A. Rebello

Large reversible magnetocaloric effect in La0.7-xPrxCa0.3MnO3

We report magnetocaloric effect in polycrystalline La0.7−xPrxCa0.3MnO3 (x = 0.0, 0.25, 0.3, 0.35, 0.4, and 0.45) samples. The magnetic entropy change (ΔSm) was measured using a differential scanning calorimeter (DSC) working in a magnetic field environment. The ΔSm shows a peak around the ferromagnetic Curie temperature (TC), and the magnitude of the peak decreases with increasing x (ΔSm = 8.15, 7.27, 6.92, 6.73, 6.41, and 5.84 Jkg−1K−1 for x = 0, 0.25, 0.3, 0.35, 0.4, and 0.45, respectively, for a field change of ΔH = 5 T). We have studied electrical, magnetoresistance, and magnetic properties of x = 0.35 compound in detail. The paramagnetic-ferromagnetic transition in this compound is found to be first order in nature. Magnetization isotherms show a field-induced metamagnetic transition in the paramagnetic (PM) state, and it is accompanied by a change in latent heat, as evidenced by the DSC data. Magnetization data give ΔSm = −7.09 Jkg−1K−1 at T = 190 K and a relative cooling power of 306.5 Jkg−1 for ΔH...

Huge ac magnetoresistance of La0.7Sr0.3MnO3 in subkilogauss magnetic fields

We report the electrical and magnetotransport behavior of a ferromagnetic metallic oxide, La0.7Sr0.3MnO3, in response to radio frequency current passing through the sample. We have studied the temperature dependence of the ac resistance (R) and inductive reactance (X) under different dc bias magnetic fields (Hdc=0–1 kOe) for different frequencies (f) of radio frequency current from f=0.1 to 5 MHz. 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, shifts downward in temperature as the bias field increases, and is completely suppressed under Hdc=1 kOe when f=0.5 MHz. The ac magnetoresistance and magnetoinductance exhibit a peak close to the TC. A huge low-field ac magnetoresistance (ΔR/R=40%) and magnetoinductance (ΔX/X=12%) are found in a field of Hdc=700 Oe and f=2 MHz. It has been suggested that the observed ac magnetoresistance has its origin in the suppression of spin fluctuation n...

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

We show that with increasing magnitude of current (I), insulator-metal (I-M) transition in Sm0.6Sr0.4MnO3 becomes discontinuous and shifts down in temperature, which is accompanied by an abrupt decrease in temperature of the sample while cooling. The I-M transition exhibits a hysteresis while warming. A magnetic field of μ0H=0.5 T results in −99% magnetoresistance for I=11 mA compared to −32% for I=1 mA. Our results underscore the importance of inhomogeneous Joule heating that leads to coexistence of the high temperature paramagnetic phase with the low temperature ferromagnetic phase over a wide temperature range.

Unusual field dependence of radio frequency magnetoimpedance in La0.67Ba0.33MnO3

Magnetic-field dependence of the ac impedance Z(f,H)=R(f,H)+jX(f, H) in La0.67Ba0.33MnO3 carried out over a wide frequency range (f=0 to 30 MHz) reveals a huge low-field ac magnetoresistance, ΔR/R=-55% at f=15 MHz and magnetoreactance, ΔX/X=-80% at f=3 MHz in μ0H=100 mT at room temperature. We show contrasting evolution of ΔR/R and ΔX/X with increasing magnetic field and frequency. While ΔR/R is negative and shows a single peak at μ0H=0 T for all but f=30 MHz, the single peak in ΔX/X transforms into a valley at the origin and a double peak at H=±Hk which shifts upward in H with increasing frequency. The ΔX/X eventually changes sign from negative to positive above 25 MHz. The observed features in ΔX/X suggest possible occurrence of ferromagnetic resonance in MHz range.

Magnetic and calorimetric studies of magnetocaloric effect in La0.7-xPrxCa0.3MnO3

We report magnetocaloric effect in La0.7 − xPrxCa0.3MnO3 (x = 0, 0.2, 0.3, and 0.4). All these compounds undergo first-order paramagnetic to ferromagnetic transition upon cooling and show field-induced metamagnetic transition (FIMMT) in the paramagnetic state. The FIMMT is accompanied by a release of latent heat and change in temperature of the sample as evidenced from differential scanning calorimetry and thermal analysis data for x = 0.3. The magnetic entropy decreases (−ΔSm = 8.23, 8.1, 7, and 5.38 Jkg−1 K−1 for a field change of ΔH = 5 T, for x = 0, 0.2, 0.3, and 0.4, respectively) and refrigeration capacity (RC) increases with increasing x (RC = 197, 215, 240, and 259 J/kg for x = 0, 0.2, 0.3, and 0.4, respectively). We suggest that collapse of magnetic polarons in the paramagnetic state and magnetovolume effect are responsible for the observed FIMMT and large −ΔSm values.

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 bilevel and multilevel resistivity switching can be induced by a sequence of pulses of varying pulse width at a fixed voltage amplitude. Resistivity increases abruptly (≈55% at 300 K) upon reducing pulse width from 100 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 cannot explain the magnitude of the resistivity change.

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 μ0H=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.

Robust electric-field tunable opto-electrical behavior in Pt-NiO-Pt planar structures

Capacitor-like metal-NiO-metal structures have attracted large interest in non-volatile memory applications based on electric field control of resistance, known as resistive switching (RS). Formation of conducting nanofilaments by the application of an electric field (electroformation) is considered an important pre-requisite for RS. Besides RS, due to the wide band gap and p-type semiconducting nature, NiO has been used to fabricate heterojunctions for photodetector applications. However, very little is known about the electrical and opto-electrical properties of NiO films in planar structure. Here, we demonstrate intriguing photoresponse and electrical behavior in electroformed Pt-NiO-Pt planar structures. While the pristine devices show ohmic electrical behavior and negligible photoresponse, the electroformed devices exhibit a nonlinear rectification behavior and a remarkable photoresponse at low voltage biases. More interestingly, the devices show a dramatic change of sign of rectification under light illumination at higher voltage biases. A polarity dependent and robust gain phenomenon is demonstrated in these devices. The large sensitivity, fast response, simple design and ease of preparation of these planar structures make them attractive for integration with current circuit technologies and various novel opto-electrical applications.

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

We report four probe impedance (Z=R+iX) 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 (ΔX/X) exhibits a sharp peak close to TP and its magnitude (≈60% in H=1 kG) exceeds that of the ac magnetoresistance (ΔR/R=5%). It is suggested that the magnetoreactance arises from changes in the self inductance of the sample rather than the capacitance.

Large and robust resistive switching in co-sputtered Pt-(NiO-Al2O3)-Pt devices

We have systematically investigated the resistive switching and electroresistance behavior in Pt-[NiO-Al2O3]-Pt (PNAP) capacitor-like structures. The PNAP devices show a large ON-OFF ratio (∼107), which is strongly dependent on the rate of the voltage sweep. Interestingly, the devices exhibit a robust electroresistance behavior in the high resistance OFF state and show an intriguing change of sign of rectification with increasing end voltage. Our direct measurement of the surface temperature of the sample during resistive switching indicates that RESET process is assisted by Joule heating effects. The results are explained on the basis of plausible interplay between Schottky barrier modification due to the trapped charge carriers at the metal–oxide interface and percolation effects of conducting nanofilaments.