Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session S24: Spin Frustration and DisorderFocus

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Sponsoring Units: GMAG DMP Chair: Shalinee Chikara, Auburn University Room: LACC 403A 
Thursday, March 8, 2018 11:15AM  11:51AM 
S24.00001: A further classification of glassy magnets: spin jam and spin glass Invited Speaker: Anjana Samarakoon Nonergodic behaviour can be easily found in many of the complex systems ranging from polymers to brain activities, to social networks and glass transitions due to their complex energy landscapes. Depending on the topology of these energy landscapes, the characteristics of these systems can be diverse in nature. Understanding their differences and classification remains a major scientific challenge. Recently, we presented a couple of experimental approaches combined with computer simulations to classify magnetic glassy materials into distinct classes. [13] The systems studied are hightemperature superconductorrelated materials, spinorbit Mott insulators, frustrated magnets, and dilute magnetic alloys. Our magnetic memory effect experiments reveal a distinct class of glassiness emerge in the densely populated magnets with a relaxation exponent of 1n ≈ 0.6(1) which is different from 1n ≈ 1/3 as of dilute magnetic glasses and from 1n ≈ 1(Debye limit) as expected for a spin solid. Furthermore, we study the freezing temperature as a function of relaxation time spanning over ten orders of magnitude combining bulk susceptibility and several neutrons scattering experiments such as Neutron Spin Echo, Backscattering and direct geometry time of flight spectroscopies. Our experimental data reveals different relaxation profiles for different classes of glasses. [1] A. M. Samarakoon, et al. Scaling of Memories and Crossover in Glassy Magnets, Scientific Reports 7, 12053 (2017). [2] A. Samarakoon, et al. Aging, memory and nonhierarchical energy landscape of a spin jam, PNAS 113, 11806 (2016). [3] J. Yang, et al. Spin jam induced by quantum fluctuations in a frustrated magnet, PNAS 112, 11519 (2015). 
Thursday, March 8, 2018 11:51AM  12:03PM 
S24.00002: Jammed Spin Liquid in the BondDisordered Kagome Heisenberg Antiferromagnet Thomas Bilitewski, Mike Zhitomirsky, Roderich Moessner We study a class of disordered continuous classical spin systems including the kagome Heisenberg magnet. While each term in its local Hamiltonian can be independently minimised, we find discrete degenerate ground states whose number grows exponentially with system size. These states do not exhibit zeroenergy `excitations' characteristic of highly frustrated magnets but instead are local minima of the energy landscape, albeit with an anomalously soft excitation spectrum. This represents a spin liquid version of the phenomenon of jamming familiar from granular media and structural glasses. Correlations of this jammed spin liquid, which upon increasing the disorder strength gives way to a conventional spin glass, may be algebraic (Coulombtype) or exponential. 
Thursday, March 8, 2018 12:03PM  12:15PM 
S24.00003: Randomfieldinduced disordering mechanism in a disordered ferromagnet: Between the ImryMa and the standard disordering mechanism Juan Carlos Andresen, Helmut Katzgraber, Moshe Schechter Random fields disorder Ising ferromagnets by aligning single spins in the direction of the random field in three space dimensions, or by flipping large ferromagnetic domains at dimensions two and below. While the former requires random fields of typical magnitude similar to the interaction strength, the latter ImryMa mechanism only requires infinitesimal random fields. Recently, it has been shown that for dilute anisotropic dipolar systems a third mechanism exists, where the ferromagnetic phase is disordered by finitesize glassy domains at a random field of finite magnitude that is considerably smaller than the typical interaction strength. Using largescale Monte Carlo simulations and zerotemperature numerical approaches, we show that this mechanism applies to disordered ferromagnets with competing shortrange ferromagnetic and antiferromagnetic interactions, suggesting its generality in ferromagnetic systems with competing interactions and an underlying spinglass phase. Furthermore, we look into the microstructure of the finitesize glassy domain structure at zero and finitetemperature. 
Thursday, March 8, 2018 12:15PM  12:27PM 
S24.00004: Effects of Disorder on Monopole Crystallization in Nd_{2}ScNbO_{7} Cole Mauws, Haidong Zhou, Matthew Stone, ZahraSadat Yamani, Christopher Wiebe The pyrochlore series Ln_{2}ScNbO_{7} (Ln = rare earth) present an excellent means of investigating the effect of chemically induced disorder on the frustrated magnetic sublattice without altering the stoichiometry of the magnetic cations. The Nd_{2}ScNbO_{7} member of this series acts as a structurally sound analogue to Nd_{2}Zr_{2}O_{7} which has attracted attention due to the experimental realization of simultaneous spin ice and Ising antiferromagnetic order signatures in neutron scattering experiments. This has been explained as magnetic charge fractionalization, leading to the condensation of magnetic monopoles. Here we report the synthesis and characterization of single crystalline Nd_{2}ScNbO_{7}, which orders magnetically beneath T_{N} = 0.34 K. The magnetic ground state is compared to Nd_{2}Zr_{2}O_{7} (T_{N} ~ 0.3 K) to observe the effects of Bsite disorder on the magnetic fragmentation. 
