Reversing the training effect in exchange biased CoO/Co bilayers

We performed a detailed study of the training effect in exchange biased CoO/Co bilayers. High-resolution measurements of the anisotropic magnetoresistance (AMR) are consistent with nucleation of magnetic domains in the antiferromagnetic CoO layer during the first magnetization reversal. This accounts for the enhanced spin rotation observed in the ferromagnetic Co layer for all subsequent reversals. Surprisingly, the AMR measurements as well as magnetization measurements reveal that it is possible to partially reinduce the untrained state by performing a hysteresis measurement with an in plane external field perpendicular to the cooling field. Indeed, the next hysteresis loop obtained in a field parallel to the cooling field resembles the initial asymmetric hysteresis loop, but with a reduced amount of spin rotation occurring at the first coercive field. This implies that the antiferromagnetic domains, which are created during the first reversal after cooling, can be partially erased.Comment: 4 pages, 4 figure

Wafer-scale, epitaxial growth of single layer hexagonal boron nitride on Pt(111)

Single-layer hexagonal boron nitride is produced on 2 inch Pt(111)/sapphire wafers. The growth with borazine vapor deposition at process temperatures between 1000 and 1300 K is in situ investigated by photoelectron yield measurements. The growth kinetics is slower at higher temperatures and follows a tanh2 law which better fits for higher temperatures. The crystal-quality of hexagonal boron nitride (h-BN)/Pt(111) is inferred from scanning low energy electron diffraction (x-y LEED). The data indicate a strong dependence of the epitaxy on the growth temperature. The dominant structure is an aligned coincidence lattice with 10 h-BN on 9 Pt(1 × 1) unit cells and follows the substrate twinning at the millimeter scale

4-Channel C-band WDM transmitter based on 10 GHz graphene-silicon electro-absorption modulators

We demonstrate three 4-channelWDMtransmitters, each based on four graphenesilicon electro-absorption modulators with passivated graphene, achieving similar to 2.6dB insertion loss, similar to 5.5dB extinction ratio for 8V voltage swing and similar to 10GHz 3dB-bandwidth at 0V DC bias

New materials for modulators and switches in silicon photonics

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Case studies of electrical characterisation of graphene by terahertz time-domain spectroscopy

Graphene metrology needs to keep up with the fast pace of developments in graphene growth and transfer. Terahertz time-domain spectroscopy (THz-TDS) is a non-contact, fast, and non-destructive characterization technique for mapping the electrical properties of graphene. Here we show several case studies of graphene characterization on a range of different substrates that highlight the versatility of THz-TDS measurements and its relevance for process optimization in graphene production scenarios

Magnetization reversal and domain structure in the polycrystalline Co/CoO exchange bias system

