753 research outputs found
Hoogwaterafvoer voorspelling Borgharen: Een bijdrage uit deelstroomgebieden
No full textDe Hoogwatervoorspelling voor de Maas is tot op heden altijd gemaakt Vanuit het looptijden beginsel. Het huidige model correleert de waterstanden van de Maas te Jambes, bij Namen, en de Ourthe bij Comblain-au-Pont aan de waterstand, 7 uur later, bij Borgharen. Een station aan de Ourthe is niet zonder reden gekozen omdat het totale strcomgebied van de Ourthe (3626 [km2]) een maatgevende afvoer in volume en tijd voor de Maas geeft. De voorspellende waarde van het huidige model is, numeriek gezien, hoog. De waterstanden worden inderdaad nauwkeurig voorspeld. Het grote nadeel is d korte voorspellingstijd. Dit maakt de operationele waarde, met het oog op het uitgeven van waarschuwingen, gering. Naar een verbetering van de hoogwatervoorspelling wordt al sinds 1926 gezocht. Aanleiding was toen het Winter Hoogwater van 1925/26. Sindsdien zijn er een aantal pcgingen gedaan om tot een goedwerkend waarschuwingssysteem te komen. Breekpunt hierin is vaak een gebrekkige informatie uitwisseling geweest. De benodigde gegevens kwamen niet altijd op tijd binnen. Een andere oorzaak van de steeds teruglopende waarde van de modellen is de bouw van stuwen op de Maas en de Sambre geweest. Nu is dat de bouw van stuwmeren. Ingrepen in het stroomgebied hebben de opgestelde relaties om een Hoogwater te voorspellen. de looptijden, altijd verstoord. Dit rapport geeft een model voor de afvoervoorspelling voor de Maas te Borgharen. De voorspellingstijd voor Borgharen is van 7 uur vergroot tot 30 uur. Het model voor de Hoogwatervoorspelling bestaat uit de superpositie, na een translatie voor de looptijd, van voorspelde afvoeren van een twaalf-tal deelstroomgebieden. Een dertiende deelstroomgebied, at van de Franse Maas, wordt vertegenwoordigd door een randvoorwaarde: het debiet bij Chooz.Hydraulic EngineeringCivil Engineering and Geoscience
Device properties of the spin-valve transistor and the magnetic tunnel transistor
Get PDFSpin electronics is a new research area, which not only uses the electron’s charge but also its spin. By using the electron spin dependent properties of magnetic materials one can make devices with a new functionality. This has lead to magnetoresistive devices that can change their resistance by 10 to 50% in small magnetic fields, such as giant magnetoresistance (GMR) devices and the magnetic tunnel junction (MTJ). This large resistance change can be used in applications such as read heads or serve as memory elements in a magnetic random access memory (MRAM). This thesis describes two devices: the spin-valve transistor (SVT) and the magnetic tunnel transistor (MTT). The SVT has an unique property, namely its huge relative collector current change of more than 300% in small magnetic fields at room temperature. This unique property by itself is not enough to warrant the applicability of the SVT. The other properties that are important for the applicability of the SVT are described in this thesis. An alternative to the SVT, the MTT, will also be discussed. The SVT is a hybrid device that generally has an n-Si/ Pt / Ni82Fe18/ Au/ Co/ Au/ n-Si structure. The Pt / Ni82Fe18/ Au/ Co/ Au multi layer is the base and the two semiconductors on each side are the emitter and the collector respectively. The SVT is used in the common base configuration, where the emitter barrier (Si/Pt) is forward biased and the collector diode is zero or reversed biased. A flow of electrons from the silicon over the Schottky barrier into the metal base starts when the emitter is forward biased. These electrons have an excess energy compared to the Fermi level of the base and move in the direction of the collector. The electrons that are scattered in the base will lose their energy or momentum and make up the base current. Only those electrons that reach the collector with the right momentum and a high enough energy can enter the collector. The collector current is thus extremely sensitive to the scattering conditions in the base. The scattering conditions in the Ni82Fe18 and the Co layer are different for the spin-up and spin-down electrons. This makes the total scattering dependent on the relative magnetizations of the Ni82Fe18 and the Co layer. The collector current is largest when the magnetizations are parallel (I P C ) and smallest when the magnetizations are anti-parallel aligned (I AP C ). The relative change in collector current is called the magnetocurrent (MC = (I P C - I AP C )/I AP C ). This PhD research started with the development of a reliable process for fabricating spin-valve transistors. The introduction of this process together with the introduction of an ultra-high vacuum metal-evaporation system and the right choice of materials resulted in the SVT’s that exhibit