Room Temperature In La0 Research Articles

Electronic structure and room temperature colossal magnetodielectric effect in La0.4Dy0.1Ca0.5MnO3 manganite

Present communication reports the possible origin of the colossal magnetodielectric (MD) effect observed at room temperature in La0.4Re0.1Ca0.5MnO3 (with Re = Eu, Gd, and Dy) polycrystalline manganites. This study reveals that all the samples are half–doped (% Mn3+ = % Mn4+ = 50 %) and room temperature X-ray absorption measurement at Mn L-edges confirms the presence of both Mn3+ and Mn4+ ions in these oxides. All these specimens are paramagnetic at room temperature with the presence of metastable charge–ordered domains. The application of a low constant magnetic field of 0.6 T and 1.2 T collapses some of the charge-ordered domains that lead to the charge delocalization and colossal values of the MD effect. The Dy–based sample exhibits the strongest charge ordering state, ascribed to the important cationic mismatch effect.

Phase Transformations of Perovskite La0.64(Ba1−xCax)0.36FeO3−δ Investigated by Mechanical Spectroscopy

Herein, perovskite La0.64(Ba1−x Ca x )0.36FeO3−δ () samples are synthesized and investigated by X‐ray diffraction and mechanical spectroscopy. The room‐temperature structure of La0.64Ba0.36FeO3−δ is a mixture of orthorhombic and cubic phases. The structures of calcium‐doped La0.64(Ba1−x Ca x )0.36FeO3−δ are orthorhombic at room temperature. A modulus dip is observed above room temperature in La0.64(Ba1−x Ca x )0.36FeO3−δ () samples. This corresponds to a phase transition from orthorhombic to rhombohedral structure with increasing temperature. The effect of oxygen nonstoichiometry on the room‐temperature structure and phase transition is observed by mechanical spectroscopy.

Broad band magnetotransport at room temperature in La0.7Sr0.3−Ca MnO3: Electrically detected magnetic resonances

Abstract We report the room temperature magnetoimpedance in bulk samples of La0.7Sr0.3−xCaxMnO3 (0 ≤ x ≤ 1) subjected to ac current excitation of frequency f = 1 MHz to 3 GHz and dc magnetic fields (−5 kOe 700 MHz. A plot of Hdc corresponding to the double peak/critical field versus frequency suggest that the curves are linear for PM samples and non-linear for FM samples. We suggest that the observed anomalies in the ac magnetoresistance are due to electron paramagnetic resonance for x ≥ 0.24 and ferromagnetic resonance for x ≤ 0.20. Thus, we are able to electrically detect paramagnetic and ferromagnetic resonances in these conducing magnetic oxides through a simple magnetoimpedance measurement.

Phenomenological model of the magnetocaloric effect and its correlation with critical behavior near room temperature in La0.7Ca0.2Sr0.1MnO3 manganite

In this paper, we investigate the field dependence of the magnetocaloric properties of La0.7Ca0.2Sr0.1MnO3 powder sample using a phenomenological model. Our compound was elaborated by the conventional solid state reaction. The model parameters were determined from the magnetization data and were used to give better fits to magnetic transition and to calculate the magnetocaloric quantities. The magnetocaloric parameters such as the maximum of the magnetic entropy change \(\Delta S_M^{max}\) and the relative cooling power RCP, have been determined from the calculation of the magnetization as a function of temperature under several magnetic applied field. Thus, from the magnetocaloric results, such as RCP ≈ b(μ0H)1+1/δ and Tpeak − TC ≈ b (µ0H)1/Δ, the critical exponents values related to the magnetic transition have been determined. The estimated results are close to those expected by the tricritical mean-field model. Furthermore, the values of the ferromagnetic transition temperature TC, as well as the critical exponents β, γ and δ obtained by the theoretical model, are compared with those obtained by other various techniques (such as the modified Arrott plots, the Kouvel–Fisher method and the critical isotherm analysis). A good agreement has been found in the vicinity of the Curie temperature.

