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Physics in nanometer-thick metallic films has been attracting many researchers because of abundant spintronics nature. In ultrathin films, some physical properties, such as the spin-charge conversion [1] and ferromagnetism [2], are gate-tunable. To extend further potential of ultrathin metallic films in spintronics, we demonstrate ferromagnetic resonance (FMR) in a 1 nm-thick ultrathin Co film [3], which exhibits sufficiently small FMR resonance field and narrow FMR linewidth simultaneously, for future realization of tunable magnon excitation. We prepared SiO2 (capping layer)/Co (tCo )/SiO2 (Sample A), where tCo was set to be 1, 2, 3 and 5 nm, and SiO2 (capping layer)/Co (1 nm)/Ta (3 nm)/SiO2 (Sample B). The Co was grown by using magnetron sputtering. The sample was placed in the center of an electron spin resonance (ESR) cavity (TE011) and FMR spectra were measured. The external dc magnetic field (Hex) and ac magnetic field (hrf) were applied along the in-plane direction of the film as shown in Fig. 1. Figure 2 shows comparison of the FMR spectra between Sample A (tCo = 1 nm) and Sample B. Whereas the thickness of the Co of both samples are the same, the clear FMR signal was observed only from Sample B, where the half-width at half-maximum and the resonance field were comparable to that of the bulk Co system. The results directly indicate that the insertion of 3 nm-thick Ta plays a dominant role for generating FMR in the ultrathin Co film. Furthermore, the narrow linewidth and the low resonance field indicate that the ultrathin Co film simultaneously possesses uniform and strong magnetization, which has not been achieved previously [4,5]. The successful detection of FMR from ultrathin Co enables efficient magnon excitation that is gate-tunable. More detailed discussion will be given in the presentation.References: [1] S. Dushenko, M. Shiraishi et al., Nat. Commun. 9, 3118 (2018). [2] D. Chiba et al., Nat. Mater. 10, 853 (2011). [3] S. Yoshii, M. Shiraishi et al., submitted. [4] S. T. Purcell et al., J. Appl. Phys. 69, 5640 (1991) [5] R. Gieniusz et al., J. Magn. Magn. Mater. 272-276, E911 (2004)

MMM 2020

Realization of Ferromagnetic Resonance in 1nm Co ultrathin film

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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Recently, we experimentally demonstrated that the nucleation and annihilation of µm-sized skyrmions in Pt/Co/AlOx trilayers can be controlled by tuning the perpendicular magnetic anisotropy with electric field gating [1]. Here, we show our present efforts to extend this concept to smaller skyrmions, down to 50 nm lateral size. Two trilayer systems, in which sub-100 nm skyrmions were observed at room temperature, were studied: Pt/Co/MgO [2] and Pt/Co/TbOx. The stacks were patterned by electron beam lithography and ion beam etching into stripes (1 – 50 µm width) then covered with a 10 nm thick ZrO2 dielectric layer and a 6 nm thick Pt top electrode to form capacitor-like structures, Fig (a). Using Magnetic Force Microscopy, Fig (b-c), we show that in both stacks, starting from a stable skyrmion network, a positive electric field (EF) leads to the increase of the PMA and to the annihilation of the skyrmion phase. We attribute this to the EF-induced migration of oxygen towards the top Co interface. This result is confirmed by the variation of the hysteresis loops taken with polar Magneto-Optical Kerr Effect, Fig (d). This new state persists after removing the EF i.e. EF-gating leads to a non-volatile effect. In the Pt/Co/MgO trilayer, the application of a negative EF shows no effect on the magnetic anisotropy, preventing us to renucleate the skyrmions. This irreversible process is probably due to the stabilization of the stochiometric MgO phase induced by oxygen ion migration towards the Co/MgO interface when a positive gating is applied. On the contrary, for Pt/Co/TbOx trilayers, the application of a negative EF triggered the oxygen ion migration away from the top Co interface. This allows us to tune reversibly the magnetic anisotropy, and therefore to reproducibly nucleate and annihilate 50 nm size skyrmions. This is an easily integrable and energetically efficient solution that might ease the use of magnetic skyrmions as information carriers in future spintronic devices.References: [1] Schott et al., Nano Lett 17, 3006 (2017) [2] Boulle et al., Nat Nano 11, 449 (2016)

