Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session E48: Frustrated Magnetism: PyrochloresFocus
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Sponsoring Units: GMAG DMP Chair: Kate Ross, Colorado State Room: 395 |
Tuesday, March 14, 2017 8:00AM - 8:12AM |
E48.00001: Magnetic excitations from a pyrochlore Heisenberg antiferromagnet Kemp Plumb, Allen Scheie, Jason Krizan, Jose Rodriguez-Rivera, Yiming Qiu, Robert Cava, Collin Broholm We present single crystal inelastic neutron scattering measurements for a new pyrochlore antiferromagnet, NaCaNi$_2$F$_7$. In this material S=1 carrying Ni ions occupy the B-site pyrochlore sublattice while Na$^{+}$ and Ca$^{2+}$ are completely disordered on the A-site. Our results show that while the infinite time magnetic correlations are dominated by the influence of this Na-Ca charge disorder, the energy scale set by this disorder is small compared with the magnetic exchange interactions. On shorter time scales (higher energies) spin liquid correlations dominate. These are the first such measurement of the full excitation spectra from the spin-liquid phase in a pyrochlore Heisenberg antiferromagnet and provide new insight into the interplay between disorder and magnetic exchange interactions in frustrated magnets. [Preview Abstract] |
Tuesday, March 14, 2017 8:12AM - 8:24AM |
E48.00002: $\mu$SR study of NaCaNi2F7 in zero field and applied longitudinal magnetic field Yipeng Cai, Murray Wilson, Alannah Hallas, Lian Liu, Benjamin Frandsen, Sarah Dunsiger, Jason Krizan, Robert Cava, Yasutomo Uemura, Graeme Luke Rich physics of abundant magnetic ground states has been realized in the A$_2$B$_2$X$_7$ geometrically frustrated magnetic pyrochlores. Recently, a new spin-$1$ Ni$^{2+}$ pyrochlore, NaCaNi$_2$F$_7$, was synthesized and shown to have spin freezing at 3.6~K with a frustration index of $f\sim36$ and antiferromagnetic exchange interactions [1] . This structure has chemical disorder on the A site caused by randomly distributed Ca and Na ions, which causes bond disorder around the magnetic Ni sites. We present Zero Field (ZF) and Longitudinal Field (LF) muon spin rotation ($\mu$SR) measurements on this single crystal pyrochlore. Our data shows that the Ni$^{2+}$ spins start freezing around 4~K giving a static local field of $\sim$140 G. The data show no oscillations down to 75 mK which indicates no long range magnetic order. They are well described by the dynamic Gaussian Kubo-Toyabe function with a non-zero hopping rate that is not easily decoupled with an applied longitudinal field, which implies persistent spin dynamics down to 75 mK. [Preview Abstract] |
Tuesday, March 14, 2017 8:24AM - 8:36AM |
E48.00003: Color ice states, weathervane modes, and fluctuation-driven phase transition in a pyrochlore Heisenberg antiferromagnet Yuan Wan, Michel Gingras We expose a new example of fluctuation-driven phase transition in the pyrochlore bilinear-biquadratic Heisenberg antiferromagnet, $H = \sum_{\langle ij\rangle}J\mathbf{S}_i\cdot\mathbf{S}_j+B(\mathbf{S}_i\cdot\mathbf{S}_j)^2$, with positive biquadratic exchange interaction ($B>0$), in the semi-classical limit ($S\gg 1$, $BS^2/J \sim O(1)$). We will show that this model possesses remarkable properties. First of all, the ground state manifold contains an extensively large family of non-coplanar spin states known as ``color ice states'', which are generalization of the familiar Ising spin ice states. Furthermore, the color ice states support two-dimensional analog of the weathervane modes in the classical kagome Heisenberg antiferromagnet. Finally, even though the bilinear and the biquadratic interactions admit a common ground state manifold, they produce different quantum fluctuations. As a result, the quantum order-by-disorder mechanism selects different states as $BS^2/J$ changes, resulting in a phase transition purely driven by fluctuations. The talk is based on arXiv:1607.02185. [Preview Abstract] |
