Magnetic Devices and Nanostructures

Goals

Brant Cage next to a magnetometer modified to simultaneously measure magnetic moment and highfrequency EPR spectra on nanomagnets.

This project develops measurements and standards for magnetic 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. Work is focused on novel methods of measuring and studying nanoscale magnetic materials and spin-electronic devices (see section on Spin Electronics). Broadband electrical measurements are being developed to characterize nanoscale devices based on giant magnetoresistance (GMR), spin-dependent tunneling (SDT), and spinmomentum transfer (SMT). New methods are being developed to quantitatively measure the magnetic moment of magnetic thin films, characterize spin transport in magnetic multilayers, and determine the high-frequency properties of nanomagnets. The project is researching magnetic nanostructures, such as molecular nanomagnets, for potential use in nanoscale magnetic data storage, new magnetoelectronic devices, and biomedical imaging.

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 an 80-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 magnetic-resonance imaging contrast agents and defense applications.

Advances in technology are dependent on the discovery and characterization of new effects such as GMR, SDT, 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 SMT 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

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 that allow the characterization of small magnetic devices at frequencies up to 40 gigahertz. The response of submicrometer magnetic devices such as spin valves, magnetic tunnel junctions, and GMR devices with current perpendicular to the plane (CPP), are measured in both the linear-response and 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 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 using 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.

Magnetic Noise and Low-Field Magnetic Sensors — 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 using 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.

In-Situ Magnetoconductance and Quantitative Magnetometry — We develop 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 are studied. Further, we are developing a new technique to quantitatively measure the moment of magnetic thin films, whose moments are typically on the order of 100 nanojoules per tesla. This quantitative magnetometer will provide measurements that are traceable to fundamental International System (SI) quantities.

Nanomagnetism — We are developing new methods to characterize the magnetic properties of nanomagnetic structures such as molecular nanomagnets. One method is high-frequency electron paramagnetic resonance (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 transition-metal 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.

Accomplishments

Image of fields above a coplanar waveguide obtained using a high-bandwidth spin-valve recording head: (a) optical image, (b) low-frequency magnetic image, (c) 1 gigahertz capacitive image, (d) 1 gigahertz magnetic image.

Lorentz microscope image of cross-tie walls and Bloch lines in an amorphous Co-Fe-B-Si magnetic thin film.

Magnetoresistive Scanning System Implemented — We built a high-frequency magnetoresistive (MR) scanning system to probe highfrequency electric and magnetic fields above highspeed circuits. The system can use either commercial recording heads or NIST-fabricated high-frequency MR sensors. The spatial resolution can be as small as 100 nanometers depending on the type of probe. The system has two 40 gigahertz microwave probes to energize the circuit, and the through power can be monitored simultaneously with the field mapping to determine the invasiveness of the high-frequency probing. A special set of high-frequency structures — which included co-planar waveguide tapers, shorts, and opens — were fabricated to allow imaging of structures that had spatially varying microwave fields in three dimensions. The structures were imaged in three modes: low frequency magnetic fields (1 kilohertz), high-frequency electric fields (1 to 4 gigahertz), and highfrequency magnetic fields (1 to 4 gigahertz).
Low-Frequency Noise Measurements on Commercial Magnetoresistive Magnetic Field — Low-frequency noise was measured in the frequency range from 0.1 hertz to 10 kilohertz on a variety of commercially available magnetic sensors. The types of sensors investigated include those implemented with anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR), and tunnel magnetoresistance (TMR) effect 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. Maximum noise occurs at the bias field that gives maximum sensitivity. The noise of TMR-based sensors is primarily due to resistance fluctuations in the tunnel barrier, having little to no field dependence. The best low-field detectivity of the sensors that has been measured is on the order of 100 picotesla per root-hertz at 1 hertz. These magnetic sensor noise data are part of a database of low-field sensor performance.
Domain-Wall Pinning by Nanoscale Defects in Amorphous Magnetic Thin Films Using Lorentz Microscopy — We worked with the Materials Science and Engineering Laboratory on the development of a dynamic Lorentz electron-microscope imaging facility. High-resolution Lorentz images were obtained using a new 200 kilo-electronvolt transmission electron microscope (TEM). The TEM was outfitted with a high-resolution camera capable of recording images at 15 frames per second. Magnetic domain-wall motion in soft magnetic thin films was observed in response to a varying in-plane magnetic field. Domain walls became trapped at nanoscale defects and were then studied using standard TEM imaging. Several different types of films were studied, including Ni-Fe alloys, Ni-Fe-Co-Si-B amorphous films, and Co-Fe-B amorphous films.
Lorentz Imaging of Magnetic Structures — We successfully fabricated and imaged arrays of magnetic elements using Lorentz microscopy. The arrays consist of elements of different designs that are being considered for magnetic sensor devices. The domain patterns are generally in good agreement with those predicted by micromagnetic simulations. Particular areas near domain walls in some of the elements show indications of fluctuations in the magnetization and are potential sources of noise in the elements.
In-Situ Observation of Nano-Oxide Formation in Magnetic Thin Films — In-situ conductance and reflection high-energy electron diffraction (RHEED) measurements were taken during the oxidation of 20-nanometer-thick Co and Co-Fe layers. The conductance shows an initial drop with exposure to oxygen followed by a period of increasing conductance. This increase in conductance clearly indicates an increase in specular reflection of electrons at the oxide interfaces. The amount by which conductance increased varied with deposition conditions. The sample with the highest increase in conductance showed an increase in specularity of at least 0.05. RHEED measurements show a blurring of the (111) face-centeredcubic (FCC) texture with exposure to oxygen, indicating the formation of an amorphous oxide during the initial conductance drop and conductance increase. After the conductance begins to decrease again, a new diffraction pattern appears in the RHEED data, indicating the formation of CoO with a FCC (111) texture but with a different lattice spacing. These studies shed light on the physical mechanisms for the giant magnetoresistive effect used in magnetic sensors.

