Matching Items (6)

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Spin Transport in Metallic Films with Strong Spin-Orbital Coupling

Description

In a pure spin current, electrons of opposite spins flow in opposite directions, thus information is conveyed by spin current without any charge current. This process almost causes no power

In a pure spin current, electrons of opposite spins flow in opposite directions, thus information is conveyed by spin current without any charge current. This process almost causes no power consumption, which has the potential to realize ultra-low-power-consumption electronics. Recently, thermal effects in magnetic materials have attracted a great deal of attention because of its potential to generate pure spin currents using a thermal gradient (∇T), such as the spin Seebeck effect. However, unlike electric potential, the exact thermal gradient direction is experimentally difficult to control, which has already caused misinterpretation of the thermal effects in Py and Py/Pt films. In this work, we show that a well-defined ∇T can be created by two thermoelectric coolers (TECs) based on Peltier effect. The ∇T as well as its sign can be accurately controlled by the driven voltage on the TECs. Using a square-wave driven potential, thermal effects of a few μV can be measured. Using this technique, we have measured the anomalous Nernst effect in magnetic Co/Py and Py/Pt layers and determined their angular dependence. The angular dependence shows the same symmetry as the anomalous Hall effect in these films.
This work has been carried out under the guidance of the author’s thesis advisor, Professor Tingyong Chen.

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Date Created
  • 2015-05

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Growth and characterization of novel thin films for microelectronic applications

Description

I studied the properties of novel Co2FeAl0.5Si0.5 (CFAS), ZnGeAs2, and FeS2 (pyrite) thin films for microelectronic applications ranging from spintronic to photovoltaic. CFAS is a half metal with theoretical spin

I studied the properties of novel Co2FeAl0.5Si0.5 (CFAS), ZnGeAs2, and FeS2 (pyrite) thin films for microelectronic applications ranging from spintronic to photovoltaic. CFAS is a half metal with theoretical spin polarization of 100%. I investigated its potential as a spin injector, for spintronic applications, by studying the critical steps involved in the injection of spin polarized electron populations from tunnel junctions containing CFAS electrodes. Epitaxial CFAS thin films with L21 structure and saturation magnetizations of over 1200 emu/cm3 were produced by optimization of the sputtering growth conditions. Point contact Andreev reflection measurements show that the spin polarization at the CFAS electrode surface exceeds 70%. Analyses of the electrical properties of tunnel junctions with a superconducting Pb counter-electrode indicate that transport through native Al oxide barriers is mostly from direct tunneling, while that through the native CFAS oxide barriers is not. ZnGeAs2 is a semiconductor comprised of only inexpensive and earth-abundant elements. The electronic structure and defect properties are similar in many ways to GaAs. Thus, in theory, efficient solar cells could be made with ZnGeAs2 if similar quality material to that of GaAs could be produced. To understand the thermochemistry and determine the rate limiting steps of ZnGeAs2 thin-film synthesis, the (a) thermal decomposition rate and (b) elemental composition and deposition rate of films were measured. It is concluded that the ZnGeAs2 thin film synthesis is a metastable process with an activation energy of 1.08±0.05 eV for the kinetically-limited decomposition rate and an evaporation coefficient of ~10-3. The thermochemical analysis presented here can be used to predict optimal conditions of ZnGeAs2 physical vapor deposition and thermal processing. Pyrite (FeS2) is another semiconductor that has tremendous potential for use in photovoltaic applications if high quality materials could be made. Here, I present the layer-by-layer growth of single-phase pyrite thin-films on heated substrates using sequential evaporation of Fe under high-vacuum followed by sulfidation at S pressures between 1 mTorr and 1 Torr. High-resolution transmission electron microscopy reveals high-quality, defect-free pyrite grains were produces by this method. It is demonstrated that epitaxial pyrite layer was produced on natural pyrite substrates with this method.

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Date Created
  • 2013

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Magnetic field sensing and nanoparticle induced ferromagnetism in graphene towards spintronics

Description

Graphene has been extensively researched for both scientific and technological interests since its first isolation from graphite. The excellent transport properties and long spin diffusion length of graphene make

Graphene has been extensively researched for both scientific and technological interests since its first isolation from graphite. The excellent transport properties and long spin diffusion length of graphene make it a promising material for electronic and spintronic device applications. This dissertation deals with the optimization of magnetic field sensing in graphene and the realization of nanoparticle induced ferromagnetism in graphene towards spintronic device applications.

Graphene has been used as a channel material for magnetic sensors demonstrating the potential for very high sensitivities, especially for Hall sensors, due to its extremely high mobility and low carrier concentration. However, the two-carrier nature of graphene near the charge neutrality point (CNP) causes a nonlinearity issue for graphene Hall sensors, which limits useful operating ranges and has not been fully studied. In this dissertation, a two-channel model was used to describe the transport of graphene near the CNP. The model was carefully validated by experiments and then was used to explore the optimization of graphene sensor performance by tuning the gate operating bias under realistic constraints on linearity and power dissipation.

