Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session Y40: Disordered and Novel Low Dimensional Magnetic MaterialsFocus Live

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Sponsoring Units: GMAG DMP Chair: Clarina Dela Cruz, Oak Ridge National Lab 
Friday, March 19, 2021 11:30AM  11:42AM Live 
Y40.00001: Effect of exchangebond randomization in a highly twodimensional quantum Heisenberg antiferromagnet Paul Goddard, Fan Xiao, William Blackmore, Benjamin Michael Huddart, Matjaz Gomišek, Thomas J Hicken, Chris Baines, Peter J Baker, Francis L Pratt, Stephen Blundell, Helen Lu, John Singleton, Dariusz Gawryluk, Mark Turnbull, Karl W Krämer, Tom Lancaster We present an investigation of the effect of randomizing exchange coupling strengths in the S = 1/2 square lattice quasitwodimensional quantum Heisenberg antiferromagnet (QHAF) (QuinH)_{2}Cu(Cl_{x}Br_{1−x})_{4}●2H_{2}O (QuinH = Quinolinium, C_{9}H_{8}N^{+}), with 0 ≤ x ≤ 1. Pulsedfield magnetization allows us to estimate an effective inplane exchange strength in a regime where exchange fosters shortrange order, while the temperature at which longrange order occurs (T_{N}) is found using muonspin relaxation, allowing us to construct a phase diagram for the series. We evaluate the effectiveness of disorder in suppressing the T_{N} and the ordered moment size and find an extended disordered phase in the region 0.4 ≤ x ≤ 0.8 where no magnetic order occurs. The observed critical substitution levels are accounted for by an energeticsbased competition between different local magnetic orders. We demonstrate experimentally that the groundstate disorder is driven by quantum effects of the exchange randomness, a feature that has been predicted theoretically and has implications for other disordered quasitwodimensional QHAFs. 
Friday, March 19, 2021 11:42AM  11:54AM Live 
Y40.00002: Phase Boundary of Diluted Hexaferrites Near the Magnetic Percolation Transition Gaurav Khairnar, Cameron J. Lerch, Thomas Vojta The magnetic phase boundary of diluted hexagonal ferrites, specifically, PbF_{12−x}Ga_{x}O_{19} has attracted attention recently because of its unusual shape [Phys. Rev. 96, 020407 (2017)]. The critical temperature as a function of dilution varies as T_{c} ∝ (1 − x/x_{c})^{2/3} over the entire concentration range from x = 0 to x_{c}, where x_{c} is very close to the percolation threshold of the lattice. To explain this behavior we employ a classical percolation scenario. We perform largescale Monte Carlo simulations to determine the magnetic phase boundary of diluted XY and Heisenberg models on simple cubic and hexaferrite lattices. Our findings are twofold. (i)In the asymptotic critical region close to x_{c}, the phase boundary follows T_{c} ∼ (1 − x/x_{c })^{φ} with φ = 1.09(2) in agreement with the percolation theory. (ii) For smaller dilutions, φ takes smaller values but the simulation does not reproduce the 2/3 power law. We also discuss the effects of additional weak couplings which frustrate the ferrimagnetic order on the shape of the phase boundary. 
Friday, March 19, 2021 11:54AM  12:06PM Live 
Y40.00003: Demonstrating the AffleckKennedyLiebTasaki spectral gap on 2D degree3 lattices TzuChieh Wei, Nicholas Pomata We report results on solving a long outstanding problem—whether the twodimensional spin3/2 antiferromagnetic valencebond model of Affleck, Kennedy, Lieb, and Tasaki (AKLT) possesses a nonzero gap above its ground state. We exploit a relation between the anticommutator and sum of two projectors and apply it to groundspace projectors on regions of the honeycomb lattice. After analytically reducing the complexity of the resultant problem, we are able to use a standard Lanczos method to establish the existence of a nonzero gap. This approach is also successfully applied to spin3/2 AKLT models on other degree3 semiregular tilings, namely, the squareoctagon, star, and cross lattices, where the complexity is low enough that exact diagonalization can be used instead. In addition, we close the previously open cases of hybrid AKLT models on the singly decorated honeycomb and singly decorated square lattices. 
