Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session J42: Spin Textures at Magnetic MultilayersFocus
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Sponsoring Units: GMAG DMP Chair: Zhaochu Luo Room: 709/711 |
Tuesday, March 3, 2020 2:30PM - 2:42PM |
J42.00001: Ultra-low-power switching of IrMn/CoFeB/MgO structure with voltage-controlled spin-orbit torque Shouzhong Peng, Jiaqi Lu, Weixiang Li, Lezhi Wang, He Zhang, Wang Kang, Zhaohao Wang, Xiang Li, Kang-Lung Wang How to efficiently manipulate magnetization remains one of the key challenges for spintronic devices and magnetic random-access memories[1-3]. Here we experimentally demonstrate the ultra-low-power and field-free switching of perpendicular magnetization by a combination of spin-orbit torque (SOT) and voltage-controlled magnetic anisotropy effect. Firstly, field-free SOT switching is achieved with the aid of an in-plane exchange bias (EB) generated in the perpendicularly magnetized IrMn/CoFeB/MgO structure. Then we explore the VCMA effect and observed a clear reduction of perpendicular magnetic anisotropy (PMA) when a gate voltage is applied. A VCMA coefficient of 35 fJ/Vm is obtained in this structure, comparable to that of the Ta/CoFeB/MgO structure. By applying a gate voltage of 0.6 V on this film, the SOT switching current can be significantly reduced, resulting in a critical switching current of 6.2 MA/cm2. Finally, an ultra-low-power and high-density spintronic memory array based on voltage-controlled SOT is proposed and demonstrated by both experiments and hybrid CMOS/MTJ simulations. |
Tuesday, March 3, 2020 2:42PM - 2:54PM |
J42.00002: Electrical noise in magnetic tunnel junctions due to magneto-structural transitions of CoFeB/MgO interfaces Arezoo Etesamirad, David Nelson, Keanuhea Dailey, Jordan A Katine, Ilya Krivorotov, Igor Barsukov Magnetic tunnel junctions (MTJs) play a central role in spintronics research as memory elements, nanoscale microwave oscillators, local magnetic sensors, and neuromorphic network components. Understanding and controlling electric noise in MTJs is a prerequisite for employing them in next-generation applications. Here, we study MTJ nanopillars of 50 nm diameter consisting of CoFeB free layer, MgO tunnel barrier, and CoFe based synthetic antiferromagnet. We observe random telegraph noise which is frequently found in MTJs and yet not fully understood. Varying the temperature in the range of 80-300 K for an MTJ in the parallel state, we encounter anomalous device resistance (steps) attributed [1,2] to magneto-structural phase transitions of iron oxide clusters at the CoFeB/MgO interface. At temperatures of these anomalies, telegraph noise shows a significant increase. This correlation suggests that the oxide clusters have a significant impact on MTJ noise characteristics. |
Tuesday, March 3, 2020 2:54PM - 3:06PM |
J42.00003: Finite-size Kosterlitz-Thouless transition in Fe/W(001) ultrathin films David Venus, Jordan Atchison, Amanjot Bhullar, Bryce Norman The Kosterlitz-Thouless (KT) transition involves the unbinding of topological excitations (vortex-antivortex pairs) in an infinite, isotropic, 2DXY system. In real 2D ferromagnetic films, it is not clear that such a transition occurs in the presence of crystalline anisotropy and finite size effects. We report MOKE magnetic susceptibility measurements χ(T) of 4-fold in-plane ultrathin Fe/W(001) films, where a prominent peak is observed. Above the peak temperature, the paramagnetic susceptibility is in excellent agreement with the form expected for a vortex-antivortex gas: χ(T) = χ0 exp[B/(T/TKT -1)a] , with a = 0.50±0.03 and B = 3.48±0.16 . In accord with finite size KT theory, the KT transition temperature,TKT, is tens of K below the susceptibility peak, giving a “finite size” consistent with μm magnetic domains. Fitting instead to a power law, as would be consistent with a 2nd order transition, gives unphysical parameters γeff = 3.7±0.7 and a Curie temperature Tγ far below the peak temperature. Below the peak, the measured susceptibility has a complicated behaviour qualitatively consistent with the re-emergence of 4-fold anisotropy and magnetic domains. |
Tuesday, March 3, 2020 3:06PM - 3:18PM |
