Spin freezing in the pyrochlore antiferromagnet Pr2Zr2O7

Emergence of weak pyrochlore phase and signature of field induced spin ice ground state in Dy2- x La x Zr2O7; x = 0, 0.15, 0.3

The pyrochlore oxides Dy2Ti2O7 and Ho2Ti2O7 are well studied spin ice systems and have shown the evidences of magnetic monopole excitations. Unlike these, Dy2Zr2O7 is reported to crystallize in a distorted fluorite structure. We present here the magnetic and heat capacity studies of La substituted Dy2Zr2O7. Our findings suggest the absence of spin ice state in Dy2Zr2O7 but the emergence of the magnetic field induced spin freezing near T ≈ 10 K in ac susceptibility measurements which is similar to Dy2Ti2O7. The magnetic heat capacity of Dy2Zr2O7 shows a shift in the peak position from 1.2 K in zero field to higher temperatures in the magnetic field, with the corresponding decrease in the magnetic entropy. The low temperature magnetic entropy at 5 kOe field is R ln2 - (1/2)R ln(3/2) which is the same as for the spin ice state. Substitution of non-magnetic, isovalent La3+ for Dy3+ gradually induces the structural change from highly disordered fluorite to weakly ordered pyrochlore phase. The La3+ substituted compounds with less distorted pyrochlore phase show the spin freezing at lower field which strengthens further on the application of magnetic field. Our results suggest that the spin ice state can be stabilized in Dy2Zr2O7 either by slowing down of the spin dynamics or by strengthening the pyrochlore phase by suitable substitution in the system.

Instabilities of a U(1) Quantum Spin Liquid in Disordered Non-Kramers Pyrochlores

Quantum spin liquids (QSLs) are exotic phases of matter exhibiting long-range entanglement and supporting emergent gauge fields. A vigorous search for experimental realizations of these states has identified several materials with properties hinting at QSL physics. A key issue in understanding these QSL candidates is often the interplay of weak disorder of the crystal structure with the spin liquid state. It has recently been pointed out that in at least one important class of candidate QSLs-pyrochlore magnets based on non-Kramers ions such as Pr^{3+} or Tb^{3+}-structural disorder can actually promote a U(1) QSL ground state. Here we set this proposal on a quantitative footing by analyzing the stability of the QSL state in the minimal model for these systems: a random transverse field Ising model. We consider two kinds of instability, which are relevant in different limits of the phase diagram: condensation of spinons and confinement of the U(1) gauge fields. Having obtained stability bounds on the QSL state, we apply our results directly to the disordered candidate QSL Pr_{2}Zr_{2}O_{7}. We find that the available data for currently studied samples of Pr_{2}Zr_{2}O_{7} are most consistent with it a ground state outside the spin liquid regime, in a paramagnetic phase with quadrupole moments near saturation due to the influence of structural disorder.

Coulomb spin liquid in anion-disordered pyrochlore Tb2Hf2O7

The charge ordered structure of ions and vacancies characterizing rare-earth pyrochlore oxides serves as a model for the study of geometrically frustrated magnetism. The organization of magnetic ions into networks of corner-sharing tetrahedra gives rise to highly correlated magnetic phases with strong fluctuations, including spin liquids and spin ices. It is an open question how these ground states governed by local rules are affected by disorder. Here we demonstrate in the pyrochlore Tb₂Hf₂O₇, that the vicinity of the disordering transition towards a defective fluorite structure translates into a tunable density of anion Frenkel disorder while cations remain ordered. Quenched random crystal fields and disordered exchange interactions can therefore be introduced into otherwise perfect pyrochlore lattices of magnetic ions. We show that disorder can play a crucial role in preventing long-range magnetic order at low temperatures, and instead induces a strongly fluctuating Coulomb spin liquid with defect-induced frozen magnetic degrees of freedom.

Experimental studies of frustrated spin systems such as pyrochlore magnetic oxides test our understanding of quantum many-body physics. Here the authors show experimentally that Tb₂Hf₂O₇ may be a model material for investigating how structural disorder can stabilize a quantum spin liquid phase.

Single crystal growth, structure and magnetic properties of Pr2Hf2O7 pyrochlore

Large single crystals of pyrochlore [Formula: see text] were successfully grown by the floating zone technique using an optical furnace equipped with high power xenon arc lamps. Structural investigations were carried out via powder synchrotron x-ray and neutron diffraction to establish the crystallographic structure of the materials produced. The magnetic properties of the single crystals were determined for magnetic fields applied along different crystallographic axes. The results revealed that [Formula: see text] is an interesting material for further investigation as a frustrated magnet. The high quality of the crystals produced makes them ideal for detailed investigation, especially using neutron scattering techniques.

