Spin-resolved Appearance Potential Spectroscopy Research Articles

Nanostructuring of NiMnSb(110): Influence on surface magnetic properties

The magnetic properties, crystal structure, and topography at the (110) surface of the ferromagnetic half-Heusler alloy NiMnSb have been investigated by means of magneto-optical Kerr effect, spin-resolved appearance potential spectroscopy, low-energy electron diffraction, and atomic force microscopy. A standard sputter-anneal cleaning procedure leads to a nanostructuring of the surface with consequences for the magnetic properties. This finding sheds light on the reduced spin polarization measured by surface-sensitive techniques on NiMnSb surfaces.

Reduced surface magnetization of NiMnSb(001)

The spin-dependent electronic structure at the (001) surface of the ferromagnetic half-Heusler alloy NiMnSb has been investigated by spin-resolved appearance potential spectroscopy. This surface-sensitive technique provides access to the spin- and element-resolved local density of unoccupied states and, therefore, probes the spin polarization above the Fermi level. In combination with low-energy electron diffraction and magneto-optical Kerr effect measurements, we characterized the sample with respect to its crystal structure, electronic structure, and magnetic properties. Our spectroscopic experiments were accompanied by a quantitative theoretical analysis. The theoretical appearance potential spectra are based on self-consistent electronic structure results for the bulk. They have been obtained in a fully relativistic framework as a self-convolution of the matrix element-weighted, orbitally resolved unoccupied density of states. The spin asymmetry in the experimental results, obtained from the surface region, is found to be significantly reduced compared with the calculations based on the bulk electronic structure of NiMnSb. Possible reasons for the reduced surface magnetization are discussed.

Spin-resolved appearance potential spectroscopy investigations on Fe/Cu(001) overlayers

The theoretical description of spin-resolved appearance potential spectroscopy (SR-APS) developed recently to deal with bulk transition-metal systems has been extended to free surfaces of ${\mathrm{Fe}}_{n}/\mathrm{Cu}(001).$ Results of previous experimental work on fcc-Fe on top of a Cu(001) substrate were interpreted to indicate the coexistence of a few ferromagnetically coupled Fe layers on top of nonferromagnetic Fe layers for $T=110$ K. Recent theoretical work, on the other hand, found for $n>2$ that the ground-state of this system corresponds to antiferromagnetically coupled Fe layers. In order to clarify this situation the SR-APS spectra of ${\mathrm{Fe}}_{n}/\mathrm{Cu}(001)$ have been calculated assuming the metastable ferromagnetic (FM) as well as antiferromagnetic (AFM) configuration. While for the FM configuration pronounced deviations from experiment were found, a nearly perfect agreement was obtained for the AFM ground-state configuration. This means that the experimental SR-APS spectra are not in conflict with the theoretical prediction but, on the contrary, they strongly support it.

Surface and interface magnetometry of layered structures by appearance potential spectroscopy

The magnetic properties of ultra-thin Cr/Fe layers on W(110) were investigated by means of spin-resolved appearance potential spectroscopy (APS). The surface and element sensitivity of APS was used to study the surface and interface effects on the magnetization. We observed induced magnetization in a single Cr layer on Fe(110), which is aligned antiparallel to the Fe substrate. The magnetic moment of Fe at the Cr/Fe interface is reduced compared with the bulk moment, whereas the surface magnetic moment of an uncovered Fe film is enhanced. The modification of the Fe surface and interface moment is described within a simple model.

Surface magnetic properties of ultrathin Fe films on W(110) studied by spin-resolved appearance potential spectroscopy

The surface sensitivity of spin-resolved appearance potential spectroscopy (APS) was exploited to investigate the magnetic properties of ultrathin Fe layers on W(110). The spin asymmetry, used to monitor the magnetic moment within the probing depth of APS, was studied as a function of the Fe film thickness. The spin asymmetry data reveal a pronounced maximum at a film thickness of $\ensuremath{\approx}6$ monolayers (ML), which is attributed to the complex film morphology in this thickness range. For thicker, structurally relaxed films, APS measurements with variable sampling depth show the Fe surface magnetic moments enhanced compared with the moments of the sublayers. The spin asymmetry data are analyzed within a simple model.

