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When applied to C2V ferromagnetic/heavy metal interfaces, Moriya’s rules do not deterministically predict the orientation of interatomic DM vectors. Low-symmetry magnetic systems can exhibit both Rashba (DR) and Dresselhaus (DD) forms of the Dzyaloshinskii-Moryia interaction (DMI), as both spin-orbit couplings are generally present [1]. We assert that the spatial variation of the DM vectors—and thus that of the strength and direction of the effective DMI field—is determined by the DMI’s character (i.e., ||DD||/||DR||), as well as the unit cell’s aspect ratio [2]. Overall, the DMI is thus nominally anisotropic and hybridized—potentially stabilizing antiskyrmions and Bloch-Néel (i.e. mixed) skyrmions [2-3]. Ascertaining the existence and tunability of [anti]skyrmions in such systems is of technological interest and would elucidate the nature of DMI [2]. Employing the Wulff construction [3], ξ-vector formalism [4], and an augmented wall energy that accounts for DMI character, this work models the equilibrium shape and wall magnetization profile of bubble domains and skyrmions in thin films with C2v symmetry. We show that for sufficiently large DMI, square-shaped antiskyrmions are stable for certain DM vector orientations while lens- and oval-shaped mixed skyrmions are favored otherwise. A symmetry-breaking applied in-plane field stabilizes domain shapes resembling ovals and Reuleaux triangles. The impact of the unit cell’s aspect ratio, the DM vector’s orientation, and other material parameters on the shape of bubble domains and skyrmions are discussed. Moreover, our results reveal unusual relationships between faceting behavior and wall stiffness under rather reasonable conditions. Elucidating the interplay between the different effective fields present, we explore the validity of these results based on pedal plots, discontinuities in the magnetization profiles, and local extrema in the wall energy.References: U. Güngördü et al., Physical Review B., 93, 064428 (2016) M. Hoffman et al., Nature Communications., 8, 308 (2017) J. P. Pellegren, D. Lau, and V. Sokalski, Physical Review Letters., 119, 027203 (2017) A. A. Wheeler, Journal of Statistical Physics., 95, 1245–1280 (1999)

MMM 2020

Energetics, equilibrium shape, and vorticity of skyrmions and antiskyrmions stabilized by the anisotropic Dzyaloshinskii-Moriya interaction in thin films with C2v symmetry

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The 65th Annual Conference on Magnetism and Magnetic Materials (MMM 2020) was held in a virtual form. The COVID-19 pandemic, with its associated uncertainty about international travel and the ability to gather in large groups, meant that the Steering Committee decided to cancel the face-to-face meeting that had been planned in Palm Beach, Florida November 2-6, 2020.

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Authors: F. Ajejas, W. Legrand, Y. Sassi, S. Collin, A. Vecchiola, K. Bouzehouane, N. Reyren, V. Cros, A. Fert, Unite Mixte de Physique CNRS/Thales, Palaiseau, Île-de-France, FRANCE|S. Pizzini, Institut NEEL, Grenoble, Auvergne-Rhône-Alpes , FRANCE|

