Magnetization of the frustrated S = 1/2 chain compound LiCuVO4, focusing on high magnetic field phases, is reported. Besides a spin-flop transition and the transition from a planar spiral to a spin modulated structure observed recently, an additional transition was observed just below the saturation field. This newly observed magnetic phase is considered as a spin nematic phase, which was predicted theoretically but was not observed experimentally. The critical fields of this phase and its dM/dH curve are in good agreement with calculations performed in a microscopic model (M. E. Zhitomirsky and H. Tsunetsugu, preprint, arXiv:1003.4096v2).PACS numbers: 75.50. Ee, 75.10.Jm, 75.10.Pq Unconventional magnetic orders and phases in frustrated quantum spin chains are attractive issues, because they appear under a fine balance of the exchange interactions and are sometimes caused by much weaker interactions or fluctuations. 1-4

From a heat capacity (C p ) measurement on a single crystal sample of the S 1 quasi-onedimensional (Q1D) Heisenberg antiferromagnet (HAF) Ni͑C 5 H 14 N 2 ͒ 2 N 3 ͑PF 6 ͒ in applied magnetic fields, we found an anomaly which is indicative of a magnetic long-range ordering. We were able to follow how the position of the anomaly in C p changes with temperature ͑T͒ and magnetic field (H). An experimental H-T phase diagram of an S 1 Q1D HAF is obtained and compared with that of the corresponding classical system. [S0031-9007(98)07105-1] PACS numbers: 75.30. Kz, 75.10.Jm, 75.40.Cx, 75.50.Ee Although the theoretical study of one-dimensional (1D) magnetism began in the 1930s [1], several decades passed before suitable model compounds became available [2]. Quasi-1D magnets, in which the magnetic interaction in one direction dominates, with much weaker interactions in other directions, exhibit a short-range ordering over a wide temperature range and usually show a long-range ordering (LRO) at finite temperature due to the interchain coupling (J 0 ). The situation is largely altered in the case of quasi-1D Heisenberg antiferromagnet (HAF) with integer spin quantum number (S). As has been predicted by Haldane [3], there is an energy gap (Haldane gap) between the singlet ground state and first excited triplet in an S 1 1D HAF. The effects of J 0 on the Haldane gap have been studied theoretically [4,5] and the results show that the Haldane gap survives even at T 0 K, if J 0 is small [͑z 0 J 0 ͞J & 0.05͒ z 0 : number of adjacent chains; J: intrachain coupling]. Therefore, quasi-1D S 1 HAF compounds with small J 0 become nonmagnetic at low temperatures and no LRO occurs. On the other hand, strong magnetic fields destroy the Haldane gap and the system recovers magnetism [6]. Then, we expect a magnetic ordering to occur in a quasi-1D S 1 HAF under high fields and at low temperatures.The magnetic ordering in a quasi-1D classical HAF has been studied both theoretically [7,8] and experimentally [9,10]. These experiments showed that the Néel temperature (T N ) of the quasi-1D S 5͞2 HAF compound ͑CH 3 ͒ 4 NMnCl 3 (TMMC) which exists already in zero field increases with increasing field.In a previous paper [11], we reported experimental evidence for the field induced magnetic ordering in the S 1 quasi-1D HAF compound Ni͑C 5 H 14 N 2 ͒ 2 N 3 ͑ClO 4 ͒, abbreviated NDMAZ. From a heat capacity (C p ) measurement on a single crystal sample of NDMAZ, we observed an anomaly at about 0.6 K and at 12 T which indicated that a magnetic ordering occurred there. Because of limitations in our calorimeter, we were unable to follow how the position of the anomaly in C p changes with temperature ͑T ͒ and magnetic field (H). We then tried to synthesize a new quasi-1D HAF compound with a weaker intrachain exchange interaction in which an LRO is expected to be induced at a lower field. In this paper, we report the first experimental H-T phase diagram of a quasi-1D S 1 HAF which might be interesting to broad audiences. It is also interesting to compare t...

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Orbital and magnetic ordering in La 0.5 Sr 1.5 MnO 4 have been studied with resonant soft x-ray scattering at the Mn L 2,3 edges and for the first time azimuthal angle scans and polarization analysis are presented. The azimuthal angle dependencies are well described by the occurrence of a quadrupole ͑orbital͒ ordering below T OO and an additional dipole ͑magnetic͒ contribution below T N . There are no indications that there is an enhanced Jahn-Teller distortion at T N as reported in a previous study. Subsequently, it is shown that there is simultaneous ferro-and antiferromagnetic ordering along the c-direction.

