Weakening Of Double-exchange Interaction Research Articles

Evidence of structural and two magnetic phase transitions in Cu doped La2FeMnO6 double perovskites

In this report, we have addressed a comprehensive study on the structural phase transitions with the help of a combination of high temperature x-ray diffraction and temperature dependence Raman spectroscopy. We also report the magnetic ground state of all the samples at room temperature and 5 K. The combined high temperature x-ray diffraction and temperature dependence Raman study suggests that the samples undergo structural phase transitions from orthorhombic to tetragonal to cubic dominated mixed phase. With the help of x-ray photoelectron spectroscopy, we confirmed the presence of multi-oxidation states of Fe and Mn ions. The temperature evolution of magnetization data has been recorded at a field of 100 Oe and temperature range between 2 K and 300 K, and reveals the presence of two magnetic phase transitions for pristine, 10% and 20% doping of Cu in the composition La 2 FeMn 1−x Cu x O 6 . The magnetic study of all the samples reveal that the magnetic ground state of all the samples shows paramagnetic nature at room temperature, whereas at lower temperature all the samples are ferromagnetic except the highest doped sample which is purely paramagnetic. The law of approach to saturation method is imposed in the saturation region of M-H curve observed at 5 K. The transition temperature decreases with the amount of Cu doping, due to the substitution of Cu in Mn-site resulting in destroy of the Mn 3+ -O 2- - Mn 4+ bridges which is responsible for weakening of double exchange interaction between Mn 3+ and Mn 4+ . • Temperature dependent XRD has shown a structural phase transition from orthorhombic to tetragonal to cubic space group. • Temperature evolution of the phononic modes predominantly influenced by the lattice degrees of freedom. • Magnetic studies have shown two phase transitions confirming low temperature FM phase with different ordering mechanisms.

Ferromagetic-paramagnetic transformation in hydrogenated ferrous titanate

Abstarct Ferrous titanate (FeTiO3) nanoparticles with hexagonal disc structure were prepared by a simple hydrothermal method. The VSM results show that ferromagnetic-paramagnetic transition of FeTiO3 caused by high temperature hydrogenation. The morphology and lattice structure of FeTiO3 was characterized in detail by XRD, Raman, SEM and TEM. It was found that the magnetic transition of FeTiO3 nanoparticles after hydrogenation could be attributed to the change of lattice structure and surface effect. Further research by XPS found that the high temperature hydrogenation treatment could cause the change of elemental valence states and oxygen vacancies concentration for FeTiO3. The weakening of double exchange interaction (Fe2+-O-Fe3+) and the reduction of uncompensated surface spin were the main cause of the ferromagnetic-paramagnetic transition. This experiment would provide a new idea for regulating the magnetic property of materials.

Enhancement of Magnetoresistance by Co Substitution in Pr0.6Sr0.4Mn1−<italic>x</italic>Co<italic>x</italic>O3

We investigate the effect of Co substitution on magnetism, electrical resistivity, and magnetoresistance (MR) in Pr0.6Sr0.4Mn1− x Co x O3( $x=0-0.2$ ). The parent compound Pr0.6Sr0.4MnO3 is a double exchange (DE) ferromagnet with a ferromagnetic (FM) transition at $T_{C} = 305$ K. The long-range FM order persists for $x=0.05$ with a radical downshift in $T_{C}$ (260 K). A decrease in magnetization as well as a drastic increase in electrical resistivity ( $\rho \sim ~0.02~\Omega \cdot \text {cm}$ for $x=0$ to $\sim 10^{7}~\Omega \cdot \text {cm}$ for $x=0.2$ at 10 K) with Co doping indicates the weakening of DE interaction and the evolution of glassy magnetic ground state. For both $x=0$ and 0.05, $\rho $ shows a prominent peak at their respective $T_{C}$ , whereas such a feature is absent for $x\ge 0.1$ . A large enhancement in MR at low temperatures occurs with increasing Co doping (from −45% for $x=0$ to −85% for $x=0.2$ at 10 K at 7 T magnetic field). The occurrence of such a huge MR at low temperatures for Co-doped compounds could be either due to spin-dependent tunneling between FM clusters with the spin-glass-like interface within grains or suppression of spin fluctuations.

Structural, Magnetic and Magnetocaloric Properties of Pr0.68Ca0.32−x Bi x MnO3 (x = 0, 0.1, 0.18, 0.26 and 0.32) Compounds

The structural, magnetic and magnetocaloric properties of the polycrystalline Pr0.68Ca0.32−xBixMnO3 (x = 0, 0.1, 0.18, 0.26 and 0.32) samples were studied. X-ray analyses indicated that all samples have a single phase of orthorhombic symmetry. The charge-order transition was observed for the x = 0 sample at TCO= 200 K, and it disappears with further Bi doping of the sample. The Curie temperature, saturation magnetisation and magnetocaloric effect decrease with increasing Bi content. The decrease in Curie temperature and saturation magnetisation was attributed to magnetic inhomogeneity and weakening of double-exchange interaction in the Bi-doped samples. The decrease in saturation magnetisation and the changed nature of phase transition give rise to a decrease in magnetic entropy change.