Abstract Body: Magnetic multilayers (MML) with large perpendicular magnetic anisotropy (PMA) and Dzyaloshinskii-Moriya interaction (DMI) have attracted great attention in recent years owing to the possibility of stabilizing non-collinear magnetic textures such as chiral domain walls (DW) [1], spin spirals [2] or skyrmions [3-4] with Neeel chiral spin rotation. The latter are promising candidates as carriers of information for next-generation race-track magnetic memories or logic devices. To this aim, the accurate experimental determination of DMI is an important challenge allowing the engineering of future devices with tailored properties. The determination of the effective DMI amplitude D in MML is still a current subject of research, in particular for metallic multilayers with a high number of repetitions. In this study, we have performed a series of measurements to determine DMI values by two different techniques: domain size periodicity (λ) [5] of aligned-stripe domains after in-plane (IP) demagnetization by magnetic force microscopy (MFM), and asymmetric expansion of domains in the presence of an in-plane magnetic field, by Kerr microscopy [6-7]. For this purpose, we selected different materials to build asymmetric trilayers, with the general structure is [Pt/Co/M]xN with M = Ru, Ni, Pd, Al, Al/Ta and N = 2, 3, 4, 5, 6 the number of repetitions. We observe a correlation between Ds = D/t (t is the Co thickness), the interfacial DMI obtained in the previous methods and intrinsic material parameters such as atomic number (Z), Pauling electronegativity (χ) [8] work function (Wf) [9] of Co/M interfaces. We find a clear linear behaviour between Ds and Wf. This correlation points to Rashba-like interfacial fields, leading to the modulation of the effective interfacial DMI. French ANR grant TOPSky TOPSKY (ANR-17-CE24-0025) and DARPA TEE program grant (MIPR#HR0011831554) and EU grant SKYTOP (H2020 FET Proactive 824123) are acknowledged for their financial support.References: [1] A. Thiaville et al. EuroPhys. Lett, 100, 57002 (2012) [2] P. Ferriani et al. Phys. Rev. Lett. 101, 27201 (2008). [3] A. Fert, N. Reyren, V. Cros, Nat. Rev. Mat. 2, 17031 (2017). [4] C. Moreau-Luchaire et al. Nature Nanotechnology 11, 444 (2016) [5] I. Lemesh et al. Physical Review B 95, 174423 (2017) [6] F. Ajejas et al. Appl. Phys. Lett. 111, 202402 (2017); [7] T. Ha Pham et al EPL 113 67001 (2016) [8] H. Jia et al. Phys. Rev. M 4, 024405 (2020) [9] Y.-K. Park et al. NPG Asia Materials 10, 995 (2018)

GMW Best Poster Award 1 - Experimental correlation of Interfacial Dzyaloshinskii-Moriya interaction amplitude and Work Function in magnetic multilayers

Authors:  A. Semiannikova, V. Irkhin, Y.A. Perevozchikova, P. Korenistov, V. Marchenkov, M N Mikheev Institute of Metal Physics of the Ural Branch of the Russian Academy of Sciences, Ekaterinburg, Sverdlovskaâ, RUSSIAN FEDERATION|V. Marchenkov, Ural Federal University named after the first President of Russia B N Yeltsin, Ekaterinburg, Sverdlovskaa, RUSSIAN FEDERATION|

Abstract Body: Half-metallic ferromagnets (HMFs) and spin gapless semiconductors (SGSs) are promising materials for spintronics with almost 100% spin polarization of charge carriers. They possess a gap near the Fermi energy for the current carriers with spin down. However, for the opposite spin projection, these materials have a difference: the energy gap is absent in HFMs [1], while it is zero in SGSs. Parameters of the energy gap and, consequently, physical properties can vary quite strongly in different X2YZ Heusler alloys [2, 3, 4], where Y-elements are 3d-transition metals and Z-components are s- and p-elements. Thus, the aim of the work is to follow the changes in the electrical, magnetic and galvanomagnetic properties and to establish their interconnection and basic behavior patterns of the Co2YSi (Y = Ti, V, Cr, Mn, Fe, Co, Ni) and Co2MnZ Heusler alloys (Z = Si, Al, Ge, Sb) in the HMF and/or SGS states. The electrical resistivity was measured in a wide temperature range from 4.2 to 300 K, the field and temperature dependences of magnetic and galvanomagnetic properties being studied too. Significant changes in the values of the coefficients of the normal and anomalous Hall Effect, magnetization, residual resistivity, type, and concentration of current carriers and their mobility were found. At the same time, the correlation between the changes in these electronic and magnetic characteristics depending on the number of valence electrons are observed. The work was performed within the framework of the state assignment of the Ministry of Science and High Education of Russia (the themes “Spin”, No. AAAA-A18-118020290104-2-2 and “Quantum” No. AAAA-A18-118020190095-4) with partial support from the RFBR (project No. 18-02-00739 and 20-32-90065) and the Government of the Russian Federation (Decree No. 211, Contract No. 02.A03.21.0006).References: [1] M.I. Katsnelson, V.Yu. Irkhin, L. Chioncel et al., Rev. Mod. Phys., Vol. 80, p. 315 (2008). [2] V.V. Marchenkov, Yu.A. Perevozchikova, N.I. Kourov et al., J. Magn. Magn. Mat., Vol. 459, p. 211 (2018). [3] Yu.A. Perevozchikova, A.A. Semiannikova, A.N. Domozhirova, P.B. Terentyev, E.B. Marchenkova, E.I. Patrakov, M. Eisterer, P.S. Korenistov and V.V. Marchenkov, Low Temp. Phys., Vol. 45, p. 789 (2019). [4] Yu.A. Perevozchikova, A.A. Semiannikova, P.B. Terentyev, M. Eisterer, P.S. Korenistov and V.V. Marchenkov, Journal of Physics: Conf. Series, Vol. 1389, p. 012110 (2019). 

