The properties of surfaces and interfaces have drawn considerable interest in recent years. Phenomena such us giant magneto-resistance, interlayer coupling, and interface anisotropy are current topics of discussion. Nuclear resonance scattering makes a handy experimental tool which is capable of contributing to the field on an atomistic scale. The scattering process proceeds via nuclear resonances lying in the hard X-ray regime, with a very high cross-section and remarkable optical activity. This high cross-section opens the possibility of studying monolayer systems even when they are buried. Additionally, due to selection rules and optical activity, the technique is very sensitive for the determination of internal fields (hyperfine fields), their magnitudes and directions. Depending on the chosen geometry of the wave vector k, the polarisation e of the X-rays, and the external and internal magnetic fields, contributions of different transitions such as m = ±1 and m = 0 might give very pronounced and different spectra. Being a time resolved technique, it benefits from the excellent time structure of synchrotron radiation and an outstanding signal-to-noise ratio. Each type of magnetism (ferro-, ferri-, antiferro-, para-, dia-magnetism), either static or dynamic, can be studied.

The following examples were studied at ID18 utilising the 57Fe resonance at 14.4 keV. The beamline offers the corresponding sample environment: a 6-8 circle diffractometer and cryo-magnet systems mounted on circles for grazing incidence scattering experiments.

Non-collinear Magnetism in fcc-Fe/Co (100) Films

A fundamental issue in solid state physics is the relation between atomic geometry and magnetism. Being at the borderline between antiferromagnetically and ferromagnetically-ordered transition metals Fe is an exemplary case for the influence of structural effects on the magnetic properties. It is well known that the magnetic structure of Fe can be altered by modifying the structural parameters. In particular, while metallic Fe is a simple ferromagnet in its normal bcc structure, it is theoretically expected to be ordered antiferromagnetically in the fcc structure for a certain range of lattice parameters.

We investigated the magnetic structure of thin, 2-10 atomic layers (ML), epitaxial fcc-Fe films sandwiched between fcc-Co(100) layers on a Cu(100) single crystal substrate at room temperature. Two different kinds of samples were grown: (i) a fcc-57Fe film with a wedge of 2-10 ML (wedge slope 1 ML/mm), and (ii) a 7 ML Fe film with a single 57Fe monolayer at various positions within the 56Fe film (see Figure 70a).

By scanning the X-ray beam over the wedge, one measures the quantum beat pattern in the time spectrum of nuclear specular reflection originating from different thicknesses and different depths of the sample, respectively. The interference pattern demonstrates that the Fe films are magnetically ordered at room temperature for all thicknesses. This implies an enhanced ordering temperature with respect to the Neél temperature and the surface Curie temperature reported for fcc-Fe films on Cu(100).

Further analysis of sample (i) gave an unexpected magnetic structure which could not be explained by any simple pattern. However, the analysis of the quantum beat spectrum from sample (ii) (Figure 70 (b,c)) gave a clear result of the magnetic structure at different depths. The hyperfine field vector at the interface lies close to the film plane, whereas it points at an angle of more than 60° from the film plane in inner atomic layers. These results also permit an explanation of the magnetic structure of sample (i).

The results provide evidence for an unexpected non-collinear magnetic structure at the interface formed by these two materials, where the orientation of the Fe moments coherently rotates in successive atomic layers. This appears to be the first established case of non-collinear magnetism among all ultrathin films of itinerant magnetic systems.


Anti-ferromagnetic Coupling and Spin Fluctuations in Thin Fe3O4 Films

Thin epitaxial layers of Fe3O4 on MgO(100) show a number of surprising features in their magnetic behaviour. It is difficult to reach magnetic saturation, which is related to the fact that the magnetic moments point out of the plane in zero field. Ultrathin layers (d < 5 nm) loose their magnetisation completely at room temperature because of spin fluctuations. It has been argued that these phenomena are caused by the presence of antiphase boundaries between structural domains that are bound to form in the Fe3O4 layer because its lattice constant is twice that of the MgO substrate.

A 15 nm thick 57Fe3O4 layer was investigated at 280 K in magnetic fields up to 5 T, using a grazing incidence geometry. Some representative spectra are displayed on the left of Figure 71. The spectra can be fitted with the two components known from bulk Fe3O4. The A-site (tetrahedral) Fe3+ ions have a positive hyperfine field whereas the B-site (octahedral) Fe ions, which have an average charge of +2.5, have the field in the opposite direction. In a field of 5 T only the m = ±1 transitions contribute to the spectrum, in agreement with the fact that the spins are aligned in the magnetic field, which was oriented in the plane of the sample and perpendicular to the photon beam.

