Session N57: Magnetization and Spin Dynamics I

Author not Attending

Implementation of spin waves (SWs) in the magnonic devices is desirable as they propagate without Joule heating and thus ensure low energy consumption. One of the ways is to use nonreciprocity in the propagation SWs in opposite directions. This can be achieved in chiral magnets with breaking inversion symmetry or in nonchiral magnets by applying an external static electric field [1-2]. Potentially, the latter option may have a wider choice of magnetic materials. For practical applications, it is important to figure out how to tune SW`s dynamics by a static electric field generating the Dzyaloshinskii-Moriya interaction (DMI). In this context, we study the two-sublattice antiferromagnet within the phenomenological approach based on the Landau-Lifshitz-Gilbert equation. It is shown that the DMI makes the SWs spectra asymmetrical concerning the wave vector, and the asymmetry level grows with an increasing electric field. The obtained results may be important for describing periodic magnetic structures and constructing a spin wave field-effect transistor [3].

[1] Tianyu Liu and G. Vignale, Phys. Rev. Lett. 106, 247203 (2011).

[2] A.S. Savchenko and V.N. Krivoruchko, J. Magn. Magn. Mater., 474, 9-13 (2019).

[3] R. Cheng et al., Sci Rep 6, 24223 (2016).   

Optically induced ultrafast switching dynamics in antiferromagnets

The dynamics of magnetic materials on ultrafast timescales is a complex area of research, as it deals with nonlinear processes far from equilibrium. The potential for applications however, in particular in the development of fast and effecient magnetic storage devices, is promising. While first investigations into ultrafast demagnetisation [1] and all-optical switching [2] have focused on ferro- and ferrimagnets, more recent research in spintronics has put forward antiferromagnets as advantageous alternatives. Their vanishing net magnetisation makes them less susceptible to perturbations by external magnetic fields and their intrinsic dynamics are exchange-enhanced leading to eigenfrequencies in the terahertz range. Here, we investigate systematically how the demagnetisation and switching dynamics differ depending on the type of magnetic order. Using spin dynamics simulations in combination with density functional theory, we show how ultrafast laser-induced switching can be realised in antiferromagnets [3]. In contrast to ferro- and ferrimagnets, where a heat-induced quenching of the magnetic order is necessary to facilitate switching, we find that in antiferromagnets, the inverse Faraday effect can induce a coherent rotation of the Néel vector. We also demonstrate how a series of laser pulses can be used to reliably switch between two perpendicular magnetic states.

[1] E. Beaurepaire et al., Phys. Rev. Lett. 76, 4250 (1996).

[2] C. D. Stanciu et al, Phys. Rev. Lett. 99, 047601 (2007).

[3] T. Dannegger et al., Phys. Rev. B 104, L060413 (2021).   

Optical detection of ultra-fast magnetization dynamics in 2D ferromagnet Cr2Ge2Ge6

Two-dimensional magnetic materials provide a unique platform for the study of magnetism in reduced dimensions. Moreover, they provide the opportunity of coupling with other 2D materials in heterostructures exploiting magnetic exchange through proximity. Understanding the magnetization dynamics of these thin materials on ultra-short time scales is vital, both for designing applications and for understanding the fundamentals of magnetism in the two-dimensional limit. Here, we study the magnetization dynamics of the layered magnetic material Cr₂Ge₂Te₆ using an all-optical technique based on time-resolved Faraday rotation, and change the magnetic properties using electrostatic gating and heterostructure engineering. By heating the two-dimensional magnet with an ultrashort intense laser pulse and probing the magnetization using a second, weaker laser pulse, we measure the demagnetization and subsequent remagnetisation at sub-picosecond timescales. Moreover, we measure a damped oscillation on top of this signal, which is due to precession of the magnetization around the externally applied magnetic field. Our first results show a fast, two-step demagnetization, consistent with a type-II demagnetization, followed by a recovery time in the order of nanoseconds. From these measurements, the interaction between the electron, phonon and spin degrees of freedom can be extracted using the microscopic three-temperature model. The magnetic field dependence of the frequency and decay time of the precession is well described by the Landau-Lifshitz-Gilbert equation, and from this we extract the magnetization dynamics parameters such as the gilbert damping. The possibility of tuning these  parameters with a gate voltage or heterostructure engineering makes two-dimensional magnets an appealing system for the development of novel magnetic devices for data processing and storage at ultrafast timescales.

    

Photo-excited Magnetization Precession in Co/Pd Multilayer Films at Low Laser Fluence Regime

In this study, we present optically induced precession of magnetization at low laser fluence regime, as a function of magnetic field, in three Co/Pd multilayer (ML) systems. The Co/Pd MLs studied here have three different regimes of magnetic anisotropy (In-Plane, weak out-of-plane, and out-of-plane) and our results provided a better understanding of the relationship between optically excited precession of magnetization and the interface anisotropy, of Co/Pd multilayers. We will discuss the connection of precession frequency and amplitude, Gilbert damping, gyromagnetic ratio, in our MLs, to their thickness. We have identified that spin-orbit interactions, affecting perpendicularly spin-polarized electrons at the Co/Pd interface, to be the dominant mechanism for our observed variation in the oscillation's amplitude of locally excited magnetization. There has been little study on the low fluence regime (< 10 µJ/cm²) and our approach could be a practical method for developing a non-thermal all-optical magnetic switching toward low-power magnetic memory applications.

