Over the past two decades, the magnetic ground states of all rare earth titanate pyrochlores have been extensively studied, with the exception of Sm2Ti2O7. This is, in large part, due to the very high absorption cross-section of naturally-occurring samarium, which renders neutron scattering infeasible. To combat this, we have grown a large, isotopically-enriched single crystal of Sm2Ti2O7. Using inelastic neutron scattering, we determine that the crystal field ground state for Sm 3+ is a dipolar-octupolar doublet with Ising anisotropy. Neutron diffraction experiments reveal that Sm2Ti2O7 orders into the all-in, all-out magnetic structure with an ordered moment of 0.44(7) µB below TN = 0.35 K, consistent with expectations for antiferromagnetically-coupled Ising spins on the pyrochlore lattice. Zero-field muon spin relaxation measurements reveal an absence of spontaneous oscillations and persistent spin fluctuations down to 0.03 K. The combination of the dipolar-octupolar nature of the Sm 3+ moment, the all-in, all-out ordered state, and the low-temperature persistent spin dynamics make this material an intriguing candidate for moment fragmentation physics.Rare earth titanate pyrochlores of the form R 2 Ti 2 O 7 have long been a centerpiece in the study of geometricallyfrustrated magnetism [1]. In this family of materials, the magnetism is carried by the R 3+ rare earth ions, which decorate a network of corner-sharing tetrahedra.The study of this family has led to the discovery of a range of fascinating ground states such as the dipolar spin ice state, which was first observed in Ho 2 Ti 2 O 7 and Dy 2 Ti 2 O 7 [2-4]. Here local Ising anisotropy combines with dominant dipolar interactions, which are ferromagnetic at the nearest neighbour level on the pyrochlore lattice [5]. The spin ice state is characterized by individual tetrahedra obeying two-in, two-out "ice rules", wherein two spins point directly towards the tetrahedron's center and the other two spins point outwards (left inset of Fig. 1). This configuration can be achieved in six equivalent ways for a single tetrahedron, giving rise to a macroscopic degeneracy for the lattice as a whole. In other titanates, where the rare earth moments are smaller than in Ho 2 Ti 2 O 7 and Dy 2 Ti 2 O 7 , dipolar interactions become less important and exchange interactions tend to dominate. This is exactly the case when R = Sm 3+ (∼ 1 µ B ), where the magnetic moment is reduced by a factor of ten from R = Ho 3+ and Dy 3+ (∼ 10 µ B ), corresponding to dipolar interactions that are weaker by two orders of magnitude.In this letter we show that anitferromagnetically coupled Ising spins with negligible dipolar interactions give rise to an all-in, all-out (AIAO) magnetic ground state in Sm 2 Ti 2 O 7 . The AIAO structure is characterized by adjacent tetrahedra alternating between all spins pointing inwards and all spins pointing outwards (right inset of Fig. 1). Unlike the ferromagnetic spin ice state, the antiferromagnetic AIAO state does not give rise to a macroscopic degenerac...

The crystal structure of KRuO is refined at both 280 and 3.5 K from neutron powder data, and magnetic properties are reported for the first time. The scheelite structure, I4/a, is confirmed at both temperatures. Atomic positions of greater accuracy than the original 1954 X-ray study are reported. The rare Ru ion resides in a site of distorted tetrahedral symmetry with nominal electronic configuration 4d(e). Curie-Weiss parameters are near free ion values for the effective moment and θ = -77 K, indicating dominant antiferromagnetic (AF) correlations. A broad susceptibility maximum occurs near 34 K, but long-range AF order sets in only below 22.4 K as determined by magnetization and heat capacity data. The entropy loss below 50 K is only 44% of the expected R ln 2, indicating the presence of short-range spin correlations over a wide temperature range. The Ru sublattice consists of centered, corner-sharing tetrahedra which can lead to geometric frustration if both the nearest-neighbor, J, and the next-nearest-neighbor, J, exchange constants are AF and of similar magnitude. A spin dimer analysis finds J/J ≈ 25, indicating weak frustration, and a (d) ground state. A single, weak magnetic reflection was indexed as (110). The absence of the (002) magnetic reflection places the Ru moments parallel to the c axis. The Ru moment is estimated to be 0.57(7) μ, reduced from an expected value near 1 μ. A recent computational study of isostructural, isoelectronic KOsO predicts a surprisingly large orbital moment due to spin-orbit coupling (SOC). However, the free ion SOC constant for Ru is only ∼30% that of Os, so it is unclear that this effect can be implicated in the low ordered moment for KRuO. The origin of the short-range spin correlations is also not understood.

The question of structural disorder and its effects on magnetism is relevant to a number of spin liquid candidate materials. Although commonly thought of as a route to spin glass behaviour, here we describe a system in which the structural disorder results in long-range antiferromagnetic order due to local symmetry breaking. Nd2ScNbO7 is shown to have a dispersionless gapped excitation observed in other neodymium pyrochlores below T N = 0.37 K through polarized and inelastic neutron scattering. However the dispersing spin waves are not observed. This excited mode is shown to occur in only 14(2)% of the neodymium ions through spectroscopy and is consistent with total scattering measurements as well as the magnitude of the dynamic moment 0.26(2) μ B . The remaining magnetic species order completely into the all-in all-out Ising antiferromagnetic structure. This can be seen as a result of local symmetry breaking due disordered Sc+3 and Nb+5 ions about the A-site. From this work, it has been established that B-site disorder restores the dipole-like behaviour of the Nd+3 ions compared to the Nd2B2O7 parent series.

Magnetic monopoles are hypothesised elementary particles connected by Dirac strings that behave like infinitely thin solenoids [Dirac 1931 Proc. Roy. Soc. A 133 60]. Despite decades of searches, free magnetic monopoles and their Dirac strings have eluded experimental detection, although there is substantial evidence for deconfined magnetic monopole quasiparticles in spin ice materials [Castelnovo, Moessner & Sondhi 2008 Nature 326 411]. Here we report the detection of a hierarchy of unequally-spaced magnetic excitations via high resolution inelastic neutron spectroscopic measurements on the quantum spin ice candidate Pr₂Sn₂O₇. These excitations are well-described by a simple model of monopole pairs bound by a linear potential [Coldea et al. Science 327 177] with an effective tension of 0.7(1) K/Angstrom. The success of the linear potential model suggests that these low energy magnetic excitations are direct spectroscopic evidence for the confinement of magnetic monopole quasiparticles in the quantum spin ice candidate Pr₂Sn₂O₇.