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In dense media positrons and positronium atoms are bound in self-trapped states. These states are density fluctuations stabilised by the light quantum particles. The positronium atoms are trapped in a bubble, while positrons are trapped in a cluster. Transitions to these states at changes of the density or temperature of the medium produce an essential effect on the annihilation rate. It can be considered as a local phase transition in the vicinity of the positron (or positronium). The present article is a review of the present state of the physics of positron and positronium self-trapped states in dense gases and liquids. Basic properties of the positronium bubbles and positron clusters are considered, as well as the conditions under which they exist and their effect on the annihilation rate. Experiments in which the self-trapping was detected are discussed. The positron and positronium self-trapping is considered to be closely related to other similar electronic phenomena, known for dense gases, liquids and other dense media. The main results from the study of positron and positronium interactions with isolated atoms are also presented. The density effects in moderately dense gases anticipating self-trapping are considered.

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The magnetization reversal of ferromagnetic Co-based amorphous microwires carrying ac current is studied. The frequency spectra of the voltage Vφ induced in the pick-up coil wound around the microwires were analysed and were found to depend on the ac current amplitude I0 and external longitudinal dc magnetic field He. The even harmonics are shown to dominate in the frequency spectra of Vφ if the ac current amplitude exceeds some threshold value, which strongly depends on He. A high sensitivity of the even harmonic amplitudes to the longitudinal dc field is demonstrated. The experimental data are described in terms of the Faraday law and the quasi-static Stoner-Wohlfarth model. The results obtained may be of importance for the design of weak magnetic field sensors.