This project develops measurements and standards for the magnetic data storage and magneto-electronics industries. These measurements and standards assist industry in the development of magnetic thin-film materials and devices required for advanced magnetic recording systems, magnetic solid-state memories, magnetic sensors, and magnetic microwave devices. The emphasis is on the performance of nanoscale devices, consisting of multilayer and multicomponent thin-film systems, operating at microwave frequencies. Project members have successfully devised better methods to measure and control the dynamical properties of magnetic devices operating in the gigahertz regime. They have fabricated magnetic nanostructures to measure new spin-dependent transport phenomena and to determine the resolution of magnetic imaging systems. In addition, the project is developing new combinatorial materials techniques for magnetic thin films and new types of on-wafer magnetic metrology. Long-term goals include the development of metrology for advanced magnetic data storage on the nanometer size scale, metrology for emerging spin-electronics technologies, and novel electron spin resonance techniques (down to the single-spin limit).
The data storage and magneto-electronics industries are pushing toward smaller and faster technologies that require sub-micrometer magnetic structures to operate in the gigahertz regime. New techniques are required to characterize the magnetic structure on nanometer size scales and over a wide range of time scales varying from picoseconds to years. For example, the response of a 100 nanometer magnetic device may be determined by a 5 nanometer region that is undergoing thermal fluctuations at frequencies of 1 hertz to 10 gigahertz. These fluctuations give rise to noise, non-ideal sensor response, and long-term memory loss. Further, new calibration artifacts, such as magnetic imaging reference standards, are required to help characterize metrology tools that will be needed to develop nanoscale magnetic data storage technologies.
Magnetic thin-film systems have become increasingly complicated, often containing quaternary alloys or multilayer systems with up to 12 layers and 20 elements. Fabrication of these multilayer systems requires atomic-level control of the layers. New techniques are required to characterize these multilayer structures in situ, while the structures are being grown. New ex-situ measurement techniques are required to efficiently and systematically characterize the magnetic, electronic, and mechanical properties of these advanced thin-film systems. In particular, new metrological systems are required that will be capable of making on-wafer measurements on a large number of sites over large regions of parameter space.
Finally, advances in technology are dependent on the discovery and characterization of new effects, such as giant magnetoresistance (GMR) and spin-dependent tunneling. A detailed understanding of spin-dependent transport is required to optimize these effects and to discover new phenomena that will lead to new device concepts. New effects, such as spin-momentum transfer and coherent spin transport in semiconductor devices, may lead to new classes of devices that will be useful in data storage, computation, and communications applications. The study of molecular nanomagnets may lead to data storage on the nanometer scale and to a better understanding of the fundamental limits of magnetic data storage.
We are developing several new techniques to address the needs of U.S. industries that require characterization of magnetic thin films and device structures on nanometer-size scales and gigahertz frequencies.
Device Magnetodynamics and Noise
We have fabricated test structures that allow the characterization of small magnetic devices at frequencies up to 10 gigahertz. The response of submicrometer magnetic devices, such as spin-valves, magnetic tunnel junctions, and GMR devices with current perpendicular to the plane (CPP), have been characterized both in the linear-response and the nonlinear switching regimes. The linear-response regime is used for magnetic recording read sensors and high-speed isolators, whereas the switching regime is used for writing or storing data in magnetic random-access memory (MRAM) devices. We measured the sensors using microwave excitation fields and field pulses with durations down to 100 picoseconds. MRAM devices have been switched with field pulses down to 200 picoseconds. We compared measured data to numerical simulations of the device dynamics to determine the ability of current theory and modeling to predict the behavior of magnetic devices. We developed new techniques to control and optimize the dynamic response of magnetic devices. These include the engineering of magnetic damping using rare-earth doping and precessional switching, which controls switching using the timing of the pulses rather than pulse amplitude.
We are developing new techniques to measure the high-frequency noise and the effects of thermal fluctuations in small magnetic structures. Understanding the detailed effects of thermal magnetization fluctuations will be critical in determining the fundamental limit to the size of magnetic sensors, magnetic data bits, and MRAM elements. High-frequency noise has been measured in our fabricated structures and in commercial read heads. High-frequency noise spectroscopy directly measures the dynamical mode structure in small magnetic devices. Devices with dimensions of 200 nanometers have been measured and the technique can be extended to measure the dynamical modes in structures with dimensions down to 20 nanometers. Further, the stochastic motion of the magnetization during a thermally activated switching process can be directly measured and can lead to a better understanding of the long-time stability of high-density magnetic memory elements.
We are exploring new physical effects to create the foundation to develop entirely new technologies relying on spin-dependent transport at the quantum level. We are investigating the use of spin-momentum transfer (SMT) to induce a dynamical response for microwave and high-speed signal processing systems. We are investigating methods of measuring small numbers of spins in semiconductor devices and spin traps. Developing this metrology will be essential to the development of methods to control and manipulate small numbers of spins in a spin circuit.
