Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session W40: Disordered Systems |
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Sponsoring Units: GMAG Chair: Tao Qu, University of Minnesota Room: 705 |
Friday, March 6, 2020 8:00AM - 8:12AM |
W40.00001: Using structural disorder to design terahertz optical response of ferromagnetic metal alloys Ming Lei, Sinisa Coh We study the intrinsic anomalous Hall conductivity of Fe-Co-Al alloys in the terahertz (THz) region by first-principles calculations. We find that alloying is a feasible way to tune the THz optical feature of ferromagnetic metals. The optical conductivity (sigma_xy) response of these alloys generically shows a lot of features in the range between 2 THz and 20 THz as it is driven by the spin-orbit interaction. In addition to sharp features in the spectrum, we also find that in some alloys sigma_xy can even change the sign in the THz regime. |
Friday, March 6, 2020 8:12AM - 8:24AM |
W40.00002: Evolution of physical properties of RE3Ni5Al19 family (RE = Y, Nd, Sm, Gd, Tb, Dy, Ho, Er) Zuzanna Ryzynska, Michal J. Winiarski, Weiwei Xie, Tomasz Klimczuk Single crystals of RE3Ni5Al19 series (RE = Y, Nd, Sm, Gd, Tb, Dy, Ho, Er) were grown using a self-flux method. The crystal structure was examined with x-ray powder diffraction measurements and single crystal refinement. Physical properties were studied for the first time for RE3Ni5Al19 (RE = Y, Nd, Gd, Tb, Dy, Ho and Er) by means of magnetic susceptibility, electrical resistivity and heat capacity measurements. Complex magnetic behavior for all of the studied compounds will be briefly discussed. |
Friday, March 6, 2020 8:24AM - 8:36AM |
W40.00003: Morphology of the glassy phase of a disordered ferromagnet Juan Carlos Andresen, Moshe Schechter Previous work established the existence of a novel ferromagnetic disordering mechanism in ferromagnetic disordered systems with random fields [Phys. Rev. Lett. 111, 177202 (2013)]. This mechanism lies in between the standard random field ferromagnetic disordering mechanism and the Imry-Ma disordering mechanism. The interplay of the quenched disorder and the small but finite random fields favors domains with a spin-glass like spin alignment destroying the ferromagnetic phase. Making use of large-scale Monte Carlo simulations of disordered spin models in a random field at different dimensionalities we study the morphology of the spin-glass like domains at different points of the phase diagram and unveil the microscopic structure of the quasi-spin glass phase. |
Friday, March 6, 2020 8:36AM - 8:48AM |
W40.00004: High Curie temperature in Eu-doped GaN caused by Ga-vacancies Akira Masago, Hikari Shinya, Tetsuya Fukushima, Kazunori Sato, Hiroshi Katayama-Yoshida The present study computationally demonstrates that room-temperature ferromagnetism, which has been experimentally observed in Eu-doped GaN, is induced by holes in N 2p states (i.e., Zener’s double exchange interaction) that arise on the assumption that Ga vacancies appear as a result of the introduction of Eu ions (i.e., volume compensation). The calculated Curie temperature (TC) suddenly increases over a certain range of Ga vacancy concentrations and gradually increases with increasing concentration of Eu ions. High TC above room temperature is dominated by Zener’s double exchange mechanism in partially occupied N 2p hole-states which itinerate throughout the whole crystals, and low TC is dominated by Zener’s p-f exchange mechanism in Eu 4f and N 2p hybridization. We can explain reasonably a surprising experimental data of 4000 μB per Gd atom in Gd-doped GaN reported by Dhar et al. [S. Dhar et al., Phys. Rev. Lett. 94, 037205 (2005)] |
Friday, March 6, 2020 8:48AM - 9:00AM |
W40.00005: Defect-related Optical and Electronic Properties of Ion-bombarded Hexagonal Boron Nitride Gabriel López-Morales, Mingxing Li, Harishankar Jayakumar, Nicholas Proscia, Daniela Pagliero, Gustavo E Lopez, Vinod M Menon, Carlos Meriles Hexagonal boron nitride (hBN), a well-known 2-dimensional (2D) Van der Waals material, is emerging as an attractive platform for spin-based nanoscale applications, owing to its defect-related single-photon emitters (SPE) and their recently shown magneto-optical response. Unfortunately, only a small fraction of emitters displays possible spin-selective transitions, while their atomic structures remain elusive. We report on work towards circumventing some of these limitations by implanting rare-earth impurities. Cerium (Ce3+) bombarded hBN flakes show a distribution of isolated defects with broadened spectral features (centered at ~ 575 nm), good optical stability and strong magneto-optical response when excited via circularly polarized light. These results help pave the way towards defect engineering in low-dimensional materials for applications in opto-electronics, nanoscale sensing and quantum information processing. |
Friday, March 6, 2020 9:00AM - 9:12AM |
W40.00006: Slowing down of spin glass correlation length growth: simulations meet experiments Raymond Orbach, Qiang Zhai, Victor Martin-Mayor, Deborah Schlagel The aging rate, d ln tw /d ln ξ = (Tc/T) zc(T,ξ), of the spin glass correlation length ξ has been measured in a single crystal of CuMn (6 at.%) as a function of the waiting time, tw, where Tc is the critical temperature. Dynamic slowing down is observed with ξ reaching a value of |
