From the discovery of GMR effect since 1988,spintronic has been extensively developed. Research and application of relative GMR, CMR and TMR topics have progressed rapidly. In the year of 2007, the Nobel prize of physics was awarded to Albert Fert and Peter Grünberg for the great achievement on the research of GMR. The application of development on MRAM is a very hot subject recently and the main operating constructions of MRAM are MTJ. In this thesis, La0.67Ca0.33MnO3¡BLa0.67Sr1.33MnO4 and La0.8Ba0.2MnO3 of CMR materials were growth to multilayer to investigate the properties and characters of the films, in order to built up the MRAM based on CMR MTJ¡¦s.

No description available.

Spintronics discusses about fundamental physics and material science in mostly nanometer size structures. Spintronics also delivers many promising technologies for now and the future. One of the interesting spintronic structures is called “Magnetic Tunnel junction” (MTJ). A typical MTJ consists of a thin (1-3nm) insulator layer sandwiched between two ferromagnetic layers. In this work, I present MTJ with perpendicular magnetic anisotropy (PMA) using an MgO tunnel barrier. The effect of different heavy metals (HMs) adjacent to the ferromagnets (FMs) on tunneling magnetoresistance (TMR) and PMA of the junctions are discussed. Namely, Ta, Mo, Ta/Mo, W, Ir, and Hf have been utilized in HM/FM/MgO structures, and magneto-transport properties are explored. It is shown that when Ta/Mo is employed, TMR values as high as 208%, and highly thermally stable PMA can be obtained. Some physical explanation based on electronic band structure and thermochemical effects are discussed. In the last part of this work, the newly discovered tunneling anisotropic magnetoresistance (TAMR) effect in antiferromagnets is studied, and clear TAMR is demonstrated for NiFe/IrMn/MgO/Ta structures.

Magnetic sensors are deployed in many applications such as automotive, consumer electronics, navigation and data storage devices. Their market’s growth is driven by demands of higher performance; primarily to assist in the advancement of the Internet of Things (IoT) and smart systems. Challenging obstacles of miniaturization and power consumptions must be overcome. A leading sensor that has the potential to accelerate the development is the magnetic tunnel junction (MTJ) devices. Corrosion causes catastrophic consequences for industries. Preventive measures could save up to 35% of annual corrosion-related costs. An advanced corrosion sensing technique is developed based on iron nanowires. The iron nanowires are magnets which lose their magnetization when corroded. Their magnetization loss is monitored using sensitive MTJ sensor. Combined, the nanowires and the MTJ sensor realize a highly integrated sensor concept that enables corrosion sensing with an ultra-low power consumption of less than 1 nW, a sensitivity of 0.1 %/min, a response time of 30 minutes and an area of 128 μm2. Surgical tool development is accelerating in the healthcare sector. Cardiac catheterization specifically is a minimally invasive surgery that relies heavily on x-ray imaging and contrast dyes. A flexible tri-axis MTJ sensor is developed to help minimizing the need for x-ray imaging during the procedure. The flexible sensor can bend to a diameter of 500 μm without compromising the performance and can endure over 1000 bending cycles without fatigue. Three flexible sensors are mounted onto the tip of a 3 mm cardiac catheter, realizing a novel sensor-on-tube (SOT) tri-axis sensor concept. The sensor has a high sensitivity of 9 Ω/° and an MR ratio of 29%. It weighs 16 μg only, adds 5 μm to the catheter’s diameter and a total size 300 μm2. The prototype system estimated the heading angle with an RMS error value of 7° and tracked the orientation of the sensor with an acceptable accuracy. However, the sensor has a misalignment issue caused by the manual placement of the sensors. A high precision tool is needed for the assembly, and any further misplacement -within a reasonable margin of error- could be corrected by calibration algorithms.

