On-line SPICE-SPIN+X Seminars

On-line Seminar: 29.06.2022 - 15:00 German Time

How to engineer non-equilibrium crystal and magnetic structures with light

Ankit Disa, MPSD

The crystal structure is a key ingredient determining the macroscopic properties of any solid. In the context of magnetism, the local moments, anisotropy, and exchange parameters are all strongly dependent on the bonding environment and the symmetry of the lattice, which can be tuned, for example, by chemical composition or external pressure. Such quasi-static approaches are limited in their speed and efficacy, however. In this talk, I describe how we can instead manipulate the crystal structure of materials dynamically using light, which enables one to induce, enhance, and control magnetic states in ways not possible in equilibrium. The approach is based on selectively exciting optical phonons with resonant THz pulses and exploiting nonlinearities of the crystal lattice [1]. We used this approach to realize a light-induced transition to a ferrimagnetic phase in the antiferromagnet CoF2 [2]. The resultant non-equilibrium magnetization is optically switchable and has a magnitude 100-fold larger than achievable via strain. More recently, we demonstrated the ability to strengthen magnetic order in the strongly correlated ferromagnet YTiO3 [3]. By driving specific lattice distortions, the low-temperature moment was enhanced and a non-equilibrium ferromagnetic state was stabilized even at temperatures well in excess of the equilibrium Tc. These experiments show that optically engineering the crystal structure provides a versatile and powerful tool for emerging magnetic and spintronic technologies.

[1] A.S. Disa, T.F. Nova, A. Cavalleri, Nature Phys. 17, 1087 (2021)
[2] A.S. Disa, et al., Nature Phys. 16, 937 (2020)
[3] A.S. Disa, et al., arXiV: 2111.13622

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 15.06.2022 - 15:00 German Time

Nonreciprocal transport and topological band structure through interactions of magnonic multilayers

Luqiao Liu, MIT

Dipolar interaction, one of the most classical interaction mechanisms among magnetic moments, acts as a ‘spin-orbit-coupling’ like mechanism on magnons, by coupling their precession handedness with the propagation direction. This has given rise to the well-known surface magnetostatic wave (Damon-Eshbach mode) in thin film ferromagnet with chiral surface states. Recently, dipolar interaction induced magnon couplings between different magnetic layers have drawn interests, for realizing non-reciprocal transmission of microwave signals and non-Hermitian Hamiltonian systems. In a recent work, via the coupling of magnons between a ferrimagnetic insulator Y3Fe5O12 (YIG) and a magnetic alloy NiFe, we showed that tunable, nonreciprocal propagation can be realized in spin Hall effect-excited incoherent magnons, whose frequencies cover the spectrum from a few gigahertz up to terahertz [1]. In the diffusion transport, the nonreciprocity is reflected as asymmetric magnon diffusion lengths, which are unequal along opposite transmission directions. The diffusive nonreciprocity is closely related to the change of the magnon damping through the chiral dipolar coupling. Extending this bi-layer structure into an antiparallelly aligned magnetic multilayer, we show theoretically that the interlayer dipolar interaction generates bulk bands with non-zero Chern integers and magnonic surface states carrying chiral spin currents [2]. The surface states are highly localized and can be easily toggled between non-trivial and trivial phases through an external magnetic field. Our experimental and theoretical findings enrich knowledge on diffusive transport of magnons and provide an easy-to-implement system for topological magnonic states, which can be used for the design of passive signal isolation devices.

[1] J. Han, Y. Fan, B. C. McGoldrick, J. Finley, J. T. Hou, P. Zhang, and L. Liu, Nano Letters, 21, 7037 (2021)
[2] Z. Hu, L. Fu, L. Liu, “Tunable Magnonic Chern Bands and Chiral Spin Currents in Magnetic Multilayers,” arXiv:2201.00312 (2022)

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 01.06.2022 - 15:00 German Time

