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Register a new userMagnetic Frustration in Lead Pyrochlores
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The rich phase diagrams of magnetically frustrated pyrochlores have maintained a high level of interest over the past 20 years. To experimentally explore these phase diagrams requires a means of tuning the relevant interactions. One approach to achieve this is chemical pressure, that is, varying the size of the non-magnetic cation. Here, we report on a new family of lead-based pyrochlores A$_2$Pb$_2$O$_7$ (A = Pr, Nd, Gd), which we have characterized with magnetic susceptibility and specific heat. Lead is the largest known possible B-site cation for the pyrochlore lattice. Thus, these materials significantly expand the phase space of the frustrated pyrochlores. Pr$_2$Pb$_2$O$_7$ has an absence of long-range magnetic order down to 400 mK and a spin ice-like heat capacity anomaly at 1.2 K. Thus, Pr$_2$Pb$_2$O$_7$ is a candidate for a quantum spin ice state, despite weaker exchange. Nd$_2$Pb$_2$O$_7$ transitions to a magnetically ordered state at 0.41 K. The Weiss temperature for Nd$_2$Pb$_2$O$_7$ is $theta_{text{CW}}$ = $-$0.06 K, indicating close competition between ferromagnetic and antiferromagnetic interactions. Gd$_2$Pb$_2$O$_7$ is a Heisenberg antiferromagnet that transitions to long-range magnetic order at 0.81 K, in spite of significant site mixing. Below its ordering transition, we find a $T^{3/2}$ heat capacity dependence in Gd$_2$Pb$_2$O$_7$, confirmation of a ground state that is distinct from other gadolinium pyrochlores. These lead-based pyrochlores provide insight into the effects of weakened exchange on highly frustrated lattices and represent further realizations of several exotic magnetic ground states which can test theoretical models.
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We present bulk and neutron scattering measurements performed on the isotopically enriched $^{154}mathrm{Sm_2Ti_2O_7}$ and $^{154}mathrm{Sm_2Sn_2O_7}$ samples. Both compounds display sharp heat capacity anomalies, at 350 mK and 440 mK, respectively. Inelastic neutron scattering measurements are employed to determine the crystalline electric field (CEF) level scheme, which includes transitions between the ground-state and first excited $J$ multiplets of the $mathrm{Sm}^{3+}$ ion. To further validate those results, the single-ion magnetic susceptibility of the compounds is calculated and compared with the experimental DC-susceptibility measured in low applied magnetic fields. It is demonstrated that the inclusion of intermultiplet transitions in the CEF analysis is fundamental to the understanding of the intermediate and, more importantly, low temperature magnetic behaviour of the Sm-based pyrochlores. Finally, the heat capacity anomaly is shown to correspond to the onset of an all-in-all-out long-range order in the stannate sample, while in the titanate a dipolar long-range order can be only indirectly inferred.
Magnetic frustration in metals is scarce and hard to pinpoint, but exciting due to the possibility of the emergence of fascinating novel phases. The cubic intermetallic compound HoInCu$_4$ with all holmium atoms on an fcc lattice, exhibits partial magnetic frustration, yielding a ground state where half of the Ho moments remain without long-range order, as evidenced by our neutron scattering experiments. The substitution of In with Cd results in HoCdCu$_4$ in a full breakdown of magnetic frustration. Consequently we found a fully ordered magnetic structure in our neutron diffraction experiments. These findings are in agreement with the local energy scales and crystal electric field excitations, which we determined from specific heat and inelastic neutron scattering data. The electronic density of states for the itinerant bands acts as tuning parameter for the ratio between nearest-neighbor and next-nearest-neighbor interactions and thus for magnetic frustration.
We have studied polycrystalline Yb4LiGe4, a ternary variant of the R5T4 family of layered compounds characterized by a very strong coupling between the magnetic and crystallographic degrees of freedom. The system is mixed valent, with non-magnetic Yb2+ and magnetic Yb3+ present, and is characterized by coexisting ferromagnetic and antiferromagnetic correlations. We present measurements of resistivity, AC-susceptibility, specific heat, and muon spin relaxation (muSR), below 1 K. The low temperature measurements suggest a transition to a mesoscopically inhomogeneous magnetically ordered state below 2 K characterized by fluctuations well below the ordering temperature. This unusual state is believed to result from the enhanced two-dimensionality produced by Li substitution and frustration effects inherent in the Yb sub-lattice geometry.
The crystal structures of the A2B2O7-x Niobium-based pyrochlores Y2(Nb0.86Y0.14)2O6.91, CaYNb2O7, and Y2NbTiO7 are reported, determined by powder neutron diffraction. These compounds represent the first observation of B-site displacements in the pyrochlore structure: the B-site ions are found to be displaced from the ideal pyrochlore positions, creating electric dipoles. The orientations of these dipoles are fully analogous to orientations of the magnetic moments in Ising spin based magnetically frustrated pyrochlores. Diffuse scattering in electron diffraction patterns shows that the displacements are only short range ordered, indicative of geometric frustration of the collective dielectric state of the materials. Comparison to the crystal structure of the Nb5+ (d0) pyrochlore La2ScNbO7 supports the prediction that charge singlets, driven by the tendency of Nb to form metal-metal bonds, are present in these pyrochlores. The observed lack of long-range order to these singlets suggests that Nb4+-based pyrochlores represent the dielectric analogy to the geometric frustration of magnetic moments observed in rare earth pyrochlores.
Localized spins and itinerant electrons rarely coexist in geometrically-frustrated spinel lattices. We show that the spinel CoV2O4 stands at the crossover from insulating to itinerant behavior and exhibits a complex interplay between localized spins and itinerant electrons. In contrast to the expected paramagnetism, localized spins supported by enhanced exchange couplings are frustrated by the effects of delocalized electrons. This frustration produces a non-collinear spin state and may be responsible for macroscopic spin-glass behavior. Competing phases can be uncovered by external perturbations such as pressure or magnetic field, which enhance the frustration.
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