Study of Magnetization Switching in Coupled Magnetic Nanostructured Systems

A study of magnetization dynamics experiments in nanostructured materials using the rf susceptibility tunnel diode oscillator (TDO) method is presented along with a extensive theoretical analysis. An original, computer controlled experimental setup that measures the change in susceptibility with the variation in external magnetic field and sample temperature was constructed. The TDO-based experiment design and construction is explained in detail, showing all the elements of originality. This experimental technique has proven reliable for characterizing samples with uncoupled magnetic structure and various magnetic anisotropies like: CrO2 , FeCo/IrMn and Co/SiO2 thin films. The TDO was subsequently used to explore the magnetization switching in coupled magnetic systems, like synthetic antiferromagnet (SAF) structures. Magnetoresistive random access memory (MRAM) is an important example of devices where the use of SAF structure is essential. To support the understanding of the SAF magnetic behavior, its configuration and application are reviewed and more details are provided in an appendix. Current problems in increasing the scalability and decreasing the error rate of MRAM devices are closely connected to the switching properties of the SAF structures. Several theoretical studies that were devoted to the understanding of the concepts of SAF critical curve are reviewed. As one can notice, there was no experimental determination of SAF critical curve, due to the difficulties in characterizing a magnetic coupled structure. Depending of the coupling strength between the two ferromagnetic layers, on the SAF critical curve one distinguishes several new features, inexistent in the case of uncoupled systems. Knowing the configuration of the SAF critical curve is of great importance in order to control its switching characteristics. For the first time a method of experimentally recording the critical curve for SAF is proposed in this work. In order to overcome technological limitations, a new way of recording the critical curve by using an additional magnetic bias field was explored

Magnetic Hysteretic Characterization of Ferromagnetic Materials with Objectives towards Non-Destructive Evaluation of Material Degradation

Structurele componenten en machineonderdelen vervaardigd uit staal worden vaak onder veeleisende omstandigheden uitgebaat. Dit kan aanleiding geven tot materiaaldegradatieprocessen zoals verbrossing en metaalvermoeiing. Mettertijd kunnen deze microstructurele processen leiden tot een graduele verslechtering van de mechanische eigenschappen, en eventueel tot scheurgroei en breuk. Om dit te vermijden is het monitoren van de materiaalintegriteit van groot belang, wat uitgevoerd kan worden met behulp van niet-destructieve evaluatietechnieken. In dit doctoraatsonderzoek bestuderen we de mogelijkheden van magnetische hysteretische karakteriseringstechnieken voor het niet-destructief monitoren van toenemende materiaaldegradatie van ferromagnetisch constructiestaal. Dergelijke aanpak is gemotiveerd door de kennis dat magnetisch hysteretisch gedrag beïnvloed is door microstructurele materiaaleigenschappen. Anders geformuleerd, de verandering in de vorm van de magnetische hysteresislussen, experimenteel waargenomen op verschillende momenten tijdens de materiaaldegradatie, reflecteert de onderliggende microstructurele veranderingen en de degradatie van de mechanische eigenschappen. Een van de onderzochte onderwerpen is de verbrossing van ferritisch staal door neutronenbestraling. Dit effect kan schadelijk zijn voor het drukvat van een nucleaire reactor. De maximum-permeabiliteit vertoont een significante dalende trend met stijgende neutronendosis en met stijgende vloeigrens, hetgeen het potentieel aangeeft van de magnetische hysteretische evaluatie van verbrossing door neutronenbestraling. Een andere topic is het continu monitoren van metaalvermoeiing aan de hand van een magnetomechanische methode. Deze methode resulteert in informatie over de verschillende vermoeiingsstadia, alsook over het finale vermoeiingsstadium. De ontwikkelde magnetische en magnetomechanische karakteriseringstechnieken kunnen gebruikt worden voor de niet-destructieve evaluatie van mechanische en microstructurele eigenschappen, met als doelstelling de beoordeling van de materiaalintegriteit tijdens uitbating en/of voor de kwaliteitscontrole tijdens materiaalproductieprocessen

