Synthesis and magnetic study of Co-Al substituted calcium hexaferrite

A series of calcium substituted polycrystalline ferrite ceramics with magnetoplumbite structures were synthesized using perfect stoichiometric mixtures of oxides with chemical composition, CaAl_xCo_xFe_12−2xO₁₉ (x = 2−5), by standard ceramic technique. The variation in the values ofH _candM _s,which depends on the additive content and the temperature, was studied by means of a vibration magnetometer. The strong variation observed in coercivity, saturation magnetization and Curie temperature with chemical composition give rise to the possibility of controlling these properties and hence applying these compounds in the millimeter— microwave range.     

Acoustic study of nano-crystal embedded PbO-P2O5 glass

Nano-crystal embedded PbO-P₂O₅ glass has been prepared and characterized by XRD and TEM measurements. The ultrasonic velocity and attenuation measured within the temperature range 80–300 K show significant structure and interesting feature with the presence of nano-crystalline region. The glass samples were prepared by melt-quench method and nano-crystals of different sizes were produced by heat treatment of the glasses for different durations of heating. All the processes were carried out at or above glass transition temperature. A theoretical model that takes account of the effects of thermally activated relaxation, anharmonicity as well as microscopic elastic inhomogeneities arising out of fluctuations has been successfully applied to interpret the variation of ultrasonic velocity and attenuation data. An interesting outcome of this application has been to propose a method for the determination of the size of nano-crystals from the ultrasonic attenuation data.     

Estimation of CE-CVM energy parameters from miscibility gap data

The powerful framework of cluster expansion- cluster variation methods (CE- CVM) expresses alloy free energy in terms of energy (model) parameters, macroscopic variables (composition and temperature) and microscopic variables (correlation functions). A simultaneous optimization of thermodynamic and phase equilibria data using CE- CVM is critically dependent on giving good initial values of energy parameters, macroscopic and microscopic variables, respectively. No standard method for obtaining the initial values of the energy parameters is available in literature. As a starting point, a method has been devised to estimate the values of energy parameters from consolute point (miscibility gap maximum) data. Empirical relations among energy parameters, temperature (T _c), composition (x _c) andd ² T/dx² at the consolute point, have been developed using CE- CVM free energy functions for bcc and fcc structures in the tetrahedron and tetrahedronoctahedron approximations, respectively. Thus from the observed data ofT _c,x _c andd ² T/dx² in the above relations, good initial values of energy parameters can be obtained. Further, a necessary modification to the classical NR method for solving simultaneous nonlinear/transcendental equations with a double root in one variable and a simple root in the other has been presented.     

Theoretical study of superconductivity in MgB2 and its alloys

Using density-functional-based methods, we have studied the fully-relaxed, fulltronic structure of the newly discovered superconductor, MgB ₂, and BeB₂, NaB₂ and AlB₂. Our results, described in terms of (i) total density of states (DOS) and (ii) the partial DOS around the Fermi energy, E_F, clearly show the importance of B p-electrons for superconductivity. For BeB₂ and NaB₂, our results indicate qualitative similarities but significant quantitative differences in their electronic structure due to differences in the number of valence electrons and the lattice constantsa andc. We have also studied Mg _1-xM_xB₂ (M = Al, Li or Zn) alloys using coherent-potential to describe disorder, Gaspari-Gyorffy approach to evaluate electron-phonon coupling, and Allen-Dynes equation to calculate the superconducting transition temperature, T_c. We find that in Mg_1-xM_xB₂ alloys (i) the way Tc changes depends on the location of the added/modified k-resolved states on the Fermi surface and (ii) the variation of Tc as a function of concentration is dictated by the Bp DOS.     

Optical properties of samarium doped zinc-tellurite glasses

Glasses with the composition, (Sm₂ O ₃) _x (ZnO)_(40-x)(TeO ₂)₍₆₀₎, were prepared by conventional melt quenching method. The density, molar volume, and optical energy band gap of these glasses have been measured. The refractive index, molar refraction and polarizability of oxide ion have been calculated by using Lorentz-Lorentz relations. Optical absorption spectra of these glasses were recorded in the range 300–700 nm at room temperature. The oxide ion polarizabilities deduced from two different quantities, viz. refractive index and optical energy band gap, agree well compared with other glasses. The nonlinear variation of the above optical parameters with respect to samarium dopant has been explained.     

