Conference Programme

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Overview
Session
S-05: Topological Insulators & Permanent Magnets
Time:
Tuesday, 20/Jun/2017:
4:00pm - 6:15pm

Session Chair: Xiufeng Han, Institute of Physics, Chinese Academy of Sciences (CAS)
Location: Rm 326

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Presentations
4:00pm - 4:30pm
Invited

Topological Insulator-based Magnetic Heterostructures

J. Raynien KWO1, Chinan WU1, Yuchi LIU1, Yuting Jason FANCHIANG2, Shangrong Roy YANG1, K. S. Mandy CHEN1, C. Mark TSENG1, Minghwei HONG2, Shang fan LEE3

1Department of Physics, National Tsing Hua University, Taiwan; 2Department of Physics, National Taiwan University, Taiwan; 3Institute of Physics, Academia Sinica, Taiwan

The emergent quantum materials such as topological insulators (TIs) host topologically protected surface states, with dissipationless transport prohibiting backscattering. Strong spin-orbit coupling along with time reversal symmetry ensures that the electrons on surface states are spin-momentum locked, hence implying efficient spin-charge interconversion for spintronic devices. Recently, we have undertaken comprehensive investigations on magnetic heterostructures based on 3D TIs. High quality TI films of Bi2Se3, Bi2Te3, and (BiSb)2Te3 have been grown by van der Waals epitaxy. The current shunting problem previously encountered in TI/FM heterostructures were circumvented via the growth of TI/FI ferrimagnetic insulator Y3Fe5O12 (YIG) thin films with very low damping constant. Further, we showed that the direction of magnetic anisotropy can be altered from in-plane to perpendicular when adopting the TmIG films. Importantly, our capabilities of interfacial control on the atomic scale and the implementation of electrical field gating have enabled systematic studies to correlate the structural and chemical properties of the FI/TI interface with the spin-charge conversion (SCC) efficiency, thus to elucidate the underlying coupling mechanism. In this talk the results of spin pumping and spin transfer torque will be presented, along with low temperature magnetoresistance measurement of anomalous Hall effect (AHE) to address the proximity effect in the vicinity of the TI/FI interface.


4:30pm - 5:00pm
Invited

Role of Bulk and Surface Conductions on Spin-Orbit Torque in Topological Insulators

Yi WANG, Dapeng ZHU, Hyunsoo YANG

National University of Singapore, Singapore

Bi2Se3 is expected to exhibit a high spin orbit torque (SOT) efficiency due to the spin-textured topological surface states. However, very few studies have reported the SOT phenomena in ferromagnet/TI by electrical measurements. Here we investigate the SOT efficiency in ferromagnet/Bi2Se3 structures at different temperatures and thicknesses by two different techniques such as spin torque ferromagnetic resonance (ST-FMR) and spin pumping.

In the ST-FMR measurements on Co40Fe40B20 (5 nm)/Bi2Se3 (20 nm), where the charge currents directly flow through devices, we find that the SOTs show a steep increase as the temperature decreases to 50 K, and the SOT efficiency increases abruptly from 0.047 at 300 K to 0.42 below 50 K. Moreover, we observe a significant out-of-plane SOT efficiency in the low temperature range. By decreasing the Bi2Se3 thickness, we observe an enhancement of the spin-orbit torque efficiency by more than 5 times below 8 quintuple layers and the spin-orbit torque efficiency reaches a maximum value of ~1.75 at 5 quintuple layers at room temperature. Our results illustrate the roles of bulk and surface conductions in Bi2Se3 on the spin-orbit torque efficiency of Bi2Se3/CoFeB, suggesting that the thinner topological insulators possess a strong capability for generating spin currents at room temperature.

We also report the SOT efficiency and spin diffusion length at different temperatures in Ni81Fe19/Bi2Se3 by the spin pumping technique, where no direct charge current flows thru the device. The SOT efficiency of Bi2Se3 is found to be ~0.01 at room temperature. In addition, the spin diffusion length in Bi2Se3 is evaluated to be 6.2 nm at room temperature. Both the SOT efficiency and spin diffusion length in Bi2Se3 increase at low temperatures. We will discuss possible reasons for the difference between the SOT efficiency from ST-FMR and that from spin pumping measurements.