Thursday, March 8, 2018 12:27PM  12:39PM 
S24.00005: Tunable Quantum Dynamics in a Disordered Magnet Christopher Tang, Daniel Silevitch, Gabriel Aeppli, Thomas Rosenbaum Quantum memories depend on minimallyinteracting, localized qubits. One approach is via localized manybody excitations, such as those found in the dilute Ising magnet, LiHo_{0.045}Y_{0.955}F_{4}. Here clusters of several hundred spins bind together and can be resonantly excited. We use a nonlinear pumpprobe magnetic susceptibility technique, combining a strong longitudinal ac magnetic pump field to drive the system out of the linear regime and a weak probe field, to study the localized excitations. The pump field excites the clusters into resonance, the detailed shape of which provides insight into the lifetimes and the coherent quantum interference between different excitation channels. We tune the dynamics of the quantum degrees of freedom by sweeping the quantum mixing parameter through zero via the amplitude of the ac pump as well as a static external transverse field. The zerocrossing is associated with a dissipationless response at the drive frequency. The identification of points where localized degrees of freedom are minimally mixed with their environment in a dense and disordered, dipolarcoupled spin system implies control over the bath coupling of qubits emerging from strongly interacting manybody systems. 
Thursday, March 8, 2018 12:39PM  12:51PM 
S24.00006: Effects of bond randomness on the spin1/2 J_{1}J_{2} triangular Heisenberg model. HanQing Wu, HanQing Wu, Shoushu Gong, Donna Sheng We study the effects of bond randomness on the antiferromagnetic J_{1}J_{2 }triangular Heisenberg model using exact diagonalization (ED) and densitymatrix renormalization group (DMRG). With increasing randomness, we identify a gapless randomsinglet (RS) phase without any magnetic order and spinglass order. Using torus geometries up to 48 sites, we characterize the RS phase using both static and dynamical correlations. The vanishing magnetic order with growing randomness shows the phase boundaries between the magnetic orders (120^{0 }Neel and stripe phases) and the RS phase. In addition, We study the effects of randomness in the highly frustrated spin liquid phase of the J_{1}J_{2} model. We show some signatures of the entanglement spectrum on cylinder geometry that may characterize the phase transition between the two disorder phases. Our numerical results may help to understand the spin liquid behaviors observed in the real materials such as the recentlydiscovered material YbMgGaO_{4} where the mixing of Mg^{2+} and Ca^{3+} ions may introduce randomness to the superexchange. 
Thursday, March 8, 2018 12:51PM  1:03PM 
S24.00007: Effect of Quenched Disorder on a Spin1/2 TriangularLattice Antiferromagnet Ba_{2.8}Sr_{0.2}CoSb_{2}O_{9} Qing Huang, Haidong Zhou, Jie Ma, Eun Choi, Zhilun Lu, Tao Hong The novel upupdown (UUD) ground state, which has been predicted to be stabilized by quantum fluctuations only, is a good example of exotic magnetism in the triangularlattice antiferromagnet (TAF). This quantum UUD state, which manifests itself with a constant magnetization (1/3 of the saturation magnetization) over a finite field range, has been reported in the S=1/2 TAF Ba3CoSb2O9. Recently, we present a systematic study on the effect of quenched disorder on Ba2.8Sr0.2CoSb2O9 using DC and AC susceptibilities and advanced neutron scattering. DC susceptibility at zero field confirms an antiferromagnetic ordering below 2.8 K with an easyplane anisotropy in Ba2.8Sr0.2CoSb2O9. And AC susceptibility measurement suggests that the field extent for the UUD phase of Ba2.8Sr0.2CoSb2O9 is larger than that of Ba3CoSb2O9. We also report the neutronscattering study on the evolution of the ordering temperature and magnetic structure as a function of applied magnetic field. 
Thursday, March 8, 2018 1:03PM  1:15PM 
S24.00008: Random field mechanism in the diluted J1J2 model Rajesh Narayanan, Shashikant Singh Kunwar, Arnab Sen, Thomas Vojta The classical ferromagnetic Ising model in the presence of competing antiferromagnetic interactions hosts a very interesting phase diagram: At low temperatures and for g =J_{2}/ J_{1}<1/2 the system is in a ferromagnetic phase whereas for g>1/2 and at low temperatures a stripe phase obtains. At high temperature one finds the disordered paramagnetic phase for all values of $g$. We investigate the effects of site dilution on the various phases highlighted above for the clean J_{1}J_{2} model, and also analyze its impact on the intervening phase transitions. We show that the stripe phase is extremely susceptible to randomness very much in contrast to the ferromagnetic phase. The reasoning behind the extreme sensitivity of the stripe order to randomness will be highlighted during the talk. Impact of disorder on the nature of transition in this model and its universality class will also be discussed in this presentation. 
