  A 1/2 magnetization plateau in magnetic fields above 23 T and
antiferromagnetic (AF) long-range order (AFLRO) in low fields were found in
Cu$_2$CdB$_2$O$_6$. Experimental results agree with quantum Monte Carlo results
for an expected spin system. There are two kinds of Cu sites [Cu(1) and Cu(2)],
which are located adjacent to each other. Unexpectedly, spins on the Cu(1) and
Cu(2) sites are in a nearly spin-singlet state and form AFLRO, respectively,
although interactions between the Cu(1) and Cu(2) spins cannot be ignored.
Cu$_2$CdB$_2$O$_6$ is the first material which shows such coexistence in an
atomic scale.
