T. Geetha Kumary

Intricacies of strain and magnetic field induced charge order melting in Pr0.5Ca0.5MnO3 thin films

Thin films of the half-doped manganite Pr0.5Ca0.5MnO3 were grown on (1 0 0) oriented MgO substrates by pulsed laser deposition technique. In order to study the effect of strain on the magnetic field induced charge order melting, films of different thicknesses were prepared and their properties were studied by X-ray diffraction, electrical resistivity and magnetoresistance measurements. A field induced charge order melting is observed for films with very small thicknesses. The charge order transition temperature and the magnetic field induced charge order melting are observed to depend on the nature of strain that is experienced by the film.

Thickness controlled proximity effects in C-type antiferromagnet/superconductor heterostructure.

Modulation of the superconducting state possessing a C-type antiferromagnetic phase in the Nd0.35Sr0.65MnO3/YBa2Cu3O7 heterostructure is investigated, with the Nd0.35Sr0.65MnO3 thickness (t) varying from 40 to 200 nm. Both the superconducting transition temperature and the upper critical field along the c-axis decrease with increasing t; while the in-plane coherence length increases from 2.0 up to 3.6 nm. Meanwhile, the critical current density exhibits a field-independent behavior, indicating an enhanced flux pinning effect. Furthermore, low-temperature spin canting induces a breakdown and re-entrance of the superconductivity, demonstrating a dynamic completion between the superconducting pairing and the exchange field. An unexpected colossal magnetoresistance is observed below the superconducting re-entrance temperature at t = 200 nm, which is attributed to the dominant influence of the exchange field over the pairing energy.

Tuning of the superconducting and ferromagnetic transitions by Cu doping for Ru in GdSr2RuCu2O8

Abstract In order to explore the possibility of tuning superconducting and ferromagnetic transitions by Cu doping (for Ru) in GdSr2RuCu2O8, we have carried out synthesis and characterization of GdSr2Ru1−xCu2+xO8 (x=0, 0.05, 0.1 and 0.2) and studied their physical properties. Coexistence of superconductivity and ferromagnetism is observed in all the Cu doped samples studied here. The zero field susceptibility data suggests formation of a spontaneous vortex phase. Cu doping decreases the ferromagnetic Curie temperature, whereas the superconducting transition temperature increases until an optimal concentration x∼0.1. This reflects an increase in hole transfer to the CuO2 planes and reduction of ferromagnetic order within the ruthenate layers.

Evolution of ferromagnetic clustering in Pr0.5Ca0.5MnO3 and its effect on the critical temperature of YBa2Cu3O7 thin film

A thin film bilayer of Pr0.5Ca0.5MnO3 and YBa2Cu3O7 has been deposited on LaAlO3 substrate by pulsed laser deposition. XRD pattern confirms the formation of the bilayer. Current and magnetic field dependent resistance measurements have been carried out to study the proximity effect. These measurements have shown that the superconducting transition temperature (Tc) of YBa2Cu3O7 is suppressed to lower temperatures with a progressive increase in current. In addition, a hump like feature appears in resistivity near Tc with increase in applied current. The results are explained on the basis of the formation and growth of ferromagnetic clusters on account of charge order melting in Pr0.5Ca0.5MnO3 upon application of high current and magnetic field.

The pressure induced insulator to metal transition in FeSb2

The evolution of the ground state properties of FeSb(2) has been investigated via temperature (4.2-300 K), magnetic field (0-12 T) and pressure (0-8.8 GPa) dependent electrical resistivity studies. The temperature dependence of the resistivity follows activated behavior in the high temperature (HT) regime (T > 60 K), while variable range hopping (VRH) dictates the transport in the intermediate temperature (IT) regime (10 K > T > 45 K) and power law behavior is observed in the low temperature (LT) regime (T < 10 K). The pressure profoundly affects the resistivity in all the temperature regimes. The energy gap (Δ) extracted in the HT regime initially increases with pressure and then decreases, while the VRH parameter T(0) deduced in the IT regime is seen to decrease monotonically and vanish beyond 5 GPa leading to an insulator to metal transition (MIT) on account of delocalization of the electronic states in the gap. The analysis of the logarithmic derivative of the conductivity indicates the MIT to occur at ~6 GPa. The magnetoresistivity is found to be positive. The analysis of the resistivity behavior under pressure and magnetic field indicates that the former induces delocalization, while the latter tends to assist localization of the defect states inside the gap of FeSb(2).

