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11.
  • Persson, Anders, 1982-, et al. (författare)
  • Low-frequency picotesla field detection with planar Hall effect bridge sensors
  • Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
    • The applicability of miniaturized magnetic field sensors are being explored in several fields of magnetic field detection, due to their integratability, low mass, and potentially low cost. In this respect, different thin-film technologies, especially those employing magnetoresistance, show great potential, being compatible with micro- and nanotechnology batch processing. For low-frequency magnetic field detection, sensors based on the planar Hall effect, especially planar Hall effect bridge (PHEB) sensors, show promising performance given their inherent low-field linearity, limited hysteresis and moderate noise figure. In this work, the applicability of such PHEB sensors to different areas is investigated. An analytical model was constructed, to estimate the performance of an arbitrary PHEB in terms of e.g. sensitivity and detectivity. The model incorporates a number of approximations and, to validate the results, modelled data is compared to measurements on actual PHEBs. It is concluded that the model slightly underestimated the detectivity, especially at low frequencies and when demagnetizing effects becomes apparent. The model is also sensitive to fabrication process induced variations of the material parameters of the sensors. Nevertheless, accounting for these discrepancies, the modelled data is typically within 10% from the experimental data and the model can be used to estimate the performance of a particular PHEB design. The model is also used to establish a design process for optimizing a PHEB to a particular set of requirements on the bandwidth, detectivity, compliance voltage and amplified signal-to-noise ratio. By applying this design process, the size, sensitivity, resistance, bias current and power consumption of the PHEB can be calculated. The model shows that PHEBs are applicable to several different science areas including archaeological surveying, satellite attitude determination, scientific space missions, and magnetic bead detection in lab-on-a-chip applications.
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12.
  • Persson, Anders, 1982- (författare)
  • Magnetoresistance and Space : Micro- and Nanofeature Sensors Designed, Manufactured and Evaluated for Space Magnetic Field Investigations
  • 2011
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • In recent years, the interest for miniaturization of spaceborne instruments and subsystems has increased steadily, as this enables development of small and lightweight satellite classes as well as more versatile payloads on traditional spacecraft. In essence, this thesis work is an investigation of the applicability of magnetoresistive technology to a magnetometer intended for space. Two types of magnetoresistive sensors, promising with respect to performance competiveness also after considerable miniaturization, were developed and evaluated, namely magnetic tunnel junctions and planar Hall effect bridge sensors. In the case of the magnetic tunnel junctions, much effort was put on the micromanufacturing process. Two schemes were developed and evaluated for sensor contouring: one employing focused ion beam processes for rapid prototyping, and the other combining sputtering and x-ray photoelectron spectroscopy for precise etch depth monitoring during ion etching. For the former, the resulting implantation damages were investigated with chemical analysis and correlated to the sensor properties. In the latter, the depth of the etching was monitored live with a resolution sufficient to stop the etching in the 1 nm thick tunneling barrier. The effect and extent of redeposition were investigated by transmission electron microscopy and micromagnetic analysis. With the knowledge so gained, the tunneling magnetoresistance of the manufactured junctions could be improved significantly and their inherent noise could be reduced. As a step in space flight qualification, the magnetic tunnel junctions were subjected to both g and particle radiation, leaving them unaffected by the first, but rendering them a reduced tunneling magnetoresistance ratio and an increased coercivity by the latter. In the case of the planar Hall effect bridge sensors, their inherent noise was thoroughly investigated, revealing both electric and magnetic 1/f noise at low frequencies along with thermal noise at higher frequencies. In addition, an analytical model of the magnetic properties of the planar Hall effect bridges was developed, and a design process, based on the model, was established to optimize the bridges for a particular application. In conclusion, both types of sensors show great promises for use in space. Of the two, the planar Hall effect bridge sensors had a better detection limit at low frequencies, whereas the magnetic tunnel junctions were more precise at higher frequencies. However, both sensors had a bandwidth greatly exceeding that of traditional spaceborne magnetometers. A magnetometer employing the magnetic tunnel junctions from this work is currently included as payload onboard the Vietnamese satellite F-1 scheduled for launch this year. A magnetometer using magnetoresistive sensors – planar Hall effect sensors, magnetic tunnel junctions, or both – enables a mass reduction of more than two orders of magnitudes compared with traditional systems.
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13.
  • Persson, Anders, 1982-, et al. (författare)
  • Micro- and nanostructured magnetic field sensor for space applications
  • 2009
  • Ingår i: 15th International Conference on Solid-State SensorsTransducers 2009. ; , s. 1190-1193
  • Konferensbidrag (refereegranskat)abstract
    • Magnetometers are popular payloads on scientific space missions. Here, the design and fabrication process of a miniaturized magnetometer based on tunneling magnetoresistance is presented. The process is capable of making magnetic tunnel junctions in a wide size range, by employing both UV lithography and focused ion beam milling and deposition. Ga implantation in the ferromagnetic electrodes of the junction is studied in more detail. It was shown that Ga implantation may harm the magnetometer if the irradiation dose exceeds 1014 Ga+ cm-2.
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14.
