Browsing by Author "Horng, Lance"

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Anisotropic pinning in Nb thin films with triangular pinning arrays
(2007-07-01)
Cao, R.
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Wu, T. C.
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Kang, P. C.
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Wu, J. C.
;
Yang, T. J.
A superconducting thin film with regular triangular arrays has been fabricated to explore the anisotropic vortex pinning behavior. We found that the critical currents for these films depend strongly on the current directions. Some interesting temperature-dependent phenomena are observed and discussed. We also made molecular dynamic simulations to study the dynamics of the vortex motion. The simulation results confirm the anisotropic features we found in the experiments. (c) 2007 Elsevier Ltd. All rights reserved.
Low-Frequency Noise Characterization of CoFeB/MgO/CoFeB MTJ-Based Perpendicular Field Sensor
(2016-07)
Das, Bipul
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Lee, Y. C.
;
Li, L. C.
;
Yi-Shiou, Liu
;
Suen, Y. W.
We report low-frequency electrical resistance noise of Co40Fe40B20/MgO/Co20Fe60B20-based magnetic tunnel junction field sensors with reference and sensing layer magnetization directions along the out-of-plane and in-plane directions, respectively. The devices are fabricated using the sputter deposition and conventional lithography techniques with a short axis of similar to 6 mu m long and long axes of similar to 6-12 mu m long. Noise power spectra under different bias currents indicate increase in noise power with increasing bias current. A close 1/ f dependence of the noise according to Hooge\'s relation, S-V (f) = AV(2)/f(alpha) (A is Hooge\'s like constant) is observed with an average value of alpha = 1.03 +/- 0.08. In smaller devices, the magnetic field dependence of noise amplitude follows similar trend of magnetoresistance (MR) behavior without any distinct peak near antiparallel-parallel states transition of the devices, which is generally observed for magnetic defects-induced resistance fluctuation. The experimental observations infer the resistance noises in those devices that are related to the defects associated with the spin independent charge trapping at structural defects near or in the barrier layer. The device with the smallest lateral size exhibits the Lorentzian contribution, whereas the device with the largest size shows magnetic contribution in total noise. Numerically deduced magnetic fluctuations Delta B-rms of all the devices shows an average magnetic fluctuation of similar to 10 nT except for one device. The devices show the highest MR% of similar to 27% under perpendicular magnetic field with a dynamic range of similar to +/- 25 Oe and a sensitivity of similar to 0.3%/Oe. The observed low-frequency noise levels are suitable for the perpendicular field detection application and can be reduced more by improving the crystallinity of the device films stacks and their interfaces.
Low-Frequency Noise Characterization of CoFeB/MgO/CoFeB MTJ-Based Perpendicular Field Sensor
(2016-07-01)
Das, Bipul
;
Lee, Y. C.
;
Li, L. C.
;
Yi-Shiou, Liu
;
Suen, Y. W.
We report low-frequency electrical resistance noise of Co40Fe40B20/MgO/Co20Fe60B20-based magnetic tunnel junction field sensors with reference and sensing layer magnetization directions along the out-of-plane and in-plane directions, respectively. The devices are fabricated using the sputter deposition and conventional lithography techniques with a short axis of similar to 6 mu m long and long axes of similar to 6-12 mu m long. Noise power spectra under different bias currents indicate increase in noise power with increasing bias current. A close 1/ f dependence of the noise according to Hooge's relation, S-V (f) = AV(2)/f(alpha) (A is Hooge's like constant) is observed with an average value of alpha = 1.03 +/- 0.08. In smaller devices, the magnetic field dependence of noise amplitude follows similar trend of magnetoresistance (MR) behavior without any distinct peak near antiparallel-parallel states transition of the devices, which is generally observed for magnetic defects-induced resistance fluctuation. The experimental observations infer the resistance noises in those devices that are related to the defects associated with the spin independent charge trapping at structural defects near or in the barrier layer. The device with the smallest lateral size exhibits the Lorentzian contribution, whereas the device with the largest size shows magnetic contribution in total noise. Numerically deduced magnetic fluctuations Delta B-rms of all the devices shows an average magnetic fluctuation of similar to 10 nT except for one device. The devices show the highest MR% of similar to 27% under perpendicular magnetic field with a dynamic range of similar to +/- 25 Oe and a sensitivity of similar to 0.3%/Oe. The observed low-frequency noise levels are suitable for the perpendicular field detection application and can be reduced more by improving the crystallinity of the device films stacks and their interfaces.
