United States

Recently, possible applications for thermoelectrics and spin caloritronics utilizing topological materials have been studied. Thermoelectric (TE) conversion that can convert heat into electricity has been attracting attention as a clean power generation technology without greenhouse gases in energy harvesting. The conversion efficiency is evaluated by the dimensionless figure of merit ZT, which is needed to be larger than 1 (energy conversion efficiency is 10%) in practice. Recently, a bismuth chalcogenide which shows the best TE conversion on the nanoscale has been also known as a three-dimensional topological insulator (3D TI). TI has a linear electronic dispersion on the surface. In the presence of a surface gap induced by magnetic dopants, the surface electronic transport strongly depends on the chemical potential, promising very large TE conversion efficiency [1]. Here, we propose a TE energy conversion device based on the surface of 3D TIs that is magnetically gap-opened and ionically disordered [2]. This device can achieve the thermocouple p-n junction by a single TI film, in which a pair of top and bottom surfaces of a 3D TI takes a role of the vertical p-n junction, which can be considered a thermocouple consisting of two dissimilar TE materials (see Fig. 1). Based on the Boltzmann theory at finite temperatures, we compute TE transport coefficients taking into account electron scattering by screened disorder potentials on the surface. By tuning the surface carrier screening by means of electric gating, we find that ZT of the present device achieves more than 1 in the wide temperature-regime from 100 K to 300 K.References: [1] T. Chiba and S. Takahashi, J. Appl. Phys. 126, 245704 (2019). [2] T. Chiba, S. Takahashi, and T. Komine, Appl. Phys. Lett. 115, 083107 (2019).

MMM 2020

Spin caloritronic device based on asymmetric surfaces of magnetic topological insulators

The 65th Annual Conference on Magnetism and Magnetic Materials (MMM 2020) was held in a virtual form. The COVID-19 pandemic, with its associated uncertainty about international travel and the ability to gather in large groups, meant that the Steering Committee decided to cancel the face-to-face meeting that had been planned in Palm Beach, Florida November 2-6, 2020.

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poster

Ferromagnetic semiconductors (FMSs) are promising materials for spintronics applications because they are compatible with the highly established semiconductor technology. Although the prototypical FMS (Ga,Mn)As was intensively investigated, it shows low Curie Temperature (TC ≤ 200 K) with only p-type conduction [1]. Recently we have successfully grown Fe-doped III-V FMSs with high TC and both n-type and p-type conduction; p-type (Ga,Fe)Sb (TC = 400 K) [2] and n-type (In,Fe)Sb (TC = 385 K) [3]. In this work, we extend our study to a quaternary-alloy FMS, (In1-x-y,Gax,Fey)Sb. This new FMS allows us to widen the degree of freedom in designing the material properties, including lattice constant, band structure, carrier type and magnetic anisotropy. We investigate the epitaxial growth, transport and magnetic properties of (In1-x-y,Gax,Fey)Sb thin films with different Fe concentration y and Ga concentration x. The sample structure consists of (from top to bottom) InSb (2 nm) / (In1-x-y,Gax,Fey)Sb (15 nm) / AlSb (100 nm) / AlAs (6 nm) / GaAs (100 nm) on a semi-insulating GaAs(001) substrate, grown by low-temperature molecular-beam epitaxy (LT-MBE). Fig.1 shows an X-ray diffraction (XRD) spectrum of the (In0.74,Ga0.1,Fe0.16)Sb sample. The XRD and other characterizations including scanning transmission electron microscopy and magnetic circular dichroism (MCD) indicate that the (In,Ga,Fe)Sb layers grown by LT-MBE maintain the zinc-blende crystal and band structure without any other second phases. Carrier concentrations and TC are estimated from Hall measurements and Arrott plots of the MCD intensity vs. magnetic field (MCD - H) curves, respectively. In (In0.84-x,Gax,Fe0.16)Sb samples (x = 2, 6, 8, 10%), the carrier type can be changed between p-type and n-type by changing x while high TC > 300 K is maintained [Fig.2]. These materials are promising for device applications such as spin diodes and spin transistors. This work was partly supported by Grants-in-Aid for Scientific Research (Nos. 16H02095, 18H05345), the CREST Program (JPMJCR1777) and PRESTO Program (JPMJPR19LB) of JST, and the Spin-RNJ.References: [1] L. Chen et al, Nano Lett., Vol. 11, p.2584 (2011). [2] S. Goel et al, PRB., Vol. 99, p.014431 (2019). [3] N. T. Tu et al, APEX., Vol.12, p.103004 (2019).

