Browsing by Subject "Micromagnetics"

Now showing 1 - 3 of 3
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    Micromagnetic analysis of magnetic nanoparticles for hyperthermia cancer treatment and of transition characteristics for recording media
    (2013-02) Sohn, Hweerin
    Research has recently focused on magnetic nanoparticles due to fascinating properties that could see great potential employment in biomedicine as well as data storage devices. Micromagnetic analysis was utilized in order to predict the dynamic motion for the magnetization vector of magnetic nanoparticles in biomedical application (hyperthermia cancer therapy) and magnetic information storage (hard disk drive). In this dissertation, the heating properties of magnetic nanoparticles for hyperthermia and the characteristics of magnetic recording media (both conventional perpendicular media and exchange coupled composite media) with a soft underlayer and an antiferromagnetic soft underlayer are presented. Magnetic nanoparticles have great potential as heating elements for use in hyperthermia. One of the critical issues with widely used iron-oxide compounds such as magnetite and mag-hematite nanoparticles is the relatively low magnetic moment, which results in low heating efficiency. To overcome this demerit, nonoxide high moment Fe70Co30 nanoparticles were considered. The mean size of particles was 12nm with 13.6% standard deviation. Micromagnetic simulation of particles’ experimental hysteresis loop suggests that their behavior is dominated by a uniaxial anisotropy. In order to understand the source of energy loss in hyperthermia, magnetic anisotropy and applied field have been optimized for iron cobalt nanocrystalline particles using numerical micromagnetics. The optimized anisotropy energy is 7.6 kBT at 500 kHz and the hysteresis loss at this optimized energy is approximately 120 x 106 ergs/s/g for a very small oscillating field of magnitude 10 Oe. We have also investigated the effects of varying the applied field and find that the addition of a 20 Oe static field applied perpendicular to the oscillating field approximately doubles the energy loss without subjecting the patient to additional radiation. This is an important benefit for magnetic hyperthermia. To achieve higher areal density in magnetic recording media, the general method is to reduce and make more uniform the grain size, while augmenting the media anisotropy in order to maintain stability. Transition jitter and shape have been studied for “soft” exchange coupled composite (ECC) media and conventional perpendicular media at equal grain size using micromagnetic simulation. A realistic medium having nonuniform grain size has been employed. Media anisotropies are optimized to reduce the high density jitter for ECC and conventional media. Surprisingly, jitter is slightly decreased at high temperature for both media types. Eye diagrams show that short bit length amplitude is higher for ECC by approximately 10 % at room temperature. This indicates that sharper transitions were obtained for ECC media particularly at 300 K where the thermal stability of ECC media presumably aids the write process. A key component of perpendicular recording has long been the soft underlayer. Conventional perpendicular media and “soft” exchange coupled composite (ECC) media with a conventional soft underlayer (SUL) and an antiferromagnetic soft underlayer (AF-SUL) have been investigated using micromagnetic simulation. The fast Fourier transform (FFT) technique and graphics processing unit (GPU) based computing have been used to reduce the intensive computation time for magnetostatic interactions between the head, SUL, and recording layer. Interestingly, the jitter is always less dependent on reader offset from track center with the AF-SUL. Jitter for ECC media is also shown to depend less strongly on reader offset than for conventional media. The transition center deviation at the optimal anisotropy for both recording layers is lower with the AF-SUL at both linear densities considered. We further find that the track center moves alternately with direction of fringing field as expected from magnetostatic considerations.
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    Micromagnetic tests of techniques for reducing pole tip remanence of high density perpendicular write heads.
    (2010-09) Patwari, Mohammed Shariat Ullah
    A multi-scale fast Fourier transform (FFT) based micromagnetic model has been developed to simulate erase after write (EAW) for a 2.4 T FeCo solid pole writer. The simulated remnant state of the writer shows vortices at the pole tip, break and paddle regions that qualitatively matches an experimental MFM image. Dynamic responses show that EAW worsens with a longer breakpoint. Sensitivity of EAW with breakpoint is in good agreement with the experimental data. Modeling suggests that cross track anisotropy reduces EAW risks; however, perpendicular anisotropy is found to be detrimental to EAW. Simulations show that EAW risks are substantially reduced when a uni-polar demagnetization pulse of polarity opposite to that of the last write is applied to the writer. Using the model, the response functions of uni-polar demagnetization pulses have been modeled for reducing EAW events. Simulations show that the value of the initial field created by the demagnetization current is the most effective parameter in reducing pole tip remanence. To avoid driving the head in the opposite direction with the demagnetization pulse, it is important to ramp down quickly with a time constant of about 500 psec. In-plane exchange is found to affect EAW quite significantly; just 25% lower exchange reduces EAW fields by 30%. Modeling shows that introduction of antiferromagnetic coupling in the write pole reduces EAW significantly. Modeling also suggests that non-magnetic holes with in plane dimensions of 35 nm in the middle of the breakpoint region reduce EAW by ~35%. The underlying mechanism to reduce EAW is to make vortex formation in the pole tip energetically inexpensive. Micromagnetics is used to design an unshielded perpendicular writer for an areal density of 1 Tb/in 2 . The head consists of a probe-type tip protruding from a collar. The tip has saturation magnetization ( M S ) of 24 kG while the collar has lower M S . The magnitude and orientation of anisotropy field ( H k ) in the tip is varied to obtain the best recording performance. The combination of high anisotropy write tip with low M S collar is shown to produce effective write fields in excess of 19 kOe and less than 20% track erasure for 10 7 passes. Introduction of in-plane anisotropy within the pole tip reduces head remanence sufficiently that on-track erasure exceeds a benchmark of 10 8 passes. The damping constant in the Landau-Lifshitz-Gilbert equation is varied to improve the frequency response. With damping constant equal to 1.0, simulations show that the head is capable of switching in approximately 0.15 ns.
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    Understanding Magnetic Hysteresis in Cubic Materials
    (2017-04) Dabade, Vivekanand
    Hysteresis is the limiting criterion in many applications of functional materials. Recent understanding and development in shape memory alloys have lead to very low hysteresis materials. Low hysteresis shape memory alloys with unusual magnetoelectric properties have found new and interesting applications. In this thesis, we try to understand magnetic hysteresis in cubic ferromagnets using the framework of micromagnetics. We look at two cubic materials: Galfenol (Fe$_{74}$Ga$_{26}$ and Fe$_{83}$Ga$_{17}$) and Permalloy (Fe$_{21.5}$Ni$_{78.5}$). The material parameters of Galfenol show that it belongs to a new parameter regime in micromagnetics that has not been explored before. We study the macroscopic properties and try to understand its magnetic microstructure. The main tools used to study the macroscopic properties are: Weak convergence and Young measures. Theoretical predictions of the macroscopic properties match well with results obtained from experiments. By including the exchange energy and minimizing the total micromagnetic energy of Galfenol we show that its magnetic microstructure has lower energy than other commonly observed magnetic microstructures. This paves the way for obtaining optimal energy scaling laws for cubic ferromagnets in general. We also touch upon the well known Permalloy problem in this thesis. Permalloy has very low coercivity at a puzzling material composition. We make few interesting observations about the magnetic microstructure of the Permalloy. Finally, we shall report the results of some novel experiments that were aimed to synthesize an elusive hard ferromagnet known as Tetrataenite.