The manipulation of spin in graphene that is desired for spintronic applications is limited by its weak spin-orbit coupling (SOC). Proximity induced ferromagnetism (PIFM) from an adjacent ferromagnetic insulator (FMI) provides a method for enhancing SOC in graphene without degrading its transport properties. However, suitable FMIs are uncommon and difficult to integrate with graphene. In this dissertation, PIFM in graphene from an adjacent Fe3O4 magnetic nanoparticle (MNP) array was demonstrated for the first time. Observation of the anomalous Hall effect (AHE) in the device structures provided the signature of PIFM. Comparison of the test samples with different control samples conclusively proved that exchange interaction at the MNP/graphene interface was responsible for the observed characteristics. The PIFM in graphene was shown to persist at room temperature and to be gate-tunable, which are desirable features for electrically controlled spintronic device applications.

The observation of PIFM in the MNP/graphene devices indicates that the spin transfer torque (STT) from spin-polarized current in the graphene can interact with the magnetization of the MNPs. If there is sufficient STT, spin torque oscillation (STO) could be realized in this structure. In this dissertation, three methods were employed to search for signatures of STO in the devices. STO was not observed in our devices, most likely due to the weak spin-polarization for current injected from conventional ferromagnetic contacts to graphene. Calculation indicates that graphene should provide sufficient spin-polarized current for exciting STO in optimized structures that miniaturize the device area and utilize optimized tunnel-barrier contacts for improved spin injection.

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Date Created
  • 2019

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Andreev reflection spectroscopy: theory and experiment

Description

A theoretical study of a three-dimensional (3D) N/S interface with arbitrary spin

polarization and interface geometry is presented. The 3D model gives the same intrinsic

spin polarization and superconducting gap dependence as

A theoretical study of a three-dimensional (3D) N/S interface with arbitrary spin

polarization and interface geometry is presented. The 3D model gives the same intrinsic

spin polarization and superconducting gap dependence as the 1D model. This

demonstrates that the 1D model can be use to t 3D data.

Using this model, a Heusler alloy is investigated. Andreev reflection measurements

show that the spin polarization is 80% in samples sputtered on unheated MgO(100)

substrates and annealed at high temperatures. However, the spin polarization is

considerably smaller in samples deposited on heated substrates.

Ferromagnetic FexSi􀀀x alloys have been proposed as potential spin injectors into

silicon with a substantial spin polarization. Andreev Reflection Spectroscopy (ARS) is

utilized to determine the spin polarization of both amorphous and crystalline Fe65Si35

alloys. The amorphous phase has a significantly higher spin polarization than that of

the crystalline phase.

In this thesis, (1111) Fe SmO0:82F0:18FeAs and Pb superconductors are used to

measure the spin polarization of a highly spin-polarized material, La0:67Sr0:33MnO3.

Both materials yield the same intrinsic spin polarization, therefore, Fe-superconductors

can be used in ARS. Based on the behavior of the differential conductance for highly

spin polarized LSMO and small polarization of Au, it can be concluded that the Fe-Sc

is not a triplet superconductor.

Zero bias anomaly (ZBA), in point contact Andreev reflection (PCAR), has been

utilized as a characteristic feature to reveal many novel physics. Complexities at a

normal metal/superconducting interface often cause nonessential ZBA-like features,

which may be mistaken as ZBA. In this work, it is shown that an extrinsic ZBA,

which is due to the contact resistance, cannot be suppressed by a highly spin-polarized

current while a nonessential ZBA cannot be affected the contact resistance.

Finally, Cu/Cu multilayer GMR structures were fabricated and the GMR% measured

at 300 K and 4.5 K gave responses of 63% and 115% respectively. Not only

do the GMR structures have a large enhancement of resistance, but by applying an

external magnetic eld it is shown that, unlike most materials, the spin polarization

can be tuned to values of 0.386 to 0.415 from H = 0 kOe to H = 15 kOe.

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Created

Date Created
  • 2015

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Monte Carlo studies of electron transport in semiconductor nanostructures

Description

ABSTRACT An Ensemble Monte Carlo (EMC) computer code has been developed to simulate, semi-classically, spin-dependent electron transport in quasi two-dimensional (2D) III-V semiconductors. The code accounts for both three-dimensional (3D)

