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The properties of superconductor/ferromagnet multilayers and junctions have been intensively studied from the standpoint of the interplay between superconductivity and ferromagnetism. Though in a special case superconductivity and ferromagnetism can coexist and exhibit novel phenomena such as π state in superconductor/ferromagnet/superconductor junctions [1], in general, the superconducting order parameter is expected to suppress in the superconducting layer but to penetrate into the ferromagnetic layer. A number of the studies have been carried out with ferromagnetic component consisting of 3&lt;i&gt;d&lt;/i&gt; transition metals like Fe, Co and Ni [2]. In contrast, only few reports exist for epitaxial NbN/Heusler alloy layered films and their devices so far; for example, giant coercivity enhancement in NbN/Co&lt;sub&gt;2&lt;/sub&gt;FeSi bilayers [3], efficient injection of pure spin currents in NbN/Co&lt;sub&gt;2&lt;/sub&gt;MnSi devices [4], superconductivity in NbN/Co&lt;sub&gt;2&lt;/sub&gt;(Fe,Mn)Si/Au trilayers [5,6], and spin polarization measurements of Co&lt;sub&gt;2&lt;/sub&gt;FeSi by the epitaxial contact Andreev reflection spectroscopy [7]. Therefore, we focus on superconducting properties of epitaxial films of a superconducting NbN layer attached to a highly spin polarized Heusler alloy Co&lt;sub&gt;2&lt;/sub&gt;FeSi layer, where the superconductivity of the NbN layer penetrates into the ferromagnetic Co&lt;sub&gt;2&lt;/sub&gt;FeSi layer. We investigate how the Co&lt;sub&gt;2&lt;/sub&gt;FeSi layer with the high spin polarization affects the superconductivity of the NbN layer in epitaxial NbN/Co&lt;sub&gt;2&lt;/sub&gt;FeSi/Au films through their interface.&lt;br/&gt;We have fabricated NbN/Co&lt;sub&gt;2&lt;/sub&gt;FeSi/Au epitaxial films on an MgO-substrate by an ultrahigh-vacuum (UHV)-compatible magnetron sputtering system. The thickness of the CFMS film was 5 nm while the thickness of the NbN film was fixed to 100 nm. Considering the NbN film was thicker enough than its coherence length &lt;i&gt;ξ&lt;/i&gt;&lt;sub&gt;NbN&lt;/sub&gt;(0) at 0 K, the superconductivity of the layered film should be in the three-dimensional (3D) regime, due to satisfying &lt;i&gt;d&lt;/i&gt;&lt;sub&gt;NbN&lt;/sub&gt; &amp;gt;&amp;gt; &lt;i&gt;ξ&lt;/i&gt;&lt;sub&gt;NbN&lt;/sub&gt;(0). Superconducting properties of the epitaxial films were evaluated by resistivity measurements with the standard four-terminal technique under both parallel and perpendicular magnetic fields up to 17 T.&lt;br/&gt;The superconducting transition temperature &lt;i&gt;T&lt;/i&gt;&lt;sub&gt;c&lt;/sub&gt; of NbN/Co&lt;sub&gt;2&lt;/sub&gt;FeSi/Au trilayer films was determined to be 16.1 K in the absence of magnetic field. Temperature dependence of the resistivity &lt;i&gt;ρ&lt;/i&gt;(&lt;i&gt;T&lt;/i&gt;) was measured in both magnetic fields parallel and perpendicular to the surface of NbN/Co&lt;sub&gt;2&lt;/sub&gt;FeSi/Au trilayer films. Figure 1 shows the field dependence of the pinning potential &lt;i&gt;U&lt;/i&gt;&lt;sub&gt;0&lt;/sub&gt;(&lt;i&gt;H&lt;/i&gt;) for both applied magnetic field directions deduced from the Arrhenius plots for the &lt;i&gt;ρ&lt;/i&gt;(&lt;i&gt;T&lt;/i&gt;,&lt;i&gt;H&lt;/i&gt;) curves. As shown in Fig. 1, &lt;i&gt;U&lt;/i&gt;&lt;sub&gt;0&lt;/sub&gt;(&lt;i&gt;H&lt;/i&gt;) was well described by &lt;i&gt;H&lt;/i&gt;&lt;sup&gt;−&lt;/sup&gt;&lt;i&gt;&lt;sup&gt;α&lt;/sup&gt;&lt;/i&gt;(1−&lt;i&gt;H&lt;/i&gt;/&lt;i&gt;H&lt;/i&gt;*)&lt;i&gt;&lt;sup&gt;β&lt;/sup&gt;&lt;/i&gt; above 5 T by the similar model with the exponents in the field dependence of the pinning force density, where &lt;i&gt;H&lt;/i&gt;* is a characteristic field representing the irreversibility field at 0 K. Temperature dependence of the upper critical field &lt;i&gt;μ&lt;/i&gt;&lt;sub&gt;0&lt;/sub&gt;&lt;i&gt;H&lt;/i&gt;&lt;sub&gt;c2&lt;/sub&gt;(&lt;i&gt;T&lt;/i&gt;) was evaluated from the &lt;i&gt;ρ&lt;/i&gt;(&lt;i&gt;T&lt;/i&gt;) measurements. Figure 2 represents the upper critical fields &lt;i&gt;μ&lt;/i&gt;&lt;sub&gt;0&lt;/sub&gt;&lt;i&gt;H&lt;/i&gt;&lt;sub&gt;c2&lt;/sub&gt;&lt;sup&gt;||&lt;/sup&gt;(&lt;i&gt;T&lt;/i&gt;) and &lt;i&gt;μ&lt;/i&gt;&lt;sub&gt;0&lt;/sub&gt;&lt;i&gt;H&lt;/i&gt;&lt;sub&gt;c2&lt;/sub&gt;&lt;sup&gt;⊥&lt;/sup&gt;(&lt;i&gt;T&lt;/i&gt;) of a NbN/Co&lt;sub&gt;2&lt;/sub&gt;FeSi/Au trilayer film in two magnetic field directions applied parallel and perpendicular to the surface of the trilayer From Fig. 2, the upper critical field &lt;i&gt;H&lt;/i&gt;&lt;sub&gt;c2&lt;/sub&gt;(0) at 0 K was also deduced to be &lt;i&gt;μ&lt;/i&gt;&lt;sub&gt;0&lt;/sub&gt;&lt;i&gt;H&lt;/i&gt;&lt;sub&gt;c2&lt;/sub&gt;&lt;sup&gt;||&lt;/sup&gt;(0) = 23.2 T for the parallel magnetic field and &lt;i&gt;μ&lt;/i&gt;&lt;sub&gt;0&lt;/sub&gt;&lt;i&gt;H&lt;/i&gt;&lt;sub&gt;c2&lt;/sub&gt;&lt;sup&gt;⊥&lt;/sup&gt;(0) = 16.2 T for the perpendicular magnetic field, respectively. The experimental results under magnetic fields revealed the superconductivity of the NbN layer was affected by the interplay between the superconducting NbN layer and the highly spin polarized Co&lt;sub&gt;2&lt;/sub&gt;FeSi layer.

