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A major obstacle to reliable and fast STT-induced switching is a counterintuitive phenomenon: the back-hopping (BH). We generally expect that the switching probability should always increase with the applied voltage. However the contrary can sometimes occur: in many instances, the MTJ resistance can back-hop to its original state after the apparent successful switching of the FL magnetization, as in the STT loop in Figure 1 at large positive voltage bias. We analyze the BH phenomenon in STT induced switching of perpendicularly magnetized tunnel junctions mimicking state-of-the art STTMRAM cells. The analysis is based on single-shot nanosecond time-resolved conductance measurements of the pulse-induced BH. The protocol allows to assign the conductance dynamics to the sole magnetization dynamics by avoiding the contributions from the bias dependence of the conductance and from current-induced heating. Studying several material variants reveals that the back-hopping is a feature of the nominally fixed system of the tunnel junction. This rules out earlier proposed scenarios based on the switching back of the free layer as a result of some anomalous voltage dependence of the STT acting on the free layer. The back-hopping is found (figure) to proceed by two sequential switching events that lead to a final state P&#39; of conductance close to --but distinct from-- that of the conventional parallel state. The P&#39; state does not exist at remanence. It generally relaxes to the conventional antiparallel state if the current is removed. The P&#39; state involves a switching of the sole spin-polarizing part of the fixed layers, i.e. the barrier-facing part of the fixed layers. The analysis of our several material variants indicates that back-hopping occurs only when the spin-polarizing layer is too weakly coupled to the rest of the fixed system. This provides mitigation strategies of back-hopping for next generation STTMRAMs.References: T. Devolder et al, arXiv:2006.05108.

MMM 2020

Back-hopping in Spin-Transfer-Torque switching of perpendicularly magnetized tunnel junctions

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A major obstacle to reliable and fast STT-induced switching is a counterintuitive phenomenon: the back-hopping (BH). We generally expect that the switching probability should always increase with the applied voltage. However the contrary can sometimes occur: in many instances, the MTJ resistance can back-hop to its original state after the apparent successful switching of the FL magnetization, as in the STT loop in Figure 1 at large positive voltage bias. We analyze the BH phenomenon in STT induced switching of perpendicularly magnetized tunnel junctions mimicking state-of-the art STTMRAM cells. The analysis is based on single-shot nanosecond time-resolved conductance measurements of the pulse-induced BH. The protocol allows to assign the conductance dynamics to the sole magnetization dynamics by avoiding the contributions from the bias dependence of the conductance and from current-induced heating. Studying several material variants reveals that the back-hopping is a feature of the nominally fixed system of the tunnel junction. This rules out earlier proposed scenarios based on the switching back of the free layer as a result of some anomalous voltage dependence of the STT acting on the free layer. The back-hopping is found (figure) to proceed by two sequential switching events that lead to a final state P' of conductance close to --but distinct from-- that of the conventional parallel state. The P' state does not exist at remanence. It generally relaxes to the conventional antiparallel state if the current is removed. The P' state involves a switching of the sole spin-polarizing part of the fixed layers, i.e. the barrier-facing part of the fixed layers. The analysis of our several material variants indicates that back-hopping occurs only when the spin-polarizing layer is too weakly coupled to the rest of the fixed system. This provides mitigation strategies of back-hopping for next generation STTMRAMs.References: T. Devolder et al, arXiv:2006.05108.