Le contenu de ce rapport vise à résumer l'ensemble des activités de recherche que j'ai menées durant ces dernières 10 années ainsi que les perspectives et les projets pour les années à venir. Le dossier se scinde en plusieurs parties. La première partie de mon rapport expose mes travaux scientifiques effectués. Leur contenu s'intègre dans le cadre de l'électronique de spin et repose essentiellement sur l'étude du magnétisme et du transport polarisé en spin dans des systèmes de jonctions tunnel magnétiques. Ce travail m'a amené à élaborer des systèmes type jonctions tunnel magnétiques et à étudier la corrélation entre leurs propriétés magnétiques et leurs propriétés de transport polarisé en spin à des échelles macroscopiques et microscopiques. Les travaux sur les JTMs epitaxiees ont démontré qu'une physique au-delà du modèle des électrons libres gouverne le transport électronique dans les systèmes cristallins. La mise en évidence directe de la corrélation entre la structure électronique et chimique de l'interface métal oxyde et le transport tunnel polarisé en spin montre un fort potentiel pour le contrôle des caractéristiques magnéto-électriques de dispositifs spintroniques. Les études actuelles sur les effets de cohérence électronique dans des structures epitaxiees à multiples barrières tunnel et le contrôle de propriétés magnétiques par des courants de spin hors-équilibre ouvrent la voie vers une nouvelle physique et de nouvelles applications. D'une part, la complexité des mécanismes de transport dans les systèmes épitaxies a généré un important investissement personnel dans les techniques de calcul de structure électronique ab-initio ainsi que dans la création des outils de modélisation des propriétés magnétiques et de transport tunnel. D'autre part, du point de vue expérimental, l'ensemble des techniques que j'ai utilise regroupe l' épitaxie par jets moléculaires et la pulvérisation cathodique en ce qui concerne l' élaboration des échantillons, ainsi que des méthodes de caractérisation in situ (diffraction d' électrons RHEED, spectroscopie Auger) ou ex-situ (AFM pour la structure, MFM pour le micro magnétisme, VSM et e et Kerr pour le magnétisme macroscopique, et des diverses techniques de mesure électriques sous champ pour le magneto-transport). Une deuxième partie du rapport résume des résultats récents obtenus sur des systèmes tunnel complexes. Une autre partie du rapport résumé mes objectifs et projets de recherche pour les années à venir. Ils consistent a poursuivre les études de magnétisme et transport polarise en spin dans des hétéro-structures complexes de faible dimensionnalité avec un accent tout particulier vers le développement d'un axe théorique orienté vers la modélisation du transport électronique par des techniques type ab-initio. Dans un rapport annexe , je présente mon Curriculum Vitae, ma production scientifique, et un résumé des activités que j'ai effectuées dans l'enseignement, l'administration de la recherche, la participation a des contrats et des responsabilités collectives.

Thermoelectricity can directly convert a temperature difference into a voltage or charge current. Recently, the development of spin caloritronics has introduced spin as another degree of freedom in traditional thermoelectrics. This discovery bodes a new generation of magnetic random access memories (MRAMs), where thermal spin-transfer torque (TSTT) rather than voltage driven spin-transfer torque (STT) is used to switch the magnetization in magnetic tunnel junctions (MTJs). To advance the rising trend of spin caloritronics, the coupling of charge, spin, and heat flow during electron transport in MTJs was systematically studied in this thesis. To begin with, the static transport properties of MTJs were studied by observing current dependent tunnel magnetic resistance (TMR). The observed decrease of TMR with a biased current is attributed to the change in spin polarization of the free ferromagnetic layer. A phenomenological model has been built based on the current dependent polarization, which agrees with our experimental results. Next, the Seebeck rectification effect in MTJs was studied. By applying microwave currents to MTJs, an intrinsic thermoelectric coupling effect in the linear response regime of MTJs was discovered. This intrinsic thermoelectric coupling contributes a nonlinear correction to Ohm's law. In addition, this effect can be controlled magnetically since the Seebeck coefficient is related to magnetization configuration. Finally, TSTT in MTJs was systematically studied. A laser heating technique was employed to apply a temperature difference across the tunnel barrier and ferromagnetic resonance (FMR) spectra were measured electrically through spin rectification. By analyzing the FMR spectra, TSTT in MTJs was observed and the angular dependence of TSTT was found to be different from dc-biased STT. By solving the Landau-Lifshitz-Gilbert equation including STT, the experimental observations were well explained. The discovery of Seebeck rectification refines the previous understanding of magneto-transport and microwave rectification in MTJs and provides a new possibility for utilizing spin caloritronics in high-frequency applications. The study of TSTT in MTJs shows clear experimental evidence of TSTT in MTJs. Further optimization of the design of MTJs may succeed in decreasing the necessary switching fields strength or even achieve a switching by only TSTT in MTJs. / February 2017

