Ultrathin Iron Films Research Articles

Brillouin scattering from ultra thin iron films

The spin-wave excitations of ultrathin iron films (1 to 5 nm) on sapphire substrates have been studied by inelastic light scattering using Brillouin spectroscopy. The room temperature magnetization J, magnetic anisotropy field B an, and the g-factor have been determined by fitting the measured ω- B results for surface spin waves to the related Damon-Eshbach theory. For thicknesses below 4 nm the film magnetization J f decreases linearly with film thickness and is found substantially smaller than J f values determined by static magnetization measurements. Equivalent reductions in J f , however, were also obtained in light scattering studies of ultrathin Fe films on GaAs substrates reported previously in the literature [2].

Fe/MgO(001) model catalysts: Electron microscopic and spectroscopic studies

An ultrathin iron film deposited on a MgO single crystal in UHV is heated up to 500°C. AES/LEED measurements combined with TEM observations performed on small single crystal platelets show that the film is transformed into small particles below 300°C. Around 380°C, surface diffusion induces a spectacular growth of some particles. Upon further heating, the evolution of the system depends on the shape of the particles. In most cases, the particles become square and orient themselves along the MgO(110) direction (epitaxial configuration); however, they are unstable: they progressively dissolve in the MgO matrix, leading to a substitutional (Mg x Fe 1− x )O solid solution. In other cases, the iron particles remain shapeless and stable, suggesting that dissolution only occurs when the particles grow epitaxially.

Diminished magnetization in ultrathin MBE-grown iron films (abstract)

We have performed direct SQUID measurements of the average magnetization M̄ of a series of ultrathin (18–130 Å) Fe single crystal films grown by molecular beam epitaxy (MBE) on GaAs substrates and overcoated with Al. These measurements were motivated by earlier FMR measurements [G. A. Prinz, G. T. Rado, and J. J. Krebs, J. Appl. Phys. 53, 2087 (1982); J. J. Krebs, F. J. Rachford, P. Lubitz, and G. A. Prinz., J. Appl. Phys. 53, 8058 (1982)] on the identical films which showed a monotonic increase in average resonance field with decreasing thickness. In-plane M̄ data were taken over the ranges 0.5≤H≤30 kOe and 5≤T≤295 K and corrected for the (significant) substrate diamagnetism. The data clearly indicate that M̄ is diminished for the thinnest films and, for a film of thickness L, the behavior of M̄ is given by M̄=M0[(η+1/2(1−η)L/L0)] to a good approximation, where M0 is the bulk magnetization of Fe, L0 is a characteristic decay depth, and ηM0 is the magnetization at the interface where the Fe film is compressed by 1.4% due to lattice mismatch. At 295 K we find η=0.49 and L0=91 Å with only modest changes at 5 K. The absolute M̄ values are known to ±10% due largely to the uncertainty in L as determined by x-ray fluorescence and Dektak measurements. We conclude that this effect is not due to a nonuniform film thickness (voids or islands) nor to interdiffusion at the interface. Rather it is thought to be due to the depth dependent lattice compression arising from mismatch although the mechanism is not yet clear.

Conversion electron Mossbauer spectroscopy of very thin iron films; Ultra-high vacuum equipment and preliminary measurements

The apparatus for preparation of ultra-thin iron films, heat and ion beam treatment, SIMS and LEED analysis and in situ CEMS measurements at vacuum better than 10−8 Torr, has been constructed and tested. Preliminary data for thin iron films (3 A - 30 A), obtained by evaporation of57Fe atoms on hot Si and Mo substrates are discussed.

The structure of ultrathin iron films on tungsten single-crystal surfaces

Ultrathin iron films vapour deposited onto the surface of a cylindrical tungsten single crystal are discussed. Results from work function change, Auger electron spectroscopic and low energy electron diffraction techniques are combined for a comparison of the initial stages of film growth on four low index planes. Advantage is taken of the opportunity to evaporate onto and simultaneously to make measurements on all surface orientations of the 〈110〉 zone.

Clarification of the different roles of surface anisotropy for thermal spin waves and FMR modes

Measurements by Mössbauer spectroscopy of the position dependence of the hyperfine field in monocrystalline iron films show that the fractional deviation of the spontaneous magnetization at temperature T from its value at T=0 K is larger by a factor of about two at a film surface than in the film’s interior. This result agrees with an early theoretical prediction of a factor of exactly two which is based on the assumption that the surface anisotropy is zero. In contrast, the results of recent ferromagnetic resonance experiments on ultra-thin films of monocrystalline iron were shown to be dominated by a surface anistropy which is nonzero. This discrepancy is reconciled for measurements at T=300 K by making use of the general boundary condition which contains the exchange stiffness A and some component(s) of the surface anisotropy Ksurf. The crucial argument is that at 300 K the thermally excited spin wavelengths are so short that at the film surfaces the normal derivative 2A∂m↘/∂n of the oscillating magnetization m↘ is very much larger than Ksurfm↘. Thus Ksurfm↘ is neglible for thermal spin waves even though it is comparable to 2A∂m↘/∂n for the long decay distances (or wavelengths) occurring in ferromagnetic resonance.

Temperature and thickness dependence of the magnetic hyperfine field in ultrathin iron films

Temperature and thickness dependence of the magnetic hyperfine field in ultrathin iron films

Mössbauer effect study of ultra-thin iron films

Mössbauer spectra of discontinous Fe films with average thickness between 6 ± 1 and 35 ± 1 Å were studied at temperatures varying from 300 down to 12 K. The disappearance of the long-range order at liquid helium temperatures in films thinner than 13 Å is attributed to an intrinsic paramagnetic state in which the spin-flip rates is due to zero-point motion. Films with thickness between 35 and 16 Å have complex spectra suggesting the coexistence of several magnetic states. One assumes that because of the low effective Curie temperatures of the films, large magnetization fluctuations could be responsible for the population of the metastable and unstable equilibrium states predicted by the molecular field theory. The Debye--Waller factor of multilayer absorbers with total Fe thickness of 20 000 Å has been studied at 300 and 80 K as a function of the thickness ( d) of the individual Fe layers. This factor decrease substantially when d is decreased, although the changes in the shape of the spectra, as demonstrated, have no influence on the total absorption area. It seems that crystal instabilities occur at the same film thickness which are responsible for the magnetization instabilities as revealed by the Mössbauer spectra.

Magnetic Ordering and Critical Thickness of Ultrathin Iron Films

Received 15 October 1964DOI:https://doi.org/10.1103/PhysRevLett.13.800©1964 American Physical Society