Inverse Magnetoresistance In Manganite/SrTiO 3 /Co Tunnel Junctions
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ABSTRACT In Lao.7Sr 0 .3MnO 3/SrTiO 3/Co tunnel junctions, the half-metallic nature of Lao. 7Sro3MnO 3 allows probing the spin polarization of Co. For applied voltage bias around zero volts, an inverse tunnel magnetoresistance is found, indicating the negative spin polarization of Co at the Fermi level as expected from the density of states of the "d" band in Co. The bias dependence of the magnetoresistance reflects the structure of the "d" band density of states of Co. In this article we underline the important consequences for the knowledge of the spin-dependent tunneling in solids brought by these results and describe in detail the effect of temperature and high magnetic field on the magnetoresistance. INTRODUCTION In the seventies, Tedrow and Meservey studied the spin polarization of Co in heterostructures of the type Superconductor/A120 3/Co [1]. The spin splitting of the quasi-particle density of states (DOS) of the superconductor induced by a magnetic field can be used to analyze the spin polarization of the tunneling current. They obtained a positive spin polarization for Co as well as for all the studied ferromagnetic metals. However, a negative spin polarization could be expected for metals like Co and Ni, in which the majority spin d subband is below the Fermi level and the minority spin d subband is predominant at the Fermi level. Magnetic tunnel junctions are heterostructures of the type Ferromagnet/Insulator/Ferromagnet. By applying a magnetic field, the angle formed by the magnetizations of the electrodes can be changed, which brings about a tunnel magnetoresistance (TMR). The largest TMR ratios are obtained when the magnetic field switches the magnetizations between the parallel and antiparallel configurations, which can be used for devices in the so-called Spin Electronics [2]. Most of the present research in magnetic tunnel junctions is carried out with transition metals as the ferromagnetic electrodes and A120 3 barriers, which leads to large TMR ratios at room temperature (around 25%) [3]. From a basic point of view, it has always been found that the resistance is smaller in the parallel than in the antiparallel magnetic configuration. From now on, we will call this effect a "normal" TMR, which is consistent with the same sign for the spin polarization of both electrodes. In fact, in the simplest model to explain the TMR, the TMR ratio depends only on the spin polarization of the electrodes [4] TMR(%)=l 00*AR/R= 100*(RAp-Rp)/(RAp)=I 00*2P 1P2/( +PIP 2)
(1)
where Rp and RAp are the resistances in the parallel and antiparallel configurations and P, and P2 are the spin polarizations of the first and second electrode respectively. The normal TMR 293 Mat. Res. Soc. Symp. Proc. Vol. 574 ©1999 Materials Research Society
effect found when Fe, Co, Ni and their alloys are used as ferromagnetic electrodes and A120 3 as the insulating barrier are coherent with the positive spin polarization found by Tedrow and Meservey for all of them in the above-mentioned experiments. To explain these results, it is c
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