Magnetization dynamics in ordered spin structures revealed by diffractive and reflectometry ferromagnetic resonance

AIP Advances AIP Publishing 11 (2021) 015327

DM Burn, S ZHANG, G van der Laan, T HESJEDAL

Synchrotron radiation based techniques provide unique insight into both the element and time resolved magnetization behavior in magnetic spin systems. Here, we highlight the power of two recent developments, utilizing x-ray scattering techniques to reveal the precessional magnetization dynamics of ordered spin structures in the GHz regime, both in diffraction and reflection configurations. Our newly developed diffraction and reflectometry ferromagnetic resonance (DFMR and RFMR) techniques provide novel ways to explore the dynamics of modern magnetic materials, thereby opening up new pathways for the development of spintronic devices. In this paper we provide an overview of these techniques, and discuss the new understanding they provide into in the magnetization dynamics in the chiral magnetic structure in Y-type hexaferrite and the depth dependence to the magnetization dynamics in a [CoFeB/MgO/Ta]4 multilayer.

Controlling spin current polarization through non-collinear antiferromagnetism.

Nature communications 11 (2020) 4671-

T Nan, CX Quintela, J Irwin, G Gurung, DF Shao, J Gibbons, N Campbell, K Song, S-Y Choi, L Guo, RD Johnson, P Manuel, RV Chopdekar, I Hallsteinsen, T Tybell, PJ Ryan, J-W Kim, Y Choi, PG Radaelli, DC Ralph, EY Tsymbal, MS Rzchowski, CB Eom

The interconversion of charge and spin currents via spin-Hall effect is essential for spintronics. Energy-efficient and deterministic switching of magnetization can be achieved when spin polarizations of these spin currents are collinear with the magnetization. However, symmetry conditions generally restrict spin polarizations to be orthogonal to both the charge and spin flows. Spin polarizations can deviate from such direction in nonmagnetic materials only when the crystalline symmetry is reduced. Here, we show control of the spin polarization direction by using a non-collinear antiferromagnet Mn3GaN, in which the triangular spin structure creates a low magnetic symmetry while maintaining a high crystalline symmetry. We demonstrate that epitaxial Mn3GaN/permalloy heterostructures can generate unconventional spin-orbit torques at room temperature corresponding to out-of-plane and Dresselhaus-like spin polarizations which are forbidden in any sample with two-fold rotational symmetry. Our results demonstrate an approach based on spin-structure design for controlling spin-orbit torque, enabling high-efficient antiferromagnetic spintronics.

Optically and microwave-induced magnetization precession in [Co/Pt]/NiFe exchange springs

ACS Applied Materials and Interfaces American Chemical Society 12 (2020) 52116-52124

M Dabrowski,, A Frisk, D Burn, D Newman, C Klewe, A N'Diaye, P Shafer, E Arenholz, G Bowden, T Hesjedal, G van der Laan, G Hrkac, R Hicken

Microwave and heat assisted magnetic recording are two competing technologies that have greatly increased the capacity of hard disk drives. The efficiency of the magnetic recording process can be further improved by employing non-collinear spin structures that combine perpendicular and in-plane magnetic anisotropy. Here, we investigate both microwave and optically excited magnetization dynamics in [Co/Pt]/NiFe exchange spring samples. The resulting canted magnetization within the nanoscale [Co/Pt]/NiFe interfacial region allows for optically stimulated magnetization precession to be observed for an extended magnetic field and frequency range. The results can be explained by formation of an imprinted domain structure, which locks the magnetization orientation and makes the structures more robust against external perturbations. Tuning the canted interfacial domain structure may provide greater control of optically excited magnetization reversal and optically generated spin currents, which are of paramount importance for future ultrafast magnetic recording and spintronic applications.

Kerr effect anomaly in magnetic topological insulator superlattices

Nanotechnology IOP Publishing 31 (2020) 434001

J Liu, A Singh, B Kuerbanjiang, C Barnes, T Hesjedal

We report the magneto-optical Kerr effect (MOKE) study of magnetic topological insulator superlattice films with alternating transition-metal and rare-earth doping. We observe an unexpected hump in the MOKE hysteresis loops upon magnetization reversal at low temperatures, reminiscent of the topological Hall effect(THE) reported in transport measurements. The THE is commonly associated with the existence of magnetic skyrmions, i.e., chiral spin textures originating from topological defects in real space. Here, the observation of the effect is tied to ferromagnetic ordering in the rare-earth-doped layers of the superlattice. Our study may provide a new approach for the non-invasive optical investigation of skyrmions in magnetic films, complementary to electrical transport measurements, where the topological Hall signal is often the only hint of non-trivial magnetization patterns.

Magnetic order in 3D topological insulators - wishful thinking or gateway to emergent quantum effects?

