EEEL, Magnetic Technology Division Banner

Project Leader:
Tom Silva

Staff-Years:
2 professionals
1 research associate

Funding:
NIST (75%)
Other (25%)

Previous Reports:
2001

Magnetic Technology Division
325 Broadway
Boulder, Colorado 80305
Phone 303-497-5477
Fax 303-497-5316

magtech@boulder.nist.gov

May 14, 2002

Magnetodynamics

Goals

Electronics engineer Tony Kos operating the pulsed inductive microwave magnetometer (PIMM)

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 memory (MRAM), and the growing spintronics community. Recent achievements 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.

Customer Needs

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 gigabit 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 develop electronic components (e.g., heads, disks, or MRAM) that can fully exploit the bandwidth potential of the material.

We are 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.

Technical Strategy

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 microwave 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.

Based upon the pioneering work at NIST in developing the inductive time-domain permeablility measurement, I have implemented the same measurement capability at Seagate Technology. We use this measurment technology daily for measuring the high frequency properties of magnetic materials. The ongoing efforts by the NIST-Boulder group to develp new magnetic measurements often have the added benefit of creating new technologies which may be incorporated into actual products.

Dr Thomas Crawford
Seagate Research
Seagate Technology LLC

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 80 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 16 gigabits per square centimeter (100 gigabits per square inch). How much farther can longitudinal media (with in-plane magnetization) 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 provide 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. All these systems rely upon radio-frequency (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 magnetization 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 measure 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 magnetization precession.
In our pulsed inductive microwave magnetometer (PIMM), the changing magnetic state of a specimen is deduced from the change in inductance of a waveguide. This technique is fast, inexpensive, and easily transferable to industry. It may also be used as a time-domain permeameter to characterize magnetic materials.

Tom Silva's technical insight and ability to effectively communicate ideas are always welcome attractions.

Dr Randy Rannow
Chair, Rocky Mountain Chapter
IEEE Magnetics Society

Your lecture was really a tour de force.

Prof. Carl Patton
Department of Physics
Colorado State University

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 instruments.

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 semiconductor's 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.

The Magnetic Technology Division has started a new program sponsored by the Defense Advanced Research Projects Agency (DARPA) to explore the use of electron-spin-based devices for applications in high-frequency communications. The four-year, $7.1 million program is a collaboration among NIST, Motorola, and Cornell University

The motivation to use electron-spin-based devices for high-frequency communication applications comes from the fact that the electron spin degree-of-freedom forms the most fundamental quantum oscillator. Unlike devices that are based on charge transfer, whose frequency performance is limited by electron velocities and charge-transfer times, the electron spin has no fundamental frequency limitations.

Recent advances in spin-based semiconductor devices have demonstrated that coherent spin precession can be maintained for hundreds of nanoseconds in III-V semiconductors and hundreds of microseconds in Si. The precession frequency can be controlled by applied magnetic fields, gate voltages, and modulation doping techniques. Terahertz precession has been observed in Mn-doped InAs heterostructures with no applied magnetic fields. Modulation of the electron g-factor has been observed in the presence of electric fields that move the spin packets between regions of different g-factors, e.g., GaAs and AlGaAs.

Motorola, a leading producer of high-frequency semiconductor components, will be fabricating high-mobility GaAs/GaAlAs and InGaAs/InAlAs heterostructures specifically designed for spin-based devices. NIST will lead the effort to develop new techniques to measure and control spin precession in small spin-based devices. The goal will be to characterize precessing spin packets with one million spins, using high-speed electrical and optical techniques. The NIST team hopes to extend the metrology down to systems consisting of a single spin.

In addition to exploring spin dynamics in semiconductors, the we will look at metallic devices that use spin-momentum transfer to induce coherent precession. Recent theoretical work has predicted that a spin-polarized direct current injected into a small magnetic structure (50 nanometers x 100 nanometers x 2.5 nanometers) can generate coherent precession of the magnetization. The precession frequency can be tuned from 1 gigahertz to 50 gigahertz by changing the current amplitude or the polarization angle.

