Lattice expansion and noncollinear to collinear ferrimagnetic order in a MnCr₂O₄ nanoparticle

Abstract

We report the magnetic behavior of spinel chromite MnCr₂O₄. Bulk MnCr₂O₄ shows a sequence of magnetic states, i.e., paramagnetic (PM) to collinear ferrimagnetic (FM) state below T_C~45 K and collinear FM state to noncollinear FM state below T_S~18 K. Decrease of particle size reduces the noncollinear spin structure and consequently, magnetic transition at T_S decreases in nanoparticle samples. However, ferrimagnetic order is still dominating in nanoparticles, except the observation of superparamagnetic-like blocking and decrease of spontaneous magnetization. This, according to the core-shell model of ferrimagnetic nanoparticles, may be due to surface disorder effects of nanoparticles. The system also shows the increase of T_C in nanoparticle samples, which is not consistent with the core-shell model. The analysis of the M(T) data, applying spin wave theory, has shown an unusual Bloch exponent value 3.35 for bulk MnCr₂O₄, which decreases and approaches 1.5, a typical value for any standard ferromagnet, with decreasing particle size. We have also observed the lattice expansion in MnCr₂O₄ nanoparticles. The present work shows the correlation between a systematic increase of lattice parameter and the gradual decrease of B site noncollinear spin structure in MnCr₂O₄.