Browsing by Author "Kabos, Pavel, committee member"

Now showing 1 - 4 of 4

Open Access
Brillouin light scattering study of linear and nonlinear spin waves in continuous and patterned magnetic thin films
(Colorado State University. Libraries, 2014) Liu, Hau-Jian Jason, author; Buchanan, Kristen S., advisor; Gelfand, Martin P., committee member; Kabos, Pavel, committee member; Neilson, James R., committee member
This thesis focuses on the use of the Brillouin light scattering (BLS) technique to measure spin waves or magnons in thin films. BLS is an experimental technique that measures the inelastically scattered light from photon-magnon interactions. Broadly, three different experiments are presented in this thesis: the measurements of spin wave properties in iron cobalt (FeCo), yttrium iron garnet (YIG), and microstructures involving Permalloy (Ni80Fe20) and cobalt nickel (CoNi). First, conventional backward scattering BLS was used to measure the spin waves in a set of Fe65Co35 films that were provided by Seagate Technologies. By fitting the spin wave frequencies that were measured as a function of the external magnetic field and film thickness, the quantum mechanical parameter responsible for short range order, known as the exchange parameter, was determined. Second, nonlinear spin waves were measured in YIG using conventional forward scattering BLS with time resolution. Two nonlinear three wave processes were observed, namely, the three magnon splitting and confluence. The nonlinear power threshold, the saturation magnetization, and the film thickness were determined independently using network analyzer measurements. The spin wave group velocities were determined from the space- and time-resolved BLS data and compared to calculations from the dispersion relations. Back calculations showed the location where the three magnon splitting process took place. Lastly, spin waves in Permalloy and CoNi microstrips were measured using a recently developed micro-BLS. The micro-BLS, with a spatial resolution of 250 nm, allows for measuring the effects on the lateral confinement of spin waves in microstrips. The confinement of spin waves led to modifications to the dispersion relations, which were compared against the spin wave frequencies obtained from the micro-BLS. The Permalloy experiments shows non-reciprocity in surface spin wave modes with opposite wavevectors and provides a quantitative measure of the difference in excitation efficiency between the surface spin wave and the backward volume spin wave modes. Measurements were also conducted in the Permalloy microstrips at zero external magnetic field, showing evidence that propagating spin waves can be observed by exploiting the effects of shape anisotropy. Finally, preliminary measurements were done on CoNi microstrips with perpendicular anisotropy. A magnetic signal was detected, however further investigation will be needed to determine the exact origin of the observed signal and to definitively answer the question as to whether or not BLS can be used to measure spin waves in perpendicularly magnetized films. Overall, the experiments and results presented in this thesis show that BLS is a useful tool for measuring spin wave properties in magnetic thin films.
Open Access
Damping mechanisms in magnetic recording materials & microwave-assisted magnetization reversal
(Colorado State University. Libraries, 2014) Lu, Lei, author; Wu, Mingzhong, advisor; Gelfand, Martin P., committee member; Kabos, Pavel, committee member; Marconi, Mario C., committee member; Patton, Carl E., committee member
Understanding the damping of magnetization precession in magnetic recording materials is of both fundamental and practical significance. From the practical perspective, the relaxation processes not only set a natural limit to the time of magnetization switching which determines recording data rates, but also play critical roles in advanced magnetic recording techniques such as microwave-assisted magnetic recording and two-dimensional magnetic recording. Experimental and theoretical studies of magnon-electron scattering and two-magnon scattering (TMS) contributions to magnetization relaxations in magnetic recording head and media materials were conducted for the first time in this dissertation. The accuracy of ferromagnetic resonance (FMR) measurements was increased by the use of vector network analyzer (VNA) FMR techniques. Working equations of the grain-to-grain TMS and grain boundary TMS processes were developed based on the TMS models of Krivosik and Mo, and were applied to understand the relaxation mechanisms in various recording-related thin film materials. The dependences of the FMR behavior and relaxation rates on the external field orientation, the microwave frequency, and the temperature were investigated experimentally in the following three domains: the exchange-coupled composite media, the free layers of tunnel magneto-resistance readers, and FeCo alloy films for future writers. The theoretical models were used to analyze the experimental data and to understand the relaxation mechanisms. Microwave-assisted magnetization reversal (MAMR) is considered as a promising mechanism for further increasing the recording area density and pushing it beyond the super-paramagnetic limit. The MAMR operation was demonstrated with a 700-Gbit/in2 perpendicular media sample in this thesis study. For microwaves with frequencies close to the FMR frequency of the media, MAMR was observed for microwave power higher than a certain threshold. For microwaves with certain high power, MAMR was observed for a broad microwave frequency range which covers the FMR frequency and is centered below the FMR frequency.
