  The effects of Ni doping in Eu(Co{1-x}Ni{x})2As2 single crystals with x =0 to
1 grown out of self flux are investigated via crystallographic, electronic
transport, magnetic, and thermal measurements. All compositions adopt the
body-centered-tetragonal ThCr2Si2 structure with space group I4/mmm. We also
find 3-4% of randomly-distributed vacancies on the Co/Ni site. Anisotropic
magnetic susceptibility chi(T) data versus temperature T show clear signatures
of an antiferromagnetic (AFM) c-axis helix structure associated with the Eu{+2}
spins-7/2 for x = 0 and x = 1 as previously reported. The chi(T) data for x =
0.03 and 0.10 suggest an anomalous 2q magnetic structure containing two helix
axes along the c axis and in the ab plane, respectively, whereas for x = 0.75
and 0.82, a c-axis helix is inferred as previously found for x = 0 and 1. At
intermediate compositions x = 0.2, 0.32, 0.42, 0.54, and 0.65 a magnetic
structure with a large ferromagnetic (FM) c-axis component is found from
magnetization versus field isotherms, suggested to be an incommensurate FM cone
structure associated with the Eu spins, which consists of both AFM and FM
components. In addition, the chi(T) and heat capacity data for x = 0.2--0.65
indicate the occurrence of itinerant FM order associated with the Co/Ni atoms
with Curie temperatures from 60 K to 25 K, respectively. Electrical resistivity
measurements indicate metallic character for all compositions with abrupt
increases in slope on cooling below the Eu AFM transition temperatures. In
addition to this panoply of magnetic transitions, {151}Eu Mossbauer
measurements indicate that ordering of the Eu moments proceeds via an
incommensurate sine amplitude-modulated structure with additional transition
temperatures associated with this effect.
