142 research outputs found
Studying individual magnetic nanoparticles with X-ray PEEM
No full textThe thesis addresses a subject with broad implications in various scientific and technical areas. It presents unique direct observations of the magnetic state of single particles of iron (Fe), cobalt (Co) and nickel (Ni) with nanoscopic dimensions by means of spatially-resolved X-ray magnetic circular dichroism (XMCD). The X-ray photoemission electron microscopy (PEEM) data are complemented with in situ reflection high energy electron diffraction (RHEED) investigations, ex situ scanning electron microscopy (SEM) and atomic force microscopy (AFM) measurements. This approach enabled to correlate the magnetic character of the particles with their individual size. The experimental findings are compared with calculated magnetic anisotropy contributions of the three different types of deposited nanoparticles (NPs). It was found that despite their different atomic structure, the body-centered cubic (bcc) iron and face-centered cubic (fcc) cobalt nanoparticles have a similar behavior and can exist in a state which demonstrates an unexpected ferromagnetic (FM) behavior with sizes down to 8 nm at room temperature (RT), while nickel particles only exhibit the expected superparamagnetic (SPM) behavior. This ferromagnetic state is assigned to an energetically excited, metastable structure which has a remarkably long life time before it decays into the expected superparamagnetic state. Combining PEEM with XMCD measurements allowed for the first time to follow the spontaneous transition from ferromagnetic to superparamagnetic behavior in single nanoparticles.
Detailed calculations of all magnetic anisotropy contributions for different sizes and types of particles indicate that the reported high anisotropy state can be associated with a meta-stable structural state due to the presence of local defects within the NPs, independent of the particle atomic structure and size.
These observations shed new light on the mechanisms which establish the size-dependent evolution of magnetic properties at the nanoscale
Thermal relaxation and ground state ordering in artificial spin ice
No full textWe have studied the thermal relaxation of artifcial spin ice in its two main geometries,
namely artificial square ice and artificial kagome spin ice. Using synchrotron based photoemission
electron microscopy we are able to directly observe how artificial square ice
systems find their way from an energetically excited state to one of the two degenerate
ground state configuration. On plotting vertex type populations as a function of time, we
can characterize the relaxation, which occurs in two stages, namely a string and a domain
regime. Kinetic Monte Carlo simulations agree well with the temporal evolution of the
magnetic state when including disorder, and the experimental results can be explained by
considering the effective interaction energy associated with the separation of pairs of vertex
excitations.
While a simple thermal annealing procedure, that involved one cycle of heating and
cooling the sample above and below the blocking temperature (T = 320-330 K), proved
to be very effective in achieving long-range ordered ground state configurations in artificial
square ice, the ability of achieving the same goal in artificial kagome spin ice is shown to
become increasingly difficult with increasing system size. By first focusing on the so-called
building block structures of artificial kagome spin ice, with system sizes ranging from a
single ring up to seven-ring structures, we proved that the abilitiy to access the ground
state is lost at a system size comprising seven kagome rings. Extrapolating the result
to extended arrays of artificiall kagome spin ice, we conclude that a long-range ordered
gound state is unlikely to be achieved in an infinite array of artificial kagome spin ice.
This conclusion is later confirmed by investigating thermal annealing on extended arrays
of artificial kagome spin ice.
Finally, we explored a potential optimization of thermal annealing on artificial kagome
spin ice. For this purpose, we patterned artificial kagome spin ice arrays with lower blocking
temperatures (T = 160K), hoping that the blocking temperature to be below the
predicted temperatures for phase transitions into ordered configurations. Both continuous
and stepped cooling from temperatures around 370 K down to 140 K proved to be
inefficient in achieving ground state configurations. We then applied an annealing procedure
that involved repeated heating and cooling just slightly around the blocking point
(T = 160K), thus allowing the system slow attempts in accessing ground state configurations,
while above the blocking point, and capture such configurational changes by cooling
back down below the blocking point. So far, this procedure represents the only known way
to access local ground state configurations in artificial kagome spin ice and paves the way
to explore even more sophisticated annealing procedures that remain to be discovered in
future work
Neutrons to probe nanoscale magnetism in perpendicular magnetic recording media
No full textMagnetic recording media refers to the disc shaped thin film magnetic medium present inside the hard disk drive of a computer. Magnetic recording is an important function of the hard disk drive by which information such as text, pictures, audio and videos are stored. Information is broken down to a simple binary format and is stored as magnetised bits along the tracks of the disk forming the hard drive. Over the years advancements in research on the type of magnetic materials used has allowed increased data storage capacities by reducing
magnetic bit sizes.