Properties of quaternary-alloy ferromagnetic semiconductor (In,Ga,Fe)Sb

Two-dimensional magnetic materials are of great interest for fundamental science and advanced applications while dilute magnetic semiconductors captured worldwide interest by combining magnetism with electronics. MnGaN-2D is an ultimately-thin, 2D-DMS material, consisting of a dense, well-ordered layer of Mn, Ga, and N atoms. MnGaN-2D shows room-temperature ferromagnetism as previously demonstrated with spin-polarized scanning tunneling microscopy as a function of applied magnetic field. This was also predicted by first-principles theoretical calculations which reveal the origins of the ferromagnetism through its highly spin-split and spin-polarized electronic structure.[1] The SP-STM results are recently confirmed by SQUID magnetometry which reveals perpendicular magnetic anisotropy and a high spin-polarization of ∼79% at room temperature.[2] New results for this novel system also suggest the possibility of magneto-elasticity. Theoretical calculations reveal sensitivity of the spin polarization to lattice strain. Spin-orbit coupling is included in the calculations, which indicate either out-of-plane or in-plane anisotropy, dependent on the type of strain whether compressive or tensile. Clear evidence for both compressive and tensile local lattice strains is found by detailed analysis of atomic resolution STM images which reveal a wide, non-Gaussian range of lattice spacings (-18% to + 18% variations), unlike normal materials. Furthermore, scanning tunneling spectroscopy measurements, showing the spin-polarized filled states manganese peak, find fluctuations in the electronic position of the peak from spectrum to spectrum. The FSMP position is also found to fluctuate, depending on lattice strain, in the theoretical calculations, thus indicating a connection between electronic states and magnetic anisotropy in this RT ferromagnetic 2D layer.References: [1] Yingqiao Ma, Abhijit V. Chinchore, Arthur R. Smith, María Andrea Barral, and Valeria Ferrari, A Two-Dimensional Manganese Gallium Nitride Surface Structure Showing Ferromagnetism at Room Temperature, Nano Letters, Vol. 18, p. 158 (2018) [2] Yingqiao Ma, Diego Hunt, Kengyuan Meng, Tyler Erickson, Fengyuan Yang, María Andrea Barral, Valeria Ferrari, and Arthur R. Smith, Local strain-dependent electronic structure and perpendicular magnetic anisotropy of a MnGaN 2D magnetic monolayer, Physical Review Materials, Vol. 4, p. 064006 (2020)

Room-Temperature Ferromagnetism and Strain-dependent Magnetic Anisotropy in 2D MnGaN

Spin transfer (ST) effect is a consequence of angular momentum conservation [1-3], one of the three fundamental conservation laws in nature. We use fully quantum simulations of scattering of spin-polarized electron wavepacket by a ferromagnet modelled as a 1d spin-1/2 chain [4], as illustrated in Fig.1, to show that energy and momentum conservation laws impose strong restrictions on the characteristics of magnetic excitations generated by spin transfer. We found that two groups of magnons are generated by ST in this 1d system, forward-propagating magnons with large energy Ef, and backward-propagating magnons with small energy Eb . Analysis shows that the backward-propagating magnons are generated by the reflected electron component, while the forward-propagating magnons -by the transmitted electron component, with the values of energy and momenta of the generated magnons determined by the changes of the corresponding characteristics of the electron wave [Fig.2]. We also show a converse effect – dependence of the scattered electron’s characteristics on the dynamical characteristics of the magnetic system. In particular, variations of magnetic anisotropy result in the variations of energy of the scattered electrons, and of the probability of electron reflection. For large anisotropy, electron reflection is completely suppressed, and only one group of magnons is generated. Our results demonstrate that quantum magnetism plays an important role in magnetoelectronic phenomena. This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award # DE-SC0018976.References: 1. J. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996). 2. L. Berger, Phys. Rev. B 54, 9353 (1996). 3. D. Ralph and M. Stiles, J. Magn. Magn. Mater. 320, 1190 (2008). 4. P. Mondal, et al, Phys. Rev. B 99, 094431 (2019).

Energy and momentum conservation in spin transfer.