The Cornell group has recently shown that spin-polarized currents can switch a small magnetic element, an effect predicted by the theory. We have further developed these ideas and proposed using this effect as a source of precessing spins for semiconductor devices and as the basis for a novel spin amplifier. Motorola, starting from a process developed at Cornell and NIST, will fabricate 50 nanometer to 100 nanometer magnetic multilayer structures on 20 centimeter Si wafers using magnetic processing techniques developed for their MRAM program. Motorola will use precision chemical-mechanical polishing to contact the devices and expects to obtain large numbers of devices with highly controlled magnetic properties. NIST will characterize the dynamics induced by spin-momentum transfer in these devices using high-speed electrical and optical techniques.

Deliverables

View through the bore of an optical cryostat and magnet assembly with 4 kelvin, 8 tesla capability. It will be used to study spin dynamics in various spintronic devices, including magnetic precession induced by spin-polarized currents and spin coherence in semiconducting media.

Magnetic Recording Diagnostics

In 2002, we will further develop the inductive current probe for the characterization of write-current pulses used in magnetic disk drives. The current probe uses a magnetic thin film as a noninvasive inductive transformer to sense the current pulses sent from a write driver to the recording head. In the next iteration, the current probe will be implemented as part of a trace suspension assembly. The work will be done in collaboration with two recording-head companies.

Nanotechnology

In FY 2002, we will collaborate with researchers at the University of Idaho, University of Alabama, and Washington University in Saint Louis on nanotechnology studies using magnetic materials. We will use inductive and optical methods to investigate the high-speed switching properties of patterned arrays of magnetic dots. We will investigate iron-nitride films, which have promise as a high-performance head material. These films will have engineered nanostructures for optimal recording properties. We will use the PIMM to search for correlation between nanostructural properties and the damping of precessional ringing.

Ferromagnetic Damping

In FY 2002, we will measure the dependence of damping on substrate sound velocity. If magnon-phonon coupling is the primary means of damping, there should be some dependence on sound velocity according to one theory. Our intent is to find whether the acoustic properties of a substrate can be used to improve the performance of recording heads in high-performance disk drives.

In FY 2002, we will study the dependence of damping on magnetic anisotropy in Permalloy (Ni80Fe20) thin films. It is possible to vary the anisotropy in Permalloy from 80 to 400 amperes per meter by annealing thin films in a magnetic field. It can be argued that damping torque and anisotropy torque should be related by conservation of angular momentum; this study should clarify the validity of that argument. A correlation between damping and permeability will establish a fundamental limit to the operational bandwidth of magnetic recording.

In FY 2002, we will complete a study of dynamic anisotropy in Permalloy films of thickness ranging from 10 to 500 nanometers. We have found that the anisotropy measured by dynamic measurements are significantly higher than the anisotropy found with a quasi-static measurement. Mechanisms for the discrepancy are under review. The goal is to find a means to increase the dynamic permeability of recording-head materials, thereby improving data-writing efficiency.

In FY 2003, we will investigate the generation of standing spin-waves through the thickness of 200-300 nanometer Permalloy films. Preliminary PIMM data indicate that such standing waves occur during the switching process. Of particular interest is how these waves couple to the uniform mode to act as a source of damping. Understanding how such modes arise, whether it is due to eddy-current generation or surface-pinning of spins, will influence current models of magnetic recording-head performance.

During FY 2002-2003, we will continue to investigate the mechanisms that lead to nonuniform magnetization dynamics during the switching process when the magnetic spins reorient into a new equilibrium direction. We will use both linear and nonlinear optical methods to measure both the surface and subsurface spin dynamics during the switching process. Results should help us understand whether such nonuniform dynamics will have a deleterious effect on the high-speed performance of recording heads.

Spintronics

Optical bridge assembly for sensitive meaurements of polarized light. It can be used for both time-resolved measurments of magnetization dynamics in thin-film recording head materials and for the study of spin precession in seminconductors with spintronic applications.

In FY 2002, we will search for a direct signature of spin-wave generation from the injection of a spin-polarized current in magnetic multilayers. Experiments will rely upon a point-contact geometry to produce the requisite high current densities of 108 amperes per square centimeter. Measurements will rely on both static and time-resolved magneto-optic contrast using a laser beam focused in proximity to the point contact. These results should help select among the many theoretical explanations that have been forwarded to explain the exchange of angular momentum between a spin-polarized current and a ferromagnet.

