Spin-resolved inverse photoemission of ferromagnetic surfaces

Inverse photoemission (IPE) with spin-polarized electrons provides a way to measure separately the exchange-split majority and minority bands in ferromagnets above the Fermi level. Consequently spin-resolved IPE turns out to be an outstanding technique for obtaining information on surface magnetism: the magnetization of the topmost atomic layer may be investigated by measuring the exchange splitting of electronic states that are localized within the surface layer. Theoretical models of ferromagnetism may be tested by observing the temperature behavior of bands which contribute to the ferromagnetism of the material. The magnetic coupling of an adsorbate to the ferromagnetic substrate may be studied by probing adsorbate-induced bands. Results for the Ni(110) surface serve as an illustration to discuss the status of spin-resolved IPE and its application to the field of surface magnetism.     

Structural and magnetic properties of the Ce/Fe(001) interface: a spin resolved inverse photoemission study

We report a spin-resolved inverse photoemission study on the early stages of the Ce/Fe(001) interface formation in the coverage range 0–10 Å. The interface has been grown in two different fashions (i.e., keeping the substrate temperature either at room temperature or at 600 °C), leading to quite different morphologic and magnetic properties. The room-temperature-grown interface shows the formation of a non-intermixed disordered Ce phase, giving rise to quite featureless spectra and to the absence of detectable spin-dependent effects. The hot-grown interface shows instead the formation of a stable and ordered Ce---Fe compound, with the clear appearance of new structures which show a sizeable spin-dependent behavior. A relevant Ce 4f contribution to them is ruled out on the basis of their angular dependence and of cross-section arguments, indicating their band-like character.     

Spin-polarized photoemission study on the temperature dependence of the exchange splitting of Ni

Using spin-polarized photoemission with high energy- and angle resolution (E=100 meV, =±3°) we have investigated the temperature dependence of the exchange splitting of Ni in the temperature range 0.5T/T _c0.94. At room temperature we find _ex=0.18 eV for the exchange splitting of theS ₄ band at theX point of the Brillouin zone. With increasing temperature the total (spin-averaged) energy distribution shows a narrowing and merges into one peak. The spin-resolved energy distribution curves approach each other and are strongly broadened. A discussion of the data within current theories of itinerant electron magnetism is given. The spectra indicate that neither the pure Stoner model nor the prediction of local band theory, assuming a temperature independent exchange splitting are justified for Ni. We conclude that the exchange splitting decreases with increasing temperature and that transverse as well as longitudinal spin fluctuations are responsible for the broadening of the spin-resoived energy distribution curves.Experimental part of the work performed at Institut für Festkörperforschung of the KFA Jülich and supported by the Deutsche Forschungsgemeinschaft through SFB 125     

The epitaxial growth of Fe on GaAs(110): Development of the electronic structure and interface formation

The epitaxial growth of ultra-thin Fe films on GaAs(110) at a substrate temperature of 175° C has been studied by spin-resolved and spin-integrated photoemission with synchrotron radiation. Formation and evolution of the interface region have been followed for incremental Fe coverages Θ between 0.1 and 75 ML. The ordered growth of the overlayer is accompanied by reactive intermixing for metal coverage up to 15 ML followed by further As outdiffusion. The surface is ferromagnetically ordered by 6 ML.     

Chemisorption and surface ferromagnetism: Theoretical analysis of spin-resolved photoemission data from Ni(1 1 0)(2×1)−O

To extract information from recent experimental spin-resolved photoemission data from Ni(1 1 0)(2×1)?O, one-step theory calculations were performed for several geometrical and magnetic model structures. The observed strong influence of oxygen chemisorption on the normal photoemission spectra at room temperature is quantitatively reproduced assuming a (2×1) saw-tooth reconstruction with the third and fourth Ni layers magnetically dead, while the magnetization of the topmost two Ni layers is almost irrelevant for the spectra. It is concluded that bulk-like deeper layers are strongly quenched at least with regard to the near-X₂ exchange splitting.     

