Theses of Graduated Students

Mohammad Javad Kamali Ashtiani

Mohammad Javad Kamali's Thesis

Magnetic Coupling of spin valve nanolayers with rough interfaces
Nowadays there is a close connection between scientifc researches and nanotechnology application. according to the increasing development of electronic facilities in two recent decays, a new branch of science and technology has been born that named nanomagnetism and Spintronics provides fundamental changes in magnetic memories and magnetic recording industry. 50 billion dollars has been spent for science and technology on magnetic materials in data storage feld and 800 percent growth of data volumes till 5 next years, so data storage industry is very important. Spintronics scientists attempt to fnd new methods for calculating and data storing systems equivalent with electronics circuits with using of regulation that govern on interaction and changing in electron spin. Of our interests is sandwich magnetic sensors or “reading heads” that can store data in Nano-bit scale like spinvalves in magnetic media include ferromagnetic nanolayers thin flms. In this research different types of magnetic coupling for spinvalves structure (two ferromagnetic nanolayers that separated by a nonmagnetic nanolayer) in abbreviation (FM/NM/FM) has been studied. then magnetostatic coupling arising from surface roughness that has particular importance discussed. Luis Neel (Physics Nobel prize at 1970) investigated this kind of coupling for sinusoidal interfaces already. That is famous in Orange peel coupling, However, with inspiration of statistical nature of rough surfaces and roughness characteristics the Neel problem has been reviewed and extended. this thesis investigated the roughness effect on spinvalve structure. And assumed that the interface between ferromagnetic nanolayers and nonmagnetic nanolayers has self-afne roughness. With applying FFT and some complicated calculation on this rough surfaces and assuming a cross correlation between two interface, the component of coupling tensor has been calculated. In this trend showed that if interfaces of between nonmagnetic layer and two magnetic layer be in phase then antiferromagnetic coupling and if out of phase then ferromagnetic coupling cause minimum the energy in system. Also at investigating correlation length and roughness exponent observed in particular region, there is a maximum of coupling. This maximum due to so many magnetic charges and cause more stray feld as expected. Additionally, this result can be a beginning point for other similar structures. It is necessary to say that this research can be used at designing and manufacturing of some facilities like spinvalves. Keywords: Magnetic Copuling, GMR, Spinvalve,Surface Roughness

Mohamad Hamdi

Mohammad Hamdi's Thesis

Perturbative Solution to Interfacial Dzyaloshinskii-Moriya Interaction in Ultrathin Magnetic Films with Effective Hamiltonian Approach
The origin of the Dzyaloshinskii-Moriya Interaction in antiferromagnetic insulating systems and ferromagnetic alloys of B20 group ( such as FeGe, MnSi ) has been investigated. Although the existence of this interaction at the interface of a heavy metal layer and a ferromagnetic metal layer has been proved, a complete model for describing the effective mechanisms of observed behaviours remains unexplored. In this thesis, we propose a model to describe the possible mechanisms of existance of this interaction in Ferromagnetic metal/Heavy metal bilayer systems based on the effective Hamiltonian theory. For this purpose, first we review the effects of DMI on magnonic and transport properties of Ferromagnetic/Heavy metal bilayer systems and magnetic skyrmions in these nanostructures. We represent the Bloch wave functions of each layer based on tight biding model. Then we calculate the total Hamiltonian of bilayer structure taking Bloch wave functions as basis. Considering hybridization between two layers, we obtain the effective Hamiltonian of the magnetic layer in presence of the nonmagnetic layer. Based on this effective Hamiltonian approach, the suggested mechanisms of DMI is as below: The hybridization of the two layers at the interface causes mixing of the Bloch states with different spins in two layers. Therefore the spin-orbit interaction is induced by a Bloch state in the nonmagnetic layer on a electron in a Bloch state of magnetic layer. Calculating effective Wannier functions using Fourier transformation of effective Bloch wave functions of magnetic layer and implementing Moriya’s formulation, the interfacial Dzyaloshinskii-Moriya interaction constant can be calculated. Keywords: Spintronics, Dzyaloshinskii-Moriya Interaction, Spin-Orbit Coupling, Skyrmion, Spin Transport, Magnetization Dynamics

Ehsan Faridi

Ehsan Faridi's Thesis

The effect of surface plasmon at magnetic and magneto-optical properties of magnetic nano structure
The magneto-optic Kerr effect (MOKE), discovered by John Kerr in 1877 as the polarization change of reflected light from the polished pole of an electromagnet, is a common and well-known technique to understand surface magnetic properties of materials and thin films. Surface plasmon polariton as charge oscillation in materials and thin films is recently found to be interesting in the field of surface physics, photonics and material science and is called plasmonics. This effect can localize the light e.g. at the boundary of metal and dielectric, to see for example enhancing the MOKE signal from magnetic layers. Therefore, nanostructures with combined magnetic and plasmonic functionalities have become an active topic. By an adequate internal architecture of the constituting components, the magneto-optical activity of such structures can be greatly increased due to the electromagnetic field enhancement associated with the plasmon resonance. In this thesis we employed the transfer matrix method to calculate the electromagnetic field in the layered media and optimized the thickness of the layers for enhancement the magneto-optical activity. In this thesis we measured the longitudinal kerr signal for Glass/Au/Co/Au and Si/Au/Py/Ir-Mn And Glass/Au/Py/Ir-Mn then by comparing the experimental data with the theoretical calculations we obtained the Ir-Mn refractive index.