Domain walls and skyrmions: From ferromagnets to ferrimagnets

Geoffrey Stephen Beach, MIT

Tremendous progress has been made in engineering highly mobile domain walls and skyrmions in room temperature materials for racetrack-based applications. Most recent efforts have focused on heavy-metal/ferromagnet heterostructures with Dzyaloshinskii-Moriya interactions and spin-orbit torques, in which chiral domain walls and skyrmions can be stabilized at room temperature and readily manipulated [1,2]. However, ferromagnets possess fundamental limitations on spin texture speed and size owing to stray fields and precessional dynamics [3]. Antiferromagnets, on the other hand, possess no stray fields, and are angular-momentum-compensated, yielding extremely fast dynamics. Ferrimagnets exhibit similar behaviors at compensation, but are more readily probed since the individually sublattices are detectible and addressable owing to the fact that the electronic and optical properties of the elements on these sites are typically different. Here, I describe ferrimagnetic spin textures and dynamics in metallic and insulating ferrimagnets. Using Pt/GdCo/TaOx films with sizable Dzyaloshinskii-Moriya interaction, we realize current-driven domain wall motion with a speed of 1.3 km/s near the angular momentum compensation temperature (TA) and room-temperature stable skyrmions with minimum diameters close to 10 nm near magnetic compensation (TM) [4]. By using temperature as a knob, the roles of compensation on the dynamics can be clearly extracted. I then describe recent work on insulating magnetic garnets with perpendicular anisotropy, in which we have discovered an interfacial Dzyaloshinskii-Moriya interaction [5,6] which, combined with low damping and pure spin current injection mediated by a Pt overlayer [7], leads to exceptionally fast motion at extremely low current densities [8]. Finally, I will discuss all-optical manipulation of skyrmions using ultrafast laser excitations, including picosecond generation of topological charge [9] tracked in real time via single-shot soft x-ray scattering, and all-optical writing, deleting, and two-dimensional steering of skyrmions by light alone [10]. Recent progress and future directions in these areas will be discussed.

[1] S. Emori, et al., Nature Mater. 12, 611 (2013)
[2] S. Woo, et al., Nature Mater. 15, 501 (2016)
[3] F. Büttner, et al., Sci. Rep. 8, 4464 (2018)
[4] L. Caretta, et al., Nature Nano. 13, 1154 (2018)
[5] C. O. Avci, et al., Nat. Mater. 14, 561 (2019)
[6] L. Caretta, et al., Nat. Comm. 11, 1090 (2020)
[7] C.O. Avci, et al., Nature Mater. 16, 309 (2017)
[8] L. Caretta, et al., Science 18, 1438 (2020)
[9] F. Buttner, et al., Nat. Mater. 20, 30 (2021)
[10] L. Caretta, et al., to be submitted (2020)

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 08.06.2022 - 15:00 German Time

Spontaneous anomalous Hall response and altermagnetism explored in MnTe and Mn5Si3

Helena Reichlova, TU Dresden

Anomalous Hall effect is a prime example of electronic responses commonly associated with the spontaneous time-reversal symmetry breaking in the electronic structure of ferromagnets. More recently, the anomalous Hall effect and other time-reversal symmetry breaking responses were also theoretically and experimentally established in crystals with a vanishing net magnetization and a non-collinear magnetic order [1,2]. The experimental work presented in this talk has been inspired by theoretical predictions that the time-reversal symmetry breaking electronic structure and responses can occur within an abundant family of materials with a compensated collinear magnetic order [2-4]. The underlying unconventional magnetic phase forms in crystals whose opposite-spin sublattices are connected by a rotation symmetry-transformation [5]. Its characteristic feature is an alternating spin polarization in both real-space crystal structure and momentum-space electronic structure, which has suggested to term this emerging phase altermagnetism [5].

We will first present our experimental observation of a spontaneous anomalous Hall response in the absence of an external magnetic field in an epitaxial film of MnTe, which is a semiconductor with a collinear antiparallel magnetic ordering of Mn moments [6]. The anisotropic crystal environment of magnetic Mn atoms due to the non-magnetic Te atoms is essential for establishing the unconventional phase and generating the anomalous Hall effect in MnTe [5,6]. In the second part of the talk, we will present our observation of the spontaneous anomalous Hall effect in epitaxial thin-film Mn5Si3 [7]. We have studied Mn5Si3 epilayers grown on Si(111) substrate. We observe that epitaxial constraints stabilize a hexagonal unit cell in the magnetic state distinct from previously described phases in bulk Mn5Si3 crystals [8]. We observe a sizable spontaneous anomalous Hall conductivity of 5-20 S/cm, accompanied by a negligible net magnetization. Combined with the theoretical symmetry analysis and microscopic ab initio calculations, we conclude that our thin-film Mn5Si3 is a candidate for a d-wave form of the altermagnetic phase, generated by a compensated collinear ordering of magnetic moments on the lattice of Mn atoms [4,6].

