News

-Sub-nanosecond switching in a cryogenic spin-torque spin-valve memory element with a dilute permalloy free layer, L. Rehm, V. Sluka, G. E. Rowlands, M.-H. Nguyen, T. A. Ohki, and A. D. Kent, Appl. Phys. Lett 114, 212402 (2019).
We present a study of pulsed current switching characteristics of spin-valve nanopillars with in-plane magnetized dilute permalloy and undiluted permalloy free layers in the ballistic regime at low temperatures. The dilute permalloy free layer device switches much faster: the characteristic switching time for a permalloy (Ni0.83Fe0.17) free layer device is 1.18 ns, while that for a dilute permalloy ([Ni0.83Fe0.17]0.6Cu0.4) free layer device is 0.475 ns. A ballistic macrospin model can capture the data trends with a reduced spin-torque asymmetry parameter, reduced spin polarization, and increased Gilbert damping for the dilute permalloy free layer relative to the permalloy devices. Our study demonstrates that reducing the magnetization of the free layer increases the switching speed while greatly reducing the switching energy and shows a promising route toward even lower power magnetic memory devices compatible with superconducting electronics.
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-Multiple magnetic droplet soliton modes, Nahuel Statuto, Christian Hahn, Joan Manel Hernàndez, Andrew D. Kent, and Ferran Macià, Phys. Rev. B 99, 174436 (2019).
Droplet solitons are large amplitude localized spin-wave excitations that can be created in magnetic thin films with uniaxial anisotropy by a spin-polarized current flowing through an electrical nanocontact. Here, we report a low-temperature (4 K) experimental study that shows there are multiple and, under certain conditions, combinations of droplet modes, each mode with a distinct high-frequency spin precession (tens of gigahertz). Low-frequency (≲1 GHz) voltage noise is used to assess the stability of droplet modes. It is found that droplets are stable only in a limited range of applied field and current, typically near the current and field at which they nucleate, in agreement with recent predictions. Applied fields in the film plane favor multiple droplet modes, whereas fields perpendicular to the film plane tend to stabilize a single droplet mode. Micromagnetic simulations are used to show that spatial variation in the energy landscape in the nanocontact region (e.g., spatial variation of magnetic anisotropy or magnetic field) can lead to quantized droplet modes and low-frequency mode modulation, characteristics observed in our experiments.
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-Voltage-Controlled Topological Spin Switch for Ultralow-Energy Computing: Performance Modeling and Benchmarking, Shaloo Rakheja, Michael E. Flatté, and Andrew D. Kent, Physical Review Applied 11, 054009 (2019).
A voltage-controlled topological spin switch (VTOPSS) that uses a hybrid topological insulator– magnetic insulator multiferroic material is presented that can implement Boolean logic operations with sub-10-aJ energy per bit and an energy-delay product on the order of 10−26 J s. The device uses a topological insulator, which has the highest efficiency of conversion of the electric field to spin torque yet observed at room temperature, and a low-moment magnetic insulator that can respond rapidly to a given spin torque. We present the theory of operation of the VTOPSS, develop analytic models of its performance metrics, elucidate performance scaling with dimensions and voltage, and benchmark the VTOPSS against existing spin-based and CMOS devices. Compared with existing spin-based devices, such as allspin logic and charge-spin logic devices, the VTOPSS offers 10–70 times lower energy dissipation and 70–1700 times lower energy-delay product. With experimental advances and improved material properties, we show that the energy and energy-delay product of the VTOPSS can be lowered to a few attojoules per bit and 10−28 J s, respectively. As such, the VTOPSS technology offers competitive metrics compared with existing CMOS technology. Finally, we establish that interconnect issues that dominate the performance in CMOS logic are relatively less significant for the VTOPSS, implying that highly resistive materials can indeed be used to interconnect VTOPSS devices.
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-Asymmetric Magnetization Switching in Perpendicular Magnetic Tunnel Junctions: Role of the Synthetic Antiferromagnet’s Fringe Field, M. Lavanant, P. Vallobra, S. Petit Watelot, V. Lomakin, A.D. Kent, J. Sun, and S. Mangin, Physical Review Applied 11, 034058 (2019).
The field- and current-induced magnetization reversal of a Co-Fe-B layer in a perpendicular magnetic tunnel junction (pMTJ) is studied at room temperature. The magnetization switching probability from the parallel (P) state to the antiparallel (AP) state and from AP to P is found to be asymmetric. We observe that this asymmetry depends on the magnetic configuration of the synthetic antiferromagnetic (SAF). The state diagram and the energy landscape are compared for two SAF configurations. We conclude that the asymmetry is due to the inhomogeneity of the fringe field generated by the SAF. Our study highlights the role of the reference-layer fringe field on the free layer’s energy barrier height, which has to be carefully controlled for memory applications.
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-A cryogenic spin-torque memory element with precessional magnetization dynamics, G. E. Rowlands, C.A. Ryan, L.Ye, L. Rehm, D. Pinna, A. D. Kent and T.A. Ohki, Scientific Reports 9, 803 (2019).
