Phase transitions and magnetocaloric and transport properties in off-stoichiometric GdNi2Mnx

The structural, magnetic, magnetocaloric, transport, and magnetoresistance properties of the rare-earth intermetallic compounds GdNi2Mnx (0.5 ≤ x ≤ 1.5) have been studied. The compounds with x = 0.5 and 0.6 crystallize in the cubic MgCu2 type phase, whereas samples with x ≥ 0.8 form a mixed MgCu2 and rhombohedral PuNi3 phase. A second order magnetic phase transition from a ferromagnetic to paramagnetic state was observed near the Curie temperature (TC). The GdNi2Mnx (0.5 ≤ x ≤ 1.5) compounds order in a ferrimagnetic structure in the ground state. The largest observed values of magnetic entropy changes (at TC for ΔH = 5T) were 3.9, 3.5, and 3.1 J/kg K for x = 0.5, 0.6, and 0.8, respectively. The respective relative values of the cooling power were 395, 483, and 220 J/kg. These values are greater than some well-known prototype magnetocaloric materials such as Gd (400 J/kg) and Gd5Si2Ge2 (240 J/kg). Analysis of the resistivity data showed a T2 dependence at low temperatures, suggesting strong electron-phonon interactions, whereas at higher temperatures s-d scattering was dominated by the electron-phonon contribution, resulting in a slow increase in resistivity. Magnetoresistance values of ∼-1.1% were found for x = 0.5 near TC, and -7% for x = 1.5 near T = 80 K

Magnetostructural phase transitions and magnetocaloric effects in as-cast Mn1-xAlxCoGe compounds

The structural and magnetic properties of as-cast Mn1-xAlxCoGe (0 ≤ x ≤ 0.05) have been studied by X-ray diffraction, differential scanning calorimetry, and magnetization measurements. The partial substitution of Al for Mn in Mn1-xAlxCoGe results in a decrease in the martensitic transition temperature TM. For the concentration range 0 ≤ x ≤ 0.01, TM was found to coincide with ferromagnetic transition temperature (TC) resulting in a first-order magnetostructural transition (MST). A further increase in aluminum concentration resulted in a splitting of the phase transition temperatures, which included a drastic decrease in the martensitic temperature. The compounds with x \u3e 0.02 showed a single transition at TC. The maximum values of the magnetic entropy changes (-ΔSM) were ∼18 J/kgK, 12 J/kgK, and 7 J/kgK for ΔH = 5T at 313 K (x = 0.00), 286 K (x = 0.01), and 220 K (x = 0.02), respectively. The maximum value of the relative cooling power (RCP) was found to be 303 J/kg for x = 0.01 at T = 286 K for ΔH = 5T. It has been established that as-cast samples of this system show large value of MCE near room temperature making this system a promising material for magnetic cooling technologies

Comparing magnetostructural transitions in Ni50Mn18.75Cu6.25Ga25 and Ni49.80Mn34.66In15.54 Heusler alloys

The crystal structure, magnetic and transport properties, including resistivity and thermopower, of Ni50Mn18.75Cu6.25Ga25 and Ni49.80Mn34.66In15.54 Heusler alloys were studied in the (10-400) K temperature interval. We show that their physical properties are remarkably different, thereby pointing to different origin of their magnetostructural transition (MST). A Seebeck coefficient (S) was found to pass minimum of about -20 μV/K in respect of temperature for both compounds. It was shown that MST observed for both compounds results in jump-like changes in S for Ga-based compound and jump in resistivity of about 20 and 200 μΩ cm for Ga and In -based compounds, respectively. The combined analyzes of the present results with that from literature show that the density of states at the Fermi level does not change strongly at the MST in the case of Ni-Mn-In alloys as compared to that of Ni-Mn-Ga

Mechanisms Of Transmembrane Signaling By Bacterial Chemoreceptors: Studies Of Dynamics And Cofactor Reactivity

