Invited Paper
Spin polarised tunnelling as a probe of half metallic ferromagnetism in mixed-valence manganites

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Abstract

We report on spin-dependent tunneling measurements in all-oxide spin valves in which mixed valence manganites La0.7Sr0.3MnO3 electrodes are separated by a thin insulating layer. Information about the density of states and polarisation properties of LaSrMnO can be inferred from the variations of the differential conductance with applied voltage and magnetic field. Our measurements are consistent with the existence of a 1.5 eV wide eg band almost fully polarized at the Fermi level. High levels of spin polarisation lead to a large magnetoresistance at low fields and a high-field contribution is attributed to the presence of canted interfacial manganite phases. A low-field magnetoresistance ratio of 450% below 100 Oe was obtained on a sample with a SrTiO3 barrier, indicating a spin polarisation value in excess of 0.83 for the manganite.

Introduction

Mixed-valent manganese perovskites with general formula A1−xBxMn3+1−xMn4+xO2−3 (where A is a trivalent ion like La, Nd or Pr, and B a divalent ion like Ca, Ba or Sr) have been increasingly studied in the past few years [1]. These compounds show a very interesting interplay between magnetism and transport whose most dramatic evidence is the “colossal magnetoresistance” (CMR) observed in the vicinity of the Curie temperature TC. In the Zener “double exchange” mechanism [2] the conductivity is due to the transfer of eg electrons between neighbouring Mn3+ and Mn4+ cations through filled O2− shells which is favoured when the spins of the two Mn ions are parallel. Spin ordering (obtained by applying high fields or lowering the temperature) allows for delocalisation of the 3d eg electrons. The resulting band is expected to be fully spin-polarised near the Fermi level if the Hund splitting J between the spin up and spin down states is larger than the bandwidth W. However, the extent to which hybridisation of the Mn 3d eg levels with spin-unpolarised oxygen 2p levels decreases the polarisation of the manganite at the Fermi level is still a subject of controversy.

CMR behaviour near TC has been the centre of focus for experiments and theory. However, the high field (in the Tesla range) needed to induce magnetic order makes the use of manganite compounds for applications difficult. the obtained low-field sensitivities 1/RdR/dH do not exceed those of conventional metallic GMR multilayers or magnetoresistive tunnel junctions. The latter are layered FM1/B/FM2 systems, where FM1 and FM2 are ferromagnetic transition metals, and B a thin insulating barrier through which electrons can tunnel. If the magnetisations of the two ferromagnets can be switched between parallel and antiparallel configurations, the relative difference of resistance TMR=(RantiparRpar)/Rpar between both configurations is given by the Julliére [3] formulaTMR=2P1P2/(1−P1P2),where P1 and P2 are the spin polarisation of the carriers in the two ferromagnets, as defined by Tedrow and Meservey [4]. Tunnel junctions made with conventional ferromagnets show maximum low-field TMR of 34% at room temperature [5], because the spin polarisation of the transition metals and alloys commonly used (Co, NiFe, CoFe) does not exceed 40%.

The expected 100% spin polarisation of the manganites in the half-metallic temperature range (far below Tc) can be utilised to generate large magnetoresistive effects in FM/I/FM structures with adjustable relative magnetic configuration (Fig. 1). Ideally, full spin polarisation should lead to zero conductance at T=0 in the antiparallel configuration, and hence to an infinite TMR. Sun et al. obtained low-field MR values near 120% in layered LSMO/STO/LSMO tunnel devices structured by conventional lithography and etching techniques for current perpendicular to plane (CPP) measurements [6].

Section snippets

Sample preparation

We report here on large magnetoresistance results obtained on planar tunnel junctions consisting of pulsed-laser-deposited epitaxial LSMO/I/LSMO trilayers structured by optical lithography for CPP measurements. The compound La1−xSrxMnO3 with x=0.3 was chosen because it is the manganite compound with the highest Curie temperature (TC=370K) [1]; this should result to a high-spin polarisation at room temperature. Three insulators epitaxially compatible with LSMO (lattice constant: a=0.380 nm) were

Field effects

Magnetic hysteresis loops demonstrate that the top and bottom LSMO electrodes are decoupled. This allows us to obtain two different configurations where the electrodes have their magnetisations parallel and antiparallel (Fig. 2).

Large magnetoresistance ratios (400 to 450%) are obtained for the three types of barriers. The magnetoresistance curves are characterised by plateaux with steep edges at low-fields and a continuous decrease of resistance at high fields (Fig. 3).

The low-field MR effects

Temperature effects

In Fig. 5, we show a typical junction resistivity curve in two magnetic configurations. The resistance maximum observed at about 170 K could correspond to the ordering temperature of the canted interface.

The increase of the tunnel resistance between 4.2 K and the maximum could be due to the reduction of carrier concentration as the spin disorder increases. This temperature is lower as the manganite is more canted, and the maximum resistivity point for the STO junction (obtained at about 190 K) is

Voltage effects

At low temperature all samples show non-linear I(V) curves typical of tunnel conduction [7]. The inferred conductances are well fitted with parabola as expected for tunneling [4]. The obtained barrier heights are quite low of the order of 100 meV. For SrTiO3, this value is much lower than that of the pure compound, providing evidence for poor insulating properties. However, non-uniformity of the barrier can induce significant errors in the estimation of the barrier height, and this value should

Conclusions

We have measured the transport properties of all-oxide tunnel junctions with nominally identical La0.67Sr0.33MnO3 electrodes and three kinds of insulating barriers. Magnetoresistance values of more than 400% were measured in all cases. This gives a minimum value of 83% for the spin polarisation of La0.67Sr0.33MnO3 at the Fermi level. Our results indicate the presence of more or less oxygen deficient and canted interface layers in the three systems. All three systems lead to similar absolute

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