Spin dynamics in Al-substituted yttrium iron garnets is investigated using broadband ferromagnetic resonance measurements in the temperature range T=200-360 K. Using the measured data, the resonance field and linewidth as well as their temperature dependence are determined, with implications for the uniformity and overall quality of the samples prepared via different chemical fabrication routes. These key parameters governing the spin dynamics in the material are important for its applications in high-speed spintronic and magnonic devices.

Spin dynamics in Al-substituted yttrium iron garnets is investigated using broadband ferromagnetic resonance measurements in the temperature range T = 200-360 K. Using the measured data, the resonance field and linewidth, as well as their temperature dependence, are determined, with implications for the uniformity and overall quality of the samples prepared via different chemical fabrication routes. These key parameters governing the spin dynamics in the material are important for its applications in high-speed spintronic and magnonic devices.

This thesis is a study of the static and dynamic magnetic properties of Heusler alloys and substituted yttrium iron garnets (YIG) using magnetometry, ferromagnetic resonance (FMR), and Brillouin light scattering (BLS) measurement techniques, in combination with other structural and chemical analysis methods. The following main research results were obtained.

Epitaxial films of magnetic shape memory alloy Ni(Co)MnSn were found to undergo at 270 K a martensitic phase transformation from the cubic austenitic phase to a twinned orthorhombic martensitic phase, which is accompanied by a 3-fold splitting of the FMR in the system. The additional resonance lines were explained as due to weak antiferromagnetic coupling of the ferromagnetic twins across twin boundaries, which makes the submicron-twinned martensite phase resemble an artificial antiferromagnetic superlattice. The elastic energy balance of the film/substrate interface was found to result in the formation of a submicron, stripe-like, periodical structure of twins, which is of interest for spintronic and magnonic applications. 

Epitaxial films of full Heusler alloy Co₂FeGe were investigated in terms of their structure and magnetic properties. Improved quality of epitaxy, formation of an atomically ordered L2₁ phase, increased magnetization, and reduced spin damping were observed for films processed at elevated temperature. BLS and FMR experiments showed that room temperature deposition followed by annealing yields the highest magnetization, exchange stiffness, and lowest spin damping in the material, which are important characteristics for high-speed spintronic devices. Strong hybridization of the spin waves modes in the Damon-Eshbach geometry was found in the BLS dispersion data, which is promising for applications in magnonic waveguides and logic devices.

Magnetometry of Al-substituted YIG (Y₃AlFe₄O₁₂) revealed a sharp rise in magnetic anisotropy below 150 K, while the saturation magnetization, exchange and spin-wave stiffness change only slightly. Using FMR we determined the effective magnetization, anisotropy, resonance field, spin damping, as well as their temperature dependence -- key parameters governing the spin dynamics in the material, important for its applications in high-speed circuits operating at room and cryogenic temperatures.

Detta avhandlingsarbete är en studie av de statiska och dynamiska magnetiska egenskaperna hos Heusler-legeringar och substituerade yttriumjärngranater (YIG) med hjälp av magnetometri, ferromagnetisk resonans (FMR), och Brillouin-ljusspridning (BLS) mättekniker, i kombination med andra strukturella och kemiska analysmetoder. Följande huvudsakliga forskningsresultat erhölls.

Epitaxiella filmer av magnetisk formminneslegering Ni(Co)MnSn befanns genomgå en martensitisk fastransformation vid 270 K från den kubiska austenitiska fasen till en tvillingortorombisk martensitisk fas, som åtföljs av en 3-faldig delning av FMR i systemet . De ytterligare resonanslinjerna förklarades bero på svag antiferromagnetisk koppling av de ferromagnetiska tvillingarna över tvillinggränserna, vilket gör att den submikron tvilling-martensitfasen liknar ett artificiellt antiferromagnetiskt supergitter. Den elastiska energibalansen för film/substrat-gränsytan visade sig resultera i bildandet av en submikronisk, randliknande periodisk struktur hos tvillingar, vilket är av intresse för spintroniska och magnoniska tillämpningar.

Strukturen och magnetiska egenskaperna hos epitaxiella Heusler-lege-ringsfilmer Co₂FeGe deponerade på MgO-substrat undersöktes. Förbättrad kvalitet på epitaxi, bildning av en atomiskt ordnad L2₁-fas, ökad magnetisering och minskad spinndämpning observerades för filmer som glödgades vid förhöjd temperatur. BLS- och FMR-mätningar visade att deponering vid rumstemperatur följt av glödgning ger högsta magnetisering, exchange-stiffness och lägsta spinndämpning i filmerna, vilket är önskvärda egenskaper för spinntroniska höghastighetskretsar. Stark hybridisering av spinnvågor i Damon-Eshbach-geometrin observerades, vilket är attraktivt för tillämpningar i magnoniska signalbehandlingskretsar.

