Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session S41: Emergent magnetism in correlated electron systems II |
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Sponsoring Units: GMAG DMP DCOMP Chair: Bruce Gaulin, McMaster University Room: 707 |
Thursday, March 5, 2020 11:15AM - 11:27AM |
S41.00001: Scanned-probe study of spin pumping from Y3Fe5O12 into two-dimensional materials using ferromagnetic resonance force microscopy Guanzhong Wu, Dongying Wang, Ryan Muzzio, Yang Cheng, Guixin Cao, Kenji Watanabe, Takashi Taniguchi, Chun Ning Lau, Fengyuan Yang, Simranjeet Singh, Jyoti Katoch, Marc Bockrath, P Chris Hammel Driving a ferromagnet to resonance will generate spin current normal to the interface with an adjacent normal metal due to spin pumping. Spin angular momentum dissipation in normal metal will lead to increasing damping in the ferromagnet. This phenomenon is less understood in the case of the ferromagnet/two-dimensional materials interface. Graphene, the best-known two-dimensional material, has a spin diffusion length of order of 1um, making it a poor spin sink. However, there are reports of detectable spin pumping from Y3Fe5O12 (YIG) into CVD grown graphene that is attributed to large Rashba-Edelstein effect at the YIG/Graphene interface. Here we report a study of spin pumping in YIG/graphene heterostructures using local, force detected ferromagnetic resonance (FMR). This technique can detect FMR in an area of a few microns and, therefore, enables study of exfoliated two-dimensional materials, which have better crystal quality than CVD grown materials. We discuss spin pumping in pristine graphene as well as graphene/transition metal dichalcogenide bilayers studied using scanned ferromagnetic resonance force microscopy. We will also discuss the surprising discovery of a magnetic uniaxial anisotropy induced in a thin YIG film by the transition metal dichalcogenide overlayer. |
Thursday, March 5, 2020 11:27AM - 11:39AM |
S41.00002: Localized gap states in oxides and their anti-doping Oleksandr Malyi, Alex Zunger In standard unreactive doping, adding charge carrier to a compound results in a shift of the Fermi level towards the conduction band for electron doping and towards the valence band for hole doping. We point out a curious case of anti-doping where p-type (n-type) doping results in band gap opening, moving the previously occupied (unoccupied) bands to the principal conduction (valence) band and reducing conductivity. We find that this is a generic behavior for a class of materials and find inverse-design principles for detecting such materials. For instance, early transition metal oxides where the sum of composition-weighed formal oxidation states is positive (e.g., TiO2-x, CeO2-x, Ba2Ti6O13 and Ba4Ti12O27) tend to form in the band gap an intermediate trapped electron band localized on reduced cation orbitals. Upon p-type doping of such materials, hole annihilates a trapped electron on just one cation; consequently, electronically equivalent cation sublattices in the undoped compound become electronically distinct after doping, i.e., symmetry breaking. We give specific theoretical predictions for target compounds where hole and electron anti-doping might be observed experimentally for the first time. |
Thursday, March 5, 2020 11:39AM - 11:51AM |
S41.00003: Charge disorder induced magnetic order Jinning Hou, Yuting Tan, Wei Ku We propose a scenario of inducing magnetic order via charge disorder in systems with coexisting local moment and itinerant degrees of freedom. By disrupting quantum magnetic fluctuation originated by the itinerant carriers, the long-range magnetic order of the local moment can emerge. We demonstrate this mechanism using realistic spin-model for the undoped FeSe as an example and demonstrate the appearance of anti-ferromagnetic order with enough disruption. This mechanism provides a possible resolution to the observation in many strongly correlated materials, for example the ruthenates [1] in which clean non-magnetic samples turns magnetic with introduction of small number of impurities during synthesis. |
Thursday, March 5, 2020 11:51AM - 12:03PM |
S41.00004: Magnetic avalanches induced by magnetic sweep rate in a cluster glass Shalinee Chikara, Gia-Wei Chern, Neil Harrison, John Singleton, Leonardo Civale, John Mitchell, Vivien Zapf We study magnetic avalanches in La1-xSrxCoO3 and show that we can induce these avalanches for certain magnetic field sweep rates. La1-xSrxCoO3 with 0.1 ≤ x ≤ 0.15 are magnetic cluster glasses consisting of magnetic Co clusters seeded by Sr dopant atoms, surrounded by non-magnetic Co ions. For certain ranges of magnetic field sweep rates, large and random steps are observed in the magnetization, consistent with near-system-size magnetic avalanches. Beyond this range, magnetic field sweep rates produce smooth curves of magnetization vs magnetic field. We develop a model effectively showing that the magnetic interactions between the clusters are controlled by the magnetic field sweep rate. |
