Discovery of the linear magnetoelectric effect in magnetic ferroelectric BiFeo₃ in a strong magnetic field

Abstract

The discovery of a new magnetic field induced phase transition between the space-modulated spin structure (SMSS) and the homogeneous antiferromagnetic state (HAFS) is reported. This phase transition is accompanied by large magnetoelectric (ME) anomalies. It is well known that BiFeO, shows the ferroelectric ordering (T _c = 1083 K) and antiferromagnetic ordering (T _N = 673 K). ME properties of BiFeO₃ so far remain a challenging experimental and theoretical problem. While the crystal symmetry of BiFeO₃ allows a linear ME effect, it cannot be observed because the antiferromagnetic order of BiFeO₃ is SMSS (cycloid). We show that there is a Lifshits invariant, which decreases the ground state energy and induces the SMSS. In a high magnetic field the energy of the HAFS decreases more rapidly than the energy of the SMSS. Hence the field induced phase transition between SMSS and HAFS is possible. We stress that a large variation of the ME effect is expected at the transition due to a rising linear ME contribution. To study the breaking of the SMSS by a magnetic field we measured the ME effect induced by a pulsed field up to 280 kOe at T = 10–180 K. Electric polarization components were measured with the help of electrodes attached to the faces of a BiFeO₃ single crystal. The measured dependence of P(H) is quadratic at H < H, and has a jump at H = H _c when the SMSS is evidently destroyed. The critical field H _c is of the order 200 kOe. These conclusions agree well with the theoretical views.

Key Words:

• Magnetoelectric effect
• phase transition
• bismuth ferrite.