Magnetic domain structure of micro-patterned PtMn/NiFe exchange bias bilayers

The local modification of magnetic properties by means of ion irradiation has been demonstrated for perpendicularly magnetized Co/Pt [1], for amorphous soft magnetic alloys [2] and for exchange-bias systems [3-5]. In all cases the magnetic features are not determined by the patterning itself, but by the micromagnetic feature sizes. Therefore, if the pattern sizes are close to the intrinsic length scales of the system under investigation a change in the overall magnetic behaviour is expected. For an exchange bias system this intrinsic length scale is in the range between 100 – 1000 nm [5].
The aim of the present study is to investigate the domain structure of a micro-patterned exchange bias system during magnetization reversal. Special attention is paid to the interaction between adjacent features changing the overall magnetic behaviour of the system. As a test system we chose a bottom pinned exchange bias bilayer consisting of 20 nm PtMn and 6 nm NiFe, capped with 5 nm of Ta, prepared by ion beam and physical vapour deposition. PtMn is chosen as the antiferromagnetic material due to its technological relevance and due to the chemically ordered L10-phase which is required in order to obtain a large exchange bias field. This phase can easily be disordered by means of ion irradiation leading to a very low ion fluence required to suppress the exchange bias effectively [6]. Thus sputtering effects are minimized. Using a 25 keV Ga+ focused ion beam (FIB) a stripe array consisting of 1 µm wide lines exposed to a fluence 21014 Ga/cm2 separated by 1 µm spacing is created. The long axes of the lines are oriented parallel to the exchange bias direction.
The global magnetic properties of the stripe array were analyzed by means of magneto-optic Kerr effect magnetometry (MOKE) and Kerr microscopy. It was found, that the overall loop shift in the patterned region is reduced to ~30 Oe compared to an exchange bias field of 180 Oe in the unpatterned area. After saturation in an applied field of +800 Oe there is no domain pattern observed in remanence by Kerr microscopy. With increasing negative field first the magnetization in the irradiated stripes reverse. This leads to an alignment of the soft magnetic (irradiated) stripes parallel to the applied field direction while the magnetization of the non-irradiated stripes remains antiparallel. After saturation at -500 Oe and increase of the applied field a hysteretic behaviour was observed that is consistent with the MOKE-measurements leading to a remaining domain contrast in remanence.
At an applied field between -12 Oe and -80 Oe a sharp contrast originating from Néel walls between irradiated and non-irradiated regions is observed by magnetic force microscopy. Moreover, a successive increase of the domain width in the irradiated areas at the expense of those in the non-irradiated areas is observed. At an applied magnetic field of about -100 Oe the magnetization in the non-irradiated area is nearly completely reversed due to the influence of the adjacent soft magnetic areas which are reversed at lower field values already. A magnetization ripple as a remainder of the non-irradiated stripes persists up to large negative field values consistent with the large exchange bias field in the completely unpatterned region.

References
[1]J. Lohau, A. Moser, C. T. Rettner, M. E. Best, B. D. Terris, Appl. Phys. Lett. 78, 990 (2001).
[2]J. McCord, J. Fassbender, M. Frommberger, M.O. Liedke, E. Quandt, L. Schultz, Appl. Phys. Lett., submitted.
[3]A. Mougin, S. Poppe, J. Fassbender, B. Hillebrands, G. Faini, U. Ebels, M. Jung, D. Engel, A. Ehresmann, H. Schmoranzer, J. Appl. Phys. 89, 6606 (2001).
[4]J. McCord, R. Schäfer, K. Theis-Bröhl, H. Zabel, J. Schmalhorst, V. Höink, H. Brückl, T. Weis, D. Engel, A. Ehresmann, J. Appl. Phys. accepted.
[5]S. Blomeier, D. McGrouther, S. McVitie, R. O’Neill, J. N. Chapman, M. C. Weber, B. Hillebrands, J. Fassbender, J. Magn. Magn. Mater. accepted.
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