K. Theis-Bröhl

The growth and structure of epitaxial niobium on sapphire

Abstract A review of the experimental and theoretical work on the growth and structure of epitaxial niobium on sapphire is presented. The review focuses on growth methods by molecular beam epitaxy (MBE) and sputtering techniques. The experimental results come from a broad spectrum of techniques, including electron microscopy, electron and X-ray diffraction, and surface microscopy. Theoretical studies provide models for the structure of the Nb–Al 2 O 3 interface and the nature and distribution of misfit dislocations. The results from growth of niobium on a number of sapphire surface planes are critically discussed and analysed, from the first stages of growth to the ensemble structure and as functions of the substrate. The resulting film characteristics are discussed as a function of substrate quality and temperature and of film thickness. The discussion attempts to define some outstanding questions that must be answered to fully understand this system.

Quantitative description of the azimuthal dependence of the exchange bias effect

While the principal features of the exchange bias between a ferromagnet and an antiferromagnet are believed to be understood, a quantitative description is still lacking. We show that interface spin disorder is the main reason for the discrepancy of model calculations versus experimental results. Taking into account spin disorder at the interface between the ferromagnet and the antiferromagnet by modifying the well known Meiklejohn and Bean model, an almost perfect agreement can be reached. As an example this is demonstrated for the CoFe/IrMn exchange biased bilayer by analysing the azimuthal dependence of magnetic hysteresis loops from MOKE measurements. Both exchange bias and coercive fields for the complete 360° angular range are reproduced by our model.

Polarized neutron reflectivity and scattering studies of magnetic heterostructures

The current interest in the magnetism of ultrathin films and multilayers is driven by their manifold applications in the magneto-and spin-electronic areas, for instance as magnetic field sensors or as information storage devices. In this regard, there is a large interest in exploring spin structures and spin disorder at the interface of magnetic heterostructures, to investigate magnetic domains in thin films and superlattices, and to understand remagnetization processes of various laterally shaped magnetic nanostructures. Traditionally neutron scattering has played a dominant role in the determination of spin structures, phase transitions and magnetic excitations in bulk materials. Today, its potential for the investigation of thin magnetic films has to be redefined. Polarized neutron reflectivity (PNR) at small wavevectors can provide precise information on the magnetic field distribution parallel to the film plane and on layer resolved magnetization vectors. In addition, PNR is not only sensitive to structural interface roughness but also to the magnetic roughness. Furthermore, magnetic hysteresis measurements from polarized small angle Bragg reflections allows us to filter out correlation effects during magnetization reversals of magnetic stripes and islands. An overview is provided on most recent PNR investigations of magnetic heterostructures.

AFM and x-ray studies on the growth and quality of Nb(110) on

We report the effect of annealing of sapphire substrate surfaces and explain the development of step patterns with large terraces. Furthermore, the epitaxial growth of Nb(110) on annealed and unannealed sapphire substrates is studied by AFM and x-ray methods. For the explanation of the well known two component line shape of transverse x-ray scans across Nb(110) Bragg peaks, a structural model is suggested.

Domain propagation in He-ion-bombarded magnetic wires with opposite exchange bias

Exchange-biased IrMn/CoFe full films are magnetically structured with He-ion bombardment into stripes with antiparallel-aligned loop shift. The patterning results in a two-step magnetization loop corresponding to two regions of oppositely aligned exchange bias. The longitudinal magnetization reversal through head-on domain-wall motion and partial penetration of magnetization from neighboring strips is highly asymmetric involving ripplelike domain structures and incoherent rotation of magnetization. In addition, Neel-wall-like structures with a preferred sense of rotation are formed at the edges of the strips. Along the transverse direction the reversal is dominated by the switching of the magnetic border structures between the strips. Complicated domain patterns are generated under other external field angles.

