Study of Spin Polarized Tunneling in Magnetoresistance and Low Temperature Anomaly in Nanoparticles of La0.5Ca0.5MnO3

Abstract

Perovskite oxides are typical correlated electron systems and important functional materials, owning fascinating physical properties and promising potential for applications [1– 4]. Half doped manganites with generic formula R 0.5 A 0.5 MnO 3 (where R, trivalent rare earth and A, divalent alkaline earth element) are topic of significant interest in both theoretical and experimental frontiers. The ground state of half doped manganites show contrasting behavior where the charge ordering (CO), orbital ordering (OO) coupled with antiferromagnetic insulating (AFI) state favored by the localizing effects and ferromagnetic metallic (FMM) state driven by the delocalizing effects. The low-temperature electronic transport in some manganites presents a feature of weak insulating/semiconducting behavior and has been explained using different mechanisms [5– 9]. Furthermore, physical properties are also modified to a considerable extent subject to the reduction of particle size. This occurs mainly due to the finite size effect, surface and interface effect in magnetic nanoparticles. In this digest, the low temperature resistivity and magnetoresistance (MR) in polycrystalline half doped La 0.5 Ca 0.5 MnO 3 of ~25.9 nm size nanoparticle synthesized using citrate route has been investigated in the absence and presence of the magnetic field (H = 0 T, 4T and 8T). As shown in Fig 1 (a). X-ray diffraction data along with fitting using Rietveld Fullprof software [10] to the orthorhombic unit cell structure crystallizes in Pnma space group (No. 62).The lower value of χ2 = 1.48 indicated a good agreement between observed and calculated patterns. The average crystallite size calculated to be 25.9 nm using the Scherrer's formula [11] after correcting for instrumental contribution to line broadening. Fig. 1 (b) and (c) depicts images elucidating the surface morphology of the sample obtained using Scanning Electron Microscopy at 1 μm and 3 μm respectively taken 15KeV. It can be seen that, the grains are in average spherical shaped nano size possessing fine and clear grain boundaries with a small neck between two adjacent grains. From the temperature dependence of resistivity plot shows metal–insulator (MI) transitions at temperature (T MI ) = 89 K at 0T as shown in Fig. 2. With the application of magnetic field, the decrease in resistivity and shifting of T MI towards higher temperatures suggests consolidation of Double Exchange mechanism [12]. The significant decreases in resistivity with field over a wide temperature range, for example, from lowest temperature of measurement to ~ 250 K indicates that FM interaction starts dominating well above observed T MI . This is further supported by isothermal MR measurement which shows PM like behaivour at 300 K and FM like below 200 K discussed later. Based on the phenomenon of phase coexistence in ferromagnetic metallic and paramagnetic insulating regions in the doped manganites, the temperature variation of the resistivity between 50 K and 300 K are in agreement with combined contributions of empirical [13], [14] and Small Polaronic Conduction (SPC) model written as; ${{\rho }_{whole}}(T)=({{\rho }_{0}}+{{\rho }_{1}}{{T}^{2}}+{{\rho }_{2}}{{T}^{4.5}}).f+(1-f)B.T.\text{ }exp({{E}_{a}}/{{k}_{B}}T\ldots ..(1)$ where $\text{f}=\{{{[\exp (\text{T}-\text{T}_{c}^{M}/delta)+1]}^{-1}}$ is the Boltzmann's distribution function of charge carriers in the metallic state. And the characteristic T MI is denoted as $\mathrm {T}_{c}^{M}$ and delta as the effective transition width over the interval of $\mathrm {T}_{c}^{M}$. At temperature below 40 K, a shallow minimum (T min ) appears in resistivity plot. The application of the magnetic field suppresses the effect and gradually decreases T min with increasing magnetic field. However application of even 8T could not diminish the effect and is explained considering intergrain tunneling of the charge carriers between anti-ferromagnetically coupled grains. Inset of Fig. 2 shows the magnetic field (H) dependent magnetoresistance [MR % = ρ (H) ρ(0)ρ(0) × 100] isotherms. At 5 K and H = 0.5 T ( MI appears to be due to nanoparticle nature of the sample where surface could be insulating. The evidence of metallic seem in isothermal MR as at 8T field MR is about 30%, which is expected due to spin polarized tunneling between two FM grains. Similar such behavior was suggested by Rawat et al. [15] for GdPd 2 Si and Lee et. al. [16] for perovskite manganites. The disappearance of the MR at higher temperature regime in insulating state can be ascribed to the weakening of the DE mechanism because of the paramagnetic (PM) state. The present results are discussed and possible explanations are given based on the allied theory.