Magnetism plays an important role in today’s technology and has a very broad range of applications. In the medical industry and research, high resolution medical scanners use magnetism and magnetic nanoparticles can be useful for therapeutic drug delivery, contrast enhancement agents for magnetic resonance imaging applications, magnetic separation of labelled cells, … Also in the information technology industry, it’s impossible to imagine life today without magnetic materials. Examples are VCR’s, credit cards, hard disks, … Due to the current and new promising applications, magnetism remains in the forefront of current fundamental and applied research. In this thesis, the exchange bias effect is investigated. Exchange bias arises from the coupling at the interface between a ferromagnetic and an antiferromagnetic layer. The antiferromagnetic layer is almost insensitive to external magnetic fields because it has no macroscopic magnetization and as a result, the coupling between an antiferromagnetic and ferromagnetic layer stabilizes the magnetization of the ferromagnetic layer in a certain direction. This “pinning”, which results in a shift of the ferromagnetic hysteresis loop, is referred to as the exchange bias effect and has been discovered 50 years ago by Meiklejohn and Bean. A detailed and reliable theoretical understanding of the exchange bias effect is, however, still lacking. This lack of understanding has its roots in the difficulty of visualizing the magnetic configuration of the antiferromagnetic layer and the uncertainty about the state of the common interface region between the ferromagnetic and antiferromagnetic layer. As a result, a wide variety of models exist today with different physical parameters, including roughness, grain size, compensated or uncompensated antiferromagnetic interfacial spins, perpendicular coupling, anisotropy, … Apart from the horizontal shift, other phenomena are related to the exchange bias effect, such as coercivity enhancement, training effect, asymmetric hysteresis loops, positive exchange bias, magnetic relaxation and a vertical shift of the hysteresis loop. The origin of some of these effects is still under debate and await complete understanding. One of these phenomena, the training effect, is the main research topic of this work. The training effect is a reduction of the horizontal shift of the hysteresis loop when cycling the bilayer through consecutive hysteresis loops. It has been widely accepted that the training effect is related to a change of the spin structure in the antiferromagnetic layer when cycling the exchange bias system in an external field. However, the microscopic origin of the training effect is still under debate. This is reflected by the large amount of models with different physical backgrounds that try to explain the training effect. The aim of this thesis is twofold. First, we want to gain further insight in the training effect and the associated domain structure in exchange biased polycrystalline Co (ferromagnetic layer)/CoO (antiferromagnetic layer) bilayers. The bilayer system is selected for its pronounced training effect and asymmetry of the first hysteresis loop after field cooling. Second, we want to reverse the training effect or, in other words, re-initialize the bilayer to the original state obtained after field cooling. In order to investigate (the reversal of) the training effect, several techniques are applied, including anisotropic magnetoresistance (AMR), low temperature magnetic force microscopy (LT-MFM), vibrating sample magnetometry (VSM), superconducting quantum interference device (SQUID) magnetometry and polarized neutron reflectivity (PNR). Polycrystalline Co/CoO bilayers were grown by magnetron sputtering or molecular beam epitaxy and subsequent in-situ oxidation at room temperature. After field cooling, the system showed a horizontal shift of the hysteresis loop and a considerable increase in coercivity. The first magnetization reversal after field cooling is abrupt and characterized by domain wall motion, while the following reversals are more rounded and dominated by a rotation of the magnetization. The asymmetric behavior of the first hysteresis loop after field cooling is a consequence of the training effect. A theoretical description of the training effect, based on the model of Fulcomer and Charap, shows that the granular structure of the oxide layer is responsible for the pronounced training effect in Co/CoO bilayers. Each antiferromagnetic grain has an uncompensated magnetic moment and, dependent on the crystalline anisotropy, some grains are stable while other grains rotate during a hysteresis loop due to the interaction with the ferromagnetic layer. The stable grains cause the exchange bias shift and the non stable rotatable grains are responsible for the increase in coercivity. The training effect is caused by the non stable grains because their uncompensated magnetization does not rotate back to the initial position after a complete Hysteresis loop. Low temperature magnetic force microscopy measurements have shown that a ferromagnetic magnetization reversal after field cooling creates ferromagnetic domains with a typical wavelength around 1 μm at remanence. The ferromagnetic domains, once created, remain present and detectable at saturation. The ferromagnetic roughness, which is formed after the first magnetization reversal, is a direct consequence of the misalignment between the uncompensated magnetization of the granular antiferromagnet and the applied magnetic field. In order to reverse the training effect or, in other words, re-induce the untrained state, the uncompensated magnetization of the unstable antiferromagnetic grains had to be more aligned along the cooling field. A straightforward approach is the application of a very large field aligned parallel to the cooling field direction. A DC magnetic field of 12 T had only little effect on the trained hysteresis loop. Furthermore, a pulsed magnetic field of 34.5 T reversed only very partially the training effect. The increase of the exchange bias shift after the application of a pulsed magnetic field directed parallel to the cooling field is an indirect evidence of the ferromagnetic nature of the interaction between the uncompensated spins of the antiferromagnetic grains and the spins of the ferromagnetic layer. The pulsed magnetic field measurements are not simulated by the extended model of Fulcomer and Charap, because the suggested energy equation does not contain a term that directly links the uncompensated moments of the antiferromagnetic grains to the external field. We also reported on the surprising possibility to largely re-induce the untrained state and asymmetry by performing a hysteresis measurement with an in plane external field oriented perpendicular to the cooling field direction, without changing the temperature of the bilayer. The next hysteresis loop along the cooling field obtained after the perpendicular hysteresis loop resembles the initial asymmetric hysteresis loop with a reduced amount of spin rotation occurring at the first coercive field. The reversal of the training effect is measured with several techniques: SQUID, AMR, LT-MFM and neutron reflectivity. It is even more surprising that the reversal of the training effect is strongly dependent on the magnitude of the applied perpendicular magnetic field. If the perpendicular field has a maximum value around 150 mT, the initial situation after field cooling (large training effect and asymmetry) is largely recovered. If the perpendicular field is increased above 400 mT, the training effect and asymmetry could no longer be recovered. It is possible to explain the recovery and disappearance of the training effect within the framework of the extended model of Fulcomer and Charap. If the perpendicular field is too low, the interfacial magnetization of the antiferromagnetic grains remains unaffected. If on the other hand, the external field becomes too high, the interfacial antiferromagnetic magnetization vectors rotate beyond their initial field cooling position, and the asymmetry again disappears. Since the model is able to consistently explain the evolution of the training effect and the asymmetry of the hysteresis loop, including the peculiar influence of a perpendicular magnetic field, we are confident to have identified the microscopic origin of the asymmetry and training effect in Co/CoO bilayers. Furthermore, it is shown, to our knowledge for the first time, that the AMR effect can be used to determine the average rotation sense of the ferromagnetic magnetization. This is accomplished by applying the in plane magnetic field under an angle of 45° with respect to the current direction. The measurements demonstrate that a cooling field direction exists along which the in plane magnetization rotates in one direction. This can probably be explained by taking into account the anisotropy of the ferromagnetic Co layer. The sense of the magnetization rotation can be changed with a perpendicular hysteresis loop that ends in the direction opposite to rotation direction of the magnetization reversal during a hysteresis loop after field cooling.status: publishe

Towards High-Quality Large-Area Single-Crystalline Monolayer Graphene: the Critical Role of the Metal Template

Oral presentation by Ken Vergutsstatus: publishe

Origin of the training effect and asymmetry of the magnetization in polycrystalline exchange bias systems

The training effect and asymmetry in exchange-coupled polycrystalline CoO/Co bilayers with in-plane magnetization has been investigated. This system is selected for its large training effect and initial asymmetry of the magnetic hysteresis after field cooling, which is removed after training. Applying an in-plane magnetic field perpendicular to the cooling field largely restores the untrained state with its pronounced asymmetry. The possibility to reinduce the asymmetry strongly depends on the magnitude of the perpendicular field, providing the key to identify the physical origin of training and removal of the asymmetry. These effects result from misalignment between the ferromagnetic magnetization and the uncompensated magnetization of the granular antiferromagnet.status: publishe