GMW Best Poster Award 3 - Electroresistivity, magnetic and galvanomagnetic properties of Co-based Heusler alloys in the states of half-metallic ferromagnet and spin gapless semiconductor

Unlike the common expectation that superconducting spintronics and long-range triplet (LRT) proximity effects require complex ferromagnetic multilayers, relying on noncollinear/spiral magnetization or half-metals, we propose a new platform compatible with commercial spintronics. Recently, we have experimentally demonstrated that interfacial spin-orbit coupling (SOC) and symmetry-filtering in all-epitaxial V/MgO/Fe junctions, cooled below the critical temperature of vanadium, provide a thousand-fold increase in tunneling anisotropic magnetoresistance, supporting the LRT formation [1]. Here we report the converse effect: transformation of the magnetocrystalline anisotropy of a Fe(001) layer driven by the superconductivity of an underlying V layer through a spin-orbit coupled MgO interface [2]. We attribute this to an additional contribution to the free energy of the ferromagnet arising from the controlled generation of triplet Cooper pairs, which depends on the relative angle between the exchange field of the ferromagnet and the spin-orbit field. This observation offers the ability to fundamentally tune magnetic anisotropies using superconductivity - a key step in designing future cryogenic magnetic memories.References: [1] I. Martínez et al; Phys. Rev. Appl. 13, 014030 (2020). [2] C. González-Ruano et al; accepted for publication as Rapid Communication in Phys. Rev. B, Cond Mat. ArXiv https://arxiv.org/abs/2006.02118

Superconductivity-induced change in magnetic anisotropy in epitaxial ferromagnet-superconductor hybrids with spin-orbit interaction (B)

Antiferromagnets (AFMs) possess great potential in spintronics because of their immunity to external magnetic disturbance, the absence of a stray field, or the resonance in the terahertz range. The coupling of insulating AFMs to spin-orbit materials enables spin transport via AFM magnons. Particularly, spin transmission over several micrometers occurs in some AFMs with easy-axis anisotropy [1]. Easy-plane AFMs with two orthogonal, linearly polarized magnon eigenmodes own unique advantages for low-energy control of ultrafast magnetic dynamics. However, it is commonly conceived that these magnon modes are less likely to transmit spins because of their vanishing angular momentum. In this work, we report experimental evidence that an easy-plane insulating AFM, a α-Fe2O3 thin film, can efficiently transmit spins over micrometer distances (Fig. 1) [2]. Using nonlocal transport device geometry, we find that the spin transmission reaches maximum (zero) when the injected spin polarization is collinear with (perpendicular to) the Néel vector of the AFM, indicating that spins are carried by the excitations of the Néel vector rather than the residual magnetic moment. The spin decay length measured by varying the injector-detector distances shows an unconventional temperature dependence which cannot be captured considering solely thermal magnon scatterings. We interpret our observations in terms of an interference of two linearly polarized, propagating magnons in analogy to the birefringence effect in optics. Furthermore, our devices can realize a bi-stable spin-current switch with a 100% on/off ratio under zero remnant magnetic field. These findings provide additional tools for nonvolatile, low-field control of spin transport in AFM systems.References: 1. Lebrun, R. et al. Tunable long-distance spin transport in a crystalline antiferromagnetic iron oxide. Nature 561, 222 (2018). 2. Han, J. et al. Birefringence-like spin transport via linearly polarized antiferromagnetic magnons. Nat. Nanotechnol. https://doi.org/10.1038/s41565-020-0703-8 (2020).