Tuning magnetic anisotropy and skyrmion stability with electric field

Magneto-ionics has emerged as a promising approach to manipulate magnetic properties in magnetic heterostructures, energy-efficiently. While this has led to exciting magneto-ionically controlled functionalities in primarily oxygen-based systems, such as perpendicular magnetic anisotropy, magnetization, exchange interaction, and superconductivity, there are challenges in oxygen-based magneto-ionics, in terms of switching speed and reversibility [1-3]. There have been keen interests in exploring other ionic systems which may overcome these issues [4]. Here we explore the use of α-Co(OH)2 for hydroxide-based magneto-ionics, which undergoes a reversible paramagnet to ferromagnet transition under electrolyte gating. The α-Co(OH)2 films are grown by electrochemical methods. Grazing incidence x-ray diffraction (XRD) confirmed the formation of α-Co(OH)2 without traces of any other hydroxide or oxide phase. After voltage application in an electrolyte-gating configuration, a ferromagnetic signal can be induced in the paramagnetic as-grown sample under negative voltage due to the reduction of α-Co(OH)2 into metallic Co (Fig. 1). Magneto-electric experiments revealed that the turn-on voltage, i.e. minimum voltage needed to generate the ferromagnetic response, is as low as -2V, among the lowest turns-on voltages reported for propylene carbonate gated experiments [5]. Its paramagnetic character can be fully recovered by reversing the applied voltage polarity. This reversible transition is due to the ionic diffusion of hydroxyl groups into the employed electrolyte. By tuning the voltage magnitude and sample area we demonstrate that the speed of the induced ionic effect can be drastically boosted, up to a factor 60 at -8V (Fig. 2). This work opens new avenues in exploring the benefits of layered materials and hydroxyl groups towards fast magneto-ionic processes. This work has been supported in part by the SRC/NIST SMART Center, the NSF (ECCS-1933527), NSF-REU (DMR-1659532) and NSF-MRI (DMR-1828420).References: [1] U. Bauer et al., Nat. Mater., 14, 174 (2015). [2] D. A. Gilbert et al., Nat Commun 7, 12264, (2016). [3] P. D. Murray et al., ACS Appl. Mater. Interfaces 12, 4741 (2020). [4] A. J. Tan et al., Nat. Mater. 18, 35 (2019). [5] C. Navarro-Senent et al., APL Mater. 7, 030701 (2019).

Hydroxide-based Magneto-Ionics: paramagnetic-to-ferromagnetic transition in electrolyte-gated α-Co(OH)2

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

Atomistic simulations play a huge part in our understanding of static and dynamic magnetic states in current state-of-the-art research. Motivated by the recent interest in magnetic Heusler alloys, a model based on the LLG equation was developed for investigating magnetisation dynamics in near-compensated ferrimagnets. These materials, including Mn2RuxGa (MRG), are predicted and often experimentally verified to possess key properties, including THz resonance frequencies and high spin-polarisation at the Fermi level [1, 2]. Another hot topic of study are magnetic skyrmions, topological defects in the spin texture of non-centrosymmetric crystals with chiral interactions. There is much interest in the dynamics of skyrmions, with fast skyrmionic magnetic storage one potential application [3, 4]. In this work, the stability of skyrmion states in a 1D ferrimagnet was investigated as a function of material parameters such as anisotropy and Dzyaloshinskii-Moriya interaction (DMI), see Fig. 1. Additionally, skyrmion propagation speed and breathing mode frequency [5] are investigated under the influence of an external magnetic field and/or spin-polarised current. The model has also been applied as a computationally efficient method to obtain magnon frequency dispersions. By applying a perturbing field pulse and taking the Fourier Transform of the resulting dynamics, the entire magnon dispersion can be obtained. A simple example for a 1D ferrimagnetic chain with Heisenberg exchange, DMI and uniaxial anisotropy is shown in Fig. 2. MRG shows compensation of its ferrimagnetic moment at room temperature and as a result is predicted to support sustained spin oscillations in the THz region [6]. Our simple model, once adjusted to accommodate the symmetry and dimensionality of the real material, will represent an efficient method to theoretically investigate the existence of such oscillations and of potential chiral ground states.References: [1] H. Van Leuken and R.A. De Groot, Physical review letters., Vol. 74(7), p.1171 (1995) [2] S. Lenne et al., arXiv preprint arXiv:1903.04432. (2019) [3] W. Legrand et al., Nano letters., Vol. 17(4), pp.2703-2712 (2017) [4] A. Fert, N. Reyren and V. Cros, Nature Reviews Materials., Vol. 2(7), p.17031 (2017) [5] Y. Onose et al., Physical review letters., Vol. 109(3), p.037603 (2012) [6] R. E. Troncoso et al., Physical Review B., Vol. 99(5), p.054433 (2019)