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Recent observations of long-ranged magnetic ordering in van der Waals bonded, layered magnetic materials down to the single layer limit has led to a plethora of research dedicated to the study of two-dimensional (2D) magnets, with plenty of opportunities to investigate fundamental physics and potential quantum applications. With these materials, the properties are typically strongly correlated to the number of layers, with new physics occurring in the few-layer (~few nm) regime. In this sense, common techniques used to measure magnetic behaviors such as neutron scattering and SQUID are at a disadvantage due to sample size requirements. On the other hand, Raman spectroscopy, which has diffraction-limited spatial resolution, is a powerful, non-destructive optical method to probe magnetism in 2D layered materials through inelastic scattering as a function of temperature, laser energy, polarization, and magnetic field. One material of particular interest is chromium tri-iodide (CrI3), a ferromagnet at bulk thicknesses below the Curie temperature but with the remarkable property of layered antiferromagnetism in thin multilayers. Here, we will report on a magneto-Raman spectroscopy study on multi-layered CrI3 [1], focusing on two new features in the spectra which appear at temperatures below the magnetic ordering temperature and were previously assigned to high frequency magnons. We observe a striking evolution of the Raman spectra with increasing magnetic field applied perpendicular to the atomic layers in which clear, sudden changes in intensities of the modes are attributed to the interlayer ordering changing from antiferromagnetic to ferromagnetic at a critical magnetic field. In addition, through DFT-calculations and symmetry arguments, we conclude that the new modes are not magnons as previously believed, but instead are zone-folded phonons.References: [1] A. McCreary et al. arXiv:1910.01237.

Distinct magneto-Raman signatures of spin-flip phase transitions in CrI3

Spin pumping refers to the transfer of spins from precessional moments in a ferromagnet to a non-magnetic material. In a ferromagnetic/non-magnetic bi-layered system, spin pumping manifests itself as a damping enhancement in the ferromagnetic layer and a spin current in the non-magnetic layer. This effect can be strong if the non-magnetic layer is a heavy metal thin film, because such a film can work as a spin sink and produce a very weak spin backflow. The effect can be even stronger if the non-magnetic component is a topological insulator, because it can efficiently convert a spin current to a charge current and thereby serve as a very efficient spin absorber, due to the spin-momentum locking of the topological surface states (TSS). In principle, TSS should also be present in other topological materials, such as Dirac or Weyl semimetals. These TSS are also expected to exhibit spin-momentum locking, but their interplays with magnetism are largely unexplored. This presentation reports strong damping enhancement in a ferromagnetic NiFe thin film due to a neighboring α-Sn thin film. Multiple evidences suggest that the damping enhancement is associated with TSS in the topological Dirac semimetal (TDS) phase of α-Sn. The TDS α-Sn films are realized on InSb substrates. The measurements used α-Sn(6nm)/Ag(2nm)/NiFe(20nm) layered structures where Ag works as a spacer to physically separate α-Sn and NiFe. Ferromagnetic resonance studies show that the damping in these structures is ~4.8 bigger than in the bare NiFe film. Control measurements indicate that this damping enhancement is mostly due to the TSS in the α-Sn, rather than the bulk of the α-Sn film or the Ag spacer; it is absent in β-Sn(6nm)/Ag(2nm)/NiFe(20nm) where the Sn film is topologically trivial. This work demonstrates that TDS may be as promising as topological insulators in terms of spintronics applications.

Topological Surface States-Caused Large Damping Enhancement in Dirac Semimetal-Ferromagnetic Metal Layered Structures

Electric-field control of magnetization is essential for the next-generation spintronic technologies such as nonvolatile magnetic memory. In particular, the voltage-control of the magnetic anisotropy (VCMA) in ferromagnets promises an energy-efficient reversal of magnetization based on the clocking scheme, which earlier has been demonstrated by using a pulsed voltage in magnetic tunnel junction (MTJ). However, the VCMA method in MTJ still requires a high gate voltage and a constant-bias magnetic field, which is a disadvantage for practical applications. In contrast, as another important method for magnetization reversal, current-induced spin-orbit torque (SOT) in topological insulator (TI) based magnetic heterostructures has been studied in recent experiments [1], which succeeded in reducing the switching power consumption by current pulse injected parallel to a bias magnetic field [2]. In this research, inspired by the VCMA and SOT approaches for magnetization control, we combine them and present two distinct clocking methods for the magnetization switching in TI based magnetic heterostructures. First, we analytically formulate the uniaxial magnetic anisotropy in magnetic TIs as a function of the Fermi energy. Next, as shown in Fig. 1, we propose a transistor-like device with the functionality of a nonvolatile magnetic memory adopting the VCMA writhing method that require no external magnetic fields and the anomalous-Hall-effect based readout scheme. For the magnetization reversal, the estimated source-drain current density and gate voltage are the orders of 104 - 105 A/cm2 and 0.1V, respectively, which are much smaller than those of traditional heavy-metal/ferromagnet heterostructures as well as the MTJs. We also discuss the possibility of using the proposed method for magnetization reversal in TI/ferromagnetic-insulator bilayers with magnetic proximity at the interface.References: [1] Y. Fan et al., Nat. Nanotechnol. 11, 352 (2016). [2] K. Yasuda et al., Phys. Rev. Lett. 119, 137204 (2017).

