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- ItemMagnetization dynamics of magnetic domain wall imprinted magnetic films(Bristol : IOP, 2014) Hamann, C.; Mattheis, R.; Mönch, I.; Fassbender, J.; Schultz, L.; McCord, J.The influence of micromagnetic objects on the dynamic magnetic excitation in magnetic thin films is studied by imprinting periodic domain wall patterns through selective ion irradiation in exchange biased Ni81Fe 19/IrMn structures. For high domain wall densities an increased precessional frequency is achieved. The zero field resonance of the domain wall state hereby depends directly on the stripe period, showing a pronounced increase with decrease of domain wall spacing. With the abrupt annihilation of magnetic domain walls with an applied bias field a jump-like decrease in precessional frequency takes place. The experimental data and micromagnetic simulations prove that the characteristic collective dynamic mode for the domain wall configurations is attributed to strongly coupled tilted magnetization structure. This is evidenced by an overlapping Néel wall structure for the narrowly spaced imprinted antiparallel unidirectional anisotropy state. The controlled introduction of high density frozen-in micromagnetic objects is a novel way to control the dynamic magnetic properties of continuous magnetic thin films.
- ItemIntroducing artificial length scales to tailor magnetic properties(Milton Park : Taylor & Francis, 2009) Fassbender, J.; Strache, T.; Liedke, M.O.; Markó, D.; Wintz, S.; Lenz, K.; Keller, A.; Facsko, S.; Mönch, I.; McCord, J.Magnetism is a collective phenomenon. Hence, a local variation on the nanoscale of material properties, which act on the magnetic properties, affects the overall magnetism in an intriguing way. Of particular importance are the length scales on which a material property changes. These might be related to the exchange length, the domain wall width, a typical roughness correlation length, or a length scale introduced by patterning of the material. Here we report on the influence of two artificially created length scales: (i) ion erosion templates that serve as a source of a predefined surface morphology (ripple structure) and hence allow for the investigation of roughness phenomena. It is demonstrated that the ripple wave length can be easily tuned over a wide range (25–175 nm) by varying the primary ion erosion energy. The effect of this ripple morphology on the induced uniaxial magnetic anisotropy in soft magnetic Permalloy films is studied. Only below a ripple wavelength threshold (≈60 nm) is a significant induced magnetic anisotropy found. Above this threshold the corrugated Permalloy film acts as a flat film. This cross-over is discussed in the frame of dipolar interactions giving rise to the induced anisotropies. (ii) Ion implantation through a lithographically defined mask, which is used for a magnetic property patterning on various length scales. The resulting magnetic properties are neither present in non-implanted nor in homogeneously implanted films. Here new insight is gained by the comparison of different stripe patterning widths ranging from 1 to 10 μm. In addition, the appearance of more complicated magnetic domain structures, i.e. spin-flop domain configurations and head-on domain walls, during hard axis magnetization reversal is demonstrated. In both cases the magnetic properties, the magnetization reversal process as well as the magnetic domain configurations depend sensitively on the artificially introduced length scale.
- ItemMagnetically induced reorientation of martensite variants in constrained epitaxial Ni-Mn-Ga films grown on MgO(001)(Milton Park : Taylor & Francis, 2008) Thomas, M.; Heczko, O.; Buschbeck, J.; Rößler, U.K.; McCord, J.; Scheerbaum, N.; Schultz, L.; Fähler, S.Magnetically induced reorientation (MIR) is observed in epitaxial orthorhombic Ni-Mn-Ga films. Ni-Mn-Ga films have been grown epitaxially on heated MgO(001) substrates in the cubic austenite state. The unit cell is rotated by 45° relative to the MgO cell. The growth, structure texture and anisotropic magnetic properties of these films are described. The crystallographic analysis of the martensitic transition reveals variant selection dominated by the substrate constraint. The austenite state has low magnetocrystalline anisotropy. In the martensitic state, the magnetization curves reveal an orthorhombic symmetry having three magnetically non-equivalent axes. The existence of MIR is deduced from the typical hysteresis within the first quadrant in magnetization curves and independently by texture measurement without and in the presence of a magnetic field probing micro structural changes. An analytical model is presented, which describes MIR in films with constrained overall extension by the additional degree of freedom of an orthorhombic structure compared to the tetragonal structure used in the standard model.
- ItemHyper-domains in exchange bias micro-stripe pattern(Milton Park : Taylor & Francis, 2008) Theis-Bröhl, K.; Westphalen, A.; Zabel, H.; Rücker, U.; McCord, J.; Höink, V.; Schmalhorst, J.; Reiss, G.; Weis, T.; Engel, D.; Ehresmann, A.; Toperverg, B.P.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.
- ItemThe magneto-optical gradient effect in an exchange-biased thin film: Experimental evidence for classical diffraction theory(Milton Park : Taylor & Francis, 2010) Schäfer, R.; Hamann, C.; McCord, J.; Schultz, L.; Kamberský, V.The magneto-optical gradient effect decorates the boundaries of in-plane domains even at perpendicular incidence of light in an optical polarization microscope. For its explanation, the classical magneto-optical diffraction theory was previously used to derive the effect from the same gyrotropic interaction as the Kerr effect. In order to explain the symmetry of the experimentally observed contrast on bulk ferromagnetic crystals, planar as well as perpendicular subsurface gradients in the magnetization had to be assumed. This was particularly needed when the surface magnetizations in neighboring domains pointed head-on and a gradient contrast appeared also in conditions of vanishing gyrotropic interaction at the surface. The gradient contrast in such conditions should not appear in very thin films where perpendicular magnetization gradients are not enforced by reduction of magnetostatic energy. Here we present the first experimental confirmation of this expectation, thus closing an experimental gap in verifying the predictions of the diffraction theory.
- ItemConverse Magnetoelectric Composite Resonator for Sensing Small Magnetic Fields(London : Nature Publishing Group, 2019) Hayes, P.; Jovičević Klug, M.; Toxværd, S.; Durdaut, P.; Schell, V.; Teplyuk, A.; Burdin, D.; Winkler, A.; Weser, R.; Fetisov, Y.; Höft, M.; Knöchel, R.; McCord, J.; Quandt, E.Magnetoelectric (ME) thin film composites consisting of sputtered piezoelectric (PE) and magnetostrictive (MS) layers enable for measurements of magnetic fields passively, i.e. an AC magnetic field directly generates an ME voltage by mechanical coupling of the MS deformation to the PE phase. In order to achieve high field sensitivities a magnetic bias field is necessary to operate at the maximum piezomagnetic coefficient of the MS phase, harnessing mechanical resonances further enhances this direct ME effect size. Despite being able to detect very small AC field amplitudes, exploiting mechanical resonances directly, implies a limitation to available signal bandwidth along with the inherent inability to detect DC or very low frequency magnetic fields. The presented work demonstrates converse ME modulation of thin film Si cantilever composites of mesoscopic dimensions (25 mm × 2.45 mm × 0.35 mm), employing piezoelectric AlN and magnetostrictive FeCoSiB films of 2 µm thickness each. A high frequency mechanical resonance at about 515 kHz leads to strong induced voltages in a surrounding pickup coil with matched self-resonance, leading to field sensitivities up to 64 kV/T. A DC limit of detection of 210 pT/Hz1/2 as well as about 70 pT/Hz1/2 at 10 Hz, without the need for a magnetic bias field, pave the way towards biomagnetic applications.