Session L41: Magnetic dynamics and magnetic switching

Attosecond Spintronics

The coupling between electronic and magnetic phenomena was one of the riddles propelling the development of modern electromagnetism. Today, the fully controlled electric field evolution of ultrashort laser pulses permits the direct and ultrafast control of electronic properties of matter and is the cornerstone of light-wave electronics. In sharp contrast, because there is no first order interaction between light and spins, the magnetic properties of matter can only be affected indirectly on the much slower tens-of-femtosecond timescale in a sequence of optical excitation followed by the rearrangement of the spin structure.
I will discuss an experiment recording an orders of magnitude faster magnetic switching with sub-femtosecond response time by initiating optical excitations with near-single-cycle laser pulses in a ferromagnetic layer stack. The unfolding dynamics are tracked in real-time by a novel attosecond time-resolved magnetic circular dichroism (atto-MCD) detection scheme revealing optically induced spin and orbital momentum transfer (OISTR) in synchrony with light field driven charge relocation. In tandem with ab-initio quantum dynamical modelling, we show how this mechanism provides simultaneous control over electronic and magnetic properties that are at the heart of spintronic functionality. This first incarnation of attomagnetism observes light field coherent control of spin-dynamics in the initial non-dissipative temporal regime and paves the way towards coherent spintronic applications with Petahertz clock rates.
  

Unification of ultrafast demagnetization and switching

All-optical spin switching and demagnetization are two sides of the same physics. We find a way to unify
them through the same model that takes into account the Heisenberg exchange between spins and spin-orbit
coupling. We include both the kinetic energy term and potential energy term, so that we can account for the
realistic interaction between the laser and the system. To have efficient spin switching, the electron initial
momentum direction must closely follow the spin's orientation, so that the orbital angular momentum is
transverse to the spin, and consequently the spin-orbit torque lies in the same direction as the spin.
Both left and right circularly polarized light have stronger but different effects in both uniform spin domains and
The demagnetization can be understood from the spin-orbit coupling-mediated spin-wave
excitation. The efficiency of laser-induced demagnetization is very high. We also observe the collapse of the
spin-spin correlation length within 20 fs, consistent with the experimental findings.

[1] G. P. Zhang and M. Murakami, Journal of Physics: Condensed Matter 31, 345802 (2019).
[2] G. P. Zhang , M. Murakami , Y. H. Bai, Thomas F. George, and X. S. Wu, J. Appl. Phys. 126, 103906 (2019).   

Ultrafast Magnetic Spectroscopy, Scattering and Full-Field Imaging Using Tabletop High Harmonic Sources

The observation and manipulation of spin dynamics on the nanoscale is a crucial ability in our quest to engineer quantum materials. We present spectroscopy, scattering, and full field imaging of magnetic nanostructures using EUV high harmonic sources. Our control of EUV polarization, spin and orbital angular momentum, along with high spatial and temporal coherence, allows us to uniquely control and probe spin dynamics on few-femtosecond timescales on up.   

Revealing angular momentum transfer channels and timescales in the ultrafast demagnetization process of ferromagnetic semiconductors

Ultrafast control of magnetic order by light provides a promising realization for spintronic devices beyond Moore’s Law. Here, we unravel the laser-induced demagnetization mechanism of ferromagnetic semiconductor GaMnAs, using an efficient time-dependent density functional theory approach that enables the direct real-time snapshot of the demagnetization process. Our results show a clear spin-transfer trajectory from the localized Mn-d electrons to itinerant carriers within 20 fs, illustrating the dominant role of sp−d interaction. We find that the total spin of localized electrons and itinerant carriers is not conserved in the presence of spin-orbit coupling (SOC). Immediately after laser excitation, a growing percentage of spin-angular momentum is quickly transferred to the electron orbital via SOC in about 1 ps, then slowly to the lattice via electron–phonon coupling in a few picoseconds. The spin-relaxation time via SOC is about 300 fs for itinerant carriers and about 700 fs for Mn-d electrons. These results provide a quantum-mechanical microscopic picture for the long-standing questions regarding the channels and timescales of spin transfer, as well as the roles of different interactions underlying the GaMnAs demagnetization process.   

Magnetic Entropy Dynamics in Ultrafast Demagnetization

Development of femtosecond laser sources and magneto-optical pump-probe techniques enabled measurements of ultrafast demagnetization in ferromagnets. It is necessary to measure two quantities to thermodynamically describe a magnetic state. In particular, measuring the magnetic entropy dynamics of a material in addition to its magnetization dynamics is crucial for fully understanding the transient magnetic state.