5:00pm - 5:15pm
Oral

New Perspectives for the Large Rashaba Splitting in Ag/ Bi2Se3 Heterostructure

Aizhu WANG, Shuyuan SHI, Lei SHEN, Hyunsoo YANG, Yuan Ping FENG

National University of Singapore, Singapore

Inspired by discovering that three-dimensional (3D) topological insulator (TI) bismuth selenide (Bi2Se3) exhibits a high charge to spin current conversion efficiency due to the existence of the topological insulator surface states (TSSs), similar phenomena induced by Rashba splitting in Bi2Se3 also have drawn considerable interest. However, the generating and dominating behaviors of the above transports remain unclear. Using first-principles calculations, we have determined that the manipulated Rashaba splitting in the systems of Ag/Bi2Se3 heterostructure mainly comes from the TI layer neighbored by Ag layer, and the calculated splitting is proportional to the thickness of Ag layers within a certain range. At the same time, the significant interaction between TI and Ag lifts the energy degeneracy of the TSSs at the surface and interface of TI. Interesting, we demonstrated that the contributions of the TI TSSs to the transport are lower than those from the TI Rashba splitting. Our calculations were confirmed by experimental results, suggesting that the Ag/TI could be promising candidates as highly efficient spin current sources for exploring the next generation of spintronic applications.


5:15pm - 5:30pm
Oral

Mechanism of Microwave Based Chemical Synthesis of Nd-Fe-Co-B Magnetic Nanoparticles

Xiao TAN1, Harshida PARMAR2, Yaoying ZHONG1, Varun CHAUDHARY2, Raju V. RAMANUJAN1

1School of Materials Science and Engineering, Nanyang Technological University, Singapore; 2Rolls-Royce@NTU Corporate Lab, Nanyang Technological University, Singapore

Modern high energy product permanent magnets, such as those based on the Nd-Fe-B alloy composition, are of high interest in the context of cutting-edge electrical machines, electric vehicles, wind turbines etc. The enhanced magnetic properties of these hard magnets are highly influenced by the nanostructure of the magnet. Therefore, further development of nanostructured hard magnetic materials requires the study of novel synthesis techniques to produce unique nanostructures. In this work, microwave assisted combustion, followed by reduction diffusion, was found to be a successful technique to synthesize Nd-Fe-Co-B magnetic nanoparticles. Instead of using costly elemental rare earth metal, this technique uses low cost metal nitrates to greatly reduce the production cost. Nd-Fe-Co-B mixed oxide nanoparticles were prepared by a microwave assisted combustion technique using Fe(NO3)3, Nd(NO3)3, Co(NO3)2, boric acid and glycine. The oxides were subsequently reduced by CaH2 through the reduction diffusion process to form Nd2(FeCo)14B. The mechanism of oxide formation and Nd2(FeCo)14B alloy formation were studied for studied for a range of process parameters, including microwave power. The effect of these process parameters on the ignition time, crystal size and magnetic properties was determined and the formation mechanism of the oxide powders and Nd2(FeCo)14B was identified.


5:30pm - 5:45pm
Oral

Mechanochemical Synthesis of Cr Alloyed Nd2(Fe,Co)14B Permanent Magnets

Varun CHAUDHARY1,2, Yaoying ZHONG2, Harshida PARMAR1,2, Xiao TAN2, Raju V RAMANUJAN2

1Rolls-Royce@NTU Corporate Lab, Nanyang Technological University, Singapore; 2School of Materials Science and Engineering, Nanyang Technological University, Singapore

Nd-Fe-B magnets which have the highest energy product, are the pillars for several cutting-edge applications, including advanced electrical machines. The traditional synthesis routes involve multi-step processes, high temperature reactions as well as expensive and hazardous starting materials. Instead, we report an environmentally benign and cost effective method of production of Cr alloyed Nd2(Fe,Co)14B magnetic nanoparticles using dry mechanochemical process. The process includes the grinding of oxide powders, unlike the processes of elemental melting or production from solutions at high temperatures. Mechanochemical synthesis involves the reduction of mixed oxides of Nd, Fe, Co, B and Cr using Ca in an Ar atmosphere. The effect of Cr content on Nd2(Fe,Co)14B magnetic nanoparticles was investigated in terms of structural and magnetic properties. The process parameters involved in the mechanochemical reaction, e.g., milling speed, milling time, role of dispersant (CaO) and their effects on the magnetic properties will also be discussed. Our method to produce Cr alloyed Nd2(Fe,Co)14B magnetic nanoparticles is solvent free, simple, efficient and could processing for industrial applications.


