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Register a new userIn-plane Dielectric and Magnetoelectric Studies of BiFeO3
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In-plane temperature dependent dielectric behavior of BiFeO3 (BFO) as-grown thin films show diffuse but prominent phase transitions near 450 (+/-10) K and 550 K with dielectric loss temperature dependences that suggest skin layer effects. The 450 K anomalies are near the transition first reported by Polomska et al. [Phys. Stat. Sol. 23, 567 (1974)]. The 550 K anomalies coincide with the surface phase transition recently reported [Xavi et al. PRL 106, 236101 (2011)]. In addition, anomalies are found at low temperatures: After several experimental cycles the dielectric loss shows a clear relaxor-like phase transition near what was previously suggested to be a spin reorientation transition (SRT) temperature (~ 201 K) for frequencies 1 kHz < f < 1MHz which follow a nonlinear Vogel-Fulcher (V-F) relation; an additional sharp anomaly is observed near ~180 K at frequencies below 1 kHz. As emphasized recently by Cowley et al. [Adv. Phys. 60, 229 (2011)], skin effects are expected for all relaxor ferroelectrics. Using the interdigital electrodes, experimental data and a theoretical model for in-plane longitudinal and transverse direct magnetoelectric (ME) coefficient are presented.
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The presence of superlattice reflections and detailed analyses of the powder neutron and x-ray diffraction data reveal that La rich (BF$_{0.50}$-LF$_{0.50}$)$_{0.50}$-(PT)$_{0.50}$ (BF-LF-PT) has ferroelectric rhombohedral crystal structure with space group textit{$R3c$} at ambient conditions. The temperature dependence of lattice parameters, tilt angle, calculated polarization $(P_{s})$, volume, and integrated intensity of superlattice and magnetic reflections show an anomaly around 170 K. Impedance spectroscopy, dielectric and ac conductivity measurements were performed in temperature range $473K leq T leq 573K$ to probe the origin of large remnant polarization and frequency dependent broad transitions with large dielectric constant near $T_c^{FE}$. Results of impedance spectroscopy measurements clearly show contributions of both grain and grain boundaries throughout the frequency range ($10^{3}$ Hz$leq fleq 10^{7} $ Hz). It could be concluded that the grain boundaries are more resistive and capacitive as compared to the grains, resulting in inhomogeneities in the sample causing broad frequency dependent dielectric anomalies. Enhancement in dielectric constant and remnant polarization values are possibly due to space charge polarization caused by piling of charges at the interface of grains and grain boundaries. The imaginary parts of dielectric constant ($epsilon^{primeprime}$) Vs frequency data were fitted using Maxwell-Wagner model at $T_c^{FE}(sim 523$K) and model fits very well with the data up to $10^{5}$ Hz. Magnetodielectric measurements prove that the sample starts exhibiting magnetoelectric coupling at $sim 170$ K, which is also validated by neutron diffraction data.
Multiferroics are materials where two or more ferroic orders coexist owing to the interplay between spin, charge, lattice and orbital degrees of freedom. The explosive expansion of multiferroics literature in recent years demon-strates the fast growing interest in this field. In these studies, the first-principles calculation has played a pioneer role in the experiment explanation, mechanism discovery and prediction of novel multiferroics or magnetoelectric materials. In this review, we discuss, by no means comprehensively, the extensive applications and successful achievements of first-principles approach in the study of multiferroicity, magnetoelectric effect and tunnel junc-tions. In particular, we introduce some our recently developed methods, e.g., the orbital selective external potential (OSEP) method, which prove to be powerful tools in the finding of mechanisms responsible for the intriguing phe-nomena occurred in multiferroics or magnetoelectric materials. We also summarize first-principles studies on three types of electric control of magnetism, which is the common goal of both spintronics and multiferroics. Our review offers in depth understanding on the origin of ferroelectricity in transition metal oxides, and the coexistence of fer-roelectricity and ordered magnetism, and might be helpful to explore novel multiferroic or magnetoelectric materi-als in the future.
Clear anomalies in the lattice thermal expansion (deviation from linear variation) and elastic properties (softening of the sound velocity) at the antiferromagnetic-to-paramagnetic transition are observed in the prototypical multiferroic BiFeO3 using a combination of picosecond acoustic pump-probe and high-temperature X-ray diffraction experiments. Similar anomalies are also evidenced using first-principles calculations supporting our experimental findings. Those calculations in addition to a simple Landau-like model we also developed allow to understand the elastic softening and lattice change at T_N as a result of magnetostriction combined with electrostrictive and magnetoelectric couplings which renormalize the elastic constants of the high-temperature reference phase when the critical T_N temperature is reached.
MnCr2O4 that exhibits spin frustration and complex spiral spin order is of great interest from both fundamental as well as application-oriented perspectives. Unlike CoCr2O4 whose ground state presents the coexistence of commensurate spiral spin order (CSSO) and ferroelectric order, MnCr2O4 shows no multiferroicity. One reason is that the spiral spin order is highly sensitive to the oxygen concentration in MnCr2O4. Here, we have successfully grown high-quality single-crystalline MnCr2O4 by the chemical vapor transport method. We observe a new first-order magnetic transition from the incommensurate spiral spin order (ICSSO) at 19.4 K to the CSSO at 17.4 K. This magnetic transition is verified by magnetization, specific heat, and magnetoelectric measurements, which also confirm that the ground state exhibits the coexistence of the CSSO and magnetoelectricity below 17.4 K. Interestingly, the temperature evolution of Raman spectra between 5.4 and 300 K suggests that the structure remains the same. We also find that the phase-transition temperature of the CSSO decreases as applied magnetic field increases up to 45 kOe.
How the magnetoelectric coupling actually occurs on a microscopic level in multiferroic BiFeO3 is not well known. By using the high-resolution single crystal neutron diffraction techniques, we have determined the electric polarization of each individual elements of BiFeO3, and concluded that the magnetostrictive coupling suppresses the electric polarization at the Fe site below TN. This negative magnetoelectric coupling appears to outweigh the spin current contributions arising from the cycloid spin structure, which should produce a positive magnetoelectric coupling.
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