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

The magnetic structures of tetragonal La0.8Y0.2Mn2Si2 have been investigate d by neutron diffraction measurements over the temperature range similar to 2-513 K. This compound corresponds to the critical concentration x(c) simi lar to 0.2 in the La1-xYxMn2Si2 series for the transition from the antiferr omagnetism of YMn2Si2 to the predominant ferromagnetism of LaMn2Si2. The hi gh temperature antiferromagnetic region of La0.8Y0.2Mn2Si2, similar to 300 K < T < T-N1 similar to 410 K, exhibits the planar antiferromagnetism commo n to the La-rich compounds in the La1-xYxMn2Si2 series, with c-axis canted antiferromagnetic and ferromagnetic structures coexisting in the region sim ilar to 150 K < T < T-N2,T-C similar to 300 K. Below similar to 150 K, La0. 8Y0.2Mn2Si2 exhibits only the single phase canted antiferromagnetic structu re. Rietveld refinements of the data demonstrate that the canted magnetic s tructures maintain the same angle of tilt with respect to the c-axis and th e same total sublattice moment in the two phase region similar to 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 Delta a similar to 0.012 Angs trom and Delta c similar to 0.007 Angstrom respectively. This simple descri ption of these magnetic structures leads to a monotonic variation of the to tal Mn magnetic moment below TN1 similar to 410 K With the onset of the c-a xis component occurring below T-N2,T-C similar to 300 K The canted antiferr omagnetic ground state of La0.8Y0.2Mn2Si2 can be described by the Mn moment values: mu(Mn) (2 K) = 2-25(4) mu(B), with the z-component of the moment m u(z) (2 K) = 1.87(4) mu(B), corresponding to a tilt angle theta (2 K) = 33. 7(9)degrees.