| 1. |
- Akansel, Serkan, et al.
(författare)
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Effect of seed layers on dynamic and static magnetic properties of Fe65Co35 thin films
- 2018
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Ingår i: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 51:30
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Tidskriftsartikel (refereegranskat)abstract
- Fe65Co35 thin films have been deposited on SiO2 substrates using sputtering technique with different choices of seed layer; Ru, Ni82.5Fe17.5, Rh, Y and Zr. Best soft magnetic properties were observed with seed layers of Ru, Ni82.5Fe17.5 and Rh. Adding these seed layers, the coercivity of the Fe65Co35 films decreased to values of around 1.5 mT, which can be compared to the value of 12.5 mT obtained for films deposited without seed layer. Further investigations were performed on samples with these three seed layers in terms of dynamic magnetic properties, both on as prepared and annealed samples, using constant frequency cavity and broadband ferromagnetic resonance measurements. Damping parameters of around 8.0X10-3 and 4.5X10-3 were obtained from in-plane and out-of-plane measurements, respectively, for as prepared samples, values that were reduced to 6.5X10-3 and 4.0X10-3 for annealed samples.
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| 2. |
- Akansel, Serkan, et al.
(författare)
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Enhanced Gilbert damping in Re-doped FeCo films : Combined experimental and theoretical study
- 2019
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Ingår i: Physical Review B. - 2469-9950 .- 2469-9969. ; 99:17
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Tidskriftsartikel (refereegranskat)abstract
- The effects of rhenium doping in the range 0-10 at.% on the static and dynamic magnetic properties of Fe65Co35 thin films have been studied experimentally as well as with first-principles electronic structure calculations focusing on the change of the saturation magnetization (M-s) and the Gilbert damping parameter (alpha). Both experimental and theoretical results show that M-s decreases with increasing Re-doping level, while at the same time alpha increases. The experimental low temperature saturation magnetic induction exhibits a 29% decrease, from 2.31 to 1.64 T, in the investigated doping concentration range, which is more than predicted by the theoretical calculations. The room temperature value of the damping parameter obtained from ferromagnetic resonance measurements, correcting for extrinsic contributions to the damping, is for the undoped sample 2.1 x 10(-3), which is close to the theoretically calculated Gilbert damping parameter. With 10 at.% Re doping, the damping parameter increases to 7.8 x 10(-3), which is in good agreement with the theoretical value of 7.3 x 10(-3). The increase in damping parameter with Re doping is explained by the increase in the density of states at the Fermi level, mostly contributed by the spin-up channel of Re. Moreover, both experimental and theoretical values for the damping parameter weakly decrease with decreasing temperature.
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| 3. |
- Akansel, Serkan, et al.
(författare)
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Enhanched Gilbert Damping in Re doped FeCo Films : A combined experimental and theoretical study
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The effects of rhenium doping in the range 0 – 10 at% on the static and dynamic magnetic properties of Fe65Co35 thin films have been studied experimentally as well as with first principles electronic structure calculations focussing on the change of the saturation magnetization (Ms ) and the Gilbert damping parameter (α ). Both experimental and theoretical results show that Ms decreases with increasing Re doping level, while at the same time α increases. The experimental low temperature saturation magnetic induction exhibits a 29% decrease, from 2.31T to 1.64T, in the investigated doping concentration range, which is more than predicted by the theoretical calculations. The room temperature value of the damping parameter obtained from ferromagnetic resonance measurements, correcting for extrinsic contributions to the damping, is for the undoped sample 2.7X103, which is close to the Gilbert damping parameter extracted from the theoretical calculations. The room temperature experimental value for the damping parameter increases to 9X103 when doping with 10 at% Re; the corresponding increase of the Gilbert damping parameter obtained from theoretical calculations is 7.3X103. Both experimental and theoretical values for the damping parameter weakly decrease with decreasing temperature.
