The Ground state of BiFeO3: Low temperature magnetic phase transitions revisited

TL;DR

This study investigates the low-temperature magnetic phases of BiFeO3, clarifying which transitions are intrinsic and how doping affects magnetic behavior, revealing coexistence of spin glass and antiferromagnetic phases.

Contribution

It provides new experimental evidence that certain magnetic transitions in BiFeO3 are intrinsic, and demonstrates the impact of MnO2 doping on magnetic properties and defect-related transitions.

Findings

Transitions at 25 K, 110 K, and 250 K are intrinsic to BiFeO3.

The transition at 50 K is defect-induced and absent in doped samples.

The ground state includes coexistence of spin glass and antiferromagnetic phases.

Abstract

Recent neutron diffraction and NMR studies suggest that the incommensurately modulated spin cycloid structure of BiFeO3 is stable down to 4.2 K, whereas DC [M(T)] and AC [\c{hi} ({\omega}, T)] magnetization, and caloric studies have revealed several magnetic transitions including a spin glass transition around 25 K. The two sets of observations are irreconcilable and to settle this, it is important to first verify if the low temperature magnetic transitions are intrinsic to BiFeO3 or some of them are offshoots of oxygen vacancies and the associated redox reaction involving conversion of Fe3+ to Fe2+. We present here the results of M (T) and \c{hi} ({\omega}, T) measurements on pure and 0.3 wt% MnO2 doped BiFeO3 samples in the 2 to 300 K temperature range. It is shown that MnO2 doping increases the resistivity of the samples by three orders of magnitude as a result of reduced oxygen vacancy concentration. A comparative study of the M (T) and AC \c{hi} ({\omega}, T) results on two types of samples reveals that the transitions around 25 K, 110 K and 250 K may be intrinsic to BiFeO3. The widely reported transition at 50 K is argued to be defect induced, as it is absent in the doped samples. We also show that the spin glass transition temperature TSG is less than the spin glass freezing temperature (Tf), as expected for both canonical and cluster glasses, in marked contrast to an earlier report of TSG > Tf which is unphysical. We have also observed a cusp corresponding to the spin glass freezing at Tf in ZFC M (T) data not observed so far by previous workers. We argue that the ground state of BiFeO3 consists of the coexistence of the spin glass phase with the long range ordered AFM phase with a cycloidal spin structure.