Dynamics Of Nanomagnets

The study of magnetic materials, particularly of films made of nanomagnets, sometimes called mesoscopic magnetism, is driven by the desire to increase storage space on magnetic storage devices such as hard drives in computers. The basic information

Figure 7.6. Dependence of the coercive field fit (i.e., hQ on the granular particle size d of a Nd-B-Fe permanent magnet. [Adapted from A. Manaf et al., J. Magn. Magn. Mater. 101, 360(1991).]

storage mechanism involves alignment of the magnetization in one direction of a very small region on the magnetic tape called a byte. To achieve a storage of 10 gigabytes (1010 bytes) per square inch, a single bit would be approximately 1 nm wide and 70 nm long. The film thickness could be about 30 nm. Existing magnetic storage devices such as hard drives are based on tiny crystals of cobalt chromium alloys. One difficulty that arises when bits are less than lOnm in size is that the magnetization vector can be flipped by random thermal vibrations, in effect erasing the memory. One solution to this is to use nanosized grains, which have higher saturation magnetizations, and hence stronger interactions between the grains. A group at IBM has developed a magnetic nanogram, FePt, which has a much higher magnetization. The FePt particles were made in a heated solution of platinum acetylacetonate and iron carbonyl with a reducing agent added. Oleic acid was also added as a surfactant to prevent aggregation of the particles by coating them with it. The solution is then spread on a substrate and allowed to evaporate, leaving behind the coated particles on the substrate. The resulting thin films are then baked at 560°C

d, NANOMETERS

Figure 7.7. Dependence of the saturation magnetization Ms of zinc ferrite on the granular particle size d normalized to the value M,(9Q) for a 90-nm grain. [Adapted from C. N. Chinnasamy, J. Phys. Condons. Matter 12, 7795 (2000).]

d, NANOMETERS

Figure 7.7. Dependence of the saturation magnetization Ms of zinc ferrite on the granular particle size d normalized to the value M,(9Q) for a 90-nm grain. [Adapted from C. N. Chinnasamy, J. Phys. Condons. Matter 12, 7795 (2000).]

for 30min, forming a carbonized hard mist containing 3-nm particles of FePt. This size of magnetic nanoparticle would result in a storage density of 150 gigabytes per square inch, which is about 10 times higher than commercially available magnetic storage units.

When length scales of magnetic nanoparticles become this small, the magnetic vectors become aligned in the ordered pattern of a single domain in die presence of a DC magnetic field, eliminating the complication of domain walls and regions having the magnetization in different directions. The Stone-Wohlferth (SW) model has been used to account for the dynamical behavior of small nanosized elongated magnetic grains. Elongated grains are generally die type used in magnetic storage devices. The SW model postulates that in the absence of a DC magnetic field ellipsoidal magnetic particles can have only two stable orientations for their magnetization, either up or down with respect to the long axis of the magnetic particles, as illustrated in Fig. 7.8. The energy versus orientation of the vectors is a symmetric double-well potential with a barrier between the two orientations. The particle may flip its orientation by

Down'

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