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| 4. |
- Akansel, Serkan, 1983-
(författare)
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Magnetization Dynamics in Ferromagnetic Thin Films : Evaluation of Different Contributions to Damping in Co2FeAl and FeCo Film Structures
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Static and dynamic magnetic properties of Co2FeAl and Fe65Co35 alloys have been investigated. Co2FeAl films were deposited at different temperatures and the deposition parameters were optimized with respect to structural and magnetic properties. As a result, a film with B2 crystalline phase was obtained without any post-annealing process. A lowest magnetic damping parameter of was obtained for the film deposited at 573K. This obtained low value is comparable to the lowest values reported in research literature. After optimizing the deposition parameters of this alloy, different seed layers and capping layers were added adjacent to the Co2FeAl layer and the effect of these layers on the magnetic relaxation was investigated. In addition to adding nonmagnetic layers to Co2FeAl, the dependence of the magnetic damping parameter with respect to the thickness of Co2FeAl was investigated by depositing films with different thicknesses. A temperature dependent study of the magnetic damping parameter was also performed and the measured damping parameters were compared with theoretically calculated intrinsic Gilbert damping parameters. Different extrinsic contributions to the magnetic damping, such as two magnon scattering, spin pumping, eddy-current damping and radiative damping, were identified and subtracted from the experimentally obtained damping parameter. Hence, it was possible to obtain the intrinsic damping parameter, that is called the Gilbert damping parameter.In the second part of the thesis, Fe65Co35 alloys were investigated in terms of static and dynamic magnetic properties. Fe65Co35 films were deposited without and with different seed layers in order to first understand the effect of the seed layer on static magnetic properties of the films, such as the coercivity of the films. Then the films with seed layers yielding the lowest coercivity were investigated in terms of dynamic magnetic properties. Fe65Co35 films with different rhenium dopant concentrations and with ruthenium as the seed and capping layer were also investigated. The purpose of this study was to increase the damping parameter of the films and an increase of about ~230% was obtained by adding the dopant to the structure. This study was performed at different temperatures and after subtraction of the extrinsic contributions to the damping, the experimental values were compared with theoretically calculated values of the Gilbert damping parameter. During the thesis work, magnetic looper and superconducting quantum interference device magnetometers set-ups were used for static magnetic measurements and cavity, broadband in-plane and broadband out-of-plane ferromagnetic resonance set-ups were used for dynamic measurements.
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| 5. |
- Akansel, Serkan, 1983-, et al.
(författare)
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Thickness dependent enhancement of damping in Co2FeAl/β-Ta thin films
- 2018
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 97:13
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Tidskriftsartikel (refereegranskat)abstract
- In the present work Co2FeAl (CFA) thin films were deposited by ion beam sputtering on Si (100) substrates at the optimized deposition temperature of 300°C. A series of CFA films with different thickness (tCFA ); 8, 10, 12, 14, 16, 18 and 20 nm were prepared and all samples were capped with a 5 nm thick b-Ta layer. The thickness dependent static and dynamic properties of the films were studied by SQUID magnetometry, in-plane as well as out-of-plane broadband VNA-FMR measurements and angle dependent cavity FMR measurements. The saturation magnetization and the coercive field were found to be weakly thickness dependent and lie in the range 900 – 950 kA/m and 0.53 – 0.87 kA/m, respectively. The effective damping parameter ( αeff) extracted from in-plane and out-of-plane FMR results reveal a 1/tCFA dependence, the values for the in-plane αeff being larger due to two-magnon scattering (TMS). The origin of the αeff thickness dependence is spin pumping into the non-magnetic b-Ta layer and in case of the in-plane αeff also a thickness dependent TMS contribution. From the out-of-plane FMR results, it was possible to disentangle the different contributions to αeff and to the extract values for the intrinsic Gilbert damping (αG ) and the effective spin-mixing conductance (g_eff^↑↓ ) of the CFA/ b-Ta interface, yielding αG=1.1X10-3 and g_eff^↑↓=2.90x1019 m-2.
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| 6. |
- Husain, Sajid, et al.
(författare)
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Anomalous Hall effect in ion-beam sputtered Co2FeAl full Heusler alloy thin films
- 2017
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Ingår i: Journal of Magnetism and Magnetic Materials. - : Elsevier. - 0304-8853 .- 1873-4766. ; 442, s. 288-294
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Tidskriftsartikel (refereegranskat)abstract
- Investigations of temperature dependent anomalous Hall effect and longitudinal resistivity in Co2FeAl (CFA) thin films grown on Si(1 0 0) at different substrate temperature Ts are reported. The scaling of the anomalous Hall conductivity (AHC) and the associated phenomenological mechanisms (intrinsic and extrinsic) are analyzed vis-à-vis influence of Ts. The intrinsic contribution to AHC is found to be dominating over the extrinsic one. The appearance of a resistivity minimum at low temperature necessitates the inclusion of quantum corrections on account of weak localization and electron-electron scattering effects whose strength reduces with increase in Ts. The study establishes that the optimization of Tsplays an important role in the improvement of atomic ordering which indicates the higher strength of spin-orbit coupling and leads to the dominant intrinsic contribution to AHC in these CFA full Heusler alloy thin films.
