arXiv subjects

R. Mahendiran

Publications and source records attributed to R. Mahendiran.

At least 37 records · Page 2Linked to original sources

arXiv2015

Large adiabatic temperature and magnetic entropy changes in EuTiO3

We have investigated the magnetocaloric effect in single and polycrystalline samples of quantum paraelectric EuTiO3 by magnetization and heat capacity measurements. Single crystalline EuTiO3 shows antiferromagnetic ordering due to Eu2+ magnetic moments below TN = 5.6 K. This compound shows a giant magnetocaloric effect around its Neel temperature. The isothermal magnetic entropy change is 49 Jkg-1K-1, the adiabatic temperature change is 21 K and the refrigeration capacity is 500 JKg-1 for a field change of 7 T at TN. The single crystal and polycrystalline samples show similar values of the magnetic entropy change and adiabatic temperature changes. The large magnetocaloric effect is due to suppression of the spin entropy associated with localized 4f moment of Eu2+ ions. The giant magnetocaloric effect together with negligible hysteresis, suggest that EuTiO3 could be a potential material for magnetic refrigeration below 20 K.

cond-mat.mtrl-sci↗

arXiv2013

Giant magnetothermopower in charge ordered Nd0.75Na0.25MnO3

We report magnetization, resistivity and thermopower in the charge-orbital ordered antiferromagnet Nd0.75Na0.25MnO3. Magnetic-field induced collapse of antiferromagnetism is found to be accompanied by a giant negative magnetothermopower (= 80-100% for a field change of 5T) over a wide temperature (T = 60-225K) and giant magnetoresistance. While the field-induced metamagnetic transition in magnetization is reversible upon field-cycling at T > 40 K, it is irreversible at lower temperatures and this has impact on magnetoresistance, magnetothermopower as well as change in the temperature of the sample. Our results indicate high sensitivity of thermopower to changes in the magnetic state of the sample.

cond-mat.mtrl-sci↗

arXiv2013

Giant magnetocaloric effect in magnetoelectric Eu1-xBaxTiO3

We report the magnetic entropy change (Delta Sm) in magnetoelectric Eu1-xBaxTiO3 for x = 0.1- 0.9. We find - delta Sm = 11 (40) J/kg.K in x = 0.1 for a field change of 1 (5) Tesla respectively, which is the largest value among all Eu-based oxides. Delta Sm arises from the field-induced suppression of the spin entropy of Eu2+:4f7 localized moments. While -delta Sm decreases with increasing x, -DeltaSm = 6.58 J/kg.K observed in the high spin diluted composition x = 0.9 is larger than that in many manganites. Our results indicate that these magnetoelectrics are potential candidates for cryogenic magnetic refrigeration.

cond-mat.mtrl-sci↗

arXiv2012

Tunable spin reorientation transition and magnetocaloric effect in Sm0.7-xLaxSr0.3MnO3 series

We report electrical resistivity, magnetic and magnetocaloric properties in Sm0.7-xLaxSr0.3MnO3 series for x= 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.65, and 0.7. All the compounds show second order paramagnetic to ferromagnetic transition at T = Tc which is tunable anywhere between 83 K and 373 K with a proper choice of the doping level (x). The insulating ferromagnet x= 0 transforms to ferromagnetic metal below Tc for x= 0.1 and the insulator-metal transition temperature shifts up with increasing x. The magnetization (M) exhibits an interesting behavior as a function of temperature and doping level. The field-cooled M(T) of all but x= 0.7 compound show a cusp at a temperature T* much below Tc. While the Tc increases monotonically with increasing x, T* increases gradually, attains a maximum value (T*= 137 K) for x= 0.6 and decreases rapidly thereafter. It is suggested that the decrease of M(T) below T* is due to ferrimagnetic interaction between Sm(4f) and Mn(3d) sublattices that promotes spin-reorientation transition of the Mn-sublattice. The observed anomalous feature in M(T) does not have impact on the dc resistivity. Magnetic entropy change (delta Sm) was estimated from magnetization isotherms. The sign of delta Sm is found to change from negative above T* to positive below T* indicating the coexistence of normal and inverse magnetocaloric effects. delta Sm is nearly composition independent (-delta Sm is about 1.5 J/Kg K for delta H = 1 Tesla) and refrigeration capacity lies between 50 and 80 J/kg K for x = 0.1-0.6. The tunability of Curie temperature with a nearly constant delta Sm value along with high refrigeration capacity makes this series of compounds interesting for magnetic refrigeration over a wide temperature range.

cond-mat.mtrl-sci↗

arXiv2012

Magnetocaloric effect and Magnetothermopower in the room temperature ferromagnet Pr0.6Sr0.4MnO3

