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The mineral having chemical compositional formula MgAl2O4 is called “spinel”. The ferrites crystallize in spinel structure are known as spinel-ferrites or ferro-spinels. The spinel structure has an fcc cage of oxygen ions and the metallic cations are distributed among tetrahedral (A) and octahedral (B) interstitial voids (sites). A compound of Co0.85Ca0.15-yMgyFe2O4 (y=0.05, 0.10, 0.15) is synthesized in polycrystalline form, using the stoichiometric mixture of oxides with conventional standard ceramic technique and characterized by X-ray diffraction (XRD).The XRD analysis confirmed the presence of cubic structure. The intensity of each Bragg plane is sensitive to the distribution of cations in the interstitial voids of the spinel lattice. The computer program Powder X software for XRD analysis has been utilized for this purpose. The compositions of Co0.85Ca0.15-yMgyFe2O4 (y=0.05, 0.10, 0.15) ferrites have been prepared by standard ceramic method with double sintering at 950°C, 1100°C. In present study, we report the structural, electrical and magnetic properties of above said compound.

The consequences of 50 MeV Li 3+ ion irradiation on the structural and magnetic behaviour of polycrystalline samples of spinel ferrite system Li 0.5+0.5 x Ti x Al 0.1 Fe 2.4−1.5 x O 4 are investigated by using X-ray diffraction (XRD), magnetization and 57 Fe Mossbauer spectroscopy. The Mossbauer spectra exhibit central enhancement after SHI-irradiation, which is attributed to the paramagnetic centres created due to the irradiation. The reduction observed in the saturation magnetic moment after SHI-irradiation is ascribed to the partial formation of paramagnetic centres and rearrangement of the cations in the lattice corroborated by XRD and Mossbauer Loretnzian intensity analysis. The presence of amorphous phase in the irradiated samples is ruled out.

The structural and bulk magnetic properties of the spinel system Zn x Mg 1.5− x Mn 0.5 FeO 4 with x =0.0–0.6 have been investigated by means of X-ray diffraction, magnetization and AC susceptibility measurements. The distribution of the cations has been deduced through XRD intensity analysis and the non-linear variation of lattice constant with Zn-content has been explained on the basis of distribution of the cations. The three sublattice models analysis of the system based on the uniform and localized spin canting approach have been used to explain the variation of magnetization with Zn-concentration. The Neel temperature obtained from thermal variation AC susceptibility measurements decreases with Zn-content ( x ).

The percentage substitution of large cation like Ca2+ for Mg2+ in MgFe2O4 without altering the cubic symmetry and its effect on structural, bulk magnetic and electrical properties have been investigated for Mg1−xCaxFe2O4 (x=0.0–0.35) spinel ferrite system by means of X-ray diffraction (XRD), magnetization, ac susceptibility and dc resistivity measurements. It is found that a maximum of 23% of Ca2+ can be substituted for Mg2+. The variation of magneton number with x can be explained on the basis of Neel’s collinear spin model. The Neel temperature determined through ac susceptibility and dc resistivity measurements agree with those calculated theoretically. The variation of electrical resistivity go hand in hand with the variation of the activation energy with the Ca-content.

Based on 57 Fe Mossbauer spectroscopy studies conducted at 300 K, the magnetic behaviour of the spinel ferrite system MgAl x Cr x Fe 2-2 x O 4 ( x =0.0–0.5) has been investigated. The Mossbauer spectra for x =0.0–0.2 exhibit two Zeeman sextets, one due to the Fe 3+ octahedral ions and the other due to the Fe 3+ tetrahedral ions, while the compositions x =0.3, 0.4 and 0.5 showed central paramagnetic doublet superimposed on the magnetic sextets. The variation of hyperfine parameters and origin of the central doublet on the magnetic sextets have been discussed.

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.

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 Neel's collinear spin model. The Neel temperatures calculated theoretically are in good agreement to those deduced experimentally.

The structural and superconducting properties of Nd- and Ca-substituted (Nd1-xCax)(Ba2-yNdy)Cu3Oz samples prepared under identical conditions have been investigated by x-ray diffraction, resistivity, ac susceptibility and oxygen content measurements. The effect of increasing Nd concentration in NdBa2-yNdyCu3Oz lowers the carrier concentration and decreases Tc, which is attributed to magnetic pair breaking by local moments and hole filling by Nd. This suppression in Tc has been compensated by appropriate hole doping with Ca in (Nd1-xCax)(Ba2-yNdy)Cu3Oz, which shows that for a fixed Nd content at the Ba site Tc increases dramatically as the Ca content increases. For instance, in the oxide (Nd1-xCax)(Baz-yNdy)Cu3Oz, the Tc increases from 0.0 K (x = 0.0, non-superconductor) to 77.5 K (x = 0.4, compensated oxide) as the magnetic pair breaking and hole filling by Nd is nearly balanced by hole doping from Ca.

