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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).&lt;br/&gt;2. S. Ikeda, J. Hayakawa, and Y. Ashizawa, Appl. Phys. Lett. 93 (8), 082508 (2008).&lt;br/&gt;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).

INTERMAG 2021

 Direct Observation of Spin Polarization in Amorphous CoFeB Thin Film


![](https://assets.underline.io/uploads/markdown_image/1/image/2e10c515564e9ec3287ee4c664011c20.png)

The energy cost of storing and moving data is enormous, and is growing at such a rapid pace that within a few years the power demand may overwhelm some developed nations. Non-volatile memory (NVM) and neuromorphic computing are enabling critical advances in this area, but there are many scientific and technological challenges. This conference will cover the materials challenges of different memory technologies, from the fundamental processes and mechanisms, to industry perspectives on materials specifications and manufacturing.

Materials Challenges for Memory

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).

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We report simulation results of vortex domain wall (VDW) dynamics pinning behavior magnetic wire with 20-nm-thick 100-nm, 200-nm, and 300-nm-wide wires with a triangular notch that depth ratio(Rd) is 30%. The Rd is the depth of notch divide by wire width. The transverse domain wall(TDW) dynamics pinning behavior in 100-nm wide wire has reported by Ref. [2]. Ref. [2] shows the pinning and depinning of DW in the Walker breakdown state(WB). Therefore we are interested in VDW depinning behavior and probability. The method of our study are running the simulation software Mumax3. Our simulation parameters of saturation magnetization(Ms), damping constant, exchange stiffness (A), and cell size of simulation parameters are 860 kA/m, 0.01, 13 pJ/m, and 5*5*5 nm3, respectively. Our results show the width of wire influence dynamic pinning probability, as shown in Fig. 1 and Fig. 2. Figure 1 and Fig. 2 show the relation between dynamics VDW pinning probability and applied field. We employed the different separations from notch sharp to VDW staring positions to simulate the DW initial states in the WB. The data points of Figures 1 and 2 are averaged fields per 5 Oe, this averaged fields mean DW passing notch probability in a field range. The insert of Fig. 1 shows the type of DW pinning at notch. The type of DW pinning is influenced by pinning potential energy barrier or side well of the notch. It indicates that the shape of the notch influences the DW dynamics pinning probability. The applied field gives the magnetic energy to overcome the notch pinning potential. Figure 1 shows VDW in the 100-nm-wide wire is easier pinning as TDW type than 200-nm and 300-nm wide wire. The wire width influences the DW energy density, DW structure, and notch pinning potential energy. To make compassion with Fig. 1, we run the simulation with different wire widths, as shown in Fig. 2. Figure 2 shows the VDW type pinning probability of 200-nm and 300-nm wide wire has more than 100-nm-wide wire. It is caused by the wire width influences. The DW energy density influences by wire width and DW energy density also influences by a notch. The DW potential energy landscape is different at the differently wide wire. The wide wire has more space to release DW energy, and more space can lead DW to transform the structure of itself. In figure 2, 200-nm-wide wire shows unique behavior in the range of 200 Oe. In 200-nm-wide wire and 300-nm-wide wire, wide wires caused by wire width is not large enough to lead DW release metastable state DW energy, the DW energy is lower in the VDW chirality. The topological defects moving are also important for DW chirality changing in DW depinning behavior. The pinning potential of notch also influenced the DW depinning behavior. Those results show the applications of DW structures controlling and placing using notch.

