No description available.

In addition to being useful for some nondestructive evaluation techniques, interactions between magnetic fields and defects in solids may also alter material properties. To explore this possibility, Maxwell's equations were coupled with a continuum mechanics model for elastic strain to formulate analytical expressions for the interaction of a magnetic field with several crack geometries. The influence of crack velocity and a realistic (nonlinear) magnetic susceptibility were included into a model of this type for the first time and shown to introduce unexpected trends in the magneto-elastic stress intensity. Singularities magneto-elastic stresses appear at different combinations of magnetic field strength and crack velocity, and the stresses at the crack tip switch sign. In a related study, the interaction of an alternating magnetic field with elastic stress through was explored through a coupling effect known as magneto-acoustic resonance. A model for the phenomena, in which magnetic waves excite elastic waves and vice versa, was formulated and used to explore the spin (magnon) and anti-plane elastic (phonon) interactions in piecewise homogeneous ferromagnetic spaces with two different sets of properties. The model suggests some combinations of magnetic field and frequency can produce a new kind of wave to appear. These new waves, which we call Accompanying Surface Magnetoelastic (ASM) waves, are localized at the interface between the two ferromagnetic media and they accompany reflection and transmission waves. It is shown that the amplitudes of the reflected, transmitted, and ASM waves depend strongly on magnetic field strength, frequency, and the angle of the incident wave, as well as on the physical properties of ferromagnetic media. / Ph. D.

Transport of spin angular momentum (spin currents) in magnetic thin films is important for non-volatile spin-based memory devices and other emerging information technology applications. It is especially important to understand how a spin current propagates across interfaces and how a spin current interacts with magnetic moments. The great interest in devices based on ferromagnetic metals generated intensive theoretical and experimental studies on the basic physics of spin currents for the last few decades. Of particular interest recently is the so-called "pure" electronic spin current, which is carried by electrons and yet unaccompanied by net charge flow, in part because of the prospect of transporting spin with minimal Joule heating. However, in contrast to ferromagnetic metals, spin transport in antiferromagnetic metals, which are promising materials for next-generation magnetic information technology, is not well understood yet. This dissertation addresses the mechanisms of transport by pure spin current in thin-film multilayers incorporating metals with antiferromagnetic order. We focus on two specific materials: (1) CoGd alloys with ferrimagnetic sublattices, which resemble antiferromagnets near the compensation composition, and (2) elemental antiferromagnetic Cr, which can be grown as epitaxial films and hence serve as a model system material. For both the CoGd and Cr studies, spin-valve-like structures of NiFe/Cu/CoGd and NiFe/Cu/Cr/CoFe are prepared to conduct ferromagnetic resonance spin pumping experiments. Precessing magnetization in the NiFe "spin source" pumps a transverse spin current to the adjacent layers. We measure the loss of the spin angular momentum in the "spin sink" layer by measuring the broadening of the resonance linewidth, i.e., the non-local damping enhancement, of the spin source. The antiparallel magnetic moments of Co and Gd sublattices partially cancel out the dephasing of a transverse spin current, thereby resulting in a long spin dephasing length of ≈ 5-6 nm near the magnetic compensation point. We find evidence that the spin current interacts somewhat more strongly with the itinerant transition-metal Co magnetism than the localized rare-earth-metal Gd magnetism in the CoGd alloy. We also examine spin transport via structurally clean antiferromagnetic Cr, epitaxially grown with BCC crystal order. We observe strong spin reflection at the Cu/Cr interface, which is surprising considering that thin layers of Cu and Cr individually are transparent to spin currents carried by electrons. Further, our results indicate other combinations of electrically conductive elemental metals (e.g., Cu/V) can form effective spin-reflecting interfaces. Overall, this thesis advances the basic understanding of spin transport in metallic thin films with and without magnetic order, which can aid the development of next generations of efficient spintronic devices. This work was supported in part by the National Science Foundation, Grant No. DMR-2003914. / Doctor of Philosophy / Manipulation of electronic flow, i.e., net charge flow, underlies modern electronic devices such as computers, mobile phones, and electric cars. However, the conventional charge transport inevitably results in wasted energy, due to resistive (Joule) heating in the devices. A new research area which uses the electron's spin has recently emerged, namely spintronics. Spintronics uses the spin of electrons rather than just the charge, thereby reducing the dependence on charge flow. The flow of spin angular momentum carried by electrons, i.e., "electronic spin current," underpins numerous phenomena in condensed matter physics. An important example is switching and excitation of magnetic order driven electrically by spin current rather than external magnetic field. Spin currents can interact not only with ferromagnetic order consisting of parallel magnetic moments – but also with antiferromagnetic order consisting of alternating magnetic moments that cancel the net magnetization of the material. Indeed, experiments from the last few years demonstrate the ability to rotate antiferromagnetic order (a.k.a. Néel vector) by spin current, which offers new physics not achievable in ferromagnets, such as ultrafast spin dynamics in the THz regime and superfluid spin transport analogous to superconducting electronic transport. However, interaction of a spin current with antiferromagnetic order is not well understood yet. The aim of this thesis is to build a better understanding of spin currents in antiferromagnetic metals. Specifically, we experimentally study basic spin-current physics in a ferrimagnet (CoGd) and an antiferromagnet (Cr). We choose CoGd because adjusting its chemical composition allows us to easily tune its magnetism from ferromagnet-like (uncompensated magnetization) to antiferromagnet-like (compensated magnetization). In antiferromagnet-like CoGd, we find that the oppositely oriented Co and Gd magnetic moments partially cancel the scrambling (dephasing) of spins, so that the spin current is able to propagate over a longer distance - about 3-4 times more than in ferromagnetic metals. The mechanisms behind the longer spin propagation is somewhat akin to the spin "rephasing" technique for lengthening the lifetime of spin-based qubits for quantum computers, but what is remarkable is that we observe this effect in rather disordered magnetic alloys at room temperature. In the other major project of this thesis, we investigate spin transport through multilayers that contain Cr, a structurally and chemically clean antiferromagnetic material. We find that Cr by itself is a good spin transmitter, i.e., effectively allowing a pure spin current to pass through. Surprisingly, when Cr and Cu (another good spin transmitter) are stacked together, we observe strong reflection of a pure spin current at the interface of Cr and Cu. We find that the antiferromagnetic order in Cr is not responsible for this peculiar spin reflection and that other pairs of spin-transmitting metals (for example, V and Cu) can form spin-reflecting interfaces as well. Our work shows an interesting example of "emergent" phenomena where the interface behaves in a way that is not intuitively expected from the properties of the constituent materials. The basic scientific findings from this thesis may help the development of more efficient information-technology devices that run on spin currents.

