Complex perovskite oxides exhibit a rich spectrum of properties, including magnetism, ferroelectricity, strongly correlated electron behaviour, superconductivity and magnetoresistance, which have been research areas of great interest among the scientific and technological community for decades. There exist very few materials which exhibit multiple functional properties; one such class of materials is called the multiferroics. Multiferroics are interesting because they exhibit simultaneously ferromagnetic and ferroelectric polarizations and a coupling between them. Due to the nontrivial lattice coupling between the magnetic and electronic domains (the magnetoelectric effect), the magnetic polarization can be switched by applying an electric field; likewise the ferroelectric polarization can be switched by applying a magnetic field. As a consequence, multiferroics offer rich physics and novel devices concepts, which have recently become of great interest to researchers. In this review article the recent experimental status, for both the bulk single phase and the thin film form, has been presented. Current studies on the ceramic compounds in the bulk form including Bi(Fe,Mn)O3, REMnO3 andthe series of REMn2O5 single crystals (RE =  rare earth) are discussed in the first section and a detailed overview on multiferroic thin films grown artificially (multilayers and nanocomposites) is presented in the second section.

Recently, multiferroic materials have provoked a renaissance because of the prospect of controlling both the dielectric and magnetic properties of these materials using a magnetic or an electric field. [1][2][3][4][5][6] Yet there are few materials that are simultaneously ferroelectric and ferromagnetic in the same phase. [7,8] The orthorhombic TbMnO 3 phase has attracted much attention owing to its intriguing coupling between spin and charge degrees of freedom, but its ferroelectricity emerges at temperatures below 27 K.[1] Here, we report on the multiferroic properties of a hexagonal TbMnO 3 metastable phase that was epitaxially stabilized in thin-film form on substrates with hexagonal in-plane symmetry. In contrast to the bulk orthorhombic phase, [1] the hexagonal TbMnO 3 films display ca. 20 times larger remnant polarization with the ferroelectric ordering temperature shifted to ca. 60 K. In addition, a newly discovered antiferroelectric-like phase and a clear signature of magnetoelectric effects suggest its uniqueness in the class of hexagonal manganites. Here we demonstrate a promising way to synthesize new multiferroic materials that do not exist in bulk form.Among the known multiferroic materials, the rare-earth manganites RMnO 3 are very intriguing material systems that can have two kinds of crystal structure. Depending on the size of the R ion, [9] RMnO 3 forms either an orthorhombic (R = La-Dy) or a hexagonal (R = Ho-Lu) structure. All of the hexagonal rare-earth manganites show multiferroic behaviors with high ferroelectric ordering temperatures, T C , (typically, above 590 K) and magnetic ordering temperatures T N ∼ 70-120 K.[3] The origin of the ferroelectric (FE) ordering in hexagonal manganites is related to the tilting of the rigid MnO 5 trigonal bipyramid.[10] By contrast, among the orthorhombic RMnO 3 , only the three compounds containing rare-earth elements near Ho (i.e., R = Dy, Tb, and Gd) show multiferroic behavior with a relatively low ferroelectric ordering temperature (ca. 27 K). [1,11] In this case, the ferroelectricity originates from the magnetic-frustration-induced lattice modulation. These facts imply that there is a possibility of controlling the multiferroic properties by modifying the structural phase of the rare-earth manganites.Since the orthorhombic TbMnO 3 is near the hexagonal RMnO 3 series, the formation energy difference between the orthorhombic and hexagonal TbMnO 3 could be small. Therefore, it is a worthwhile attempt to stabilize it in a hexagonal phase and explore its multiferroic properties. We fabricated TbMnO 3 in a new hexagonal phase by laser ablating the bulk orthorhombic materials into thin films on either Pt(111)//Al 2 O 3 (0001) or YSZ(111) (YSZ: yttria-stabilized zirconia) substrates. Note that the atomic arrangement on the surfaces of both substrates has hexagonal in-plane symmetry. In bulk, the TbMnO 3 exists in the distorted orthorhombic (GdFeO 3 -type) structure, as shown schematically in Figure 1a. The hexagonal in-plane symmetry on the substrate surface...