**References:**

[1] T. Kontos, M. Aprili, J. Lesueur, F. Genêt, B. Stephanidis, R. Boursier, Phys. Rev. Lett. 89 (2002) 137007.&lt;br/&gt;[2] L. Lazar, K. Westerholt, H. Zabel, L. R. Tagirov, Y. V. Goryunov, N. N. Garif&#39;yanov, I. A. Garifullin, Phys. Rev. B 61 (2000) 3711.&lt;br/&gt;[3] A. Gupta, G. Singh, D. Kumar, H. Kishan, and R. C. Budhani, Appl. Phys. Lett. 103 (2013) 182602.&lt;br/&gt;[4] D. Kumar, P. C. Joshi, Z. Hossain, and R. C. Budhani, Appl. Phys. Lett. 102 (2013) 112409.&lt;br/&gt;[5] I. Shigeta, T. Kubota, K. Makise, S. Kimura, S. Awaji, B. Shinozaki, K. Koyama, K. Takanashi, and M. Hiroi, IEEE Magn. Lett. 8 (2017) 3305605.&lt;br/&gt;[6] I. Shigeta, T. Kubota, Y. Sakuraba, S. Kimura, S. Awaji, K. Takanashi, and M. Hiroi, Physica B 536 (2018) 310.&lt;br/&gt;[7] I. Shigeta, T. Kubota, Y. Sakuraba, C. G. Molenaar, J. N. Beukers, S. Kimura, A. A. Golubov, A. Brinkman, S. Awaji, K. Takanashi, and Masahiko Hiroi, Appl. Phys. Lett. 112 (2018) 072402.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/33cb1ef7173a0adf01a17dd4170bd8e5.jpg)
Fig. 1. Pinning potential &lt;i&gt;U&lt;/i&gt;&lt;sub&gt;0&lt;/sub&gt;(&lt;i&gt;H&lt;/i&gt;) as a function of magnetic field &lt;i&gt;H&lt;/i&gt;. The blue circles are &lt;i&gt;U&lt;/i&gt;&lt;sub&gt;0&lt;/sub&gt;(&lt;i&gt;H&lt;/i&gt;) parallel magnetic fields &lt;i&gt;H&lt;/i&gt;&lt;sup&gt;||&lt;/sup&gt; to the surface for the NbN/Co&lt;sub&gt;2&lt;/sub&gt;FeSi/Au trilayer film and the red squares are perpendicular magnetic fields &lt;i&gt;H&lt;/i&gt;&lt;sup&gt;⊥&lt;/sup&gt;. Inset: Relationship between ln &lt;i&gt;ρ&lt;/i&gt;&lt;sub&gt;0f&lt;/sub&gt; and &lt;i&gt;U&lt;/i&gt;&lt;sub&gt;0&lt;/sub&gt; for both magnetic fields &lt;i&gt;H&lt;/i&gt;&lt;sup&gt;||&lt;/sup&gt; and &lt;i&gt;H&lt;/i&gt;&lt;sup&gt;⊥&lt;/sup&gt;.

![](https://assets.underline.io/uploads/markdown_image/1/image/508049c8022712d9aedc6040944a89ff.jpg)
Fig. 2.    Temperature dependence of the upper critical field &lt;i&gt;H&lt;/i&gt;&lt;sub&gt;c2&lt;/sub&gt;(T) for both directions deduced from &lt;i&gt;ρ&lt;/i&gt;(&lt;i&gt;T&lt;/i&gt;) measurements at various magnetic fields.

INTERMAG 2021

 Superconducting properties of epitaxial films of superconducting NbN and highly spin polarized Co2FeSi under high magnetic fields

