Kalipada Das

Giant enhancement of magnetoresistance in core-shell ferromagnetic-charge ordered nanostructures

We have achieved giant enhancement of magnetoresistance (MR) by the formation of La0.67Sr0.33MnO3 (LSMO)–Pr0.67Ca0.33MnO3 (PCMO) core-shell nanostructure. Astonishingly, 1143% enhancement of MR in the core-shell nanostructure has been observed with respect to the parent PCMO nanoparticles at 100 K and 2 T magnetic field. The observed giant enhancement is the result of significantly weaken the charge ordered state in the created ferromagnetic-charge ordered core-shell nano structure. Our study clearly indicates a method to achieve huge enhancement of magnetoresistance that can eventually give rise artificially created superior materials important for magnetic field sensor technology.

The effect of artificial grain boundaries on magneto-transport properties of charge ordered-ferromagnetic nanocomposites

Nanocomposites of charge ordered insulating Pr0.67Ca0.33MnO3 (PCMO) and ferromagnetic metallic La0.67Sr0.33MnO3 (LSMO) nanoparticles have been prepared by chemical synthesis. Transport and magneto-transport properties of nanocrystalline LSMO and the PCMO–LSMO nanocomposites have been studied in detail. At a low temperature region, upturn of resistivity for both the compounds was observed. The upturn of resistivity is strongly influenced by the external magnetic field. The results are analyzed considering intergranular spin polarized tunneling model. Our study reveals that spin polarized tunneling (SPT) is the dominant mechanism leading to the rise in resistivity especially below T = 60 K. It also indicates that SPT through the grain boundary is significantly modified in nanocomposite compounds, which leads to the enhancement of magnetoresistance and low field magnetoconductance compared to LSMO nanoparticles.

The metal–insulator transition in nanocrystalline Pr0.67Ca0.33MnO3: the correlation between supercooling and kinetic arrest

The transition and hysteresis widths of a disorder broadened first order magnetic transition vary in H-T space which influences the co-existing phase fraction at low temperature arising due to kinetic arrest of the first order transition. We explored the role of change in the relative width of the supercooling/superheating band and kinetic arrest band for a ferromagnetic metallic to antiferromagnetic insulating transition. It is shown that for a correlated kinetic arrest and supercooling bands, the topology of the devitrification curves (or transformation across the (H(K),T(K)) band during warming) changes with the change in the relative width of these two bands. In addition to this, for a broader kinetic arrest band, the transformation temperature across the superheating band under constant H now depends on the arrested phase fraction. These predictions have been tested on nanocrystalline Pr(0.67)Ca(0.33)MnO(3), which is known to show a large variation in hysteresis width in H-T space. This is the first report where correlation between the kinetic arrest band and the supercooling band has been shown experimentally, in contrast to the universal observation of anticorrelation reported so far.

Observation of large magnetocaloric effect in HoRu2Si2

Detailed magnetic, magnetotransport, and magnetocaloric measurements on HoRu2Si2 have been performed. In this Letter, we report presence of spin reorientation transition below paramagnetic to antiferromagnetic transition temperature (TN = 19 K). Large magnetic entropy change 9.1 J/kg K and large negative magnetoresistance ∼21% in a magnetic field of 5 T has been observed around TN, which is associated with field induced spin-flip metamagnetic transition.

Giant enhancement of magnetocaloric effect at room temperature by the formation of nanoparticle of La0.48Ca0.52MnO3 compound

Magnetocaloric effect (MCE) is the change in temperature or entropy of a material due to the application of external magnetic field. The temperature dependence of MCE shows maximum value near the ferromagnetic transition of the material. Material with large MCE near room temperature will make revolutionary change in cooling technology and will have large impact in human society. We have transformed antiferromagnetic bulk manganite material La0.48Ca0.52MnO3 to ferromagnetic material by the reduction of the particle size in nanometer scale. MCE, as well as the Relative cooling power (RCP) of the material at room temperature (RT) systematically increase with the reduction of the particle sizes. The RCP value for the 45 nm particle size sample at RT is about 400% larger compared with the bulk counterpart. Our detail study on the nano form of the compound highlights an effective way to enhance the MCE. It opens up a vast possibility of achieving giant enhancement of MCE of several already known materials just ...