Thursday, March 5, 2020 12:03PM - 12:15PM |
S41.00005: The Role of Charge Doping in High Entropy Perovskite Oxides Alessandro Mazza, Yogesh Sharma, Elizabeth Skoropata, Wenrui Zhang, Thomas Heitmann, Timothy R Charlton, John William Freeland, Thomas Zac Ward Disorder is an important aspect of correlated quantum systems. As examples, it can be used to manipulate superconductivity, magnetic ordering, and enable scaling of non-fermi liquid responses. While synthesis of new quantum materials is generally focused on creating perfect crystals comprised of only a few elemental building blocks, we will present our recent efforts to create high quality single crystals with a high degree of configurational elemental disorder on sublattice sites. Complex crystal structures comprising two or more sublattices, such as those in the perovskite family, are particularly promising. This is due to the nearest neighbor cations on one configurationally disordered sublattice being tied together by an intermediate common and uniform anion sublattice. We will present our recent stabilization of single crystal epitaxial films of the ABO3 perovskite, La(Cr0.2Mn0.2Fe0.2Co0.2Ni0.2)O3. Hole doping the A site with Sr is shown to give access to unexpected functional responses, as demonstrated by transport, magnetometry, x-ray spectroscopy, and neutron studies. |
Thursday, March 5, 2020 12:15PM - 12:27PM |
S41.00006: Classification of materials with phonon angular momentum and microscopic origin of angular momentum Sinisa Coh We group materials into five symmetry classes and determine in which of these classes will phonons generically carry angular momentum. In some classes of materials, phonons acquire angular momentum via the forces induced by relative displacements of atoms out of their equilibrium positions. However, for other materials, such as ferromagnetic iron, phonon angular momentum arises from the forces induced by relative velocities of atoms. These latter effects are driven by the spin-orbit interaction. |
Thursday, March 5, 2020 12:27PM - 12:39PM |
S41.00007: The Role of Oxygen Vacancies in the Magnetic and Electronic Structure of La0.7Sr0.3MnO3 Zachary Romestan, A. C. Garcia-Castro, Xu He, Mikel Holcomb, Aldo H Romero Perovskite manganites host a variety of phenomena such as (anti)ferromagnetism, charge and orbital order, metal-insulator transitions, and colossal magnetoresistance that are highly sensitive to charge carrier density and crystal structure. As with cation substitution, oxygen vacancies can provide control over these parameters through electronic doping and steric effects. In our investigation, we consider the role of oxygen vacancies on the magnetic, electronic, and structural properties of La0.7Sr0.3MnO3-δ within the DFT+U formalism. In order to isolate the effects of the vacancies, we utilize the Virtual Crystal Approximation to treat the La/Sr content. We report the cases when δ = 0.125, 0.25, and 0.5. For each system, we consider all symmetry inequivalent vacancy sites. We optimize each structure to determine the energetically favorable vacancy sites and magnetic ordering. For the low energy structures, we determine the vacancy formation energy, electronic structure, magnetic structure, magnetic exchange coupling parameters, and conductivity. From the results, we correlate the calculated properties with the effects of the vacancies on the orbital character. |
Thursday, March 5, 2020 12:39PM - 12:51PM |
S41.00008: Insulating antiferromagnetism in VTe David Parker, Liurukara D Sanjeewa, Xiaoping Wang, Valentino Cooper, Yaohua Liu, Athena S. Sefat We report a comprehensive theoretical and experimental study on the vanadium monotelluride VTe, which crystallizes in the NiAs hexagonal structure. First principles calculations reveal a complex hierarchy of magnetic interactions and energy scales, with the ground state theoretically determined as an (1/2,0,1) antiferromagnetic ordering with insulating character and a band gap of 0.5 eV. These interactions vary significantly from a nearest-neighbor or extended Heisenberg-like model, suggesting a similarity to the parent state of the iron-based superconductors and the possibility of superconductivity if appropriately doped. Experimental synthesis and characterization efforts find a substantially off-stoichiometric orthorhombic structure (a defect NiAs structure) with composition V$_{0.85}$Te, and an apparent Neel point of some 45 K. Our first principles calculations find the stoichiometric phase VTe to have a negative Vanadium defect formation energy, thus explaining the formation of the off-stoichiometric phase. |