Friday, March 6, 2020 9:12AM - 9:24AM |
W40.00007: Response Time Reveals Temperature Chaos in Spin Glasses Qiang Zhai, Raymond Orbach, David Harrison We have measured the zero field cooled magnetization MZFC(t) for Cu86.5Mn13.5 multilayer thin films of different thickness using a one-step quench protocol and two-step temperature drop protocol. The response time, tweff, was extracted from the relaxation rate S(t) = dMZFC(t)/dlnt. We found that in the one-step temperature quench to T2 experiments, tweff was a monotonic function of waiting time tw until the system reached a quasi-equilibrium state. In the two-step temperature drop experiments, the sample was aged at an intermediate temperature T1 until the quasi-equilibrium state established. The temperature was then lowered to T2, the same final temperature in one-step quench experiment. After the system regained its quasi-equilibrium state, the response time tweff was compared with the maximum response time twref in one-step quench protocol. The results are rather counterintuitive: there exists a temperature range ΔT in which the two-step effective waiting times tweff are either smaller or larger than twref . Above the range ΔT, tweff recovers to twref. The existence of a ΔT and the recovery to reference response time may be indicative of temperature chaos in spin glasses. |
Friday, March 6, 2020 9:24AM - 9:36AM |
W40.00008: Probing spin glass energy landscapes with 1/f noise David Harrison, E. Dan Dahlberg, Raymond Orbach We have measured the 1/f noise in the electrical resistance of thin (<10nm) CuMn wires in order to explore the spin glass state. Consistent with previous measurements, the magnitude of the 1/f noise rises rather abruptly when the system is cooled through its freezing temperature. In thin spin glass films, at temperatures near the freezing temperature, the time-dependent correlation length can grow to reach the thickness of the film on experimental timescales. This fixes the maximum barrier height, which determines the subsequent dynamics. We compare our 1/f noise data, as well as our data on the second spectrum (the noise in the 1/f noise), with previously published results on thicker CuMn films. We attribute differences to accessible time scales (frequencies) associated with the barrier heights of the thinner films. |
Friday, March 6, 2020 9:36AM - 9:48AM |
W40.00009: Experimental Determination of the Critical Spin Glass Correlation Length Gregory Kenning We have made detailed measurements of the waiting time effect of the thermoremanent magnetization (TRM) near the spin glass transition temperature Tg. In previous measurements we found that the timescale tweff associated with the decay, rapidly decreases near Tg (where Tg is defined by the onset of remanence). This effect has been systematically mapped out for waiting times ranging from 300 s to 100,000 s. Using tweff to determine the length scale of the glassy correlations, ξglassy, (observed in both numerical studies and experiment) we see both growth of this correlation length as Tg is approached (from lower temperatures), and then a rapid reduction of this correlation length very close to Tg. We interpret this reduction in ξglassy, for all waiting times, as being governed by the critical correlation length ξcrit . The data from all waiting times collapse on a master curve that can be fit to (T-Tc)-zn. |
Friday, March 6, 2020 9:48AM - 10:00AM |
W40.00010: Demonstration and quantitative characterization of effective random exchange fields in ferromagnet/antiferromagnet bilayers Dylan James Collette, Guanxiong Chen, Sergei Urazhdin It was conjectured over 30 years ago that some of the unusual magnetic properties of ferromagnet/antiferromagnet bilayers are associated with the effective random field arising due to the frustration of exchange interaction at their interface [1]. This conjecture was supported by recent measurements demonstrating a correlated spin glass state in these systems [2], but has not yet been directly confirmed. |
Friday, March 6, 2020 10:00AM - 10:12AM |
W40.00011: Demagnetizing field dependent dynamic spin freezing model with random easy axes orientation in nanoparticles ensembles Korobi Konwar, Som Datta Kaushik, Debasis Sen, Pritam Deb We investigate the dynamic freezing of magnetic ensembles with varied self-arrangement of constituent Zinc Ferrite nanoparticles in their hierarchical ensembles. The small angle X-ray scattering and small angle neutron scattering experiments provide the overall information of the arrangement of primary nanoparticles in their secondary hierarchical ensembles. The extensive study of dc magnetization reveals the prevalence of strength of dipolar interaction among the nanoparticles on the magnetic memory effect [arXiv: 1907.11116]. The enhancement in magnetic memory effect in both the Field-Cooling and Zero Field-Cooling conditions is observed in an ensemble having higher demagnetizing field resulting higher degree of randomness in easy axes orientation with spin-glassy state at low temperature range. |