Magneto-optical study of the magnetization reversal process of Fe nanowires

We discuss results of magneto-optical Kerr effect (MOKE) measurements performed on a thin Fe film of 13 nm thickness, which has been patterned into a periodic arrangement of nanowires by means of optical interference lithography. The resulting array of nanowires consist of stripes having a width of 150 nm and a periodicity of 300 nm. MOKE hysteresis loops are measured within magnetic fields which are aligned in different directions, both parallel and perpendicular with respect to the direction of the nanowires as well as for various angles in between. A particular arrangement of the longitudinal Kerr effect measurement allows us to identify both the longitudinal and the transverse component of the magnetization of Fe nanowires. From this both the angle and the magnitude of the magnetization vector are derived. For a non-parallel alignment of the nanowires with respect to the direction of the external magnetic field, the hysteresis loops consist of a plateau region with two coercive fields Hc1 and Hc2, which is discussed as resulting from an anisotropic pinning behaviour of magnetic domains in directions along and perpendicular to the nanowires.

Experimental Observation of the Spin Screening Effect in Superconductor/Ferromagnet Thin Film Heterostructures

We have studied the nuclear magnetic resonance (NMR) of 51V nuclei in the superconductor/ferromagnet thin film heterostructures Pd_{1-x}Fe_{x}/V/Pd_{1-x}Fe_{x} and Ni/V/Ni in the normal and superconducting state. Whereas the position and shape of the NMR line in the normal state for the trilayers is identical to that observed in a single V layer, in the superconducting state the line shape definitely changes, developing a systematic distortion of the high-field wing of the resonance line. We consider this as the first experimental evidence for the penetration of ferromagnetism into the superconducting layer, a phenomenon which has been theoretically predicted recently and dubbed the spin screening effect.

Erratum to: Possible reconstruction of the ferromagnetic state under the influence of superconductivity in epitaxial V/Pd1−xFex bilayers

Epitaxial V/Pd1−xFex (001) bilayers with a V thickness of the order of 40 nm and with a Pd1−xFex thickness in the range from 0.8 nm to 4.4 nm were prepared by molecular beam epitaxy techniques. The Curie temperature of the Pd1−xFex layers varies between 90 and 250 K. For a bilayer with a Pd1−xFex thickness of 1.2 nm the ferromagnetic resonance measurements revealed a decrease of the effective magnetization 4πMeff of the ferromagnetic layer below the superconducting transition temperature of V. As a possible explanation for this decrease we suggest a spatial modulation of the ferromagnetic order in the Pd1−xFex layer due to modifications of the indirect exchange interaction of magnetic ions via conduction electrons in the superconducting state. A comparison with a recent theoretical investigation supports this possibility.

Growth of fcc(111) on bcc(110): new type of epitaxial transition observed for Pd on Cr

Abstract Depending on film thickness, t , and growth temperature, T , LEED and in-plane X-ray measurements show Kurdjumov—Sachs (KS) as well as Nishiyama-Wassermann (NW) epitaxial relationships for Pd(111) on Cr(110). This is a surprising result, because theoretical models definitely expect the KS orientation for this system. In a systematic study of this phenomenon we determined areas of different epitaxial orientations in a t T diagram. During Pd growth, we observed a continuous transition from the KS to the NW epitaxial relationship, which is accompanied by an enormous reorientation of material. Qualitative considerations, concerning the Pd island structure developing during growth, help to explain the observed behaviour.

Hyper-domains in exchange bias micro-stripe pattern

A combination of experimental techniques, e.g. vector-MOKE magnetometry, Kerr microscopy and polarized neutron reflectometry, was applied to study the field induced evolution of the magnetization distribution over a periodic pattern of alternating exchange bias (EB) stripes. The lateral structure is imprinted into a continuous ferromagnetic/antiferromagnetic EB bilayer via laterally selective exposure to He-ion irradiation in an applied field. This creates an alternating frozen-in interfacial EB field competing with the external field in the course of the re-magnetization. It was found that in a magnetic field applied at an angle with respect to the EB axis parallel to the stripes the re-magnetization process proceeds via a variety of different stages. They include coherent rotation of magnetization towards the EB axis, precipitation of small random (ripple) domains, formation of a stripe-like alternation of the magnetization, and development of a state in which the magnetization forms large hyper-domains comprising a number of stripes. Each of those magnetic states is quantitatively characterized via the comprehensive analysis of data on specular and off-specular polarized neutron reflectivity. The results are discussed within a phenomenological model containing a few parameters, which can readily be controlled by designing systems with a desired configuration of magnetic moments of micro- and nano-elements.