exempli®ed by the much studied system Fe/Cu(001). Fe is b.c.c. in bulk at temperatures below the b.c.c.±f.c.c. transition at 917°C; the Curie temperature in b.c.c. is at 770°C, and f.c.c. Fe is overall non-magnetic, although the calculated details depend very sensitively on the lattice constant. However, when Fe is deposited on a cold (77 K) Cu(001) substrate, and warmed to room temperature, the magnetization is perpendicular to the ®lm for a coverage ,5 ML, and is parallel for .5 ML. But below 10 ML, Fe is not b.c.c., but is pseudomorphic with the Cu(001) and is nominally f.c.c.

The detailed structure is actually f.c.t. (face-centered tetragonal), where the expansion parallel to the ®lm plane causes compression along the normal direction. This type of distortion is very common, occurring in the opposite sense for Ge/Si(001) as discussed in section 7.3.3. In the magnetic case, the tetragonality induces uniaxial anisotropy favoring perpendicular magnetization, which overcomes the shape eVects favoring parallel alignment, if the ®lm is thin enough. The particular system Fe/Cu(001) is in fact rather complicated, because in deposition even at room temperature, we can get exchange diVusion of Fe into the Cu, since, on surface energy grounds, the Cu wants to cover the Fe layer. There are now several similar examples (Fe/Ag, Fe/Au, Co/Cu, etc.), which have been seen by Auger spectroscopy, STM and other methods. The extreme sensitivity of the magnetism to the exact lattice parameter and micro-struc- tural condition of the ®lm means that there are (too many) contradictory results in the literature. These points have started to become clear in recent research papers; they are discussed further here in sections 5.5.3 and 8.3.

6.3.3Magnetic surface techniques

Investigation of magnetic surfaces and thin ®lms proceeds at two levels: structural and microstructural examination can be done using the same techniques as for non-mag- netic materials, as described in chapter 3, where some relevant examples were given. Some techniques which are speci®c to magnetism are described here in outline. These include optical rotation (Faraday and Kerr eVects plus magnetic circular dichroism (MCD)) and spin-polarized electron techniques. For analysis of domain structures, several microscope based methods have been developed which display magnetic contrast. These include SEMPA, SMOKE microscopy, TEM (Lorentz or holography, described in the last section), spin-polarized LEEM and magnetic force microscopy (MFM), which are explained below.

Optical techniques work in magnetic materials via the rotation of the plane of polarized light. The dielectric constant « is a tensor, with oV-diagonal terms of the form

6i«xy, in addition to the usual diagonal terms, so that rightand left-handed circularly polarized light behave diVerently. The eVects are called the Faraday eVect in transmission and the Kerr eVect in re¯ection. The most commonly used technique is called MOKE (magneto-optic Kerr eVect) and the acronym SMOKE is used when this technique is applied to surfaces. Depending on the light polarization with respect to the magnetization of the sample, one can measure diVerent Kerr signals which have components perpendicular and parallel to the sample. By varying the magnetic ®eld cyclically, one can obtain hysteresis loops to characterize the magnetic state of the sample.

214 6 Electronic structure and emission processes

Figure 6.17. The MIDAS column and preparation chamber arranged for magnetic studies. The column shows the position of the sample (S), objective lens (OL), electron parallelizers

(P) and secondary electron detector (SE), all of which are shown in more detail in ®gure 3.24, plus the ®eld emission gun (FEG). The sample can be transported from the airlock to the sample preparation station, which has multiple ports used as shown plus those for sample heating (H) and extra Knudsen cell evaporators (K1±K3); the YAG screen (Y) is for viewing the RHEED pattern. After preparation, the copper sample (C) can be transported to the SMOKE station before being examined by high resolution SEM and analysis (after Heim et al. 1993, reproduced with permission).

A diagram of the geometry of the preparation chamber of MIDAS at Arizona State University, and typical Kerr loops, are shown in ®gures 6.17 and 6.18. This con®guration has enabled in situ comparisons of structural and magnetic properties of a range of thin ®lm magnetic systems.