Size effect on the structural, magnetic, and magnetotransport properties of electron doped manganite La0.15Ca0.85MnO3

Nanocrystalline La0.15Ca0.85MnO3 samples of various grain sizes ranging from ∼17 to 42 nm have been prepared by sol-gel technique. Phase purity and composition were verified by room temperature x-ray diffraction and SEM-EDAX analysis. The bulk La0.15Ca0.85MnO3 is known to order antiferromagnetically around 170 K and to undergo a simultaneous crystal structural transition. DC magnetization measurements on 17 nm size La0.15Ca0.85MnO3 show a peak at ∼130 K (TN) in zero-field-cooled (ZFC) state. Field-cooled magnetization bifurcates from ZFC data around 200 K hinting a weak ferromagnetic component near room temperature due to surface moments of the nanoparticle sample. Low temperature powder neutron diffraction experiments reveal that the incomplete structural transition from room temperature orthorhombic to low temperature orthorhombic-monoclinic state also occurs in the nanoparticle sample as in the bulk. Magnetization in the ordered state decreases as particle size increases, thus indicating the reduction ...

Magnetic phase separation in Nd0.5(Ca,Sr)0.5MnO3

The phase separation of Nd0.5Ca0.5−ySryMnO3, for 0≤y≤0.5, is studied via electron spin resonance (ESR) in the temperature range 80–400 K. Two types of magnetic phases are found to coexist in the samples with y>0 below a phase separation temperature T∗. The ESR line width exhibits a minimum near the charge order transition temperature TCO for samples with y<0.25, and near T∗ for y≥0.25. A systematic decrease in line width is observed as y increases from 0 to 0.5. The g factors increase slightly and the ESR intensities increase exponentially when the temperature decreases in the paramagnetic region. Our results demonstrate presence of two local magnetic structures due to the intrinsic phase separation.

Modulation of superconductivity by spin canting in a hybrid antiferromagnet/superconductor oxide

The proximity effect of a C-type antiferromagnet with the spin canting at low temperature is investigated in the hybrid Nd0.35Sr0.65MnO3/YBa2Cu3O7 oxide system through magnetic and transport measurements. It is found that the onset of a spin-canted state destroys partially the superconducting order parameter. Interestingly, due to the instability of this spin-canted state, zero resistivity recovers at the offset of spin canting. Our result demonstrates clearly the high sensitivity of superconducting order parameter to a modulation of internal field.

Strain enhanced spin polarization in Nd0.43Sr0.57MnO3/YBa2Cu3O7 bilayers

Electrical and magnetic properties of antiferromagnetic/superconducting bilayers Nd0.43Sr0.57MnO3/YBa2Cu3O7 (NSMO/YBCO) are investigated as functions of NSMO thickness (d) and current density (J). The superconducting transition temperatures (Tc) of bilayers decrease monotonically with decreasing d, and the suppression rate of Tc with J (dTc/dJ) is enhanced by three orders of magnitude in NSMO(d)/YBCO compared with that in pure YBCO. Based on the analysis on the d dependencies of Tc and dTc/dJ, it is suggested that the strain enhanced spin polarization is responsible for the great suppression of superconductivity in NSMO(d)/YBCO with d=40 and 80 nm.

On unusual temperature dependence of the upper critical field in YNi2−xFexB2C

Abstract Measurement of upper critica field in YNi 2− x Fe x B 2 C is reported for x = 0, 0.05, 0.10, and 0.15. An anomalous positive curvature is observed for a range of temperatures close to T c , for all x . As x is increased, the temperature interval over which the curvature in H c2 ( T ) is positive, is reduced and the system shows a tendency to go to the usual behaviour exhibited by conventional low temperature superconductors. Most of the theories based on a Fermi liquid normal state seem to be inadequate to understand this anomalous behaviour. It is speculated that this anomalous behaviour of H c2 ( T ) signifies the presence of strong correlations in the pristine YNi 2 B 2 C and that strong correlation effects become less and less important upon substitution of Ni with Fe.