Pyrochlore Spin Ice Research Articles

Quantum anomalous Hall effect in kagome ice

An anomalous Hall insulator without magnetic long-range ordering is theoretically reported in the absence of the relativistic spin-orbit coupling. It is realized in itinerant electrons coupled with the Ising spins on a <111> kagome plane of pyrochlore spin ice in applied magnetic field. We find that the kagome-ice type local spin correlation in the magnetization plateau state opens a charge gap without magnetic ordering, which results in quantization of the Hall conductivity. By Monte Carlo simulation, we identify the anomalous Hall insulating region in the magnetic phase diagram, in addition to another anomalous Hall insulator in a fully-saturated state.

Crystal growth of spin-ice pyrochlores by the floating-zone method

Single crystals of RE2Ti2O7 (RE=Y, Tb, Dy and Ho) have been grown by the floating-zone technique in an image furnace in a controlled atmosphere. The effect of annealing the crystals in different atmospheres was studied and found to be very important. Small amounts of Y were doped on the Dy site to study its effect on the spin-ice behaviour. Results for the structural parameters obtained by X-ray diffraction and magnetic properties of the crystals are reported.

The temperature evolution of the magnetic correlations in pure and diluted spin ice Ho2−xYxTi2O7

Diffuse polarized neutron scattering studies have been carried out on single crystals of pyrochlore spin ice Ho2−xYxTi2O7 (x=0, 0.3, and 1) to investigate the effects of doping and anisotropy on spin correlations in the system. The crystals were aligned with the (1−10) orientation coincident with the direction of neutron polarization. For all the samples studied the spin flip (SF) diffuse scattering (i.e. the in-plane component) reveals that the spin correlations can be described using a nearest-neighbour spin ice model (NNSM) at higher temperatures (T=3.6K) and a dipolar spin ice model (DSM) as the temperature is reduced (T=30mK). In the non-spin flip (NSF) channel (i.e. the out-of-plane component), the signature of strong antiferromagnetic correlations is observed for all the samples at the same temperature as the dipolar spin ice behaviour appears in the SF channel. Our studies show that the non-magnetic dopant Y does not significantly alter SF or NSF scattering for the spin ice state, even when Y doping is as high as 50%. In this paper, we focus on the experimental results of the highly doped spin ice HoYTi2O7 and compare our results with pure spin ice Ho2Ti2O7. The crossover from a dipolar to a nearest-neighbour spin ice behaviour and the doping insensitivity in spin ices are briefly discussed.

Magnetic correlations in pyrochlore spin ice as probed by polarized neutron scattering

Polarized neutron diffuse scattering experiments have been carried out on (1–10) and (111) oriented single crystals of the pyrochlore spin ice [1] compound Ho2Ti2O7 in order to investigate the complex spin correlations in this material at low temperature. The incident neutron beam was polarized perpendicular to the horizontal scattering plane. Thus, the corresponding components of the spin correlations in and out of the scattering plane can be probed with spin-flip (SF) and non-spin-flip (NSF) scattering respectively via neutron polarization analysis. A strong anisotropy of the in-plane and out-of-plane spin scattering has been observed in both samples. The temperature dependence of magnetic diffuse scattering has also been measured. Here we concentrate on reporting the experimental results. In addition, a brief discussion of the possible nature of spin correlations in this pyrochlore spin ice will also be given.

A lattice dynamical investigation of the Raman and the infrared frequencies of the Dy 2Ti 2O 7 pyrochlore spin ice compound

A short range force constant model has been applied for the first time to investigate the Raman and the infrared frequencies in Dy 2Ti 2O 7 pyrochlore spin ice compound in its cubic phase of space group Fd3 m. The calculations of zone center phonons have been made with four stretching and two bending force constants. The calculated values of Raman and infrared frequencies are in good agreement with the observed ones. The contribution of the nearest atomic interaction between Dy and O has been found to be negligible and is quite likely to play an important role in spin ice compound.

Temperature-dependent Raman and x-ray studies of the spin-ice pyrochloreDy2Ti2O7and nonmagnetic pyrochloreLu2Ti2O7

We present here temperature-dependent Raman, x-ray diffraction, and specific heat studies between room temperature and 12 K on single crystals of spin-ice pyrochlore compound ${\text{Dy}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$ and its nonmagnetic analog ${\text{Lu}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$. Raman data show a ``new'' band not predicted by factor group analysis of Raman-active modes for the pyrochlore structure in ${\text{Dy}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$, appearing below a temperature of ${T}_{c}=110\text{ }\text{K}$ with a concomitant contraction of the cubic unit cell volume as determined from the powder x-ray diffraction analysis. Low-temperature Raman experiments on ${\text{O}}^{18}$-isotope substituted ${\text{Dy}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$ confirm the phonon origin of the new mode. These findings, absent in ${\text{Lu}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$, suggest that the room-temperature cubic lattice of the pyrochlore ${\text{Dy}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$ undergoes a ``subtle'' structural transformation near ${T}_{c}$. We find anomalous redshift of some of the phonon modes in both the ${\text{Dy}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$ and the ${\text{Lu}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$ as the temperature decreases, which is attributed to strong phonon-phonon anharmonic interactions.