Hossein Falizkaran Yazdi

Hossein Falizkaran Yazdi's Thesis

Micromagntic simulation Magnetic droplet soliton in spin valve with perpendicular magnetic anisotropy
we report on the dynamic and nucleation of a specific solitonic spin waves, known as magnetic droplet solitons in spin valve structures with perpendicular magnetic anisotropy. Numerical simulation performed based on magneto dynamic equation with droplet nucleation boundaries taken into account using MuMax framework. Simulation results show magnetic droplet solitons formation in the frequency lower than ferromagnetic resonance frequency, which is well consistent with previously reported theoretical works. Our results are capable of implementation in applied spintronics and magnonics such as magnonic communication systems.

Niusha Vesali

Niusha Vesali's Thesis

Fabrication metallic and magnetic flexible remote control microrobats kits
Nowadays investigation of various structures in nano and micro scale has attracted attention among biophysicists. This is due to the increasing demand for controllable nano/microstructures which have various capabilities to performing special missions in the body such as drug delivery. The drug delivery through these microrobats has advantages such as reducing side effects and damage to healthy tissues. Recently, inspired by bacterias, a variety of micro-swimmers have been proposed for biological application, which can move in high-adhesion liquid medias such as blood in a human vein. The propulsion of microrobats is provided by ultrasonic, electric and magnetic fields which can carry drugs. Making microrobats is very challenging with its complications. A magnetic microswimmer consists of a magnetic head and a flexible tail. There are different methods for making microswimmers and one of them is the chemical electrodeposition in aluminum, polycarbonate and silicon templates in nano and micro scales. In this research, we fabricated copper/nickel two-segments flexible magnetic microswimmer by electrodeposition process in porous silicon (silicon template). There are many manufacturing methods for construction an array of microspores of silicon and we used the anodizing process for simplicity and low costs. The adjustability of shapes, depth and diameter size of pores during an anodize, make us able to prepare them in appropriate shape for their applications. Porous silicon has many applications in photonics, medicine, solar cells, sensors and etc, which has been well received by researchers and can be used in various fields of technology. We used SEM, XRD, EDX, and VSM to characterize the prepared microwires. Finally, we examined different types of microswimmers stimulation mechanisms, in particular, magnetic stimulation, various magnetic fields, and production mechanisms. Also we studied the hydrodynamics of microorganisms in a Newtonian fluid with a low Reynolds number.

Milad Mohammadbeigi

Milad Mohammadbeigi's Thesis

Synthesis and Optical and electrical characterization of two-dimensional layers MoS2
A great contribution to advances in nanotechnology has been due to the advancement of two-dimensional materials in recent years. Meanwhile, twodimensional semiconductors have attracted the most attention, due to their many applications in electronic, optic and optoelectronics devices. Of the most widely used two-dimensional semiconductors, one can refer to the transition metal dichalcogenides with the general formula MX2. The most famous member of this family is the molybdenum disulfide (MoS2) which its single-layer structure has favorite features like a direct band gap of 1.9ev, high optical absorption in the visible light range, and spin-orbit splitting of 150 meV in the valence band. These properties made MoS2 a promising material for electronics, optoelectronics and spintronics. In this study, MoO3 crystals are grown at a scalable size and the effect of sulfurization of these crystals on surface morphology, color, crystalline structure and optical absorption was investigated at different temperature. Increasing the temperature in the sulfurization process increases the conductivity and decreases the optical gap of the crystals. In the next step, we used this crystal as precursors to growth MoS2 in CVD oven on the substrate based on sulfurization MoO3 crystals at higher temperature from the previous step. Utilizing Raman spectroscopy, photoluminescence and SEM techniques, the lateral size and thickness of the samples are investigated. In addition, the characteristic curve of the currentvoltage and the effect of light on this curve are examined, which indicates that thelight can increases the intensity of the carrier concentration. Finally, the effect of MoS2 nanosheets, which prepared whit mechanical exfoliation on the ferromagnetic resonance, and magnetoresistance of NiFe magnetic layer and magnetoimpedance of CoFeB ribbon are investigated.

Behnam Tork

Behnam Tork's Thesis

Synthesis and characterization of graphene doped with magnetic nano-crystals
Two-dimensional heterostructures are novel building blokes conveying multifunctional research area for potential applications. Graphene has shown to be useful for magetoelectronics application because of it small spin orbit interaction as a medium to let the spin of electron transports thought the structure for couple of microns. To apply this bright character of Graphene for practical applications, we demonstrate a new fabrication procedure with output heterostructure products of graphene/magnetic crystal hybrids nano-flakes with tunable electrical conductivity, exfoliation rate and magnetization. Our method is a facile fabrication based on electrochemically exfoliation/deposition technique that Graphite gets exfoliated in Graphene flakes and simultaneously Ni, NiO and Ni(OH)2 magnetic crystals is being nucleated and grown between graphene flakes. The final product is a hybrid element including Gr nano-flakes with magnetic nanocrystals as a new heterostructure showing giant magnetoresistance (GMR) effect. We analyze the final products and report their systematic characterization by TEM, XPS, XRD, SEM, FTIR, UV-Visible, DLS and I~V. The GMR effect is modeled based on Boltzman electron transport which is quite consistence with experimental results. Our results open path ways for versatile application of Gr/magnetic crystal flakes hybrid for magnetoelectronics.