We report the development of a broadband ferromagnetic resonance (FMR) system operating in the frequency range from 10 MHz to 70 GHz using a closed-cycle He refrigeration system for measurements of thin films and micron/nano structures. The system is capable of carrying out measurements in frequency and field domain. Using two coplanar waveguides, it is capable of simultaneously measuring two samples in the out of plane and in plane FMR geometries. The system operates in the temperature range of 27-350 K and is sensitive to less than one atomic monolayer of a single crystal Fe film.

The issue of mixed-valency has been intensively studied on transition metal oxide materials such as, manganites, rare earth metals and a variety of compounds. An interesting phonemenon occurs, surface valence transition, which has recently been used to explain the electronic structure of the Sm(0001) surface [1]. Eu metal is divalent whereas its valency in compounds is 2 + or 3 + . The Eu 2+ ions exhibit a magnetic moment of 7 µ B whereas Eu 3+ ion has a non-magnetic ground state (S = 3, L = 3, J = 0). EuF 3 shows a weak . Eu in EuF3 is trivalent, but the core level peaks in surface sensitive photoemission studies show an additional structure at lower binding energies relative to the main photoemission (PE) lines. Cho and Oh [3] have found that the appearance of these lines is connected with the formation of the divalent Eu states on the surface of the trivalent Eu compounds (pressed powder of EuF 3 , EuCl 3 , Eu 2 O 3 ). A similar effect has been reported for metallic EuPd 3 and EuPt 5 [4]. Surface valence transition has been attributed to the reduced coordination number and modified atomic geometry relative to the bulk structure. On the other hand, it is known that PE from some light rare earth and 3d transition metal compounds shows "shake-down" satellites which are related to the photoemission final state. In the case of EuF 3 , the additional final state would have Eu 2+ character due to the increased occupation of the Eu 4f level. The satellites then have the charge transfer character. The theory given by van der Laan et al. [5] and Asada et al. [6] could explain the core level PE in some of the 3d transition metal compounds. One has to also mention the Gunnarsson -Schönhammer model developed for mixedvalence systems, such as La and Ce compounds [7]. The model relates the increased 4f occupancy to the hybridization between the 4f and conduction states. In this work, we prove the origin of the Eu 2+ originated lines by performing the photoemission from various kinds of samples including single crystal. We have clearly shown that the final state effects are not responsible for the Eu 2+ structure in the spectra. Moreover, we have found that the ultrathin buried layers exclusively exhibit a 3 + valency.

Molecular beam epitaxy was used to grow a ferromagnet/antiferromagnet (Fe/KCoF 3 ) system. A series of structural, surface, and magnetic characterization techniques were used to understand the correlation between microstructural and magnetic properties in this exchange bias system. Depending on deposition conditions, the fluoride grew either in a single crystal or a polycrystalline form, which was initially confirmed by reflection high energy electron diffraction patterns and x-ray studies. The crystalline form of the KCoF 3 significantly affected the magnetic properties, in particular the exchange bias and the magnetocrystalline anisotropy of the Fe layer. Transmission electron microscopy ͑TEM͒ studies were carried out to shed more light on the microstructure of the fluoride and on the interface between Fe and KCoF 3 layers. Single crystals KCoF 3 layers grown at elevated temperature on ͑001͒ Fe template have a ͑001͒ orientation. On the other hand, the cross-sectional TEM images of the polycrystalline fluoride deposited at room temperature show columnar structure of the grains with a column diameter of about 10 nm. In addition, planar defects were observed in the Fe layer due to the slight mismatch between Fe and KCoF 3 lattices. These defects and grain boundaries in the antiferromagnet are responsible for considerable modification of magnetic properties of the structures with polycrystalline fluoride compared to those with the single crystal KCoF 3 . Magnetic anisotropy and the exchange bias were measured using ferromagnetic resonance and superconducting quantum interference device magnetometry, respectively. The exchange bias and blocking temperature in the samples with polycrystalline fluoride were significantly reduced, however, the low-temperature fourfold anisotropy was enhanced by a factor of 3 for the samples with 1-nm-thick Fe and polycrystalline fluoride compared to the samples with the same thickness of Fe but single crystal fluoride.

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Exchange bias was studied in the Fe/KCoF3 ferromagnet/antiferromagnet system. KCoF3 can be deposited onto single crystal of Fe, either in the polycrystalline or single crystal form, depending on growth conditions. The samples were grown by molecular beam epitaxy on Ga-terminated GaAs (100) wafers. We study effects of the crystal state of the fluoride, thickness of the Fe film, crystallographic orientation of the Fe, and temperature on exchange bias. The structures with single crystal KCoF3 show that the exchange bias is well correlated with the coercivity at low temperatures and vanishes at a temperature close to the Néel temperature. Both the magnitude of the exchange bias and the blocking temperature of the samples with the polycrystalline fluoride were significantly reduced compared to the single crystal structures. As the Fe film thickness was increased, the exchange bias decreased for all samples. In contrast, the blocking temperature remained unchanged for the samples with the single-crystal fluoride. The exchange bias measured along the easy anisotropy axis of the Fe was slightly larger than that measured along the hard axis. In addition, all samples exhibited a weak training effect.