Electrical & Computer Engineering Research Works
Permanent URI for this collectionhttp://hdl.handle.net/1903/1658
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Item Spin-stand imaging of overwritten data and its comparison with magnetic force microscopy(American Institute of Physics, 2001-06-01) Mayergoyz, I. D.; Tse, C.; Krafft, C.; Gomez, R. D.A new technique of magnetic imaging on a spin-stand [Mayergoyz et al., J. Appl. Phys. 87, 6824 (2000)] is further developed and extensively tested. The results of successful imaging of digital patterns overwritten with misregistration ranging from 0.3 to 0.07 mm are reported. The results are compared with magnetic force microscopy (MFM) images and the conclusion is reached that the spin-stand imaging technique can provide (at least) the same level of resolution and accuracy as the MFM imaging technique.Item Slow magnetization dynamics of small permalloy islands(American Institute of Physics, 2000-05-01) Koo, H.; Luu, T. V.; Gomez, R. D.; Metlushko, V. V.The conditions that lead to specific domain configurations and the associated switching characteristics of small permalloy islands were studied by using magnetic force microscopy. By measuring a large number of particles, it was established that islands that have nonzero remanent moments ~nonsolenoidal! exist in one of three distinct configurations, namely: ~a! true single domain, ~b! quasisingle domain with edge closure patterns, and ~c! multidomain with nonuniform internal magnetization. The configuration depended upon the island width as well as the aspect ratio. Islands that are 310 nm wide or less are true single domain particles at low aspect ratios (;1.87) and higher, while islands wider than 500 nm always exhibited edge closure domains even for very large aspect ratios. In the range between 310 and 500 nm, the onset of single domain behavior was a function of the aspect ratio and thickness. Our studies involving in situ applied field similarly revealed the mechanisms of the reversal processes for each of the configurations, which correlated quite well with the values of the switching fields.Item Dependence of the perpendicular anisotropy in Co/Au multilayers on the number of repetitions(American Institute of Physics, 2003-05-15) Gubbiotti, G.; Carlotti, G.; Albertini, F.; Casoli, F.; Bontempi, E.; Depero, L. E.; Mengucci, P.; Di Cristoforo, A.; Koo, H.; Gomez, R. D.The correlations between structure and magnetism in [Co(0.9 nm)/Au(5 nm)]XN multilayer films with different number of repetitions N510, 30, and 50, have been studied by the combined use of complementary structural and magnetic techniques, such as x-ray reflectivity, x-ray diffraction, and transmission electron microscopy, alternating gradient force magnetometry, magnetic force microscopy and Brillouin light scattering. On increasing the value of N, an overall improvement of the multilayer quality is observed which corresponds to a change in the micromagnetic structure and to an enhancement of the perpendicular anisotropy. These effects have been attributed to a reduction of the magnetostatic energy associated with the formation of perpendicular magnetic domains in multilayers with increasing number of layers repetitions.Item Low temperature behavior of magnetic domains observed using a magnetic force microscope(American Institute of Physics, 2001-06-01) Chung, S. H.; Shinde, S. R.; Ogale, S. B.; Venkatesan, T.; Greene, R. L.; Dreyer, M.; Gomez, R. D.A commercial atomic force microscope/magnetic force microscope ~MFM! was modified to cool magnetic samples down to around 100 K under a high vacuum while maintaining its routine imaging functionality. MFM images of a 120 nm thick La0.7Ca0.3MnO3 film on a LaAlO3 substrate at low temperature show the paramagnetic-to-ferromagnetic phase transition. Evolution of magnetic domains and magnetic ripples with decreasing temperature are also observed near the edge of a 20 nm thick patterned Co film on a Si substrate.Item Ultrahigh vacuum scanning tunneling microscopy/magnickel oxide filmsnetic force microscopy study of ultrathin iron films grown on polycrystalline nickel oxide films(American Institute of Physics, 2002-05-15) Dreyer, M.; Hwang, D. G.; Gomez, R. D.The thickness dependence of the topographic and magnetic structure of ultrathin Fe films grown on polycrystalline NiO films under ultrahigh vacuum ~UHV! conditions was studied to investigate the growth mechanism of the ferromagnetic film and the corresponding magnetic interaction with the antiferromagnetic substrate. Externally prepared NiO films of 60 nm thickness were cleaned by heating in UHV. Ultrathin layers of Fe in the range of 1–27 nm were deposited on top of the NiO film and were analyzed at specific coverages. Iron grows as a polycrystalline film with the grains increasing in size with the thickness. The contours of the underlying NiO crystallites were evident at low coverages but gradually disappeared as the Fe grains coalesced at thicker coverages. Magnetic force microscopy images of