Magnetic Devices and Nanostructures
Goals
 NMR system (7 teslas, 300 megahertz) for use in |
The Magnetic Thin Films and Nanostructures Project develops measurements and standards for nanomagnetic materials and devices used in the magnetic data storage, magnetoelectronics, and biomedical industries. These measurements and standards assist industry in the development of advanced magnetic recording systems, magnetic solid-state memories, magnetic sensors, magnetic microwave devices, and biomedical materials and imaging systems. Broadband electrical measurements are being developed to characterize nanoscale devices based on giant magnetoresistance (GMR), tunneling magnetoresistance (TMR), and spin-momentum transfer (SMT). Magnetic resonance techniques, such as high-frequency electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMRM) are being used to study the properties of nanomagnets to improve magnetic resonance imaging (MRI) or find applications in nanotagging. We are developing dynamic nanoscale magnetic imaging, such as time-resolved Lorentz microscopy in collaboration with the Materials Science and Engineering Laboratory, to better understand the operation of nanoscale magnetic structures and devices.
Customer Needs
The data storage and magnetoelectronics industries are developing smaller and faster technologies that require sub-hundred-nanometer magnetic structures to operate in the gigahertz regime. New types of spintronic devices with increased functionality and performance are being incorporated into data storage and magnetoelectronic technologies. New techniques are required to characterize these magnetic structures on nanometer-size scales and over a wide range of time scales varying from picoseconds to years. For example, the response of a 50-nanometer magnetic device, used in a read head or a magnetic random-access memory (MRAM) element, 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. Spintronic devices and nanomagnetic materials are finding applications in other areas such as homeland security and biomedical imaging. These industries require better low-power magnetic field sensors for weapons detection, chemical detection, and magnetocardiograms, and require novel nanomagnetic materials for MRI contrast agents and defense applications.
Advances in technology are dependent on the discovery and characterization of new effects such as GMR, TMR, and SMT. Detailed understanding of spin-dependent transport is required to optimize these effects and to discover new phenomena that will lead to new spintronic 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. Many technologies require, or are enabled by, the use of magnetic nanostructures such as molecular nanomagnets. The study of magnetic nanostructures will enable data storage on the nanometer scale, a better understanding of the fundamental limits of magnetic data storage, and new biomedical applications.
Technical Strategy
 Carbon nanotubes growing from Fe nanodots for |
We are developing several new techniques to address the needs of U.S. industries for characterization of magnetic thin films and device structures on nanometer size scales and gigahertz frequencies.
Device Magnetodynamics — We fabricate test structures for characterizing small magnetic devices at frequencies up to 40 gigahertz. The response of submicrometer magnetic devices such as spin valves, magnetic tunnel junctions (MTJs), and GMR devices with current perpendicular to the plane is measured in both the linear-response and nonlinear-switching regimes. The linear-response regime is used for magneticrecording read sensors and high-speed isolators, whereas the switching regime is used for writing or storing data in MRAM devices. We measure the sensors using microwave excitation fields and field pulses with durations down to 100 picoseconds. We compare 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 develop new techniques to control and optimize the dynamic response of magnetic devices. These include the engineering of magnetic damping by use of rare-earth doping and precessional switching, which controls switching by use of the timing of the pulses rather than pulse amplitude. This research is aimed at developing high-frequency magnetic devices for improved recording heads and for imaging of microwave currents in integrated circuits and microwave devices. Novel device structures that incorporate magnetic materials with other nanostructures, such as carbon nanotubes, are being investigated in collaboration with the Materials Science and Engineering Laboratory and the Radio-Frequency Electronics Group.
Magnetic Noise and Low-Field Magnetic Sensors — In collaboration with the Quantum Electrical Metrology Division, we develop new techniques to measure both the low-frequency and 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 is measured in our fabricated structures and in commercial read heads. High-frequency noise spectroscopy directly measures the dynamical mode structure in small magnetic devices. The technique can be extended to measure the dynamical modes in structures with dimensions as small as 20 nanometers. The stochastic motion of the magnetization during a thermally activated switching process is measured directly, which will lead to a better understanding of the long-time stability of high-density magnetic memory elements. New methods are being developed to dynamically image thermal fluctuations by use of time-resolved Lorentz and scanned probe microscopies. These new metrologies will be essential to study and control thermal fluctuations and 1/f noise in magnetic and spintronic devices.
 Magneto-optical Kerr effect microscope. |
Nanomagnetism — We are developing new methods to characterize the magnetic properties of nanomagnetic structures such as patterned media and molecular nanomagnets. Patterned magnetic nanodots are fabricated with sputter deposited magnetic multilayers and electron-beam lithography. The magnetic properties are studied with magnetic force microscopy, magneto-optical Kerr effect, and Lorentz microscopy. An important focus of this work is the ability to characterize dynamics, such as magnetic recording bit reversal, in single nanostructures at high frequencies. Magnetic nanostructures are also characterized with high-frequency EPR, based on a superconducting quantum interference device (SQUID) magnetometer, which can simultaneously measure low-frequency magnetic properties and high-frequency characteristics, such as resonant absorption/emission of microwaves in the frequency range of 95 to 141 gigahertz over a temperature range of 1.8 to 400 kelvins. Molecular nanomagnets, which are the smallest well defined magnetic structures that have been fabricated, exhibit quantum and thermal fluctuation effects that will necessarily be encountered as magnetic structures shrink into the nanometer regime. These systems, which contain from 3 to 12 transitionmetal atoms, form small magnets with Curie temperatures of 1 to 30 kelvins. We are investigating new methods of manipulating these nanomagnets by varying the ligand structure and binding them to various films. We are looking at new applications by incorporating the nanomagnets into molecular devices and exploring how the nanomagnets relax nuclear spins in biological systems.
