Observation of enhanced ferromagnetic spin-spin correlations at a triple point in quasi-two-dimensional magnets
Yugang Zhang, Zefang Li, Jing Zhang, Ning Cao, Long Zhang, Yonghong Li, Shuang Liu, Xiaoyuan Zhou, Young Sun, Wenhong Wang, Yisheng Chai
Abstract
The magnetic field-temperature phase diagrams and spin-spin correlations of quasi-two-dimensional ferromagnetic ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ and ${\mathrm{Cr}}_{2}{\mathrm{Si}}_{2}{\mathrm{Te}}_{6}$ and antiferromagnetic $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$ are investigated and compared by a sensitive composite magnetoelectric method. The phase diagrams of ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ and ${\mathrm{Cr}}_{2}{\mathrm{Si}}_{2}{\mathrm{Te}}_{6}$ disclose a triple point around the Curie temperature where three phases coexist, for both $H\ensuremath{\parallel}c$ axis and $ab$ plane configurations. In contrast, a triple/tricritical point is found in $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$ only for the $H\ensuremath{\parallel}c$ axis configuration. Anomalous enhancement of magnetoelectric signals is observed near the triple points of ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ and ${\mathrm{Cr}}_{2}{\mathrm{Si}}_{2}{\mathrm{Te}}_{6}$, suggesting significant enhancement of ferromagnetic spin-spin correlations at this point. Nevertheless, no short-range spin-spin correlation is found in $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$ above the N\'eel temperature.