Birefringence-like spin transport via linearly polarized antiferromagnetic magnons (B)

Magnetic skyrmions and antiskyrmions are topologically protected non-collinear spin textures. They have opposite topological charges of +1 and – 1 owing the differently winding of the in-plane spin component about a fixed direction of the core1. The existence of them in a material system mostly depends on the symmetry of the underlying crystal structure. For example, cubic B20 compounds host Bloch skyrmion of isotropic spin texture2, whereas D2d symmetry compounds have anisotropic spin texture named 'antiskyrmion'3,4. Antiskyrmions comprise of alternating chiral boundaries of helicoids and cycloids. They found in two varieties of shape, namely round and square-shaped antiskyrmion5. Here for the first time, using the Lorentz microscopy technique, we have discovered elliptical shape skyrmions in the same material which comprises of Bloch walls only 5. We named them as "elliptical Bloch skyrmions". The elliptical skyrmion's major axis is oriented along the helical propagation directions, [100] and [010], and have opposite chiralities. They are found as spare and lattice states. Interestingly, both topological structure phases can also be stabilized in zero biased fields and below, over wide temperature ranges. It is found that in D2d long-range dipole-dipole interactions play a significant rule for the stabilization of those different opposite topological spin textures. Micromagnetic simulations also confirms these nano-objects. Antiskyrmions are mostly found in room temperature and above, and in lower temperatures, elliptical Bloch skyrmion is observed. At a critical temperature of 268 K, both these nano-objects are stabilized. We also performed experiment at room temperature on the nano-stripes. The transformation and density variation of antiskyrmions and elliptical Bloch skyrmions occur here that depend on the degree of tilting6. Notably, those nano-objects in the nano-tracks produce as a single chain form in the middle of the track even without giving an in-plane field that was indeed needed in the broad width lamellae. The novel and unanticipated finding of opposite topological charges of 1in one material system make the family of D2d material exceptional.References: 1. A. Bogdanov, U. Roeßler and K.-H. Mueller, Phys. Rev. B, 66, 214410 (2002) 2. S. Muehlbauer, B. Binz and C. Pfleiderer, Science, 323, 915-919 (2009) 3. A. K. Nayak, V. Kumar and S. S. P. Parkin, Nature, 548, 561 (2017) 4. J. Jena, R. Stinshoff and S. S. P. Parkin, Nano Lett., 20, 59−65 (2019) 5. J. Jena, B. Goebel and S. S. P. Parkin, Nat. Commun., 11, 1115 (2020) 6. J Jena, B. Goebel and S. S. P. Parkin, Submitted., XX,XXX (2020)

Observation of opposite topological charges in single D2d Heusler compound (B)