ABSTRACT An Ensemble Monte Carlo (EMC) computer code has been developed to simulate, semi-classically, spin-dependent electron transport in quasi two-dimensional (2D) III-V semiconductors. The code accounts for both three-dimensional (3D) and quasi-2D transport, utilizing either 3D or 2D scattering mechanisms, as appropriate. Phonon, alloy, interface roughness, and impurity scattering mechanisms are included, accounting for the Pauli Exclusion Principle via a rejection algorithm. The 2D carrier states are calculated via a self-consistent 1D Schrödinger-3D-Poisson solution in which the charge distribution of the 2D carriers in the quantization direction is taken as the spatial distribution of the squared envelope functions within the Hartree approximation. The wavefunctions, subband energies, and 2D scattering rates are updated periodically by solving a series of 1D Schrödinger wave equations (SWE) over the real-space domain of the device at fixed time intervals. The electrostatic potential is updated by periodically solving the 3D Poisson equation. Spin-polarized transport is modeled via a spin density-matrix formalism that accounts for D'yakanov-Perel (DP) scattering. Also, the code allows for the easy inclusion of additional scattering mechanisms and structural modifications to devices. As an application of the simulator, the current voltage characteristics of an InGaAs/InAlAs HEMT are simulated, corresponding to nanoscale III-V HEMTs currently being fabricated by Intel Corporation. The comparative effects of various scattering parameters, material properties and structural attributes are investigated and compared with experiments where reasonable agreement is obtained. The spatial evolution of spin-polarized carriers in prototypical Spin Field Effect Transistor (SpinFET) devices is then simulated. Studies of the spin coherence times in quasi-2D structures is first investigated and compared to experimental results. It is found that the simulated spin coherence times for GaAs structures are in reasonable agreement with experiment. The SpinFET structure studied is a scaled-down version of the InGaAs/InAlAs HEMT discussed in this work, in which spin-polarized carriers are injected at the source, and the coherence length is studied as a function of gate voltage via the Rashba effect.

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Agent

Created

Date Created
  • 2011

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Tuning anomalous Hall effect and spin polarized current in magnetic ultrathin films

Description

In this dissertation I studied the anomalous Hall effect in MgO/Permalloy/Nonmagnetic Metal(NM) based structure, spin polarized current in YIG/Pt based thin films and the origin of the perpendicular magnetic anisotropy(PMA)

In this dissertation I studied the anomalous Hall effect in MgO/Permalloy/Nonmagnetic Metal(NM) based structure, spin polarized current in YIG/Pt based thin films and the origin of the perpendicular magnetic anisotropy(PMA) in the Ru/Co/Ru based structures.

The anomalous Hall effect is the observation of a nonzero voltage difference across a magnetic material transverse to the current that flows through the material and the external magnetic field. Unlike the ordinary Hall effect which is observed in nonmagnetic metals, the anomalous Hall effect is only observed in magnetic materials and is orders of magnitude larger than the ordinary Hall effect. Unlike quantum anomalous Hall effect which only works in low temperature and extremely large magnetic field, anomalous Hall effect can be measured at room temperature under a relatively small magnetic field. This allows the anomalous Hall effect to have great potential applications in spintronics and be a good characterization tool for ferromagnetic materials especially materials that have perpendicular magnetic anisotropy(PMA).

In my research, it is observed that a polarity change of the Hall resistance in the MgO/Permalloy/NM structure can be obtained when certain nonmagnetic metal is used as the capping layer while no polarity change is observed when some other metal is used as the capping layer. This allows us to tune the polarity of the anomalous Hall effect by changing the thickness of a component of the structure. My conclusion is that an intrinsic mechanism from Berry curvature plays an important role in the sign of anomalous Hall resistivity in the MgO/Py/HM structures. Surface and interfacial scattering also make substantial contribution to the measured Hall resistivity.

Spin polarization(P) is one of the key concepts in spintronics and is defined as the difference in the spin up and spin down electron population near the Fermi level of a conductor. It has great applications in the spintronics field such as the creation of spin transfer torques, magnetic tunnel junction(MTJ), spintronic logic devices.

In my research, spin polarization is measured on platinum layers grown on a YIG layer. Platinum is a nonmagnetic metal with strong spin orbit coupling which intrinsically has zero spin polarization. Nontrivial spin polarization measured by ARS is observed in the Pt layer when it is grown on YIG ferromagnetic insulator. This result is contrary to the zero spin polarization in the Pt layer when it is grown directly on SiO2 substrate. Magnetic proximity effect and spin current pumping from YIG into Pt is proposed as the reason of the nontrivial spin polarization induced in Pt. An even higher spin polarization in the Pt layer is observed when an ultrathin NiO layer or Cu layer is inserted between Pt and YIG which blocks the proximity effect. The spin polarization in the NiO inserted sample shows temperature dependence. This demonstrates that the spin current transmission is further enhanced in ultrathin NiO layers through magnon and spin fluctuations.

Perpendicular Magnetic Anisotropy(PMA) has important applications in spintronics and magnetic storage. In the last chapter, I study the origin of PMA in one of the structures that shows PMA: Ru/Co/Ru. By measuring the ARS curve while changing the magnetic field orientation, the origin of the PMA in this structure is determined to be the strain induced by lattice mismatch.

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Created

Date Created
  • 2018