Spin-based electronics has evolved into a major field of research that broadly encompasses different classes of materials, magnetic systems, and devices. This review describes recent advances in spintronics that have the potential to impact key areas of information technology and microelectronics. We identify four main axes of research: nonvolatile memories, magnetic sensors, microwave devices, and beyond-CMOS logic. We discuss state-of-the-art developments in these areas as well as opportunities and challenges that will have to be met, both at the device and system level, in order to integrate novel spintronic functionalities and materials in mainstream microelectronic platforms.Conventional information processing and communication devices work by controlling the flow of electric charges in integrated circuits. Such circuits are based on nonmagnetic semiconductors, in Technologies based on GMR and MTJ devices are now firmly established and compatible with CMOS fab processes. Yet, in order to meet the increasing demand for high-speed, high-density, and low power electronic components, the design of materials, processes, and spintronic circuits needs to be continuously innovated. Further, recent breakthroughs in basic research brought forward novel phenomena that allow for the generation and interconversion of charge, spin, heat, and optical signals.Many of these phenomena are based on non-equilibrium spin-orbit interaction effects, such as the spin Hall and Rashba-Edelstein effects 6,8,23 or their thermal 24 and optical 25,26 analogues. Spin-orbit torques (SOT), for example, can excite any type of magnetic materials, ranging from metals to semiconductors and insulators, in both ferromagnetic and antiferromagnetic configurations 6 . This versatility allows for the switching of single layer ferromagnets, ferrimagnets, and antiferromagnets, as well as for the excitation of spin waves and auto-oscillations in both planar and vertical device geometries 10,11 . Charge-spin conversion effects open novel pathways for information processing using Boolean logic, as well as promising avenues for implementing unconventional neuromorphic 27,28,29 and probabilistic 30 computing schemes. Finally, spintronic devices cover a broad bandwidth ranging from DC to THz 31,32 , leading to exciting opportunities for the on-chip generation and detection of high frequency signals.

Electrical currents at the surface or edge of a topological insulator are intrinsically spin-polarized. We show that such surface/edge currents can be used to switch the orientation of a molecular magnet weakly coupled to the surface or edge of a topological insulator. For the edge of a two-dimensional topological insulator as well as for the surface of a three-dimensional topological insulator the application of a well-chosen surface/edge current can lead to a complete polarization of the molecule if the molecule's magnetic anisotropy axis is appropriately aligned with the current direction. For a generic orientation of the molecule a nonzero but incomplete polarization is obtained. We calculate the probability distribution of the magnetic states and the switching rates as a function of the applied current.

The spin chemical potential characterizes the tendency of spins to diffuse. Probing this quantity could provide insight into materials such as magnetic insulators and spin liquids and aid optimization of spintronic devices. Here we introduce single-spin magnetometry as a generic platform for nonperturbative, nanoscale characterization of spin chemical potentials. We experimentally realize this platform using diamond nitrogen-vacancy centers and use it to investigate magnons in a magnetic insulator, finding that the magnon chemical potential can be controlled by driving the system's ferromagnetic resonance. We introduce a symmetry-based two-fluid theory describing the underlying magnon processes, measure the local thermomagnonic torque, and illustrate the detection sensitivity using electrically controlled spin injection. Our results pave the way for nanoscale control and imaging of spin transport in mesoscopic systems.

