Drastic Change of Magnetic Phase Diagram in Doped Quantum   Antiferromagnet TlCu$_{1-x}$Mg$_x$Cl$_3$
  TlCuCl$_3$ is a coupled spin dimer system, which has a singlet ground state with an excitation gap of $\Delta/g\mu_{\mathrm B}$ = 5.5 T. TlCu$_{1-x}$Mg$_x$Cl$_3$ doped with nonmagnetic Mg$^{2+}$ ions undergoes impurity-induced magnetic ordering. Because triplet excitation with a finite gap still remains, this doped system can also undergo magnetic-field-induced magnetic ordering. By specific heat measurements and neutron scattering experiments under a magnetic field, we investigated the phase diagram in TlCu$_{1-x}$Mg$_x$Cl$_3$ with $x\sim 0.01$, and found that impurity- and field-induced ordered phases are the same. The gapped spin liquid state observed in pure TlCuCl$_3$ is completely wiped out by the small amount of doping. 
M. Fujisawa, T. Ono, H. Fujiwara, H. Tanaka, V. Sikolenko, M.   Meissner, P. Smeibidl, S. Gerischer and H. A. Graf