Fully-Vector Simulation Framework for the Investigation of Magnetisation Dynamics and Skyrmions

Assessment of local magnetization in 3D objects is indispensable to performance estimation of magnetic recording processes and electric power devices. It was shown that an elementary Stoner-Wohlfarth-like hysteresis operator may be constructed using two-node Hopfield neural network (HNN) having internal positive feedback [1]. The approach was further utilized to simulate vector hysteresis in arbitrary 2D geometries ([2, 3]). This paper presents an approach using which local magnetization in 3D objects exhibiting hysteresis may be assessed. It utilizes a four-node tetrahedron-shaped HNN (Fig. 1.a) with activation functions “f” constructed using a weighted superposition of a step and sigmoidal functions in accordance with the M-H curve squareness. Vector orientation of the activation function a neuron is assumed to be along the unit vector ev initiated from the tetrahedron center point to the vertex corresponding to that neuron (Fig. 1.b). Feedback kij between any two neurons is given by: Kij=kcopl (evi . evj), where kcopl is a constant identified by the material coercivity. Given an applied field H orientated along the unit vector eH, activation function outputs of the tetrahedron HNN implicitly settles in accordance with the minimum of the energy E: E=- ∑i= 1 to 4 (HeH . f evi + ½ ∑j=1 to 4, j≠i kij fi fj ). (1) Overall vector magnetization of the tetrahedron M eM is given by: M eM = ∑i = 1 to 4 fi evi . (2) Geometrical (shape) configuration of the tetrahedron affects the internal mutual node feedback factors and consequently the overall vector magnetization components. To demonstrate the applicability of the approach, an ellipsoid whose diameters were in the ratio of 2:1:1 was constructed using 120 tetrahedra. Main M-H curve of the material was assumed and corresponding activation function specifics and kcopl factor were consequently set (Figs. 1 and 2). More theoretical and computational details are given in the paper.References: [1] A.A. Adly and S.K. Abd-El-Hafiz, Journal of Advanced Research, Vol. 4, p. 403 (2013). [2] A.A. Adly and S.K. Abd-El-Hafiz, Journal of the Applied Computational Electromagnetics Society, Vol. 31, p. 765 (2016). [3] A.A. Adly and S.K. Abd-El-Hafiz, Journal of Magnetism and Magnetic Materials, Vol. 434, p. 151 (2017).

Utilizing Four-Node Tetrahedra-Shaped Hopfield Neural Network Configurations in the Local Magnetization Assessment of 3D Objects Exhibiting Hysteresis

We discuss localized spin modes in the model of monoaxial chiral helimagnet. Folllowing the idea suggested in Ref. [1] we address the so-called phase of forced ferromagnetism, where in the ground state all spins align with an external magnetic field applied along the chiral axis [2]. Our analysis shows that the intrinsic localized modes, or discrete breathers, exist and the Dzyaloshinskii-Moryia (DM) interaction does not hinder their emergence. We consider the spin Hamiltonian including the Heisenberg exchange coupling, the easy-plane anisotropy, the Zeeman coupling with an external magnetic field directed along the chiral axis, and the Dzyaloshinskii-Moryia interaction term along the chiral axis. It is supposed that the external field exceeds a critical value to stabilize the forced ferromagnetic phase. By using equations of motion for spin operators of a finite chain we find an approximate analytical solution of the discrete breather, establish necessary conditions for the breather frequency and the easy-plane anisotropy strength. The discrete breather accumulates not only magnon density but a topological charge. The topological charge depends on the balance between a scale of breather localization and a pitch of the spiral determined by the DM interaction. We calculate the energy of the discrete breather. It includes a part that may be obtained in the continuum limit and a pinning part that takes into account discreteness of the spin chain. The work is supported by the RFFI grant (project 20-02-00213)References: [1] R.F. Wallis, D.L. Mills and A.D. Boardman, Physical Review B, Vol. 52, p. R3828, (1995). [2] J. Kishine, A.S. Ovchinnikov, Solid State Physics, Vol. 66, Pp. 1-130, (2015).