We investigate the origin of the spin Seebeck effect in yttrium iron garnet (YIG) samples for film thicknesses from 20 nm to 50 μm at room temperature and 50 K. Our results reveal a characteristic increase of the longitudinal spin Seebeck effect amplitude with the thickness of the insulating ferrimagnetic YIG, which levels off at a critical thickness that increases with decreasing temperature. The observed behavior cannot be explained as an interface effect or by variations of the material parameters. Comparison to numerical simulations of thermal magnonic spin currents yields qualitative agreement for the thickness dependence resulting from the finite magnon propagation length. This allows us to trace the origin of the observed signals to genuine bulk magnonic spin currents due to the spin Seebeck effect ruling out an interface origin and allowing us to gauge the reach of thermally excited magnons in this system for different temperatures. At low temperature, even quantitative agreement with the simulations is found. The thermal excitation of a spin current by a temperature gradient is commonly called the spin Seebeck effect (SSE) which is detected by the inverse spin Hall effect (ISHE) [1,2], leading to a thermovoltage similar to the charge analogue, the Seebeck effect. Experimental evidence of the SSE, first in ferromagnetic metals [3], and later, both in semiconductors [4] and in insulators [5][6][7][8], has brought up the question about the origin of the SSE. Of particular interest for spin caloritronics is the observation of the SSE in insulators, which allows us to generate pure spin currents in insulating systems.However, the underlying mechanism, properties, and the origin of the observed signals have been highly controversial. Thermally induced magnonic spin currents have been suggested as the origin [9,10], based on the presence of the effect in magnetic insulators, which excludes charge currents as the source. Despite this explanation of the origin of the effect, direct experimental evidence has not been reported. While parasitic interface effects [11] were suggested as an alternative source of the SSE due to a polarization of the paramagnetic detector layer [12], generally, the observed effects are now primarily attributed to magnonic spin currents [13,14].Time resolved experiments trying to address the problem by probing the temporal evolution of the SSE have obtained contradictory results: For film thickness up to 61 nm, no cut-off frequency due to an intrinsic limitation by the SSE was observed [15]. In contrast, for μm thick films, a characteristic rise time was found, and a finite magnon propagation length of the order of several 100 nm was put forward as a possible explanation [16,17]. This clearly calls for study to reveal the origin of this discrepancy as it underlies the fundamental mechanism of the SSE and to determine the intrinsic length scale.To clarify the origin of the measured SSE signals, we present a detailed study of the relevant length scales of the longitudinal SSE (LSSE) coveri...

Introducing magnetic order in a topological insulator (TI) breaks time-reversal symmetry of the surface states and can thus yield a variety of interesting physics and promises for novel spintronic devices. To date, however, magnetic effects in TIs have been demonstrated only at temperatures far below those needed for practical applications. In this work, we study the magnetic properties of Bi2Se3 surface states (SS) in the proximity of a high Tc ferrimagnetic insulator (FMI), yttrium iron garnet (YIG or Y3Fe5O12). Proximity-induced butterfly and square-shaped magnetoresistance loops are observed by magneto-transport measurements with out-of-plane and in-plane fields, respectively, and can be correlated with the magnetization of the YIG substrate. More importantly, a magnetic signal from the Bi2Se3 up to 130 K is clearly observed by magneto-optical Kerr effect measurements. Our results demonstrate the proximity-induced TI magnetism at higher temperatures, an important step toward room-temperature application of TI-based spintronic devices.

Yttrium iron garnet (YIG, Y 3Fe5O12) films have been epitaxially grown on Gadolinium Gallium Garnet (GGG, Gd3Ga5O12) substrates with (100) orientation using pulsed laser deposition. The films were single-phase, epitaxial with the GGG substrate, and the root-mean-square surface roughness varied between 0.14 nm and 0.2 nm. Films with thicknesses ranging from 17 to 200 nm exhibited low coercivity (<2 Oe), near-bulk room temperature saturation moments (∼135 emu cm−3), in-plane easy axis, and damping parameters as low as 2.2 × 10−4. These high quality YIG thin films are useful in the investigation of the origins of novel magnetic phenomena and magnetization dynamics.

