  The rare earth magnetic order in pure and doped
Er$_{(1-x)}$Ho$_{x}$Ni$_2$B$_2$C (x~=~0,~0.25,~0.50,~0.75,~1) single crystal
samples was investigated using magnetization and neutron diffraction
measurements. Superconducting quaternary borocarbides, $R$Ni$_2$B$_2$C where
R~=~Er, Ho , are both magnetic intermetallic superconductors with the
transition temperatures $\sim$ 10 K. These compounds also develop magnetic
order in the vicinity of this temperature. Depending on the rare earth
composition the coupling between superconductivity and magnetism creates
several phases, ranging from a reentrant superconductor with a mixture of
commensurate and incommensurate antiferromagnetism to a total incommensurate
antiferromagnetic spin modulation with a weak ferromagnetic state. All of these
phases coexist with superconductivity. RKKY magnetic interactions are used to
describe the magnetic orders in the pure compounds. However, the doping of Ho
on Er (or Er on Ho) sites which have two strong magnetic moments with two
different easy directions creates new and complicated magnetic modulations with
possible local disorder effects. One fascinating effect is the development of
an induced magnetic state resembling the pure and doped $R_2$CuO$_4$, R~=~Nd
and Pr.
