Browsing by Subject "Hall Effect"
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Item Open Access Design and applications of A Z-gradient array in magnetic resonance imaging(Bilkent University, 2019-01) Ertan, Niyazi KorayArray of gradient coils driven by independent power amplifiers can generate gradient fields with dynamically changing gradient field profiles. Nine channel prototype z-gradient coil array with a diameter of 25 cm is designed and manufactured. Previously designed gradient power amplifiers with maximum voltage of 50 V and maximum current of 20 A are used to independently drive the coils. Mutual coupling between gradient coils are investigated to maintain high time fidelity in the gradient waveform. A first-order circuit model including the mutual couplings is provided to analytically calculate the input voltages and minimum achievable rise times for a given set of gradient array currents and amplifier limitations. Mutual impedance of the system is measured which is in a good agreement with the first order circuit model inside the operating bandwidth of the amplifiers (<10kHz). An example z-gradient profile is optimized and used in Magnetic Resonance Imaging (MRI) phantom experiment as a readout gradient. After validating the proper functioning of the hardware with current measurements and MRI experiments, advantages of dynamically arrangeable field profiles generated by z-gradient array are investigated. Firstly, linear gradient in variable Volume of Interests (VOIs) with variable linearity errors are optimized with four different performance parameters such as maximization of gradient strength for unit amplifier current limits, maximization of slew rate for unit amplifier voltage limits, minimization of current norm and peak vector B-field for a unit gradient strength. Decreasing the size of the gradient VOI and allowing more linearity error increases all performance parameters more than five times among the sweep ranges. The advantage of dynamic field optimization is demonstrated in Diffusion Weighted Imaging (DWI). Maximization of gradient fields only inside the slice volume rather than entire coil volume results in 4 times higher gradient strength which decreases the diffusion encoding gradient duration 3 times and halves the echo time. Increased signal to noise ratio (SNR) of the diffusion weighted images results in better estimate for the apparent diffusion coe cient (ADC) values inside the phantom. Secondly, gradient array is also capable of generating nonlinear gradient field distributions. In addition to many applications of nonlinear gradients in MRI, two novel applications based on nonlinear gradients are proposed. In the first application, nonlinear gradients are used to encode multiple slice locations to the same frequency. Excitation of multiple slices is achieved with a single band radio frequency (RF) pulse in contrast to multi-band RF pulses with higher specific absorption rate (SAR) and peak power. Two different field design method is presented and both of them are analyzed in terms of slice thickness error, center location variation, gradient strength per unit norm current, power dissipation. Two and three slices are excited with a single band RF pulse in phantom experiments. In the second application, a single channel nonlinear gradient field is simultaneously used with linear gradients during spoke excitation to mitigate the B+ 1 inhomogeneity. Excitation k-space with increased dimension are introduced for simultaneous use of linear and nonlinear gradients by including independent k-space variables for nonlinear gradient channel. Simulations are performed for 1D, 2D, RF power limited and RF power unlimited cases to demonstrate the enhanced B+ 1 homogeneity for simultaneous use of linear and nonlinear gradients compared to using only linear or only nonlinear gradients. Proposed method results in 2.3 times more decrease in the excitation inhomogeneity compared to using only linear gradients in MRI experiments.Item Open Access Determination of the LO phonon energy by using electronic and optical methods in AIGaN/GaN(Springer, 2012) Celik, O.; Tiras, E.; Ardali, S.; Lisesivdin, S. B.; Özbay, EkmelThe longitudinal optical (LO) phonon energy in AlGaN/GaN heterostructures is determined from temperature-dependent Hall effect measurements and also from Infrared (IR) spectroscopy and Raman spectroscopy. The Hall effect measurements on AlGaN/GaN heterostructures grown by MOCVD have been carried out as a function of temperature in the range 1.8-275 K at a fixed magnetic field. The IR and Raman spectroscopy measurements have been carried out at room temperature. The experimental data for the temperature dependence of the Hall mobility were compared with the calculated electron mobility. In the calculations of electron mobility, polar optical phonon scattering, ionized impurity scattering, background impurity scattering, interface roughness, piezoelectric scattering, acoustic phonon scattering and dislocation scattering were taken into account at all temperatures. The result is that at low temperatures interface roughness scattering is the dominant scattering mechanism and at high temperatures polar optical phonon scattering is dominant.Item Open Access Room temperature scanning Hall probe microscopy of localized magnetic field fluctuations on the surfaces of magnetic recording media, permanent magnets and crystalline garnet films in external bias fields(Elsevier B.V., 2002-04) Sandhu, A.; Iida, N.; Masuda, H.; Oral, A.; Bending, S. J.A sub-micron room temperature scanning Hall probe microscope (RT-SHPM) was used for real-time imaging of surface magnetic domains of floppy disks, Sr ferrite magnets and Bi-substituted iron garnets placed in large external bias fields. Domain wall nucleation was observed in the garnets where bubble lattices expanded, collapsed and transformed into stripe domains in cyclic bias fields. Evolution of RT-SHPM images was compared with conventional vibrating sample magnetometer measurements.Item Open Access Room Temperature Scanning Micro-Hall Probe Microscopy Under Extremely Large Pulsed Magnetic Fields(IEEE, 2003) Sandhu, A.; Masuda, H.; Oral, A.Abstract The versatility of a room-temperature scanning Hall probe microscope system with an integrated minicoil capable of generating pulsed magnetic fields up to 2.9 T was demonstrated by imaging magnetic structures on the surface of 1.4-MB floppy disks and demagnetized strontium ferrite permanent magnets. Vibration isolation between the sample and minicoil was achieved by using a combination of quartz glass plates and silicone gel layers and enabled extremely fast measurements under fields as high as 2.9 T, without detrimental effects on a GaAs-AlGaAs micro-Hall probe sensor located at a height of 0.5 mum above-the sample surface.