United States

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)

MMM 2020

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

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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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Magnetic rogue waves, like ocean rogue waves, are large deviations from equilibrium which are localized in time and space. They can be created in thin magnetic films by a time-reversal process consisting of a recording step and a reconstruction step. During the recording step, an initial rogue wave is allowed to decay and the microwave fields at various sites are recorded. In reconstruction step, these fields are played backwards in time at the recording sites, ultimately recreating the initial rogue wave as seen in Fig. 1. This technique gives the ability to control the magnitude and position of the rogue waves, which may make them suitable for application in spin wave logic devices and for encryption and secure communications. We study the efficiency of constructing rogue waves. For a Permalloy film which is 30 nm thick, the peak to noise ratio (PNR) of the reconstructed rogue waves can vary by a factor of 5 depending on the location of sensors with respect to the magnetic field. This is caused by the emission of caustic beams as the initial wave evolves in time. The best location is with the sensors perpendicular to the applied field. We also study the relative importance of dipolar and exchange coupling in creating rogue waves. We explore a geometry of a square magnetic film (side length 300 nm) separated from a surrounding magnetic rim by a gap of 10 nm. This gap prevents exchange coupling between the two regions. The initial rogue wave, a small square (25 nm on a side) where the magnetization is raised out of plane by a maximum of 2 degrees, is established in the inner region, but all the recording sensors are in the outer region. It is clear in Fig. 2 that the system in the middle of the recording phase is dominated by short wavelength modes, where exchange is important. However, in the playback, it is the long wavelength dipolar fields which are important in reconstructing the rogue wave.

Theoretical Studies of Dipolar and Exchange Effects in Creating Magnetic Rogue Waves

The effect of Mn doping of a Cr84Re16 alloy are reported, as well as the possible existence of a spin glass (SG) state which were investigated using electrical resistivity and magnetisation measurements. Mn is an electron donor resulting in a concomitant increase in the size of the electron sheet and a subsequent increase in Néel temperature, TN [1]. The temperature dependence of magnetisation, M(T), for the (Cr84Re16)100−xMnx alloys with x = 0.4, 0.6 and 3.1 showed bifurcation of the M(T) data at the pinning temperature, Tp, in the zero field cooled (ZFC) and field cooled (FC) curves. The splitting of the ZFC and FC curves is not in itself a signature of SG but is an indication of magnetic anisotropy in the system [2]. In the (Cr84Re16)100−xMnx alloy system, there is a sharp increase in magnetisation on increasing T after ZFC. A prominent peak is observed close to 40 K above which the behaviour is identical to that observed in the FC state as shown in the figure for the x = 0.4 alloy. Other signatures associated with canonical SG namely the time dependence of isothermal remanent magnetisation (IRM) and the increase of magnetic stiffness upon increasing the wait time in thermoremanent magnetisation (TRM) were also observed in the (Cr84Re16)100−xMnx alloys [3]. The frequency dependence of the real component of AC-susceptibility shows the SG characteristic cusp close to Tp [4]. However, there is no dependence of the maximum value of cusp or the temperature corresponding to the cusp on frequency which is contrary to the SG state [5]. As only some of the SG signatures were observed in the (Cr84Re16)100−xMnx alloy system, it cannot be categorised as canonical SG. These alloys can be classified as a new type of SG with frustration of itinerant spins at the interfaces between antiferromagnetic (AFM) domains established around each Mn local magnetic moment.References: [1] E. Fawcett, H.L. Alberts, V.Y. Galkin, D.R. Noakes and J.V. Yakhmi, Rev. Mod. Phys., Vol. 66, p.25 (1994) [2] P.A. Joy, P.S. Anil Kumar and S.K. Date, J. Phys. Condens. Matter., Vol. 10, p.11049 (1998) [3] B. Li, H.L. Alberts, A.M. Strydom, B.M. Wu, A.R.E. Prinsloo and Zh.J. Chen, J. Magn. Magn. Mater., Vol. 321, p.61 (2009) [4] C.A.M. Mulder, A.J. Van Duyneveldt and J.A. Mydosh, Phys. Rev. B., Vol. 25, p.515 (1982) [5] J.A. Mydosh, “Spin glasses: an experimental introduction”, Taylor and Francis, London (1993)