Time-resolved magneto-optical Kerr effect measurements of magnetization and magnetic entropy dynamics were made for ferromagnetic Co/Au/glass thin films. An intense 1030 nm pump pulse excited the film and a weak 515 nm time-delayed probe pulse was detected after reflecting off the sample. An external magnetic field was applied along the film surface. Different powers of the pump were used, increasing the film temperature to Curie temperature. Measurements revealed unexplored properties of the magnetic state at ultrafast timescales. Results will be compared to predictions of the Landau model and computer simulations.
  

Analytic modeling of switching time dynamics of monodomain ferromagnets with biaxial energy landscape

Assuming the macrospin model, we develop analytic models to describe the magnetization dynamics of an in-plane-anisotropy ferromagnet driven by spin-transfer-torque with spin polarization collinear to the easy axis orientation. Thus far, the physics of in-plane magnets has been analyzed using numerical solution of the Landau Lifshitz Gilbert (LLG) equation, while analytic expressions of switching time probability, which are needed for memory design and optimization, are lacking. In the limit of small torque, low damping and zero temperature, we construct an average energy flow equation to describe the dynamics of the in-plane magnet. We approximate the elliptic integrals in the flow equation with rational functions and obtain the switching time of the magnetization as a function of energy landscape, material parameters, and input spin current. We also evaluate analytical expressions for switching time probability and cumulative distribution functions assuming a Boltzmann equilibrium distribution of magnetization in the initial energy basin. Good agreement between the model and numerically evaluated results based on Monte Carlo simulations of the LLG equation is demonstrated.   

Elastic Properties of Encapsulation Epoxy for Vanadium Tetracyanoethylene Devices Measured Using Brillouin Light Scattering (BLS)

The organic-based ferrimagnetic coordination compound vanadium tetracyanoethylene (VTCNE) shows promise for microwave applications because it has low damping, similar to that of that of yttrium iron garnet (YIG), it exhibits conformal deposition on a variety of substrates, and the deposition process does not require high temperatures, which allows for simple integration into large scale semiconductor fabrication processes. Like many organic materials, however, it is sensitive to oxygen. The use of organic-friendly epoxy for encapsulation is a common method for protecting the organic that can easily be integrated into device fabrication processes on a large scale, and epoxies that protect the VTCNE without compromising the damping have been identified. Recent measurements suggest, however, that the magnetic properties, particularly the anisotropy, of VTCNE may be sensitive to strain. Consequently, understanding the mechanical properties of the encapsulating epoxy under typical device conditions is important. Here, we have used Brillouin light scattering (BLS) spectroscopy to probe the elastic properties of the cured encapsulating epoxy via phonon spectra measurements as a function of the angle of incidence and light polarization.   

Overcoming the spectroscopic limitations of inelastic light scattering by sub-diffraction light confinement

The small momentum of light strongly limits inelastic light scattering techniques. For instance, only long-wavelength spin waves are accessible to Brillouin light spectroscopy (BLS) widely utilized in studies of magnetic materials and nanostructures [1,2]. We overcome this limitation by utilizing a nanoscale metallic antenna on yttrium iron garnet (YIG) film. The antenna facilitates sub-diffraction confinement of light, generating momentum significantly larger than that of free-space light, and simultaneously enhances the local field due to a combination of geometric phase matching and optical reflection. We also present evidence for the plasmonic effects further enhancing the sensitivity and the spectral range. Our approach can be extended to other types of excitations and light scattering techniques. The demonstrated momentum enhancement can also facilitate light absorption in indirect-gap semiconductors, improving the efficiency of solar cells and optical detectors.

[1] S.O. Demokritov and V.E. Demidov, IEEE Trans. Magn. 44, 6 (2008).
[2] V.E. Demidov et al. Phys. Rep. 673, 1 (2017).   

Terahertz emission from circular photogalvanic effect in bismuth thin films

When circularly polarized light is shined onto a sample, optical selection rules lead to spin-dependent excitations. This can cause helicity-dependent spin-polarized photocurrents within the sample, referred to as the circular photogalvanic effect (CPGE). Bismuth is a highly expected material to host spin-dependent photocurrents, due to its large spin Hall angle and Rashba-like surface states. Recently, helicity dependent photocurrents have been demonstrated in bismuth/copper heterostructures by dc transport measurements [1]. Correspondingly, ultrafast transient photocurrents are expected to occur under the illumination of femtosecond laser pulses, resulting in the emission of terahertz (THz) pulse radiation.
Here, we present the observation of THz emission from bismuth thin films under near-infrared (800 nm, 1.55 eV) pulse excitation. A polarization dependent THz emission is observed in both bismuth thin films and bismuth/metal heterostructures. The opposite polarities of the emitted THz pulse with the left/right-hand circular polarized excitation evidences the transient photocurrent with the opposite direction due to CPGE.

