Large Magnetoresistance at Room Temperature in Ferromagnetic Thin Film Tunnel Junctions
Moodera, J.S., Kinder, L.R., Wong, T.M. & Meservey, R., 1995. Large Magnetoresistance at Room Temperature in Ferromagnetic Thin Film Tunnel Junctions. Phys. Rev. Lett. 74, 3273
Essay about this article
Long before GMR per se was discovered, there existed, by 1972, another magnetoresistive effect that resembles Current Perpendicular to the Plane Magnetoresitance (CPP-MR); that is tunneling magnetoresistance (TMR). The difference between them is the spacer layer between the magnetic layers. In GMR it’s a nonmagnetic metal whereas for TMR it’s an insulator. The difference is important because it determines the type of conduction process that transmits the current between the magnetic entities [grains or layers]. For a metallic spacer, transmission takes place by conduction electrons at the Fermi level; whereas for an insulating spacer there are no electrons at the Fermi level as the insulator falls in a gap between conduction and valence bands: therefore electrons “tunnel”, in the quantum mechanical sense, between the magnetic entities. Conduction electrons have wavefunctions that oscillate between positive and negative amplitudes with a frequency related to the wavelength at the Fermi level, e.g., for a typical 3d transition-metal this is on the order of 1 Å. This is a rapid oscillation so that minute details of the roughness of the interfaces [of this lengthscale] between the spacer and magnetic layers affect the electrical conduction process. Indeed this is why the details of the roughness and diffusion at the interfaces are crucial for predictions of ab-initio calculations of GMR in metallic multilayers. Electrons that tunnel between magnetic entities do not have oscillatory wavefunctions; rather they decay exponentially. In this case details about the interfaces with the magnetic entities are less important. This is the primary reason ab-initio calculation had a far greater success in predicting TMR behavior (see Article17).
TMR was first observed in the tunneling between grains in granular nickel films by Gittleman et al in 1972. Michel Jullière was the first to observe it in the more conventional multilayer geometry in 1975 known as Magnetic Tunnel Junctions (MTJ) where he found 14% TMR at low temperatures for Fe(iron)/Ge(germanium) /Co(cobalt); this was followed by Maekawa and Gäfvert’s observations, in1982, of TMR by using nickel, iron and cobalt electrodes across nickel oxide barriers. Then, in 1995, Miyazake and Moodera both observed reproducible TMR in MTJ’s. Their work came at a propitious time when there was increased interest in magnetoresistive elements and it gave rise to a flurry of activity in this field.
The first phenomenological models of TMR were provided by Gittleman et al. and Jullière, and theoretical work on MTJ’s was first done by John Slonczewski; see the next section. ab-initio calculations came close on the heels of the findings of Miyazake and Moodera and were based on the Landauer-Büttiker (LB) formalism of conduction. This formalism, which is suitable for ballistic transport, was previously used for the contribution of band structure to the GMR in metallic multilayers. Transport in metallic systems is usually described as diffusive; this is in large part due to the oscillatory wave functions at the Fermi surface which are the carriers in metallic structures (of course, impurity scattering is also necessary). However, while the transport in the ferromagnetic electrodes may be diffusive, the tunneling across the insulating barrier is through evanescent states and this part of the conduction can be ballistic, in which case one can apply a Landauer-Büttiker-like analysis to TMR. Also, as tunneling currents are small compared to currents in metals, the role of current-driven charge and spin accumulation do not have a big effect on the resistivity of MTJs, i.e., their neglect does not change one’s predictions for the TMR of MTJs.
M. Jullière, Phys. Lett. A 54, 225 (1975); S. Maekawa and U. Gäfvert IEEE Trans. Magn. 18, 707 (1982); T. Miyazaki and N. Tezuka, J. Magn. Magn. Mater. 139, L231 (1995); J.L. Gittleman, Y. Goldstein and S. Bozowski, Phys. Rev. B 5, 3609 (1972).
a. Is there spin accumulation in magnetic tunnel junctions; if so where?
b. Is it important to include these accumulations when determining the conductance of a magnetic tunnel junction?
The above article is reprinted with permission from the author(s) of Moodera, J.S., Kinder, L.R., Wong, T.M. & Meservey, R., 1995. Large Magnetoresistance at Room Temperature in Ferromagnetic Thin Film Tunnel Junctions. Phys. Rev. Lett. 74, 3273 Copyright (1995) by the American Physical Society. Readers may view, browse, and/or download material for temporary copying purposes only, provided these uses are for noncommercial personal purposes. Except as provided by law, this material may not be further reproduced, distributed, transmitted, modified, adapted, performed, displayed, published, or sold in whole or part, without prior written permission from the American Physical Society .
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