Magnetic tunnel junction based out-of-plane field sensor with perpendicular magnetic anisotropy in reference layer
(2015-05-07)
Lee, Y. C.
;
Chao, C. T.
;
Li, L. C.
;
Suen, Y. W.
;
Horng, Lance
A magnetic tunnel junction (MTJ) with orthogonal magnetic anisotropy and consisting of Ta X/Co40Fe40B20 1.2 (reference)/MgO 2.0/Co20Fe60B20 2.3 (sensing)/Ta 5/Ru 5 (thickness in nanometers), where X ranges from 15 to 30, is proposed and investigated in response to the demand for out-of-plane field sensors. The reference layer with perpendicular magnetic anisotropy (PMA) demonstrates tuneable coercivity ranging from 72 Oe to 175 Oe. The sensing layer exhibits in-plane anisotropy with the avoidance of exchange coupling from the PMA reference layer because of a thick MgO barrier layer. The magnetization reversal behavior of micron scale devices not only corresponds well to the sheet film, but is also independent in terms of shape and size. The magnetoresistance curve exhibits a ratio of similar to 27% in the presence of a perpendicular field and is insensitive to the in-plane field. For perpendicular field sensing, the dynamic range with a sensitivity of similar to 0.3%/Oe can achieve +/- 25 Oe with a coercive field of less than 3 Oe. Additionally, even when bias is applied up to 9.1 mV, magnetic fluctuation still stays below 0.15 mOe. (C) 2015 AIP Publishing LLC.
Matching effect and flux pinning force in a Nb superconducting thin film with a triangular lattice of artificial pinning centers
(2004-05-01)
Horng, Lance
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Wu, J. C.
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Kang, P. C.
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Wu, T. C.
;
Yang, T. J.
A strong pinning force in a Nb superconducting thin film is formed by a triangular lattice of defects due to the structural corrugation that acts as pinning centers. The magnetoresistance measurement close to the transition temperature shows a set of minima of magnetoresistance at equal external field intervals. A crossover in current-voltage relations with matching field and half-integer matching field occurs for currents exceeding a certain threshold value. The phenomenon is related to the presence of pinning structures at matching fields. (C) 2003 Elsevier B.V. All rights reserved.
Properties of vortex propagation in a niobium film with spacing-graded density of pinning sites
(2007-05-01)
Horng, Lance
;
Wu, T. C.
;
Wu, J. C.
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Cao, R.
;
Yang, T. J.
The vortex propagation has been investigated in Nb superconductors with one dimension spacing-graded density of submicrometer-scaled holes in this research. A rectification effect can be discerned because the applied ac is transformed into a net dc voltage. These results demonstrate that the rectification is mainly characterized by the asymmetric pinning landscape and the rectified voltage depends considerably on temperature and the amplitude of applied ac. The dc voltage increases steeply up to the maximum value and then decreases to zero dc voltage with the increase of ac. The shape of peaks in the dc voltage signal become steeper and move to higher applied ac regime with the decrease of temperature.
Vortex ratchet effect in a niobium film with spacing-graded density of pinning sites
(2007-08-01)
Wu, T. C.
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Horng, Lance
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Wu, J. C.
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Cao, R.
;
Kolacek, Jan
The vortex propagation exhibits rectification effect in Nb superconductors with spacing-graded density of holes. A rectified dc voltage is obtained when the vortex lattice is driven by ac current. The asymmetric geometry of the pinning array produces a significant influence on the vortex motion. The rectified voltage depends considerably on the amplitude of the applied ac current and the magnetic field. The experimental results reveal a drastic change of the vortex rectification for magnetic field above/below the first matching field. The reason may be that the interstitial vortices are formed in the film above the first matching field. A reversible vortex motion is induced by the interstitial vortices for the field above the first matching field. (c) 2007 American Institute of Physics.