[1] Hirose et al., Appl. Phys. Lett. 113, 222404 (2018)   

Ultrafast Spin Seebeck Measurements on Rare-Earth Iron Garnets

Understanding the thermal generation of spin currents in magnetic materials is an important goal for the field of spin caloritronics. Among magnetic materials, rare earth iron garnets (REIG) display intriguing magnetic transport properties as result of strong antiferromagnetic exchange interactions and low magnetic damping. We report on ultrafast longitudinal spin Seebeck effect (LSSE) experiments on thin film REIG / heavy metal (HM) heterostructures (RE: Y, Tm, Eu, Tb; HM: Au). We use time-resolved magneto optic Kerr effect measurements to directly observe the transfer of magnetization from the REIG into the HM on femto-second timescales, allowing us to selectively probe the interfacial SSE. We observe a factor of 4 difference in the magnitude of the LSSE among the different REIG samples. Our results provide insight regarding the different contributions to the spin current from the different REIGs and the relevance of the interface between the REIG and the HM layers.   

Controlled nonlinear magnetic damping in spin-Hall nano-devices

One of the fundamental dynamical phenomena in magnetic systems is the nonlinear damping enhancement, which imposes strict limitations on the operation and efficiency of magnetic nanodevices. For instance, nonlinear damping prevents coherent magnetization auto-oscillations driven by the spin injection into spatially extended magnetic regions [1].
We utilize micro-focus Brillouin light spectroscopy (BLS) to demonstrate that nonlinear damping can be controlled by the ellipticity of magnetization precession. By balancing the demagnetizing field with the magnetic anisotropy in a Pt/Co/Ni heterostructure, we minimize ellipticity and achieve coherent magnetization oscillations in a microscopic CoNi disk, driven by spatially extended injection of spin current generated in Pt by the spin Hall effect. Micromagnetic simulations show that the mechanism responsible for the nonlinear damping is non-resonant parametric pumping enabled by the precession ellipticity. Our results provide a novel route for the implementation of efficient active spintronic and magnonic devices driven by spin current.

[1] V. E. Demidov, S. Urazhdin, B. Divinskiy, V. D. Bessonov, A. B. Rinkevich, V. V. Ustinov and S. O. Demokritov, Nature Comm. 8, 1579 (2017)   

Conductivity-Like Gilbert Damping due to Intraband Scattering in Epitaxial Iron

Confirming the origin of Gilbert damping by experiment has remained a challenge for many decades, even for some of the simplest ferromagnetic metals. Here, we experimentally identify Gilbert damping that increases with decreasing electronic scattering in thin films of BCC Fe. The observed conductivity-like damping, which cannot be accounted for by classical eddy current loss, is in excellent quantitative agreement with theoretical predictions of Gilbert damping due to intraband scattering. These results resolve the longstanding question since the 1970s about the role of intraband scattering in Gilbert damping. Our results also indicate that – somewhat counterintuitively – disorder can reduce intrinsic damping at low temperatures in ferromagnetic metals, such that optimally disordered films may be well suited for cryogenic spintronic and quantum applications.   

Manipulation of terahertz spectrum using microfabricated magnetic heterostructures

Terahertz (THz) radiation with sub-micrometer wavelength falls in the gap between the optical and radio frequency range. Conventional THz emitters rely only on the electron's charge. However, recently it was found that spin-based effects occur on the ultrafast time scale. Upon excitation with a femtosecond laser pulse a diffusive spin current is created in a ferromagnet that leads to THz transients in an adjacent heavy metal layer due to a conversion by the inverse spin Hall effect. Here, we demonstrate generation and control of THz radiation from microstructured Fe/Pt bilayers. We compare the THz spectrum of different patterns and an extended film using time-domain THz spectroscopy. The microstructures are fabricated using optical lithography and sputtering deposition. The THz spectrum is experimentally observed and interpreted in terms of a simplified multi-slit diffraction model, which captures the main experimental features. Our results show an efficient control of the emitted THz light. This is a crucial step forward for the design and realization of directional spin-based THz sources.