Quantum spin ice: a search for gapless quantum spin liquids in pyrochlore magnets

The spin ice materials, including Ho2Ti2O7 and Dy2Ti2O7, are rare-earth pyrochlore magnets which, at low temperatures, enter a constrained paramagnetic state with an emergent gauge freedom. Spin ices provide one of very few experimentally realized examples of fractionalization because their elementary excitations can be regarded as magnetic monopoles and, over some temperature range, spin ice materials are best described as liquids of these emergent charges. In the presence of quantum fluctuations, one can obtain, in principle, a quantum spin liquid descended from the classical spin ice state characterized by emergent photon-like excitations. Whereas in classical spin ices the excitations are akin to electrostatic charges with a mutual Coulomb interaction, in the quantum spin liquid these charges interact through a dynamic and emergent electromagnetic field. In this review, we describe the latest developments in the study of such a quantum spin ice, focusing on the spin liquid phenomenology and the kinds of materials where such a phase might be found.

Quantum fluctuations in spin-ice-like Pr2Zr2O7

Spin ice is a magnetic analog of H2O ice that harbors dense static disorder. Dipolar interactions between classical spins yield a frozen frustrated state with residual configurational Pauling entropy and emergent magnetic monopolar quasiparticles. Introducing quantum fluctuations is of great interest as this could melt spin ice and allow coherent propagation of monopoles. Here, we report experimental evidence for quantum dynamics of magnetic monopolar quasiparticles in a new class of spin ice based on exchange interactions, Pr2Zr2O7. Narrow pinch point features in otherwise diffuse elastic neutron scattering reflects adherence to a divergence-free constraint for disordered spins on long time scales. Magnetic susceptibility and specific heat data correspondingly show exponentially activated behaviors. In sharp contrast to conventional ice, however, >90% of the neutron scattering is inelastic and devoid of pinch points furnishing evidence for magnetic monopolar quantum fluctuations.

Dimensional analysis, spin freezing and magnetization in spin ice

Dimensional analysis is shown to give an insight into the non-ergodic behaviour of spin ice below its apparent 'spin freezing' temperature. Expressions are derived for the temperature-dependent magnetic susceptibility that are found to be highly consistent with the previously reported field cooled and zero field cooled magnetization of the spin ice dysprosium titanate, Dy(2)Ti(2)O(7), as well as with the theory of a 'magnetolyte', including Debye-Hückel screening and Wien dissociation. The spin freezing is inferred to reflect the inability of the quasi-free magnetic charges or 'monopoles' that comprise the magnetolyte to fully screen an applied magnetic field on the timescale of an experiment. The apparent freezing temperature (T(f)≈0.65 K) is identified as the point where the Debye screening length becomes greater than the Bjerrum association distance for charge pairs. Combining these dimensional arguments with Onsager's theory of the Wien effect, it is shown that magnetization data at relatively high field (Snyder et al 2004 Phys. Rev. B 69 064414) may be used to estimate the elementary magnetic charge of spin ice, as well as the temperature-dependent monopole density. Evidence is presented of a non-equilibrium population of monopoles below T≈0.2 K. It is also shown how Onsager's microscopic theory of field-induced monopole pair separation naturally suggests the 'magnetization jumps' in Dy(2)Ti(2)O(7) observed at applied fields of the order of ∼ 0.1 T. It is concluded that the results of dimensional analysis, when combined with Onsager's theory, provide an accurate, albeit approximate, description of the properties of Dy(2)Ti(2)O(7), that could be improved by the development of a lattice theory of the Wien effect, or tested on other spin ice materials.

Quantum melting of spin ice: emergent cooperative quadrupole and chirality

Quantum melting of spin ice is proposed for pyrochlore-lattice magnets Pr2TM2O7 (TM=Ir, Zr, and Sn). The quantum superexchange Hamiltonian having a nontrivial magnetic anisotropy is derived on the basis of atomic non-Kramers magnetic doublets. The ground states exhibit a cooperative ferroquadrupole and pseudospin chirality, forming a magnetic analog of smectic liquid crystals. Our theory accounts for dynamic spin-ice behaviors experimentally observed in Pr2TM2O7.