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The properties of superconductor/ferromagnet multilayers and junctions have been intensively studied from the standpoint of the interplay between superconductivity and ferromagnetism. Though in a special case superconductivity and ferromagnetism can coexist and exhibit novel phenomena such as π state in superconductor/ferromagnet/superconductor junctions [1], in general, the superconducting order parameter is expected to suppress in the superconducting layer but to penetrate into the ferromagnetic layer. A number of the studies have been carried out with ferromagnetic component consisting of 3d transition metals like Fe, Co and Ni [2]. In contrast, only few reports exist for epitaxial NbN/Heusler alloy layered films and their devices so far; for example, giant coercivity enhancement in NbN/Co2FeSi bilayers [3], efficient injection of pure spin currents in NbN/Co2MnSi devices [4], superconductivity in NbN/Co2(Fe,Mn)Si/Au trilayers [5,6], and spin polarization measurements of Co2FeSi by the epitaxial contact Andreev reflection spectroscopy [7]. Therefore, we focus on superconducting properties of epitaxial films of a superconducting NbN layer attached to a highly spin polarized Heusler alloy Co2FeSi layer, where the superconductivity of the NbN layer penetrates into the ferromagnetic Co2FeSi layer. We investigate how the Co2FeSi layer with the high spin polarization affects the superconductivity of the NbN layer in epitaxial NbN/Co2FeSi/Au films through their interface. We have fabricated NbN/Co2FeSi/Au epitaxial films on an MgO-substrate by an ultrahigh-vacuum (UHV)-compatible magnetron sputtering system. The thickness of the CFMS film was 5 nm while the thickness of the NbN film was fixed to 100 nm. Considering the NbN film was thicker enough than its coherence length ξNbN(0) at 0 K, the superconductivity of the layered film should be in the three-dimensional (3D) regime, due to satisfying dNbN &gt;&gt; ξNbN(0). Superconducting properties of the epitaxial films were evaluated by resistivity measurements with the standard four-terminal technique under both parallel and perpendicular magnetic fields up to 17 T. The superconducting transition temperature Tc of NbN/Co2FeSi/Au trilayer films was determined to be 16.1 K in the absence of magnetic field. Temperature dependence of the resistivity ρ(T) was measured in both magnetic fields parallel and perpendicular to the surface of NbN/Co2FeSi/Au trilayer films. Figure 1 shows the field dependence of the pinning potential U0(H) for both applied magnetic field directions deduced from the Arrhenius plots for the ρ(T,H) curves. As shown in Fig. 1, U0(H) was well described by H−α(1−H/H*)β above 5 T by the similar model with the exponents in the field dependence of the pinning force density, where H* is a characteristic field representing the irreversibility field at 0 K. Temperature dependence of the upper critical field μ0Hc2(T) was evaluated from the ρ(T) measurements. Figure 2 represents the upper critical fields μ0Hc2||(T) and μ0Hc2⊥(T) of a NbN/Co2FeSi/Au trilayer film in two magnetic field directions applied parallel and perpendicular to the surface of the trilayer From Fig. 2, the upper critical field Hc2(0) at 0 K was also deduced to be μ0Hc2||(0) = 23.2 T for the parallel magnetic field and μ0Hc2⊥(0) = 16.2 T for the perpendicular magnetic field, respectively. The experimental results under magnetic fields revealed the superconductivity of the NbN layer was affected by the interplay between the superconducting NbN layer and the highly spin polarized Co2FeSi layer.

**References:**

[1] T. Kontos, M. Aprili, J. Lesueur, F. Genêt, B. Stephanidis, R. Boursier, Phys. Rev. Lett. 89 (2002) 137007. [2] L. Lazar, K. Westerholt, H. Zabel, L. R. Tagirov, Y. V. Goryunov, N. N. Garif'yanov, I. A. Garifullin, Phys. Rev. B 61 (2000) 3711. [3] A. Gupta, G. Singh, D. Kumar, H. Kishan, and R. C. Budhani, Appl. Phys. Lett. 103 (2013) 182602. [4] D. Kumar, P. C. Joshi, Z. Hossain, and R. C. Budhani, Appl. Phys. Lett. 102 (2013) 112409. [5] I. Shigeta, T. Kubota, K. Makise, S. Kimura, S. Awaji, B. Shinozaki, K. Koyama, K. Takanashi, and M. Hiroi, IEEE Magn. Lett. 8 (2017) 3305605. [6] I. Shigeta, T. Kubota, Y. Sakuraba, S. Kimura, S. Awaji, K. Takanashi, and M. Hiroi, Physica B 536 (2018) 310. [7] I. Shigeta, T. Kubota, Y. Sakuraba, C. G. Molenaar, J. N. Beukers, S. Kimura, A. A. Golubov, A. Brinkman, S. Awaji, K. Takanashi, and Masahiko Hiroi, Appl. Phys. Lett. 112 (2018) 072402.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/33cb1ef7173a0adf01a17dd4170bd8e5.jpg)
Fig. 1. Pinning potential U0(H) as a function of magnetic field H. The blue circles are U0(H) parallel magnetic fields H|| to the surface for the NbN/Co2FeSi/Au trilayer film and the red squares are perpendicular magnetic fields H⊥. Inset: Relationship between ln ρ0f and U0 for both magnetic fields H|| and H⊥.

![](https://assets.underline.io/uploads/markdown_image/1/image/508049c8022712d9aedc6040944a89ff.jpg)
Fig. 2. Temperature dependence of the upper critical field Hc2(T) for both directions deduced from ρ(T) measurements at various magnetic fields.

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Using LaAlO3 (100) substrates and a 17-nm thick Ca0.96Ce0.04MnO3 (CCMO) buffer layer, we grew BiFeO3 (BFO) thin films with various thicknesses (18nm, 23nm, and 44nm) via pulsed laser deposition (PLD). X-Ray diffraction (XRD), Reciprocal Space Mapping (RSM) and Atomic Force Microscopy (AFM) techniques are employed to characterize the thickness, structure and surface morphology of the films. The large misfit of lattice constants between BFO films and LAO substrates exerts a large compressive strain on the BFO film, which initially stabilizes BFO in the tetragonal (T-) phase. A mixture of T-phase and rhombohedral (R-) phase emerges when the BFO thickness exceeds 20nm. As the BFO film thickness increases, the proportion of R-phase BFO increases as verified by AFM and XRD. Using DC magnetron sputtering, magnetic multilayers of Pt(4nm)/Co(0.8nm)/Pt(1.5nm) were deposited on top of these BFO films. Both Anomalous Hall Effect (AHE) and SQUID-VSM measurements reveal that the mixed-phase BFO favours a perpendicular magnetic anisotropy (PMA) of the upper Pt/Co/Pt multilayers, while the pure T-phase BFO film with smaller thickness suppresses the PMA (Fig.1 and Fig.2). Considering the R-phase BFO at the surface of the mixed-phase BFO film can change reversibly to T-phase BFO when applying electric field across BFO, this finding may provide a new route for realizing the electrical control of PMA in hybrid ferromagnetic-multiferroic structures.

**References:**

[1] Zuhuang Chen, et al., Phys. Rev. B 84, 094116 (2011). [2] He, Q., et al., Nat Commun 2, 225 (2011). [3] R. J. Zeches, et al., Science 326, 977-980 (2009) [4] Huang, Y., et al., Nat Commun 11, 2836 (2020).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/fbcd28badc4d5c9e655779a945f8e454.jpg)
Room temperature anomalous Hall resistance Rxy for Co(0.8nm)/Pt(1.5nm)/BFO(44nm), Co(0.8nm)/Pt(1.5nm)/BFO(23nm), and Co(0.8nm)/Pt(1.5nm)/BFO(18nm).

