12 research outputs found
Assessing the use of Circumferentially Notched Tension specimen for fracture toughness determination of ductile offshore steels
No full textThe Circumferentially Notched Tension (CNT) specimen is a potential candidate for determining the fracture toughness of highly constrained cracks, theoretically approaching plane strain conditions, even for small test specimen dimensions. This research aims to experimentally evaluate the use of the CNT test specimen geometry to fracture toughness test ductile steels. The process involves development of a fatigue precracking method designed to minimize eccentricity values, experimentally verifying a number of claims made in literature regarding the behaviour of the CNT specimen and investigating specimen dimensioning. All with the purpose to move toward implementing the CNT specimen as an established fracture toughness test geometry. Initiating a fatigue precrack by compressive fatiguing followed by tensile fatiguing to the required fatigue crack length proved to lead to ligament eccentricity values larger than those encountered in literature created using rotating bending. The prefatiguing method also proved sensitive to the material type. The resulting eccentricity values had no significant effect on the result of fracture toughness tests on S690QT steel. It is believed that the uncracked ligament realigns with the loading line without causing excessive damage on this ductile steel type. The bending distortion is thereby effectively cancelled. SEM photography on a specimen with a commonly encountered eccentricity magnitude of 0.2 mm showed that fracture is still initiated at the position of the longest fatigue crack length despite the relatively low eccentricity value. Comparing the results of fracture toughness tests on CNT specimen under two stress intensification rates of 3 MPa√m/s and 571 MPa√m/s showed an insensitivity of the CNT specimen to varying crosshead speeds. The conclusion is that the combined machine and specimen stiffness increase the displacement of the crack area during crack growth beyond the speed at which the crosshead moves. It is suggested that larger CNT specimens are required to approximate plane strain conditions for the S690QT steel tested. Experimental results of fracture toughness tests and critical stretch zone widths using SEM on 10 mm major diameter CNT specimens were comparable to SENB specimens with a width of 10 mm. These SENB specimen are known to be undersized and do not provide a plane strain fracture toughness. It is thought that areas of increased stress emerge at the crack tips and converge at the centreline of the CNT specimen. Literature source identified the linking up of these so called stress bands decreased crack tip constraint. The linking up of stress bands at experimental loads was confirmed to take place by 2-dimensional FEM simulations. An addition to the decrease in crack tip constraint may be the presence of a bending load. Further research is required to quantify the effect of bending on crack tip constraint levels. Critical stress intensity values obtained experimentally at the point of 2% crack extension proved to be consistent with little spread. This proves the potential of the CNT geometry to provide consistent fracture toughness values despite the presence of eccentric ligaments. More research is recommended in order to introduce the CNT specimen as a robust and efficient fracture toughness specimen.Mechanical, Maritime and Materials EngineeringMaterials Science and Engineering (MSE
Thermal spin transport and spin transfer torque in ferromagnetic/non-magnetic nanoscale devices
No full textDit proefschrift beschrijft fundamentele experimenten waarin de koppeling tussen de elektrische, magnetische en thermische eigenschappen van elektronen transport in metalen wordt onderzocht. Om dit doel te bewerkstelligen zijn verschillende zeer kleine structuren, zogenaamde devices, gemaakt met behulp van elektronenbundel- en optische-lithografie. Deze structuren zijn lateraal, dat wil zeggen dat ze parallel aan het oppervlak liggen van het Silicium substraat waarop de devices gemaakt zijn. Het onderzoek verbind de twee bestaande individuele onderzoeksgebieden van de ‘spin-elektronica’ en de ‘caloritronica’ tot een nieuw onderzoeksgebied: de spin-caloritronica.
Het proefschrift is in twee delen opgedeeld. In het eerste deel worden experimenten beschreven waarin de verbinding tussen de elektrische, magnetische en thermische eigenschappen van elektronen transport in metalen wordt onderzocht. In het tweede deel worden experimenten en theorie beschreven waarin de invloed van spin-stromen op de magnetisatie dynamica wordt onderzocht. Voor alle experimenten maken we gebruik van magnetische geheugenelementen, die uit de magnetische legering Permalloy en koper bestaan.
This dissertation describes fundamental experiments which investigate the coupling between the electric, magnetic and thermal properties of electron transport. Several devices were constructed to accomplish this goal. The devices were created with the aid of electron-beam and optical lithography. The fabricated structures are lateral, which means they are oriented parallel to the Silicon substrate on which they are fabricated. This research connects the two existing research areas of spin-based electronics and caloritronics to a new research area: spin caloritronics.