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| 7. |
- Husain, Sajid, et al.
(författare)
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Growth of Co2FeAl Heusler alloy thin films on Si(100) having very small Gilbert damping by Ion beam sputtering
- 2016
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 6
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Tidskriftsartikel (refereegranskat)abstract
- The influence of growth temperature T-s (300-773 K) on the structural phase ordering, static and dynamic magnetization behaviour has been investigated in ion beam sputtered full Heusler alloy Co2FeAl (CFA) thin films on industrially important Si(100) substrate. The B2 type magnetic ordering is established in these films based on the clear observation of the (200) diffraction peak. These ion beam sputtered CFA films possess very small surface roughness of the order of subatomic dimensions (<3 angstrom) as determined from the fitting of XRR spectra and also by AFM imaging. This is supported by the occurrence of distinct Kiessig fringes spanning over the whole scanning range (similar to 4 degrees) in the x-ray reflectivity (XRR) spectra. The Gilbert damping constant alpha and effective magnetization 4 pi M-eff are found to vary from 0.0053 +/- 0.0002 to 0.0015 +/- 0.0001 and 13.45 +/- 00.03 kG to 14.03 +/- 0.04 kG, respectively. These Co2FeAl films possess saturation magnetization ranging from 4.82 +/- 0.09 to 5.22 +/- 0.10 mu(B)/f.u. consistent with the bulk L2(1)-type ordering. A record low alpha-value of 0.0015 is obtained for Co2FeAl films deposited on Si substrate at T-s similar to 573 K.
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| 8. |
- Husain, Sajid, et al.
(författare)
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Observation of Skyrmions at Room Temperature in Co2FeAl Heusler Alloy Ultrathin Film Heterostructures
- 2019
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 9
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Tidskriftsartikel (refereegranskat)abstract
- Magnetic skyrmions are topological spin-textures having immense potential for energy efficient spintronic devices. Here, we report the observation of stable skyrmions in unpatterned Ta/Co2FeAl(CFA)/MgO thin film heterostructures at room temperature in remnant state employing magnetic force microscopy. It is shown that these skyrmions consisting of ultrathin ferromagnetic CFA Heusler alloy result from strong interfacial Dzyaloshinskii-Moriya interaction (i-DMI) as evidenced by Brillouin light scattering measurements, in agreement with the results of micromagnetic simulations. We also emphasize on room temperature observation of multiple skyrmions which can be stabilized for suitable combinations of CFA layer thickness, perpendicular magnetic anisotropy, and i-DMI. These results provide a significant step towards designing of room temperature spintronic devices based on skyrmions in full Heusler alloy based thin films.
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| 9. |
- Husain, Sajid, et al.
(författare)
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Spin pumping in ion-beam sputtered Co2FeAl/Mo bilayers: Interfacial Gilbert damping
- 2018
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 97:6
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Tidskriftsartikel (refereegranskat)abstract
- The spin-pumping mechanism and associated interfacial Gilbert damping are demonstrated in ion-beam sputtered Co2FeAl (CFA)/Mo bilayer thin films employing ferromagnetic resonance spectroscopy. The dependence of the net spin-current transportation on Mo layer thickness, 0 to 10 nm, and the enhancement of the net effective Gilbert damping are reported. The experimental data have been analyzed using spin-pumping theory in terms of spin current pumped through the ferromagnet/nonmagnetic metal interface to deduce the real spin-mixing conductance and the spin-diffusion length, which are estimated to be 1.56(±0.30)×1019m−2 and 2.61(±0.15)nm, respectively. The damping constant is found to be 8.8(±0.2)×10−3 in the Mo(3.5 nm)-capped CFA(8 nm) sample corresponding to an ∼69% enhancement of the original Gilbert damping 5.2(±0.6)×10−3 in the Al-capped CFA thin film. This is further confirmed by inserting the Cu dusting layer which reduces the spin transport across the CFA/Mo interface. The Mo layer thickness-dependent net spin-current density is found to lie in the range of 1−4MAm−2, which also provides additional quantitative evidence of spin pumping in this bilayer thin-film system.
