Temperature anisotropy

Table 9.1 Room temperature anisotropy constants of some important ferrites...

Whereas the room-temperature anisotropy is mainly determined by KA it becomes necessary to consider higher order terms when going to lower temperatures. The concentration dependence of K3, K2 and K3 for Pr2(Fej xCox)14B is shown in fig. 20. The minimum of KL still corresponds to the minimum of HA shown for this series in fig. 19. 

The slightly reduced values of the room-temperature anisotropy fields when relatively small amounts of Fe in R2Fe14B are replaced by Co (see fig. 19) are not the only reason why Co substitution does not lead to improved hard magnetic properties. This is true for the temperature coefficient of the coercive force in particular. A probable reason for this is that not only JIA itself but also its temperature dependence becomes less favourable upon Co substitution. This may be inferred already from a comparison of the temperature dependences of HA in the pure ternaries Nd2Fe14B and Nd2Co17B shown in fig. 21, where it can be seen that the slope of the HA(T) curve for the latter compound around room temperature is much steeper than that of the former. In fact, the HA(T) curve in Nd2Co14B tends to approach the horizontal axis at a temperature of about 540 K, which is still far below the corresponding Curie temperature (Tc = 1007 K). The most obvious... 

The substitution of C for B in R2Fe14B presents considerable metallurgical difficulties. Extensive studies of the possibility of this type of substitution were made by Liu et al. (1987) and results of their investigations have already been reproduced in fig. 6. The effect of C substitution is to increase the hard magnetic properties (Liu and Stadelmaier 1986, Bolzoni et al. 1985). The latter authors reported that the room-temperature anisotropy field in Nd2Fe14B1 xCx increases... 

The first quantitative evidence for the temperature anisotropy of CMBR was provided by the COBE (Cosmic Background Explorer) satellite in 1992. The angular resolution of its detectors was 7°. This enabled the collaboration to determine the first 20 multipole moments of the fluctuating part of CMBR beyond its isotropic component. It has been established that the degree of anisotropy of CMBR is one part in one hundred thousand (10 ). There are two questions of extreme importance related to this anisotropy ... 

Figure 8.6 shows the X-ray structure of Fe(NCS)2(bpp)2 2(benzene) measured at 223 K. It proved that iron was coordinated by four N atoms of bpp and two N atoms of NCS in the trans position, as observed in Fe(NCS)2(bpp)2. There is one crystallographically independent iron ion that is, the conformers of bpp were anti-gauche and anti-gauche. In enclathrated benzene molecules, carbon atoms have a high temperature anisotropy and 12 atoms per molecule were observed, showing 30° rotation. The host framework has ID chain structure, and benzene molecules are located at the interstitial space located between two ID chains. The spin state of iron showed temperature-independent high-spin Fe" state. 

Meet DVD, Reimann P Temperature anisotropy in a driven granular gas, EPL (Europhys Lett) 74 384, 2006. 

Small molecules in low viscosity solutions have, typically, rotational correlation times of a few tens of picoseconds, which means that the extreme narrowing conditions usually prevail. As a consequence, the interpretation of certain relaxation parameters, such as carbon-13 and NOE for proton-bearing carbons, is very simple. Basically, tlie DCC for a directly bonded CH pair can be assumed to be known and the experiments yield a value of the correlation time, t. One interesting application of the measurement of is to follow its variation with the site in the molecule (motional anisotropy), with temperature (the correlation... 

Fig. 4. The effect of temperature for Mng 6 Zng 3 Fe Fe on (a) initial magnetic permeabiUty, )J., measured on a polycrystalline toroid appHed as a core for a coil driven by a low (B <0.1 mT) ampHtude, low (10 kHz) frequency sinusoidal signal and (b) magnetocrystalline anisotropy constant, measured on a monocrystalline sphere showing the anisotropy/compensation temperature Tq and the Curie temperature, T. To convert joules to calories, divide by...

From the write and read process sketched so far, some requirements for MO media can be derived (/) a high perpendicular, uniaxial magnetic anisotropy K in order to enable readout with the polar Kerr effect (2) a magnetoopticady active layer with a sufficient figure of merit R 0- where R is the reflectivity and the Kerr angle (T) a Curie temperature between 400 and 600 K, the lower limit to enable stable domains at room temperature and the upper limit because of the limited laser power for writing. 

Change of compensation temperature with composition is Positive value of K indicates perpendicular anisotropy. 

None of the biaary compounds with this composition is well matched to the needs of MO recording. Gd—Fe has too high a Curie temperature and has an in-plane anisotropy. Tp is too low for binary alloys such as Tb—Fe and Dy—Fe. Co-based alloys which exhibit a close to room temperature have... 

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