an MC of more than 300% at room temperature. This thesis starts with a study on the size dependence of the magnetic and electrical properties of the SVT. We extended the previously mentioned process by using silicon on insulator (SOI) wafers, a combination of dry and wet etching techniques and SU8 (a negative tone photoresist) as an insulator layer. We were successful in producing SVTs with lateral dimensions that ranged from 300µm by 300µm to 10µm by 10µm. As expected, we saw no influence of the dimensions on the Schottky barrier height. Moreover the reverse current scaled down linearly with area. Both observations show that we have high-quality Schottky diodes. The key property of the SVT, its MC, showed no size dependence and remained constant around 240% for all dimensions. The transfer ratio is the ratio between the applied emitter current and the measured collector current. This ratio showed a slight decrease for transistors with dimensions below 25µm by 25µm. This is attributed to a deterioration of the emitter efficiency. The maximum possible emitter current decreases with transistor dimensions. The limiting factor is the maximum possible current density in the spin-valve base, which is 1.7 × 10 7 A/cm 2 . This value agrees with electromigration failure of spin valves. We have shown that it is possible to scale the lateral dimensions of the SVT down to 10µm by 10 µm. In my view further scaling down is limited to the physical height needed for the emitter, which includes the depletion width for the Schottky barrier and the doping profile needed for the Ohmic contact. To characterize the noise sources of the SVT we studied the frequency spectrum of three types of transistors that differed only in the type of metal base. The measurement showed that the frequency spectrum of the transistor with only non-magnetic layers in the base was completely dominated by shot noise in the frequency range of the measurement (10 Hz to 100kHz). The inclusion of one or more magnetic layers lowered the collector current and thus the level of the shot noise. It did not however change the nature of the noise or add noise (of magnetic origin) to the collector current. The collector current spectral density (SI) changes linearly with IC in a quasi-static magnetic field as expected for shot noise. We have however not observed 1/f noise in our measurements, not even at the switching fields of the spin valve. With this knowledge we can calculate the signal to noise ratio (SNR) of the SVT. The SNR increases with increasing MC and also with the absolute value collector current. From the basic relation IC = α IE we see that we can increase the collector current by either increasing the emitter current (IE) or the transfer ratio (alpha). We saw before that the emitter current has an upper limit imposed by device breakdown, therefore the way to enlarge IC is to improve the transfer ratio. We started to improve α by enlarging the energy difference between the emitter and collector barrier. The transfer ratio increased with increasing energy difference due to the larger number of states available at the collector semiconductor when electrons arrive with a higher energy. The transfer ratio also improves when materials with longer attenuation lengths are used in the base, i.e. Au instead of Pt. The influence of the SVT’s structural quality on the transfer ratio is demonstrated by the optimum in collector current versus Pt layer thickness. Furthermore, by varying the thickness of the NiFe layer we were able to prove that there is a maximum in the absolute current change for a certain thickness, due to the trade-off between transfer ratio and MC. The same study yielded a value for the attenuation of an interface, which is a factor of 0.55. The influence of crystal orientation on the transfer ratio was found to be negligible. Temperature effects on the transfer ratio are weak and are due to the spatial distribution of Schottky barrier heights and thermal spin wave scattering. Summarizing, we improved the transfer ratio by a factor of 118 from a Si/Pt/NiFe/Au/Co/Pt/Si SVT compared with a Si/Au/NiFe/Au/Co/Cu/Si transistor, while the MC remained constant above 200% and showed only small and non-systematic changes. The latter implies that the collection of both the spin-up and spin-down electrons can be improved, resulting in an increase in collector current without affecting the MC. The best results so far for SVTs are with a Si/ Au (20Å)/ Ni82Fe18 (30Å)/ Au (70Å)/ Co (30Å)/ Au (40Å)/ Si SVT, it has a transfer ratio of 1.2 × 10 4 and an MC of 230%. Further improvement of the transfer ratio might result from better control over the quality of the complete SVT structure. Another option is to use a tunnel barrier on the emitter side. This not only allows one to further enlarge the energy