We have investigated magnetization(M), magnetocaloric effect(MCE) and magnetothermopower(MTEP) in polycrystalline Pr0.6Sr0.4MnO3, which shows a second-order paramagnetic to ferromagnetic transition near room temperature (TC = 305 K). However, field-cooled M(T) within the long range ferromagnetic state shows an abrupt decrease at TS = 86 K for H < 3 T. The low temperature transition is first-order in nature as suggested by the hysteresis in M(T) and exothermic/endothermic peaks in differential thermal analysis for cooling and warming cycles. The anomaly at TS is attributed to a structural transition from orthorhombic to monoclinic phase. The magnetic entropy change is negative at TC but changes to positive at TS. Thermopower (Q) is negative from 350 K to 20 K, shows a rapid decrease at TC and a small cusp around TS in zero field. The MTEP reaches a maximum value of 25% for deltaH = 3 T around TC which is much higher than 15% dc magnetoresistance for the same field change. A linear relation between MTEP and magnetoresistance, and between delta Sm and Delta Q are found near TC. Further, ac magnetotransport in low dc magnetic fields (H less than or equal to 1 kOe), critical analysis of the paramagnetic to ferromagnetic transition and scaling behavior of the magnetic entropy change versus a reduced temperature under different magnetic fields are also reported.

cond-mat.str-el↗

arXiv2011

Inverse magnetocaloric effect in ferromagnetic Sm0.6-xLaxSr0.4MnO3 due to 4f-3d exchange interaction

We report magnetic and magnetocaloric properties of Sm0.6-xLaxSr0.4MnO3 (x = 0-0.6). A rapid increase around TC and an anomalous peak at a temperature T*<<TC occur in magnetization which lead to normal and inverse magnetocaloric effects (MCE), respectively. While TC increases with increasing x (TC=118 K for x=0 and TC=363 K for x=0.6), T* increases from 30 K (x=0) to 120 K (x=0.4) and then decreases to 105 K (x=0.5). The \DeltaSm reaches +1.07 Jkg-1K-1 at 10 K and -4 Jkg-1K-1 around TC in x=0.4 for ΔH=5T. The inverse MCE is attributed to antiferromagnetic coupling between Sm-4f and Mn-3d magnetic moments.

cond-mat.mtrl-sci↗

arXiv2009

Magnetothermal cooling with a phase separated manganite

We show that temperature of a current (I = 20 mA) carrying manganite (Nd0.5Ca0.5Mn0.93Ni0.07O3) in presence of a magnetic field (H) decreases abruptly as much as deltaT = 45 K (7 K) accompanied by a step like decrease in magnetoresistance at a critical value of H when the base temperature is 40 K (100 K). The magnitude of deltaT and the position of magnetoresistance step decrease towards lower H with decreasing amplitude of the current. We discuss possible origins of the current and magnetic- field driven temperature change which may find applications in magnetothermal refrigeration besides magnetocaloric effect.

cond-mat.mtrl-sci↗

arXiv2009

Current driven discontinuous insulator-metal transition and colossal low-field magnetoresistance in Sm0.6Sr0.4MnO3

It is shown that with increasing magnitude of current (I), resistivity of Sm0.6Sr0.4MnO3 transforms from a smooth to a discontinuous insulator-metal transition which is also accompanied by an abrupt decrease in temperature of the sample. We report colossal low-field magnetoresistance under a high current bias (-99% at H = 0.5 T and 70 K) and electroresistance (-8000 % at H = 0 T and 60 K) for I = 11 mA. We interpret our observations in terms of current induced supercooling of the high temperature paramagnetic phase and enlargement of volume fraction of the ferromagnetic phase under a magnetic field.

cond-mat.mtrl-sci↗

arXiv2009

Influence of length and measurement geometry on magnetoimpedance in La0.7Sr0.3MnO3

We show that ac magnetoresistance at room temperature in La0.7Sr0.3MnO3 is extremely high (= 47% in H = 100 mT, f = 3-5 MHz), and magnetic field dependence of reactance exhibits a double peak behavior. However, magnitudes of the ac magnetoresistance and magnetoreactance for a fixed length of the sample (li) decrease with decreasing separation (lv) between voltage probes unlike the dc magnetoresistance. On the contrary, change in li has a negligible influence on magnetoimpedance when lv is fixed. Our results indicate that high frequency electrical transport is sensitive to local variations in the magnetic permeability.

cond-mat.mtrl-sci↗

arXiv2009

Electric field-induced colossal electroresistance and its relaxation in multiferroic La2NiMnO6

