G.J. Baldha

Mössbauer study of the spinel system GexCu1−xFe2O4☆

Abstract The magnetic properties of the spinel series Ge x Cu 1− x Fe 2 O 4 ( X = 0 to 0.8) have been investigated by means of Mossbauer spectroscopy. Mossbauer spectra for X = 0.0 to 0.6 suggest the existence of two hyperfine fields, one due to the Fe 3+ tetrahedral ions ( A -sites) and the other due to Fe 3+ octahedral ions ( B -sites), while for X = 0.8 it shows the superposition of hyperfine field split spectra from A - and B -site ions and a broad central line spectrum. For 0.2 ⩽ X ⩽ 0.4, fast electron exchange among octahedral iron ions occurs as in Fe 3 O 4 . The variations of nuclear magnetic fields at the A - and B -sites are explained on the basis of A ue5f8 B and B ue5f8 B supertransferred hyperfine interactions.

Study of cation distribution and macro-magnetic properties of magnesium and aluminum co-substituted lithium ferrite

Abstract The structural and bulk magnetic properties of mixed spinel ferrite system Mg x Al 2 x Li 0.5(1− x ) Fe 2.5(1− x ) O 4 ( x =0.0, 0.1, 0.2, 0.3, 0.4 and 0.5) have been investigated by means of X-ray diffraction, magnetization and a.c. susceptibility measurements. The cation distribution determined through X-ray data are supported by the magnetization and susceptibility measurements. The study has revealed the strong octahedral site preference of Li + and Al 3+ as compared to Mg 2+ . It is also found that the magnetic ordering in the system is of collinear type and the variation of magnetization can be explained on the basis of Neels collinear spin model. The Neel temperatures calculated theoretically are in good agreement to those deduced experimentally.

Influence of hole filling by Co and hole doping by Ca on the superconductivity of ErBa2Cu3O7−δ

Abstract The structural and superconducting properties of (Er1−yCay)Ba2 (Cu1−xCox)3Oz samples are investigated using X-ray diffraction, resistivity and oxygen content measurements. The effect of increasing Co concentration in ErBa2(Cu1−xCox)3Oz lowers the carrier concentration and decreases Tc, which is attributed to hole filling by Co. This suppression of Tc has been compensated by appropriate hole doping with Ca, for instance, in the oxide (Er1−yCay)Ba2(Cu0.94Co0.06)3Oz, the Tc increases from 34 K (y = 0.0) to 85 K (y = 0.18, compensated oxide) as the hole filling by Co is completely balanced by hole doping from Ca.

Mössbauer study of the CaxCo1−xFe2O4 system

Abstract The magnetic properties of Ca x Co 1− x Fe 2 O 4 with x varying from 0.0 to 0.4 have been investigated by means of Mossbauer spectroscopy and magnetization measurements. Analysis of X-ray diffraction intensity data and Mossbauer data suggests that this system remains in single phase upto x ⩽0.15 then it shows the mixture of two phases for 0.2⩽ x ⩽0.4. Well defined hyperfine Zeeman spectra are observed for samples with x ⩽0.15 at 300 K and resolved into two sextets corresponding to octahedral and tetrahedral sites. The samples with x = 0.2 to 0.4 show Mossbauer patterns corresponding to two separate phases. The magnetization and Mossbauer data suggest Neels configuration upto x ⩽0.15.

Structural properties of magnesium and aluminium co-substituted lithium ferrite

The structural properties of Mg2+ and Al3+ co-substituted Li0.5Fe2.5O4 are studied by synthesizing the spinel solid solution series MgxAl2xLi0.5(1−x)Fe2.5(1−x)O4. Polycrystalline samples of this series with x=0.0, 0.1, 0.2, 0.3, 0.4 and 0.5 have been prepared by double-sintering ceramic method. The structural details like: lattice constant and distribution of cations in the tetrahedral and octahedral interstitial voids have been deduced through X-ray diffraction (XRD) data analysis. The x dependence of bond length, oxygen positional parameter, site ionic radii, bulk density, porosity and shrinkages have also been determined.

