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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...

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ESR measurements in the double-chain compound NH 4 CuCl 3 , which has magnetization plateaus at one-quarter and three-quarters of the saturation magnetization, have been performed in magnetic fields up to 30 T in the frequency range ϳ95 762 GHz. It is found that the frequency versus field diagrams for H k a and H k b coincide when normalized by the g factors. In each plateau region, a pair of excitations expressed as v g͑H 2 H lc ͒ and v g͑H hc 2 H͒ are observed, where H lc and H hc are the lower and higher edge fields of the plateau, respectively. These two modes correspond to the quantum gap in the lower-and higher-field halves in the plateau region. [S0031-9007(98)08295-7] PACS numbers: 75.30.Cr, Recently, the plateau of the magnetization curve in a quantum spin system has been attracting considerable attention. The magnetization plateau is predicted at one-third of the saturation magnetization M s in a spin- 2Heisenberg chain with ferromagnetic-ferromagneticantiferromagnetic interactions [1,2] and at half of M s in a spin-1 2 antiferromagnetic alternating Heisenberg chain with the next-nearest-neighbor interactions [3,4] and a spin-1 antiferromagnetic alternating Heisenberg chain [5,6]. The plateau results from the quantum gap which opens between the ground state and the lowest excited state in the appropriate magnetic field range. Since the magnetization plateau appears irrespective of the magnetic field direction, the plateaus in these systems are different from those observed in the metamagnetic systems. Recently, Oshikawa et al. [7] demonstrated that the magnetization curves of general quantum spin chains with axial symmetry can have plateaus, and that the magnetization is quantized at the plateaus as n͑S 2 m͒ an integer, where n is the period of the ground spin state, S is the magnitude of spin, and m is the magnetization per site in the unit of gm B . The excitation gap responsible for the plateau can exist only when this condition is satisfied. All of the above-mentioned plateaus satisfy this quantization condition. Experimentally, the magnetization plateau has been observed at ͑ 1 2 ͒M s in an S 1 antiferromagnetic alternating chain system ͓Ni 2 ͑methyl-bis͑3-aminopropyl͒amine͒ 2 ͑m-ox͒ ͑m-N 3 ͔͒ClO 4 ? 0.5H 2 O [8].In our previous paper [9], we have reported that the magnetization curve of NH 4 CuCl 3 has two plateaus at one-quarter and three-quarters of M s . NH 4 CuCl 3 is isostructural with KCuCl 3 at room temperature [10,11]. The feature of the crystal structure is the double chain of edge-sharing CuCl 6 octahedra along the crystallographic a axis. From the chemical point of view, the present system seems to be described as an S 1 2 Heisenberg double-spin chain system.In NH 4 CuCl 3 , two plateaus are clearly observed at ͑ 1 4 ͒M s and ͑ 3 4 ͒M s . It was confirmed that the magnetization curves for H k a and H k b coincide when normalized by the g factor, and that the plateaus appear in every external field direction. Therefore, the origin of the plateaus is attributed not to the magnetic aniso...