Phase transitions of bulk and nanocrystalline La1−xSrxMnO3 (x = 0.35 and 0.37) perovskite manganite materials using in situ ultrasonic studies

La_1−xSr_xMnO₃ perovskite manganite materials with two compositions, namely x = 0.35 and 0.37, both at bulk and nanoscale, were prepared by solid-state reaction and sonochemical reactor methods, respectively. The magnetic phase transition temperature of the prepared bulk and nanosamples was evaluated by in situ ultrasonic velocity and attenuation measurements. A home-made experimental setup was used for in situ measurement of ultrasonic velocities and attenuation over a wide range of temperatures (from 300 to 400 K). The observed anomalous lattice-softening behavior in the ultrasonic parameters was used to study the phase transition temperature (Curie temperature, T_C), i.e., from paramagnetic to ferromagnetic phase, both in bulk and nanostructured perovskite samples. Further, the ultrasonic measurements confirmed that sharp and broad transitions occur in bulk and nanostructured perovskite manganite materials, respectively. The Curie temperature for nanostructured perovskite samples was lower than that for the corresponding bulk perovskite sample, which was clearly identified by ultrasonic measurements.     

New magnetic implant material for interstitial hyperthermia

Magnetic properties and heat generation characteristics of a bio-compatible ceramic Mg_1+xFe_2–2xTi_xO₄ system have been investigated as an implant material for the magnetic induction hyperthermia. Curie temperature (T_c) of the ceramic decreases with increasing x; and become T_c, 350 K at x ç0:35 and, 315 K at x ç 0:38; which is suitable Curie temperature for implant material. The temperature of ceramic as a function of time under the high frequency alternating magnetic field is self-controlled at T_c.

The surface temperature of a powder injection sphere cancer model, which was a mixture of the agar phantom and the ceramic powder implant, and the temperature distribution around the sphere set in the pure agar phantom matrix have been measured. The result is in good agreement with calculation using a finite element method (FEM). It was found that the temperature distribution inside of the sphere and the minimum quantity of Mg_1+xFe_2–2xTi_xO₄ necessary for hyperthermia could be estimated by the FEM calculation.     

On the electronic structure and equation of state in high pressure studies of solids

We discuss the high pressure behaviour of zinc as an interesting example of controversy, and of extensive interplay between theory and experiment. We present its room temperature electronic structure calculations to study the temperature effect on the occurrence of its controversial axial ratio (c/a) anomaly under pressure, and the related electronic topological transition (ETT). We have employed a dense 63 X 63 X 29 k-point sampling of the Brillouin zone and find that the small (c/a) anomaly near 10 GPa pressure persists at room temperature. A weak signature of the anomaly can be seen in the pressure-volume curve, which gets enhanced in the universal equation of state, along with that of K-point ETTs. We attribute the change of slope in the universal equation of state near 10 GPa pressure, mainly to hybridization effects. The temperature effect in fact enhances the possibility of L-point ETT. We find that the L-point ETT is very sensitive to exchange correlation terms, and hence we suggest that further refinements in the theoretical techniques are needed to resolve the controversies on the ETT in Zn.     

Polarization Effect in ESR of Co Doped CuGeO<Subscript>3</Subscript>

Electron spin resonance of a single crystal of CuGeO₃ doped with 2% of Co has been studied at f = 99 GHz in temperature range 1.8–50 K. Contributions to ESR absorption from Cu²⁺ chains and from Co²⁺ ions were derived. It is found that functions obtained for ESR integrated intensities: Curie-Weiss for Cu²⁺ (χCu ∼ C _Cu/(T + Θ), with Θ = 92 K) and Curie for Co²⁺ (χCo ∼ C _Co/T) are well consistent with temperature dependence of static magnetic susceptibility. Strong dependence of ESR absorption on polarization of oscillating magnetic field was discovered for Co²⁺ contribution. Polarization effect was studied for magnetic field applied along a, b and c directions. Values of g-factors of resonance lines are presented.     

Spectroscopic investigations of Cu2+ in Li2O-Na2O-B2O3-Bi2O3 glasses

Pure and copper doped glasses with composition,x Li ₂ O-(40-x)Na ₂ O-50B ₂ O ₃-10Bi ₂ O ₃,have been prepared over the range 0 ≤ x ≤ 40. The electron paramagnetic resonance (EPR) spectra of Cu²⁺ ions of these glasses have been recorded in the X-band at room temperature. Spin Hamiltonian parameters have been calculated. The molecular bonding coefficients, α² and β², have been calculated by recording the optical absorption spectra in the wavelength range 200–1200 nm. It has been observed that the site symmetry around Cu²⁺ ions is tetragonally distorted octahedral. The density and glass transition temperature variation with alkali content shows non-linear behaviour. The IR studies show that the glassy system contains BO₃ and BO₄ units in the disordered manner.     