The structural and superconducting properties of (Gd 1− x − y Ca y Mo x )Ba 2 Cu 3 O z samples are investigated using X-ray diffraction, ac susceptibility, electrical resistivity and oxygen content measurements. The effect of increasing the Mo concentration in (Gd 1− x Mo x )Ba 2 Cu 3 O z changes the structure from orthorhombic to tetragonal accompanied by an increase in the normal state resistivity and decreases both the oxygen content and T c which is attributed to hole filling by Mo. The substitution of Ca for Gd in (Gd 0.85− y Ca y Mo 0.15 )Ba 2 Cu 3 O z provides proper matching between the ionic radius and valence of Gd 3+ (0.94 A) and the average ionic radius and valence of Mo 4+ (0.70 A) and Ca 2+ (0.99 A). As the Ca content increases, the T c increases from 58.5 K ( y = 0) to 86 K ( y = 0.15, compensated oxide), closer to the value of 91 K for pure GdBa 2 Cu 3 O 7−δ due to the balance between the hole filling by Mo and hole doping by Ca.

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.

A study has been made of the linear attenuation and mass attenuation coefficients of 662 and 1170 keV gamma radiations, from 137Cs and 60Co, respectively, for varying concentrations of solutions of CoSO4-7H20. Comparison has been made with predictions, based upon the mixture rule applied to these particular solutions.

The structural and superconducting properties of (Er1-xCax)Ba2-ySry(Cu2.76Co0.24)Oz are investigated using X-ray diffraction, resistivity, ac susceptibility, and oxygen content measurements. Increasing the Sr content in (Er0.76Ca0.24)Ba2-ySry(Cu2.76Co0.24)Oz lowers the oxygen content and decreases theT c, which is attributed to the rearrangement of oxygen atoms in the basal plane. This suppression ofT c aty =0.6 cannot be improved by appropriate hole doping with Ca in (Er1-xCax)Ba1.4Sr0.6(Cu2.76Co0.24)Oz forx = 0.24-0.48.

The alternating current (a.c.) low field susceptibility vs temperature, magnetization and57Fe Mossbauer effect measurements are reported for the spinel solid solution series Mg x Co1−x Fe2O4 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 Mossbauer spectra and lower saturation magnetization values confirm small particle ferrite behaviour. Especially, Mossbauer 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.

The structural and superconducting properties of CuCo and coupled ErCa CuCo substitution in ErBa2Cu3O7 − δ have been studied using X-ray diffraction, a.c. susceptibility, resistivity and oxygen content measurements. The oxides (Er1 − yCay)Ba2(Cu1 − xCox)3Oz have been synthesized for 0 ≤ x ≤ 0.15 and 0 ≤ y ≤ 0.45. The effect of increasing the Co-concentration in ErBa2(Cu1 − xCoxOz for 0 ≤ x ≤ 0.15 is to lower the carrier concentration and decrease Tc, which is attributed to the hole filling by Co. This suppression in Tc has been compensated for by appropriate hole doping with Ca, in (Er1 − yCay)Ba2(Cu1 − xCox)3Oz, which shows that for a fixed Co-content Tc increases dramatically as the Ca-content increases, leading, for instance, to the oxide (Er0.55Ca0.45)Ba2(Cu0.85Co0.15)3O6.81 (to a Tc of 71 K), whereas the non-substituted phase ErBa2(Cu0.85Co0.15)3O6.70 does not superconduct. As a result, the ‘compensated oxide’ (Er1 − yCay)Ba2(Cu − xCox)3Oz exhibits an almost x-independent Tc ranging from 86 to 84 K for 0.12 ≤ y = 3x ≤ 0.24.

The Al-substituted disordered spinel series CuFe 2- x Al x O 4 ( x = 0.0−1.6) has been synthesized and studied by X-ray diffraction, magnetization, ac susceptibility and Mossbauer effect measurements. The lattice constants are determined and the applicability of Vegard's law has been tested. The variation of the saturation magnetic moment per formula unit measured at 77 and 300 K with the Al content is satisfactorily explained on the basis of Neel's collinear spin ordering model for x = 0.0−1.0. The Mossbauer spectra at 300 K have been fitted with two sextets in the ferrimagnetic state corresponding to Fe 3+ at the tetrahedral (A) and octahedral (B) sites for x ≤ 0.8. Mossbauer results confirm a collinear ferrimagnetic structure for x = 0.0−0.6 and suggest non-collinear behaviour for x ≥ 0.8. The Curie temperature decreases nearly linear with increase of Al concentration from x = 0.0 to 1.2.

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.

The structural and superconducting properties of (Er1−y−yCayHfx)Ba2Cu3Oz samples are investigated using X-ray diffraction, resistivity, AC susceptibility and oxygen-content measurements. The effect of increasing the Hf concentration in Er−xHfxBa2Cu3Oz lowers the oxygen content and decreases Tc which is attributed to hole filling by Hf. The substitution of Ca for Er in (Er0.85−yCayHf0.15)Ba2Cu3Oz provides a proper matching between the ionic radius and valence of Er3+ (0.88 A) and the average ionic radius and valence of Hf4+ (0.78 A) and Ca2+ (0.99 A). As the Ca content increases, the Tc increases from 70 K (y = 0.05) to 86 K (y = 0.25, compensated oxide), closer the value of 91 K for pure ErBa2Cu3O7−δ due to the balance between the hole filling by Hf and hole doping by Ca.

