Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session S37: Spin Glasses, Spinels and Other Frustrated SystemsFocus
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Sponsoring Units: GMAG DCMP DMP Chair: Adam Aczel, Oak Ridge National Laboratory Room: BCEC 206A |
Thursday, March 7, 2019 11:15AM - 11:27AM |
S37.00001: Role of Anisotropy in Spin Glass 1/f Noise Measurements David Harrison, E. Dan Dalhberg, Raymond Orbach We have measured the 1/f noise in the resistance of the spin glass state of Cu1-x-yMnxAuy and Ag1-x-yMnxAuy thin films. The magnitude of 1/f noise shows a relatively abrupt increase as the system is cooled below its spin glass transition temperature. Doping of both the Ag and Cu based alloys with Au, allows us to systematically change the strength of the Dzyaloshinskii-Moriya interaction (DMI), a unidirectional anisotropy believed to give metallic spin glasses a more Ising-like characteristic. Witness films fabricated with the noise samples allow us to make a direct comparison of the temperature dependence of our noise measurements and conventional SQUID magnetometry measurements. We also compare our data with previously published results. |
Thursday, March 7, 2019 11:27AM - 11:39AM |
S37.00002: A statistical approach to rejuvenation and memory in spin glasses Raymond Orbach Rejuvenation and memory in spin glasses are approached from a statistical perspective. The analysis is based on temperature chaos being a "rare-event-driven phenomenon." Three regions are identified with increasing magnitudes of negative temperature shifts -δT (δT > 0). The first region exhibits reversibile dynamics for δT sufficiently small. The second is the chaotic regime for larger δT where the dynamics re-start, termed "rejuvenation." Only a few spatial regions of size of the order of the spin glass correlaton length go chaotic. The large number of separated correlated regions for correlation lengths much less than the sample thickness or crystallie size (e.g. thin films or bulk samples) suggests that, as the temperature is lowered further, the probability that those regions that went chaotic in the rejuvenation regime would go chaotic again is statistically small. The growth of free energy barriers in these "rare" regions as the temperature is lowered further (δT increasing) "freezes" their dynamics. As the temperature is raised back to the rejuvenation regime, their dynamics are "revived" and return to those initially measured. This is the origin of "memory." |
Thursday, March 7, 2019 11:39AM - 11:51AM |
S37.00003: Heisenberg domain state in thin-film ferromagnet/antiferromagnet bilayers is a magnetic glass Weijie Li, Sergei Urazhdin Random exchange interaction at the interface between ferromagnetic (F) and antiferromagnetic (AF) films results in the formation of a multidomain state. It was predicted by Malozemoff that for sufficiently thin films, domains become smaller than the domain wall width, forming a “Heisenberg domain state” (HDS) [1]. The nature and properties of HDS remain unexplored. |
Thursday, March 7, 2019 11:51AM - 12:03PM |
S37.00004: Investigation of the Magnetic Ground State of Ni-Mn-Sn-based Shape Memory Alloy: a combined muSR and Powder Neutron Diffraction Study Subham Majumdar, Jhuma Sannigrahi, D. T. Adroja, J Lord, Dmitry Khalyavin, Adrian Hillier We performed muon spin relaxation (muSR) and neutron powder diffraction (NPD) measurements as a function of temperature (T) on a metamagnetic shape memory alloy of nominal composition Ni51Mn35Sn14. The initial asymmetry (A0) of the muSR data shows some unusual T variation. A0 falls rapidly below the ferromagnetic (FM) transition at TCA = 320 K, indicating the onset of bulk magnetic order. A0 regains its full asymmetry value below the structural transition at TMS = 290 K suggesting the collapse of the FM order into a fully disordered paramagnetic state. A0 again falls below the second magnetic transition at TCM = 240 K. Interestingly, A0 increases sluggishly below the exchange bias blocking temperature TB = 120 K. This indicates that the system attains a disordered/glassy magnetic phase below TB, which is responsible for the exchange bias and anomaly in the ac susceptibility data. The NPD does not show any magnetic superlattice reflection ruling out the possibility of a long range antiferromagnetic state at low temperatures. The ground state is likely to be comprising of a spin-glass phase in the backdrop of an FM state. |
Thursday, March 7, 2019 12:03PM - 12:15PM |