ΔE-Effect Magnetic Field Sensors

Many conceivable biomedical and diagnostic applications require the detection of small-amplitude and low-frequency magnetic fields. Against this background, a magnetometer concept is investigated in this work based on the magnetoelastic ΔE effect. The ΔE effect causes the resonance frequency of a magnetoelastic resonator to detune in the presence of a magnetic field, which can be read-out electrically with an additional piezoelectric phase. Various microelectromechanical resonators are experimentally analyzed in terms of the ΔE effect and signal-and-noise response. This response is highly complex because of the anisotropic and nonlinear coupled magnetic, mechanical, and electrical properties. Models are developed and extended where necessary to gain insights into the potentials and limits accompanying sensor design and operating parameters. Beyond the material and geometry parameters, we analyze the effect of different resonance modes, spatial property variations, and operating frequencies on sensitivity. Although a large ΔE effect is confirmed in the shear modulus, the sensitivity of classical cantilever resonators does not benefit from this effect. An approach utilizing surface acoustic shear-waves provides a solution and can detect small signals over a large bandwidth. Comprehensive analyses of the quality factor and piezoelectric material parameters indicate methods to increase sensitivity and signal-to-noise ratio significantly. First exchange-biased ΔE-effect sensors pave the way for compact setups and arrays with a large number of sensor elements. With an extended signal-and-noise model, specific requirements are identified that could improve the signal-to-noise ratio. The insights gained lead to a new concept that can circumvent previous limitations. With the results and models, important contributions are made to the understanding and development of ΔE-effect sensors with prospects for improvements in the future

Magnetic-field control of near-field radiative heat transfer and the realization of highly tunable hyperbolic thermal emitters

We present a comprehensive theoretical study of the magnetic field dependence of the near-field radiative heat transfer (NFRHT) between two parallel plates. We show that when the plates are made of doped semiconductors, the near-field thermal radiation can be severely affected by the application of a static magnetic field. We find that irrespective of its direction, the presence of a magnetic field reduces the radiative heat conductance, and dramatic reductions up to 700% can be found with fields of about 6 T at room temperature. We show that this striking behavior is due to the fact that the magnetic field radically changes the nature of the NFRHT. The field not only affects the electromagnetic surface waves (both plasmons and phonon polaritons) that normally dominate the near-field radiation in doped semiconductors, but it also induces hyperbolic modes that progressively dominate the heat transfer as the field increases. In particular, we show that when the field is perpendicular to the plates, the semiconductors become ideal hyperbolic near-field emitters. More importantly, by changing the magnetic field, the system can be continuously tuned from a situation where the surface waves dominate the heat transfer to a situation where hyperbolic modes completely govern the near-field thermal radiation. We show that this high tunability can be achieved with accessible magnetic fields and very common materials like n-doped InSb or Si. Our study paves the way for an active control of NFRHT and it opens the possibility to study unique hyperbolic thermal emitters without the need to resort to complicated metamaterials.Comment: 21 pages, 10 figure

Theory and Simulation of the Microwave Response of Concentric Ferromagnetic Shells

Structured ferromagnetic metal-based metamaterials comprised of spherical particles exhibit properties that are attractive for microwave applications, such as a broad frequency bandwidth and higher working frequencies when compared with bulk ferrimagnetic oxides. In this thesis, the dynamical properties of ferromagnetic spherical shells are studied using a combination of analytical and numerical methods, to further understanding and enhance the permeability of these materials towards higher frequencies. Using linearised micromagnetic equations, saturated spherical shells are investigated in the exchange-dominated regime when assuming that surface anisotropy is present at both the inner and outer boundaries. This configuration is amenable to exact solutions for the resonance eigenvalues and to investigate the size/thickness dependence of the resonance frequencies. It is found that the mode frequency can increase with decreasing shell thickness or is driven rapidly towards the ferromagnetic resonance frequency depending on the choice of the surface anisotropy constant at each boundary. Moreover, surface anisotropy introduces a dependence of the zeroth mode on shell thickness, removing the degeneracy with the ferromagnetic resonance and leading to a pronounced size dependence of this mode for thin shells. A generalised resonance theory is further outlined for a multilayered spherical nanoparticle comprised of exchange-coupled concentric layers. It can be used to compute the resonance spectra of core-shell nanoparticles, as in the case of a solid spherical ferromagnetic core surrounded by an outer oxide shell. Detailed micromagnetic modelling of two- and three-dimensional ferromagnetic particles was carried out to study the role of long-range magnetostatic interactions between concentric rings and the influence of realistic domain structures on the dynamic susceptibility. Micromagnetic modelling of such structures demonstrates that a family of higher-order flexural modes is present for spherical shells relaxed into the vortex state, which can reach high-frequencies 20-25 GHz under weak-field excitations. These simulations provide an alternative and more plausible interpretation of observed high-frequency resonance modes in measured permeability spectra of spherical shell particle composites, and aid in the design of high-frequency, light-weight composite materials. The dynamical properties of three-dimensional permalloy elements supporting vortex domain structures were also investigated with micromagnetic simulations and compared with experiment. This is to study the influence of nonuniform field gradients and three-dimensional static magnetisation configurations on 1 the dynamical behaviour. It is found that the permalloy elements support domain walls with perpendicular out-of-plane components which can be switched dynamically in response to specific magnetic pulse parameters. This project further aimed to incorporate the fundamental nonlinear micromagnetic and electromagnetic details, including exchange and magnetocrystalline anisotropy, within the finite-difference time-domain (FDTD) method. This is to study the interaction between magnetic materials and electromagnetic waves in the presence of current and magnetic sources at microwave frequencies. Results are presented for conducting semi-infinite permalloy pillars in the micrometer and sub-micrometer size range. It is found that microwave absorption results primarily from edge modes localised at the boundaries of the pillar in accordance with the skin depth, which appear at a lower frequency than the ferromagnetic resonanceEngineering and Physical Sciences Research Council (EPSRC

