Shilpi Karmakar

Magnetocaloric effect in charge ordered Nd0.5Ca0.5MnO3 manganite

The magnetocaloric effect (MCE) has been observed in three well-defined temperature intervals (region I: 2–115 K, region II: around charge order transition TCO, and region III: around room temperature) in polycrystalline Nd0.5Ca0.5MnO3 (NCMO) manganite system showing a first order (CO) transition at a temperature, TCO∼250 K. The magnitude of ΔSM(H) increases monotonically with applied magnetic field but does not reach saturation even at fields as high as 60 kOe. In the three given temperature regions, negligible magnetic and thermal hysteresis are found, which satisfies the requirements of using NCMO as an effective magnetocaloric material for magnetic refrigeration.

Origin of low temperature memory and aging effects in spin glass like La0.7Ca0.3MnO3 nanomanganite

Interesting low temperature memory phenomena have been observed from equilibrium and out of equilibrium magnetic measurements on the La0.7Ca0.3MnO3 nanomanganite system. The observed phenomenon were screened for atomic spin glass (SG), super spin glass (SSG), cluster glass (CG), and superparamagnetic behavior. The results evidences of SG like behavior at low temperature (<40 K) in this manganite system consisting of ferromagnetic nanoparticles. In the temperature region between 40 K and the ferromagnetic Curie point TC∼217 K, a ferromagnetic CG state develops with a relatively weaker interparticle interaction than that of the low temperature SG phase. The dynamic magnetization shows aging, chaos and memory effects. Moreover, we have also noticed asymmetric response in magnetic relaxation in response to positive and negative temperature cycling protocols. The origin and nature of the low-temperature SG state in this system is discussed within the framework of hierarchical organization of metastable states....

Conduction mechanism and dielectric relaxation in high dielectric KxTiyNi1−x−yO

Complex impedance spectroscopic study has been made to elucidate the conductivity mechanism and dielectric relaxations in a low loss giant dielectric (e′∼104) KxTiyNi1−x−yO (KTNO) system with x=0.05–0.30 and y=0.02 over a wide temperature range (200–400K). Below ambient temperature (300K), dc conductivity follows variable range hopping mechanism. The estimated activation energy for dielectric relaxation is found to be higher than the corresponding polaron hopping energy, which is attributed to the combined effect of K-doped grains and highly disordered grain boundary (GB) contributions in KTNO. Observed sharp fall of e′ below ∼270K is ascribed to the freezing of charge carriers. Comparatively lower value of relaxation time distribution parameter β of KTNO than that of the CaCu3Ti4O12 (CCTO) system reveals more disorder in KTNO. It is also found that KTNO is structurally more stable compared to the CCTO system, both having giant e′ value.

Ferro-antiferromagnetic coupling and unusual transport properties of ferro-antiferromagnetic (100−x) La0.7Pb0.3MnO3+xPr0.63Ca0.37MnO3 (x=0–85wt%) composites

We report a low field (H=0.5T) magnetoresistance of colossal magnetoresistive (CMR) composites prepared with ferromagnetic (FM)—antiferromagnetic (AFM) coupled system (100−x) La0.7Pb0.3MnO3(LPMO)+xPr0.63Ca0.37MnO3 with x=0–85wt%. A large increase (∼16% maximum) of magnetoresistance (MR) (∼10% for pure LPMO) is observed at a relatively higher optimum concentration (x=70% AFM). Interesting double peaks are observed for lower AFM concentrations (x⩽35%) in the resistivity-temperature [ρ(T)] curve: one due to the usual intrinsic double exchange interaction (between Mn3+ and Mn4+) mechanism and the other due to the stronger FM-AFM coupling for lower concentration. Unlike the usual ceramic CMR material, thermoelectric power (TEP) in this system is also nonadditive in character, indicating the presence of additional defects and strain in this composites. A two-channel (conducting and semiconductor) model has been used which fit well with the ρ(T) data near the optimum concentration x=70%. Around this highest AFM ...

Small polaron hopping conduction and magnetic frustration in the electron-doped charge ordered Ca0.85La0.15MnO3 system

The electron-doped Ca0.85La0.15MnO3 system exhibits a charge ordering transition at ~135 K (TCO) with the antiferromagnetic Neel temperature at 160 K (TN). The resistivity data above and below TN (down to 90 K) are found to follow the small polaron hopping conduction mechanism. The corresponding activation energy, estimated from the theoretical fitting of resistivity data, shows an anomalous increase below TN, supporting the aforementioned charge ordering phenomenon. This result is further confirmed from fitting of the current–voltage (I–V) data with the small polaron hopping model. As revealed from the field cooled and zero field cooled magnetization measurements, this electron-doped system exhibits the signature of magnetic frustration, below TCO associated with phase separation behavior.

