Search

The influence of magnetic field on the crystal structure of colossal magnetoresistance (CMR) material La0.5Sr0.5MnO3+delta has been investigated. Low temperature high-magnetic field powder X-ray diffraction (XRD) measurements were carried out on La0.5Sr0.5MnO3+delta in temperature range from 4.2 to 300 K and magnetic field range from 0 to 8 T. A first order structural phase transition from tetragonal I4/mcm to orthorhombic Fmmm phase, coupled with reported electronic and magnetic phase transition from ferromagnetic metal to antiferromagnetic insulator state has been started at similar to 200 K and completed at similar to 100 K. On the application of 8 T magnetic field this magneto-structural phase transition shifted to lower temperature (similar to 162 K) with phase coexistence regime down to 4.2 K. Isothermal X-ray diffraction measurements at 150 K infer the evidence of magnetic field driven first order structural phase transition from Fmmm to I4/mcm. These results consistent with the resistivity and magnetoresistance results; present the microscopic evidence of strong spin-lattice coupling and reveal the magnetic field driven structural phase transition as the origin of observed colossal magnetoresistance in this material. (C) 2016 Published by Elsevier B.V.

Manganites with the formula La1−x Ca x MnO3 for 0.2

We report electrical transport experiments on the colossal magnetoresistance compound (La,Ca)MnO3 over a wide range of composition and temperature. Comparison of thermopower and electrical resistivity measurements above the metal-insulator transition indicate a transport mechanism not dominated by spin disorder, but by small polaron formation. Additionally, we find that in the high-temperature limit the thermopower corresponds to backflow of spin entropy, expected from motion of positively charged particles in a rigid S=2 system, showing a remarkable independence of S=3/2 particle density.

A detailed comparative Mossbauer study of La(0.67)Ca(0.33)MnO(3) and La(0.67)Sr(0.33)MnO(3) samples doped with 1% Sn is presented. The spectra at 5 K can be analyzed by a single ferromagnetic component. However, at higher temperatures two separate ferromagnetic components and a paramagnetic one are required to fit the spectra. The ferromagnetic component with the lower hyperfine field is attributed to clusters of weakened double-exchange interactions due to competition by localization mechanisms. Clear differences between the two samples in regard to the critical behavior arise: (i) The hyperfine field of La(0.67)Ca(0.33)Mn(0.99)Sn(0.01)O(3) disappears abruptly at T(c) in contrast to that of La(0.67)Sr(0.33)Mn(0.99)Sn(0.01)O(3) which varies continuously and goes to zero smoothly at the critical temperature. (ii) The second ferromagnetic phase appears above 0.35T(c) in the Ca sample but only above 0.83T(c) in the Sr sample. The paramagnetic component appears for T > 0.67T(c) in the former compared to T > 0.9T(c) in the latter. The results support recent studies that show differences in the polaronic character, and critical behavior thereof, between the two materials. Physical Review B

A detailed comparative Mossbauer study of La0.67Ca0.33MnO3 and La0.67Sr0.33MnO3 samples doped with 1% Sn is presented. The spectra at 5 K can be analyzed by a single ferromagnetic component. However, at higher temperatures two separate ferromagnetic components and a paramagnetic one are required to fit the spectra. The ferromagnetic component with the lower hyperfine field is attributed to clusters of weakened double-exchange interactions due to competition by localization mechanisms. Clear differences between the two samples in regard to the critical behavior arise: (i) The hyperfine field of La0.67Ca0.33Mn0.99Sn0.01O3 disappears abruptly at T-c in contrast to that of La0.67Sr0.33Mn0.99Sn0.01O3 which varies continuously and goes to zero smoothly at the critical temperature. (ii) The second ferromagnetic phase appears above 0.35T(c) in the Ca sample but only above 0.83T(c) in the Sr sample. The paramagnetic component appears for T > 0.67T(c) in the former compared to T > 0.9T(c) in the latter. The results support recent studies that show differences in the polaronic character, and critical behavior thereof, between the two materials. Physical Review B

Nanocrystalline La0.67Ca0.33MnO3 powders were prepared from organic precursors using a microwave synthesis technique. A variation of the precursor solution pH and annealing temperature was found to change the effective Mn4+ concentration which leads to variations in electronic behavior. In all the cases however, a single phase microstructure was observed showing the stability of orthorhombic manganite. The nanocrystalline manganite prepared from a solution with a pH of 10.5 has the lowest Mn4+ and accordingly low electrical and magnetic transition temperatures. The electrical transport behavior of powders prepared from precursor solutions with pH of 11.5 and 12.5 could be modeled using an effective medium approach across the whole temperature range, 30 K to 300 K. The magnetoresistance in these nanocrystalline manganites increases with decreasing temperature, an effect of increased grain boundaries. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Compositionally modulated structures consisting of La2/3Ca1/3MnO3 ferromagnetic (FM) layers and La1/3Ca2/3MnO3 antiferromagnetic (AF) layers, with layer thickness 1.5 nm less than or equal to t(FM),t(AF)less than or equal to 6 nm, were grown on (100)LaAlO3 by pulsed laser deposition. Thermomagnetic and isothermal magnetic measurements reveal an exchange-biasing mechanism with the same blocking temperature T(B)approximate to 70 K for all the examined combinations of t(FM) and t(AF). The ratio of colossal magnetoresistance increases in multilayers with larger t(FM), whereas the exchange-biasing field increases in samples with larger t(AF). The independence of T-B on t(FM) and t(AF) shows that the artificially induced La1-xCaxMnO3 (x approximate to 0.5) compositional modulation at the interfaces is of critical importance for exchange biasing in La-Ca-Mn-O based multilayers. (C) 2000 American Institute of Physics. [S0021-8979(00)01708-4]. Journal of Applied Physics

An explanation is given for the charge order, orbital order and insulating state observed in half-doped manganese oxides, such as Nd$_{1/2}$Sr$_{1/2}$MnO$_{3}$. The competition between the kinetic energy of the electrons and the magnetic exchange energy drives the formation of effectively one-dimensional ferromagnetic zig-zag chains. Due to a topological phase factor in the hopping, the chains are intrinsically insulating and orbital-ordered. Most surprisingly, the strong Coulomb interaction between electrons on the same Mn-ion leads to the experimentally observed charge ordering. For doping less than 1/2 the system is unstable towards phase separation into a ferromagnetic metallic and charge-ordered insulating phase., To appear in Phys. Rev. Lett., 4 pages, 4 figures

We report electrical transport experiments on the colossal magnetoresistance compound (La,Ca)MnO3 over a wide range of composition and temperature. Comparison of thermopower and electrical resistivity measurements above the metal-insulator transition indicate a transport mechanism not dominated by spin disorder, but by small polaron formation. Additionally, we find that in the high-temperature limit the thermopower corresponds to backflow of spin entropy, expected from motion of positively charged particles in a rigid S=2 system, showing a remarkable independence of S=3/2 particle density.