Jean-Eric Wegrowe

Insights into the orbital magnetism of noncollinear magnetic systems

The orbital magnetic moment is usually associated with the relativistic spin-orbit interaction, but recently it has been shown that noncollinear magnetic structures can also be its driving force. This is important not only for magnetic skyrmions, but also for other noncollinear structures, either bulk-like or at the nanoscale, with consequences regarding their experimental detection. In this work we present a minimal model that contains the effects of both the relativistic spin-orbit interaction and of magnetic noncollinearity on the orbital magnetism. A hierarchy of models is discussed in a step-by-step fashion, highlighting the role of time-reversal symmetry breaking for translational and spin and orbital angular motions. Couplings of spin-orbit and orbit-orbit type are identified as arising from the magnetic noncollinearity. We recover the atomic contribution to the orbital magnetic moment, and a nonlocal one due to the presence of circulating bound currents, exploring different balances between the kinetic energy, the spin exchange interaction, and the relativistic spin-orbit interaction. The connection to the scalar spin chirality is examined. The orbital magnetism driven by magnetic noncollinearity is mostly unexplored, and the presented model contributes to laying its groundwork.

Anomalous optical properties of Rashba conductor (Conference Presentation)

We theoretically explore the optical properties of a bulk Rashba conductor by calculating the transport coefficients at finite frequencies. It is demonstrated that the combination of direct and inverse Edelstein effects leads to a softening of the plasma frequency for the electric field perpendicular to the Rashba field, resulting in a hyperbolic electromagnetic metamaterial. In the presence of magnetization, a significant enhancement of anisotropic propagation (directional dichroism) is predicted because of the interband transition edge singularity. On the basis of an effective Hamiltonian analysis, the dichroism is demonstrated to be driven by toroidal and quadratic moments of the magnetic Rashba system. The effective theory of the cross-correlation effects has the same mathematical structure as that of insulating multiferroics.

Eigenmodes of semiconductor spin-lasers with local linear birefringence and gain dichroism (Conference Presentation)

We present a general method for modeling spin-lasers such as spin-polarized vertical cavity surface emitting laser (spin-VCSELs) with multiple quantum wells including anisotropic effects such as i) the emission of elliptically-polarized photons and originating from unbalanced spin-up and spin-down pumps, ii) the linear gain dichroism originating from the reduction from Td to C2v symmetry group at the III-V ternary interfaces and iii) the locally linear birefringence due to the anisotropic strain field at surface of ½ VECSELs an optical birefringence of quantum wells from the Henry’s factor. New recurrence calculations, together with analytically gain tensor derived from Maxwell-Bloch equations, enable to model emission from multiple quantum well active zones to find the laser resonance conditions and properties of eigenmodes. The method is demonstrated on real semiconductor laser structures. It is used for the extraction of optical permittivity tensors of surface strain and of quantum wells (QWs). The laser structures are also experimentally studied via ellipsometry methods by measurement of the rotation spectra of complete Mueller matrix in the reflection geometry. The anisotropic optical permittivity constants in the spectral range from 0.73 to 6.4 eV are modeled in order to disantangle surface and QWs contributions to the linear optical birefringence of the structures.

Phonon-induced superconductivity in a self-consistent Hubbard model (Conference Presentation)

We have studied an infinite Hubbard chain with a spin-orbit coupling term, submitted to a uniform magnetic field as well as local phonons. By means of a Lang-Firsov transformation, we show that an effective interacting fermion model emerges. Moreover, a self-consistent mean-field theory of this model, formulated in terms of thermal Greens functions, shows that a BCS term emerges, thus leading to a superconducting phase transition at zero temperature. We find analytical expressions for the phase boundary, that agree well with exact numerical diagonalization of the Hamiltonian.

Injection of sub-picosecond ultrashort spin current pulses in semiconductors (Conference Presentation)

The origin of the ultrafast demagnetisation has been a mystery for a long time. Recently we have proposed an approach based on spin dependent electron superdiffusion. [1-3] A number of experimental works have confirmed the importance and the amplitude of the superdiffusive spin transport for ultrafast magnetisation dynamics [4-7]. In particular the spin superdiffusion model predicted the transfer of magnetisation in the non-magnetic substrate and the possibility of increasing the magnetisation: both phenomena were experimentally confirmed. [4-5] We predict the possibility of injecting ultrashort (sub-picosecond) spin current pulses from a ferromagnetic metallic layer undergoing ultrafast demagnetisation into a semiconducting substrate. [8] After laser excitation, energetic carriers can overcome the semiconductor bandgap. We address the complex interplay of spin diffusion, the formation of high electric fields at the metal/semiconductor interface, and the concomitant thermalisation of the laser excited carriers by ad hoc numerical techniques. We show that spin currents pulses hundreds of femtoseconds long are injected in the semiconductor and present a record spin polarisation. Such spin current pulses have the possibility of becoming the carriers of information in future spintronics running at unprecedented frequencies above the THz regime. [1] M. Battiato, K. Carva, P.M. Oppeneer, Phys Rev. Lett. 105, 027203 (2010). [2] M. Battiato, K. Carva, P.M. Oppeneer, Phys Rev. B 86, 024404 (2012). [3] M. Battiato, P. Maldonado, P.M. Oppeneer, J. Appl. Phys. 115, 172611 (2012). [4] A. Melnikov et al., Phys. Rev. Lett. 107, 076601 (2011). [5] D. Rudolf, C. La-O-Vorakiat, M. Battiato et al., Nature Comm. 3, 1037 (2012). [6] A. Eschenlohr,* M. Battiato,* et al., Nature Mater. 12, 332 (2013). [7] T. Kampfrath, M. Battiato, et al, Nature Nanotechnol. 8, 256 (2013). [8] M. Battiato and K. Held, Phys Rev. Lett. 116, 196601 (2016).

