Strain induced modifications of the magnetic moment in epitaxial multiferroic BiFeO₃ and enhanced magneto-electric coupling in SrRuO₃/BiFeO₃ heterostructures

Multiferroic materials have drawn a lot of attention in recent times due to its promising potential applications in energy efficient magnetization switching. As these materials exhibit coupling between electric and magnetic order parameters, in principle, the magnetoelectric (ME) coupling can be used to tune the electric polarization by external magnetic field and vice-versa. Among the known magnetoelectric materials, BiFeO₃ shows spontaneous polarization and an ordered magnetic state at room temperature. Ever since it has been established that BiFeO₃ could be a multiferroic, efforts are on to improve the magnetic properties and establish a strong magneto-electric coupling. In the first part of this lecture, the possibilities of tuning the magnetic characteristics of BiFeO₃ films by structural control are discussed. Epitaxial BiFeO₃ films on SrTiO₃ (001) and NdGaO₃ were prepared with different thicknesses and they exhibit variation of crystallographic structures owing to epitaxy and strain. In the case of samples grown on SrTiO₃ (001), the low thickness samples exhibit "R-like" phase while the higher thickness films show both "R-like" and "T-like" phases. Interestingly, an enhancement of MS is observed in "R-like" phase. The BiFeO₃ films grown on (001) oriented orthorhombic NdGaO₃ substrate gets stabilized in Cm phase close to the interface and a highly distorted "T-like" phase appears in the film if the thickness is increased. DC magnetization measurement at room temperature confirms that the Cm phase has very high magnetic moment (upto ∼3.5 mB/f.u) compared to other crystallographic phases. A BiFeO₃ film with Cm crystal structure has almost 2.5 times higher saturation magnetization compared to a film of identical thickness grown on SrTiO₃ substrate. Out-of-plane piezo force microscopy shows that the coercive voltage of the phase vs. bias voltage hysteresis loop is less in "T-like" phase in comparison to the Cm phase. Then the interlayer magneto-electric coupling in BiFeO₃/SrRuO₃ heterostructure investigated by impedance spectroscopy over a temperature range of 80 K - 260 K will be discussed. In-plane impedance measurements were performed using interdigitated gold electrodes fabricated on the BiFeO₃ layer. A pronounced dip in the temperature coefficient of equivalent-capacitance and a distinct increase in the temperature coefficient of equivalent-resistance of the BiFeO₃ layer were observed on cooling across ferromagnetic T_C of the bottom SrRuO₃ layer. Temperature dependent capacitance (at 0 T magnetic fields) and magneto-capacitance (at 5 T magnetic fields) plots showed anomalies near 160 K. These observations suggest a strong magneto-electric coupling between the BiFeO₃ and SrRuO₃ layers of this heterostructure. (author)