Commensurate to incommensurate magnetic phase transition in the type-II multiferroic YBaCuFeO₅

The simultaneous existence of magnetic and ferroelectric ordering is a characteristic of multiferroic materials. The search for new multiferroics is partly motivated by the need for non-volatile random access memories for which the electric polarisation (magnetisation) is controlled by magnetic field (electric field) or vice versa. The use of such materials would be extremely beneficial for the next generation of electronic devices (mobile phones, tablets etc) whereby compact size constraints are important as well as optimising reading/writing speeds and power consumption. YBaCuFeO₅ has been classified as a type-II multiferroic due to its complex magnetic interactions and low temperature feroelectricity. Two magnetic phase transitions of antiferromagnetic nature have been found near T_N1 = 450 K and T_N2 = 170 K. The first represents the ordering of the Fe³⁺ ions into a commensurate antiferromagnetic state, while the second represents the ordering of the Cu²⁺ ions giving an overall incommensurate antiferromagnetic ground state. For the first time, using modified traveling solvent floating zone growth method, we have been able to grow a centimeter-sized, high-quality, singlecrystal of YBaCuFeO₅. Magnetisation and neutron diffraction results indicate a complex magnetic phase diagram in this material with a strong hysteresis effect and a chiral magnetic ground state.