Table of contents

The present position of our knowledge of antiferromagnetism, including ferrimagnetism, is reviewed, and some very interesting phenomena concerning the magnetic behaviour of certain ferrites and of pyrrhotite are described and explained.

The theory of ferromagnetism is often developed on the assumption that the exchange energy responsible for the magnetization is adequately represented as a sum of exchange integrals between pairs of electrons with mutually parallel spins. This is equivalent to using for the total wave function a determinant built up of one-electron functions. For the magnetized states of interest in ferromagnetism a single determinant wave function must be regarded as inadequate on account of the so-called spin or exchange degeneracy, which requires us to use instead appropriate linear combinations of the degenerate determinants. In the present paper a model is presented for which the results of neglecting or taking account of this spin degeneracy are identical. The model can be handled exactly and leads to equations for the free energy, magnetization, etc., which are a generalization of Stoner's theory of ferromagnetism to a system with two energy bands; these could be the overlapping 3d and 4s bands in the iron transition series. The calculations are not necessarily restricted to ferromagnetic metals and would apply equally well to those that are paramagnetic.

Proton magnetic resonance widths and relaxation times have been measured in ammonium chloride, bromide and iodide, methyl alcohol and dimethyl sulphate from 90°k to 290°k. Line width transitions were observed in NH₄Cl and NH₄Br at 135°k and 99°k respectively. There were no sharp changes in either the relaxation time or line width at the temperatures of the λ-specific heat anomalies of the ammonium salts. Evidently, there is no marked increase in the rate of rotation of the ammonium ion at these transitions. The results for NH₄Br suggest that rotation of the NH₄⁺ ion is hindered by a potential barrier of about 3 500 cal/mole. The N-H distance is 1.03±0.01 Å. The line width and relaxation time show pronounced changes at the λ-point of methyl alcohol, indicating a sudden change in the motion of the CH₃OH molecule. In solid dimethyl sulphate, a narrow line of small amplitude appears at about 170°k.

For an isotropic two-band conductor in a transverse magnetic field, the various galvanomagnetic, thermomagnetic and thermoelectric coefficients are derived by elementary arguments in terms of those of the individual bands. The expressions are applicable to quasi-free electrons of any degree of degeneracy: for fully degenerate electrons they reduce to those already given by Sondheimer and Wilson. When the square of the thermoelectric power is comparable with the Lorenz number, as may happen in semiconductors, the expressions for the adiabatic thermomagnetic and thermoelectric effects become extremely complicated, though those for the isothermal effects remain reasonably simple.

Samples containing elements of even Z have been examined for possible beta-activity using a photographic emulsion technique. For the elements Cr, Fe, Ni, Zn, Ge and Cd the results show that the effective half-value period for double beta-decay for the elements as a whole is in each case greater than 10¹⁷ years if the disintegration energy is 2MeV or more (or greater than 10¹⁶ years if the disintegration energy is 0.35 MeV or more). For Ca, Mo, Sr, Sn, Te, Ba, W, Os and Pt lower limits between 2×10¹⁴ and 6×10¹⁶ years are similarly obtained on the assumption of 2 MeV disintegration energy, the residual uncertainty in most cases being due to the slight contamination of the samples with natural radio-elements. A possible weak beta-activity shown by samples containing Mo requires further examination.

Expressions for the angular distribution of tritons emitted from nuclei bombarded with deuterons have been obtained using Born's approximation and the non-perturbation method of Butler. Discussion of the effect of different forms of the triton wave function on the angular distribution is given. The theory is shown to explain the experimental angular distributions satisfactorily.

Born's approximation is used to calculate the cross section associated with resonance charge exchange between protons and hydrogen atoms, it being pointed out that in earlier treatments of this problem unjustified simplifying assumptions were made. An energy range of up to 250 keV is covered. The predicted energy variation of the cross section is in harmony with the results of Ribe's recent laboratory work on collisions of protons with hydrogen molecules.

The transition curve for cosmic-ray bursts under lead at sea level is analysed to find the contributions from stars, extensive showers, electromagnetic interactions of μ-mesons, and single high-energy electrons and photons. When this is done, a substantial fraction of the bursts under lead thicknesses less than about 2.5 cm remains unaccounted for. It is suggested that they are produced by `narrow air showers' consisting predominantly of photons of about 2.5×10⁸ eV and having a sideways spread of less than 0.3 m.

A modified definition of the Riesz potential for the meson field is introduced It is shown that the new definition is a generalization, in the α-plane, of the meson potential in the interaction representation. The relations satisfied by the meson potential in the interaction and in the Schrödinger representations are compared.

An inquiry is made into the factors which cause shifts in the ultra-violet spectra of aromatic molecules on substitution. The method of molecular orbitals is used, and the discussion is confined to π-electrons. It is shown that there are two chief effects: the first is essentially electrostatic, or inductive; the second is essentially conjugative, or mesomeric. In alternant hydrocarbons the second predominates and causes a red shift. In other molecules (e.g. azulene, quinoline) the first effect is considerably larger and now predominates over the second. Different positions of substitution will lead to shifts, sometimes to the red and sometimes to the blue. Electron donors will cause shifts in opposite directions to electron acceptors. The importance of these variations in characterizing the electronic transitions is stressed. Detailed investigation of the various methyl azulenes confirms the assignment of the long wavelength band with almost complete certainty.

The half-lives, mass assignments and radiation characteristics of three neutron-excess gold isotopes have been determined as follows:

	200Au	201Au	203Au
Half-life	48 min	26 min	55 sec
Max. β-energy (MeV)	2.2	1.5	1.9
γ-energy (MeV)	0.39, 1.13	0.55	0.69

Corrections to 1952 Proc. Phys. Soc. A 65 773