**References:**

[1]. L. J. Chang, Y.D.Yao, Pang Lin, IEEE Transactions on Magnetics., Vol. 47, No 10, (2011) [2]. Burn, D. M., & Atkinson, D. Scientific reports, 6, 34517 (2016). [3]. L. K. Bogart and D. Atkinson, Physics Review B., Vol. 79, p.054414 (2009) [4]. Y. Gao, B. You, X. Z. Ruan, Scientific Reports., Vol. 6, Article number: 32617 (2016) [5]. J. Brandão, R. L. Novak, H. Lozano., Journal of Applied Physics., Vol. 116, p.193902 (2014) [6]. D. M. Burn, D. Atkinson, Scientific Reports., Vol. 6, Article number: 34517 (2016) [7]. K. A. Omari, T. J. Hayward, Scientific Reports., Vol. 7, Article number: 17862 (2017) [8]. Y. Li a, Z. Lu, C. Chen, M. Cheng, Journal of Magnetism and Magnetic Materials., Vol. 456, p.341 (2018)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/3af7b314acd1d21a2cc7c1fff5262cbc.jpg)
Fig. 1. Dynamics domain wall pinning probability of 100-nm-wide and 200-nm-wide wires. Insert shows the pinning types image of vortex domain wall(VDW) pinning and transverse domain wall(TDW).

![](https://assets.underline.io/uploads/markdown_image/1/image/f8fdc5b72b6336859504ed83f1e68734.jpg)
Fig. 2. The curve shows VDW and TDW types pinning probability in 200-nm-wide wire and 300-nm-wide. The square symbol shows the 200-nm-wide wire simulation results. The circle symbol shows the 300-nm-wide wire simulation results. The open symbols show the probability of DW depinning.

 Domain wall pinnning probability in different width of sub-micron-wire with a notch

Antiferromagnetic spintronics have been blooming in both fundamental science and practical application in recent few years due to the low sensitivity to external perturbation and ultrafast spin dynamics in antiferromagnetic materials. How to detect and manipulate the magnetic state of antiferromagnets efficiently has been a long-term outstanding question (1, 2). Among various regulation methods, electric-field control of antiferromagnetism is a promising way for realizing ultrafast and high density antiferromagnetic spintronics without considering Joule heating effect. Based on ferroelectric P(VDF-TrFE) copolymer, low-voltage-pulse modulation of ferromagnetic La0.7Sr0.3MnO3 (FM-LSMO) thin films via oxygen vacancy has been demonstrated (3, 4). Here, we investigated low-voltage-pulse modulation of antiferromagnetic La1-xSrxMnO3 (x = 0.65) (AFM-LSMO) thin films. Positive 10-volt pulses can increase the resistance at low temperature (Fig. 1), which is ascribed to the oxygen vacancies induced by positive voltage pulses at 300K. This effect was further demonstrated to be related with oxygen vacancy by x-ray photoelectron spectra (XPS) results (Fig. 2). The XPS results suggest that positive pulses can induce more oxygen vacancy and reduce the valence state of Mn ions in AFM-LSMO thin films, thus changing the electrical and magnetic properties of antiferromagnetic layer. Exchange bias effect was established in bilayer FM-LSMO/AFM-LSMO structure and regulated by low-voltage pulses. Both exchange bias field and coercivity show voltage-polarity dependence, suggesting that low-voltage-pulse induced oxygen vacancy dominates the tuning process. Our findings provide a new way for exploring electric-field modification of antiferromagnetic spintronics.

**References:**

1. T. Jungwirth, X. Marti, P. Wadley, J. Wunderlich. Nat Nanotechnol 11, 231-241 (2016). 2. P. Němec, M. Fiebig, T. Kampfrath, A. V. Kimel. Nat Phys 14, 229-241 (2018). 3. H. F. Wong, S. M. Ng, W. F. Cheng et al. Solid-State Electronics 138, 56-61 (2017). 4. H. F. Wong, S. M. Ng, W. Zhang et al. ACS Appl Mater Interfaces 12, 56541-56548 (2020).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/1ee895f3894fb907fdfc80dbd9e20a61.jpg)
Fig. 1 Temperature dependent resistance of 7-nm AFM-LSMO thin film at pristine, positive pulses gated and negative pulses gated states, respectively. Inset: resistance change as a function of temperature. Rpulses and Rpristine represent the resistance of pulses gated state and pristine state, respectively. Note that all pulses are applied at 300K in vacuum by Kiethley 2400 multimeter and the resistance is recorded after removing pulses.

