Diamagnetic Substances Research Articles

The thermodynamics of magnetization.

An accurate analysis of the thermodynamics of magnetic systems has to our knowledge never been made. The accounts given in the usual sources of reference are slight. Quite recently Stoner has tried to remedy this deficiency by a paper full of interest, but there are several points in his treatment which seem to us not altogether free from objection. We are therefore attempting in the present paper to give an accurate and fairly comprehensive treatment of the subject, in which the method of approach is somewhat different. This treatment seems to clarify the situation considerably. In particular we give a much more thorough analysis of pressure-volume effects than has previously been attempted. At the end of our paper we consider the relation between some of Stoner's formulae and our own. The thermodynamics of magnetic systems may be regarded as an extension of the electrodynamics of such systems to take account of thermal changes and volume changes of the magnetic matter. The chief difficulty encountered was that of finding a logically consistent treatment of magnetic energy from a purely electrodynamic aspect. In most of the recognized text-books either the treatment is not self-consistent or else it is assumed that the ratio of the magnetic induction to the magnetic field intensity is a constant. In a thermodynamic treatment this assumption may not be made even for diamagnetic and paramagnetic substances, because even supposing it to be valid at constant temperature it will not be valid when the temperature varies. Nor ca it generally be accurate both for variations at constant volume and for variations at constant pressure. It is therefore essential to start with a formula for magnetic energy which is independent of any assumed relation between the magnetic induction and the magnetic field intensity other than that each is a single valued continuous function of the other. This subject has been discussed in another paper where a formula of general validity for magnetic energy was obtained. We shall make this the basis of the present treatment.

On magnetic and electrostatic energy

The present paper is preliminary to another dealing with the thermodynamics of magnetization. The treatment of magnetic energy in most of the standard text-books is unsatisfactory, because the relations deducible directly from Maxwell's equations are inextricably mixed up with the assumption that the ratio of the magnetic induction B to the magnetic field intensity H is a constant. In order to extend the dynamical treatment of magnetic energy to a thermodynamic treatment which takes account of temperature and pressure variations it is essential not to assume that the ratio B/H is a constant. For even supposing that B/H may be assumed constant in the electrodynamic treatment, this merely means that its value is independent of B and H, but it will usually not be independent of temperature and pressure. Suppose, for instance, that for isothermal variation the ratio B/H remains constant, then for adiabatic variations it will usually not remain constant. Similarly if B/H is constant at constant pressure, it will not be constant at constant volume. In order to deduce the accurate thermodynamic relations, it is therefore essential to be clear about the eletrodynamic formulae for magnetic energy for the general case that B is any single valued continuous function of H. Such formulae will be applicable not only to diamagnetic and paramagnetic substances but also to ferromagnetic substances and even permanent magnets provided we exclude hysteresis. The treatment presented here is an elaboration of that of Cohn. Some of the errors made by other authors are discussed in a later section. 2—Notation Our notation agrees, so far as possible, with that of the S. U. N. Committee and with that of Cohn. In particular we leave open the question whether ε 0 and μ 0 are identically equal to unity. The most important symbols used are the following:—

The magnetostriction of bismuth single crystals

The problem of magnetostriction has been recently discussed by Kapitza, who by his well-known method of large magnetic fields was able to show its existence in a number of diamagnetic substances. The effect was most marked in bismuth single crystals, but only longitudinal measurements could be conveniently made with the method used, and formal analysis showed that these were not sufficient to provide a complete description ( i.e. ,the scheme of magnetostriction moduli) of the phenomenon. The present paper describes an experimental investigation of transverse magnetostriction in bismuth crystals, showing how this completes the scheme of magnetostriction moduli, and what contribution this scheme makes to an understanding of the anomalous behaviour of bismuth. The work had also the object of confirming the general features of the magnetostriction that were found by Kapitza in large fields, for the region of much lower fields available in an ordinary electromagnet. A rough estimate showed that the magnetostriction change of length in these fields (~ 15,000 gauss) would be extremely small (~ 5 × 10 -7 cm for a crystal 10 cm long), and the first part of the paper describes a new magnification method which had to be developed for the measurement of such small changes of length. This is followed by an account of the results obtained and a discussion of their significance.

