A. K. Pramanik

Temperature evolution of magnetic and transport behavior in 5d Mott insulator Sr2IrO4: significance of magneto-structural coupling

We have investigated the temperature evolution of magnetism and its interrelation with structural parameters in the perovskite-based layered compound Sr2IrO4, which is believed to be a J(eff) = 1/2 Mott insulator. The structural distortion plays an important role in this material and induces a weak ferromagnetism in an otherwise antiferromagnetically ordered magnetic state with a transition temperature around 240 K. Interestingly, at low temperatures, below around 100 K, a change in the magnetic moment has been observed. Temperature dependent x-ray diffraction measurements show that sudden changes in structural parameters around 100 K are responsible for this. Resistivity measurements show insulating behavior throughout the temperature range across the magnetic phase transition. The electronic transport can be described with Motts two-dimensional variable range hopping (VRH) mechanism, however, three different temperature ranges are found for VRH, which is a result of varying the localization length with temperature. A negative magnetoresistance (MR) has been observed at all temperatures in contrast to positive behavior generally observed in strongly spin-orbit coupled materials. The quadratic field dependence of MR implies the relevance of a quantum interference effect.

Flux dynamics and avalanches in the 122 pnictide superconductor Ba0.65Na0.35Fe2As2.

In this work we present the results of the bulk magnetization measurements in a superconducting state of single crystals of Ba0.65Na0.35Fe2As2. The isothermal magnetic field (Hxa0∥xa0c axis) dependent magnetization (M) loops exhibit a second peak (SP) or fishtail effect, as well as remarkable flux jumps at low temperatures. The critical current density Jc obtained from the M(H) loops is rather high, of the order of 10(6)xa0Axa0cm(-2). The analysis of the temperature-xa0and field-dependent Jc implies that high Jc is mainly due to collective (weak) pinning of vortices by dense microscopic point defects with some contribution from a strong pinning mechanism. Pronounced magnetic instabilities in terms of flux jumps depend strongly on temperature as well as on the field sweep rate. The field for the first flux jump as calculated from an adiabatic model, however, is much lower than the experimentally observed values, and this enhanced stability is attributed to a flux creep phenomenon. The analysis of field-dependent magnetic relaxation data additionally supports a collective pinning model. The data further suggest that SP in M(H) is likely related to the crossover in creep dynamics from an elastic to a plastic mechanism. We have constructed the vortex phase diagram on the field-temperature plane.In this work we present the results of the bulk magnetization measurements in a superconducting state of single crystals of Ba0.65Na0.35Fe2As2. The isothermal magnetic field (H ||c axis) dependent magnetization (M) loops exhibit a second peak (SP) or ‘fishtail effect’, as well as remarkable flux jumps at low temperatures. The critical current density Jc obtained from the M(H) loops is rather high, of the order of 10 A/cm. The analysis of the temperature and field dependent Jc implies that high Jc is mainly due to collective (weak) pinning of vortices by dense microscopic point defects with some contribution from a strong pinning mechanism. Pronounced magnetic instabilities in terms of flux jumps depend strongly on temperature as well on the field sweep rate. The field for first flux jump as calculated from an adiabatic model, however, is much lower than the experimentally observed values, and this enhanced stability is attributed to a flux creep phenomenon. The analysis of field dependent magnetic relaxation data additionally supports a collective pinning model. The data further suggests that SP in M(H) is likely related to crossover in creep dynamics from elastic to plastic mechanism. We have constructed the vortex phase diagram on field-temperature plane.

Insulating phase in Sr2IrO4: An investigation using critical analysis and magnetocaloric effect

Abstract The nature of insulating phase in 5d based Sr2IrO4 is quite debated as the theoretical as well as experimental investigations have put forward evidences in favor of both magnetically driven Slater-type and interaction driven Mott-type insulator. To understand this insulating behavior, we have investigated the nature of magnetic state in Sr2IrO4 through studying critical exponents, low temperature thermal demagnetization and magnetocaloric effect. The estimated critical exponents do not exactly match with any universality class, however, the values obey the scaling behavior. The exponent values suggest that spin interaction in present material is close to mean-field model. The analysis of low temperature thermal demagnetization data, however, shows dual presence of localized- and itinerant-type of magnetic interaction. Moreover, field dependent change in magnetic entropy indicates magnetic interaction is close to mean-field type. While this material shows an insulating behavior across the magnetic transition, yet a distinct change in slope in resistivity is observed around Tc. We infer that though the insulating phase in Sr2IrO4 is more close to be Slater-type but the simultaneous presence of both Slater- and Mott-type is the likely scenario for this material.