We present a study of precessional magnetization switching in orthogonal spin-torque spin-valve devices at low temperatures. The samples consist of a spin-polarizing layer that is magnetized outof-the flm plane and an in-plane magnetized free and reference magnetic layer separated by nonmagnetic metallic layers. We fnd coherent oscillations in the switching probability, characterized by high speed switching (~200ps), error rates as low as 10−5 and decoherence efects at longer timescales (~1 ns). Our study, which is conducted over a wide range of parameter space (pulse amplitude and duration) with deep statistics, demonstrates that the switching dynamics are likely dominated by the action of the out-of-plane spin polarization, in contrast to in-plane spin-torque from the reference layer, as has been the case in most previous studies. Our results demonstrate that precessional spin-torque devices are well suited to a cryogenic environment, while at room temperature they have so far not exhibited coherent or reliable switching.
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-Increased energy efficiency spin-torque switching of magnetic tunnel junction devices with a higher order perpendicular magnetic anisotropy, Marion Lavanant, Sebastien Petit-Watelot, Andrew D. Kent, and Stephane Mangin, Applied Physics Letters 114, 012404 (2019).
We study the influence of a second order magnetic anisotropy on magnetization reversal by spin transfer torque in perpendicularly magnetized magnetic tunnel junctions (pMTJs). Using a macrospin model to describe the dynamics of the free layer, analytical solutions for the switching voltage and the voltage threshold for precession are determined as a function of the first and second order magnetic anisotropies. To compare the spin-transfer-torque energy efficiency to that of a classical pMTJ, a junction without the second order anisotropy term, we compare these cases at a fixed energy barrier to thermally activated reversal. We show that the critical voltage for switching can be reduced by a factor 0.7 when the ratio of the second to the first order magnetic anisotropy is 1/3. Importantly, the switching time can be reduced by nearly a factor of two for this magnetic anisotropy ratio. These results highlight an important and practical method to increase the spin-torque efficiency, while reducing the energy dissipation and switching time in magnetic random access memory devices.
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-First harmonic measurements of the spin Seebeck effect, Yizhang Chen, Debangsu Roy, Egecan Cogulu, Houchen Chang, Mingzhong Wu, and Andrew D. Kent, Applied Physics Letters 113, 202403 (2018).
We present measurements of the spin Seebeck effect (SSE) by a technique that combines alternating currents (AC) and direct currents (DC). The method is applied to a ferrimagnetic insulator/heavy metal bilayer, Y3Fe5O12 (YIG)/Pt. Typically, SSE measurements use an AC current to produce an alternating temperature gradient and measure the voltage generated by the inverse spin-Hall effect in the heavy metal at twice the AC frequency. Here, we show that when Joule heating is associated with AC and DC bias currents, the SSE response occurs at the frequency of the AC current drive and can be larger than the second harmonic SSE response. We compare the first and second harmonic responses and show that they are consistent with the SSE. The field dependence of the voltage response is used to distinguish between the damping-like and field-like torques. This method can be used to explore nonlinear thermoelectric effects and spin dynamics induced by temperature gradients.
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-Ferromagnetic resonance linewidth in coupled layers with easy-plane and perpendicular magnetic anisotropies, Jun-Wen Xu, Volker Sluka, Bartek Kardasz, Mustafa Pinarbasi, and Andrew D. Kent, Journal of Applied Physics 124, 063902 (2018).
Magnetic bilayers with different magnetic anisotropy directions are interesting for spintronic applications as they offer the possibility to engineer tilted remnant magnetization states. We investigate the ferromagnetic resonance (FMR) linewidth of modes associated with two interlayer exchange-coupled ferromagnetic layers, the first a CoNi multilayer with a perpendicular magnetic anisotropy, and the second a CoFeB layer with an easy-plane anisotropy. For antiferromagnetic interlayer exchange coupling, a local maximum in the FMR linewidths is observed below a characteristic field. This is in contrast to what is found in uncoupled, ferromagnetically coupled and single ferromagnetic layers in which the FMR linewidth increases monotonically with field. We show that the existence of the local maximum as well as the characteristic field below which there is a dramatic increase in FMR linewidth can be understood using a macrospin model with Heisenberg-type exchange coupling between the layers.
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-Generation and stability of dynamical skyrmions and droplet solitons, Nahuel Statuto, Joan Manel Hernàndez, Andrew D Kent and Ferran Macià, Nanotechnology 29, 325302 (2018).
A spin-polarized current in a nanocontact to a magnetic film can create collective magnetic oscillations by compensating the magnetic damping. In particular, in materials with uniaxial magnetic anisotropy, droplet solitons have been observed—a self-localized excitation consisting of partially reversed magnetization that precesses coherently in the nanocontact region. It is also possible to generate topological droplet solitons, known as dynamical skyrmions (DSs). Here, we show that spin-polarized current thresholds for DS creation depend not only on the material’s parameters but also on the initial magnetization state and the rise time of the spin-polarized current. We study the conditions that promote either droplet or DS formation and describe their stability in magnetic films without Dzyaloshinskii–Moriya interactions. The Oersted fields from the applied current, the initial magnetization state, and the rise time of the injected current can determine whether a droplet or a DS forms. DSs are found to be more stable than droplets. We also discuss electrical characteristics that can be used to distinguish these magnetic objects.
