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Structural, dielectric, ferroelectric (FE), 119Sn Mössbauer, 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 Mössbauer 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 based applications.

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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.

The nature of the magnetic transition, critical scaling of magnetization, and magnetocaloric effect in Mn 1+x fe 4−x Si 3 (x = 0 to 1) are studied in detail. Our measurements show no thermal hysteresis across the magnetic transition for the parent compound which is in contrast with the previous report and corroborate the second order nature of the transition. The magnetic transition could be tuned continuously from 328 K to 212 K with Mn substitution at the Fe site. The Mn substitution leads to a linear increase in the unit cell volume and a slight reduction in the effective moment. A detailed critical analysis of the magnetization data for x = 0.0 and 0.2 is performed in the critical regime using the modified Arrott plots, Kouvel-Fisher plot, universal curve scaling, and scaling analysis of magnetocaloric effect. The magnetization isotherms follow modified Arrott plots with critical exponent (β  0.308, γ  1.448, and δ  5.64) for the parent compound (x = 0.0) and (β  0.304, γ  1.445, and δ  5.64) for x = 0.2. The Kouvel-Fisher and universal scaling plots of the magnetization isotherms further confirm the reliability of our critical analysis and values of the exponents. These values of the critical exponents are found to be same for both the parent and doped samples which do not fall under any of the standard universality classes. The exchange interaction decays as J(r) ~ r −3.41 following the renormalization group theory and the observed critical exponents correspond to lattice dimensionality d = 2, spin dimensionality n = 1, and the range of interaction σ = 1.41. This value of σ(<2) indicates long-range interaction between magnetic spins. A reasonable magnetocaloric effect ΔS m  −6.67 J/Kg-K and −5.84 J/Kg-K for x = 0.0 and 0.2 compounds, respectively, with a huge relative cooling power (RCP ~ 700 J/Kg) for 9 T magnetic field change is observed. The universal scaling of magnetocaloric effect further mimics the second order character of the magnetic transition. The obtained critical exponents from the critical analysis of magnetocaloric effect agree with the values deduced from the magnetic isotherm analysis.

Novel thermal effects across the first order antiferromagnetic (AFM) -ferromagnetic (FM) transition in an intermetallic alloy are reported. They show instances of warming when heat is extracted from the sample, and cooling when heat is provided to the sample across AFM-FM transition in Ta doped Hf F e2, thus providing indisputable evidence of metastable supercooled AFM and superheated FM states, respectively. Such thermal effects in a magnetic solid prepared from commercially available materials has been reported for the first time. The transition proceeds in multiple steps which is interpreted in the framework of quenched disorder broadening of AFM-FM transition and classical nucleation theory. Measurements in the presence of magnetic field conform to above frame work.PACS numbers: 75.30. Kz, 72.15.Gd, 75.60.Nt, 75.50.Bb First order transitions are characterized by latent heat and transformation from one phase to another phase occurs via nucleation and growth process. If the heat (∆Q) extracted from the system is less than its total latent heat (L), then only a fraction (∝ ∆Q/L) of the total system can transform to low temperature phase. However, this fraction cannot be reduced to an arbitrarily small value as the nuclei below a critical size will be unstable. In the classical nucleation theory, the critical size of the stable nuclei (r*) is proportional to (γ sl × T C )/(∆T * × ∆L), where γ sl is the interfacial energy, T C is the equilibrium transition temperature, ∆T * is undercooling temperature and ∆L is the latent heat of the transition of nuclei. Therefore, the transformation from one phase to other phase occurs in a quantum of steps, the size of which is dictated by r*. With increased undercooling, i.e., with increased metastability of supercooled state, r* decreases. Higher the metastability smaller is the perturbation required to transform this state into a stable state. For such transformation during cooling (warming), if the heat removed from (supplied to) the adiabatic system is less than the latent heat of transformation, then system shows warming, when heat is removed from the system (cooling when heat is added to the system). Such thermal effects known as recalescence are commonly observed during liquid to solid transformation (e.g. water to ice transition, solid-liquid transition in NiAl by Kulkarni et al.[1], Al-Nb alloys by Munitz et al. [2]). However, observation of similar thermal effects across first order * E-mail : rrawat@csr.res.in magnetic transitions in solids are rare. Generally, first order transitions in solids are broadened due to presence of quenched disorder. For such materials, there exists a spatial distribution of transition temperatures on the length scale of correlation length and for a macroscopic system, it leads to quasi-continuous distribution of transition temperatures leading to apparent gradual change in physical properties [3,4]. Though locally (on the length scale of correlation length), transition remains discontinuous [5][6][7]. Till date, there are only two ex...

The magnetic field-pressure-temperature (H-P-T) phase diagram for first order antiferromagnetic (AFM) to ferromagnetic (FM) transition in Fe49(Rh0.93Pd0.07)51 has been constructed using resistivity measurements under simultaneous application of magnetic field (up to 8 Tesla) and pressure (up to 20 kbar). Temperature dependence of resistivity (ρ-T) shows that with increasing pressure, the width of the transition and the extent of hysteresis decreases whereas with the application of magnetic field it increases. Consistent with existing literature the first order transition temperature (TN) increases with the application of external pressure (∼ 7.3 K/ kbar) and decreases with magnetic field (∼ -12.8 K/Tesla). Exploiting these opposing trends, resistivity under simultaneous application of magnetic field and pressure is used to distinguish the relative effect of temperature, magnetic field and pressure on disorder broadened first order transition. For this a set of H and P values are chosen for which TN (H1 , P1) = TN (H2 , P2). Measurements for such combinations of H and P show that the temperature dependence of resistivity is similar i.e. the broadening (in temperature) of transition as well as extent of hysteresis remains independent of H and P. The transition width decreases exponentially with increasing temperature. Isothermal magnetoresistance measurement under various constant pressure show that even though the critical field required for AFM-FM transition depends on applied pressure, the hysteresis as well as transition width (in magnetic field) both remains independent of pressure, consistent with our conclusions drawn from ρ-T measurements.

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