The dissertation is split up in two parts. In the first part, we describe experiments which are aimed to demonstrate the coupling between the electrical, magnetic en thermal properties of electron transport. In the second part we describe experiments and theory which investigates the influence of spin-currents on spin-transfer torque. For all experiments, we make use of metallic spin-valves, which exist from the magnetic alloy Permalloy and copper.
Magnetization oscillations induced by spin current in a paramagnetic disc
No full textWhen electron spins are injected uniformly into a paramagnetic disk, they can precess along the demagnetizing field induced by the resulting magnetic moment. Normally this precession damps out by virtue of the spin relaxation, which is present in paramagnetic materials. We propose a mechanism to excite a steady-state form of this dynamics by injecting a constant spin current into this paramagnetic disk. We show that the rotating magnetic field generated by the eddy currents provide a torque that makes this possible. Unlike the ferromagnetic equivalent, the spin-torque oscillator, the oscillation frequency is fixed and determined by the dimensions and intrinsic parameters of the paramagnet. The system possesses an intrinsic threshold for spin injection, which needs to be overcome before steady-state precession is possible. The additional application of a magnetic field lowers this threshold. We discuss the feasibility of this effect in modern materials. Transient analysis using pump-probe techniques should give insight in the physical processes which accompany this effect.
Thermally driven spin injection from a ferromagnet into a non-magnetic metal
No full textCreation, manipulation and detection of spin-polarized carriers are the key elements of spin-based electronics. Most practical devices use a perpendicular geometry in which the spin currents are accompanied by charge currents. In recent years, new sources of pure spin currents (that is, transport of spin angular momentum without charge currents) have been demonstrated and applied. Here we demonstrate a conceptually new source of pure spin current driven by the flow of heat across the interface between a ferromagnet and a non-magnetic metal. This spin current is generated because, in a ferromagnet, the Seebeck effect—which describes the generation of a voltage as a result of a temperature gradient—is spin dependent. We studied this new source of spin currents experimentally in a non-local lateral geometry and developed a three-dimensional model that describes the heat, charge and spin transport in this geometry, enabling us to quantify this process. We obtain a spin-dependent Seebeck coefficient for Permalloy of -3.8 µV K-1, suggesting that thermally driven spin injection is a feasible alternative for electrical spin injection in, for example, spin-transfer-torque experiments.
Anomalous Nernst and anisotropic magnetoresistive heating in a lateral spin valve
No full textWe measured the anomalous Nernst effect and anisotropic magnetoresistive heating in a lateral multiterminal permalloy/copper spin valve using all-electrical lock-in measurements. To interpret the results, a three-dimensional thermoelectric finite-element model is developed. Using this model, we extract the heat profile which we use to determine the anomalous Nernst coefficient of permalloy RN = 0.13 and also determine the maximum angle θ = 8° of the magnetization prior to the switching process when an opposing noncollinear 10° magnetic field is applied.
Direct observation of the spin-dependent Peltier effect
No full textThe Peltier coefficient describes the amount of heat that is carried by an electrical current when it passes through a material. When two materials with different Peltier coefficients are placed in contact with one another, the Peltier effect causes a net flow of heat either towards or away from the interface between them. Spintronics describes the transport of electric charge and spin angular momentum by separate spin-up and spin-down channels in a device. The observation that spin-up and spin-down charge transport channels are able to transport heat independently of each other has raised the possibility that spin currents could be used to heat or cool the interface between materials with different spin-dependent Peltier coefficients. Here, we report the direct observation of the heating and cooling of such an interface by a spin current. We demonstrate this spin-dependent Peltier effect in a spin-valve pillar structure that consists of two ferromagnetic layers separated by a non-ferromagnetic metal. Using a three-dimensional finite-element model, we extract spin-dependent Peltier coefficients in the range -0.9 to -1.3 mV for permalloy. The magnetic control of heat flow could prove useful for the cooling of nanoscale electronic components or devices.
Interplay of Peltier and Seebeck Effects in Nanoscale Nonlocal Spin Valves
No full textWe have experimentally studied the role of thermoelectric effects in nanoscale nonlocal spin valve devices. A finite element thermoelectric model is developed to calculate the generated Seebeck voltages due to Peltier and Joule heating in the devices. By measuring the first, second, and third harmonic voltage response nonlocally, the model is experimentally examined. The results indicate that the combination of Peltier and Seebeck effects contributes significantly to the nonlocal baseline resistance. Moreover, we found that the second and third harmonic response signals can be attributed to Joule heating and temperature dependencies of both the Seebeck coefficient and resistivity.