difference between the injected electrons and the collector Schottky barrier, but also opens up the possibility to remove layers from the base if a ferromagnetic emitter electrode is used, as in an MTT. Magnetic tunnel transistors have been successfully realized with the use of in situ shadow mask technology. Already we achieved a transfer ratio equal to that of SVTs, while the MCof the MTT is above 100%. The MTT has a Si/ Co/ Al2O3/ CoFe/ IrMn/ Ta structure. We have shown that the MTT can be used to determine a lower limit for the tunnel spin polarization of a ferromagnet/insulator interface. With a MTT this lower limit can be determined in a large temperature and tunnel-barrier bias range. The transfer ratio measured versus tunnel-barrier bias continues to increase, due to the larger number of available states at the collector at higher energies. More research is needed to explain the tunnelbarrier bias dependence of the MC. We expect that MTTs can be improved by using evaporation techniques rather than sputter techniques. Furthermore the quality of the collector diode can be improved with a corresponding increase in transfer ratio by choosing the right materials. The comparison of the SVT and MTT with tunnel junctions in terms of signal, noise, scalability, frequency response, robustness and of course the ability to study the properties of spin-polarized hot-electrons in magnetic materials justifies the further research of SVTs and MTTs. Last modified: May 16, 2002 by Hans
Interface engineering of spin-tunnel contacts to silicon - towards silicon-based spintronic devices
Get PDFDit proefschrift gaat over spin-elektronica componenten gebaseerd op silicium en beschrijft onderzoek naar de belangrijkste aspecten en knelpunten voor de ontwikkeling van silicium spintronica componenten, en de oplossingen daarvoor. Door eigenschappen van magnetische materialen en halfgeleiders te combineren en gebruik te maken van de spin van het elektron, geeft halfgeleider spintronica nieuwe mogelijkheden voor de ontwikkeling van elektronische componenten. Een voorbeeld is de Si spin-MOSFET, een spintransistor met een Si kanaal en twee ferromagnetische contacten, waarbij de geleiding door het kanaal tussen de contacten kan worden beïnvloed via de relatieve oriëntatie van de magnetisatie van beide contacten. Silicium is het aangewezen materiaal voor een spin-transitor, daar het de bestaande halfgeleider technologie domineert, en bovendien kan worden verwacht dat de levensduur van de spin van elektonen in Si zeer lang is
Canticlever - Planar fabrication of probes for magnetic imaging
Get PDFMagnetic force microscopy (MFM) is a scanning probe technique that is used to generate a high–resolution image of stray fields above a magnetic thin film, with very little sample preparation. This makes the MFM the instrument of choice for the analysis of bit patterns in hard disks. The tremendous increase in data density of magnetic storage systems has pushed the bit size down into the nanometer range, close to the maximum resolution of current MFM’s. This resolution, which is currently limited by the geometry of the magnetic tip, has to be improved, or the hard disk research community will loose a very powerful imaging technique. In this thesis a new probe for magnetic microscopy and recording, called the CantiClever, is presented. This probe is designed to address the issue of the tip geometry of conventional MFM tips. The new probe also allows for integration of other sensors besides an MFM tip with relative ease due to its planar fabrication process
Sputtered thin films for high density tape recording
Get PDFThis thesis describes the investigation of sputtered thin film media for high density tape recording. As discussed in Chapter 1, to meet the tremendous demand of data storage, the density of recording tape has to be increased continuously. For further increasing the bit density the key factors are: increasing media coercivity and reducing the magnetic film thickness. To meet further requirements for recording, tape media with high magnetic anisotropy are desired. Since the shape anisotropy is limited, magnetocrystalline anisotropy is more suitable. Therefore, the motivation of this thesis is to produces a magnetic layer on a polymer substrate with high magnetic anisotropy and low film thickness. We applied the sputtering technology since this technique offers higher energy for the arriving atoms than vacuum evaporation. In addition, deposition of alloys with complex composition can also be made easily by sputtering. In Chapter 2, we describe the main experiment methods that we used in the thesis. The followed chapter presents some relevant aspects as well as results of oblique Co and Co alloy directly sputtered on polymer substrate. The first part of this chapter is dedicated to describe