Andrew Boothroyd

Abstract:

The quantum many-body interactions in one-dimensional spin- 1 2 systems are subject to Tomonaga-Luttinger liquid (TLL) physics, which predict an array of multi-particle excitations that form continua in momentum-energy space. Here we use inelastic neutron spectroscopy to study the TLL spin dynamics in SrCo2V2O8, a compound which contains weakly coupled spin- 1 2 chains of Co atoms, at 0.05 K and in a longitudinal magnetic field up to 9.0 T. The measurements were performed above 3.9 T, where the ground state N´eel antiferromagnetic (AFM) order is completely suppressed, and the multi-particle excitations are exclusively of TLL type. In this region and below 7.0 T, the longitudinal TLL mode – psinon/antipsinon (P/AP) – is unexpectedly well described by a damped harmonic oscillator (DHO) while approaching the zone center defining the static spin-spin correlations. A non-DHO-type, continuum-like signal is seen at higher fields, but deviations from the ideal one-dimensional TLL still remain. This change in the P/AP mode coincides with the phase transition between the longitudinal spin density wave (LSDW) and transverse AFM order. Inside the LSDW state, the DHO-type P/AP spectral weight increases and the linewidth broadens as the magnetic order parameter decreases. These results reveal the impact of three-dimensional magnetic order on the TLL spin dynamics; they call for beyond the mean-field treatment for the interchain exchange interactions.

Abstract:

The magnetic properties of the S=1/2 two-dimensional square-lattice antiferromagnets Sr2NiO3X (X=Cl, F) with the trivalent nickel ions in a low-spin state were studied by magnetic susceptibility, heat capacity, neutron powder diffraction, high-field electron spin resonance (ESR), muon spin rotation and relaxation (μ+SR) measurements, and density functional theory (DFT) calculations. Both oxyhalides are isostructural to an ideal quantum square-lattice antiferromagnet Sr2CuO2Cl2, but the chlorine/fluorine anion exclusively occupies an apical site in an ordered/disordered manner with an oxygen anion, resulting in the formation of highly distorted NiO5X octahedra with an off-center nickel ion. Magnetic susceptibility measurements revealed a remarkable difference between these two compounds: the magnetic susceptibility of Sr2NiO3Cl exhibited a broad maximum at approximately 35 K, which is typical of low-dimensional antiferromagnetic behavior. In contrast, the magnetic susceptibility of Sr2NiO3F exhibited spin-glass-like behavior below 12 K. No anomaly associated with long-range magnetic ordering was observed in the heat capacity, ESR, and neutron powder diffraction experiments. However, μ+SR measurements revealed the emergence of a static magnetic ordered state below TN=28K in Sr2NiO3Cl and a short-range magnetic state below TN=18K in Sr2NiO3F. The DFT calculations suggested that the unpaired electron occupied a d3z2-r2 orbital, and ferromagnetic couplings between the nearest-neighbor nickel spins were energetically favored. The mechanism of ferromagnetic superexchange interactions and the reason for the difference between the magnetic ground states in these nickel oxyhalides are discussed.

Abstract:

According to previous theoretical work, the binary oxide CuO can become a room temperature multiferroic via tuning of the superexchange interactions by application of pressure. Thus far, however, there has been no experimental evidence for the predicted room-temperature multiferroicity. Here, we show by neutron diffraction that the multiferroic phase in CuO reaches 295 K with the application of 18.5 GPa pressure. We also develop a spin Hamiltonian based on density functional theory and employing superexchange theory for the magnetic interactions, which can reproduce the experimental results. The present study provides a stimulus to develop room-temperature multiferroic materials by alternative methods based on existing low temperature compounds, such as epitaxial strain, for tunable multifunctional devices and memory applications.