  • Persson, Anders, et al. (författare)
  • Modelling and design of planar Hall effect bridge sensors for low-frequency applications
  • 2013
  • Ingår i: Sensors and Actuators A-Physical. - : Elsevier BV. - 0924-4247 .- 1873-3069. ; 189, s. 459-465
  • Tidskriftsartikel (refereegranskat)abstract
    • The applicability of miniaturized magnetic field sensors is being explored in several areas of magneticfield detection due to their integratability, low mass, and potentially low cost. In this respect, differentthin-film technologies, especially those employing magnetoresistance, show great potential, being compatible with batch micro- and nanofabrication techniques. For low-frequency magnetic field detection,sensors based on the planar Hall effect, especially planar Hall effect bridge (PHEB) sensors, show promising performance given their inherent low-field linearity, limited hysteresis and moderate noise figure. Inthis work, the applicability of such PHEB sensors to different areas is investigated. An analytical modelis constructed to estimate the performance of an arbitrary PHEB sensor geometry in terms of, e.g., sensitivity and detectivity. The model is valid for an ideal case, e.g., disregarding shape anisotropy effects, andalso incorporates some approximations. To validate the results, modelled data was compared to measurements on actual PHEBs and was found to predict the measured values within 13% for the investigatedgeometries. Subsequently, the model was used to establish a design process for optimizing a PHEB to aparticular set of requirements on the bandwidth, detectivity, compliance voltage and amplified signalto-noise ratio. By applying this design process, the size, sensitivity, resistance, bias current and powerconsumption of the PHEB can be estimated. The model indicates that PHEBs can be applicable to severaldifferent areas within science including satellite attitude determination and magnetic bead detection inlab-on-a-chip applications, where detectivities down towards 1 nT Hz−0.5at 1 Hz are required, andmaybeeven magnetic field measurements in scientific space missions and archaeological surveying, where thedetectivity has to be less than 100 pT Hz−0.5at 1 Hz.
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15.
  • Persson, Anders, et al. (författare)
  • Radiation tolerance of a spin-dependent tunnelling magnetometer for space applications
  • 2011
  • Ingår i: Measurement science and technology. - : IOP Publishing. - 0957-0233 .- 1361-6501. ; 22:4, s. 045204-
  • Tidskriftsartikel (refereegranskat)abstract
    • To meet the increasing demand for miniaturized space instruments, efforts have been made to miniaturize traditional magnetometers, e. g. fluxgate and spin-exchange relaxation-free magnetometers. These have, for different reasons, turned out to be difficult. New technologies are needed, and promising in this respect are tunnelling magnetoresistive (TMR) magnetometers, which are based on thin film technology. However, all new space devices first have to be qualified, particularly in terms of radiation resistance. A study on TMR magnetometers' vulnerability to radiation is crucial, considering the fact that they employ a dielectric barrier, which can be susceptible to charge trapping from ionizing radiation. Here, a TMR-based magnetometer, called the spin-dependent tunnelling magnetometer (SDTM), is presented. A magnetometer chip consisting of three Wheatstone bridges, with an angular pitch of 120 degrees, was fabricated using microstructure technology. Each branch of the Wheatstone bridges consists of eight pairs of magnetic tunnel junctions (MTJs) connected in series. Two such chips are used to measure the three-dimensional magnetic field vector. To investigate the SDTM's resistance to radiation, one branch of a Wheatstone bridge was irradiated with gamma rays from a Co-60 source with a dose rate of 10.9 rad min(-1) to a total dose of 100 krad. The TMR of the branch was monitored in situ, and the easy axis TMR loop and low-frequency noise characteristics of a single MTJ were acquired before and after irradiation with the total dose. It was concluded that radiation did not influence the MTJs in any noticeable way in terms of the TMR ratio, coercivity, magnetostatic coupling or low-frequency noise.
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16.
  • Persson, Anders, et al. (författare)
  • Rapid prototyping of magnetic tunnel junctions with focused ion beam processes
  • 2010
  • Ingår i: Journal of Micromechanics and Microengineering. - : IOP. - 0960-1317 .- 1361-6439. ; 20:5, s. 055039-
  • Tidskriftsartikel (refereegranskat)abstract
    • Submicron sized Magnetic tunnel junctions (MTJs) are most often fabricated by time-consuming and expensive e-beam lithography. From a research and development perspective, a short lead time is one of the major concerns. Here, a rapid process scheme for fabrication of micrometer size MTJs with focused ion beam processes is presented. The magnetic properties of the fabricated junctions is investigated in terms of magnetic domain structure, tunnelling magnetoresistance (TMR) and coercivity, with extra attention to the effect of Ga implantation from the ion beam. In particular, the effect of the implantation on the minimum junction size and the magnetization of the sensing layer are studied. In the latter case, magnetic force microscopy and micromagnetic simulations, with the Object Oriented Micromagnetic Framework (OOMMF), are used to study the magnetization reversal. The fabricated junctions show considerable coercivity both along their hard and easy axes. Interestingly, the sensing layer exhibit two remanent states: one with a single and one with a double domain. The hard axis TMR loop has kinks at about ±20 mT which is attributed to a non-uniform lateral coercivity, where the rim of the junctions, which is subjected to Ga implantation from the flank of the ion beam, is more coercive than the unirradiated centre. The width of the coercive rim is estimated to 160 nm from the hard axis TMR loop. The easy axis TMR loop shows more coercivity than an unirradiated junction and, this too, is found to stem from the coercive rim, as seen from the simulations. It is concluded that the process scheme has three major advantages. Firstly, it has a high lateral and depth resolution – the depth resolution is enhanced by end point detection – and is capable of making junctions of sizes down towards the limit set by the width of the irradiated rim. Secondly, the most delicate process steps are preformed in unbroken vacuum enabling the use of materials prone to forming oxides in the MTJ film stack. Thirdly, the scheme is both uncomplicated and quick and makes it possible to go from design to characterization in the order of hours.
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