Observation of enhanced ferromagnetic spin-spin correlations at a triple point in quasi-two-dimensional magnets

The magnetic field-temperature phase diagrams and spin-spin correlations of quasi-two-dimensional fer-romagnetic Cr2Ge2Te6 and Cr2Si2Te6 and antiferromagnetic MnBi2Te4 are investigated and compared by a sensitive composite magnetoelectric method. The phase diagrams of Cr2Ge2Te6 and Cr2Si2Te6 disclose a triple point around the Curie temperature where three phases coexist, for both H II c axis and ab plane configurations. In contrast, a triple/tricritical point is found in MnBi2Te4 only for the H II c axis configuration. Anomalous enhancement of magnetoelectric signals is observed near the triple points of Cr2Ge2Te6 and Cr2Si2Te6, suggest-ing significant enhancement of ferromagnetic spin-spin correlations at this point. Nevertheless, no short-range spin-spin correlation is found in MnBi2Te4 above the Neel temperature.