MOTION' OF INDlViniAL DOMAIN' WALLS 1040 



TT + COS l(/\/3) on the other. Then, 



r+cos-l(l/-v/3) 



/ 



•'C( 



loie) - g%)]''dd = om VrKTl. (35) 



co.s-l(l/v'3) 



In pcrformino- this integration, rare must be taken to use the positive 

 ^•alue of the square root over the whole interval. It should be noted that 

 in using (8) and (10) to evaluate (13) we are assuming that the wall is 

 moving slowly enough so that its shape is the same as that of the wall 

 at rest. A is best evaluated from a fundamental relation derived by 

 Herring and Kittel^^ between A and the Bloch constant: 



A = [SoM"'[k/l3.^C"\ (36) 



where k is Boltzmann's constant, C is Bloch's constant as used in the 

 relation Ms = il/o(l — CT^'^), So is the atomic spin, and fi is the atomic 

 volume. (*So/fi) is equal to the saturation magnetization at 0°K divided 

 by twice the Bohr magneton. 



For Fe304 , we find C = 4 X 10""' by fitting the Bloch T"- law to 

 the saturation magnetization measurements of Weiss and Forrer." 

 From (3G), assuming Ms at O^iv is 505 c.g.s. units, we then find A — 

 1.24 X 10~^ Furthermore, 7^1 = -1.1 X lO' as given by Bickford,'' and 



7 = (1.76 X 10')^/2 = 1.865 X lO', 



where we have used Bickford's^^ value (2.12) of g.^^ Now from (13) we 

 find 



X = 5.5 X 10' 



in Fe304 at room temperature. 



For (NiO)o.75(FeO)o.25Fe203 , we assume that .!/« , while different 

 from that for Fe304 , varies in the same way with temperature so that 

 C = 4 X 10" . From our measurement of M^ at room temperature 

 (322.5 c.g.s. units) and this assumption about the variation of Ms with 

 T, we find that Ms at 0°K is 342 c.g.s. units. This leads by (36) to A = 

 1.09 X 10~ . Ferromagnetic resonance experiments" done by W. A. Yager 

 and F. R. Merritt in collaboration with the author on spherical single 

 crystals of the same ferrite material as that used in the present research 

 give a g value of 2.14 at all the temperatures mentioned in Table II 

 except 77°K, where g = 2.19. These values are used in determining the 

 value of y[^ 1.76 X 10 g/2] in (13). The anisotropy values in Table II 

 are also taken from the results of these ferromagnetic resonance experi- 