The stress-strain state of ferromagnetic plane with a moving crack has been investigated in this study. The model considers a soft magnetic ferroelastic body and incorporates a realistic (nonlinear) susceptibility. A moving crack is present in the body and is propagating in a direction perpendicular to the magnetic field. Assuming that the processes in the moving coordinates are stationary, a Fourier transform method is used to reduce the mixed boundary value problem to the solutions of a pair of dual integral equations yielding to a closed form solution. As a result of this investigation, the magnetoelastic stress intensity factor is obtained and its dependency upon the crack velocity, material constants and nonlinear law of magnetization are highlighted. It has been shown that stress result around the crack essentially depend on external magnetic field, speed of the moving crack, nonlinear law of magnetization, and other physical parameters. The results presented in this work show that when cracked ferromagnetic structure is under the influence of magnetic field it is necessary to take into account the interaction effects between deformation of the body and magnetic field and that such interaction can bring to a new conditions for strengthening the materials. Closed form solutions for the stress-strain state are obtained, graphical representations are supplied and conclusions and prospects for further developments are outlined. / Master of Science

The objective of this thesis is to introduce and demonstrate a novel magnetic bead detector based on inductive detection at the ferromagnetic resonance (FMR) frequency for use in bio-sensing applications. Detection ability is demonstrated through theoretical arguments, numerical computer simulations, and experimental characterization of micro-fabricated detectors. The detector is composed of two uniplanar rf waveguides (coplanar waveguide and slotline) terminated together at a short-circuit junction, which serves as the sensitive area. Experimental characterization of a micro-fabricated junction gives a signal ranging between 1 microvolt/volt and 12 microvolts/volt, depending on the number of beads at the junction as well spatial distribution of the beads. The locations around the tips of the CPW were shown to be the most sensitive. A more complex rf circuit design was created employing the detection junction, and detection of magnetic beads was successfully shown at rf frequencies around 6 GHz in this configuration. Due to lack of FMR characterization data for magnetic beads in the literature, several varieties of magnetic beads were characterized using a CPW transmission line and custom apparatus to determine FMR properties. Finally, successful detection of magnetic beads was demonstrated in a system-level integration experiment employing the detector junction in combination with microfluidics and bio-chemical surface modifications. / Graduation date: 2010

