Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session R41: Chiral Interactions and Phase TransitionsFocus
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Sponsoring Units: GMAG DMP Chair: Hans Nembach, NIST Room: 707 |
Thursday, March 5, 2020 8:00AM - 8:36AM |
R41.00001: Magnetic textures with particle properties beyond skyrmions: chiral bobbers, globules, and hopfions Invited Speaker: Nikolai S. Kiselev Magnetic crystals with competing interactions allow the existence of localized magnetic textures possessing a lot of similarities to ordinary particles meaning that they can move and interact with each other: attract, repel, or mutually annihilate. In my presentation, I discuss a wide variety of different types of such textures existing in magnetic crystals. |
Thursday, March 5, 2020 8:36AM - 8:48AM |
R41.00002: Magnetic structures in 1D spin chains with long-range RKKY and Dzyaloshinskii-Moriya interactions Edson Vernek, Oscar Avalos-Ovando, Sergio E Ulloa Magnetic order in 1D magnetic chains has gained considerable interest in recent years. Absent in one dimensional systems with only short-range interaction (SRI), magnetic order is still possible if long-ange interaction (LRI) is present, without violating the celebrated Mermin-Wagner theorem. Indeed, interesting quantum phase transitions have been investigated in a Heisenberg chain with LRI [1]. We investigate the role of spin-orbit effects by considering spin-1/2 chains in the presence of both RKKY and Dzyaloshinskii-Moriya (DM) LRIs. Our results show an interestingly rich phase diagram in which the system transitions from collinear to transverse magnetic correlated order as the ratio between RKKY and DM coupling strength changes. We analyze the new ground states and explore their possible physical implementation in different systems. |
Thursday, March 5, 2020 8:48AM - 9:00AM |
R41.00003: Magnetic field induced spin liquids and quantum phase transitions in spin orbit coupled quantum magnets. Minseong Lee, Vivien Zapf, Haidong Zhou When an external magnetic field suppressed the magnetic ordering in spin-orbit coupled system, a new magnetic phase is often observed. For example, controversial theoretical and experimental results propose spin liquid behavior above in-plane 7 T where antiferromagnetic order is suppressed in a-RuCl3. Theoretical studies also show that out-of-plane magnetic field can induce spin liquid phase in this system as well. In this work, we present the analysis of inelastic neutron scattering data of a-RuCl3 and extracted g-factor and the Γ/J using linear spin wave theory. In addition, we present the out-of-plane pulsed field data of a-RuCl3 to study a new phases predicted by theoretical studies by drawing the full field vs temperature phase diagram. We also explore the how the magnetic field breaks the orbital assisted spin dimerized phase in one dimensional spin chains, and new magnetic phases are induced to discuss the role of the orbital degrees of freedom in rescale the Hund coupling and on-site interorbital Coulomb interactions in spin-orbit coupled systems. |
Thursday, March 5, 2020 9:00AM - 9:12AM |
R41.00004: Exchange Coupling Torque in Ferrimagnetic Bi-layer Systems at the Angular Momentum Compensation Temperature Robin Bläsing, Tianping Ma, See-Hun Yang, Chirag Garg, Stuart Parkin Within the last decade, the efficiency of current-induced motion of magnetic domain walls (DWs) has been enhanced tremendously by utilizing the exchange coupling torque (ECT) in synthetic antiferromagnets. In this talk we show that this mechanism also applies to ferrimagnetic layers consisting of a transition metal layer and a rare earth metal layer which couple antiferromagnetically. The motion is most efficient when the angular momenta of both layers compensate each other and the ECT is maximized. Additionally, we demonstrate that at angular momentum compensation a magnetic field, which is applied along the effective Dzyaloshinskii-Moriya interaction field, has no influence on the DW velocity. By changing the temperature of a Co/Gd ferrimagnetic bi-layer, we can to tune the ratio of angular momenta of Co and Gd and identify the angular momentum compensation temperature TA. Since the device temperature is significantly increased by the current pulses, taking into account Joule heating is of major importance. The presented findings can be used for the development of novel storage devices and improving their efficiency. |
Thursday, March 5, 2020 9:12AM - 9:24AM |
R41.00005: Spontaneous rotation of ferrimagnetism driven by antiferromagnetic spin canting Anuradha Vibhakar, Dmitry Khalyavin, Pascal Manuel, Alexei Belik, Roger Johnson Spin-reorientation phase transitions that involve the rotation of a crystal’s magnetisation have been well characterised in distorted perovskite oxides such as the orthoferrites. In these systems spin reorientation occurs due to competing rare earth and transition metal anisotropies coupled via f − d exchange. Here, we demonstrate an alternative paradigm for spin reorientation in distorted perovskites. We show that the R2CuMnMn4O12 (R = Y or Dy) triple A-site columnar ordered quadruple perovskites have three ordered magnetic phases and up to two spin reorientation phase transitions. Unlike the spin reorientation phenomena in other distorted perovskites, these transitions are independent of rare earth magnetism, but are instead driven by an instability towards AFM spin canting originating in frustrated Heisenberg exchange interactions, and the competition between Dzyaloshinskii-Moriya and single-ion anisotropies. |
