Evidence of Strong Orbital-Selective Spin-Orbital-Phonon Coupling in CrVO_{4}.

Coupling of orbital degree of freedom with a spin exchange, i.e., Kugel-Khomskii-type interaction (KK), governs a host of material properties, including colossal magnetoresistance, enhanced magnetoelectric response, and photoinduced high-temperature magnetism. In general, KK-type interactions lead to deviation in experimental observables of coupled Hamiltonian near or below the magnetic transition. Using diffraction and spectroscopy experiments, here we report anomalous changes in lattice parameters, electronic states, spin dynamics, and phonons at four times the Néel transition temperature (T_{N}) in CrVO_{4}. The temperature is significantly higher than other d-orbital compounds such as manganites and vanadates, where effects are limited to near or below T_{N}. The experimental observations are rationalized using first-principles and Green's function-based phonon and spin simulations that show unprecedentedly strong KK-type interactions via a superexchange process and an orbital-selective spin-phonon coupling coefficient at least double the magnitude previously reported for strongly coupled spin-phonon systems. Our results present an opportunity to explore the effect of KK-type interactions and spin-phonon coupling well above T_{N} and possibly bring various properties closer to application, for example, strong room-temperature magnetoelectric coupling.

Magnetoelastic coupling and spin contributions to entropy and thermal transport in biferroic yttrium orthochromite.

Direct engineering of material properties through exploitation of spin, phonon, and charge-coupled degrees of freedom is an active area of development in materials science. However, the relative contribution of the competing orders to controlling the desired behavior is challenging to decipher. In particular, the independent role of phonons, magnons, and electrons, quasiparticle coupling, and relative contributions to the phase transition free energy largely remain unexplored, especially for magnetic phase transitions. Here, we study the lattice and magnetic dynamics of biferroic yttrium orthochromite using Raman, infrared, and inelastic neutron spectroscopy techniques, supporting our experimental results with first-principles lattice dynamics and spin-wave simulations across the antiferromagnetic transition atTN∼ 138 K. Spectroscopy data and simulations together with the heat capacity (Cp) measurements, allow us to quantify individual entropic contributions from phonons (0.01 ± 0.01kBatom-1), dilational (0.03 ± 0.01kBatom-1), and magnons (0.11 ± 0.01kBatom-1) acrossTN. High-resolution phonon measurements conducted in a magnetic field show that anomalousT-dependence of phonon energies acrossTNoriginates from magnetoelastic coupling. Phonon scattering is primarily governed by the phonon-phonon coupling, with little contribution from magnon-phonon coupling, short-range spin correlations, or magnetostriction effects; a conclusion further supported by our thermal conductivity measurements conducted up to 14 T, and phenomenological modeling.

Large positive magnetoresistance and Dzyaloshinskii-Moriya interaction in CrSi driven by Cr 3d localization.

Spin chiral systems with Dzyaloshinskii-Moriya (DM) interaction due to broken inversion symmetry are extensively studied for their technological applications in spintronics and thermoelectrics. Here, we report an experimental study on the magnetization, magnetoresistance (MR) and electronic structure of a non-centrosymmetric compound CrSi with B20 crystal structure. Both magnetization and MR shows competing ferromagnetic (FM) and antiferromagnetic (AFM) correlations with the FM correlations being comparatively weaker indicating the presence of DM interaction in CrSi. A large positive MR [Formula: see text] obtained at 5 K and 5 T magnetic field arises due to the stronger AFM correlations. Resonant photoemission shows both localized and itinerant nature of Cr 3d electrons to be present in CrSi and this is supported by the temperature dependence of magnetic susceptibility. Drastic variation in the density of states along with valence band broadening at low temperature indicates the increase in hybridization between Cr 3d and Si 3s-3p states which enhances the localization effects. Spin polarized itinerant Cr 3d electrons give rise to AFM spin density wave in CrSi. Magnetic interaction between the localized and itinerant Cr 3d electrons are found to be crucial for realizing DM interaction in this system. Spectral density of states derived from high resolution valence band measurements provides evidence of electronic topological transition in CrSi. Large density of polarized itinerant electrons which varies with temperature and the large positive MR with AFM correlations suggests CrSi as a potential candidate for both the thermoelectric and spintronics applications.

Renormalization of electron-phonon coupling in the Mott-Ioffe-Regel limit due to point defects in the V1-x Ti x alloy superconductors.