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-Generation and annihilation time of magnetic droplet solitons, Jinting Hang, Christian Hahn, Nahuel Statuto, Ferran Macià & Andrew D. Kent, Scientific Reports 8, 6847 (2018).
Magnetic droplet solitons were first predicted to occur in materials with uniaxial magnetic anisotropy due to a long-range attractive interaction between elementary magnetic excitations, magnons. A non-equilibrium magnon population provided by a spin-polarized current in nanocontacts enables their creation and there is now clear experimental evidence for their formation, including direct images obtained with scanning x-ray transmission microscopy. Interest in magnetic droplets is associated with their unique magnetic dynamics that can lead to new types of high frequency nanometer scale oscillators of interest for information processing, including in neuromorphic computing. However, there are no direct measurements of the time required to nucleate droplet solitons or their lifetime– experiments to date only probe their steady-state characteristics, their response to dc spin-currents. Here we determine the timescales for droplet annihilation and generation using current pulses. Annihilation occurs in a few nanoseconds while generation can take several nanoseconds to a microsecond depending on the pulse amplitude. Micromagnetic simulations show that there is an incubation time for droplet generation that depends sensitively on the initial magnetic state of the nanocontact. An understanding of these processes is essential to utilizing the unique characteristics of magnetic droplet solitons oscillators, including their high frequency, tunable and hysteretic response.
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-Tunable magnetic textures: From Majorana bound states to braiding, Alex Matos-Abiaguea, Javad Shabani, Andrew D. Kent, Geoffrey L. Fatin, Benedikt Scharf, Igor Žutić, Solid State Communications 262, 1-6 (2017).
A versatile control of magnetic systems, widely used to store information, can also enable manipulating Majorana bounds states (MBS) and implementing fault-tolerant quantum information processing. The proposed platform relies on the proximity-induced superconductivity in a two-dimensional electron gas placed next to an array of magnetic tunnel junctions (MTJs). A change in the magnetization configuration in the MTJ array creates tunable magnetic textures thereby removing several typical requirements for MBS: strong spinorbit coupling, applied magnetic field, and confinement by one-dimensional structures which complicates demonstrating non-Abelian statistics through braiding. Recent advances in fabricating two-dimensional epitaxial superconductor/semiconductor heterostructures and designing tunable magnetic textures support the feasibility of this novel platform for MBS.
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-Effect of Temperature on Magnetic Solitons Induced by Spin-Transfer Torque, Sergi Lendínez, Jinting Hang, Saül Vélez, Joan Manel Hernández, Dirk Backes, Andrew D. Kent, and Ferran Macià, Physical Review Applied 7, 054027 (2017).
Spin-transfer torques in a nanocontact to an extended magnetic film can create spin waves that condense to form dissipative droplet solitons. Here we report an experimental study of the temperature dependence of the current and applied field thresholds for droplet soliton formation, as well as the nanocontact’s electrical characteristics associated with droplet dynamics. Nucleation requires lower current densities at lower temperatures, in contrast to typical spin-transfer-torque-induced switching between static magnetic states. Magnetoresistance and electrical noise measurements (10 MHz-1 GHz) show that droplet solitons become more stable at lower temperature. These results are of fundamental interest in understanding the influence of thermal noise on droplet solitons and have implications for the design of devices using the spin-transfertorque effects to create and control collective spin excitations.
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-Spin transport in antiferromagnetic NiO and magnetoresistance in Y3Fe5O12/NiO/Pt structures, Yu-Ming Hung, Christian Hahn, Houchen Chang, Mingzhong Wu, Hendrik Ohldag, and Andrew D. Kent, AIP Advances 7, 055903 (2017).
We have studied spin transport and magnetoresistance in yttrium iron garnet (YIG)/NiO/Pt trilayers with varied NiO thickness. To characterize the spin transport through NiO we excite ferromagnetic resonance in YIG with a microwave frequency magnetic field and detect the voltage associated with the inverse spin- Hall effect (ISHE) in the Pt layer. The ISHE signal is found to decay exponentially with the NiO thickness with a characteristic decay length of 3.9 nm. This is contrasted with the magnetoresistance in these same structures. The symmetry of the magnetoresistive response is consistent with spin-Hall magnetoresistance (SMR). However, in contrast to the ISHE response, as the NiO thickness increases the SMR signal goes towards zero abruptly at a NiO thickness of ≃ 4 nm, highlighting the different length scales associated with the spin-transport in NiO and SMR in such trilayers.