A study of the magnetic, thermal, and optical absorption properties in Ho(OH)3 crystal

This paper reports the result of our experimental study on the mean susceptibilities K and also the molar anisotropies K∥–K⊥ between 300 and 80 K in pure Ho(OH)3 crystals. In our previous study we had shown that for Ho3+ compounds having C3h symmetry, the thermal variation of the magnetic anisotropies from 300–80 K are sensitively dependent on the detailed pattern of the crystal‐field levels of the lowest 5I8 term, i.e., on the crystal‐field parameters (CFP). Thus by fitting the experimental results of K, K∥–K⊥, K∥ and K⊥ K⊥ simultaneously in the temperature region studied, we determined the CFP for Ho(OH)3 more accurately than was possible from an analysis of spectral data which are not very informative in this crystal. We obtained B02 = 200 cm−1, B04 = −57 cm−1, B06 = −40 cm−1, and B66 = 400 cm−1. The values of B02 and B06 differed from the corresponding values for the dilute crystal, i.e., Ho3+:Y(OH)3, as reported from spectral studies. A similar type of difference in the values of CFP for dilute and...

Observation of large low temperature magnetocaloric effect in HoCu2

We report the observation of large low temperature magnetocaloric effect and magnetoresistance in the rare-earth based intermetallic compound HoCu2. The compound undergoes an antiferromagnetic type ordering below about TN = 10.5 K, which is second order in nature. The magnetocaloric effect in terms of entropy change under the application of 50 kOe of field is found to have a maximum value of −19.3 J kg−1 K−1 peaking around TN, and an appreciable value of relative cooling power of 268 J kg−1 was associated with it. The sample also shows giant negative magnetoresistance with its value as high as −36.5% around TN for 50 kOe of field. Field induced second order metamagnetic transition is found to be responsible for the observed magnetocaloric and magnetoresistance behaviors in the sample. The sample is devoid of any thermal or field hysteresis by virtue of the second order nature of the transitions, which enables us to exploit large reversible magnetic cooling at cryogenic temperatures.

Anomalous thermal properties and local lattice effects in low doped manganites La0.875Ca0.125MnO3 and La0.875 Ca0.0625 Sr0.0625MnO3

The results of a comparative study of the magnetic, transport and thermal properties of two ferromagnetic La0.875Ca0.125MnO3(LCMO) and La0.875Ca0.0625Sr0.0625MnO3 (LCSMO) samples with the same Mn3+/Mn4+ ratio and identical crystallographic structure have been presented. While LCSMO exhibits magnetic transition accompanied by an insulator to metal transition, no such transition is observed in LCMO. Moreover, unlike LCMO, only in the case of LCSMO, is a sudden drop in the thermoelectric power S(T) along with an enhancement in the value of thermal conductivity λ(T) observed below the Curie temperature (TC). This confirms that not only the Mn3+/Mn4+ ratio but also local lattice effects and carrier transport play a vital role in determining in the physical properties of these low doped manganite systems.

Low-temperature metallic behavior of amorphous MoO3–TeO2 thin films

We report that vacuum-deposited amorphous MoO3–TeO2 films of different thickness (469–1185 nm) and compositions show metal-insulator transition (MIT) in the temperature (Tp) range 341–231 K (depending on thickness and composition). Interestingly, the corresponding thick bulk glass plates (∼0.2–0.5-mm thickness) of same compositions behave like the usual semiconducting transition metal oxide glasses over the entire range of temperature (80–400 K) and follow small polaron hopping (SPH) conduction mechanism. Temperature-dependent resistivity curves of the films showed peaks (maxima) around the respective MIT temperature Tp, which shifted to the lower-temperature region with the increase of MoO3 content. Fitting of the conductivity data of the film in the high-temperature (T>Tp) semiconducting phase shows a crossover around Tc(>Tp) from the Mott variable range hopping to SPH behavior, which is in sharp contrast to the behavior of the bulk glass. In these films, the estimated localization length (ξ) diverges a...

Universal field dependence of conventional and inverse magnetocaloric effects in DyCo2Si2

The rare-earth intermetallic compound DyCo2Si2 orders antiferromagnetically below TN = 23 K followed by a second magnetic anomaly at Tt = 9 K. The sample is known to show multiple metamagnetic transitions, which are reproduced in our present study. Our investigations on this sample indicate that the magnetocaloric effect (MCE) calculated from the magnetization data (in terms of change in entropy, ΔSM) is quite fascinating, and it is characterized by multiple sign reversals around TN and Tt. The MCE is found to be conventional (i.e., ΔSM is negative) above TN and below Tt, while it is inverse (i.e., ΔSM is positive) between TN and Tt. We performed a comprehensive analysis of the field dependence of the observed MCE, and a universal quadratic variation is observed at temperatures above and below TN (including the region below Tt) as long as the applied field is lower than the critical field for metamagnetic transition. The present work is able to show that the field dependence of the MCE in this antiferroma...