Electric-Field Control of Spin-Orbit Torque and Magnetic Anisotropy in Topological Insulator Heterostructures

The recent discovery that van der Waals-bonded magnetic materials retain long range magnetic ordering down to a single layer[1,2] stimulates a thorough Raman spectroscopic study of one such material, FePS3, a large spin (S = 2) Mott insulator where the Fe atoms form a honeycomb lattice. Bulk FePS3 was shown to be a quasi-2D Ising antiferromagnet, with additional features in the Raman spectra emerging below the Neel temperature (TN ~ 120 K). Using temperature- and magnetic field-dependent Raman spectroscopy as an optical probe of magnetic structure, we demonstrate[3] that one of these Raman-active modes (ψ4) below TN is a magnon with a frequency of ~ 3.7 THz (~122 cm-1). Contrary to previous work[4], which interpreted this feature as a phonon, our Raman data[3] shows the expected frequency shifting and splitting of the magnon as a function of temperature and magnetic field, respectively, were we find the g-factor ~2. Other Raman-active modes emerging below TN are attributed to zone-folded phonons in the antiferromagnetic state. The anomalous temperature dependence of these modes along with an analysis of temperature-dependent magnetization data, suggests a persistence of short-range magnetic order above TN.References: [1] C. Gong, L. Li, Z. Li, et al., Nature 546, 265 (2017). [2] B. Huang, G. Clark, E. Navarro-Moratalla, et al., Nature 546, 270 (2017). [3] A. McCreary, J. R. Simpson, T. T. Mai, et al., Phys. Rev. B 101, 064416 (2020). [4] J.-U. Lee, S. Lee, S. H. Ryoo, et al., Nano Lett. 16, 7433 (2016).

Temperature- and Magnetic Field-Dependent Raman Spectroscopy of Layered, Antiferromagnetic FePS3

Material structures containing tetrahedral FeAs bonds, depending on their density and geometrical distribution, can host several competing quantum ground states ranging from superconductivity to ferromagnetism. In Fe-based superconductors where the FeAs bonds are confined in 2D monolayers (MLs), the magnetic ground state of the Fe spins is antiferromagnetic1. On the other hand, in Fe-based ferromagnetic semiconductors such as (In,Fe)As where the FeAs bonds are distributed randomly in a three-dimensional (3D) InAs matrix, the Fe spins are coupled ferromagnetically via interactions with electron carriers2. Here we examine structures of quasi two-dimensional (2D) layers of tetrahedral FeAs bonds embedded with a regular interval in a semiconductor InAs matrix, which resemble the crystal structure of Fe-based superconductors. The samples were grown by low-temperature molecular beam epitaxy on InAs (001) or semi-insulating GaAs (001) substrates. Lattice images by scanning transmission electron microscopy (STEM) reveal that the zinc-blende structure of the host InAs is well maintained, and the Fe atoms are mainly confined within a thickness of 2-3 MLs (Fig. 1a). We study magnetic and transport properties of a series of FeAs/InAs superlattices (thickness is fixed at 12 nm) containing (1ML of FeAs / t ML of InAs) x N periods (t = 20, 10, 7, 5; N = 1, 3, 5, 7) (the inset of Fig. 1(b)). All the samples exhibit intrinsic ferromagnetism, and the TC value changes rapidly as t-3 (Fig 1b). Furthermore, extremely large magnetoresistance (MR) (~500%, Fig. 1c) are observed, whose origin can be attributed to the giant MR occurring in these multilayer structures. These interesting features indicate that the FeAs/InAs superlattice structures are promising for functional spintronic devices3. This work was partly supported by Grants-in-Aid for Scientific Research (17H04922, 18H05345, 19H05602, 19K21961), CREST program (JPMJCR1777) and PRESTO program (JPMJPR19LB) of JST, and Spin-RNJ.References: [1] H. Hosono et al. Materials Today 21, 278-302 (2018). [2] P. N. Hai, L. D. Anh et al., Appl. Phys. Lett. 101, 182403 (2012). [3] L. D. Anh et al, ArXiv:2005.10181 (2020).

