Nanomagnetics
Transport in single domain ferromagnets
| We are currently fabricating and measuring the magnetotransport properties of submicron scale "spin valve" devices. The idea is that one can transport a nonequilibrium spin current, i.e., a magnetization current, into a paramagnet from a ferromagnet. The resulting spin current can then be detected via a second ferromagnet connected to the system. The figure on the right shows one of the devices we have fabricated and measured . The two small elliptcal particles are nickel whereas the rest of the structures is gold. The direction of the spins is determined by the magnetization of the ferromagnetic particles and so the spin direction can be switched via magnetization flipping of the particles. By measuring the magnetoresistance of such a device, one can directly measure the hysteretic magnetization curves of the smaller particle. One can also measure the transport properties of a single submicron ferromagnetic particle. These types of measurements also show that it is quite feasible to do submicron ferromagnetic particle dynamics studies using transport instead of SQUID magnetometry. | |
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| We have also investigated quantum interference phenomena in single magnetic particles. We observe reproducible fluctuations in the resistance of a single ferromagnetic particle as a function of magnetic field and voltage bias, indicating the presence of quantum interference in the ferromagnet. The four-terminal resistance of the ferromagnet obeys the Landauer-Buttiker symmetries when both the externally applied magnetic field and the magnetization are taken into account. | |
Spin dynamics in single particles and arrays of particles
Current hard disk storage media consist of thin continuous magnetic films on which information is stored as the orientation of the magnetization of individual magnetic domains, with each domain corresponding to one bit (either up or down). The size of each domain is determined by the size of the heads used to read and write the data. The recent increase in storage capacity obtained on computer hard disks is due to advances in the design of read-write heads. Ultimately, however, there is a limit to the minimum size that can be obtained by this technique.
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Another way of obtaining higher storage densities is to use an array of small magnetic dots which are closely packed together. The figure above left shows a scanning electron micrograph of a Ni particle array, and the figure on the right shows an atomic force microscope (AFM) image of a section of a array of nickel dots taken with a Thermomicroscopes SPM, along with a Magnetic Force Microscope (MFM) image taken at the same time. Each dot consists of a single magnetic domain whose magnetization can be oriented either up or down, corresponding to one bit of information.
At these small size scales, it is important to understand the dynamics of magnetization reversal and the role of particle-particle interactions on the magnetic properties of the individual dots. At present, we are characterizing the static and dynamic properties of such arrays using a variety of scanning probes, magnetization measurements, and ferromagnetic resonance (FMR) measurements. The movies above show simulations of the response of a circular permalloy particle of diameter 0.5 microns to an external microwave field. The first movie corresponds to the uniform mode, and the second to a spin-wave mode.
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