Realization of a spin-polarized two-dimensional electron gas via image-potential-induced surface states

Electrons in image-potential-induced surface states form a two-dimensional electron gas in front of the surfaces. In the case of ferromagnets, their binding energies as well as lifetimes depend on the orientation of their spin magnetic moment with respect to the magnetization direction. Various experiments with inverse photoemission and two-photon photoemission to detect the spin dependence of image states are reviewed. A new and successful approach to achieve and detect a spin-polarized two-dimensional electron gas is presented, namely polarization-dependent and spin-resolved two-photon photoemission. Additional time resolution opens the way to study spin-dependent electron dynamics.     

Temperature dependence of the Pd susceptibility

Experimental results on the temperature dependence of the magnetic susceptibility of pure Pd are compared with the prediction of Stoner theory using accurate energy bands.     

Observation of the self-convolved spin-split valence band density of states in MVV Auger decay from Fe(100)

By spin-resolved photoemission with synchrotron radiation we have investigated the Auger electrons from Fe(100) above the 3 p-3d resonance. The observed majority- and minority-spin Auger electrons have the expected kinetic energy independent on photon energy and without noticeable reduction because of correlation effects. The spin-resolved Auger electron energy distributions resemble convolutions of theoretical density of states curves of appropriate spin direction.     

Thickness dependence of the spin- and angle-resolved photoemission of ultrathin,epitaxial Ni(111)/W(110) layers

The thickness dependence of the magnetic band structure of ultrathin, epitaxial Ni(111)/W(110) layers has been studied by spin and angle-resolved photoemission spectroscopy. The changes of the spin-resolved photoemission intensities upon reducing the layer thickness depend strongly on the wavevector along the -L line of the Brillouin zone. The measured exchange splitting atk 1/3(-L) andk 1/2(-L) is found to be independent of the layer thickness for layers consisting of 3 or more atomic layers, while decreases rapidly with the layer thickness atk2/3(-L). This behavior is very similar to the temperature dependence of the spin-resolved photoemission spectra of bulk Ni(111) at the samek-points.     

The origin of enhanced magnetization in strained gadolinium

From combined spin-resolved photoemission and spin-polarized inverse photoemission, the experimental spin-resolved band structure of gadolinium on Mo(112) has been constructed. The occupied spin dependent electronic structure near the Fermi level is dominated by shallow dispersion of a spin minority band with considerable surface weight. There is an occupied spin majority bulk band straddling the Fermi level whose spin minority counterpart remains largely on the unoccupied side of the Fermi level. This results in large spin majority weight in the occupied band structure relative to spin minority.     

Exchange splitting of an oxygen 2p-derived band at Ni(100)

Employing spin-resolved inverse photoemission we have observed an exchange splitting of the unoccupied oxygen-induced band in the chemisorption system (2 × 1)-O/Ni(110). At the centre of the surface Brillouin zone the splitting between the oxygen 2p-derived majority and minority band— referred to the magnetization vector of nickel-was found to be 80±20 meV. This effect is a manifestation of the strong magnetic correlation between the oxygen and nickel bands. The size of the splitting is surprising as earlier experimental work indicated a substantial reduction of the surface magnetization due to chemisorption. For the empty 2π1-derived band of CO on Ni(110) no such splitting has been observed.     

Study of magnetotransport properties of interacting quantum dot systems using the ECA method

We study the magnetotransport of the interacting QD system in a magnetic field using the numerical method of embedded-cluster approximation (ECA). The spin-resolved conductances display different magnetic field dependences for different transport regimes. Through comparison of conductance polarization, the mixed-valence regime shows the largest polarization. The spin-resolved conductance as a function of the ratio between the magnetic field and Kondo temperature H/TK is found to exhibit an approximate universal behavior in the Kondo regime. We also investigate conductance dependence on interaction strength and find interesting inversion of sign of polarization in some cases.     