J42.00004: Spin-Orbit-Torque Field-Effect Transistor (SOTFET): A New Magnetoelectric Memory Xiang Li, Phillip Dang, Joseph Casamento, Zexuan Zhang, Olalekan Afuye, Alyssa B. Apsel, Darrell Schlom, Debdeep Jena, Daniel C. Ralph, Huili Xing Spin-based memories are attractive for their non-volatility and high durability but provide modest resistance changes, whereas semiconductor logic transistors are capable of large resistance changes but lack memory function with high durability. The recent availability of multiferroic materials provides an opportunity to directly couple the change in spin states of a magnetic memory to a charge change in a semiconductor transistor. In this work, we propose and analyze the spin-orbit-torque field-effect transistor (SOTFET), a device with the potential to significantly boost the energy efficiency of spin-based memories, and to simultaneously offer a palette of new functionalities. Analysis of the memory aspect indicates that the SOTFET can offer orders of magnitude increase in the on-off resistance ratio compared to existing magnetic memories, which can potentially lower the operation energy significantly. We establish a quantitative model of the operations of the SOTFET. From the model, the materials needs for the successful operation are identified and the feasibility of the SOTFET is proved in a properly designed CoFe/BiFeO3 gate stack. |
Tuesday, March 3, 2020 3:18PM - 3:30PM |
J42.00005: Pulsed or Microwave Currents Induced Random Telegraph Signal in a Magnetic Tunnel Junction Rajapaksayalage Rajapakse, Nicholas Penthorn, Xiaojie Hao, Zihui Wang, Yiming Huai, HongWen Jiang Recent work has shown creation and dynamics of magnetic skyrmions, without the Dzyaloshinskii-Moriya interaction, in the free layer of a magnetic tunnel junction (MTJ) at a temperature of 4K [1]. In order to study how skyrmions, the topological excitations, can be protected against disorders and temperature fluctuations, we have extended our experimental investigation to higher temperatures, up to 200K, for various device sizes. For the intermediate resistance state, initiated by pulse or microwave currents, we have observed stochastic switching between two resistance states, commonly referred as the random telegraph signal (RTS). Statistics of the observed RTS depends on device temperature and microwave stimulation while the corresponding relations have been studied. In conjunction with micro-magnetic simulations, we discuss the interplay of the topological skyrmion and disorder-induced domains in MTJs. |
Tuesday, March 3, 2020 3:30PM - 3:42PM |
J42.00006: Controlling long-range skyrmion lattices using field and temperature in Fe/Gd multilayers Lisa DeBeer-Schmitt, Ryan Desautels, Nan Tang, Sergio Montoya, W L. N. C. Liyanage, Sheena Patel, Michael Fitzsimmons, Eric Fullerton, Julie Ann Borchers, Dustin Gilbert Ordered magnetic skyrmion lattices presents a playground of new and interesting physics to explore. This opportunity is provided by the topological nature of the skyrmion. We have recently fabricated thin-films of amorphous Fe and Gd multilayers that support skyrmions and skyrmion lattices at room temperature and zero applied magnetic field. These skyrmions are stabilized by dipolar interactions, rather than the Dzyaloshinskii–Moriya interaction (DMI). By varying the film thickness and alloy composition we can alter the dipole interactions relative to the exchange and anisotropy and thus control the skyrmion size, pitch, and stability. Using small angle neutron scattering (SANS), we demonstrated that once formed these dipole skyrmions are stable over a large field and temperature range including zero field and room temperature. We observe temperature and field dependent changes in the scattering vector (Q) for peak scattering related to the skyrmion lattice spacing. The origin of this change has led us to identify a key requirement necessary for the range of stability that we have observed in these amorphous thin films. |
Tuesday, March 3, 2020 3:42PM - 3:54PM |
J42.00007: Multi-probe, laser-scanning optical microscopy for investigation of novel magnetic domain structures and boundaries Austin Kaczmarek, Liuyan Zhao Spin ordering in a magnetic material leads to the breaking of time-reversal symmetry along with some crystallographic symmetries, defining the magnetic symmetry group of the magnetic phase. Multiple degenerate ground states always exist, related through these broken symmetries, leading to interesting magnetic domain structures in macroscopic samples, which potentially host novel low-dimensional phases at the domain boundaries. In this talk, I will present our development of a spatially resolved, diffraction limited, ultrafast