France



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.&lt;br/&gt;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.&lt;br/&gt;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.&lt;br/&gt;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)&lt;br/&gt;[2] J. R. Petta et al., Phys. Rev. Lett., 93, 136601 (2004)&lt;br/&gt;[3] W. Wang et al.,, Appl. Phys. Lett. 89, 153105 (2006)&lt;br/&gt;[4] M. Galbiati et al., Adv. Mater. 24, 6429 (2012)&lt;br/&gt;[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

INTERMAG 2021

 Magnetic Tunnel Junctions Based on Photoswitchable Self Assembled Monolayers

#### Welcome to the 67th Annual Conference on Magnetism and Magnetic Materials, MMM 2022!

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MMM 2022

This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

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

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Current induced domain wall motion is a crucial part of spintronics that has received large attention in the last 15 years and has resulted in memory as well as logic applications. Spin tranfer torques(STT) and Spin orbit torques(SOT) are the two mechanism through which the current induced domain wall motion takes place. In the case of STT, the electron spins get polarized through exchange interaction within the ferromagnetic layer and then transfers its angular momentum into the local magnetic moment of the domain walls thereby resulting in its motion. On the other hand, in the case of spin orbit torque, the spin current is generated by Spin Hall effect and Rashba effect in an adjacent heavy metal layer or interface and then diffuses into the magnetic layer applying a torque on the domain wall magnetic moments. Here, we will focus on spin transfer torque driven domain wall motion in ferrimagnetic Ni doped Mn4N thin films. Recent work on domain wall motion has focused on ferrimagnets in which angular or spin moment compensation can be obtained by either changing the temperature of the material or the composition. Very fast domain wall velocities has been shown at the angular momentum compensation point where the precessional motion of the local spin is neglible thus resulting in faster domain wall motion. Mn4N is a class of rare-earth free ferrimagnetic material with a low magnetization and a high perpendicular magnetic anisotropy. It has an anti-perovskite crystal structure with antiferromagnetically coupled magnetic Mn atoms at the corner sites Mn(I) and at the face centered sites Mn(II). Very high current driven domain wall velocities has already been demostrated in thin Mn4N films epitaxially grown on SrTiO3 [1]. Since in these system domain walls have Bloch internal structure, the driving mechanism is the classical spin-transfer torque. The compensation point is not achieved by varying the temperature of the material. However, by doping of Ni in Mn4N the magnetic compensation can be achieved at room temperature [2]. The Ni atoms replace the Mn(I) atom at the corner sites and their moments align parallel to that of Mn(II) leading to a decrease in the net magnetization and a magnetic compensation or around 3.6% of Ni concentration [3]. On these epitaxially grown Mn4-xNixN thin films on SrTiO3 substrates, due to the reduction of the angular momentum originating from the Ni doping and the large spin-polarization of conduction electrons, very high velocities approaching 3000 m/s have been achieved for compositions close to the magnetic compensation point. Moreover, a reversal of the domain wall motion direction was observed after crossing the angular momentum (and magnetization) compensation point where the domain walls move in the opposite to the electron flow. This unique phenomenon is in agreement with the analytical 1D model applied to ferrimagnetic systems and is due to the switch of the sign of the angular moment with respect to that of the spin polarization after the compensation point. This is supported by the results of ab-initio calculations.

**References:**

1. T. Gushi, M.J. Klug, J.P. Garcia, NanoLetters.,19, 8716–8723(2019) 2. T. Komori, A. Anzai, T. Gushi, Journal of Crystal Growth., 507, 163-167 (2019) 3. T. Komori, T. Hirose, T Gushi, Journal of Applied Physics 127, 043903 (2020)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/987f325efaa3e71c344195d0784bddf9.jpg)
Fig 1: Anomalous Hall Effect curves showing a change of sign from negative to positive when crossing the magnetic compensation point. The magnetization decreases towards the compensation point and has been shown as negative after the crossing the compensation point.

![](https://assets.underline.io/uploads/markdown_image/1/image/5aa6153307025ed27b85f769a243db10.jpg)
Fig 2: Domain wall velocity vs current density for Mn4-xNixN thin films of different compositions below and above the compensation point; full symbol show velocities parallel to the electron flow before the compensation point and empty symbols show velocity after the compensation point in the opposite direction. Here, the highest velocity of 3000 m/s is achieved for the lowest magnetization with a composition after the compensation point.

 Very fast current driven and reverse domain wall motion in a rare-earth free compensated ferrimagnetic Mn4-xNixN.