It is with this advancement in magnetic data storage, that we have today’s hard disk drive
technology, which uses a perpendicular magnetic medium to store data. A perpendicular magnetic medium is a multi-layered magnetic thin film structure with the topmost layer comprising nanoscale magnetic grains of high perpendicular anisotropy. The topmost recording layer (RL) is mapped into individual bits of 80-100 nm² area that consist of 5-10 nm diameter CoCrPt grains, embedded in an oxide matrix. A bit area is defined to ensure a significant number of stable grains allowing data to be stored in each bit as a ‘0’ or a ‘1’
depending on its switched magnetic state. The magnetic grains if sputtered below a threshold grain size tend to suffer from thermal fluctuation and instability due to super-paramagnetic effects, hence bringing limitations to grain size. As a result of this, research in recent years has been directed at introducing a softer magnetic exchange coupled composite (ECC) layer
above the recording layer. This layer facilitates the delicate balance of switching smaller grains with strong magneto-crystalline anisotropy at lower magnetic fields, by exchange coupling with the CoCrPt grains in the recording layer. However this technique of increasing the efficiency in the perpendicular magnetic medium by introducing ‘facilitating’ layers is an area that is still being widely researched and understood.
Although numerous surface and bulk analysis techniques exist to study magnetic and surface properties of these materials, there is limited information on the structural and magnetic properties of these materials at the nanoscale level. The reported work investigates the structural and magnetic properties of the magnetic grains and multi-layers in the perpendicular magnetic medium using polarised neutron scattering and reflectivity techniques. The work investigates the structural and magnetic properties of the CoCrPt grains, apart from understanding the CoCrPt magnetic grain switching. The work also investigates the
magnetisation in the layers of the thin film perpendicular media structure using polarised
neutron reflectivity (PNR).
Using polarised small angle neutron scattering (PolSANS), it has been shown that
ferromagnetic ordered core region of the CoCrPt grain in the recording layer is smaller than the physical CoCrPt granular structure. The magnetic switching behaviour of the CoCrPt grain at different magnetic fields is also analysed and the experimental PolSANS data is fitted with non-interacting size-dependent analytical grain switching models. This result provides significant evidence that the magnetic anisotropy increases with grain size, with larger magnetic grains having larger magnetic anisotropy.
Polarised neutron scattering experiments are carried out with the magnetically softer
exchange coupled composite (ECC) layer included in the thin film magnetic structure. The first experiments investigate if the ECC layer contributes to the nuclear and magnetic interference scattering term in the experimenting scattering data. The experiments clearly show that there is no contribution from the ECC layer in the nuclear and magnetic scattering interference term. The role of the ECC layer in the magnetic switching process is then investigated at different magnetic fields. The ECC layer was found to influence the size-dependent magnetic grain switching of the CoCrPt grains in the recording layer and a detailed investigation is presented in the reported work.
Polarised neutron reflectivity (PNR) experiments have also been carried out with the ECC layer on the perpendicular magnetic media samples. These experiments investigate the composition and thickness of the thin film structure, while also providing information on the magnetic state of the thin films under the influence of an in-plane magnetic field.The in-plane
magnetisation in the recording and ECC layer is determined at different in-plane magnetic
fields. The magnetisation values determined for the ECC layer and the recording layer (RL) at different in-plane magnetic fields help better understand the differences in their magnetic properties
Double-sided fresnel zone plates as high performance optics in X-ray microscopy
No full textX-ray microscopy describes a range of analytical techniques, specialized for the characterization of organic and inorganic samples using high energy photons. It takes advantage of the high penetration depth, high resolution and chemical sensitivity of X-rays and allows for the study of extended samples in their native environment without extensive sample preparation. Many of these experimental methods employ diffractive X-ray optics, like Fresnel zone plate lenses to obtain high spatial resolution or the better utilization of the incoming flux. Since improving the efficiency of zone plates can increase the throughput, quality and resolution of measurements, there is a constant demand for high efficiency and high resolution X-ray optics.
