S. Chakraverty

Size-dependent magnetic properties of Mn0.5Zn0.5Fe2O4 nanoparticles in SiO2 matrix

Mn0.5Zn0.5Fe2O4 ferrite nanoparticles (<100 nm) in SiO2 matrix have been prepared by the sol-gel method. The particle size was varied by changing the duration of heat treatment above crystallization temperature. An x-ray diffraction study indicates the presence of single-phase spinel ferrite in the sample. The particle size was estimated by the x-ray diffraction method as well as from the micrograph taken by a transmission electron microscope. The magnetic properties of the samples were studied by a vibrating sample magnetometer. The samples show superparamagnetic behavior when the particle size is below 20 nm, which is confirmed by Mossbauer spectroscopy measurements. The average particle size in the superparamagnetic state was also estimated from the low-field magnetization measurement by considering the samples as consisting of noninteracting single domain particles.

BaFeO3 cubic single crystalline thin film: A ferromagnetic insulator

We have synthesized BaFeOx thin films by pulsed-laser deposition followed by low temperature (200 °C) oxidation to BaFeO3. X-ray diffraction indicates that fully oxidized single crystalline BaFeO3 has been formed with a lattice parameter of 3.97 A, the bulk stoichiometric value. The oxidation state of Fe is confirmed to be 4+ by using hard x-ray photoemission spectroscopy. A ferromagnetic ground state is found with saturation magnetization and Curie temperature of 3.2 μB/formula unit and 115 K, respectively. Unusually, for a uniform cubic ferromagnet, the films are insulating with an optical gap of ∼1.8 eV.

Epitaxial Structure of (001)- and (111)-Oriented Perovskite Ferrate Films Grown by Pulsed-Laser Deposition.

We report epitaxial growth and structures of SrFeO2.5 (SFO) films on SrTiO3 (STO) (001) and (111) substrates by pulsed-laser deposition. Reflection high-energy electron diffraction intensity oscillations were observed during the initial growth on both substrates, reflecting a layer-by-layer growth mode of the formula unit cell. It was found that the films were stabilized with a monoclinic structure that was derived from the original orthorhombic structure of bulk Brownmillerite. Using an X-ray reciprocal space mapping technique, in-plane domain structures and the orientation relationship were investigated. In addition, the impact of laser spot area on the epitaxial structures was studied. For the films grown on the (001) STO, the orientation relationship was robust against the change of the laser spot area: SFO(001)//STO(001) and SFO(100)//STO(100) for the out-of-plane and the in-plane, respectively, with the [001] axis tilted toward the 4-fold a- and b-axes by ∼1.4°, whereas nearly (111)-oriented films were obtained on the (111) STO, exhibiting a complicated manner of tilting that depended on laser spot area. The observed variation in tilting configurations can be understood in terms of possible atomic arrangements at the SFO/STO interface. These results present a guide to control the heteroepitaxial growth and structure of (111)-oriented noncubic perovskites.

Spontaneous atomic ordering and magnetism in epitaxially stabilized double perovskites

We have studied the atomic ordering of B-site transition metals and magnetic properties in the pulsed laser deposited films of La 2 CrFeO 6 (LCFO) and La 2 VMnO 6 (LVMO), whose bulk materials are known to be single perovskites with random distribution of the B-site cations. Despite similar ionic characters of constituent transition metals in each compound, the maximum B-site order attained was surprisingly high, ∼90% for LCFO and ∼80% for LVMO, suggesting a significant role of epitaxial stabilization in the spontaneous ordering process. Magnetization and valence state characterizations revealed that the magnetic ground state of both compounds was coincidently ferrimagnetic with saturation magnetization of ∼2 μ B per formula unit, unlike those predicted theoretically. In addition, they were found to be insulating with optical band gaps of 1.6 and 0.9 eV for LCFO and LVMO, respectively. Our results present a wide opportunity to explore novel magnetic properties of binary transition metal perovskites upon epitaxial stabilization of the ordered phase.

Engineered spin-valve type magnetoresistance in Fe3O4-CoFe2O4 core-shell nanoparticles

Naturally occurring spin-valve-type magnetoresistance (SVMR), recently observed in Sr2FeMoO6 samples, suggests the possibility of decoupling the maximal resistance from the coercivity of the sample. Here we present the evidence that SVMR can be engineered in specifically designed and fabricated core-shell nanoparticle systems, realized here in terms of soft magnetic Fe3O4 as the core and hard magnetic insulator CoFe2O4 as the shell materials. We show that this provides a magnetically switchable tunnel barrier that controls the magnetoresistance of the system, instead of the magnetic properties of the magnetic grain material, Fe3O4, and thus establishing the feasibility of engineered SVMR structures.

Magnetic Properties of NiFe2O4 Nanoparticles in SiO2 Matrix

Inverse spinel NiFe2O4 ferrite nanoparticles (2.5 nm to 25 nm) in SiO2 matrix were prepared by sol–gel method. X-ray diffraction method and Transmission Electron Microscope were used to identify the phase and average particle diameter of the samples. The size dependent magnetic properties were investigated using a vibrating sample magnetometer. Superparamagnetic behaviour was observed at room temperature when the average particle size is less than 6 nm. The Mossbauer spectra change from two-peak to six-peak pattern with the increase of particle size and decrease in temperature. Two sets of sextets in the Mossbauer spectra indicate the presence of Fe ion in tetrahedral as well as octahedral sites. The Langevin function used to explain paramagnetism is found to describe well the magnetization behaviour of these samples consisting of weakly-interacting single domain particles in the superparamagnetic state.

Controlled B-site ordering in Sr2CrReO6 double perovskite films by using pulsed laser interval deposition

We have investigated the effects of growth interval in pulsed laser deposition on structural, magnetic, and electrical transport properties of (111)-oriented Sr2CrReO6 double perovskite films. It was found that the degree of B-site ordering varied in a wide range by just changing the growth interval. Magnetization under 1 T and metallicity (the ratio of room-temperature resistivity to minimum resistivity at low temperatures) monotonically increased with increasing degree of ordering and reached values nearly identical to bulk. This technique, called pulsed laser interval deposition, may provide more efficient way to study relations between structural and physical properties in double perovskites than controlling growth parameters, such as temperature and oxygen partial pressure.

Magnetic properties of Sr2FeTaO6 double perovskite epitaxially grown by pulsed-laser deposition

We have investigated the magnetic properties of B-site-ordered Sr2FeTaO6 double perovskite thin films grown on (111) SrTiO3 substrates by using pulsed-laser deposition. High degree of ordering of Fe and Ta ions has been confirmed using x-ray diffraction and atomically resolved high-angle annular dark-field scanning transmission electron microscopy. We have found clear signatures of magnetic frustration in {111} planes of local spin moment arising from next-nearest-neighbor interaction in triangular FeO6 sublattice below 50 K. It is also found that this behavior is suppressed by increasing disorder and eventually leads to antiferromagnetic state.

X-ray spectroscopic study of BaFeO3 thin films: An Fe4+ ferromagnetic insulator

We investigated the electronic and magnetic properties of fully oxidized

Atomic-scale structure and electronic property of the La2FeCrO6/SrTiO3 interface

{\mathrm{BaFeO}}_{3}