Inductive current probe in microwave test station
This project develops instruments, techniques, and theory for the understanding of the high-speed response of commercially important magnetic materials. Techniques used include linear and nonlinear magneto-optics and inductive response. Emphasis is on broadband (above 1 gigahertz), time-resolved measurements for the study of magnetization dynamics under large-field excitation. Research concentrates on the nature of coherence and damping in ferromagnetic systems and on the fundamental limits of magnetic data storage. Exploratory research on spintronic systems and physics is underway. The project provides results of interest to the magnetic-disk-drive industry, developers of magnetic random-access memories (MRAM), and the growing spintronics community. Recent results include the observation of deleterious magnetic turbulence during the magnetic switching process, evanescent flux-pulse propagation in metallic films, and anisotropic coupling ("damping") between uniform excitations and the crystal lattice. Coherent-control methods have been used to switch magnetization without unwanted precessional ringing. Recently, an inductive current probe was developed to assess trace-suspension interconnects for disk-drive recording heads.
Our primary customers are the magneto-electronics industries. These include the magnetic-disk-drive industry, the magnetic-sensor industry, and those companies currently developing MRAM. As commercial disk drives approach data-transfer rates of 1 gigabits per second, there is increased need for an understanding of magnetization dynamics. In addition, measurement techniques are needed that can quantify the switching speeds of commercial materials. Once the response of a material has been benchmarked, the engineer can then develop electronic components (e.g., heads, disks, or MRAM) that can fully exploit the bandwidth potential of the material. We are also providing novel metrology for the burgeoning spintronics industry. The spin precession of charge carriers in semiconductor hosts has significant potential for telecommunications applications. Unlike the case of conventional semiconductor switching, the frequency of spin precession is not fundamentally limited by the physical thickness of dielectric spacers. We plan to investigate novel magnetic/semiconductor heterostructures of interest to the telecommunications industry.
The focus of this project is the measurement of switching time of magnetic materials for applications in data storage. This has led to the development of cutting-edge instrumentation and experiments using magneto-optics and micro-wave circuits. Microwave coplanar waveguides are used to deliver magnetic-field pulses to materials under test. In response, the specimen's magnetization switches - but not smoothly. Rather, the magnetization vector undergoes precession, much as a spinning top precesses in the Earth's gravitational field. Sometimes, the magnetization can precess nonuniformly, resulting in the generation of spin waves or, in the case of small devices, incoherent rotation. Our technical strategy is to identify future needs in the data-storage and other important industries, develop new metrology tools, and do the experiments and modeling to provide data and theoretical underpinnings.
We concentrate on two major problems in the magnetic-data-storage industry: (1) data-transfer rate, the problem of gyromagnetic effects, and the need for large damping without resorting to high fields; and (2) storage density and the problem of thermally activated reversal of magnetization.
Data-transfer rates are increasing at 40 percent per year (30 percent from improved linear bit density and 10 percent from greater disk rotational speed). The maximum data-transfer rate is currently 50 megabytes per second. In five years, frequencies for writing and reading will be in the microwave region, which raises the question, "How fast can magnetic materials switch?"
The current laboratory demonstration record for storage density is 9 gigabits per square centimeter (60 gigabits per square inch). How much farther can longitudinal media (with in-plane magnetiza-tion) be pushed? Can perpendicular recording or discrete data bits extend magnetic recording beyond the superparamagnetic limit at which magnetization becomes thermally unstable? As the data-storage industry seeks its own answers to these pressing questions, we must strive to pro-vide the necessary metrology to benchmark the temporal performance of new methods of magnetic data storage.
We have sought to extend magneto-optics for the quantitative measurement of magnetization dynamics in practical ferromagnetic films. Methods include time-resolved generalized magneto-optic ellipsometry (TRe-GME), time-resolved second-harmonic magneto-optic Kerr effect (TRe-SHMOKE), and quantitative wide-field Kerr microscopy (our "MOKEroscope"). All these systems rely upon rf waveguide technology for the delivery of fast magnetic field pulses to excite magnetization switching in specimens. We use several methods to detect the state of mag-netization as a function of time. These include the following:
- The magneto-optic Kerr effect (MOKE) makes use of the rotation of polarization of light upon reflection from a magnetized film. We have used MOKE with an optical microscope to meas-ure equilibrium and nonequilibrium decay of magnetization in recording media.