A comparative study of magnetic training effect in bulk and nanocrystalline La0.46Ca0.54MnO3 compound

Transport, magneto-transport, and magnetic properties of La0.46Ca0.54MnO3 compounds having average grain size down to ∼15 nm have been studied. A magnetic training effect due to the external magnetic field cycling was distinctly observed in charge ordered antiferromagnetic bulk compound. Our present study indicates that the training effect was markedly modified along with the modification of the charge ordering due to the reduction of the grain size, and eventually both phenomenons disappeared in case of our lowest particle size sample (∼15 nm). Enhanced ferromagnetic correlation with the reduction of particle size plays the key role for the gradual diminishing of the training effect in the region of nanometer length scale.

Magnetic field induced insulator-metal transition in nanocrystalline Pr0.67Ca0.33MnO3 compounds: Evidence of large temperature co-efficient of resistance

We report the electronic transport, magneto-transport, and magnetic properties of nanocrystalline Pr0.67Ca0.33MnO3 compound. A magnetic field induced insulator—metal transition appears for the external magnetic field higher than 50 kOe. We have obtained large value of the temperature coefficient of resistance (TCR) along with magnetoresistance and field coefficient of resistance (FCR). The value of TCR is 135%/K at 48 K. The calculated magnetoresistance is about −9.8 × 107% for 70 kOe and maximum FCR is about 320%/kOe around 75 K. Due to the application of the external magnetic field, charge ordered state of the compound is destabilized leading to such large values of TCR and FCR. Large values of TCR and FCR along with the large magnetoresistance exhibited by the material is interesting from the application point of view.

Experimental and theoretical investigation of magnetocaloric behavior in HoRu2Si2

The magnetic and magnetocaloric properties of intermetallic compound HoRu2Si2 have been investigated from experimental as well as theoretical point of view. Magnetization and magnetocaloric measurements on polycrystalline sample indicate an antiferromagnetic phase transition at 18 K. The theoretical investigation has done using a microscopic model and analyzed as a function of model parameters.

Magnetic and magnetoresistive properties of half-metallic ferromagnetic and charge ordered modified ferromagnetic manganite nanoparticles

In our present study, we address in detail magnetic and magneto-transport properties of well known half metallic La0.67Sr0.33MnO3 (LSMO) and charge order suppressed ferromagnetic La0.48Ca0.52MnO3 (LCMO) nanoparticles. The average particle size for LSMO and LCMO is ∼20 nm and ∼25 nm, respectively. With respect to their magnetic properties, both compounds exhibit ferromagnetic behavior, whereas they markedly differ in their magneto-transport characteristics. The magnetoresistive properties of LSMO nanoparticles indicate low field magnetoresistance and tendency for saturation at higher field values. In addition to the sharp low field magnetoresistance, we have achieved significantly large magnetoresistance at higher values of external magnetic field for the ferromagnetic LCMO nanoparticles. To address such anomalous behavior in these two different classes of ferromagnetic materials, we introduce the re-entrant core-shell type structure formation in charge ordered nanoparticles (LCMO) when charge ordering is ...

Giant enhancement of magnetoresistance of nanocrystalline La0.45Ca0.55MnO3

The enhancement of magnetoresistance (MR) has been observed in case of nanocrystalline La0.45Ca0.55MnO3 in comparison with its polycrystalline bulk form. It appears that the robust charge ordered state (COS) formed in bulk sample becomes unstable, when the sample is prepared in nanocrystalline form and can be destabilized in the presence of magnetic field giving rise to enormous MR.