Magnetic Resonance And Coupling In Nano-Structure Materials

The investigation of magnetic materials is one of the most important issues in magnetism. Soft magnetic materials (SMM), due to their high saturation magnetization and low coercivity, have been widely applied in many aspects. In recent years, a lot of attention has been paid towards SMM to meet the fast development of spintronics. Bilayer structures of SMM and heavy metals are mostly investigated to study many different effects in spintronics, such as spin pumping effect, spin Hall effect and spin Seebeck effect, etc. Compared to pure ferromagnetic materials, permalloy (Py) presents lower coercivity and magnetic anisotropy, which makes it the most popular material for spintronics devices. However, the spin-orbital coupling (SOC) in Py is relatively weak because the orbital moment is largely quenched in such 3d transition-metal, and the magnetization dynamic damping is accordingly small. This shortcoming restricts the application of Py.Rare earth materials (RE) are usually recognized as national strategic resources due to their high demand in both hard and soft magnetic materials. Most rare earth materials are weakly magnetic at room temperature, and due to their 4f electrons, strong SOC is observed in RE materials such as gadolinium (Gd), terbium (Tb), neodymium (Nd), etc. Multilayer structures of RE and Py are proposed to modify the magnetic dynamic damping while preserving the soft magnetic properties. This will be helpful to set up spintronic devices like spin transfer torque oscillators. Besides that, the topic of strong SOC materials is crucial in spintronics research, including spin pumping and spin Hall effect. In fact, SOC can be divided into two types: intrinsic and extrinsic, distinguished from their mechanism. Intrinsic SOC is derived from the material spin-orbital interaction itself, while extrinsic SOC results from the impurities doped into the materials. The dopant will cause the scattering of the spin up and spin down electrons, leading to enhancement of the SOC. Extrinsic SOC can also be divided into two kinds according to the electron scattering orientation:skew scattering and side jump. Numerous research indicates that the extrinsic SOC may strengthen the spin Hall effect, but will not affect the spin pumping effect.For the evolution of military and telecommunication, soft magnetic materials are fundermentally required to work in high frequency range. Classical ferromagnetic (alloy) metal SMM and ferrite SMM both have some disadvantages which may generate high hysteresis losses and eddy current losses, limiting their high frequency application. An iron-based amorphous nanometer ribbon/film is created to overcome these drawbacks mentioned before, where the ribbon or film shape is beneficial to reduce eddy current losses. Optimal coercivity can be obtained when changing the annealing condition. Moreover, annealing treatment of the sample enlarges its resistivity, which is helpful for high frequency performance. In one word, we can achieve soft magnetic materials with high permeability, high saturation magnetization and high resistivity by this ribbon/film fabrication.In this thesis, we fabricated Py/RE (Tb and Nd) multilayer films, investigated their ferromagnetic resonance information, and discussed spin pumping effect under the influence of multi Py/RE interfaces. We also focused on extrinsic SOC by proposing a bilayer structure of Py and Cu-Ta, where copper was the host metal while tantalum was the dopant. Spin Hall effect measurement was performed to confirm the prediction that impurities enhance the extrinsic SOC. Finally, to achieve high frequency soft magnetic materials, we prepared amorphous Fe-Hf-Cr-B alloy ribbons with different Cr component, and studied the magnetic properties, resistivity and permeability after annealing. The results are as follows:1. Spin pumping in multi-interface system——Py/RE multilayer filmsTwo series of Py/RE and Py/RE/Py flims were prepared for the research, where the RE elements chosen in this work were Tb and Nd. By comparing the spin pumping effect, we figured out the influence of the extra RE/Py interface between Py/RE and Py/RE/Py:(1) For the Py-Tb samples, the coercivity (Hc) of bilayer samples increased from 2 Oe to 9 Oe with the thickness of Tb (dTb) increasing from 0 to 10 nm, and then kept stable when dTb>10nm. While the saturation magnetization (Ms) decreased with dTb increasing, from 790 emu/cm3 to 700 emu/cm3. In the trilayer, the general evolutions of He and Ms were similar to the bilayer: He increased from 2 Oe to 8 Oe and Ms dropped from 820 to 680 emu/cm3. However, double He phenomenon was observed in trilayer samples when dTb≥12nm. In our opinion, the differences between Py/Tb and Tb/Py interfaces may cause the two Py layers to show different Hc.(2) For the Py-Nd samples, He and Ms almost kept the same in both bilayer and trilayer samples with increasing dNd. He remained around 2 Oe in bilayer, while slightly changed from 3 Oe to 1 Oe and then kept around 2 Oe in trilayer. Ms oscillated around 770 and 790 emu/cm3 respectively. The reason was that Nd showed lower Curie temperature, and presented nonmagnetic at room temperature. No magnetic moment could be induced by the adjacent Py layer. Nevertheless, Tb would present an opposite moment to Py’s, resulting in Ms reduction in Py/Tb films.