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Spintronics has traditionally relied on ferromagnetic materials to provide non-volatility and to generate and detect spin currents. However, magnetization reversal by spin transfer and spin orbit torques is a power-consuming process. This is driving research on multiferroics to achieve low-power electric-field control of the magnetization, but practical materials are scarce and magnetoelectric switching remains difficult to control. In this presentation we will focus on an alternative strategy: use ferroelectricity to obtain the remanent control of the spin-charge interconversion. At the interface between a ferroelectric and an ultrathin spin orbit coupling (SOC) systems -such as 2DEG- electrons are accumulated or depleted depending on the polarization direction. This modifies the electric field in the interface region, and in the ideal case changes its sign. If a Rashba state is present in the SOC system at the interface with the ferroelectric, reversing the sign of the local electric field reverses the chirality of the spin textures in both split Fermi contours as depicted in figure 1.This mechanism offers the possibility to design a wealth of devices such as bipolar memories and can also be the basis of logic devices akin to the magnetoelectric spin–orbit device proposed by Intel A highly efficient conversion was evidenced in the two-dimensional electron gas (2DEG) obtained at the interface between SrTiO3 (STO) and LaAlO3 (LAO). Moreover, thanks to the high dielectric constant of STO at cryogenic temperature the spin current generation and detection can be tuned using gate voltage [1,2]. A similar 2DEG can be obtained by depositing a thin layer of a reducing metal as Al on top of STO alleviating the need for the high-temperature growth of crystalline LAO and allowing the modulation of the carrier density through the adjustment of the Al thickness. In this system the gate tunability of the spin to charge conversion and its link to the bandstructure of STO was evidenced [3]. But despite the possibility to tune the conversion using gate voltage in STO based Rashba system, the remanent control of the 2DEG was still not achieved. While STO is not a ferroelectric it is possible to induce ferroelectricity in STO based structure either by doping it with Calcium [4] or by applying high electric field. In this presentation we will evidence that it is possible to control the 2DEG carrier density and the sign of the spin to charge conversion in a remanent way in ferroelectric-like STO based structures [5]. After applying a high enough electric field to induce a ferroelectric state in STO It is possible to modulate the sign of the conversation with positive and negative voltage pulses as depicted in figure 2.References: [1] E. Lesne, Y. Fu, S. Oyarzun et al., Nature Materials 15,1261–1266(2016) [2] F. Trier, D.C. Vaz, P. Noël et al., Nano Lett. 20, 1, 395–401 (2020) [3] D. C. Vaz,P. Noël, A. Johansson et al., Nature Materials 18, 1187–1193(2019) [4] J. Bréhin, F. Trier, P. Noël et al., Phys. Rev. Materials 4, 041)002(R) (2020) [5] P. Noël, F. Trier, L. M. Vicente Arche et al., Nature 580, 483–486(2020)

MMM 2020

Non-volatile electric control of spin–charge conversion in a SrTiO3 Rashba system INVITED

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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technical paper

In the racetrack or similar technologies, information (bit) stored in the direction of the magnetic moment of domains placed next to each other in a wire is shifted when electrical current passes along or parallel to the wire. While using electrons as ‘carrier’, these technologies sometimes lack the deterministic motion of the bits causing data integrity issues [1]. In this work, we are proposing a more deterministic, ‘carrier’ free transmission of data in an insulating ferromagnetic wire where the data is encoded in the oscillation frequencies of magnetic moments. Fig. 1 shows the schematic view of the device, including a thin transmission strip made of soft magnetic material. An Antiferromagnet (AF)/Ferromagnet (F) coupled perpendicular spin torque oscillator (STO) [2] is used to generate the full 360o in-plane magnetization rotation in a small confined region of the transmission strip that is also the free layer of the STO. This precession at STO end generates a traveling rotational magnetization wave along the wire, as seen in Fig. 1. While the spatial wavelength is increasing with distance, the rotational frequency along the wire remains constant, which is a key for our data transmission method. By changing the current amplitude according to the transmission bits, STO frequency can be tuned. Even at 800 nm away from the STO end (Fig. 2), the variation of the rotational frequency with time matches the variation of input STO frequency (or the input current pulse), affirming reliable data transmission. For this micromagnetic simulation, archetypal values for F such as α = 0.01 and Ms = 800 emu/cc are considered. 1 μm wire with a width of 10 nm and thickness of 2 nm is discretized using a grid size of 4nm x 2nm x 2nm.References: [1] Foerster M. et al., Domain Wall Memory Device (2016). In: Xu Y., Awschalom D., Nitta J. (eds) Handbook of Spintronics. Springer, Dordrecht [2] J.-G. Zhu, A. Shadman, IEEE Trans. Magn., vol. 55, no. 3, 2019.