Colossal magnetocapacitance effect in BiFeO3/La5/8Ca3/8MnO3 epitaxial films

A colossal magnetocapacitance in magnetic fields was observed near the Curie temperature T_c = 220 K of La_5/8Ca_3/8MnO₃ for the BiFeO₃/La_5/8Ca_3/8MnO₃ epitaxial film. It was found that the magnetocapacitance increases with increasing magnetic fields and reaches a maximum up to 1100% enhancement around T_c at 10 T. From the analysis of the dielectric relaxation, one can see that the behavior of relaxation time τ above T_c differs from that below T_c, and the value of τ decreases with increasing magnetic fields. This colossal magnetocapacitance effect near room temperature in BiFeO₃/La_5/8Ca_3/8MnO₃ may have potential applications in multifunctional microelectronic device.     

Critical-region thermal expansion in CuK2Cl4 · 2H2O

The Curie-point anomaly in thea-axis linear thermal expansion coefficient of CuK₂Cl₄ · 2H₂O (T _c=0.88° K) has been observed using the three-terminal capacitance technique. Length changes of the 3.8-mm single-crystal sample were determined to within approximately 0.1 Å. In the critical region our data suggest a logarithmic singularity as found previously for the specific heat. However, imperfections in the sample limit the divergence of the expansion coefficient at temperatures closer than 0.01T _c to the transition. From a comparison of the linear expansion coefficient with the specific heat in the critical region, the stress dependence of the Curie temperature is calculated. We find that the temperature derivative of the spin-correlation function describing nearest neighbor magnetic ions is not proportional to the temperature derivative of the spin-correlation function describing next nearest neighbors. Furthermore, the exchange parameters characterizing nearest and next nearest neighbor interactions do not have equal stress dependences. Between 1.5 and 2.5° K the thermal expansion coefficient is proportional to the inverse square of the temperature. Comparison of the expansion coefficient with the specific heat in this temperature range indicates that the temperature derivative of both spin-correlation functions is proportional toT ^–2. The stress dependence of the Curie temperature calculated from data in this region agrees within experimental error with the value found from different considerations using data in the critical region.Work supported by National Science Foundation.     

Spin-Entropy Wave Propagation in 3He A1 in Magnetic Fields up to 12 Tesla

The propagation of spin-entropy wave has been studied in ³ He A ₁ in magnetic fields up to 12 tesla and at a pressure of 22.9 bar. The superfluid fraction at T _c2 extracted from the propagation velocity increases linearly with magnetic field and reduced temperature. The anomalous attenuation previously found near T _c2 in lower magnetic fields was also observed in the present maximum field. The pore size in the oscillating superleak transducer does not affect the anomalous attenuation. Outside the anomalous attenuation region, the dissipation coefficient is found to vary linearly with frequency. This frequency dependence is contrary to that of dissipations owing to viscous losses at walls and to bulk losses.     

Magnetic properties of Sm2Fe14−xCoxAl3 compounds

Compounds in the series Sm₂Fe_14–x Co _x Al₃ (x = 0, 1,2, 3, 4 and 5) have been shown to be of the Th₂Zn₁₇ structure. The Curie temperature is found to increase monotonically from a critical temperature,T _c equal to 471 K for thex=0 sample, toT _c=681 K for thex=5 sample. X-ray diffraction measurements of magnetic field oriented powders showed that all compounds exhibit a room temperature uniaxial anisotropy. Magnetization measurements show that the magnetic anisotropy of Sm₂Fe₁₄Al₃ can be increased substantially by the substitution of even a small quantity of Co for Fe. Results are discussed in terms of possible applications of these compounds as particulate recording media.     

