Magnetization Dynamics in Ferromagnetic Thin Films : Evaluation of Different Contributions to Damping in Co2FeAl and FeCo Film Structures

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

Magnetization Dynamics in Ferromagnetic Thin Films : Evaluation of Different Contributions to Damping in Co2FeAl and FeCo Film Structures

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

Kompleks Hidrit Nanoparçacıkların Sentezi ve Hidrojen Kinematiğinin Araştırılması

Today, also in conjunction with the advancements in technology, demand for energy is rapidly increasing. Fossil fuels are the most commonly used energy sources. However, we are running out of fossil fuel sources and fossil fuels have hazardous effects on nature, as well. Therefore, clean and renewable energy sources must be discovered. Hydrogen is a possible candidate that would fulfill our requirements in terms of energy. However, hydrogen must be stored to make it possible to use as an energy source. Storing hydrogen in solid state is the best way to store it in a safe way and with high capacity. Hydrogen can be stored in solid state as hydride structures. Hydride is a chemical compound, formed by hydrogen that bind chemically to the atoms of storage material. Solid state hdrogen storage materials must have a capacity of wt 5.5% and they must work at temperatures less than 85 oC (USDOE, 2009). There is a wide range of materials that scientists research for solid state hydrogen storage. However, none of these materials have displayed a satisfying performance up to now. So it?s still not possible to use hydrogen as an energy source for daily appllications. LiNH2/MgH2 mixture is an attractive material in the last years with it?s better storage performance compared to other materials. Researches show that there are materials with storage capacity more than wt 5.5%, but these materials must be heated up to temperatures more than 300 oC in order to desorb all of the stored hydroge. On the other hand, there are hydrides that desorb hydrogen at room temperatures, but their storage capacities are less than wt 5,5%. So, LiNH2/MgH2 mixture is a favorable material with it?s wt 5.6% storage capacity and hydrogen desorpion temperature about 200 oC. In this study, different catalysts are added to the LiNH2/MgH2 mixture and the effects of these catalysts on the hydrogen storage properties of the samples are investigated. Samples were synthesized via ball milling technique. Structural analysis of the samples were made by utilizing x-ray powder diffraction (XRD) and Fourier transform infrared (FTIR) methods. For thermodynamic analysis, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) methods were used. Kinetic analysis were conducted with a volumetric Sievert?s type apparatus. In the first part of the study, LiNH2/MgH2 mixture is milled with different milling parameters and according to the FTIR measurements of the milled samples, ideal milling parameters were determined. Then LiNH2/MgH2 were mixed with a stoichiometric ratio of 2/1.1 and wt 5% Ca(BH4)2 was added to mixture. This mixture was milled with ideal milling parameters. Beside this, the same mixture was milled with same parameters without adding any catalyst. When we compare the samples with and without catalyst, we have observed that the sample milled with catalyst desorbs hydrogen at a temperature about 16 oC lower than the sample without catalyst. Kinetic measurements were also performed for the samples. Measurements for hydrogen desorption were conducted at 180 oC under 1 bar hydrogen pressure and for hydrogen absorption, measurements were conducted at 150 oC and under 30 bar hydrogen pressure. After kinetic measurements, it was observed that, under same conditions, sample