Efficient spin injection into semiconductors remains a formidable and elusive challenge even after almost three decades of major scientific effort, studded by obstacles and only partial workarounds. Few years ago, I predicted the possibility of injecting massive ultrashort spin current pulses across a ferromagnetic metal/semiconductor interface [1-2]. We have now proved experimentally the prediction [3]. By injecting strongly out-of-equilibrium sub-picosecond spin current pulses across a bare ferromagnet/semiconductor interface, we have overcome the crippling problem of impedance mismatch and obtained a massive spin transfer. We demonstrated this by producing ultrashort spin current pulses into cobalt and injecting them into monolayer MoS2. The semiconducting MoS2 layer also acts as a selective converter of the spin current into a charge current, whose THz emission is then measured. As predicted, we measured a giant spin current, orders of magnitude larger than typical injected spin current densities in modern devices.References: [1] M. Battiato, K. Held, Phys. Rev. Lett. 116, 196601 (2016) [2] M. Battiato J. Phys. Condens. Matter 29, 174001 (2017). [3] L. Cheng et al., Nature Physics 15, 347 (2019).

Far out-of-equilibrium spin populations trigger giant spin injection into atomically thin MoS2

Tunneling spin injection on graphene is possible by using different materials as barriers1–3. However, the growth of ultra-thin tunnel barriers leads to the creation of pinholes, which in turn causes direct ohmic contact and a large suppression of spin polarization. This occurs due to a fundamental limitation known as the resistance mismatch problem4, which limits spin injection when the resistance of an injector contact, , is lower than the spin resistance of the graphene channel, . This is the result of contact-induced spin relaxation, which greatly decreases the measured spin signals5. In this work, we demonstrate an alternative approach to grow ultra-thin tunnel barrier contacts made by oxidizing aluminium on top of previously functionalized graphene. We show that molecular functionalization of graphene allows the growth of smooth, high quality tunnel barriers, leading to contacts with increased spin injection efficiency and enhanced spin signals by one order of magnitude. The functionalization process consists on the laser activated physisorbtion of benzoyl peroxide (BPO) molecules on graphene. This laser controlled photoreaction enables us to control both the spatial location and the degree of functionalization, as visualized by Raman spectroscopy maps via the intensity ratio of and peaks (ID/IG). We assess the quality of the oxide barriers using atomic force microscopy, charge transport and spin transport measurements. These results open the path towards engineering spintronic devices with spatially controlled functionalization and efficient tunnel contacts.References: Tombros, N. et al, Nature 448, 571–574 (2007). Tombros, N. et al, Phys. Rev. Lett. 101, 2–5 (2008). Han, W. et al, Phys. Rev. Lett. 105, 167202 (2010). Schmidt, G. et al, Phys. Rev. B 62, R4790–R4793 (2000). Maassen, T. et al, Phys. Rev. B 86, 235408 (2012).

Spin Injection Enhancement via ultra-thin Oxide barriers into Molecularly Functionalized Graphene

The non-local spin valve (NLSV) is an important tool in spintronics, primarily due to its ability to isolate pure spin currents1. As such, NLSVs have been instrumental in spin transport measurements of a wide range of materials2. Yet, even in relatively simple systems, questions remain on the impact of interfaces, defects and impurities3–5. Of particular interest is an increase in the “spin-signal”, ΔRNL, under large magnetic fields (up to 9 T) in all-metallic NLSVs6, in stark contrast to the key tenet that ΔRNL is invariant under a magnetic field. This effect, which appears only for certain combinations of FM and normal metals, has been a long-standing mystery and, thus far, has been a challenge to describe with existing models. Our earlier works on similar materials have shown that a low temperature upturn in spin-relaxation rate, 1/τs, originates from magnetic impurity (MI) scattering near the NLSV interfaces5,7, underlining the importance of the Kondo effect in determining spin-relaxation, particularly in Cu devices. In this study we demonstrate MI scattering as the origin of the field enhancement of ΔRNL, and we provide a theoretical framework. By correlating the magnitude of the field enhancement, δRNL, with the NLSV material pairings, we ascertain that δRNL is only present in those materials that can host MI moments (Figure 1). Next, focusing on Cu/Fe NLSVs, we extract spin-transport parameters from the field-enhanced ΔRNL. Under zero-field, we observe the same increase in 1/τs at low temperatures (Figure 2), due to Kondo scattering from MIs, in agreement with our previous work5. Importantly, we show that this mechanism is quenched by a magnetic field and we successfully model the data using a magnetic-field-modified Kondo expression8. This work not only highlights a systematic overestimation of 1/τs in materials containing MIs, including a clear impact on Hanle spin relaxation measurements, but also provides a simple means to quantify and suppress this scattering mechanism.References: 1 F.J. Jedema, M.S. Nijboer, A.T. Filip, and B.J. van Wees, Phys. Rev. B 67, 085319 (2003). 2 I. Zutić, J. Fabian, and S. Das Sarma, Rev. Mod. Phys. 76, 323 (2004). 3 H. Idzuchi, Y. Fukuma, L. Wang, and Y. Otani, Appl. Phys. Lett. 101, 022415 (2012). 4 E. Villamor, M. Isasa, L.E. Hueso, and F. Casanova, Phys. Rev. B 87, 094417 (2013). 5 L. O’Brien, M.J. Erickson, D. Spivak, et al., Nat. Commun. 5, 3927 (2014). 6 K. Výborný, G. Mihajlović, A. Hoffmann, and S.I. Erlingsson, J. Phys. Condens. Matter 25, (2013). 7 J.D. Watts, L. O’Brien, J.S. Jeong, et al., Phys. Rev. Mater. 3, 124409 (2019). 8 V.I. Litvinov, Phys. Status Solidi 77, 71 (1976).

