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Register a new userUnusual magnetic relaxation behavior in La0.5Ca0.5MnO3
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We present an extensive study of the time dependence of the magnetization in a polycrystalline and low temperature charge ordered La0.5Ca0.5MnO3 sample. After application and removal of a 5 T magnetic field, a systematic variation of the magnetic relaxation rate from 10 K to 245 K was found. At 195 K, the magnetization decreases in a very short time and after that it increases slowly as a function of time. Moreover, between 200 and 245 K, an increase in magnetization, above the corresponding value just after removing the 5 T magnetic field, was measured. This unusual behavior was tested in several other relaxation procedures. PACS: 70, 74.25 Ha, 75.60.-d, 76.60.Es
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We have carried out a systematic magnetic relaxation study, measured after applying and switching off a 5 T magnetic field to polycrystalline samples of La0.5Ca0.5MnO3 and Nd0.5Sr0.5MnO3. The long time logarithmic relaxation rate (LTLRR), decreased from 10 K to 150 K and increased from 150 K to 195 K in La0.5Ca0.5MnO3. This change in behavior was found to be related to the complete suppression of the antiferromagnetic phase above 150 K and in the presence of a 5 T magnetic field. At 195 K, the magnetization first decreased, and after a few minutes increased slowly as a function of time. Moreover, between 200 K and 245 K, the magnetization increased throughout the measured time span. The change in the slope of the curves, from negative to positive at about 200 K was found to be related to the suppression of antiferromagnetic fluctuations in small magnetic fields. A similar temperature dependence of the LTLRR was found for the Nd0.5Sr0.5MnO3 sample. However, the temperature where the LTLRR reached the minimum in Nd0.5Sr0.5MnO3 was lower than that of La0.5Ca0.5MnO3. This result agrees with the stronger ferromagnetic interactions that exist in Nd0.5Sr0.5MnO3 in comparison to La0.5Ca0.5MnO3. The above measurements suggested that the general temperature dependence of the LTLRR and the underlying physics were mainly independent of the particular charge ordering system considered. All relaxation curves could be fitted using a logarithmic law at long times. This slow relaxation was attributed to the coexistence of ferromagnetic and antiferromagnetic interactions between Mn ions, which produced a distribution of energy barriers.
We reported a systematic change in the average magnetic relaxation rate, after the application and removal of a 5 T magnetic field, in a polycrystalline sample of La0.5Ca0.5MnO3. Magnetic relaxation measurements and magnetization versus field curves were taken from 10 K to 160 K. The long time behavior of the relaxation curves was approximately logarithmic in all cases. Keywords: Charge Ordering, Relaxation, Magnetic measurements
We report unusual magnetic behavior in iron oxide hollow nanospheres of 9.3 $nm$ in diameter. The large fraction of atoms existing at the inner and outer surfaces gives rise to a high magnetic disorder. The overall magnetic behavior can be explained considering the coexistence of a soft superparamagnetic phase and a hard phase corresponding to the highly frustrated cluster-glass like phase at the surface regions.
Traces of superconductivity (SC) at elevated temperatures (up to 65 K) were observed by magnetic measurements in three different inhomogeneous sulfur doped amorphous carbon (a-C) systems: (a) in commercial and (b) synthesized powders and (c) in a-C thin films. (a) Studies performed on commercial (a-C) powder which contains 0.21% of sulfur, revealed traces of non-percolated superconducting phases below Tc = 65 K. The SC volume fraction is enhanced by the sulfur doping. (b) a-C powder obtained by pyrolytic decomposition of sucrose did not show any sign for SC above 5 K. This powder was mixed with sulfur and synthesized at 400 C (a-CS). The inhomogeneous products obtained, show traces of SC phases at TC= 17 and 42 K. (c) Non-superconducting composite a-C-W thin films were grown by electron-beam induced deposition. SC emerged at Tc = 34.4 K only after heat treatment with sulfur. Other parts of the pyrolytic a-CS powder, show unusual magnetic features. (i) Pronounced irreversible peaks around 55-75 K appear in the first zero-field-cooled (ZFC) sweep only. Their origin is not known. (ii) Unexpectedly these peaks are totally suppressed in the second ZFC runs measured a few minutes later. (iii) Around the peak position the field-cooled (FC) curves cross the ZFC plots (ZFC>FC). These peculiar magnetic observations also ascribed to a-CS powder prepared from the commercial a-C powder and are connected to each other. All SC and magnetic phenomena observed are intrinsic properties of the sulfur doped a-C materials. It is proposed that the a-CS systems behave similarly to well known high TC curates and/or pnictides in which SC emerges from magnetic states.
We report the thermodynamic, magnetic, and electronic transport properties of the new ternary intermetallic system (Ce,La)3Pt4In13. Ce3Pt4In13 orders antiferromagnetically at 0.95 K while the non-magnetic compound La3Pt4In13 is a conventional 3.3 K superconductor. Kondo lattice effects appear to limit the entropy associated with the Neel transition to (1/4)Rln2 as an electronic contribution to the specific heat of gamma = 1 J/mole-Ce K2 is observed at TN; roughly 35% of this gamma survives the ordering transition. Hall effect, thermoelectric power, and ambient-pressure resistivity measurements confirm this interpretation. These results suggest that RKKY and Kondo interactions are closely balanced in this compound (TN = TK). Contrary to expectations based on the Doniach Kondo necklace model, applied hydrostatic pressure modestly enhances the magnetic ordering temperature with dTN/dP = +23 mK/kbar. As such Ce3Pt4In13 provides a counterexample to Kondo systems with similar Kondo and RKKY energy scales wherein applied pressure enhances TK at the expense of the ordered magnetic state.
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