Reconfigurable logic device represented by field-programmable gate array (FPGA) has been widely used in various fields of applications including digital signal processing, image processing, cryptography and so on [1]. The logic programmability makes it possible to deal with varieties of different computing tasks with limited number of devices. Because SRAM-based FPGA suffers from static power dissipation and volatility issues, numerous studies have been conducted on nonvolatile reconfigurable spintronic logic devices [2][3]. Although the logic devices proposed in these studies could perform multiple Boolean logic operations, they could not cover all logic functions. In order to perform all combinational logic functions in a reconfigurable way, look-up tables (LUTs) are used in FPGA and the function of LUT could be realized by cascading multiplexers. Hence, reconfigurable spin logic covering all combinational logic functions could be realized if spintronic multiplexers are demonstrated. Here, we proposed a nonvolatile spintronic 2-to-1 multiplexer based on propagation of domain wall in Ta/CofeB/MgO multilayer with perpendicular magnetic anisotropy (PMA). The device could operate at room temperature with no external magnetic field required for the function of multiplexer. The schematics of the nonvolatile spintronic multiplexer were shown in the inset of Fig. 1 and Fig. 2. Ta/CoFeB/MgO multilayer with PMA was patterned with “H” shape. The magnetization of the magnetic multilayer located at the vertical wires of “H” was manipulated by external out-of-plane magnetic field. The magnetization of the H-shaped magnetic film was first saturated by an external magnetic field applied on all parts of the film. After removing the saturation field, magnetic field of opposite direction was applied only on the left vertical wire of “H” to reverse the magnetization. Thus, a domain wall was generated. When current was applied in the horizontal wire, the domain wall would propagate and the direction of the domain wall movement was determined by the direction of the applied current. No external field was required for the current-driven domain wall motion. When the domain wall propagated from one end to the other end of the horizontal wire, the magnetization of the magnetic multilayer passed through by the domain wall would be reversed. Thus, the magnetization of the horizontal wire was determined by two factors: the initial magnetization state of the device and the direction of the write current applied. The experiment results of the device were shown in Fig. 1 and Fig. 2. The magnetization of the horizontal wire was represented by anomalous Hall voltage. When the write current flew leftwards, the magnetization of the horizontal wire would be the same with the left vertical wire, and vise versa when the write current flew rightwards. If the magnetization states of the two vertical wires were considered as two data inputs and the direction of write current as the selection input, the magnetization of the horizontal wire would be the output of the 2-to-1 multiplexer. In our proposed device, both the data inputs and the output were in the form of spin directions of ferromagnetic multilayer which would retain after power off. Such property not only made this device nonvolatile, but also provided the possibility for cascading because the output could be used as the data input of the multiplexer in the next step. The cascadable feature was necessary for reconfigurable combinational logic. Above all, the proposed device consisted of H-shape ferromagnetic multilayer Ta/CoFeB/MgO with PMA realized the function of nonvolatile 2-to-1 multiplexer experimentally at room temperature and was expected to performing full sets of combinational logic with programmability by cascading.

**References:**

[1] C. Katherine and H. Scott, ACM Computing Surveys., Vol. 34, p.171-210 (2002) [2] Z. Luo, Z. Lu, X. Zhang, et al., Advanced Materials., Vol. 29, p.1605027(2017) [3] X. Wang, C. H. Wan and X. F. Han, et al., Advanced Materials., Vol. 30, p.6 (2018)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/298fb3102649d4def3b1ad1ad77ebeac.jpg)
Fig. 1 Current-driven domain wall propagation when the left vertical wire was magnetized downward and right vertical wire magnetized upward. When current flew leftwards in the horizontal wire, the domain wall moved rightwards and the magnetization of the horizontal wire was reversed to downward. When current flew rightwards, the propagation of domain wall and the reversal of magnetization in the horizontal wire were both in the opposite direction.

![](https://assets.underline.io/uploads/markdown_image/1/image/de61849619a96fd5b196fad6e633e902.jpg)
Fig. 2 Current-driven domain wall propagation when the left vertical wire was magnetized upward and right vertical wire magnetized downward. Compared with the case in Fig. 1, the direction of applied write current was still opposite to the direction of domain wall propagation. As a result, the chirality of magnetization switching was reversed. The magnetization state of the horizontal wire was determined by both write current and the magnetization of vertical wires on the left and right side.

 Nonvolatile Spintronic 2-to-1 Multiplexer Based on Current-driven Domain Wall Propagation

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

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.

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