In the next decade, technology trends--smaller dimension, lower voltage, higher operating frequency--introduce new technical considerations and challenges for radiation effects in integrated circuits. Semiconductor based circuits and traditional dynamic random-access memories will malfunction when exposed to extreme environments, such as space and nuclear reactor. The mechanisms for radiation effect are mainly attributed to the radiation-induced charging of the oxide in a CMOS device. Spintronics is an emerging area of nanoscale electronics involving the detection and manipulation of electron spin. The magnetic tunnel junctions (MTJs), based on the intrinsic spin of the electron, can be used as the storage elements in non-volatile magnetoresistive random-access memories (MRAMs). In this effort, we study radiation tolerance of MTJs by exposing the devices in gamma and neutron radiation environment. Theoretical model for the radiation-induced defects is analyzed in this work. Experiments of the MgO-based MTJs under the conditions of pre- and post-radiation are concluded. MTJs were irradiated with gamma ray to a total dose of 10 Mrad. During the neutron irradiation, total epithermal neutron fluence up to 2.9��10�����/cm�� was obtained. The experimental results show that neither the electrical nor the magnetic properties of MTJs are affected by the radiation. / Graduation date: 2013 / Access restricted to OSU community at author's request from Sept. 11, 2012 - Sept. 11, 2014

Orientador: Guillermo Gerardo Cabrera Oyarzún / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-18T12:28:58Z (GMT). No. of bitstreams: 1 Fernandes_ImaraLima_M.pdf: 9006612 bytes, checksum: accaacf2ec8bab2326612b97e18b9b16 (MD5) Previous issue date: 2011 / Resumo: Neste trabalho, apresentamos um estudo do tunelamento e do transporte quântico em sistemas mesoscópicos, particularmente em junções de tunelamento magnéticas, visando esclarecer a polarização magnética da corrente de tunelamento. Nos dispositivos de tunelamento, um filme isolante é crescido entre os eletrodos ferromagnéticos. Nesse sistema a condutância é controlada pelo coeficiente de transmissão do efeito túnel. Nos metais de transição (Fe, Co, Ni), as bandas s, p e d contribuem para a condução eletrônica, entretanto a magnetização deve-se à polarização das bandas d. Resultados experimentais mostram que essa polarização da corrente pode ser muito diferente da polarização do volume no nível de Fermi, podendo até estar invertida. Qualitativamente sabe-se que os elétrons da banda d apresentam menor probabilidade de tunelamento do que os elétrons s ou p. Os elétrons de condução do tipo s são representados por ondas planas com vetores de onda pequenos (centro da zona de Brillouin). Já os elétrons d possuem maior massa efetiva e um caráter localizado, portanto, são representados por pacotes de muitas componentes de ondas planas com vetores de onda maiores. Estudamos o tunelamento desses elétrons por barreiras de potencial que representam o material isolante entre eletrodos metálicos. Propomos um modelo simples para a corrente de tunelamento e estimamos o efeito da magnetoresitência / Abstract: This work introduces a detailed study of tunneling and quantum transport in mesoscopic systems, particularly in tunneling magnetic junctions, to understand the magnetic polarization of the tunneling current. These systems consist of two ferromagnetic metal layers separated by a thin insulating barrier layer. The conductance is controlled by the transmission coeficient of the tunnel effect. In the transition metal (Fe, Co, Ni), the bands s, p and d contribute to the electronic conduction, however, to the magnetization only the d-band contributes. Experimental results show that the current polarization may be different of the bulk polarization in the Fermi level and may be reversed. Qualitatively it is known that tunneling probability of the d-like electrons is lower than the s-like and p-like electrons. The s-electrons are represented by wave planes with small wave vector (center of the Brillouin zone). Since the d-electrons have higher effective mass and they are localized states, they are represented by wave packet with many components of wave planes with larger wave vectors. We investigate the tunneling of these electrons through potential barriers, which represent the insulating layer between the ferromagnetic electrodes. We propose a simple model for the tunneling current and estimated the effect of the magnetoresistance / Mestrado / Física da Matéria Condensada / Mestra em Física