![](https://assets.underline.io/uploads/markdown_image/1/image/3c1ebadb60c428cb21edc6d8984bc5f5.jpg)
Room temperature out-of-plane hysteresis curves for Co(0.8nm)/Pt(1.5nm)/BFO(44nm), Co(0.8nm)/Pt(1.5nm)/BFO(23nm), and Co(0.8nm)/Pt(1.5nm)/BFO(18nm).

 Influence of BiFeO3 phase on perpendicular magnetic anisotropy in Co/Pt

The electric field manipulation of magnetization [1,2] became a subject of acute interest, since it provides a low energy consumption and sub-nanosecond switching time. The yield of such process depends on the anisotropy energy variation in response to the applied electric field and therefore a wide variety of studies have been dedicated to this topic. In this work we present a combined experimental-theoretical study of the perpendicular magnetic anisotropy (PMA) electric field modulation in magnetron-sputtered epitaxial Pt(111)/Co/MgO type systems. Ab-initio calculations of voltage-controlled PMA have been performed using the Full Potential Linear Augmented Plane Wave FP-LAPW code Wien2k [3]. A supercell model has been used to describe the multilayer configuration and the electric field has been applied using a zig-zag additional potential in the Hamiltonian. The anatomy of PMA and its electric field dependence has been theoretically analyzed within the framework of two main mechanisms: the surface charge doping which alters the relative occupation of electronic orbitals at the interface and the intrinsic interfacial dipole field. The main issue of our calculation is the prediction that the adjunction of Pt at top Co/MgO interface in Pt/Co/MgO ultrathin structures enhances the perpendicular anisotropy by about 84% and its electric field variation rate (Fig.1 (a), (b)). These theoretical predictions have been further used to design, grow by sputtering samples and pattern spintronic devices dedicated for the experimental analysis of the voltage response of magnetic anisotropy. As expected, in both standard Pt/Co(1nm)/MgO and top interface tailored Pt/Co(1nm)/Pt/MgO samples, the room-temperature magnetometry experiments demonstrated the out-of-plane remanent magnetization configuration related to surface PMA. The electric field influence on the PMA has been then studied on Hall-bridge patterned samples with a top-gating insulator (Fig.2 (a)), suitable for Anomalous Hall Effect magnetometry. In these devices, a qualitative agreement between the theoretical predictions and experimental results has been observed. The asymmetric variation of the anisotropy energy dependent on the polarity of the electric field, which was deduced from the ab-initio calculations is also reflected in the asymmetric variation of the coercive field as a response to the applied voltage (Fig.2 (b)). Supplementary insight into the anatomy of the mechanisms involved in the electric field effect on the coercive field, driven by the electric field modulation of the surface anisotropy, is further provided within a macrospin approach.

**References:**

References: [1] W-G. Wang et al, Nature Mater., vol. 11, pp. 64-68, 2012. [2] Y. Shiota et al, Nature Mater., vol. 11, pp. 39-43, 2012. [3] P. Blaha et al, J. Chem. Phys., vol. 152, p. 074101, 2020.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/8aa1914679751d7ed802354b310f9987.jpg)
Fig.1 Ab-initio calculation of electric field variation of the surface perpendicular anisotropy Ks for Pt/Co/MgO (a) and Pt/Co/Pt(1ML)/MgO (b) supercells

![](https://assets.underline.io/uploads/markdown_image/1/image/6a2fc3c369a03e3f0c26127a23d3fb4c.jpg)
Fig.2 (a) Hall-bridge UV lithographically patterned devices; (b) Variation of coercive field, surface anisotropy with the applied electric field.

 Electric field modulation of interfacial magnetic anisotropy in magnetron-sputtered Pt/Co/MgO ultra-thin structure with chemically tailored top Co interface

Voltage-controlled magnetic anisotropy (VCMA) has attracted broad interests due to its high efficiency in switching magnetization. Large VCMA was experimentally observed in Cr/Fe/MgO junction with ultrathin Fe layer [T. Nozaki et al., Phys. Rev. Applied 5, 044006 (2016)] , whose underlying mechanism was still not clear however. The Cr/Fe/MgO/Fe magnetic tunnel junction (MTJ) is also well known for its quantum-well (QW) states and as-induced spin-dependent resonant tunneling [F. Greullet et al, Phys. Rev. Lett. 99, 187202 (2007)]. Here, in order to uncover the relation between the large VCMA and the QW states, we developed a k-resolved VCMA calculation method combined with the second-order perturbation theory to investigate it. We find the VCMA coefficient reaches -290 fJ/Vm matching well with the previous experiment with 3 monolayers (MLs) of Fe. The coefficient oscillates strongly and even changes its sign with increasing the number of Fe MLs. Comparing the k-resolved VCMA with Fermi surface of the interfacial Fe atom, the screening charges theory for VCMA was verified. For 2~9 MLs Fe, interestingly, the QW states of Δ1 electron at Γ point provide large (no) contribution to the VCMA with odd (even) MLs. Moreover, the change of the orbital-resolved Fermi surface at the interfacial Fe atom also plays an important role on VCMA oscillation, which as well as the QW states result in the largest VCMA for 3 MLs Fe. Our results deepen understanding of the large VCMA in Cr/Fe/MgO junction, which would be helpful to design practical MTJ with large VCMA.

**References:**

[1] W. Z. Chen, L. N. Jiang, Z. R. Yan, Y. Zhu, C. H. Wan, and X. F. Han, Phy. Rev. B 101, 144434 (2020)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/f4ddb052ca46a844fc55a82c39826ddf.jpg)
Fig. 1. (a) MCA and (b) VCMA for Cr/Fe/MgO junctions with different number of Fe MLs. The blue stars are the VCMA coefficient measured in Ref [Takayuki Nozaki .etc PRA 5, 044006 (2016)].

![](https://assets.underline.io/uploads/markdown_image/1/image/d2e4e1ae8a17960dc0529c0c461d6a4a.jpg)
Fig. 2. k-resolved VCMA in the two dimensional Brillouin zone for Cr/Fe/MgO junctions with different number of Fe MLs from 2MLs to 9MLs (a)~(h).

