Impurity band broadening as the origin of the positive magnetoresistance in the dilute magnetic semiconductor zinc manganese selenide

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PetznickSteve_2016_04_29.pdf (16.37 MB)

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2015

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DOI:
http://dx.doi.org/10.22029/jlupub-9731

Abstract

This work is dedicated to give a novel explanation for the large positive magnetoresistance effect in the n-type dilute magnetic semiconductor zinc manganese selenide. This effect has been observed in experiment in the hopping transport regime. The appearance of hopping transport is a necessary requirement for this effect to occur, as is the presence and the amount of magnetic atoms in the material. In literature, no conclusive explanation for this large positive magnetoresistance effect is known. The nature of the hopping transport requires a precise knowledge of the energetic position of each donor level, as an thermally activated electron has to overcome the energy difference from one donor level to another. Another aspect of the transport mechanism is that each donor-bound electron interacts only with the manganese atoms in its vicinity. Therefore, our explanation is based on the statistical distribution of magnetic atoms in the crystal. The random distribution of manganese leads to fluctuations of the donor levels according to their local manganese contents. Due to the bowing of the conduction band edge a donor level with a high local manganese content has a low energy. The fluctuating donor levels are represented by their density of states. This donor energy distribution is called impurity band and can be described by its width and its shape. An external magnetic field leads to a downshift of the preferred spin state of each donor, which is proportional to the Brillouin function and to its local manganese content. The energetic position inside the impurity band at zero magnetic field and the magnetic field induced energetic downshift, both depending on the local manganese content, are relevant. For example, a donor level with a high local manganese content is low in energy and experiences a large downshift in energy. The combination of these two aspects leads to an increasing of the width of the impurity band, while its shape remains unaffected in a first approximation. Using only basic hopping rates, this increasing impurity band-width results in an increasing resistivity, i.e. a positive magnetoresistance. To quantify this process two theoretical approaches are presented, i.e. a scaling approach and simulations of the effect.

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