R. Rawat

Electro-caloric effect in lead-free Sn doped BaTiO3 ceramics at room temperature and low applied fields

Structural, dielectric, ferroelectric (FE), 119Sn Mossbauer, and specific heat measurements of polycrystalline BaTi1–xSnxO3 (x = 0% to 15%) ceramics are reported. Phase purity and homogeneous phase formation with Sn doping is confirmed from x-ray diffraction and 119Sn Mossbauer measurements. With Sn doping, the microstructure is found to change significantly. Better ferroelectric properties at room temperature, i.e., increased remnant polarization (38% more) and very low field switchability (225% less) are observed for x = 5% sample as compared to other samples and the results are explained in terms of grain size effects. With Sn doping, merging of all the phase transitions into a single one is observed for x ≥ 10% and for x = 5%, the tetragonal to orthorhombic transition temperature is found close to room temperature. As a consequence better electro-caloric effects are observed for x = 5% sample and therefore is expected to satisfy the requirements for non-toxic, low energy (field) and room temperature b...

Room temperature giant baroresistance and magnetoresistance and its tunability in Pd doped FeRh

We report room temperature giant baroresistance (≈128%) in Fe49(Rh0.93Pd0.07)51. With the application of external pressure (P) and magnetic field (H), the temperature range of giant baroresistance (≈600% at 5 K, 19.9 kilobars and 8 T) and magnetoresistance (≈ −85% at 5 K and 8 T) can be tuned from 5 K to well above room temperature. It is shown that under external pressure, antiferromagnetic state is stabilized at room temperature and shows giant magnetoresistance (≈−55%). Due to coupled magnetic and lattice changes, the isothermal change in resistivity at room temperature under pressure (at constant H) as well as magnetic field (at constant P) can be scaled together to a single curve, when plotted as a function of X = T + 12.8 × H − 7.3 × P.

Concentration dependence in kinetic arrest of the first-order magnetic transition in Ta doped HfFe2

Magnetic behavior of the pseudo-binary alloy Hf(1-x)Ta(x)Fe(2) has been studied, for which the zero-field ferromagnetic (FM) to antiferromagnetic (AFM) transition temperature is tuned near to T = 0 K. Our studies show that such composition lies around x = 0.230. Detailed magnetization studies on x = 0.225, 0.230 and 0.235 show thermomagnetic irreversibility at low temperature due to kinetic arrest of the first-order AFM-FM transition. All three compositions studied show a reentrant transition in the zero-field-cooled warming curve and non-monotonic variation of the upper critical field. The region in H-T space where these features of kinetic arrest manifest themselves increases with increasing Ta concentration.

Magnetocaloric effect and magnetoresistance correlation in Ge-doped Mn2Sb

Magnetocaloric effect (MCE) and magnetoresistance (MR) in Ge-doped Mn2Sb systems with near-room-temperature, first-order antiferromagnetic (AFM) to ferrimagnetic (FRI) transitions have been studied. They show an inverse MCE with a 3.2 J/kg-K isothermal change in entropy (ΔS), and a refrigeration capacity that varies linearly up to 130 J kg−1 for a 13 Tesla magnetic field change. MR (dominated by change in electronic structure) and ΔS (dominated by change in magnetic entropy) are shown to have similar temperature dependence but with opposite signs due to coupled electronic and magnetic changes across the transition. The ratio of the peak values of MR (%) and ΔS is found to be −5.6 (J/kg-K), which remains nearly constant for the studied range of magnetic field change. Existing data of MR and MCE in other Mn2Sb systems (with substitution elements other than Ge) with nearby transition temperatures also show nearly same value for this ratio. Therefore, this ratio can be related to coupling between magnetic and electronic changes and will be an useful parameter for systems with such transitions.

Oxygen‐deficiency‐induced suppression of JT distortion and stabilization of charge ordering in La0.2Sr0.8MnO3−δ

Structural phase transition studies employing low-temperature transmission electron microscopy, and low-temperature x-ray diffraction have been carried out on nearly stoichiometric ({\delta}=0.01) and off-stoichiometric ({\delta}=0.12) versions of La0.2Sr0.8MnO3-{\delta} manganite. The nearly stoichiometric La0.2Sr0.8MnO3-{\delta} under goes a cubic to Jahn-Teller distorted C-type antiferromagnetic tetragonal phase-transition at 260K. For off-stoichiometric La0.2Sr0.8MnO3-{\delta} the Jahn-Teller distorted tetragonal phase transition gets totally suppressed and the basic perovskite lattice remains cubic but shows a charge ordered phase. Stabilization of charge ordered phase in the absence of cooperative Jahn-Teller distortion has been attributed to coulomb-repulsion and Hunds coupling energy. The off-stoichiometric sample shows characteristically different physical properties. This has been realized through transport, magnetic and calorimetric measurements. A smooth crossover from variable range hopping transport to power-law dependence of resistivity ({\rho}) on temperature (T) i.e. {\rho}=C.T-{\alpha} has been realized. This has been attributed to multistep inelastic tunneling through channels involving localized states around oxygen vacancy sites.