doped with catalyst manifest faster both hydrogen absorption and desorption than the undoped sample. Doped sample desorbed 90% of its total desorbed hydrogen in 45 minutes. However, undoped sample desorbed only 65% of its total desorbed hydrogen in 45 minutes. Regarding absorption, doped sample absorbed 90% of its total absorbed hydrogen in 4.3 hours. On the other hand, undoped sample absorbed only 50% of it?s total absorbed amount of hydrogen in 4.3 hours. After kinetic measurements, it was determined that doped sample desorbed wt 3.45% hydrogen at 180 oC under 1 bar pressure. Undoped sample desorbed only wt 3.10% hydrogen under same conditions. Doped sample absorbed wt 2.72% hydrogen at 150 oC, under 30 bar hydrogen pressure. Undoped sample absorbed only wt 1% hydrogen under same conditions. Theoretical hydrogen storage capacity of the LiNH2/MgH2 mixture is wt 5.6%. However both doped and undoped samples couldn?t absorb or desorb hydrogen in this amount. Moreover, it was observed that doped sample could absorb and desorb hydrogen with higher capacities than undoped sample. In addition, after recycling both hydrogen absorption and desorption capacity of the doped sample increased. On the other hand, sorption kinetics of the doped sample slowed down after recycling. During this study, CaH2 was also added to LiNH2/MgH2 mixture as another catalyst, but this catalyst didn?t drastically effect the performance of the samples. Structural analysis of the samples after milling, desorption and absorption of hydrogen were made by XRD and FTIR measurements. According to these measurements, it was determined that the phases in the mixture were consistent with those provided in the litrature.Günümüzde, teknolojinin oldukça hızlı bir şekilde ilerlemesiyle birlikte, enerjiye olan ihtiyaç da hızla artmaktadır. Buna karşın, yaygın enerji kaynağı olarak kullanılan fosil yakıtlar hem hızla tükenmekte, hem de çevreye zarar vermektedir. Dolayısıyla, artık temiz ve tükenmez enerji kaynakları bulunması gerekmektedir. Gelecekte enerji ihtiyacımızı karşılayabileceği düşünülen kaynaklar arasında, hidrojen, sahip olduğu ideal bir enerji kaynağında bulunması gereken özellikler ile ön plana çıkmaktadır. Fakat hidrojenin enerji kaynağı olarak kullanılabilmesi için depolanması gerekmektedir. Enerji kaynağı olarak kullanılabilecek miktarlarda ve güvenli bir şekilde depolanabilmesi ise ancak katı fazda depolanması ile mümkündür. Hidrojenin katı fazda depolanma yöntemlerinden bir tanesi, hidrojen atomlarının katı malzeme ile kimyasal bağlar oluşturarak hidrit yapıda depolanmasıdır. Katı fazda hidrojen depolayan bir malzemenin, pratik uygulamalarda kullanılabilmesi için, hidrojen depolama kapasitesinin ağırlıkça %5,5 olması ve çalışma sıcaklığının 85 oC'yi geçmemesi gerekmektedir (USDOE, 2009). Hidrojenin hidrit yapılarda depolanabilmesi için üzerinde çalışılan pek çok malzeme bulunmaktadır. Fakat bu malzemelerden hiç biri henüz hidrojenin enerji kaynağı olarak kullanılabilmesini sağlayacak performansa ulaşamamıştır. Son yıllarda üzerinde çalışılan ve diğerlerine oranla daha iyi depolama özelliklerine sahip olan malzemelerden bir tanesi de LiNH2/MgH2 karışımıdır. Bugüne kadar yapılan çalışmalarda kapasite olarak ağırlıkça %5,5'ten daha yüksek oranlarda hidrojen depolayan malzemeler bulunmaktadır. Fakat bu malzemelerin depoladıkları hidrojenin tamamını salabilmeleri için 300 oC üzerindeki sıcaklıklara çıkılması gerekmektedir. Buna karşılık oda sıcaklığında da hidrojen salabilen hidrit yapılar vardır. Fakat bu yapıların depolama kapasiteleri ağırlıkça %5,5'in altındadır. Buna göre ağırlıkça %5,6 depolama kapasitesine sahip olması ve depoladığı hidrojeni 200 oC civarındaki sıcaklıklarda salabilmesi nedeniyle, LiNH2/MgH2 karışımı