MAGNETIC FIELD SENSOR UTILIZING MAGNETO IMPEDANCE IN THIN FILM MULTI-LAYERS

Since the discovery of the Magneto Impedance (MI) effect in 1994 there has been a global increase in the research devoted to understanding the effect. In certain magnetic materials, the impedance change, often referred to as the MI ratio, is in the range of 50 to 100% for an excitation current in the MHz frequency range for external magnetic fields of a few Oe. The use of thin film multilayer structures allows the increase of sensitivity and the reduction of size for MI effect to be integrated with micro magnetic sensor technologies. In the present work, we explain the origins of the MI effect and its versatile nature for the development of sub nano Tesla magnetic field sensors. The matrix like nature of the MI effect allows a variety of MI characteristics to be implemented in a thin film, which allows the structure to be tailored for maximum sensitivity in the chosen field sensing application. In the case of a simple transverse magnetic anisotropy, the diagonal components of the MI matrix are symmetric and the off diagonal components are anti-symmetric with respect to the dc longitudinal field. The asymmetry in the MI behaviour can be related to either a certain asymmetric arrangement of the dc magnetization (crossed an isotropy), or a contribution to the measured voltage due to the ac cross-magnetization process, which is represented as an off-diagonal component. These asymmetrical characteristics are useful in producing linear bi-directional field sensors without DC biasing. In attempt to find optimal film systems with respect to relative impedance change, sensitivity, linearity, operational frequency range, and dimensions, thin film multi-layers, consisting of a magnetic / conductor / magnetic material configuration were fabricated. Variations in magnetic compositions, geometries, structures and magnetic configurations (transverse, longitudinal or cross anisotropy) and additional insulations layers were produced. Planar coil thin film multi-layers were constructed to utilize the more magnetic complex asymmetric characteristics of the MI effect. An experimental configuration was developed in order to measure all components of the MI matrix within the thin films and standardise their sensitivity using the MI ratio. Two sub nano Tesla magnetic field sensors were developed during the course of this work based on the fabricated thin films. The first sensor concentrates on utilizing two asymmetrical Magneto Impedance (AMI) elements combined differentially. The sensor is driven by a sinusoidal current of 90 MHz biased with a dc bias current. For AMI film element of 5mm long, 40µm wide and having anisotropy angle of 45° the field detection resolution is in the magnitude of 1µ Oe for both ac and dc for fields of ~ 20e magnitude. The maximum response speed is in the order of 1MHz. The use of MI to the measurement low frequency fields such as bio-medical signals drove the design of the second sensor. Extensive research was undertaken to improve the phase noise of the oscillator and sensitivity of the detection mechanism using novel RF techniques to improve the sensitivity at high frequencies, and secondly a method to improve the low frequency sensitivity by AC biasing the MI element with a magnetic field. A thin film multilayer MI sensor was produced based on the measurement of the modulation of the incident reflected power due to an external AC magnetic field. Direct field measurement performance at 1kHz produced a resolution of 3.73 x 10-7 Oe. AC biased performance at 5kHz of a 20Hz field was a resolution of 5.27 x 10-6 Oe, and at 10Hz of 9.33 x 10-6 Oe. With continued improvement of the electronic components utilized in this novel method of Magneto Impedance sensor presented in this work, the possibility of measuring bio magnetic signals of the human body at room temperature becomes a distinct reality