Neutron diffraction and inelastic spectroscopy is used to characterize the magnetic Hamiltonian of SrHo 2 O 4 and SrDy 2 O 4 . Through a detailed computation of the crystal-field levels we find site-dependent anisotropic single-ion magnetism in both materials, and diffraction measurements show the presence of strong one-dimensional spin correlations. Our measurements indicate that competing interactions of the zigzag chain, combined with frustrated interchain interactions, play a crucial role in stabilizing spin-liquid type correlations in this series.

Magnetic frustration and low dimensionality can prevent long range magnetic order and lead to exotic correlated ground states. SrDy 2 O 4 consists of magnetic Dy 3+ ions forming magnetically frustrated zig-zag chains along the c-axis and shows no long range order to temperatures as low as T = 60 mK. We carried out neutron scattering and AC magnetic susceptibility measurements using powder and single crystals of SrDy 2 O 4 . Diffuse neutron scattering indicates strong one-dimensional (1D) magnetic correlations along the chain direction that can be qualitatively accounted for by the axial next-nearest neighbour Ising (ANNNI) model with nearestneighbor and next-nearest-neighbor exchange J 1 = 0.3 meV and J 2 = 0.2 meV, respectively. Three-dimensional (3D) correlations become important below T * ≈ 0.7 K. At T = 60 mK, the short range correlations are characterized by a putative propagation vector k 1/2 = (0,2 ). We argue that the absence of long range order arises from the presence of slowly decaying 1D domain walls that are trapped due to 3D correlations. This stabilizes a low-temperature phase without long range magnetic order, but with well-ordered chain segments separated by slowly-moving domain walls.

Despite remarkable progress in developing multifunctional materials, spin-driven ferroelectrics featuring both spontaneous magnetization and electric polarization are still rare. Among such ferromagnetic ferroelectrics are conical spin spiral magnets with a simultaneous reversal of magnetization and electric polarization that is still little understood. Such materials can feature various multiferroic domains that complicates their study. Here we study the multiferroic domains in ferromagnetic ferroelectric Mn2GeO4 using neutron diffraction, and show that it features a double-Q conical magnetic structure that, apart from trivial 180o commensurate magnetic domains, can be described by ferromagnetic and ferroelectric domains only. We show unconventional magnetoelectric couplings such as the magnetic-field-driven reversal of ferroelectric polarization with no change of spin-helicity, and present a phenomenological theory that successfully explains the magnetoelectric coupling. Our measurements establish Mn2GeO4 as a conceptually simple multiferroic in which the magnetic-field-driven flop of conical spin spirals leads to the simultaneous reversal of magnetization and electric polarization.

Inelastic neutron scattering has been used to study the magneto-elastic excitations in the multiferroic manganite hexagonal YMnO3. An avoided crossing is found between magnon and phonon modes close to the Brillouin zone boundary in the (a, b)-plane. Neutron polarization analysis reveals that this mode has mixed magnon-phonon character. An external magnetic field along the c-axis is observed to cause a linear field-induced splitting of one of the spin wave branches. A theoretical description is performed, using a Heisenberg model of localized spins, acoustic phonon modes and a magneto-elastic coupling via the single-ion magnetostriction. The model quantitatively reproduces the dispersion and intensities of all modes in the full Brillouin zone, describes the observed magnonphonon hybridized modes, and quantifies the magneto-elastic coupling. The combined information, including the field-induced magnon splitting, allows us to exclude several of the earlier proposed models and point to the correct magnetic ground state symmetry, and provides an effective dynamic model relevant for the multiferroic hexagonal manganites.

We report on the evolution of the magnetic structure of BiFeO 3 thin films grown on SrTiO 3 substrates as a function of Sm doping. We determined the magnetic structure using neutron diffraction. We found that as Sm increases, the magnetic structure evolves from a cycloid to a G-type antiferromagnet at the morphotropic phase boundary, where there is a large piezoelectric response due to an electric-field induced structural transition. The occurrence of the magnetic structural transition at the morphotropic phase boundary offers another route towards room temperature multiferroic devices.As one of the only single-phase multiferroic materials with ferroelectric and magnetic transition temperatures well above room temperature, BiFeO 3 (BFO) has been extensively studied. It possesses a robust ferroelectric polarization which is closely tied to its rhombohedral structure, and microstructural properties of BFO thin films (such as the stress state, grain size and its orientation, etc.) can sensitively affect its local ferroelectric properties. Magnetoelectric coupling between the local ferroelectric polarization and magnetism inside BFO thin films can serve as the basis for heterostructured multiferroic devices, 1,2 but their antiferromagnetic properties are known to display complex variations depending delicately on the local microstructural properties. We have previously used neutron diffraction to probe the nature of antiferromagnetic domains in epitaxial BFO thin films. 3,4 Chemical substitution in BFO has been explored in order to improve the ferroelectric, piezoelectric, and dielectric properties of the material. [5][6][7][8] It has been demonstrated that the substitution of rare earth elements into the A-site of BFO thin films results in a structural phase transition from a ferroelectric rhombohedral phase to a paraelectric orthorhombic phase. 9,10,11 In the case of Sm the transition occurs at ~14% doping 12 at which point films exhibit a Morphotropic Phase Boundary (MPB). Earlier studies showed that in the vicinity of the MPB, an electric field can be used to drive the transition from the paraelectric orthorhombic phase to the rhombohedral ferroelectric phase, resulting in a very large piezoelectric effect d 33