MRI Standards And Contrast Agent Metrology — We develop new techniques to characterize nanomagnetic materials for improved MRI contrast and to fabricate quantitative MRI phantoms. Nanomagnetic materials in solid, liquid, or gel phases are studied with SQUID magnetometry, EPR, and NMR. We have installed an NMR relaxometer that can measure the nuclear relaxation induced in biological systems by neighboring nanomagnets. The relaxometer can measure over a field range of 0.5 to 7 teslas and NMR frequency range of 20 to 300 megahertz. Traceable standards are being developed to help monitor MRI stability and intercomparability and to enable quantitative MRI.
Accomplishments
- Low-Frequency Noise Measurements
on Magnetic Field Sensors
— Low-frequency
noise was measured in the frequency range from
0.1 hertz to 10 kilohertz on a variety of commercially
available magnetic sensors and custom sensors fabricated at NIST. The types of sensors investigated include those based on anisotropic magnetoresistance (AMR), GMR, TMR, giant magnetoimpedance (GMI), and fluxgate devices. The 1/f noise components of electronic and magnetic origin were identified by measuring sensor noise and sensitivity at various applied magnetic fields. For the GMR sensors, both electronic and magnetic components contribute to the overall sensor noise. MTJ bridge sensors, which operate on the tunneling magnetoresistance effect, were fabricated at NIST. MTJ devices consist of two magnetic layers: a free layer and a fixed layer separated by a thin insulator. The tunneling current through the insulator is dependent on the relative orientation of the two magnetic layers. These devices are used in magnetic recording read sensors, MRAM bits, and magnetic field sensors. Two different sensors are currently under development: a low power sensor (less than 100 microwatts) with a field detectivity of 1 nanotesla per root hertz was demonstrated this year, and a low field sensor with field detectivity of 1 picotesla per root hertz will be demonstrated next year. These sensors are small, low in cost, and compatible with complementary metal oxide semiconductors (CMOS), and will enable many applications for which there are currently no sensor solutions.
Low frequency noise measured in
different types of magnetic field sensors. - Dynamic Lorentz Imaging of Magnetic
Tunnel Junctions
— In collaboration with the
Materials Science and Engineering Laboratory, we
studied disorder in MTJs using dynamic Lorentz
microscopy. Lorentz microscopy is a form of
transmission electron microscopy (TEM) that uses
the defection of electrons as they pass through a
magnetic sample to image nanoscale magnetic
structure. The advantages of Lorentz microscopy
are that it can image the interior of a complicated
device stack, it acquires data in parallel allowing
rapid image acquisition needed for dynamic imaging,
and it can image nanoscale structural defects
that can give rise to the magnetic disorder. Considerable
disorder in the free layer of the MTJ was
observed during free layer rotation and switching.
The disorder was a function of the MTJ preparation
technique. By incorporating a nano-oxide just below
the tunnel barrier (a technique developed in the
Materials Science and Engineering Laboratory) the
observed disorder was reduced. Time dependent
fluctuations in the magnetic structure, which are a prime source of 1/f noise, were directly observed with dynamic Lorentz microscopy.
Magnetic force microscope image of 100-nanometer nanodots
showing partial magnetic switching. Each nanodot is a stack
of 8 layers of 0.35 nanometer of Co alternating with 8 layers
of 1.02 nanometers of Pd. - Narrow Switching Distributions in Nanoscale Patterned Media — Magnetic nanodots have been fabricated by electron-beam lithography to study their suitability for magnetic media. The nanodots are fabricated from multilayers of Co and Pd either using sputtering or molecular beam epitaxy. The perpendicularly magnetized nanodots have switching fields that range between 0.5 and 1.5 teslas. The switching field distributions were measured for both polycrystalline and epitaxial structures with a variety of underlayers to determine the effect of the microstructure on switching. The key results were the observation of a very narrow switching distribution in nanodots with Ta underlayers and that the epitaxial structures did not show significantly narrower switching distributions than the polycrystalline structures. This study shows that the polycrystalline nature of the nanodots, which affects both the magnetic and lithographic uniformity, is not responsible for the variation in observed switching properties.
- Evaluation of Fe8 for use as an MRI contrast agent — In collaboration with the University of Colorado, we completed a study of the potential use of Fe8 molecular nanomagnets as an MRI contrast agent. NMR relaxivity data were obtained over a broad range of concentrations. At low concentrations the relaxivity, which is proportional to MRI contrast, was shown to be comparable to that of existing contrast agents. At high concentrations the relaxivity decreased. Based on a SQUID magnetometer evaluation of the decomposition of Fe8 in aqueous solutions, the concentration dependence of the relaxivity was attributed to the concentration dependence of the decomposition rate. The excitation spectra of Fe8 were characterized using SQUID high-frequency EPR. This novel EPR technique uses a SQUID magnetometer to quantitatively measure the spin excitation in response to microwave radiation. This work is our first effort to correlate the ESR fluctuation spectrum of a nanomagnet to the NMR relaxivity.
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Award
U.S. Department of Commerce Silver Medal (Bill Rippard, Stephen Russek, and Tom Silva) and EEEL Distinguished Associate Award (Matt Pufall and Shehzaad Kaka) for the discovery of mutual phase-locking, external frequency-locking, and frequency modulation of spin-transfer nano-oscillators, 2006.