Low temperature high magnetic field 57Fe Mössbauer study of kinetic arrest in Ta doped HfFe2

Low temperature high magnetic field (57)Fe Mössbauer measurements were carried out on the inter-metallic compound Hf0.77Ta0.23Fe2 by following novel paths in H-T space. The ferromagnetic (FM) fraction at 5 K and zero magnetic field is shown to depend on the cooling field, i.e., the higher the field is, the higher the FM fraction is. Mössbauer spectra collected in the presence of a 4 T magnetic field show that the antiferromagnetic (AFM) spins are canted with respect to the applied magnetic field and hence contribute to the total bulk magnetization in this compound. The data also show an induced magnetic moment even at the 2a site of the AFM phase. Mössbauer spectra collected using the CHUF (cooling and heating in un-equal magnetic fields) protocol show a reentrant transition when the sample is cooled in zero field and measured during warming in 4 T, showing the FM state as the equilibrium state. This work is the first microscopic experimental evidence for the de-vitrification of the kinetically arrested magnetic state.

Influence of thermal annealing and magnetic field on first order magnetic transition in Pd substituted FeRh

Influence of successive thermal annealing and magnetic field on First order antiferro (AFM) to ferromagnetic (FM) transition in the Pd substituted FeRh has been studied. With successive thermal annealing CsCl type bcc phase increases at the expense of fct (pseudo fcc) phase. Resistivity measurements do not show any transition in as-cast sample in contrast to annealed samples. AFM to FM transition temperature (TN)is found to decrease with higher annealing temperature. With the application of magnetic field, TN shift to lower temperature. These measurements show anomalous thermomagnetic irreversibility besides showing giant magnetoresistance across magnetic field induced first order AFM to FM transition.

Note: Inverted heat pulse method to measure heat capacity during cooling: A counterpart of conventional quasi-adiabatic heat pulse method

A simple method to extract known amount of heat from a sample within a given time interval has been proposed. Using this method, which we call inverted heat pulse (IHP) method, absolute values of heat capacity (C(P)) during cooling can be measured in a manner similar to conventional quasi-adiabatic heat pulse method of measuring C(P) during warming. Absolute accuracy of the measured C(P) using IHP method is found to be better than 2% in the temperature range 100-300 K. Applicability of this method to a broad and hysteretic first order transition is tested by measuring C(P) of Rh doped FePt sample, which shows a antiferromagnetic to ferromagnetic first order transition with a transition width of ∼35 K and hysteresis of ∼6 K. The peak value of the measured C(P) using IHP during cooling as well as entropy change calculated from measured data is found to be in good agreement with that measured during warming using conventional quasi-adiabatic heat pulse method.

Study of magnetoresistance in Rh-doped FePt

First order antiferromagnetic (AFM) to ferromagnetic (FM) transition in FePt0.78Rh0.22 has been studied by resistivity and magnetoresistance (MR) measurements. With the application of magnetic field, AFM-FM transition temperature (TN) decreases monotonically at the rate of ∼4.5K/Tesla. Around TN AFM to FM transition can be induced with the application of magnetic field which shows a large MR of ∼16%, at 8 Tesla magnetic fields. At room temperature in the FM state it shows ∼5% negative MR.

Study of magnetic anisotropy in Co-doped Mn2Sb

We have grown single crystals of Mn2−xCoxSb with x =0.20 and 0.15. Magnetization studies show a first order ferrimagnetic to antiferromagnetic transition with lowering temperature at TN ≈ 160 K and ≈ 117 K for x = 0.20 and 0.15, respectively. TN is found to be independent of applied magnetic field (1000 Oe) direction. Besides TN, signature of a spin reorientation transition (TSR) has also been observed near room temperature for x = 0.15. Below spin reorientation transition, magnetization behavior is highly anisotropic and easy direction of magnetization are different for x = 0.20 and 0.15.