Data Transmission in an Insulator utilizing Rotational Magnetization Wave

It has been shown both theoretically [1] and experimentally [2] that a spectrum analyzer based on an externally driven spin torque nano-oscillator (STNOs) can perform high-speed, wide-band spectrum analysis in a microwave frequency band. In such a spectrum analyzer, an STNO generates a signal whose frequency is tuned in a linear manner over a scanning bandwidth Δf. Theoretical estimations show that when employing an STNO in spectrum analysis, the following can be achieved: a Δf of 10 GHz, a linear scanning rate ρ=Δf/T (where T is the scanning period) of up to 2 GHz/ns, all in a frequency range of operation between 200 MHz and 70 GHz [3]. The fast speed of frequency analysis in a STNO-based spectrum analyzer can be achieved mainly due to the nano-scale size of the STNO, and the consequent ns characteristic scanning times. At a sufficiently large scanning rate ρ, the accuracy of the frequency determination ΔF (defined as the minimum separation required to distinguish two neighboring frequencies) of an STNO-based spectrum analyzer is fundamentally limited by the bandwidth theorem. In contrast, at low scanning rates, this accuracy is limited by the STNO generation linewidth [4], which is determined by the thermal (or random-walk) noise that is causing the jitter of the STNO generation frequency. Fig. 1 shows the frequency accuracy ΔF as a function of the scanning rate ρ for the STNO-based spectrum analyzer [2], where the gray solid line is the result of numerical modeling, dash-dotted line is a result of the experimental measurement [2], and dashed line shows the limit due to the “bandwidth” theorem. It is evident that at sufficiently large scanning rate, ΔF approaches the fundamental limit ΔF =1/T, whereas at smaller ρ the ΔF is limited by the magnitude of the STNO linewidth.References: [1] S. Louis, et al., Appl. Phys. Lett., 113, p. 112401, (2018). [2] A. Litvinenko, et al., “Experimental demonstration of a rapid sweep-tuned spectrum analyzer with temporal resolution based on a spin-torque nano-oscillator,” arXiv preprint arXiv:2004.03508, (2020). [3] S. Louis, “Development of a spectrum analyzer using a spin torque nano-oscillator,” Ph.D. dissertation, Department of Electrical and Computer Engineering, Oakland University, (2020). [4] Joo-Von Kim, et al., Phys. Rev. Lett. 100, 017207 (2008).

The influence of a thermal noise on the accuracy of a spectrum analyzer based on a spin torque nano-oscillator