Origin of the Magnetic Field Enhancement of the Spin Signal in Metallic Non-Local Spin Transport Devices

Semiconductors nanowires (NWs) with strong spin-orbital interaction, like InSb and InAs nanowires, are a leading platform for realizing future computing devices, such as Datta-Das spin-transistor1 and topological-superconductor quantum computing devices2 using Majorana zero modes. Coupling nanowires to electrical spin-selective contacts, therefore, is an important step as it can, on the one hand, provide a setting to study spin transport in quantum wires, and on the other hand could enable the realization of Majorana devices that do not require an applied magnetic field, by taking advantage of the magnetic exchange interaction or spin accumulation. Here, we will discuss our measurements on InSb NW devices with iron (Fe) electrical contacts3 (see Figure 1, scanning electron microscopy picture). Conductance measurements show conductance quantization and Fabry–Pérot interference, clearly demonstrating phase-coherent and ballistic electronic transport with spin-polarized currents. Our magnetoconductance study clearly demonstrates spin injection/ transport across the NWs by the observation of hysteretic spin-valve signals (Figure 2a). Moreover, we show that electrostatic gating tunes the observed hysteretic signal and reveals a transport regime where the device acts as a spin filter (Figure 2b). *Supported primarily by the Department of Energy under Award No. DE-SC-0019274. Nanowire growth was supported by the European Research Council (ERC HELENA 617256), and the Dutch Organization for Scientific Research (NWO).References: 1. Datta, S. & Das, B. Electronic analog of the electro-optic modulator. Appl. Phys. Lett., Vol.56, p.665–667 (2016) 2. Mourik, V. et al. Signatures of Majorana Fermions in Hybrid Superconductor-Semiconductor Nanowire Devices. Science., Vol.336, p.1003–1007 (2012) 3. Yang, Z. et al. Spin Transport in Ferromagnet-InSb Nanowire Quantum Devices. Nano Lett., Vol 20, p.3232-3239 (2020)

Ballistic Spin Transport in InSb Nanowires with Strong Rashba Spin-Orbital Interaction

It has been demonstrated that the Kondo effect from iron impurities substantially enhances the spin relaxation rate at low temperature in submicron copper channels of nonlocal spin valves (NLSV) [1,2]. In this work, we explore a long standing question about the physical existence of Kondo screening clouds [3] by examining the relationship between Kondo spin relaxation rate and Kondo resistivity. A systematic method is utilized to extract spin and charge transport parameters from each of the 20 NLSV devices investigated. Figure 1 (a) shows that the spin relaxation length of a NLSV reaches its maximum of 3.0 microns at 30K. Low temperature upturns are clearly observed in the spin relaxation rate versus temperature (Figure 1 (b)) and the Cu resistivity versus temperature (inset of Figure 1 (b)) plots. Both upturns are attributed to the Kondo effect from Fe impurities of several ppm. The Kondo spin relaxation rate and Kondo resistivity are extracted by fitting and plotted in Figure 1 (c) for all 20 NLSVs. Surprisingly, the Kondo spin relaxation rate remains almost constant, while the Kondo resistivity varies by nearly a factor of 10. This is significantly different from the linear relationship predicted by the Elliott-Yafet model. The effective Kondo spin flip probability, shown in Figure 1 (d), is greatly suppressed as the Kondo resistivity (proportional to impurity density) increases. The data can be explained by considering the spin dephasing processes through the Kondo screening clouds around Fe impurities, and support the physical existence of Kondo screening clouds.References: [1] L. O’Brien, et.al, Nat. Commun. 5, 3927 (2014). [2] J. T. Batley, et.al, Phys. Rev. B 92, 220420(R) (2015). [3] V. Borzenets, et.al, Nature 579, 210–213 (2020).