Magnetic and transport properties of bilayered perovskite manganites La<sub>1.3-<italic>x</italic></sub>Gd<sub><italic>x</italic></sub>Sr<sub>1.7</sub>Mn<sub>2</sub>O<sub>7</sub>(<italic>x</italic>=0,0.05)

Samples La_{1.3-<italic>x</italic>}Gd_{<italic>x</italic>}Sr_1.7Mn₂O₇(<italic>x</italic>=0,0.05) were prepared by solid state reaction method. The effect of Gd substitution for La site in layered manganese oxides La_1.3Sr_1.7Mn₂O₇on the magnetic and electrical properties has been investi-gated. The spin-glass behavior was observed at the temperature below 50 K for <italic>x</italic> = 0.05 sample. The doping of Gd ions induces the decrease of magnetization, 2D magnetic transition temperature, 3D magnetic transition temperature and metal-insulator transition temperature as well as the increase of resistance. These phenomenons are probably ascribed to the weakening of double exchange interaction as a result of crystal lattice mismatch with Gd ions doping.

Research on charge-ordering state in La 0.5Sr 0.5MnO 2.88 and La 0.5Sr 0.5Mn 0.5Ti 0.5O 3 systems

The structural, magnetic and transport properties of La 0.5Sr 0.5MnO 2.88 and La 0.5Sr 0.5Mn 0.5Ti 0.5O 3 samples have been investigated systematically. Indeed, this series has been considered to understand the influence of physical parameters such as oxygen deficiency and titanium doping effect in undoped La 0.5Sr 0.5MnO 3 sample. Ceramic material based on La 0.5Sr 0.5MnO 2.88 exhibits interesting behaviours of charge-ordering (CO), ferromagnetic (FM) states and a good conductivity down to the lowest temperatures. The substitution of Ti for Mn destroyed drastically the CO, damaged the motion of itinerant e g electrons and changed the local parameters of perovskite cell. A change of the structure from tetragonal to rhombohedral symmetry is observed causing a weakening of double-exchange interaction. The experiment results show that the suppression of the CO is sensitive to the variety of Mn 3+/Mn 4+ ratio. In a field of 8 T at 10 K, FM and CO phase can be evaluated to be ∼20:80 according to the μ exp/μ cal ratio for La 0.5Sr 0.5MnO 2.88, whereas the CO state is suppressed for La 0.5Sr 0.5Mn 0.5Ti 0.5O 3 sample, FM and anti-ferromagnetic (AFM) phase are coexisted and evaluated to be ∼54:46, respectively.

Local structure and electron configuration effects on Curie temperature in La 0.7Ca 0.3Mn 1−xTi xO 3 lanthanum manganites

X-ray diffraction, X-ray absorption near edge structure (XANES) and magnetization studies of La 0.7Ca 0.3Mn 1− x Ti x O 3 ( x=0, 0.05) manganites are presented. The substitution of Ti for Mn damaged a part of motion of itinerant e g electrons and changed the local parameters of perovskite cell causing a weakening of double-exchange interaction and a decrease in Curie temperature, T C, to about 103 K. According to XANES study, the Ti occupy the B(=Mn, Ti)-site in the tetravalent form. The cell parameters increased with x because the Ti 4+ ion is larger than the Mn 4+ ion.

Local structure and destruction of electron pathway effects on Curie temperature of B-site-substituted lanthanum manganites

A study of B-site-substituted La0.7Ca0.3Mn1−xScxO3 lanthanum manganites is presented. Substitution of Sc3+ ion for Mn3+ ion damages a path of motion of itinerant eg electrons, thus causing a weakening of double-exchange interaction and a decrease of Curie temperature, TC (about 35K for each 1% Sc doping from x=0 to 0.03). We suggest that the observed strong decrease of the TC with x is mostly caused by a difference in an electron configuration of the Mn3+ and Sc3+ ions and partially due to a change in the local parameters of the MnO6 octahedra because of the larger Sc3+ ion than the Mn3+ one. A method for distinguishing the effect of the above two factors on Curie temperature is supposed.

Mn-site-substituted lanthanum manganites: destruction of electron pathway and local structure effects on Curie temperature

Curie temperature of La0.7Ca0.3Mn1−xScxO3 decreases for about 35K for each 1% Sc doping from x=0 to 0.03 because the weakening of double-exchange interaction. It is mostly caused by the damaging of pathway for the itinerant eg electrons due to a difference in electron configuration between Mn3+ and Sc3+ ions and partially due to the change in the local parameters of MnO6 octahedra because of the larger Sc3+ ion than the Mn3+ one.