Applied Physics Letters AIP Publishing 117 (2020) 150502

AI Figueroa, T Hesjedal, N-J Steinke

Three-dimensional topological insulators (TIs) are a perfectly tuned quantum-mechanical machinery in which counter-propagating and oppositely spin-polarized conduction channels balance each other on the surface of the material. This topological surface state crosses the bandgap of the TI, and lives at the interface between the topological and a trivial material, such as vacuum. Despite its balanced perfection, it is rather useless for any practical applications. Instead, it takes the breaking of time-reversal symmetry (TRS), and the appearance of an exchange gap to unlock hidden quantum states. The quantum anomalous Hall effect, which has first been observed in Cr-doped (Sb,Bi)2Te3, is an example of such a state in which two edge channels are formed at zero field, crossing the magnetic exchange gap. The breaking of TRS can be achieved by magnetic doping of the TI with transition metal or rare earth ions, modulation doping to keep the electronically active channel impurity free, or by proximity coupling to a magnetically ordered layer or substrate, in heterostructures or superlattices. We review the challenges these approaches are facing in the famous 3D TI (Sb,Bi)2(Se,Te)3 family, and try to answer the question whether these materials can live up to the hype surrounding them.

Magnetic skyrmions

MagNews UK Magnetics Society 2019 (2020) 19-21

G van der Laan, T Hesjedal

Skyrmions getting an X-ray

MagNews UK Magnetics Society 2019 (2020) 22-22

S Zhang, T Hesjedal, G van der Laan

Electron beam lithography of magnetic skyrmions

Advanced Materials Wiley (2020)

Y Guang, Y Peng, Z Yan, Y Liu, J Zhang, X Zeng, S Zhang, S Zhang, DM Burn, N Jaouen, J Wei, H Xu, J Feng, C Fang, G van der Laan, T Hesjedal, B Cui, X Zhang, G Yu, X Han

The emergence of magnetic skyrmions, topological spin textures, has aroused tremendous interest in studying the rich physics related to their topology. While skyrmions promise high-density and energy-efficient magnetic memory devices for information technology, the manifestation of their non-trivial topology through single skyrmions, ordered, and disordered skyrmion lattices could also give rise to many fascinating physical phenomena, such as the chiral magnon and skyrmion glass states. Therefore, generating skyrmions at designated locations on a large scale, while controlling the skyrmion patterns, is key to advancing topological magnetism. Here, we present a new, yet general, approach to the ‘printing’ of skyrmions with zero-field stability in arbitrary patterns on a massive scale in exchange-biased magnetic multilayers. By exploiting the fact that the antiferromagnetic order can be reconfigured by local thermal excitations, we use a focused electron beam with a graphic pattern generator to ‘print’ skyrmions, which we refer to as skyrmion lithography. Our work provides a route to design arbitrary skyrmion patterns, thereby establishing the foundation for further exploration of topological magnetism.

Depth-resolved magnetization dynamics revealed by x-ray reflectometry ferromagnetic resonance

Physical Review Letters American Physical Society 125 (2020) 137201

D Burn, S Zhang, G Yu, Y Guang, H Chen, X Qiu, G van der Laan, T Hesjedal

Magnetic multilayers offer diverse opportunities for the development of ultrafast functional devices through advanced interface and layer engineering. Nevertheless, a method for determining their dynamic properties as a function of depth throughout such stacks have remained elusive. By probing the ferromagnetic resonance (FMR) modes with element-selective soft x-ray resonant reflectivity, we gain access to the magnetization dynamics as a function of depth. Most notably, using reflectometry ferromagnetic resonance (RFMR), we find a phase lag between the coupled ferromagnetic layers in [CoFeB/MgO/Ta]4 multilayers, which is invisible to other techniques. RFMR enables the time- and layer-resolved probing of the complex magnetization dynamics of a wide range of functional magnetic heterostructures with absorption edges in the soft x-ray wavelength regime.

Absolute crystal and magnetic chiralities in the langasite compound Ba3NbFe3Si2O14 determined by polarized neutron and x-ray scattering

Physical Review B American Physical Society 102 (2020) 54417

N Qureshi, A Bombardi, S Picozzi, P Barone, E Lelièvre-Berna, X Xu, C Stock, D McMorrow, A Hearmon, F Fabrizi, P Radaelli, S-W Cheong, L Chapon

We present a combined polarized neutron and x-ray scattering study on two enantiopure langasite single crystals aimed at the determination of their absolute structural and magnetic chiralities and the coupling between them. Our respective data sets unambiguously reveal two samples of opposite structural chirality, where the magnetic handedness is pinned by the structural one. Simple energy considerations of the magnetic exchange and single-ion anisotropy parameters reveal that it is not the Dzyaloshinskii-Moriya interaction but the local single-ion anisotropy on a triangular plaquette which plays a key role in stabilizing one of the two magnetic helices.