<p>Probabilistic computing has been proposed as an attractive alternative for bridging the computational gap between the classical computers of today and the quantum computers of tomorrow. It offers to accelerate the solution to many combinatorial optimization and machine learning problems of interest today, motivating the development of dedicated hardware. Similar to the ‘bit’ of classical computing or ‘q-bit’ of quantum computing, probabilistic bit or ‘p-bit’ serve as a fundamental building-block for probabilistic hardware. p-bits are robust classical quantities, fluctuating rapidly between its two states, envisioned as three-terminal devices with a stochastic output controlled by its input. It is possible to implement fast and efficient hardware p-bits by modifying the present day magnetic random access memory (MRAM) technology. In this dissertation, we evaluate the design and performance of low-barrier magnet (LBM) based p-bit realizations.<br> </p> <p>LBMs can be realized from perpendicular magnets designed to be close to the in-plane transition or from circular in-plane magnets. Magnetic tunnel junctions (MTJs) built using these LBMs as free layers can be integrated with standard transistors to implement the three-terminal p-bit units. A crucial parameter that determines the response of these devices is the correlation-time of magnetization. We show that for magnets with low energy barriers (Δ ≤ k_BT) the circular disk magnets with in-plane magnetic anisotropy (IMA) can lead to correlation-times in <i>sub-ns</i> timescales; two orders of magnitude smaller compared to magnets having perpendicular magnetic anisotropy (PMA). We show that this striking difference is due to a novel precession-like fluctuation mechanism that is enabled by the large demagnetization field in mono-domain circular disk magnets. Our predictions on fast fluctuations in LBM magnets have recently received experimental confirmation as well.<br></p> <p>We provide a detailed energy-delay performance evaluation of the stochastic MTJ (s-MTJ) based p-bit hardware. We analyze the hardware using benchmarked SPICE multi-physics modules and classify the necessary and sufficient conditions for designing them. We connect our device performance analysis to systems-level metrics by emphasizing problem and substrate independent figures-of-merit such as flips per second and dissipated energy per flip that can be used to classify probabilistic hardware. </p>