diğer depolama malzemelerine göre daha avantajlı bir malzemedir. Yapılan çalışma kapsamında LiNH2/MgH2 karışımına farklı katalizörler eklenerek, bu katalizörlerin, örneklerin hidrojen depolama performansı üzerindeki etikleri araştırıldı. Malzeme hazırlama tekniği olarak mekanik öğütme yöntemi kullanılmıştır. Mekanik öğütme sonrası örneklerin nitel yapı analizleri x-ışınları toz kırınımı (XRD) ve Fourier dönüşümlü kızılötesi (FTIR) spektrometresi ölçümleri ile yapıldı. Örneklerin termodinamik özelliklerinin analizleri için diferansiyel taramalı kalorimetre (DSC) ve termogravimetrik analiz (TGA) ölçüm sistemleri kullanıldı. Kinetik ölçümler ise hacimsel ölçüm yapan Sievert sistemi ile gerçekleştirildi. Çalışmanın ilk kısmında LiNH2/MgH2 karışımı farklı öğütme parametreleri ile öğütüldü ve öğütme işleminden sonra, öğütülen karışımın FTIR ölçümleri karşılaştırılarak ideal öğütme parametreleri belirlendi. Daha sonra, 2/1,1 sitokiyometrik oranında karıştırılan LiNH2/MgH2 karışımına katalizör olarak ağırlıkça %5 Ca(BH4)2 eklenerek öğütüldü ve aynı öğütme koşulları ile katalizörsüz olarak öğütülen karışıma göre 16 oC daha düşük sıcaklıkta hidrojen salım reaksiyonunu gerçekleştirdiği gözlendi. Bununla beraber yapılan kinetik ölçümlerde katkılı örneğin, hem 1 bar hidrojen basıncı altında ve 180 oC sıcaklıktaki hidrojen salma reaksiyonunu hem de 30 bar hidrojen basıncı altında ve 150 oC sıcaklıktaki hidrojen soğurma reaksiyonunu, katkısız örneğe göre daha hızlı gerçekleştirdiği gözlendi. Katalizör katkılı örnek reaksiyon sonunda saldığı hidrojen miktarının %90'ını reaksiyonun ilk 45 dakikasında salmıştır. Katalizörsüz örnek ise, 45 dakika içinde, reaksiyon sonunda salmış olduğu hidrojenin sadece %65'ini salabildiği gözlendi. Soğurma reaksiyonlarında ise, katalizör katkılı malzeme, reaksiyonun sonunda soğurduğu hidrojenin %90'ını 4,3 saatte soğurduğu gözlendi. Katkısız malzeme ise 4,3 saatlik sürede, reaksiyon sonunda soğurmuş olduğu hidrojenin sadece %50'sini soğurabildiği belirlendi. Hacimsel yöntemle yapılan kinetik ölçümler esnasında katalizör katkılı malzemenin 180 oC sıcaklıkta ve 1 bar hidrojen basıncı altında ağırlıkça %3,45 hidrojen saldığı belirlendi. Bu değer katkısız malzemede %3,10'da kalmıştır. Katkılı malzemenin 30 bar hidrojen basıncı altında ve 150 oC sıcaklıkta hidrojen soğurma kapasitesi ağırlıkça %2,72 olarak ölçülürken, katkısız malzemede bu değerin %1 olduğu belirlendi. LiNH2/MgH2 karışımının hidrojen depolama kapasitesi teorik olarak ağırlıkça %5,6'dır. Yapılan ölçümlerde örnekler bu seviyede hidrojen soğurumu ya da salımı gerçekleştiremedi. Fakat katkılı malzemenin hem hidrojen soğurma hem de hidrojen salma kapasitesinin katkısız malzemeden daha iyi olduğu belirlendi. Bununla beraber katkılı malzeme ardışık olarak hidrojen soğurma ve salma reaksiyonlarını tekrarladığında hidrojen soğurma ve salma kapasitesi artarken reaksiyon kinetiğinin yavaşladığı gözlendi. Yine çalışma kapsamında LiNH2/MgH2 karışımına katalizör olarak CaH2'de eklendi fakat yapılan ölçümler sonucunda malzemenin performansında önemli bir değişim gözlenmedi. Hazırlanan örneklerin hem mekanik öğütme sonrası, hem de hidrojen soğurma ve salma reaksiyonları sonrasında, FTIR ve XRD ölçümleri ile yapısal değişiklikleri incelendi. Karışımda oluşan fazların, bu malzemelerin literatürde belirtilen reaksiyon ilerleyiş mekanizmaları ile uyum içinde olduğu belirlendi

Magnetization Dynamics in Ferromagnetic Thin Films : Evaluation of Different Contributions to Damping in Co2FeAl and FeCo Film Structures

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

Growth of Co2FeAl Heusler alloy thin films on Si(100) having very small Gilbert damping by Ion beam sputtering

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