Charge-transfer-driven enhanced room-temperature ferromagnetism in BiFeO3/Ag nanocomposite.

We report observation of more than an order of magnitude jump in saturation magnetization in BiFeO3/Ag nanocomposite at room temperature compared to what is observed in bare BiFeO3nanoparticles. Using transmission electron microscopy together with energy dispersive x-ray spectra (which maps the element concentration across the BiFeO3/Ag interface) and x-ray photoelectron spectroscopy, we show that both the observed specific self-assembly pattern of BiFeO3and Ag nanoparticles and the charge transfer between Ag and O are responsible for such an enormous rise in room-temperature magnetization. The BiFeO3/Ag nanocomposites, therefore, could prove to be extremely useful for a variety of applications including biomedical.

Shape dependent multiferroic behavior in Bi2Fe4O9nanoparticles.

Ferroelectric and magnetic properties are investigated for Bi2Fe4O9nanoparticles with different shapes (cuboid and sphere-like) synthesized by hydrothermal and sol-gel method. The magnetic study reveals that coercivity, Neel temperature and remanent magnetization strongly depend on shape of the particle. The nanoparticle with sphere-like shape exhibits magnetization curve of antiferromagnetic (AFM) ordering with ferromagnetic (FM) component. As the particle shape changes from sphere-like to cuboid, the AFM component is dominating over the ferromagnetic component. A small exchange bias is also observed at low temperature in both the sphere-like and cuboid nanoparticle. The coercivity, remanent magnetization and Neel temperature of sphere-like nanoparticle is greater than cuboid nanoparticle. Ferroelectric measurement shows the remanent polarization of cuboid is greater than sphere-like nanoparticle but the coercivity is almost same. This Bi2Fe4O9nanoparticle shows a small change in polarization under magnetic field. The polarization value decreases with magnetic field increases. The magnetoelectric coupling-measured by change of remanent polarization under magnetic field are found to be greater in Bi2Fe4O9sphere-like nanoparticles. These shape dependent magnetic and ferroelectric properties are coming because of shape anisotropy.

Superspin glass mediated giant spontaneous exchange bias in a nanocomposite of BiFeO3-Bi2Fe4O9.

We observe an enormous spontaneous exchange bias (~300-600 Oe)--measured in an unmagnetized state following zero-field cooling--in a nanocomposite of BiFeO(3) (~94%)-Bi(2)Fe(4)O(9) (~6%) over a temperature range 5-300 K. Depending on the path followed in tracing the hysteresis loop--positive (p) or negative (n)--as well as the maximum field applied, the exchange bias (H(E)) varies significantly with | - H(Ep) | > | H(En) |. The temperature dependence of H(E) is nonmonotonic. It increases, initially, till ~150 K and then decreases as the blocking temperature T(B) is approached. All these rich features appear to be originating from the spontaneous symmetry breaking and consequent onset of unidirectional anisotropy driven by "superinteraction bias coupling" between the ferromagnetic core of Bi(2)Fe(4)O(9) (of average size ~19 nm) and the canted antiferromagnetic structure of BiFeO(3) (of average size ~112 nm) via superspin glass moments at the shell.

A training effect on electrical properties in nanoscale BiFeO3.

We report our observation of the training effect on dc electrical properties in a nanochain of BiFeO3 as a result of large scale migration of defects under the combined influence of electric field and Joule heating. We show that an optimum number of cycles of electric field within the range zero to ~1.0 MV cm(-1) across a temperature range 80-300 K helps in reaching the stable state via a glass-transition-like process in the defect structure. Further treatment does not give rise to any substantial modification. We conclude that such a training effect is ubiquitous in pristine nanowires or chains of oxides and needs to be addressed for applications in nanoelectronic devices.

Large Magnetoelectric Coupling in the Thin Film of Multiferroic CuO.

We report observation of large magnetoelectric coupling in an epitaxial thin film of multiferroic CuO grown on the (100)MgO substrate by the pulsed laser deposition technique. The film is characterized by X-ray diffraction, transmission electron microscopy, and Raman spectrometry. The crystallographic structure of the film turns out to be monoclinic (space group C2/c) with [111]CuO||[100]MgO "out-of-plane" epitaxy and "in-plane" domain structure. The lattice misfit strain is found to vary within ±1-3%. The dc resistivity, magnetization, dielectric spectroscopy, and remanent ferroeletric polarization have been measured across 80-300 K. The dielectric constant is found to decrease by >20% under a moderate magnetic field of ∼18 kOe while the remanent ferroelectric polarization, emerging at the onset of magnetic transition (T N ∼ 175 K), decreases by nearly 50% under ∼18 kOe field. These results could assume importance as the strain-free bulk CuO does not exhibit magnetoelectric coupling within such magnetic field regime. The strain-induced large magnetoelectric coupling in the CuO thin film would generate new possibility of further strain tuning to observe room-temperature magnetoelectric multiferroicity suitable for scores of applications such as memories, sensors, energy-harvesting devices, generators, amplifiers, and so forth.