In FY 2002, we will develop a simple and inexpensive method to measure spin precession in GaAs substrates using time-resolved magneto-optic techniques. The methods will rely on a low-cost laser diode system that is tuned to the electron band gap of GaAs. Development of such a system should greatly expand the research opportunities in this important segment of spintronics research and development.

In FY 2004, we will investigate the possibilities to use the spin-momentum-transfer effect for telecommunications applications. Device possibilities include high-frequency oscillators and amplifiers based on spin-wave amplification by stimulated emission of radiation (SWASER). We will seek to establish whether the SWASER effect is real and what parameters exist to manipulate the effect.

In FY 2003, we will seek to manipulate coherent spin populations in a semiconductor medium using pulsed RF signals. The spins will be induced by optical orientation methods. We will use waveguide structures to apply the RF signals to the spin packets. Optical methods will then be used to monitor the spin polarization during the RF manipulations. With such a technique, it should be possible to determine the degree to which inhomogeneities determine the measured coherence times. Long coherence times are desirable for the development of high-quality-factor (high-Q) oscillators and amplifiers.

Accomplishments

Point-contact assembly for the study of the spin-momentum-transfer (SMT) effect, whereby spin-polarized carriers can manipulate the magnetic state of a nanometer-scale element. It will be used with a cryostat and magnet assembly capable of operation at 4 kelvins and 8 teslas. Experiments performed with this instrument will elucidate the role of spin-wave generation in the SMT effect.

Pulsed Inductive Microwave Magnetometer - As part of our program in high-speed magnetics, we have developed an automated, pulsed inductive microwave magnetometer (PIMM) to characterize magnetic thin films. The PIMM is designed to measure the magnetodynamic properties of materials used in recording heads for magnetic data storage. The data-storage industry is developing new magnetic alloys with high saturation magnetization to use in write heads. The magnetic damping behavior of these new alloys will determine their usefulness for high-speed recording.

The PIMM uses a coplanar waveguide as both a source of fast, pulsed magnetic fields and as an inductive flux sensor. Magnetic field pulses are provided by a 10 volt, 55 picosecond rise-time pulse generator. Orthogonal Helmholtz coils provide the magnetic bias and saturating fields required for the measurement. A 20 gigahertz digital sampling oscilloscope is used to acquire the data. The system can measure dynamical behavior as a function of several variables, including applied bias field, pulsed field amplitude and width, and sample orientation. Using fast Fourier transforms, the PIMM can determine the frequency dependence of the complex magnetic permeability, as well as the step and impulse responses of magnetic systems.

The PIMM includes components necessary for completely automated magnetodynamic measurements. No user intervention is required to insert or remove attenuators on the front end of the high-bandwidth sampling oscilloscope. This reduces the chance of an electrostatic discharge that could damage the sensitive front-end circuitry of the oscilloscope. It also increases the system sensitivity, since the attenuators can be set in finer increments to give the largest signal as the pulse amplitude is adjusted.

In order to generate a precise impulse with repeatable characteristics, the system can automatically insert a passive impulse-forming network into the output pulse path. This allows the impulse response of the magnetic system to be measured, which is useful for characterizing the system's transfer function. The automatic operation allows measurements of pulsed magnetic switching dynamics at different pulse amplitudes interspersed with measurements of system impulse response.

This unique magnetometer has been used by visiting scientists from university data-storage research centers and disk-drive manufacturers.

Vector Magnetodynamic Measurements at the Surface and Interior of Films for Recording Heads - We have made quantitative, vectorial measurements of magnetization dynamics of Ni-Fe magnetic thin films, simultaneously at the surface and approximately 50 nanometers in the interior. The measurements were performed using the linear and nonlinear magneto-optic Kerr effects. The linear effect (MOKE) is sensitive to the magnetization in the interior of the film, whereas the nonlinear effect (SHMOKE) is sensitive to the magnetization of only the first few atomic layers.

These measurements address the problem of inhomogeneous magnetization response of magnetic materials when subjected to rapidly changing magnetic fields. When the rate of change of the applied field approaches the characteristic response time of the material (the precessional frequency, typically several gigahertz), the magnetization's response can become complicated. At these frequencies, the magnetization does not simply align with the field, but instead swings toward the field and oscillates (precesses) around it before finally settling into the field direction. However, it was not known whether the surface and the interior of the magnetic material reacted to the applied field in the same way.

