Markayendeyulu G

We report on structural, spin glass behaviour and competing magnetic interactions in xNiFe2O4 - (1-x)BaTiO3 (x = 0.2 and 0.3) nanocomposites prepared by sol-gel method. The structure and surface morphology of the NiFe2O4-BaTiO3 composites were examined by X-ray diffraction (XRD) and scanning electron microscope (SEM). XRD measurements of composite materials show the presence of both NiFe2O4 and BaTiO3 phases. FESEM and energy dispersive X-ray (EDX) measurements indicate uniformly distributed nano size particles and presence of Ni, Fe, O, Ba and Ti elements respectively. Temperature variation Raman spectra indicates large structural disordering (Peak broadening and shift) around 395 K due to the structural transformation of BaTiO3. Magnetization, zero field cooled (ZFC) and field cooled (FC) measurements were carried out from 2 K to 320 K for both composites. ZFC and FC measurements reveal the spin glass and Superparamagnetism like behaviour along with competing magnetic interaction in both composites. Saturation magnetization value is seen to increase from 6.7 to 10 emu/g with increasing Ni ferrite composition from x = 0.2 to x = 0.3. Distribution function associated with magnetocrystalline anisotropy energy barrier exhibits broad peaks due to the random distribution of spins associated with different particle size in the x = 0.3 composition.

An in house designed magnetron sputtering source were described, which in turn attached to a radio-frequency (RF) power supply and used to prepare the NiFe2−xLuxO4 (x = 0, 0.075) thin films. The effects of RF power during sputtering and annealing on the structural, optical and magnetic properties of NiFe2O4 and NiFe1.925Lu0.075O4 thin films were reported. Both annealing and increasing the RF power has increased the crystallinity of the thin films. Upon annealing the films, the saturation magnetization (Ms) values of NiFe2O4 films were increased and those of NiFe1.925Lu0.075O4 films were decreased. The coercive field (Hc) values were reduced upon annealing and with increasing RF power. Both Ms and Hc values of NiFe1.925Lu0.075O4 thin films were lesser than those of NiFe2O4 thin films.

Magnetoimpedance (MI) has been investigated in the laser and microwave annealed Fe66Ni7Si7B20 ribbons. The largest MI [(MI)m] values of the ribbons annealed using laser with energies of 150, 200 and 250 mJ/pulse (mJp) are 25% (at 7 MHz), 30% (at 5 MHz) and 21% (at 7 MHz) respectively. The effect of domain wall pinning on MI was observed as field insensitive regions in the MI profiles in the ribbon annealed using 150 mJp energy. Flower shaped grains in amorphous matrix in the ribbon annealed with 200 mJp energy are responsible for highest (MI)m due to the least anisotropy. In the ribbons annealed for 40, 45 and 50 min at 400 °C using microwaves, (MI)m values are 35% (at 5 MHz), 46% (at 6 MHz) and 29% (at 7 MHz) respectively. The large DC conductivity and the least anisotropy (smallest Hk values) in the ribbon microwave annealed for 45 min at 400 °C resulted in (MI)m reaching its highest value in the ribbons investigated.

Magnetic properties of intermetallic MnFe0.25Sb through magnetization and heat capacity measurements with the support of powder XRD patterns and electronic structure calculations are presented in this paper. Fe atoms occupy the vacant 2d sites in MnSb and as a result, the a-parameter increases and the c-parameter decreases. Both the magnetic ordering temperature and the saturation magnetization are smaller than those of MnSb. An A-type antiferromagnetic behaviour, where the Mn moments order ferromagnetically in the ab-plane (intra-layer) and the interlayer (along the c-direction) exchange is antiferromagnetic, was observed below 25 K. Specific heat data too revealed a small change in entropy at 28 K, consistent with the observations from the magnetization data. Furthermore, at 241 K a ferrimagnetic to paramagnetic transition was observed. Electronic structure calculations showed that MnFe0.25Sb stabilized in an A-type antiferromagnetic (AFM) like ground state. In this state, the exchange interactions between the moments of (i) Mn atoms along the c direction and (ii) Mn and Fe atoms are antiferromagnetic and is ferromagnetic between the moments of Mn atoms in the ab-plane. The competition between these interactions leads to magnetic frustration, which also emphasizes the near-degeneracy of different magnetic states.

Magnetism in Mn1.25Sb is discussed using the combined results of measured structural and magnetic properties and spin-polarized density functional theory calculations. Mn1.25Sb crystallizes in NiAs-type hexagonal structure, where the Mn-atoms occupy the 2a-sites and 2d-sites. The lattice parameter values are a = b = 4.2134 (1) Å and c = 5.6890 (4) Å. From the dc-magnetization measurements, it was understood that the compound exhibits a typical R-type ferrimagnetic behavior. The total magnetic moment follows predominantly T3/2 behavior up to ~60 K and predominantly T2 behavior above 90 K. The values of critical exponents (β = 0.60 ± 0.01, γ = 1.38 ± 0.04 and δ = 3.32 ± 0.01) indicate that the magnetization conforms to the mean-field theory as well as to the 3D- Heisenberg model with local or short-range magnetic interactions. From the DFT calculations, it was understood that the magnetic moment of Mn-atom at the 2d-site (μMn2d=4.09μB) is aligned antiparallel to the magnetic moment of the Mn-atom at the 2a-site (μMn2a=4.47μB). The exchange interaction strength parameter (J) is found to be positive for μMn2a-μMn2a and negative for the μMn2a-μMn2d confirming the presence of competing magnetic interactions in the system.

Sm28Fe72 and Sm32Fe68 films of 100 nm thickness were grown using DC magnetron sputter deposition and their structure, magnetization, electrical and Hall resistance characteristics were investigated. An increase in electrical resistivity from 4.75×10 -6 to 5.62×10-6 Ω m and from 2.26×10-6 to 2.84×10-6 Ω m for Sm 28Fe72 and Sm32Fe68 films, respectively, with decrease in temperature from 300 to 40 K is attributed to the strain induced anisotropy that dominates at lower temperatures. The positive extraordinary Hall coefficients (RS) are observed for both films at 300 and 80 K. The existence of hysteresis indicates that Sm28Fe 72 and Sm32Fe68 films possess perpendicular anisotropy at 300 K. Hysteresis loop becomes narrow at 80 K for both Sm 28Fe72 and Sm32Fe68 films. Magnetization measurements at 300 K exhibiting small coercive field values of 31 and 49 Oe for Sm28Fe72 and Sm32Fe68 films, respectively, confirm the existence of perpendicular anisotropy at 300 K. © 2010 Elsevier B.V. All rights reserved.