2- 2 composite Curie temperature

There is often a wide range of crystalline soHd solubiUty between end-member compositions. Additionally the ferroelectric and antiferroelectric Curie temperatures and consequent properties appear to mutate continuously with fractional cation substitution. Thus the perovskite system has a variety of extremely usehil properties. Other oxygen octahedra stmcture ferroelectrics such as lithium niobate [12031 -63-9] LiNbO, lithium tantalate [12031 -66-2] LiTaO, the tungsten bron2e stmctures, bismuth oxide layer stmctures, pyrochlore stmctures, and order—disorder-type ferroelectrics are well discussed elsewhere (4,12,22,23). 

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... 

Some of the composition adjustments in the Alnicos result in a high Curie temperature so that the decomposition reaction can take place relatively rapidly below This is particularly tme for Co, which is 24 wt % or greater for the anisotropic magnets. Another important consideration is the suppression of nonmagnetic fee y-phase which may appear at 1000—1100°C in this regard, the amount of Al, which is a y-suppressor, is critical. The formation of y is pronounced if the Al content falls much below 7—8 wt %. 

Tantalum and niobium are added, in the form of carbides, to cemented carbide compositions used in the production of cutting tools. Pure oxides are widely used in the optical industiy as additives and deposits, and in organic synthesis processes as catalysts and promoters [12, 13]. Binary and more complex oxide compounds based on tantalum and niobium form a huge family of ferroelectric materials that have high Curie temperatures, high dielectric permittivity, and piezoelectric, pyroelectric and non-linear optical properties [14-17]. Compounds of this class are used in the production of energy transformers, quantum electronics, piezoelectrics, acoustics, and so on. Two of... 

Fig. 95 shows the change in cell parameters, density and Curie temperature for ceramics with initial compositions of Li(Tai xMgx)03.3xF3x (where 0 < x < 0.2) versus x value. It should also be mentioned that the pyroelectric coefficient for x = 0.05 was found to be 4.0 nC cm 2 K l. 

Maximum density was achieved for the ceramic material that corresponds to the composition Li(Tao.92sMgo 075)02 775F0.i69(OH)o.o56> with a Curie temperature of 1060 10 K. 

In an amorphous ferromagnet (a sputtered him with the composition Tb0.33Fe0 67) Rhyne et al (1972) used neutron diffraction to demonstrate a random direction of moments with average ferromagnetic orientation. This material has a fairly sharp Curie temperature in the range 380-390 K, the moment below this temperature reaching saturation at about 50kOe. 

Ferroelectricity has also been found in certain copolymer compositions of VF2 with trifluoroethylene, F3E, [6-11] and tetrafluoroethylene, F4E, [12-15] and in nylon 11 [16]. Specifically, copolymers of vinylidene fluoride and trifluoroethylene (VF2/F3E) are materials of great interest because of their outstanding ferroelectricity [9,17-18], together with a parallel strong piezo- [7] and pyroelectricity [19]. These copolymers exhibit, in addition, an important aspect of ferroelectricity that so far has not been demonstrated in PVF2 the existence of a Curie temperature at which the crystals undergo reversibly a ferroelectric to a paraelectric phase transition in a wide range of compositions [9, 17-18],... 

Yamada et al. [9,10] demonstrated that the copolymers were ferroelectric over a wide range of molar composition and that, at room temperature, they could be poled with an electric field much more readily than the PVF2 homopolymer. The main points highlighting the ferroelectric character of these materials can be summarized as follows (a) At a certain temperature, that depends on the copolymer composition, they present a solid-solid crystal phase transition. The crystalline lattice spacings change steeply near the transition point, (b) The relationship between the electric susceptibility e and temperature fits well the Curie-Weiss equation, (c) The remanent polarization of the poled samples reduces to zero at the transition temperature (Curie temperature, Tc). (d) The volume fraction of ferroelectric crystals is directly proportional to the remanent polarization, (e) The critical behavior for the dielectric relaxation is observed at Tc. 

Neutron diffraction studies under pressure [84] on the 70/30 composition have revealed that transitions in this copolymer are displaced towards higher temperature with increasing pressure, as can be seen in the phase diagram of Fig. 11. In addition, it is worth noting the non-linear increase of the Curie temperature with pressure. By considering the Clausius-Clapeyron relation dTc/dP = TCAVC/Ahc, this effect can be related to a decrease in the volume... 

Table 10.2 Composition, lattice parameters, unit cell volume, magnetization (M), and Curie temperature (Tc) for Sm2(Fe1 J.Co )17C>22...

Fig. 10. Compositional variation of the Curie temperature (Tc) vs the TM content (x) for Co- and Fe-based binary RE-TM alloys. In the range of practical interest (about 75 atomic % TM), Tc is nearly independent of the Fe content, but strongly dependent on the Co content (about 7 K/atomic %).

Phase diagrams show the compositions for which ordering is possible. With increased temperature, the range of compositions decreases to the stochiometric composition at the Curie temperature. The phase diagram for the CuAu system is given in Figure 8.5. 

Fig. 29. CMR at 5 T on cooling for different compositions 0 x 0.7 in the system Lao.7 xPr Cao3MnC>3. Arrows indicate the Curie temperatures Tq after Hwang et al. (1995).

At the critical Ni2.30Mn0.70Ga composition Tm and Tc are no longer coupled, which results in a drastic increase of the martensitic transformation temperature up to 530 K, and in a decrease of Tc down to 350 K. In the alloys with the higher Ni excess, the martensitic transformation occurs at temperatures above 600 K, whereas the Curie temperature continues to decrease. Considering the empirical correlation between the electron concentration e/a and the martensitic transformation temperature Tm [2], it can be suggested that further increase in Tm of the 0.30 < x < 0.36 alloys can be attained by the substitution of Ga for Ni or Mn. 

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