Magnetothermodynamics of the Ising antiferromagnetDy2Ge2O7

We report systematic low-temperature measurements of the dc magnetization, ac susceptibility, and heat capacity of dysprosium pyrogermanate $({\text{Dy}}_{2}{\text{Ge}}_{2}{\text{O}}_{7})$ single crystal and powder samples. Our results confirm that ${\text{Dy}}_{2}{\text{Ge}}_{2}{\text{O}}_{7}$ is an anisotropic antiferromagnet. The isothermal field dependent magnetization and the integrated magnetic entropy both indicate that the ${\text{Dy}}^{3+}$ ions behave as Ising-like spins, analogous to those in the pyrochlore spin ice materials. Both single-spin and collective spin-relaxation phenomena appear to lead to spin freezing in this material, again in analogy to observations in the spin ice materials, suggesting that such phenomena may be generic to a broader class of magnetic materials.

Structure and magnetic properties of theHo2Ge2O7pyrogermanate

We report the anisotropic magnetic properties of ${\text{Ho}}_{2}{\text{Ge}}_{2}{\text{O}}_{7}$ determined from dc and ac magnetization, specific heat and powder neutron-diffraction experiments. The magnetic lanthanide sublattice, seen in our refinement of the tetragonal pyrogermanate crystal structure, is a right-handed spiral of edge-sharing and corner-sharing triangles; the local Ho-O coordination indicates that the crystal field is anisotropic. Susceptibility and magnetization data indeed show that the magnetism is highly anisotropic, and the magnetic structure has the Ho moments confined to the plane perpendicular to the structural spiral. The ordered moment of ${\text{Ho}}^{3+}$, as determined from refinement of the neutron-diffraction data, is $9.0\text{ }{\ensuremath{\mu}}_{B}$. Magnetic ordering occurs around 1.6 K. Temperature and field dependent ac susceptibility measurements show that this compound displays spin relaxation phenomena analogous to what is seen in the spin ice pyrochlore system ${\text{Ho}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$.

Direct observation of the ice rule in an artificial kagome spin ice

Recently, significant interest has emerged in fabricated systems that mimic the behavior of geometrically frustrated materials. We present the full realization of such an artificial spin ice system on a two-dimensional kagome lattice and we demonstrate rigid adherence to the local ice rule by directly counting individual pseudospins. The resulting spin configurations show not only local ice rules and long-range disorder, but also correlations consistent with spin ice Monte Carlo calculations. Our results suggest that dipolar corrections are significant in this system, as in pyrochlore spin ice, and that they open a door to further studies of frustration in general.

Coexisting Ferromagnetic Order and Disorder in a Uniform System of HydroxyhalideCo2(OH)3Cl

Order or disorder often exists in a uniform spin system consisting of one kind of magnetic ion. Nevertheless, they rarely coexist in normal conditions. Our thermodynamic and microscopic magnetic studies of Co2(OH)3Cl, a distorted tetrahedral lattice compound with uniform Co2+ spin, demonstrate that the spins located on one corner of the tetrahedron are periodically ordered, but those on the other three are disordered below a ferromagnetic transition at TC=10.5 K. The partial order resembles that of the field-induced "kagomé-ice" state in spin ice pyrochlore compounds. Evidence suggests that a distortion in the tetrahedron is responsible for this partial ferromagnetic order in a zero field.

Far infrared optical properties of the pyrochlore spin ice compoundDy2Ti2O7

Near normal incidence far infrared reflectivity spectra of [111] dysprosium titanate(Dy2Ti2O7) single crystal have been measured at different temperatures. Seven phononmodes (eight at low temperature) are identified at frequencies below1000 cm−1. Optical conductivity spectra are obtained by fitting all the reflectivity spectra with thefactorized form of the dielectric function. Both the Born effective charges and the staticoptical permittivity are found to increase with decreasing temperature. Moreover, phononlinewidth narrowing and a phonon mode shift with decreasing temperature are alsoobserved, which may result from enhanced charge localization. The redshift of several lowfrequency modes is attributed to the spin–phonon coupling. All observed optical propertiescan be explained within the framework of the nearest neighbour ferromagnetic (FM) spinice model.

Magnetocaloric Effect Study on the Pyrochlore Spin Ice Compound Dy2Ti2O7in a [111] Magnetic Field

Field variation of the entropy S ( H ) of the spin ice compound Dy 2 Ti 2 O 7 has been examined by means of magnetocaloric effect measurements in a magnetic field H ∥[111], with attention focused o...

Observation of a Liquid-Gas-Type Transition in the Pyrochlore Spin Ice CompoundDy2Ti2O7in a Magnetic Field

Low temperature magnetization measurements on the pyrochlore spin ice compound Dy2Ti2O7 reveal that the ice-rule breaking spin flip, appearing at H approximately 0.9 T applied parallel to the [111] direction, turns into a novel first-order transition for T<0.36 K which is most probably of a liquid-gas type. T-linear variation of the critical field observed down to 0.03 K suggests the unusual situation that the entropy release across the transition remains finite [approximately 0.5 (J/K) x mol-Dy] as T-->0, in accordance with a breaking of the macroscopic degeneracy in the intermediate "kagomé ice" state.