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

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 similar to 45 K and collinear FM state to noncollinear FM state below T-S similar to 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.