Mo-substituted samples with stoichiometric composition YBa2(Cu1−xMox)3O7−δ, where 0 < x < 0.04, prepared under identical conditions have been characterized by d.c. magnetization and thermogravimetric analysis measurements. D.C. magnetization measurements on randomly oriented powdered systems, as a function of temperature and field, exhibit an M(H) hysteresis loop, indicative of moderate flux pinning even as T approaches Tc. We feel that this flux pinning arises due to the preferential substitution of Mo on the Cu-site, which acts as a pinning centre. Intragrain critical current density, Jc, values deduced from magnetization measurements for x = 0.01−0.04 at H = 5 kOe and T = 15 K decrease with increasing Mo concentration due to decreasing free carrier concentration, p.

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.

Pr substituted at constant Ca concentration for Y in (Y1−x−yPrxCay)Ba2Cu3O7−δ superconductors have been prepared under identical conditions and the temperature dependence of the electrical resistivity of these samples are measured. The resistively determined values of Tc decrease linearly with increasing x (0 ≤ x ≤ 0.2) for constant y = 0.10 and 0.15 which provides convincing evidence that the suppression of superconductivity by Pr is mainly due to magnetic pair breaking. The suppression of superconductivity can also be correlated to the observed changes in oxygen content determined by iodometric analysis and to the average Cu-valences. However, it is found that the observed suppression of Tc cannot be compensated by appropriate hole doping with Ca.

Pr- and Ca-substituted (Er1-x-yPrxCay)Ba2Cu3O7- delta superconductors have been prepared under identical conditions and characterized according to resistivity and AC susceptibility measurements. The observed reduction of Tc with x in (Er1-xPrx)Ba2Cu3O6.90 for 0

The real and imaginary (lossy) components of a.c. susceptibility of high-Tc superconductor YBa1·5Ca0·5Cu3O7−δ were measured as a function of temperature for different a.c. fields and frequencies. From the lossy component of the a.c. susceptibility the transport critical current density and its temperature dependence have been computed on the basis of Kim’s critical state model.

The authors report on the successful synthesis of single-phase Zr-substituted YBa{sub 2}(Cu{sub 1-x}Zr{sub x})O{sub 7-{delta}} superconductors up to 5 at.% (x = 0.0-0.05) and characterized them according to resistivity and ac susceptibility measurements. The authors did not observe any significant change in orthorhombicity and in {Tc} similar to Ni{sup 2+}- and Nb{sup 5+}-doped systems.

We have prepared and identified as a single-phase the new high- T c 2-2-3 type superconductor Y 2 Ba 1.5 Ca 0.5 Cu 3 O 8+δ by resistance and a.c.-susceptibility measurements. The resistive superconducting onset occurs at 82 K and “zero resistance” at 78 K. Diamagnetism and Meissner effect sets in at low fields and magnetic superconducting onset starts at 82 K. The systematic investigation of the real and imaginary components of a.c.-susceptibility reveals that the magnetic behavior is that of a granular type superconductor.

A systematic study of (Er1−xMx)Ba2Cu3O7−d with M=Hf4+, Pr3+→4+, Ca2+ has been caried out with an objective of investigating the effect of dopant valency and magnetic moment on hole filling, pair breaking and hole doping mechanisms in influencing the superconductivity. It is found that non magnetic Hf-substitution up to x=0.2 shows that the reduction in Tc is mainly due to hole filling, whereas the substitution of Pr (up to x=0.2) displays suppression of Tc due to both hole filling and pair breaking mechanisms, as Pr possesses fluctuating valence and magnetic moment. In the case of non magnetic Ca-substitution, the Tc decreases due to hole doping and pair breaking mechanisms. The comparative results on these three systems will be discussed in the light of pair breaking, hole doping and hole filling mechanisms through empirical relations.

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 Neel's configuration upto x ⩽0.15.

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  B and B  B supertransferred hyperfine interactions.

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.

High- T c YBa 1.5 Ca 0.5 Cu 3 O 7−δ superconductor has been prepared by solid-state reaction method. Low field a.c.-magnetic properties have been measured and are explained in relation to the microstructure of the material. From the lossy component of the a.c.-susceptibility the transport critical current J c and its temperature dependence have been computed. It is concluded that the magnetic and transport behaviour is that of a granular type superconductor in which strongly superconducting grains are coupled via weak superconducting links.

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 (SDSP) transition and the overall behaviour shows a cluster spin glass type of ordering.

The X-ray Debye temperatures, θ M , of spinel ferrite composition Ni0.5Zn0.5Fe2O4, mechanically milled upto 9 hrs, were determined from integrated intensities of selected Bragg reflections. The θ M was found to increase with milling time. The results are explained in the light of milling induced grain orientation and surface effect. The values of θ M were found to be lower as compared to the Debye temperatures obtained from infrared spectral data analysis. The difference can be explained on the basis of increase in an excess free volume in the form of vacancies and vacancy clusters associated with grain boundaries.