Spin-Polarized Scanning Tunneling Microscopy

Literature Review by Dmitri Y. Petrovykh

Physics Department, University of Wisconsin, Madison, WI, USA (1997)

Scanning-probe microscopies revolutionized surface science research, providing the possibility of direct observation and manipulation of atomic structure of surfaces with sub-nm precision. Can this awesome power be extended into studies of magnetic properties? Spin-polarized scanning tunneling microscopy proposes to use spin-dependent tunneling to do just that. Here details and preliminary results are reviewed for the two currently developed approaches: magnetic (metallic) tips and optically pumped GaAs tips. Each of the two techniques is beneficial for a specific class of measurements and may ultimately achieve atomic resolution.

I. Introduction

The field of surface magnetism is the natural extension of the bulk magnetism studies. Surface science in general was inhibited for a long time because of the lack of sample preparation conditions adequate for performing reproducible experiments on well-defined systems. Since the ultra-high vacuum (UHV) equipment became widespread, the surface science research flourished. Techniques were developed to achieve atomic resolution in real-space imaging including the recent surge of scanning-probe microscopies, started with the invention of the Scanning Tunneling Microscope¹ (STM). The availability of structural data on the atomic level led to deeper understanding of the macroscopic phenomena and allowed the direct observation of microscopic effects. In magnetic measurements similar advances can be expected when imaging of magnetic moments of individual atoms or, at least direct observation of magnetic domain boundaries, becomes possible. Behavior of such domain boundaries is important both for "pure" research, such as the near phase transition phenomena, and for practical applications, such as imaging bits on magnetic storage devices.

Few magnetic microscopies² are available for imaging at progressively diminishing scale. Optical microscopes can be adapted to produce contrast using Kerr rotation of linearly polarized light reflected off domains with different magnetization³. The diffraction limit of the resolution is about 500 nm for optical techniques and can be somewhat overcome by near-field optical microscopy⁴. Magnetic Force Microscopy (MFM) measures the force that stray magnetic field of the surface exerts on the small magnetic tip. Magnetization then is probed indirectly via the magnetic poles of the sample⁵, and the resolution obtained is on the order of 10-50 nm. Electron-based microscopies push resolution further. Secondary-electron microscopy with polarization analysis (SEMPA) measures the spin polarization of secondary electrons emitted from a magnetized sample⁶ with resolution of about 20 nm. High-energy electron microscopes, such as Lorentz microscopy, where the deflection of the beam by magnetic domains is detected, can achieve resolution of 2 nm and can be used both as reflection and transmission techniques⁷. So far the only technique potentially capable of atomic resolution is spin-polarized scanning tunneling microscopy (SPSTM). While spin contrast is not yet obtainable on a routine basis, the recent advances described in the following sections suggest that SPSTM may become the ultimate magnetic microscopy technique.

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