Ferromagnetism and giant magnetoresistance in zinc-blende FeAs/InAs superlattice structures

The proximity effect in two-dimensional materials opens ways to achieve important functions for future spintronic devices. In van der Waals heterostructures (vdWHs), transition metal dichalcogenides (TMD) can be used to enhance the spin-orbit coupling of graphene leading to highly efficient spin-to-charge conversion (SCC) by spin Hall effect (SHE) that is predicted to be controllable by a gate voltage. Here, we report for the first time the observation of the SHE in graphene proximitized with WSe2 in a vdWHs. These kind of vdWHs are a promising platform to study a variety of spin-dependent phenomena [1,2]. By Hanle precession measurements, we quantify the spin transport and SCC parameters from 10 K up to room temperature. Exceptional for graphene/TMD devices, the sole mechanism is the SHE for all measurements and no Rashba-Edelstein effect is observable. Importantly, we are able to amplify and turn off the SCC by applying a back-gate voltage, demonstrating the long-awaited milestone of an electrically-tunable SHE. The amplified SCC shows a high efficiency, measured with an unprecedented SCC length of larger than 20 nm, defined as the product of the spin Hall angle (ΘSH) and the spin diffusion length (λs). These results show the capability of two-dimensional materials to advance towards the implementation of novel spin-based devices and future applications.References: [1] W. Yan et al., Nat. Commun. 7, 13372 (2016). [2] C. K. Safeer et al., Nano Lett. 19, 1074–1082 (2019).

Proximity-Induced Spin Hall Effect in Graphene/WSe2 van der Waals Heterostructures with Tunable, Highly Efficient Spin-to-Charge Conversion

Recently, van-der-Waals (vdW) magnets have attracted much attention and been researched extensively [1]. The present study aims to elucidate the magnetic properties of a vdW magnet, namely CeTe3. CeTe3 is a cleavable magnetic material in the family of rare earth tritellurides. Its crystal structure consists of a CeTe slab, separated by two Te layers. The material undergoes two magnetic phase transitions at TN1 = 3 K and TN2 = 1.2 K. For bulk CeTe3, the first transition is understood to be to an antiferromagnetic (AFM) state with an in-plane easy axis [2, 3]. The second transition is known to be to a different AFM state with still an in-plane easy axis, but a nonparallel axis to the first AFM state [4]. However, details of these AFM states are still unknown, and thin film transport measurements have not been performed yet. In this work, we have performed magnetoresistance measurements from 0.4 to 4 K on 40 nm CeTe3 thin film devices [5]. Figure 1 shows magnetoresistance at different temperatures. We have observed a clear Shubnikov-de-Haas oscillation from well above TN1, which has never been reported so far. Such a large oscillation implies a highly two-dimensional (2D) nature of the electrical conduction. Moreover, we have observed a unique hysteresis and peak structure below TN1 which is retained even below TN2, as shown in Fig. 2. This implies a possible canting of the magnetic moments along the stacking direction induced through the thin film fabrication. Further studies could lead to understanding the possible enhancement of perpendicular magnetic anisotropy in magnetic thin films. Moreover, the coexistence of quantum oscillation and magnetic hysteresis is extremely unique, and the material would pave the way for understanding the interaction between magnetic moments and highly 2D conduction electrons.References: [1] Y. Deng et al., Nature 563, 94 (2018) [2] Y. Iyeiri et al., Phys. Rev. B 67, 144417 (2003). [3] K. Deguchi et al., J. Phys.: Conf. Ser. 150, 042023 (2009). [4] D. A. Zocco et al., Phys. Rev. B 79, 134428 (2009). [5] M. Watanabe et al., arXiv: 2006.14955.