the mechanisms of oblique deposition such as shadowing and steering effect. In the second part of this chapter, experimental results of obliquely sputtered Co and Co-alloys on polymer substrate are presented. The pure Co films mainly consists of fcc structure and this gives a low anisotropy. In this case, the most important source of anisotropy is the shape anisotropy of the columns. At high angle of incidence (70), thick films exhibit well-separated columnar structure and due to the shape anisotropy this results in a moderate anisotropy which con- fine an in-plane easy axis in the longitudinal direction (Hc = 50 kA/m and Ha = 230 kA/m). In contradiction, at small thickness, the elongated nuclei caused by steering effect switches the in-plane easy axis to the transverse direction. At intermediate incident angle (50 degrees), the anisotropy is very small and directed in the longitudinal direction. As the consequence of the strong exchange coupling, stripe domains following the longitudinal direction are observed. We also investigated the effect of a commercial metal coating deposited on top of the polymer substrate on the properties of a sputtered Co layer. It has been found that such coating layer cannot alter the crystallographic structure of the Co. Thus also in this case the shape anisotropy is still the main source. Nevertheless, the coating layer has a certain effect in modifying the microstructure of the films, i.e the size of the nuclei. If we sputter a CoCrPt alloy directly on polymer, the film consists of mainly hcp phase, and consequently the magnetic anisotropy is much improved (Hc = 150 kA/m and Ha = 600 kA/m). In addition, the crystal size of the CoCrPt films is much smaller than that of Co films at the same thickness. This result indicates that composition modification is also an effective way to obtain films with desired properties. Chapter 4 deals with the properties of Co and Co alloys obliquely sputtered on different underlayers. In this chapter, we have shown that the introduction of Cr underlayer have a significant effect on the formation of hcp Co. When the thickness of Cr is high enough, the anisotropy and coercivity of the film is much higher than in the case of Co sputtered directly on polymer substrate. A good epitaxy relationship between Co and Cr is also found. The growth mode of Cr prefers (110) texture, which results in Co (10.1) texture with results in a c-axis orientation tilted out of the film plane. This texture is more pronounced in thick Co films. In case of thin films, the in-plane anisotropy is confined in the transverse direction. This effect is explained by the elongation of Cr grains in this direction. The epitaxial growth of Co on Cr then induced also an elongated structure of the Co grains. As a result, highly orientation of easy axis of Co in this direction is found. In the static configuration of sputtering, high coercivity (Hc of 200 kA/m) and high anisotropy constant (Ha of 4.6x105 J/m3) are obtained in the film sputtered at incident angle of 70with Co thickness of 20 nm and Cr thickness of 180 nm. Since the high anisotropy Co film can be obtained with low thickness it is interesting to realize this type of film as media for tape recording. We have attempted to produce experimental tapes with a high coercivity of 160 kA/m, low thickness of 20 nm and bi-directional recording behavior. The recording properties can also be further improved by the refinement of grains using NiAl seed layer. It is shown that the recording performances of our experimental tapes are closed with that of MICROMV tape, which is the most advanced ME tape commercially available. In the last part of this chapter, we also presented a study of CoCrPt/CoCrMn bilayer. This type of films exhibits excellent properties with a coercivity of 300 kA/m at a thickness of 30 nm, very small and well-defined crystal structure with crystal sizes of less than 10nm. This type of film can be a very promising candidate for high-density tape recording in the near future. Another somewhat different material, FePt, has been studied in Chapter 5. This material with a huge crystalline anisotropy has been chosen since it is also a potential candidate for ultra high-density hard disk recording. It shows by using proper sputtering conditions the hard mag- netic fct phase. Even at moderate annealing temperatures of 350 C - 400 C we proofed the presence of the fct phase. This hard magnetic phase acts at pinning sites preventing the domain wall movement of soft fcc phase. Therefore, high coercivities of 600 - 700 kA/m can be achieved. We also show that different factors such as Ar pressure and film thickness strongly influence the formation of fct phase. Increasing the Ar pressure promotes the formation of the fct phase. However, when the Ar pressure is too high the