The Meyer-Neldel (MN) rule, also known as entropy-enthalpy compensation, is a peculiar phenomenon that has been reported across diverse fields of the natural sciences, in semiconductors, chemical reactions, biology, etc [1]. In magnetism, it was very recently observed in the decay of the skyrmion lattice [2]. Consider a family of thermally activated processes whose rates k obey an Arrhenius-type law, i.e., k = f0 exp(-βΔE), in which ΔE is the internal energy barrier, β = (kBT)-1 is Boltzmann's factor, and f0 is the Arrhenius prefactor — the so-called "attempt frequency". If the Meyer-Neldel rule applies, the prefactor scales like an exponential of the energy barrier. In this work, we compute the mean waiting time between thermally activated magnetization reversals in a nanodisk, with parameters that resemble that of a free CoFeB layer, as used in magnetic random access memories [3]. We use two independent approaches, namely Langer's theory [4] and forward flux sampling [5]. By varying the perpendicular anisotropy, and the interfacial Dzyaloshinkii-Moriya interaction (DMI), we find that the Arrhenius prefactor can take extreme, seemingly non-physical values up to 1021 Hz, which is orders of magnitude beyond the typically assumed value of 109 Hz, and vary drastically as a function of material parameters. We show that it behaves like an exponential of the energy barrier, ΔE, which stems from a linear variation of the activation entropy, ΔS, with the energy barrier, thereby demonstrating a new case of the Meyer-Neldel rule (Fig. 1). Our results show that the Arrhenius prefactor is not an attempt frequency, because it also contains an important entropic contribution which cannot be neglected in complex systems. This suggests that modelling information retention times with a barrier-independent prefactor in magnetic storage elements is not justified [6].References: [1] A. Yelon, B. Movaghar, and R. Crandall, Reports on Progress in Physics, Vol. 69, p.1145 (2006) [2] J. Wild, T. N. Meier, S. Pollath et al., Science Advances, Vol. 3, p. e1701704 (2017) [3] J. Sampaio, A. Khvalkovskiy, M. Kuteifan et al., Applied Physics Letters, Vol. 108, p.112403 (2016) [4] J. S. Langer, Annals of Physics, Vol. 54, p. 258 (1969) [5] R. J. Allen, P. B. Warren, and P. R. ten Wolde, Physical Review Letters, Vol. 94, p. 018104 (2005) [6] L. Desplat and J.-V. Kim, submitted (2020)

Entropy-Reduced Retention Times in Magnetic Memory Elements: A Case of the Meyer-Neldel Compensation Rule

Current-perpendicular-to-plane (CPP) giant magnetoresistance (GMR) devices using Co-based Heusler alloys (Co2YZ, where Y is usually a transition metal and Z is a main group element) have attracted much interest as reading heads for next-generation hard disk drives [1-5]. To take full advantage of the half-metallic character of Co-based Heusler alloys, it is important to suppress the antisite of Co atoms, where Co atoms replace Y or Z site [6]. To do this increasing Y or Z composition in Co2YZ is highly effective, and we recently demonstrated the enhancement of MR ratios in off-stoichiometric Mn-rich Co2MnSi-based MTJs [7] and CPP-GMR devices [4]. In this study we systematically investigated the Ge-composition dependence of MR ratios in Co2Fe(Ga,Ge) (CFGG)-based CPP-GMR devices, and achieved the highest MR ratios of 88% at room temperature. We prepared CFGG-based CPP-GMR devices with various Ge composition α in Co2Fe1.03Ga0.41Geα electrodes ranging from 0.56 to 1.25. The layer structures of the fabricated CPP spin valves were as follows: (from the substrate side) MgO (10 nm)/Co50Fe50 (10)/Ag (100)/CFGG lower electrode (10)/NiAl (0.21)/Ag (5)/NiAl (0.21)/CFGG upper electrode (8)/Ru (5), grown on MgO(001) substrates. Ultrathin (0.21 nm) NiAl layers were aimed to enhancing the MR ratio [3]. Just after the deposition of CFGG upper electrodes, the layer structure was in-situ annealed at 550°C. We fabricated CPP spin valves with the nominal junction sizes ranging from 70 × 120 nm to 400 × 640 nm by using EB lithography and Ar ion milling. The MR characteristics were measured using a dc four-probe method at 290 K. We observed the MR ratio enhanced from 21% to 88% as α increased from 0.56 (Ge-stoichiometric CFGG) to 1.25 (Ge-rich CFGG). To the best of our knowledge, the MR ratio of 88% is the highest value ever reported among CPP-CMR devices. This result suggests that Ge-rich CFGG is effective in improving the half-metallic character of CFGG possibly due to the suppression of Co antisite with the increase of Ge composition.References: [1] Y. Sakuraba et al., Appl. Phys. Lett. 101, 252408 (2012). [2] H. Narisawa et al., Appl. Phys. Express 8, 063008 (2015). [3] Y. Du et al., Appl. Phys. Lett. 107, 112405 (2015). [4] M. Inoue et al., Appl. Phys. Lett. 111, 082403 (2017). [5] J. W. Jung et al., Appl. Phys. Lett. 108, 102408 (2016). [6] S. Picozzi et al., Phys. Rev. B 69, 094423 (2004). [7] H.-x. Liu et al., Appl. Phys. Lett. 101, 132418 (2012).

