S. B. Roy

First order magnetic transition in doped CeFe2 alloys: phase coexistence and metastability.

First order ferromagnetic (FM) to antiferromagnetic (AFM) phase transition in doped CeFe2 alloys is studied with the micro-Hall probe technique. Clear visual evidence of magnetic phase coexistence on micrometer scales and the evolution of this phase coexistence as a function of temperature, magnetic field, and time across the first order FM-AFM transition is presented. Such phase coexistence and metastability arise as a natural consequence of an intrinsic disorder-influenced first order transition. The generality of these phenomena involving other classes of materials is discussed.

Reproducible room temperature giant magnetocaloric effect in Fe?Rh

We present the results of magnetocaloric effect (MCE) studies in polycrystalline Fe?Rh alloy over a temperature range of 250?345?K across the first order antiferromagnetic to ferromagnetic transition. By measuring the MCE under various thermomagnetic histories, contrary to the long held belief, we show here explicitly that the giant MCE in Fe?Rh near room temperature does not vanish after the first field cycle. In spite of the fact that the virgin magnetization curve is lost after the first field cycle near room temperature, reproducibility in the MCE under multiple field cycles can be achieved by properly choosing a combination of isothermal and adiabatic field variation cycles in the field-temperature phase space. This reproducible MCE leads to a large effective refrigerant capacity of 324.42?J?kg?1, which is larger than that of the well-known magnetocaloric material Gd5Si2Ge2. This information could be important as Fe?Rh has the advantage of having a working temperature of around 300?K, which can be used for room temperature magnetic refrigeration.

First order magneto-structural phase transition and associated multi-functional properties in magnetic solids.

We show that the first order magneto-structural phase transitions observed in various classes of magnetic solids are often accompanied by useful multi-functional properties, namely giant magneto-resistance, magneto-caloric effect and magneto-striction. We highlight various characteristic features associated with a disorder influenced first order phase transition namely supercooling, superheating, phase-coexistence and metastability, in several magnetic materials and discuss how a proper understanding of the transition process can help in fine tuning of the accompanied functional properties. Magneto-elastic coupling is a key element in this first order phase transition, and methods need to be explored for maximizing the contributions from both the lattice and the magnetic degree of freedom while simultaneously minimizing the thermomagnetic hysteresis loss. An analogy is also drawn with the first order phase transition observed in dielectric materials and vortex matter of type-II superconductors.

Relating supercooling and glass-like arrest of kinetics for phase separated systems: Doped Ce Fe 2 and ( La , Pr , Ca ) Mn O 3

Coexisting ferromagnetic and antiferromagnetic phases over a range of temperature as well as magnetic field have been reported in many materials of current interest, showing disorder-broadened first-order transitions. Anomalous history effects observed in magnetization and resistivity are being explained invoking the concepts of kinetic arrest akin to glass transitions. From magnetization measurements traversing unusual paths in field-temperature space, we obtain the intriguing result that the regions of the sample which can be supercooled to lower temperatures undergo kinetic-arrest at higher temperatures, and vice versa. Our results are for two diverse systems viz., the inter-metallic doped

Magnetocaloric effect in CeFe2 and Ru-doped CeFe2 alloys

\mathrm{Ce}{\mathrm{Fe}}_{2}

Imaging of time evolution of the first-order magneto-structural transition in Fe-Rh alloy using magnetic force microscopy

which has an antiferromagnetic ground state, and the oxide La-Pr-Ca-Mn-O which has a ferromagnetic ground state, indicating the possible universality of this effect of disorder on the widely encountered phenomenon of glass-like arrest of kinetics.

Contrasting the magnetic response between a magnetic glass and a reentrant spin glass

The magnetocaloric effect (MCE) is studied in CeFe2 and Ru-doped CeFe2 alloys with dc magnetization measurements. The pure CeFe2 compound shows a distinct peak in MCE around the paramagnetic to ferromagnetic transition. In Ru-doped CeFe2 alloys there is a further transition from the ferromagnetic to antiferromagnetic state at low temperatures. This latter magnetic transition gives rise to a relatively large inverse MCE. A comparative study of the MCE associated with two different magnetic transitions is made and the possible role of a structural transition in the inverse MCE is discussed.

On the reliable determination of the magnetic field for first flux-line penetration in technical niobium material

Cooperative lattice distortion can lead to phase coexistence at micrometer scales across a first-order magneto-structural transition. Using magnetic force microscopy we show such a phase coexistence across an antiferromagnetic to ferromagnetic transition coupled with a structural distortion in the intermetallic alloy of Fe-Rh. Our results provide direct evidence of strong coupling between the elastic and electronic degrees of freedom seen across this first-order magneto-structural transition. Theoretical frameworks based on long-range strain disorder coupling proposed originally for explaining phase coexistence in manganese oxide compounds with colossal magnetoresistance also appear to be applicable for other systems, like intermetallics, undergoing magneto-structural transitions.

The magnetocaloric performance in pure and mixed magnetic phase CoMnSi

Magnetic glass is a recently identified phenomenon in various classes of magnetic systems undergoing a first-order magnetic phase transition. We shall highlight here a few experimentally determined characteristics of magnetic glass and the relevant set of experiments, which will enable to distinguish a magnetic glass unequivocally from the well-known phenomena of spin glass and reentrant spin glass.

Low-temperature magnetic response of MnSi: Thermomagnetic history effects, metamagnetic transition and the kinetic arrest of the reverse transition

We present a way to reliably determine the field for first penetration HP in various kinds of bulk samples of niobium material used in technical applications like the fabrication of superconducting RF cavities. Special attention is given to the role of flux-line pinning in the determination of HP. It is observed that the pinning properties and HP can be altered significantly with the chemical treatment of bulk niobium. A correlation is proposed between HP of the niobium materials and the anomalous high-field Q drop observed in the superconducting RF cavities fabricated using such niobium material.