Investigation of the critical properties near room temperature in La0.7Ca0.2Ba0.1MnO3 manganite

In this paper, we studied in detail the critical behavior of La0.7Ca0.2Ba0.1MnO3 manganite sample synthesized by the solid state reaction. The critical properties near the ferromagnetic–paramagnetic phase transition have been analyzed from the magnetic data, using various techniques such as the modified Arrott plots, the Kouvel fisher method, critical isotherm analysis and the field dependence of magnetocaloric results. Moreover, the validity of the critical exponents using the various methods has been confirmed by the scaling equation of state and all data fall on two distinct branches, one for T TC, indicating that the critical exponents obtained in this work are accurate. The exponents determined in this study have revealed that the β value is in between the theoretically expected values for 3D-Ising and tricritical mean field model. While, the γ value is close to the mean field theory. The third exponent δ was determined independly from the critical isotherms satisfying the Widom scaling relation. Our systematic result reflects the coexistence of a long range and short range ferromagnetic order in our sample. The temperature variation in the effective exponents βeff and γeff resemble with those for disordered ferromagnet around Curie temperature.

Improvements of dielectric properties of Cu doped LaTiO3+δ

The ceramic composites of Cu-doped La1−xCuxTiO3+δ (x=0.05, 0.15, 0.3, 0.5) were synthesized by conventional solid-state reaction. The complex dielectric properties of the composites were investigated as a function of temperature (77K≤T≤320K) and frequency (100Hz≤f≤1MHz) separately. In all composites, the dielectric constants increase monotonously and the dielectric loss undulates with temperature. And it is clearly observed that extraordinarily high low-frequency dielectric constant (~104) appear at room temperature in La0.5Cu0.5TiO3+δ, which is ~100 times larger than that of La0.95Cu0.05TiO3+δ. Interestingly, the dielectric constants increase remarkably with the doped Cu contents, meanwhile the dielectric loss for all samples is ideal lower than 1 at room temperature in the measured frequency range. By means of complex impedance analysis, the improvements of dielectric properties are attributed to both bulk contribution and grain boundary effect, in which the bulk polaronic relaxation and the Maxwell–Wagner relaxation due to grain boundary response are heightened remarkably with the high doped Cu contents.

Coexistence of tunneling magnetoresistance and electroresistance at room temperature in La0.7Sr0.3MnO3/(Ba, Sr)TiO3/La0.7Sr0.3MnO3 multiferroic tunnel junctions

Tunnel junctions composed of two ferromagnetic electrodes separated by a ferroelectric barrier were fabricated from epitaxial La0.7Sr0.3MnO3/Ba0.95Sr0.05TiO3/La0.7Sr0.3MnO3 trilayers. Typical R−H curves with sharp-switched resistance states (magnetic parallel and antiparallel) of magnetic tunnel junctions have been observed up to room temperature. After applying a poling voltage, which reverses the barrier polarization, both the parallel and antiparallel resistance states will switch to different values. Clear tunneling magnetoresistance and tunneling electroresistance, hence the four resistance states have been observed at room temperature.

Large magnetic entropy change near room temperature in La0.7(Ca0.27Ag0.03)MnO3 perovskite

In this paper, the magnetic properties and magnetocaloric effect (MCE) of La0.7(Ca1−xAgx)0.3MnO3 (x=0, 0.1, 0.2, 0.7, and 1) powder samples are reported. Our polycrystalline compounds were synthesized using the solid state reaction method at high temperature. Magnetization measurements versus temperature showed that all our samples exhibited a paramagnetic to ferromagnetic transition with decreasing temperature. The Curie temperature, TC, has been found to increase from ∼250K for x=0–270K for x=1. Ag doping weakens the first order phase transition, and at higher Ag doping, the phase transition is of second order. For the La0.7(Ca0.27Ag0.03)MnO3 composition, the maxima of the magnetic entropy changes from the applied magnetic field (ΔSM) at 2 and 5T are about 4.5 and 7.75J/kgK, respectively, at the Curie temperature of ∼263K. The relative cooling power (RCP) values without hysteresis loss are about 102 and 271J/kg for the applied fields of 2 and 5T, respectively. Due to the large ΔSM, large RCP, and high Curie temperature, La0.7(Ca0.27Ag0.03)MnO3 is promising for application in potential magnetic refrigeration near room temperature.

Influence of length and measurement geometry on magnetoimpedance in La0.7Sr0.3MnO3

We show that ac magnetoresistance at room temperature in La0.7Sr0.3MnO3 is extremely high (≈−47% in μ0H=100 mT, f=3–5 MHz), and magnetic field dependence of reactance exhibits a double peak behavior. However, magnitudes of the ac magnetoresistance and magnetoreactance for a fixed length of the sample (li) decrease with decreasing separation (lv) between voltage probes unlike the dc magnetoresistance. On the contrary, change in li has a negligible influence on magnetoimpedance when lv is fixed. Our results indicate that high frequency electrical transport is sensitive to local variations in the magnetic permeability.