Stacking is an established concept for extending the capabilities of zone plate optics. By stacking two zone plates in each other's optical near field, they act as a single zone plate with combined optical transmission profile, that would be infeasible to make as a single optical element. Yet the existing implementations of stacking suffer from issues regarding complexity and stability.
This work presents the development of an alternative solution to conventional zone plate stacking, that circumvents most of its drawbacks. By patterning two zone plates on the front and back sides of a membrane, double-sided zone plates can deliver the advantages of stacked zone plates as inherently monolithic, single-chip optical elements.
Double-sided blazed zone plates with two complementary binary zone plates on the two sides of the membrane were produced to provide an effective four level transmission profile. This allowed to bypass the fundamental limitations of binary zone plates by providing up to 54.7% diffraction efficiency at 6.2 keV while having 200 nm smallest half-pitch and a reasonable working distance.
For high resolution zone plates, structure height is the main limiting factor. Therefore by patterning two identical zone plates on the two sides of the membrane, one can double the effective structure height. This provided us with a significant gain in focusing efficiency at high photon energies, as we have successfully measured 9.9% focusing efficiency at 9 keV with 30 nm smallest half-pitch, while preserving diffraction limited optical performance.
Stacking two complementary zone plates for multiplying their spatial frequencies opens the possibility for ultra-high resolution zone plate optics. We have successfully produced and tested interlaced zone plate optics down to 7 nm smallest half-pitch while still maintaining practical aperture sizes.
This thesis is a comprehensive summary of the work performed for the fabrication and characterization of the high performance zone plates representing each concept and provides possible examples for their future use
Organic semiconductors in the limit of a few monolayers : molecular surface doping of pentacene thin film transistors
No full textSpin dynamics in ferroic materials
No full textThe work presented in my thesis delivers new insights into the magnetoelectric coupling in strain mediated artificial multiferroics, and into laser-induced magnetisation processes in ferromagnetic and ferrimagnetic materials. The work mainly consists of three independent experimental results.
Firstly, I have investigated magnetoelectric coupling in Ni nanopatterned islands deposited on a PMN-PT ferroelectric single crystal. The results constitute the first experimental proof of a 90° uniform magnetisation rotation by the application of an electric field. Since the rotation of the magnetisation is complete, the corresponding magnetoelectric coupling coefficient is among the highest measured so far. I have found that the multidomain structure of the ferroelectric single crystal leads to a complex strain-mediated magnetoelectric coupling. This suggests that realising the full magnetoelectric stack at the nanoscale, in order to achieve a single domain configuration in the ferroelectric as well as in the ferromagnet, is of primary importance not only to fulfil large scale integration requirements but also to achieve a reliable magnetisation manipulation by an electric field. Since the electric field induced magnetisation reorientation is related to the strain generated in the ferroelectric as the polarisation switches, the ultimate speed of the reorientation process is limited by the propagation of ferroelectric domain walls which is significantly slower than ferromagnetic switching.
Next, I have investigated laser-induced magnetisation switching in nanopatterned GdFeCo nanostructures. Heat pulse induced switching was observed in nanostructures at different length scales down to a 200 nm out-of-plane domain located in the centre of a 400 nm wide nanostructure. Due to the structuring process, all nanostructures had in-plane edge domains that are found to play no particular role in the efficiency of the switching thanks to the strength of the driving force by which the switching occurs.
Lastly, I developed a novel experimental set-up that allows one to obtain a real time measurement of the transient magnetisation state induced by the excitation of a magnetic sample with an ultrashort near infrared (IR) pulse. My approach relies on ultrashort extreme ultraviolet (XUV) pulses from the free electron laser FLASH and an off-axis Fresnel zone plate to obtain a spatial encoding of the XUV-IR delay over a time window of 1500 fs. Using a single pump-probe event, we succeeded in capturing the ultrafast demagnetisation process of a cobalt thin layer with a time resolution comparable to laser based repetitive pump-probe experiments. The analysis of the statistical distribution of the demagnetisation time obtained from subsequent shots, suggests that no stochastic contribution are present in the early time period of the demagnetisation process.
