SEMPA Studies of Thin Films, Structures, and Exchange Coupled Layers

Abstract

Scanning electron microscopy with polarization analysis (SEMPA) has developed into a powerful technique to study domains in ultrathin films. In this chapter, we discuss from a very general point of view the instrumental aspects of the method. Examples of thin film investigations are given that demonstrate unique features of SEMPA. New solutions around apparent limitations of the technique are presented at the end, i.e., analyzing samples with contaminated surfaces and imaging in external fields. IntroductionIn 1982, triggered by the investigations of the energy dependence of the spinpolarization of secondary electrons (SE), the idea emerged to use this effect in a microscope to image magnetic structures [1,2]. The combination of a conventional Scanning Electron Microscope (SEM) and a spin-polarization analyzer promised the potential of investigating magnetic microstructures with high spatial resolution. In 1984, the first spin-SEM was realized by Koike and coworkers [3], followed less than one year later by a microscope built at NIST [4]. The latter group introduced the abbreviation SEMPA, which stands for Scanning Electron Microscope with Polarization Analysis. We will use both acronyms interchangeably. The next instruments followed in Europe [5,6]. A sketch of SEMPA is given in Fig. 7.1. Due to the low depth of information, ultra-clean surfaces are essential, requiring ultrahigh vacuum (UHV) conditions. Since conventional SEMs usually do not operate in UHV, appropriate UHV-compatible columns (or guns) are rare and expensive. Hooked on is the detector system that consists of an electron optic and a spin-polarization analyzer. The optic has to focus the secondaries into the polarization analyzer. The most important feature of the electron optic is the acceptance angle for SE. It is extremely important that the optic collects electrons emitted in the full 2π solid angle.Several microscopes have been realized [7][8][9][10][11][12][13][14], and a few more have been proposed. Basically, all the systems look very similar. Some have attachments for surface

Magnetic imaging with spin-polarized low-energy electron microscopy

“…Contrary to what is observed during dewetting that is initiated at step-bunches (Fig. 7d), no magnetic contrast is detected along the edges of the grooves, and the magnetization of the continuous film remains aligned along the [1][2][3][4][5][6][7][8][9][10] direction (Figs. 7h and 7i).…”

“…Fe films become magnetic at a thickness of about 1.5 ML [113] with an easy axis of magnetization parallel to the [1][2][3][4][5][6][7][8][9][10] direction, due to a strong surface anisotropy. This magnetization axis remains stable up to about 50 ML thickness [114], above which a SRT occurs and the [001] direction becomes the new easy axis of magnetization (i.e.…”

“…As the annealing proceeds, Fe removed from the grooves accumulates on the remaining Fe ribbons, increasing locally the film thickness. The corresponding magnetic images for a spin polarization of the incident electron beam along the [1][2][3][4][5][6][7][8][9][10] and [001] directions are given in Figures 7c and 7d, respectively. Away from the dewetted regions, magnetization lies along the [1-10] direction (large black magnetic domain), as observed for continuous films before annealing (see Fig.…”

“…Mapping the surface magnetization reveals that it follows the edges of the voids, and is not necessarily oriented exactly along the [001] crystal axis. In other words, while the magnetization of the continuous films is aligned along the [1][2][3][4][5][6][7][8][9][10] direction of the W substrate, the direction of magnetization in the elongated Fe islands may or may not be aligned along the [001] direction, depending on how these islands are oriented after the annealing is complete. Reduction of surface anisotropy does not appear to be a key driving force in the observed SRT.…”

“…-optical microscopies, where magneto-optic Kerr microscopy is one of the most widely known techniques [3,4]; -electron microscopies, including scanning electron microscopy with polarization analysis (SEMPA) [5,6], Lorentz microscopy [7,8], and electron holography [9,10]; -scanning probe microscopies such as magnetic force microscopy (MFM) [11,12], spin-polarized scanning tunneling microscopy (SP-STM) [13], or ballistic emission magnetic microscopy (BEMM) [14,15]; -soft X-rays microscopies, including X-ray photoemission electron microscopy (X-PEEM) [16,17] and scanning transmission X-ray microscopy (STXM) [18,19].…”