Friday, March 19, 2021 12:06PM  12:18PM Live 
Y40.00004: The AKLT models on the singly decorated diamond lattice and two degree4 planar lattices are gapped Wenhan Guo, Nicholas Pomata, TzuChieh Wei Recently various 2D AKLT models have been shown to be gapped, including the one on the hexagonal lattice. Here we report on a nontrivial 3D AKLT model which consists of spin2 entities on the diamond lattice sites and one single spin1 entity between every neighboring spin2 site. Although the nonzero gap problem for the uniformly spin2 AKLT models on the diamond and square lattices is still open, we are able to establish the existence of the gap for two planar lattices, which we call the inscribed square lattice and the triangleoctagon lattice, respectively. So far, these latter two models are the only two uniformly spin2 AKLT models that have a provable nonzero gap above the ground state. We also discuss some attempts in proving the gap existence on both the square and kagome lattices. In addition, we show that if one can solve a finitesize problem of a weighted AKLT Hamiltonian and if the gap is larger than certain threshold, then the model on the square lattice is gapped in the thermodynamic limit. The threshold of the gap scales inversely with the linear size of the finitesize problem. 
Friday, March 19, 2021 12:18PM  12:30PM Live 
Y40.00005: Structural, magnetic and electrontransport properties of Ti_{2}MnAl Heusler alloys Zachary J Lehmann, Jace Waybright, Bishnu Dahal, Yung Huh, Paul Michael Shand, Pavel Lukashev, Parashu Kharel Magnetic materials with a high degree of spin polarization have attracted much attention due to their potential applications in spintransportbased devices. In addition, roomtemperature halfmetals with a small or no magnetic moments are beneficial for nanoelectronic devices due to the absence of stray magnetic field, which otherwise interferes with the neighboring magnetic elements. We have synthesized one such predicted halfmetallic alloy Ti_{2}MnAl in cubic Heusler structure with B2type disorder using arcmelting and annealing. The samples show very small magnetic moment with the Curie temperature of about 305 K. In this presentation, we will also discuss the electrontransport properties of the synthesized alloys and a theoretical investigation on the effect of disorder in the magnetic properties of Ti_{2}MnAl. 
Friday, March 19, 2021 12:30PM  12:42PM Live 
Y40.00006: Quenching of magnetic moments due to defect states hybridization in V doped WS_{2} Boyang Zheng, Yuanxi Wang, Vincent Henry Crespi Dilute magnetic semiconductors, achieved through substitutional doping of spinpolarized transition metals into semiconducting systems, enable experimental modulation of spin dynamics in ways that hold great promise for novel magnetoelectric or magnetooptical devices. A roomtemperature 2D dilute magnetic semiconductor can be achieved by doping vanadium into WS_{2}^{[1]}. Ferromagnetism peaks at an intermediate vanadium concentration of a few atomic percent and decreases for higher concentrations. The experimentally observed nonmonotonicity of the magnetization as a function of the doping level can be explained by hybridization between dopant defect states that quenches the magnetic moments when they approach too closely. An effective Zeeman shift at valleys in this system corresponds to an external magnetic field of ~100 T. 
Friday, March 19, 2021 12:42PM  12:54PM Live 
Y40.00007: Effect of Doping on the Physical Properties of La_{5}Co_{2}Ge_{3} Atreyee Das, Scott M Saunders, Elena Gati, Tyler Slade, Sergey Budko, Paul C Canfield The ternary crystal structure R_{5}Co_{2}Ge_{3} was discovered in 2017 [1]. When R is moment bearing, the lowtemperature magnetic state(s) can be complex. For nonmoment bearing R=La, the situation is simpler and intriguing; La_{5}Co_{2}Ge_{3} is a small moment, itinerant ferromagnetic compound having a transition temperature of 3.8K and an effective moment of 0.1µ_{B} [2]. In order to better understand the nature and possible tunability of this ferromagnetic transition, we report the substitution of Co with Ni, Fe and Rh as well as Ge with Si. Single crystals of La_{5}(Co_{1x}A_{x})_{2}Ge_{3} (A=Ni, Fe, Rh) and La_{5}Co_{2}(Ge_{1x}Si_{x})_{3} were grown and characterized by magnetic and transport measurements. A comparison of the transition temperature, magnetic and transport properties will be discussed. 
