Study of the effective magnetic anisotropy in the MnCr2O4 cubic sinel

DINA TOBIA; JULIÁN MILANO; ELIN WINKLER; MARÍA TERESA CAUSA

Barcelona

Conferencia; 20th International Conference on Magnetism; 2015

The cubic spinel systems with general formula AB2O4, where A, B or both sites are occupied by transition metal ions, have rich phase diagrams due to the correlations between their magnetic, electrical, and structural properties.  In the particular case of the chromium spinel MnCr2O4, the Cr3+ ions occupy the octahedral sites in a pyrochlore-like arrangement. As a consequence of this spatial array, this system exhibits magnetic frustration and unusual low temperature magnetic states. At TC=42K the MnCr2O4 spinel presents long-range ferrimagnetic order; as the temperature decreases below TH=18K, this magnetic phase coexists with a short-range helical order, where the spin rotation axis does not coincide with the helicoidal propagation vector. In this complex scenario, the ferromagnetic resonance (FMR) spectroscopy emerges as a suitable technique because it provides microscopic information related to the exchange and magnetic anisotropy and allows extending the knowledge of the nature of the long-range ferrimagnetic order and the spiral short-range state. Here we present a study of MnCr2O4 polycrystalline magnetic phases through dc magnetization and FMR. We describe the temperature evolution of the system by a phenomenological model that considers the different terms that contribute to the free energy density. Below TC, the FMR spectra were modeled by a cubic magnetocrystalline anisotropy to the second order, with K1 and K2 anisotropy constants that define the easy magnetization axis along the <110> direction. At lower temperatures, the formation of a helicoidal phase was considered by including a uniaxial anisotropy axis along the [1-10] propagation direction, with a Ku anisotropy constant. The values obtained from the fittings at 5K are K1=−2.3x10^4 erg/cm3, K2=6.4x10^4 erg/cm3 and Ku=7.5x10^4 erg/cm3.