Enhancement of magnetoresistance characteristics of Ge-rich Co2Fe(Ga,Ge)-based current-perpendicular-to-plane giant magnetoresistance devices

Ferromagnetic semiconductors (FMSs) which show both ferromagnetic and semiconducting characteristics are promising materials for spintronics applications. Recently, we have successfully grown Fe-doped FMSs; p-type (Ga,Fe)Sb with Curie temperature (TC) ≈ 340 K2 and n-type (In,Fe)Sb with TC ≈ 335 K3, which are promising for devices operating at room temperature. One of the important steps towards practical applications is to realize spin-valve effects in heterostructures containing Fe-doped FMSs. In this work, we demonstrate spin-valve magnetoresistance (MR) in all-semiconductor heterostructures containing Fe-doped FMS (Ga,Fe)Sb layers with TC ≈ 260 K. The samples examined here consist of (Ga0.75,Fe0.25)Sb (40 nm) / InAs (thickness tInAs = 0, 3, 6, 9 nm) / (Ga0.8,Fe0.2)Sb (40 nm) grown by low temperature molecular beam epitaxy (LT-MBE) [Fig. 1(a) and (b)]. In these structures, electrical current flows mainly in InAs due to the high resistivity of (Ga,Fe)Sb layers at low temperature (≤ 20 K)4. A clear spin-valve effect with an MR ratio of ~ 1.6% with an open minor loop is observed in the structure with tInAs = 3 nm at 3.7 K [Fig. 2], whose peaks (≈ ±0.1 T) correspond to the coercive forces in the magnetization characteristics of the (Ga,Fe)Sb layers obtained with superconducting quantum interference device (SQUID) magnetometry and anomalous Hall effect (AHE) measurements. We found that the spin-valve MR (MRSV) increases (from 0.03 to 1.6%) with decreasing tInAs (from 9 to 3 nm), which reflects the enhancement of spin-dependent scattering at the InAs/(Ga,Fe)Sb interfaces. This demonstration is the first important step towards device application of these promising high-TC FMSs.References: [1] L. Chen et al, Nano Lett., Vol. 11, p.2584 (2011). [2] N. T. Tu, et al, Appl. Phys. Lett., Vol. 108, p.192401(2016). [3] N. T. Tu, et al, Appl. Phys. Express, Vol. 11, p.063005(2018). [4] N. T. Tu, et al., Phys. Rev. B, Vol. 92, p.144403 (2015). [5] K. Sriharsha, et al., APL Mater., Vol. 7, p.021105 (2019).

Current-in-plane spin-valve magnetoresistance in ferromagnetic semiconductor (Ga,Fe)Sb heterostructures with high Curie temperature

Chirality-induced spin selectivity (CISS), with which structural chirality leads to different electric conductivities for electrons of opposite spins, is of significant current interest for its underlying physics and device potentials.1 The spin filtering effect has been reported for a number of chiral molecules, however, the microscopic origin and transport mechanisms remain controversial. In particular, the Onsager relation is expected to preclude linear-response detection of CISS by a ferromagnet in a two-terminal spin valve device. Here, we report observation of CISS in vertical heterojunctions of (Ga,Mn)As/AHPA-L molecules/Au. The robust CISS effect in such devices enables a detailed examination of its bias dependence, which shows the presence of both linear- and nonlinear-response magnetoconductance. The experiment directly verifies spin filtering by the AHPA-L molecules and spin detection by the (Ga,Mn)As. The results constitute definitive signature of CISS-induced two-terminal spin-valve effect, in apparent violation of the Onsager reciprocity. As a new twist in spintronics, the CISS effect on semiconductors has the potential of facilitating spin injection into GaAs at room temperature without the use of any ferromagnet. This work is supported by NSF grant DMR-1905843.References: 1R. Naaman and D.H. Waldeck, Annu. Rev. Phys. Chem. 66, 263 (2015).