crystallinity of the films is degraded and the fct phase can not be formed. In our system, we found an optimum pressure of around 2x10-2 mbar. The thickness of the film also influences the ordering process: the transformation from fcc to fct structure takes place easier in thicker films. When the thickness of FePt is reduced to 10 nm, annealing temperature of 400 C is not sufficient to allow the formation of fct phase. In addition, the effect of Ag underlayer has been studied. Although Ag films sputtered at room temperature have (111) texture and consequently the (001) texture of FePt layer has not been promoted, it is obvious that the degree of ordering can be strongly enhanced by introduction of Ag underlayer. Ag underlayer can also provide pinning sites, which increase the coercivity and reduce the inter-grain exchange coupling. Although at this current state, our sputtered FePt films are not suitable for recording due to the existence of a strong exchange coupling, the obtained results would be benefit for further development of sputtered tape with very high density using novel FePt material. Finally, in Chapter 6 the main conclusions are given and the recommendations for future study are discussed. Last modified: Aug 27, 2004 by Hans
Patterned magnetic thin films for ultra high density recording
Get PDFThis thesis describes the results of a research project in the field of high bit-density data-storage media. More specifically, the material aspects of the novel recording technique using patterned media have been studied. The aim of the work was the design, realization and characterization of such a patterned medium. Chapter 1 provides a general introduction to the field of data storage. The incredible progress in bit density of today's data storage devices is highlighted. Moreover, an outlook to future developments is given. It is shown that patterned media, due to their discrete and single domain nature, have superior recording properties and allow much higher bit densities than conventional hard disk media. In Chapter 2 the material requirements for a prototype patterned medium are given. A proper patterned medium should consist of magnetic dots which have a strong intergranular exchange coupling, large uniaxial magnetic anisotropy and low switching field distribution. Using these guidelines several candidate materials are proposed: bariumferrite, Co or Fe based alloys with L10 phase, amorphous rare earth - transition metal alloys and Co based multilayers. All four materials have a large intrinsic uniaxial magnetic anisotropy, which guarantees a sufficiently large switching field and a long-term thermal stability. Moreover, the dots of the patterned medium can be shaped in such a way that magnetostatic interactions are suppressed. In this respect, single-element Co, Ni or Fe patterned media are disadvantageous because they suffer from too large magnetostatic interactions in a densely packed 2D dot-array and therefore will limit the ultimate bit density. With respect to the amorphous alloys some caution is required, because the origin of their magnetic anisotropy is not undisputed. In addition to these qualitative considerations, the single- to two-domain transition and the switching field of Co50Ni50/Pt multilayer dots have been predicted by analytical and micromagnetic calculations: dcrit » 70 nm and Hsw ³ 500 kA/m. However, it is also shown that the latter is strongly dependent on dot shape and can be considerably smaller. In Chapter 3 the patterning technology for our studies on submicron patterned magnetic thin films is motivated and discussed. A review of existing submicron patterning technologies shows that laser interference lithography uniquely combines simplicity, large areas and sub 100 nm dimensions. With the present process sub-100 nm resist structures at a period of 200 nm can be prepared. However, at these dimensions the process latitude is limited. With a higher contrast resist in combination with an antireflective coating considerable improvement is achieved. Although further progress may be limited by instabilities of the present exposure setup, lasers with smaller wavelength and improved (substrate) stability allows the succesfull patterning of dot sizes smaller than 50 nm with our technique. In Chapter 4 an extensive study into the relation between deposition conditions, microstructure and magnetic anisotropy of sputtered Co50Ni50/Pt multilayers is presented. The degree of texture, as determined with X-Ray Diffraction, appeared to be strongly dependent on the total layer geometry and deposition pressure. At low deposition pressure the multilayers have a low atomic roughness and a strong preferred (111) texture, while at high deposition pressure the multilayers have a large atomic roughness and an additional (200) texture is