Discrete Magnetic Breathers In Monoaxial Chiral Helimagnet

Thin films with high saturation magnetization deposited on semiconducting substrates have the potential to enable monolithic microwave integrated circuits (MMICs) operating at higher frequencies than conventional devices based on ferrites [1]. The ability to control the anisotropy in these films is a key requirement for use in high frequency devices. To demonstrate this ability, heterostructures of 2.5 nm Ru/50 nm FM/2.5 nm Ru, (FM = Co or Fe60Co40), were deposited by magnetron sputtering on native-oxide coated silicon substrates. The samples were deposited in the presence of a 20 mT in-plane bias field. The Ru underlayer is known to reduce the interfacial perpendicular anisotropy in transition metal films compared to SiO2 [2] and the Ru overlayer protects the film from oxidizing in air. In addition to quasistatic magnetization reversal curves broadband ferromagnetic resonance measurements (FMR) and in-plane angle dependent FMR at 20GHz were used to determine the magnetic properties of the samples. This includes the saturation magnetization, the gyromagnetic ratio and the field induced in-plane anisotropy. We find that the bias field produces an easy axis along the bias field direction characterized by a uniaxial anisotropy field of 2.9 ± 0.1 mT for Co and 3.1 ± 0.1 mT for Fe60Co40. We also find a low effective damping parameter of 9.8 × 10-3 in Co and 3.4× 10-3 in Fe60Co40.References: 1. Y. D. Zhang, S. H. Wang, J. I. Xiao et al., IEEE Trans. Mag. 37, 4 (2001). 2. J. B. Mohammado, G. Mankey, C. K. A. Mewes et al., J. Appl. Phys. 125, 023901 (2019).

Field Induced Uniaxial Anisotropy in Ferromagnetic Thin Films

Recently, there has been a significant amount of interest in the areas of ultrafast magnetism and spintronics due to the search of faster methods to process and magnetically record information [1]. One of the most promising materials in the field of ultrafast magnetism and spintronics are heterostructures consisting of ferromagnetic and antiferromagnetic layers. In such structures, spin-polarized current can be excited by femtosecond laser pulses [2]. One example of such materials is an intermetallic superlattice of exchange-bonded nanolayers TbCo2/FeCo, in which interlayer interaction shows ferrimagnetic pattern. Such structure possesses giant magnetostriction, a large coefficient of magnetomechanical coupling, controlled magnetic anisotropy and spin reorientation transitions, which can be generated by a magnetic field [3]. In this paper we investigated the spin-polarized photocurrent in a TbCo2/FeCo superlattice by the THz time domain spectroscopy. The sample was grown on a SiO2 substrate by cathode sputtering in magnetic field, which resulted in an in-plane anisotropy [4]. Optical pump beam illuminates the structure through the substrate and is focused on the superlattice. This allowed to excite spin-polarized currents in the superlattice, which generated an electromagnetic field in the THz range. The generated THz radiation was detected by electro-optical sampling [5]. The generation of THz radiation from a spintronic emitter based on TbCo2/FeCo in the range from 0.1 to 3 THz was shown. The total phase change of the THz signal after switching the magnetization of the spintronic emitter was observed (Fig.1). The change of THz phase took place at the application of coercive field with the value ~ 10 mT. The work is supported by State Task for Universities FSFZ-0706-2020-0022 and RFBR (#18-52-16021)References: 1. Stanciu C. D. et al., Physical review letters, Vol. 99, p. 047601. (2007) 2. Huisman T. J. et al. Applied physics letters, Vol. 110, p. 072402.(2017) 3. A. Klimov et al, IEEE Trans. Magn., Vol. 42, p. 3090-3092 (2006). 4. N.Tiercelin et al. Journ of Mag. & Mag. Mat., Vol 249, p. 519-523 (2002). 5. Kovalev, S. P., G. Kh Kitaeva., JOSA B, Vol.30, p. 2650-2656 (2013)

Spin-polarized Photocurrent in TbCo/FeCo Superlattice Observed via THz Time Domain Spectroscopy