 Origin of the large voltage-controlled magnetic anisotropy in Cr/Fe/MgO junction with an ultrathin Fe Layer: First-principles investigation

Electric field-driven ion migration can significantly modulate the electric- and magnetic- properties of solids, which creates new opportunities for electromagnetic devices with ultralow power consumption. Earlier works used vertically stacked structures, e.g., ferromagnet/oxide heterostructures, in which the magnetic properties of ferromagnet are controlled by the electric-field-driven oxygen ion migration along the vertical direction [1,2]. However, the existence of an interface obscures the detailed understanding of how the oxygen ion migration occurs and how it affects the electrical and magnetic properties of a ferromagnet. Instead of stacked structures, here we investigate the oxygen ion migration effect in a single ferromagnetic GdOx wire with lateral electric field application [3]. We directly visualize the electric-field-driven lateral motion of oxygen ions by an optical microscope (Fig. 1). The oxygen ions are found to migrate along the electric field direction by thermal activation. By exploiting the electric-field-driven oxygen ion migration, we succeed to increase or decrease the resistivity of GdOx in a controllable way, which allows mimicking the functional behavior of biological synapse (Fig. 2). Large negative magnetoresistance is observed in GdOx wire of which magnitude is significantly enhanced up to 20% by 9 T by oxygen ion control. Our results suggest that the electrical and magnetic properties of a single GdOx can be efficiently controlled by an electric field, which paves the way towards fully functional electromagnetic devices such as artificial synapses.

**References:**

1. Uwe Bauer, Lide Yao and Geoffrey S. D. Beach, Nature Materials, Vol. 14, p.174 (2015) 2. Mantao Huang, Aik Jun Tan and Geoffrey S. D. Beach, Scientific Report, Vol. 7, p.7452 (2017) 3. Jun-Ho Kang, Soogil Lee and Kab-Jin Kim, NPG Asia Materials, Vol. 12, p.44 (2020)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/ff1482f1cba10df19a5fb53bf372fcf2.jpg)
Fig. 1. Real-time images of the migration of accumulated oxygen ions in a GdOx device.

![](https://assets.underline.io/uploads/markdown_image/1/image/f264f61a5f3af3055a5ccc4d3e50e5df.jpg)
Fig. 2. Longitudinal resistance (Rxx) with repeated switching of the current (I) direction.

 Control of electrical resistance and magnetoresistance by electric field-driven oxygen ion migration in a single GdOx wire

Magnetic tunnel junctions (MTJs) are known to be one of the main building blocks of spintronics. In these devices, the integration of molecular layers as tunnel barrier is envisioned as an opportunity to allow the engineering of spintronics at the molecular scale. Actually, thanks to the spin-dependent hybridization at ferromagnet/molecule interfaces, spin polarization and thus tunnel magnetoresistance can now be tailored by molecules. For example, it was shown that spin polarization could be inverted or enhanced depending on the hybridization coupling strength [1]. In this direction, among molecular systems, self-assembled monolayers (SAMs) appear as one of the most promising tool to tailor MTJs properties. Indeed, the anchoring group, body and head of molecules can be changed to tune the coupling strength of both ferromagnetic electrodes independently and at will. However, up to now, only basic “passive” molecules such as alkane chain have been integrated into MTJs [2-4]. These molecules could be defined as “passive” since their electronic properties cannot be switched by external stimulus. In this talk we will present molecular MTJs integrating “active” molecules from the diarylethene molecule family. These molecules can be switched between two stable states corresponding to an open and a closed form of its central part. As the electron delocalization, energy gap and coupling strength to the electrodes depend on the molecule state, the tunnel resistance and tunnel magnetoresistance (TMR) is expected to be tuned by switching the molecule. We will then discuss the magnetotransport properties of NiFe/diarylethene/Co MTJs (cf. Figure 2(a)). After presenting the fabrication process of the molecular MTJs, we will first show that the diarylethene molecules grafting over the NiFe ferromagnetic bottom electrode do not alter its surface properties. We will in particular show X-Ray Photoelectron Spectroscopy (XPS) measurements with spectra recorded at Ni and Fe 2p edges (figure 1) ruling out any trace of oxidization and validating our surface recovery protocol [5]. We will then focus on the transport results. We will show that we have been able to obtain non-linear I(V) and parabolic conductance curves in both open and closed forms. In addition, we have performed inelastic electron tunneling spectroscopy (IETS) in order to detect molecular vibrations inside the tunnel barrier. Those measurements confirm that these active molecules act as an effective tunnel barrier in our MTJs. We will also show magnetoresistance curves that have been recorded at different bias voltages, temperatures and magnetic field directions. We have obtained magnetoresistance effects in both open and closed formed of diarylethene molecules (result shown for the open form in figure 2(b)). The angular dependence of the magnetoresistance reveals that both TMR and tunneling anisotropic magnetoresistance effects coexist and allows us to disentangle them. All these studies demonstrate for the first time that switchable active molecules can be successfully integrated into MTJs to build multifunctional spintronic devices electrically and/or optically controllable.

**References:**

[1] C. Barraud et al., Nat. Phys. 6, 615 (2010) [2] J. R. Petta et al., Phys. Rev. Lett., 93, 136601 (2004) [3] W. Wang et al.,, Appl. Phys. Lett. 89, 153105 (2006) [4] M. Galbiati et al., Adv. Mater. 24, 6429 (2012) [5] M. Galbiati et al., AIP Advances. 5, 057131 (2015)]

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/8c88e8460872455a93231591ad25f5da.jpg)
Figure 1: XPS spectra at nickel and iron 2p edges of diarylethenes SAMs grafted on a NiFe electrode.

