NMR study on the stability of the magnetic ground state in MnCr2O4

Abstract

The canting angles and fluctuation of the magnetic ion spins of spinel oxide MnCr2O4 were studied by nuclear magnetic resonance at low temperatures, which has a collinear ferrimagnetic order below T C and a ferrimagnetic spiral order below Ts< 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° and 110°, 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. © 2010 The American Physical Society.