強磁性金属酸化物接合における界面磁気異方性の磁電効果

九州工業大学博士学位論文 学位記番号:情工博甲第351号 学位授与年月日:令和2年9月25日1. Introduction|2. Methods: Experimental and Simulations|3. Control of the symmetry of magnetic anisotropy in CoFeB/MgO junctions using magnetic annealing|4. Micromagnetic simulations on the effect of magnetic anisotropy on switching of an easy cone magnetized free layer|5. Electric-field induced magnetization dynamics in an in-plane magnetized CoFeB/MgO junctions|6. Efficient generation of spin current and excitation of parametric resonance using electric fields九州工業大学令和2年

強磁性金属酸化物接合における界面磁気異方性の磁電効果

九州工業大学博士学位論文（要旨）学位記番号：情工博甲第351号 学位授与年月日：令和2年9月25

Simulations on the Effect of Magnetic Anisotropy on Switching of an Easy Cone Magnetized Free Layer

The easy cone state of magnetization that arises as a result of a competition between the second- and fourth-order perpendicular magnetic anisotropies has the advantage of a non-zero electric-field torque without the application of a bias magnetic field and is thus a potential candidate for purely voltage-driven magnetic storage devices. In this article, the onset of the easy cone state of magnetization is simulated in a ferromagnet film. Subsequently, the switching field and time for voltage-controlled magnetization switching process are studied as a function of the inclination angle of the easy cone state from the film normal in the range of 0°- 45°. The switching field is found to decrease with decreasing anisotropy. The switching time is found to become faster for higher inclination angle of the easy cone state due to an increase in its torque

Modulation of Magnetization Precession Trajectories by Perpendicular Magnetic Anisotropy in CoFeB Thin Films

Precession trajectories during excitation of magnetization dynamics play an important role in determining spin wave propagation, emission power of spin torque oscillator, and spin current generated by spin pumping. However, comparatively little information has emerged on the effect of perpendicular magnetic anisotropy (PMA) on dynamical magnetization trajectories. The effect of PMA on magnetization dynamics has become particularly important since the discovery of voltage control of magnetic anisotropy in CoFeB/MgO junctions. This motivates us to investigate the effect of PMA on dynamical magnetization trajectories of CoFeB thin films using micromagnetic simulations. The ellipticity of the trajectories is found to increase monotonically with PMA. On the other hand, the area of such elliptical trajectories, which determine the spin current generation, shows a non-monotonous change with respect to PMA. This area can be expected to be maximum for the case where ellipticity is ~0.5

Room temperature charge-to-spin conversion from q-2DEG at SrTiO3-based interfaces

Interfacial two-dimensional electron gas (2DEG), especially the SrTiO3-based ones at the unexpected interface of insulators, have emerged to be a promising candidate for efficient charge-spin current interconversion. In this article, to gain insight into the mechanism of the charge-spin current interconversion at the oxide-based 2DEG, we focused on conducting interfaces between insulating SrTiO3 and two types of aluminium-based amorphous insulators, namely SrTiO3/AlN and SrTiO3/Al2O3, and estimated their charge-spin conversion efficiency, {\theta}_cs. The two types of amorphous insulators were selected to explicitly probe the overlooked contribution of oxygen vacancy to the {\theta}_cs. We proposed a mechanism to explain results of spin-torque ferromagnetic resonance (ST-FMR) measurements and developed an analysis protocol to reliably estimate the {\theta}_cs of the oxide based 2DEG. The resultant {\theta}_cs/t, where t is the thickness of the 2DEG, were estimated to be 0.244 nm-1 and 0.101 nm-1 for the SrTiO3/AlN and SrTiO3/Al2O3, respectively, and they are strikingly comparable to their crystalline counterparts. Furthermore, we also observe a large direct current modulation of resonance linewidth in SrTiO3/AlN samples, confirming its high {\theta}_cs and attesting an oxygen-vacancy-enabled charge-spin conversion. Our findings emphasize the defects' contribution to the charge-spin interconversion, especially in the oxide-based low dimensional systems, and provide a way to create and enhance charge-spin interconversion via defect engineering

Electric field induced parametric excitation of exchange magnons in a CoFeB/MgO junction

Inspired by the success of field-effect transistors in electronics, electric field controlled magnetization dynamics has emerged as an important integrant in low-power spintronic devices. Here, we demonstrate electric field induced parametric excitation for CoFeB/MgO junctions by using interfacial in-plane magnetic anisotropy (IMA). When the IMA and the external magnetic field are parallel to each other, magnons are efficiently excited by electric field induced parametric resonance. The corresponding wavelengths are estimated to be tuned down to exchange interaction length scales by changing the input power and frequency of the applied voltage. A generalized phenomenological model is developed to explain the underlying role of the electric field torque. Electric field control of IMA is shown to be the origin for excitation of both uniform and parametric resonance modes in the in-plane magnetized sample, a crucial element for purely electric field induced magnetization dynamics. Electric field excitation of exchange magnons, with no Joule heating, offers a good opportunity for developing nanoscale magnonic devices and exploring various nonlinear dynamics in nanomagnetic systems