![](https://assets.underline.io/uploads/markdown_image/1/image/d3b94195d50c63cf8755c10a625fbf11.jpg)
Figure 2: (a) Schematic representation of the molecular MTJ (b) Magnetoresistance curve of a NiFe/diarylethene/Co MTJ at 50 K obtained for the open form of the diarylethene molecules

 Magnetic Tunnel Junctions Based on Photoswitchable Self Assembled Monolayers

The areal density of hard disk drives (HDD) must be increased to meet the needs of today’s society and its ever-increasing data creation. This requires both the writability of smaller bits and the ability to read these bits. This can only be achieved by reducing the size of conventional tunnelling magnetoresistance (TMR) sensors. A possible solution to this is tunnel anisotropic magnetoresistance (TAMR) which arises due to a change of density of states at the Fermi level when a magnetic sublattice rotates with respect to the crystal field. It is particularly present in materials which possess large spin-orbit (SO) interactions such as CoPt [1]. Very large TAMR signals have been observed at low temperatures consisting of an IrMn based magnetic tunnel junction (MTJ) [2] which if optimised at room temperature would remove the requirement for a second ferromagnetic electrode and thus reducing the size of the sensor. VAMPIRE [3] brings together a simple user interface through the use of plain text files to produce an open-source atomistic spin simulation software. The software is used in conjunction with vibrating sample magnetometry (VSM) and ferromagnetic resonance (FMR) to investigate the bilayer system of Ni80Fe20/Co70Fe30 deposited via magnetron sputtering, in order to optimise the ferromagnetic layer of the MTJ. Simulation parameters are achieved directly from experiment and supplied to VAMPIRE. Integration of simulation and experiment in this manner can yield insight into the magnetisation dynamics on the atomic scale and guide experimental development. The inclusion of the thin layer of Co70Fe30 within simulations of a permalloy thin film has been found to enhance the coercive field of ferromagnetic layer of the MTJ which can be seen in figure 1. This has been experimentally confirmed in addition to an increase in magnetisation saturation. Building on this, we found that there is an increase in exchange with 1nm of CoFe on top of NiFe through FMR analysis as shown in figure 2. Further research has been carried out on Ta, Ru and Pd seed layers to assess whether or not they would make viable seed layers for the FM layer of the MTJ. VSM measurements, FMR analysis and X-Ray diffraction (XRD) scans have been carried out to further analyse the stacks. As each layer of the TAMR structure is simulated, experimentally confirmed and finally optimised; it will pave the way for the future of data storage.

**References:**

[1] - Phys. Rev. Letts 100, 087204 (2008) [2] - Nature Mat. 10, 347 (2011) [3] - R. F. L. Evans et al., Appl. Phys. Lett. 104, 082410 (2014)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/333fae02046ce4def0303705c39e4ebd.jpg)
Fig.1 VAMPIRE hysteresis loop Ni80Fe20(Y)/Co70Fe30(X) of normalised magnetisation against applied field. Inset is the visual representation of the generated system with a total z height of x + y

![](https://assets.underline.io/uploads/markdown_image/1/image/03e12db268de7ee7a1ade20819f602a3.jpg)
Fig.2 Spin wave stiffness, D against CoFe thickness, x for Ta(5)/NiFe(70)/CoFe(x)/Ta(5) sample set.

 Atomistic Simulations for TAMR Applications

CoFeB-based magnetic tunnel junctions (MTJ) is one of the most studied spintronic devices[1] due to its high tunneling magnetoresistance (TMR)[2] and low switching current density[3]. Here, using a direct characterization technique of spin-resolved photoemission, the spin polarization of an amorphous Co40Fe40B20 thin film prepared by magnetron sputtering is obtained. The spin-resolved photoemission measurement consists of a UVS 300 unpolarized helium lamp () and a SPECS PHOIBOS 150 hemisphere energy analyzer equipped with 3D Micro-Mott detector operated at a scattering energy of 25 kV. As shown in Fig. 1, the measured spin polarization at Fermi level is as high as 60%, which is comparable to the values obtained by other techniques. It is shown that our experimental value of spin polarization at is positive while the calculated spin polarization at ground state using first-principle calculation has a negative spin polarization at . Fig. 2 displays the density of states after considering the photoionization cross section, thermal broadening and experimental resolution broadening. The modified calculated spin polarization indicates a positive spin polarization at which is consistent to the experimental results. Moreover, we find that the maximum spin polarization is located at ~0.2 eV above , supported by the modified calculated spin polarization spectrum.

**References:**

1. C. Fu, X. Zhang, and Y. Duan. Magn. Magn. Mater. 492, 165657 (2019). 2. S. Ikeda, J. Hayakawa, and Y. Ashizawa, Appl. Phys. Lett. 93 (8), 082508 (2008). 3. B. Liu, X. Ruan, Z. and Y. Xu, Appl. Phys. Lett. 109 (4), 042401 (2016).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/dd331884eb1494b055cdc054f6e0d160.jpg)
Fig. 1 (a) Spin-resolved photoemission spectra of the CoFeB thin film. (b) Corresponding spin polarization curve, with the black line is a guide to eyes.

![](https://assets.underline.io/uploads/markdown_image/1/image/961e789d99472be07ad7f2f0828f2dbc.jpg)
FIG. 2 Calculated density of states after considering (a) the photoionization cross sections, (b) temperature and (c) experimental resolution. (d) Spin polarization curve corresponds to (c).