Coexistance of quantum oscillation and magnetic hysteresis in CeTe3 thin films

The use of double magnetic tunnel junctions (DMTJ) is a well-known strategy to enhance STT-MRAM efficiency by increasing thermal stability (Δ) while reducing the critical switching current (Ic). In these structures, the Storage layer (SL) is sandwiched between two tunnel barriers, each with its corresponding fixed polarizer. For the case of perpendicular DMTJ, the SL Δ is increased due to the two interfacial anisotropy contributions arising from both magnetic metal-oxide interfaces. If the top (TP) and bottom (BP) polarizers are set antiparallel, the Ic is found both theoretically [1] and experimentally [2] to be reduced due to additive spin-transfer torques. Another feature of DMTJ is the possibility of switching one of the polarizers, allowing to tune the STT depending on the polarizers parallel (Read Mode) or antiparallel (Write Mode) configuration [3]. Fig.1 (a) and (b) show the structure of such p-DMTJ with switchable TP and the two operation modes, where the polarizers are coupled to a hard Co/Pt synthetic antiferromagnetic multilayer (SAF). In this study, we investigate the possibility of replacing the TP SAF by a single magnetic layer realizing a p-DMTJ structure as depicted in Fig.1.(c). The structure proposed comprises a SAF based BP acting also as a fixed reference layer, a composite SL and the top free polarizer called assistance layer (ASL). The ASL is designed to have an out-of-plane anisotropy and a coercivity lower than that of the SL. In standby, the magnetic coupling between the SL and the ASL sets the magnetization of these two layers in parallel configuration, reinforcing the Δ of the SL. During write, the ASL acts as a polarizer inducing an additional STT, thus reducing the Ic. To investigate this concept, we fabricated, characterized and modeled these devices to understand the ASL working conditions. We demonstrate an increase by a factor of 5 in the figure of merit Δ/Ic of these structures compared to conventional MTJ stack for STT-MRAM with double barriers and a non-magnetic ASL Fig.2.References: [1] Daniel C. Worledge,IEEE MAGNETICS LETTERS, Volume 8 (2017) [2] G. Hu, J. H. Lee, J. J. Nowak, IEEE International Electron Devices Meeting (IEDM), (2015) [3] P.-Y. Clément, C. Baraduc, M. Chshiev, B. Diény,IEEE 6th International Memory Workshop (IMW), (2014) [4] K. Nishioka, H. Honjo, S. Ikeda,Symposium on VLSI Technology Digest of Technical Papers, (2019)

Simplified Double Magnetic Tunnel Junction with switchable Assistance layer for improved STT-MRAM performance