present. Moreover, if deposited without underlayer, the first several nm's of the multilayer stack have a lack of texturing, even at low deposition pressure. The magnetic anisotropy consists of interface, shape and magneto-elastic anisotropy. The magneto-elastic anisotropy contributes to the perpendicular direction, but is only 10-20% of the shape anisotropy. Therefore, interface anisotropy is the most significant contribution to the effective perpendicular magnetic anisotropy. Thin multilayers have a lower average interface anisotropy because of the 'initial layer effects'. Besides, due to an increase in interfacial mixing by high-energy bombardment, the interface anistropy decreases towards lower deposition pressure. On the other hand, due to the increase of atomic roughness, the effective area and herewith the effective interface anisotropy decreases towards higher deposition pressure. Therefore, in order to optimize perpendicular magnetic anisotropy, the multilayers should be as thick as possible and be deposited at an intermediate pressure. Finally, based on these results and the constraints of the patterning process, the deposition conditions of the Co50Ni50/Pt multilayers for the research on patterned media were selected. This multilayer has 26 bilayers of Co50Ni50 (6 Å) / Pt (6 Å) and is deposited at 12 mbar. It has a large perpendicular magnetic anisotropy (HK,eff = (4.7±0.2)·102 kA/m) and a strong intergranular exchange coupling. Therefore, the basic requirements for a patterned storage medium have been fullfilled. In Chapter 5 the magnetization reversal in submicron and micron-sized Co50Ni50/Pt multilayer dots is described. It was checked that the patterning process (deposition-resist spinning and patterning-ion beam etching) does not deteriorate the magnetic properties of the multilayers. Instead, an improvement of the perpendicular magnetic anisotropy was found. The magnetization reversal of Co50Ni50/Pt multilayer dots is strongly dependent on dot size (i.e. diameter). In fact, over the studied range of sizes, i.e. 60 nm - 215 mm, three main types of reversal can be distinguished. For dots larger than 20 mm the reversal is dominated by small structural defects in the as deposited multilayer. These dots have a broad nucleation field distribution and domains are formed during reversal. Dots with a size between 140 nm and 20 mm are actually multi domain as well, but no domains are formed during the perpendicular (easy-axis) reversal. Finally, dots smaller than 140 nm can not be demagnetized and appear as single domain dots. Their mode of reversal is incoherent rotation. The most important conclusion of Chapter 5 is that with the preparation of 70 nm Co50Ni50/Pt multilayer dots over a large area (order of cm2 and 200 nm center-to-center spacing) a prototype of patterned storage medium has been realized. This 16 Gbit/in2 medium consists of truly single domain dots with large uniaxial magnetic anisotropy. The thermal stability is not sufficient for long-term stability, but by an optimization of the patterning process this can be enormously improved without the need for another material. Due to the lack of magnetostatic interactions between dots and the presence of a strong intrinsic perpendicular magnetic anisotropy, this patterned medium has superior storage properties compared to single-element (Co or Ni) patterned media reported in literature so far. In Chapter 6 the experimental values of properties such as critical dot diameter and switching field, as presented in Chapter 5, are compared with the (theoretical) predictions made in Chapter 2. In particular, the switching field seems to be extremely sensitive to effects of shape, etching damage and/or quality of layer growth. This leads to a switching field distribution, which is hard to control. More in general, Chapter 6 shows that in this relatively new field of research many challenges are still present. This applies both for the development of the application of magnetic dots as patterned media and for the improvement of present models of the magnetization reversal of single domain particle
Magnetotransport of hot electrons and holes in the spin-valve transistor
Get PDFThe conventional electronics uses the charge property of the electrons and holes. The building blocks are semiconductors which can be tuned to change the properties of the devices. In the field of spintronics, the spin property of the charge carriers is added to the functionality of the devices. The spin-valve transistor (SVT) is one of the spintronics devices which allows us to study the spindependent transport characteristic of non-equilibrium electrons and holes in semiconductor/ferromagnetic hybrid structures. The SVT uses Schottky barriers to inject and collect hot-carriers. In the metal base of the device, two ferromagnetic metals separated by a nonmagnetic metal are utilized to analyze the spin-dependent transport of the electrons and holes. In this thesis, one of the motivations is to understand the role of interfaces in the hot-electron transport. Another motivation is to study the spin relaxation in metals which is crucial for adding the functionality of spin property. Finally, it is important to understand the hot-hole transport since the devices working with holes and electrons offer better performances