Spin-torque nano-oscillators (STNOs) are prospective successors of transistor-based emitters and receivers of radio-frequency signals in commonly used remote communication systems. In comparison to the conventional electronic oscillators, STNOs offer the advantage of being tunable over a wide range of frequencies simply by adjusting the applied current, the smaller lateral size (up to 50 times) and the lower power consumption as the lateral size of the device is reduced. It has already been demonstrated that the output signal characteristics of STNOs are compatible with the requirements for applications: they can provide output powers in the µW range, frequencies of the order of GHz, quality factors Q (equal to df/f, where f is the frequency, and df is the linewidth) up to several thousands (e.g., 3 200), and can be integrated into Phase-Locked Loop (PLL) circuits. The most promising type of spin-torque oscillators is the hybrid geometry STNOs utilizing an in-plane magnetized fixed layer, an out-of-plane magnetized free layer and the MgO tunnel barrier as a spacer. This geometry maximizes the output power, since the full parallel-to-antiparallel resistance variation can be exploited in the limit of large magnetization precession angle (i.e., when the magnetization oscillates fully within the plane of the STNO stack). Moreover, the considered hybrid geometry allows for the reduction of the critical currents, enables functionality regardless of the applied magnetic or current history and requires a simplified fabrication process in comparison to the opposite hybrid geometry, consisting of an in-plane magnetized free layer and an out-of-plane reference layer, which requires an additional read-out layer. Simultaneously, the choice of the spacer material in considered STNOs is motivated by the increase of both the output power (via large magnetoresistance ratios) and the power conversion rate ('output power to input power' ratio), compared to their fully metallic counterparts. Despite the many advantages of MgO-based hybrid geometry STNOs, unexplained issues related to the physics behind their principle of operation remained. In this thesis, the main focus is put on the two key aspects related to the out-of-plane steady-state precession in hybrid STNOs: the precession mechanism (combined with the analysis of the influence of the bias dependence of the tunnel magnetoresistance) and the zero-field oscillations stabilized by an in-plane shape anisotropy. State-of-the-art theoretical studies demonstrated that stable precession in hybrid geometry STNOs can only be sustained if the in-plane component of the spin-transfer torque (STT) exhibits an asymmetric dependence on the angle between the free and the polarizing layer (which is true for fully metallic devices, but not for the MgO-based magnetic tunnel junctions (MTJs)). Nevertheless, recent experimental reports showed that spin-transfer driven dynamics can also be sustained in MgO-based STNOs with this particular configuration. In this thesis, a phenomenological and straightforward mechanism responsible for sustaining the dynamics in considered system is suggested. The mechanism is based on the fact that, in MgO-based MTJs, the strong cosine-type angular dependence of the tunnel magnetoresistance, at constant applied current, translates into an angle-dependent voltage component, which results in an angle-dependent spin-transfer torque giving a rise to the angular asymmetry of the in-plane STT and, thus, enabling steady-state precession to be sustained. Subsequently, the bias dependence of the tunnel magnetoresistance (TMR), which has been so far neglected in similar calculations, is taken into account. According to the results of analytical and numerical studies, the TMR bias dependence brings about a gradual quenching of the dynamics at large applied currents. The theoretical model yields trends confirming our experimental results. The most important conclusion regarding to this part of the thesis is that, while the angular dependence of the tunnel magnetoresistance introduces an angular asymmetry for the in-plane spin transfer torque parameter (which helps maintain steady-state precession), the bias dependence of the resistance works to reduce this asymmetry. Thus, these two mechanisms allow us to tune the asymmetry of the in-plane STT as function of current and to control the dynamical response of the actual device. Except for the precession mechanism, the thesis is also focused on the issue of zero-field oscillations, which would be especially desirable from the point of view of potential applications. According to the state-of-the-art theoretical studies, for hybrid geometry devices with circular cross-section (i.e., exhibiting no other anisotropy terms), current-driven dynamics cannot be excited at zero applied field. Indeed, zero-field oscillations have only been experimentally observed for systems having the free layer magnetization slightly tilted from the normal to the plane, which has usually been achieved by introducing an in-plane shape anisotropy. In the thesis, the influence of the in-plane shape anisotropy of the MTJ on zero-field dynamics in the hybrid geometry MgO-based STNOs is analytically and numerically investigated. In agreement with the previous reports, no zero-field dynamics for circular nano-pillars is observed; however, according to the numerical data, an additional in-plane anisotropy smaller than the effective out-of-plane anisotropy of the free layer enables zero-field steady-state precession. Accordingly, the lack of an in-plane anisotropy component (e.g., for circular cross-section nano-pillars), or the presence of an in-plane shape anisotropy equal or greater than the out-of-plane effective anisotropy, inhibits the stabilization of dynamics in the free layer at zero field. The results of analytical and numerical studies and the general trends identified in the corresponding experimental data are found to be in excellent qualitative agreement.:1. Introduction 1.1. Short history of magnetotransport applications 1.2. Spin-transfer torque induced effects and devices 1.3. Goals of the thesis 2. Fundamentals 2.1. Electronic transport in single transition metal layers 2.2. Tunnel magnetoresistance (TMR) 2.2.1. Electronic transport in magnetic tunnel junctions 2.2.2. Tunnel magnetoresistance versus structural properties of the multilayer 2.2.3. Bias voltage and temperature dependence of tunnel magnetoresistance 2.2.4. Angular dependence of tunnel magnetoresistance 2.3. Spin-transfer torque in GMR/TMR structures 2.3.1. Spin-transfer torque 2.3.2. Landau-Lifshitz-Gilbert (LLG) equation 2.3.3. LLG equation and spin-transfer torques 2.3.4. Bias voltage dependence of spin-transfer torques in MTJs 2.3.5. Angular dependence of spin-transfer torque 2.4. Spin-torque-based phenomena 2.4.1. Current-induced switching 2.4.2. Current-induced dynamics 3. Experimental 3.1. General characteristics of MgO-based magnetic tunnel junctions 3.2. STNO samples 3.2.1. Samples by AIST (Tsukuba, Japan) 3.2.2. Samples by HZDR / SINGULUS (Dresden / Kahl am Main, Germany) 3.3. Magnetotransport measurements 3.3.1. Experimental setup and data analysis 3.3.2. Experimental results 3.4. Aspects to be explained 4 Numerical and analytical calculations 4.1 Out-of-plane steady-state precession in hybrid geometry STNO 4.1.1 Angular dependence of tunnel magnetoresistance as a mechanism of stable precession 4.1.2. Influence of the bias dependence of tunnel magnetoresistance 4.1.3. Comparison with the experimental data 4.1.4. Comparison with the GMR-type counterpart 4.1.5. Summary 4.2. Zero-field dynamics in hybrid geometry STNO stabilized by in-plane shape anisotropy 4.2.1. Effect of the in-plane shape anisotropy 4.2.2. Zero-field dynamics 4.2.3. Summary 5. Conclusions 6. Outlook Appendix Bibliography