Papri Dasgupta

Significant enhancement of magnetoresistance with the reduction of particle size in nanometer scale

The Physics of materials with large magnetoresistance (MR), defined as the percentage change of electrical resistance with the application of external magnetic field, has been an active field of research for quite some times. In addition to the fundamental interest, large MR has widespread application that includes the field of magnetic field sensor technology. New materials with large MR is interesting. However it is more appealing to vast scientific community if a method describe to achieve many fold enhancement of MR of already known materials. Our study on several manganite samples [La1−xCaxMnO3 (x = 0.52, 0.54, 0.55)] illustrates the method of significant enhancement of MR with the reduction of the particle size in nanometer scale. Our experimentally observed results are explained by considering model consisted of a charge ordered antiferromagnetic core and a shell having short range ferromagnetic correlation between the uncompensated surface spins in nanoscale regime. The ferromagnetic fractions obtained theoretically in the nanoparticles has been shown to be in the good agreement with the experimental results. The method of several orders of magnitude improvement of the magnetoresistive property will have enormous potential for magnetic field sensor technology.

Role of inhomogeneous cation distribution in magnetic enhancement of nanosized Ni0.35Zn0.65Fe2O4: A structural, magnetic, and hyperfine study

In this paper, we report the structural, microstructural, and magnetic properties of nanosized (particle size ranging from 20 to 30 nm) Ni0.35Zn0.65Fe2O4 (MA4) system synthesized via mechanochemical route followed by annealing. The Rietveld refinement is used for the first time to precisely resolve the crystal structure of a ferrite system at nanoscale. MA4 is a cubic spinel of Fd3¯m symmetry. According to XRD and HRTEM studies, it is a well crystalline sample which possesses large microstrain. In spite of its nanometric size, MA4 has displayed some notably distinct magnetic properties like, enhancement of magnetization (64 emu g−1 at 15 K), magnetic order, magnetic ordering temperature, coercivity (1000 Oe at 15 K), magnetic anisotropy energy, and reduction of superparamagnetic relaxation compared with its counterparts synthesized by chemical route. It exhibits clear hysteresis loop (HC = 50 Oe) at 300 K and ferrimagnetic ordering below the blocking temperature (∼250 K). These improvements in magnetic pr...

Giant spontaneous exchange bias effect in Sm1.5Ca0.5CoMnO6 perovskite

We report here on the enormously large value of the spontaneous exchange bias (SEB) of ~5.1 kOe in the Sm1.5Ca0.5CoMnO6 bulk double perovskite system after zero-field cooling from an unmagnetized state. Depending on the path used for measuring the magnetic hysteresis loop, P-type or N-type, the EB field varies significantly. Dc and ac magnetization reveals a super-spin-glass-like (SSG) state below 19 K. The EB effect has been observed below the glassy transition temperature. The conventional exchange bias (CEB) as well as the SEB increase with a decrease in temperature and show monotonic variation. All these interesting features can be explained through the multimagnetic phase of double perovskite and the onset of unidirectional anisotropy driven by interface exchange coupling between the SSG, turned isothermally into a field-induced superferromagnetic shell and antiferromagnetic core.

Tuning magnetization, blocking temperature, cation distribution of nanosized Co0.2Zn0.8Fe2O4 by mechanical activation

The study on structural, microstructural, magnetic, and hyperfine properties of nanosized Co0.2Zn0.8Fe2O4 having particle size ∼18 nm (CZM) synthesized by high energy ball milling of Co0.2Zn0.8Fe2O4 nanoparticles of size ∼20 nm (CZ) produced by flow rate controlled coprecipitation method has revealed that the inclusion of strain induced anisotropy produced by mechanical treatment and escalation of oxygen mediated intersublattice exchange interaction of spinel ferrites by tuning cation distribution properly, can improve the magnetic quality of nanosized ferrites significantly. This upshot will be of immense help in promoting the technological application of nanostructured ferrites. The Rietveld refinement of powder x-ray diffraction pattern and the analysis of transmission electron micrographs, energy dispersive x-ray spectrum, and FTIR spectrum of the sample have confirmed that CZM is single phase cubic nanometric spinel ferrite of Fd3¯m symmetry and it possesses large microstrain within its crystal latti...