Emergent helical texture of electric dipoles

Science American Association for the Advancement of Science 369 (2020) 680-684

D Khalyavin, R Johnson, F Orlandi, P Radaelli, P Manuel, AA Belik

Proximity-induced odd-frequency superconductivity in a topological insulator

Physical Review Letters American Physical Society 125 (2020) 026802

J Krieger, A Pertsova, S Giblin, M Döbeli, T Prokscha, C Schneider, A Suter, T Hesjedal, A Balatsky, Z Salman

At an interface between a topological insulator (TI) and a conventional superconductor (SC), superconductivity has been predicted to change dramatically and exhibit novel correlations. In particular, the induced superconductivity by an s-wave SC in a TI can develop an order parameter with a p-wave component. Here we present experimental evidence for an unexpected proximity-induced novel superconducting state in a thin layer of the prototypical TI, Bi2Se3 proximity coupled to Nb. From depthresolved magnetic field measurements below the superconducting transition temperature of Nb, we observe a local enhancement of the magnetic field in Bi2Se3 that exceeds the externally applied field, thus supporting the existence of an intrinsic paramagnetic Meissner effect arising from an odd-frequency superconducting state. Our experimental results are complemented by theoretical calculations supporting the appearance of such a component at the interface which extends into the TI. This state is topologically distinct from the conventional Bardeen-Cooper-Schrieffer state it originates from. To the best of our knowledge, these findings present a first observation of bulk odd-frequency superconductivity in a TI. We thus reaffirm the potential of the TI-SC interface as a versatile platform to produce novel superconducting states.

Exchange bias in magnetic topological insulator superlattices

Nano Letters American Chemical Society (2020)

J Liu, A Singh, T Hesjedal, Et al.

Magnetic doping and proximity coupling can open a band gap in a topological insulator (TI) and give rise to dissipationless quantum conduction phenomena. Here, by combining these two approaches, we demonstrate a novel TI superlattice structure that is alternately doped with transition and rare earth elements. An unexpected exchange bias effect is unambiguously confirmed in the superlattice with a large exchange bias field using magneto-transport and magneto-optical techniques. Further, the Curie temperature of the Cr-doped layers in the superlattice is found to increase by 60 K compared to a Cr-doped single-layer film. This result is supported by density-functional-theory calculations, which indicate the presence of antiferromagnetic ordering in Dy:Bi2Te3 induced by proximity coupling to Cr:Sb2Te3 at the interface. This work provides a new pathway to realizing the quantum anomalous Hall effect at elevated temperatures and axion insulator state at zero magnetic field by interface engineering in TI heterostructures.

Polarizing an antiferromagnet by optical engineering of the crystal field (June, 10.1038/s41567-020-0936-3, 2020)


AS Disa, M Fechner, TF Nova, B Liu, M Foerst, D Prabhakaran, PG Radaelli, A Cavalleri

Polarizing an antiferromagnet by optical engineering of the crystal field

Nature Physics Nature Research 16 (2020) 937-941

AS Disa, M Fechner, T Nova, B Liu, M Foerst, D Prabhakaran, P Radaelli, A Cavalleri

Strain engineering is widely used to manipulate the electronic and magnetic properties of complex materials. For example, the piezomagnetic effect provides an attractive route to control magnetism with strain. In this effect, the staggered spin structure of an antiferromagnet is decompensated by breaking the crystal field symmetry, which induces a ferrimagnetic polarization. Piezomagnetism is especially appealing because, unlike magnetostriction, it couples strain and magnetization at linear order, and allows for bi-directional control suitable for memory and spintronics applications. However, its use in functional devices has so far been hindered by the slow speed and large uniaxial strains required. Here we show that the essential features of piezomagnetism can be reproduced with optical phonons alone, which can be driven by light to large amplitudes without changing the volume and hence beyond the elastic limits of the material. We exploit nonlinear, three-phonon mixing to induce the desired crystal field distortions in the antiferromagnet CoF2. Through this effect, we generate a ferrimagnetic moment of 0.2 μB per unit cell, nearly three orders of magnitude larger than achieved with mechanical strain.

