On-line SPICE-SPIN+X Seminars

On-line Seminar: 13.07.2022 - 15:00 German Time

Magneto-ionics: using ionic motion to control magnetism

Liza Herrera Diez, CNRS and Université Paris-Saclay

Reliable and dynamic control of magnetic properties in technologically relevant magnetic materials is at the heart of a variety of emerging practical applications in spintronics. Gate voltage-controlled ionic diffusion in magnetic devices has shown to provide non-volatile control of perpendicular magnetic anisotropy, the Dzyaloshinskii Moriya interaction, as well as the velocity and pinning of magnetic domain walls, opening a solid path towards novel multifunctional spintronics devices. In this talk, I will present an overview of this exciting field, what it means for practical applications, and discuss the physical mechanisms involved.

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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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On-line SPICE-SPIN+X Seminars

On-line Seminar: 09.03.2022 - 15:00 German Time

Unraveling Proximity and Topology at Interfaces with Next Generation Neutron Reflectometry

Alexander J. Grutter, NIST

Whether acting as a platform for quantum transport effects or ultra-efficient spintronics, heterostructures incorporating topologically nontrivial materials are among the most exciting playgrounds in condensed matter physics. Despite their promise, topological spintronic devices represent a difficult materials engineering challenge wherein the need to introduce magnetic order must be balanced with ensuring that topologically nontrivial conduction channels dominate the transport behavior. These competing requirements are highlighted by the first reported quantum anomalous Hall (QAH) insulator, Cr-doped (Bi,Sb)2Te3, where gap-inhomogeneity is thought to suppress the quantization temperature.[1,2] While this issue may be mitigated by increasing dopant density, defect channels rapidly come to dominate the conduction and prove equally detrimental to observing the physics of interest.
It is in this context that magnetic proximity effects have drawn considerable interest. By growing an ordered magnetic material in direct contact with the relevant electronic states, magnetic order may be induced through proximity without the introduction of additional defects. Despite the successful realization of a proximity-induced QAH effect, this approach has yielded no improvement in quantization temperature, highly inconsistent reports of ordering temperatures, and even disagreement over the existence of proximity effects in many systems.[3-5] A proper understanding of magnetic proximity effects at topologically nontrivial interfaces hinges critically on our ability to precisely isolate the properties of the interface from the bulk of the system. By decomposing the magnetic and electronic properties on a layer-by-layer and element-resolved basis, new quantum material systems may be robustly understood and designed. In this talk, we will examine approaches for accurately identifying magnetic proximity effects and other forms of magnetic interface coupling in systems such as (Bi,Sb)2Te3 and Cd3AS2, with a special emphasis on combining polarized neutron reflectometry with X-ray scattering, spectroscopy and electron microscopy.[6-8] We will conclude with a discussion on the future of ultra-sensitive probes of magnetic interfaces and the potential impact from highly multiplexing neutron instrumentation.

[1] C.-Z. Chang et al., Science 340, 167 (2013)
[2] E. O. Lachman et al., Science Advances 1, e150074 (2015)
[3] F. Katmis et al., Nature 533, 513 (2016)
[4] A. I. Figueroa et al., Physical Review Letters 125, 226801 (2020)
[5] R. Watanabe et al., Applied Physics Letters 115, 102403 (2019)
[6] Q. L. He et al., Nature Materials 16, 94 (2017)
[7] C.-Y. Yang et al., Science Advances 6, eaaaz8463 (2020)
[8] W. Yanez et al., Physical Review Applied 16, 054031 (2021)

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 23.03.2022 - 15:00 German Time

Ultrafast magnetization reversal driven by optical phonons

Andrei Kirilyuk, Radboud University

Identifying an efficient pathway to change the order parameter via a subtle excitation of the coupled high-frequency mode is the ultimate goal of the field of ultrafast phase transitions [1,2]. This is an especially interesting research direction in magnetism, where the coupling between spin and lattice excitations is required for magnetization reversal [3]. Despite several attempts [4,5] however, the switching between magnetic states via resonant pumping of phonon modes has not yet been demonstrated.
To provide resonant excitation of the phonon modes, we use pulses from FELIX (Free Electron Lasers for Infrared eXperiments, Nijmegen, The Netherlands). The IR/THz light with photon energy ranging between 25 meV and 124 meV (wavelength 10−50 μm) is typically used.
And thus we show how an ultrafast resonant excitation of the longitudinal optical phonon modes in magnetic garnet films switches magnetization into a peculiar quadrupolar magnetic domain pattern, unambiguously revealing the magneto-elastic mechanism of the switching [7]. In contrast, the excitation of strongly absorbing transverse phonon modes results in thermal demagnetization effect only. The mechanism appears to be very universal, and is shown to work in samples with very different crystallographic symmetry and magnetic properties, including weak ferromagnets and antiferromagnets [7].

[1] T. Kubacka et al, Science 343, 1333 (2014).
[2] A. Kirilyuk, A.V. Kimel, T. Rasing, Rev. Mod. Phys 82, 2731 (2010).
[3] N. Li et al., Rev. Mod. Phys. 84, 1045 (2012).
[4] T. F. Nova et al., Nature Physics 13, 132 (2017).
[5] S. F. Maehrlein et al., Science Advances 4, 5164 (2018).
[6] G.M.H. Knippels et al, Phys. Rev. Lett. 83, 1578 (1999).
[7] A. Stupakiewicz et al, Nature Physics 17, 489 (2021).
[8] P. Stremoukhov et al, New J. Physics 24, 023009 (2022).

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 11.05.2022 - 15:00 German Time

Magnetic Chirality

Sang-Wook Cheong, Rutgers University

The term of “chiral” has been extensively, in fact, almost abusively, used in Physics community in recent years. Chirality refers the situation where an object and its mirror image cannot overlap to each other by spatial rotation. In addition, chirality should not change under time reversal. Magnetic chirality means chirality in spin ordered states or spin textures. Chirality prime (chirality′) means that all of mirror and time reversal symmetries are broken even if spatial rotation is freely allowed. We can have magnetic chirality or chirality′ in three different situations: [1] in centrosymmetric magnetic lattices while their crystallographic lattices are chiral, [2] in chiral magnetic lattices while their crystallographic lattices are also chiral, and [3] in centrosymmetric crystallographic lattices. We will discuss a number of examples of magnetic chirality and chirality′, and also their emergent physical phenomena.

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