CoFeB-MgO-based magnetic tunnel junctions (MTJs) with perpendicular magnetic anisotropy (PMA) (p-MTJs) [1-3] have become the de-facto standard of current STT-MRAM. In the STT-MRAM, the bottom pinned structure [4,5], and in the SOT-MRAM, the top pinned structure has been employed [6,7]. The bottom pinned structure exhibits high PMA, whereas the top pinned structure is difficult to obtain a high PMA of Co/Pt due to the unavailability of a thick buffer layer under Co/Pt. In the bottom pinned p-MTJs, as the Pt buffer layer thickness increases, the PMA of Co/Pt increases [8]. However, Pt thickness cannot exceed 2 nm to obtain sufficient magnetic coupling between ferromagnetic layers [9]. Therefore, there is a trade-off between PMA and magnetic coupling in the top pinned structure. To overcome this trade-off, we developed a high PMA Co/Pt pinned layer for top pinned p-MTJs, using surface modification treatment (SMT). In Stack A without SMT, Pt seed layer (SL) thickness varied from 0.5 to 2.5 nm. In Stack A with SMT, a 6 nm-thick Pt SL was exposed to Ar ion plasma. After SMT, the residual Pt SL thickness was varied from 0.5 to 2.5 nm. We also prepared top pinned p-MTJs (Stack B) with a synthetic ferrimagnetic structure with and without SMT (Fig.2(a)). Annealing temperature is 300, 350, and 400°C for 1 h. In Stack A with SMT, the values of magnetic anisotropy energy density Keff*t are larger than those without SMT (Fig.1). In Stack B with SMT, TMR ratio increases up to 400°C, whereas in Stack B without SMT, TMR ratio degrades at 400°C (Fig.2(b)). The employment of SMT is very effective for achieving both higher perpendicular anisotropy and robustness against annealing for top pinned structure. This work is supported by CIES’s Industrial Affiliation on STT MRAM program, CIES Consortium, JST-OPERA, and CAO-SIP.References: [1] S. Ikeda et al., Nature Mater. 9, 721 (2010). [2] H. Sato et al., Appl. Phys. Lett. 101, 022414 (2012). [3] H. Sato et al., IEEE Trans. Magn. 49, 4437 (2013). [4] H. Honjo et al., J. Appl. Phys., 126, 113902 (2019). [5] K. Nishioka et al., IEEE Trans Electron Devices 67, 995 (2020). [6] H. Honjo et al., IEDM, 28.5.1. (2019). [7] M. Natsui et al., VLSI, CM2.2 (2020). [8] H. Honjo et., al, VLSI, T160 (2015). [9] H. Honjo et., al, IEEE Trans. Magn. 50, 1401904 (2014).

Effect of surface modification treatment on top pinned MTJ with perpendicular easy axis

Graphene has been known as an excellent material for long-distance spin transport due to its weak spin-orbit coupling (SOC). However, the same reason makes graphene an adverse candidate for different spintronics applications in which strong SOC is required, such as the Datta and Das spin transistor proposal or spin-charge interconversions. It has recently been predicted theoretically that SOC can be induced in graphene so that spin-orbit phenomena such as spin Hall effect (SHE) or Rashba-Edelstein effect can be obtained. In our work, by using van der Waals heterostructure-based lateral spin valves, we experimentally demonstrate spin-to-charge conversion (SCC) due to SHE in graphene via spin-orbit proximity with MoS2, a transition metal dichalcogenide (TMD)1. The combination of long-distance spin transport and large spin-to-charge conversion in a van der Waals heterostructure gives rise to a hitherto unreported efficiency for the spin-to-charge voltage output. We performed similar experiments in graphene in proximity with WSe22. Here we observed gate-tunable SHE with SCC efficiency larger than in some of the best SCC materials such as topological insulators. Similarly, we observed efficient SHE in graphene combined with an insulator, Bi2O33. Here, the SHE can be due to extrinsic mechanisms such as skew scattering in graphene decorated with adatoms. Using a similar approach, we observed large multidirectional SCC in Weyl semimetal MoTe22. Here, due to the low symmetry of MoTe2 crystal, we detect, along with the conventional SCC, an unconventional SCC where the spin polarization and the charge current are parallel. Our finding enables the simultaneous conversion of spin currents with any in-plane spin polarization in one single experimental configuration. All in all, these exceptional effects obtained by the unique properties of 2D materials open exciting opportunities in a variety of future spintronic applications.References: 1. C. K. Safeer, et al. Nano Letters, 19, 2, 1074-1082 (2019). 2. F. Herling, et al. APL Materials 8, 071103 (2020). 3. C. K. Safeer, et al. Nano Letters, 20, 6, 4573-4579 (2020). 4. C. K. Safeer, et al. Nano Letters, 19, 12, 8758-8766 (2019).

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Distinct magneto-Raman signatures of spin-flip phase transitions in CrI3

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