A New Highly Anisotropic Rh-Based Heusler Compound for Magnetic Recording

The development of high-density magnetic recording media is limited by superparamagnetism in very small ferromagnetic crystals. Hard magnetic materials with strong perpendicular anisotropy offer stability and high recording density. To overcome the difficulty of writing media with a large coercivity, heat-assisted magnetic recording was developed, rapidly heating the media to the Curie temperature T_c before writing, followed by rapid cooling. Requirements are a suitable T_c, coupled with anisotropic thermal conductivity and hard magnetic properties. Here, Rh₂CoSb is introduced as a new hard magnet with potential for thin-film magnetic recording. A magnetocrystalline anisotropy of 3.6 MJ m⁻³ is combined with a saturation magnetization of μ₀M_s = 0.52 T at 2 K (2.2 MJ m⁻³ and 0.44 T at room temperature). The magnetic hardness parameter of 3.7 at room temperature is the highest observed for any rare-earth-free hard magnet. The anisotropy is related to an unquenched orbital moment of 0.42 μ_B on Co, which is hybridized with neighboring Rh atoms with a large spin–orbit interaction. Moreover, the pronounced temperature dependence of the anisotropy that follows from its T_c of 450 K, together with a thermal conductivity of 20 W m⁻¹ K⁻¹, make Rh₂CoSb a candidate for the development of heat-assisted writing with a recording density in excess of 10 Tb in.⁻².     

Rings, chains and planes: Variation ofT g with composition in chalcogenide glasses

We propose a microscopic, phenomenological model for the decrease in the viscosity observed at glass transition. Our model is primarily applicable to chalcogenide glasses. According to this model, the decrease in the viscosity at glass transition is mainly due to the breaking of the Van der Waals bonds in the chalcogenides. Using this model, we derive a relationship between the glass transition temperature,T _g ,and the molar volume V _m.The validity of this relation is checked using experimental data available in the literature for two binary systems (Ge-Se and As-S) and a pseudo-binary system (As ₄₀ Se _x Te _60-x .     

Upper limit ofTc for the copper oxides

The value of the critical temperature of the cuprates correlates with the doping level and is affected by the interplay of two competing factors: (1) the increase in carrier concentration, and (2) the pair-breaking effect of magnetic impurities. An analysis of the temperature dependence of the critical field leads to the conclusion that magnetic impurities are present even in a sample with the maximum observed value ofT _c.A new parameter, intrinsicT _c (T _cintr), which is its value in the absence of magnetic impurities, is introduced. The maximum value ofT _cintr, which corresponds to the maximum doping level, appears to be similar for different cuprates and to be equal to 160–170 K. This is the upper limit ofT _c in the cuprates.     

Magnetic Field Influence on the Low Temperature Heat Capacity and Heat Release of (TMTSF)2PF6

We have measured under applied magnetic field the heat capacity c _p and heat release of the quasi-1D conductor (TMTSF)₂PF₆ in its spin density wave (SDW) ground state. The low-temperature heat capacity (T < 0.5 K) is dominated by a Schottky anomaly contribution. A corresponding term was also found for the heat release. In this T-range both properties are strongly sensitive to moderate magnetic fields (i.e., below 0.5 T): their amplitudes have a sharp maximum for H _c=0.2 T. We show that the corresponding density of two-level states N _S undergoes a sharp maximum at H _c, which rules out an extrinsic origin for the two-level states. Instead we suppose an intrinsic origin, such as defects of commensurability of the SDW, as the (slow) excitations related to the domain walls. This effect is found to be independent on the field orientations H a and H a.     

Local lattice anomalies in underdoped YBa2Cu3O7-Σ probed by X-ray absorption fine structure

Temperature-dependent local lattice anomalies in underdoped YBa₂Cu₃O_7-Σ are studied by extended X-ray absorption fine structure (EXAFS) in a fluorescence mode. The in-plane polarized Cu A^-EXAFS is measured on underdoped epitaxial YBa₂Cu₃O_7-Σ films (100-120 nm) prepared onto SrTiO₃ substrates using a pulsed laser ablation technique. The Fourier transform (FT) results for a sample withT _c = 55K (△T = 4K) shows an in-plane lattice anomaly atT* = 83 K (~ 1.5T _c ) below which the in-plane oxygen distribution consists of the two Cu-O peaks separated by 0.2-0.3 A. The contribution of the elongated Cu-O bond increases on loweringT, indicating either the in-plane rhombic distortion of the square-planar CuO₄ or the tilting of the CuO₅ pyramid. Contrary to the anomalies in spin susceptibility and transport measurements indicating the presence of a spin gap, the onset temperature of the in-plane lattice anomaly (T*) shifts to the lowerT _c on decreasing the carrier density. The results suggest that the in-plane local lattice anomaly is anextrinsic lattice effect which is not directly related to the spin-phonon interaction.