Electric Field Control of Non‑Volatile Magnetism in Co₂FeAl / PMN‑PT Heterostructures at Room Temperature

Abstract

We demonstrate that a half‑metallic Heusler alloy, Co₂FeAl, deposited on a Pb(Mg₁/₃Nb₂/₃)O₃–PbTiO₃ (PMN‑PT) piezoelectric substrate, exhibits non‑volatile, electric‑field‑induced changes in its magnetic properties at room temperature. By applying pulsed electric fields along the [100] and [01–1] crystal axes, we achieve two stable, giant remanent magnetization states that persist after the field is removed. The effect originates from the large piezostrain generated in the PMN‑PT layer, offering a viable pathway for magnetoelectric memory devices.

Background

The growing demand for high‑speed, low‑power, and non‑volatile information storage has spurred interest in magnetoelectric (ME) coupling within ferromagnetic/ferroelectric (FM/FE) heterostructures. ME coupling arises primarily through three mechanisms: piezostrain, charge, and exchange bias. The piezostrain effect, in particular, enables an applied electric field to generate strain in the ferroelectric layer, which in turn modulates the magnetic anisotropy of an adjacent ferromagnet. PMN‑PT, with a large d₃₃ piezoelectric coefficient, is widely employed as the ferroelectric component because its strain can be efficiently transferred to the magnetic film. Co₂FeAl (CFA), a half‑metallic Heusler alloy, is an attractive FM choice due to its low damping, high spin polarization, and Curie temperature above 1000 K, making it ideal for spintronic applications. Prior work has shown that electric‑field‑induced strain can rotate the magnetic easy axis in FM/FE bilayers, but a clear demonstration of non‑volatile, reversible magnetization control at room temperature remains elusive. In this study, we fill that gap by showing that a CFA/PMN‑PT (011) heterostructure retains two distinct remanent magnetization states after the electric field is removed, a hallmark of non‑volatile operation suitable for memory technologies.

Methods

The heterostructure consists of a 40 nm CFA film sputtered onto a (011)‑oriented PMN‑PT substrate. DC magnetron sputtering was carried out at 600 °C under an Ar pressure of 0.1 Pa (10 SCCM). The base pressure was 2 × 10⁻⁵ Pa. Platinum electrodes (10 nm top, 50 nm bottom) were deposited from a 2 mm target. Cu wires were bonded to the electrodes with conductive adhesive. Static magnetic properties were measured with a vibrating sample magnetometer (VSM, MicroSense EV9) while a DC voltage from a Keithley 2410 source was applied to the piezoelectric layer. Magnetic domain images were acquired by magnetic force microscopy (MFM) using an Asylum Research© MFP‑3D system with soft magnetic tips magnetized perpendicular to the sample plane. All measurements were performed at ambient temperature.

Results and Discussions

The CFA/PMN‑PT heterostructure is illustrated in Fig. 1a, with the in‑plane coordinate system shown in Fig. 1b. The effective electric‑field‑induced strain field (Hσ) is orthogonal to the magnetic anisotropy field (Hk). We define the [100] direction as 0° and [01–1] as 90° (see Fig. 1b). The PMN‑PT polarization–electric field loop (P–E, 1 Hz) and strain–electric field curve (S–E) measured with ferroelectric and strain gauges are presented in Fig. 1c, revealing a saturation polarization of ≈25 µC cm⁻² and a coercive field of ≈100 V (2.5 kV cm⁻¹). MFM imaging after removing a 1000 Oe field (Fig. 1d) shows a stripe‑domain pattern, indicative of strong perpendicular magnetic anisotropy. Hysteresis loops measured along [100] and [01–1] under electric fields of ±0 and ±5 kV cm⁻¹ (Fig. 2a) confirm clear in‑plane anisotropy. The easy axis lies along [100] in the remnant polarization state (−0 kV cm⁻¹), while a 90° rotation occurs when the substrate is polarized to +5 kV cm⁻¹ (Fig. 2b). The anisotropy disappears in the +0 kV cm⁻¹ state (Fig. 2c) and re‑emerges along [100] when the field is switched to –5 kV cm⁻¹ (Fig. 2d). Rotational VSM measurements (Fig. 2e) reveal that the relative remanent magnetization (Mr/Ms) follows a sinusoidal dependence on the azimuthal angle, with the easy axis rotating by 90° as the electric field is varied. The electric‑field dependence of remanent magnetization was further examined by sweeping E from +10 to –10 kV cm⁻¹ after demagnetizing the sample (Fig. 3a,c). The resulting butterfly‑shaped Mr/Ms curves mirror the strain–E behavior, confirming that strain dominates the magnetization control. Residual magnetization in the remnant polarization state (±0 kV cm⁻¹) differs from that after ±10 kV cm⁻¹, reflecting the intrinsic residual stress of the PMN‑PT substrate. To demonstrate non‑volatile control, pulsed electric fields of ±5 or ±10 kV cm⁻¹ were applied while the sample was first saturated at 1200 Oe and then brought to zero field (Fig. 4). The resulting Mr/Ms values (labeled A–H) show four distinct, stable states that persist after the pulse is removed, both along [100] and [01–1]. This multistate behavior illustrates the potential of piezostrain‑mediated control for polymorphic memory devices.

Conclusions

We have shown that a CFA/PMN‑PT heterostructure exhibits non‑volatile, electric‑field‑induced modulation of magnetic anisotropy and remanent magnetization at room temperature. The stripe‑domain structure observed by MFM and the 90° rotation of the easy axis under ±5 kV cm⁻¹ confirm piezostrain‑mediated control. Moreover, pulsed electric fields generate four stable remanent states, offering a route to multilevel magnetoelectric memory devices.

Abbreviations

CFA

Co₂FeAl

DC

Direct current

FM/FE

Ferromagnetic/ferroelectric

ME

Magnetoelectric

MFM

Magnetic force microscopy

PMN‑PT

Pb(Mg₁/₃Nb₂/₃)O₃–30%PbTiO₃

VSM

Vibrating sample magnetometer