The Division's program in spin electronics and nanoscale spin dynamics develops new measurement techniques to characterize the high
frequency properties and performance of nanomagnetic structures and devices.
Hard-disk drives in personal computers and data centers push the limits of
technology, with current data bit densities of 100 billion per square
centimeter. The program investigates the high-frequency behavior of nanoscale
magnetic materials and devices. The switching of magnetization in write heads,
read heads, recording media, and innovative memory elements at frequencies in
the hundreds of megahertz to hundreds of gigahertz will be the foundation for
future magnetic data storage systems and microwave integrated circuits. These
technologies will depend on newly discovered properties and limitations of
magnetic materials and devices that appear only at the nanoscale.
data demands of cloud computing, expanded Internet use, mobile device support,
and other applications have prompted the creation of large, centralized
computing facilities at hundreds of thousands of sites around the world. Even
if the power needs for all U.S. data centers can be met, the inherent
constraints of semiconductor electronics will still impose scaling and
clock-rate limits on future processing capacity at a time when the digital
information is increasing exponentially. Electron-spin torque may be used to
switch future, nonvolatile, magnetic memory elements. Compared to switching
memory bits with magnetic fields, this method would offer higher speed, greater
reliability, lower power, and would be scalable to smaller device dimensions. The
division investigates theoretical and experimental aspects of spin transport
and the transfer of spin angular momentum to magnetic structures.
The Spin Electronics Group
develops metrology to determine how spin currents can be generated and used to
control and manipulate magnetization in new ways at timescales less than 1 ns. We
provide and disseminate advanced high frequency measurements, analysis, and
fabrication of nanoscale magnetic structures and materials to enable the
development of novel spin-based devices. Focus areas include (1) the
fundamental understanding of the interactions between spin and magnetic materials,
superconductors, and materials with large spin-orbit scattering; (2) the nonlinear
dynamics of both individual and interacting nanoscale magnetic systems; and (3)
the role of thermal noise in nanomagnetic systems. We study how these effects
can be controlled with applied spin currents and electric fields. Applications
include nonvolatile magnetic data storage, non-Boolean computation
architectures, active magnetic nanoscale devices, and novel radiofrequency
We have developed new
metrology to characterize and understand the behavior of nanoscale magnetic
memory elements in disparate environments. For instance, we developed ways to monitor
individual switching events in spintronic devices, perform ferromagnetic-resonance
spectroscopy of individual devices, and quantitatively determine the roles that
stray fields and exchange coupling play in hybrid spin-valve/Josephson-junction
cryogenic memory. This work has direct relevance to companies developing memory
for both conventional and superconducting supercomputers. We also developed new
techniques to characterize the nonlinearities that cause phase-locking and
phase noise in nanoscale magnetic oscillator arrays. These measurements are helping
companies develop bio-inspired, non-Boolean computing architectures for
low-power and approximate computing.
NANOSCALE SPIN DYNAMICS
The Nanoscale Spin
Dynamics Group develops metrology for magnetodynamic effects such as resonance,
switching, and damping. We apply our measurement tools and expertise to
materials and structures commonly used in nanomagnetic devices. Collaborations
with industry leaders such as Intel and Hitachi Global Storage Technologies
have led to new understanding of damping in advanced materials and to
replication of a NIST measurement tool at Intel's development facility for
magnetic memory. We also investigate fundamental aspects of spin transfer in
materials and structures that offer improved performance in future devices.
These studies include spin-orbit torques, spin pumping and the spin Hall effect
at ferromagnet/non-magnet interfaces, spin transport in two-dimensional
materials such as graphene, and the chiral magnetic properties of
feromagnet/heavy-metal interfaces, all of which play a role in proposed
magnetic memory and logic devices with low energy consumption.
Spin-Transfer-Torque Magnetic Random-Access Memory
The ubiquitous dynamic
random-access memory (DRAM) for
computers must be refreshed every 50 ms or so and therefore requires power to retain
information. A promising alternative to DRAM is a nonvolatile, radiation-hard,
low-energy-loss, nanoscale, magnetic memory: spin-transfer-torque magnetic random-access
memory (STT-MRAM). An STT-MRAM
element consists of two ferromagnetic layers separated by a thin (about 1 nm)
MgO layer, which allows the two layers to behave independently and contributes
to the electrical transport through the device. The resistance of such a
structure depends on the relative orientations of the magnetization of the two
layers; it changes by roughly a factor of two when their magnetizations are
switched between parallel (P) and
antiparallel (AP), thus storing "0"
and "1" bit states. The structures are fabricated so that the two ferromagnetic
layers in the three-layer sandwich are asymmetric, which allows one of the layers
to be switched freely while the other is fixed.
