Session F54: Terahertz Spintronics and Spin-Torque Driven Magnetization Dynamics

Interplay of Spin-Torque and Nonlinearity: Disentangling and Controlling Magnon Processes in Nanomagnets

Nanoscale magnets are the building blocks of many spintronics technologies. However, various aspects of spin dynamics in the nanoscale geometrical confinement have remained elusive. For instance, untangling damping contributions and engineering spin-torque response in nanomagnets is often a challenging task.

Using magnetic tunnel junctions as a sample platform, we investigate the discrete magnon spectrum of individual zero-dimensional magnets and identify the inherent inter-magnon processes. While manipulation of magnetization by spin-torque is a key functionality of today’s spintronics, we find that resonant magnon processes redefine and invert a nanomagnet's response to spin-torque [1]. We discuss the mechanisms of this counter-intuitive interplay of nonlinearity and spin-torque, which has far-reaching implications for the performance of magnetic memory and oscillators.

Controlling magnon processes and thus forging the nonlinearity of a nanomagnet would add decisive functionality to existing and emerging technologies, in particular to magnetic neuromorphic systems where tunability of the nonlinear response is instrumental. We develop an approach for engineering magnon interaction by means of symmetry-breaking fields with nanoscale nonuniformity [2]. In a proof-of-concept, we employ a nanoscale synthetic antiferromagnet as a switchable source of such fields and achieve tunability of magnon coupling by at least one order of magnitude. The results open up avenues for controlling magnon processes by external stimuli at nanoscale and show prospects for spin-torque applications and quantum information technologies.

[1] Giant nonlinear damping in nanoscale ferromagnets, I. Barsukov et al., Sci. Adv. 5, eaav6943 (2019)

http://dx.doi.org/10.1126/sciadv.aav6943

[2] Controlling magnon interaction by a nanoscale switch, A. Etesamirad et al., ACS Appl. Mater. Interfaces 13, 20288 (2021)

https://doi.org/10.1021/acsami.1c01562   

Injection Locking of Antiferromagnetic Oscillators

Antiferromagnetic oscillators are fascinating devices that behave like room temperature Josephson Junction oscillators [1].  These nonlinear auto-oscillators operate through the spin-hall effect, spin transfer torque, and spin pumping.  Antiferromagnetic oscillators (AFMOs) can theoretically produce coherent low range THz signals.  One challenge with AFMOs as THz signal generators is their power due to their nanoscale construction.  A solution to this limitation is the synchronization of many AFMOs to increase aggregate power generation.  To start investigating synchronization coupling options we analyze the behavior of the AFMO in a simple injection locking regime.  In this regime driving currents far above the threshold current are avoided because the AFMO becomes insensitive to anisotropy and thus synchronization cannot be achieved. Even with large damping to avoid hysteretic behavior near the threshold current the pulse train generated is unstable bordering on chaotic like behavior.  Thus injection locking in this manner is not desirable for achieving synchronization of AFMOs.

[1] R. Khymyn, I. Lisenkov, V. Tyberkevych, B. A. Ivanov, & A. Slavin, Scientific Reports 7:43705, DOI: 10.1038/srep43705    

Spin-orbit driven terahertz optical response in ferromagnetic Fe-Co-Al alloys

We study the optical conductivity response of simple ferromagnetic alloys driven by spin-orbit coupling. The spin-orbit coupling leads to a band splitting near the Fermi surface in the range of 10-100 THz. This band-splitting energy range provides an opportunity to study the optical conductivity in the THz regime. The optical conductivity response is sensitive to the details of the structure and chemical composition. For simple alloys, like Fe-Co alloys, we find that there is not much dependence of optical conductivity response in the THz range. However, we find the optical conductivity response changes sign from positive in Fe to negative in Fe-Al alloys. Furthermore, those features are washed away at 10 THz but remain only at 100 THz because of the short electron lifetime.   

Nanoscale transient gratings of magnetization induced by extreme ultraviolet light

We describe a methodology for generating and probing periodic nanoscale patterns of magnetization using femtosecond extreme ultraviolet (EUV) pulses from a free electron laser. Two EUV pulses crossed at the sample surface excite a magnetization grating with a period in the tens of nanometers range; the dynamics of this grating are monitored via diffraction of a time-delayed EUV pulse tuned to the M-edge of a transition metal element. The magnetization grating emerges on a subpicosecond time scale as the sample is demagnetized at the maxima of the EUV pump intensity and decay on the time scale of tens of picoseconds. Following the initial report [1], we improved the setup by adding a polarizing mirror analyzer that enables the separation of magnetic and non-magnetic responses of the sample.  We will describe experiments conducted on Co-Ni and Co-Pt multilayers as well as on a CoGd alloy with perpendicular magnetic anisotropy and discuss the dependence of the observed dynamics on the grating period and the pump fluence. We will propose several avenues for studying ultrafast magnetic dynamics on the nanoscale using the transient magnetic gratings approach.

 

[1] D. Ksenzov et al., Nano Lett. 21, 2905 (2021).   

