Magnetic Thin Films and Devices 2001
Cryogenic microwave probe station for testing magnetoresistive devices.
This project develops measurements and standards for magnetic thin-film materials and devices for the magnetic data storage and magneto-electronics industries, with applications in magnetic recording, 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, at microwave frequencies. Project members have successfully devised ways to control the dynamical properties of magnetic devices. In addition, they fabricate magnetic nanostructures to determine the resolution of magnetic imaging systems and develop new combinatorial materials and on-wafer metrology. Recently, the project has begun to develop new techniques to measure spin-dependent electron transport at surfaces and interfaces in advanced magnetic device structures. Long-term goals include the development of metrology that will be required to develop quantum spin-based electronics for data storage and terahertz information processing.
Our project serves the needs of U.S. industries that use and develop magnetic thin-film and magnetic-device technologies. These industries include magnetic-hard-disk recording, magnetic tape recording, magnetic random-access memories (MRAM), and magneto-electronics (including sensors, isolators, and microwave devices). 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 measure and characterize these magnetic structures. Advances in technology are dependent on the discovery and characterization of new effects such as giant magnetoresistance 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.
Magnetic thin film systems have become increasingly complicated, often containing quaternary alloys or multilayer systems with 4 to 10 elements that require atomic-level control of the layers. New techniques are required to efficiently and systematically develop and 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 a large region of parameter space.
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.
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 recording 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 have fabricated test structures that allow the characterization of small magnetic devices at frequencies up to 10 gigahertz. The response of sub-micrometer magnetic devices, such as spin-valves, magnetic tunnel junctions, and giant-magnetoresistive devices with current perpendicular to the plane, have been characterized both in the linear-response and the non-linear 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. Measured data have been compared to numerical simulations of the device dynamics to determine the ability of current theory to predict the behavior of magnetic devices.
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 sub-monolayer additions of oxygen, noble metals, and rare earths on giant magnetoresistance have been studied.
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 with a large number of sites with systematic variation of materials properties such as composition, or process parameters such as growth temperature. In addition to library fabrication, the combinatorial process involves the development of high-throughput on-wafer metrologies that can systematically characterize the libraries and scan for desirable materials properties.
Finally, 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 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 an essential ingredient to the development of methods to control and manipulate small numbers of spins in a spin circuit.
- In 2001, characterize gigahertz noise and thermal-fluctuation-induced dynamics in 100 nanometer spin valves and tunnel junctions.
- During 2001-2002, investigate the electronic scattering from nano-oxides in giant magnetoresistive (GMR) systems and determine if electron confinement techniques can significantly enhance read-head sensor performance.
- By 2002, develop techniques to make on-wafer measurements of saturation magnetization and magnetostriction.
- By 2002, measure spin-transfer-induced dynamics in nanoscale current-perpendicular-to-plane (CPP) devices in collaboration with Cornell University and Motorola.
- In 2001, measure electron spin resonance (ESR) in sub-micrometer spin-polarized two-dimensional degenerate electron-gas devices.
We have measured the ferromagnetic resonance in micrometer-sized spin-valve devices. The ferromagnetic resonance was measured using both time-domain and frequency-domain magnetoresistance techniques. This work represents the measurement of ferromagnetic resonance in the smallest single particle to date. The work provides a clear measurement of the response of magnetic sensors similar to those used in magnetic recording read heads, up to 6 gigahertz. Knowledge of the high-frequency performance of these devices will be required in the next few years when the data transfer rates exceed 1 gigahertz. This work has further stimulated the use of these sensors in other magneto-electronic applications such as high-speed isolators working in the gigahertz regime.
We have completed a study of switching probabilities in small spin-valve devices designed for MRAM applications. The switching probability was measured as a function of magnetic field pulse height and pulse width. At long pulse widths (above 1 nanosecond) reproducible switching, with unity switching probability, was observed. As the pulse width was decreased, a rapid transition occurred in which the switching probability decreased from 1 to 0 over a span of 100 picoseconds. Metastable states were observed in this transition region. The metastable states were found to have a very wide spread in lifetimes, ranging from a few nanoseconds to several milliseconds.
Using micromagnetic simulations of rotations in spin-valve devices, we have shown that the inclusion of disorder causes a non-uniform damping, which agrees qualitatively with the measured data from small spin-valve devices. The simulations showed that during the initial large-angle motions there was a larger transfer of energy, mediated by the disorder, from the uniform mode to short-wavelength magnetic modes. As the motion amplitude decreased, fewer spin waves were produced and the damping decreased. The exact type of the disorder in giant-magnetoresistive devices remains to be determined. Possible candidates include edge rough-ness, Néel coupling with the pinned layer, varia-tions in local anisotropy, or stress.
We discovered a new method of engineering the high-speed dynamical properties of technologically relevant magnetic thin films. Doping Ni-Fe films with a few percent of Tb will increase the damping of the films by an order of magnitude while not appreciable changing other magnetic properties. Gd doping was also explored and showed less increase in damping. Gd has only a spin moment whereas Tb has both a spin and an orbital moment. The results are consistent with the hypothesis that the orbital moment of Tb, in combination with the high anisotropy of the orbitals, allows the magnetization to directly couple to the lattice. Both uniform and modulation doping were explored. Temperature-dependent measurements of the magnetization and damping were made, along with high-resolution x-ray measurements, to determine the correlation of the damping to the magnetic properties and microstructure. This work represent the first effort to engineer the intrinsic dynamical properties of materials used in magnetic data storage applications.
We measured magnetic-force microscope line scans of 100 nanometer Co-Pt dots and compared them with calculated field and field-gradient profiles. The calculations show that there should be sharp features in the field gradi-ents at the edge of the dots if the dots were ideal, with uniform perpendicular magnetization. The features should have peak widths of about 20 nanometers, which were not resolved in our measurements. Therefore, the calculations indicate that these samples may be useful for magnetic imaging reference samples, since they should have features at or beyond the resolution of current magnetic-force microscopes.
We completed fabrication of the first set of magnetic combinatorial libraries in collaboration with the Materials Science and Engineering Laboratory. The libraries cover a large range of the Tb-Ni-Fe-Co phase diagram and contain several magnetic and crystalline phases. The magnetic structure ranges from in-plane ferromagnetic to perpendicular ferrimagnetic to isotropic ferrimagnetic, to paramagnetic. These libraries were designed to explore giant-magnetostrictive materials and to provide test systems for developing on-wafer magnetic metrology.
We completed a study of the high-speed magnetic switching properties of Co-Fe-Hf-O in collaboration with Nonvolatile Electronics, Inc. Co-Fe-Hf-O is a new material developed by NVE
with potential applications in high-speed mag-netic devices. Co-Fe-Hf-O has a tunable resistivity and anisotropy and has dynamic properties that exceed those of Ni-Fe for thicknesses greater than 100 nanometers.