The study of the magnetic properties of matter in strong magnetic fields. Part III.—Magnetostriction

Magnetostriction may be defined in general as the change of shape of a substance when it is magnetised. The phenomenon may originate from various causes, but there is one which appears to us to be of major importance. From our present conceptions of the origin of cohesion between the atoms forming a crystal lattice it appears that a considerable part of this cohesion is due to forces of electrodynamical origin; we may therefore expect to influence these forces by means of a magnetic field, and thus produce a change of shape of the body. In ferromagnetic substances magnetostriction is easily observed in ordinary magnetic fields and a number of theoretical investigations have been carried out to explain the general aspects of the phenomenon. With para- and diamagnetic substances, however, no magnetostriction has been observed.

Diamagnetism of Liquid Mixtures

AN investigation of binary mixtures of diamagnetic substances (one of the substances being water) made in this laboratory has shown a small but definite departure from additivity. The measurements are made with an improved Curie-Cheneveau magnetic balance.1 In view of the controversy concerning chloroform-acetone mixtures, it has been of interest to examine this system.

The study of the magnetic properties of matter strong magnetic fields.—I.—The balance and its properties

In several previous communications the author has described a method by which magnetic fields up to 300,000 gauss could be obtained for a duration of time of the order of 1/100 of a second. It was shown that these magnetic fields, in spite of the shortness of their duration, can be applied to the study of different phenomena such as the change of resistance, the Zeeman effect, and others. The present paper describes a number of investigations which have been made on different substances, extending the application of intense magnetic fields to the study of magnetic susceptibility and magnetostriction. The interest in measuring the susceptibility of different substances in strong magnetic fields lies mainly in seeing whether the linear law of magnetisation for ordinary para- and diamagnetic substances holds for higher fields, and also in the investigation of the saturation of paramagnetic bodies at low temperatures, with a view to determining the elementary magnetic moments. In the present communication a method of measuring the magnetic susceptibility is described and experimental results are given which verify the linear law of magnetisation for several paramagnetic and diamagnetic substances. The saturation of iron and nickel in strong fields is also studied. As will be seen later, the possibility of making these measurements in such a small fraction of time results from the increased magnitude of the phenomenon itself. The most direct method for measuring the magnetic susceptibility is to record the force on a magnetised body in an inhomogeneous magnetic field. In the usual experiments at room temperature this force is only a few hundred dynes, but when fields reach the magnitude of 300 kilogauss the force becomes several grams, and is then sufficiently large to be measured with fair accuracy even in short times of the order of 1/100 of a second. In this paper a special type of balance will be described by which these measurements are made possible.

The study of the magnetic properties of matter in strong magnetic fields. Part II.—The measurement of magnetisation

The balance described in Part I of this paper was found to be quite suitable for measuring the magnetisation of most substances. The experimental arrangement, by which it was used for measuring magnetisation in strong magnetic fields, is shown in fig. 4. A thin-walled quartz or glass tube (19) is suspended below the diaphragm (4) of the balance; the tube is divided into two parts by a small neck, the top part being filled with the substance to be investigated (20). The lower part of the tube was introduced merely to compensate the magnetic force which acts on the top part; in this way the corrections due to the magnetic properties of the suspension were reduced to a minimum. If the substance (20) was diamagnetic it was placed below the centre of the coil; if paramagnetic it was placed above the centre; thus the force to be measured by the balance always acts downwards. This is necessary as in most cases, even with weakly diamagnetic substances, the force was larger than the weight of the glass tube with the substance.

Magnetostriction of Diamagnetic Substances in Strong Magnetic Fields

THE change in shape of a body, when magnetised, may be accounted for by two causes. First the stresses produced by the magnetic forces act on the magnetic poles of the magnetised body. This effect, which we shall call the classical magnetostriction, is in a given magnetic field completely determined by the magnetic susceptibility and the elastic constants of the body. In general it will result in a contraction for diamagnetic substances and an expansion for paramagnetic ones. The classical magnetostriction is very small even for a strongly diamagnetic substance like bismuth, where δl/l in a field of 300 kilogauss is only 1.3 × 10-6. It is evident that the discovery of an effect of this order would amount to a verification of the classical theory of magnetisation and would afford no new information on the property of the substance.

