J.-H. Kang

Enhanced ferromagnetic properties in Ho and Ni co-doped BiFeO3 ceramics

The magnetic properties of polycrystalline Bi1-xHoxFe1-yNiyO3 (x = 0, 0.1; y = 0, 0.03), which were prepared by the solid-state method, have been investigated. The powder X-ray diffraction reveals that all the samples are polycrystalline and show rhombohedral perovskite structure. The micro-Raman scattering studies confirm that Bi0.9Ho0.1Fe0.97Ni0.03O3 has a compressive lattice distortion induced by the simultaneous substitution of Ho and Ni ions at A and B-sites, respectively. From the magnetization dependences at room temperature, Bi0.9Ho0.1Fe0.97Ni0.03O3 has enhanced magnetization (0.2280 emu/g) and low coercive field (280 Oe). It was revealed that the Ni dopant plays an important role for the improved ferromagnetic properties and the Ho dopant favors the magnetic exchange interactions in the co-doped ceramic.

Spin-glass behavior of Cr-doped YMnO3 compounds

Structural and magnetic properties of polycrystalline YMn1−xCrxO3 with x = 0, 0.05, and 0.1, which was prepared by solid-state method, have been investigated. The x-ray diffraction patterns reveal that all the samples are in single phase and show hexagonal structure with P63cm space group. The temperature dependence of magnetization presents that the Cr-doped samples exhibit increase of the ferromagnetic transition temperature due to the double-exchange interaction between Cr3+ and Mn3+ ions. The magnetic hysteresis loops show weak ferromagnetic behavior. The long-time relaxation of magnetization indicates that the Cr dopants favor the spin-glass phase.

High ferromagnetic transition temperature in multiferroic BiFe0.95Ni0.05O3 compound

We have prepared BiFe0.95Ni0.05O3 by the standard solid-state reaction and rapid sintering. The structural analysis by x-ray diffraction for BiFe0.95Ni0.05O3 shows rhombohedral perovskite structure (R3c), and lattice constants a and c of BiFe0.95Ni0.05O3 are slightly increased as compared to those of BiFeO3. The Raman studies present the increase of oxygen vacancy by the Ni2+ substitution. The magnetic properties of BiFe0.95Ni0.05O3 are greatly improved, owing to the enhanced ferromagnetic interaction caused by the Fe/Ni coupling. BiFe0.95Ni0.05O3 sample is ferromagnetic with a cluster-glass state unlike the antiferromagnetic BiFeO3-based compounds.

Flux pinning and the paramagnetic Meissner effect in MgB2 with TiO2 inclusions

The mixed-state superconducting properties of bulk MgB2+2?at.%TiO2, prepared by the in?situ solid state reaction, have been investigated. The high-resolution transmission electron microscopy study reveals that the sample has combined microstructures, consisting of regions with small-?and large-size MgB2 grains, and the MgO inclusions. The particular grains are separated by thin TiB2 interlayers along the c axis. Analysis on the mixed-state parameters, such as the upper critical field, the coherence length and the Ginzburg?Landau parameter ?, proves that the MgB2+2?at.%?TiO2 belongs to a high-? type-II superconductor in the dirty limit. The non-uniformly scaled microstructure results in an unusual (close to the double peak) magnetic field dependence of the pinning force density. The field-cooled temperature dependence of magnetic moment exhibits a transition of the sample to the paramagnetic state at certain applied magnetic fields, which is treated as a manifestation of the paramagnetic Meissner effect. The experimental results are discussed on the basis of modern theoretical approaches.