Neutron diffraction investigation of the magnetic structure and magnetoelastic effects in NdMnO(3).

We investigated the evolution of the magnetic structure and magnetoelastic effects of NdMnO(3) by neutron powder diffraction. We confirmed the A-type antiferromagnetic (AF) structure of NdMnO(3) below T(N)≈82 K with magnetic moments parallel to the b axis of the orthorhombic crystal structure (space group Pbnm). We found that the magnetic moments of Nd order below about T(N)(Nd)≈20 K in a ferromagnetic structure with moments parallel to the c axis. At the same temperature the magnetic moments of Mn develop a ferromagnetic component parallel to the c axis. We found strong magnetoelastic effects associated with the AF transition at T(N)≈82 K. The effect is very prominent for the b lattice parameter. All three lattice parameters and therefore the unit cell volume contracts at T(N)≈82 K. We treated quantitatively the magnetoelastic effect of the b lattice parameter and found that the extra change in the lattice parameter Δb due to the magnetoelastic effect is proportional to the magnetic moment of the Mn ion. We also determined the critical exponent of the AF phase transition to be ß = 0.296 ± 0.008.

Direct evidence for the Nd magnetic ordering in NdMnO(3) from the hyperfine field splitting of Nd nuclear levels.

We investigated the low energy excitations in NdMnO(3) in the µeV range by a backscattering neutron spectrometer. The energy spectra on polycrystalline NdMnO(3) samples revealed inelastic peaks at E = 2.15 ± 0.01 µeV at T = 2 K on both energy gain and energy loss sides. The inelastic peaks move gradually towards lower energy with increasing temperature and tend to merge with the elastic peak at the electronic magnetic ordering temperature of Nd, T(Nd)≈20 K. However, at temperatures higher than T(Nd)≈20 K the energy of the inelastic peak decreases at a much slower rate and remains finite up to T = 55 K, the highest temperature investigated. We interpret the inelastic peaks to be due to the transition between the hyperfine-split nuclear level of the (143)Nd and (145)Nd isotopes with spin I = 7/2 caused by the magnetic ordering of Nd electronic moment below T(Nd)≈20 K. We ascribe the finite energy of the inelastic peak and its much smaller temperature dependence at T>20 K to be due to the polarization of the Nd magnetic moment by the field of Mn moments that order below T(N)≈78 K.

Nonmonotonic particle-size-dependence of magnetoelectric coupling in strained nanosized particles of BiFeO3.

Using high resolution powder x-ray and neutron diffraction experiments, we determined the off-centered displacement of the ions within a unit cell and magnetoelectric coupling in nanoscale BiFeO3 (≈20-200 nm). We found that both the off-centered displacement of the ions and magnetoelectric coupling exhibit nonmonotonic variation with particle size. They increase as the particle size reduces from bulk and reach maximum around 30 nm. With further decrease in particle size, they decrease precipitously. The magnetoelectric coupling is determined by the anomaly in off-centering of ions around the magnetic transition temperature (T N ). The ions, in fact, exhibit large anomalous displacement around the T N which is analyzed using group theoretical approach. It underlies the nonmonotonic particle-size-dependence of off-centre displacement of ions and magnetoelectric coupling. The nonmonotonic variation of magnetoelectric coupling with particle size is further verified by direct electrical measurement of remanent ferroelectric hysteresis loops at room temperature under zero and ∼20 kOe magnetic field. Competition between enhanced lattice strain and compressive pressure appears to be causing the nonmonotonic particle-size-dependence of off-centre displacement while coupling between piezo and magnetostriction leads to nonmonotonicity in the variation of magnetoelectric coupling.

Fabrication of zinc ferrite nanocrystals by sonochemical emulsification and evaporation: observation of magnetization and its relaxation at low temperature.

A new ultrasound assisted emulsion (consisting of rapeseed oil and aqueous solution of Zn(2+) and Fe(2+) acetates) and evaporation protocol has been developed for the synthesis of zinc ferrite (ZnFe(2)O(4)) nanoparticles with narrow size distribution. The as-synthesized sample consisted of crystalline zinc ferrite particles with an average diameter of approximately 4 nm, whereas the average size of the heat-treated ferrite particles increases to approximately 12 nm. To remove the small amount of oil present on the surface of the as-synthesized ferrite sample, heat treatment was carried out at 350 degrees C for 3 h. The as-synthesized and heat-treated ferrites were characterized by X-ray diffraction (XRD), infrared spectroscopy (IR), TGA/DTA, transmission electron microscopy (TEM), and energy dispersion X-ray spectroscopy (EDS) techniques. Magnetic measurements show that the nanocrystalline ZnFe(2)O(4), prepared through this technique, is either at par with those obtained in other cases or even more improved. Both the as-synthesized and heat-treated samples reveal relaxation of magnetization. Our study also shows that one can tailor the magnetization and relaxation pattern by suitably controlling the particle size of the nanocrystalline ZnFe(2)O(4). The key features of this method are avoiding (a) the cumbersome conditions that exist in the conventional methods, (b) the usage of necessary additive components (stabilizers or surfactants, precipitants), and (c) calcination requirements. In addition, rapeseed oil has replaced organic nonpolar solvents used in earlier studies. As a whole, this simple straightforward sonochemical approach results in a better pure phase system of nanoferrite with improved magnetic properties.