Enhanced room temperature coefficient of resistance and magnetoresistance of Ag-added La0.7Ca0.3−xBaxMnO3 composites

In this paper we report an enhanced temperature coefficient of resistance (TCR) close to room temperature in La0.7Ca0.3−xBaxMnO3 + Agy (x = 0.10, 0.15 and 0 ⩽ y ⩽ 1.0) (LCBMO + Ag) composite manganites. The observed enhancement of TCR is attributed to the grain growth and opening of new conducting channels in the composites. Ag addition has also been found to enhance intragranular magnetoresistance. Intergranular MR, however, is seen to decrease with Ag addition. The enhanced TCR and MR at/near room temperature open up the possibility of the use of such materials as infrared bolometric and magnetic field sensors, respectively.

Manipulating Magnetoresistance Near Room Temperature in La0.67Sr0.33MnO3/La0.67Ca0.33MnO3 Films Prepared by Polymer Assisted Deposition

Manipulating Magnetoresistance Near Room Temperature in La_0.67Sr_0.33MnO₃/La_0.67Ca_0.33MnO₃ Films Prepared by Polymer Assisted Deposition

Enhanced magnetoresistance at room temperature in La0.7Sr0.3Mn1−xScxO3perovskites

The uncommon Sc ions are employed to partially replace Mn in La0.7Sr0.3MnO3perovskite. X-ray diffraction patterns indicate a pure single phase withrhombohedral lattice symmetry for La0.7Sr0.3Mn1−xScxO3 (0 ≤ x ≤ 0.1). The Curietemperature (TC)is tuned to near room temperature by Sc substitution withx = 0.03–0.05,and consequently a significant enhancement in magnetoresistance (MR) at roomtemperature is obtained in the Sc-doped compounds. Magnetic measurementshows no spin coupling between Sc and Mn ions. We suggest that the reduction inTCand the enhancement in MR are ascribable to the suppression of ferromagneticorder due to the structural distortion caused by the large Sc3+ ions.

Large domain wall magnetoresistance up to room temperature in La0.7Sr0.3MnO3 bridges with nanoconstrictions

To obtain low field magnetoresistance (MR) in manganites, we have introduced a geometrically constrained magnetic domain wall (DW) in La0.7Sr0.3MnO3 micrometric devices. Nanoconstrictions artificially induced using high resolution e-beam lithography are shown to effectively reduce the DW width leading to strongly enhanced DW resistance. Sharp and large resistance switches result from the appearance and annihilation of the DWs. Room temperature sharp resistance switches, with a MR of 16%, are evidenced in a manganite-based device.

Response to “Comment on ‘Ferromagnetism at room temperature in La0.8Ca0.2MnO3 thin films’ ” [Appl. Phys. Lett. 76, 1209 (2000)

Abstract

Ferromagnetism at room temperature in La0.8Ca0.2MnO3 thin films

Anomalously high metal–insulator transition temperature Tp and ferromagnetic transition temperature Tc have been observed in thin films of La0.8Ca0.2MnO3. Ferromagnetic resonance signals from microwave studies are observed at room temperature (well above Tc) suggesting the presence of multiple magnetic spin systems. The Tp and Tc values are higher than that of La0.67Ca0.33MnO3 which have the highest Tp and Tc according to the bulk phase diagram. These results suggest that properties well beyond those seen in the bulk phase diagram may be achievable in thin films of the manganites.

Giant magneto radio frequency absorption in magneto-resistive materials La0.7(Sr,Ca)0.3MnO3

The dc magnetic field, frequency, and temperature dependence of electromagnetic absorption at rf frequencies of 335 kHz and 10 MHz are measured in the as sintered samples of colossal magneto resistive materials La0.7Sr0.3MnO3 and La0.7Ca0.3MnO3 by measuring the penetration depth using the method of Schawlow and Devlin. Very large magneto-electromagnetic absorption effects, much larger than the magneto resistive effects, are observed. A dc magnetic field of 1000 G changes the penetration depth at 350 kHz by 53% at room temperature in La0.7Sr0.3MnO3, whereas the resistance of the sample only decreases by 0.75% for the same field. The effects are primarily a result of the decrease of the permeability of the sample with dc magnetic field in the ferromagnetic state. A simple dc magnetometer employing the as sintered La0.7Sr0.3MnO3 contained in the coil of an LC oscillator as the sensing element is described.