In conventional MRAM,
switching between these states is achieved with applied magnetic fields. Spin-transfer
torque, on the other hand, allows switching between the P and AP states to be achieved
with low-loss, spin-polarized currents, which impart their angular momentum to the
magnetization of the free layer in the film stack. We developed new methods to
measure switching statistics for over one million events per device, which showed
that many experimental devices do not necessarily switch reliably at the levels
required by the microelectronics industry. We found that certain devices (or
bits) behave differently than their seemingly identical counterparts, and we
developed a frequency-domain method to quickly identify these individual outliers.
Bill Rippard (left) and Eric Evarts measuring error rates in STT-MRAM
- R. Heindl, W. H. Rippard, S. E. Russek, and M. R.
Pufall, "Time-domain analysis of spin-torque induced switching paths in
nanoscale CoFeB/MgO/CoFeB magnetic tunnel junction devices," J. Appl. Phys. 116, 243902 (Dec. 2014); doi: 10.1063/1.4905023.
- E. R. Evarts, R. Heindl, W. H. Rippard, and M. R.
Pufall, "Correlation of anomalous write error rates and ferromagnetic resonance
spectrum in spin-transfer-torque-magnetic-random-access-memory devices
containing in-plane free layers," Appl.
Phys. Lett. 104, 212402 (May
2014); doi: 10.1063/1.4879847.
Ferromagnet-Based Josephson-Junction Memory for Superconducting
will increasingly be used to model complex systems such as weather and climate,
biology, astronomy, finance and economics, weapons simulations, and
cryptography. However, completely new computer architectures will be required
to achieve high clocking speeds and low energy dissipation. The Intelligence
Advanced Research Projects Activity (IARPA)
is considering superconducting computing as a promising technology for
large-scale computing and has started a multiyear program to investigate its
viability. One major hurdle is the lack of a nonvolatile memory element that is
scalable (down to nanometers), fast (on the order of nanoseconds), and energy efficient
In collaboration with the
Quantum Voltage Group, we are incorporating nanoscale, nonvolatile, ferromagnetic
memory elements similar to STT-MRAM devices into Josephson junctions (JJs) for superconducting supercomputing.
This combines two disparate disciplines that are generally mutually exclusive since
spin polarized currents are generally not compatible with Cooper-pair electron transport
in superconductors. Because of this, the field has a significant need for basic,
cross-disciplinary metrological advances. For instance, the JJ critical current
in these structures depends on the state of the memory element (P or AP), and
hence the critical current can be used to interrogate the memory state.
However, the microscopic magnetic reasons for this were, until recently, unclear.
Through a series of careful measurements we determined that the change of the
JJ critical current depended on the ferromagnetic exchange interactions in the
memory element and not stray magnetic fields. This is a fundamental advance in
this emerging field because the exchange interaction is scalable to the
nanoscale whereas the effects of stray fields are not. We recently showed that
the same energy-efficient STT effect exploited in room temperature memory can
also be utilized in their superconducting counterparts.
- B. Baek, W. H. Rippard, M. R. Pufall, S. P. Benz, S.
E. Russek, H. Rogalla, and P. D. Dresselhaus, "Spin-transfer torque switching
in nanopillar superconducting-magnetic hybrid Josephson junctions," Phys. Rev. Appl. 3, 011001 (Jan. 2015);
- B. Baek, W. H. Rippard, S. P. Benz, S. E. Russek, and
P. D. Dresselhaus, "Hybrid superconducting-magnetic memory device using
competing order parameters," Nature
Commun. 5, 3888 (May 2014); doi: 10.1038/ncomms4888.
Nanoscale Oscillators for Non-Boolean Logic
We work with the Defense
Advanced Research Projects Agency (DARPA) to support their efforts to develop spin-based
devices for low power, non-Boolean computing. Both industry and the government
are investigating alternative computer architectures to efficiently solve the
"best-match" pattern-recognition problem, which has applications in the search of
very large databases and the real-time analysis of high-definition video
streams. Pattern matching requires the calculation of the "distance" between a
test item and each item in a database, and accepting the closest matches. The high
precision of conventional complementary-metal-oxide-semiconductor (CMOS) logic
wastes energy and time when performing such imprecise calculations. A
bio-inspired alternative to this calculation is to use arrays of coupled, phase-locking,
nonlinear oscillators to perform the distance calculation in a quasi-parallel
fashion, with the relative phases of
the oscillators representing the degree of match. In effect, the physics of the
phase locking performs the distance "calculation."