Spin glass behaviour of the spinel system GexCu1−xFe2O4☆

Abstract Ac-susceptibility measurements at various temperatures and hysteresis studies at various fields indicate that the system Ge x Cu 1− x Fe 2 O 4 ( x ⩽ 0.6) contains mixed multidomain, single domain or superparamagnetic states. Sharp susceptibility maxmima, resembling those of spin glasses, are observed for x = 0.1 and 0.2, which may be attributed to the single domain-superparamagnetic (SDue5f8SP) transition and the overall behaviour shows a cluster spin glass type of ordering.

Magnetic properties of magnesium-cobalt ferrites synthesized by co-precipitation method

The alternating current (a.c.) low field susceptibility vs temperature, magnetization and57Fe Mössbauer effect measurements are reported for the spinel solid solution series MgxCo1−xFe2O4 synthesized by a wet-chemical method before and after high temperature annealing. The observed features for the wet samples, such as the coexistence of paramagnetic doublet and magnetic sextets in Mössbauer spectra and lower saturation magnetization values confirm small particle ferrite behaviour. Especially, Mössbauer spectra of wet samples reveal the presence of superparamagnetic particles which exist simultaneously with ferrimagnetic regions in the materials well supported by a.c. susceptibility data. The high temperature annealing changes the wet-prepared ferrites into the ordered magnetic structure of ceramic ferrites.

Superconductivity of the La3.5−x–yRyCa2xBa3.5−xCu7Oz system (R = Gd, Nd and Dy)

Abstract The structural and superconducting properties of single phase La 3.5− x – y R y Ca 2 x Ba 3.5− x Cu 7 O z (0 ≤ x = y ≤ 0.70) (LRCB with R = Gd, Nd and Dy compounds with tetragonal triple-perovskite structure are investigated using X-ray diffraction, resistivity, AC susceptibility and oxygen-content measurements. Increasing Ca and Gd substitution for La and Ca for Ba resulted in a decrease in unit cell axes and an increase in oxygen stoichiometry. Samples with 0.2 ≤ x = y ≤ 0.70 are superconducting and T c R = 0 is between 31 and 79 K. With increasing x = y , both the hole concentration in the Cu2-O layers and T c R = 0 increase to a maximum value of 0.313 and 79 K at x = y = 0.6, respectively. A correlation between the T c and the hole concentration is suggested. Similar superconducting behaviour has been observed for R = Nd and Dy in LRCB compounds.

Enhanced flux pinning and critical currents by Hf and Hf-Ca substitution in YBa2Cu3O7 − δ

Abstract Hf and Ca substituted samples with stoichiometric compositions (Y1 − xHfx)Ba2Cu3Oz with x = 0.0, 0.1, 0.2 and (Y0.85 − yCayHf0.15)Ba2Cu3Oz with y = 0.1, 0.2, 0.4, respectively, prepared under identical conditions have been characterized by d.c. magnetization and thermogravimetric analysis measurements. Detailed magnetization measurements on randomly orientated powdered samples as a function of temperature and field exhibit pronounced M(H) hysteresis loops, indicative of large flux pinning, even as T approaches Tc. It is believed that the flux pinning arises due to the preferential substitution of Hf or Hf-Ca on the Y site, which acts as a pinning centre. The critical current density Jc has been deduced from hysteresis data which is discussed in the light of the flux creep model.

Superconductivity in the system YBa2−xCaxCu3O7−δ

Abstract High temperature superconductivity was observed in the system YBa 2− x Ca x Cu 3 O 7− δ for 0 ⩽ x ⩽ 1 by means of resistance and a.c.- usceptibility measurements. It was found that 50% Ca is substituted for Ba in YBa 2 Cu 3 O 7− gd with transition temperatures at zero resistance vary between 90.0K ( x = 0) and 78.0K ( x = 1), which may be explained on the basis of deviation from linearity of resistance.