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Register a new userHuge magnetoresistance and ultra-sharp metamagnetic transition in polycrystalline ${Sm_{0.5}Ca_{0.25}Sr_{0.25}MnO_3}$
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Large magnetoresistive materials are of immense interest for a number of spintronic applications by developing high density magnetic memory devices, magnetic sensors and magnetic switches. Colossal magnetoresistance, for which resistivity changes several order of magnitude (${sim10^4 %}$) in an external magnetic field, occurs mainly in phase separated oxide materials, namely manganites, due to the phase competition between the ferromagnetic metallic and the antiferromagnetic insulating regions. Can one further enhance the magnetoresistance by tuning the volume fraction of the two phases? In this work, we report a huge colossal magnetoresistance along with the ultra-sharp metamagnetic transition in half doped ${Sm_{0.5}Ca_{0.25}Sr_{0.25}MnO_3}$ manganite compound by suitably tuning the volume fraction of the competing phases. The obtained magnetoresistance value at 10 K is as large as $sim10^{13}%$ in a 30 kOe external magnetic field and $sim10^{15}%$ in 90 kOe external magnetic field and is several orders of magnitude higher than any other observed magnetoresistance value reported so far. Using model Hamiltonian calculations we have shown that the inhomogeneous disorder, deduced from tunneling electron microscopy, suppresses the CE-type phase and seeds the ferromagnetic metal in an external magnetic field.
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We investigate the ultra-sharp jump in the isothermal magnetization and the resistivity in the polycrystalline $Sm_{0.5}(Ca_{0.5-y}Sr_{y})MnO_3$ $(y = 0, 0.1, 0.2, 0.25, 0.3, 0.5)$ compounds. The critical field $(H_{cr})$, required for the ultra-sharp jump, decreases with increase of `Sr concentration, i.e. with increase of average A-site ionic radius $langle r_Arangle$. The magnetotransport data indicate that the phase separation increases with the increase of $langle r_Arangle$, i.e. with $y$. The dependency of $H_{cr}$ with magnetic field sweep rate reveals that the ultra-sharp jump from antiferromagnetic (AFM) state to the ferromagnetic (FM) state is of martensitic in nature. Our two-band double exchange model Hamiltonian calculations show that the `Sr doping induces the ferromagnetic clusters in the antiferromagnetic insulating phase and in turn reduces the critical field. In the end we present a phenomenological picture obtained from our combined experimental and theoretical study.
$La_{0.7}Ca_{0.3}MnO_3$ samples were prepared in nano- and polycrystalline forms by sol-gel and solid state reaction methods, respectively, and structurally characterized by synchrotron X-ray diffraction. The magnetic properties determined by ac susceptibility and dc magnetization measurements are discussed. The magnetocaloric effect in this nanocrystalline manganite is spread over a broader temperature interval than in the polycrystalline case. The relative cooling power of the poly- and nanocrystalline manganites is used to evaluate a possible application for magnetic cooling below room temperature.
The temperature dependence of ultrafast photoinduced reflectivity transients is reported in Nd$_{0.5}$Sr$_{0.5}$MnO$_{3}$ thin film. The photoinduced reflectivity shows a complex response with very different temperature dependences on different timescales. The response on the sub-ps timescale appears to be only weakly sensitive to the 270K-metal-insulator phase transition. Below $sim 160$ K the sub-ps response displays a two component behavior indicating inhomogeneity of the film resulting from the substrate induced strain. On the other hand, the slower response on the 10-100 ps timescale is sensitive only to the metal-insulator phase transition and is in agreement with some previously published results. The difference in the temperature dependences of the responses on nanosecond and $mu $s timescales indicates that thermal equilibrium between the different degrees of fredom is established relatively slowly - on a nanosecond timescale.
In this work we demonstrate that the polycrystalline ribbons of (Ni48Co6)Mn26Al20 with B2 structure at room temperature show a magnetic behavior with competing magnetic exchange interactions leading to frozen disorders at low temperatures. It is established that by considering the presence of both antiferromagnetic and ferromagnetic sublattices, we can explain the observed magnetic behavior including the metamagnetic transition observed in these samples. From the Arrott plots, the Neel temperature of (Ni48Co6)Mn26Al20 is deduced to be ~170 K and the broad ferro to para like magnetic phase transition is observed at ~ 200 K. Based on Neel theory, a cluster model is used to explain the presence of ferromagnetic and anti-ferromagnetic clusters in the studied ribbons. Formation of ferromagnetic clusters can be understood in terms of positive exchange interactions among the Mn atoms that are neighboring to Co atoms which are located on the Ni sites.
We have studied the critical behaviour in $La_{0.5}Sr_{0.5}CoO_{3}$ near the paramagnetic-ferromagnetic transition temperature. We have analysed our dc magnetisation data near the transition temperature with the help of modified Arrott plots, Kouvel-Fisher method. We have determined the critical temperature $T_c$ and the critical exponents, $beta$ and $gamma$. With these values of $T_c$, $beta$ and $gamma$, we plot $M/(1-T/T_c)^{beta}$ vs $H/(1-T/T_c)^{gamma}$. All the data collapse on one of the two curves. This suggests that the data below and above $T_c$ obeys scaling, following a single equation of state.
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