To this end, we are
developing new methods understand the physics of novel, spin-based, nonlinear,
nanoscale oscillators, which are promising candidates for the large arrays
needed in such computation schemes, due to the oscillators' nanoscale size, greater
than 10 gigahertz frequencies, and inherent nonlinearities. We are exploring
ways to efficiently couple these oscillators, quantifying the nonlinearities
that mediate the coupling, and developing metrology to characterize their phase
behavior and understand the time scales over which locking occurs. We have also
started to apply these techniques to measure the properties of stochastic, spin-based,
pulsed oscillators for use as potentially more efficient relaxation oscillators
for neural networks based on "memristors."
- W. H. Rippard, M. R. Pufall, and A. B. Kos, "Time
required to injection-lock spin torque nanoscale oscillators," Appl. Phys. Lett. 103, 182403 (Oct. 2013); doi: 10.1063/1.4821179.
- M. R. Pufall, W. H. Rippard, S. E. Russek, and E. R.
Evarts, "Anisotropic frequency response of spin-torque oscillators with applied
field polarity and direction," Phys. Rev.
B 86, 094404 (Sep. 2012); doi:
- G. Csaba, M. R. Pufall, W. H. Rippard, and W. Porod,
"Modeling of coupled spin torque oscillators for applications in associative
memories," Int. Conf. Nanotech., Birmingham, U.K., Aug. 2012, IEEE, New York;
Mode-Dependent Damping Measured with Heterodyne Magneto-Optic Microwave
Future computer hard disk
drives are likely to require patterned media composed of uniform,
perpendicularly magnetized nanodots instead of present-day continuous magnetic
films. Similarly, STT-MRAM utilizes the magnetization state of a "free layer"
nanodot to encode a digital bit. The damping of gyromagnetic precession is a
critical figure of merit for the performance of both patterned media and
STT-MRAM. Determination of how the damping scales with nanomagnet size is
essential to predict future device performance. We have developed a new, highly
sensitive, magneto-optic instrument to measure the dynamics of magnetic
nanodots as a function of frequency, with the goal of evaluating nanodot
quality and homogeneity via a rapid spectroscopic analysis of individual nanodots.
measurement techniques can be applied to characterize the dynamics of other
nanoscale magnetic devices, such as the "free layer" in spin torque oscillators.
We determined that the
damping for in-plane oriented nanomagnets in the 100-400 nm size range depends strongly
on the particular spin-dynamics eigenmode that is excited: the more uniform the
eigenmode, the lower the damping. The measurements were accomplished in 2013 with
an entirely novel optical microscope system that utilizes heterodyne mixing of continuous-wave
laser beams to both generate microwaves to excite spin dynamics and to detect
the resultant dynamics via the magneto-optic Kerr effect. Since then, we have
repeated our measurements, but now with nanostructures patterned from films ranging in thickness from 3 to 15 nm.
Here, the mode-dependent damping is strongly dependent on film thickness, with
the strongest effect in the thinnest films. These results suggest that the
fundamental mechanism that causes the mode-dependent damping is driven by spin
relaxation at the ferromagnetic layer interfaces. These results have
significant implications for the write-current requirements for STT-MRAM.
Specifically, it appears that mode-uniformity is an important figure of merit
for minimization of the write current.
Tom Silva (left) and Hans Nembach examining the
heterodyne microwave magneto-optic microwave microscope.
- H. T. Nembach, J. M. Shaw, C. T. Boone, and T. J.
Silva, "Mode- and size-dependent Landau-Lifshitz damping in magnetic
nanostructures: Evidence for nonlocal damping," Phys. Rev. Lett. 110,
117201 (Mar. 2013); doi: 10.1103/PhysRevLett.110.117201.
Measurement of Spin-Orbit Torques at Microwave Frequencies
Spin-orbit torques at
ferromagnet/normal metal (FM/NM)
interfaces promise to be much more efficient as a means of switching magnetic
memory elements than spin-polarized charge currents. The reason is intrinsic to
the geometry employed for spin-orbit torques: an electric field applied
parallel to the FM/NM interface results in a flow of angular momentum across
the interface. Hence, even if the NM layer is extremely thin, the flux of
angular momentum remains constant. Since the ohmic heating scales as the square
of the charge current, this becomes a powerful means of minimizing the energy
required to switch a FM memory element.