Thursday, March 8, 2018 1:15PM  1:27PM 
S24.00009: Magnetic Field Effects on Spin Glass Dynamics Qiang Zhai, David Harrison, Raymond Orbach We have measured magnetic field effects on the dynamics of Cu_{0.89}Mn_{0.11 }thin film spin glasses of 20 nm thickness arranged in a multilayer structure. We extract the maximum free energy barrier height, Δ_{max }, by allowing the perpendicular correlation length to reach the film thickness L. This fixes the correlated region in the spin glass, and results in activated dynamics with activation energy Δ_{max}. The magnitude of Δ_{max }is extracted by measuring the time dependence of the irreversible magnetization after the perpendicular correlation length has reached L. A new experimental protocol was invoked that accounts for the uncertainty in the time dependence of the field cooled magnetization. The change in the magnitude of Δ_{max }with change in magnetic field generates the number of correlated spins. We find the correlated region to be of a pancakelike shape because of d = 2 dynamics after the perpendicular correlation length has reached L. 
Thursday, March 8, 2018 1:27PM  1:39PM 
S24.00010: Measurements of 1/f Noise in CuMn and AgMn Thin Films David Harrison, E Dahlberg, Raymond Orbach We have measured the 1/f noise in the spin glass state of CuMn and AgMn thin films. The Mn concentrations we have investigated are approximately 11% Mn for the CuMn samples studied and 12% in the case of AgMn. Our CuMn results are consistent with previous measurements that have demonstrated lowfrequency resistance fluctuations increase in magnitude as a spin glass is cooled below the glass temperature T_{g}, but with some significant differences. Similar to the CuMn, we find a rather abrupt increase in the 1/f noise when the AgMn is cooled to below the glass temperature. A comparison of the noise between the two systems and previous measurements will be presented. 
Thursday, March 8, 2018 1:39PM  1:51PM 
S24.00011: De AlmeidaThouless Line in the Reentrant SpinGlass Phase of a FeZr_{ }Alloy P. R. T. Ribeiro, D. R. Ratkovski, F. L. A. Machado, E. Dan Dahlberg Differences in the Tdependence of the zerofield (ZFC) and fieldcooled (FC) magnetizations (M) measured for different values of H is a signature of spinglass (SG) phases. The magnetic field dependent irreversibility transition is given by the de AlmeidaThouless line (ATline): H = H_{0 }(1T/T_{G})^{Φ/2}, where T_{G}_{ }is the glass temperature and Φ a critical exponent. Determining the ATline in a reentrant SG is complicated by the ferromagnetism (FM). Fe_{90}Zr_{10} is FM below 225 K and enters a SGphase at T_{G} = 15.5 K. The ATline was obtained from the Tdependence of M measured for 2.5 ≤ H ≤ 60 Oe in the temperature range 2 ≤ T ≤ 30 K after cooling in 85 kOe. Two regimes were observed: (1) near T_{G} a fit to the data yielded Φ = 3.6 while (2) below about 11 K one obtains Φ = 5.2. Exchangebias (EB) was also observed in the hysteresis loops measured after cooling the sample in a field of 85 kOe for 2 ≤ T ≤ 300 K. The EB was found to rapidly increase below T_{G} following the trend of the coercivity data. The overall results are in good agreement with the χac and GMI data used for probing the dynamics of the spins in the reentrant SGphase both below 10 kHz and in the MHz regime. 
Thursday, March 8, 2018 1:51PM  2:03PM 
S24.00012: Aging as Domain Growth in the Spin Glass Copper Manganese Daniel Tennant, Gregory Kenning, Qiang Zhai, David Harrison, Raymond Orbach We report on dynamical measurements on a 4.5nm thin film of the spin glass, Cu_{0.88}Mn_{0.12}, as well as a bulk sample of Cu_{0.96}Mn_{0.04}. Systematic measurements of the waiting time effect on the thermoremanent magnetization (TRM) of both samples at temperatures approaching their freezing and glass temperature respectively were performed. The process of aging between the film and the bulk is found to differ substantially. In the thin film sample, the S(t) character of the TRM is absent at temperatures between 14K and T_{f}. By the use of the value of the TRM after a standard measuring time, it is found that the waiting time effect is absent at temperatures between 0.9T_{f}  T_{f}. This is in contrast with aging in the bulk sample. While the S(t) character of the TRM is evident all temperatures in the bulk, the S(t) curves gradually collapse onto one another between the temperatures of 0.90T_{g} – 0.96T_{g}. These observations are examined in light of the spin glass correlation length, ξ(t, T), from nucleation to length scales comparable to either the thin film thickness L in the film or the size of crystallites in the bulk. 
Thursday, March 8, 2018 2:03PM  2:15PM 
S24.00013: Critical behavior of fourdimensional Ising spin glasses with bimodal disorder Andrew Ochoa, Amin Barzegar, Christopher Pattison, Ian Campbell, Helmut Katzgraber We perform Monte Carlo simulations of the bimodal Ising spinglass model 
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