| 1. |
- Zamani, Atieh
(författare)
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Metallic Amorphous Thin Films and Heterostructures with Tunable Magnetic Properties
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The primary focus of this thesis is to study the effect of doping on magnetic properties in amorphous Fe100−xZrx alloys. Samples with compositions of x = 7,11.6 and 12 at.% were implanted with different concentrations of H. Moreover, the samples with a composition of x = 7 at.% were also implanted with He, B, C and N. Magnetic measurements were performed, using SQUID magnetometry and MOKE, in order to compare the as-grown and the implanted films. The Curie temperature (Tc) increases and the coercivity (Hc) decreases, with increasing dopant volume. We also found that Hc increases with temperature for B and C doped samples. Magnetization curves at low temperature validate the presence of non-collinear spin configurations in the as-grown films, which is suppressed after doping, resulting in films with tunable soft magnetic properties. We have also studied the effect of interlayer mixing and finite size effects on FeZr in Fe92Zr8/AlZr multilayer films, and found an anomalous increase of Tc with decreasing thickness.Strain induced changes in the magnetization of an amorphous Co95Zr5 film at the orthorhombic phase transition of the BaTiO3 substrate, was also studied. The results show that structural modifications of the substrate increases the stress and hence changes the magnetic anisotropy in the amorphous Co95Zr5 layer.Finally, the magnetization reversal of Co and CoX heterostructures, with X being Cr, Fe, Ni, Pd, Pt and Ru, has been studied. For this purpose a synthetic antiferromagnet structure, FM/NM/FM, was used, where FM is a ferromagnetic Co or CoX layer and NM is a nonmagnetic Ru spacer layer. The FM layers are coupled antiferromagnetically across the NM layer. For a range of FM layer thicknesses, the exchange stiffness parameter Aex and the interlayer coupling (JRKKY ) of the Co or CoX layers were obtained. This is done by fitting M(H) curves, measured by SQUID magnetometry, to a micromagnetic model. The alloying in CoX resulted in a decreasing Aex and also a reduced MS. The experimental results are in a good agreement with DFT calculations.
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| 2. |
- Gupta, Rahul
(författare)
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Spin Current Generation in Magnetic Heterostructures and its Impact on Terahertz Emission : A Spin Dynamics Perspective
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The transfer of knowledge from one generation to another is key to the intellectualness of mankind. In the present information age, digital technology provides easy access to knowledge and information. However people across the globe simultaneously generate an enormous digital footprint, which demands to store and process the information in a modish way. Spin-based electronics is being considered a prospective candidate beyond complementary metal-oxide-semiconductor technology with several applications in data storage and data communication. The key concept of this technology is the generation, transportation, and detection of spin currents in magnetic heterostructures consisting of ferromagnetic (FM) and non-ferromagnetic (NFM) bilayer thin films.In this thesis, I describe the concepts of spin dynamics at the nano- to femtosecond timescales and experimental techniques used to extract the spin dynamics properties of magnetic heterostructures. In this regard, we have shown that the Gilbert damping parameter and the number of quantum conductance channels (QCCs) can be enhanced by doping the FM layer with Re in the Ru/Fe65Co35/Ru heterostructure. The same heterostructure was used to evidence superdiffusive spin transport and a proximity induced magnetic moment in the Ru layer. It has also been shown that the number of QCCs can be enhanced by inserting a Cu layer at the interface between the FM and NFM layers in the Co2FeAl/β-Ta heterostructure where the Gilbert damping parameter of Co2FeAl depends on its chemical ordering. Further, we have found that the spin torque (SOT) efficiency in the 2D-transition metal dichalcogenide, 1T-TaS2, based heterostructure is one order larger as compared to Co2FeAl/β-Ta and Fe/Pd heterostructures. Moreover, it has been shown that crystalline quality and strain engineering can significantly impact the SOT efficiency and emission of terahertz radiation in Fe/Pd and Fe/Pt heterostructures, respectively. Finally, a full Heusler (Co2FeAl) based spintronic terahertz emitter is presented, which utilizes an optically induced spin current and the inverse spin Hall effect phenomenon. This thesis provides useful insights in the pathway towards power efficient spin logic devices.
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| 3. |
- Jana, Somnath, et al.