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-Time-resolved studies of the spin-transfer reversal mechanism in perpendicularly magnetized magnetic tunnel junctions, Christian Hahn, Georg Wolf, Bartek Kardasz, Steve Watts, Mustafa Pinarbasi, and Andrew D. Kent, Physical Review B 94, 214432 (2016).
Pulsed spin-torque switching has been studied using single-shot time-resolved electrical measurements in perpendicularly magnetized magnetic tunnel junctions as a function of pulse amplitude and junction size in 50 to 100nm-diameter diameter circular junctions. The mean switching time depends inversely on pulse amplitude for all junctions studied. However, the switching dynamics is found to be strongly dependent on junction size and pulse amplitude. In 50-nm-diameter junctions the switching onset is stochastic but the switching once started, is fast; after being initiated it takes less than 2 ns to switch. In larger diameter junctions the time needed for complete switching is strongly dependent on the pulse amplitude, reaching times less than 2 ns at large pulse amplitudes. Anomalies in the switching rate versus pulse amplitude are shown to be associated with the long-lived (>2ns) intermediate junction resistance states.
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-Domain wall fringe field coupled spin logic, Yu-Ming Hung and Andrew D. Kent, AIP Advances 6, 125118 (2016).
A class of spin logic devices based on the spin-orbit induced spin-transfer torques requires magnetic coupling between electrically isolated ferromagnetic elements. Here we use micromagnetic modeling to study the magnetic coupling induced by fringe fields from chiral domainwalls in perpendicularly magnetized nanowires. These domains can be displaced using spin-orbit torques from a proximal heavy metal layer. For a 16 nm width wire that is 1 nm thick, we find that spin-orbit torques induced domain wall propagation can reliably switch a proximal 16 nm diameter 1 nm thick nanomagnet. These results show a promising means of implementing spin logic with spin-orbit torques using elements with perpendicular magnetization, which does not require an applied magnetic field.
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-Interlayer exchange coupling between layers with perpendicular and easy-plane magnetic anisotropies, Lorenzo Fallarino, Volker Sluka, Bartek Kardasz, Mustafa Pinarbasi, Andreas Berger, and Andrew D. Kent, Applied Physical Letters 109, 082401 (2016).
Interlayer exchange coupling between layers with perpendicular and easy-plane magnetic anisotropies separated by a non-magnetic spacer is studied using ferromagnetic resonance. The samples consist of a Co/Ni multilayer with perpendicular magnetic anisotropy and a CoFeB layer with easy-plane anisotropy separated by a variable thickness Ru layer. At a fixed frequency, we show that there is an avoided crossing of layer ferromagnetic resonance modes providing direct evidence for interlayer coupling. The mode dispersions for different Ru thicknesses are fit to a Heisenbergtype model to determine the interlayer exchange coupling strength and layer properties. The resulting interlayer exchange coupling varies continuously from antiferromagnetic to ferromagnetic as a function of the Ru interlayer thickness. These results show that the magnetic layer single domain ground state consists of magnetizations that can be significantly canted with respect to the layer planes and the canting can be tuned by varying the Ru thickness and the layer magnetic characteristics, a capability of interest for applications in spin-transfer torque devices.
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-Reliable spin-transfer torque driven precessional magnetization reversal with an adiabatically decaying pulse, D. Pinna, C. A. Ryan, T. Ohki, and A. D. Kent, Physical Review B 93, 184412 (2016).
We show that a slowly decaying current pulse can lead to nearly deterministic precessional switching in the presence of noise. We consider a biaxial macrospin, with an easy axis in-plane and a hard axis out-of-plane, typical of thin film nanomagnets patterned into asymmetric shapes. Out-of-plane precessional magnetization orbits are excited with a current pulse with a component of spin polarization normal to the film plane. By numerically integrating the stochastic Landau-Lifshitz-Gilbert-Slonczewski equation we show that thermal noise leads to strong dephasing of the magnetization orbits. However, an adiabatically decreasing pulse amplitude overwhelmingly leads to magnetization reversal, with a final state dependent on the pulse polarity. We develop an analytic model to explain this phenomena and to determine the pulse decay time necessary for adiabatic magnetization relaxation and thus deterministic magnetization switching.
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-Towards a six-state magnetic memory element, Yevgeniy Telepinsky, Vladislav Mor, Moty Schultz, Yu-Ming Hung, Andrew D. Kent, and Lior Klein, Applied Physics Letters 108, 182401 (2016).
We pattern permalloy films into three crossing elongated ellipses with an angle of 60! between the major axes of any pair of ellipses. Planar Hall effect measurements show that the magnetization in the area of overlap of the ellipses has six stable magnetic orientations parallel to the major axes of the three ellipses. We determine the effective anisotropy field for small magnetic deviations from the easy axis and the switching field between the easy axes as a function of magnetic field orientation. We compare our results with micromagnetic simulations and present an effective Hamiltonian that captures the magnetic response. We show how such magnetic structures in a magnetic tunnel junction would result in a magnetic memory element with six distinct resistance states that could be written using spin-orbit torques.
