Polarized neutron reflectometry study on a magnetic film with an ion beam imprinted stripe pattern
Introduction
High package densities in magnetic recording media can be achieved by scaling the magnetic elements to smaller sizes of magnetic grains, clusters or structures patterned by lithographic methods. However, approaching very small feature sizes, the superparamagnetic limit may be reached where the long-term thermal stability of the magnetic elements is lost [1]. To avoid this problem patterning of the recording media into magnetically separated areas can be performed. It may be realized either by planar magnetic patterning, by using lithography methods [2], or by direct fabrication of topographically isolated, single-domain magnetic nanostructures [3]. We used ion bombardment induced magnetic patterning (IBMP) for changing the exchange-bias (EB) [4] between ferromagnetic (F) and antiferromagnetic (AF) layers in one part of the film, while keeping the properties of the untreated regions unchanged. For the resulting configuration of the magnetization in the sample both, the interfacial exchange bias properties but also intralayer exchange between the moments in the different regions have to be considered. For a detailed study of the magnetic interactions between neighbouring magnetic elements in a continuous film we designed a model system which allowed the efficient use of different experimental techniques for complementary analysis. Therefore, we did not approach very small length scales but designed our magnetic elements to be in the micrometer range and used methods as the magneto-optical Kerr effect (MOKE), Kerr microscopy (KM) and polarized neutron reflectometry (PNR). We have chosen a film with imprinted in-plane stripe-like magnetic domains. The stripes are expected to display alternating magnetization directions in the remanent demagnetized state.
Section snippets
Experimental
The sample studied is a EB Co70Fe30(28 nm)/Mn83Ir17(15 nm) F/AF bilayer with buffer layers Cu(28 nm)/SiO2(50 nm) on Si(111) and a TaO-Ta(9 nm) top layer prepared by magnetron sputtering. The initial EB direction was set by field cooling in a magnetic in-plane field of 1 kOe after annealing at 275 ∘C which is above the blocking temperature. Subsequently the film was covered by a photoresist, which was patterned into equally spaced stripes with a width of d=2.5 μm and a periodicity of Λ=5 μm.
Results and discussion
KM results of the present sample are published recently [5]. It was shown that starting from saturation magnetization the reversal proceeds through an antiparallel alignment of magnetization. Here we present in Fig. 1 one KM image taken at a field at which the magnetization in neighboring regions is in this state. The longitudinal MOKE hysteresis loop along the EB axis together with results of fits to specular PNR data is displayed Fig. 1. A two-step reversal, corresponding to the two regions
Acknowledgements
We gratefully acknowledge financial support by the DFG via SFB 491 and BMBF O3ZA6BC1.
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