In-situ conductance measurements of a spin-valve taken during growth. The data can be fit to determine the local current density.

Measurements of molar susceptibility of the EPR standard DPPH using SQUID HF-EPR. The open triangles, for no applied microwaves, show the expected constant value of susceptibility as a function of magnetic field. Upon microwave irradiation at 95.510 gigahertz, a minimum and a small shoulder in susceptibility appear at specific magnetic fields (blue squares). This structure is due to g-tensor anisotropy, which gives rise to two resonance peaks. This splitting cannot be resolved using low-frequency EPR. The inset shows the corresponding susceptibility as a function of swept field at 141 gigahertz, demonstrating a resolution of 1.8 milliteslas in an applied field of 5.0460 teslas.

Current Density Distribution in a Spin Valve Determined through In-Situ Conductance Measurements — The sheet conductances of toppinned spin valves and single-material films were measured in situ as the thin-film layers were grown. The data were fit to a Boltzmann transport calculation. The electrical conductivity and electron mean free paths were determined for each material by measuring the in-situ conductance of thick, singlematerial films. The electron transmission probabilities were deduced for each interface from the theoretical fits to the multilayer data. From these interfacial transport parameters the ratio of current density to electronic field, or effective conductivity, was calculated as a function of position for the completed spin valve. The distribution of current in the spin valve was not very sensitive to the overall amount of diffuse scattering at the interfaces. Spin valves are used in modern magnetoresistive read heads in disk drives.
Broadband SQUID-Detected Electron Paramagnetic Resonance Probe — High-frequency electron paramagnetic resonance (HFEPR) is a powerful technique for the characterization of magnetic materials. Measurements at 100 gigahertz and above allow greater resolution in the determination of magnetic energy levels, which are useful for the development of new materials for nanomagnetic data storage, spin electronics, and biomagnetism. However, a serious shortcoming of conventional HF-EPR is its inability to quantitatively measure magnetic moment. We have developed a new technique to make quantitative measurements using a commercial SQUID magnetometer.
We are able to directly measure the change in magnetic moment of a specimen as microwave stimulation causes resonance at different values of applied magnetic field. The apparatus uses a 95 or 141 gigahertz klystron microwave source followed by an isolator, an attenuator for saturation studies, and a directional coupler. A detector/mixer monitors the frequency and both reflected and incident power. A square-to-round waveguide transition to thin-wall tubing is used to deliver the microwaves to a sample located inside the magnetometer. The probe assembly and sample are mechanically oscillated through the SQUID pick-up loops to obtain the magnetic moment. Quantitative measurement of the degree of saturation at any value of magnetic field allows spin-lattice relaxation times to be calculated in the low-power, saturation regime. (Conventional EPR studies require approaching or exceeding the high-power, nonlinear regime.) This is important at high frequencies where the available microwave power is usually limited.
Characterization of Energy Levels and Saturation in Fe-8 Molecular Nanomagnets Using SQUID HF-EPR — We completed a study of Fe-8 molecular nanomagnets using SQUID HFEPR. This work is the first to quantitatively measure the magnetization suppression of molecular nanomagnets under microwave illumination. By resonantly pumping low lying energy levels at 95 gigahertz it was possible to suppress the magnetization by 80 percent, indicating the presence of an efficient "spin cascade" that allows energy to be efficiently transported from the low-lying spin levels to high-energy levels. The exact mechanism of this spin cascade is still under investigation.

Structure of Fe-8 molecular nanomagnets and SQUID high-frequency EPR spectra of Fe-8 showing resonant absorption corresponding to transitions between the quantized energy levels.