Markayendeyulu G

NiFe2O4 & NiFe1.925R0.075O4 (R = Lu, Yb & Y) samples conductivity response for alternating current (σac) in the high temperature region (300 K to 570 K), and for direct current conductivity (σdc) response in the temperature range 80 K – 300 K were investigated. For all the investigated samples σac increases with raising frequency owing to transfer of charge carriers at the two octahedral sites containing Fe3＋ or Ni2+. The total σac was observed to follow σ = D + Bωn where the frequency exponent n, initially decreases and then increases following overlapping large polaron model. The behaviour of σac with temperature reveals the semiconducting behaviour of the investigated samples. The Arrhenius plots of σac shows two regions of conductivity suggesting two types of relaxation phenomena. The σdc, also increases with increasing temperature, owing to the raise in the thermally stimulated drift mobility of charge carriers. The decrease in the activation energy with decreasing temperature corresponds to small polaron hopping mechanism.

Composites of NiFe2O4 (NFO)-BaTiO3 (BTO) and NiGd0.01Fe1.99O4 (G0.01)-BTO were investigated by x-ray diffraction, magnetization, transmission electron microscopy, magnetocapacitance, and ferroelectric studies. NFO and G0.01 nanoparticles were synthesized by the sol-gel method. The crystallite size of the nanoparticles estimated from the x-ray diffraction patterns is 20-22 nm. The average crystallite sizes of NFO and G0.01 nanoparticles were estimated from the transmission electron micrographs as 26 (1) nm and 22.3 (0.3) nm, respectively. These nanoparticles were encapsulated in a BTO shell, resulting in the formation of nanocomposites. Room temperature magnetization (at 60 kOe) of G0.01 nanoparticles was found to be slightly higher than that of NFO nanoparticles, due to the larger moment of Gd3+ than that of Fe3+. Also, the magnetization of G0.01-BTO is more than that of NFO-BTO nanocomposites. The magnetoelectric effect was observed with a magnetocapacitance value of approximately -10% at 10 kHz in both the composites.

Magnetic properties of intermetallic MnMn0.25Sb crystallizing in the NiAs-type hexagonal structure are reported. A spin-glass transition at Tf ≈ 10.7 K was identified from Ac susceptibility measurements. The value of the Mydosh parameter (φ≈0.06). was further indicative of cluster spin glass like state. The frequency dependence of Tf was investigated by critical slowing-down model and Vogel-Fulcher law. The characteristic time constant of τ0∗≈0.18×10−7s was analysed using power-law for a single spin-flip and a critical exponent of zv=4.37±0.14. The characteristic time constant was obtained from the Vogel-Fulcher law for a single spin-flip τ0≈7.96×10−8s, and T0=6.49±0.21K. The cluster spin-glass behaviour in the current system is due to the existence of both competing interactions and atomic disorder at 2d site.

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.

Entropy stabilization was attempted through the addition of 5 different rare earth cations, 5 different transition metal cations and all these 10 cations in the form of three compounds based on NiFe2O4. Equiatomic ratios of these cations were used to maximize the entropy stabilization effect. The compounds attempted and investigated were NiFe1.9(Dy0.02Er0.02Gd0.02Ho0.02Tb0.02)O4, (Co0.2Cr0.2Fe0.2Mn0.2Ni0.2)Fe2O4 and (Co0.2Cr0.2Fe0.2Mn0.2Ni0.2)Fe1.9(Dy0.02Er0.02Gd0.02Ho0.02Tb0.02)O4. Synthesis of these compounds were carried out by sol–gel technique and all the compounds were found to crystallize in [Formula presented] spinel structure indicating the entropy stabilization. An appropriate calculation of configurational entropy of mixing employing the sublattice model is demonstrated for such entropy stabilized oxides.

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.