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 μm, and the inner diameter is varied from 0 (Disk) to 1.6 μm in steps
of 0.4 μm. We observed a systematic change in the hysteresis loops by increasing
the inner diameter. For narrow rings, sharp transitions from the so-called
“onion” state to the “vortex” 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 μm-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.
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