Tuesday, March 14, 2017 8:36AM - 8:48AM |
E48.00004: Long-Range Anti-ferromagnetic Order in Sm$_{2}$Ti$_{2}$O$_{7}$ Cole Mauws, Paul Sarte, Alannah Hallas, Andrew Wildes, Jeffrey Quilliam, Graeme Luke, Bruce Gaulin, Christopher Wiebe The spin ice state has been a key topic in frustrated magnetism for decades. Largely due to the presence of monopole-like excitations, leading to interesting physics. There has been a consistent effort in the field at synthesising new spin ice phases that possess smaller moments in the hopes of increasing the density of magnetic monopoles. As well as investigating the phase when quantum fluctuations dominate over dipolar interactions. Initially Sm$_{2}$Ti$_{2}$O$_{7}$ was thought to be a candidate for a quantum spin ice, possessing a low moment of 1.5 $\mu _B$ in the high-spin case and crystal fields may reduce it to a true spin-1/2 system. However anti-ferromagnetic interactions as well as a lambda-like heat capacity anomaly pointed towards long-range antiferromagnetic order. An isotopically enriched samarium-154 single crystal was taken to the D7 polarized diffuse scattering spectrometer at the ILL. Long-range antiferromagnetic order was observed and indexed onto the all-in all-out structure. This agrees with theoretical predictions of Ising pyrochlore systems with sufficiently large anti-ferromagnetic coupling. [Preview Abstract] |
Tuesday, March 14, 2017 8:48AM - 9:00AM |
E48.00005: Experimental Evidence for field induced emergent clock anisotropies in the XY pyrochlore Er$_2$Ti$_2$O$_7$ Jonathan Gaudet, Alannah M. Hallas, Jacques thibault, Nicholas P. Butch, Hanna Dabkowska, Bruce Gaulin The XY pyrochlore Er$_2$Ti$_2$O$_7$, with its $\psi_2$ magnetic ground state, has garnered much attention due to the possibility that its ground state selection could originate from an order-by-disorder mechanism[1,2]. However, recently, theoretical work has exploited the fact that the symmetry breaking in this system is a rare case of high discrete symmetry (Z$_6$) [3]. This work studied the effect of a magnetic field on the Z$_6$ symmetry breaking and predicted rich and controllable magnetothermodynamic properties. Indeed, the authors predict numerous domains transitions in the low field regime that strongly depends on the field direction. In this talk, I will present neutron scattering data on Er$_2$Ti$_2$O$_7$ with a magnetic field applied along different high symmetry direction. Our experimental study has provided the first experimental evidence for this rich Z$_6$ domain phase behaviour. Lastly, I will address how our results could shed light on the putative ground state selection mechanism in Er$_2$Ti$_2$O$_7$. [1]L. Savary et al., Phys. Rev. Lett.,109,167201 (2012) [2]M. E. Zhitomirsky et al., Phys. Rev. Lett.,109,077204 (2012) [3]V. S. Maryasin et al., Phys. Rev. B., 93,100406(R) (2016) [Preview Abstract] |
Tuesday, March 14, 2017 9:00AM - 9:12AM |
E48.00006: Tuning Frustration in Rare Earth Pyrochlores by Platinum Substitution Alannah Hallas, Jonathan Gaudet, Arzoo Sharma, Murray Wilson, Yipeng Cai, Makoto Tachibana, Chris Wiebe, Bruce Gaulin, Graeme Luke A successful mechanism for exploring the rich physics of rare earth pyrochlores, R$_2$B$_2$O$_7$, is to substitute the non-magnetic B-site. Varying the ionic radius of the B-site induces an internal chemical pressure. Some rare earths are robust to substitutions; for example, the holmium-based pyrochlores all exhibit a dipolar spin ice state. In the case of other rare earths such as ytterbium, the ground states are remarkably fragile to chemical pressure. In this talk, I will introduce two materials with a new non-magnetic B-site: