arXiv subjects

R. Rawat

Publications and source records attributed to R. Rawat.

At least 37 records · Page 2Linked to original sources

arXiv2016

Low field magnetoelectric effect in Fe substituted Co4Nb2O9

Co4Nb2O9 (CNO) having α-Al2O3 crystal structure with Co chains along c-direction shows gigantic magnetoelelctric coupling below antiferromagnetic ordering temperature of 27 K but above a spin flop field of 1.6 T. We have investigated structural, magnetic and magnetoelectric properties of Fe substituted (10% and 20%) samples and compared with the parent one. In fact magnetic and specific heat measurements have revealed an additional magnetic transition below 10 K and presence of short range magnetic ordering above ~ 50 K in parent as well as in Fe substituted samples. Linear magnetoelelctric and ferroelectric behaviours are evidenced in the Fe substituted samples where an electric field of 5 kV/m is sufficient to align the dipoles and the magnetoelelctric coupling is ensured for magnetic fields as low as 0.25 T, far below the spin flop field.

cond-mat.mtrl-sci↗

arXiv2015

Novel thermal effects across first order magnetic transition in Ta doped $HfFe_2$ intermetallic

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 $HfFe_2$, 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.

cond-mat.str-el↗

arXiv2014

Temperature evolution of magnetic and transport behavior in 5\textit{d} Mott insulator Sr$_2$IrO$_4$: Significance of magneto-structural coupling

We have investigated the temperature evolution of magnetism and its interrelation with structural parameters in perovskite-based layered compound Sr$_2$IrO$_4$, which is believed to be a $J_{eff}$ = 1/2 Mott insulator. The structural distortion plays an important role in this material which induces a weak ferromagnetism in otherwise antiferromagnetically ordered magnetic state with transition temperature around 240 K. Interestingly, at low temperature below around 100 K, a change in magnetic moment has been observed. Temperature dependent x-ray diffraction measurements show sudden changes in structural parameters around 100 K are responsible for this. Resistivity measurements show insulating behavior throughout the temperature range across the magnetic phase transition. The electronic transport can be described with Mott's two-dimensional variable range hopping (VRH) mechanism, however, three different temperature ranges are found for VRH, which is a result of varying localization length with temperature. A negative magnetoresistance (MR) has been observed at all temperatures in contrast to positive behavior generally observed in strongly spin-orbit coupled materials. The quadratic field dependence of MR implies a relevance of a quantum interference effect.

cond-mat.str-el↗

arXiv2014

Room temperature giant baroresistance and magnetoresistance and its tunability in Pd doped FeRh

We report room temperature giant baro-resistance ($\approx$128\%) in $Fe_{49}(Rh_{0.93}Pd_{0.07})_{51}$. With the application of external pressure and magnetic field the temperature range of giant baro-resistance ($\approx$600\% at 5K and 19.9 kbar and 8 Tesla) and magnetoresistance ($\approx$-85\% at 5K and 8 tesla) can be tuned from 5 K to well above room temperature. As the AFM state is stabilized at room temperature under external pressure, it shows giant room temperature magnetoresistance ($\approx$-55\%) with magnetic field. Due to coupled magnetic and latticel changes, the isothermal change in room temperature resistivity with pressure (in the absence of applied magnetic field) as well as magnetic field (under various constant pressure) can be scaled together to a single curve when plotted as a function of X = T + 12.8*H - 7.2*P.

cond-mat.str-el↗

arXiv2014

Field induced large magnetocaloric effect and magnetoresistance in ErNiSi

Large magnetocaloric effect (MCE) and magnetoresistance (MR) together with negligible hysteresis loss has been observed in ErNiSi compound, which undergoes metamagnetic transition at low temperatures. Magnetization, heat capacity and resistivity measurements confirm the metamagnetic transition. The maximum value of isothermal entropy change and MR for a field change of 50 kOe are found to be 19.1 J/kg K and -34 %. Large MCE with negligible magnetic hysteresis loss could make this material promising for low temperature magnetic refrigeration.

cond-mat.str-el↗

arXiv2014

Oxygen deficiency induced suppression of JT-distortion and stabilization of chargeordering in La0.2Sr0.8MnO3-d