laser-based optical scanning microscope, which includes measurement capabilities of the magneto-optic Kerr effect (MOKE), a direct measure of broken time-reversal symmetry, and the optical second harmonic generation rotational anisotropy (SHG RA), a sensitive probe to broken spatial symmetries. I will present a comparison between this scanning-based microscope and a more traditional wide-field microscope. I will then discuss our experimental results on resolving magnetic domain structures in two types of antiferromagnetism, Ising and Heisenberg, on a honeycomb and a triangle lattice respectively. Finally, I will discuss the possibility of incorporating time-resolution into the aforementioned static probes. |
Tuesday, March 3, 2020 3:54PM - 4:06PM |
J42.00008: Chirality-driven growth of stripe domains in Co/Ni/Pt-based multilayers Jeffrey Brock, Rajasekhar Medapalli, Eric Fullerton The interfacial Dzyaloshinskii-Moriya interaction (iDMI) arising from inversion asymmetry at interfaces between magnetic materials and heavy metals with large spin-orbit coupling can transform DWs from Bloch to chiral Néel, leading to asymmetric DW propagation in the presence of a symmetry-breaking in-plane magnetic field. However, stripe domain growth in systems with iDMI is not yet well understood. Here we present an experimental study of [Co(0.7 nm)/Ni(0.5 nm)/Pt(0.7 nm)]N (1 ≤ N ≤ 5) multilayers designed to have asymmetric Pt/Co and Ni/Pt interfaces and perpendicular anisotropy. Kerr microscopy reveals that for N ≥ 3, reversal occurs via stripe domains and that when a static in-plane magnetic field is applied, the growth directionality is strongly linked to the field strength. For low fields, the domains grow roughly perpendicular to the field – a directionality not predicted by current understanding of the iDMI. The propagation direction becomes strongly collinear to the field at higher field magnitudes, as expected from the iDMI. Possible explanations in terms of a mixed domain wall with a mixed chiral Neel and chiral Bloch character will be discussed. |
Tuesday, March 3, 2020 4:06PM - 4:18PM |
J42.00009: High tunability of synthetic antiferromagnets applied in magnetic field sensing and memory devices Kang Wang, Yiou Zhang, Gang Xiao We investigated magnetic configurations in synthetic antiferromagnets (SAFs) of CoFeB/Ta/CoFeB by tuning interlayer exchange coupling (IEC, the energy density is denoted by Jex) and perpendicular magnetic anisotropy (PMA, Ku). Two magnetic layers are either ferromagnetic or antiferromagnetic coupling, depending on both the non-magnetic layer thickness (dNM) and the ferromagnetic layer thickness. Besides spin-flip transitions in SAFs with large PMA, we observed the canted magnetic configurations and spin-flop transitions when Ku < Jex/dNM. Most interestingly, the transfer curve of anomalous Hall resistance (magnetizations) becomes linear when tuning IEC and PMA to moderate values. This allows for applications in anomalous Hall effect (AHE) sensors with ultrahigh magnetic field detectability. Noise spectra reveal the magnetic field detectability reaches 100 nT/√Hz (at 1 Hz) at room temperature and decreases to sub-µT/√Hz (at 1 Hz) at 150 K. The studies indicate that the synthetic antiferromagnets are good candidates for both the magnetic memory devices and ultra-detectable magnetic field sensors with high thermal stability. |
Tuesday, March 3, 2020 4:18PM - 4:30PM |
J42.00010: Dynamic Skyrmion-Mediated Switching of Perpendicular MTJs: Scaling to 20 nm with Thermal Noise Md Mahadi Rajib, Walid Al Misba, Dhritiman Bhattacharya, Jayasimha Atulasimha One method of creating and annihilating skyrmions in confined geometries is to use Voltage-Controlled Magnetic Anisotropy (VCMA) [1, 2, 3]. Previous study shows that robust voltage controlled ferromagnetic reversal from “up” to “down” state in the soft layer of a p-MTJ can be achieved by creating and subsequently annihilating an intermediate skyrmion state [4] in the presence of room temperature thermal noise and anisotropy variation across grains [4]. However, when scaling to 20 nm, thermal noise can annihilate the skyrmions, for example, by randomly moving the core towards the boundary of the nanostructure. Thus, a region of different anisotropy in the skyrmion core may be needed to stabilize the skyrmion state allowing competitive scaling. Here, we will investigate the extent to which the skyrmion mediated switching scheme can scale to lateral dimensions ~ 20 nm in ferromagnets. |
