At low temperatures, Tb2Ti2O7 enters a spin liquid state, despite expectations of magnetic order and/or a structural distortion. Using neutron scattering, we have discovered that in this spin liquid state an excited crystal field level is coupled to a transverse acoustic phonon, forming a hybrid excitation. Magnetic and phononlike branches with identical dispersion relations can be identified, and the hybridization vanishes in the paramagnetic state. We suggest that Tb2Ti2O7 is aptly named a "magnetoelastic spin liquid" and that the hybridization of the excitations suppresses both magnetic ordering and the structural distortion. The spin liquid phase of Tb2Ti2O7 can now be regarded as a Coulomb phase with propagating bosonic spin excitations.

Very small superparamagnetic iron oxide nanoparticles were characterized by innovative synchrotron X-ray total scattering methods and Debye function analysis. Using the information from both Bragg and diffuse scattering, size-dependent core–shell magnetite–maghemite compositions and full size (number- and mass-based) distributions were derived within a coherent approach. The magnetite core radii in 10 nm sized NPs well match the magnetic domain sizes and show a clear correlation to the saturation magnetization values, while the oxidized shells seem to be magnetically silent. Very broad superstructure peaks likely produced by the polycrystalline nature of the surface layers were experimentally detected in room temperature oxidized samples. Effective magnetic anisotropy constants, derived by taking the knowledge of the full size-distributions into account, show an inverse dependence on the NPs size, witnessing a major surface contribution. Finally, an additional amorphous component was uncovered within the diffuse scattering of the “ordered” magnetite–maghemite NPs. Under the hypothesis that this material may form an external dead layer, an additional thickness varying between 0.3 and 1.0 nm should be added to the overall core–shell NPs size.

Spirals and helices are common motifs of long-range order in magnetic solids, and they may also be organized into more complex emergent structures such as magnetic skyrmions and vortices. A new type of spiral state, the spiral spin-liquid, in which spins fluctuate collectively as spirals, has recently been predicted to exist. Here, using neutron scattering techniques, we experimentally prove the existence of a spiral spin-liquid in MnSc2S4 by directly observing the 'spiral surface' -a continuous surface of spiral propagation vectors in reciprocal space. We elucidate the multi-step ordering behavior of the spiral spin-liquid, and discover a vortex-like triple-q phase on application of a magnetic field. Our results prove the effectiveness of the J1-J2 Hamiltonian on the diamond lattice as a model for the spiral spin-liquid state in MnSc2S4, and also demonstrate a new way to realize a magnetic vortex lattice.Magnetic frustration, where magnetic moments (spins) are coupled through competing interactions that cannot be simultaneously satisfied 1 , usually leads to highly cooperative spin fluctuations 2,3 and unconventional longrange magnetic order 4,5 . An archetypal ordering in the presence of frustration is the spin spiral. Competing interactions and spiral orders give rise to many phenomena in magnetism, including the multitudinous magnetic phases of rare earth metals 6 , domains with multiferroic properties 7,8 , and topologically non-trivial structures such as the emergent skyrmion lattice 9,10 .Recently, a new spiral state -a spiral spin-liquid in which the ground states are a massively degenerate set of coplanar spin spirals -was predicted to exist in the J 1 -J 2 model on the diamond lattice (see Fig. 1a) [11][12][13] . Although the diamond lattice is bipartite, and therefore unfrustrated at the near-neighbour (J 1 ) level, the second-neighbour coupling (J 2 ) can generate strong competition. For classical spins, mean-field calculations show that when |J 2 /J 1 | > 0.125 the spiral spin-liquid appears, and that it is signified by an unusual continuous surface of propagation vectors q in reciprocal space (see Fig. 1b for the spiral surface of |J 2 /J 1 | = 0.85). At finite temperature, thermal fluctuations might select some specific q-vectors on the spiral surface 11 , resulting in an orderby-disorder transition 14,15 .Until now, several series of A-site spinels, in which the magnetic A ions form a diamond lattice, have been investigated, including: the cobaltates Co 3 O 4 and CoRh 2 O 4 16 ; the aluminates M Al 2 O 4 with M = Fe, Co, Mn 17-20 ; and the scandium thiospinels M Sc 2 S 4 with M = Fe, Mn 21 . For the spinels with Fe 2+ at the A-site, the e g orbital angular momentum of Fe 2+ is active, making the pure spin J 1 -J 2 model inadequate 22 . Among the other compounds, CoAl 2 O 4 and MnSc 2 S 4 manifest the strongest frustration. For CoAl 2 O 4 , the ratio of |J 2 /J 1 | has been identified as 0.109 19 , which is near, but still lower than, the 0.125 threshold for the spiral spin-liquid state. Many expe...

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Anisotropic magnetic properties of a layered kagome-like system Cu 3 Bi(SeO 3) 2 O 2 Br have been studied by bulk magnetization and magnetic susceptibility measurements as well as powder and single-crystal neutron diffraction. At T N = 27.4 K the system develops an alternating antiferromagnetic order of (ab) layers, which individually exhibit canted ferrimagnetic moment arrangement, resulting from the competing ferro-and antiferro-magnetic intralayer exchange interactions. A magnetic field B C ∼ 0.8 T applied along the c axis (perpendicular to the layers) triggers a metamagnetic transition, when every second layer flips, i.e., resulting in a ferrimagnetic structure. Significantly higher fields are required to rotate the ferromagnetic component towards the b axis (∼7 T) or towards the a axis (∼15 T). The estimates of the exchange coupling constants and features indicative of an XY character of this quasi-2D system are presented.