Low-energy spin excitations of the frustrated ferromagnetic J1-J2 chain material linarite PbCuSO4(OH)2 in applied magnetic fields parallel to the b axis

We report a study of the spin dynamics of the frustrated ferromagnetic J1 -J2 chain compound linarite, PbCuSO4(OH)2, in applied magnetic fields up to field polarization. By means of an extreme-environment inelastic neutron scattering experiment, we have measured the low-energy spin excitations of linarite in fields up to 8.8 T for H II b axis. We have recorded the spin excitation spectra along h, k, and l for the field-induced magnetic phases IV, V, and the field-polarized state close to saturation. By employing first-principles calculations, we estimate the leading magnetic exchanges out of the bc plane, and we model the dispersion relations using linear spin-wave theory. In this way, we find evidence for a (very weak) residual magnetic exchange coupling out of the bc plane. Together with the previously established dominant intrachain couplings J1 and J2 and the interchain coupling J3, we derive an effective set of exchange couplings for a microscopic description of linarite. Further, we find that the peculiar character of phase V manifests itself in the measured spin dynamics.