Structural phase transition studies employing low-temperature transmission electron microscopy, and low-temperature x-ray diffraction have been carried out on nearly stoichiometric (δ=0.01) and off-stoichiometric (δ=0.12) versions of La0.2Sr0.8MnO3-δ manganite. The nearly stoichiometric La0.2Sr0.8MnO3-δ under goes a cubic to Jahn-Teller distorted C-type antiferromagnetic tetragonal phase-transition at 260K. For off-stoichiometric La0.2Sr0.8MnO3-δ the Jahn-Teller distorted tetragonal phase transition gets totally suppressed and the basic perovskite lattice remains cubic but shows a charge ordered phase. Stabilization of charge ordered phase in the absence of cooperative Jahn-Teller distortion has been attributed to coulomb-repulsion and Hunds coupling energy. The off-stoichiometric sample shows characteristically different physical properties. This has been realized through transport, magnetic and calorimetric measurements. A smooth crossover from variable range hopping transport to power-law dependence of resistivity (ρ) on temperature (T) i.e. ρ=C.T-α has been realized. This has been attributed to multistep inelastic tunneling through channels involving localized states around oxygen vacancy sites.

cond-mat.str-el↗

arXiv2014

Effect of oxygen off-stoichiometry on coupled structural and magnetic phase-transitions in La0.15Sr0.85MnO3-d (d = 0.02, 0.14)

Effect of oxygen off-stoichiometry on the structural and magnetic properties of La0.15Sr0.85MnO3-d (d = 0.02,0.14) has been studied employing low-temperature x-ray diffraction, calorimetry, electrical-transport and magnetization. The nearly stoichiometric composition under goesfirst order phase-transition from cubic to tetragonal P4/mmm at 238 K accompanied by magnetic and electrical phase-transitions. On the contrary the off-stoichiometric composition does not show any transition and remains cubic Pm-3m down to 80 K. Offstoichiometry induced oxygen-vacancy seems to suppress the cooperative JT distortion and possibly stabilize a G-type antiferromagnetic phase with localized cantedspins. Thus we demonstrate that controlling oxygen stoichiometry is a way of structure/property engineering in electron-doped-like manganites.

cond-mat.mtrl-sci↗

arXiv2013

Effect of Strain disorder on the magnetic glassy state in La5/8-yPryCa3/8MnO3 (y = 0.45) thin films

Present study reveals that the free energy landscape of the La5/8-yPryCa3/8MnO3 (LPCMO) system could be modified by elastic strain interaction in the epitaxial thin films. Epitaxial LPCMO thin films of various thicknesses are grown on LaAlO3 substrate by pulsed laser deposition. With increasing thickness, by virtue of island growth morphology, strain disorder is invoked in thin films during strain relaxation process. The length-scale of phase separation is found to be highly correlated with strain disorder. Magneto-transport measurements demonstrate that coherent strain stabilizes charge ordered insulating phase while strain disorder stabilizes metallic phase. Resistivity under cooling and heating in unequal field (CHUF) protocol exhibits lower value of freezing temperature for strain disordered films compared to bulk system. Raman spectroscopy reveals that the charge ordered insulating and ferromagnetic metallic phases are structurally dissimilar and possess monoclinic and rhombohedral like symmetries respectively. Interfaces between two phases strongly influence low temperature glassy metastable state resulting in different phase separation states in the LPCMO thin films.

cond-mat.str-el↗

arXiv2013

Crossover from injection to tunneling conduction mode and associated magneto-resistance in a single $Fe_{3}O_{4}$(111)/$Alq_{3}$/Co spin-valve device

We demonstrate interface energy level engineering, exploiting the modification in energy band structure across Verwey phase transition of $Fe_{3}O_{4}$ electrode, in a $Fe_{3}O_{4}$(111)/$Alq_{3}$/Co vertical spin-valve (SV) device. Experimental results on device characteristics I-V) study exhibit a transition in conduction mode from carrier injection to tunneling across Verwey transition temperature ($T_{V}$) of $Fe_{3}O_{4}$ electrode. Both giant magneto-resistance (GMR) and tunneling MR (TMR) have been observed in a single SV device as a function of temperature, below and above $T_{V}$, respectively. Appearance of GMR, accompanied by injection limited natural Schottky-like I-V characteristics, provide evidences of spin injection at electrode/$Alq_{3}$ interface and transport through molecular orbitals in this SV device. Features of TMR exhibit significant differences from that of GMR. This is due to the dominant hyperfine-field interaction in the multi-step tunneling regime. We have achieved room-temperature SV operation in our device. A phenomenological model for device operation has been proposed to explain the transition in the conduction mode and associated MR features across $T_{V}$. We propose that the tuning of charge gap at Fermi level across Verwey transition due to charge ordering on the octahedral iron sites of $Fe_{3}O_{4}$ results in a corresponding tuning of conduction mode causing this unique cross over from GMR to TMR in this ferrite-based organic SV.