Thursday, March 5, 2020 9:24AM - 9:36AM |
R41.00006: Using structural phase transitions to enhance the coercivity of ferromagnetic films Ryan Need, Joshua P Lauzier, Logan Sutton, Brian James Kirby, Jose De La Venta Heat-assisted magnetic recording (HAMR) is a promising magnetic information storage technology that uses a heating step to lower the coercivity of the recording media and decrease the energy of each writing operation. However, HAMR currently requires temperature increases of several hundred Kelvin, which can cause heat spreading and limit recording rates. Here, we describe a mechanism for tuning the coercivity of ferromagnetic films over small temperature ranges by coupling them to a layer that undergoes a structural phase transition. The method is demonstrated in Ni/FeRh bilayers where Ni layers were deposited either above or below the FeRh metamagnetic transition at 370 K. When the Ni layer is grown at high temperatures, the 1 % FeRh lattice expansion alters the Ni’s crystallographic texture and leads to a 500 % increase in coercivity upon cooling through the FeRh’s metamagnetic transition. Analysis suggests this effect is due to domain wall pinning across grain boundaries with different orientations and strain states. Our work highlights a means to increase the thermal coercivity response of ferromagnetic materials through structural coupling to underlying films, which could enable simplified heatsink designs and expand the selection of materials compatible with HAMR. |
Thursday, March 5, 2020 9:36AM - 9:48AM |
R41.00007: Parity-Time Symmetric Spintronics and Spin Cavitronics Yunshan Cao, Huanhuan Yang, Chen Wang, Tianlin Yu, Peng Yan Non-Hermitian Hamiltonian obeying parity-time (PT) symmetry can exhibit real spectra and a spontaneous symmetry breaking accompanied by a real-to-complex spectral phase transition at the ‘exceptional point (EP)’. Here, we predict that a ferromagnet with gain (loss) is equivalent to an antiferromagnet with an equal amount of loss (gain), and show a ferromagnet to antiferromagnet phase transition in PT-symmetric magnetic bilayers when the balanced gain-loss parameter exceeds a critical value. We then generate the idea to a spin cavitronic device that allows the strong coupling between photons and magnons. We demonstrate a “Z”-shape transmission spectrum in the exact PT phase. The spectrum evolves to a step function when the polariton touches the third-order exceptional point. The estimated magnetic sensitivity can approach 10-15 T Hz-1/2 around the third-order EP. Our work paves the way for investigating the parity-time symmetry in spintronics and spin cavitronics, and for designing ultrasensitive magnetometers. |
Thursday, March 5, 2020 9:48AM - 10:00AM |
R41.00008: Ultrafast Control of Charge Density and Spin Density waves in Chromium Louis Ponet, Oleg Gorobtsov, Andrej Singer, Sergey Artyukhin Experimental advances in ultrafast physics have allowed to monitor structural and electronic processes and even phase transitions on their natural timescales. Here we model recent experiments on ultrafast control of spin density wave phase in elemental Chromium with a sequence of optical pulses. The strain wave and CDW, induced by the spin density modulation via exchange striction, are monitored using x-ray diffraction. Results show order parameter oscillations and a partial melting of the SDW in response to optical pulses. Interestingly, depending on the exact delay between two sequential optical pulses, one can increase or decrease the oscillation amplitude, allowing for optimal control. We use Landau theory and heat transfer equations to describe the dynamics of the interacting charge and spin density waves. All details of the experiment are replicated to a high degree by the model. |
Thursday, March 5, 2020 10:00AM - 10:12AM |
R41.00009: Realizing corner states in artificial crystals based on topological spin textures Peng Yan, Zhixiong Li, Yunshan Cao, Xiang Rong Wang The recent discovery of higher-order topological insulators (HOTIs) has significantly extended our understanding of topological phases of matter. Here, we predict that second-order corner states can emerge in the dipolar-coupled dynamics of topological spin textures in two-dimensional artificial crystals. Taking a breathing honeycomb lattice of magnetic vortices as an example, we derive the full phase diagram of collective vortex gyrations and identify three types of corner states that have not been discovered before. We show that the topological "zero-energy" corner modes are protected by a generalized chiral symmetry in the sexpartite lattice, leading to particular robustness against disorder and defects, although the conventional chiral symmetry of bipartite lattices is absent. We propose the use of the quantized Z_6 Berry phase to characterize the nontrivial topology. Full micromagnetic simulations confirm the theoretical predictions with good agreement. Our findings open up a promising route for realizing higher-order topologically protected corner states in magnetic systems and finally achieving topological spintronic memory and computing. |