We successfully measured
spin-orbit torques in FM/NM bilayers, where the FM is Permalloy (Ni80Fe20)
and the NM layers are Pt, Cu/Pt, Pd, Ta, Nb, and Cu/Au. We specifically measured
the inverse forms of the spin-Hall effect (iSHE)
and the Rashba-Edelstein effect (iREE)
in these systems, where microwave stimulation of the FM layer gives rise to an
ac voltage in the NM layer.
With the iSHE, the spin
chemical potential, generated by spin precession at the FM/NM interface, is
converted to a charge current. On the other hand, the iREE directly converts
the spin precession into a charge current, and with a phase that is shifted by
90 degrees relative to that of the iSHE. Microwave magnetic fields were applied with a
proximate coplanar waveguide assembly, and the generated electrical signals in
the multilayer samples were extracted by use of an electrically isolated,
terminated waveguide structure. Our measurement method employs phase-sensitive
detection to separate the two spin-orbit contributions to the measured
microwave signal. Furthermore, a calibration reference component was included
in the device design to permit separation of the spin-orbit torque
contributions from any inductive signals.
Our measurement results
indicate that the Rashba-Edelstein contribution to the net spin-orbit torque is
comparable to the spin-Hall contribution. These results will have significant
implications for the utility of employing spin-orbit torques in three-terminal
magnetic memory designs. Such designs have been proposed as a means of greatly
improving the switching efficiency of spin-transfer torque magnetic random-access
memory, a promising nonvolatile alternative to conventional CMOS cache memory in
Illustration of the inverse spin-Hall effect. A non-equilibrium
gradient in the spin accumulation along one axis gives rise to a charge current
along an orthogonal axis. The dynamic spin current Js due to the time
variation of the transverse magnetization My gives rise to an ac
electric field EiSHE along x.
- M. Weiler, J. M. Shaw, H. T. Nembach, and T. J. Silva, "Phase-sensitive
detection of spin pumping via the ac inverse spin Hall effect," Phys. Rev. Lett. 113, 157204 (Oct. 2014); doi: 10.1103/PhysRevLett.113.157204.
- M. Weiler, J. M. Shaw, H. T. Nembach, and T. J. Silva, "Detection of the
dc inverse spin Hall effect due to spin pumping in a novel meander-stripline
geometry," IEEE Magn. Lett. 5, 3700104 (Oct. 2014); doi: 10.1109/LMAG.2014.2361791.
Measurement of the Dzyaloshinskii-Moriya Interaction
Thermal fluctuations are
the main limitation to the scalability to nonvolatile magnetic memory. To
prevent thermal erasure of a bit in a magnetic memory element, it is imperative
that the magnetization be in a uniform "single-domain" state. However, a novel
physical mechanism at play in certain types of FM/NM multilayers might
compromise the uniformity of magnetization: the Dzyaloshinskii-Moriya
interaction (DMI). The DMI favors
chiral magnetization orientation rather than a uniform state, and a
sufficiently large DMI is expected to greatly reduce the thermal stability of
magnetic memory. Hence, a quantitative measurement technique is required to
characterize the DMI at different FM/NM interfaces to ensure that the
particular FM/NM combination does not result in an excessively large DMI.
With the DMI, the usual
exchange interaction is modified to favor chiral spin orientation rather than
either uniform (ferromagnetic) or antiparallel (antiferromagnetic)
configurations. The DMI requires a combination of spatial symmetry breaking and
the spin-orbit coupling. These requirements are satisfied at ferromagnet/normal
metal interfaces, such as that between Permalloy and Pt. We demonstrated the
ability to accurately measure the DMI at the Permalloy/Pt interface by use of
Brillouin light scattering (BLS).
With BLS, we use inelastic photon scattering to measure the asymmetry in the
dispersion of spin waves propagating perpendicular to the Permalloy
magnetization. Spin waves have an inherent chirality associated with the
precession of the spin orientation. For the case where the spin wave chirality
is favored by the DMI, the waves propagate faster.
It had been theoretically
predicted that BLS could be used to measure the DMI, but the effect is rather
subtle, and the measurement required several stages of data processing and
instrument calibration to avoid measurement artifacts. Nevertheless, we were
successful in measuring the DMI for films with varying Permalloy thickness, and
we were able to correlate the DMI with the exchange energy that gives rise to
ferromagnetic order in the bulk of the Permalloy film. A direct proportionality
between exchange and the DMI was originally predicted by Moriya, but had never
been confirmed until now. This result is of immediate technological relevance
for those working to develop STT-MRAM as a nonvolatile alternative to DRAM.