We report here the temperature dependence of electrical resistivity ([Formula: see text](T)), heat capacity (C(T)) and thermal conductivity ([Formula: see text](T)) of superconducting V1-x Ti x alloys in the absence and presence of external applied magnetic fields. The [Formula: see text](T) changes from positive temperature coefficient of resistivity (TCR) to negative TCR at about x = 0.7 indicating that many of these alloys lie close to the Mott-Ioffe-Regel (MIR) limit. The jump in the C(T) across the superconducting transition temperature ([Formula: see text]) indicates that these alloys are in the intermediate coupling limit. The [Formula: see text](T) increases in certain V1-x Ti x alloys as the temperature decreases below the [Formula: see text] indicating that the phonons dominate the heat conduction in the superconducting state, whereas we found that the electrons are the major carriers of heat in the normal state. Our analysis suggests that the unusual features of thermal conductivity have origin in (i) the electron mean free path approaching the inter atomic distances (MIR limit) and (ii) the renormalization of the phonon mean free path due to the presence of point defects and the electron-phonon interaction.

Magnetotransport and magnetothermal properties of the ternary intermetallic compound TbFe2Al10.

We have studied the temperature and field dependences of electrical resistivity and heat capacity of TbFe2Al10, and have also complimented the above studies with low field magnetization measurements. In zero magnetic field, TbFe2Al10 exhibits paramagnetic (PM) to ferrimagnetic (Ferri-I) and Ferri-I to antiferromagnetic (AFM) phase transitions below 17.6 and 10 K respectively. We have found that the electrical resistivity of TbFe2Al10 exhibits a sharp rise across the PM to Ferri-I phase transition in this compound. Our analysis indicates that this sharp rise of electrical resistivity is related to the formation of new zone boundaries (across the PM to Ferri-I phase transition) that reduce the area of the Fermi surface. We have found that TbFe2Al10 exhibits large magnetoresistance (MR) below 100 K. Overall, the MR behaviour of TbFe2Al10 below 17.6 K in different magnetic fields reveals strong competition between AFM and ferromagnetic (FM) correlations, which seems to be quite intrinsic to the magnetic structure of the compound. Our analysis indicates that the large MR and magnetocaloric effect persisting deep inside the PM regime of TbFe2Al10 is mainly related to the presence of FM spin fluctuations and the formation of a Griffiths like (GL) phase consisting of FM clusters within the PM regime. The formation of the GL phase may be mediated by the static crystal defects in the midst of the competing inter and intra layer magnetic interactions.

Electronic structure of Mo1-x Re x alloys studied through resonant photoemission spectroscopy.

We studied the electronic structure of Mo-rich Mo1-x Re x alloys ([Formula: see text]) using valence band photoemission spectroscopy in the photon energy range 23-70 eV and density of states calculations. Comparison of the photoemission spectra with the density of states calculations suggests that, with respect to the Fermi level E F, the d states lie mostly in the binding energy range 0 to -6 eV, whereas s states lie in the binding energy range -4 to -10 eV. We observed two resonances in the photoemission spectra of each sample, one at about 35 eV photon energy and the other at about 45 eV photon energy. Our analysis suggests that the resonance at 35 eV photon energy is related to the Mo 4p-5s transition and the resonance at 45 eV photon energy is related to the contribution from both the Mo 4p-4d transition (threshold: 42 eV) and the Re 5p-5d transition (threshold: 46 eV). In the constant initial state plot, the resonance at 35 eV incident photon energy for binding energy features in the range E F (BE = 0) to -5 eV becomes progressively less prominent with increasing Re concentration x and vanishes for x > 0.2. The difference plots obtained by subtracting the valence band photoemission spectrum of Mo from that of Mo1-x Re x alloys, measured at 47 eV photon energy, reveal that the Re d-like states appear near E F when Re is alloyed with Mo. These results indicate that interband s-d interaction, which is weak in Mo, increases with increasing x and influences the nature of the superconductivity in alloys with higher x.

Evidence of multiband superconductivity in the ß-phase Mo1-xRex alloys.

We present a detailed study of the superconducting properties in the ß-phase Mo(1-x)Re(x) (x = 0.25 and 0.4) solid solution alloys pursued through magnetization and heat capacity measurements. The temperature dependence of the upper critical field H(C2)(T) in these binary alloys shows a deviation from the prediction of the Werthamer-Helfand-Hohenberg (WHH) theory. The temperature dependence of superfluid density estimated from the variation of lower critical field H(C1) with temperature, cannot be explained within the framework of a single superconducting energy gap. The heat capacity also shows an anomalous feature in its temperature dependence. All these results can be reasonably explained by considering the existence of two superconducting energy gaps in these Mo(1-x)Re(x) alloys. Initial results of electronic structure calculations and resonant photoelectron spectroscopy measurements support this possibility and suggest that the Re-5d like states at the Fermi level may not intermix with the Mo-5p and 5s like states in the ß-phase Mo(1-x)Re(x) alloys and contribute quite distinctly to the superconductivity of these alloys.

The magnetotransport properties of the intermetallic compound GdCu6.