In this work, we report direct as well as pulsed electric field-induced resistivity switching and its relaxation in a multiferroic insulator La2NiMnO6. At a fixed base temperature (Tb), the dc resistivity switches abruptly from a high to a low value, which is manifested as an upward jump in the dc current density (J) when the electric field (E) exceeds a threshold value Eth. The fractional change in the resistance is as much as 70 % at room temperature for Eth = 95 V/cm. The Eth increases with lowering Tb and follows the relation Eth(Tb) = Eth(0)exp[-Tb/T0], as similar to the behavior found in charge density wave systems. It is shown that the abrupt jump in J vanishes under pulsed electric fields if the period between pulses is long enough. Surprisingly, a step-like increase in J also occurs at a fixed dc electric field (Ec) and T = Tb, above a threshold waiting time (tth). The tth decreases with increasing Ec and Tb. Simultaneous measurement of surface temperature during the J-E sweep and temporal studies suggest that conductive channels are created in an insulating matrix due to the local self heating, and the coalescence of these channels above a threshold E- field or time causes the observed anomalies in J. However, the dissipated Joule power (P = Ith2R) at the transition from high to low resistive state in the sample decreases with lowering temperature, which suggests that the Joule heating is the consequence of transition from the high to low resistance state rather than itself a driving force of the non linear electrical transport. In addition, non linear J-E characteristics is also found even with a pulsed voltage sweep, which suggests that intrinsic mechanisms other than self heating is still active in this material.

cond-mat.mtrl-sci↗

arXiv2009

Effect of Fe substitution on magnetocaloric effect in La0.7Sr0.3Mn1-xFexO3 (x = 0 to 0.2)

We have studied the effect of Fe substitution on magnetic and magnetocaloric properties in La0.7Sr0.3Mn1-xFexO3 (x = 0.05, 0.07, 0.10, 0.15, and 0.20) over a wide temperature range (T = 10-400 K). It is shown that substitution by Fe gradually decreases the ferromagnetic Curie temperature (Tc) and saturation magnetization up to x = 0.15 but a dramatic change occurs for x = 0.2. The x = 0.2 sample can be considered as a phase separated compound in which both short-range ordered ferromagnetic and antiferromagnetic phases coexist. The magnetic entropy change (DSm) was estimated from isothermal magnetization curves and it decreases with increase of Fe content from 4.4 J/KgK at 343 K (x = 0.05) to 1.3 J/KgK at 105 K (x = 0.2), under H = 5 T. The La0.7Sr0.3Mn0.93Fe0.07O3 sample shows negligible hysteresis loss, operating temperature over 60 K range around room temperature with refrigerant capacity of 225 J/Kg, and magnetic entropy of 4 J/kgK which will be an interesting compound for application in room temperature refrigeration.

cond-mat.mtrl-sci↗

arXiv2009

A large magnetoinductance effect in La0.67Ba0.33MnO3

We report four probe impedance of La0.67Ba0.33MnO3 at f = 100 kHz under different dc bias magnetic fields. The ac resistance (R) exhibits a peak around Tp = 325 K which is accompanied by a rapid increase and a peak in the reactance (X) in a zero field. The magnetoreactance exhibits a sharp peak close to Tp and its magnitude (= 60% in H = 1 kG) exceeds that of the ac magnetoresistance (= 5 % inH = 1 kG). It is suggested that the magnetoreactance arises from changes in the self inductance of the sample rather than the capacitance.

cond-mat.mtrl-sci↗

arXiv2009

Electrical, magnetic, magnetodielectric and magnetoabsorption studies in multiferroic GaFeO3

We report electrical, magnetic, magnetodielectric and magnetoabsorption properties of a polycrystalline GaFeO3. The resistivity measurement shows that the sample is highly insulating below 200 K and the resistivity above 200 K obey the Arrhenius law with an activation energy of Ea = 0.67 eV. An anomaly occurs in the temperature dependence of permittivity (e) near the ferrimagnetic transition temperature (TC = 228 K) in a zero magnetic field and it is suppressed under H = 60 mT which indicates a possible magnetoelectric coupling in GaFeO3 with a fractional change of de/e = -1.8% at 60 mT around TC. The coercivity (HC) of the sample increases dramatically with lowering temperature below 200 K from 0.1 T at 200 K to 0.9 T at 5 K. Magnetoabsorption was studied with a LC resonance technique and we found a close correlation between the shift in the resonance frequency due to applied magnetic field and the coercive field measured using dc magnetization measurements. Our results obtained with multiple techniques suggest that GaFeO3 is an interesting ferrimagnet with potential applications in future multiferroic devices.