Magneto-optical Kerr switching properties of (CrI3)2 and (CrBr3/CrI3) bilayers

ACS Applied Electronic Materials American Chemical Society 2 (2020) 1373-1380

K Yang, W Hu, H Wu, M-H Whangbo, P Radaelli, A Stroppa

We explore the magneto-optical Kerr effect (MOKE) for different spin configurations of the (CrI3)2 bilayer and (CrBr3/CrI3) mixed bilayer using symmetry arguments and first-principles electronic structure calculations. Starting from CrX3 (X = I, Br) monolayers, we considered collinear ferromagnetic (FM) and layered antiferromagnetic (AFM) states for (CrI3)2 and (CrBr3/CrI3) bilayers. The AFM (CrI3)2 bilayer does not show MOKE, consistent with the presence of a symmetry operator combining inversion (I) and time reversal (T) symmetries. The FM state preserves I symmetry but breaks the T symmetry, thus allowing a nonzero Kerr angle, which is reversible by switching the FM spins. The (CrBr3/CrI3) bilayer breaks both the I and T symmetries and thus exhibits MOKE both in the FM state and, remarkably, in the AFM state. In both FM and AFM configurations, the Kerr angle switches by reversing the spins in both layers. Our study demonstrates that the MOKE spectra can help to characterize different magnetic configurations in these emerging two-dimensional (2D) magnetic materials due to a different stacking of the monolayers, even in the AFM case. Note that the present symmetry analyses and MOKE properties apply to more general 2D magnetic van der Waals heterostructures. Furthermore, we propose the (CrBr3/CrI3) bilayer as a promising candidate for AFM spintronics since the two time-reversed AFM states are associated with opposite Kerr rotation, i.e., they could be used as memory elements.

Coherent transfer of spin angular momentum by evanescent spin #Waves within Antiferromagnetic NiO

Physical Review Letters American Physical Society 124 (2020) 217201

M Dąbrowski, N Takafumi, D Burn, A Frisk, D Newman, C Klewe, Q Li, M Yang, P Shafer, E Arenholz, T Hesjedal, G van der Laan, Z Qiu, R Hicken

Insulating antiferromagnets have recently emerged as efficient and robust conductors of spin current. Element-specific and phase-resolved x-ray ferromagnetic resonance has been used to probe the injection and transmission of ac spin current through thin epitaxial NiO(001) layers. The spin current is found to be mediated by coherent evanescent spin waves of GHz frequency, rather than propagating magnons of THz frequency, paving the way towards coherent control of the phase and amplitude of spin currents within an antiferromagnetic insulator at room temperature.

Direct observation of the energy gain underpinning ferromagnetic superexchange in the electronic structure of CrGeTe3

Physical Review B: Condensed Matter and Materials Physics American Physical Society 101 (2020) 205125

M Watson, I Markovic, F Mazzola, A Rajan, E Morales, D Burn, T Hesjedal, G van der Laan, S Mukherjee, T Kim, C Bigi, I Vobornik, M Hatnean, G Balakrishnan, P King

We investigate the temperature-dependent electronic structure of the van der Waals ferromagnet, CrGeTe3. Using angle-resolved photoemission spectroscopy, we identify atomic- and orbital-specific band shifts upon cooling through TC. From these, together with x-ray absorption spectroscopy and x-ray magnetic circular dichroism measurements, we identify the states created by a covalent bond between the Te 5p and the Cr eg orbitals as the primary driver of the ferromagnetic ordering in this system, while it is the Cr t2g states that carry the majority of the spin moment. The t2g states furthermore exhibit a marked bandwidth increase and a remarkable lifetime enhancement upon entering the ordered phase, pointing to a delicate interplay between localized and itinerant states in this family of layered ferromagnets.

Magnetic skyrmion interactions in the micromagnetic framework

Physical Review B: Condensed Matter and Materials Physics American Physical Society 101 (2020) 134422

R Brearton, G van der Laan, T Hesjedal

Magnetic skyrmions are localized swirls of magnetization with a nontrivial topological winding number. This winding increases their robustness to superparamagnetism and gives rise to a myriad of novel dynamical properties, making them attractive as next-generation information carriers. Recently the equation of motion for a skyrmion was derived using the approach pioneered by Thiele, allowing for macroscopic skyrmion systems to be modeled efficiently. This powerful technique suffers from the prerequisite that one must have a priori knowledge of the functional form of the interaction between a skyrmion and all other magnetic structures in its environment. Here we attempt to alleviate this problem by providing a simple analytic expression that can generate arbitrary repulsive interaction potentials from the micromagnetic Hamiltonian, using it to provide a correction to the interaction between a skyrmion and the boundary of its material. We also discuss a toy model of the radial profile of a skyrmion, which is accurate for a wide range of material parameters.

Element- and Time-Resolved Measurements of Spin Dynamics Using X-ray Detected Ferromagnetic Resonance

Synchrotron Radiation News Informa UK Limited 33 (2020) 12-19

C Klewe, Q Li, M Yang, AT N’Diaye, DM Burn, T Hesjedal, AI Figueroa, C Hwang, J Li, RJ Hicken, P Shafer, E Arenholz, G van der Laan, Z Qiu