Hot-electron transport in the spin-valve transistor
Get PDFThis thesis discusses research on the hot-electron transport in the spin-valve transistor (SVT). This 3-terminal device consists of a silicon emitter and collector with in between a base consisting of magnetic (NiFe and Co) and non-magnetic (Au) metal layers, a so-called spin-valve multilayer. Furthermore, the base includes thin layers of Pt and Au to form two different Schottky barriers with the Si emitter and collector. The collector current is dependent on the amount of current that is injected from the emitter into the Pt/NiFe/Au/Co/Au base, and on the magnetic state of the spin-valve multilayer. When the NiFe and Co layers are magnetized in the same direction (parallel), more collector current is measured, than when the layers are magnetized oppositely (anti-parallel). As described in this thesis, the spin-valve transistor can operate at room temperature and shows a large relative change in collector current (magnetocurrent > 300%) within small magnetic fields of only some Oe’s. Therefore, the spin-valve transistor is extremely suited to measure magnetic fields
Magnetic media patterned by laser interference lithography
Get PDFThe scope of this thesis consists on the fabrication and magnetic characterization of CoNi/Pt and Co/Pt patterned films with perpendicular anisotropy with possible applications as an information storage medium. We have realized patterned magnetic nanodots arranged in square and hexagonal lattices with periodicites ranging from 600 nm down to 300 nm, which corresponds to bit densities from 1.8 Gb/sqi up to 7.2 Gb/sqi. The main aim has been focused towards the understanding of the problems involved in the fabrication of large samples (1 × 1 cm till 3 × 3 cm) and their write-ability. We discuss about the magnetic properties of an idealized magnetic pattern medium. Issues as the thermal stability, required magnetic anisotropy, bit to bit interaction and its consequences for the material’s switching field distribution are considered. Laser Interference Lithography has been the technique chosen to produce the patterned media samples studied in this work. A discussion about different photoresist stacks which were used in the course of this work is presented. We also discuss on the etching process and the photoresist removal afterwards. Regarding the magnetic analysis of the samples, three issues are mainly discussed: The possible damage to the magnetic properties of the material due to the fabrication process, in particular the ion beam etching step and the subsequent removal of the residual photoresist. The switching field distribution displayed by the samples and possible ways to reduce it are also discussed. Finally we examine the thermostability of our samples. Difficulties in writing and retrieving actual information are commented and possible lines for further research are sketched
Oblique evaporation of CO80Ni20 films for magnetic recording
No full textTape and disc media have their own specific fields of application in magnetic, magneto-optic and optical recording. Tape media are commonly used where a large storage capacity is needed. To meet the increasing demands for larger storage capacities industry has investigated the possibility to use the method of vacuum evaporation for the production of magnetic recording tape. This has resulted in a commercial product called Metal Evaporated tape. In ME-tape production one takes advantage of the special properties of magnetic films evaporated under oblique vapour incidence, a method which has been discovered in 1959 in laboratory experiments. The difference however between industrial production of ME-tape and laboratory scale evaporation of thin film is extreme. The results obtained in the 35 years of experiments with oblique evaporation cannot be used unconditionally for the industrial production of ME-tape. Therefore the necessity for a model relating process conditions to structural and magnetic film properties is evident. In this thesis such a model is constructed using theoretical and experimental results already published in literature, combined with new theory and experiments. A number of experiments with Co80Ni20 evaporated at high angles of incidence are presented, including the first results on experiments with small-scale ME-tape