Spin Glass Behaviour in (Cr84Re16)100−xMnx Alloys

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)

Antiferromagnets have considerable potential as spintronic materials [1]. Notably, they manifest resonant modes at frequencies higher than can be observed in conventional ferromagnetic materials. Synthetic Antiferromagnets (SAFs), engineered structures of antiferromagnetic (AF) RKKY exchange coupled ferromagnetic(FM)/non-magnetic(NM)/ferromagnetic trilayers, mimic the properties of single phase antiferromagnets whilst possessing greater compatibility with current device technologies such as those used for STT-MRAM production [2,3]. In addition to the conventional acoustic (Kittel) mode, SAFs also possess a higher order resonant (optic) mode [3]. The structure of a SAF is shown in Fig.1a. We report the dynamic properties of SAFs with structure Ta(2 nm)/CoFeB(5 nm)/Ru(tRu)/CoFeB(5 nm)/Pt(4 nm) where tRu is the thickness of the Ru layer, acting as the NM spacer layer. The samples were fabricated using magnetron sputtering and characterized by X-ray Reflectivity (XRR) to measure layer thickness, vector Vibrating Sample Magnetometry (VSM) to verify AF interlayer coupling and a Vector Network Analyser – Ferromagnetic Resonance (VNA-FMR) setup to explore dynamic properties. Fig.1b shows the resonant dynamics of a single CoFeB layer and a CoFeB SAF. The data demonstrate the highest zero field frequencies observed in SAFs to date [4] with resonances of 18 GHz and 21 GHz at the 1st and 2nd instances of AF RKKY coupling respectively. A three part analytical model can reproduce the major features of the measured spectra.References: [1] T. Jungwirth, X. Marti, P. Wadley et al., Nat. Nanotechnol., Vol. 11, p. 231–241 (2016) [2] R. Hao, L. Wang and T. Min, IEEE Trans. Magn., Vol. 55, p. 1-6 (2019) [3] S. Li, C. Wang, X. Chu et al., Adv. Funct. Mater., Vol. 26, p.3738-3744 (2016) [4] H. Waring, N.A.B. Johansson, I. Vera-Marun et al., Phys. Rev. Appl., Vol. 13, p. 034035 (2020)


Zero Field Optic Mode Beyond 20 GHz in a Synthetic Antiferromagnet (B)

Interface Characterisation on a MTJ Using TEM and SEM

Structural and magnetic properties of a quasi-one-dimensional spin-1/2 compound NaVOPO4 are explored by x-ray diffraction, magnetic susceptibility, high-field magnetization, specific heat, electron spin resonance, and 31P nuclear magnetic resonance measurements, as well as complementary ab initio calculations. Whereas magnetic susceptibility of NaVOPO4 may be compatible with the gapless uniform spin chain model, a detailed examination of the crystal structure reveals a weak alternation of the exchange couplings with the alternation ratio α ~ 0.98 and the ensuing zero-field spin gap Δ0/kB ~ 2.4 K directly probed by field-dependent magnetization measurements. No long-range order is observed down to 50 mK in zero-field. However, applied fields above the critical field Hc1 ~1.6 T give rise to a magnetic ordering transition with the phase boundary TN ∝ (H − Hc1 )1/φ, where φ ~1.8 is close to the value expected for Bose-Einstein condensation of triplons. With its weak alternation of the exchange couplings and small spin gap, NaVOPO4 lies close to the quantum critical point.References: [1] P. K. Mukharjee et. al., Phys. Rev. B 100, 144433 (2019) [2] S. Sachdev, Nat. Phys. 4, 173 (2008) [3] Mukhopadhyay et. al, Phys. Rev. Lett. 109, 177206 (2012) [4] Zapf et. al., Rev. Mod. Phys. 84, (2014)

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