Imaging, Manipulation, and Spectroscopic Measurements of Nanomagnets by Magnetic Force Microscopy
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Imaging,
Manipulation, and Spectroscopic Measurements of Nanomagnets by Magnetic Force Microscopy Xiaobin Zhu and Peter Grütter
Abstract Magnetic force microscopy (MFM) is a well-established technique for imaging the magnetic structures of small magnetic particles. In cooperation with external magnetic fields, MFM can be used to study the magnetization switching mechanism of submicrometer-sized magnetic particles. Various MFM techniques allow the measurement of a hysteresis curve of an individual particle, which can then be compared to ensemble measurements. The advantage of using MFM-constructed hysteresis loops is that one can in principle understand the origin of dispersion in switching fields. It is also possible to directly observe the correlation between magnetic particles through careful imaging and control of the external magnetic field. In all of these measurements, attention needs to be paid to avoid artifacts that result from the unavoidable magnetic tip stray field. Control can be achieved by optimizing the MFM operation mode as well as the tip parameters. It is even possible to use the tip stray field to locally and reproducibly manipulate the magnetic-moment state of small particles. In this article, we illustrate these concepts and issues by studying various lithographically patterned magnetic nanoparticles, thus demonstrating the versatility of MFM for imaging, manipulation, and spectroscopic measurements of small particles. Keywords: magnetic force microscopy, nanomagnets, scanning probe microscopy.
Introduction Since its invention in 1987,1,2 magnetic force microscopy (MFM) has become a powerful tool for characterizing magnetic structures.3,4 Magnetic storage media and recording heads3,5 as well as magnetic domain structures can be investigated with a routine spatial resolution of 50 nm. More recent, nonstandard developments include magnetic dissipation imaging to investigate
MRS BULLETIN/JULY 2004
magnetization dynamics through studying the energy transfer between the cantilever and the magnetic sample,6 low-temperature measurements to investigate magnetic vortices or local variations in the magnetic penetration length in superconductors7–10 and colossal magnetoresistance materials,11 magnetic resonance force microscopy with the aim of measuring single nuclear and
electron spins,12 and microcantilever magnetometery13–15 with exquisite magnetization sensitivity that allows for the direct measurement of de Haas–van Alphen oscillations of the magnetization16 and of hysteresis loops.17 Among these applications, characterizing small magnetic structures is one that is growing in significance.18–21 Small magnetic structures are currently widely studied both from a fundamental research standpoint and for their potential applications in ultrahigh-density storage,19–22 spintronic devices,23 and magnetic logic devices.24–26 This booming interest requires techniques for characterizing these small structures individually. Due to its high spatial resolution and sensitivity, MFM has become one of the most
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