[1] Nakatsuji and Arita, Annual Review of CMP 13, 119-142 (2022)
[2] Smejkal et al. Nat. Rev. Mater., on-line 30 March (2022)
[3] Smejkal et al. Science Advances 6, 23 (2020)
[4] Mazin et al. PNAS 118 42 (2021)
[5] Smejkal et al. arXiv:2105.05820, arXiv:2204.10844
[6] Gonzales-Betancourt et al. arXiv:2112.06805
[7] Reichlova et al., arXiv:2012.15651
[8] Gottschilch et al., J. Mater. Chem., 22, 15275 (2012)

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 18.05.2022 - 15:00 German Time

Ferrimagnetic spintronics and self-torque

J. Carlos Rojas-Sanchez, Institut Jean Lamour UL-CNRS

Two-dimensional spin-textured materials or interfaces are expected to exhibit high efficiency for the interconversion of spin current into charge current, even more efficient than conventional non-magnetic heavy metals [1,2]. Additionally, magnetic materials possessing strong spin-orbit coupling such as GdFeCo can also efficiently generate spin currents of different symmetries, spin anomalous Hall effect SAHE-like, and spin Hall effect SHE-like [3-5]. And one such symmetry, SHE-like, could produce what we have coined "self-torque" [6].
In the first part of the talk, I will show an example of “standard” or “external” spin-orbit torque in a W/CoTb/AlOx system. We use the strong spin-orbit coupling from W to exert spin-orbit torque and manipulate the perpendicular magnetization of CoTb amorphous ferrimagnetic layer [2] .
In the second part, I will show our study on ferrimagnetic FiM GdFeCo alloys in which the 5d band of Gd induces large spin-orbit coupling. We demonstrate the giant spin current emission (SAHE+SHE) by GdFeCo from the current-induced modulation of the ferromagnetic resonance linewidth of NiFe in GdFeCo/Cu/NiFe. Overall efficiency is 25 times more important in GdFeCo/Cu/NiFe than in Pt/Cu/NiFe [3].
The study of the self-torque is carried out by harmonic Hall voltage measurements in samples where the GdFeCo layer exhibits out-of-plane magnetization. We compare the self-torque in GdFeCo/Cu with torques induced by Pt or Ta in Pt/Cu/GdFeCo and Ta/Cu/GdFeCo [3]. Thus, These “self-torques” can be tuned by adjusting the spin absorption outside the GdFeCo layer. Moreover, taking advantage of the different characteristics temperatures in ferrimagnets [2,6,7], we show the features that differentiate self-torque from what we know so far, the "external" spin-orbit torque [6].
These results pave the way for new architectures to achieve switching by self-SOT and skyrmions manipulation.
Work performed with co-authors in Refs. 1,2,6 and 7. This work was supported from Agence Nationale de la Recherche (France) under contract ANR-19-CE24-0016-01 (TOPTRONIC), and related projects in Ref. 2,6-7.

[1] Rojas-Sánchez, J. C. & Fert, A. Compared Efficiencies of Conversions between Charge and Spin Current by Spin-Orbit Interactions in Two- and Three-Dimensional Systems. Phys. Rev. Appl. 11, 054049 (2019).
[2] Pham, TH et al, Thermal contribution to the spin-orbit torque in metallic-ferrimagnetic systems. Phys. Rev. Appl. 9, 064032 (2018).
[3] Taniguchi et al. Spin-Transfer Torques Generated by the Anomalous Hall Effect and Anisotropic Magnetoresistance. Phys Rev Appl. 3, 044001 (2015).
[4] Amin et al., Intrinsic spin current in ferromagnets. Phys. Rev. B 99, 220405 (2019)
[5] Kim & Lee, Generalized Spin Drift-Diffusion Formalism in the Presence of Spin-Orbit Interaction of Ferromagnets. Phys. Rev. L 125, 207205 (2020)
[6] Céspedes-Berrocal D., Damas H. et al. Current-Induced Spin Torques on Single GdFeCo Magnetic Layers. Adv. Materials 33, 2007047 (2021)
[7] Damas H. et al. PSS-Rapid Res. Lett., https://doi.org/10.1002/pssr.202200035

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 04.05.2022 - 15:00 German Time