- The second-harmonic magneto-optic Kerr effect (SHMOKE) is especially sensitive to surface and interface magnetization. We have used SHMOKE for time-resolved, vectorial measurements of magnetization dynamics and to demonstrate the coherent control of magnetiza-tion precession.
While the aforementioned instruments have immediate use for the characterization of magnetic data-storage materials, they are also powerful tools for the elucidation of magnetodynamic theory. The primary mathematical tools for the analysis of magnetic switching data are essentially phenomenological. As such, they have limited utility in aiding industry in its goal to control the high-speed switching properties of heads and media. We have sought to provide firm theoretical foundations for the analysis of time-resolved data, with special emphasis on those theories that provide clear and unambiguous predictions that can be tested with our instru-ments.
We are committed to supporting new magnetic technologies as they emerge in the 21st century. Spintronics is a novel direction in electronics that promises to revolutionize telecommunications and information processing. The essential idea behind spintronics is the manipulation and control of the quantum-mechanical spin of a semi-conductor charge carrier. The extension of electronic manipulation toward the spin degree-of-freedom has intrinsic advantages that warrant further exploration. For example, the fundamental problem with high-frequency semiconducting devices is nonzero resistance R coupled with gate capacitance C. In essence, the RC time constant limits the maximum frequency attainable. A key feature of spin-based rf circuitry is the fundamentally quantum-mechanical nature of spin precession. Spin precession frequencies are not intrinsically limited by loss mechanisms such as carrier mobility, as long as coherence can be preserved. Spintronics technology holds the promise of extending telecommunications frequencies into the terahertz regime.
- During 2001-2003, improve understanding of ferromagnetic switching processes.
- By 2002, continue development of non-invasive inductive current probe for measurement of current rise-times in trace-suspension interconnects between disk-drive heads and write-current drivers.
- During 2001-2003, develop methods for the quantitative study of high-speed switching in ferromagnetic films.
- Upgrade the PIMM to a self-contained user facility for use by industrial collaborators to measure switching speeds of standard and proprietary materials.
- By 2002, measure precessional dynamics in spintronic components using time-resolved magneto-optics.
- By 2003, investigate practical applications of spin-momentum transfer effect in magnetic heterostructures.
New Field Sources
We built two com-puter-controlled field sources that allowed our inductive current-probe measurements to be automated and improved the accuracy and consistency of our measurements. We have made dozens of measurements in a tenth of the time previously required. A bandwidth of 6 gigahertz has been achieved in the deconvolved results for the step-current rise time when using a supple-mental bias field to improve the Ni-Fe response.
New Theory for Damping in Ferromagnetic Resonance
We have developed a theoretical framework for understanding damping in ferro-magnets. We analyzed the damping mechanism in the case of direct coupling between the electron spins and the crystal lattice within the context of the quantum-mechanical magnetodynamic equation originally proposed by Herbert Callen in 1958. Callen's landmark work analyzed the process of ferromagnetic relaxation as the sum of three distinct processes: direct coupling between the uniform precession and the lattice, dissipation via spin wave generation, and coupling of spin waves to the crystal lattice. We found that one can calculate the direct coupling term in the relaxation equation using conservation of angular momentum and a quantum-mechanical description of the fundamental spin relaxation process. An implication of the calculation is that the direct relaxation process is a function of the magnetization angle relative to the crystalline anisotropy axis. For magnetization oriented along the easy axis, direct relaxation is a maximum. For orientation along the hard axis, direct relaxation is forbidden by simple symmetry considerations. Thus, relaxation (damping) is dominated by spin-wave generation when the magnetization is orthogonal to the anisotropy axis.