Friday, March 19, 2021 12:54PM  1:06PM Live 
Y40.00008: Thermodynamic and Transport Properties of RCuAs_{2} Single Crystals (R = Y, LaNd, Sm, GdYb) David Evans, Eundeok Mun The RCuAs_{2} (R = rareearth) compounds crystallize in HfCuSi_{2}type tetragonal structure and compounds with moment bearing Rions order antiferromagnetically at low temperatures. In particular, resistivity curves on polycrystal samples for R = Sm, Gd, Tb, and Dy show an unexpected resistivity minimum above their magnetic ordering temperatures [1]. To study the origin of the minimum with negligible 4fhybridization, single crystals of RCuAs_{2} have been grown and characterized by XRD, specific heat, resistivity, and magnetization measurements. Magnetization measurements for this family clearly indicate local moment behavior with a large degree of anisotropy between H  ab and H  c due to the CEF effect. All physical property measurements clearly indicate antiferromagnetic ordering at low temperatures for R = Sm  Yb. Although the resistivity value of single crystals is significantly smaller than that for polycrystalline samples, resistivity data for R = Sm, Gd, Tb, and Dy exhibit a pronounced minimum above their magnetic ordering temperatures. In this talk we will present physical property measurements for this family and discuss the possible origin of the resistivity minimum by considering the Kondo and RKKY interactions. 
Friday, March 19, 2021 1:06PM  1:18PM Not Participating 
Y40.00009: Colossal Magnetoresistance in a Layered Phosphide EuCd_{2}P_{2} ZhiCheng Wang, Jared Rogers, Xiaohan Yao, Renee J Nichols, Kemal Atay, Philip James Ryan, Fazel Tafti Colossal magnetoresistance (CMR) is a sudden drop in the electrical resistance in response to an external magnetic field – a desirable effect for magnetic sensing and recording technologies. The established paradigm of CMR is based on the manganese oxide materials (perovskite manganates) where a mixed valence of Mn^{3+}/Mn^{4+} leads to a ferromagnetic (FM) doubleexchange (DE) interaction and a dynamical JahnTeller (JT) distortion, which cooperatively induce CMR. Prior efforts to find different materials or mechanisms for CMR resulted in a much smaller effect. Here we show an enormous CMR at low temperatures in EuCd_{2}P_{2} without manganese, oxygen, mixed valence, or cubic perovskite structure. EuCd_{2}P_{2} has a layered trigonal lattice and exhibits Atype antiferromagnetic ordering at 11 K. The magnitude of CMR (10^{4} percent) in asgrown crystals of EuCd_{2}P_{2} rivals the magnitude in optimized thin films of manganates. Our magnetization, transport, and synchrotron Xray data suggest that strong magnetic fluctuations are responsible for this phenomenon. The realization of CMR at low temperatures without heterovalency leads to a new regime for materials and technologies related to antiferromagnetic spintronics. 
Friday, March 19, 2021 1:18PM  1:30PM Live 
Y40.00010: Disorder and Itinerant Magnetism in Full Heusler Pd2TiIn Guanhua Qin, Wei Ren, David Singh We report the electronic and magnetic properties of full Heusler Pd2TiIn based on first principles 
Friday, March 19, 2021 1:30PM  1:42PM Live 
Y40.00011: Magnetic Correlations Close to Quantum Critical Point in the Disordered Ferromagnetic Alloys NiV Observed with SANS Hind Adawi, Shiva Bhattarai, Adane Gebretsadik, Jean Guy L Lussier, Almut Schroeder, Kathryn Lynn Krycka We present a smallangle neutron scattering (SANS) analysis of the ferromagnetic (FM) alloys Ni_{1x}V_{x }where the FM order is destroyed towards a paramagnetic phase by sufficient substitution of Ni by V. The critical temperature T_{c }vanishes at x_{c}=0.116 indicating a quantum critical point (QCP) with signatures of disorder [1]. We performed polarized SANS experiments on different Ni_{1x}V_{x }polycrystalline samples close to x_{c }at NG7SANS, NCNR, NIST utilizing a ^{3}He cell within the wave vector regime Q=0.061nm^{1}. We could resolve magnetic scattering at different length scales within the FM state well below T_{c}<50K [2]. We find isotropic magnetic shortrange correlations as well as anisotropic magnetic contributions that reveal large scale magnetic domains and an indication of local magnetic defects that relates to the V concentration. We show how these different contributions respond to high magnetic fields and evolve with V concentration x, characterizing a disordered FM alloy, close to the QCP. 
