Temperature evolution of the effective magnetic anisotropy in the MnCr$_2$O$_4$ spinel

TL;DR

This study investigates the temperature-dependent magnetic phases of MnCr₂O₄ spinel using magnetization and FMR, revealing characteristic transition temperatures and anisotropy behaviors with a phenomenological model.

Contribution

It provides a detailed phenomenological description of magnetic anisotropy evolution in MnCr₂O₄ across different phases, including the helicoidal transition.

Findings

Identification of key transition temperatures T_C and T_H.

Modeling of FMR spectra with cubic and uniaxial anisotropy.

Quantitative anisotropy constants at 5 K.

Abstract

In this work we present a study of the low temperature magnetic phases of polycrystalline MnCr$_2$O$_4$ spinel through dc magnetization and ferromagnetic resonance spectroscopy (FMR). Through these experiments we determined the main characteristic temperatures: T$_C$ $\sim$41 K and T$_H$ $\sim$18 K corresponding, respectively, to the ferrimagnetic order and to the low temperature helicoidal transitions. The temperature evolution of the system is described by a phenomenological approach that considers the different terms that contribute to the free energy density. Below the Curie temperature the FMR spectra were modeled by a cubic magnetocrystalline anisotropy to the second order, with $K_1$ and $K_2$ 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 uniaxial anisotropy axis along the [1-10] propagation direction of the spiral arrange, with a $K_u$ anisotropy constant. The values obtained from the fittings at 5 K are $K_1$=-2.3x10$^4$ erg/cm$^3$, $K^2$=6.4x10$^4$ erg/cm$^3$ and $K_u$=7.5x10$^4$ erg/cm$^3$.