Magnetic hyperfine field at surfaces

Magnetic Moment (and Magnetic Hyperfine Field) at Surfaces and in Ultrathin Films... 

Experimental Methods Probing Magnetic Hyperfine Field at Surfaces... 

The goethite consists of fairly monodisperse acicular crystals about 20-25 nm long, 10-15 nm wide and 4 10 nm thick (Fig. 5-4 bottom, left). The diamond-shaped cross section indicates that the crystals are again essentially bounded by 101 faces. The surface area is ca. 130 m /g and the magnetic hyperfine field at RT is 38-39 T (for further properties... 

Koch et al. (1985) produced feroxyhyte in the same way as described above, except that the 0.1 M FeCb solution was produced by dissolving Ferrum reductum (a mixture of iron metal and Fe304) in HCl. They obtained platy crystals 15-90 nm across and 2-3 nm thick with a surface area of 110 m /g and a magnetic hyperfine field at 5 K of 52.5 T. The magnetic properties of this material are described in Bender-Koch et al. (1995). 

Method 3 produces ca. 3 g of hematite. The sample prepared in 2 10 M HCl consists of subrounded crystals between 30-50 nm across (Fig. 10-1 c) with a surface area of around 30 m /g. In 10 M HCl the crystal size is around 150-200 nm (Fig. 10-1 d) and the surface area is only a few m /g. The X-ray peaks of the 0.002 M HCl product are somewhat broader than those of the material produced in 10 M HCl owing to the smaller crystal size. The Mossbauer spectrum at RT (Fig. 10-4) shows a sextet corresponding to a magnetic hyperfine field of 53.3 T. 

An other important point is related to the surface hyperfine field which may be a priori smaller or larger than the bulk hyperfine field at low temperature in the case of ultrathin films, according to the substrate and the material coating the surface. Mossbauer studies of a-Fe nanoparticles have revealed that the hyperfine field values of inner Fe nuclei are similar to those of bulk crystalline a-Fe at low temperature (i.e. static blocked magnetic regime), but those characteristic of the superficial atomic layer are found either lower or higher [91-96]. Such conclusions are well supported by some examples of the literature with 2 nm a-Fe particles in organic liquids [93],... 

B. SURFACE HYPERFINE FIELD. The demagnetizing field and the Lorentz field are not defined for the atoms at the surface. Calculations of the magnetic dipole fields at atoms near the surface in fine particles and thin films of a-Fe show that only the first surface layer is perturbed, with variations of the order of lOkOe depending on the position at the surface. Studies of thin and ultrathin metal films by Mossbauer spectroscopy (see, e.g.. Refs. 31, 224, 238, and 239) show that one or two atomic layers are perturbed, as for the magnetization (see Section B.3). The surface hyperfine field may be larger or smaller than in the bulk at... 

Structure develops close to the surface. A number of theoretical studies on magnetic transition-metal films have predicted the enhancement of their surface magnetic moment with respect to their bulk values. Theoretical calculations are coherent with the simple picture that the properties of the atoms at surfaces are closer to the properties of isolated atoms compared to atoms in the bulk. Consequently, the magnetic moment is predicted to be a monotonic function of the dimensionality. The principia of magnetic hyperfine field (Btf) calculation are basically the same, and the same methods can be applied. 

A more promising quantity to be measured in this aspect seems to be the magnetic hyperfine field (Bhf). which can be probed layer by layer because of the isotope specificity of the Mossbauer spectroscopy (MS) (at least in the case of Fe films and surfaces). This is important in view of the expected oscillating character of Bhf close to the surface and its dependence on temperature, which exactly follows the temperature dependence of magnetization. 

The temperature for Bose-Einstein condensation varies with density as n20. Because density is limited by three-body recombination, the search for the transition leads naturally to lower temperatures. Unfortunately, at temperatures below 0.1 K, adsorption rapidly becomes prohibitive. To avoid this problem, Hess [4] suggested confinirig the atoms in a magnetic trap without any surfaces. The states confined are the "low-field seeking" states, (HT, electron spin "up"). These are the hyperfine states (F-l,m-l) and (F=l,m=0). 

This implies that the magnetic field at the iron is produced by the short-range core-polarisation effects, and that the conduction-electron polarisation is negligible because of there being little 4 density at the Fermi surface. This is consistent with the successful interpretations of the hyperfine field values using a 3(/-model only. 

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