Conference Programme

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S-03: Memory & Topological Insulators
Tuesday, 20/Jun/2017:
10:30am - 12:00pm

Session Chair: Yihong Wu, National University of Singapore
Location: Rm 326

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10:30am - 11:00am

Magnetism-Induced Massive Dirac Spectra and Topological Spin Textures in the Surface State of Magnetic Topological Insulators

Nai-Chang YEH1, Chien-Chang CHEN1, Marcus L. TEAGUE1, Liang HE2, Xufeng KOU2, Murong LANG2, Kang L. WANG2

1California Institute of Technology, United States; 2University of California, Los Angeles, United States

Proximity-induced magnetic effects on the surface Dirac spectra of topological insulators are investigated by scanning tunneling spectroscopic (STS) studies of MBE-grown bilayer structures consisting of undoped Bi2Se3 on top of (Bi1-xCrx)2Se3.[1] For thickness of the top Bi2Se3 layer equal to or smaller than 3 quintuple layers, a spatially inhomogeneous surface spectral gap Δ opens up below a characteristic temperature Tc2D, which is much higher than the bulk Curie temperature Tc3D determined from the anomalous Hall resistance. The mean value and spatial homogeneity of Δ increase with increasing c-axis magnetic field (H) and increasing Cr doping level (x), suggesting that the gap is due to proximity-induced c-axis ferromagnetism, whereas the temperature (T) dependence of Δ is non-monotonic, showing initial increase below Tc2D followed by a “dip” and then reaching maximum at T << Tc3D. These phenomena may be attributed to proximity magnetism induced by two types of contributions with different temperature dependences: a three-dimensional (3D) contribution from the bulk magnetism that dominates at low T, and a two-dimensional (2D) contribution associated with the RKKY interactions mediated by surface Dirac fermions, which dominates at Tc3D << T < Tc2D.[1] In addition, spatially localized sharp resonant spectra are found along the boundaries of gapped and gapless regions.[1] These spectral resonances are long-lived at H = 0, with their occurrences being most prominent near Tc2D and becoming suppressed under c-axis magnetic fields. We attribute the spectral resonances to magnetic impurity-induced topological spin textures of the surface Dirac fermions, which are in stark contrast to the nearly T-independent non-magnetic impurity resonances found in undoped Bi2Se3.[2] The long-term stability of these topologically protected two-level states may find potential applications to quantum information technology.


[1] C.-C. Chen et al., New J. Phys. 17, 113042 (2015).

[2] M. L. Teague et al., Solid State Commun. 152, 747 (2012).

11:00am - 11:30am

Initialization-free Multilevel States Driven by Spin-orbit Torque Switching

Chih-Huang LAI, Kuo-Feng HUANG, Hsiu-Hau LIN

National Tsing Hua University, Taiwan

Among non-volatile memories, MRAM is one of the most promising candidates for its excellent endurance and speed. In recent developments, the so-called spin-orbit torques (SOT) has been proposed as an alternative writing mechanism for the next generation MRAM. Because the writing and reading procedures in the 3-terminal design are separated, SOT-MRAM possesses better endurance for massive writing cycles and fewer disturbances on the stored bits for reliable reading. However, the 3-terminal setup of the SOT-MRAM limits its smallest cell size. To reduce the effective cell size, multilevel storage has been developed in various non-volatile memories. When the reported multilevel states are realized, an extra initialization process is essentially needed, which prolongs write latency and wastes power consumption.

Here we demonstrate how multilevel storage can be achieved in Co/Pt multilayers without extra initialization steps. By stacking Co/Pt repeatedly to N layers, the magnetic property can be tailored with large SOT as in the Co/Pd multilayers. It is appealing that the SOT switching behavior strongly depends on the number of repeated Co/Pt layers. For N=1, the applied electric pulses cause sharp binary switching. When the layer number increases to N=4, multi-domain formation becomes energetically favorable and the initial magnetization dissolves into the demagnetized state upon applying electric pulses. For intermediate layer number N=2, the multi-domain configurations settle into multilevel states with different net magnetizations, well controlled by modulating the electric pulses and completely independent of its initial states. We perform the read/write test on the N=2 Co/Pt multilayers and demonstrate four well-separated multilevel states without initialization steps. Moreover, by generalizing the modified Landau-Lifshitz-Gilbert equation to the multi-domain configurations, the theoretical predictions provide nice agreement with the experimental findings, rendering the SOT switching mechanism in Co/Pt multilayers well understood. The SOT switching mechanism we discovered here paves a promising avenue for further developments.

