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Spin Electronics and Nanoscale Spin Dynamics


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.

The 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 communications.

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.


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.

Major Accomplishments:

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 devices
Bill Rippard (left) and Eric Evarts measuring error rates in STT-MRAM devices.


  • 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 Computing

Large-scale computing 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 (consuming femtojoules).

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); doi: 10.1103/PhysRevApplied.3.011001.
  • 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: 10.1103/PhysRevB.86.094404.
  • 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; doi: 10.1109/NANO.2012.6322201.

Mode-Dependent Damping Measured with Heterodyne Magneto-Optic Microwave Microscope

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. The same 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 
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 system-on-chip applications.

Illustration of the inverse spin-Hall effect
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.

Wafer-scale Growth 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 Permalloy-graphene-copper sample
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 medium.

Damping of Permalloy (Py) films deposited on Cu and graphene-Cu substrates
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. 


Coupled spin-torque oscillators.
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.                                     

End Date:


Lead Organizational Unit:



Spin Electronics

Bill Rippard
Burm Baek
Eric Evarts
Emilie Jué
Matt Pufall
Mike Schneider

Nanoscale Spin Dynamics
Tom Silva
Mark Keller
Hans Nembach
Javier Pulecio
Justin Shaw

Magnetics Group
Ron Goldfarb
Tony Kos
Ted Stauffer

Office Manager
Mildred Obermiller