In this thesis, an inductive ferromagnetic resonance (FMR) technique is developed to measure the magnetisation dynamics in thin films across a wide range of frequencies and fields. In particular, this project concentrates on measuring higher order exchange dominated modes to observe surface and interface effects in bilayer films. The experimental technique was first developed as a time domain technique, utilising a fast rise time (~50 ps) step pulse to disturb the equilibrium position of the magnetisation. The subsequent precessional damped decay was measured at different applied fields to observe the resonant modes. The data is Fourier transformed to extract a frequency dependent susceptiblity, and results are presented for the frequency and linewidth dependence of excitations of a permalloy film as a function of applied field. This technique is limited to a frequency range dictated by the rise time of the pulse. The technique was then extended so as to use a continuous wave perturbation, utilising a network analyser as both the excitation source and the measurement device. The scattered wave parameters of both the transmission and reflection from the sample were measured, and a magnetic susceptibility is extracted. This method has a frequency range which is dictated by the bandwidth of the network analyser and the microwave circuit. In this project, results are presented for frequencies up to 15 GHz. The signal to noise ratio was also found to be lower than the pulsed technique. Fundamental resonant mode studies are presented for a Fe/MnPd exchange bias bilayer film. Crystalline and exchange anisotropies are extracted from angular measurements, and the behaviour of the magnetisation is investigated during its reorientation to a hard axis direction. Information about the distribution of the local exchange field strength and direction is predicted. Fundamental mode studies are also presented for a Py/Co exchange spring bilayer film. Two modes are observed, approximating an optical and acoustical excitation. Film systems were also designed with suitable thicknesses to observe in the experimentally available frequency range non-uniform exchange dominated excitations through the thickness of the film. The broadband nature of the experiment allowed the frequency of the modes to be measured as a function of field. Results from a single permalloy layer showed two observable modes, the fundamental and the first exchange mode. Measurements were also taken of bilayer films where permalloy is coupled to cobalt. In this system the effect of the cobalt is seen to shift the single layer Py mode frequencies, as well as introduce new modes. The relative intensities of the modes also change with the addition of cobalt. Results are shown for a Pt/Co multilayer coupled to a permalloy layer through a Cu spacer of varying thickness. The observation of excitations through the thickness of the film motivated the development of a suitable theory. A system of integro-differential equations were derived which account for dipole and exchange coupling in the film as well as the field screening by the metal of the coplanar line. The conductivity of the sample and the finite wavevector excitation of the stripline are also included. Numerical solution of the equations results in a spectrum of acoustical, optical and higher-order modes. Fitting of the model to the experimental results allowed extraction of the film parameters including; the exchange constants in the film; the surface pinning from any surface layer anisotropy; as well as the interlayer exchange coupling across the interface.

This thesis investigates the recently developed technique of spin-torque-driven ferromagentic resonance (ST-FMR). Contrary to conventional FMR techniques where the magnetodynamics are excited by torques on the magentic moments produced by microwave fields, ST-FMR uses the spin-transfer torque acting on a nanomagnet. Here we present two experiments which exploit ST-FMR in the standard AFM/FM/NM/FM exchange-biased spin valve (EBSV) structures, where the ferromagnetic (FM) layers are separated by a nonmagnetic (NM) spacer, and one ferromagnet is pinned with an antiferromagnetic (AFM) layer. In our experiments microwave currents are applied to a mechanical point contact between a sharpened Cu tip and a SV (IrMn/Py/Cu/Py) multilayer film. While most ST-FMR experiments require noncollinear orientation of FM-layer magnetizations, we studied ST-FMR in SVs above saturation, where the two FM layers have parallel magnetizations. The resulting magnetodynamics are detected electrically by a small rectified dc voltage, which appears across the structure during resonance. Studies of the resonance frequencies, amplitudes, line widths, and line shapes as a function of microwave power, microwave frequency, dc current and magentic field are presented. The results are analyzed in terms of ST-FMR and rectification based on GMR. However the origin of the observed voltage cannot be fully explained by the resistance changes which come from the giant magnetoresistance (GMR) effect of the spin valve. To investigate other sources of rectified dc voltages at resonance we have performed the second set of measurements with lithographically patterned pairs of (Py/Cu/Co/IrMn)-SV microstripes. These measurements also revealed a resonance in the rectified voltage at FMR frequencies, and showed additional structures which might be related to spin wave excitations. The observations can be tentatively attributed to additional rectification effects due to anisotropic magnetoresistance (AMR). The line pair structure allows us to use different measurement geometries to investigate the magnetodynamics in the SV. In this experiment FMR can either be excited by spin transfer or by a rf magnetic field created by the microwave current, depending on the used geometry. Qualitative studies of the FMR dependencies and characteristics are presented for different measurement geometries. / text