 Direct Observation of Spin Polarization in Amorphous CoFeB Thin Film

At the interface of a heavy metal (HM) with a strong spin-orbit coupling (SOC) and a ferromagnetic material (FM), a break in inversion symmetry arises leading to the appearance of several phenomena. One of them is the antisymmetric exchange interaction called the Dzyaloshinskii-Moriya Interaction (iDMI). It is responsible for the stabilization of chiral magnetic structures such as skyrmions and for the non-reciprocity of spin wave propagation. Another interface phenomenon, subject of extensive research in magnetic heterostructures (e.g. for HM /FM / metal oxide (MOx) type stacks) is interfacial Perpendicular Magnetic Anisotropy (iPMA) [2]. This originates from the level of oxidation, from the interface roughness and from the SOC [3]. These two quantities are of considerable interest because of their crucial importance for applications in spintronic devices, such as non-volatile magnetic memories and hard disk recording [4]. As an example, an MgO layer is often used as a tunnel barrier in magnetic tunnel junctions (MTJs), providing high spin transfer torque (SST), or in tunnel magnetoresistance (TMR), which are key properties for such devices. In order to optimize the magnetic properties at the interfaces of thin films, a thermal treatment is often used because it yields structural changes and atomic rearrangement and thus induces phase transition or alloys formation. However, annealing at high temperatures can also induce oxidation thus reducing the volume magnetic moment or increasing the magnetic anisotropy. In this work, we investigate the effect of the annealing temperature on iPMA and iDMI in Pt/Co/MgO stacks, with variable Co thickness (1≤tCo≤6 nm). These stacks were deposited by magnetron sputtering on a Si substrate and annealed at different temperatures (Ta), up to 400°C. For all the studied films, the saturation magnetic moment per unit area was measured by Vibrating Sample magnetometry (VSM) for each Co thickness. This allowed us to accurately determine the saturation magnetization Ms and the dead layer thickness td which are necessary to evaluate iDMI and iPMA. These quantities have been measured using the Brillouin light scattering (BLS) in the Damon-Eshbach (DE) configuration. Effective magnetizations (Meff) were deduced from measurements with fixed wave vector (ksw = 8.08 µm-1) and variable magnetic fields. The presence of the DMI in the studied structures induces a frequency shift between the two Stokes and anti-Stokes modes, this allowed us to quantify the iDMI effective constant (Deff) which is proportional to this shift. The variation of Meff as a function of the inverse of the Co thickness, shows the presence of two different linear regimes separated by a critical thickness (tc), which depends on Ta. This is generally observed in the case of epitaxial growth with a lattice misfit between the constituents, the strain can contribute to Kv⊥ (regime I: t &lt; tc), or to Ks⊥ (regime II: tc &lt; t). In order to separate the different surface/interface anisotropies (magnetocrystalline, magnetoelastic and Neel type) from the volume anisotropies, we have used an adapted model [1]. In figure (1), we can see the evolution of these different anisotropies as a function of the annealing temperature. For Ta above 200°C, a variation in the different contributions of the surface and volume anisotropies is observed. Similar to the iPMA, the evolution of the iDMI interaction was investigated as a function of the annealing temperature and the thickness of the effective magnetic layer. The measurements show that the effective iDMI constant (Deff) is proportional to the inverse effective magnetic thickness. Therefore, for low thicknesses we find that the values of Deff deviate from the linear regime probably due to the reduction of magnetic thickness (effect of dead layer). The iDMI surface constant Ds is then deduced form Deff wich decreases linearly (with absolute value) with the annealing temperature Ta (see figure(2)). It varies from (-1.9pJ/m) at room temperature to (-0.7pJ/m) at 400°C annealing temperature. We then conclude the possibility to modulate the strength of the iDMI by changing the annealing temperature.

**References:**

[1] M. T. Johnson, P. J. H. Bloemenz, F. J. A. den Broeder, and J. J. de Vries, Rep. Prog. Phys. 59, 1409 (1996). [2] B. Dieny and M. Chshiev, Rev. Mod. Phys. 89, 025008 (2017). [3] N.-H. Kim, et al , AIP Adv. 7, 035213 (2017). [4] Xueying Zhang, et al ACS Applied Materials & Interfaces 2018 10 (19), 16887-16892 [5] J. H. Moon, et al, Phys. Rev. B 88, 184404 (2013)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/dd67d9db22ce764185c90945ff722646.jpg)
Variation of the different anisotropy constants magnetocristalline (Kmc⊥), volume (KIme,v⊥) magnetoelastic , Neel-type interface (KN⊥) and the surface (KIIme,s⊥) magnetoelastic as a function of annealing temperature.

![](https://assets.underline.io/uploads/markdown_image/1/image/a29103b80c55a2aa457283b58a111134.jpg)
iDMI surface constant Ds as a function of annealing temperature.

 Annealing temperature and thickness dependencies of perpendicular magnetic Anisotropy and Dzyaloshinskii-Moriya Interaction of Pt/Co/MgO thin film.

