Effect of low Zn doping on the Verwey transition in magnetite single crystals: Mossbauer spectroscopy and x-ray diffraction
Abstract
To observe, by microscopic probe, how low Zn doping in Fe3-xZnxO4 (x is below 1%) changes the Verwey transition, we have performed Mossbauer spectroscopy measurements on three single crystalline samples with various Zn doping. In spectra analysis we used the recently published model of Mossbauer data treatment formulated as a result of ab initio calculations for a low-temperature monoclinic structure (of Cc symmetry) of magnetite.
It was suggested there that the hyperfine parameters for all 24 Fe distinct positions in the lattice can be grouped into four major components with very similar hyperfine parameters within each set. Using these parameters as starting values, very good fits were obtained for magnetite with low doping level, while for higher doping, x = 0.03, where the Verwey transition changes its character, one component is significantly different.
In particular, low hyperfine field B-eff = 36 T, considered as a characteristic feature of the Cc phase spectrum, is absent here. Also, in this case, the high-temperature spectra are different from those for lower doped magnetite showing more pronounced continuous alteration with temperature.
This might be due to crystal structure of lower than Fd-3m symmetry, a fact suggested by our x-ray synchrotron studies. All this triggered a discussion about an experimental fingerprint for the difference between these two classes of magnetite, frequently referred to as magnetite of first- and second-order Verwey transition, and about the electronic structure of both kinds of systems.