5:45pm - 6:00pm
Oral

Study of Kinetics and Energy Transfer during Mechano-chemical Synthesize of Nd2(Fe,Co)14B Nanoparticles

Yaoying ZHONG1, Varun CHAUDHARY2, Harshida PARMAR2, Xiao TAN1, Raju V. RAMANUJAN1

1School of Materials Science and Engineering, Nanyang Technological University, Singapore; 2Rolls-Royce@NTU Corporate Lab, Nanyang Technological University, Singapore

With increasing demand of high performance permanent magnets in energy generation and conversion systems, the quest to develop and understand novel processing routes to produce better permanent magnets becomes more urgent. Nd-Fe-B based permanent magnets, due to their superior magnetic properties, have attracted intensive attention. In this alloy, cobalt substitution of iron can effectively increase the Curie temperature. However, conventional physical synthesis methods for this alloy are usually associated with high cost, and novel chemical synthesis methods usually have limited scalability. We report a novel, cost-effective and scalable mechanochemical process for the synthesis of high coercivity Nd2(Fe,Co)14B particles. This process involves the reduction of neodymium oxide, iron oxide, cobalt oxide and boron anhydride in the presence of calcium. The transformation mechanism during this mechanochemical process was investigated to allow deeper understanding of the mechanochemical process as well as to facilitate optimization of the process parameters. The reduction kinetics of the milling process were experimentally studied by determining the change in the concentration of reactants and products as a function of milling time. The impact energy of the mechanical milling can be used to activate the chemical reactions plays a role analogous to that of the thermal energy in the conventional thermally activated process, a theoretical model was proposed to explain the kinetics of the mechanically assisted reduction process. Our experimental data fits well with the theoretical model. These results will be elucidated in the presentation.


6:00pm - 6:15pm
Oral

Chemically Synthesized Exchange Coupled Nd2Fe14B/α-Fe Hard Magnetic Nanoparticles and Mechanism

Harshida PARMAR1,2, Xiao TAN2, V. CHAUDHARY1,2, Yaoying ZHONG2, R.V. RAMANUJAN2

1Rolls-Royce@NTU Corporate Lab, Nanyang Technological University, Singapore; 2School of Materials Science and Engineering, Nanyang Technological University, Singapore

Modern high energy product permanent magnets are based on nanostructured magnetic materials. Exchange-coupled hard/soft nanocomposite magnets have been predicted toexhibit remarkably high maximum energy product (BH)max values. However, such high (BH)max values have not yet been achieved. The main reason is the large gap between the ideal and actual nanostructures. To achieve the ideal nanostructure novel synthesis techniques are required, such as chemical synthesis. Microwave assisted combustion followed by reduction diffusion was found to be a successful technique to synthesize Nd-Fe-B based magnetic nanoparticles. Nd-Fe-B mixed oxide nanoparticles were prepared using Fe(NO3)3, Nd(NO3)3, boric acid and glycine by a microwave assisted combustion approach. The oxides were subsequently reduced by CaH2 through reduction diffusion process to form Nd2Fe14B. The formation mechanism was studied for a range of process parameters. The formation of Nd2Fe14B during reduction diffusion was investigated by differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The reduction of Fe2O3 to α-Fe and the reduction of NdFeO3 to Fe and Nd2O3 occurs at ~ 350°C, while reduction of Nd2O3 to NdH2 occurs at ~ 700°C. The desired Nd2Fe14B phase was formed at 800°C and a holding time of 2 h. The removal of CaO byproduct was studied. It was found that the CaO byproduct was removed with NH4Cl (dissolved in methanol). The magnetization doubled after CaO removal. These results of the chemical synthesis of Nd2FezB will be described in this presentation.


6:15pm - 6:30pm
Oral

Magnetic Properties of Ni Doped Ferrihydrite Nanoparticles

Chandni RANA, S D TIWARI

Thapar University, India

Magnetic nanoparticles have gained much interest in recent years, in different disciplines of science and engineering, due to their novel behaviour and important applications. Superparamagnetism is one such unique behaviour exhibited only by small particles of magnetic materials. Superparamagnetic behaviour of ferro and ferrimagnetic nanoparticles are well studied. However, there have not been much work on magnetic behaviour antiferromagnetic nanoparticles. Ferrihydrite is a nanocrystalline material and known to be antiferromagnetic in nature. Its Néel transition temperature is about 350 K. This system is found in many environmental and biological systems. Pure phase of this material can also be synthesized by rapid hydrolysis of a solution containing Fe3+ ions. It is classified as two lines and six lines ferrihydrite, depending on number of peaks in x-ray diffraction pattern.

In this work, undoped and two nickel doped (5 and 10 atomic percent) samples of two line ferrihydrite nanoparticles are synthesized. Structural characterization of these samples is done with x-ray diffraction and transmission electron microscopy. The average crystallite size and average particle size for all samples are found to be about 2 nm. Magnetization of samples as function of temperature (10 - 300 K) and applied magnetic field (0 - 5 T) are measured using a SQUID magnetometer. We study the effect of Ni doping on blocking temperature, particle magnetic moment and anisotropy constant of ferrihydrite nanoparticles. These parameters are found to decrease with increasing concentration of Ni doping. Possible reason for the observed behaviour is discussed in details.



 
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