cond-mat.mes-hall↗

arXiv2013

Low temperature high magnetic field $^{57}Fe$ M$\ddot{o}$ssbauer study of kinetic arrest in $Hf_{0.77}Ta_{0.23}Fe_2$

Low temperature high magnetic field $^{57}Fe$ M$\ddot{o}$ssbauer measurements were carried out on inter-metallic $Hf_{0.77}Ta_{0.23}Fe_2$ compound by following novel paths in H-T space. The ferromagnetic (FM) fraction at 5K and zero magnetic field is shown to depend on cooling field i.e., higher the field higher is the FM fraction. M$\ddot{o}$ssbauer spectra collected in the presence of 4 Tesla magnetic field shows that the antiferromagnetic (AFM) spins cant with respect to the applied magnetic field and hence, contribute to the total bulk magnetization in this compound. The data also show induced magnetic moment even at the 2a site of AFM phase. M$\ddot{o}$ssbauer spectra collected using CHUF (cooling and heating in un-equal magnetic fields) protocol shows re-entrant transition when sample is cooled in zero field and measured during warming in 4 Tesla showing FM state as the equilibrium state. The present work is the first microscopic experimental evidence for the de-vitrification of kinetically arrested magnetic state.

cond-mat.str-el↗

arXiv2012

Influence of local structure on magnetic properties of Layered Cobaltites PrBaCo_2O{5+δ}, δ> 0.5

The effect of local structure on the magnetic and transport properties of the layered perovskites has been investigated. The samples PrBaCo$_{2}$O$_{5+δ}$, ($δ$ = 0.80 and 0.67) though crystallize in the same 112 type tetragonal structure have different magnetic ground states. Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy has been employed to explain the changes in magnetic interactions based on the rotation and tilting of CoO polyhedra in these oxygen rich double perovskites.

cond-mat.str-el↗

arXiv2012

Concentration dependence in kinetic arrest of first order magnetic transition in Ta doped $HfFe_2$

Magnetic behavior of the pseudo-binary alloy $Hf_{1-x}Ta_xFe_2$ has been studied, for which the zero field ferromagnetic (FM) to antiferromagnetic (AFM) transition temperature is tuned near to T=0 K. These alloys show anomalous thermomagnetic irreversibility at low temperature due to kinetic arrest of the first order AFM-FM transition. All the three studied compositions show re-entrant transition in zero field cooled warming curve and anomalous non-monotonic variation of upper critical field in isothermal magnetization. The region in H-T space, where these features of kinetic arrest manifest themselves, increases with increasing Ta concentration.

cond-mat.str-el↗

arXiv2010

Effect of strain on phase separation and de-vitrification of magnetic glass state in thin films of La5/8-yPryCa3/8MnO3 (y=0.45)

We present our study of effect of substrate induced strain on the La5/8-yPryCa3/8MnO3 (y=0.45), thin films grown on LaAlO3, NdGaO3 and SrTiO3 susbtrates that show large scale phase separation. It is observed that unstrained films grown on NdGaO3 behave quite similar to bulk material but the strained film grown on SrTiO3 show melting of insulating phase to metallic phase at low temperatures. However, the large scale phase separation and metastable glass-like state is observed in all the films despite difference in substrate induced strain. The measurements of resistivity as a function of temperature under cooling and heating in unequal field (CHUF) protocol elucidates the presence of glass-like meta-stable phase generated due to kinetic arrest of first order transformation in all the films. Like structural glasses, these magnetic glass-like phase shows evidence of devitrification of the arrested charge order-antiferromagnetic insulator (CO-AFI) phase to equilibrium ferromagnetic metallic (FMM) phase with isothermal increase of magnetic field and/or iso-field warming. These measurements also clearly brought out the equilibrium ground state of this system to be FMM and metastable glass-like phase to be AFI phase.

