3642.     ---------------.  [WEISS, P., AND FORRER, R.]  [Absolute Saturation of Ferromagnetic Substances and the Law of Approach as a Function of the Field and of the Temperature.]  Ann. Physik, vol. 12, No. 10, 1929, pp. 279-374; Chem. Abs., vol. 24, 1930, p. 2345.

        Question of ferromagnetism is discussed in detail.  The mathematical theory and the experimental work on Fe, Ni, magnetite, ferromagnetic sesquioxide of Fe, Fe₂B, cementite, pyrrhotine, and a series of ferrocobalts are described.  Conclusions:  The atomic moments of Fe and Ni are, respectively, 11 and 3 times 1125.6 c.g.s., with a precision of about 1 in 1,000.  This value of the experimental magneton is approximately 0.19% higher than the older value (1,123.5) and 0.63% less than 1/5 of the Bohr magneton.  It is very close to the value, 1,126, which was deduced in 1924 from the moments of the ions of the Fe family, calculated from the Curie constant by the classical Langevin formula.  This is regarded as an experimental justification of that formula.  For ferric Fe in magnetite, the moment is 10.10 magnetons.  A part of this excess over an integer is attributed to the paramagnetism that accompanies ferromagnetism in a molecule containing both ferric and ferrous Fe.  It is very difficult to obtain C. P. ferromagnetic sesquioxide, cementite, Fe₂B, and pyrrhotine, and the last 3 are extremely hard, magnetically.  The experimental values found for the magnetic moments are 8.88, 9.10, 8.84, and 1.83, respectively, which, considering the difficulties just mentioned, are compatible with the integral moments 9, 9, 9, and 2, respectively.  The frequency with which the moment 9 occurs here, as well as in a-Co in the ferrocobalts and γ-Co of the Ni-Co series, is remarkable.  The value 2 for pyrrhotine is the smallest moment known.  In the Fe-co and Ni-Co alloys, the atomic moment varies linearly with the concentration in certain intervals, thus confirming the law of mixtures.  The results also indicate that the metals exist in different states, characterized by different magnetic moments.  Thus in a-Fe-Co rich in Fe, the moment of Co is 17, whereas in a-Fe-Co rich in Co, the Fe has a moment of 15 and the Co 9 magnetons.  In γ-Fe-Co, the moment of Co is 8 2/3.  In γ-Fe-Co, the moment of Co is 8 2/3.  In γ-Ni-Co, the Co moment is 9 magnetons, whereas in H-Ni-Co it is 8 ½.  In the Fe-Co alloys containing 13-50% Co, there is indication of at least 3 different atomic moments.  For 35-45% Co, the saturation value at ordinary temperatures is about 12.4% higher than for pure Fe.  The law of approach to saturation as a function of the field is given by the equation _σH,T=_σooT [1=(a/H)].  The constant a measures the magnetic hardness in strong fields and is sensibly independent of the temperature in the internal investigated.  It differs from one specimen to another and is small for pure metals that crystallize in the cubic system.  It is large for noncubic substances such as Fe₂B, cementite, and pyrrhotine.  This law is valid for Fe and N at ordinary temperatures down to about 1,000 gausses, and this limit is displaced toward higher values as the temperature is lowered.  The law of approach to saturation toward the limit of O abs. is given by the equation _σooT=_σooooK (1-AT²--BT⁴-- . . . .).  Certain conditions that these 2 laws impose on the atomic model are not yet satisfactorily explained.  The maximum magnetization at saturation of pyrrhotine is at 160° K., as already obtained by Zeigler.  2 new phenomena were observed.  In most of the specimens of Ni, a feeble magnetization, proportional to the field, was found superimposed upon the regular phenomenon of approach to saturation.  This parsitic magnetization is zero at 0° K.  At 120° K., magnetite has a singular change in state.  This does not affect its magnetic moment or the law of thermal variation of saturation but manifests itself at increasing temperatures by a discontinuous variation in magnetization, owing to an abrupt decrease in the coefficient of magnetic hardness, a.

        WEISSBERGER, E.  See abs. 1525.

        WEITKAMP, A. W.  See abs. 420a, 420b, 3767, 3768.