Spin-resolved photoemission form ferromagnetic Ni(110)c(2 × 2)-S: Theory and comparison with experiment

Changes in spin-resolved photoemission spectra, which had been observed experimentally upon chemisorption of a c(2 × 2)-S layer on ferromagnetic Ni(110), are investigated theoretically by means of dynamical one-step-model photoemission calculations. Two large peaks produced about 1 and 2 eV below the clean Ni peak can, with the aid of the bulk band structure along ΓX and along the “surface-Umklapp” direction LW, be explained as due to Umklapp coupling in the initial state and in the final state, respectively.     

Angle resolved UV photoemission spectra from a Bi(0001) surface

Angle resolved photoemission spectra taken from a Bi(0001) surface for hv = 16.9 and 21.2 eV exhibit (1) broad features which appear to be associated with one dimensional density of states, rather than band like features, and (2) a weak dependence upon the direction of incident light These effects can be explained by a large number of nondegenerate final state bands available for optical emission and by a uniform mixing of atomic orbitals in the initial states.     

Spin- and angle-resolved photoemission from ferromagnetic Fe(001)

For near-normal photoemission from ferromagnetic Fe(001) excited by linearly polarized synchrotron radiation, energy-resolved spin polarization and intensity distribution have been measured at 60 eV photon energy. Calculations using a one-step theory of photoemission consistently reproduce. the present spin-resolved data as well as earlier spin-averaged measurements. The quasi-particle exchange splitting deduced from the data is 2 eV. The agreement with band structure calculations is suggested to be coincidental due to a compensation of real and imaginary self-energy corrections.     

Two- to three-dimensional crossover in the electronic structure of (Bi, Pb)2(Sr, La)2CuO(6+delta) from angle-resolved photoemission spectroscopy

The hole-concentration (x) dependence of the three-dimensional energy-momentum dispersion in (Bi, Pb)2(Sr, La)2CuO(6+delta) has been investigated by angle-resolved photoemission spectroscopy. For a heavily overdoped sample of T(c) < or = 0.5 K, an energy dispersion of approximately 10 meV in width is observed in the vicinity of the (pi, 0) point with varying momentum along the c axis (k(z)). This k(z) dispersion is zero for underdoped, optimally doped, and slightly overdoped samples up to a doping level corresponding to T(c) = 22 k. At higher doping levels we observe significant dispersion of the order of 10 meV (sample with T(c) < or = 0.5 K). This is clear evidence that at a doping value corresponding to T(c) = 22 K, a crossover from two- to three-dimensional electronic structure occurs.     

Measurement of electronic structure in van der Waals ferromagnet Fe5-xGeTe2

As a van der Waals ferromagnet with high Curie temperature, Fe_5-xGeTe₂ has attracted tremendous interests recently. Here, using high-resolution angle-resolved photoemission spectroscopy (ARPES), we systematically investigated the electronic structure of Fe_5-xGeTe₂ crystals and its temperature evolution. Our ARPES measurement reveals two types of band structures from two different terminations with slight k_z evolution. Interestingly, across the ferromagnetic transition, we observed the merging of two split bands above the Curie temperature, suggesting the band splitting due to the exchange interaction within the itinerant Stoner model. Our results provide important insights into the electronic and magnetic properties of Fe_5-xGeTe₂ and the understanding of magnetism in a two-dimensional ferromagnetic system.     

Intrinsic impurity-band Stoner ferromagnetism in C60Hn

We identify Stoner ferromagnetism in fcc C60H(n) (n=odd) by using a local density approximation in the framework of the density functional theory. Hydrogen chemisorption on fullerenes creates quasilocalized π electrons on the fullerene surface, overlapping of their wave functions giving rise to a narrow half filled impurity band in the fcc C60H(n). The Stoner-type ferromagnetic exchange between the itinerant electrons leads to spin-split impurity bands. The magnetic moment per C60H(n) molecule is 1 μ(B) (for n=odd) or 0 (for n=even, including zero), only one of the hydrogens contributing to the spin-split states. Direct overlapping of the quasilocalized π-electron orbitals is essential for the ferromagnetism.