Electronic States in Magnetic Quantum Wells
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Electronic States in Magnetic Quantum Wells
J. E. ORTEGA*, F. J. HIMPSEL*, G. J. MANKEY**, and R. F. WILLIS** *IBM T. .. Watson Research Center, P.O. Box 218 Route 134, Yorktown Heights, NY 10598, USA "**Physics Department, Penn State University, University Park, PA 16802 ABSTRACT
We have searched for the electronic states that mediate oscillatory magnetic coupling in superlattices, and have found strong evidence that these are quantum well states, which are created by quantizing the momentum of s,p-band states perpendicular to the interfaces. The quantum well picture also explains how quantum well states in noble metals become spin-polarized, due to a spin-dependent electron reflectivity at the interface with the ferromagnet. The resulting implications for magnetoresistance are discussed. MAGNETIC COUPLING VIA QUANTUM WELL STATES
Magnetic multilayers have become very popular recently since they exhibit a "giant" magnetoresistance, which has an impact on the development of magnetoresistive reading heads in magnetic storage.'- 6 Here we explore the nature of the electronic states that mediate magnetic coupling across a non-magnetic spacer layer. In this first section we will provide evidence that these are quantum well states, confined to the spacer layer by Bragg reflection at the interfaces. In the second section the implications of the quantum well model onto magnetoresistance will be discussed, in particular the spin dependent interface reflectivity that is a natural consequence of the model. One of the strong clues of a connection between magnetic coupling and quantum well states is shown in Fig. 1. It compares magnetic oscillations7,r with oscillations in the density of states at the Fermi level, found by inverse photoemission. 9,1" Both exhibit the same periodicity of about 6 layers. The oscillations in the density of states are due to thickness-dependent quantum well states crossing the Fermi level, as shown in Fig. 2.9,10 Here we have plotted a series of inverse photoemission spectra, taken for Cu films on fcc Fe(100) at thickness intervals of about two monolayers (the exact thickness can be read from Fig. I, top curve). These spectra represent the density of unoccupied electronic states at a momentum parallel to the surface ki, = 0. Compared to the bulk spectrum of Cu(100) top we find that the s,p-band continuum has been discretized into several quantum well states in the thin films. This is exactly what one expects theoretically, as shown in Fig. 3. Looking at the band structure of Cu(100) at k11 = 0 we find a continuum up to the band edge at X' 4, given by the E(k-L) dispersion of the s,p-band (line in Fig. 3, bottom). For a thin film this continuum is expected to split up into a number of discrete states (numbered dots in Fig. 3, bottom). This effect may be viewed as a quantization of the momentum perpendicular to the film, k1-, due to the matching conditions for the wavefunction at the confining interfaces (Fig. 3, top). Roughly-speaking, a discrete number of halfperiods of the so-called envelope fu
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