cond-mat.str-el↗

arXiv2009

Excess specific heat and evidence of zero point entropy in magnetic glassy state of half-doped manganites

We show that specific heat C$_p$ has non-Debye behavior for glassy states in half-doped manganites. Irrespective of the magnetic order or electronic states, these magnetic glasses have higher C$_p$ compared to their equilibrium counterparts. The excess C$_p$ contributed by the glassy state varies linearly with temperature similar to conventional glasses indicating tunneling in the two-level systems. These glassy states show signature of zero point entropy. Magnetic glasses can be produced simply by different field cooling protocols and may be considered ideal magnetic counterpart of the conventional glass.

cond-mat.str-el↗

arXiv2009

Variation of T$_g$ with magnetic field in a magnetic glass

Glass-like arrest has recently been reported in various magnetic materials. As in structural glasses, the kinetics of a first-order transformation is arrested while retaining the higher-entropy phase as a non-ergodic state. We show visual mesoscopic evidence of the irreversible devitrification of the arrested antiferromagnetic-insulating phase in $Pr_{0.5}Ca_{0.5}Mn_{0.975}Al_{0.025}O_3$ to its equilibrium ferromagnetic-metallic phase with isothermal increase of magnetic field, similar to its iso-field devitrification on warming. The slope of $T_g$ vs magnetic field in half-doped manganites is shown to have a sign governed by Le Chatelier's Principle.

cond-mat.stat-mech↗

arXiv2008

Enhancement of Jc by Hf -Doping in the Superconductor MgB2: A Hyperfine Interaction Study

Measurements of the critical current density (Jc) by magnetization and the upper critical field (Hc2) by magnetoresistance have been performed for hafnium-doped MgB2. There has been a remarkable enhancement of Jc as compared to that by ion irradiation without any appreciable decrease in Tc, which is beneficial from the point of view of applications. The irreversibility line extracted from Jc shows an upward shift. In addition, there has been an increase in the upper critical field which indicates that Hf partially substitutes for Mg. Hyperfine interaction parameters obtained from time differential perturbed angular correlation (TDPAC) measurements revealed the formation of HfB and HfB2 phases along with the substitution of Hf. A possible explanation is given for the role of these species in the enhancement of Jc in MgB2 superconductor.

cond-mat.supr-con↗

arXiv2008

Magnetoresistance behavior of a ferromagnetic shape memory alloy: Ni_1.75Mn_1.25Ga

A negative-positive-negative switching behavior of magnetoresistance (MR) with temperature is observed in a ferromagnetic shape memory alloy Ni_1.75Mn_1.25Ga. In the austenitic phase between 300 and 120 K, MR is negative due to s-d scattering. Curiously, below 120K MR is positive, while at still lower temperatures in the martensitic phase, MR is negative again. The positive MR cannot be explained by Lorentz contribution and is related to a magnetic transition. Evidence for this is obtained from ab initio density functional theory, a decrease in magnetization and resistivity upturn at 120 K. Theory shows that a ferrimagnetic state with anti-ferromagnetic alignment between the local magnetic moments of the Mn atoms is the energetically favoured ground state. In the martensitic phase, there are two competing factors that govern the MR behavior: a dominant negative trend up to the saturation field due to the decrease of electron scattering at twin and domain boundaries; and a weaker positive trend due to the ferrimagnetic nature of the magnetic state. MR exhibits a hysteresis between heating and cooling that is related to the first order nature of the martensitic phase transition.

cond-mat.mtrl-sci↗

arXiv2007

Electronic and structural transition in $La_{0.2} Sr_{0.8} Mn O_3$

We investigate the interplay of the electronic and structural transition in La$_{0.2}$Sr$_{0.8}$MnO$_{3}$. The transport and specific heat measurements exhibit unusual evolutions and signature of a first order phase transition around 265 K. Mn K-edge extended $x$-ray absorption fine structure results reveal distortion in the MnO$_6$ octahedra even in the cubic phase and a remarkable evolution of the distortion across the phase transition. These results manifest the importance of fluctuations in Mn 3$d$ orbital occupancy and disorder in their electronic properties, which may help in understanding the orbital and spin ordering proposed in these systems.

cond-mat.str-el↗