- H. T. Nembach, J. M. Shaw, M. A. Weiler, E. M. Jué,
and T. J. Silva, "Linear relation between Heisenberg exchange and interfacial Dzyaloshinskii–Moriya interaction in metal films," Nature Phys., online
(Aug. 2015); doi:10.1038/nphys3418.
of Epitaxial Graphene on Cu and Spin Pumping
Graphene should have outstanding properties for spintronics
applications. The predicted long spin
lifetime and spin diffusion length
should make graphene a superlative interconnect for spintronic circuits. However, the measured
lifetime is much shorter than expected and a definitive understanding of the spin transport
properties of graphene in a device environment is still lacking.
As part of our ongoing effort to develop measurement tools to
assess the viability of two-dimensional materials for spintronics applications,
we developed a chemical-vapor deposition (CVD) process for wafer-scale
growth of graphene on Cu thin films, rather than the more common Cu foils. A prerequisite for this accomplishment was
the development of a sputtering and annealing process to grow single-crystal Cu(111) films on sapphire
wafers that remained continuous during CVD at temperatures near the Cu melting
point. We used electron microscopy and low-energy electron diffraction (LEED)
to show the graphene grows epitaxially on the Cu(111) surface across length
scales of at least 3 mm.
In order to fabricate devices combining graphene and FM
materials, we developed a method for sputtering that does not damage the
graphene but also yields high quality FM films with low damping. We found that
deposition of an initial layer of 5 nm of Permalloy at grazing incidence
preserved the integrity of the graphene, as verified with Raman spectroscopy,
although the magnetic quality of the metallic film was compromised. Subsequent
deposition of more Permalloy at nearly normal incidence was sufficient to
restore the nominal magnetic properties of the Permalloy film without damaging
the graphene under the initial layer.
Transmission electron microscope cross-section image of a
Py/graphene/Cu sample with 10 nm of Py initially deposited at θ
= 90 degrees with respect to the normal axis and then 10 nm of Py deposited at θ = 30 degrees.
With this two-angle
sputtering method, we compared spin pumping from Permalloy (Ni80Fe20)
films deposited on graphene/Cu(111) substrates and on bare Cu(111) substrates.
In a spin pumping measurement, an alternating magnetic field excites spin
precession in the FM film. Relaxation of the spin excitations at the FM/NM
interface generates a pure spin current that flows from the FM into the
nonmagnetic conductor. The loss of angular momentum due to the spin current
increases the damping of the FM by an amount that is inversely proportional to
the FM thickness. We used our highly accurate ferromagnetic resonance
spectrometer to measure the damping as a function of thickness and thus
quantify the spin current generated at the interface for co-deposited
Permalloy/graphene/Cu and Permalloy/Cu samples. The bare Cu samples showed the
expected increasing in damping for the thinnest films, but this effect was
entirely absent for the graphene/Cu samples. Importantly, magnetometry on these
samples showed that the Permalloy properties were nearly identical, so the
difference in spin pumping can be attributed to the graphene itself.
Our finding that graphene prevents the flow of spin current into
the underlying Cu is consistent with the known contact resistance between
graphene and Cu. This result opens the door to fabrication of graphene
spintronic devices without the need to transfer the graphene to another
substrate, thus avoiding a major source of contamination in graphene devices.
This may be a key step toward realizing graphene's potential as a spin transfer
Damping of Permalloy (Py)
films deposited on Cu and graphene/Cu substrates. The Py/Cu films show the
expected increase in damping for very thin films due to a pure spin current
pumped from the Py and absorbed in the Cu. The absence of this trend in the
Py/graphene/Cu films indicates the graphene monolayer blocks spin pumping by
preventing the spin current from reaching the Cu.
- W. Gannett, M. W. Keller, H. T. Nembach, T. J. Silva, and
A. N. Chiaramonti, "Suppression of spin pumping between Ni80Fe20
and Cu by a graphene interlayer," J.
Appl. Phys. 117, 213907 (June
2015); doi: 10.1063/1.4921425.
D. L. Miller, M. W. Keller, J. M. Shaw, K. P. Rice, R.
R. Keller, and K. M. Diederichsen, "Giant secondary grain growth in Cu films on
sapphire," AIP Advances 3, 082105 (Aug. 2013); doi: 10.1063/1.4817829.
Micromagnetic simulations showing the spin-wave interaction between two spin-transfer oscillators. Each device emits spin-waves towards the other, causing the devices to synchronize.
Lead Organizational Unit:
Nanoscale Spin Dynamics
Ted StaufferOffice Manager