Intergrain Magnetoresistance via Second-Order Tunneling in Perovskite Manganites

We have investigated the effect of nanometric grain size on magnetic properties of single phase, nanocrystalline, granular La 0.7 Ca 0.3 MnO 3 (LCMO) sample. We have considered core-shell structure of our LCMO nanoparticles, which can explain its magnetic properties. From the temperature dependence of field cooled (FC) and zero-field cooled (ZFC) dc magnetization (DCM), the magnetic properties could be distinguished into two regimes: a relatively high temperature regime T > 40 K where the broad maximum of ZFC curve (at T = T max ) is associated with the blocking of core particle moments, whereas the sharp maximum (at T = T S ) is related to the freezing of surface (shell) spins. The unusual shape of M (H) loop at T = 1.5 K, temperature dependent feature of coercive field and remanent magnetization give a strong support of surface spin freezing that are occurring at lower temperature regime (T < 40 K) in this LCMO nanoparticles. Additionally, waiting time (t w ) dependence of ZFC relaxation measurements at T = 50 K show weak dependence of relaxation rate [S(t)] on t w and dM/dln(t) following a logarithmic variation on time. Both of these features strongly support the high temperature regime to be associated with the blocking of core moments.2 At T = 20 K, ZFC relaxation measurements indicates the existence of two different types of relaxation processes in the sample with S(t) attaining a maximum at the elapsed time very close to the wait time t w = 1000 sec, which is an unequivocal sign of glassy behavior.This age-dependent effect convincingly establish the surface spin freezing of our LCMO nanoparticles associated with a background of superparamagnetic (SPM) phase of core moments.

Section: Introductionmentioning

confidence: 96%

Enhanced grain surface effect on magnetic properties of nanometric La0.7Ca0.3MnO3 manganite: Evidence of surface spin freezing of manganite nanoparticles

Dey

Nath

Manna

et al. 2008

“…As known from the analysis of the MR in highly spinpolarized half-metallic maganites, 10,11 intergrain tunneling MR ͑ITMR͒ exhibits a low and high field regime. This phenomenon is characterized by an initial drop of the resistivity with applied magnetic field and a following slower decrease at higher fields.…”

mentioning

confidence: 99%

Extrinsic and intrinsic magnetoresistance contributions of CrO2 thin films

Rüdiger

Rabe

Samm

et al. 2001

The growth of (010)-oriented CrO2 thin films on Al2O3(0001) substrates leads to a higher grain boundary density than the growth of (100)-oriented CrO2 thin films on isostructural TiO2(100) substrates. For both types of films an intrinsic linear contribution to the high field magnetoresistance (MR) due to spin disorder has been determined at T=300 K. This contribution does not depend on the crystalline quality of the films and supports the suggested intrinsic double exchange mechanism for CrO2. At low temperature (T=10 K) intergrain tunneling MR and Lorentz MR appear, which strongly depend on the crystalline properties of the CrO2 films.

“…They concluded that multistep tunneling via a number of localized states within the barrier had to be added to the elastic tunneling conductivity contribution. Several different inelastic processes have been suggested in order to explain experimental data, most of them emphasizing scattering at magnetic intra-barrier states [13,14,15].…”

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confidence: 99%

Stoner–Wohlfarth model applied to bicrystal magnetoresistance hysteresis

Gunnarsson

Hanson

Dubourdieu

2004

We calculate numerically the magnetization direction as function of magnetic field in the StonerWohlfart theory and are able to reproduce the shape of the low-field magnetoresistance hysteresis observed in manganite grain boundary junctions. Moreover, we show that it is necessary to include biaxial magnetocrystalline anisotropy to fully describe the grain boundary magnetoresistance in La0.7Sr0.3MnO3/SrTiO3 bicrystal tunnel junctions.Can the fairly simple model presented by Stoner and Wohlfart in 1948 [1] provide insights in the processes that occur in manganite bicrystal grain boundary junctions? In the following we show how the coherent rotation of the magnetization vector can explain the characteristic features in the field dependence of the magnetoresistance.Recently, a substantial low-field magnetoresistance was observed in grain boundaries of perovskite manganites [2,3] and other half-metallic ferromagnets [4,5]. In order to understand and exploit this effect several studies have used bicrystal grain boundary junctions [6,7,8]. Since bicrystals are samples with well defined crystal orientations with respect to an interface, they are in a way ideal systems for studies of the behaviour of magnetic tunnel junctions. Traditionally, the conductivity of magnetic tunnel junctions is compared with Julliere's model [9], in which the tunneling magnetoresistance is determined by the spin polarization of the electrodes. A more realistic scenario was considered by Slonczewski [10], who used a method to match the wave functions across the tunneling barrier. He derived an expression for direct tunneling through the interface which includes the angle between the directions of the magnetization of the electrodes.