}_{N1}\ensuremath{\sim}9.5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ${T}_{N2}\ensuremath{\sim}7.2\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Neutron diffraction reveals an incommensurate magnetic order with a wave vector ${k}_{\text{ICM}}=(\ensuremath{\sim}0.46,\ensuremath{\sim}0.27,\ensuremath{\sim}0.24$) between ${T}_{N1}$ and ${T}_{N2}$ and a commensurate magnetic order with a wave vector ${k}_{\text{CM}}=(0.5,0.25,0.25)$ below ${T}_{N2}$. The magnetic periodicity in the commensurate phase is four times larger than the nuclear unit cell length along the $b$- and $c$-axis directions with a saturated magnetic moment equal to $\ensuremath{\approx}2.92\phantom{\rule{4pt}{0ex}}{\ensuremath{\mu}}_{B}$. Below ${T}_{N1}$, the ICM wave vector for ${\mathrm{Li}}_{2}\mathrm{Co}{({\mathrm{WO}}_{4})}_{2}$ varies with decreasing temperature and locked into commensurate at ${T}_{N2}$, whereas weaker temperature dependence of ${k}_{\text{ICM}}=(0.46,0.17,0.33)$ is observed for ${\mathrm{Li}}_{2}\mathrm{Ni}{({\mathrm{WO}}_{4})}_{2}$.