Flagellated bacteria constantly and actively search for optimum niches for their survival and proliferation by swimming in the environment. A family of receptor proteins, named chemoreceptors or methyl-accepting chemotaxis proteins (MCPs), along with cellular energy sensor Aer and sensory rhodopsins (SR) guide the cells towards optimal chemical and spectral compositions by controlling the activity of intracellular kinase CheA and thereby regulating cellular taxes (or movements). These receptors also serve as model systems for transmembrane signal transduction and cell-environment interaction. Although the receiver domains in these receptors vary greatly in sequence, function, and even in disposition relative to the membrane, the cytoplasmic domains of all of these transmembrane proteins share high homology and signal through the same kinase CheA. We engineered two variants of the cytoplasmic domain of E. coli aspartate receptor Tar, H1-Tar and H1-2-Tar, that mimic the ligand unbound and bound states of the full length native receptor, respectively, in order to uncover the general principle behind kinase activity regulation. As translated, H1-Tar stimulates the kinase, whereas H1-2-Tar deactivates the kinase both in vivo and in vitro, despite similar binding to CheA. These variants also respond to the modifications in four conserved glutamate and glutamine residues as the native Tar: mutation of two glutamines to glutamates renders H1-Tar deactivating, whereas H1-2-Tar stimulates the kinase upon mutation of the two glutamates to glutamines. Continuous wave and pulsed dipolar electron spin resonance spectroscopic studies of the spin labeled variants reveal dynamical coupling throughout the cytoplasmic domain of MCPs and that dynamics in the two ends of the domain are inversely correlated. The dynamics in the membrane distal region of the domain, named protein interaction region (PIR) that interacts with the kinase via an adaptor protein CheW correlate with the activating state of the receptors: conformationally dynamic PIR deactivates CheA and relatively static   PIR activates the kinase. Thus, we find that these receptors regulate the kinase activity based on the receiver domain state and modifications in those conserved residues by altering the dynamics throughout the cytoplasmic domain. Of these receptor proteins, the state of the receptive PAS domain of the energy sensor Aer relevant for CheA activity modulation is not particularly well described. We have, for the first time, purified bacterial Aer and have confirmed that Aer PAS domain binds FAD as a cofactor. In vitro sodium dithionite treatment reversibly reduces FAD in Aer from fully oxidized state to the single electron reduced anionic semiquinone (ASQ) state supporting the notion that Aer FAD samples different redox states to modulate the kinase activity. We show that Aer in fully oxidized FAD state activates the kinase, whereas in ASQ state inhibits the kinase. These results suggest that Aer senses the electron flow in the electron transport chain through FAD bound at the PAS domain and modulates the kinase activity based on the FAD redox state. Overall, this dissertation gains insights into the mechanisms of how these transmembrane receptors regulate the kinase activity in order to govern bacterial motion in response to the environmental stimuli

On the visibility of singularities in general relativity and modified gravity theories

We argue that the global causal structure of the singularity is not a purely geometric property but also depends on the collapsing matter-field leading to its formation. To show this, we investigate the global visibility of the end state of a spherically symmetric marginally bound Lemaitre-Tolman-Bondi collapsing cloud (which is well studied in general relativity) in the framework of modified gravity having the generalized Lagrangian $R+\alpha R^2$ in the Einstein-Hilbert action. Here $R$ is the Ricci scalar, and $\alpha>0$ is a constant. As an example, we depict that for the same LTB metric governing the spacetime formed due to two different matter-fields, i.e., dust in general relativity and imperfect viscous fluid in $f(R)$ gravity, the singularity is locally visible in the former case, and globally visible in the latter case.Comment: 10 pages, 4 figure

The effects of substituting Ag for In on the magnetoresistance and magnetocaloric properties of Ni-Mn-In Heusler alloys

The effect of substituting Ag for In on the structural, magnetocaloric, and thermomagnetic properties of Ni50Mn35In15−xAgx (x = 0.1, 0.2, 0.5, and 1) Heusler alloys was studied. The magnitude of the magnetization change at the martensitic transition temperature (TM) decreased with increasing Ag concentration. Smaller magnetic entropy changes (ΔSM) were observed for the alloys with larger Ag concentrations and the martensitic transition shifted to higher temperature. A shift of TM by about 25 K to higher temperature was observed for an applied hydrostatic pressure of P = 6.6 kbar with respect to ambient pressure. A large drop in resistivity was observed for large Ag concentration. The magnetoresistance was dramatically suppressed due to an increase in the disorder of the system with increasing Ag concentration. Possible mechanisms responsible for the observed behavior are discussed

The effects of substituting Ag for In on the magnetoresistance and magnetocaloric properties of Ni-Mn-In Heusler alloys

The effect of substituting Ag for In on the structural, magnetocaloric, and thermomagnetic properties of Ni50Mn35In15−xAgx (x = 0.1, 0.2, 0.5, and 1) Heusler alloys was studied. The magnitude of the magnetization change at the martensitic transition temperature (TM) decreased with increasing Ag concentration. Smaller magnetic entropy changes (ΔSM) were observed for the alloys with larger Ag concentrations and the martensitic transition shifted to higher temperature. A shift of TM by about 25 K to higher temperature was observed for an applied hydrostatic pressure of P = 6.6 kbar with respect to ambient pressure. A large drop in resistivity was observed for large Ag concentration. The magnetoresistance was dramatically suppressed due to an increase in the disorder of the system with increasing Ag concentration. Possible mechanisms responsible for the observed behavior are discussed