The results presented in my thesis stress the importance of further investigating magnetisation processes at nanometer length scales and femtosecond time scales. Although this two aspects may seems apparently unrelated, the velocity of electrons in an itinerant ferromagnet is approximately 1 nm/fs. In this regard, further novel and exciting results are expected from experiments combining reciprocal space techniques with ultrashort pulses from free electron lasers in the soft x-ray regime
Ferroelectric control of magnetism in artificial multiferroic composites
No full textIn this thesis, we studied ferromagnet/ferroelectric heterostructures, so-called artificial multiferroic composites, which exhibit magnetoelectric coupling between different ferroic order parameters. For a range of material combinations, we found that electrical switching of the ferroelectric polarization induces non-volatile reversible magnetization changes in the magnetic constituent and we contributed to the understanding of the underlying interface coupling mechanisms.
The ferromagnet/ferroelectric system La_{0.7}Sr_{0.3}MnO_{3}/ [Pb(Mg_{1/3}Nb_{2/3})O_{3}]_{0.68}-[PbTiO_{3}]_{0.32} (011) (LSMO/PMN-PT) enables magnetoelectric control of the double exchange interaction via strain. Reversible electrical switching of the ferroelectric polarization induces a 10 K shift of the magnetic Curie temperature Tc. A similar magnitude in Tc change has been previously only observed under applied electric fields. Sweeping between oppositely out of plane (OOP) poled ferroelectric polarization directions, PMN-PT (011) may exhibit an in-plane (IP) poled state where the ferroelectric polarization lies in the surface plane. OOP and IP poled configurations are stable at remanence and reciprocal space maps highlight the accompanying lattice parameter changes which impose a biaxial strain on the manganite thin film. The magnetic response to the strain changes is probed by temperature dependent Mn L_{3,2} x-ray magnetic circular dichroism (XMCD) providing quantitative values of the Mn spin and orbital moment. X-ray natural linear dichroism spectra for both strain states probe changes in the valence charge anisotropy. Multiplet and density functional theory calculations support the picture that the existing population imbalance between out of plane and in plane oriented orbitals increases further with tensile strain, favoring orbital occupation in the surface plane. An increase in tensile in-plane strain leads to an increased energy difference between the two e{_g} orbitals and a larger Mn-O-bond length. Increasing the electron-lattice coupling and reducing the e{_g} electron itinerancy that leads to ferromagnetism due to the double exchange interaction, results ultimately in lower Tc values in agreement with the Millis model.
In Co/PMN-PT (011), we disentangle the strain and charge contributions to the magnetic response upon electrical switching, using XMCD at the Co L_{3,2} edges as the main probe.
Our results evidence the coexistence of two coupling mechanisms leading to three distinct magnetization states upon electrical switching. If the ferroelectric polarization is switched to the IP poled state, the corresponding lattice parameter changes in the PMN-PT exert a strain on the Co layer and induce an anisotropy change with higher remanent magnetization along the [011-] direction.
When comparing oppositely OOP poled ferroelectric polarization configurations, an additional Co anisotropy change is observed. Since the structure of PMN-PT in the two OOP poled states is equivalent, this dependence of the anisotropy must stem from the substrate polarity. The bound charge at the interface is expected to be screened by the cobalt metal within the Thomas Fermi screening length of a few Angstroms. We use a Co wedge geometry to study the magnetic response as a function of Co layer thickness employing XMCD with surface sensitive total electron yield detection. Consequently, the anisotropy change induced by the charged substrate is observed for the thinner part but absent in the thicker part of the Co wedge.
Lattice parameter values for cobalt and PMN-PT obtained by x-ray diffraction as well as domain distributions obtained from atomic force microscopy serve as an input for density functional theory calculations which reproduce the experimentally observed anisotropy behaviour for fcc (111) textured cobalt as a function of the lateral strain and charge.
Our investigation unravels how magnetoelasticity and interfacial charge density trigger changes in the magnetic anisotropy. The observed coexistence of multiple coupling mechanisms opens up the possibility to tune and enhance the cross-coupling between layers in heterostructures.
