Long-range magnetic ordering in R2BaNiO5 (R=magnetic rare earth) quasi-1-dimensional mixed-spin antiferromagnets is described by a simple mean-field model that is based on the intrinsic staggered magnetization function of isolated Haldane spin chains for the Ni-subsystem, and single-ion magnetization functions for the rare earth ions. The model is applied to new experimental results obtained in powder diffraction experiments on Nd2BaNiO5 and NdYBaNiO5, and to previously published diffraction data for Er2BaNiO5. From this analysis we extract the bare staggered magnetization curve for Haldane spin chains in these compounds. 75.25.+z,75.10.Jm,75.50.Ee The quantum-disordered ground state and the famous Haldane energy gap in the magnetic excitation spectrum [1] have kept one-dimensional (1-D) integer-spin Heisenberg antiferromagnets (HAF) at the center of attention of condensed matter physicists for the last 15 years. Among the more recent developments are studies of such systems in external uniform magnetic fields [2][3][4]. It was found that in sufficiently strong fields one of the three Haldane-gap modes undergoes a complete softening at some particular wave vector. The result is a transition to a new phase with long-range antiferromagnetic correlations (see for example Refs. [5,6]). The effect of a staggered field H π , to which a Haldane chain is most susceptible, is expected to be no less dramatic. Unfortunately, this problem has been given much less attention in literature, simply because such conditions are almost impossible to realize in an experiment. The only chance of producing a magnetic field modulated on the microscopic scale is to make use of some periodic modulation that is intrinsic to the system under investigation. This can be, for example, a structural modulation, as is the case in NENP, one of the best-known Haldane-gap compounds (Refs. [7,8] and references therein). The S = 1 chains in this material consist of alternating crystallographically non-equivalent Ni 2+ ions with slightly different gyromagnetic ratios. A weak effective staggered field can thus be induced in NENP by applying a uniform external field [9]. Unfortunately, the effect of the staggered component is obscured by the response of the system to the strong uniform field itself [9,10].A more direct approach is to use an intrinsic magnetic modulation in a material that, in addition to integer-spin Heisenberg chains, has other magnetic ions. Should the latter become ordered magnetically with an appropriate propagation vector, they will project an effective staggered exchange field on the Haldane spin chains. The magnitude of the staggered field can be varied in an experiment indirectly, by changing the temperature, and thus the magnitude of the magnetic order parameter. By measuring the induced moment on the Haldane chains one can hope to directly measure the staggered magnetization curve for a Haldane spin system. This function, that we shall denote as M(H π ), is one of the principal characteristics of a quantum-disordered intege...
4SEASONS is a Fermi chopper spectrometer in operation at the Materials and Life Science Experimental Facility (MLF) in the Japan Proton Accelerator Research Complex (J-PARC). 4SEASONS is expected to facilitate highefficiency measurements of weak inelastic signals on novel spin and lattice dynamics using thermal neutrons. The spectrometer is equipped with a coupled moderator and sophisticated components such as an elliptical converging neutron guide with high-critical-angle supermirrors, long (2.5 m) position-sensitive detectors, and a Fermi chopper appropriate for multiple-incident-energy (multi-E i ) measurements by the repetition-rate multiplication technique. Herein, we discuss in detail the design and performance of the spectrometer, and present some examples of the measurements obtained using this spectrometer.
We succeeded in experimentally demonstrating that a series of two-dimensional maps of a dynamical structure factor in momentum-energy space with multiple incident energies can be simultaneously obtained by one measurement. This method reduces the dead time of time-of-flight measurement, and thus, it markedly increases the measurement efficiency. Our achievement realized using the Fermi chopper spectrometer 4SEASONS in J-PARC is expected to open up new possibilities of inelastic neutron scattering measurements.KEYWORDS: pulsed neutron source, Fermi chopper spectrometer, inelastic neutron scattering, multi-E i measurement, J-PARC DOI: 10.1143/JPSJ.78.093002A ''Fermi chopper'' spectrometer is a typical instrument for inelastic neutron scattering (INS) measurement at a pulsed neutron source. The Fermi chopper is used for monochromating the incident neutron beam. Inelastic scattering from a sample is measured by a time-of-flight (TOF) technique over a neutron source period. The accumulated TOF data can be converted to a dynamical structure factor. In fact, the INS measurements conducted using Fermi chopper spectrometers have revealed various mysteries of materials science, and thus, a Fermi chopper spectrometer should remain to be a flagship instrument in new-generation pulsed neutron sources such as the Japan Proton Accelerator Research Complex (J-PARC), which is one of the world's highest intensity pulsed neutron sources.A conventional Fermi chopper has a set of curved slits. The curvature is adjusted to the passage of neutrons with a selected velocity, which means that neutron transmission is optimized for a specific single incident energy (E i ). From another point of view, the curved slits would bring about the inefficiency of an INS measurement. Chopper rotation frequency (100 -600 Hz) is typically much higher than the frequency of the pulsed source (25 Hz in J-PARC), and the utilized TOF region of INS measurement with single E i is approximately less than 10% of the source period (40 ms in J-PARC). To reduce the dead time of TOF measurement, we have proposed the method of utilizing multiple E i beams. 1,2)We apply a set of straight and wide slits to a Fermi chopper, and consequently, a high transmission of neutrons can be achieved over a wide E i range, though the energy resolution is relaxed. Thus, this type of Fermi chopper can select multiple E i beams within the source period, and the simultaneous INS measurements with multiple E i 's can be realized. We refer to this method as Multi-E i measurement.Another technique, the so-called ''repetition rate multiplication (RRM) method'', for reducing the dead time of TOF measurement has also been developed, 3,4) and recently its first implementation has been reported. 5) The RRM method is mainly applied to measurements with cold neutrons, such as those involving quasi-elastic neutron scattering. It requires several disk choppers to suppress the contamination of slow neutrons and produce the symmetric pulse shape of incident neutrons. On the other hand, Multi-E i m...
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