NMR study of successive magnetic transitions in the A-site ordered perovskite LaMn₃Cr₄O₁₂

We have investigated the successive magnetic phase transitions of the A-site ordered perovskite LaMn₃Cr₄O₁₂ by measuring ¹³⁹La nuclear magnetic resonance (NMR) spectra. The successive magnetic transitions are revealed by a very small but clear increase in the full width at half maximum (FWHM) of the ¹³⁹La NMR signal at T _M2 = 150 K and by the disappearance of the signal due to the line broadening at temperatures below T _M1 = 50 K. These two magnetic transitions are associated with independent orderings within the Cr-site sublattice at T _M2 and within the Mn-site sublattice at T _M1. Regarding the magnetic structure of the Cr-site sublattice at temperatures below T _M2, the cancellation of internal fields at the La site, ruling out ferromagnetic and multi-q antiferromagnetic structures, is consistent with the typical antiferromagnetic structure for perovskites, such as G-, A- and C-type orders. The uncanceled internal field at the La site at temperatures below T _M1 is also consistent with these typical antiferromagnetic structures in the Mn-site sublattice. The differences in the temperature dependences of the Knight shift K(T) and the bulk susceptibility χ(T) are consistently explained by the independent magnetic orderings of the Cr- and the Mn-site sublattices and the cancellation of internal field originating from the Cr-site sublattice at the La site at temperatures below T _M2.