  Defects intentionally introduced into magnetic materials often have a
profound effect on the physical properties. Specifically tailored neutron
spectroscopic experiments can provide detailed information on both the local
exchange interactions and the local distances between the magnetic atoms around
the defects. This is demonstrated for manganese dimer excitations observed for
the magnetically diluted three- and two-dimensional compounds KMn(x)Zn(1-x)F(3)
and K(2)Mn(x)Zn(1-x)F(4), respectively. The resulting local exchange
interactions deviate up to 10% from the average, and the local Mn-Mn distances
are found to vary stepwise with increasing internal pressure due to the Mn/Zn
substitution. Our analysis qualitatively supports the theoretically predicted
decay of atomic displacements according to 1/r**2, 1/r, and constant (for
three-, two-, and one-dimensional compounds, respectively) where r denotes the
distance of the displaced atoms from the defect.