(författare)
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Revisiting Goodenough-Kanamori rules in a new series of double perovskites LaSr1-xCaxNiReO6
- 2019
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322 .- 2045-2322. ; 9:1
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Tidskriftsartikel (refereegranskat)abstract
- The magnetic ground states in highly ordered double perovskites LaSr1-xCaxNiReO6 (x = 0.0, 0.5, 1.0) are studied in view of the Goodenough-Kanamori rules of superexchange interactions in this paper. In LaSrNiReO6, Ni and Re sublattices are found to exhibit curious magnetic states separately, but no long range magnetic ordering is achieved. The magnetic transition at similar to 255 K is identified with the independent Re sublattice magnetic ordering. Interestingly, the sublattice interactions are tuned by modifying the Ni-O-Re bond angles through Ca doping. Upon Ca doping, the Ni and Re sublattices start to display a ferrimagnetically ordered state at low temperature. The neutron powder diffraction data reveals long range ferrimagnetic ordering of the Ni and Re magnetic sublattices along the crystallographic b-axis. The transition temperature of the ferrimagnetic phase increases monotonically with increasing Ca concentration.
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| 4. |
- Malik, Rameez Saeed, 1987-
(författare)
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Ultrafast Magnetization Dynamics : Element-selective studies of magnetic alloys using ultra short XUV pulses
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis, I investigate the ultrafast magnetization dynamics in 3d ferromagnets and their alloys with ultrashort laser pulses. The high harmonics generation (HHG) setup provides extreme-ultraviolet photons with energies 35-72 eV, which is the energy range where 3d metals have their M2,3 absorption edges. By employing HHG with the transverse magneto-optical Kerr effect, the magnetization of multiple elements in a magnetic system is probed and their dynamics are resolved separately on femtoseconds time scales.The magneto-optical response of elemental Fe and Ni during demagnetization is investigated. This magneto-optical response is measured as an asymmetry in the intensity of reflected light for two opposite sample magnetization directions. Experiment and density functional theory calculations show that for Fe, the asymmetry is strongly dependent on the particular type of magnetic excitation. However, for Ni, it is relatively insensitive to the magnetic excitation. Next, the element-specific magnetization dynamics of FeNi alloys are investigated. A time delay in the Ni demagnetization relative to Fe is observed for all alloy compositions. This Ni-delay depends on the alloy composition and is related to changes in the exchange interactions.Co2FeAl (CFA) Heusler alloys are unique due to their peculiar electronic structure and because they can exhibit very low damping. Experimentally, CFA films show a decreased damping with an increase in structural ordering. The demagnetization times of Fe and Co in CFA samples with different amount of ordering are similar for all samples. However, the remagnetization times exhibit a dependence on the structural ordering. Both the theoretical and experimental damping parameters correlate well with the remagnetization times. In FeCo alloys, the damping can be changed by doping with heavy metals. Here, the magnetization dynamics of Fe65Co35 films as a function of Re doping are investigated. We find no observable change in the demagnetization times for samples with increased damping. However, when increasing the Re doping and the damping, the remagnetization time becomes faster. Also, a fast increase of the asymmetry signal is observed at the Ru-edge during the demagnetization of FeCo. This effect is attributed to a super-diffusive spin current going from the FeCo layer to the Ru capping layer.Last, the magnetization dynamics of a ferrimagnetic insulator is studied. The NiFe2O4 asymmetry shows oscillatory dynamics after an ultrashort laser pulse excitation. With 1.55 eV pump, these oscillations are strong. For 3.1 eV pump, demagnetization becomes dominant and the oscillations diminish.
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| 5. |
- Unikandanunni, Vivek, 1993-
(författare)
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Ultrafast spin dynamics and relaxation in metallic ferromagnets
- 2019
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- We performed ultrafast demagnetization measurements on epitaxial hcp-cobalt thin film with the easy axis in the plane of the film by pumping the sample with 400 nm radiation and probing the magneto-optic effects in the longitudinal magneto optic Kerr effect geometry using 800 nm radiation. The in-plane magnetic anisotropy of the sample enabled us to measure the demagnetization dynamics along two different crystalline directions, namely along the easy axis and at 45 degrees between the easy and hard axes. The spin relaxation is found to be systematically faster in the 45 degrees axis direction compared to the easy axis of magnetization. The observed effect can be related to the anisotropic electron-phonon coupling in cobalt. Our results emphasize the importance of the lattice symmetry in ultrafast demagnetization dynamics.We also performed transient reflectivity measurements on platinum and silver with THz/400 nm pump and 800 nm probe. The optically induced reflectivity is usually explained using the phenomenological two temperature model (2TM). We could see that the reflectivity induced by 400 nm could be explained in terms of the two temperature model whereas the THz induced reflectivity could not be explained with it since the microscopic mechanism of pump excitation is not dealt within the 2TM. We could qualitatively explain the deviation from the 2TM by considering the effect of pump on the band structure of the materials under investigation.
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