Large magnetocaloric effect and critical behavior in Sm0.09Ca0.91MnO3 electron-doped nanomanganite

The magnetic properties and magnetocaloric effect (MCE) in electron-doped Sm0.09Ca0.91MnO3 nanomanganite have been investigated in detail by magnetization and heat capacity measurements. The maximum magnetic entropy change and the relative cooling power (RCP) are found to be, respectively, and for a 7 T magnetic field change along with a negligible hysteresis loss, making of this material a promising candidate for magnetic refrigeration at low temperature. The maximum value of occurs close to the Curie temperature . The values of and RCP are comparable to a few hole-doped manganite. To investigate the nature of the paramagnetic-to-ferromagnetic phase transition, critical exponent study has been carried out. Based on the modified Arrott plot, we have determined the values of critical parameters ( and δ) and conclusively shown that Sm0.09Ca0.91MnO3 has nearly mean-field–like long-range interaction. The calculated values of critical exponents not only obey the scaling hypothesis, but also corroborate the results obtained employing the Kouvel-Fisher method. The re-scaled magnetic entropy change curves for different applied magnetic fields collapse into a single master curve for this electron-doped manganite indicating clearly a second-order magnetic phase transition. Such noticeably large at low magnetic field makes this material a potential candidate for low-temperature magnetic refrigeration.

Strain modulated large magnetocaloric effect in Sm0.55Sr0.45MnO3 epitaxial films

Epitaxial Sm0.55Sr0.45MnO3 thin films were deposited on LAO (001), LSAT (001), and STO (001) single crystalline substrates by pulsed laser deposition technique to investigate the correlation between the substrate induced film lattice strain and magnetocaloric effect (MCE). The film on LAO substrate (S_LAO), which is under compressive strain, undergoes ferromagnetic → paramagnetic transition at TC ∼ 165 K. The films on STO (S_STO) and LSAT (S_LSAT) substrates are under tensile strain and have TC ∼ 120 K and 130 K, respectively. At T < TC, the zero field cooled and field cooled magnetization curves of all the films show huge bifurcation. In case of S_STO and S_LSAT films, hysteresis is also observed between field cooled cooling and warming cycle in magnetization versus temperature measurement at low magnetic field similar to first order-like magnetic phase transition. No signature of first order magnetic phase transition has been observed in the case of S_LAO film. Most interestingly, both normal (i.e., neg...

Magnetocaloric and phase coexistence in La0.5Ca0.5–xSrxMnO3 manganites

Structural, magnetic, and magnetocaloric properties of the phase separated La0.5Ca0.5–xSrxMnO3 (x = 0, 0.2, 0.3, 0.4, and 0.5) manganites have been studied. The results show that the phase coexistence state can be investigated by magnetocaloric studies (especially the field dependence of magnetic entropy change at constant temperature). Magnetic entropy change (△SM) shows positive (negative) peak at the vicinity of TN (TC). However, in the intermediate temperatures between TN and TC, both the sign and magnitude of △SM are strongly dependent on temperature and magnetic field, manifesting the competition of ferromagnetic (FM) (negative △SM) and non-FM (positive △SM) phases. This behavior is more pronounced in the parent compound, La0.5Ca0.5MnO3, in which the intermediate phase separation ranges between 160 and 225 K. The substitution of Ca by Sr enhances the ferromagnetic state, weakens the phase separation, and thus narrows the temperature range in which the field related effects (such as the sign change o...

Role of the stability of charge ordering in exchange bias effect in doped manganites

In this work we have carried out an elaborate study on the magnetic properties and investigated the exchange bias phenomena of some charge-ordered (CO) manganites. The detailed study of Sm1−xCaxMnO3 (x = 0.5, 0.55, 0.6, 0.65, 0.7) compounds shows that Sm0.4Ca0.6MnO3, which is the most robust charge ordered material studied here, shows significantly large exchange bias field (HE) as compared to the other compounds. Our experimental results and analysis indicate that TCO, which reflects the stability of the charge-ordered state, is one of the key parameters for the exchange bias effect. Similar behaviour is found in other rare-earth analogues, viz., La1−xCaxMnO3 and Pr1−xCaxMnO3 compounds as well. We also found that with increasing stability of CO states in Sm1−xCaxMnO3 compounds, HE enhances due to increase in number and reduction in size of ferromagnetic clusters.