Magnetic order and electronic structure in thin films

The magnetic order in ultrathin films depends critically on a variety of film parameters. Therefore, to understand the magnetic properties of thin films on the basis of the spin-dependent electronic structure is a challenging task for both experimentalists and theoreticians. Experimentally, spin-resolved electron spectroscopies probe directly either the spin-dependent density of states or specific energy states, often at a defined wave vector. In this paper, two surface-sensitive techniques, that provide complementary information about the unoccupied electronic states, are described. Spin-resolved appearance potential spectroscopy gives element-specific access to the spin-dependent local density of states. Spin-resolved inverse photoemission permits detailed investigations of electron states characteristic of the surface and the layers underneath as a function of the wave vector. Both techniques are sensitive also to the film structure. A variety of magnetic film properties is discussed in the light of the electronic structure. The examples described in this paper include Fe films on W(110) and Cu(001) as well as Gd films on W(110).

Spin-polarised Electron Studies of Low-dimensional Magnetic Systems

Spin-polarised electrons provide unique experimental access to magnetic properties of surfaces and layered structures. The combined use of different techniques allows us to develop a microscopic picture of the physics underlying the macroscopic magnetic properties, e.g. magnetic phase transitions, magnetic coupling phenomena, exceptional surface magnetic properties. In this paper, two techniques are described together with the kind of questions addressed by them. Spin-resolved appearance potential spectroscopy gives local magnetic information about multi-component systems by probing the spin-dependent local density of unoccupied states. Spin-resolved inverse photo-emission measures specific electron states above the Fermi level. In particular, two-dimensional states serve as magnetic sensors at surfaces. Examples from surfaces as well as thin-film structures of band and local-moment ferromagnets are presented.

Theoretical investigations of spin-resolved appearance potential spectroscopy for Fe/Cu(001) overlayers

Abstract We present spin-resolved appearance potential spectroscopy (APS) calculations at the 2p3/2 edge of Fe in the high-moment ferromagnetic phase of fcc Fe/Cu(001) overlayers, based on a single-particle description of the underlying electronic structure. Within the framework of the real-space Korringa-Kohn-Rostoker method, the spectra were calculated in a layer-resolved mode. This allows one to investigate in detail the dependence of the total APS signal–a superposition of weighted layer-signals–as a function of the Fe overlayer thickness.

Theoretical investigations of spin-resolved appearance potential spectroscopy for Fe/Cu(001) overlayers

Theoretical investigations of spin-resolved appearance potential spectroscopy for Fe/Cu(001) overlayers

Temperature-dependent surface magnetization of studied by spin-resolved appearance potential spectroscopy

The temperature-dependent magnetic properties of the surface were investigated by means of spin-resolved appearance potential spectroscopy. Within the temperature range studied, from 100 K to 1100 K, two phase transitions occur in the bulk: a compositional order-disorder phase transition between 770 and 785 K and a ferromagnetic-paramagnetic phase transition with a Curie temperature of 863 K. The first one does not show up in the appearance potential signals of Fe and Ni, while the latter is reflected in the spin-asymmetry signals. Both the Fe and the Ni signals follow the bulk magnetization curve of with no evidence of any exceptional surface magnetic properties. Furthermore, our data show that, to a good approximation, the spin asymmetry of the appearance potential signal is proportional to the magnetization.

Theoretical description of spin-resolved appearance potential spectroscopy

A theoretical description of spin-resolved appearance potential spectroscopy is presented on the basis of a single-particle description of the underlying electronic structure. The final expression for the signal intensity turns out to be essentially a cross-section-weighted self-convolution of the density of states above the Fermi energy, in close analogy to the result for core-valence-valence Auger electron spectroscopy. Application of the formalism presented to bcc Fe and fcc Ni leads to results in very satisfying agreement with corresponding experimental data. Because this is achieved only by treating the involved transition matrix elements in a proper way, their properties are discussed in some detail.