Electric field driven destabilization of the insulating state in nominally pure LaMnO3.

We report an electric field driven destabilization of the insulating state in nominally pure LaMnO3 single crystal with a moderate field which leads to a resistive state transition below 300 K. The transition is between the insulating state in LaMnO3 and a high resistance bad metallic state that has a temperature independent resistivity. The transition occurs at a threshold field (Eth) that shows a steep enhancement on cooling. While at lower temperatures the transition is sharp and involves a large change in resistance, it softens on heating and is eventually absent above 280 K. When the Mn(4+) content is increased by Sr substitution up to x = 0.1, the observed transition, although observable in a certain temperature range, softens considerably. This observation has been explained as a bias driven percolation type transition between two co-existing phases, where the majority phase is a charge and orbitally ordered polaronic insulating phase and the minority phase is a bad metallic phase. The mobile fraction f of the bad metallic phase deduced from the experimental data follows an activated kinetics as f = fo(E)exp(-Δ/kBT) with the activation energy Δ ≈ 200 meV, and the pre-factor fo(E) is a strong function of the field that leads to a rapid enhancement of f on application of field, leading to the resistive state transition. We suggest likely scenarios for such co-existing phases in nominally pure LaMnO3 that can lead to the bias driven percolation type transition.

Magnetoelastic effects in multiferroic YMnO3.

We have investigated magnetoelastic effects in multiferroic YMnO(3) below the antiferromagnetic phase transition, T(N) ≈ 70 K, using neutron powder diffraction. The a lattice parameter of the hexagonal unit cell of YMnO(3) decreases normally above T(N), but decreases anomalously below T(N), whereas the c lattice parameter increases with decreasing temperature and then increases anomalously below T(N). The unit cell volume also undergoes an anomalous contraction below T(N). By fitting the background thermal expansion for a non-magnetic lattice with the Einstein-Grüneisen equation, we determined the lattice strains Δa, Δc and ΔV due to the magnetoelastic effects as a function of temperature. We have also determined the temperature variation of the ordered magnetic moment of the Mn ion by fitting the measured Bragg intensities of the nuclear and magnetic reflections with the known crystal and magnetic structure models and have established that the lattice strain due to the magnetoelastic effect in YMnO(3) couples with the square of the ordered magnetic moment or the square of the order parameter of the antiferromagnetic phase transition.

Ultrasonic cavitation induced water in vegetable oil emulsion droplets--a simple and easy technique to synthesize manganese zinc ferrite nanocrystals with improved magnetization.

In the present investigation, synthesis of manganese zinc ferrite (Mn(0.5)Zn(0.5)Fe(2)O(4)) nanoparticles with narrow size distribution have been prepared using ultrasound assisted emulsion (consisting of rapeseed oil as an oil phase and aqueous solution of Mn(2+), Zn(2+) and Fe(2+) acetates) and evaporation processes. The as-prepared ferrite was nanocrystalline. In order to remove the small amount of oil present on the surface of the ferrite, it was subjected to heat treatment at 300 °C for 3h. Both the as-prepared and heat treated ferrites have been characterized by X-ray diffraction (XRD), infrared spectroscopy (IR), TGA/DTA, transmission electron microscopy (TEM) and energy dispersion X-ray spectroscopy (EDS) techniques. As-prepared ferrite is of 20 nm, whereas the heat treated ferrite shows the size of 33 nm. In addition, magnetic properties of the as-prepared as well as the heat treated ferrites have also been carried out and the results of which show that the spontaneous magnetization (σ(s)) of the heat treated sample (24.1 emu/g) is significantly higher than that of the as-synthesized sample (1.81 emu/g). The key features of this method are avoiding (a) the cumbersome conditions that exist in the conventional methods; (b) usage of necessary additive components (stabilizers or surfactants, precipitants) and (c) calcination requirements. In addition, rapeseed oil as an oil phase has been used for the first time, replacing the toxic and troublesome organic nonpolar solvents. As a whole, this simple straightforward sonochemical approach results in more phase pure system with improved magnetization.