Field induced incommensurate-to-commensurate magnetic phase transition in Ca₃Co_1.8Fe_0.2O₆: a neutron diffraction study

Neutron powder diffraction experiments have been performed to investigate the nature of magnetic ordering, as a function of temperature (1.5–100 K) and magnetic field (0, 2 and 4 T), in the compound Ca₃Co_1.8Fe_0.2O₆. In zero applied field, the compound orders magnetically in the incommensurate spin density wave (SDW) structure (T_N ∼ 20 K). Under an applied field of ∼2 T, an incommensurate-to-commensurate magnetic phase transition has been observed. With a further increase in the magnetic field (∼4 T), the commensurate magnetic structure transforms into a ferrimagnetic structure. In zero applied field, magnetic short-range ordering (SRO) coexists with the SDW long-range ordering (LRO) at all temperatures below T_N. In an applied magnetic field (2 and 4 T), SRO is converted into LRO only over the temperature range 12–20 K; however, below ∼12 K, an increase in the volume fraction of the SRO has been observed. The correlation length for the SRO (below ∼12 K) also gets affected by the application of a field