SPIN

We report magnetic susceptibility, specific heat and muon spin relaxation (μSR) experiments on the triangular antiferromagnet La₂Ca₂MnO₇ which develops a genuine two-dimensional (2D), three-sublattice magnetic order below T_N = 2.8 K. From the susceptibility and specific heat an estimate of the exchange interaction is derived. A value for the spin-wave gap is obtained from the latter data. The analysis of a previously reported inelastic neutron scattering data yields values for the exchange and spin-wave gap compatible with the results obtained from macroscopic measurements. An appreciable entropy is still missing at 10 K that may be ascribed to intense short-range correlations. The critical paramagnetic fluctuations extend far above T_N, and can be partly understood in terms of two-dimensional spin-wave excitations. While no spontaneous μSR field is observed below T_N, persistent spin dynamics is found. Short-range correlations are detected in this temperature range. Their relation to a possible molecular spin substructure and the observed exotic spin fluctuations is discussed.

The neutron diffraction and measurements of specific heat and magnetization were carried out on single crystals of a quantum spin ice system Yb₂Ti₂O₇. The thermal hysteresis loops of temperature dependence of the magnetization and neutron diffraction intensity indicate a first-order ferromagnetic transition, where the used single crystals are confirmed to have a sharp peak structure in the specific heat (C)–temperature (T) curve at T_C ∼ 0.2 K. The magnetic structure analysis of neutron diffraction results revealed that the Yb³⁺ moments have a ferromagnetic collinear structure along [001] directions with ordered moment value 1.1 ± 0.1 μ_B at 0.03 K. The mechanism of ferromagnetic transition of Yb₂Ti₂O₇ will be discussed.

The hidden ordered state of the frustrated pyrochlore oxide Tb_2+xTi_2-xO_7+y is possibly one of the two electric multipolar, or quadrupolar, states of the effective pseudospin-1/2 Hamiltonian derived from crystal-field ground state doublets of non-Kramers Tb³⁺ ions. These long-range orders are antiparallel or parallel alignments of transverse pseudospin components representing electric quadrupole moments, which cannot be observed as magnetic Bragg reflections by neutron scattering. However, pseudospin waves of these states are composite waves of the magnetic-dipole and electric-quadrupole moments, and can be partly observed by inelastic magnetic neutron scattering. We calculate these spin-quadrupole waves using linear spin-wave theory and discuss previously observed low-energy magnetic excitation spectra of a polycrystalline sample with x = 0.005(T_c = 0.5 K).

We have studied the magnetic response of itinerant spin ice, by applying the cluster dynamical mean field theory (CDMFT) to the Ising Kondo lattice model on a pyrochlore lattice. As a result, we found a characteristic interplay between magnetization curve and spin ice correlation developed at low temperatures. The magnetization develops a kink-like structure at the 2/3 of its saturation value, reminiscent of kagome ice plateau. Accompanied with the magnetization process, the monopole density shows nonmonotonic magnetic field dependence with a clear minimum, reflecting a subtle energetics of spin configurations. The spin ice correlation also affects the transport properties of the system, and brings about negative magnetoresistivity with its slope strongly dependent on the magnitude of spin ice correlation. We discuss these behaviors in comparison with the magnetic response observed in Pr₂Ir₂O₇.

If spin liquids have been famously defined by what they are not, i.e., ordered, the past years have seen the frontier between order and spin liquid starting to fade, with a growing number of materials whose low-temperature physics cannot be explained without co-existence of (partial) magnetic order and spin fluctuations. Here, we study an example of such co-existence in the presence of magnetic dipolar interactions, related to spin ice, where the order is long range and the fluctuations support a Coulomb gauge field. Topological defects are effectively coupled via energetic and entropic Coulomb interactions, the latter one being stronger than for the spin-ice ground state. Depending on whether these defects break the divergence-free condition of the Coulomb gauge field or the long-range order, they are respectively categorized as monopoles — as in spin ice — or monopole holes, in analogy with electron holes in semiconductors. The long-range order plays the role of a fully-occupied valence band, while the Coulomb spin liquid can be seen as an empty conducting band. These results are discussed in the context of other lattices and models which support a similar co-existence of Coulomb gauge field and long-range order. We conclude this work by explaining how dipolar interactions lift the spin-liquid degeneracy at very low energy scale by maximizing the number of flippable plaquettes, in light of the equivalent quantum dimer model.

We review recent theoretical progress in our understanding of electron-driven novel magnetic phases on frustrated lattices. Our specific focus is on Kondo-lattice or double-exchange models assuming finite magnetic moments localized at the lattice sites. A salient feature of systems with SU(2) symmetric local moments is the emergence of noncoplanar magnetic ordering driven by the conduction electrons. The complex spin textures then endow the electrons a nontrivial Berry phase, often giving rise to a topologically nontrivial electronic state. The second part of the review is devoted to the discussion of metallic spin ice systems, which are essentially frustrated Ising magnets with local spin ordering governed by the so-called ice rules. These rules are similar to those that describe proton configurations in solid water ice, hence the name "spin ice". The nontrivial spin correlations in the ice phase give rise to unusual electron transport properties in metallic spin-ice systems.