Nanoscale modulated magnetization patterns for reproducible configurational and switchable static and dynamic properties films

Domain wall (DW) imprinting in spatially varying magnetic property thin films is a novel method to obtain thin films with new effective magnetic characteristics ranging from static to dynamic applications. In order to generate such effective material systems, light ion irradiation is an advantageous method allowing for, e. g. laterally modified exchange bias (EB) directions, making the imprinting of artificial magnetic DW patterns possible. The magnetically hybrid structures with different unidirectional anisotropy directions are unique as, e.g. a domain pattern and domain walls can be imprinted directly into the magnetic material. A reproducible nucleation and positioning of magnetic domain walls in a high density arrangement is achieved. In dependence of the applied magnetic field amplitude, the system allows for an additional defined adjustment of the magnetic configuration with varying effective magnetic anisotropy.
Extended Ni19Fe81(50nm)/Ir23Mn77(7nm) thin films with an initial unidirectional anisotropy are patterned by local He-ion irradiation in the presence of a magnetic field, which is aligned perpendicular to the initial unidirectional anisotropy, and by which hybrid magnetization patterns with two different types of modulated directions of EB are obtained. The stripe width of the patterned thin films is varied down to 500 nm by one order of magnitude, being well below the distance of the magnetic Néel wall tails. The influence of the overlapping DW structures on the effective static and dynamic magnetization properties of the thin films are investigated by complementary inductive static and dynamic methods, magneto-resistive measurement techniques, by magneto-optical microscopy, and are as well supported by matching micromagnetic simulations. By this, a complete picture of spatial and temporal evolution of magnetization is derived.
In a zig-zag or head-to-tail configuration a folded magnetization forms (Fig.1), in which the magnetization is modulated along the magnetic net direction perpendicular to the stripes. In an alternating head-to-head/tail-to-tail configuration charged DWs form, favoring a modulated magnetization with low angle head-to-head and tail-to-tail configurations. In all cases the remanent magnetization along the net-magnetization of the films increases with decreasing feature size. The fixed position of the artificial DWs leads to pronounced two staged quasi-static magnetization reversal and, accordingly, different dynamic magneto-static modes are excited in the magnetic meta-material (Fig. 2).
We show that precessional frequencies of magnetic thin films can be directly influenced by means of high density DW imprinting (up to 104 DWs/sample). The static and dynamic magnetic response is tuned by imprinting periodic DW patterns with overlapping DW structures through selective ion irradiation. Mode coupling via dynamic magnetic charges in the periodically modulated magnetization patterns is directly provoked by adjusting the micromagnetic interface density. At the transition from the saturated magnetic phase to the domain wall phase the permeability spectra exhibit a pronounced discontinuous jump in the dynamic response, making an abrupt switch between two different dynamic states achievable. We show that the controlled introduction of micromagnetic DW objects is a unique way to tailor the effective magnetic properties of magnetic thin films. Dependencies of magnetic field angle, stripe-width and orientation of EB will be discussed.

Funding from the German Science Foundation DFG through (MC9/7-2; FA314/3-2) and the Heisenberg programme of the DFG (MC9/9-1) is highly acknowledged.