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- Warnicke, Peter, 1978-
(author)
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Tailored Properties of Ferromagnetic Thin Films
- 2008
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Doctoral thesis (other academic/artistic)abstract
- Magnetic thin films and patterned nanostructures have been studied with respect to their magnetic properties using SQUID-magnetometry, magnetic force microscopy, electrical measurements, and micromagnetic calculations. Properties of vortex domain walls, trapped in Permalloy nanowires with artificial constrictions, were investigated experimentally and by numerical calculations. In particular, the geometrical extent and strength of the pinning potential were evaluated. In these wires, long-range vortex domain wall displacement induced by spin polarized alternating currents was obtained numerically at reduced threshold current densities as compared with the direct current case. Due to the asymmetry of the energy potential, the long-range displacement direction is determined by the vortex chirality. Strained FeCo/Pt superlattices with strong perpendicular anisotropy were investigated experimentally. The strain was controlled by varying the thickness of each alternating layer with monolayer precision and was found to have a dominating effect on the total anisotropy. Epitaxial films of the diluted magnetic semiconductor (Ga,Mn)As were studied with focus on how the ferromagnetic transition temperature could be controlled by post-growth annealing. The ferromagnetic transition temperature was enhanced by approximately 85% for a Mn-doping concentration of 6% under certain conditions. A method to manipulate micrometer sized magnetic particles on patterned arrays of elliptical Permalloy microstructures was studied. Controlled motion and separation of the magnetic particles were obtained using applied rotating magnetic fields. The domain structure of the elliptical elements was studied numerically.
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| 6. |
- Knut, Ronny, et al.
(author)
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Interface characterization of Co2MnGe/Rh2CuSn Heusler multilayers
- 2013
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 88:13, s. 134407-
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Journal article (peer-reviewed)abstract
- To address the amount of disorder and interface diffusion induced by annealing, all-Heusler multilayer structures, consisting of ferromagnetic Co2MnGe and nonmagnetic Rh2CuSn layers of varying thicknesses, have been investigated by means of hard x-ray photoelectron spectroscopy and x-ray magnetic circular dichroism. We find evidence for a 4 angstrom thick magnetically dead layer that, together with the identified interlayer diffusion, are likely reasons for the unexpectedly small magnetoresistance found for current-perpendicular-to-plane giant magnetoresistance devices based on this all-Heusler system. We find that diffusion begins already at comparably low temperatures between 200 and 250 degrees C, where Mn appears to be most prone to diffusion.
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| 12. |
- Andersson, Gabriella, et al.
(author)
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Perpendicular magnetocrystalline anisotropy in tetragonally distorted Fe-Co alloys
- 2006
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In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 96:3
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Journal article (peer-reviewed)abstract
- We report on the experimental realization of tetragonal Fe-Co alloys as a constituent of Fe(0.36)Co(0.64)/Pt superlattices with huge perpendicular magnetocrystalline anisotropy energy, reaching 210 mu eV/atom, and a saturation magnetization of 2.5 mu(B)/atom at 40 K, in qualitative agreement with theoretical predictions. At room temperature the corresponding values 150 mu eV/atom and 2.2 mu(B)/atom are achieved. This suggests that Fe-Co alloys with carefully chosen combinations of composition and distortion are good candidates for high-density perpendicular storage materials.
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| 13. |
- Boulle, Olivier, et al.
(author)
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Non-adiabatic spin transfer torque in high anisotropic magnetic nanowires with narrow domain walls
- 2008
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In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 101:21, s. 1-4
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Journal article (peer-reviewed)abstract
- Current induced domain wall (DW) depinning of a narrow DW in out-of-plane magnetized (Pt/Co)3/Pt multilayer elements is studied by magnetotransport. We find that for conventional measurements Joule heating effects conceal the real spin torque efficiency and so we use a measurement scheme at a constant sample temperature to unambiguously extract the spin torque contribution. From the variation of the depinning magnetic field with the current pulse amplitude we directly deduce the large nonadiabaticity factor in this material and we find that its amplitude is consistent with a momentum transfer mechanism.
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| 16. |
- Warnicke, Peter
(author)
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Asymmetric energy potential of pinned domain walls at constrictions
- 2008
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In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 93:17, s. 172508-
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Journal article (peer-reviewed)abstract
- The pinning properties of magnetic vortex domain walls were experimentally investigated in patterned nanowires with artificial constrictions. Domain walls were nucleated and depinned by applied magnetic fields and observed with magnetic force microscopy. It was found that the pinning strength is dependent on the chirality of the vortex wall. Micromagnetic simulations confirm these results and reveal the energy potential of the pinned domain wall, including the width of the pinning site.
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| 18. |
- Warnicke, Peter, et al.
(author)
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Long-range vortex domain wall displacement induced by an alternating current : Micromagnetic simulations
- 2008
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 78:1, s. 012413-
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Journal article (peer-reviewed)abstract
- A magnetic vortex domain wall, confined in a Permalloy nanowire with periodically varying width, is brought to resonance and long-range motion by means of a spin-polarized alternating current in micromagnetical simulations. The long-range direction of motion is found to be dependent on the vortex chirality but independent of the vortex polarity. Compared to the case where a direct current is used to move the vortex domain wall, the threshold current density for long-range motion is significantly reduced.
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| 19. |
- Warnicke, Peter, et al.
(author)
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Magnetic anisotropy of tetragonal FeCo/Pt (001) superlattices
- 2007
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In: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 19:22, s. 226218-
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Journal article (peer-reviewed)abstract
- The magnetic properties of tetragonally strained Fe0.36Co0.64 alloys in the form of FeCo/Pt(001) superlattices have been investigated experimentally. The strain was controlled by varying the individual constituent thicknesses in the range 3–10 atomic monolayers (ML), resulting in lattice-parameter ratios c/a between 1.18 and 1.31. The sample with a c/a ratio of 1.18 has a preferred out-of-plane magnetization direction and magnetic stripe domains, revealed by MFM, while samples with higher c/a ratios prefer in-plane magnetization. The strain-induced contribution to anisotropy was found to have a dominating effect. Saturation-magnetization values up to 2.31 T at room temperature were measured.
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| 21. |
- Wei, Yajun, et al.
(author)
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Magnetic coupling in asymmetric FeCoV/Ru/FeNi trilayers
- 2014
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In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550.
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Conference paper (peer-reviewed)abstract
- We have investigated the magnetic anisotropy and interlayer coupling in trilayer films of permendur(100 angstrom)/Ru/permalloy(100 angstrom), with the thickness of the Ru spacer varying from 0 to 200 angstrom. While the permendur/permalloy sample exhibits a small in-plane uniaxial magnetic anisotropy with H-u = 27 Oe, all trilayers are magnetically isotropic in-plane. Results from hysteresis loop and ferromagnetic resonance measurements were fitted to a micromagnetic model, with the results indicating that all the films are ferromagnetically coupled except the one with 10 angstrom Ru spacer, which shows antiferromagnetic coupling. The trilayers with Ru spacer layer thickness larger than 20 angstrom exhibit only very weak ferromagnetic coupling. (C) 2014 AIP Publishing LLC.
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