If I have a semiconductor that shows negative magnetoresistance at low temperature, what does it mean? It is an oxide.
While I'm not an expert in inorganic semiconductors, it means that either the concentration of charge carriers or their mobility increases with the application of a magnetic field (in organic semiconductors the increase of charge carrier population is known as field-doping). That said, you might want to have a read of the following papers:
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.32.1360
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.26.773
https://www.sciencedirect.com/science/article/pii/B978012805256300009X
Article Giant Magnetotransport Phenomena in Filling-Controlled KONDO...
Srinivasan RADHA Krishnan Thank you, how can I know if I have a spin polarization?
In non-magnetic materials, a negative magnetoresistance can be due to an additional Hall-effect which in turn is due to a transverse current, which in turn is due to surface recombination.
Negative MR at low temperatures is usually a consequence of disorder-induced localization. For 2D weakly-localized systems, it should be possible to describe your MR data using the Hikami-Larkin-Nagaoka model; the temperature dependence of the resistivity may also exhibit a logarithmic divergence at low temperature. For a strongly-localized (i.e. highly disordered) material, R(T) will follow a variable-range hopping law and the negative MR can be much larger (assuming that Coulomb repulsion hasn't opened a gap at the Fermi energy).
I thoroughly recommend reading Efros and Shklovskii's (excellent) book to obtain a deeper understanding of your data.
https://www.springer.com/gp/book/9783662024058
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