Magneto-ionics (MI) provides a nonvolatile EF modulation of magnetism through the migration of ions toward/away from the magnetic interfaces that allows for low-power spintronics devices. However, the ionic mechanism involved still needs to be explored, as key properties such as reversibility have been shown to strongly depend on the chemical composition and oxidation level of the ferromagnet (FM)/oxide interface, (1–3), and on the degree of crystallinity of the system (4). We have recently shown that the gate-voltage driven degree of oxidation at the FM/oxide interface in Ta/Co20Fe60B20/HfO2 defines two MI regimes: (I) underoxidized to optimally oxidized, and (II) optimally oxidized to overoxidized, accompanied by a change in the magnetization easy axis from in-plane (IP) to out-of-plane (OOP) in regime I and from OOP to IP in regime II (3). Only regime II showed reversibility, which was linked to the different binding strength of the oxygen species in regimes I and II.
In this work, we present the MI behavior in Pt/Co60Fe20B20/HfO2, where also two MI regimes have been identified. In this system, both regime I and II are reversible, but the stability of the nonvolatile magnetic states was found to be significantly different in regime I and II. Magnetic states in regime I was found to be highly stable, with no significant loss of remanence in the hysteresis loops over several weeks after applying the gate voltage. In this regime, a significant variation of the Dzyaloshinskii-Moriya interaction was also observed. In contrast, the magnetic states in regime II (between OOP and IP) were found to evolve with time back to the OOP state within one day after applying the gate voltage. Interestingly, only the final overoxidized IP state is fully stable in regime II.
This disparity in the stability of the MI states in regimes I and II could be attributed to differences in the gate-voltage driven chemical reactions and ionic transport, where also the composition of CoFeB may play an important role. Our results show the importance of studying the link between the ionic mechanisms and the MI state stability, in view of practical applications in spintronics.<br>

**References:**<br>
(1) L. Herrera Diez, Y. T. Liu, M. Belmeguenai, et al., Physical Review Applied, 12, 034005 (2019);<br>
(2) A. Fassatoui, L. Ranno, S. Pizzini, et al., Physical Review Applied, 14, 064041 (2020),<br> 
(3) R. Pachat, D. Ourdani, L. Herrera Diez. et al., Physical Review Applied, 15, 064055 (2021),<br>
(4) R. Pachat, D. Ourdani, L. Herrera Diez, et al., Adv. Mat. Inter., Accepted (2022)<br>

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/ea145b4e40c9304eb75f27a8883b090a.png)

Stability of under/over oxidized magneto ionic states in Pt/Co60Fe20B20/HfO2

Strain-induced modification of magnetic properties in hybrid piezoelectric/ferromagnetic heterostructures is being intensively investigated due to its potential for low-power
spintronics applications based on magnetic domain wall (DW) motion ([1], [2]).
In this study, we present PMN-PT(011)/Ta/CoFeB/MgO-based devices where both out-of-plane and in-plane strain configurations have been exploited to control DW dynamics. In PMNPT (011), the application of a gate voltage along the out-of-plane (OOP) direction of the sample leads to a non-volatile polarisation state which can be reverted back to the un-poled state by
annealing at temperatures above the Curie temperature ([3]), which was confirmed in our system by piezoresponse force microscopy. In this configuration, we observe a non-volatile
increase in the coercivity in 10µm wires in the poled state of about 90% with respect to the unpoled state. This response can be linked not only to a strain-control of magnetic anisotropy but
also to a strain-controlled change in the homogeneity of the magnetic landscape. The multiple polarisation directions of the domains in the un-poled state of the PMN-PT can translate into
a wider anisotropy distribution in the magnetic system, compared to the poled state. This can significantly affect DW nucleation/depinning fields, which opens the door to using strain to
control magnetic disorder. We also combine this non-volatile effect with local, volatile, in-plane control of DW dynamics, dominated by strain-induced changes in magnetic anisotropy.
In conclusion, we present a multifunctional strain-controlled device where both disorder and anisotropy can be manipulated. We show that multiple strain configurations in
piezoelectric/ferromagnetic hybrid structures can open new opportunities for the electrical
manipulation of DW dynamics.<br>
**References**<br> [1] Na Lei, Thibaut Devolder, Guillaume Agnus, et al., Nature Communications, 4, 1-4 (2013)
[2] P. M. Shepley, A. W. Rushforth, M. Wang, et al., Scientific Reports, 5, 1-5 (2015)
[3] V.S. Kathavate, B. Praveen Kumar, I. Singh, et al., Ceramics International, 46, 12876-12883 (2020)

Exploring multiple strain geometries in PMN PT/Ta/CoFeB/MgO magneto

COA: Magneto-ionics

technical paper

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Stability of under/over oxidized magneto ionic states in Pt/Co60Fe20B20/HfO2

Exploring multiple strain geometries in PMN PT/Ta/CoFeB/MgO magneto

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