We induced rapid, near-90 degree rotations of the magnetization of Ni-Fe films in a geometry similar to that of the ferromagnetic cores of magnetic recording write heads. The rotation of the magnetization vector was probed with a 50 femtosecond laser pulse. The system had an overall temporal sensitivity of 50 picoseconds and a sensitivity to magnetization angle of about 3 degrees. We found that, contrary to the expectations of some models for magnetic response, the surface and the interior region responded identically. Detailed measurements showed that the magnetization exhibited a fast rotational response over about one nanosecond followed by a smaller, slow response over tens of nanoseconds.

These effects will soon be important in magnetic data storage devices as data rate increases. Disk drives store information by switching small regions of magnetic material to represent binary data. It is necessary to have a detailed understanding of the induced precessional magnetodynamics in order to optimize the recording process. Modeling of these large, rapid magnetization motions in real materials is difficult, so direct dynamic measurements are important for continued device development.

Work is ongoing to study the dependence of the response on the thickness of the film to find when the response of the surface might deviate markedly from the interior due to eddy currents.

Relative Rates of Spin-Spin and Spin-Lattice Relaxation Measured for Thin-Film Permalloy - We completed an investigation of damping in a Ni-Fe thickness series ranging from 10 to 100 nanometers. Films in this thickness range do not exhibit significant damping as a result of eddy-current generation. The damping parameter was extracted by fitting of the data to the Landau-Lifshitz-Gilbert (LLG) phenomenological model for damped precessional dynamics. The resultant damping parameter was found to decrease monotonically with increasing longitudinal bias field from 0 to 2000 amperes per meter. A theory was developed that considers the competing role of spin-lattice and linear magnon-magnon coupling. It was presumed that the spin-lattice interaction was independent of applied field to first order, whereas the magnon-magnon coupling was proportional to the density of states (DOS) for spin-wave modes that have the same frequency as the uniform precessional mode. Conventional spin-wave dispersion theory predicts that the DOS of degenerate modes is a strong function of applied field in the range measured. Fitting of the data to the theory finds that the spin-lattice relaxation path is 10 times faster than the formation of degenerate magnons in the absence of any applied bias field along the anisotropy axis. This result was substantiated by vector-resolved dynamics measured with the time-resolved second-harmonic magneto-optic Kerr effect (TRe-SHMOKE), where coherent dynamics were observed in Ni-Fe films, 50 nanometers thick, similar to those used for the present study.

Relative Role of Nonlinear Effects Elucidated for Large-Angle Magnetic Motion - We measured the dependence of damping in Ni-Fe films, 50 nanometers thick, on the amplitude of an applied field step. The measurements were conducted with the pulsed inductive microwave magnetometer (PIMM) using a pulse generator that produces field steps with 50 picosecond rise-time and 10 nanosecond duration. The pulse bandwidth is sufficient to induce underdamped precessional dynamics in the Permalloy film. The pulse amplitude ranged from 20 to 200 amperes per meter. The film anisotropy was 320 amperes per meter. For this range of pulses, the magnetization was rotated over a range from 3.6 to 39 degrees away from the ambient magnetization direction along the easy axis. It was found that the magnetization response was linear over this response range, with the response for a pulse of 200 amperes per meter scaling onto the response for a pulse of 20 amperes per meter. The conclusion may be drawn that the damping measured with the PIMM system for such films is the result of linear relaxation processes.

This result is important because commonly used ferromagnetic-resonance (FMR) measurements of precessional response are limited to excitations that induce magnetization oscillations of less than a degree before the damping is dominated by nonlinear processes. These measurements suggest that nonlinear processes are suppressed when the magnetization is stimulated in such a manner that the precessional dynamics are allowed to decay before the application of the next excitation pulse.

Coplanar waveguide used to deliver high-speed magnetic field pulses to thin-film samples. Field pulses with rise times as short as 50 picoseconds can be produced with such a waveguide. Magnetic response is obtained using time-resolved magneto-optic methods with vector measurement capability.

External Recognition

Tom Silva served as IEEE Magnetics Society Distinguished Lecturer. Presenting his talk entitled "Consideration of the Spherical Cow: The Realities of Magnetodynamics in an Imperfect World," he represented NIST's research and its impact at more than 20 research institutions throughout the world. Many attendees commented on the novel optical techniques for accurate time resolution of ultrafast spin dynamics.