    

Ultrafast Spin Dynamics in Heusler Alloys

We present new ultrafast spin and electron dynamics for Heusler alloys uncovered using an extreme ultraviolet (EUV) probe.  Heusler alloys represent a diverse group of materials whose constituent elements can be chosen to tune electronic and magnetic properties.  By addressing M-edge resonances with an extreme UV probe we can perform element specific magneto-optical Kerr effect (MOKE) measurements on the natural timescales of the shortest processes in Heusler alloys.  The ultrafast responses of these materials exhibit unique signatures, such as ultrafast spin transfer, that depends on the band structure of their magnetic sublattices.   

Ultrafast Magnetization Dynamics of CoFe Alloys

Understanding the ultrafast magnetization dynamics in a metal after laser heating is critical for many engineering applications, including nanoscale thermal engineering, photocatalysis, and spintronics. Despite the topic’s importance, and decades of study, fundamental questions about ultrafast magnetization dynamics remain unanswered.  In this talk, I discuss my group’s experimental work to answer the following question. Can we control ultrafast magnetization dynamics in metal alloys by using the effect of alloy composition on electronic band structure to control the strength of quasi-particle interactions? We performed a series of pump/probe experiments on Co_1-xFe_x alloys of their ultrafast magnetization dynamics, Gilbert damping, thermal conductivity, and more. We find the ultrafast magnetization dynamics in these alloys depends strongly on composition. Alloy compositions with weak electron-phonon interaction strengths are also observed to have distinct ultrafast dynamics and low Gilbert damping parameters. I conclude my talk by discussing the implications of our findings for ultrafast magnetic switching and THz spintronics.

    

Symmetry-dependent ultrafast manipulation of nanoscale magnetic domains

The quest towards controlling magnetism at the femtosecond timescales is driven by the demand for energy efficient and fast magnetic storage devices¹. Many studies have focused on switching the magnetization in different material, but few have discussed the role of the spatial evolution of the magnetization.

Here, we study the ultrafast response of magnetic multilayers with domain patterns with different local symmetries. Through time-resolved small angle X-ray scattering at the European XFEL and an accurate 2D fitting routing, we find a symmetry-dependent behavior of the ultrafast response. By observing an ultrafast shift in the scattering peak position for labyrinth domains, without translational symmetry, but no such shift for stripe domains, with translational symmetry, we confirm the results of previous studies². We also study regions of the sample where both labyrinth and stripe characters are present. By isolating the isotropic and anisotropic components of the scattering, we find that only the labyrinth domains exhibit an ultrafast shift in the isotropic diffraction peak position, even in a mixed domain pattern. Our findings reveal the unpredicted dependence of the ultrafast spin dynamics on the configuration of the magnetic domains and invites further investigation on far-from-equilibrium spin transport.

¹Kirilyuk, Kimel, Rasing, RevModPhys 82 (2010)

²Pfau et al., Nat Comm 3 (2012); Vodungbo et al., Nat Comm 3 (2012), Zusin et al, accepted PRB (2022); Hennes et al., PRB 102 (2020)

Optical Detection of Magnetoelastic Dynamics in an Antiferromagnetic Insulator

Antiferromagnets are promising for spintronic applications due to their high resonant frequencies and small or vanishing net magnetization. To fully realize the advantageous properties of antiferromagnets, control and read-out of the Neel vector orientation is crucial. Accordingly there is much interest in reliable Neel vector switching. The often large magnetoelastic coupling in antiferromagnets can be used to manipulate the Neel vector in insulating materials. Here we present a scheme for exciting antiferromagnetic dynamics in NiO and α-Fe₂O₃ using surface acoustic waves generated electrically by an interdigitated transducer on the piezoelectric substrate PMN-PT. By modulating the strain-induced magnetic anisotropy below the antiferromagnetic resonance frequency, we expect to generate dynamics that reduce the switching threshold. The resulting dynamics are detectable using paramagnetic nitrogen-vacancy centers in diamond, which enable noninvasive optical detection of magnetic field fluctuations produced by spin dynamics. Insight into the nature of magnetoelastically-generated dynamics is fundamental for engineering devices capable of robust control of the Neel vector.

    

Coherent manipulation of electronic order in a correlated insulator via sub-gap optical excitations

Correlated transition-metal oxides allows for coherent manipulation of electronic order via sub-gap excitations through strong optical fields. Degeneracy between d-electron levels is often broken by the formation of local atomic moments. In nickel oxide, these moments order anti-ferromagnetic below about 525K. The magnetic ordering temperature is governed by the interatomic exchange-interaction, typically mediated through hybridization between nickel d-states and oxygen p-states. By upsetting the balance between Coulomb repulsion and kinetic effects with the strong electromagnetic fields of a sub-gap optical laser, we may influence the microscopic interactions governing electronic and magnetic order. The ability to influence Coulomb repulsion, as encompassed in the Hubbard U, and interatomic exchange is investigated. We conclude that the Hubbard U is robust even for fields up to 0.22V/Å, whereas exchange interactions may be modulated with sub-gap excitations, providing an avenue for coherent control of magnetic order.

