Control of magnetization states in micro-structured ferromagnetic rings

Marcus Steiner and Junsaku Nitta
Materials Science Laboratory

@It is difficult to detect the magnetization of a single micro-magnet even by using a high sensitive susceptibility-meter; therefore, ensemble averaged magnetization processes is measured in an array including several thousand micro-structured magnets. We have shown that a fringe-field-induced local Hall effect (LHE) device can detect the magnetization process of a single micro-structured magnet [1]. It has been predicted that a flux closure state (vortex state) is stable in ferromagnetic small ring structures. In the vortex state, almost no stray field is generated, that offers a potential application for high integration of the magnetoresistive random access memory (MRAM). We have investigated the magnetization processes of micro-structured ferromagnetic rings using the LHE device and have found that the magnetic transition strongly depends on the inner diameter of the ring.
@The inset of Fig. 1 shows an SEM image of a fabricated sample. A cross-shape is a semiconductor Hall device. A NiFe micro-structured ferromagnetic ring is placed near the Hall cross to detect a fringe field. An external magnetic field is applied in parallel to the semiconductor two-dimensional electron gas in order that it does not affect the Hall resistance. Figure 1 shows hysteresis loops of the Hall resistance. The outer diameter of the rings is fixed to 2.0 m, and the inner diameter is varied from 0 (Disk) to 1.6 m in steps of 0.4 m. We observed a systematic change in the hysteresis loops by increasing the inner diameter. For narrow rings, sharp transitions from the so-called gonionh state to the gvortexh state were observed. In rings with smaller inner diameter, the transitions are broad and more complex [2]. A comparison between the hysteresis loop of the Hall resistance of a 0.4 m-diameter ring and numerical calculation is shown in Fig. 2. Starting from the onion state (i), the ring reversibly enters a wave-like state (ii). The global vortex state (iii) is then irreversibly reached and forms a stable configuration that produces a plateau in the hysteresis. Via an irreversible transition into a single local vortex state (iv), the saturation configuration is reached again. The measured Hall resistance loop is well reproduced by the numerical simulation.
@These results indicate that the transition field to the vortex state can be controlled by the inner diameter.

[1] J. Nitta, et al. Jpn. J. Appl. Phys. 41 (2002) 2479.
[2] M. Steiner and J. Nitta, Appl. Phys. Lett. 84 (2004) 939.

Fig. 1. Local Hall resistance measurements.
Fig. 2. Measured and simulated hysteresis loops.