To find out which electrons are producing these effects we have investigated their energy levels by inverse photoemission. Thereby, low energy electrons impinge onto the surface and drop into unoccupied energy levels emitting ultraviolet photons. This study discovered quantized electronic states in magnetic multilayers that are connected to their special properties.
The figure shows periodic changes in the density of electron states when the thickness of a copper film is changed, just a couple of atomic layers at a time (top panel). A way of understanding the experiment is shown on the right: Electrons in the Cu film are acting like waves and give maximum intensity when an integer number of oscillations fits into the film. These structures can be viewed as the smallest man-made interferometers, right at the atomic limit. They allow us to map out the wave function of electrons.
It turns out that the density-of-states maxima are correlated with oscillations of the magnetic coupling in multilayers (bottom panel, from Qiu et al., Phys. Rev. B 46, 8659 (1992)). The magnetoresistance oscillates with the same period. This gives us the clues for understanding the magnetic phenomena as the effects of quantized electron levels in nanostructures. Current efforts are directed towards manipulating the interfaces in magnetic multilayers to enhance the spin-dependent reflectivity and optimize the magnetic effects.
Review of magnetic nanostructures:
Himpsel et al., Advances in Physics 47, 511 (1998).
Basic principles of magnetic recording:
Grochowski and Thompson, IEEE Trans. Magn. 30, 3797 (1994)
Magnetic Quantum Well States::
Ortega et al., Phys. Rev. Lett. 69, 844 (1992);
Ortega et al., Phys. Rev. B 47, 1540 (1993);
Himpsel, Science 283, 1655 (1999).
Beyond the GMR effect in reading heads there are several other magnetic phenomena that can be incorporated into electronic devices. For example, spin-polarized tunneling lies at the core of magnetic random access memory (MRAM). A new field of magnetoelectronics is developing, where spin currents are used instead of charge currents. For that, it is important to know how to produce spin-polarized electrons, how to filter them, and how to detect their spin. Electrical and magnetic measurements provide practical information, but they integrate over all momenta, that is, they do not distinguish between electrons moving in different directions and at different speeds.
The cleanest technique to resolve the momenta of these electrons is angle-resolved photoelectron spectroscopy, which is able to measure the complete set of quantum numbers of electrons in solids. The figure below shows a theoretical plot of the quantum numbers energy (E) and momentum (k) for the two spin directions (red and green). The photoemission experiment on the right zooms in on the electrons that are relevant to magnetoelectronics. These are those close to the Fermi level (E=0). The intensity of these electrons is plotted versus their momentum in the two panels at the bottom right. From the k-separation of the two spin peaks one can infer the magnetic moment, from their relative intensity the spin-polarization, and from their width the mean free path of the electrons.
Spin currents can be manipulated by spin doping, where magnetic atoms are introduced as dopants for selecting one type of spin carrier. For example, adding iron to nickel suppresses the green spins, but not the red spins in the figure above. The Fe/Ni ratio of 20/80 corresponds to permalloy, the dominant material in magnetoelectronics.
Introduction: Himpsel et al., J. Magn. Magn. Mat. 200, 456 (1999).
Details:
Petrovykh et al., Appl. Phys. Lett. 73, 3459 (1998)
Altmann et al., Phys. Rev. B 61, 15661 (1998)
and Phys. Rev. Lett 87, 137201 (2001)
Supported by NSF-DMR