Neutron and Mössbauer studies of FeCr2Se4

FeCr2Se4 has been studied with x-ray and neutron diffraction techniques, superconducting quantum interference device magnetometer, and Mossbauer spectroscopy. The crystal structure of FeCr2Se4 at 295K is monoclinic (space group I2∕m), with the lattice parameters a=6.267A, b=3.617A, c=11.822A, and β=90.69°, and its magnetic structure is found to be C-type ordering. The Mossbauer spectra were obtained at various temperatures ranging from 4.2to295K. Magnetic hyperfine and electric quadrupole interactions at 4.2K have been fitted, yielding the following results: Hhf=108.8kOe, θ=72°, φ=90°, η=0, ΔEQ=(1∕2)e2qQ[1+(1∕3)η2]1∕2=−1.65mm∕s, and R=−2.25. Our study strongly suggests that the quadrupole interaction of FeCr2Se4 is larger than the magnetic dipole interaction.

Reinforced magnetic properties of Ni-doped BiFeO3 ceramic

Multiferroic materials attract considerable interest because of the wide range of potential applications such as spintronic devices, data storage devices and sensors. As a strong candidate for the applications among the limited list of single-phase multiferroic materials, BiFeO3 (BFO) is a quite attractive material due to its multiferroic properties at room temperature (RT). However, BFO is widely known to have large leakage current and small spontaneous polarization due to the existence of crystalline defects such as oxygen vacancies. Furthermore, the magnetic moment of pure BFO is very weak owing to its antiferromagnetic nature. In this paper, the effects of Ni2+ substitution on the magnetic properties of bulk BFO were investigated. BFO, and BiFe0.99Ni0.01O3, BiFe0.98Ni0.02O3 and BiFe0.97Ni0.03O3 (BFNO: Ni-doped BFO) ceramics were prepared by solid-state reaction and rapid sintering, and analyzed by structural and magnetic-property measurements. The leakage current density was measured at RT by using a standard ferroelectric tester. All the Ni-doped BFO samples exhibited the similar rhombohedral perovskite structure (R3c) to that of BFO. The magnetic properties of Ni-doped BFO were much enhanced with respect to BFO prepared at the same conditions, because the enhanced ferromagnetic interaction is caused by the Fe/Ni coupling.

Flux pinning and vortex dynamics in MgB2 doped with TiO2 and SiC inclusions

The mixed-state superconducting properties of bulk MgB2+2at.%TiO2 and +8at.%SiC, prepared by in situ solid state reaction, are investigated. Analysis of the mixed-state parameters, such as the upper critical field, the coherence length, and the Ginzburg-Landau parameter, proves that MgB2+2at.%TiO2 is a high-κ type-II superconductor in the dirty limit, while MgB2+8at.%SiC corresponds to that in the moderately clean limit. It is shown that the grain-boundary pinning realized in fine-grained doped MgB2 polycrystals is of the anisotropic rather than the electron-scattering type. The field-cooled temperature dependences of the magnetic moment reveal a transition of the samples to the paramagnetic state at certain applied magnetic fields, which is treated as manifestation of the paramagnetic Meissner effect. The experimental results are discussed on the base of modern theoretical approaches.

Magnetic phase diagram and structural separation of La0.7(Ca1−ySry)0.3MnO3 thin films

The structural, magnetic, and transport properties of La0.7(Ca1?ySry)0.3MnO3 films, deposited on a LaAlO3?(001) single crystalline substrate by rf-magnetron sputtering using “soft” (or powder) targets, have been investigated. It was found that at 0.3 ? y ? 0.5 both the rhombohedral (R3c) and the orthorhombic (Pnma) crystal phases in the form of nanoscale clusters are coexistent at room temperature. The observed structural clustering is accompanied by two-stage magnetic and electronic transitions, and governed by a nonuniform distribution of the lattice strain through the film. It was shown that for the films with 0 ? y ? 0.5 the nonlinear (almost parabolic) MR(H) dependence is typical while, for 0.65 ? y ? 1.0, the linear MR(H) behavior is observed at room temperature. The magnetotransport properties of films are explained within the framework of field-dependent activation-energy model. The magnetic phase diagram for La0.7(Ca1?ySry)0.3MnO3 thin-film system is also presented.