Thursday, March 5, 2020 12:51PM - 1:03PM |
S41.00009: On-chip terahertz characterization of the antiferromagnet CaFe2O4 Daniel Heligman, Rachel Resnick, Lunyong Zhang, Jae Wook Kim, Sang-Wook Cheong, Rolando Valdes Aguilar We report on the magnetic excitations of the antiferromagnet CaFe2O4 as measured with time-domain terahertz spectroscopy via an on-chip device. This material has been previously measured with a free-space terahertz apparatus, showing a magnon excitation at ~725 GHz below the material’s Neel temperature of 200 K. We have observed a possible first-order phase transition between 60 K and 120 K, indicating the coexistence of two different magnetic phases. For this experiment the material was measured with a device that generates a terahertz pulse confined to a transmission line. We report on the results of this experiment. |
Thursday, March 5, 2020 1:03PM - 1:15PM |
S41.00010: Ferrimagnetism in High Entropy Transition Metal Spinel Oxides Graham Johnstone, Alannah Hallas High entropy oxides (HEOs) have a crystal structure comprised of chemically ordered oxygen anions and an equimolar proportion of five metal cations randomly distributed over their sublattice. The imposition of large disorder allows configurational entropy to overcome the enthalpy of formation and stabilize a new high entropy phase. HEOs have potential applications ranging from ferroic multifunctionality to reversible energy storage. Studies of the magnetic properties of HEOs to date have been limited to (MgCoNiCuZn)0.2O, which has a rock-salt structure. This material undergoes a long-range antiferromagnetic ordering transition at TN=113K despite intense disorder and a 40% magnetic dilution. Here, we present the discovery of three 3d-transition metal based ferrimagnetic spinel type HEOs. The high entropy phase is confirmed by Rietveld refinement of powder x-ray diffraction data and elemental microprobe analysis. Magnetic susceptibility measurements of these HEOs suggest that all three transition into ferrimagnetic ordered states between T=300K and 400K, followed by a spin reorientation around 100K. In this talk we will present a robust characterization of their magnetic and electronic ground states via heat capacity and resistivity measurements. |
Thursday, March 5, 2020 1:15PM - 1:27PM |
S41.00011: Giant Electric Field Modulation of Magnetism in Ferrimagnetic Heusler Heterostructures QILONG SUN, Sohee Kwon, Nicholas Kioussis The demand for high efficient magnetoelectric random access memory (MeRAM) requires the search of novel materials and magnetic tunnel junction stacks with voltage-controlled magnetic anisotropy (VCMA) efficiency greater than the 1000 fJ/(Vm) challenge. In this talk, we will present predictions of ab initio electronic structure calculations of the VCMA of Ir/Mn3Ga/MgO heterostructures, in which the ultrathin ferrimagnetic Mn3Ga has the tetragonal DO22 structure. We find that the MnII-MnII- (MnI-Ga-) terminated interfaces exhibit large out-of-plane (in-plane) magnetic anisotropy (MA). More importantly, the calculations reveal colossal PMA and VCMA efficiency of about one order of magnitude higher than the values reported today. We demonstrate that both the MA and VCMA depend critically on the heavy metal thickness. We also predict a sign reversal of the VCMA efficiency from the MnII-MnII to the MnI-Ga-terminated interface. Our results show that the dominant contribution to both the PMA and VCMA arises from the strong spin-orbit-coupling of Ir and the E-field induced shift of the Ir d-derived orbitals. These findings provide useful guiding rules in the design of more energy-efficient ferrimagnetic-based MeRAM devices. |
Thursday, March 5, 2020 1:27PM - 1:39PM |