Thursday, March 5, 2020 10:12AM - 10:24AM |
R41.00010: 3D Topological solitons in chiral magnets Robert Voinescu, Ivan I Smalyukh, Jung-Shen Tai Three dimensional knotted solitons are continuous field configurations classified by a Hopf topological invariant. These quasi-particles are known to arise in many laboratory systems and more exotic settings ranging from theories in particle physics to cosmology. By exploiting the external magnetic field and crystalline anisotropy couplings in a micromagnetic hamiltonian we numerically model a new class of 3D topological solitons embedded in a helical background. We determine the parameter region of stability afforded by the non-trivial twisting and windings present in our particle-like excitation. Furthermore, we analyze individual topological solitons and an ensemble of such in the bulk with multiparticle simulations to determine their interaction and self-assembly properties. Finally, we discuss how such topological excitations may find uses in racetrack magnetic memory devices and spintronics applications. |
Thursday, March 5, 2020 10:24AM - 10:36AM |
R41.00011: Unprecedented Electronic Structure, Magnetism, and Anisotropy in Rare Earth Intermetallics Durga Paudyal, Renu Choudhary, Ralph Skomski We investigate magnetic rare earth intermetallics with quantum properties. Many quantum features of rare earth intermetallics are consistent with the past research, but some of the findings are seemingly contradictory. The disagreement is not caused by numerical errors or accidental mistakes but reflects how the many-electron nature of the rare-earth 4f electrons is interpreted by crystal-field and local spin density approximation with onsite electron correlation and spin orbit coupling theories [Das et. al, PRB 100, 024419 (2019)]. The two-sublattice crystal-field theory describes a broad variety of physical properties exhibited by rare earth transition metals, such as the temperature dependence of magnetization and anisotropy, but it is not a first-principles approach. By contrast, our first-principles approach yields a substantial orbital-moment quenching, which violates Hund's rules and is contradictory to conventional knowledge accumulated over decades of rare-earth research. Rationalizing the orbital-moment quenching in terms of the dependence of the 4f charge distribution on the magnetization angle, we argue that medium- and long-run future research will be necessary to reconcile experiment, sublattice models, and first-principle calculations in rare earth intermetallics. |
Thursday, March 5, 2020 10:36AM - 10:48AM |
R41.00012: Nanoscale Spatial Dependence of Photoinduced Structural Phase Transition in FeRh Youngjun Ahn, Mathew J. Cherukara, Zhonghou Cai, Donald A Walko, Michael Bartlein, Tao Zhou, Jan-Ulrich Thiele, Eric Fullerton, Martin Holt, Paul G. Evans, Haidan Wen Simultaneous temporal and spatial characterization of materials with high resolution provides multidimensional data essential for a deeper understanding of nanoscale heterogeneities that arise from complex interactions during solid-solid phase transitions.Time-resolved hard x-ray microscopy has unique capabilities to provide a direct structural probe. Here, using newly developed x-ray diffraction microscopy with unprecedented 100 ps and 30 nm resolution at the Advanced Photon Source, we have observed the nanoscale dependence of the structural relaxation dynamics following ultrafast laser excitation of an FeRh thin film. The spatial dependence is linked to variations in spatially inhomogeneous strain, resulting in different thresholds for driving the antiferromagnetic-ferromagnetic phase transition. In addition, we have found the spatially different degree of the broadening of transition in temperature near nano-island regions. The observed relationship between lattice parameter and transition threshold gives new insight on the photoinduced phase transition at the nanoscale and has important implications in heat-assisted magnetic recording. |
Thursday, March 5, 2020 10:48AM - 11:00AM |
R41.00013: Electric-field control of the interlayer exchange coupling Shehrin Sayed, Cheng-Hsiang Hsu, Niklas Roschewsky, See-Hun Yang, Sayeef Salahuddin Magnetization switching with electric-field is of great current interest to achieve high density and low energy non-volatile memory devices. We propose a new mechanism for electric-field controlled magnetization switching, assisted solely by the interlayer exchange coupling (IEC) between the fixed and the free magnets. Using non-equilibrium Green’s function (NEGF) method, we show that the resonant tunneling mechanism can tune the IEC with an electric-field, which in turn can switch the free magnet to have either a parallel or antiparallel configuration with respect to the fixed magnet, depending on the sign of the IEC. Such bi-directional switching can be achieved with the same voltage polarity but different magnitudes. Due to the 'conservative' nature of the exerted torque by IEC, the switching threshold is decoupled from the speed, while the conventional spin-torque devices exhibit a trade-off due to the 'non-conservative' nature of the exerted torque. |
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