Magnetic phase transition under hydrostatic pressure in TlCu0.988Mg0.012Cl3 was investigated by magnetization measurements. The parent compound TlCuCl3 is a coupled spin dimer system, which undergoes a pressure-induced quantum phase transition from a gapped ground state to an antiferromagnetic state at Pc = 0.42 kbar due to the shrinkage of the gap. At ambient pressure, the present doped system exhibits impurity-induced magnetic ordering at TN = 2.5 K. With increasing pressure, TN increases. This is because the effective exchange interaction J eff between unpaired spins is enhanced by the shrinkage of the gap. With a further increase in pressure, the present system undergoes a phase transition to a uniform antiferromagnetic phase due to the closing of the triplet gap in the intact dimers. The crossover from the impurity-induced ordered phase to the uniform antiferromagnetic phase occurs at P ≃ 1.3 kbar.
Magnetic phase transitions under hydrostatic pressures in spin gap systems TlCu0.988Mg0.012Cl3 and KCu0.973Mg0.027Cl3 were investigated by magnetization measurements. The present doped systems exhibit impurity-induced magnetic orderings. With increasing pressure, ordering temperature TN increases. With a further increase in pressure, the present systems undergo phase transitions to uniform antiferromagnetic phases due to the closing of the triplet gap in the intact dimers. The crossover from the impurity-induced ordered phase to the uniform antiferromagnetic phase occurs at P ≃ 1.3 kbar for TlCu0.988Mg0.012Cl3. c 2017 Elsevier B.V. All rights reserved. Gapped ground states have been observed in many magnetic insulators. Most gapped ground states are composed of singlet spin dimers. Therefore, when a small number of nonmagnetic ions are substituted for magnetic ions, singlet spin dimers are partially broken, so that unpaired spins are produced near the nonmagnetic ions. Unpaired spins can interact through the effective exchange interaction J eff that is mediated by the triplet excitations in intact dimers [1,2]. This effective exchange interaction can cause the ordering of unpaired spins, which leads to the small staggered magnetic order in intact dimers. Such impurity-induced magnetic ordering is observed in many gapped spin systems doped with nonmagnetic impurities [3,4,5].The present study is concerned with impurity-and pressure-induced magnetic orderings in ACu 1−x Mg x Cl 3 with A= Tl and K. The parent compounds TlCuCl 3 and KCuCl 3 are S = 1/2 coupled spin dimer systems with antiferromagnetic intradimer exchange interactions J/k B = 65.9 K and 50 K, respectively [6,7]. The magnetic ground states are spin singlets with excitation gaps ∆/k B = 7.5 K and 30.5 K [8]. The gap decreases under hydrostatic pressure, and both systems undergo magnetic ordering [9,10]. The critical pressures for TlCuCl 3 and KCuCl 3 are P c = 0.42 kbar and 8.2 kbar, respectively. Because the effective interaction J eff between unpaired spins is enhanced * Corresponding author.Email address: tanaka@lee.phys.titech.ac.jp (H. Tanaka).as the gap decreases [1,2], we expect an increase in T N with pressure in ACu 1−x Mg x Cl 3 . To investigate the systematic change in the magnetic ordering in ACu 1−x Mg x Cl 3 , we carried out magnetization measurements under hydrostatic pressure.Doped ACu 1−x Mg x Cl 3 crystals were prepared by the vertical Bridgman method. The magnesium concentration x was analyzed by ICP−OES. In the present study, we used samples with x = 0.012 and 0.027 for A=Tl and K, respectively. The magnetizations were measured at temperatures down to 1.8 K under magnetic fields of up to 7 T using a SQUID magnetometer. Pressures of up to ∼9 kbar were applied using a cylindrical high-pressure cramp cell. A magnetic field was applied along the [2, 0, 1] direction. As pressure-transmitting fluid, a mixture of Fluorinert FC70 and FC77, and Daphne oil 7373 were used. The pressure was calibrated with the superconducting transition...
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