Robust two-terminal spin-valve effect from chirality-induced spin selectivity

The recent emergence of novel spintronics studies focuses on the generation, transmission, and control of a pure spin current through spin-orbit effects by converting the pure spin current into the charge current or vice versa. The emergence of novel electronic and magnetic phenomena at the interfaces between polar and non-polar crystalline transition metal complex oxides such as high mobility two-dimensional electron gases (2DEG) provides an ideal platform for the design of spin-to-charge (StC) convertors. At these interfaces, an interface-driven spin-orbit coupling mechanism - the Rashba effect - shows great promise due to its unprecedented StC interconversion efficiency, so-called inverse Rashba-Edelstein effect (IREE). Here we report a new type of 2DEG formed at the interface between the antiferromagnetic LaCrO3 and insulating SrTiO3 heterostructure with efficient StC conversion efficiency. The measured IREE length is up to 0.3 nm which can be modulated by a gate voltage. The frequency and field orientation dependence of IREE response are both consistent with the spin pumping model, from which the spin relaxation time (~30ps) is derived. Our findings launch a new class of oxide-based 2DEGs for future spin orbitronic applications. Device preparation was partially supported by ECCS-1933297 and NC State-Nagoya Collaboration Grant. Sample growth was supported by NSF DMR-1751455.

Room temperature gate tunable spin-to-charge conversion in LaCrO3/SrTiO3 heterostructures

Spintronic devices operating with pure spin currents represent a new paradigm in nanoelectronics, with higher energy efficiency and lower dissipation as compared to charge currents. This technology, however, will be viable only if the amount of spin current in a nanochannel can be tuned while guaranteeing electrical compatibility with other device elements, to which it should be integrated in high-density three-dimensional architectures. Here, we address these two crucial milestones and demonstrate that pure spin currents can effectively propagate in metallic nanochannels with a three-dimensional curved geometry [1]. We report the first measurements of pure spin currents in curved metallic channels. The curved channel is realized by e-beam evaporation of aluminium thin films over predefined profiles in the form of focussed ion beam-etched trenches (Fig. 1). Remarkably, the geometric design of the nanochannels can be used to reach an independent tuning of spin transport and charge transport characteristics. These results put the foundation for the design of efficient pure spin current based electronics, which can be integrated in complex three-dimensional architectures.References: [1] Das, K. S. et al., Nano Letters, Vol. 19, p. 6839–6844 (2019)

Independent Geometrical Control of Spin and Charge Resistances in Curved Spintronics

Manganites have shown potential in spintronics due to their low damping and insulating characteristics. Here, we studied the damping properties of La0.67Sr0.33MnO3/Pt bilayer samples which are prepared using oxide molecular beam epitaxy. We have observed decrease in damping coefficient (α) with increase in Pt thickness. Further, we investigated inverse spin Hall effect (ISHE) and observed that ISHE signal gets enhanced with Pt thickness. We observed maximum spin pumping voltage of 20.05 µV for the sample with Pt thickness of 3 nm. We have evaluated spin Hall angle for these samples. Introduction: Pure spin current based devices are potential for faster and low power consumption [1]. Generation of pure spin current has been demonstrated by ferromagnetic resonance (FMR) through spin pumping mechanism [2]. This pure spin current can loss their spin angular momentum in the presence of high spin orbit coupling (SOC) material e.g. Pt, W, Ta. The loss of spin angular momentum can develop voltage by asymmetric scattering of spins, which is known as inverse spin Hall effect (ISHE) [3]. So far studies have been concentrated mostly on Pt and FM metals [4]. However, magnetic oxides are less explored. La0.67Sr0.33MnO3 (LSMO) is well known FM oxide for exhibiting high Curie temperature (TC=350 K) and nearly 100% spin polarization (in bulk) [5]. In this work, in order to get high spin Hall angle, high resistive Pt film is grown on LSMO thin film. Experimental details: LSMO (20 nm)/Pt (tPt = 0, 3 and 10 nm) bilayer samples have been prepared on SrTiO3(001) substrate using an oxygen plasma assisted molecular beam epitaxy system. ISHE measurements are performed using home modified coplanar wave-guide (CPW) based ferromagnetic resonance (FMR) spectroscopy [6]. Results: We have studied the static and dynamic properties of the LSMO/Pt systems. A decrease in α has been observed with increase in Pt thickness. We performed angle dependent ISHE at frequency of 7 GHz. From angle dependent ISHE measurement spin Hall angle were calculated 0.033 and 0.014 for samples with 3 and 10 nm of Pt, respectively.References: [1]. C. Chappert, A. Fert, and F. N. Van Dau, Nature Materials 6, 813 (2007). [2]. O. Mosendz, J. Pearson, F. Fradin, Phys. Rev. Lett. 104, 046601 (2010). [3]. J. Hirsch, Phys. Rev. Lett. 83, 1834 (1999). [4]. J.-C. Rojas-Sanchez, N. Reyren, P. Laczkowski, Phys. Rev. Lett. 112, 106602 (2014). [5]. J.-H. Park, E. Vescovo, H.-J. Kim, Nature 392, 794 (1998). [6]. B. B. Singh, S. K. Jena, M. Samanta, Phys. Status Solidi Rapid Res. Lett. 13, 1800492 (2019).