Strontium Ferrite magnetic nano-powder is being studied to be applied in magnetic 3D printer filament. Here we report on a study of the time dependence of its magnetic properties to better understand the permanent magnetic moment per unit volume of a nanocomposite containing the material [1,2,3]. Samples were made by loading up to 30 mg of OP-71 from Dowa Electronics Materials Company in a cylindrical custom design powder sample holder. Isotropic and anisotropic samples were made by loading the material in 0 field or in a field of 22,000 Oe. After loading the powder, the samples were compacted by compressing the powder along the cylindrical access. The typical samples were cylinders with a diameter of 2.5 mm and a length of 3 mm. The magnetic hysteresis curves of the samples were measured with a biaxial MicroSense Vibrating Sample Magnetometer (VSM) as a function of the field angle. For the isotropic samples, the average coercivity and the Mr/Ms were approximately constant as a function of the field angle for the Mx signal and were respectively 3402.35 Oe and 0.49. The shape of the Mx-hysteresis curves was independent of the field angle. For the anisotropic samples, the coercivities were around 3455 Oe (Mx curves). The Mr/Ms of the anisotropic samples depends on the field angle and varies from 0.41 along the hard axis to 0.64 along the easy axis. The coercivity of the My curves is around 4373 Oe. The coercivity of the anisotropic data seems to be larger than that of the isotropic sample due to the orientation of the crystallites. The time dependence of the Mx /Ms and My/Ms signals after a field step from saturation were measured for both the isotropic and anisotropic samples. The time derivative of the magnetization normalized on the Ms (S) depend on the field magnitude and shows a maximum. For the Mx signal this maximum coincides with the sample’s coercivity. The S(H) for the anisotropic curve for the My signal is shifted to larger fields as shown in the graph below. This work was supported in part by NSF through an DMR-MRI Grant under Award 1726970.References: [1] reference of strontium ferrite, maybe original philips patent. [2] R. Street, J. C. Wooley, Proc. Phys. Soc. London, Sect. A 62 (1949) 562. [3] L. Neel , Ann. Geophys. 5 (1949) 99.

Time Dependence of Magnetic Moment of Strontium-Ferrite Powder Measured by Biaxial VSM

Recently, to achieve faster, smaller, and more energy efficient devices for information and communications technologies, much attention have been focused on Fe-Ga films. Until now, we investigated the magnetization dynamics of Fe-Ga polycrystalline films, and suggested that further study to appreciably reduce damping constant (α) for these highly magnetostrictive films are strongly desired from technological standpoints because of wide FMR widths and high α (1) for these films. Herein, to improve the soft magnetic properties and high-frequency magnetic properties of Fe-Ga films, we prepare 10-nm-thick B-doped Fe-Ga films with different Ga composition (x) and discuss the structures and magnetic properties of films in detail. As for static magnetic properties of every thin film, the uniaxial anisotropy appears and coercivity (Hc) are much lower than those of Fe-Ga films(1). As shown in Fig. 1(a), the saturation magnetostriction (λs) increases for x ≤ 21.8 at.%, decreases for 21.8 at.% ≤ x ≤ 24.5 at.%, and further increases again for x ≥ 24.5 at.%. Their values becomes approximately 49 - 67 ppm, and are higher than that of amorphous (Fe80Ga20)0.85B15 films (50 ppm)(2). On the other hand, as for high-frequency magnetic properties of these films, the damping constant (α) increases slightly for x ≤ 20.8 at.%, decreases for 20.8 at.% ≤ x ≤ 24.5 at.%, and further increases for x ≥ 24.5 at.% (Fig. 1(b)). This trend is similar to that of λs. The α values becomes 0.006 - 0.010 and are much lower than those of Fe-Ga binary alloys films (1), (3), which might be attribute to both the structural characteristics and the static magnetic properties. Therefore, these results demonstrate that B-doped Fe-Ga thin films possess good soft magnetic properties and high-frequency magnetic properties compared to the conventional Fe-Ga films.References: (1)Y. Kawabe, Y. Endo and T. Miyazaki, T.Magn. Soc. Jpn. (Special Issues), 3, 34-38 (2019). (2)J. Lou, R. E. Insignares and Z. Cai, APPLIED PHYSICS LETTERS 91, 182504 (2007). (3)Daniel B. Gopman, Vimal Sampath and Hasnain Ahmad, IEEE TRANSACTIONS, VOL. 53, No.11, (2017).

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Realization of Ferromagnetic Resonance in 1nm Co ultrathin film

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