Prospects of voltage-controlled spin Hall nano-oscillators for neuromorphic computing

Mutually synchronized nano-constriction-based spin Hall nano-oscillators (SHNOs) are promising energy-efficient devices for high-quality microwave signal generation and an attractive alternative to conventional spin-torque nano-oscillators [1]. They have recently shown great potential to closely emulate the brain-inspired processes and thus open the way for ultra-fast oscillator-based neuromorphic computing [1,2]. However, interfacing of such SHNO networks and tunability of individual oscillators to perform complex tasks in large oscillator networks remain challenging tasks.

In my talk, I will describe a state-of-the-art voltage-controlled nano-constriction based W(5nm)/ CoFeB(1.7nm)/MgO(2nm) SHNOs, which combine a nano-scale footprint, CMOS compatibility, and energy-efficient fine individual oscillator control in mutually synchronized chains [3,4]. I will further discuss how voltage-induced moderate changes in perpendicular magnetic anisotropy across the CoFeB/MgO interface strongly tune the auto-oscillation frequency and spin-wave localization in the constriction region. As a result, we observe a substantial voltage modulation in threshold current (22%) and a substantial 50 MHz frequency tunability (12 MHz/V) in these SHNOs [3]. Finally, I will demonstrate how such strong voltage-tunability in both frequency and threshold current can lead to the non-volatile tuning of the synchronization state in a chain of four mutually synchronized SHNOs [4]. The demonstrated energy-efficient tuning approach is promising to train large SHNO networks for cognitive tasks and scale oscillator-based neuromorphic computing schemes to more extensive network sizes.   

Ultrafast terahertz field control of the emergent electronic and magnetic interactions at the CaMnO3/LaNiO3 ferromagnetic interface

Ultrafast electric-field control of electronic and magnetic dynamics in oxide materials has great potential as a strategy for the future development of faster and more efficient logic and memory devices. LaNiO₃/CaMnO₃ superlattices display emergent ferromagnetism at the interface of paramagnetic LaNiO₃ and antiferromagnetic CaMnO₃ which can be tuned via control of the thickness-dependent metal-insulator transition in LaNiO₃. We utilized temperature- and fluence-dependent time-resolved magneto-optic Kerr effect, optical reflectivity, and transmissivity spectroscopies of variable-thickness LaNiO₃/CaMnO₃ superlattices to disentangle multiple interrelated electronic and magnetic processes driven by ultrafast high-field terahertz pulses. Detailed analysis of temperature- and field-dependent amplitudes and time constants for the various electronic and magnetic components of the dynamics are presented. These findings suggest an avenue for efficient electric-field switching of two-dimensional ferromagnetic states at oxide interfaces.

[1] Grutter et al., Phys. Rev. Lett. 111, 087202 (2013); [2] Flint et al., Phys. Rev. Materials 1, 024404 (2017); [3] Chandrasena et al., Phys. Rev. B 98, 155103 (2018).   

Spin-Orbit Torques in Mn3Sn and FeSn Topological Semimetals

Recently discovered topological materials are promising candidates for efficient manipulation of magnetic memory and logic devices due to their exotic transport features. Topological band structures of these materials provide spin momentum locking which can generate large spin-orbit torques (SOT). Recent studies have shown that topological insulators (TIs) can generate significantly larger SOT in comparison with that by common heavy metals, due to their surface Dirac state with spin-momentum locking (1). However, SOT’s observed in these materials thus far are limited by their symmetry to lie in the film plane (2) and cannot efficiently manipulate magnetic moments of ferromagnets with perpendicular magnetic anisotropy, which are required for high-density applications.

A new class of topological materials called topological semimetals (TSMs), have not only topological bulk states but also surface Fermi arc states with unusual spin-momentum locking. Both bulk and surface states can generate large SOT whose characteristics are altogether different from that in common heavy metals and possibly TIs (3). Our work on Mn₃Sn and FeSn Topological semimetals have shown additional contributions to the signal coming from SOTs in harmonic hall measurements. The results suggest that these signals may be generated from unusual spin polarizations.

 

 

 

Photogalvanic spin current of magnons in magnetic insulators

Photogalvanic responses in noncentrosymmetric semiconductors are a powerful tool for studying various aspects of low-energy electronic properties, including topological electronic states. In recent years, several different mechanisms for the spin current analog of the photogalvanic effect has been theoretically proposed. However, neither candidate materials nor a general formula for calculating the photogalvanic spin current is known. In this work, we develop a generic theory for the photogalvanic spin current through a magnetic resonance process.

 

Using a nonlinear response formalism [1,2], we find the nonlinear conductivity consists of two contributions: contribution involving one magnon band, and that involving two bands. The former is related to those studied recently, whereas the latter is a contribution unknown to date. We argue that the two-band process produces a large photogalvanic spin current in the antiferromagnetic phase of bilayer CrI3, whose resonance frequency can be tuned between GHz-THz range by an external magnetic field. Our finding opens a route to studies on the photogalvanic effect of spin-angular momentum in realistic setups.

 

[1] H. Ishizuka and M. Sato, Phys. Rev. Lett. 122, 197702 (2019).

[2] H. Ishizuka and M. Sato, Phys. Rev. B 100, 224411 (2019).