There is an increasing understanding of the mechanisms underlying the development of magnetoelectric coupling and multiferroic order in both single-phase and composite materials. The investigations underlying this advance include a range of studies on thin films, which are expected to play an important role in the development of novel magnetoelectric devices. The properties of both single-phase and composite systems are widely studied. While single-phase materials can exhibit rich spin-charge coupling physics, the magnetizations, polarizations, and transition temperatures are often too small to be innately useful for device design. Conversely, a number of ferromagnetic–piezoelectric composites can show strong magnetoelectric coupling at ambient temperatures, which develops as a product-property mediated by elastic deformation, making these systems more directly amenable to fabricating devices. In this review, we provide a short overview of the mechanisms for magnetoelectric coupling in multiferroics, together with a discussion of how this magnetoelectric coupling is relevant for designing new multiferroic devices, including magnetic field sensors, dual electric and magnetic field tunable microwave and millimetre wave devices and miniature antennas. We present a brief summary of some of the significant results in studies on thin-film multiferroics, with an emphasis on single-phase materials, and covering systems where the magnetic and ferroelectric transitions fall at the same temperature as well as systems where they fall at different temperatures.

https://iopscience.iop.org/article/10.1088/0022-3727/44/24/243001/pdf

Introduction to magnetoelectric coupling and multiferroic films

Effect of A site and B site doping on structural, thermal, and dielectric properties of BiFeO3 ceramics

In this study we report the synthesis of BiFeO3 nanorods using a sonochemical technique. The nanorods had a diameter of 20–50 nm, a length of 100–500 nm and exhibit aspect ratios in the range of 5–10. However, after doping, the TEM images of Bi0.9Ba0.1Fe0.9Mn0.1O3 and Bi0.9Ca0.1Fe0.9Cr0.1O3 samples show that the aspect ratios of both the double doped samples have reduced considerably, while retaining the crystallinity of the particles. BiFeO3 nanorods show a weak ferromagnetic order at room temperature, which is quite different from the linear M–H relationship reported for bulk BiFeO3. The saturation magnetization of these BiFeO3 nanostructures has been found to increase on doping with various metal ions (Ba2+, Ca2+, Mn2+, Cr3+), reaching a maximum value of 1.35 emu g−1 for the Bi0.9Ba0.1Fe0.9Mn0.1O3 nanostructures. However, saturation of electric polarization was observed only in case of the Bi0.9Ca0.1Fe0.9Cr0.1O3 nanostructures.

Effect of doping on the morphology and multiferroic properties of BiFeO3 nanorods

Pr modified Bi0.9−xLa0.1PrxFeO3 (BLPFO-x, x = 0, 0.1 and 0.2) ceramics were prepared by the conventional method based on the solid state reaction of mixed oxides and a detailed study of electrical and magnetic properties of Pr modified bismuth ferrite (BLPFO) is reported. X-ray analysis shows the formation of a bismuth ferrite rhombohedral phase. Pr doping significantly increases the resistivity and leads to a successful observation of electrical polarization hysteresis loops. All the samples have been found to possess a spontaneous magnetic moment at room temperature which increases further at low temperatures. The strong dependence of remnant polarization and dielectric constant on the strength of magnetic field is a direct evidence of magnetoelectric coupling in BLPFO-2 ceramics.

https://iopscience.iop.org/article/10.1088/0953-8984/21/40/405901/pdf

Pr doped bismuth ferrite ceramics with enhanced multiferroic properties

Bi(Fe0.95Co0.05)O3 bulk ceramics were prepared by rapid sintering using sol-gel derived fine powders. Bi(Fe0.95Co0.05)O3 crystallized in a rhombohedrally distorted BiFeO3 structure with compressive lattice distortion induced by the Co substitution at Fe sites from Raman study. Compared with BiFeO3 prepared under similar conditions, the magnetic properties were significantly enhanced, with saturate magnetization of 1.6 emu/g and remnant magnetization of 0.7 emu/g at 300 K. Clear metamagnetism was observed in Bi(Fe0.95Co0.05)O3.

The magnetic properties of Bi(Fe0.95Co0.05)O3 ceramics

We have synthesized a range of transition-metal-doped BiFeO3 thin films on conducting silicon substrates using a spin-coating technique from metal–organic precursor solutions. Bismuth, iron and transition-metal–organic solutions were mixed in the appropriate ratios to produce 3% transition-metal-doped samples. X-ray diffraction studies show that the samples annealed in a nitrogen atmosphere crystallize in a rhombohedrally distorted BiFeO3 structure with no evidence for any ferromagnetic secondary phase formation. We find evidence for the disappearance of the 404 cm−1 Raman mode for certain dopants indicative of structural distortions. The saturation magnetization of these BiFeO3 films has been found to increase on doping with transition metal ions, reaching a maximum value of 8.5 emu cm−3 for the Cr-doped samples. However, leakage current measurements find that the resistivity of the films typically decreases with transition metal doping. We find no evidence for any systematic variation of the electric or magnetic properties of BiFeO3 depending on the transition metal dopant, suggesting that these properties are determined mainly by extrinsic effects arising from defects or grain boundaries.

http://iopscience.iop.org/article/10.1088/0953-8984/21/3/036001/pdf

M S R Rao

Papers

Structural, magnetic, and electrical studies on polycrystalline transition-metal-doped BiFeO3 thin films