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-Observation of droplet soliton drift resonances in a spin-transfer-torque nanocontact to a ferromagnetic thin film, S. Lendínez, N. Statuto, D. Backes, A. D. Kent, and F. Macià, Physics Review B 92, 174426 (2015).
Magnetic droplet solitons are nonlinear dynamical modes that can be excited in a thin film with perpendicular magnetic anisotropy with a spin-transfer torque. Although droplet solitons have been proved to be stable with a hysteretic response to applied currents and magnetic fields at lowtemperature,measurements at room temperature indicate less stability and reduced hysteresis width. Here, we report evidence of droplet soliton drift instabilities, leading to drift resonances, at room temperature that explains their lower stability. Micromagnetic simulations show that the drift instability is produced by an effective-field asymmetry in the nanocontact region that can have different origins.
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-Direct Observation of a Localized Magnetic Soliton in a Spin-Transfer Nanocontact, D. Backes, F. Macià, S. Bonetti, R. Kukreja, H. Ohldag, and A. D. Kent, Physical Review Letters 115, 127205 (2015).
We report the direct observation of a localized magnetic soliton in a spin-transfer nanocontact using scanning transmission x-ray microscopy. Experiments are conducted on a lithographically defined 150 nm diameter nanocontact to an ultrathin ferromagnetic multilayer with perpendicular magnetic anisotropy. Element-resolved x-ray magnetic circular dichroism images show an abrupt onset of a magnetic soliton excitation localized beneath the nanocontact at a threshold current. However, the amplitude of the excitation ≃25° at the contact center is far less than that predicted (⪅180°), showing that the spin dynamics is not described by existing models.
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NYU News: “Physicists Catch a Magnetic Wave that Offers Promise for More Energy-Efficient Computing”

-X-ray Detection of Transient Magnetic Moments Induced by a Spin Current in Cu, R. Kukreja, S. Bonetti, Z. Chen, D. Backes, Y. Acremann, J. A. Katine, A. D. Kent, H. A. Dürr, H. Ohldag, and J. Stöhr, Physical Review Letters 115, 096601 (2015).
We have used a MHz lock-in x-ray spectromicroscopy technique to directly detect changes in magnetic moment of Cu due to spin injection from an adjacent Co layer. The elemental and chemical specificity of x rays allows us to distinguish two spin current induced effects.We detect the creation of transient magnetic moments of 3 × 10−5μB on Cu atoms within the bulk of the 28 nm thick Cu film due to spin accumulation. The moment value is compared to predictions by Mott’s two current model. We also observe that the hybridization induced existing magnetic moments at the Cu interface atoms are transiently increased by about 10% or 4 × 10−3μB per atom. This reveals the dominance of spin-torque alignment over Joule heat induced disorder of the interfacial Cu moments during current flow.
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Physics News and Commentary: X Rays Expose Transient Spins.

-Thermal Stability of Magnetic States in Circular Thin-Film Nanomagnets with Large Perpendicular Magnetic Anisotropy, Gabriel D. Chaves-O‘Flynn, Georg Wolf, Jonathan Z. Sun, and Andrew D. Kent, Physical Review Applied 4, 024010 (2015).
The scaling of the energy barrier to magnetization reversal in thin-film nanomagnets with perpendicular magnetization as a function of their lateral size is of great current interest for high-density magnetic random-access memory devices. Here we determine the micromagnetic states that set the energy barrier to thermally activated magnetization reversal of circular thin-film nanomagnets with large perpendicular magnetic anisotropy. We find a critical length in the problem that is set by the exchange and effective perpendicular magnetic anisotropy energies, with the latter including the size dependence of the demagnetization energy. For diameters smaller than this critical length, the reversal occurs by nearly coherent magnetization rotation and the energy barrier scales with the square of the diameter normalized to the critical length (for fixed film thickness), while for larger diameters, the transition state has a domain wall, and the energy barrier depends linearly on the normalized diameter. Simple analytic expressions are derived for these two limiting cases and verified using full micromagnetic simulations with the string method. Further, the effect of an applied field is considered and shown to lead to a plateau in the energy barrier versus diameter dependence at large diameters.
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-Quasistatic and Pulsed Current-Induced Switching With Spin-Orbit Torques in Ultrathin Films With Perpendicular Magnetic Anisotropy, Yu-Ming Hung, Laura Rehm, Georg Wolf, and Andrew D. Kent, IEEE Magnetics Letters 6, 3000504 (2015).
Spin-orbit interaction derived spin torques provide a means of reversing the magnetization of perpendicularly magnetized ultrathin films with currents that flow in the plane of the layers. A basic and critical question for applications is the speed and efficiency of switching with nanosecond current pulses. Here, we investigate and contrast the quasistatic (slowly swept current) and pulsed current-induced switching characteristics of micrometer scale Hall crosses consisting of very thin (<1 nm) perpendicularly magnetized CoFeB layers on β-Ta. While complete magnetization reversal occurs at a threshold current density in the quasistatic case, short duration (≤10 ns) larger amplitude pulses (≃10 times the quasistatic threshold current) lead to only partial magnetization reversal and domain formation. We associate the partial reversal with the limited time for reversed domain expansion during the pulse.
