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= 1 2 | ↑↓> −| ↓↑> α α c J αJ 0 1 QD LRO J J a b J leg J rung J int b c a Cu Br N C H J

The notion of a quasiparticle, such as a phonon, a roton or a magnon, is used in modern condensed matter physics to describe an elementary collective excitation. The intrinsic zero-temperature magnon damping in quantum spin systems can be driven by the interaction of the one-magnon states and multi-magnon continuum. However, detailed experimental studies on this quantum many-body effect induced by an applied magnetic field are rare. Here we present a high-resolution neutron scattering study in high fields on an S=1/2 antiferromagnet C9H18N2CuBr4. Compared with the non-interacting linear spin–wave theory, our results demonstrate a variety of phenomena including field-induced renormalization of one-magnon dispersion, spontaneous magnon decay observed via intrinsic linewidth broadening, unusual non-Lorentzian two-peak structure in the excitation spectra and a dramatic shift of spectral weight from one-magnon state to the two-magnon continuum.

We report a neutron diffraction study of the multiferroic mechanism in (ND4)2FeCl5•D2O, a molecular compound that exhibits magnetically induced ferroelectricity. This material exhibits two successive magnetic transitions on cooling: a long-range order transition to an incommensurate (IC) collinear sinusoidal spin state at TN =7.3 K, followed by a second transition to an IC cycloidal spin state at TF E =6.8 K, the later of which is accompanied by spontaneous ferroelectric polarization. The cycloid structure is strongly distorted by spin-lattice coupling as evidenced by the observations of both odd and even higher-order harmonics associated with the cycloid wave vector, and a weak commensurate phase that coexists with the IC phase. The appearance of the 2nd-order harmonic coincides with the onset of the electric polarization, thereby providing unambiguous evidence that the induced electric polarization is mediated by the spin-lattice interaction. Our results for this system, in which the orbital angular momentum is expected to be quenched, are remarkably similar to those of the prototypical TbMnO3, in which the magnetoelectric effect is attributed to spin-orbit coupling.

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Electronic phase separation has been increasingly recognized as an important phenomenon in understanding many of the intriguing properties displayed in transition metal oxides. It is believed to produce fascinating functional properties in otherwise chemically homogenous electronic systems, e.g. colossal magnetoresistance manganites and high-Tc cuprates. While many well-known electronically phase separated systems are oxides, it has been argued that the same phenomenon should occur in other electronic systems with strong competing interactions. Here we report the observation of electronic phase separation in molecular (ND4)2FeCl5•D2O, a type-II multiferroic. We show that two magnetic phases, one of which is commensurate and the other of which is incommensurate, coexist in this material. Their evolution under applied magnetic field produces emergent properties. In particular, our measurements reveal a field-induced exotic state linked to a direct transition from a paraelectric/paramagnetic phase to a ferroelectric/antiferromagnetic phase, a collective phenomenon that hasn't been seen in other magnetic multiferroics.

The Spherical Neutron Polarimetry (SNP) Technique probes complex magnetic structures which are inaccessible by other methods. This technique is achieved by combining a zero-field sample chamber with full control of incoming and outgoing neutron polarization. While wide-angle SNP capability has been fully implemented in Europe, it is still in development in the US. At Oak Ridge National Laboratory, we are developing a wide-angle SNP capability suited for multiple neutron beamlines. The design is based on the basic concept of SNP while utilizing high-Tc superconducting films and Mu-metal. In this article we report our recent development of the SNP capability and its future applications.