V. B. Naik

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 μ0H=60 mT which indicates a possible magnetoelectric coupling in GaFeO3 with a fractional change of Δe/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 sugge...

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...

Magnetic and calorimetric investigations of inverse magnetocaloric effect in Pr0.46Sr0.54MnO3

We investigated magnetic entropy change (ΔSm) in the A-type antiferromagnet Pr0.46Sr0.54MnO3 by magnetic and differential scanning calorimetry (DSC) methods. The field-induced antiferromagnetic to ferromagnetic transition is first-order in nature and is accompanied by a large change in the latent heat as evidenced by the DSC data. The ΔSm shows an inverse magnetocaloric effect (ΔSm=+9 J kg−1 K−1 for ΔH=7 T) around the Neel temperature (TN=210±2 K) by magnetic measurement, which closely agrees with the calorimetric results. It is suggested that the large positive ΔSm results from a field-induced structural transition that accompanies the destruction of antiferromagnetism.

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...

Normal and inverse magnetocaloric effects in ferromagnetic Sm0.6−xLaxSr0.4MnO3

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 ΔSm is negative around TC and it decreases from ΔSm = −6.2 Jkg−1K−1 for x = 0 to −4.2 Jkg−1K−1 for x = 0.6 for ΔH = 5 T. The inverse MCE occurs below T* and shows a maximum value of ΔSm = +1.07 Jkg−1K−1 at T = 10 K for ΔH = 5 T at x = 0.4. This composition also shows a significant normal MCE (ΔSm = −4 Jkg−1K−1) around TC = 320 K, along with a high refrigeration capacity of 214 Jkg−1, which makes it an interesting compound for room temperature magnetic refrigeration. The inverse MCE has been suggested to be caused by the antiferromagnetic coupling between Sm 4f and Mn 3d mo...

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.

Magnetically tunable rf wave absorption in polycrystalline La0.67Ba0.33MnO3

We investigated temperature and magnetic field dependent radio-frequency electromagnetic absorption in La0.67Ba0.33MnO3 by monitoring changes in resonance frequency (fr) and current (I) through a LC resonant circuit powered by an integrated chip oscillator. The ferromagnetic to paramagnetic transition at TC in zero magnetic field is accompanied by an abrupt increase in fr and I and they are tunable by small external magnetic field. We observed fractional changes as much as 46% in Δfr/fr and 23% in ΔI/I around TC under μ0H=0.1 T that can be exploited for low-magnetic field sensors and other 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.

Dynamical magnetotransport in Ln0.6Sr0.4MnO3 (Ln = La, Sm)

We compare radio frequency electrical transport in Ln0.6Sr0.4MnO3 (Ln = La and Sm). AC resistance (R) and reactance (X) were simultaneously measured as a function of temperature (T), magnetic field (H), and frequency (f). The R(T, H = 0 G) in La0.6Sr0.4MnO3 (LSMO) shows a broad insulator-metal transition around 247 K below the Curie temperature (TC) for f ≤ 2 MHz but a steplike increase at TC for f ≥ 3 MHz. Conversely, R(T, H = 0 G) in Sm0.6Sr0.4MnO3 exhibits a single peak at TC = 116±2 K for f = 0.1 MHz which splits into two peaks accompanied by a dip at TC for higher frequencies. While the low temperature peak shifts upward rapidly with the increasing H (μ0H > 1 T) for f = 5 MHz, the high temperature peak is insensitive to the magnetic field. A large ac magnetoresistance (ΔR/R= −32%) and magnetoreactance (ΔX/X = −21%) at a low magnetic field (H = 70 mT) is observed in LSMO around TC in contrast to a positive ΔR/R (=170%) and a negative ΔX/X (= −94%) at μ0H = 7 T. We discuss the possible origins of the o...

Magnetic and Magnetoabsorption Studies in Multiferroic

We report a comparative study of magnetic and magnetoabsorption properties of bulk and nanoparticles of Ga_2-xFe_xO₃ (x ≈ 1.25) compounds synthesized using sol-gel method. The average size of Ga_2-xFe_xO₃ nanoparticles is found to be 100 nm. We observe an enhancement in the ferrimagnetic transition temperature (T_C=354 K) of Ga_2-xFe_xO₃ nanoparticles compared to T_C=298 K of the bulk Ga_2-xFe_xO₃. The bulk and nanoparticles of Ga_2-xFe_xO₃ exhibit a large coercive field (H_C) of H_C= 0.95 and 0.8 T at T= 10 K, respectively and we observe that the H_C decreases dramatically with increasing temperature. 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. We discuss the possible origins of the observed effects in both bulk and nanoparticles of Ga_2-xFe_xO₃ compounds.