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-State diagram of an orthogonal spin transfer spin valve device, Li Ye, Georg Wolf, Daniele Pinna, Gabriel D. Chaves-O’Flynn, and Andrew D. Kent, Journal of Applied Physics 117, 193902 (2015).
We present the switching characteristics of a spin-transfer device that incorporates a perpendicularly magnetized spin-polarizing layer with an in-plane magnetized free and fixed magnetic layer, known as an orthogonal spin transfer spin valve device. This device shows clear switching between parallel (P) and antiparallel (AP) resistance states and the reverse transition (AP!P) for both current polarities. Further, hysteretic transitions are shown to occur into a state with a resistance intermediate between that of the P and AP states, again for both current polarities. These unusual spin-transfer switching characteristics can be explained within a simple macrospin model that incorporates thermal fluctuations and considers a spin-polarized current that is tilted with respect to the free layer’s plane, due to the presence of the spin-transfer torque from the polarizing layer.
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-A new spin on magnetic memories, Andrew D. Kent and Daniel C. Worledge, Nature Nanotechnology 10, 187-191 (2015).
Solid-state memory devices with all-electrical read and write operations might lead to faster, cheaper information storage.
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NYU News: “Magnetic Memory Attracts Promise for Faster and More Energy Efficient Information Storage.”

-Micromagnetic study of spin transfer switching with a spin polarization tilted out of the free layer plane, Gabriel D. Chaves-O’Flynn, Georg Wolf, Daniele Pinna, and Andrew D. Kent, Journal of Applied Physics 117, 17D705 (2015).
We present the results of zero temperature macrospin and micromagnetic simulations of spin transfer switching of thin film nanomagnets in the shape of an ellipse with a spin-polarization tilted out of the layer plane. The perpendicular component of the spin-polarization is shown to increase the reversal speed, leading to a lower current for switching in a given time. However, for tilt angles larger than a critical angle, the layer magnetization starts to precess about an out-of-plane axis, which leads to a final magnetization state that is very sensitive to simulation conditions. As the ellipse lateral size increases, this out-of-plane precession is suppressed, due to the excitation of spatially non-uniform magnetization modes.
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-Nonlocal transport mediated by spin supercurrents, Hua Chen, Andrew D. Kent, Allan H. MacDonald, and Inti Sodemann, Physical Review B 90, 220401 (2014).
In thin-film ferromagnets with perfect easy-plane anisotropy, the component of total spin perpendicular to the easy plane is a good quantum number and the corresponding spin supercurrent can flow without dissipation. Here we explain how spin supercurrents couple spatially remote spin-mixing vertical transport channels, even when easy-plane anisotropy is imperfect, and discuss the possibility that this effect can be used to fabricate new types of electronic devices.
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-Stable magnetic droplet solitons in spin-transfer nanocontacts , Ferran Macià, Dirk Backes and Andrew D. Kent, Nature Nanotechnology 10, 1038 (2014).
Magnetic thin films with perpendicular magnetic anisotropy have localized excitations that correspond to reversed, dynamically precessing magnetic moments, which are known as magnetic droplet solitons. Fundamentally, these excitations are associated with an attractive interaction between elementary spin-excitations and have been predicted to occur in perpendicularly magnetized materials in the absence of damping. Although damping suppresses these excitations, it can be compensated by spin-transfer torques when an electrical current flows in nanocontacts to ferromagnetic thin films. Theory predicts the appearance of magnetic droplet solitons in nanocontacts at a threshold current and, recently, experimental signatures of droplet nucleation have been reported6. However, to date, these solitons have been observed to be nearly reversible excitations, with only partially reversed magnetization. Here, we show that magnetic droplet solitons exhibit a strong hysteretic response in field and current, proving the existence of bistable states: droplet and non-droplet states. In the droplet soliton state we find that the magnetization in the contact is almost fully reversed. These observations, in addition to their fundamental interest, are important to understanding and controlling droplet motion, nucleation and annihilation.
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NYU News: “Researchers Create and Control Spin Waves, Lifting Prospects for Enhanced Information Processing”

-Spin-torque oscillators with thermal noise: A constant energy orbit approach, D. Pinna, D. L. Stein, and A. D. Kent, Physical Review B 90, 174405 (2014).
We study the magnetization dynamics of spin-torque oscillators in the presence of thermal noise and as a function of the spin-polarization angle in a macrospin model. The macrospin has biaxial magnetic anisotropy, typical of thin film magnetic elements, with an easy axis in the film plane and a hard axis out of the plane. Using a method that averages the energy over precessional orbits, we derive analytic expressions for the current that generates and sustains out-of-plane precessional states. We find that there is a critical angle of the spin polarization necessary for the occurrence of such states and predict a hysteretic response to applied current. This model can be tested in experiments on orthogonal spin-transfer devices, which consist of both an in-plane and out-of-plane magnetized spin polarizers, effectively leading to an angle between the easy and spin-polarization axes.
