Magnetic properties of Ho2Ru2O7 and Dy2Ru2O7 pyrochlores

Physica Scripta, 2023

We investigated the structural and magnetic properties of Ni 42.5 (Fe, Co, Ni, Cu) 0.5 Mn 46 Sn 11 alloys fabricated by arc melting. Substitution of Ni by Fe, Co and Cu causes lattice expansions consistent with increasing atomic sizes. The zero-field cooled and field cooled results show second-order magnetic transition at the hightemperature austenite phase to a first-order magnetic transition in the low-temperature martensite phase. The substitution of Ni by Fe and Co increases the austenite Curie temperature T A C from 282 K to 289 K and 294 K respectively while Cu reduces it to 278 K. The martensitic transition temperature T M increased from 221 K to 241 K for Fe substitution and decreased to 210 K and 209 K for Co and Cu respectively. The coercive field H C increased significantly from 457 Oe for Ni at 100 K to 729 Oe for Co at 80 K. The increase to 763 Oe for Fe and 769 Oe for Cu occurred at the same temperature of 40 K. We attribute such increases to domain wall pinning effects due to the inclusions of Fe, Co and Cu. The H C exhibited an anomalous temperature dependence in all the samples. The exchange bias field H EX also showed a significant enhancement below 40 K from 196 Oe for Ni to 476 Oe, 430 Oe and 434 Oe for Fe, Co, and Cu substitutions respectively. The fits to the temperature dependence of the H C reveal significant changes in the competition between ferromagnetic and antiferromagnetic interactions. The peak magnetic entropy change ∆S pk M has a linear dependence on the magnetic field H 2/3. The highest value of 28.8 J kg-1 K −1 for ∆S M is obtained in the first order magnetic transition compared to 3.0 J kg-1 K −1 in the second order transition. We report an effective cooling power of 155 J kg-1 in the second order magnetic transition.

APS March Meeting Abstracts, 2004

Neutron scattering measurements on the spin-ice candidate material Ho 2 Ru 2 O 7 have revealed two magnetic transitions at T 95 and 1:4 K to long-range ordered states involving the Ru and Ho sublattices, respectively. Between these transitions, the Ho 3 moments form short-ranged ordered spin clusters. The internal field provided by the ordered S 1 Ru 4 moments disrupts the fragile spin-ice state and drives the Ho 3 moments to order. We have directly measured a slight shift in the Ho 3 crystal field levels at 95 K from the Ru ordering.

Journal of Alloys and Compounds, 2009

We report a detailed study of structural and magnetic properties of the pyrochloretype (Bi 2-x Pr x )Ru 2 O 7 series. Eleven compositions with 0 ≤ x ≤ 2 were prepared by solid state reaction at 1050°C and found to form a solid solution in the whole range of compositions. Structural refinements from X-ray powder diffraction data by the Rietveld method show that the main variation concerns the ruthenium site. Ru-O distances increase and the Ru-O-Ru angle (bridging corner-sharing Ru-O octahedra) decreases with increasing praseodymium content, in agreement with the metal-semiconductor transition observed previously in this system. Magnetic measurements show that the metallic Pauli paramagnetism of Ru in Bi 2 Ru 2 O 7 evolves to a magnetism of localized low spin moments coexisting with the magnetism of Pr 3+ cations moments. Strong magnetic correlations are present at high Pr content, as illustrated by a Néel temperature of -224 K for Pr 2 Ru 2 O 7 .

Journal of the Physical Society of Japan, 2011

We report results of NMR experiments on the ruthenium oxide Hg2Ru2O7 with the pyrochlore structure, which exhibits a metal-insulator transition at TMI = 107 K. In the metallic phase above TMI, the nuclear spin-lattice relaxation rate 1/T1 and the Knight shift at the Hg sites follow the Korringa relation, indicating the absence of substantial spatial spin correlation. At low temperatures in the insulating phase, 99,101 Ru-NMR signals are observed at zero magnetic field, providing evidence for a commensurate antiferromagnetic order. The estimated ordered moment is about 1 µB per Ru, much smaller than 3 µB expected for the ionic (4d) 3 configuration of Ru 5+. Thus, the localized spin models are not appropriate for the insulating phase of Hg2Ru2O7. We also discuss possible antiferromagnetic spin structures.

Journal of Physics: Condensed Matter, 2013

We present a model which accounts for the high-field magnetization at very low temperature in the frustrated pyrochlore compound Er 2 Ti 2 O 7 . In Er 2 Ti 2 O 7 , the Er 3+ ion has a planar crystal field anisotropy and the material undergoes a transition to antiferromagnetism at T N = 1.2 K. Our model is a mean-field self-consistent calculation involving the four rare earth sites of a tetrahedron, the building unit of the pyrochlore lattice. It includes the full crystal field Hamiltonian, the infinite range dipolar interaction and anisotropic nearest neighbour exchange described by a 4-component tensor. We discuss the equivalence of our treatment of the exchange tensor, taken to be diagonal in a frame linked to a rare earth-rare earth bond, with the pseudo-spin Hamiltonian recently developed for Kramers doublets in a pyrochlore lattice.