Friday, March 6, 2020 10:12AM - 10:24AM |
W40.00012: Tuning the Properties of Colloidal Magnetic Particles for Thermometry on the Nanoscale Adam Biacchi, Thinh Bui, Eduardo De Lima Correa, Thomas Moffat, Weston L Tew, Michael Donahue, Solomon Isaac Woods, Cindi Dennis, Angela Hight Walker Accurate, local, remote, and real-time temperature measurements are essential for many technological applications. While conventional thermometry can accurately measure the temperature of a microscale spot on a surface, magnetic nanothermometry is being developed to measure temperature at comparable spatial resolution throughout a volume. However, commercially available nanomaterials display only modest magnetic thermosensitivity; this response must be strengthened to improve the signal-to-noise ratio as a prerequisite to practical volumetric nanothermometry. Here, we engineer solution-synthesized nanoparticles and measure their magnetization dependence on the temperature of their local environment. Modulation of composition, size, and structure allows for different magnetic transition temperature regimes and sensitivities to improve these nanothermometers’ performance. Over the range 100 K to 350 K, under an applied magnetic field of 0.01 T, these structurally complex samples show considerable temperature-dependent change to their magnetization. Results collected from X-ray scattering and diffraction, Raman spectroscopy, and high-resolution electron microscopies provide correlations between the nanoscale structure of these particles with their magnetic thermosensistivity. |
Friday, March 6, 2020 10:24AM - 10:36AM |
W40.00013: Classical and Tunneling Percolation Transitions in Nanoparticle Composites Shiva P Pokhrel, Brendon Waters, Zhi Feng Huang, Boris Nadgorny Electrical conductivity in nanoparticle composites, which consist of insulating and conducting nanoparticles, is determined by two competing mechanisms, percolation in a continuous conducting network (governed by classical percolation with universal critical exponents) and tunneling between isolated conducting particles (described by tunneling percolation with non-universal critical exponents). In this work, we investigate a flexible model system based on half-metallic spherocylindrical CrO2 nanoparticles, which can be gradually converted by annealing from metallic (CrO2) to insulating (Cr2O3) state. Hence we can study a transition from classical to tunneling percolation with the variation of insulating Cr2O3 shell barrier (which can be controlled with sub-nanometer precision), through both experimental measurements and computer simulations - by utilizing a combination of hard-particle Monte Carlo and mechanical contraction methods. Our experimental and theoretical results for the percolation thresholds and critical exponents for classical percolation are in good agreement. We will also report the results of tunneling percolation, as well as magnetoresistance measurements for different tunnel barrier thicknesses. |
Friday, March 6, 2020 10:36AM - 10:48AM |
W40.00014: Spin excitation spectrum in strong disorder spin system using spectrum bifurcation renormalization group Hong-Ye Hu, Hui-Ke Jin, Yizhuang You Recently the interplay between frustration and disorder has attracted much attention. In the experiment, there is hint that YbMgGaO4 is related to a quantum spin liquid state with a spinon Fermi surface. However, it is still under debate that whether the observed spin-excitation spectrum of YbMgGaO4 is due to effect of spin liquid or disorder. We developed a numerical tool called spectrum bifurcation renormalization group(SBRG) to study strong disordered quantum magnet. Using SBRG, we calculated the spin excitation spectrum of a strong disorder spin model on triangle lattice. We found the spin excitation spectrum agrees well with the experiment, and the result is robust with respect to model parameters. |
Friday, March 6, 2020 10:48AM - 11:00AM |
W40.00015: Enhanced non-collinear magnetic phase in mixed anisotropy antiferromagnet Sophie Morley, Sujoy Roy, David Lederman Antiferromagnetic (AFM) materials and non-collinear spin textures have recently become popular for their possible technological advantages, such as THz dynamics and topological protection. We have grown AFM epitaxial thin films of (110) FexNi1-x F2 on MgF2 substrates using molecular beam epitaxy (MBE). FeF2 has a strong uniaxial anisotropy along its tetragonal c-axis, whereas NiF2 has an anisotropy in the a-b plane. Orthogonal anisotropies such as these can be described by the Dzyaloshinskii-Moriya interaction (DMI) and are responsible for interesting spin textures, such as helices and skyrmions. DMI is enhanced via coupling to a high spin-orbit material. We observe an enhancement of a secondary phase close to the AFM Néel ordering temperature in our Pt capped films. We also measured temperature-dependent X-ray resonant magnetic scattering that scales well with the magnetisation. We argue that an AFM skyrmionic-like phase is stabilised in this mixed anisotropy system which is further enhanced upon proximity to a high spin orbit heavy metal. |
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