production. In chapter 2 the relation between process conditions and film structure is discussed. The special structure of obliquely evaporated films has its origin in shadowing phenomena during film growth. Because of shadowing the film consists of bundles of inclined columns, the bundles being aligned perpendicularly to the vapour incidence direction. The column inclination angle lies in-between the film normal and the vapour incidence direction. Different models found in literature relating process parameters and film structure are discussed. It is shown that surface diffusion plays an important role, especially the difference between random and directional surface diffusion. The latter is induced by the oblique evaporation process. An attempt is made to give a quantitative expression for the relation between process conditions and surface diffusion including the influence of substrate temperature, rate and contamination with residual gasses. Using the models found in literature and adding the new calculations the relation between surface diffusion and film structure is discussed in detail and compared with measurements found in literature. In chapter 3 the relation between film structure and magnetic properties of obliquely evaporated magnetic thin films is treated. The tilted columnar structure results in a magnetic shape anisotropy. Two methods to calculate the shape anisotropy are compared, one is especially developed for this purpose. Next to the columnar structure also the anisotropic texture orientation and stress distribution contribute to the magnetic anisotropy. All these effects lead to an easy axis of magnetisation tilted out of the film plane. To determine the magnetic anisotropy in such films a measurement method using a torque-magnetometer is presented, leading to a general formulation of the anisotropy energy in arbitrary systems. The magnetic reversal is strongly affected by the magnetic anisotropy and the exchange between spins in adjacent columns. In films where the exchange is continuous throughout the film the tilted easy axis leads to specific magnetic domain structures which are only found in obliquely evaporated magnetic films. With increasing columnar separation the magnetic reversal turns to a mechanism of rotation of magnetisation in magnetically isolated particles, accompanied by a strong increase in coercivity. Furthermore the tilted easy axis causes a dependence of the recording properties on the tape running direction. In chapter 4 and 5 several experiments performed in the CNRS and MESA laboratories are discussed. Where possible a relation with the theory of chapter 2 and 3 is given. For each evaporation set-up used an estimate of the run-to-run reproducibility is made. It is concluded that the run-to-run scatter is mainly caused by small variations in the angle of incidence. This could be prevented by using an improved substrate holder. It is shown that the structural and magnetic properties of obliquely evaporated Co80Ni20 films depend on layer thickness up to a thickness of about 150 nm. Above this thickness the film properties remain constant. It was concluded that the difference is not caused by the change in substrate temperature during evaporation but by the film growth process itself. In preparation of the tape deposition set-up the rate of evaporation had to be increased. Therefore a short investigation on the effect of increase in evaporation on films prepared on Si substrates has been performed. An increase in rate from 0.5 to 20 nms-1 resulted in an increase in coercivity and anisotropy of the film. Since the columnar inclination and the apparent saturation magnetisation remain almost constant, it was concluded that the changes are mainly caused by an increase in crystal anisotropy. The influence of stress however should not be excluded. The observed change in film properties with increasing evaporation rate might be caused by the expected decrease in contamination with H2O molecules. To enable recording experiments, Co80Ni20 is evaporated on tape by the method of continuously varying angle of vapour incidence. For this a special "mini roll coater" was constructed which resembles the much larger roll-coaters used in industry for the production of ME-tape. This mini roll coater is used to study the effect of addition of oxygen on the film structure and its magnetic and recording properties. The recording performance of the tape prepared with this mini-roll coater are still bad in comparison with commercially available ME tape. The aim of the experiment however is not to produce commercial tape but to investigate the relation between process conditions, film structure, magnetic and recording properties. From this perspective the first results are encouraging
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