Electrical writing and readout of antiferromagnetic insulators is a key prerequisite in view of using this class of materials in applications. Only recently, this has been demonstrated in insulating AFM/heavy metal bilayers by utilizing the transverse spin Hall magnetoresistance effect (SMR) as the readout. In the electrical measurement, a steplike signal resulting from a magnetic SMR contribution was reported in NiO1 and α-Fe2O32, while non-magnetic contributions due to artifacts in the heavy metal layer were found to mimic the signal expected from the magnetic switching.3,4 These results have generated a debate on the magnitude of the magnetic contributions, the reliability of the SMR readout mechanism and even on the possibility of switching domains in AFMs electrically. In our work5 we use a table-top approach to demonstrate stable and reversible current-induced switching of large-area antiferromagnetic domains in NiO/Pt by direct imaging in a Kerr microscope.6 Concurrent transport and magneto-optical imaging measurements allow us to correlate the AFM domain switching fraction and magneto-transport signal response. We establish a procedure to subtract the non-magnetic contributions and extract the magnetic SMR response from the transverse resistance signal. The disentanglement of magnetic and non-magnetic contributions clarifies the presence of a significant SMR response, associated with the switching of the antiferromagnetic domains. We thus highlight the possibility to deduce details of the antiferromagnetic switching from simple transport measurements.References: 1 L. Baldrati, O. Gomonay, A. Ross, Phys. Rev. Lett. 123, 177201 (2019). 2 Y. Cheng, S. Yu, M. Zhu, Phys. Rev. Lett. 124, 027202 (2020). 3 A. Churikova, D. Bono, B. Neltner, Appl. Phys. Lett. 116, 022410 (2020). 4 C.C. Chiang, S.Y. Huang, D. Qu, Phys. Rev. Lett. 123, 227203 (2019). 5 F. Schreiber, L. Baldrati, C. Schmitt, Appl. Phys. Lett. 117, 082401 (2020). 6 J. Xu, C. Zhou, M. Jia, Phys. Rev. B 100, 134413 (2019).

Concurrent magneto-optical imaging and magneto-transport readout of electrical switching of insulating antiferromagnetic thin films (B)

New fascinating phenomena involving coupled spin waves and phonons are observed at the magneto-elastic resonance: magnetic field dependent spin Seebeck effect [1], acoustically-assisted spin pumping [2, 3] and magnetization swiching [4, 5]. In this context, it has been shown that an efficient mean to excite phonons is by surface acoustic waves (SAW). The integration of SAW-FMR (ferromagnetic resonance) mechanism in spintronic and magnonic devices are claimed to operate with very low power. However, most ferromagnetic materials have precession frequencies well above 5 GHz, i.e. above the highest frequency reached by today's SAW technologies. This restricts the research on SAW-FMR to low magnetization and/or low magnetic anisotropy materials, such as (Ga,Mn)As or (GaMn)(As,P), Ni, and YIG. Here, we show that thin films of Fe epitaxially grown on GaAs(001), a spintronic and magnonic compatible magnetoelastic and piezoelectric heterostucture, give the opportunity to obtain SAW-FMR, at room temperature, below 1 GHz despite the high Fe magnetization. This is obtained by exploiting the softening of the magnetic stiffness when the field is applied along a hard axis, at moderate intensity (60 mT). Slight variations of the magnetic field direction, less than 0.1 deg off, increase dramatically magnetization precession frequency leading to off-resonance interaction and suppressing SAW attenuation. The observed phenomena are described by adopting a very simple magnetization dynamics model that catches the physics of the interaction. The observation of resonant, low frequency and highly directional magnetoelastic coupling in Fe suggests that SAW mature technology offers the opportunity to magnonics and spintronics to handle magnetization precession in a remote way, without using any dissipative electron current. [6]References: [1] Takashi Kikkawa, et al., Phys. Rev. Lett. 117, 207203, (2016) [2] M. Weiler, et al. Phys. Rev. Lett. 108, 176601, (2012) [3] Ken ichi Uchida, et al., Solid State Communications 198, 26, (2014) [4] L. Thevenard,et al. Phys. Rev. B 93, 134430, (2016) [5] P. Kuszewski, et al. Journal of Physics: Condensed Matter 30, 244003 (2018) [6] J.-Y. Duquesne, P. Rovillain, et al. Phys. Rev. Appl. 12, 024042 (2019)