Over the past few years, spintronic devices have attracted considerable attention due to their potential application compared to the classical electronics. Recently, a new concept of racetrack memory (RM) is developed, where domain walls (DWS) are replaced by magnetic chiral spin textures (skyrmions) [1] stabilized in the presence of the interfacial Dzyaloshinskii-Moriya interaction (iDMI). Their small size, high speed, and low energy consumption make skyrmions good candidates for future spintronic data storage. Skyrmion-RM will would allow to considerably increasing storage densities and information transfer speed [1, 2]. Therefore such devices performance depends significantly on the proper modulation of the Nonmagnetic/ Ferromagnetic (FM) interface and interfacial associated phenomena. In addition to the Heavy-metal (HM)/ (FM) interfaces and related spin-orbit coupling (SOC) interaction, it has recently been demonstrated the presence of the perpendicular magnetic anisotropy (PMA) and the iDMI in the FM/ Metal Oxide (Mox) interfaces. The PMA in such interface is due to the hybridization between the ferromagnetic metal and oxygen ions [3]. Recently, density functional theory has predicted the controlling of the iDMI sign and magnitude by changing the oxygen coverage of the FM layer [4]. However, knowing that these effects are dependent on the interface configuration, the study of the iDMI in the FM/MOx is still lacking. An efficient way to determine the role and the contribution of such interface is to investigate a FM/MOx system with or without a HM underlayer. Thus, in this work we propose the investigation of the PMA and, the iDMI as a function of the annealing temperature, the PtOx thickness and the oxidation state in CoFeB/PtOx interfaces. We expect to obtain overoxidation state for the thinner Pt films, where oxygen penetrates the underlying magnetic layer, whereas underoxidazied and optimally oxidized states are obtained for thicker Pt layers. Two categories of samples were grown by DC magnetron sputtering on thermally oxidized silicon, with or without Ta (3 nm) buffer layer. In the first category, the Co8Fe72B20 (CFB) thickness was varied up to 10 nm (2nm≦ tCFB≦10 nm) and capped by TaOx (0.8 nm). In the second category, CFB thickness was kept at 1.5 nm and was capped by variable thicknesses of PtOx (0.7nm≦ tPt≦1.6 nm). Both as grown and 225 °C annealed are studied for this second category. For this experimental investigation, Vibrating sample magnetometer (VSM), Brillouin light scattering (BLS) in Damon-Esbach (DE) configuration and Microstrip line ferromagnetic resonance (MS-FMR) were combined. The VSM was used to investigate the static magnetic properties (magnetization at saturation (Ms) and magnetic dead layer thickness (td)). The FMR was used for the determination of the gyromagnetic ratio, and for the investigation of the PMA in the thicker films. The BLS was employed for the study of the PMA (thinner films) and the iDMI. For the PMA, the spectra were measured at spin wave (SW) vector ( ksw= 8.08 µm−1) under variable in-plane applied magnetic fields, while for the iDMI measurements, BLS spectra were measured as a function of the ksw under in-plane saturating magnetic field. VSM measurement revealed that td is approximately the same for the CFB/TaOx (td≈ 0.7 nm) and CFB/PtOx (td≈ 1 nm) systems. For the CFB/PtOx stackcs we notice slightly higher value for the samples with Ta buffer layer due to the supplement of intermixing at the bottom interface. The larger value of the td (≈ 1 nm) obtained for CFB/PtOx compared to CFB/TaOx samples may be due to the fact that the Pt does not oxidize, the oxygen penetrates more into the CFB films thus increasing the thickness of the magnetic dead layer. Ms is also approximately the same for all the samples (around 1510 emu/cm3) which is in good agreement for systems of CFB rich on Fe [5]. The PMA investigation in the CFB/TaOx samples has shown the degradation of the interface anisotropy by inserting a Ta buffer layer. Indeed, the surface anisotropy constant (Ks) in the Ta/CFB/TaOx is twice weaker for the samples without Ta buffer layer confirming the degradation of the interface anisotropy by the Ta buffer layer. The PMA and iDMI measurements as a function of the Pt thickness have shown that the effective magnetization (Meff) and the effective iDMI constant (Deff) have the same trend for all the systems. Deff and Meff increase with increasing tPt thus allowing characterizing the strength of the iDMI and PMA for the CFB/PtOx interface. In the as deposited CFB/PtOx films, the thinnest Pt (tPt = 0.7 nm) sample is perpendicularly magnetized. The increasing of Meff as Pt thickness increases suggested that the oxidation of CoFeB decreases. The experimental data of Meff and Deff were then fitted using exponential grow function, to extract a decay thickness (λ). It is found that, λ for the Meff is higher compared to that of Deff for as grown CFB/PtOx stacks, confirming that the iDMI is slightly more localized at the first monolayers at the interface compared to the PMA. Measurements of spin pumping induced damping have also been performed by FMR and BLS will also be presented.

**References:**

[1] R. Tomasello, E. Martine., R. Zivieri et al. . Sci Rep 4, 6784, (2014) [2] W. Kang, X. Chen, D. Zhu et al., IEEE 7th Non-Volatile Memory Systems and Applications Symposium (NVMSA), Hakodate, pp. 7-12, (2018) [3] S. Monso, B. Rodmacq, S. Auffret, et al. Applied Physics Letters, 80, no 22, pp 4157, (2002) [4] A. Belabbes, G. Bihlmayer, S. Blügel, et al. Scientific Reports 6, 24634, (2016). [5] T. Ogasawara, M. Oogane, M. Al-Mahdawi et al, AIP Adv. 9, 125053, (2019).

**Images:**

 Interfacial Dzyaloshinskii Moriya interaction and perpendicular magnetic anisotropy in CoFeB/PtOx based systems

In this work, we investigate the influence of interface parameters on the magnetic energies such as the perpendicular magnetic anisotropy (PMA) and the interfacial Dzyaloshinskii-Moriya interaction (DMI)[1, 2] of Pd(111)/Cox/CoO1-x/Pd ultrathin epitaxial films with different oxidation depths. Epitaxial Pd(3 nm)/Co(1 nm) films were grown on a Si(111)/Cu(2nm) surface by molecular beam epitaxy in ultrahigh vacuum chamber, and after that, films were exposed the procedure of artificial oxidation. We varied the oxide thickness and this way we changed ferromagnetic thickness. The lattice parameters of bulk Pd and Co materials are mismatched by about 9,7%, which leads to the appearance of significant elastic strains on the bottom interface, which induced strong perpendicular magnetic anisotropy [3]. Oxidation in heavy metal(HM)/ferromagnetic metal(FM)/Oxide polycrystalline trilayers leads to significant changes of the magnetic properties [4]. The lattice period of the metal layers during growth and their structure were analyzed by means of RHEED. Magnetic hysteresis loops of the samples were measured using a vibrating sample magnetometer (VSM). Magnetic structure was investigated by a magneto-optical Kerr effect (MOKE) microscope. The MOKE microscope was equipped with a hand-made coil applying out-of-plane magnetic fields and an in-plane electromagnet. The out-of-plane coil was used in pulse mode and produced magnetic fields with an amplitude up to 50 mT and a width down to 2 msec. Artificial oxidation of magnetic layer leads to symmetry breaking on the bottom and top magnetic interfaces, which reduces compensation of interfacial DMI. Determination of the thickness of the magnetic layers carried out by measuring the saturation magnetization with vibrating samples magnetometry. Increasing of oxidation time leads to decreasing of volume of magnetic material and to increasing of effective PMA energy. The reported study was partially funded by Council for Grants of the President of the Russian Federation (project - MK-1384.2021.1.2) and the Russian Ministry of Science and Higher Education (the state task 0657-2020-0013).

**References:**

[1] Dzyaloshinsky I., Journal of Physics and Chemistry of Solids, Vol. 4.4, p.241-255 (1958) [2] Moriya T., Physical review, Vol. 120, p. 91 (1960) [3] Davydenko A. V., et al., Phys. Rev. B, Vol. 95, 064430 (2017) [4] Feng, Junxiao, et al., Physical Review Applied, Vol. 13.4, 044029 (2020)

**Images:**

Superconducting properties of epitaxial films of superconducting NbN and highly spin polarized Co2FeSi under high magnetic fields

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