Friday, March 19, 2021 1:42PM  1:54PM Live 
Y40.00012: FirstPrinciples calculations of disorder in MnSbTe systems Swarnava ghosh, Markus Eisenbach We present first principles calculations of MnSbTe using the LocallySelf consistent Multiple Scattering (LSMS) framework, a real space first principles code for scalable first principles density functional theory calculations of materials. Simulations of disordered systems at realistic concentrations require large cells with thousands of atoms. Traditional first principles methods scale cubically with the number of atoms and is computationally prohibitive for large system sizes. This talk will present a brief overview of the realspace formalism of LSMS, which allows firstprinciples calculations of O(100,000) atoms. Next, we will present results of the influence of disorder on the energetics and magnetic properties of MnSbTe systems. 
Friday, March 19, 2021 1:54PM  2:06PM On Demand 
Y40.00013: Influence of mixing the lowvalent transition metal atoms Y = Sc, Ti, V, Cr, Mn and Fe on the properties of possible halfmetallic Heusler compounds Co_{1.5}Y_{0.5}FeSi Rabin Mahat, Shambhu K KC, Upama Karki, Sudhir Regmi, JiaYan Law, Victorino Franco, Iosif Galanakis, Arunava Gupta, Patrick LeClair The realization of physical systems whose electronic and magnetic properties can be easily manipulated by playing with valence electrons are very promising from both the fundamental and applied perspectives. Halfmetallic ferromagnets with high T_{c} are ideal candidates for spintronic applications. Co based Heusler alloys have recently attracted great interest because most of these alloys exhibit above mentioned criteria. Here, we present a combined experimental and theoretical study of quaternary Heusler alloys Co_{1.5}Y_{0.5}FeSi (Y = Sc,Ti,V,Cr,Mn,&Fe) to get a global overview of the electronic, magnetic and mechanical properties. All single phase alloys exhibit fcc crystal structure with a strong tendency towards L2_{1} ordering. The lowtemperature saturation magnetic moments agree fairly well with our theoretical results and also obey the SlaterPauling rule, a prerequisite for half metallicity^{1}. All alloys are soft ferromagnets with high T_{c}, allowing for applications at room temperature and above. Relatively high mechanical hardness values are also observed^{1,2,3}. 
Friday, March 19, 2021 2:06PM  2:18PM On Demand 
Y40.00014: Localized Spinwave Edge Modes in a Tapered V[TCNE]_{x~2} Thinfilm Kwangyul Hu, Michael Flatté In magnonics, many researches have been conducted to study spinwaves in a thin film that has a perfect rectangular form. However, it is not always possible to achieve such an ideal rectangular structure depending on patterning techniques. For instance, a V[TCNE]_{x~2 }thinfilm deposited by using a CVD method has taper around its edges. In this presentation, we investigate and report characteristics of dipole interaction induced spinwave modes in a thinfilm of a trapezoidal crosssection. We used a micromagnetic simulation program; MUMAX3[1] to simulate spinwave spectra and corresponding mode profiles of the thinfilm. For material parameters, we referred V[TCNE]_{x~2} which is a emerging magnetic material that has low damping and narrow linewidth [2]. Our results provide understanding of how tapered edges influence spinwaves and suggest possible applications of localied edge modes. 
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