Colin Rawlings

The Next Generation of Superconducting Permanent Magnets: The Flux Pumping Method

Magnets made from bulk YBCO are as small and as compact as the rare earth magnets but potentially have magnetic flux densities orders of magnitude greater than those of the rare earths. In this paper a simple technique is proposed for magnetizing the superconductors. This technique involves repeatedly applying a small magnetic field which gets trapped in the superconductor and thus builds up and up. Thus a very small magnetic field such as one available from a rare earth magnet can be used to create a very large magnetic field. This technique which is applied using no moving parts is implemented by generating a traveling magnetic wave which moves across the superconductor. As it travels across the superconductor it trails flux lines behind it which get caught inside the superconductor. With each successive wave more flux lines get caught and the field builds up and up. The wave could be generated in many different ways but the preferred way is simply to heat a material whose permeability changes with temperature at its edge. As the heat travels across the material so the permeability changes and a magnetic wave is generated. It is in effect the first novel heat pump in a very long time and one which will enable the enormous potential available from these unique and highly versatile superconducting magnets to be fully realized. Within this paper we present results showing the superconductor being progressively magnetized by sequentially applied ldquoheatrdquo pulses. We also demonstrate that the sign of the magnetization is reversed if ldquocoldrdquo pulses are applied instead of heat pulses. These experimental results are supported by modeling.

Numerical analysis of thermally actuated magnets for magnetization of superconductors

Superconductors, such as YBCO bulks, have extremely high potential magnetic flux densities, comparing to rare earth magnets. Therefore, the magnetization of superconductors has attracted broad attention and contribution from both academic research and industry. In this paper, a novel technique is proposed to magnetize superconductors. Unusually, instead of using high magnetic fields and pulses, repeatedly magnetic waves with strength of as low as rare earth magnets are applied. These magnetic waves, generated by thermally controlling a Gadolinium (Gd) bulk with a rare earth magnet underneath, travel over the flat surface of a YBCO bulk and get trapped little by little. Thus, a very small magnetic field can be used to build up a very large magnetic field. In this paper, the modelling results of thermally actuated magnetic waves are presented showing how to transfer sequentially applied thermal pulses into magnetic waves. The experiment results of the magnetization of YBCO bulk are also presented to demonstrate how superconductors are progressively magnetized by small magnetic field

Magnetization of Bulk Superconductors Using Thermally Actuated Magnetic Waves

A novel technique is proposed to magnetize bulk superconductors, which has the potential to build up strong superconducting magnets. Instead of conventionally using strong magnetic pulses, periodical magnetic waves with strength as low as that of rare-earth magnets are applied. These magnetic waves travel from the periphery to the center of a bulk superconductor and become trapped little by little. In this way, bulk superconductors can gradually be magnetized. To generate these magnetic waves, a thermally actuated magnet was developed, which is constructed by a heating/cooling switch system, a rare-earth bulk magnet, and a Gadolinium (Gd) bulk. The heating/cooling switch system controls the temperature of the Gd bulk, which, along with the rare-earth magnet underneath, can transform thermal signals into magnetic waves. The modeling results of the thermally actuated magnet show that periodical magnetic waves can effectively be generated by applying heating and cooling pulses in turn. A YBCO bulk was tested in liquid nitrogen under the magnetic waves, and a notable accumulation of magnetic flux density was observed.

Calibration of the spring constant of cantilevers of arbitrary shape using the phase signal in an atomic force microscope

The measurement of cantilever parameters is an essential part of performing a calibrated measurement with an atomic force microscope (AFM). The thermal motion method is a widely used technique for calibrating the spring constant of an AFM cantilever, which can be applied to non-rectangular cantilevers. Given the trend towards high frequency scanning, calibration of non-rectangular cantilevers is of increasing importance. This paper presents two results relevant to cantilever calibration via the thermal motion method. We demonstrate the possibility of using the AFMs phase signal to acquire the thermal motion. This avoids the challenges associated with connecting the raw photodiode signal to a separate spectrum analyser. We also describe how numerical calculations may be used to calculate the parameters needed in a thermal motion calibration of a non-rectangular cantilever. Only accurate knowledge of the relative size of the in-plane dimensions of the cantilever is needed in this computation. We use this pair of results in the calibration of a variety of rectangular and non-rectangular cantilevers. We observe an average difference between the Sader and thermal motion values of cantilever stiffness of 10%.