We have studied the temperature and magnetic field dependence of the electrical resistivity of GdCu(6) and have co-related the results with the temperature dependence of heat capacity and magnetization. The magnetoresistance of GdCu(6) is found to be positive both in the paramagnetic and antiferromagnetic regimes. Within the antiferromagnetic regime, the magnetoresistance is very high and increases to still higher values both with increasing field and decreasing temperature. In the paramagnetic regime the magnetoresistance continues to exhibit a finite positive value up to temperatures much higher than that corresponding to the antiferromagnetic to paramagnetic phase transition. We have shown through quantitative analysis that both the temperature dependences of resistivity and heat capacity indicate the presence of spin fluctuations within the paramagnetic regime of GdCu(6). The field dependence of electrical resistivity indicates that the positive magnetoresistance in the paramagnetic phase is not related to the orbital motion of the conduction electrons in a magnetic field (the Kohler rule). In contrast, our analysis indicates that these spin fluctuations are responsible for the positive magnetoresistance observed within this paramagnetic regime. The nature of the field dependence of electrical resistivity is found to be qualitatively similar both in the antiferromagnetic and paramagnetic regimes, which probably indicates that spin fluctuations in the paramagnetic regime are of the antiferromagnetic type.

The effect of external pressure on the magnetocaloric effect of Ni-Mn-In alloy.

The martensitic transition in Ni(50)Mn(34)In(16) alloy has been studied by measuring the magnetization of the alloy as a function of temperature, magnetic field and pressure. Magnetic field and pressure have opposite effects on the martensitic transition in this alloy; the martensitic transition temperature decreases with increasing magnetic field but it increases with increasing pressure. The effect of pressure on the magnetocaloric properties of this large magnetocaloric effect alloy has been investigated in detail. The magnitude of the peak in the isothermal magnetic entropy change in Ni(50)Mn(34)In(16) increases with pressure. The temperature at which the magnetocaloric effect reaches the peak value in this alloy increases from near 240 K under ambient pressure to near 280 K under an external pressure of 9.5 kbar. The temperature corresponding to the peak in the isothermal magnetic entropy change increases with increasing pressure at a rate which matches the rate of increase of the martensite start temperature with increasing pressure. The temperature dependence of the isothermal magnetic entropy change under different pressures is found to follow a universal curve for a particular magnetic field change. These results show that pressure as a control parameter can be used to tune the temperature regime of the magnetocaloric effect in the alloy. The effect of pressure on the martensitic transition also gives a clue as regards the possibility of tuning this temperature regime with elemental substitution.

Temperature and magnetic field induced multiple magnetic transitions in DyAg(2).

The magnetic properties of the rare-earth intermetallic compound DyAg(2) are studied in detail with the help of magnetization and heat capacity measurements. It is shown that the multiple magnetic phase transitions can be induced in DyAg(2) both by temperature and magnetic field. The detailed magnetic phase diagram of DyAg(2) is determined experimentally. It was already known that DyAg(2) undergoes an incommensurate to commensurate antiferromagnetic phase transition close to 10 K. The present experimental results highlight the first order nature of this phase transition, and show that this transition can be induced by magnetic field as well. It is further shown that another isothermal magnetic field induced transition or metamagnetic transition exhibited by DyAg(2) at still lower temperatures is also of first order nature. The multiple magnetic phase transitions in DyAg(2) give rise to large peaks in the temperature dependence of the heat capacity below 17 K, which indicates its potential as a magnetic regenerator material for cryocooler related applications. In addition it is found that because of the presence of the temperature and field induced magnetic phase transitions, and because of short range magnetic correlations deep inside the paramagnetic regime, DyAg(2) exhibits a fairly large magnetocaloric effect over a wide temperature window, e.g., between 10 and 60 K.

Glassy dynamics in magnetization across the first order ferromagnetic to antiferromagnetic transition in Fe0.955Ni0.045Rh.

We present the results of magnetization relaxation measurements across the ferromagnetic to antiferromagnetic transition in Fe(0.955)Ni(0.045)Rh. The transition from the high temperature ferromagnetic phase to the low temperature antiferromagnetic phase seems to be arrested by increasing the applied magnetic field. The crossover from crystallization-like dynamics to glassy dynamics can be tracked by measuring isothermal time dependent magnetization at various constant temperatures while cooling across this ferromagnetic to antiferromagnetic transition. The initial conversion from the ferromagnetic to antiferromagnetic phase as a function of time at higher temperatures follows a distinct power law relaxation. The transition is incomplete at low temperatures with the stretched exponential relaxation behaviour dominating over the power law, which is indicative of glassy dynamics or the arrest of the kinetics of the phase transition. In the intermediate temperature regime, the magnetic relaxation can be explained as a combination of both the power law and stretched exponential. The temperature dependence of the time constant of the stretched exponential follows the Arrhenius law which is usually observed in the case of strong glass-forming liquids.

Increase in oxidative stress at low temperature in an antarctic bacterium.