Combinatorial Materials, Meta-materials, and On-Wafer Metrology
We are developing combinatorial materials techniques to assist industry in the development and characterization of complicated magnetic thin-film systems. Combinatorial materials techniques involve the fabrication of libraries of materials with a systematic variation of materials properties, such as composition and growth temperature. In addition to fabrication of libraries of materials, the combinatorial process involves the development of high-throughput on-wafer metrologies that can systematically characterize the libraries and scan for desirable materials properties.
In-Situ Magnetoconductance and Magnetometry
We are developing new techniques to measure the electronic and magnetic properties of magnetic thin-film systems in situ (as they are deposited). One such technique, in-situ magnetoconductance measurements, can determine the effects of surfaces and interfaces on spin-dependent transport in a clear and unambiguous manner. The effects of submonolayer additions of oxygen, noble metals, and rare earths on GMR have been studied.
We are developing methods to characterize the magnetic properties of molecular nanomagnets. These systems contain from 3 to 12 transition-metal atoms that form small magnets with Curie temperatures of 1 to 30 kelvins. The magnetic properties will be characterized with a magnetometer based on a superconducting quantum interference device (SQUID) and high-frequency ESR. The modulation of the magnetic properties by electric and magnetic fields, and adsorption onto substrates will be studied. We will investigate ways to use these molecular nanomagnets as molecular transistors.
Magnetic Imaging Reference Samples
We have fabricated magnetic nanostructures that can be used to determine the resolution and relative merits of various magnetic-imaging systems. These structures include bits recorded on commercial media, small Co-Pt nanostructures fabricated by electron-beam lithography, and small structures fabricated by focused-ion-beam techniques. The magnetic structures must have stable, well characterized features on length scales down to 10 nanometers to allow the testing of commercial imaging systems. We are currently designing the second generation of magnetic imaging reference samples that should have repeatable magnetic structures on length scales down to 1 nanometer.
Device Magnetodynamics and Noise
Devices have submicrometer dimensions and are fabricated within a test structure that includes high-bandwidth transmission lines. One line delivers ultra-fast magnetic field pulses to the device. The other line is electrically connected to the device and carries the voltage pulse generated as the device changes state. This voltage pulse serves as a probe of the magnetization dynamics of the device.
In a spin valve, only one ferromagnetic layer, the free layer, responds to external fields. Internal magnetic fields within the device allow only two stable magnetization directions, 180 degrees apart, along an easy axis. Current implementation of MRAM requires field pulses applied for 10 to 20 nanoseconds along either the positive or negative easy axis, depending on the desired state.
We discovered a way to switch the devices using field pulses of less than 300 picoseconds duration directed perpendicular to the easy axis. The magnetization is reversed due to large-angle precessional motion. For longer-duration pulses, the device does not switch because the magnetization rotates back to its initial direction while the pulse is on. Precessional switching requires only a single-polarity pulse applied perpendicular to the device easy axis, which results in a toggle operation of the magnetic state of the device. This is a simpler and more efficient bit-setting operation than using pulsed fields along the easy axis, which requires longer pulses in both directions.
Precessional switching relies on precise timing of the pulse width. The system is driven far above its quasistatic switching threshold and, therefore, the reversal is much faster than conventional switching. If the pulse is turned off precisely at the right time, when the system is in one of its low energy states, the system will switch cleanly. If, however, the pulsed field is turned off when the magnetization is far from a low-energy state, the magnetization, influenced by nonuniform internal fields, relaxes slowly and chaotically through inhomogeneous spin-wave type modes.
The devices, with dimensions of 0.8 micrometer by 2.0 micrometers, show high-frequency noise peaks near 2 gigahertz, corresponding to the uniform magnetic precession resonance of the devices. Several devices show multiple noise peaks, which indicates that other modes, in addition to uniform precession, are excited. The noise peaks shift with the application of a longitudinal magnetic field, similar to the shift in transverse magnetic susceptibility of the device. The noise amplitude, about 0.5 nanovolts per root hertz, indicates that the intrinsic thermal magnetic fluctuations of these devices will dominate the high-frequency noise as device dimensions shrink. Thus, thermal noise will likely dictate the fundamental size and performance limitation of GMR read heads. The data further suggest that thermal magnetic noise spectroscopy will be a powerful technique to characterize magnetodynamics in small magnetic structures.
The increase in the FMR frequency with decreasing temperature is much larger than expected from the measured increase in saturation magnetization or the large change in the coupling field between the free and fixed layers, indicating the presence of other temperature-dependent anisotropies. Structure in the high-frequency noise, beyond what is predicted by a simple single-domain model, can be resolved at various temperatures and bias fields
Combinatorial Materials, Meta-materials, and On-Wafer Metrology