Magnetic structure and critical properties of FeGe2.

Abstract

A neutron study of the tetragonal antiferromagnet ${\mathrm{FeGe}}_{2}$ has shown the existence of two continuous magnetic transitions at temperatures of \ensuremath{\sim}263 and \ensuremath{\sim}289 K. The upper temperature corresponds to a transition from paramagnetism to a basal-plane spiral structure propagating along the cell edges in that plane. At the lower temperature the spiral structure is transformed into the simple collinear structure previously reported in the literature. Typical critical behavior is observed at the upper temperature for individual satellite peaks. The spiral propagation vector decreases continuously to zero at the lower critical point, exhibiting power-law behavior with an exponent of ${0.40}_{7}$\ifmmode\pm\else\textpm\fi{}0.005. Heat-capacity measurements reveal two \ensuremath{\lambda}-type anomalies with critical exponents in the expected range. The phase diagram has been analyzed using mean-field and renormalization-group considerations. A model based on zero basal-plane spin anisotropy yields a magnetic structure which agrees with the observed structure of the intermediate phase. The effect of an external field has also been treated theoretically.