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| 10. |
- Husain, Sajid, et al.
(författare)
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Static and dynamic properties of Co2FeAl thin films : Effect of MgO and Ta as capping layers
- 2017
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Ingår i: 61ST DAE-SOLID STATE PHYSICS SYMPOSIUM. - : Author(s). - 9780735415003
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Konferensbidrag (refereegranskat)abstract
- The influence of MgO and Ta capping layers on the static and dynamic magnetic properties of Co2FeAl (CFA) Heusler alloy thin films has been investigated. It is observed that the CFA film deposited with MgO capping layer is preeminent compared to the uncapped or Ta capped CFA film. In particular, the magnetic inhomogeneity contribution to the ferromagnetic resonance line broadening and damping constant are found to be minimal for the MgO capped CFA thin film i.e., 0.12±0.01 Oe and 0.0074±0.00014, respectively. The saturationmagnetization was found to be 960±25emu/cc.
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| 11. |
- Jana, Somnath, et al.
(författare)
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Atom-specific magnon driven ultrafast spin dynamics in Fe1-xNix alloys
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- By employing element specific ultrafast spectroscopy in Fe1-xNix alloys alloys, we find a composition dependent effect in the demagnetization that we relate to changes in electron-magnon scattering. In all six measured alloys of different composition, the demagnetization of Ni compared to Fe exhibits a delay, an effect which we find is inherent in alloys but not in elemental Fe and Ni. Using a model based on electron-magnon scattering, we extract a spin-wave stiffness from all alloys that show excellent agreement with values obtained from other techniques. The result establishes the atom-specific sd-exchange induced magnon generation as an underlying mechanism during ultrafast demagnetization in Fe1-xNix alloys
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| 12. |
- Jana, Somnath, et al.
(författare)
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Atom-specific magnon-driven ultrafast spin dynamics in Fe1-xNix alloys
- 2023
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Ingår i: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 107:18
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Tidskriftsartikel (refereegranskat)abstract
- By employing element-specific spectroscopy in the ultrafast time scale in Fe1-xNix alloys, we find a composition-dependent effect in the demagnetization that we relate to electron-magnon scattering and changes in the spin-wave stiffness. In all six measured alloys of different composition, the demagnetization of Ni compared to Fe exhibits a delay, an effect which we find is inherent in alloys but not in elemental Fe and Ni. Using a model based on electron-magnon scattering, we extract a spin-wave stiffness from all alloys that show excellent agreement with values obtained from other techniques.
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| 13. |
- Kumar, Ankit, et al.
(författare)
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Effect of in situ electric-field-assisted growth on antiphase boundaries in epitaxial Fe3O4 thin films on MgO
- 2018
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Ingår i: Physical Review Materials. - : American Physical Society. - 2475-9953. ; 2:5
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Tidskriftsartikel (refereegranskat)abstract
- Antiphase boundaries (APBs) normally form as a consequence of the initial growth conditions in all spinel ferrite thin films. These boundaries result from the intrinsic nucleation and growth mechanism, and are observed as regions where the periodicity of the crystalline lattice is disrupted. The presence of APBs in epitaxial films of the inverse spinel Fe3O4 alters their electronic and magnetic properties due to strong antiferromagnetic (AF) interactions across these boundaries. We explore the effect of using in-plane in situ electric-field-assisted growth on the formation of APBs in heteroepitaxial Fe3O4(100)/MgO(100) thin films. The electric-field-assisted growth is found to reduce the AF interactions across APBs and, as a consequence, APB-free thin-film-like properties are obtained, which have been probed by electronic, magnetic, and structural characterization. The electric field plays a critical role in controlling the density of APBs during the nucleation process by providing an electrostatic force acting on adatoms and therefore changing their kinetics. This innovative technique can be employed to grow epitaxial spinel thin films with controlled AF interactions across APBs.
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| 14. |
- Kumar, Ankit, et al.