This Dissertation explores the interaction between the magnetisation and an alternating current in a uniform ferromagnetic system. Diluted magnetic semiconductors (Ga,Mn)As and (Ga,Mn)(As,P) have been studied. Due to their strong spin-orbit coupling and well-understood band-structure, these materials are well-suited to this investigation. The combined effect of spinorbit coupling and exchange interaction permits the alternating current to induce an oscillating current-induced torque (CIT) on the magnetisation. In the frequency range close to the natural resonance frequency of the magnetic moments (gigahertz), CIT can excite precessional motion of the magnetisation, a process known as ferromagnetic resonance (FMR). CIT can be parameterised by an effective magnetic field. By analysing the lineshape of the measured FMR signals, the magnitude and orientation of this effective field have been accurately determined. Moreover, the current-induced fields in these ferromagnetic materials have been observed with symmetries of the Dresselhaus, and for the first time, Rashba spin-orbit coupling. A new class of device-scale FMR technique, named as CIT-FMR, has been established in this Dissertation, with the advantage of simple device structure (only a resistor is required) and scalability (measurements have been performed on devices sized from 4 μm down to 80 nm). This technique is not only limited to magnetic semiconductors, but can also be transferred to study other ferromagnetic systems such as ultrathin metal films. Finally, the CIT-FMR technique is employed to study the magnetic anisotropyin individual (Ga,Mn)As and (Ga,Mn)(As,P) micro-devices. Devices down to 80 nm in width have been measured in (Ga,Mn)(As,P), which show strong strain-relaxation-induced anisotropy, larger than any previously reported cases on (Ga,Mn)As. Furthermore, due to the tensile-strain on the (Ga,Mn)(As,P) epilayers, the anisotropy field due to patterning-induced strain-relaxation in these devices is observed to take the opposite direction compared to that in the compressively-strained (Ga,Mn)As samples.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第18060号 / 理博第3938号 / 新制||理||1567(附属図書館) / 30918 / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 石田 憲二, 教授 前野 悦輝, 教授 松田 祐司 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

Ferromagnetic (FM) thin-film heterostructures provide opportunities to investigate GHz magnetic dynamics and emerging magnetoelectric devices based on dynamic phenomena. An intriguing direction for these studies is the control of the flow of pure (chargeless) spin current flow in different magnetic systems. In this thesis, we focus on the excitation of pure spin currents using ferromagnetic resonance (FMR), also known as spin pumping, and their transport characteristics in magnetic heterostructures based on Ni₈₁Fe₁₉ (Permalloy, or Py) under a variety of circumstances. In Chapter 2, we present measurements of the anisotropy of the spin pumping effect in the Pt/Py/Pt system via variable-frequency, swept-field FMR. We find a very small anisotropy of enhanced Gilbert damping with sign opposite to a recent theory's prediction from the Rashba effect at the FM/Pt interface. In Chapter 3, we present an experimental and theoretical study of spin dynamics in the antiferromagnetically coupled Py/Ru/Py system. We show that, contrary to the behavior of the uniform mode in a saturated single-layer FM, the symmetric mode in unsaturated synthetic antiferromagnet (SAF) has approximately constant FMR linewidth as a function of frequency. This behavior can be explained mostly semiclassically by our model. In Chapter 4, we present an investigation of interfacial Gilbert damping due to the spin pumping effect in Py/W heterostructures with enriched α phase or β phase W. The spin mixing conductances (SMC) for W at interfaces with Py are found to be significantly lower than those for similar heavy metals such as Pd and Pt, but comparable to those for Ta, and independent of enrichment in the β phase. The experimental results also indicate that W, no matter of which phase, is a good spin sink in Py/W heterostructures. In Chapter 5, we describe explorations of the spin pumping effect in antiferromagnetic insulator (AFI)-based heterostructures using variable-temperature, variable-frequency FMR. We find a spin-pumping-induced damping enhancement for Py/Cu/CoO, Py/Cu/CoO/Cu/Pt and Py/Cu/NiO/Cu/Pt. Broad peaks have been observed in FMR linewidth difference as a function of temperature for Py/Cu/NiO, normalized to the linewidth of the reference Py sample. Our results indicate that some effects previously attributed to spin current flow are better described by a defect-related mechanism. Chapter 6 summarizes the various findings of spin-pumping effects in ferromagnetic thin film heterostructures and possible future work.