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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On-line SPICE-SPIN+X Seminars

On-line Seminar: 16.03.2022 - 15:00 German Time

Ferrimagnetic Spintronics

Kyung-Jin Lee, KAIST

Compensated ferrimagnets combine the best features of antiferromagnets and ferromagnets [1]. Antiferromagnets are of considerable interest because the exchange torques between the two sublattices give a time scale that is much faster than that in ferromagnets and the lack of magnetization and net angular momentum lead to minimal perturbation by stray fields and eased constraints due to angular momentum conservation. A compensated ferrimagnet has all these virtues. At the same time, the lack of symmetry between the two sublattices in a compensated ferrimagnet means that quantities like average spin currents are not zero making the systems potentially easier to manipulate and detect the consequences. We will describe calculations and measurements of domain wall and skyrmion motion at the angular momentum compensation point. At this point with no net spin density, the rotational motion of the magnetic textures (domain walls and skyrmions) is absent. As a result, domain walls move fast [2,3], the skyrmion Hall effect vanishes [4], the magnon-photon coupling enhances [5], and a relativistic domain wall motion is realized [2,6]. We will also discuss the increased efficiency of spin torques due to the weakened dephasing in compensated ferrimagnets. Combining experiments with theoretical studies, Refs. [7] and [8] show large torques for ferrimagnetic multilayers and for ferrimagnetic domain walls, respectively.

[1] S. K. Kim, G. S. D. Beach, K.-J. Lee, T. Ono, Th. Rasing, and H. Yang, Ferrimagnetic spintronics, Nat. Mater. 21, 24 (2022).
[2] T. Shiino et al., Antiferromagnetic domain wall motion driven by spin-orbit torques. Phys. Rev. Lett. 117, 087203 (2016).
[3] K.-J. Kim et al., Fast domain wall motion in the vicinity of the angular momentum compensation temperature of ferrimagnets. Nat. Mater. 16, 1187 (2017).
[4] Y. Hirata et al., Vanishing skyrmion Hall effect at the angular momentum compensation temperature of a ferrimagnet. Nat. Nanotechnol. 14, 232 (2019).
[5] J. Shim, S.-J. Kim, S. K. Kim, and K.-J. Lee, Enhanced magnon-photon coupling at the angular momentum compensation point of ferrimagnets. Phys. Rev. Lett. 125, 027205 (2020).
[6] L. Caretta et al., Relativistic kinematics of a magnetic soliton. Science 370, 1438 (2020).
[7] J. Yu et al., Long spin coherence length and bulk-like spin-orbit torque in ferromagnetic multilayers. Nat. Mater. 18, 29 (2019).
[8] T. Okuno et al., Spin-transfer torques for domain wall motion in antiferromagnetically coupled ferrimagnets. Nat. Electron. 2, 372 (2019).

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 02.03.2022 - 15:00 German Time

Towards coupling coherent femtosecond charge and spin dynamics in antiferromagnets

Davide Bossini, University of Konstanz

The pressing demand to develop novel information technology schemes has bolstered the research area of spintronics. In particular, the colossal amount of data to process and transfer, especially in the cloud, have pushed researchers to explore concepts for an ever faster and less energy-demanding method to record and handle information. A possible approach consists in employing femtosecond laser pulses to optically generate spin dynamics in magnetic materials. One of the possible outcomes of the light-spin interaction on the sub-picosecond time-scale is the generation of coherent magnons. This process has been demonstrated [1-2] even in the absence of absorption of light by the lattice [3]. Additionally both low- and high-energy magnons can be induced and coherently manipulated [4-5]. In my talk I will introduce first some aspects of the all-optical generation of coherent magnons on the ultrashort time-scale, discussing several possible mechanisms. Furthermore I will focus on the possibility to drive an hybrid electronic-magnonic transition, called exciton-magnon, which allows to arbitrarily amplify coherent THz spin oscillations. In addition, exploring this concept in a multi-domain antiferromagnet, an unprecedented coupling between different magnon modes was reported, as the ultrafast spin dynamics enters the nonlinear regime. This effect was proved both experimentally and theoretically to be due to the domain walls, which are effectively able to couple different magnon modes in different domains [6-7]. While this discovery enables perspectives of coherent transfer of energy on the picosecond time- and (sub)-micrometer length-scale, the implications for the electronic system are left open to investigate.

[1] Nature 435, 655 (2005)
[2] Physical Review Letters 105, 077402 (2010)
[3] Physical Review B 89, 060405(R) (2014)
[4] Nature Communications 7, 10645 (2016)
[5] Physical Review B 100, 024428 (2019)
[6] J. Phys. D: Appl. Phys. 54, 374004 (2021)
[7] Physical Review Letters 127, 077202 (2021)

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 16.02.2022 - 15:00 German Time