cond-mat.mtrl-sci↗

arXiv2009

Huge ac magnetoresistance in La0.7Sr0.3MnO3 in sub- kilo gauss magnetic fields

We report dynamical magnetotransport in a ferromagnetic metallic oxide, La0.7Sr0.3MnO3 using the ac impedance technique. The temperature dependence of the ac resistance(R) and the inductive reactance (X) of the complex impedance (Z = R+jX) under different dc bias magnetic fields (Hdc = 0-1 kOe) were studied for different frequencies f = 0.1 to 5 MHz of alternating current. The zero field R, which decreases smoothly around the Curie temperature TC for f = 100 kHz, transforms into a peak for f = 0.5-5 MHz. The peak decreases in amplitude, broadens and shifts downward in temperature as the bias field increases. A huge ac magnetoresistance (= 45 % at f = 2 MHz) in a field of Hdc = 1 kOe is found and we attribute it to the magnetic field- induced enhancement in the skin depth and concomitant suppression of magnetic fluctuations near TC. Our study suggests that radio frequency magnetotransport provides an alternative strategy to enhance the magnetoresistance and probe the spin-charge coupling in manganites.

cond-mat.mtrl-sci↗

arXiv2009

Current-induced electroresistance in Nd0.5Ca0.5Mn0.95Ni0.05O3

We have investigated the dc and pulsed current-induced electroresistance in phase separated manganite Nd0.5Ca0.5Mn0.95Ni0.05O3 (NCMONi05) as a function of temperature and magnetic field. It is shown that the negative differential resistance which appears above a threshold current (Ic) and hysteresis in the V-I progressively vanish with increasing period of the current pulses. However a strong non-linearity in V-I exists even for a pulse period of 6s. The peak voltage at Ic decreases in magnitude and shifts towards higher current values with increasing strength of the magnetic field. The strong nonlinear behavior and the negative differential resistance in the dc current sweep are accompanied by a rapid increase of the sample surface temperature and therefore primarily arise from the Joule heating in the sample. While the Joule heating assists electroresistance in the high dc current regime, the nonlinearity in the pulsed current sweep and the resistivity switching between a high and low value induced by controlling the width and period of pulses can not be explained solely on the basis of Joule heating.

cond-mat.mtrl-sci↗

arXiv2009

Spontaneous and low-field magnetoimpedance in La0.7Sr0.3CoO3 and La1-xSrxMnO3 (x = 0.18-0.5)

We report ac electrical transport in La0.7Sr0.3CoO3 and La1-xSrxMnO3 (x = 0.18- 0.5) as a function of temperature and frequency in H = 0 and 60 mT. Both resistive (R) and reactive (X) components of impedance (Z = R+jX) were measured. It is shown that a smooth decrease of the resistance around the Curie temperautre in La0.7Sr0.3CoO3 transforms into an abrupt increase followed closely by a peak in R with increasing frequency. Similar behavior in R was also found in metallic compositions of the La-Sr-MnO3 series. The observed anomaly at TC is suppressed by a small dc magnetic field of H = 60 mT which rrsults in a huge ac magnetoreistance (= 30 % in manganite and =7 % in cobaltite at f = 2 MHz). The anomaly in R in zero field is also accompanied by a sudden incrases of X. While the ac magnetoresitance is negative as a function of field from 100 Hz to 30 MHz, we show that the sign of X changes and a new double peak structure develops in X for frequencies above 15 MHz.

cond-mat.mtrl-sci↗

arXiv2009

Magnetic and magnetoelectric studies in pure and cation doped BiFeO3

We report magnetic and magnetoelectric studies on BiFeO3 and divalent cation (A) suvtitute Bi0.7A0.3FeO3 (A = Sr,Ba, and Sr0.5Ba0.5). It is shown that the rapid increase of magnetization at the Neel temperature (TN = 642 K) is suppressed in the co-doped compound A = Sr0.5Ba0.5. All the divalent subtituted compounds show enhanced magnetization and hysteresis loop. Both longitudinal and transverse magnetoelectric coefficients were measured using the dynamical lock-in technique. The co-doped compound shows the highest magnetoelectric coefficient at room temperature although it is not the compound with the highest saturation magnetization. It is found that as the size of the A-site cation increses, the transverse magnetoelectric coeffient increases and exceeds the longitudinal magnetoelectric coefficient. It is suggested that changes in magnetic domain structure and magnetostriction are possible reasons for the observed changes in the magnetoelectric coefficients.

cond-mat.mtrl-sci↗

arXiv2009

Pulse width controlled resistivity switching at room temperature in Bi0.8Sr0.2MnO3

We report pulsed as well as direct current/voltage induced electroresistance in Bi0.8Sr0.2MnO3 at room temperature. It is shown that bi-level and multi-level resistivity switching can be induced by a sequence of pulses of varying pulse width at fixed voltage amplitude. Resistivity increases abruptly (= 55 % at 300 K) upon reducing pulse width from 100 ms to 25 ms for a fixed electric field (E = 2 V/cm2) of 200 ms pulse period. The resistivity switching is accompanied by a periodic change in temperature which alone can not explain the magnitude of the resistivity change.

cond-mat.mtrl-sci↗