Magneto-Ionic Engineering of Antiferromagnetically RKKY-Coupled Multilayers

Abstract

Voltage-driven ion motion offers a powerful means to modulate magnetism and spin phenomena in solids, a process known as magneto-ionics, which holds great promise for developing energy-efficient next-generation micro- and nano-electronic devices. Synthetic antiferromagnets (SAFs), consisting of two ferromagnetic layers coupled antiferromagnetically via a thin non-magnetic spacer, offer advantages such as enhanced thermal stability, robustness against external magnetic fields, and reduced magnetostatic interactions in magnetic tunnel junctions. Despite its technological potential, magneto-ionic control of antiferromagnetic coupling in multilayers (MLs) has only recently been explored and remains poorly understood, particularly in systems free of platinum-group metals. In this work, room-temperature voltage control of Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions in Co/Ni-based SAFs is achieved. Transitions between ferrimagnetic (uncompensated) and antiferromagnetic (fully compensated) states is observed, as well as significant modulation of the RKKY bias field offset, emergence of additional switching events, and formation of skyrmion-like or pinned domain bubbles under relatively low gating voltages. These phenomena are attributed to voltage-driven oxygen migration in the MLs, as confirmed through microscopic and spectroscopic analyses. This study underscores the potential of voltage-triggered ion migration as a versatile tool for post-synthesis tuning of magnetic multilayers, with potential applications in magnetic-field sensing, energy-efficient memories and spintronics.

Keywords: RKKY interactions; magneto‐ionics; perpendicular magnetic anisotropy; synthetic antiferromagnets; voltage control of magnetism.

Figure 1

Voltage control of transitions between antiferromagnetic and ferrimagnetic states in perpendicularly magnetized SAF MLs. a) Schematic illustration of the composition of the ML stacks and electrolyte gating configuration. Gate voltage, V _G, is applied between the partially masked bottom electrode and a Pt foil of a similar area (counter electrode) during the gating experiments. b, c) Room‐temperature hysteresis loops of the as‐grown (top panels), voltage‐treated (+2 V for 5 min, middle panels) and recovered (−15 V for 60 min, bottom panels) [Ni/Co]_M/Ru/[Co/Ni]₅ samples (t _Ru = 0.9 nm), where M = 4 and 5 for panels (b) and (c), respectively.

Figure 2

Magnetic hysteresis behavior of the as‐grown [Ni/Co]₇/Ru/[Co/Ni]₅ stacks with variable Ru thickness, t _Ru. a) Schematics of magnetic moment reversal for the uncoupled [Co/Ni]₅ stacks (in red), and representative schematics of spin switching of top and bottom MLs in [Ni/Co]₇/Ru/[Co/Ni]₅ stacks with three‐ (t _Ru = 0.60 nm, in green) and four‐ (t _Ru = 0.95 nm, in dark brown) steps. The black arrows indicate the spins of the top and bottom ferromagnets. b) Room‐temperature hysteresis loops of the as‐grown [Co/Ni]₅ MLs and [Ni/Co]₇/Ru/[Co/Ni]₅ stacks with variable t _Ru. Applied magnetic fields are along out‐of‐plane direction with respect to the film plane, unless otherwise specified. c) Dependence of the RKKY coupling field, H _ex, and exchange coupling strength, J _ex, as a function of t _Ru.

Figure 3

Voltage control of the hysteresis loops behavior of [Ni/Co]₇/Ru/[Co/Ni]₅ SAF stacks (t _Ru = 0.40 and 0.90 nm). a) Evolution of the hysteresis loops as a function of voltage actuation time for the stack with 0.90 nm thick Ru. b) A representative hysteresis loop corresponding to the 2 V/5 min treated SAF heterostructure, where the RKKY coupling field, H _ex, the total saturation moment, m _S, as well as the saturation moment (m _{S_1} and m _{S_2}) and the loops slopes at the switching fields (χ ₁ and χ ₂) are indicated for the low‐ and high‐field loops, respectively. These parameters are quantified in c) as a function of gating time for the sample with t _Ru = 0.90 nm. d) Evolution of the hysteresis loops as a function of voltage actuation time for the stack with 0.40 nm thick Ru.

Figure 4

Voltage control of the hysteresis behavior of [Ni/Co]₇/Ru/[Co/Ni]₅ stacks (with t _Ru = 0.60 and 1.05 nm), showing the generation of additional magnetization switching events upon gating. a, b) Evolution of hysteresis loops as a function of gating time. c) Schematic representation of the formation of inhomogeneous magnetic regions in the top ML of the SAF stack with t _Ru = 0.60 nm, which could lead to the observed new magnetization states. d) Enlarged view of the final hysteresis loop in b), clearly showing the appearance of a six‐plateau step‐like loop after applying +2 V for 55 min.

Figure 5

MOKE microscopy imaging of the voltage‐treated [Ni/Co]₇/Ru/[Co/Ni]₅ SAF stack (with t _Ru = 1.05 nm), in the magnetic field range of 500–4000 Oe, showing nucleation and propagation of magnetic domains, that are otherwise not observable in the as‐grown sample. a) Typical high‐field minor hysteresis loop of this sample recorded by vibrating sample magnetometry (VSM), highlighting the approximate magnetic fields where the MOKE images were acquired. b) shows the occurrence of skyrmion‐like or circular domain bubbles during nucleation of magnetic domains. Note the scale is normalized into the range between 0 and 1. c) Enlarged view of the regions (red rectangles) highlighted in (b).

Figure 6

Structural and compositional characterizations of the as‐grown and voltage‐treated [Ni/Co]₇/Ru/[Co/Ni]₅ SAF structures (t _Ru = 1.05 nm). a, d, g) STEM images of the as‐grown and voltage‐treated (2 V/10 min and 2 V/55 min) lamellae. b, e, h) Corresponding EDX elemental mapping and chemical concentration results for both overlayed (Ni+Co+Ru+O and Ni+O) and separate Co (green), Ru (blue), Ni (yellow) and O (pink) components. c, f, i) Integrated depth profiles showing the compositional evolution for each sample. The vertical dotted lines denote the approximate boundaries between the ferromagnetic MLs and the spacer layer.

Figure 7

X‐ray absorption and X‐ray magnetic dichroism study of the as‐grown and voltage‐treated [Ni/Co]₇/Ru/[Co/Ni]₅ SAF (t _Ru = 1.05 nm). a) Co L _2,3 and b) Ni L _2,3 edges XAS and XMCD spectra for the as‐grown and gated samples (under 2 V/10 min and 2 V/55 min conditions). c,d) show the dependence of the Co and Ni magnetic moments per atom obtained from applying the sum rules to the XMCD data for the three states. The total moment, m _TOT amounts to the sum of the spin (m _S) and orbital (m _L) moments. Note the different Y‐axis scales in panels (c) and (d).