The application of gate voltages in ferromagnet/Oxide/Heavy metal thin film multilayer has been identified as one possible candidate to manipulate their anisotropy at will [3], [1], [4]. Unfortunately, this method has proven to be unreliable in terms of the reversibility: e.g, the application of a gate voltage onMgO/CoFeB/Ta [3] causes an irreversible change of the magnetic anisotropy axis from in plane to out-of-plane, while HfO2/CoFeB/Ta stacks display a more complicated behavior, with different regimes characterized by different reversibility behaviors [2]. The microscopic origin of these phenomena remains unclear and is the object of our inquiry. In this work we performed ab-initio simulations on 6 setups comprising Fe/O, HfO2/Fe, MgO/Fe interfaces each one with frontal and interstitial oxygen atomic configurations (fig.1). The pure Fe/O interface prefers interstitial oxygen configurations while the addition of oxides seems to switch this trend around. In particular, HfO2/Fe displays a strong preference for the frontal oxygen arrangement, leading us to identify a possible hypothesis for the phenomenon of the multiple magneto-ionic regimes displayed. In it’s as grown form, HfO2/CoFeB is characterized by an underoxidized ferromagnet interface [2]. The application of a gate voltage on HfO2/CoFeB pushes some of the oxygen species in proximity of the interface where they occupy the stable frontal site. Further application of a gate voltage pushes other oxygen atoms toward the still free and more unstable interstitial sites. The occupation of these interstitial sites has the effect of destroying perpendicular magnetic anisotropy as seen both in experiments and simulations [5] [1]. Once these more mobile interstitial sites are occupied, magnetic anisotropy can be switched reversibly by inverting the gate voltage and adjusting the interstitial site occupation (fig.2)

![](https://assets.underline.io/uploads/markdown_image/1/image/40c593503d5a4f3ee9c27db4ad2dc04d.jpg)

Comparison of the different interfacial configurations in the differentsamples. The bar plots represent the ground state energies and highlight themore stable configuration

![](https://assets.underline.io/uploads/markdown_image/1/image/f5dc5807d0f4552079d7a39375420a9e.jpg)

Hypothesis for the mechanism governing magneto-ionic regimes inHfO2/FM stacks. The text highlights the magnetic properties of the interface under different oxidation conditions.

Reversible anisotropy manipulation via gate voltages in ferromagnet/oxide multilayers

Magnetoionics (MI) is a much-in-demand feature for spintronics devices due to its great potential for improving energy efficiency in memory devices. The main feature of MI is to allow for the nonvolatile modulation of magnetic properties [1–5], making MI reversibility a fundamental quality for high device performance. Various studies have addressed issues impacting reversibility such as the presence of different types of ions, oxides and the degree of oxidation at the oxide/ferromagnetic interface [3, 5–8], while the influence of annealing temperature on reversibility has been largely overlooked. Annealing is commonly used to induce crystallization in CoFeB layers leading to the enhancement of perpendicular magnetic anisotropy (PMA), interfacial Dzyaloshinskii-Moriya interaction (iDMI), and tunnel magnetoresistance [9–11], therefore its impact on MI is of great interest. In this study, we show that W/CoFeB/HfO2 thin films annealed at 350°C and 390°C show a significant but irreversible dependence of PMA and DMI on gate voltage. However, an annealing program using a slow ramp that minimizes the exposure to the highest temperatures allows to induce PMA and iDMI while also showing a MI reversibility of 71% in remanence with respect to PMA. This dependence of MI reversibility on annealing profiles and temperatures could be related to the different degrees of crystallinity obtained under different annealing conditions. Such structural differences, such as size and distribution of crystallites, can greatly impact the ion mobility inside the stack and be responsible for observed modulation of the MI reversibility. Our results therefore show that annealing parameters need to be optimized to obtain enhanced magnetic properties without compromising MI performance.

