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Blocking temperatures of aggregates of Mn _0.5Zn _0.5Gd _x Fe( _{2- x})O ₄ ferrite nanoparticles, with x = 0.02, 0.05, 0.11, 0.15, and 0.2, were obtained from the zero-field-cooled (ZFC) magnetization measurements. We found a nonmontonic behavior of the blocking temperature with increasing size of the particles. The effective magnetic anisotropy was calculated and found to have two distinct rates of increase with decreasing the size of the particles. These results were attributed to the strong inter-particle interactions in the aggregated nanoparticles and to the enhanced role of surface anisotropy with the decrease of the size of the particles. In three samples, the ZFC magnetization was found to exhibit a significant negative magnetization in a considerable part of the low temperature region. To our knowledge, this is the first time that negative magnetization is reported in such nanoparticles. These peculiar result...

The influence of temperature on coercivity, H _c and saturation magnetization, M _s were investigated experimentally in Mn _0.5Zn _0.5Gd _0.02Fe _1.98O ₄ ferrite nanoparticles (average size 35 nm). Isothermal magnetization curves M ( H) were obtained in the field range from -5 kOe to +5 kOe at different temperatures after the zero field cooling (ZFC) process. The temperature dependence of the coercivity, H _c( T) deviated slightly from the classical Kneller's law. The temperature dependence of saturation magnetization, M _s( T) was found to have an excellent agreement with the Bloch's law. These results are discussed in terms of several factors such as the size and size distribution of the particles, inter-particle interactions and the surface spin.

Spin echo measurements of the ⁵⁹Co nuclear resonance in GdCo ₂ have revealed a rapid decay of the spin-echo amplitude with temperature up to 40 K followed by a further growth up to 79 K and subsequent decay above this temperature.

NMR spin echo spectra of Gd _1-xY _xAl ₂ and Gd _1-xLa _xAl ₂ (0<or=x<or=0.30) were studied in the ferromagnetically ordered state at 4.2 K. The Al resonance line profiles were analysed under the assumption of various models for the spatial extent of the conduction electron polarization and general confirmation of a RKKY like oscillatory polarization was found. Slowly decreasing nonoscillatory polarization functions are shown to be unable to explain the observed spectra. The line shape is shown to depend rather critically on the value of the Fermi wavevector k _F. The polarization seems to decrease less strongly with distance than might be expected from the RKKY function.

Spin echo measurements have been used to establish the way in which the magnetic moments in GdAl ₂ adjust themselves to the presence of dysprosium ions introduced substitutionally into the lattice. The dysprosium moments favour a (100) easy magnetisation direction in contrast to the (100) of the GdAl ₂ lattice. The results suggest that the gadolinium moment directions change gradually from the (111) direction and a model is examined for spherical volumes of gadolinium ions about each dysprosium.

Electron spin resonance measurements for the pseudobinary system Gd(Co _xAl _1-x) ₂ with x<or=0.6 are reported. The results indicate a change of sign of the Gd-conduction-electron interaction parameter J at x approximately=0.3. This is shown to be consistent with a variation of the Curie temperature with x.

The effect of adding 0.05 to 16.2 mol.% CeO ₂ on the dielectric properties and loss tangent of sintered BaTiO ₃ was investigated at different temperatures. The permittivity increased with CeO ₂ additions up to 0.5 mol%, then decreased with increasing concentrations. In the samples doped with 5.6 and 16.2 mol.% CeO ₂ the curves show a slight increase in the permittivity with temperature, without any significant tendency to a peak value, down to room temperature. Smaller additions of CeO ₂ (0.05 to 2 mol.%) lower the Curie temperature to 92 degrees C. The loss tangent maximum (tan delta ) _max at the Curie temperature was less than 0.035 for pure BaTiO ₃ and 0.0120 for samples containing 16.2 mol.% CeO ₂. X-ray measurement shows that the lattice parameters change with the concentration of additive, the a axis expanded while the c axis shrank, approaching a cubic structure at room temperature for 5.6 mol....

The crystal structure of BaTiO ₃:CeO ₂ was studied using the ESR spectrum of Fe ³⁺ ions, contaminating pure BaTiO ₃, in the temperature range -160 degrees C to +160 degrees C. The observed powder spectra are classified into three parts: pure, lightly doped (0.05-0.5 mole% CeO ₂), and heavily doped (1-3 mole% CeO ₂)BaTiO ₃. The EPR spectrum of pure BaTiO ₃ showed five resonance lines, at room temperature, around g=2. On lowering the temperature, the five lines tend to converge towards the central line. The lightly doped EPR spectra reflected that the crystal fields experienced by Fe ³⁺ ions are highly inhomogeneous and g-values in all the range g=12 to g=1.8 may be observed. The heavily doped samples reflected an obvious stabilisation of crystal structure with variation of temperature. The g-value reflected the effect of the dopant concentration and showed little or no variation with temperat...

The permittivity of BaTiO ₃ doped with small concentrations of Gd ₂O ₃ (0.01 approximately 0.1 mol.%) are found to be remarkably dependent on the type of the solid solution formed between Gd ₂O ₃ and BaTiO ₃. The decrease of permittivity at the Curie peak with decreasing grain size for BaTiO ₃ doped with concentrations of Gd ₂O ₃ from 0.5 approximately 3 mol.% is attributed to the formation of defective non-ferroelectric grain boundary layers of low permittivity.