Performing quantitative MFM measurements on soft magnetic nanostructures

We have extended our previous work (Rawlings et al 2010 Phys. Rev. B 82 085404) on simulating magnetic force microscopy (MFM) images for magnetically soft samples to include an accurate representation of coated MFM tips. We used an array of square 500 nm nanomagnets to evaluate our improved MFM model. A quantitative comparison between model and experiment was performed for lift heights ranging from 20 to 100 nm. No fitting parameters were used in our comparison. For all lift heights the qualitative agreement between model and experiment was significantly improved. At low lift heights, where the magnetic signal was strong, the difference between theory and experiment was less than 30%.

The inverse problem in magnetic force microscopy—inferring sample magnetization from MFM images

Nanomagnetic structures have the potential to surpass silicons scaling limitations both as elements in hybrid CMOS logic and as novel computational elements. Magnetic force microscopy (MFM) offers a convenient characterization technique for use in the design of such nanomagnetic structures. MFM measures the magnetic field and not the samples magnetization. As such the question of the uniqueness of the relationship between an external magnetic field and a magnetization distribution is a relevant one. To study this problem we present a simple algorithm which searches for magnetization distributions consistent with an external magnetic field and solutions to the micromagnetic equations qualitative features. The algorithm is not computationally intensive and is found to be effective for our test cases. On the basis of our results we propose a systematic approach for interpreting MFM measurements.

Single-nanometer accurate 3D nanoimprint lithography with master templates fabricated by NanoFrazor lithography

Nanoimprint lithography (NIL) is one of the most promising technology platforms for replication of nanometer and micrometer scale 3D topographies with extremely high resolution and throughput, as needed for e.g. photonic or optical applications. One of the remaining challenges of 3D NIL, however, is the fabrication of high quality 3D master originals – the initial patterns that are replicated multiple times in the NIL process. Here, we demonstrate a joint solution for 3D NIL where NanoFrazor thermal scanning probe lithography (t-SPL) is used to pattern the master templates with singlenanometer accurate 3D topographies. 3D topographies from polymer resist master templates are replicated using a HERCULES NIL system with SmartNIL technology. Furthermore, 3D patterns are transferred from the resist into a silicon substrate via reactive ion etching (RIE) and the resulting silicon master template is used for producing polymeric working stamps into OrmoStamp and, finally, replicas into optical grade OrmoClearFX material. Both replication strategies result in very high-quality replicas of the original patterns.

Fast turnaround fabrication of silicon point-contact quantum-dot transistors using combined thermal scanning probe lithography and laser writing

The fabrication of high-performance solid-state silicon quantum-devices requires high resolution patterning with minimal substrate damage. We have fabricated room temperature (RT) single-electron transistors (SETs) based on point-contact tunnel junctions using a hybrid lithography tool capable of both high resolution thermal scanning probe lithography and high throughput direct laser writing. The best focal z-position and the offset of the tip- and the laser-writing positions were determined in situ with the scanning probe. We demonstrate <100 nm precision in the registration between the high resolution and high throughput lithographies. The SET devices were fabricated on degenerately doped n-type >1020/cm3 silicon on insulator chips using a CMOS compatible geometric oxidation process. The characteristics of the three devices investigated were dominated by the presence of Si nanocrystals or phosphorous atoms embedded within the SiO2, forming quantum dots (QDs). The small size and strong localisation of electrons on the QDs facilitated SET operation even at RT. Temperature measurements showed that in the range 300 Kxa0>xa0Txa0>xa0∼100 K, the current flow was thermally activated but at <100 K, it was dominated by tunnelling.