The authors fabricated Pb(Zr0.52Ti0.48)O3–NiFe2O4 composite films consisting of randomly dispersed NiFe2O4 nanoparticles in the Pb(Zr0.52Ti0.48)O3 matrix. The structural analysis revealed that the crystal axes of the NiFe2O4 nanoparticles are aligned with those of the ferroelectric matrix. The composite has good ferroelectric and magnetic properties. The authors measured the transverse and longitudinal components of the magnetoelectric voltage coefficient, which supports the postulate that the magnetoelectric effect comes from direct stress coupling between magnetostrictive NiFe2O4 and piezoelectric Pb(Zr0.52Ti0.48)O3 grains.

The influence of oxygen vacancies on the dielectric relaxation behavior of pure and Eu-substituted BiFeO3 nanoparticles synthesized by a sol-gel technique has been studied using impedance spectroscopy in the temperature range of 90 °C to 180 °C. The electric relaxation time and activation energy of the oxygen vacancies can be calculated from the Arrhenius equation, and found to be 1.26 eV and 1.76 eV for pure and Eu-substituted BiFeO3, respectively. Substitution induces structural disorder and changes in the Fe-O-Fe bond angle, leading to alteration of the magnetic properties, observed from magnetic studies and evaluated using Rietveld refinement of the XRD patterns. X-ray photoelectron spectroscopy (XPS) confirms the shifting of the binding energy of the Bi 4f orbital, establishing Eu substitution at the Bi site. Calculation of the area under the Fe(2+)/Fe(3+) (2p) and O (1s) XPS spectra gives approximate values of the oxygen vacancies.

We investigated electronic structure of hexagonal multiferroic YMnO3 using the polarization dependent x-ray absorption spectroscopy (XAS) at O K and Mn L(2,3) edges. The spectra exhibit strong polarization dependence at both edges, reflecting anisotropic Mn 3d orbital occupation. Moreover, the O K edge spectra show that Y 4d states are strongly hybridized with O 2p ones, resulting in large anomalies in Born effective charges on off-centering Y and O ions. These results manifest that the Y d(0)-ness with rehybridization is the driving force for the ferroelectricity, and suggest a new approach to understand the multiferroicity in the hexagonal manganites.

We fabricated epitaxial thin films of hexagonal DyMnO 3 , which otherwise form in a bulk perovskite structure, via deposition on Pt(111)//Al 2 O 3 (0001) and YSZ (111) substrates: each of which has in-plane hexagonal symmetry. The polarization hysteresis loop demonstrated the existence of ferroelectricity in our hexagonal DyMnO 3 films at least below 70 K. The observed 2.2 µC/cm 2 remnant polarization at 25 K corresponded to a polarization enhancement by a factor of 10 compared to that of the bulk orthorhombic DyMnO 3 . Interestingly, this system showed an antiferroelectric-like feature in its hysteresis loop. Our hexagonal DyMnO 3 films showed an antiferromagnetic Néel temperature around 60 K and a spin reorientation transition around 40 K. We also found a clear hysteresis in the temperature dependence of the magnetization, which was measured after zero-field-cooling and field-cooling. This hysteresis may well have been of spin glass origin, which was likely to arise from the geometric frustration of antiferromagnetically-coupled Mn spins with an edge-sharing triangular lattice.

The possibility of controlling transport properties of colossal magnetoresistance manganite films using substrate-induced strain has attracted great interest. We have investigated transport properties of La0.9Ca0.1MnO3, La0.92Ba0.08MnO3, La0.8Ba0.2MnO3, and LaMnO3 films. When the films were post-annealed at proper conditions, all of them showed metal–insulator transitions. Their transition temperatures TMI were much higher than the corresponding bulk values, irrespective of the type of substrate-induced biaxial strain. This surprising fact demonstrated that strain could not be the main origin of the TMI enhancement observed in the underdoped (dopant concentration x<0.3) manganite films. We suggested that TMI enhancements should be attributed mostly to the cationic vacancies in the post-annealed films.

The authors investigated the magnetic and ferroelectric properties of hexagonally grown HoMnO3 thin films. The magnetic measurements revealed bulklike magnetic phase transitions with an additional spin-glass-like behavior feature below the Néel temperature. The ferroelectricity in the films was distinctly different from the suggested bulk behavior. Below 40K, the HoMnO3 films showed typical ferroelectric character: their remnant polarization and coercive field values at 20K were 3.7μC∕cm2 and 0.69MV∕cm. Above 40K, however, the films exhibited an unusual antiferroelectriclike behavior, with more pronounced features appearing at higher temperatures. These intriguing physical properties make HoMnO3 films a potential candidate material for numerous future applications.