S37.00005: Low-temperature and dynamic magnetism of highly frustrated 5d2 Li4MgOsO6 in comparison with 5d3 Li3Mg2OsO6 Jeremy Carlo, Shahab Derakhshan, Thomas Gredig, Graeme Luke, John Greedan Geometric magnetic frustration (GMF) has attracted substantial interest due to the exotic physics and rich phase diagrams revealed by the cancellation of normally-dominant magnetic interactions,giving impetus for the search for novel frustrated systems, most often based on antiferromagneticcorrelations between magnetic ions decorating triangular or tetrahedral lattices. We report low-temperature magnetic susceptibility and muon spin relaxation results on Li4MgOsO6 and Li3Mg2OsO6, members of the A5BO6 "rock salt ordered" family of frustrated materials. In Li3Mg2OsO6 we find spin freezing below 12K. In Li4MgOsO6, which can crystallize into either orthorhombic Fddd or monoclinic C2/m crystal symmetries depending on synthesis conditions, we find magnetism consistent with glassy-like behavior dominating below 2K, with partial ordering and evidence for dynamics at somewhat higher temperatures. |
Thursday, March 7, 2019 12:15PM - 12:27PM |
S37.00006: Spin Glass Behavior in Spinel Ni0.8Fe2.2O4 Roshan Nepal, Mohammad Saghayezhian, Jiandi Zhang, Rongying Jin Spinel oxides (AB2O4) with magnetic A and B ions are known to exhibit complex magnetic behavior that emerge from the competing AA, AB, and BB magnetic interactions. Here, we report the magnetic and thermal properties of single-crystalline Ni0.8Fe2.2O4, which orders ferrimagnetically (FI) below TFI ~ 860 K, followed by another magnetic transition at TSG ~ 14 K. The dc magnetization shows a dramatic downturn upon cooling below TSG, whereas both real and imaginary parts of ac susceptibility show frequency dependence. Further quantitative analysis reveals the presence of de Almeida-Thouless (A-T) line, dynamic magnetization behavior, memory effect, and the critical power law dependence of transition temperature to ac frequency, all indicative of a spin glass ground state. The origin of the spin glass state and its implication to other physical properties will be discussed. |
Thursday, March 7, 2019 12:27PM - 12:39PM |
S37.00007: Quadrupolar Order and Disordered States in Spin-1 Diamond Lattice Antiferromagnets Matthew Butcher, Shuyi Li, Vaideesh Loganathan, Andriy Nevidomskyy Recent experiments have demonstrated a lack of long-range magnetic order in spin-1 diamond lattice compounds such as NiRh2O4 [1]. These compounds can potentially provide a rich playground for exotic quantum states such as a proposed quantum spiral spin liquid [2]. We study the phase diagram of the Heisenberg model with bilinear and biquadratic interactions between nearest and next-nearest neighbors on the diamond lattice to search for ordered and quantum spin liquid states. The phase diagram for this frustrated spin-1 model reveals ground states that lack long-range spin order, including quadrupolar states and valence-bond solids. We use variational Monte Carlo with projected fermionic states to compare energies for different candidate states within a plausible region of the phase diagram corresponding to NiRh2O4. |
Thursday, March 7, 2019 12:39PM - 12:51PM |
S37.00008: Coexisting zero-field and field induced relaxation channels in spinel spin ice Edward Riordan, Tom Fennell, Elsa Lhotel, Oksana Zaharko, Vladimir Tsurkan, Sean Giblin Spin ice materials exhibit novel magnetic behaviour because of their atomic interactions and geometric constraints of the crystal lattice. This novel behaviour takes the form of emergent quasi-particles that behave as magnetic monopoles. We have studied the spin ice materials CdEr2Se4 and CdEr2S4 using a bespoke high frequency ac susceptometer capable of measuring at low temperatures (>2K). These ac susceptibility measurements have uncovered novel behaviour when in the presence of applied DC fields, in the form of two coexisting relaxation channels with differing time scales and temperature dependence. Such behaviour has not been so directly observed before. We have further investigated the relaxations by use of DC magnetisation and muon spin rotation techniques. |
Thursday, March 7, 2019 12:51PM - 1:03PM |
S37.00009: Spin Jahn-Teller effect in the antiferromagnet CoTi2O5 Franz Lang, Franziska Kirschner, Stephen Blundell, Roger Johnson, Dharmalingam Prabhakaran We have used a combination of neutron powder diffraction and muon spin rotation experiments, complemented with DFT calculations, to solve the magnetic structure of orthorhombic CoTi2O5, which we find adapts a long-range ordered, antiferromagnetic state below 26 K with a moment of 2.72(1)μB per Co2+ ion and propagation vector k=(±1/2,1/2,0) [1]. Interestingly, in the experimentally determined crystal structure all the magnetic exchange couplings are completely frustrated by the underlying symmetry. Therefore, we conclude that the magnetic transition must driven by a Spin Jahn-Teller effect, in which the large spin degeneracy is relieved by a distortion of the crystal structure and an associated lowering of the structural symmetry. We investigate this distortion using high resolution x-ray experiments and DFT calculations. Furthermore, we discuss recent experimental studies of FeTi2O5 and the observed similarities to CoTi2O5, which leads us to conlcude that spin-phonon coupling can induce magnetic order in lower symmetry systems than previously reported. |