The possibility to induce ferromagnetism in a per se paramagnetic system via electrical switching is explored for a Pd/Pb(Zr_{0.2}Ti_{0.8})O_{3} heterostructure.
Pd has a large magnetic susceptibility and is close to fulfilling the Stoner criterion for magnetism. According to calculations the polarity of adjacent ferroelectric layers could trigger a paramagnetic/ferromagnetic transition in paramagnetic metals by introducing shifts in the density of states.
No XMCD difference signal upon ferroelectric switching was found within the noise ratio of 0.2% at the M_{3,2} edge and of 1% at the L_{3,2} edge
Manipulation of magnetic and chemical properties of cobalt nanoparticles studied by means of x-ray photo-emission electron microscopy
No full textInvestigation of structure and magnetic properties of nanoparticles is important for application in catalysis, data storage, chemical sensing, energy conversion and drug delivery. Laser manipulation of magnetization of the nanoparticles can be promising for next generation data recording technology.
Large scattering of magnetic properties of cobalt nanoparticles makes it difficult to apply simple scaling laws. Most probably this is due to the measurement techniques averaging over a large number of the nanoparticles with different crystal structures, internal defects and morphology. In our approach we use an outstanding combination of characterization techniques that allow us to directly correlate magnetic properties in individual cobalt nanoparticles with their crystal structure and morphology. We use x-ray photo-emission electron microscopy (XPEEM) for magnetic characterization of the nanoparticles and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) for atomic resolution structural characterization. Our results show that magnetic blocking in cobalt nanoparticles occurs independently of the particles size and the orientation of the magnetization does not correlate with crystallographic axes. Our structural investigations suggest also that many of the particles have defects which modify the magnetic anisotropy. We have developed atomistic models for STEM simulations and compare them to the STEM data to prove the nature of the defects and their positions in the nanoparticle. Still ongoing is the development of a theoretical approach for calculating the magnetic properties of nanoparticles with defects.
We combine XPEEM and HAADF-STEM approach to correlate magnetic properties and chemical composition of cobalt nanoparticles with the actual morphology upon in situ oxidation. Understanding the role of the surface is important for revealing the origin of magnetically blocked states of cobalt nanoparticles smaller than 15 nm, oxidation kinetics and the products of the reaction is important for catalysis. Most of the studies rely on x-ray absorption spectroscopy and modelling of the spectra. However, early oxidation kinetics of cobalt nanoparticle remained unclear. We show that reduction of magnetic volume upon oxidation lowers the magnetic energy barrier. Our STEM data show a surprisingly complicated oxidation kinetics, which is not properly reflected in simulated x-ray absorption spectra. The early stage of oxidation leads to formation of inhomogeneous shell on the nanoparticles, dosing more oxygen improves the shell morphology, even further oxidation leads to thickening of the shell.
Purely optical magnetization orientation reversal with femtosecond laser pulses (all-optical switching) was shown in thin films and granular media of various materials. So far no such results were presented for single nanoparticles with diameter smaller than 15 nm. We combine XPEEM with femtosecond laser pulse exposure to investigate the effect of ultrashort laser pulses on cobalt nanoparticles. No deterministic switching is found independently on laser fluence and polarization. Also, no thermal switching of nanoparticles magnetization is observed. Instead, we find that laser triggers a chemical reaction with the substrate which alters magnetic energy barrier in the nanoparticles. Our results suggest that for a successful laser-induced switching of the magnetic nanoparticles, nanoparticles with lower Curie temperature TC either defined by size effects or by choosing different materials are required.
Summarizing, our investigations show that structural defects are important for magnetic properties of cobalt nanoparticles, especially for stability of their magnetization and orientation of the magnetic moment. We found complex oxidation kinetics, which is important for better understanding of catalysis and magnetic behavior. Femtosecond laser excitation of magnetic nanoparticles seems promising but for materials with lower TC. Higher resolution x-ray imaging is needed to reveal spin configuration of the individual nanoparticles time and better investigate the chemical composition and magnetic properties of oxidized cobalt nanoparticles
Scanning nanomagnetometry : probing magnetism with single spins in diamond
No full textScanning nanomagnetometry based on the electronic spin of the nitrogen vacancy (NV) center in diamond is an emerging sensing technology, which allows for the probing of magnetic fields on the nanoscale. High sensitivity, of a few tens of nT, can be achieved by exploiting the extraordinary properties of this special lattice defect. Incorporating this atomic sized sensor in the apex of all-diamond scanning probes allows controlled proximity of the NV center and a sample to be achieved. The resulting resolution of a few tens of nm in combination with an NV's sensitivity offers unique possibilities for exploring new physical properties or phenomena.