Open Access
Nonlinear spin wave instability processes in manganese substituted zinc y-type hexagonal ferrites
(Colorado State University. Libraries, 2010) Cox, Richard Garner, author; Patton, Carl E., advisor; Eykholt, Richard Eric, 1956-, committee member; Kabos, Pavel, committee member; Leisure, Robert Glenn, 1938-, committee member; Menoni, Carmen S., committee member; Robinson, Raymond S., committee member
The large magnetocrystalline anisotropy observed in hexagonal ferrites makes these materials ideally suited for high frequency millimeter-wave applications. However, the large microwave losses observed at low-power levels and the high-power handling capabilities of hexagonal ferrites need to be addressed prior to their wide acceptance in real devices. In order to address the above issues, measurements and analyses of the microwave field amplitude (hcrit) required to parametrically excite nonlinear spin wave amplitude growth were performed on single crystal easy plane disks of Mn substituted Zn Y-type hexagonal ferrites at 9 GHz and room temperatures. Plots of the hcrit dependence on the static magnetic field, termed "butterfly curves," were obtained and analyzed for the resonance saturation (RA), subsidiary absorption (SA), and parallel pumping (PP) configurations. In order to obtain the butterfly curve data and perform the analyses: (1) a state-of-the-art computer-controlled high-power microwave spectrometer was constructed, (2) the classical spin wave instability theory, originally developed by Suhl and Schloemann, was extended, and (3) instability measurements were performed on multiple Zn Y-type hexagonal ferrites samples for several pumping configurations and static field settings. The measurements and analyses performed here constitute the first time RS, SA, and PP spin wave instability butterfly curve analysis have all been performed in planar hexagonal ferrite samples. This work also corresponds to the first time that resonance saturation measurements and analyses were performed for static magnetic fields both at and in the vicinity of the ferromagnetic resonant field in a hexagonal ferrite. The data obtained as part of this work show that the microwave field amplitude required to parametrically excite nonlinear spin wave amplitude growth in hexagonal ferrites is similar to polycrystalline cubic ferrites, which are currently in use in microwave devices. Follow-up measurements, motivated by this work, revealed that hcrit can be varied by manipulating the sample dimensions. The analyses performed here indicate that two-magnon scattering is likely not the dominant source of the large low-power microwave losses observed in these hexagonal ferrites; rather that these losses may be an intrinsic property of the material. The theoretical work performed here identified a sign problem with the anti-Larmor uniform mode complex damping terms in several past publications and provides an improved methodology of treating the uniform mode anti-Larmor complex frequency damping.
Open Access
Studies of magnetization dynamics in magnetic recording media and patterned yttrium iron garnet films
(Colorado State University. Libraries, 2018) Richardson, Daniel, author; Wu, Mingzhong, advisor; de la Venta Granda, Jose, committee member; Kabos, Pavel, committee member; Krueger, David, committee member; Marconi, Mario, committee member
Exchange coupling and damping are studied in magnetic media materials for applications in current perpendicular magnetic recording (PMR) technology as well as future heat assisted magnetic recording (HAMR) media technology. Damping and exchange coupling are directly related to magnetization switching time in writing operation and the signal-to-noise ratio in reading, both critical to the performance of hard disk drives. Intergranular exchange is studied in current PMR media to see how exchange is altered in the presence of SiO2 based segregant. By varying the segregant by as much as 30%, there is strong tunability of the exchange field between the grains. The damping in future FePt-based HAMR media is studied near the curie temperature (725 K) of FePt where the writing stage in the recording media takes place. The trends of ferromagnetic resonance (FMR) linewidth varying with the sample temperature, the volume fraction of carbon in the media, and the angle of the external field indicate that the overall damping includes strong contributions from intrinsic magnon-electron scattering as well as extrinsic two-magnon scattering between the grains. Interlayer exchange coupling and damping were studied in magnetic layered systems consisting of a soft ferromagnetic transition metal or alloy layer and a hard FePt layer at room and elevated temperatures. It was found that exchange coupling and damping are strongly dependent on temperature, the soft layer thickness, and the choice of material of the soft layer. Spin waves are studied in the linear and non-linear regimes using magnonic crystals consisting of yttrium iron garnet (YIG) thin film strips with periodic etched lines or periodic metallic lines deposited on top of the YIG strip, as well as YIG strips with randomly spaced metallic lines deposited on top. The various media provide ways of controlling the dispersion by altering the interference of the spin waves, allowing for a wide range of interesting phenomenon to be observed. Spin-wave fractals are observed for the first time in a YIG strip with periodic etched lines. The etched lines serve as position dependent potentials to increase dispersion in the YIG strip large enough for fractal formation in the nonlinear regime. This is also the first time fractals of any type that have been observed without the formation of time-domain solitons. Spin-wave localization is observed in the linear regime for the first time in YIG strips with randomly spaced metallic lines where the metallic lines serve as potential barriers for causing spin wave interference. Magnonic crystals consisting of YIG strips with periodically spaced metallic lines are used to compare a standing wave state with the localized state. The localized state is much stronger and much more confined to a smaller physical space than the standing wave state.