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Neutron elastic scattering measurements have been performed under a hydrostatic pressure in order to investigate the spin structure of the pressure-induced magnetic ordering in the spin gap system TlCuCl 3 . Below the ordering temperature T N ¼ 16:9 K for the hydrostatic pressure P ¼ 1:48 GPa, magnetic Bragg reflections were observed at reciprocal lattice points Q ¼ ðh; 0; lÞ with integer h and odd l, which are equivalent to those points with the lowest magnetic excitation energy at ambient pressure. This indicates that the spin gap closes due to the applied pressure. The spin structure of the pressure-induced magnetic ordered state for P ¼ 1:48 GPa was determined.KEYWORDS: TlCuCl 3 , spin gap, pressure-induced magnetic ordering, spin structure, neutron elastic scattering DOI: 10.1143/JPSJ.72.1026The spin gap system is a magnetic system having the singlet spin liquid ground state with a finite excitation gap. 1)Recently, the magnetic ordering induced by modifying the gapped ground state by an external field or impurity doping has been energetically investigated. When a magnetic field, which is higher than the gap field H g corresponding to the energy gap Á ¼ g B H g , is applied in a spin gap system, the energy gap vanishes, and the system can undergo the magnetic ordering with the help of three-dimensional (3D) interactions. On the other hand, when nonmagnetic ions are substituted for magnetic ions in a spin gap system, the singlet ground state is disturbed, so that staggered moments are induced around the impurities. If the induced moments interact through effective exchange interactions, which are mediated by intermediate singlet spins, the 3D long-range order can arise. Such field-induced and impurity-induced magnetic orderings were observed in many spin gap systems. 2-7)The application of pressure is another method of controlling the quantum magnetism including the spin gap and the spin-Peierls transition. Some significant pressure effects have been observed in some quantum spin systems. The pressure-induced magnetic ordering was observed in Cu 2 (C 5 H 12 N 2 ) 2 Cl 4 , 8) which was first assumed to be an S ¼ 1=2 Heisenberg antiferromagnetic two-leg ladder 9) and was later characterized as a frustrated 3D spin gap system. 10) In Cu 2 (C 5 H 12 N 2 ) 2 Cl 4 , some of the singlet spin pairs are broken by the applied pressure to become paramagnetic spins, and the magnetic ordering similar to the impurity-induced antiferromagnetic ordering occurs in the rest of the spins. 8)The spin gap remains even in the ordered state.In a well-known inorganic spin-Peierls (SP) material CuGeO 3 , the SP phase was enhanced by the applied pressure, 11) so that the revival of the SP phase was observed for highly Mg-doped CuGeO 3 , in which the SP transition does not occur and the impurity-induced antiferromagnetic ordering only occurs at the low temperature for ambient pressure. 12)This paper is concerned with the pressure-induced magnetic ordering in the spin gap system TlCuCl 3 .We summarize the physical properties of TlCuCl ...

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Under the hydrostatic pressure P ¼ 1:48 GPa, polarized neutron elastic scattering experiments have been carried out on the coupled spin dimer system TlCuCl 3 with the gapped ground state at ambient pressure. Pressure-induced magnetic ordering occurs at the transition temperature T N ¼ 16:9 K. As in the field-induced and impurity-induced magnetic ordered phases, the ordered moments lie in the a-c plane just below T N . An additional spin reorientation (SR) phase transition was observed at T SR ¼ 10 K, where the ordered moments start to incline toward the b-axis. The temperature variations of the direction and the magnitude of the ordered moments were also investigated. The spin gap in interacting spin systems is a macroscopic quantum phenomenon and has been attracting considerable attention both theoretically and experimentally. Recently, unconventional magnetic orderings induced by impurity ions, applied magnetic field and pressure in the spin gap systems have been energetically investigated. The impurityinduced magnetic ordering due to the breaking of the local spin singlet pair has been well studied through the wellknown inorganic spin-Peierls system CuGeO 3 . The coexistence of the lattice dimerization associated with the spin gap and antiferromagnetic ordering have been observed. 1-4)Field-induced and pressure-induced magnetic orderings are quantum phase transitions realized by the vanishing of the spin gap. The field-induced magnetic ordering can be interpreted as the Bose-Einstein condensation of triplet magnons. [5][6][7][8] The title spin gap system TlCuCl 3 undergoes all of these magnetic orderings. [9][10][11][12][13][14][15][16][17] TlCuCl 3 has a monoclinic structure (space group P2 1 =c).18) The crystal structure of TlCuCl 3 consists of planar dimers of Cu 2 Cl 6 . The dimers form infinite double chains along the crystallographic a-axis. These chains are located at the corners and the center of the unit cell in the bc plane, and are separated by Tl þ ions. The magnetic ground state of TlCuCl 3 is the spin singlet 18) with the excitation gap Á ¼ 7:7 K. 9,19) The origin of the spin gap is the strong antiferromagnetic spin dimer in the chemical dimer Cu 2 Cl 6 , and the neighboring spin dimers couple via strong threedimensional interdimer interactions along the double chain and in the ð1; 0; À2Þ plane, in which the hole orbitals of Cu 2þ spread. 20,21) TlCuCl 3 undergoes pressure-induced magnetic ordering above P c $ 0:2 GPa.16) Unpolarized neutron elastic scattering experiments under the hydrostatic pressure P ¼ 1:48 GPa were carried out in order to investigate the spin structure of the pressure-induced magnetic ordering in TlCuCl 3 . 17) Below the ordering temperature T N ¼ 16:9 K, the magnetic Bragg reflections were observed at reciprocal lattice points Q ¼ ðh; 0; lÞ with integer h and odd l, which are equivalent to those points with the lowest magnetic excitation energy at ambient pressure. The spin structure of the pressure-induced ordered phase in TlCuCl 3 for P ¼ 1:48 GPa was determined as shown...