![](https://assets.underline.io/uploads/markdown_image/1/image/01c793136ea4c47a9354e2b42e0791eb.jpg)
Fig. 2 Mn 2p core-level spectra of AFM-LSMO thin films at pristine, positive pulses gated and negative pulses gated states, respectively.

 Low-voltage-pulse control of the transport properties of antiferromagnetic La0.35Sr0.65MnO3 thin film via ferroelectric P(VDF-TrFE) copolymer

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

Rare earth iron garnets (REIGs) are magnetic insulators that can be grown as nanometer thickness films with perpendicular magnetic anisotropy (PMA), making them attractive for studies of spin orbit torques (SOT) and chiral spin textures. PMA has been achieved in epitaxial films of several REIGs on garnet substrates due to magnetoelastic anisotropy from epitaxial lattice mismatch strain1-2. However, it is of interest to develop PMA garnet films on non-garnet substrates, such as silicon for practical spintronic devices and piezoelectric substrates to investigate the effects of modulating the piezoelectric strain. PMA garnet has been grown directly on silicon, enabling integration of garnet-based spintronics into a conventional CMOS ecosystem. The films are polycrystalline, and PMA is due to the thermal expansion mismatch between the film and substrate following a high temperature anneal to crystallize the film3. Here, we report the growth of garnet films on Si and on piezoelectric substrates. Piezoelectric substrates allow modulation of the film strain through application of a voltage. This allows for tuning of the film anisotropy through magnetoelastic anisotropy, which may be used to manipulate domain walls with voltage-induced strain4. Dysprosium iron garnet (DyIG) was chosen because its two magnetostriction coefficients have the same sign. Pulsed laser deposition is then used to deposit the DyIG, which is then crystallized by a rapid thermal anneal (850 °C/5 min). DyIG grown on Si crystallized without a seed layer for thicknesses over 25nm. Atomic force microscopy of the DyIG/Si films indicates sub-nanometer surface roughness with large grain sizes on the order of 10 µm in a 41 nm thick film. DyIG on Si has a large in plane tensile strain leading to PMA with a coercivity of 4.5kOe at 41 nm thickness compared with 2.5kOe at 75nm thickness, and a room temperature saturation magnetization of 20 emu cm-3. The film coercivity can be tuned from 2.5-6 kOe by varying the annealing temperature. The anisotropy was reduced while retaining PMA by substitution of yttrium for dysprosium. We show that thinner films of DyIG may be crystallized using an underlayer of a nonmagnetic garnet Gd3Ga5O12 coated with ~2nm Pt to prevent interdiffusion. This allowed 10 nm polycrystalline DyIG films to be formed on Si with bulk-like magnetization and PMA. Films were also grown on PMN-PT (011) substrates by first sputtering a thin layer of Pt onto the substrates to prevent interdiffusion. The modulation of the magnetic properties of DyYIG/Pt/PMN-PT will be described as a function of voltage applied to the PMN-PT which produces a strain in the garnet. Fig. 1. Out of plane VSM hysteresis loop for a 75 nm DyIG/Si film substituting the dysprosium with varying amounts of yttrium. Fig. 2. AFM image of large grains in a 41 nm DyIG/Si film. Grain boundaries are indicated by the arrows.

**References:**

1. A. Quindeau, C. Avci, W. Liu, Adv. Elec. Mater., Vol. 3, 1600376 (2017). 2. E. Rosenberg, L. Beran, C. Avci, Phys. Rev. Mater., Vol. 2, 094405 (2018). 3. J. Bauer, E. Rosenberg, C. Ross, APL, Vol. 114, 052403 (2019). 4. M. Azam, et al. Nanotechnology, Vol. 31, 145201 (2020).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/2391572cbd85d2f4dbab48fca73870b8.jpg)

![](https://assets.underline.io/uploads/markdown_image/1/image/21c847133e550ecabc6425cc4ad545ca.jpg)

 Dysprosium Iron Garnet Films on Silicon and Piezoelectric Substrates with Perpendicular Magnetic Anisotropy

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

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.

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