Damping Theory Applied to Magneto-Optic Data
We used our new theoretical description of damping to interpret our recent SHMOKE data where we found that the magnitude of the magnetization can be strongly reduced immediately after application of a strong field pulse. The reduction occurred only if the magnetization was initially oriented along the hard axis prior to application of the pulse. We determined that the Callen model can be used to fit the time dependence of the magnetization reduction with a single fitting parameter: the quantum-mechanical rate of spin-wave generation. We found that the rate of spin-wave generation greatly increases for large pulses in excess of 560 amperes per meter. This has profound implications for the disk drive industry, where very large field pulses are routinely used in the operation of a recording head.
Spins at a Surface: Are They Faster?
We are comparing surface and bulk magnetodynam-ics in thin films of Ni-Fe. Using second-harmonic and conventional forms of the magneto-optic Kerr effect (SHMOKE and MOKE) and measuring the response to a field pulse, we found that the surface and bulk dynamics are almost indistinguishable from each other. The ability to measure both effects simultaneously using the same pulsed laser source removes many possible sources of systematic error. Our error analysis will set quantitative bounds on the similarity of surface and bulk magnetodynamics. This study should be of great interest to those modeling dynamics in recording-head materials, where there is general disagreement as to the role of eddy currents.
Assessing the Performance of Chaff at RF
We extended our work on measurements of the resistance of carbon fibers to high frequencies. We are using two approaches: The first is a direct measurement of resistance calibrated against known resistors specified for 20 gigahertz opera-tion. The second is a method using Fourier analysis of time-domain reflectometry measurements to extract resistance versus frequency. This method is simpler experimentally but requires more difficult analysis.
Amplifying the Electron Spin: A Proposed Spintronics Device
In collaboration with the Magnetic Thin Films and Devices Project, Cornell University, and Motorola Corporation, we developed a novel spintronics device concept: the spin amplifier. It is based on the recent experi-mental results at Cornell, where a resonant enhancement of the giant magnetoresistance was observed in nanoscale devices in the presence of large magnetic fields. According to the theory, the spins that constitute a ferromagnet enter a massively degenerate excited state at the bottom of the spin-wave spectrum under conditions of sufficiently large dc current, reminiscent of the stimulated emission process that drives a laser. We plan to use such a device for the injection and detection of coherently precessing spins in a semiconductor host. As a spin injector, a "SWASER" (spin-wave amplification by stimulated emission of radiation) is unique in that it prepares spins in a coherent superposition of "up" and "down" states (parallel and antiparallel to an applied magnetic field) before injection into a semiconductor. Such a coherent superposition has intrinsic, statistical advantages compared to a polarized spin current.
Advanced High-Moment Head Material Benchmarked
In collaboration with the Mate-rials Science and Engineering Department at Stanford University, we measured the switching speed of Fe-Co-N films with the PIMM system. Our collaborators at Stanford discovered that they can greatly improve the uniaxial orientation of sputtered, high-moment, iron nitride films through use of Permalloy (Ni0.8Fe0.2) as a seed layer. We found that the materials exhibit an intrinsic switching speed of 200 picoseconds, more than a factor of two faster than conventional head materials. The highest-quality films were surprisingly well damped in their precessional response. Such desirable response is coincident with the observation of an anomalous second-harmonic component in the time-resolved data that may contribute to the large damping. The physics of this effect are still under investi-gation.
Flux Propagation Speed Measured in Recording Head Material
The spatial propaga-tion of magnetic flux pulses launched in thin-film Ni-Fe were measured using TRe-SHMOKE. The energy propagation velocity, or "group velocity," was found to be 105 meters per second in a film that was 400 nanometers thick. Such a fast speed is consistent with the predictions of Damon and Eshbach's magnetostatic spin-wave theory. It was also found that the decay of the flux pulse was consistent with a damping parameter of 0.02. Such a value for damping is typical for Ni-Fe films. These results suggest that the usual quasi-static calculations for the recording efficiency of disk-drive heads may be erroneous in the limit of precessional dynamics.
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