Neuromorphic computing with stochastic spintronic devices (Conference Presentation)

Spin torque magnetic memory (ST-MRAM) is currently under intense academic and industrial development, as it features non-volatility, high write and read speed and high endurance. However, one of its great challenge is the probabilistic nature of programming magnetic tunnel junctions, which imposes significant circuit or energy overhead for conventional ST-MRAM applications. In this work, we show that in unconventional computing applications, this drawback can actually be turned into an advantage. First, we show that conventional magnetic tunnel junctions can be reinterpreted as stochastic “synapses” that can be the basic element of low-energy learning systems. System-level simulations on a task of vehicle counting highlight the potential of the technology for learning systems. We investigate in detail the impact of magnetic tunnel junctions’ imperfections. Second, we introduce how intentionally superparamagnetic tunnel junctions can be the basis for low-energy fundamentally stochastic computing schemes, which harness part of their energy in thermal noise. We give two examples built around the concepts of synchronization and Bayesian inference. These results suggest that the stochastic effects of spintronic devices, traditionally interpreted by electrical engineers as a drawback, can be reinvented as an opportunity for low energy circuit design.

First-principles calculation of spin transport and relaxation in magnetic heterostructures (Conference Presentation)

Manipulation of a spin current at nanoscale is desired in many proposed spintronics devices. Magnetic multilayers consisting of ferromagnetic, ferrimagnetic and nonmagetic materials show rich phenomena when a spin current propagates through the multilayers. An interface of ferromagnetic and nonmagnetic metals has been demonstrated to play an important role in the generation and dissipation of a spin current. Using first-principles scattering calculation, we study the transport and relaxation of spin currents in typical transition metals and alloys and their interfaces. In particular, we focus on identifying the correlation of spin transport and relaxation with the specific order parameters of magnetic materials. By examining the spin-Hall conductivity and spin-flip diffusion length as a function of conductivity (resistivity), we are able to distinguish different dominant physical mechanisms of the generation and dissipation of spin currents.

Acoustic driven magnonics (Conference Presentation)

Magnonics aims to utilize magnons (quanta of spin-waves) to process and transfer digital and analog information, promising high throughput, low power computing for the post-CMOS era. Any such future magnonic circuits will require spin wave signal sources and amplifiers. We propose that acoustic pumping of spin waves provides a mechanism to implement these functions in efficient and highly localized devices. In support of this, we have developed a general theoretical model of linear and parametric magneto-acoustic interactions, covering all possible polarizations of acoustic waves and spin wave modes. The model combines the predictive power of analytical techniques with numerical micromagnetic simulations and is thus well-suited for the design of complex physical devices. Based on this, we determine the configurations most amenable to spin wave generation and amplification. As an experimental prototype we demonstrate an acoustically-pumped amplifier for spin-waves. Our device consists of an yttrium-iron-garnet (YIG) film grown on a gallium gadolinium garnet (GGG) substrate, with a bulk acoustic waves (BAW) transducer fabricated on the top of the GGG substrate. We show experimentally that the amplitude of the propagating spin-waves increases with the application of the BAW. Moreover, this scheme can be used as a signal correlator, where the modulated spin-waves and acoustic waves serve as signal inputs and the resulting modulation of the amplified spin wave serves as the output.

All-optical magnetization switching of FePt magnetic recording medium (Conference Presentation)

Magnetization manipulation is an indispensable tool for both basic and applied research [1].

Room-temperature skyrmion shift device for memory application (Conference Presentation)

Magnetic skyrmions are intensively explored for potential applications in ultralow-energy data storage and computing. To create practical skyrmionic memory devices, it is necessary to electrically create and manipulate these topologically-protected information carriers in thin films, thus realizing both writing and addressing functions. Although room-temperature skyrmions have been previously observed, fully electrically controllable skyrmionic memory devices, integrating both of these functions, have not been developed to date. In this talk, I will talk about our recent demonstration of a room-temperature skyrmion shift memory device, where individual skyrmions are controllably generated and shifted using current-induced spin-orbit torques. Particularly, it is shown that one can select the device operation mode in between: (i) writing new single skyrmions, or (ii) shifting existing skyrmions, by controlling the magnitude and duration of current pulses. Thus, we electrically realize both writing and addressing of a stream of skyrmions in the device. This prototype demonstration brings skyrmions closer to real-world computing applications.