    

Spin wave generation via acoustically-driven ferromagnetic resonance in a ferromagnetic insulator Y3Fe5O12 on LiNbO3

Spintronic devices employing spin waves, a propagating excitation of a spin lattice, are a promising platform to achieve low loss spin information transport.^1-4 To realize such devices, generation and detection of spin waves is an ongoing challenge that has been addressed in various ways. One approach to detect spin wave launching is via acoustically-driven ferromagnetic resonance (ADFMR), ferromagnetic resonance induced by surface acoustic waves (SAWs). However, most ADFMR devices are based on ferromagnetic metals which suffer from short spin coherence length.^5,6 In this study, we demonstrate the launching of spin waves via ADFMR in a ferromagnetic insulator (Y₃Fe₅O₁₂; yttrium iron garnet, YIG), which has a longer spin coherence length. Interdigitated transducers were used to excite SAWs on piezoelectric LiNbO₃ substrates patterned with YIG. The transmitted signal shows absorption at the resonant frequency when measured by a vector network analyzer. Angle-dependent measurements of ADFMR suggest this absorption is due to spin wave generation. We will also discuss detection of spin wave propagation through the YIG layer using the inverse spin Hall effect. These results open the possibility of developing spin wave materials and devices for efficient spin information transport.   

Gilbert damping in the real-space KKR method for metallic nanostructures

The ab-initio determination of Gilbert damping parameters is an important issue for accurate atomistic spin dynamics and micromagnetic calculations. Going beyond presently available methods of calculating the Gilbert damping scalar parameter in bulk materials, we implemented the torque-torque correlation formula [1] into the fully relativistic real-space Korringa-Kohn-Rostoker (KKR) method [2] using the Budapest SKKR code to be able to treat chemically inhomogeneous systems. This enables the ab-initio determination of spatially resolved on-site and non-local Gilbert damping tensors [3] in atomic nanostructures. After performing extensive tests for metallic bulk materials to identify the relevant parameter settings of the calculations, we show some examples of inhomogeneous Gilbert damping results in various metallic atomic (nano-)structures.

[1] Ebert et al. Phys. Rev. Lett. 107, 066603 (2011).

[2] Lazarovits et al. Phys. Rev. B 65, 104441 (2002).

[3] Thonig et al. Phys. Rev. Mater. 2, 013801 (2019).   

A new method for conservative dynamics in contact with a thermal bath: study of the classical Heisenberg antiferromagnet

To simulate spin systems in the canonical ensemble it is common to use Montecarlo [1]. However, Montecarlo dynamics is far from the physical one, especially when symmetries and conservation laws change the dynamic critical exponents, as in the important case of antiferromagnets. For this reason, the use of the more physical spin dynamics has become a standard in the field [2].

Yet, within micro-canonical spin dynamics it is not obvious how to add thermal fluctuations that conserve the magnetization, so that thermalization is ensured by evolving initial conditions drawn with the canonical distribution. Although progress has been done by combining Kawasaki canonical Montecarlo with microcanonical spin-dynamics [3], it would be highly convenient to have a simple spin dynamics coupled to a heat bath that respects the conservation law.

Here we introduce such an algorithm and we test it on the Heisenberg antiferromagnet in d=3. We show that at Tc the algorithm reproduces the right value of the dynamical critical exponent, z=1.5, while in the low temperature phase spin waves correctly emerge. The kinetic coefficient of the conservative heat bath is connected in a simple way to the microscopic parameters of the spin-phonon coupling, thus explaining softening of the spin-wave peaks.

[1] K. Chen, A.M. Ferrenberg and D. P. Landau, PHYSICAL REVIEW B 48, 3249 (1993)

[2] D P Landau and M Krech 1999 J. Phys.: Condens. Matter 11 R179

[3] R. Nandi and U. C. Täuber, PHYSICAL REVIEW B 99, 064417 (2019).

    

Anisotropic Gigahertz Antiferromagnetic Resonances of the Easy-Axis van der Waals Antiferromagnet CrSBr

We report measurements of the antiferromagnetic resonance modes in the van der Waals (vdW) antiferromagnet CrSBr. Weak interlayer exchange coupling between vdW layers gives resonances in the GHz range, making it easier to study their fundamental properties compared to antiferromagnets with THz-scale resonances. Unlike the other vdW magnets whose modes have been characterized previously (CrCl₃ and CrI₃), CrSBr has strong triaxial magnetic anisotropy. As a result, the quantum-mechanical hybridization between the two antiferromagnetic modes can be tuned via the angle of an in-plane magnetic field. The anisotropy also causes distinctly different types of resonance modes depending on the field direction with respect to the anisotropy axes. Fits to a two-sublattice Landau-Lifshitz model allow quantitative measurements of the anisotropy and interlayer exchange as a function of temperature.  This work sets the foundation for future experiments regarding controlled manipulation of antiferromagnetic modes.