Abstract
Magnetization and 57Fe Mossbauer effect spectroscopy (MS) studies of Ba(Fe1−xNix)2As2 single crystals (x=0 to 0.054) at temperatures (5 K to 300 K) have been performed. Magnetic measurements show that for BaFe2As2 the magnetic moment decreases below TN=136 K. TN is suppressed monotonically by Ni doping. On the other hand, for higher x values the magnetic moment increases below TN. Unexpectedly for x=0.024 (TN=67 K), the virgin zero-field-cooled (ZFC) curve is higher than that of field-cooled (FC) one below 48 K. The magnetic MS spectra of this sample are composed of a superposition of two subspectra, corresponding to commensurate and incommensurate field distributions. The average magnetic hyperfine field Heff decreases with T and becomes zero at 80 K. For higher x values, the samples become superconducting at TC=15.5 and 19 K for x=0.046 and 0.054, respectively. For both samples below TC, the FC curves are positive (the paramagnetic Meissner effect) up to applied field of H∼15 Oe and the susceptibility is inversely proportional to H. The MS spectra below and above TC are almost identical, indicating that the MS parameters are not sensitive enough to detect the superconducting state. The peculiar phenomena observed are attributed to disorder induced by the presence of Ni atoms in the Fe sublattice.
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