Magnetic circular dichroism (MCD) is a powerful recent technique which is a cross between photoemission and MOKE. By using spin-orbit split core levels, separated by 10 eV or more, the magnetism of thin ®lms, including internal interfaces, can be studied. The core levels are speci®c to particular elements, and the rotation of the plane of polarization is speci®c to the magnetism at the sites of these elements. A special merit of MCD is that it enables spin-speci®c and element-speci®c measurements to be made concurrently (Bader & Erskine 1994). This is particularly powerful, e.g. (a) in ferrimagnetic systems where diVering spin sublattices have diVerent orientations; (b) in trilayers and multilayers such as Fe/Cr/Fe(001), where the magnetic alignment is diVerent in the various layers (Idzerda et al. 1993). MCD is typically performed using

Figure 6.18. Polar and longitudinal hysteresis loops from Fe/Cu(001) grown and measured at room temperature, taken with the SMOKE setup in MIDAS shown in ®gure 6.17 with an angle of incidence of 45°: (a) and (b) 2.1 ML, no remanent magnetization; (c) and (d) 3.5 ML, remanent, mostly out of plane; (e) and (f) 4.7 ML remanent, in plane; (g) and (h) 10 ML, nonmagnetic f.c.c. Fe ®lm (after Hembree et al. 1994, reproduced with permission).

a synchrotron radiation source, and is a powerful application of display analyzers, as shown in ®gure 3.9(d) (Daimon et al. 1995).

Electron spectroscopy can be used to study magnetic domains. Electrons emitted from a magnetic material are spin polarized, because the spin up and spin down bands are populated diVerently. This is a strong eVect for low energy secondary electrons, where the polarization can reach 640%, with ,620% for Fe and ,610% for Co at

216 6 Electronic structure and emission processes

higher energies, where the spectrum re¯ects the polarization of the valence band (Kirschner 1985, Landolt 1985, Hopster 1994). Combined with SEM, this has lead to the development of scanning electron microscopy with polarization analysis (SEMPA). The extra element is the addition of a spin polarization detector. These detectors detect left±right (or up±down) asymmetries caused by spin-orbit (Mott) scattering, typically in a heavy target such as gold. Clear views of the domain structure, completely diVerent from the normal SEM image of the same area, can be obtained as shown in ®gure 6.19 (Hembree et al.1987; Scheinfein et al. 1990).

There are several versions of such detectors, and the polarization P is determined by an algorithm of the form

using the two signals L and R. The sensitivity of this technique depends on the eVective Sherman function SeV ,0.1±0.3 typically, increasing with incident electron energy in the range 10±100 keV, and the constant C,SeV21. The ®gure of merit for the detector is proportional to the ratio of the detected to the incident current (ID /I0)´SeV2 which is typically small (,1024). Typically, magnetic eVects can be enhanced by reversing the ®eld and taking diVerence signals, but this is not always necessary if one is prepared to live with an oVset signal arising from possible alignment errors in the detector system. Spin-polarized AES is also possible using an electron spectrometer in addition to a spin-polarized detector. As may be imagined, SPAES signal levels are very small, and long collection times are required to achieve an adequate SNR.

The above techniques use unpolarized electron sources, but spin polarized sources can be made using circularly polarized photoemission from spin-polarized valence bands. The most commonly used source is p-type GaAs(001), selectively exciting the heavy hole (p3/2) band with 1.4 eV photons. This puts spin-polarized electrons into the conduction band. The trick is then to activate the surface to negative electron aYnity, by coating the surface with a Cs/O layer. This strongly reduces the work function of GaAs, such that the bottom of the conduction band is above the vacuum level; electrons therefore spill out into the vacuum, and are suYciently intense to form a source, even for a microscope. Comprehensive reviews of this technique have been given by Pierce et al. (1980) and Pierce (1995).

Spin-polarized LEEM is a technique which is being developed for magnetic materials. Phase sensitive detection to eliminate unwanted background signals is possible, by modulating the laser polarization, and detecting the electrons in synchronism. This work is in its infancy at present, but progress has been reviewed (Bauer 1994); more recent results are given by Bauer et al. (1996). An example of a SPLEEM image from the last reference is shown in ®gure 6.20.

Magnetic force microscopy is a development of AFM which measures the ®eld gradient above ferromagnets; the force on a magnetic moment m in the z-direction Fz52mdBz/dz, and lateral force measurements are also possible. Typically, a cantilever with a etched Si tip of radius r ,10 nm is sputter coated with a ferromagnetic material. If the coercivity is high, this allows the magnetization distribution of the tip to remain ®xed as the ®elds on the sample are changed. The ®eld emanating from the tip falls oV