S41.00012: Hybrid Purification and Sampling Approach for Thermal Quantum Systems Jing Chen, Miles Stoudenmire We propose an algorithm combining two methods for studying finite-temperature quantum systems with tensor networks. One approach is the ancilla method, which gives high-precision results but scales poorly at low temperatures. The other is the minimally entangled typical thermal state (METTS) sampling algorithm which scales better than the ancilla method at low temperatures and can be parallelized, but requires many samples to converge. Our proposed hybrid of these two purifies physical sites in a small central spatial region with partner ancilla sites, sampling the remaining sites by the METTS. Observables measured within the purified cluster have much lower sample variance than in the METTS approach, while sampling the sites outside the cluster reduces their entanglement and the computational cost of the algorithm. The sampling steps of the algorithm remain straightforwardly parallelizable. The hybrid approach also solves an important technical issue with METTS that makes it difficult to benefit from quantum number conservation.By studying S=1 Heisenberg ladder systems, we find the hybrid method converges more quickly than both the ancilla and METTS algorithms at intermediate temperatures and for systems with higher entanglement. |
Thursday, March 5, 2020 1:39PM - 1:51PM |
S41.00013: Fermi surface investigation of intermetallic La2Pt3Ge5 Elizabeth Green, Johannes Klotz, Kathrin Goetze, Tobias Foerster, Marc Uhlarz, Alix McCollam, Jennifer Neu, Theo Siegrist, Tino Gottschall, Joachim Wosnitza, Kefeng Wang, Cedomir Petrovic Rare-earth intermetallic compounds, specifically those in the R2T3X5 family (where R is rare-earth, T is transition metal, and X is p-block element), have garnered interest due to their novel electronic and magnetic properties. Though they have been a topic of research for decades, detailed Fermi surface studies have been lacking. To fill this void, we performed de Haas-van Alphen effect measurements on La2Pt3Ge5, which was reported to have the highest critical temperature in the R2T3X5 family, Tc ~ 8.1 K [1], though the presence of superconductivity is a topic of debate [2]. Our results from a high-quality sample evidence several small pockets in the Fermi surface. Electronic band structure calculations and the implications of our results will be discussed. |
Thursday, March 5, 2020 1:51PM - 2:03PM |
S41.00014: High-field resonant torsion magnetometry as a probe of magnetic phases in honeycomb lattice Na2IrO3 Christopher Pocs, Ian Leahy, Peter Siegfried, Gang Cao, Arkady Shekhter, Ross McDonald, Minhyea Lee We probe magnetoanisotropy of the quasi-2d honeycomb antiferromagnet Na2IrO3 via resonant torsion magnetometry in pulsed fields up to 65T. In recent years, this material has received much experimental and theoretical attention as a candidate for realizing Kitaev spin-liquid physics. At low temperatures, Na2IrO3 has a robust zigzag AFM ground state and unambiguous signatures of any field-induced transition out of this state have remained elusive so far. Our analysis of the magnetoanisotropy reveals unusual field and angular dependence at high field and a possible signature of a field-driven transition to a quantum disordered state, providing strong evidence for a potential quantum spin liquid phase at high field. We will discuss how resonant torsion magnetometry can extract detailed thermodynamic information and show how it is used to map the entire magnetic free energy in real space from data at only a handful of low symmetry angles. These results are compared to the case of related Heisenberg Kitaev material RuCl3. |
Thursday, March 5, 2020 2:03PM - 2:15PM |
S41.00015: Quasi-2D magnon identification in antiferromagnetic FePS3 via magneto-Raman spectroscopy Jeffrey Simpson, Amber McCreary, Thuc Mai, Robert McMichael, Jason E Douglas, Nicholas Butch, Cindi Dennis, Angela Hight Walker, Rolando Valdes Aguilar The recent discovery that van der Waals-bonded magnetic materials retain long range magnetic ordering down to a single layer stimulates a thorough magneto-Raman study of one such material, FePS3, a large spin (S = 2) Mott insulator where the Fe atoms form a honeycomb lattice. Bulk FePS3 was shown to be a quasi-2D Ising antiferromagnet, with additional features in the Raman spectra emerging below the Neel temperature (TN ≈120 K). Using temperature and magnetic field dependent Raman spectroscopy as an optical probe of magnetic structure, we demonstrate that one of these Raman-active modes below TN is a magnon with a frequency of ≈3.7 THz (122 cm-1). Contrary to previous work, which interpreted this feature as a phonon, our Raman data shows the expected frequency shifting and splitting of the magnon as a function of temperature and magnetic field, respectively, were we find the g-factor ≈2. In addition, the symmetry behavior of the magnon is studied by polarization-dependent Raman spectroscopy and explained using the magnetic point group of FePS3. Temperature dependence of the Raman-active phonons will also be discussed. |
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