Investigations On The L-S Couplings Of Magnetic Films

The most commonly used magnetic metal in the research for spintronics applications is permalloy (Py = Ni80Fe20), due to its low coercivity, low magnetocrystalline anisotropy and low damping (a). The noble non-magnetic heavy metal elements platinum (Pt), silver (Ag), and gold(Au) exhibit a strong spin-orbital coupling and as such interact with or modify magnetic properties in thin-film-based spintronic devices. In this dissertation, the properties and applications based on Py thin films doped with these heavy metals are the main topic. We systematically report on the tunability of the thin films Py100-xMx (M = Pt, Ag, Au) and Py100-x-yPtxAgy for the use in magnonic devices. We also investigate the realization and microwave emission properties of spin Hall nano-oscillators based on those films. The main results are summarized as follows:1. The magnetodynamic properties of Py100-xMx (M = Pt, Ag, Au) show element-specific behaviors as a function of the concentration of element M.The magnetostatic and magnetodynamic properties of the Py100-xMx films have been systematically investigated by ferromagnetic resonance spectroscopy, and the study found that: (1) While the Gilbert damping a of the Py100-xAgx and Py100-xAux films has a linear dependence with concentration,the Py100-xPtx samples show a more quadratic behavior.While a increases rapidly with both Pt and Au content, it is virtually independent of the Ag content. Alloying with Pt has the strongest effect on damping, leading to a maximum value of ? = 0.035 at 30 at. %, i.e., about 4.5 times the value of the pure Py film. (2) The saturation magnetization Ms of Pyioo-xMx is found to quadratically decrease with increasing alloying, the most for Au and the least for Pt. (3) The exchange stiffness A decreases rapidly in the case of the Pyioo-xAgx and Pyioo-xAux films, but less so for Pyioo-xPtx. The ab-initio calculated trends quantitatively agree with the experimental observations.Using variable temperature broadband ferromagnetic resonance spectroscopy, the temperature (T) and dopant-concentration dependence of the magnetodynamic properties of Py and Pyioo-xMx films have been further studied. The damping increases with for all samples, but the enhancement is most pronounced for Py doped with 30 at. % Au. We extract the spin wave stiffness (D) using the first perpendicular standing spin wave (PSSW)mode. The observed temperature dependence of D is best described by a T2 law, which suggests a noticeable effect of the itinerant character of the electrons. The spin wave stiffness is also estimated using Bloch's law and we compare the extracted values to the first method. The results strongly imply that not only spin wave and Stoner excitations,but also other mechanisms contribute to the reduction of Ms.2. Py100-x-yPtxAgy films can be defined by a three-dimensional parameter space, where Ms, a, and A can be independently varied.Co-alloying with Pt, Au, and Ag in different proportions defines a three-dimensional parameter space where Ms, a, and A can be independently tuned to the desired values,within certain limits. As a proof-of-principle demonstration,we design a set of 100 nm thick Py100-x-yPtxAgy films, where the saturation magnetization Ms stays constant throughout the set and the damping can be tuned by a factor of 4, which we then confirm experimentally.3. Py100-x-yPtxAgy/Pt - SHNOs - Single nano-constrictionIn a series of nano-constriction spin-Hall nano-oscillators (SHNO) we utilized a series of tailored alloys of the form Py100-x-yPtxAgy, which have the same magnetization ?0Meff=(0.617±0.034) T but a varies by close to a factor of three. SHNOs are respectively Py84Ag16/Pt (S01), Py77.5Pt10Ag12.5/Pt (S02), Py75Pt15Ag10/Pt (S03), and Py73Pt19Ag8/Pt (S04), and the width of the single nano-constriction is 150 nm. By means of electrical microwave measurements, under the same condition of a constant field 0.5 T with the current IDC being swept, we can observe the following: (1) The increased damping does not change the fundamental nature of the excited auto-oscillation, which is evident from the similar current-frequency dependences. (2) The linewidth of our nanoconstriction-SHNOs generally decreases rapidly after the auto-oscillation onset and then levels off for higher IDC values. The low-damping SHNO S01 reached its minimum level at ?f? 9.6 MHz, while the SHNOs from higher damping materials all have similar smaller minimum linewidths of ?f?3 MHz. (3) The output power P of the four nanoconstriction-SHNOs increases almost exponentially with increasing IDC. However, with increasing Pt concentration a sharp drop in P is apparent. (4) The threshold current increases almost linearly with the initial damping in the magnetic free layer. (5) Comparing the relationship between threshold current and different applied fields, the auto-oscillations of SHNOs can be kept in the condition of lower damping and higher applied fields. Under the condition of a constant applied current IDC = 3.2 mA and the magnetic field being swept: (1) the auto-oscillation frequencies increase with the applied field, in accordance with the relation of out-plane magnetic field and oscillation frequency. Moreover, the relationship between auto-oscillation frequency and applied magnetic field for all four devices is the same,resulting from the constant magnetization among the samples. (2) The microwave power of each SHNO device as a function of the swept field is similar, firstly increasing for all samples with increasing field, then having a maximum for an intermediate field, and finally dropping relatively sharp until a point where no more oscillations are detectable,The field at which the SHNOs emit their maximum output power decreases monotonically with increasing damping. (3) The minimum linewidth decreases with increased damping.4. Py100-x-yPtxAgy/Pt - SHNOs - Double nano-constrictionThe double nano-constriction SHNOs based on Py100-x-yPtxAgy films are successfully fabricated,SHNOs are respectively Py84Agi6/Pt (D01),Py77.5PtioAg12.5/Pt (D02),Py75Pt15Ag10/Pt (D03), and Py73Pt19Ag8/Pt (D04). Under a constant magnetic field with a swept bias current, the following results are obtained: (1) at the same spacing of double nano-constriction SHNOs (dcc = 300 nm), the mutual synchronization becomes weak or disappears, the output power decreases, and the linewidth increases, all in connection with an increasing damping. The linewidth of mutually synchronized signals is not only much narrower than in the un-synchronized state, but also the output power under mutual synchronization is even bigger than the sum of the power of the two unsychronized signals.(2) We vary the spacing of double nano-constriction SHNOs (dcc = 300 nm, 600 nm,900 nm) to investigate how the mutual synchronization changes. For the low damping D01 device, we find that the synchronized state can be achieved with both spacings 300 nm and 600 nm, while for the higher damping D02 device, the synchronized state can be only found for 300 nm spacing. In the D03 and D04 devices, we could not observe synchronization in any spacing.5. Py100-x-yPtxAgy/Pt - SHNOs - Multi nano-constrictionThe lowest damping Py84Agi6/Pt structure has been chosen to make five nano-constriction SHNO aligned in a row. Compared with double nano-constriction SHNO based on the same film structure under the same constant field 6000 Oe, the same width nano-constriction, and the same spacing 300 nm, we obtained a twice as large maximum output power. At the magnetic field 7000 Oe, the five nano-constriction SHNO can reach the maximum output power of up to 17 pW,and the minimum linewidth is less than 1 MHz.