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Configurational anisotropy effects in 90 degree domain wall imprinted thin films - statics and dynamics

Trützschler, J.; Sentosun, K.; Langer, M.; Mattheis, R.; Fassbender, J.; McCord, J.

The dynamic magnetic behavior of magnetic films has gained increased attention due to the use of magnetic films for high frequency inductors and their application as microwave filters. Moreover, the excitation and modification of spin waves has led to considerable interest in the field of magnonic crystals[1]. In general, the high frequency behavior of magnetic film stacks is determined by the material’s magnetic properties and by structural patterning. Yet, dynamic magnetization modes are not only inherent to the physical structure of magnetic films, but are also strongly influenced by e.g. ripplelike magnetic domain states[2] and as well as the pure existence of domain walls (DW)[3] in magnetic films. One way to introduce DWs in a controlled way in thin films is by local ion-irradiation[4,5,6].
In order to introduce a periodic DW pattern, extended Ni19Fe81(50nm)/Ir23Mn77(7nm) films with an initial unidirectional anisotropy are patterned by local He-ion irradiation into stripe-like twodimensional structures with periodically alternating directions of exchange bias. Magnetization patterns with zigzag oriented exchange bias directions are obtained. The influence of the DW density on static and dynamic magnetization properties is investigated for a stripe period (stripe width) from 12 μm (6 μm) down to 1 μm (500 nm). By this, exactly oriented and magnetically charged 90 N´eel-type domain walls with a DW density up to 2x103/mm are imprinted in the film.
Static and dynamic magnetization properties of the thin films are analyzed by complementary methods.
In Figure 1 (a) and (c) exemplary magnetization loops are presented for a stripe period of 2 μm. Perpendicular to the stripe axis an effective exchange bias field, which is caused by the magnetic interaction of the individual exchanged biased stripes, results in a net exchange bias direction. Due to DW interactions with increasing stripe period the samples correspondingly exhibit a decrease of remanent magnetization. Applying the external magnetic field parallel to the stripe axis, a two staged reversal loop is obtained. Even down to low stripe periods and despite of the straightening of magnetization the two step magnetization process remains for low stripe widths.
The corresponding change of high frequency permeability maps (up to 5 GHz) with bias fields in accordance with the shown magnetization reversal loops are displayed in Fig. 1 (b) and (d). Increasing the external magnetic field perpendicular to the stripes two distinct precessional frequencies, corresponding to an acoustic and an optical dynamic mode, are exhibited over the whole field range (Fig. 1(b)). Applying the field parallel to the stripe axis, in the central plateaued region (Fig. 1(d)) a bi-modal dynamic behavior is observed, that transforms into a single mode with higher permeability outside the plateau region. With increasing stripe period, the precessional frequencies at zero magnetic field decrease.
The occurring magnetic configurations are verified by high resolution Kerr microscopy in the longitudinal mode, examples of which are given in Fig. 2. The displayed images for different applied field values match the situation in Fig. 1 (c) and (d). The domain imaging data proves the existence of a pronounced magnetic modulation with high stability to magnetic fields even for a highly remanent state. The domain states, shown in Fig. 2 (b, c, d), exist in a magnetic field range, which is in accordance with the plateau in the magnetization loop and the change in the permeability spectrum around zero field.
Quasi-static and dynamic behavior are explained in terms of an increased domain wall mediated configurational magnetic anisotropy that results from variable magnetic charges at the imprinted domain walls due to the zigzagged alignment of magnetization. The magnetic charges increase with the rotational magnetization process. The DW stabilization induced effect has also significant influence on the dynamic magnetic characteristics. The effect of DW orientation relative to the alignment of exchange bias will be discussed. The controlled introduction of high density and locked micromagnetic objects opens new ways to control the static and dynamic magnetic properties of continuous magnetic thin films.
Funding from the German Science Foundation DFG through the grants MC9/7-2, FA314/3-2, and the Heisenberg programme of the DFG (MC9/9-1) is highly acknowledged.
[1] A. V. Chumak, A. A. Serga, B. Hillebrands, M. P. Kostylev, Appl. Phys. Lett. 93, 022508 (2008)
[2] C. Patschureck, K. Lenz, M. O. Liedke, M. U. Lutz, T. Strache, I. M¨onch, R. Sch¨afer, L. Schultz, and J. McCord, Phys. Rev. B 86, 054426 (2012)
[3] U. Queitsch, J. McCord, A. Neudert, R. Sch¨afer, L. Schultz, K. Rott, H. Br¨uckl, J. Appl. Phys. 100, 093911 (2006)
[4] J. Fassbender, J. McCord, J. Magn. Magn. Mater. 320, 579 (2008)
[5]C. Hamann, R. Mattheis, I. M¨onch, J. Fassbender, L. Schultz, J. McCord, Magnetization dynamics of magnetic domain wall imprinted magnetic films, submitted
[6] J. Tr¨utzschler, K. Sentosun, M. Langer, I. M¨onch, R. Mattheis, J. Fassbender, J. McCord, Magnetoresistive and domain investigations of zigzag folded magnetization structures, submitted

Keywords: Magnetic Domains; Anisotropic Magneto-Resistance; Kerr-Microscopy; Ferromagnetic Resonance

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Publ.-Id: 21180