NMR study on the stability of the magnetic ground state in MnCr${}_2$O${}_4$

TL;DR

This study uses NMR to analyze the magnetic ground state stability in MnCr₂O₄, revealing a single Cr ion canting angle, temperature-dependent spin fluctuations, and phase coexistence with first-order transition characteristics.

Contribution

It provides new insights into the magnetic structure and phase transition behavior of MnCr₂O₄, challenging previous reports and detailing spin canting angles and fluctuation dynamics.

Findings

Only one Cr ion canting angle observed.

Spin fluctuations increase near the spiral transition temperature.

Coexistence of collinear and spiral phases below transition.

Abstract

The canting angles and fluctuation of the magnetic ion spins of spinel oxide MnCr${}_2$O${}_4$ were studied by nuclear magnetic resonance (NMR) at low temperatures, which has a collinear ferrimagnetic order below $T_C$ and a ferrimagnetic spiral order below $T_s < T_C$. Contrary to previous reports, only one spin canting angle of Cr ions was observed. The spin canting angles of Mn and Cr ions in the ferrimagnetic spiral obtained at a liquid-He temperature were $43\,^{\circ}$ and $110\,^{\circ}$, respectively. The nuclear spin-spin relaxation was determined by the Suhl-Nakamura interaction at low temperatures but the relaxation rate $T_2^{-1}$ increases rapidly as the temperature approaches $T_s$. This indicates that the fluctuation of the spiral component becomes faster as the temperature increases but not fast enough to leave an averaged hyperfine field to nuclei in the time scale of nuclear spin precession in the ferrimagnetic phase, which is on the order of $10^{-8}$ s. The spiral volume fraction measured for various temperatures reveals that the collinear and the spiral ferrimagnetic phases are mixed below the transition temperature of the spiral order. The temperature hysteresis in the volume fraction implies that this transition has first-order characteristics.