Sub-GHz resonant magnetoelastic coupling in epitaxial Fe thin films

Voltage control of the structure, magnetic and electronic properties of materials enables the realization of new types of nonvolatile magnetoelectric memory devices. Here, we investigate the effect of ionic liquid gating on the magnetic properties of self-assembled heterostructure films consisting of nanoscale CoyFe3-yO4 (CFO) spinel pillars embedded in a SrCo1-xFexO3-δ (SCFO) perovskite (P) or brownmillerite (BM) matrix, over a range of compositions x and y. These self-assembled nanocomposites are grown by pulsed laser deposition on SrTiO3 substrates using SrCoO3-δ-SrFeO3-δ, SrCoO3-δ-Fe3O4, or SrFeO3-δ-Co3O4 pairs of targets. We first describe the growth kinetics of these nanocomposites, focusing on how the oxygen pressure affects the distribution of Co and Fe between the two phases and the formation of P vs. BM structure for the matrix. Unusually, at high oxygen pressures, the two-phase structure can grow from the codeposition of SrCoO3-δ and SrFeO3-δ, attributed to depletion of the Sr in the film. The pillar CFO magnetism depends on y, e.g. ferrimagnetic with room temperature saturation magnetization Ms ~300 kA/m for y=1 and paramagnetic for y=0. The matrix SCFO with the BM phase has antiferromagnetic ordering irrespective of x, while the magnetism of the P phase depends strongly on x and can exhibit a room temperature Ms ~100 kA/m for x=0.5 but almost no moment for x=0 and x=1 [1]. With the gating voltage of -3V/+3V, the lattice parameter of matrix SCFO changes ±3% and induces strain transfer around ±1% to the pillar phase (Fig. 1). In particular, for x=0.5 and y=0 (Fig. 2), the negative gating inserts oxygens into the BM phase and changes it into a P phase showing ferromagnetism. Our study clarifies the manipulation of the magnetism of composite materials through an electrochemical process, expanding the applications of magnetoelectric coupling.References: [1] S. Ning, Q. Zhang and C. A. Ross, ACS Nano, https://dx.doi.org/10.1021/acsnano.0c03750 (2020)

Controlling Magnetic Properties of Epitaxial Nanocomposite Thin Films Through Ionic Liquid Gating

The focus of lab-on-a-chip technology centers on small, low power devices that can be integrated on a microscopic scale to perform laboratory functions within one unit or “chip”. In order to achieve this, one has to understand how to transfer fluid at this scale. Due to the low Reynolds number limit, such devices need to break time-reversible symmetry to produce a net flow. There has been a degree of interest and research into using magnetic materials to achieve this. In this work, we propose a new model employing ferromagnetic elements for construction of a practical microfluidic pump. The design is based on a repeated chain of magnetic discs connected together via links with hinges at the centers of the discs, which allow the discs to rotate (Fig 1a). While the model is based on that of ferromagnetic swimmers[1]. The device is surrounded by fluid and is activated by an external AC magnetic field. The magnetic and elastic forces in this device can work together against the fluidic forces to create a time-irreversible movement, which results in a net flow. While magnetisation of all the discs are oriented in the same direction, the relative rotational stiffness of the discs (or hinge stiffness) is varied to produce a lag in the response to the magnetic torque. This collective effect is similar to that of a metachronal wave, which is ultimately responsible for the flow. We found that if both discs had the same rotational stiffness, then no flow was produced. Depending on the chosen frequency, there is an optimal rotation stiffness of the two discs where a maximum flow rate is achieved. We observed as the frequency increases so does the flow, until the frequency is too large for the discs to turn to their maximum angle before the direction of the magnetic field reverses. To conclude, we demonstrated that an elasto-magnetic device can produce a net flow at low Reynolds number. The flow direction can be controlled by altering the frequency of the driving magnetic field or by using different rotational stiffness. These results could lead to new types of ferromagnetic pumping devices.References: [1] A. D. Gilbert, et al., Q. J. Mech. Appl. Math., 64, 239 (2011).

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Energetics, equilibrium shape, and vorticity of skyrmions and antiskyrmions stabilized by the anisotropic Dzyaloshinskii-Moriya interaction in thin films with C2v symmetry

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GMW Best Poster Award 1 - Experimental correlation of Interfacial Dzyaloshinskii-Moriya interaction amplitude and Work Function in magnetic multilayers

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