Association between cold stress and oxidative stress was demonstrated by measuring the activity of two antioxidant enzymes and the level of free radicals generated in two batches of cells of an Antarctic bacterium Pseudomonas fluorescens MTCC 667, grown at 22 and 4°C. Increase in oxidative stress in cells grown at low temperature was evidenced by increase in the activity of an enzyme and also in the amount of free radicals generated, in the cold-grown cells. The association between cold stress and oxidative stress demonstrated in this investigation bolsters the concept of interlinked stress response in bacteria.

A scanning Hall probe imaging study of the field induced martensite-austenite phase transition in Ni50Mn34In16 alloy.

The martensite to austenite phase transition in the off-stoichiometric Heusler alloy Ni(50)Mn(34)In(16) can be induced both by temperature change and by application of a magnetic field. We have used scanning Hall probe imaging to study the magnetic field induced martensite-austenite phase transition. The study provides clear visual evidence of the coexistence of the martensite and austenite phases across this field induced transition in both increasing and decreasing magnetic fields. Clear evidence of thermomagnetic history effects associated with the martensite-austenite phase transition is also obtained. Quantitative analysis of the magnetic field dependence of the volume fraction of the austenite phase in Ni(50)Mn(34)In(16) shows evidence of a nucleation and growth mechanism across the field induced martensite-austenite phase transition. The local M-H loops constructed from the Hall images indicate the presence of a landscape of the critical magnetic field (for the field induced transition) distributed over the sample volume and thus confirm the disorder influenced nature of this first-order magnetic phase transition.

Disorder influenced magnetic phase transition in the Ce(Fe 0.9 Ru 0.1)2 alloy.

We have studied a 10% Ru-doped CeFe(2) alloy, Ce(Fe(0.9)Ru(0.1))(2), through magnetization, magnetotransport, and heat capacity measurements. This study shows that, while this alloy is antiferromagnetic at low temperatures and paramagnetic at high temperatures, there exists evidence of ferromagnetic ordering in the intermediate temperature regime. We show here that with 10% Ru doping the first order magnetic transition observed in the Ce(Fe(1 - x)Ru(x))(2) alloys with x < 0.08 is reduced to a quasi-continuous phase transition. The characteristic thermomagnetic history effects associated with the ferromagnetic-antiferromagnetic phase transition in the Ce(Fe(1 - x)Ru(x))(2) alloys with x < 0.08 are not observed in the Ce(Fe(0.9)Ru(0.1))(2) alloy. This alloy continues to exhibit the large magnetoresistance and large magnetocaloric effect associated with this first order magnetic transition in the alloys with smaller Ru concentration, but it does not show any energy loss due to thermomagnetic hysteresis. The present work thus shows how the introduction of quenched disorder due to alloying effects may be used to tune the first order magnetic transition in a material for more efficient functional use.

The effect of substitution of Mn by Fe and Cr on the martensitic transition in the Ni50Mn34In16 alloy.

The potential shape memory alloy Ni(50)Mn(34)In(16) is studied with partial substitution of Mn with Fe and Cr to investigate the effect of such substitution on the martensitic transition in the Ni-Mn-In alloy system. The results of ac susceptibility, magnetization and electrical resistivity measurements show that while the substitution with Cr increases the martensitic transition temperature, the substitution with Fe decreases it. Possible reasons for this shift in martensitic transition are discussed. Evidence of kinetic arrest of the austenite to martensite phase transition in the Fe substituted alloys is also presented. Unlike the kinetic arrest of the austenite to martensite phase transition in the parent Ni(50)Mn(34)In(16) alloy which takes place in the presence of high external magnetic field, the kinetic arrest of the austenite to martensite phase transition in the Fe doped alloy occurs even in zero magnetic field. The Cr substituted alloys, on the other hand, show no signature of kinetic arrest of this phase transition.

Magnetic properties of the field-induced ferromagnetic state in MnSi.

We present results of dc magnetization measurements focusing on the magnetic properties of the field-induced ferromagnetic state in MnSi. The temperature dependence of saturation magnetization in this ferromagnetic state exhibits the signatures of both spin wave excitations and itinerant electron ferromagnetism. The Arrott plots obtained from the isothermal field dependence of magnetization, however, are found to be distinctly nonlinear and hence cannot be explained within a simple framework of itinerant electron magnetism.

Mechanism of bacterial adaptation to low temperature.

Survival of bacteria at low temperatures provokes scientific interest because of several reasons. Investigations in this area promise insight into one of the mysteries of life science - namely, how the machinery of life operates at extreme environments. Knowledge obtained from these studies is likely to be useful in controlling pathogenic bacteria, which survive and thrive in cold-stored food materials. The outcome of these studies may also help us to explore the possibilities of existence of life in distant frozen planets and their satellites.

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.