Modeling of magneto-thermodynamics phenomena

Oksana Chubykalo-Fesenko, Instituto de Ciencia de Materiales de Madrid

Magnetisation dynamics usually takes place at non-zero temperatures, i.e. in a thermodynamical environment. Many recent applications (e.g. heat-assisted magnetic recording) make use of temperature to influence the spin dynamics. On the contrary, others application (e.g. magnetocaloric) use magnetization changes to produce heat. In the present talk I will discuss our multi-scale framework to model both situations. I will review some recent results concerning the influence of temperature on domain wall width [1] or skyrmion size [2] and illustrate their motion under thermal gradient via the spin-Seebeck effect. Typically, domain walls or skyrmion are moving to the hot region, abeit for skyrmions with a skyrmion Hall angle. For the domain wall motion, especially in the perpendicular materials, we observed their oscillations as a function of time, particularly in the presence of pinning centers. For skyrmions, we also report a motion to the cold region in multilayered systems contrary to what happens in the ultrathin single layer case.
Since temperature can change magnetisation, there is a well-known reciprocal effect: magnetisation changes produce heat, used for example in magnetic hyperthermia for cancer treatment. These effects are typically considered through the area of hysteresis cycle, while I will underline the important role of local heating and magnetization dynamics [3]. Similarly, since the domain wall can be moved under thermal gradient, one can expect the reciprocal effect - the domain wall motion could be accompanied by a temperature release (the spin-Peltier effect for domain wall [4]). In this case the released temperature is proportional to the ratio of domain wall velocity/width. Here we consider the antiferromagnetic MnAu material, where ultra-high velocities are predicted when the domain wall is moved under current by spin-orbit torque. Importantly, when the domain wall velocities are high, its width decreases due to relativistic effects. We estimate that the domain wall motion in this material can be accompanied by a localized ultrafast heat pulse as strong as 0.1K, much higher than for coherent magnetization switching. The energy release is especially efficient under elastic collision of domain walls with the same topological charge [5].

[1] R.Moreno et al Phys. Rev. B 94, 104433 (2016)
[2] R.Tomasello et al Phys. Rev. B 97, 060402(R) (2018)
[3] C.Muñoz-Mendez, Phys. Rev.B 102, 214412 (2020)
[4] R.M.Otxoa et al Comm. Phys 3, 31 (2020)
[5] R.M.Otxoa et al Phys.Rev.Res. 3, 043069 (2021)

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 27.04.2022 - 15:00 German Time

Planar Hall Torque

Ilya Krivorotov, University of California at Irvine

Spin-orbit torques (SOTs) in bilayers of ferromagnetic (FM) and nonmagnetic (NM) materials, such as spin Hall [1] and Rashba [2, 3] torques, enable energy efficient manipulation of magnetization by electric currents. In this talk, I will discuss the discovery [4] of a damping-like SOT arising from planar Hall current in FM conductors [5, 6]. The magnitude of this planar Hall torque (PHT) is similar to that of the giant spin Hall torque in FM/Pt bilayers and strong PHT can be present in a system with negligibly small spin Hall torque such as FM/Au bilayers. We also show that PHT is large enough to cancel magnetic damping of the FM and excite auto-oscillations of the FM magnetization. The discovery of PHT expands the class of materials and systems for energy efficient manipulation of magnetization by giant SOTs.

1. Liu L. et al. (2012) Spin-torque switching with the giant spin Hall effect of tantalum. Science. 336, 555.
2. Miron I. M. et al. (2011) Perpendicular switching of a single ferromagnetic layer induced by in-plane current injection. Nature 476, 189.
3. Kurebayashi H. et al. (2014) An antidamping spin–orbit torque originating from the Berry curvature, Nature Nanotech. 9, 211.
4. Safranski C., Montoya E.A., Krivorotov I.N. (2019) Spin–orbit torque driven by a planar Hall current. Nat. Nanotech. 14, 27.
5. Taniguchi T., Grollier J., Stiles M.D. (2015) Spin-Transfer Torques Generated by the Anomalous Hall Effect and Anisotropic Magnetoresistance. Phys. Rev. Appl. 3, 1.
6. Ochoa H., Zarzuela R., Tserkovnyak Y. (2021) Self-induced spin-orbit torques in metallic ferromagnets, J. Magn. Magn. Mater. 538, 168262.

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 13.04.2022 - 15:00 German Time

Driving Exchange Mode Resonance as Adiabatic Quantum Motor with 100% Mechanical Efficiency

Ran Cheng, University of California

An insulating ferromagnet (FM)-topological insulator (TI)-FM trilayer heterostructure can be operated as an adiabatic quantum motor by virtue of the combined effect of voltage-induced torque and its reverse effect—topological charge pumping. Unlike traditional current-driven systems, such a voltage-driven system can achieve 100% mechanical efficiency because the output current is purely adiabatic which does not incur Joule heating as dissipative currents do. This mechanism enables the excitation of the high-frequency exchange mode resonance in an FM-TI-FM system, where the two FM precess with a 180° phase difference, without producing any waste heat. Even in the presence of leakage currents and other imperfections, our proposed setup can still realize a mechanical efficiency two orders of magnitude larger than current-driven magnetic resonances. On the theoretical side, the voltage-induced torque is determined by the Berry curvature jointing time and crystal momentum, which has been previously overlooked in the study of topological materials. Our findings will facilitate the development of ultrafast spintronic devices consuming extremely low power.