The diamagnetism of hydrogen and the value of the magneton

In previous work on diamagnetism the author has brought forward evidence that the magneton may prove to be a constituent of diamagnetic as well as of paramagnetic and ferro-magnetic matter. Certain properties of diamagnetic crystalline media can be interpreted as due to the effects of intense local intermolecular forces which if interpreted magnetically are comparable with the Weiss molecular field in ferro-magnetic substances. This suggests the possibility that in the molecules of diamagnetic substances there may be elements of magnetism arranged in such a way that the molecule is self-compensated, so that experimentally the effect of applying an external magnetic field gives a diamagnetic effect only.

The influence of molecular constitution and temperature on magnetic susceptibility. Part III.—On the molecular field in diamagnetic substances

The work is a continuation of that in ‘Phil. Trans.,’ A, vol. 214, pp. 109-146, which contains Parts I and II.

III. The influence of molecular constitution and temperature on magnetic susceptibility. - Part III.On the molecular field in diamagnetic substances

In the present communication an attempt is made to examine to what extent the crystalline state of diamagnetic substances (which form a very large proportion of the whole number of substances known to us) involves mutual actions between the constituent molecules and to obtain a measure of the forces which hold the molecules together in a definite space lattice characteristic of the substance. The work is thus a continuation of that published in ‘Roy. Soc. Phil. Trans.,' vol. 214, pp. 109-146, 1914, wherein it was shown how from determinations dealing with the change of diamagnetic property on crystallization information could be derived concerning the inner structure of crystalline media and the intensity of the interacting molecular forces. In the paper cited (p. 143) it was suggested that the local molecular field within diamagnetic crystalline media may be comparable with the ferro-magnetic molecular (525.) field. In the light of the experimental results on aromatic substances there described and of those given by DU BOIS, HONDA and OWEN, which are concerned with elementary substances, this suggestion has been found to lead to interesting results. It is clear that we can easily test whether such an intense local molecular field exists; for if so it must be of import in connection with the transition from the liquid to the crystalline state in general, and the changes of other physical properties which accompany this change of state must also depend directly or indirectly upon the molecular field. The object of the present communication is to test the experimental and theoretical work which has already been completed by applying the results obtained to a wider range of physical phenomena. Evidence of a change of specific diamagnetic susceptibility owing to crystallization has been obtained with about forty substances, but with some substances the change, if it exists, is exceedingly small. Ten additional substances have been investigated, and the results, in conjunction with the other physical properties, are used to extend the ideas concerning the molecular field to diamagnetic crystalline media in general.

Hughes's Induction Balance

HAVING just made a Hughes's induction balance, I have, in the course of some experiments with it, observed what was new to me, for I have not seen it mentioned in any account of the balance. I take the liberty, therefore, of asking through your columns whether the explanation resolves itself into the difference between paramagnetic and diamagnetic substances. The apertures of my bobbins are 1½ inch in diameter; my primary current is from three Daniell's, and the break is a bent steel spring whose free point just grazes the surface of a mercury cup, so that the merest touch with a finger causes a series of regular breaks. Now, if I place an iron or steel disk, or ring, such as a key-ring, inside the aperture, the telephone sounds loudly if the plane of the disk or ring is at right angles to the plane of the coils; but very very faintly if it is parallel to the plane of the coils. On the other hand, if a disk, or ring, or coil of wire, of any of the diamagnetic metals—copper, brass, zinc, silver, gold, aluminium, lead—be used, the telephone sounds loudly if the plane of the disk or ring he parallel to the plane of the coils; but very faintly, if at all, when it is perpendicular to the plane of the coils. Further, if a short bar of soft iron, or of nickel, be inserted so that the length of the bar is parallel to the plane of the coils, almost no sound is heard; but if it be turned through a right angle so as to be perpendicular to the plane of the coils, the sound is a maximum. Have we in this simple instrument the ready means of distinguishing paramagnetic from diamagnetic substances?

Residual Magnetism in Diamagnetic Substances

IN the account which Prof. Ledge gives of his very interesting experiments (NATURE, March 25, p. 484) he describes an observation which at first sight seemed to show the existence of residual diamagnetic polarity in a diamagnetic substance after exposure to a strong field, and remarks that this seemed an incomprehensible result. It appears to me that this result, should it be confirmed, is not incomprehensible on Weber's theory of diamagnetism, if we supplement that by a modification of the Ampere-Weber theory of ordinary magnetisation.

XXIX. Remarks on the forces experienced by inductively magnetized ferromagnetic or diamagnetic non-crystalline substances

XXIX. <i>Remarks on the forces experienced by inductively magnetized ferromagnetic or diamagnetic non-crystalline substances</i>