Effect of magnetic field on mechanics of nonmagnetic crystals: The nature of magnetoplasticity
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SORDER, AND PHASE TRANSITIONS IN CONDENSED SYSTEMS
Effect of Magnetic Field on Mechanics of Nonmagnetic Crystals: The Nature of Magnetoplasticity A. L. Buchachenko Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, 119991 Russia e-mail: [email protected], [email protected] Received November 15, 2005
Abstract—The magnetoplastic effect in mechanics of nonmagnetic crystals is attributed to spin evolution in the spin-selective nanoscale reactor created by electron transfer from a dislocation to a stopper. In this “dislocation + stopper” system, dislocation depinning is facilitated because the Coulomb attraction between the dislocation and the stopper is switched off. Since magnetic field stimulates the singlet-to-triplet conversion of the nanoscale reactor (the reverse electron transfer is forbidden), the nanoscale reactor with switched-off Coulomb interaction has a longer lifetime. The resulting increase in depinning rate and dislocation mobility provides a physical explanation for magnetoplasticity. PACS numbers: 61.72.Yx, 62.20.Fe, 81.40.Rs DOI: 10.1134/S1063776106050116
1. INTRODUCTION Studies of mechanical properties of nonmagnetic crystals have revealed an unexpected dependence of these properties on magnetic field strength (see reviews in [1–3]). At first glance, this phenomenon is unlikely to occur, because the magnetic-field-induced increase in the energy of a nonmagnetic crystal (such as NaCl, PbS, InSb, or LiF) is negligible. However, it has been repeatedly and reliably observed in various experiments. In one group of experiments, a sample crystal is subjected to a preliminary magnetic field treatment, and then mechanical tests are performed on the sample at zero field. The persistent effects of preliminary treatment can be called static effects or magnetic memory. In another group of experiments, mechanical testing and measurements of crystal properties are performed in nonzero magnetic field. The effects observed in this regime are called dynamic effects. The leading position in the discovery and experimental investigation of magnetic-field effects is held by science schools at the Institute of Solid-State Physics of the RAS (Osip’yan, Morgunov) [3], Shubnikov Institute of Crystallography of the RAS (Al’shits with coworkers) [1, 2, 4–7], and Tambov State University (Golovin) [8, 9]. Static magnetic-field effects on solubility and microhardness of crystals, dislocation mobility, or yield are commonly attributed to the metastable nonequilibrium states of paramagnetic impurities and defects in crystals, which can be affected by the field (even though the underlying mechanism is not known) and undergo changes due to crystal aging [1, 2]. Magnetic memory is not so easy to observe and demonstrate, because its manifestations are obscured by aging processes.
Much more conclusive results are obtained in studies of dynamic magnetic-field effects (magnetoplasticity), which have been reliably demonstrated, even if not explained. These effects include a decrease in microhardness, a lowe
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