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-Switching field distributions with spin transfer torques in perpendicularly magnetized spin-valve nanopillars, D. B. Gopman, D. Bedau, S. Mangin, E. E. Fullerton, J. A. Katine, and A. D. Kent, Physical Review B 89, 134427 (2014).

These experiments confirm the extent of applicability of an effective-barrier/temperature model for understanding the thermal stability of a nanopillar’s magnetization under spin-transfer torques.
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-Organic magnetoelectroluminescence for room temperature transduction between magnetic and optical information, Ferran Macià, Fujian Wang, Nicholas J. Harmon, Andrew D. Kent, Markus Wohlgenannt and Michael E. Flatté, Nature Communications 5, 3609 (2014).
Magnetic and spin-based technologies for data storage and processing provide unique challenges for information transduction to light because of magnetic metals’ optical loss, and the inefficiency and resistivity of semiconductor spin-based emitters at room temperature. Transduction between magnetic and optical information in typical organic semiconductors poses additional challenges, as the spin–orbit interaction is weak and spin injection from magnetic electrodes has been limited to low temperature and low polarization efficiency. Here we demonstrate room temperature information transduction between a magnet and an organic light-emitting diode that does not require electrical current, based on control via the magnet’s remanent field of the exciton recombination process in the organic semiconductor. This demonstration is explained quantitatively within a theory of spin-dependent exciton recombination in the organic semiconductor, driven primarily by gradients in the remanent fringe fields of a few nanometre-thick magnetic film.
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NYU News: “One Word for How NYU Physicists Are Setting Stage for Future Computers: Plastics”

-Partial spin reversal in magnetic deflagration, S. Vélez, P. Subedi, F. Macìa, S. Li, M. P. Sarachik, J. Tejada, S. Mukherjee, G. Christou, and A. D. Kent, Physical Review B 89, 144408 (2014).
The reversal of spins in a magnetic material as they relax toward equilibrium is accompanied by the release of Zeeman energy, which can lead to accelerated spin relaxation and the formation of a well-defined self-sustained propagating spin-reversal front known as magnetic deflagration. To date, studies of Mn12-acetate single crystals have focused mainly on deflagration in large longitudinal magnetic fields, and they found a fully spin-reversed final state.We report a systematic study of the effect of a transverse magnetic field on magnetic deflagration, and we demonstrate that in small longitudinal fields the final state consists of only partially reversed spins. Further, we measured the front speed as a function of applied magnetic field. The theory of magnetic deflagration, together with a modification that takes into account partial spin reversal, fits the transverse field dependence of the front speed but not its dependence on the longitudinal field. The most significant result of this study is the finding of a partially spin-reversed final state, which is evidence that the spins at the deflagration front are also only partially reversed.
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-Dynamics of spin torque switching in all-perpendicular spin valve nanopillars, H. Liu, D.Bedau, J.Z.Sun, S.Mangin, E.E.Fullerton, J.A.Katine, A.D.Kent, Journal of Magnetism and Magnetic Materials 358-359, 233-258 (2014).
We present a systematic experimental study of the spin-torque-induced magnetic switching statistics at room temperature, using all-perpendicularly magnetized spin-valves as a model system. Three physical regimes are distinguished: a short-time ballistic limit below a few nanoseconds, where spin-torque dominates the reversal dynamics from a thermal distribution of initial conditions; a long time limit, where the magnetization reversal probability is determined by spin-torque-amplified thermal activation; and a cross-over regime,where the spin-torque and thermal agitation both contribute. For a basic quantitative understanding of the physical processes involved, an analytical macrospin model is presented which contains both spin-torque dynamics and finite temperature effects. The latter was treated rigorously using a Fokker–Plank formalism, and solved numerically for specific sets of parameters relevant to the experiments to determine the switching probability behavior in the short-time and cross-over regimes. This analysis shows that thermal fluctuations during magnetization reversal greatly affect the switching probability over all the time scales studied, even in the short-time limit.
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-Spin-transfer switching of orthogonal spin-valve devices at cryogenic temperatures, L. Ye, D. B. Gopman, L. Rehm, D. Backes, G. Wolf, T. Ohki, A. F. Kirichenko, I. V. Vernik, O. A. Mukhanov, and A. D. Kent, Journal of Applied Physics 115, 17C725 (2014).
We present the quasi-static and dynamic switching characteristics of orthogonal spin-transfer devices incorporating an out-of-plane magnetized polarizing layer and an in-plane magnetized spin valve device at cryogenic temperatures. Switching at 12K between parallel and anti-parallel spinvalve states is investigated for slowly varied current as well as for current pulses with durations as short as 200 ps. We demonstrate 100% switching probability with current pulses 0.6 ns in duration. We also present a switching probability diagram that summarizes device switching operation under a variety of pulse durations, amplitudes, and polarities.