Physical Review B, 2020

The f-d magnetic exchange interaction is considered to be a key ingredient for many exotic topological phases in pyrochlore iridates. Here, we have investigated the evolution of structural, magnetic and electronic properties in doped pyrochlore iridate, (Y1−xPrx)2Ir2O7. Apart from geometrical frustration, pyrochlore iridates are well known for its active spin-orbit coupling effect. The substitution of Pr 3+ (4f 2) for the nonmagnetic Y 3+ (4d 0) acts as a magnetic doping, which provides an ideal platform to study f-d exchange interaction without altering the Ir-sublattice. With Pr substitution, system retains its original cubic structural symmetry but the local structural parameters show an evolution with the doping concentration x. The robust magnetic-insulating state in Y2Ir2O7 is drastically weakened, while Pr2Ir2O7 (x = 1.0) shows a paramagnetic-metallic behavior. A metal-insulator transition is observed for x = 0.8 sample. This evolution of magnetic and electronic properties are believed to be induced by an exchange interaction between localized Pr-4f and itinerant Ir-5d electrons as well as by an increased hybridization between Ir-t2g and (basal) O-p orbitals as observed in XAS study. The resistivity in insulating materials follows a power-law behavior with a decreasing exponent with x. A negative magnetoresistance is observed for present series of samples at low temperature and where the magnetoresistance shows a quadratic field dependence at higher fields.

Journal of Physics: Condensed Matter, 2000

AC and DC magnetic susceptibilities have been measured on frustrated pyrochlore ferromagnets Ho 2 Sn 2 O 7 and Ho 2 Ti 2 O 7 at temperatures down to 100 mK. In the AC magnetic susceptibility, a dramatic drop of χ (in-phase component) and a single maximum of χ (quadrature component) are observed at around 1 K. The frequency dependence of these peak positions indicates the presence of a slow magnetic relaxation at low temperature, whose relaxation time obeys the Arrhenius formula with the energy barrier E b = 19.6 K and 27.5 K for Ho 2 Sn 2 O 7 and Ho 2 Ti 2 O 7 , respectively. Strong irreversibility is also observed in the temperature dependence of the DC magnetization of Ho 2 Sn 2 O 7 at 1 kOe below T p ∼ 0.75 K; the zero-field-cooling (ZFC) curve indicates a very sharp peak at T p , whereas the field-cooling curve has no anomaly at T p and increases monotonically with decreasing T. In the DC magnetization process of the ZFC state at 0.46 K, a steep increase of the moment occurs above 2 kOe, and the irreversibility disappears at around ∼15 kOe where the moment ferromagnetically saturates. These results indicate a clustering of magnetic moment whose size is of the order of a few tetrahedra, consistent with a recently proposed 'spin ice' model.

Journal of Alloys and Compounds, 2006

The polycrystalline intermetallics DyAgAl, HoAgAl and ErAgAl were obtained from a levitated melt. X-ray powder diffraction confirmed singlephase CeCu 2 -type structure (Imma). The X-ray photoelectron spectra were obtained with a Physical Electronics PHI 5700/660 XPS spectrometer using monochromatized Al K␣ radiation. The magnetic measurements (in static fields of 35-2100 Oe) were carried out in the temperature range 4.2-600 K by the Faraday method. Above 50 K magnetic susceptibility follows Curie-Weiss law with θ parameter 35, 16, and 8 K for DyAgAl, HoAgAl and ErAgAl, respectively. The effective magnetic moments per rare earths atom are slightly higher than their free ion values. At low temperatures thermomagnetic effects between ZFC and cooled in field of 2500 Oe (FC) runs exist. The dc susceptibility versus temperature for ZFC runs shows a peak at 37, 18 and 13 K for DyAgAl, HoAgAl and ErAgAl, respectively. The ac susceptibility shows a cusp at 44, 24 and 15 K for DyAgAl, HoAgAl and ErAgAl, respectively. The electrical resistivity has been investigated in the temperature range 4.2-300 K. In the liquid helium (4.2 K) the resistivity values are large (2.69 m for DyAgAl, 2.78 m for HoAgAl and 3.39 m for ErAgAl). The temperature dependence of the resistivity of ErAgAl shows local maximum at 12.5 K (3.4 m) followed by minimum at 31 K (3.38 m). The thermomagnetic effects, cusps in ac and peaks in dc susceptibility indicate the spin glass state. HoAgAl may be considered as a cluster glass material. Disorder in nonmagnetic atoms sublattice causes the short mean path of the conduction electrons. The RKKY interactions are reduced to the closest neighbors. Randomly distributed Al and Ag atoms differ enough to disturb the symmetry in neighborhood of rare earths. In connection with reducing the range of the RKKY interactions it leads to NMAD driven spin glass state.

Physical Review Letters, 2000

Recent experiments suggest that the Ising pyrochlore magnets Ho2Ti2O7 and Dy2Ti2O7 display qualitative properties of the spin ice model proposed by Harris et al. Phys. Rev. Lett. 79, 2554 (1997). We discuss the dipolar energy scale present in both these materials and consider how they can display spin ice behavior despite the presence of long range interactions. Specifically, we present numerical simulations and a mean field analysis of pyrochlore Ising systems in the presence of nearest neighbor exchange and long range dipolar interactions. We find that two possible phases can occur, a long range ordered antiferromagnetic one and the other dominated by spin ice features. Our quantitative theory is in very good agreement with experimental data on both Ho2Ti2O7 and Dy2Ti2O7. We suggest that the nearest neighbor exchange in Dy2Ti2O7 is antiferromagnetic and that spin ice behavior is induced by long range dipolar interactions.

The temperature dependence of magnetic susceptibility of the Bi$_{2-y}$Sr$_{y}$Ir$_{2}$O$_{7}$ system was studied from 2 to 300 K. According to these measurements, the solid solution $(0\leq y\leq 0.9)$ does not show any magnetic transition; however, a noticeable deviation from the Curie-Weiss law occurs and the magnetic behavior of this system can be ascribed to short-range magnetic order that is also present in several geometrically frustrated systems. The Pauli magnetic susceptibility decreases as the Sr content increases, this can be associated with a shift towards Ir$^{5+}$ in samples; also the Ir effective magnetic moment decreases. The estimated Wilson-ratio indicates the presence of strong electron-electron correlations.