platinum. The ionic radius of platinum is comparable to that of titanium, which occupies the B-site in the most well-studied family of pyrochlores. Thus, platinum does not induce a strong chemical pressure on the lattice. Nevertheless, using Gd$_2$Pt$_2$O$_7$ and Er$_2$Pt$_2$O$_7$ as examples, I will show that platinum does affect a dramatic change on the magnetic properties. We trace this effect to platinum's empty $e_g$ orbitals, which mediate superexchange pathways not available in other rare earth pyrochlores. In Gd$_2$Pt$_2$O$_7$, this results in a striking 160\% enhancement of $T_N$ as compared to other Gd-based pyrochlores. In Er$_2$Pt$_2$O$_7$, the ordering temperature is strongly suppressed and the ground state is altered. [Preview Abstract] |
Tuesday, March 14, 2017 9:12AM - 9:48AM |
E48.00007: Ba$_3$Yb$_2$Zn$_5$O$_{11}$: A model system for anisotropic exchange on the breathing pyrochlore lattice Invited Speaker: Jeffrey Rau Competing strongly anisotropic exchange interactions can stabilize a range of unusual phenomena; this can include unusual magnetic and non-magnetic orders as well as disordered classical and quantum spin liquids. In many of such compounds, the physics is very delicate and accurate determination of the effective exchange interactions is crucial in making definite progress. In this talk we present a study of the “breathing” pyrochlore compound Ba$_3$Yb$_2$Zn$_5$O$_{11}$. Due to the nearly decoupled nature of its tetrahedral units, this compound serves as an ideal testbed for exploring the nature of anisotropic exchange in a theoretically and experimentally tractable rare-earth system. The relevant low-energy model of the Yb3+ tetrahedra is parametrized by four anisotropic exchange constants and is capable of reproducing the inelastic neutron scattering data, specific heat, and magnetic susceptibility with high fidelity. Surprisingly, the fitted exchange parameters reveal a Heisenberg antiferromagnet with a very large Dzyaloshinskii-Moriya interaction. Using this model, we predict the appearance of an unusual non-Kramers octupolar paramagnet at low temperatures. We further speculate on possible collective, inter-tetrahedron physics of these non-Kramers doublets. Finally, we will discuss what we can learn from Ba$_3$Yb$_2$Zn$_5$O$_{11}$ about anisotropic exchange in other rare-earth magnets. [Preview Abstract] |
Tuesday, March 14, 2017 9:48AM - 10:00AM |
E48.00008: Crystals of new spin ice materials Monica Ciomaga Hatnean, Romain Sibille, Michel Kenzelmann, Sylvain Petit, Elsa Lhotel, Claudia Decorse, Martin R. Lees, Oleg A. Petrenko, Geetha Balakrishnan Significant progress has been made in the past in the study of geometrically frustrated magnets due to the availability of large, high quality single crystals of rare earth titanate and molybdate pyrochlore oxides. These materials have been studied in great detail and yet their fascinating magnetic properties (such as spin ice/spin glass/spin liquid behaviour or long-range magnetic ordered states) continue to puzzle researchers. One of the most exciting avenues of future research is into systems which exhibit novel magnetic ground states, such as quantum spin liquid and quantum spin ice. As the search for frustrated magnets that display quantum effects widens, the research community has turned its attention to less studied pyrochlore systems, such as rare earth zirconates and hafnates $R_{2}M_{2}$O$_{7}$ ($R=$ Rare Earth, $M=$ Zr or Hf). Recent advances in the crystal growth of rare earth zirconates and hafnates have opened up a route to further investigations of these two novel classes of pyrochlore magnets. We present the recent developments in the synthesis of large high quality crystals of these new frustrated pyrochlore magnets and discuss briefly the interest in their magnetic properties. [Preview Abstract] |
