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 . 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 Êm, and the inner diameter is varied from 0 (Disk) to 1.6 Êm in steps of 0.4 Êm. 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 . A comparison between the hysteresis loop of the Hall resistance of a 0.4 Êm-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.
 J. Nitta, et al. Jpn. J. Appl. Phys. 41 (2002) 2479.
 M. Steiner and J. Nitta, Appl. Phys. Lett. 84 (2004) 939.
Fig. 1. Local Hall resistance measurements.
Fig. 2. Measured and simulated hysteresis loops.