11:30am - 11:45am

Pressure Induced Topological Phase Transition and Spin-Orbit Coupling Tuning in Layered Bi2S3

Yongzheng LUO1, Ming YANG2,3, Lei SHEN4, Yuan Ping FENG1,3

1Department of Physics, National University of Singapore, Singapore; 2Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), Singapore; 3Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, Singapore; 4Engineering Science Programme, National University of Singapore, Singapore

Modulating nontrivially topological states in trivial materials is of both scientific and technological interest. Inspired by the robustness to scattering of conducting edge states in quantum Hall systems, the searching of strong three-dimensional (3D) topological insulators (TIs) has successfully given rise to the Bi2Se3 family of compounds, which share a rhombohedral structure containing blocks of quintuple layers. In this study, via hybrid density functional theory based first-principles calculations, we first predict that Bi2S3, the lightest bismuth-based chalcogenide binary component, can also be realized in a similar rhombohedral structure, and it becomes a TI by a moderate compressive pressure of about 5.3 GPa with spin-orbit coupling (SOC). Interestingly, the inverted band gap is able to be tuned larger than 0.4 eV with an experimentally accessible pressure. Simultaneously, the surface states and Z2 invariants are calculated with maximally localized Wannier functions. The theoretical analyses presented here demonstrate that the topological quantum phase transition between a non-trivial phase and a trivial insulating/metallic phase can be realized by pressure, which also shed light on searching new TIs with large band gap by the tuning of SOC.

11:45am - 12:00pm

Structural Anisotropy and High Energy Product in FePt-Fe3Pt Hard-Soft Magnet Nano-Composites from ab Initio Calculations

Bheema Lingam CHITTARI1, Vijay KUMAR2,3

1Department of Physics, University of Seoul, Korea; 2Dr. Vijay Kumar Foundation, India; 3Center for Informatics, School of Natural Sciences, Shiv Nadar University, India

We report results of ab initio calculations on FePt-Fe3Pt hard-soft magnet matrix-nanoparticle composites using a supercell approach. It is found that the FePt matrix induces tetragonal anisotropy in L12 Fe3Pt nanoparticles when embedded in L10 FePt matrix. The FePt matrix and the Fe3Pt nanoparticles have the same easy axis. The magnetic moments on Fe and Pt atoms are enhanced in both the matrix as well as the nanoparticles compared with the values in FePt hard magnet even though there is a reduction in Pt contents. The enhanced magnetic moments and reduced volume in these nano-composites lead to high maximum energy product of 71.8 MGOe which is much higher than the calculated value of 46.8 MGOe for bulk FePt hard magnet. A systematic investigation of the structure and magnetic properties of these nano-composites reveals charge transfer from Fe to Pt, quantum confinement, and interface effects to play an important role.

12:00pm - 12:15pm

Spin Phonon Coupling and Magnetic Excitations in Spin Orbit Coupled Irridates: A Raman Spectroscopic Study

Dileep Kumar MISRA1,2

1Materials Science Program, Indian Institute of Technology, Kanpur, India; 2Department of Condensed Matter Physics and Materials Science (DCMP and MS), Tata Institute of Fundamental Research, Mumbai, India

In strongly correlated electronic systems, 5d- irridates are recent focus of research interest. Presence of strong spin-orbit coupling in irridates give rise to novel quantum states of matter termed as topological insulators and quantum spin liquids. The both topological phases have promise applications in spintronics and quantum computing [1] that led to flurry of intense research investigations for potential applications and fundamental physics point of view. Sr2IrO4 is a spin orbit Coupled Mott Insulator, possesses unconventional electronic and magnetic ground states. Raman Scattering is an extremely sensitive local probe to explore unconventional electronic and magnetic states.

Here, Raman scattering study under extreme temperature condition on the high quality Sr2IrO4 polycrystalline pallet is presented. The low temperature Raman measurements were carried out upto high wave numbers that reveals the existence of two magnon excitations in Sr2IrO4 which is dramatically presented well above the TN. Strong Fano assymetry is observed in some of the Raman modes which indicates towards presence of electronic or psuedospin excitation in the paramagnetic state of the system, however the psuedospin excitation are not quenched in magnetic ordered states as observed in single crystalline form of the system [2]. Spin phonon coupling is also evidenced in magnetic ordered state, the magnetic exchange interaction constant (J) has been calculated using this data.


[1] C. Nayak, A. Stern, M. Freedman, and S. Das Sarma, Rev. Mod. Phys. 80, 1083 (2008).

[2] H. Gretarsson, N. H. Sung, M. Hoeppner, B. J. Kim, B. Keimer, M. Le Tacon , Phys. Rev. Lett. 116, 136401 (2016).

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