4. on the Origin of the Magnetism Based on the Structure of Atoms

12 . Vol. 4,

4. On the Origin of the Magnetism based on the Structure of Atoms.

By Kotaro HONDA, M.I.A.

(Comm. Dec. 12, 1927.)

1. In the electron theory of magnetism it is usual to attribute the origin of magnetism to the revolving electrons round the nucleus of atoms, these electrons being at the same time the origin of the light emission. It is true that revolving electrons have each a magnetic moment corresponding to their velocity and the radius of their orbits ; but these elementary magnets cannot be magnetized by an external magnetic field acting on them. For, by the action of a magneticfield, these magnets will make the Larmor precession, the angle between the magnetic axis and the field remaining unchanged ; hence the component of the magnetic moment in the direction ofthe field does not change by the application of the field ; that is, these magnets cannot be magnetised. On the other hand, during the Larmor precession, the electrons retain their initial orbits and their velocity remains unchanged, but these electrons as a whole are revolving around the directionof the field. This last motion gives rise to the diamagnetism of the atoms. From the above, it is to be concluded that the ferro-and paramagnetisms cannot be explained by a system of revolving electrons, but these electrons produce the diamagnetic effect. This conclusion evidently applies to the case of spinning electrons. 2. Consider next the magnetization of a molecule consisting of two atoms, such as, for example, a model known as a hydrogen molecule. This model may be considered as a gyroscope, and hence its motion under the action of a magnetic field is equivalent to that of the gyroscope acted on by theforce of gravity. According to the theory of gyroscope, the model of hydrogen molecule will makea precession and a nutation by the action of the field. Now the nutational motion contributes acertain magnetic moment to the magnetization, but the precessional motion which has no relation tothe magnetization, gives rise to a certain moment of diamagnetism, as remarked above. Hence the resultant effect of magnetization is the difference of these two m tgnetizations. No. 1.] Origin of the Magnetism based on the Structure of Atoms. 13

Again the magnetic moment M and the angular momentum p of a revolving electron are related by the formula p = 2m M. e But from the theory of gyroscope, it is known that the angular momentum of the system in the direction of the field remains unchanged by the application of it; Hence the same must hold good for the component of magnetization, and therefore the model cannot be magnetized. Inother words, the magnetization due to nutation is just equal and opposite to that of precession. In this consideration it is assumed that the orbital motion of the electron is not in the least affected by the magnetic field, but actually it suffers a slight change in their velocity, such that this change results in the diamagnetism of the molecule. Hence it is further to be concluded that the model consisting of two atoms cannot be magnetized, but gives rise to the diamagnetism. 3. In the above we have shown that the optical electrons may be the cause of the diamagnetism, but not that of the ferro- or paramagnetism. What is then the origin of the latter magnetism? The answer to this question given by the writer is that it is due to the electrons revolving very rapidly in the interior of the nucleus of atoms. The atoms consist each of a nucleus and a system of revolving electrons, and the nucleus itself contains a certains number of revolving electrons, which is nearly equal to the number of the atomic weight diminished by the atomic number. These electrons, according to Rutherford, are also revolving in an extremely small orbit within the nucleus with a very great velocity approaching that of light, and hence they produce a magnetic moment of a reasonable magnitude. For example, in the case of iron atoms, we may assume,

ƒÁ =5•~10-12 cm , ƒÒ=2•~1010cm/sec.,e=1.59•~ 10-20, n=30;

and therefore M=1/2neru=2.4•~10- 20

This value nearly agrees with the observed value 2.05 x 10-20. It is thus evident that the electrons within the nucleus may explain the magnetic moment of atoms. It is here to be noted that if the electrons are revolving with such a high velocity, the angular momentum of the nucleus is so large that an extremely strong field can only produce a very minute deflection of the nucleus towards the field. To avoid this difficulty, it is assumed after 14 K. HONDA. [Vol. 4,

Rutherford that just outside these revolving electrons, still within the nucleus, a certain number of protons are revolving in an orbit in the opposite direction, so that the resultant angular momentum due to the protons and to the electrons nearly cancel with each other. Such an atom, when acted on by a magnetic field, tends to turn towards the field analogous to motion of a circuit madeof a metallic wire carrying an electric current, that is, behaves like a ferro- or paramagneticatom. 4. The nature of the ferro-or paramagnetic atom being thus explained, we now proceed to consider the case of magnetic substances. The atoms constituting solids are distributed along the space-lattice, and are executing thermal motions, which increase with the rise of temperature. Since these atoms have each a magnetic moment, they form, in the case of ferromagnetic substances, an immense number of elementary groups, in each of which the magnetic axes by virtue of their mutual action, take the same orientation corresponding to the minimum potential energy ; but the direction of these axes differs from one group to the other, its distribution being as a whole uniform in all directions. In the case of para-and diamagnetic substances, the thermal agitation is assumed to be so great that the elementary complexes cannot be formed, and at any instance, all the atoms take an at-random orientation. Hence in each case, when no external field acts on the substance, the magnetization vanishes on the whole. In the case of a ferromagnetic substance, the angular momentum of the nucleus of atoms and thermal agitation at ordinary temperature are assumed to be very small, and hence under the action of a magnetic field, the nuclei will easily turn in the direction of the field against their mutual action, thus giving rise to the ferromagnetic phenomenon. It is here to be remarked that during magnetization, the outer or optical electrons will perform Larmor's precession, though it is not so free as in the case of a single atom, and hence any appreciable polariza- tion of the orbital plane of the optical electrons cannot be produced . In the case of a paramagnetic substance, it is assumed that the resultant angular momentum of the nucleus has a greater value than in the last case, and hence under the action of a magnetic field , its gyrostatic action combined with the thermal agitation will strongly resist the turning of the nucleus in the direction of the field , thus giving rise to the paramagnetism. In the case of a diamagnetic substance, the annihilation of the angular momenta within nucleus is very incomplete, and hence its gyrostatic effect is so great that by the application of a strong field , the No. 1.] Origin of the Magnetism based on the Structure of Atoms. 15 nucleus cannot deflect towards the field by any measurable degree. On the other hand, optical electrons will perform the Larmor precession and thus give rise to the diamagnetism. In our theory, then even the diamagnetic atoms have each a definite magnetic moment comparable with the ferro-or paramagnetic atoms, but owing to their large angular momentum, they cannot be magnetised. In short, in ordinary theory, the origin of the ferro-, para-and diamagnetisms is all attributed tothe optical electrons in atoms ; but in the present theory, the ferro-and paramagnetisms are considered to be due to nuclear electrons, while the diamagnetism is as before attributed to the outer or optical electrons. So far the new theory is discussed only from the qualitative point of view, but it has been found that when this matter is considered quantitatively, many observed facts are explained quite satisfactorily without any strain.