Tuesday, March 14, 2017 10:00AM - 10:12AM |
E48.00009: Orbital Dimer Model for Spin-Glass State in Y$_2$Mo$_2$O$_7$ Joseph Paddison, Peter Thygesen, Andrew Goodwin, Ronghuan Zhang, Michael Hayward, Karena Chapman, Kevin Beyer, Helen Playford, David Keen, Matthew Tucker The formation of a spin glass generally requires that magnetic interactions are both frustrated and disordered. Consequently, the origin of spin-glass behavior in Y$_2$Mo$_2$O$_7$ -- in which magnetic Mo$^{4+}$ ions occupy a frustrated pyrochlore lattice with minimal compositional disorder -- is a longstanding question. We use neutron and X-ray pair-distribution function (PDF) analysis to develop a disorder model that resolves apparent incompatibilities between previous PDF, EXAFS and NMR studies, and provides a new and physical explanation of the exchange disorder responsible for spin-glass formation. We show that Mo$^{4+}$ ions displace according to a local ``2-in/2-out" rule on Mo$_4$ tetrahedra, driven by orbital dimerization of Jahn-Teller active Mo$^{4+}$ ions. Long-range orbital order is prevented by the macroscopic degeneracy of dimer coverings permitted by the pyrochlore lattice. Cooperative O$^{2-}$ displacements yield a distribution of Mo--O--Mo angles, introducing disorder into magnetic interactions. Our presentation shows how frustration of atomic displacements can assume the role of compositional disorder in driving a spin-glass transition, and reveals a link between ice-like and spin-glass physics. [Preview Abstract] |
Tuesday, March 14, 2017 10:12AM - 10:24AM |
E48.00010: Quantum phase transitions and anomalous Hall effect in frustrated Kondo lattices Silke Paschen, Sarah Elaine Grefe, Wenxin Ding, Qimiao Si Among the pyrochlore iridates, the metallic compound $Pr_2Ir_2O_7$ (Pr-227) has shown characteristics of a possible chiral spin liquid state [PRL \textbf{96} 087204 (2006), PRL \textbf{98}, 057203 (2007), Nature \textbf{463}, 210 (2010)] and quantum criticality [Nat. Mater. \textbf{13}, 356 (2014)]. An important question surrounding the significant anomalous Hall response observed in Pr-227 is the nature of the f-electron local moments, including their Kondo coupling with the conduction d-electrons. The heavy effective mass and related thermodynamic characteristics indicate the involvement of the Kondo effect in this system’s electronic properties. In this work, we study the effects of Kondo coupling on candidate time-reversal-symmetry-breaking spin liquid states on frustrated lattices. Representing the f-moments as slave fermions Kondo-coupled to conduction electrons, we study the competition between Kondo-singlet formation and chiral spin correlations. We derive an effective chiral interaction between the local moments and the conduction electrons and calculate the anomalous Hall response across the quantum phase transition from the Kondo destroyed phase to the Kondo screened phase. We discuss our results’ implications for Pr-227 and related frustrated Kondo-lattice systems. [Preview Abstract] |
Tuesday, March 14, 2017 10:24AM - 10:36AM |
E48.00011: A Disordered Route to the Coulomb Quantum Spin Liquid: Random Transverse Fields on Spin Ice in Pr2Zr2O7 Jiajia Wen, Seyed Koohpayeh, Kate Ross, Benjamin Trump, Tyrel McQueen, Kenta Kimura, Satoru Nakatsuji, Yiming Qiu, Daniel Pajerowski, John Copley, Collin Broholm Inelastic neutron scattering reveals a broad continuum of excitations in Pr$_2$Zr$_2$O$_7$, the temperature and magnetic field dependence of which indicate a continuous distribution of quenched transverse fields ($\Delta$) acting on the non-Kramers Pr$^{3+}$ crystal field ground state doublets. We show the magnetic excitations in Pr$_2$Zr$_2$O$_7$ are composed of two parts: a lower energy regime that is driven by inter-spin correlations, and a momentum transfer independent higher energy part driven by quenched transverse fields. A random phase approximation provides an excellent account of the data with a transverse field distribution $\rho(\Delta)\propto (\Delta^2+\Gamma^2)^{-1}$ where $ \Gamma=0.28(1)$~meV. Established during high temperature synthesis due to an underlying structural instability, it appears disorder in Pr$_2$Zr$_2$O$_7$ actually induces a quantum spin liquid. [Preview Abstract] |