the 1 nm thick film show randomly oriented magnetic grains with an average domain size of 30 nm. With an increase in film thickness the size of the domains grows to about 200 nm at 15 nm of iron. At a film thickness of 19 nm cross-tie domain walls become visible, indicating the crossover of some parts of the film from random magnetic grains into continuous domains with in-plane magnetization. A further increase in the film thickness leads to larger in-plane domains, while there are some areas with localized grains on the surface.Item Current-controlled bi-stable domain configurations in Ni81Fe19 elements: An approach to magnetic memory devices(American Institute of Physics, 2002-07-29) Koo, H.; Krafft, C.; Gomez, R. D.The discovery of current-switchable bi-stable remanent domain configurations on small ferromagnetic islands is reported. Rectangular NiFe islands with a thickness of 50 to 100 nm and lateral dimensions on the order of several microns were imaged using magnetic force microscopy after application of 10 ns current pulses through the material. The closure configuration can be set into either the 4 or 7 domain configuration by applying positive or negative current polarity at density on the order 107 A/cm2. The chirality of the closure patterns is fixed, implying that only two rather than four states are stable in these patterns. The possibility of using these configurations as a means of storing a logic state for memory applications is discussed.Item Universal Scaling of Ballistic Magnetoresistance in Magnetic Nanocontacts(The American Physical Society, 2002-12-31) Chung, S. H.; Munoz, M.; Garcıa, N.; Egelhoff, W. F.; Gomez, R. D.We show that ballistic magnetoresistance exhibits universal scaling in atomic or nanometer scale contacts. Plotting the data as conductance, we find that, if the maximum magnetoconductance is normalized to unity and the conductance is scaled with the conductivity of the bulk material, the data fall in a narrow region, independent of the nanocontact materials, for our four data sets and four from the literature. The results agree with a theory that takes into account spin-scattering within a magneticdomain wall.Item Universal scaling of magnetoconductance in magnetic nanocontacts (Invited)(American Institute of Physics, 2003-05-15) Chung, S. H.; Munoz, M.; Garcia, N.; Egelhoff, W. F.; Gomez, R. D.We present results of half-metallic ferromagnets formed by atomic nanocontact of CrO2– CrO2 and CrO2–Ni that show as much as 400% magnetoconductance. Analysis of the magnetoconductance versus conductance data for all materials known to exhibit so-called ballistic magnetoresistance strongly suggests that the magnetoconductance of nanocontacts follows universal scaling. If the maximum magnetoconductance is normalized to unity and the conductance is scaled to the resistivity of the material, then all data points fall into a universal curve that is independent of the contact material and the transport mechanism. The analysis was applied to all available magnetoconductance data of magnetic nanocontacts in the literature, and the results agree with theory that takes into account the spin scattering within a magnetic domain wall.Item Thin Al, Au, Cu, Ni, Fe, and Ta films as oxidation barriers for Co in air(American Institute of Physics, 2003-05-15) Gan, L.; Gomez, R. D.; Powell, C. J.; McMichael, R. D.; Chen, P. J.; Egelhoff, W. F. Jr.We have investigated the effectiveness of Al, Au, Cu, Ni, Fe, and Ta films with thicknesses up to 4 nm for protecting a Co surface from oxidation in air at room temperature. The distinct change in the Co 2p3/2 core-level line shape observed by x-ray photoelectron spectroscopy upon the oxidation of Co makes it a simple matter to identify the fractions of the Co that are in the metallic state and in the oxidized state. We find that the best choices for protecting Co from oxidation are Al and Ta. We found that Au, which is one of the most popular choices, is not particularly effective for protecting Co.Item Spin-current-induced magnetization reversal in magnetic nanowires with constrictions(American Institute of Physics, 2005-05-05) Florez, S. H.; Krafft, C.; Gomez, R. D.We have performed experiments on current-induced domain-wall motion sCIDWMd in the case of the domain walls sDWd trapped within the nanoscale constrictions in patterned NiFe structures. Direct observation of current-induced magnetization reversal was achieved and critical current densities jc were measured in the presence of easy-axis magnetic fields. The direction of CIDWM was found to be along the direction of the electron motion in absence of an applied magnetic field and in the direction of the field when in the presence of even relatively weak fields. Data for the field dependence of jc for both uniform and fast rising pulses suggest that the current, regardless of polarity, assists in the depinning of the DW. Only for the dc case does the data strongly reveal the influence of the electron pressure in promoting or hindering DW motion.