Tuesday, March 3, 2020 4:30PM - 4:42PM |
J42.00011: Skyrmion Breathing Modes in Synthetic Ferri- and Antiferromagnets Martin Lonsky, Axel Hoffmann Magnetic multilayers that combine strong spin-orbit interaction with lacking inversion symmetry can give rise to the presence of topologically nontrivial spin textures, so-called magnetic skyrmions. Recent studies have indicated strongly enhanced propagation velocities of skyrmions in antiferromagnets and compensated ferrimagnets [1]. At the same time, it is unclear how magnetic compensation may affect dynamic excitations of magnetic skyrmions, such as breathing modes which entail an oscillation of the skyrmion size at GHz frequencies [2]. Here, we present micromagnetic simulations of these excitations in synthetic ferri- and antiferromagnets. The observed features in the calculated power spectra show a systematic dependence on the coupling strength between the individual layers and are related to pure breathing modes as well as to hybridizations of breathing and spin wave modes that are characteristic for the considered geometry. Based on these simulations, we then discuss the impact of these results for potential skyrmion sensing. |
Tuesday, March 3, 2020 4:42PM - 4:54PM |
J42.00012: Voltage Control of magnetic Skyrmions in patterned nanodots Dhritiman Bhattacharya, Seyed Armin Razavi, Hao Wu, bingqian dai, Kang-Lung Wang, Jayasimha Atulasimha Manipulation of magnetic skyrmions that are fixed in space can lead to the implementation of computing devices with smaller footprint. Moreover, if the control is achieved solely via applying an electric field, additional advantage of energy efficiency can be obtained. We have previously shown the feasibility of such a scheme by annihilating and creating skyrmions in a thin film of IrMn/CoFeB/MgO using Voltage Control of Magnetic Anisotropy (VCMA) [1]. However, in patterned structures, due to the confinement effect, complete reversal of skyrmion polarity as well as skyrmion mediated ferromagnetic reversal can be achieved using only voltage pulses [2, 3]. Here we propose to show this behavior in patterned nanodots of CoFeB sandwiched between an oxide and a heavy metal layer. We will also study the intermediate magnetic states during the reversal by performing magnetic force microscopy (MFM) imaging under in situ voltage. Finally, micromagnetic simulations will be presented that corroborate our experimental observations. |
Tuesday, March 3, 2020 4:54PM - 5:30PM |
J42.00013: A deeper understanding of the Dzyaloshinskii-Moriya interaction in magnetic multilayers Invited Speaker: Hans Nembach The Dzyaloshinskii-Moriya Interaction (DMI) gives rise to chiral magnetic structures, which include chiral spin-chains and skyrmions. DMI requires broken inversion symmetry and can exist in the bulk as well as at interfaces. We use Brillouin Light Scattering spectroscopy (BLS) to determine the DMI from the non-reciprocal frequency-shift of Damon-Eshbach spin-waves [1]. In order to gain deeper insight into the underlying physics of DMI and explore ways, on how to tune the DMI through interface modifications, we prepared multiple sample series to study different aspects of the DMI, examples are: 1.) A Cu/Co90Fe10 and a Pt/Co90Fe10 sample series were in-situ oxidized for different times and subsequently capped to prevent any further oxidation. Density functional theory (DFT) calculations have demonstrated that the hybridization and the associated charge transfer is important for DMI at oxide interfaces. We determined that the spectroscopic splitting factor g is correlated to the DMI. This is an indirect confirmation of the theory predictions. [2] 2.) We introduced a Cu dusting layer at the interface between CoFeB and Pt to disrupt the Heisenberg exchange directly at the interface. SQUID magnetometry shows that the Cu dusting layer reduces the proximity magnetization in the Pt as well. The proximity magnetization is a direct result of the exchange coupling and can be seen as a measure for its strength. 3.) So far, most work on DMI has been carried out for highly symmetric interfaces. Low symmetry systems can have anisotropic DMI. We prepared a Pt/Fe(110) sample and found that the DMI is anisotropic with the strongest DMI along the [001] direction, which coincides with the magnetic easy axis. We compared the results for the DMI with DFT calculations. |
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