Magnetometri av Al-substituerad YIG (Y₃AlFe₄O₁₂) visade en kraftig ökning av magnetisk anisotropi under 150 K, medan mättnadsmagnetisering, exchange-stiffness och spinnvågs-stiffness endast ändras något. Med hjälp av FMR fastställde vi den effektiva magnetiseringen, anisotropikonstanten, resonansfältet, spinndämpningskonstanten, såväl som deras temperaturberoende - nyckelparametrar som styr spindynamiken i materialet, viktiga för dess tillämpningar i höghastighetskretsar som arbetar i rum och kryogena temperaturer.

Magnetic properties of Y3AlFe4O12 garnet ceramics have been studied over a wide range of temperature (3 – 370 K) and magnetic fields (up to 2 kOe). Effects of varying the temperature on some of the application-specific magnetic characteristics have been analyzed in detail. With the decrease in temperature, the effective anisotropy constant, KEff, is found to sharply rise below ∼150 K, while the saturation magnetization, Ms, changes only slightly (<20 % within 3 – 250 K). The exchange stiffness, Aex, and spin wave stiffness, D, parameters mirror the relatively smooth behavior of the magnetization at low temperatures but display a rapid drop when T approaches TC ≈ 436 K. The temperature dependence of the domain wall thickness and the critical single-domain size correlate with the corresponding values for pure Y3Fe5O12 (YIG) and are in agreement with the theoretical estimates. We conclude by discussing the ways of how to use the obtained results for tailoring the magnetic properties of YIG-based materials for technological applications.

The dynamic magnetic properties of full Heusler alloy thin films of Co₂FeGe, grown on MgO (001) substrates under different thermal conditions, were investigated. Brillouin light scattering and ferromagnetic resonance measurements revealed that depositing at room temperature followed by annealing at 300 ° C for 1 h produces the best results for maximizing magnetization, exchange stiffness, and minimizing spin-dynamic dissipation in the films, which are desirable characteristics for high-speed spintronic devices. Additionally, strong hybridization of spin waves in the Damon-Eshbach geometry was observed, which is attractive for applications in magnonic signal processing circuits.

The influence of film thickness on the formation of twinning structure, martensitic transformation and magnetoelastic properties of epitaxial films of Ni(Co)MnSn magnetic shape memory alloy is investigated by means of ferromagnetic resonance spectroscopy, synchrotron X-ray diffraction and standard magnetic measurements. It is found that constraints from the film/substrate interface block the martensitic transformation in the 20 nm thick film. The increase of the film thickness results in a progressive stress relaxation and, as a result, the martensitic transformation becomes possible starting from 50 nm. Twinning of the films is required to conserve the films surface area. The elastic energy balance between the film/substrate interface and the twin boundaries leads to the formation of a submicron wide, stripe like, periodical structure of twins, which is of interest for spintronic or magnonic applications. The width of the twin variants increases with the film thickness growth, resulting in the dramatic modification of magnetic properties.

Magnetic properties of N⁢i_46.0⁢M⁢n_36.8⁢S⁢n_11.4⁢C⁢o_5.8/MgO⁡(001) epitaxial thin film, which undergo a martensitic phase transformation from cubic austenitic phase to a twinned orthorhombic martensitic phase at 270 K, were studied by the magnetic resonance at the microwave frequency of 9.45 GHz. It was found that the single resonance line observed in the austenite splits into three lines in the martensitic phase. A theoretical approach was developed to show that the additional resonance lines are caused by the weak antiferromagnetic coupling of the ferromagnetic twin components across twin boundaries. Fitting of the experimental resonance lines to model gives an effective field of antiferromagnetic coupling of about 1.5 kOe, which is two or three orders of magnitude lower than in the conventional antiferromagnetic solids because the number of magnetic ions interacting antiferromagnetically through the twin boundary is much less than the total number of magnetic ions in the twin. This feature shows a strong resemblance between the submicron twinned martensite and artificial antiferromagnetic superlattices, whereby providing a distinctive insight into magnetism of the studied magnetic shape memory material.