In this thesis, we developed and characterized a high performance scanning NV magnetometer and we demonstrate its potential for probing magnetic fields in two applications. We implemented a procedure to fabricate single-crystal, all-diamond scanning probes and developed a highly efficient and robust approach for integrating these devices into our setup. The resulting sensitivities of nT for DC and nT for AC-magnetic fields and resolution of nm enabled real space imaging of the stray field of an antiferromagnet and the imaging of microwave magnetic fields with unprecedented spatial resolution. Both applications illustrate the potential of this powerful technique for imaging weak magnetic fields and revealing physical properties that are inaccessible with alternative approaches. Scanning NV magnetometry therefore forms an attractive, new technique, which will have a profound impact on many different research areas ranging from magnetism and advanced material sciences to spintronics and quantum computing
Artificial multiferroic heterostructures: magnetoelectric coupling and dynamics
No full textArtificial multiferroics consist of materials systems engineered to have a coupling between multiple order parameters at the interface, such as between magnetic and ferroelectric order (magnetoelectric coupling) which enable the electric field control of magnetism suitable for applications in energy efficient storage or sensor devices. In this thesis we investigate two types of magnetoelectric coupling, namely, strain-mediated and charge-mediated, with a goal of characterizing their dynamic behaviour. For strain-mediated coupling, we considered a system consisting of Co dots fabricated on a ferroelectric BaTiO3 thin film, where application of an electric field led to a change in magnetic domain structure induced by the piezo-strain; however, we find that the process is stochastic as a consequence of a strong pining of the Co magnetization induced by the high surface roughness of BaTiO3 making it unsuitable for pump and probe dynamical characterization. A second type of system investigated consists of perpendicular magnetic anisotropy (PMA) structures deposited on a silicon nitride membrane gate dielectric, where we used the charge screening effects to modulate the charge carrier density at the metallic/silicon nitride interface. We studied two types of tri-layer structure (i) Pt/Co/Pt/Si3N4 and (ii) Pt/Co/Ta/Si3N4, where the Co thickness is chosen to be at spin reorientation transition. For Pt/Co/Pt, we find the presence of a charge mediated magnetoelectric coupling in the form of domain nucleation and domain wall fluctuations dependency with the electric field; from the latter we estimate a change in energy barrier height of about 10 %. For Ta/Co/Pt heterostructures a net Dzyaloshinskii-Moriya interaction (DMI) is expected and the goal was to investigate the possibility to control the DMI and/or skyrmions with applied electric fields. For these structures we observe the presence of out-of-plane spin structures in an in-plane dominant magnetized surroundings. The out-of-plane spin structures resemble a Neel type skyrmion with a dimension from 200 nm to 2 µm at room temperature under no external magnetic field. We demonstrate that such out-of-plane spin structures can be manipulated by changing the anisotropy of the system with electric fields. The measured capacitive rise time of a 200 nm thick silicon nitride membrane is ~140 ns making it suitable for high frequency characterization; however, we find that the presence of charge traps and/or charge defects in the silicon nitride membranes preclude a systematic control of the magnetization. In this context, we characterize the dielectric time response of different dielectrics, including stoichiometric silicon nitride membranes, AlN, Al2O3, BaTiO3 and MgO grown by physical vapour deposition (PVD) methods. We find that all dielectrics have a significant density of charge defects and/or charge traps. From capacitance vs frequency characterization, we find that the capacitance decreases with increasing frequency; since the mobility of carrier charges such as electrons is independent of the measuring frequency and we measure a higher capacitance at lower frequency, it is likely that we are also moving ions or possible vacancies with the applied electric field along with bound electrons, as ionic mobility with electric field is slower than electron mobility. Our results suggest the importance of characterizing and optimizing the dielectric time response for high frequency charge mediated magnetoelectric devices
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