However, in lower magnetic fields the m = 0 transitions are also present, showing that not all magnetic moments are aligned along the magnetic field. It is possible to describe this behaviour quantitatively by the following model. Across an antiphase boundary (APB), spins at B-sites are strongly coupled antiferromagnetically. To minimise their magnetic energy, they turn perpendicular to the magnetic field while staying in the plane of the APB. Because the field is in the plane of the layer, these spins must turn perpendicular to the sample plane. The orientation of neighbouring spins is governed by the competition between the exchange interaction, which tries to keep the spins (anti) parallel and the magnetic field (external field plus shape anisotropy field), which tries to align them in the sample plane. The only adjustable parameter in this model is the average distance between the APB's.

Fits to the spectra using this model are shown in the same figure. The panels on the right show the corresponding spin profile, i.e. the orientation of the spins as a function of their distance to the APB. The resulting average size of the structural domains, 114 nm, is in reasonable agreement with estimations from electron microscope pictures.


Bulk Spin Transition in a Magnetic Superlattice

Metallic multilayers built from ferromagnetic layers separated by non-magnetic spacer layers often show antiferromagnetic (AF) interlayer coupling, a well-known example being the Fe/Cr system. Besides their interlayer interaction J, the layer magnetisations M experience the magneto-crystalline anisotropy A. Of particular interest as a model system are Fe/Cr superlattices of an even number of magnetic layers with external magnetic field Hext in the magnetocrystalline easy axis of Fe. In case of a fourfold anisotropic symmetry and with the provision that M of each Fe layer is confined to the sample plane, for Hext = 0 one has two, mutually perpendicular, initial directions of equal energy of the magnetisation axis. In nonzero external fields one of the anisotropy-stabilised configurations becomes energetically unfavourable and, at a certain critical external field depending on the J/A ratio, it moves to a more favourable state. This is an example of the bulk spin flop (BSF) transition, a well-known phenomenon in atomic antiferromagnets. Nevertheless, the first evidence of a BSF transition in a multilayer is new and the change of the direction of the layer magnetisation during a BSF transition in multilayers has not yet been directly observed.

BSF was experimentally studied on a MgO(001) / [57Fe(25Å)/Cr(14Å)]20 superlattice at 294 K. The magneto-optical Kerr effect was indicative of AF coupling with a saturation field of about 1 T and of BSF at a field about 25 mT. The epitaxial relationship on MgO(001) substrate is MgO(001)[110]//Fe(001)[100], therefore the magnetisation of the individual Fe layers points parallel or anti-parallel to either of the Fe[010] or Fe[100] axes in the film plane (cf. the blue double arrows in inset 1 of Figure 72).

First the sample was saturated in plane in one of the easy directions (inset 2) then the external field was decreased to zero (inset 3), and then the sample was turned in plane by 90° and time integral angular scans were recorded as a function of increasing external magnetic fields (insets 4 and 5) at a grazing angle between 0° and 1°. The scattering plane was vertical and the wave-vector k was perpendicular to the in-plane magnetic field. Under such conditions, no AF superreflections can be observed in the time integral scans if M is perpendicular to k but they will appear if M is parallel to k. The spectra in Figure 72 are a series of time integral scans taken in increasing magnetic fields up to 25 mT. The appearance of the 1/2 and 3/2 order AF reflections from 14 mT on is a direct evidence of the 90° rotation of the Fe layer magnetisations, i.e. of the BSF process. The peaks 0 and 1 are the total reflection peak and the first order structural Bragg reflection, the latter being used for arbitrary normalisation of the spectra. This state is preserved after the magnetic field is removed because of the fourfold symmetry of the anisotropy.

Principal Publications and Authors
C. Carbone (a), A. Dallmeyer (a), M.C. Malagoli (a), K. Maiti (a), J. Wingbermühle (a), W. Eberhardt (a), D.L. Nagy (b), L.H. Bottyán (b), L. Deák (b), E. Szilágy (b), R. Rüffer (c), O. Leupold (c), to be published.

(a) Institut für Festkörperforschung des Forschungszentrums Jülich (Germany)
(b) KFKI Research Institute for Particle and Nuclear Physics Budapest (Hungary)
(c) ESRF

L. Kalev (a), T. Hibma (a), F.C. Voogt (a), L. Niesen (a), R. Rüffer (b), O. Leupold (b), to be published.

(a) Nuclear Solid State Physics, Materials Science Centre, Groningen (The Netherlands)
(b) ESRF

L. Bottyán (a), J. Dekoster (b), L. Deák (a), B. Degroote (b), E. Kunnen (c), C. L'abbé (b), G. Langouche (b), O. Leupold (d), M. Major (a), J. Meersschaut (b), D.L. Nagy (a), R. Rüffer (d), to be published.

(a) KFKI Research Institute for Particle and Nuclear Physics Budapest (Hungary)
(b) IKS KU Leuven (Belgium),
(c) VSM KU Leuven (Belgium)
(d) ESRF