Field-induced commensurate transverse magnetic ordering is observed in the Haldane-gap compound Ni(C(5)D(14)N(2))2N(3)(PF(6)) by means of neutron diffraction. Depending on the direction of applied field, the high-field phase is shown to be either a three-dimensional ordered Néel state or a short-range ordered state with dominant two-dimensional spin correlations. The structure of the high-field phase is determined, and properties of the observed quantum phase transition are discussed.

Inelastic neutron scattering experiments on the Haldane-gap quantum antiferromagnet Ni(C5D14N2)2N3(PF6) are performed at mK temperatures in magnetic fields of almost twice the critical field Hc applied perpendicular to the spin cahins. Above Hc a re-opening of the spin gap is clearly observed. In the high-field Néel-ordered state the spectrum is dominated by three distinct long-lived excitation branches. Several field-theoretical models are tested in a quantitative comparison with the experimental data.One-dimensional (1D) integer-spin antiferromagnets (AFs) are famous for having a disordered "spin liquid" ground state and an energy gap ∆ ∼ exp(−πS) in the excitation spectrum [1]. Elementary excitations are a triplet of massive (gapped) long-lived "magnons". An external magnetic field modifies the magnon energies by virtue of Zeeman effect [2,3]. At a certain critical field H c the gap in one of the branches approaches zero [3,4]. The result is a condensation of magnons[3, 5] and the emergence of a qualitatively new ground state. What this new ground state actually is, depends on the symmetry of the problem. Theory predicts that in an axially symmetric (AS) Heisenberg or XY-like scenarios the high-field phase is a gapless "Luttinger spin liquid" with quasi-long-range order and a diffuse continuum of excitations (no sharp magnons) [6]. The high-field phase in the axially asymmetric (AA) case is expected to be totally different. Here the ground state should have true long-range Néel order ("spin solid"). A simple boson description [3] predicts a re-opening of the gap at H > H c , and implies the restoration of a single-particle excitation spectrum. Is this state then similar to a classical easyplane AF in a field, that also features long-range order and sharp gapped spin waves?Only recently did experiments, which are the key to understanding the high-field behavior, become technically feasible. This was in part due to the discovery of the very useful model material Ni(C 5 D 14 N 2 ) 2 N 3 (PF 6 ) (NDMAP) [7], where various techniques [7,8,9] confirmed a quantum phase transition at an easily accessible critical field of H c ≈ 6 T. Inelastic neutron studies were carried out in the AA geometry in the thermally-disordered phase: at H > H c , but at temperatures high enough to destroy long-range Néel order [10]. Somewhat unexpectedly, it was found that as the Haldane gap closes at H c , the spectrum retains a considerable quasielastic (gapless) component at higher fields. The theoretically predicted reopening of the gap was thus not observed. At the time, this behavior was not fully understood, though several intriguing explanations were put forward. One attributed the phenomenon to the 1D diffusion of thermally excited classical solitons [11], while another drew parallels with the incommensurate Luttinger liquid state in the AS geometry. To better understand this issue, we carried out a new series of measurements at considerably lower temperatures and in higher magnetic fields, overcoming any finite-T effects to direct...

Detailed results of heat capacity and magnetization measurements are reported on a single crystal sample of the spin Sϭ1 quasi-one-dimensional Heisenberg antiferromagnet Ni(C 5 H 14 N 2 ) 2 N 3 (PF 6 ), abbreviated NDMAP. From these results, we constructed the magnetic field (H) versus temperature phase diagram of this compound which exhibits the quantum disordered Haldane, field-induced long-range-ordered ͑LRO͒ and thermally disordered paramagnetic phases. The phase boundary curve separating the paramagnetic and LRO phases is anisotropic; the increase of the Néel temperature (T N ) with H along the a and b axes is more rapid than that along the c axis. We calculated T N as a function of H by taking into account the interchain coupling as the form of a mean field. The calculation of the staggered susceptibility of the Sϭ1 one-dimensional antiferromagnet with an easy-plane anisotropy shows quite different behavior in the different field directions, resulting in the anisotropic phase boundary curve. A good agreement is obtained between theory and experiment using the exchange and anisotropy constants obtained from the neutron scattering experiment.

Inelastic neutron scattering experiments on the Haldane-gap quantum antiferromagnet Ni(C5D14N2)2-N3(PF6) are performed in magnetic fields below and above the critical field H(c) at which the gap closes. Quasielastic neutron scattering is found for H>H(c), indicating topological excitations in the high-field phase.