Site dilution in SrRuO3: effects on structural and magnetic properties

We have investigated the effect of site dilution with substitution of nonmagnetic element in SrRu1-x Ti x O3 (xu2009u2009⩽u2009u20090.7). The nature of ferromagnetic state in SrRuO3 is believed to be of itinerant type with transition temperature [Formula: see text] K. Crystallographically, SrRuO3 has a distorted orthorhombic structure. Substitution of [Formula: see text] (3d 0) for Ru+4 (4d 4), however, does not introduce significant structural modification due to their matching ionic radii. This substitution, on the other hand, is expected to tune the electronic correlation effect and the d electron density in the system. With Ti substitution, we find that magnetic moment and Curie temperature decreases but T c remains unchanged which has been attributed to opposite tuning of electron correlation effect and density of states within the framework of itinerant ferromagnetism. The estimated critical exponent (β) related to magnetization implies a mean-field type of magnetic nature in SrRuO3. The value of β further increases with x which is understood from the dilution effect of magnetic lattice. The system evolves to exhibit Griffiths phase like behavior above T c which is usually realized in diluted ferromagnet following local moment model of magnetism. Our detail analysis of magnetization data indicates that magnetic state in SrRuO3 has contribution from both itinerant and local moment model of magnetism.

Electronic and magnetic properties in Sr1-xLaxRuO3

Here we report the structural, magnetic and transport properties in La doped SrRuO3. The doping of La3+ modifies the ionic state of Ru by converting Ru4+ to Ru+3. However, there is modification in lattice parameters as La3+ has smaller ionic radii than that of Sr2+. We find La doping weakens the ferromagnetic state in SrRuO3 in terms of lowering Tc and decreasing the magnetic moment. The electrical resistivity shows metallic behavior in whole temperature range, however, resistivity increases with doping of La.

Investigation of structural and electrical transport properties in Ti doped Sr2IrO4

Effects of Ti substitution at Ir-site on structural and electrical properties are studies in Sr2IrO4. Structural investigation has been done using x-ray powder diffraction and allied Rietveld analysis for all the samples. While there is no structural phase transition but the lattice parameters evolve with Ti doping. Resistivity measurements show an insulating behavior for all the samples. Data show resistivity increases drastically with Ti doping. The increase in resistivity is understood to arise due to dilution of Ir-O-Ir network as well as tuning of electronic correlation effect.

Effect of Y substitution in SrRuO3: Evolution from itinerant to localized type magnetism

Evolution of structural and magnetic properties have been investigated in Y doped SrRuO3. The SrRuO3 is believed to have itinerant type of magnetism with ordering temperature ∼160u2005K. The substitution of nonmagnetic Y+3, which is expected to induce structural modifications and to change the Ru ionic state, does not change Tc. The magnetic moment at low field increases which is on contrary in view of change of Ru charge state. The estimated critical exponent related to magnetization shows itinerant type magnetic state in SrRuO3 which changes to localized model with Y doing.

Critical behavior in itinerant ferromagnet SrRu1-xTixO3

Abstract SrRuO 3 presents a rare example of ferromagnetism among the 4 d based oxides. While the nature of magnetic state in SrRuO 3 is mostly believed to be of itinerant type, recent studies suggest a coexistence of both itinerant and localized model of magnetism in this material. Here, we have investigated the evolution of magnetic state in doped SrRu 1 - x Ti x O 3 through studying the critical behavior using standard techniques such as, modified Arrott plot, Kouvel-Fisher plot and critical isotherm analysis across the magnetic transition temperature T c . The substitution of nonmagnetic Ti 4 + (3 d 0 ) for Ru 4 + (4 d 4 ) would simply dilute the magnetic system apart from modifying the electron correlation effect and the density of states at Fermi level. Surprisingly, T c does not change with x . Moreover, our analysis show the exponent β related to spontaneous magnetization increases while the exponents γ and δ related to initial inverse susceptibility and critical magnetization, respectively decrease with Ti substitution. The estimated exponents do not match with any established theoretical models for universality classes, however, the exponent obey the Widom relation and the scaling behavior. Interestingly, this particular evolution of exponents in present series has similarity with that in isoelectronic doped Sr 1 - x Ca x RuO 3 . We believe that site dilution by Ti leads to formation magnetic clusters which causes this specific changes in critical exponents.