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 30.03.2022 - 15:00 German Time

Ultrafast optical excitation and probing of coherent antiferromagnetic spin dynamics

Christian Tzschaschel, Harvard University

Antiferromagnetic spintronics present a promising approach to overcome limitations of current information technology. Owing to the vanishing net magnetization, antiferromagnetic materials exhibit spin dynamics on sub-picosecond timescales potentially allowing for not only data storage and logic circuit applications that are orders of magnitude faster than their established ferromagnetic counterparts, but also the development of new paradigms for device architectures with greater functionality. The tremendous interest in the realization of antiferromagnet-based devices has triggered an ongoing exploration of tools for controlling and manipulating antiferromagnets.
In this talk, I will present recent advances in the ultrafast optical excitation and probing of antiferromagnetic spin precessions. Based on the inverse magneto-optical effects [1,2], laser pulses can act like ultrafast magnetic field pulses, thereby enabling efficient non-thermal optical excitation of coherent spin precessions in fully compensated antiferromagnets. We show that the initial phase of the spin precession contains valuable information about the excitation, which allows us not only to distinguish between different excitation mechanisms [3], but also to reveal an ultrafast damping torque which can even become the dominant excitation mechanism in antiferromagnets [4]. The ensuing coherent spin precession leads to a transient symmetry reduction. Using symmetry-sensitive nonlinear optical probes, we track the antiferromagnetic order parameter quantitatively in three dimensions [5]. We observe a strongly elliptical precession – typical for antiferromagnetic dynamics. I will conclude with an outlook on how these results can contribute to emerging topics in ultrafast magnetization dynamics [6].

[1] A.V. Kimel, A. Kirilyuk, P.A. Usachev, R.V. Pisarev, A.M. Balbashov, and Th. Rasing, Nature 435, 655 (2005)
[2] A.M. Kalashnikova, A.V. Kimel, R.V. Pisarev, V.N. Gridnev, A. Kirilyuk, and Th. Rasing, Phys. Rev. Lett. 99, 167205 (2007)
[3] C. Tzschaschel, K. Otano, R. Iida, T. Shimura, H. Ueda, S. Günther, M. Fiebig, and T. Satoh, Phys. Rev. B 95, 174407 (2017)
[4] C. Tzschaschel, T. Satoh, and M. Fiebig, Nat. Commun. 11, 6142 (2020)
[5] C. Tzschaschel, T. Satoh, and M. Fiebig, Nat. Commun. 10, 3995 (2019)
[6] J. Li, C.-J. Yang, R. Mondal, C. Tzschaschel, and S. Pal, Appl. Phys. Lett. 120, 050501 (2022)

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 20.04.2022 - 15:00 German Time

Resolving chicken-or-egg causality dilemma for magneto-structural phase transition in FeRh

Aleksei V. Kimel, Radboud University

The nature of the counter-intuitive heat induced ferromagnetism in FeRh has been a subject of ongoing debates for about 60 years, resembling a dispute about the chicken-or-egg causality dilemma. FeRh is antiferromagnetic at low temperatures and becomes ferromagnetic, when heated above 370 K. These magnetic changes are accompanied by an expansion of the unit cell. It is, however, still unknown whether this a magnetic phase transition that drives the lattice expansion or a structural phase transition that causes the magnetic changes. To resolve this magnetism-or-lattice causality dilemma, we heated FeRh with femtosecond laser pulse and traced structural and magnetic changes by measuring reflectivity and the magneto-optical Kerr effect, respectively. Alternatively, we performed ultrafast magnetometry and traced formation of ferromagnetic domains with the help of double-pulse THz emission spectroscopy. We show that while a femtosecond laser pulse indeed generates ferromagnetic nuclei in FeRh, it takes of about 10 ps before the nuclei acquire a net magnetization. We argue that this latency is intrinsic to the phase transition from collinear antiferromagnetic to ferromagnetic states and must be present even in the case when the sign of the exchange interaction changes instantaneously. Using high magnetic fields up to 25 T, we could accelerate the magnetic phase transition and eventually discovered the fastest possible emergence of ferromagnetism in step with the lattice. As a result, we show that both spins and lattice evolve simultaneously. This finding practically resolves the magnetism-or-lattice causality dilemma.

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