(författare)
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Spin transfer torque ferromagnetic resonance induced spin pumping in the Fe/Pd bilayer system
- 2017
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Ingår i: Physical Review B. - 2469-9950 .- 2469-9969. ; 95:6
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Tidskriftsartikel (refereegranskat)abstract
- Inconsistencies in estimates of the spin Hall angle (theta(SH)) and spin diffusion length (lambda(SD)) of nonmagnetic (NM) layers using the spin transfer torque ferromagnetic resonance (ST-FMR) in ferromagnetic FM/NM bilayer structures are attributed to the inverse spin Hall effect (ISHE) and interfacial parameter contributions, interface spin transparency, interfacial anisotropic magnetoresistance, and effective spin-mixing conductance. These contributions in Fe(10 nm)/Pd(2-8 nm) bilayer structures have been probed employing the simultaneous detection of ST-FMR and ISHE in conjunction with in-plane FMR measurements. The interfacial contributions are found to increase with an increase in Pd layer thickness (t(NM)), which can be linked to the spin pumping effect in conjunction with spin backflow. Correcting the t(NM) dependence of the ST-FMR spectra for the interfacial and ISHE contributions prior to estimating theta(SH) and theta(SD) of the Pd layer, the estimated values are found to be 0.10 +/- 0.03 and 5.4 +/- 1.2 nm, respectively.
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| 15. |
- Kumar, Ankit, et al.
(författare)
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Temperature-dependent Gilbert damping of Co2FeAl thin films with different degree of atomic order
- 2017
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 96:224425
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Tidskriftsartikel (refereegranskat)abstract
- Half-metallicity and low magnetic damping are perpetually sought for spintronics materials, and full Heusler compounds in this respect provide outstanding properties. However, it is challenging to obtain the well-ordered half-metallic phase in as-deposited full Heusler compound thin films, and theory has struggled to establish a fundamental understanding of the temperature-dependent Gilbert damping in these systems. Here we present a study of the temperature-dependent Gilbert damping of differently ordered as-deposited Co2FeAl full Heusler compound thin films. The sum of inter- and intraband electron scattering in conjunction with the finite electron lifetime in Bloch states governs the Gilbert damping for the well-ordered phase, in contrast to the damping of partially ordered and disordered phases which is governed by interband electronic scattering alone. These results, especially the ultralow room-temperature intrinsic damping observed for the well-ordered phase, provide fundamental insights into the physical origin of the Gilbert damping in full Heusler compound thin films.
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| 16. |
- Pogoryelov, Yevgen, et al.
(författare)
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Nonreciprocal spin pumping damping in asymmetric magnetic trilayers
- 2020
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 101:5
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Tidskriftsartikel (refereegranskat)abstract
- In magnetic trilayer systems, spin pumping is generally addressed as a reciprocal mechanism characterized by one unique spin-mixing conductance common to both interfaces. However, this assumption is questionable in cases where different types of interfaces are present. Here, we present a general theory for analyzing spin pumping in cases with more than one unique interface and where the magnetic coupling is allowed to be noncollinear. The theory is applied to analyze layer-resolved ferromagnetic resonance experiments on the trilayer system Ni80Fe20/Ru/Fe49Co49V2 where the Ru spacer thickness is varied to tune the indirect exchange coupling. It is demonstrated that the equation of motion of macrospins driven by spin pumping need to be modified in case of noncollinear coupling. Our analysis also shows that the spin pumping in trilayer systems with dissimilar magnetic layers, in general, is nonreciprocal.
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| 17. |
- Wei, Yajun, et al.
(författare)
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Exponentially decaying magnetic coupling in sputtered thin film FeNi/Cu/FeCo trilayers
- 2015
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 106:4
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Tidskriftsartikel (refereegranskat)abstract
- Magnetic coupling in trilayer films of FeNi/Cu/FeCo deposited on Si/SiO2 substrates have been studied. While the thicknesses of the FeNi and FeCo layers were kept constant at 100 angstrom, the thickness of the Cu spacer was varied from 5 to 50 angstrom. Both hysteresis loop and ferromagnetic resonance results indicate that all films are ferromagnetically coupled. Micromagnetic simulations well reproduce the ferromagnetic resonance mode positions measured by experiments, enabling the extraction of the coupling constants. Films with a thin Cu spacer are found to be strongly coupled, with an effective coupling constant of 3 erg/cm(2) for the sample with a 5 angstrom Cu spacer. The strong coupling strength is qualitatively understood within the framework of a combined effect of Ruderman-Kittel-Kasuya-Yosida and pinhole coupling, which is evidenced by transmission electron microscopy analysis. The magnetic coupling constant surprisingly decreases exponentially with increasing Cu spacer thickness, without showing an oscillatory thickness dependence. This is partially connected to the substantial interfacial roughness that washes away the oscillation. The results have implications on the design of multilayers for spintronic applications. (C) 2015 AIP Publishing LLC.
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