Kranti Kumar

Anomalous first-order transition in Nd0.5Sr0.5MnO3: an interplay between kinetic arrest and thermodynamic transitions

A detailed investigation of the first-order antiferromagnetic insulator (AFI) to ferromagnetic metal (FMM) transition in Nd 0.5 Sr 0.5 MnO 3 is carried out by resistivity and magnetization measurements. These studies reveal several anomalous features of thermomagnetic irreversibility across the first-order transition. We show that these anomalous features cannot be explained in terms of the supercooling effect alone and the H-T diagram based on isothermal M-H or R-H measurements alone does not reflect the true nature of the first-order transition in this compound. Our investigations reveal glass-like arrest of kinetics at low temperature which plays a dominant role in the anomalous thermomagnetic irreversibility observed in this system. The interplay between kinetic arrest and supercooling is investigated by following novel paths in the H-T space. It is shown that coexisting FMM and AFI phases can be tuned in a number of ways at low temperature. These measurements also show that kinetic arrest temperature and supercooling temperature are anticorrelated, i.e. regions which are arrested at low temperature have higher supercooling temperature and vice versa.

Coexisting tunable fractions of glassy and equilibrium long-range-order phases in manganites

Antiferromagnetic-insulating (AF-I) and ferromagnetic-metallic (FM-M) phases coexist in various half-doped manganites over a range of temperature and magnetic field, and this is often believed to be an essential ingredient of their colossal magnetoresistance. We present magnetization and resistivity measurements on Pr 0.5 Ca 0.5 Mn 0.975 Al 0.025 O 3 and Pr 0.5 Sr 0.5 MnO 3 showing that the fraction of the two coexisting phases at low temperature in any specified measuring field, H, can be continuously controlled by following designed protocols traversing field-temperature space; for both materials the FM-M fraction rises under similar cooling paths. Constant-field temperature variations, however, show that the former sample undergoes a first-order transition from AF-I to FM-M with decreasing T, while the latter undergoes the reverse transition. We suggest that the observed path-dependent phase-separated states result from the low-T equilibrium phase coexisting with supercooled glass-like high-temperature phase, where the low-T equilibrium phases are actually homogeneous FM-M and AF-I phases respectively for the two materials.

History-dependent nucleation and growth of the martensitic phase in the magnetic shape memory alloy Ni45Co5Mn38Sn12

We study through the time evolution of magnetization the low-temperature (T) dynamics of the metastable coexisting phases created by traversing different paths in magnetic field H and T space in a shape memory alloy system, Ni45Co5Mn38Sn12. It is shown that these coexisting phases consisting of a fraction of kinetically arrested austenite phase and a remaining fraction of low-T equilibrium martensitic phase undergo a slow relaxation to low magnetization (martensitic) state but with very different thermomagnetic history-dependent rates at the same T and H. We discovered that, when the nucleation of the martensitic phase is initiated at much lower T through the de-arrest of the glasslike arrested state contrasted with the respective first-order transformation (through supercooling at much higher T), the long-time relaxation rate scales with the nonequilibrium phase fraction but has a very weak dependence on T. This is explained on the basis of the H-T path dependent size of the critical radii of the nuclei and the subsequent growth of the equilibrium phase through the motion of the interface.

Observation of different spin behavior with temperature variation and Cr substitution in a multiferroic compound YMn2O5

Abstract In this article, the collective response of the spins is explored through low field bulk magnetic measurement for the series YMn2−xCrxO5 (x=0.0, 0.05). Low field ac susceptibility and dc magnetization of YMn2O5 shows multiple transitions in analogy to those observed in electrical measurement of the compound. Perturbing the parent compound with a small doping (2.5% Cr) causes a drastic change in the long-range magnetic behavior. It is observed that, YMn1.95Cr0.05O5 undergoes a ferrimagnetic ordering with an enhanced magnetic ordering temperature as compared to the parent, which undergoes an antiferromagnetic ordering. Appearance of spontaneous magnetization without any major change in the atomic structure is rather significant since the parent compound is an important multiferroic material. In addition, magnetic memory effect is observed in the Cr substituted compound whereas it is absent in the parent compound.

Tuning the phase transition dynamics by variation of cooling field and metastable phase fraction in Al doped Pr0.5Ca0.5MnO3

We report the effect of field, temperature and thermal history on the time dependence in resistivity and magnetization in the phase separated state of Al doped Pr(0.5)Ca(0.5)MnO(3). The rate of time dependence in resistivity is much higher than that of magnetization and it exhibits a different cooling field dependence due to percolation effects. Our analysis shows that the time dependence in physical properties depends on the phase transition dynamics, which can be effectively tuned by variation of temperature, cooling field and metastable phase fraction. The phase transition dynamics can be broadly divided into the arrested and unarrested regimes, and in the arrested regime this dynamics is mainly determined by time taken in the growth of critical nuclei. An increase in cooling field and/or temperature shifts this dynamics from the arrested to unarrested regime, and in this regime, this dynamics is determined by the thermodynamically allowed rate of formation of critical nuclei, which in turn depends on the cooling field and available metastable phase fraction. At a given temperature, a decrease in metastable phase fraction shifts the crossover from arrested to unarrested regimes towards lower cooling field. It is rather significant that in spite of the metastable phase fraction calculated from resistivity being somewhat off that of magnetization, their cooling field dependence exhibits a striking similarity, which indicates that the dynamics in arrested and unarrested regimes are so different that it comes out vividly provided that the measurements are performed around the percolation threshold.