No abstract

Neutron elastic scattering experiments have been performed on the spin gap system TlCuCl 3 in magnetic fields parallel to the b-axis. The magnetic Bragg peaks which indicate the fieldinduced Néel ordering were observed for the magnetic field higher than the gap field H g ≈ 5.5 T at Q = (h, 0, l) with odd l in the a * − c * plane. The spin structure in the ordered phase was determined. The temperature and field dependence of the Bragg peak intensities and the phase boundary obtained were discussed in connection with a recent theory which describes the field-induced Néel ordering as a Bose-Einstein condensation of magnons.KEYWORDS: TlCuCl 3 , spin gap, field-induced magnetic ordering, spin structure, neutron elastic scattering, BoseEinstein condensation of magnonsThe singlet ground state with the excitation gap (spin gap) is a notable realization of the macroscopic quantum effect in quantum spin systems. When a magnetic field is applied in the spin gap system, the gap Δ is suppressed and closes completely at the gap field H g = Δ/gμ B . For H > H g the system can undergo magnetic ordering due to three-dimensional (3D) interactions. Such field-induced magnetic ordering was studied first for Cu(NO 3 ) 2 · 5 2 H 2 O. 1) However, the magnetic properties near H = H g have not been investigated because of the very low ordering temperature, the maximum of which is 0.18 K. Recently, the study of fieldinduced magnetic ordering has been revived, because the spin gap has been found in many quantum spin systems. Field-induced 3D ordering has been observed in several quasi-one-dimensional spin gap systems. [2][3][4][5][6] This paper is concerned with field-induced magnetic ordering in TlCuCl 3 . This compound has a monoclinic structure (space group P 2 1 /c). 7) TlCuCl 3 contains planar dimers of Cu 2 Cl 6 , in which Cu 2+ ions have spin-1 2 . These dimers are stacked on top of one another to form infinite double chains parallel to the crystallographic aaxis. These double chains are located at the corners and center of the unit cell in the b−c plane, and are separated by Tl + ions. The magnetic ground state is the spin singlet with the excitation gap Δ/k B ≈ 7.5 K. 8, 9) The magnetic excitations in TlCuCl 3 were investigated by Oosawa et al., 10) who found that the lowest excitation occurs at Q = (0, 0, 1) and its equivalent reciprocal points, as observed in KCuCl 3 . 11, 12) The origin of the gap is the strong antiferromagnetic interaction J = 5.26 meV on the planar dimer Cu 2 Cl 6 in the double chain. The * E-mail: tanaka@lee.phys.titech.ac.jp neighboring dimers couple magnetically along the chain and in the (1, 0, −2) plane.Our previous magnetic measurements revealed that TlCuCl 3 undergoes 3D magnetic ordering in magnetic fields higher than the gap field H g ≈ 5.5 T. 9) The magnetization exhibits a cusplike minimum at the ordering temperature T N . The phase boundary on the temperature vs field diagram is independent of the field direction when normalized by the g-factor, and can be represented by the power lawwith φ = 2.2...

The magnetic susceptibility and high-field magnetization process of NH 4 CuCl 3 with double chains of CuCl 3 have been measured using single crystals. No anomaly indicative of the threedimensional ordering is observed in susceptibility data above 1.7 K. It is found, by magnetization measurement down to 0.5 K, that in contrast to KCuCl 3 and TlCuCl 3 , NH 4 CuCl 3 has a gapless magnetic ground state at zero field. It is observed that the magnetization curve has two plateaus at one-quarter and three-quarters of the saturation magnetization, irrespective of the external field direction. The origin of the plateaus is attributed not to the magnetic anisotropy, but to the quantum effect. The relation between the plateaus and the period of the spin state is discussed in terms of a recent theory presented by Oshikawa et al. [Phys. Rev. Lett. 78 (1997) 1984.KEYWORDS: NH 4 CuCl 3 , double chain, high field, magnetization process, magnetization plateausThe magnetization processes of low-dimensional quantum spin systems with spatial structures such as spin ladders and exchange-alternating chains, are new problems in magnetism. In particular, magnetization plateaus are of great interest, because the magnetization is quantized at the plateaus. In a spin-1/2 Heisenberg chain with ferromagnetic (J F )-ferromagnetic (J F )-antiferromagnetic (J AF ) interactions, the magnetization curve has a plateau at 1/3 of the saturation magnetization M s , when J F /J AF < 5 ∼ 6.1, 2) For a spin-1/2 antiferromagnetic alternating Heisenberg chain with the next-nearestneighbor interaction, a plateau can appear at (1/2)M s in an appropriate parameter region. 3,4) In this case the next-nearest-neighbor interaction is essential for the existence of the plateau. The period of the spin state at the plateau is twice as large as the period of the Hamiltonian.4) Thus the plateau is caused by the quantum manybody effect. For a spin-1 antiferromagnetic Heisenberg chain, a plateau exists at (1/2)M s as long as the exchange interaction alternates.5, 6) The 1/2-plateau has been observed experimentally in the nickel compound8) investigated general Heisenberg chains in a magnetic field. They showed that the magnetization can have plateaus, and that it is quantized at the plateaus aswhere n is the period of the spin state, S the magnitude of spin and m the magnetization per site in the unit of gμ B . All of the above-mentioned plateaus satisfy this quantization condition. * Author to whom correspondence should be addressed. E-mail: tanaka@lee.cme.phys.titech.ac.jp 1548In this letter we report the magnetization plateaus observed in NH 4 CuCl 3 . At room temperature, NH 4 CuCl 3 is isostructural with KCuCl 3 which belongs to the monoclinic space group P 2 1 /c. 9, 10) The crystal structure is composed of double chains of edge-sharing CuCl 6 octahedra along the a-axis. The double chains are located at the corners and center of the unit cell in the bc-plane, and are separated by NH 4+ ions. There are three kinds of nearest-neighbor-interactions, J 1 , J 2 and J 3...

No abstract

No abstract