Competing magnetic interactions in La0.8Y0.2Mn2Si2 - coexistence of canted ferromagnetism and antiferromagnetism

Abstract

The magnetic structures of tetragonal La0.8Y0.2Mn2Si2 have been investigated by neutron diffraction measurements over the temperature range ~2 - 513 K. This compound corresponds to the critical concentration xc ~ 0.2 in the La1 - xYxMn2Si2 series for the transition from the antiferromagnetism of YMn2Si2 to the predominant ferromagnetism of LaMn2Si2. The high temperature antiferromagnetic region of La0.8Y0.2Mn2Si2, ~ 300 K < T < TN1 ~ 410 K, exhibits the planar antiferromagnetism common to the La-rich compounds in the La1 - xYxMn2Si2 series, with c-axis canted antiferromagnetic and ferromagnetic structures coexisting in the region ~ 150 K < T < TN2,C ~ 300 K. Below ~ 150 K, La0.8Y0.2Mn2Si2 exhibits only the single phase canted antiferromagnetic structure. Rietveld refinements of the data demonstrate that the canted magnetic structures maintain the same angle of tilt with respect to the c-axis and the same total sublattice moment in the two phase region ~ 150 - 300 K, and that the two tetragonal phases (both with the ThCr2Si2 structure) can be distinguished crystallographically with differences in their a lattice parameters and c lattice parameters of around a ~ 0.012 Å and c ~ 0.007 Å respectively. This simple description of these magnetic structures leads to a monotonic variation of the total Mn magnetic moment below TN1 ~ 410 K with the onset of the c-axis component occurring below TN2,C ~ 300 K. The canted antiferromagnetic ground state of La0.8Y0.2Mn2Si2 can be described by the Mn moment values: µMn(2 K) = 2.25(4) µB , with the z-component of the moment µz(2 K) = 1.87(4) µB , corresponding to a tilt angle (2 K) = 33.7(9)°.