Domain Structures and Current-Controlled Switching Characteristics of Micron Sized Permalloy Structures with Varying Asp
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Domain Structures and Current-Controlled Switching Characteristics of Micron Sized Permalloy Structures with Varying Aspect Ratio S C Seah, Y S Soh, and V Ng Information Storage Materials Laboratory, Department of Electrical & Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117576, Singapore, Singapore ABSTRACT Ni80Fe20 magnetic structures were fabricated by electron beam lithography and evaporation on top of a conductor. A current passing through the conductor was used to induce a field across the structures. Magnetic force microscopy was used to study the domain changes of the remanent structures after every increment of current. The effect of changing aspect ratio in the structures was studied. Simulation was also conducted using Object-Oriented Micro-Magnetic Framework to compare with the experimental results. INTRODUCTION Magnetic patterned structures using soft materials such as Ni80Fe20 are being explored due to their applications such as MRAMs [1] in information storage. Work in magnetic domain configuration and switching behavior for various shapes such as squares [2], rectangles [3], ellipses [4] and rings [5] have been reported. However, the conventional method used to produce a field is through the application of an external magnetic field. In this work, we demonstrate the switching of magnetic structures using a current-carrying conductor to generate a field. The objective is to understand the magnetic domain configurations and properties of micron-sized magnetic structures and to observe how the properties vary according to aspect ratio. EXPERIMENT METHODS Length Au contact pad Device fabricated on undoped silicon substrate
(a)
Width (b)
Au channel 0.05 mm 0.7 mm Array of NiFe structures
0.4 mm
0.05 mm Au contact pad (200 nm thick) 0.4 mm 200 nm
Hard axis
Easy axis *not drawn to scale
Figure 1. Schematics of the experimental setup and the shape of the magnetic structures having (a) aspect ratio 2 and (b) aspect ratio 4.
Electron beam lithography was used to pattern the structures in resist. 40 nm of Ni80Fe20 was deposited by evaporation and then lifted off. The structures are shaped as shown in fig 1 with sizes 2 µm x 1 µm and 4 µm x 1 µm. Aspect ratio is defined as the length to width ratio of the structure with the first size having aspect ratio 2 (fig. 1a) and second having aspect ratio 4 (fig. 1b). Fig 2 shows the magnetic structures placed on 50 µm wide, 200 nm thick gold conductors. The structures positioned parallel (perpendicular) to the conductors are used for hard (easy) axis experiments. For each size, the two differently aligned arrays of structures are fabricated on different conductors but on the same sample. The magnets were first saturated by a field of 1000 Oe along the easy or hard axis. By passing a current along the gold channel (fig. 1), a magnetic field would be produced perpendicular to the direction of current flow. This acted as a reversed field on the magnetic structures. Current was increased in steps of 50 mA (e
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