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-Bimodal switching field distributions in all-perpendicular spin-valve nanopillars, D. B. Gopman, D. Bedau, S. Mangin, E. E. Fullerton, J. A. Katine, and A. D. Kent, Journal of Applied Physics 115, 17C707 (2014).
Switching field measurements of the free layer element of 75 nm diameter spin-valve nanopillars reveal a bimodal distribution of switching fields at low temperatures (below 100 K). This result is inconsistent with a model of thermal activation over a single perpendicular anisotropy barrier. The correlation between antiparallel to parallel and parallel to antiparallel switching fields increases to nearly 50% at low temperatures. This reflects random fluctuation of the shift of the free layer hysteresis loop between two different magnitudes, which may originate from changes in the dipole field from the polarizing layer. The magnitude of the loop shift changes by 25% and is correlated to transitions of the spin-valve into an antiparallel configuration.
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-Spin wave excitation patterns generated by spin torque oscillators, F. Macia, F. C. Hoppensteadt and A. D. Kent, Nanotechnology 25, 045303 (2014).
Spin torque nano-oscillators (STNO) are nanoscale devices that can convert a direct current into short wavelength spin wave excitations in a ferromagnetic layer. We show that arrays of STNO can be used to create directional spin wave radiation similarly to electromagnetic antennas. Combining STNO excitations with planar spin waves also creates interference patterns. We show that these interference patterns are static and have information on the wavelength and phase of the spin waves emitted from the STNO. We describe a means of actively controlling spin wave radiation patterns with the direct current flowing through STNO, which is useful in on-chip communication and information processing and could be a promising technique for studying short wavelength spin waves in different materials.
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-Thermally assisted spin-transfer Torque dynamics in energy space, D. Pinna, A. D. Kent and D. L. Stein, Physical Review B 88, 1004405 (2013).
We consider the general Landau-Lifshitz-Gilbert theory underlying the magnetization dynamics of a macrospin magnet subject to spin-torque effects and thermal fluctuations. Thermally activated dynamical properties are analyzed by averaging the full magnetization equations over constant-energy orbits. After averaging, all the relevant dynamical scenarios are a function of the ratio between hard and easy axis anisotropies. We derive analytically the range of currents for which limit cycles exist and discuss the regimes in which the constant energy orbit averaging technique is applicable.
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-Temperature dependence of the switching field in all-perpendicular spin-valve nanopillars, D. B. Gopman, D. Bedau, G. Wolf, S. Mangin, E. E. Fullerton, J. A. Katine, and A. D. Kent,. Physical Review B Rapid Communication 8, 100401(R) (2013).
We present temperature dependent switching measurements of the Co/Ni multilayered free element of 75-nm-diameter spin-valve nanopillars. Angular dependent hysteresis measurements as well as switching field measurements taken at low temperature are in agreement with a model of thermal activation over a perpendicular anisotropy barrier. However, the statistics of switching (i.e. the mean switching field and the variance of the switching field distribution) from 20 up to 400 K are in disagreement with a Néel-Brown model that assumes a temperature independent barrier height and anisotropy field. We introduce a modified Néel-Brown model that fits the experimental data in which we attribute a T3/2 dependence to the barrier height and the anisotropy field due to the temperature dependent magnetization and anisotropy energy.
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-Thermally-Assisted Spin-Transfer Torque Magnetization Reversal of Uniaxial Nanomagnets in Energy Space, D. Pinna, A. D. Kent and D. L. Stein, Journal of Applied Physics 114, 033901 (2013).
We consider the general Landau-Lifshitz-Gilbert (LLG) dynamical theory underlying the magnetization switching rates of a thin film uniaxial magnet subject to spin-torque effects and thermal fluctuations. After discussing the various dynamical regimes governing the switching phenomena, we present analytical results for the mean switching time behavior. Our approach, based on explicitly solving the first passage time problem, allows for a straightforward analysis of the thermally assisted, low spin-torque, switching asymptotics of thin film magnets. To verify our theory, we have developed an efficient Graphics Processing Unit (GPU)-based micromagnetic code to simulate the stochastic LLG dynamics out to millisecond timescales. We explore the effects of geometrical tilts between the spin-current assisted dynamics. We find that even in the absence functionally described in a form virtually identical to the collinear case.
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-Onset of a Propagating Self-Sustained Spin Reversal Front in a Magnetic System, P. Subedi, Velez, F. Macia, S. Li, M. P. Sarachik, J. Tejada, S. Mukherjee, G. Christou, and A. D. Kent, Physical Review Letters 110, 207203 (2013).
The energy released in a magnetic material by reversing spins as they relax toward equilibrium can lead to a dynamical instability that ignites self-sustained rapid relaxation along a deflagration front that propagates at a constant subsonic speed. Using a trigger heat pulse and transverse and longitudinal magnetic fields, we investigate and control the crossover between thermally driven magnetic relaxation and magnetic deflagration in single crystals of Mn12-acetate.
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