Magneto-Acoustic Waves in Magnetic Thin Films

Mathias Weiler, TU Kaiserslautern

Spin waves form the basis for the field of magnonics, where they are used for information transport and processing [1]. Surface acoustic waves (SAWs) are widely employed as frequency filters in mobile communication technology. SAWs have group velocities comparable to that of spin waves and consequently can be generated with magnon-compatible wavelengths and frequencies. In magnetic media, spin waves can interact with SAWs which defines the field of magnetoacoustics. Magneto-acoustic phenomena can be used to excite and detect magnetization dynamics acoustically and control SAW propagation magnetically. Because of the ellipticity of the magneto-acoustic driving fields, as well as the spin-wave non-reciprocity due to dipolar coupling and the Dzyaloshinskii-Moriya interactions [2,3], magneto-acoustic waves are thereby generally chiral and non-reciprocal.
I will introduce the fundamentals of magnetoacoustics and then discuss the symmetry and non-reciprocity of magneto-acoustic waves in magnetically ordered thin films and heterostructures [4-6]. We quantitatively model the SAW-spin wave interaction based on the Kalinikos-Slavin equation and spin wave excitation by elliptically polarized coherent phonons to reveal that the magnon-phonon coupling is driven not only by magneto-elastic interactions [7] but also by magneto-rotation [4,8]. Our experiments furthermore demonstrate that SAW based spin-wave spectroscopy provides a sensitive measurement of the spin-wave dispersion and the Dzyaloshinskii-Moriya interaction. Non-reciprocal magneto-acoustic waves may be useful for the implementation of miniaturized on-chip microwave isolators.

[1] A. V. Chumak et al., Nat. Phys. 11, 453 (2015).
[2] J.-H. Moon et al., Phys. Rev. B 88, (2013).
[3] H. T. Nembach et al., Nat. Phys. 11, 825 (2015).
[4] M. Küß et al., Phys. Rev. Lett., 125, 217203 (2020).
[5] M. Küß et al., Phys. Rev. Applied, 15, 034060 (2021).
[6] M. Küß et al., Phys. Rev. Applied, 15, 034046 (2021).
[7] M. Weiler et al., Phys. Rev. Lett. 106, 117601 (2011).
[8] M. Xu et al., Sci. Adv. 6, eabb1724 (2020).

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 26.01.2022 - 15:00 German Time

Iron garnet thin films for spintronic and photonic devices

Caroline A. Ross, Massachusetts Institute of Technology

Ferromagnetic insulator thin films provide unique functionality in spintronic, magnonic, and magnetooptical devices. Yttrium iron garnet has very low magnetic damping, and substitution of rare earth ions as well as the introduction of point defects such as antisite defects allows the anisotropy, magnetostriction, compensation temperature and optical properties to be tuned. We use pulsed laser deposition to produce single crystal films of rare earth garnets down to a thickness of 2.5 nm, about 2 unit cells. We show intriguing magnetic behavior in garnet/heavy metal bilayers including spin orbit torque-driven domain wall motion at room temperature at velocities exceeding 4 km/s, switching the magnetic state. Iron garnets also exhibit magnetooptical activity and high transparency in the infrared, and we show how garnets grown on silicon can be used in integrated magnetooptical isolators to control the flow of light in photonic circuits.

References: Nature Commun. 11 1090 (2020), Nature Nanotech. 14 561 (2019), Optica 6 473 (2019), ACS Photonics 5, 5010 (2018), Phys. Rev. Mater. 2, 094405 (2018), Nature Materials 16, 309–314 (2017), Adv. Electron. Mater. 3 1600376 (2017)

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 02.02.2022 - 15:00 German Time

Three dimensional spintronics: “Faster, higher, stronger”

Amalio Fernández-Pacheco, CSIC-University of Zaragoza

The expansion of spintronics to three dimensions provides exciting opportunities to explore new physical phenomena, opening great prospects to create 3D magnetic devices for future technologies [1]. To get full access to the rich phenomenology predicted to emerge when moving to 3D, we have developed a new framework for the “3D nano-printing” of materials using focused electron beam induced deposition [2], which enables the fabrication of complex-shaped 3D magnetic structures with sub-100nm resolution. Making use of this tool, in combination with advanced magneto-optical and X-ray magnetic microscopy methods, we are studying the controlled motion of domain walls along the whole space in 3D magnetic interconnectors, either via external fields [3] or geometrical effects [4]. We are also studying the magnetoelectrical signals generated in these devices, where the non-collinear configuration of magnetic states and electrical currents results in deviations from standard angular dependences normally obtained in planar devices [5]. I will also present our recent work on chiral effects in 3D helical geometries formed by interlaced nanowires, where exchange and dipolar interactions are balanced to result in a very rich phenomenology. The freedom provided to control magnetic effects in this type of geometries has been exploited to form chiral interfaces between domain walls of opposite chirality, allowing us to imprint topological spin defects at localized regions [6]. Furthermore, helical structures may also form strongly coupled domain wall pairs, which result in complex stray magnetic field configurations with topological features [7].

[1] A. Fernández-Pacheco et al, Nature Comm. 8, 1 (2017)
[2] L. Skoric, Nano Letters 20, 184 (2020)
[3] D. Sanz-Hernández et al, ACS Nano 11, 11066 (2017)
[4] L. Skoric et al, arXiv:2110.04636
[5] F. Meng et al, ACS Nano 15, 6765 (2021)
[6] D. Sanz-Hernández et al, ACS Nano 14, 8084 (2020)
[7] C. Donnelly et al, Nature Nanotechnol. (2021). https://doi.org/10.1038/s41565-021-01027-7

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