Thermoelectric Detection of Ferromagnetic Resonance of a Nanoscale Ferromagnet
No full textWe present thermoelectric measurements of the heat dissipated due to ferromagnetic resonance of a Permalloy strip. A microwave magnetic field, produced by an on-chip coplanar strip waveguide, is used to drive the magnetization precession. The generated heat is detected via Seebeck measurements on a thermocouple connected to the ferromagnet. The observed resonance peak shape is in agreement with the Landau-Lifshitz-Gilbert equation and is compared with thermoelectric finite-element modeling. Unlike other methods, this technique is not restricted to electrically conductive media and is therefore also applicable to for instance ferromagnetic insulators
Thermoelectric effects in magnetic nanostructures
No full textElektronentransport is een belangrijk natuurkundig verschijnsel dat vaak wordt gebruikt in de hedendaagse technologie. Alle elektrische apparaten, variërend van stofzuigers tot computerchips, zijn in principe gebaseerd op dit type transport. Ondanks dat elektronen meerdere eigenschappen hebben, wordt in de praktijk echter vaak alleen de lading van het elektron gebruikt. De energie en het magnetisch moment van de elektronen zijn eigenschappen die in respectievelijk thermoelektriciteit en spin-elektronica (afgekort: spintronica) worden toegepast. Spintronica richt zich op de overdracht van het magnetisch moment om bijvoorbeeld informatie te transporteren, terwijl thermoelektrische verschijnselen, zoals het Peltier en Seebeck effect, toegepast kunnen worden voor elektrische verwarming/koeling en thermokoppels.
Dit proefschrift beschrijft de fundamentele interacties tussen de drie vormen van transport (lading, warmte en spin) in magnetische nanostructuren. Het experimentele werk is een onderdeel van een breder onderzoeksgebied dat spin-caloritronica wordt genoemd. Deze onderzoeksrichting binnen de spintronica bestudeert de rol van het magnetisch moment van elektronen in warmtetransport.
De meerwaarde van deze spin-caloritronische effecten ligt, ten opzichte van reguliere thermoelektrische verschijnselen, in het gemak van het controleren van de magnetische textuur op de nanoschaal. Dit biedt daarom een sterk gelokaliseerde en programmeerbare controle over warmtestromen en zou bruikbaar kunnen zijn voor het genereren van thermo-elektrische energie of voor koeling. De gemeten effecten zijn echter klein en zijn nog ver weg van directe toepassingen. Desalniettemin zouden nieuwe ontwikkelingen binnen dit onderzoeksveld op een dag kunnen leiden tot de implementatie van spin-caloritronica in onze dagelijkse elektronische apparatuur.
Electron transport is one of the most important physical phenomena used in todays technology. All modern electrical equipment, ranging from vacuum cleaners till high-end microprocessors is essentially founded on this type of transport. However, in most cases the electron's full potential is not used and most applications only exploit the negative elementary charge that it possesses. The energy and the magnetic moment of the electrons are properties which are used in thermoelectricity and spin-electronics (spintronics), respectively. Spintronics focuses on the transfer of magnetic moments for the information transport, while thermoelectric phenomena (e.g. the Peltier and Seebeck effect) have found their application in devices for electric heating/cooling or thermocouples.
This thesis describes the fundamental interactions between the three types of transport (charge, heat, spin) in magnetic nanostructures. The experimental work described here is part of a wider research direction, called spin-caloritronics. This research branch of spintronics studies the role of magnetic moments in heat transport.
The potential advantages of spin-caloritronic effects with respect to regular thermoelectricity can be found in the easy manipulation of magnetic textures at the nanoscale. This enables very localized and programmable control of heat flow which might prove useful for thermopower energy harvesting or refrigeration. However, the previously discussed effects are weak and far from direct applications. Nonetheless, a combination of new developments in this field and by exploring novel materials it could one day lead to the implementation of spin-caloritronics in our everyday electronic devices.
Optical probing of spin dynamics of two-dimensional and bulk electrons in a GaAs/AlGaAs heterojunction system
No full textThe electron spin dynamics in a GaAs/AlGaAs heterojunction system containing a high-mobility two-dimensional electron gas (2DEG) have been studied in this paper by using pump–probe time-resolved Kerr rotation experiments. Owing to the complex layer structure of this material, the transient Kerr response contains information about electron spins in the 2DEG, an epilayer and the substrate. We analyzed the physics that underlies this Kerr response, and established the conditions under which it is possible to unravel the signatures of the various photo-induced spin populations. This was used to explore how the electron spin dynamics of the various populations depend on the temperature, magnetic field and pump-photon density. The results show that the D’Yakonov–Perel’ mechanism for spin dephasing (by spin–orbit fields) plays a prominent role in both the 2DEG and bulk populations over a wide range of temperatures and magnetic fields. Our results are of importance for future studies on the 2DEG in this type of heterojunction system, which offers interesting possibilities for spin manipulation and control of spin relaxation via tunable spin–orbit effects.
