In this Letter we discuss the observation of a pattern of concentric annuli formed as a result of thermal effects upon a light beam. To our knowledge, this is the first time such patterns have been reported. When a beam of light passes through an absorbing medium whose refractive index decreases as a function of temperature, the medium behaves as a negative lens. Gordon et al.1 observed this phenomenon by placing a cell containing an absorbing liquid between the mirrors of a helium-neon laser. The absorption constants for several liquids with very low absorbencies have been determined using this effect.2 Rieckhoff3 measured the increase in the diameter of a light beam that passed through a thermal self-defocusing liquid placed external to the laser inter-ferometer. Recently Leite et al.4 demonstrated that this effect may be used as a power limiting device.

The features of minor loop behaviour in magnetic hysteresis are examined by comparing some experimental data on a Mn‒Al‒Ge film with two phenomenological models. The ‘independent particle’ model of Preisach and Everett proves to describe qualitatively all the observed behaviour. This model is generalized slightly to allow for the interaction of the ‘particle’ through an average demagnetizing field, and a convenient method for calculating the consequences of the model is described.

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