Shape-anisotropy

Shape anisotropy Shape control Shape factors Shape-memory alloys Shape-selective catalysis Shape selectivity Sharpless catalyst Shaving cream Shaving creams... 

In the case of Co—Cr having perpendicular anisotropy there is, in principle, a competition between the uniaxial anisotropy of a hexagonal stmcture and the demagnetizing energy of the thin film. In the case of magnetically separated Co—Cr columns (particulate morphology) then also the shape anisotropy contributes to the perpendicular anisotropy. 

If there is a crystal anisotropy, with the easy axis parallel to the shape-anisotropy axis of the particle, the total anisotropy is = shape + crystal and the total switchHig field is = (A — Aj,)Af + 2Aj = crystal anisotropy constant). In the case of practical... 

Here iC is the intrinsic anisotropy constant due to the crystalline anisotropy. After the demagnetization in the longest direction, k is the shape-dependent constant (for an infinite cylinder k = 1.38), M is the exchange constant, and R the particle radius. An infinite cylinder with only shape anisotropy gives... 

Shape anisotropy is related to the magnetostatic energy of a magnet. A needle-shaped sample tends to line the atomic moments along the needle axis and a disk-shaped sample tends to line these moments parallel to the disk surface. 

It generally is accepted that the mechanism of coercivity in the Alnicos is incoherent rotation of single-domain particles of the a -phase based on shape anisotropy. As coercivity increases, the larger the aspect ratio of the tods and the smoother thek surface becomes the difference between the saturation polarizations of the two phases also increases. It is thought that Ti increases the coercivity of Alnico because of an increased aspect ratio of the rods and a smoother surface. 

The physical properties of metal nanoparticles are very size-dependent. This is clear for their magnetic properties, for which the shape anisotropy term is very important. This is also true for the optical properties of nanoparticles displaying plasmon bands in the visible range (Cu, Ag, Au) and for 111-V... 

Mirzamaani et al. [74, 75] point out that the earlier studies of the interrelationships between structure and magnetics have examined films substantially thicker than those now being used in thin-film disks. These authors have examined very thin CoP films and have studied the relative roles of shape anisotropy, stress anisotropy, and crystal anisotropy in determining the magnetic properties. For their CoP-deposition system, shape anisotropy dominated the other factors in determining the film magnetic properties. The shape anisotropy of a particular deposit was determined by the surface condition of the substrate on which the CoP was deposited. 

Fig. 5. Model of the origin of shape anisotropy in electroless CoP thin-film media [74]. (Reprinted by permission of The Electrochemical Society).

Since vertical recording is not yet being employed in disk files, the electrochemical deposition of perpendicular materials is still at an exploratory stage. Electroplated vertical media are uncommon. In some cases, the vertical media are modifications of materials used as horizontal media. In others, the vertical medium is produced by introducing a strong shape anisotropy. 

Another potential vertical medium is not a continuous thin film, but rather an assembly of metal particles deposited in well defined pores in an alumina film on Al [109-117]. The shape anisotropy of the particles gives the desired vertical anisotropy. In producing such structures, the proper conditions for the anodization of the Al disk and for the subsequent control of pore size are as important as the conditions of metal deposition. The structures of such disks are discussed in detail below. 

The structures of electroplated hard alloys have been less extensively studied than those of similar electrolessly deposited materials. Sallo and co-workers [118-120] have investigated the relationship between the structure and the magnetic properties of CoP and CoNiP electrodeposits. The structures and domain patterns were different for deposits with different ranges of coercivity. The lower-f/c materials formed lamellar structures with the easy axis of magnetization in the plane of the film. The high-Hc deposits, on the other hand, had a rod-like structure, and shape anisotropy may have contributed to the high coercivity. The platelets and rods are presumed to be isolated by a thin layer of a nonmagnetic material. 

Note 2 The molecular origins of dipolar flexo-electricity are the particular shape anisotropy (e.g., resembling a pear or banana) of the molecules, each of which must also possess a permanent dipole moment. 

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