Harnessing the electron's second fundamental property, its spin, is the basis of spintronic phenomena and devices [1] . These include recently discovered phenomena like the quantum anomalous Hall effect [2] in magnetic topological insulators [3] , spin transfer torque [4,5] effects in nonmagnetic metal / ferromagnetic metal / oxide heterostructures and spin transfer torque (SOT) switching of ferromagnets [6,7] , and ultimately of FMIs [8] . To realize novel circuit devices based on these effects a rich variety of specifically tailored magnetic materials has still to be developed. For instance, the study of exotic phenomena occurring at the boundary of topological insulators with ferromagnets requires the latter to be insulating, yet to retain magnetic properties including PMA.One of the most prominent FMIs classes is that of iron garnets, of which the most well studied is Y 3 Fe 5 O 12 (YIG). The ultra low magnon damping characteristics [9] and magneto-optical properties [10,11] of YIG are well known. The former makes YIG a suitable candidate for spin wave logic [12] and signal transmitters [13] due to the extremely large magnon decay length of several tens of millimeters. Epitaxial YIG thin films can, in principle, also possess PMA as a result of magnetization-lattice coupling [14] for thicknesses below nm [15,16] , but the fabrication of YIG films with complete out-of-plane remanence remains elusive because of its low magnetocrystalline anisotropy and magnetoelastic coefficients. In contrast, 50 nm thick thulium iron garnet (Tm 3 Fe 5 O 12 , TmIG), has been reported to show PMA [17,18] caused by magnetoelastic anisotropy when grown epitaxially on (111)-oriented gallium gadolinium garnet (Gd 3 Ga 5 O 12 or GGG) [19] .We recently demonstrated [8] reversible magnetization switching in 8 nm thick TmIG by utilizing SOT from an adjacent platinum layer, but a detailed structural and magnetic characterization of TmIG in the few-nm thickness regime is still lacking. In the present article, we provide a comprehensive description of the structural characteristics and magnetic properties of TmIG/GGG down to a thickness of nm. Furthermore we demonstrate that efficient spin transport can be achieved through the TmIG/Pt interface by measuring spin Hall magnetoresistance (SMR) in Pt. We exploit this method to measure the anisotropy field of the strained TmIG film electrically, which is inaccessible by conventional magnetometry measurements due to the dominant paramagnetic contribution of the GGG substrate. These results emphasize the potential of TmIG as a spintronic material.Structural characterization of the TmIG films are summarized in Fig.1. With xray reflectometry (XRR) scans (not shown), we measured film thicknesses of PMA epitaxial TmIG down to nm nm . To quantify the strain state of TmIG via XRD we measured a nm thick TmIG film, since thinner films could not be resolved with enough intensity using high resolution XRD. The symmetric XRD spectra shown in Fig.1a demonstrates a fully strained film with lattice spacing of ...

The dependence of the spin-pumping effect on the yttrium iron garnet (Y 3 Fe 5 O 12 , YIG) thickness detected by the inverse spin Hall effect (ISHE) has been investigated quantitatively. Due to the spin-pumping effect driven by the magnetization precession in the ferrimagnetic insulator Y 3 Fe 5 O 12 film a spin-polarized electron current is injected into the Pt layer. This spin current is transformed into electrical charge current by means of the ISHE. An increase of the ISHE voltage with increasing film thickness is observed and compared to the theoretically expected behavior. The effective damping parameter of the YIG/Pt samples is found to be enhanced with decreasing Y 3 Fe 5 O 12 film thickness. The investigated samples exhibit a spin mixing conductance of g ↑↓ eff = (3.87 ± 0.21) × 10 18 m −2 and a spin Hall angle between θ ISHE = 0.013 ± 0.001 and 0.045 ± 0.004 depending on the used spin-diffusion length. Furthermore, the influence of nonlinear effects on the generated voltage and on the Gilbert damping parameter at high excitation powers is revealed. It is shown that for small YIG film thicknesses a broadening of the linewidth due to nonlinear effects at high excitation powers is suppressed because of a lack of nonlinear multimagnon scattering channels. We have found that the variation of the spin-pumping efficiency for thick YIG samples exhibiting pronounced nonlinear effects is much smaller than the nonlinear enhancement of the damping.