Neutron diffraction study of BiFeO3 and BiFe0.98Co0.02O3 nano-particles

• G-type antiferromagnetic order with spin canting out of ab-plane in BiFeO3.
• Bunching of spins along c-axis for Cobalt doped BiFeO3.
• Anharmonicity in spin cycloid is suppressed at low temperature.
• Spin state transitions due to Cobalt doping at T<100 K.

Nano particles of BiFeO3 and BiFe0.98Co0.02O3 are studied via neutron diffraction technique. Analysis of purely magnetic peak near d ~ 4.56 Å, reveals large spin canting and anharmonic cycloidal magnetic structure i.e., bunching of the magnetic moment along c-axis at 300K. This is in contrary to that observed in bulk, where the spins are restricted to ab-plane only. The substitution of Co3+ is found to be inducing more spin canting. On decreasing the temperature, magnetic anharmonicity decreases and the magnetic cycloidal structure revives. At room temperature, the Co3+ are found to be in intermediate spin state corresponding to magnetic moment 2.828 μB, whereas that of Fe3+ as 3.848 μB. At low temperatures, signature of spin state transition from intermediate spin (S =1) to low spin (S = 0) of Co3+ ion is observed.