Multiphase magnetism in Yb2Ti2O7

Exact diagonalization for a 16-site spin-1/2 pyrochlore cluster

We find exact solutions to the Hamiltonian of a 16-site spin-1/2 pyrochlore cluster with nearest neighbour exchange interactions. The methods of group theory (symmetry) are used to completely block-diagonalize the Hamiltonian, yielding precise details about symmetry of the eigenstates, in particular those components which arespin icestates, in order to evaluate the spin ice density at finite temperature. At low enough temperatures, a 'perturbed' spin ice phase, where the '2-in-2-out' ice rule is largely obeyed, is clearly outlined within the four parameter space of the general model of exchange interactions. The quantum spin ice phase is expected to exist within these boundaries.

Dynamical Scaling as a Signature of Multiple Phase Competition in Yb_{2}Ti_{2}O_{7}

Yb_{2}Ti_{2}O_{7} is a celebrated example of a pyrochlore magnet with highly frustrated, anisotropic exchange interactions. To date, attention has largely focused on its unusual, static properties, many of which can be understood as coming from the competition between different types of magnetic order. Here we use inelastic neutron scattering with exceptionally high energy resolution to explore the dynamical properties of Yb_{2}Ti_{2}O_{7}. We find that spin correlations exhibit dynamical scaling, analogous to behavior found near to a quantum critical point. We show that the observed scaling collapse can be explained within a phenomenological theory of multiple-phase competition, and confirm that a scaling collapse is also seen in semiclassical simulations of a microscopic model of Yb_{2}Ti_{2}O_{7}. These results suggest that dynamical scaling may be general to systems with competing ground states.

Emergence of mesoscale quantum phase transitions in a ferromagnet

Mesoscale patterns as observed in, for example, ferromagnets, ferroelectrics, superconductors, monomolecular films or block copolymers^1,2 reflect spatial variations of a pertinent order parameter at length scales and time scales that may be described classically. This raises the question for the relevance of mesoscale patterns near zero-temperature phase transitions, also known as quantum phase transitions. Here we report the magnetic susceptibility of LiHoF₄-a dipolar Ising ferromagnet-near a well-understood transverse-field quantum critical point (TF-QCP)^3,4. When tilting the magnetic field away from the hard axis such that the Ising symmetry is always broken, a line of well-defined phase transitions emerges from the TF-QCP, characteristic of further symmetry breaking, in stark contrast to a crossover expected microscopically. We show that the scenario of a continuous suppression of ferromagnetic domains, representing a breaking of translation symmetry on mesoscopic scales in an environment of broken magnetic Ising symmetry on microscopic scales, is in excellent qualitative and quantitative agreement with the field and temperature dependence of the susceptibility and the magnetic phase diagram of LiHoF₄ under tilted field. This identifies a new type of phase transition that may be referred to as mesoscale quantum criticality, which emanates from the textbook example of a microscopic ferromagnetic TF-QCP. Our results establish the surroundings of quantum phase transitions as a regime of mesoscale pattern formation, in which non-analytical quantum dynamics and materials properties without classical analogue may be expected.

Magnetic order and disorder environments in superantiferromagnetic [Formula: see text] nanoparticles

Magnetic nanoparticles exhibit two different local symmetry environments, one ascribed to the core and one corresponding to the nanoparticle surface. This implies the existence of a dual spin dynamics, leading to the presence of two different magnetic arrangements governed by different correlation lengths. In this work, two ensembles of [Formula: see text] nanoparticles with mean sizes of 18 nm and 13 nm have been produced to unravel the magnetic couplings established among the magnetic moments located within the core and at the nanoparticle surface. To this end, we have combined neutron diffraction measurements, appropriate to investigate magnetically-ordered spin arrangements, with time-dependent macroscopic AC susceptibility measurements to reveal memory and aging effects. The observation of the latter phenomena are indicative of magnetically-frustrated states. The obtained results indicate that, while the [Formula: see text] magnetic moments located within the nanoparticle core keep the bulk antiferromagnetic commensurate structure in the whole magnetic state, the correlations among the surface spins give rise to a collective frustrated spin-glass phase. The interpretation of the magnetic structure of the nanoparticles is complemented by specific-heat measurements, which further support the lack of incommensurability in the nanoparticle state.

Dynamical ground state in the XY pyrochlore Yb2GaSbO7

The magnetic ground state of the pyrochlore Yb2GaSbO7 has remained an enigma for nearly a decade. The persistent spin fluctuations observed by muon spin relaxation measurements at low temperatures have not been adequately explained for this material using existing theories for quantum magnetism. Here we report on the synthesis and characterisation of Yb2GaSbO7 to elucidate the central physics at play. Through DC and AC magnetic susceptibility, heat capacity, and neutron scattering experiments, we observe evidence for a dynamical ground state that makes Yb2GaSbO7 a promising candidate for disorder-induced spin-liquid or spin-singlet behaviour. This state is quite fragile, being tuned to a splayed ferromagnet in a modest magnetic field μ 0 H c ∼ 1.5 T .