Simultaneous observation of anti-damping and inverse spin Hall effect in La0.67Sr0.33MnO3/Pt bilayers

Ferrimagnetic insulator garnet films play an important role in magnon spintronics where magnon-mediated spin currents are used to carry, transport, and process information. This wave-based computing technologies lead to energetically more efficient devices and allows a decrease in the footprint of computing elements [1]. When a magnetic insulator (MI) film is combined with a non-magnetic (NM) spin current detector material like platinum, it is possible to electrically control the MI’s magnetization via the spin-orbit torque (SOT) effect. If the MI presents a perpendicular magnetic anisotropy (PMA), the electrical current density required to switch the magnetization is lower, and the process is faster. This makes the magnetoresistive random access memories based on SOT (SOT-MRAM) challenge the MRAMs based on the spin-transfer torque effect (STT-MRAM) [2]. Moreover, ferrimagnetic garnets with PMA enables high-speed domain-wall (DW) motion that also can be driven by electrical currents via SOT and are ideal materials for developing efficient DW racetrack memories [3]. Thulium iron garnet (TIG – Tm3Fe5O12) is a good candidate due to its high critical temperature of 549 K, a Gilbert damping of 10-2, and negative magnetostriction constant that favors the control of the PMA [4]. When combined with Pt, TIG/Pt is a reach system for the study of spin waves and the observation of spin-orbit effects. We investigate the generation and detection of magnon-based spin currents driven by microwaves and heat currents via the spin pumping (SPE) and spin Seebeck (SSE) effects in TIG(60 nm)/Pt(4 nm), at room temperature. The TIG film was deposited over GGG (Gd3Ga5O12) substrate using the sputtering technique and presented a flat surface with an RMS roughness of 0.1 nm. The magnetization saturation was ~ 100 emu/cm3, and the Gilbert damping parameter was 0.02. The results showed an effective spin-to-charge conversion via the inverse spin Hall effect (ISHE). It made it possible to detect a micro-voltage at the edges of the Pt film and confirmed high spin transparency of the interface TIG/Pt. The spin Seebeck coefficient was 0.54 μV/K, whereas the ratio between the microwave-driven voltage and the microwave power was 4μV/W.References: [1] A. V. Chumak et al., Nature Physics, Vol. 11, p. 453 (2015). [2] M. Cubukcu et al., IEEE Transactions on Magnetics, Vol. 54, p. 1 (2018). [3] S. Vélez et al., Nature Communications, Vol. 10, p. 4750 (2019). [4] G. Vilela et. al., Journal of Applied Physics, Vol. 127, p. 115302 (2020).

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Non-volatile electric control of spin–charge conversion in a SrTiO3 Rashba system INVITED

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