Drastic Change of Magnetic Phase Diagram in Doped Quantum Antiferromagnet TlCu1−xMgxCl3

TlCuCl 3 is a coupled spin dimer system, which has a singlet ground state with an excitation gap of Δ/ g µ 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. Through 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 ∼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.