The electronic and magnetic character of epitaxial Fe films on Ag(001) has been studied as a function of Fe coverage by spin-and angle-resolved photoemission. At coverages well below a monolayer, the spectra exhibit a local spin-split electronic state. Although spectra for films in the monolayer coverage range display electronic structure in close agreement with calculated monoiayer-film critical-point energies, no spin polarization is observed up to 2.5 monolayers. Thicker films approach the spin-split electronic structure and spin polarization of bulk Fe(001).PACS numbers: 75.50. Bb, 75.10.Lp, 75.70.Dp, 79.60.Cn A fundamental problem of magnetism is at what dimension long-range ferromagnetic order occurs. The question of whether two-dimensional ferromagnetism is possible was raised long ago. 1 Powerful computational methods have since been developed to investigate the electronic and magnetic structure of ultrathin films within the local-density approximation. 2 "" 4 Recent calculations describing the spin-resolved band structure and consequent magnetic character of epitaxial Fe monolayers on Ag(001) predict an enhanced magnetic moment for one 3 ' 4 or two 4 such Fe monolayers (ML). This is believed to be due to a decrease in coordination number and an increase in nearestneighbor spacing experienced by the atoms comprising the monolayer, and a lack of hybridization between the electronic states of the overlayer and substrate. These studies thus address fundamental aspects of the formation and interplay between the electronic and magnetic character of a system.Spin-polarized angle-resolved photoelectron spectroscopy is well suited for investigating such effects, as it provides an independent identification and decomposition of spin-split emission features for samples exhibiting ferromagnetic order. An additional fundamental question addressed in most studies of magnetic surface layers or thin films is a determination of the onset of long-range magnetic order as a function of temperature or film thickness. There is strong evidence to suggest that the exchange splitting alone cannot be taken as an indication of spontaneous magnetization or as a measure of long-range ferromagnetic order. 5 " 7 Since spin-polarized angle-resolved photoelectron spectroscopy measures the net polarization of the photoelectrons, it provides a direct measure of longrange magnetic order (or absence thereof) required to address the predictions of enhanced moments.In this Letter, we report the results of spin-and angle-resolved photoemission studies of Fe films epitaxially grown in situ on Ag(001). The epitaxial system of Fe on Ag is well suited for these studies, since Ag has only very weak (sp) emission between the Fermi energy E F and =*3.5-eV binding energy, the energy range over which emission from the prominent Fe 3d bands occurs. With use of a photon energy of hv = 60 eV to minimize the photoelectron escape depth, optimum surface sensitivity is achieved. Use of this photon energy also permits direct comparison with previous work on...

We investigate the magnetic properties of Mn adsorbates on Fe͑100͒ in the regime up to a few monolayers.Magnetic circular dichroism in absorption shows long-range ferromagnetic order for the Mn adsorbate, with antiferromagnetic alignment with respect to the Fe substrate. Element-specific magnetic domain imaging and hysteresis measurements show that the macroscopic magnetic behavior of the Mn adlayer is fully determined by the Fe substrate. For coverages below 0.5 ML the Mn absorption spectra show rich structures that are typical for localized d states. From this the Mn ground state is identified as a mixture of atomiclike d 5 and d 6 states, with a local spin moment of 4.5 B . However, the circular dichroism is 2.4 times smaller than expected for this ground state, suggesting disorder within the Mn adsorbate with an ordered moment of 1.9 B at 120 K. The magnetic signal vanishes near 1 ML coverage, consistent with the theoretically predicted c(2ϫ2) antiferromagnetic ground state of the monolayer.

Temperature-induced changes in the electronic structure of Fe(100) have been investigated by spin-and angle-resolved photoemission for temperatures between room temperature and the Curie temperature T c . States nearly stationary in energy (T^r^) have been observed for photon energy hv = 60 eV. However, from a strong increase in minority-spin intensity for hv = 3l and 21 eV, a downwards shift of the AJ band is inferred to occur upon heating towards T c for large k vectors. PACS numbers: 75.50.Bb, 75.10.Lp, 79.60.Cn The electronic structure at finite temperatures of the 3d-transition metals Fe, Co, and Ni is currently a matter of strong theoretical interest. Spinpolarized band theory based on the self-consistent local-density-functional description gives an adequate account of the ferromagnetic ground state (e.g., cohesive energy, nonintegral moments). 1 However, controversial attempts have been made recently to describe transition-metal magnetism at finite temperatures. 2 "" 6 The basic common idea is to try to incorporate into the theory the existence of local magnetic moments even above T c . The ferromagnetic-to-paramagnetic phase transition is then governed by thermal disordering of the moments, requiring much less energy than singleparticle spin flips which would involve energy changes as large as the exchange splitting. The controversy is over the spatial extent of correlation among the magnetic moments, which is connected intimately to the present debate on the existence of spin waves above TQ?The ferromagnetic-to-paramagnetic phase transition of Fe has been studied by spin-unresolved, angle-resolved photoemission. 8 However, only by measuring the electron spin explicitly can exchange-split bands be identified unambiguously and the band dispersions be detected, as will be shown below. Furthermore, the spin dynamics at elevated temperatures, as spin rotations around the spontaneous magnetization direction or flips of local magnetic moments which currently are considered to be the driving force for the phase transition, can be observed only by this method. We have therefore, for the first time, performed a spin-, angle-, and energy-resolved photoemission experiment on temperature-induced changes in the electronic structure of Fe. Because of a predicted wave-vector (k) dependence of the temperature dependence of the exchange splitting, 5,9 we employed monochromatized tunable synchrotron radiation from the German storage ring BESSY, allowing selection of initial states with different k.The experiment is similar to a recent one on Ni(110) 10 using a resonance lamp. Total energy resolution, including the linewidth of light, was 0.4 eV at Ai/ = 60 eV and about 0.3 eV at Ai/ = 31 eV. The angular resolution was about ±3° at hv = 60 eV decreasing to about ±4° at hv = 31 eV, resulting in k resolution of about j-of the Brillouin zone. The sample was cleaned in situ by standard surfaceanalysis techniques and its surface conditions were monitored by low-energy electron diffraction and photoelectron spectroscopy. 11 T...

By spin-and angle-resolved photoemission with synchrotron radiation the electronic structure of Fe(100) has been tested between room temperature and the Curie temperature T, for photon energies in the range XI-70 eV. The spinresolved energy distribution curves (SREDCs) reflect the dispersions of the Ai'symmetry initial state bands. This manifests in an abrupt change in spin character of the peak near EF from predominantly minority spin to majority spin when tuning the photon energy across 33 eV. The non-spin-resolved EDCs thereby remain nearly unchanged. Upon heating to 0.85 T/T,, depending on photon energy, qualitative different changes in the SREDCs are observed: At hv = 60 eV, I'!& is found to be stationary in energy upon heating, and the spin-summed intensity decreases less than 5%. I?~~ becomes strongly broadened in energy and wave vector, resulting in a strong loss of intensity. Contrary, at hv = 31 and 21 eV, an increase in minority-spin (and total) photocurrent upon heating is observed. This is interpreted as resulting from a decrease of the exchange splitting with temperature near H.