![](https://assets.underline.io/uploads/markdown_image/1/image/54d50bab8c546ddbc8fd33ecfea28690.jpg)

![](https://assets.underline.io/uploads/markdown_image/1/image/bf6e6dbd3a0f9277642eaa6aab5dd034.jpg)

Effect of Annealing on Magnetoionics in W/CoFeB/HfO2

Effect of Annealing on Magnetoionics in W/CoFeB/HfO2

Controlling magnetic properties of materials with electric fields (EFs) is of remarkable importance in spintronics applications due to its potential for much lower power consumption devices. Much like the power efficiency, two other sought-after features to incorporate are non-volatility and reversibility. This has been demonstrated with magneto-ionics, where EFs induce the migration of mobile ionic-species towards/away from the magnetic interface [1–4]. However, several works conducted in a variety of magneto-ionic materials have revealed important issues like irreversibility, the need of elevated temperatures to recover the initial states and chemical and structural degradation [4–6]. Recent studies have also focused on the differences in magneto-ionic reversibility between various oxides [7], pointing out the importance of the mechanisms governing ionic conduction. In this study, we unravel a higher degree of complexity in an HfO2 based device and demonstrate the coexistence of two distinct magneto-ionic regimes. For this purpose, we chose amorphous Ta/CoFeB/HfO2 thin films grown by magnetron sputtering with an ionic-liquid gate. It is shown that a gate voltage drives the anisotropy of the system from an underoxidized state exhibiting in-plane anisotropy (IPA) to an optimum oxidation level resulting in perpendicular anisotropy (PMA) and further into an overoxidized state resulting IPA in a non-volatile manner. The IPA (underoxidized) to PMA regime is found to be highly irreversible and is significantly faster than the PMA to IPA (overoxidized) regime which is fully reversibile under the same gate voltages. In addition, the effective damping parameter also shows a marked dependence with the anisotropy evolution in different regimes; IPA to PMA regime going from 0.029 to 0.012 and only a modest increase to 0.014 in the PMA to IPA regime. The existence of two magneto-ionic regimes has been linked to a difference in the chemical environment of oxygen species added to under/over oxidized layers. Our results show that different magneto-ionic regimes can coexist and that their identification is of great importance for tuning the properties and the design of high performance spintronics devices based on magneto-ionics.

**References:**

[1] U. Bauer, S. Emori, and G. S. D. Beach, Nat. Nanotech. 8, 411 (2013) [2] U. Bauer, L. Yao, A. J. Tan, and G. S. D. Beach, Nat. Mater. 14, 174 (2015) [3] A. J. Tan, M. Huang, C. O. Avci, and G. S. D. Beach, Nat. Mater. 18, 35 (2019) [4] L. Herrera Diez, Y. Liu, D. Gilbert, M. Belmeguenai, J. Vogel, S. Pizzini, E. Martinez, A. Lamperti, J. Mohammedi, A. Laborieux, et al., Phys. Rev. Appl. 12, 034005 (2019) [5] C. Bi, Y. Liu, T. Newhouse-Illige, M. Xu, M. Rosales, J. W. Freeland, O. Mryasov, S. Zhang, S. G. E. te Velthuis, and W. G. Wang, Phys. Rev. Lett. 113, 267202 (2014) [6] D. A. Gilbert, A. J. Grutter, E. Arenholz, K. Liu, B. J. Kirby, J. A. Borchers, and B. B. Maranville, Nat. Commun. 7, 12264 (2016) [7] A. Fassatoui, Phys. Rev. Appl. 14, 064041 (2020)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/739c64ac3b9dc0fe2b2d809ba80bb186.jpg)
Fig. 1: Graphic representation of the Ta/CoFeB/HfO2 magneto-ionic stack covered with an [EMI-TFSI] ionic liquid gate. The gate voltage induces motion of oxygen species in HfO2 that drive anisotropy changes in the CoFeB layer.

![](https://assets.underline.io/uploads/markdown_image/1/image/a9cfb45146f55322b6c3cafa5270f8f3.jpg)
Fig. 2: A progressive oxidation is induced upon exposure to a gate voltage GV=-2V. Different exposure times t drive the system from IPA (a, initial state) through regime I (IPA to PMA) (b) into PMA (c) and back to IPA through regime II (PMA to IPA) (d).

 Reversible and Irreversible Magneto-ionic Regimes in Ta/CoFeB/HfO2

**Session chair:** Minori Goto, Osaka University



GOM - Voltage-controlled Magnetic Anisotropy and Switching

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Reversible anisotropy manipulation via gate voltages in ferromagnet/oxide multilayers

Effect of Annealing on Magnetoionics in W/CoFeB/HfO2

Reversible and Irreversible Magneto-ionic Regimes in Ta/CoFeB/HfO2

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