(3) Ferromagnetic resonance measurement of all the samples was performed by a vector network analyzer (VNA-FMR) set-up. Frequency-dependent FMR spectra were picked up and fitted to obtain the information of resonance field and linewidth. For Py-Tb films, with dTb increasing, resonance field decreased at the same frequency, and uniaxial anisotropy gradually increased. Resonance linewidth increased to a stable value when dTb increasing. On the other hand, for Py-Nd films, xesonance field did not change much witir dNd becoming thicker. Uniaxial anisotropy was nearly unchanged. This phenomenon was corresponding to Ms variation trend. But the resonance linewidth had the same trend like Py/Tb, increasing and getting saturated in the end.(4) Damping values were extracted from resonance linewidth. By fitting the damping-dRE curve, for Py-Tb films, it was found that damping increased from 0.006 to a stable value of 0.045 in both bilayer and trilayer, but damping grew faster in trilayer. According to the fitting result, the spin diffusion length was about 6 nm in both bilayer and trilayer, while the spin mixing conductance was 50.5 x 1015cm-2 and 58.2 x 1015cm-2 respectively. For Py-Nd films, damping showed the same trend as Py/Tb’s:increased from 0.006 to 0.020, smaller than Py-Tb’s. The spin diffusion length of Nd was fitted to be around 11nm,and the spin mixing conductance was about 25 x 1015cm-2.(5) Spin pumping effect was significant in both Tb and Nd. The SOC of Nd was stronger than the common SOC in elements like Pt and Pd, etc. As for Tb, its SOC should be even stronger because of its shorter spin diffusion length and larger spin mixing conductance. We proposed a concept of effective spin diffusion length to emphasize the role of the second RE/Py interface in the trilayer film. The extra interface would raise the probability of spin reflux, and reduce the energy dissipation. This caused the damping to increase slower, and made RE in the trilayer films to have a longer spin diffusion length.2. Enhancement of spin orbital coupling induced by Ta impurities in copperWe prepared bilayer film structure of Py and Cu doped with Ta by magnetron sputtering. Ferromagnetic resonance and planar Hall effect measurement were performed to investigate the SOC.(1) We fabricated Py/Cu-Ta bilayer films by magnetron sputtering, and took Ti and SiO2 as buffer and capping layer respectively. The thickness of Py was fixed to 2 nm while dCu-Ta was varied to investigate thickness dependence. We used co-sputtering method to deposit Cu-Ta layer, putting several Ta chips on the copper target. By changing the chips amount, different components of Ta were obtained. EDS was taken to examine the mass composition of Ta, and the result showed to be 7%,10% and 12% in this work.(2) SOC was usually divided into two types:intrinsic and extrinsic. Ferromagnetic resonance measurement of the samples showed there was no apparent spin pumping effect, and damping was kept around 0.007 for all the samples. This value was corresponding to pure Py’s damping, and no obvious enhancement took place in the measurement. This meant the intrinsic SOC in Cu-Ta was quite small to be detected.(3) Planar Hall effect experiment was carried out to study the spin Hall effect of all the different component Ta samples. For accurate measurement, Hall bar structure with electrodes was prepared by photolithography method. It turned out that spin Hall effect in Py/Cu-Ta was as the same strength as in Py/Pt. The largest spin Hall angle appeared in the 7% Ta dopant sample, where the thickness of Cu-Ta was around 2 nm. The value was 0.066, which was comparable to Pt. It was evidential that Ta doping in copper could strengthen extrinsic SOC. However, we did not distinguish whether the enhancement was from side jump or skew scattering.3. High frequency magnetic properties Fe-Hf-B-O ribbons with chromium additiveAmorphous Fe-Hf-Cr-B alloy ribbons were fabricated in this work. Chromium was added to prevent the oxidation of iron, like its role in the stainless steel. We investigated different Cr component ribbons and treated the ribbons with different annealing conditions to get high frequency soft magnetic materials.(1) The amorphous Fe-Hf-Cr-B ribbons with different Cr composition were fabricated by melt spinning method. To provide a low oxygen partial pressure annealing condition, water bubbler system was set up, and ice water mixture was used to keep the bubbler under 0-4℃.(2) The morphology and structure of Fe80Hf9Cr8B3O and Fe85Hf10Cr2B3O were detected by SEM and XRD. The VSM results of Fe85Hf10Cr2B3O showed the coercivity decreased first and then kept increasing when anneal temperature growing. The optimal sample was annealed under 600℃ for 1.5 hours, which presented a coercivity of 0.56 Oe and saturation magnetic flux density of 1.33 T. Its grain size was about 29nm, which was close to the exchange coupling length of iron.(3) Resistivity and permeability were measured to further certify these samples were good candidate for high frequency soft material. The largest resistivity was obtained in the 500℃ 1 hour annealed sample, with a value about 200 μΩ·cm. The optimal sample also showed a resistivity of 165 μΩ·cm. Although these resistivities were not as large as expected, they were still two magnitudes higher than soft magnetic metals. So it would effectively reduce the eddy current losses under high operating frequency.(4) Impedence experiment was taken place on the VNA to extract the permeability of the two high resistivity samples. It was exciting that the real part of permeability was still impressively high with a value of 250 at 10 MHz. As a result, we could conclude that theses samples were good competitor for high frequency usage.