Nanoscale Electronics and Magnetics:
to develop techniques for fabricating nanostructures
and measuring their electronic and magnetic properties.
INTENDED OUTCOME AND
BACKGROUND
The intended outcome of this work is the continuous improvement of techniques
for fabricating and characterizing nanometer-scale electronic and magnetic
structures, as required to meet current and future needs of the semiconductor
and data-storage industries. Our main tools for pursuing this program are
scanning electron microscopy with polarization analysis (SEMPA) and scanning
tunneling microscopy (STM). The Electron Physics Group in the Division has been
a leading innovator in both of these techniques, which are outgrowths of work
begun at NIST in the 1970s.
SEMPA enables us to use conventional scanning electron microscopy (SEM) to
image nanometer-scale magnetic structure, through spin-polarization analysis of
secondary electrons ejected from the sample. It has several unique capabilities
that distinguish it from other magnetic imaging techniques: it is a
surface-sensitive technique, and so is especially well suited for in situ
studies of thin-film and surface magnetization; it provides a direct
measurement of the magnetization of a material region, rather than of a
magnetic field; it has the high spatial resolution (about 10 nm), long
working distance, and large depth-of-field characteristic of SEM; and it
facilitates simultaneous measurements of the magnetization and the topography.
SEMPA studies have led to a number of breakthroughs in understanding the basic
mechanisms of magnetism on the microscopic scale, and have also addressed
near-term measurement issues faced by the magnetic data-storage industry.
Our STM program is focused on understanding the mechanisms of growth of
nanostructures on surfaces and determining their electronic properties. In
recent years the STM program has been particularly concerned with the magnetic
multilayer materials that have been investigated by SEMPA. The complementarity
of SEMPA and STM measurements has elucidated many connections between
conditions of layer growth and magnetic-device performance.
The main current direction of the STM program is the course of research made
possible by the recently-completed Nanoscale Physics Laboratory. The laboratory
was designed with the goal of measuring quantum-electronic structures with
atomic-scale imaging resolution and high electron-energy resolution. Samples
can be grown in situ and probed with magnetic fields of up to 10 T
at temperatures down to 2.5 K.
Accomplishments
Nanometer-Scale Measurements of the Vortex Lattice in a Type-II
Superconductor
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Figure 1. Vortex flux lattice in V3Si observed in STM
Fermi-level conductance image at H = 3 T and
T = 2.3 K. The peaks indicate the location of vortices with a
single flux quantum of magnetic flux. |
We have made the first real-space measurements of the hexagonal-to-square
symmetry transition in the vortex lattice of V3Si, a type-II
superconductor. The mixed state in a type-II superconductor is characterized by
the properties of the vortex or flux lattice that forms in the presence of an
applied magnetic field. Vortices are formed at points where the magnetic field
penetrates the superconductor in a flux tube through the sample. The flux
penetrates the superconductor in quantized units of the flux quantum
( 0 = h/2e),
with the region near the core of the vortex acting as a normal metal. As the
density of vortices in the material increases, a vortex lattice is formed by
the interactions between the flux tubes or vortices.
Many of the important applications of superconductors rely on the formation and
stabilization of the vortex lattice at high values of applied field. For
example, the optimization of vortex pinning in high-TC materials
controls flux creep and is responsible for the high critical currents needed
for the operation of superconducting magnets. Probing the underlying physics of
the vortex lattice is not only of technological importance, it is also the key
to understanding more complex interactions, such as the coexistence of
superconductivity and magnetism.
Real-space measurements of vortex lattices are difficult because the length
scale of the vortex unit cell is in the nanometer range for field strengths on
the order of 1 T. Such measurements are now possible using cryogenic
scanning tunneling microscopy, which probes the electronic structure of the
superconductor on the atomic scale.
Measurements of the symmetry transition in V3Si were made by
recording spatial maps of the local density of states (LDOS) of the
superconductor as a function of magnetic field, using the low-temperature
scanning tunneling microscope of the Nanoscale Physics Laboratory in the
Electron Physics Group. At the location of the vortex, the superconductor is a
normal metal and has a much higher density of states for energies inside the
superconducting gap. Thus, spatial maps of the LDOS show a bright spot at the
location of the vortex.
Figure 2. (a-d) STM Fermi-level conductance images of the vortex lattice
of V3Si as a function of applied magnetic field at 2.3 K.
(e-h) Corresponding auto-correlation images showing the unit cell
of the vortex lattice undergoing the hexagonal-to-square symmetry
transition. |
Measuring the vortex lattice as a function of magnetic field shows that
vortex-vortex interactions are important in determining the symmetry of the
vortex lattice. Moreover, the measurements reveal that symmetries in the
electronic structure of the superconductor play an important role, as they
directly link the symmetry of the vortex lattice to the underlying crystal
structure.
Magnetic Nanostructures in Patterned Ferromagnetic Rings
Small, patterned, magnetic, thin-film structures are a critical part of
emerging magneto-electronic technologies, which range from non-volatile
magnetic random-access memories to magnetic biosensor arrays. To be useful in
devices, the magnetic structure of these rings must assume only a few
well-defined magnetic states, and the switching between states must be
reproducible.