Electric-field control of interfacial in-plane magnetic anisotropy in CoFeB/MgO junctions

Magnetoelectric coupling in metal/oxide heterostructures has opened up the possibility of controlling magnetization by voltage, i.e., electric field. However, the electric-field excitation of magnetization dynamics in perfectly in-plane and out-of-plane magnetized films have not been demonstrated so far due to zero electric-field torque originating from voltage control of perpendicular magnetic anisotropy. This limits the application of voltage-controlled magnetic anisotropy in magnetic field free control of magnetization dynamics. Here we show that magnetic annealing can induce an interfacial in-plane magnetic anisotropy of CoFeB/MgO junctions, thereby controlling the symmetry of interfacial magnetic anisotropy. The magnetic anisotropy is modulated by applying voltage: a negative bias voltage increases perpendicular magnetic anisotropy, while a positive bias voltage decreases perpendicular magnetic anisotropy and increases the in-plane magnetic anisotropy. Such a control of symmetry of the interfacial magnetic anisotropy by magnetic annealing and its tunability by electric fields is useful for developing purely voltage-controlled spintronic devices

Facet controlled anisotropic magnons in Y<inf>3</inf>Fe<inf>5</inf>O<inf>12</inf> thin films

Directional specific control on the generation and propagation of magnons is essential for designing future magnon-based logic and memory devices for low power computing. The epitaxy of the ferromagnetic thin film is expected to facilitate anisotropic linewidths, which depend on the crystal cut and the orientation of the thin film. Here, we have shown the growth-induced magneto-crystalline anisotropy in 40 nm epitaxial yttrium iron garnet (YIG) thin films, which facilitate cubic and uniaxial in-plane anisotropy in the resonance field and linewidth using ferromagnetic resonance measurements. The growth-induced cubic and non-cubic anisotropy in epitaxial YIG thin films are explained using the short-range ordering of the Fe3þ cation pairs in octahedral and tetrahedral sublattices with respect to the crystal growth directions. This site-preferred directional anisotropy enables an anisotropic magnon–magnon interaction and opens an avenue to precisely control the propagation of magnonic current for spin-transfer logics using YIG-based magnonic technology

Magnetoimpedance of Epitaxial Y3Fe5O12 (001) Thin Film in Low-Frequency Regime

The atomically flat interface of the Y3Fe5O12 (YIG) thin film and the Gd3Ga5O12 (GGG) substrate plays a vital role in obtaining the magnetization dynamics of YIG below and above the anisotropy field. Here, magnetoimpedance (MI) is used to investigate the magnetization dynamics in fully epitaxial 45 nm YIG thin films grown on the GGG (001) substrates using a copper strip coil in the MHz–GHz frequency region. The resistance (R) and reactance (X), which are components of impedance (Z), allow us to probe the absorptive and dispersive components of the dynamic permeability, whereas a conventional spectrometer only measures the field derivative of the power absorbed. The distinct excitation modes arising from the resonance in the uniform and dragged magnetization states of YIG are respectively observed above and below the anisotropy field. The magnetodynamics clearly shows the visible dichotomy between two resonant fields below and above the anisotropy field and its motion as a function of the direction of the applied magnetic field. A low value of a damping factor of ∼4.7 – 6.1 × 10–4 is estimated for uniform excitation mode with an anisotropy field of 65 ± 2 Oe. Investigation of below and above anisotropy field-dependent magnetodynamics in the low-frequency mode can be useful in designing the YIG-based resonators, oscillators, filters, and magnonic devices

Nonstoichiometric FePt Nanoclusters for Heated Dot Magnetic Recording Media

Heated dot magnetic recording (HDMR) provides a path to increase the areal density of magnetic recording media beyond 4 Tb/in2. HDMR-based recording media requires ultrasmall, noninteracting, and thermally stable magnetic dots with high perpendicular anisotropy. We have synthesized nonstoichiometric Fe60Pt40 nanoclusters with and without a Pt buffer layer on silicon substrates, which shows a reduction in chemical ordering temperatures. The Fe60Pt40 nanoclusters retain the hard magnetic phase up to 1023 K with the coercive field of 1.3 Tesla due to the Pt element compensation from the buffer layer. This compensation of Pt was confirmed through X-ray diffraction (XRD) investigations where two distinct phases of Fe3Pt and FePt3 are observed at elevated annealing temperatures. Micromagnetic simulations were performed to understand the effect of magnetic anisotropy, dipolar interaction, and exchange coupling between the soft magnetic Fe3Pt and hard magnetic FePt. The results imply that nonstoichiometric Fe60Pt40 with the Pt buffer layer facilitates low chemical ordering temperatures retaining the high perpendicular anisotropy with minimal noninteracting behavior, suitable for HDMR