Sonia Sharmin

Effect of Copper Substitution on Fe 3 O 4 Particles Prepared via Coprecipitation and Flux Methods

We investigated the effect of copper substitution on the magnetic and physical properties of Fe₃O₄ prepared by coprecipitation and flux methods. The coprecipitation compositions were chosen according to the formula (Cu_x²⁺Fe_1-x²⁺) Fe₂³⁺O₄, where x varied between 0 and 1. We found that the flux treatment method is ideal for growing large particles of sub-micrometer size. The size of the final particles obtained was in the range of 200-1500 nm, and the size decreased with increasing copper content. Cubic spinel structured Cu_xFe_3-xO₄ particles were obtained by conducting hydrogen gas reduction process at 380 °C-530 °C for 1-2 h after flux treatment. From x-ray diffraction patterns, all particles were determined to be cubic spinel without any sign of the tetragonal structure expected from the Jahn-Teller effect. For these cubic spinel particles, saturation magnetization was controlled at 25-86 Am²/kg, and the value decreased linearly with increasing copper content. The coercive force remained almost constant at 13.7-19.1 kA/m, independent of the copper content.

Spin Hall magnetoresistance at the interface between platinum and cobalt ferrite thin films with large magnetic anisotropy

The recently discovered spin Hall magnetoresistance (SMR) effect is a useful means to obtain information on the magnetization process at the interface between a nonmagnetic metal and ferromagnetic insulators. We report the SMR measurements at the interface between platinum and cobalt ferrite thin films for samples with two different preferential directions of magnetization (out-of-plane and in-plane). The directional difference of the magnetic easy axis does not seem to influence the value of SMR.

RHEED oscillations in spinel ferrite epitaxial films grown by conventional planar magnetron sputtering

Real-time in situ reflection high energy electron diffraction (RHEED) observations of Fe3O4, γ-Fe2O3, and (Co,Fe)3O4 films on MgO(001) substrates grown by a conventional planar magnetron sputtering was studied. The change in periodical intensity of the specular reflection spot in the RHEED images of three different spinel ferrite compounds grown by two different sputtering systems was examined. The oscillation period was found to correspond to the 1/4 unit cell of each spinel ferrite, similar to that observed in molecular beam epitaxy (MBE) and pulsed laser deposition (PLD) experiments. This suggests that the layer-by-layer growth of spinel ferrite (001) films is general in most physical vapor deposition (PVD) processes. The surfaces of the films were as flat as the surface of the substrate, consistent with the observed layer-by-layer growth process. The observed RHEED oscillation indicates that even a conventional sputtering method can be used to control film thickness during atomic layer depositions.

Large Negative Uniaxial Magnetic Anisotropy in Epitaxially Strained Nickel Ferrite Films

We investigated the magnetic properties of NiFe2O4(NFO) epitaxial films grown on MgAl2O4(MAO) substrates by the reactive radio frequency magnetron sputtering method. The films were found to be coherently distorted to at least 61 nm thickness because of the lattice mismatch between MAO and NFO. The NFO(001) films exhibited large negative uniaxial anisotropy that can be quantitatively explained by the magneto-elastic theory despite the lattice distortion being as much as 3%. We also discovered magnetic anomalies in both the saturation magnetization and anisotropy of the thinnest film, which may because of the reconstruction of the electronic structures at NFO interfaces.

Enhanced Anisotropy in Tetragonalized (Cu,Co)Fe 2 O 4 Particles via the Jahn–Teller Effect of Cu 2+ Ions

We studied the Jahn–Teller (JT) effect of Cu²⁺ ions on the tetragonalization of CuFe₂O₄ and (Cu,Co)Fe₂O₄ particles. Sub-micrometer-sized CuFe₂O₄ and (Cu,Co)Fe₂O₄ particles were synthesized via coprecipitation and flux methods, followed by a heat-treatment process. From the X-ray diffraction patterns, all particles were found to be cubic spinel after flux treatment. The tetragonalization process was carried out by a heat-treatment process in air, where the temperature was varied between 700 °C and 900 °C, followed by furnace cooling. We confirmed that the ideal temperature for the tetragonalization of both CuFe₂O₄ and (Cu,Co)Fe₂O₄ is 900 °C. The obtained single-phase tetragonal spinel-structured CuFe₂O₄ and (Cu,Co)Fe₂O₄ particles indicate tetragonal distortion as a consequence of the JT effect of the Cu²⁺ ions. The results of saturation magnetization suggest that most of the divalent ions are in the B sites. The lengthening of the c-axis due to the JT effect increased the coercivity of the CuFe₂O₄ and (Cu,Co)Fe₂O₄ particles during the cubic–tetragonal phase transformation. The tetragonal (Cu,Co)Fe₂O₄ particles showed a saturation magnetization of 26 emu/g and a coercivity of 2200 Oe.