Figure 3. SEMPA image of a magnetic Co ring (center) along with a
micromagnetic simulation (right).
In collaboration with the Naval Research Lab and the University of Cambridge
Thin-Film Magnetism Group, we have used the NIST Scanning Electron Microscopy
with Polarization Analysis (SEMPA) facility to non-intrusively image the
magnetic nanostructure of a promising subclass of these structures: microscopic
magnetic rings. The SEMPA images provided a first direct look at these magnetic
states.
By comparing different sized thin-film elements grown under various conditions,
the SEMPA images revealed that the interplay between shape and crystalline
anisotropy can lead to unexpected magnetic configurations. In particular,
undesirable magnetic vortices can be induced in thick-walled rings during
switching. The SEMPA images were also used as a quantitative test of
micromagnetic models that, in turn, can explore regions of the switching
behavior not accessible to static SEMPA measurements.
Switching Nanomagnets
A current passing through two ferromagnets separated by a non-magnetic spacer
layer exerts a torque on each magnetization whenever the two magnetization
directions are not parallel. In the appropriate configuration, a large enough
current passing through such a multilayer can switch the relative alignment of
the magnetizations of the layers between parallel and antiparallel. In other
configurations, it appears that the magnetization of one layer rapidly
precesses around that of the other layer.
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Figure 4. Electron spin scattering from a ferromagnetic interface. The change
in spin due to scattering rotates the magnetization. |
These effects, which were observed starting in 1999, are being studied for two
device applications. If the size of the current necessary to reverse magnetic
bits can be reduced sufficiently, spin-transfer torques could provide a way to
switch bits in magnetic random-access memory (MRAM). Alternatively, if the
interpretation of recent experiments as precession is correct, it should be
possible to make controllable-frequency oscillators.
We have developed quantitative models for spin-transfer torques to help enable
the development of devices based on this effect. These models require three
types of calculations. We have carried out quantum-mechanical calculations to
determine the fate of an electron that scatters from an interface with a
ferromagnet. These calculations show that the transverse component of the
incident spin current is absorbed close to the interface.
We use this result as a boundary condition in the second type of calculation,
semiclassical transport. These calculations determine the polarization, both
magnitude and direction, of the current throughout the structure. With the
input from the first-principles calculations, our semiclassical calculations
reproduce both the transport properties of the structures and the
current-induced torques.
Analyzing the calculations in detail highlights the important physical
processes, which are difficult to access experimentally. The calculated torques
can be used as input to the third type of calculation, classical simulations of
the magnetization dynamics.
Linking the Micro and Nano Worlds with Nanolines
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Figure 5. Beating of nanolines: the superposition of two nanoscale
patterns with slightly different periodicities results in an optically visible
beating on the microscale. |
As technologies ranging from electronics and data storage to biotechnology
become more and more miniaturized, there is an increasing need for ways to make
measurements that bridge the domains of the micrometer and the nanometer.
We have collaborated with scientists from the University of Nijmegen to make an
artifact that facilitates this connection. It relies on a technology first
demonstrated in the Electron Physics Group in 1993, called laser-focused atomic
deposition. In its original form, an optical standing wave focused chromium
atoms onto a surface, creating an array of lines with a highly precise spacing
of 212.78 nm. In the new process, two arrays of lines with slightly
different spacing are superimposed, creating a beating, or
Moiré, pattern with periodicity of 44.46 µm. When
viewed through crossed polarizers, this pattern is clearly visible with an
optical microscope.
The result of this process is that nanometer-scale and micrometer-scale
patterns are created on a single substrate in coherent registration with each
other. Ongoing investigations indicate that because of the resonant nature of
the laser-focused atomic-deposition process, the dimensions of the patterns may
be accurately traced to an atomic frequency, so that the patterns can be used
as absolute length standards on the nano- and micro-scale.
Electron Beam Induced Magnetic Switching
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Figure 6. A series of SEMPA images from Fe/GaAs (110) showing the
electron-beam-induced magnetization reorientation. |
Understanding and controlling the magnetic properties of ferromagnetic thin
films grown on semiconductor substrates is a critical element in developing
hybrid magnetic/semiconductor magnetoelectronic devices. We use the exquisite
surface sensitivity provided by SEMPA to systematically explore the properties
of various ferromagnet/semiconductor combinations. As part of this work, we
discovered that electron beams can be used to induce changes in the
magnetization direction in ferromagnetic thin films.
When Fe films are grown epitaxially on the (110) crystal surface of GaAs, the
magnetization can be trapped in a metastable state, oriented along the [-110]
direction. Irradiating the sample with an electron beam causes the
magnetization to locally rotate by 90° into the stable [100] direction.
In addition to providing information regarding domain-wall dynamics in Fe/GaAs,
this phenomenon may also provide a means of locally writing magnetic
information or patterns in Fe films with electron-beam precision. Work is
underway to understand the electron-beam/sample interaction that drives the
reorientation, and to investigate its potential for electron-beam writing of
magnetic information.
First strategic focus |
Second strategic focus |
Third strategic focus
"Technical Activities 2002" - Table of Contents |