Tuesday, March 14, 2017 10:36AM - 10:48AM |
E48.00012: Quantum fluctuations in the Kagome spin ice state of Pr$_{\mathrm{2}}$Zr$_{\mathrm{2}}$O$_{\mathrm{7}}$ Huiyuan Man, Jiajia Wen, Nicholas Butch, Satoru Nakatsuji, Collin Broholm While water ice and spin ice compounds such as Dy$_{\mathrm{2}}$Ti$_{\mathrm{2}}$O$_{\mathrm{7}}$ and Ho$_{\mathrm{2}}$Ti$_{\mathrm{2}}$O$_{\mathrm{7}}$ are semi-classical, low spin pyrochlores where exchange interactions prevail over dipole-dipole interactions can have substantial quantum fluctuations and perhaps realize quantum spin ice [1-3] where emergent magnetic monopoles have quantum dynamics and an artificial electromagnetism is manifest in low energy photon-like excitations. Pr$_{\mathrm{2}}$Zr$_{\mathrm{2}}$O$_{\mathrm{7}}$ was recently found to be an excellent candidate for quantum spin ice [2]. Strong quantum fluctuations were detected in Pr$_{\mathrm{2}}$Zr$_{\mathrm{2}}$O$_{\mathrm{7}}$ [3] but also a very strong sample variability, which is tied to the non-Kramers nature of the J$=$4 Pr3$+$ multiplet and an underlying structural instability. We have conducted neutron scattering experiments new high quality crystals to study quantum fluctuations in Pr$_{\mathrm{2}}$Zr$_{\mathrm{2}}$O$_{\mathrm{7}}$, with fields along [111]. At Q$=$ (2/3, 2/3, -4/3) where classical spin ice Ho$_{\mathrm{2}}$Ti$_{\mathrm{2}}$O$_{\mathrm{7}}$ shows a pinch point, we found a peak in Pr$_{\mathrm{2}}$Zr$_{\mathrm{2}}$O$_{\mathrm{7}}$. The quantum kagome ice state thus appears to be quite different from the classical case. [1] K. A. Ross et al., Phys. Rev. X 1 (2011) 021002. [2] K. Kimura et al., JPS Conf. Proc. 3 (2014) 014027. [3] K. Kimura et al., Nat. Commun. 4 (2013) 1934. [Preview Abstract] |
Tuesday, March 14, 2017 10:48AM - 11:00AM |
E48.00013: Magnetic Ground State Selection and B-site Disorder in the Pyrochlore Magnet Tb2NbScO7 Connor Buhariwalla, Alannah Hallas, Cole Mauws, Qianli Ma, Adam Aczel, Jacob Ruff, Robert Cava, Chris Wiebe, Bruce Gaulin Tb-based cubic pyrochlores have attracted interest due to the exotic, possible quantum spin ice ground state in Tb$_2$Ti$_2$O$_7$. Replacing the homogenous B site of B=Ti$^{4+}$ with a mixture of nonmagnetic ions changes the magnetic ground state and its excitations due in part to the sensitivity of the non Kramers Tb$^{3+}$ ion. Previous results showed that a highly disordered, fluctuating ground state is obtained in isovalent B-site disordered Tb$_2$Ti$_{(2-x)}$Sn$_x$O$_7$. Our current work focuses on the mixed valence B site disordered pyrochlore Tb$_2$NbScO$_7$. In this system, the B-site is occupied by non-magnetic Nb$^{5+}$ and Sc$^{3+}$, for B$^{4+}$ on average. Surprisingly our measurements of a powder sample of Tb$_2$NbScO$_7$ using time-of-flight and triple axis neutron scattering techniques show that this disordered B-site system displays an ordered antiferromagnetic ground state with Tc≈ 1.5 K. Diffuse x-ray scattering measurements on a small single crystal reveals a rich pattern of diffuse scattering due to the underlying B-site structural disorder and concomitant oxygen displacements.We discuss these results in relation to magnetic ground state selection in the Tb pyrochlores with B=Ti,Sn and Ti$_{1-x}$Sn$_{x}$. [Preview Abstract] |
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