O. Donzelli

Static magnetization direction in fcc (111) Fe/Ni multilayers

Abstract The rotation of the static magnetization was measured by Mossbauer spectroscopy as 2.5 monolayers of resonating 57 Fe were moved from inside the Fe films to the Fe/Ni interfaces in Fe/Ni (17/200 A) multilayers produced by thermal evaporation. The results indicate a remarkable dependence of the tilt angle as a function of distance from the surface.

The Ni-γ-Fe interface: hyperfine magnetic field and magnetic anisotropy

Abstract Conversion Electrons Mossbauer Spectroscopy (CEMS) is used to study the magnetic polarization state and the magnetic anisotropy of ultrathin Fe/Ni films in multilayer samples with two, one and any Fe/Ni interface. The analysis of the distribution function of the hyperfine fields indicates a magnitude of 34 T for the Fe/Ni interface field and gives evidence of the existence of a pure, highly susceptible γ-Fe phase. Results corresponding to intermixed regions suggest enhanced hyperfine fields and magnetic moments in local environments with more than 6 and up to 9 nearest Fe neighbor atoms, and low fields in Fe sites with 10–11 Fe neighbors. Out-of-plane perpendicular anisotropy is found only in samples having an Fe/Ni interface.

Magnetic properties of (111) Cu/Fe multilayers

The magnetic properties of (111) Cu/Fe multilayers grown on Cu underlayers of several thickness (2000-500-50 Å) on cleaved mica have been investigated by Mössbauer spectroscopy and SQUID magnetometry. The analysis of experimental results suggests that 2.5 monolayers of ψ-Fe interfaced with Cu evolves partially from paramagnetic to ferromagnetic states as the Cu underlayer thickness is reduced to 50 Å and partially from weak to strong antiferromagnetism.

Development of interface-induced out-of-plane magnetic anisotropy in Ni/Fe/Ni trilayers

Abstract The magnetic properties of trilayers Ni(1000 A)/ 57 Fe( x A)/Ni(200 A) grown by evaporation in ultra-high vacuum, have been investigated using Mossbauer spectroscopy, magneto-optic Kerr vector-magnetometry, and magnetic force microscopy (MFM). The data measured from a sample with Fe thickness x =6 A show the development of an out-of-plane anisotropy that tends to align the magnetization of the 57 Fe at about 40° respect to the normal to the layer plane. For higher 57 Fe layer thickness the data indicate that the magnetization lies in the film plane. The magneto-optic measurements and the MFM images demonstrate also that when x =6 A the 57 Fe layer induces a reorientation of the magnetization of the Ni outermost layer from in-plane to out-of-plane. The results are interpreted as due to anisotropy effects induced at the interface Fe/Ni and to the γ→α transition that takes place between 6 and 16 A.

Study of the Fe−Ni interface by conversion electron Mössbauer spectroscopy

Nickel-gamma iron interfaces obtained by growing films of Fe on Ni substrates are studied using CEMS. We have found that a) intermixing is confined to two atomic layers, b) Fe atoms deposited on structural defects of the Ni (111) surfaces are magnetically polarized, c) Fe epitaxially grown on Ni is highly susceptible at room temperature. Moreover the Ni−Fe interface shows Neel-type magnetic anisotropy.

Effect of magnetic anisotropies on spin waves of multilayers with antiferromagnetic coupling

Abstract We investigate how the spin waves spectra of Fe Cr finite superlattices with antiferromagnetic coupling depend on the interface anisotropy (in-plane and out-of-plane). The magnetic ground state, evaluated within the mean field approach, exhibits a canted configuration of the static magnetization. The spin waves are calculated by considering dipole interaction, intralayer and interlayer exchange coupling, uniaxial and interface anisotropies, in presence of an external field. The calculated spectra are compared with recent experimental Brillouin scattering data which show two spin waves peaks.

LOCATION OF MUONIUM AND HYDROGEN IN C60 FULLERENE AND ASSOCIATED ELECTRONIC STRUCTURE AND HYPERFINE PROPERTIES

The unrestricted Hartree-Fock (UHF) procedure is used to investigate the locations, associated electronic structures and hyperfine interactions for muonium and hydrogen in C60 fullerene. Our results indicate that from total energy considerations, in keeping with earlier investigations, the exohedral model has the lowest energy. However, the energies of the endohedral model involving the muonium (hydrogen) inside the fullerene and bonded to one of the carbon atoms, and of the muon at the center are found to be almost equal, contrary to earlier results. The hyperfine interaction constant for the endohedral site is in good agreement with that required to explain the lower observed muon spin-rotation (μSR) frequency in the C60-muonium system. The same appears to be the case for the exohedral model. However, there seems to be some uncertainty about the theoretical result in the latter case due to significant admixtures of higher spin states in the UHF wave-function. Additionally, in solid fullerene, the calculated location of the muonium for the exohedral model is such that it could be bonded to two fullerene molecules and therefore a muonium attached to a simple fullerene may not be representative of the exohedral state. This feature as well as the difficulty for the exohedral model of explaining the observed equality of the correlation times for relaxation effects associated with both μSR and13C relaxation times in nuclear magnetic resonance (NMR) experiments suggests that the endohedral model for muonium cannot at present be ruled out as a viable model in favor of the exohedral model. Possible avenues for future investigations to resolve some of the problems for both exohedral and endohedral models are discussed. Results obtained for muonium at the center of fullerene are presented and compared to the features of the observed high frequency μSR signal, and possible improvements in theory are discussed.

MOSSBAUER SPECTROSCOPY OF (111) GAMMA -FE SINGLE LAYERS GROWN ON NI

The magnetic profile of Ni(111)/γ-Fe/Ni with different thickness is studied by CEMS Mössbauer spectroscopy. It depends both from the thickness of the Fe layer and the roughness of the (111)Ni surface. For a particular thickness of 8 Å, two magnetic regions have been pointed out, one antiferromagnetic with low spin states and the second ferromagnetic with high spin states.

Magnetic behaviour of iron-nickel multilayers

Abstract We have studied the magnetic behaviour of Ni/Fe multilayers in both high anf low magnetic fields. The results obtained in high magnetic fields suggest in plane anisotropy typical of homogeneous magnetic thin films, those ones obtained in low fields give evidence for an antiferromagnetic coupling among the Ni layers near to the room temperature.

Magnetic properties of Fe/Ni multilayers grown on mylar

Abstract Ni200/Fe16 (A) multilayers were prepared by thermal evaporation in high vacuum onto mylar substrates at room temperature (RT). CEMS spectra at RT show the coexistence of several magnetic phases and out-of-plane magnetic anisotropy of the iron layers. SQUID measurements in the 4.2–350 K temperature range show in-plane magnetization of the Ni layers and a magnetic phase transition near 300 K.