Unusual scaling of Kondo spin relaxation

Topological insulator BiSb is a promising candidate for spin-orbit-torque (SOT) magnetoresistive random-access memory (MRAM), due to its giant spin Hall effect (spin Hall angle θSH ∼ 52 for BiSb(012) surface) [1] and high electrical conductivity (σ ~ 2.5 × 105 Ω-1m-1) [2] at room temperature. Recently, we demonstrated that high-quality BiSb thin films with quasi single crystal structure could be grown by sputtering deposition, underlying its potential for mass production of SOT-MRAM [3]. In this work, we show that in all-sputtered BiSb-(Co/Pt)n multilayers structure, the BiSb layer with (110) surface has large θSH = 12.3 and high σ ~ 1.5 × 105 Ω-1m-1, which lead to ultrahigh efficient SOT magnetization switching of (Co/Pt)n multilayers. We deposited (0.4 nm Co/0.4 nm Pt)2 - 10 nm BiSb multilayers on c-plane sapphire (0001) substrate at room temperature (Fig. 1a). The (Co/Pt)2 multilayers are perpendicularly magnetized with a large anisotropy field, as shown in Fig. 1b. We employed the second harmonic Hall measurement to evaluate θSH in 50 μm × 25 μm Hall bars. A representative 2nd harmonic Hall resistance (R2ω) is plotted as a function of in-plane magnetic field (H) in Fig. 1c, and the antidamping-like field HAD is evaluated by fitting to R2ω at high field. Fig. 1d shows the HAD at various current density JBiSb, from which we obtained θSH = 12.3. Thanks to the large θSH, the magnetization of (Co/Pt)2 can be efficiently switched by a small DC threshold current density JthBiSb = 1.5 × 106 Acm-2 at H = ±2.75 kOe (Fig. 2a and 2b). Fig. 2c shows robust switching by 75 consecutive pulses at ±1.83 kOe (JBiSb = 4.4 × 106 Acm-2, tpulse = 100 μs). Our results demonstrate the feasibility of BiSb for ultralow power SOT-MRAM and other SOT-based spintronics devices.References: [1] N. H. D. Khang, Y. Ueda and P. N. Hai, Nat. Mater. 17, 808-813 (2018). [2] Y. Ueda et al., Appl. Phys. Lett. 110, 062401 (2017). [3] T. Fan et al., Jpn. J. Appl. Phys. 59, 063001 (2020).

Ultrahigh Efficient Spin-Orbit-Torque Magnetization Switching in Sputtered Topological Insulator BiSb and (Co/Pt)2 Multilayers

Spintronics has opened a new paradigm through the use of the spin variable as the vector of information and has been largely applied from hard drives read-heads to the STT-MRAMs. While very recent, the introduction of 2D materials in Magnetic Tunnel Junctions (MTJs) has already shown some promising properties[1]. Graphene and the 2D insulator h-BN have been the first 2D materials to show strong impact on spin transport in MTJs. The recent advent of the wide TMDC family of 2D semiconductors opened new opportunities for further tailoring of spintronics properties. We will present results on the scarcely studied WS2. We will detail a protocol using laser lithography technology to fabricate spin valves based on CVD grown WS2, with step by step characterizations in support (Raman spectroscopy, photoluminescence, and AFM measurements). We will finally show our first spin transport measurements obtained in a CVD WS2 based MTJ. Our measured MR signals, above state of the art for 2D semiconductor based MTJs, validates our integration approach. We observe that the spin signal extracted from a ferromagnetic electrode can be tuned by placing atomically thin WS2 on top of it. Furthermore the thickness of WS2 significantly affects the extracted spin polarization of the 2D/FM interface. We discuss that trend using the peculiar band structure of WS2, supported by DFT calculations, leading to thickness dependent spin filtering. Our work opens the way to the integration of different members of the very large TMDCs family, in order to reveal their spin transport properties in MTJs[2].References: [1] M. Piquemal-Banci, R. Galceran, M-B. Martin et al. J. Phys. D: Appl. Phys., 50 , 203002 (2017). [2] V. Zatko, M. Galbiati, S-M-M. Dubois et al. ACS Nano., 13 , 14468 (2019).

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Spin caloritronic device based on asymmetric surfaces of magnetic topological insulators

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Properties of quaternary-alloy ferromagnetic semiconductor (In,Ga,Fe)Sb

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