Thursday, March 7, 2019 1:03PM - 1:15PM |
S37.00010: Single-Crystal Neutron Scattering Studies of Breathing Pyrochlore Ba3Yb2Zn5O11 Sachith Dissanayake, William Steinhardt, Zhenzhong Shi, Stephen J Kuhn, Nicholas Butch, Matthias D Frontzek, Yiming Qiu, David E Graf, Hongcheng Lu, Casey Marjerrison, Sara Haravifard Recently a new class of materials called Breathing pyrochlore compounds have attracted much attention. These systems consist of two alternating, opposing tetrahedra expanded and contracted, leading to differing intra- and inter-tetrahedra exchange interactions and the emergence of the Dzyaloshinskii-Moriya interaction due to loss of inversion symmetry. They are predicted to host exotic physics, including quantum spin ice state and the field-tunable Weyl magnon. In this talk we will present our recent magneto-transport and field-dependent diffuse and inelastic neutron scattering results measured on single crystal of the breathing pyrochlore compound Ba3Yb2Zn5O11. |
Thursday, March 7, 2019 1:15PM - 1:27PM |
S37.00011: Powder inelastic neutron scattering of a spin-liquid candidate Ba3ZnRu2O9 Daniel Pajerowski, liurukara Sanjeewa, Matthew Brandon Stone, Zheng Gai, Joseph Kolis Ba3ZnRu2O9 (BZRO) is a 6-H perovskite compound that contains triangular lattice layers of Ru5+ dimers, and was recently reported to have no signatures of long-range-magnetic-order down to 37 mK and therefore put forward as a spin-liquid candidate.[1] We present inelastic neutron scattering measurements on BZRO powder that show a magnetic feature with a momentum dependence that is similar to the Ru-magnetic-form-factor, a peak energy intensity at 25 meV, and no obvious dispersive character. This magnetic mode is then considered in the context of density functional theory calculations and magnetization measurements. Comparisons are then made to the historical work on the analogous Ba3CaRu2O9.[2] |
Thursday, March 7, 2019 1:27PM - 1:39PM |
S37.00012: Magnetic excitations of the classical spin-liquid MgCr2O4 Martin Mourigal, Xiaojian Bai, Joseph Paddison, Eliot Kapit, Seyed Koohpayeh, Jiajia Wen, Sian E Dutton, Collin Broholm, John Chalker, Andrei T Savici, Garrett E Granroth, Alexander I Kolesnikov We report a comprehensive inelastic neutron-scattering study of the frustrated pyrochlore antiferromagnet MgCr2O4 in its cooperative paramagnetic regime. Theoretical modeling yields a microscopic Heisenberg model with exchange interactions up to third-nearest neighbors, which quantitatively explains all the details of the dynamic magnetic response. Our work demonstrates that the magnetic excitations in MgCr2O4 are faithfully represented in the entire Brillouin zone by a theory of magnon excitations propagating in a highly-correlated paramagnetic background. Our results also suggest that MgCr2O4 is proximate to a spiral spin-liquid phase distinct from the Coulomb phase, what has implications for the magneto-structural phase transition observed at low temperature in this material. |
Thursday, March 7, 2019 1:39PM - 1:51PM |
S37.00013: Possible Random Singlet phase in the S = 1/2 magnet Y2CuTiO6 Avinash Mahajan, Susanta Kundu, Jean-Christophe Orain, Michael Baenitz Y2CuTiO6 contains edge-shared triangular planes where the vertices are occupied by magnetic Cu and nonmagnetic Ti atoms in equal proportion. In spite of the large dilution of the triangular magnetic lattice, the magnetic susceptibility shows a large, AF θCW of about 240 K without any sign of ordering down to 1.8 K. No bifurcation is observed in the ZFC/FC magnetisation data in a low field of 50 Oe, down to 0.5 K. Likewise, the heat capacity shows no anomalies and the magnetic contribution has a power law behaviour at low-temperatures. Our muSR measurements down to 50 mK also do not show any signs of ordering. These observations are typical of potential quantum spin liquid systems. In the present case, however, the magnetic heat capacity Cm depends on the magnetic field H. We observed that the data could be scaled such that HγCm/T vs T/H follows a universal curve with γ = 0.6. Such a scaling behaviour has been suggested in systems with random singlet formation. The randomness of exchange coupling in Y2CuTiO6 might be due to the random occupation of Cu and Ti on the vertices of triangles. It is nevertheless surprising that in spite of the large (50%) depletion of the lattice, no spin freezing is observed. |
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