Electric field-controlled magnetization in exchange biased IrMn/Co/PZT multilayers

Electric-field modulating exchange bias and near 180° deterministic magnetization switching at room temperature are demonstrated in simple antiferromagnetic/ferromagnetic/ferroelectric (AFM/FM/FE) exchange-coupled multiferroic multilayers of IrMn/Co/PZT. A rather large exchange bias field shift up to ΔHex/Hex = 500% was obtained. This change governs mainly the electric-field strength rather than the applied current. It is explained as being realized through the competition between the electric-field induced uniaxial and unidirectional anisotropies. These results show good prospects for low-power spintronic devices.


Introduction
Modern spintronics has become increasingly important because of its potential impact on memory technologies and magnetic sensors (see [1][2][3][4][5] and references therein). Traditionally, the function of these devices is based on the magnetic-field induced magnetization switching. In nanostructures, however, this physical mechanism is not efficient enough to control the magnetic bits due to the large current. In particular, when approaching the downscaling limits (e.g. in densely packed arrays) the unavoidable distribution of writing parameters coupled to the large stray fields will lead to spreading program errors and may cause influence on neighborhood architectures. In this context, the current-induced (or spin-transfer driven) switching mode is considered to be more efficient; however, two main facts still Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
remain challenging for applications in information storage technologies. Firstly, all metal spintronic devices have low resistances; secondly, the critical current density to move domain walls is still too high for the economization of the power consumption [2]. In order to tackle these difficulties, electric (E) field-induced magnetization switching is a prospective solution [1,[3][4][5][6]. That approach includes E-field induction of carrier density in semiconductor systems [7], high k electrolyte and insulator to change interfacial electronic states [8] and magnetoelectric effect, which occurs in multiferroic-type materials consisting of both ferromagnetic and ferroelectric orders [9][10][11].
For magnetoelectric coupling systems, the main principle of E-field controlled magnetization manipulation is that ferroelectric phases are used to generate strain that is transferred to the magnetic phase and the magnetization orientation can be influenced, thanks to the inverse magnetostriction (Villary effect) [12]. In this case, the stress sensing phase is preferred with a highly magnetostrictive material where the maximal change in the magnetization direction can reach up to 90 • . This concept of the strain-driven magnetization rotation has been demonstrated in several spin-valve based multiferroic heterostructures, in which both the E-field induced magnetization and magnetoresistance were reported [13][14][15][16]. On the other hand, E-field control of exchange bias in antiferromagnetic/ ferromagnetic/ferroelectric (AFM/ FM/FE) heterostructures could lead to deterministic 180 • magnetization switching. This is of great significance for information storage such as magnetoelectric random access memories (MERAM), but has still been difficult to realize [3,[17][18][19].
In this work we report an alternative approach in achieving power-efficient E-field control of exchange bias in high-quality AFM/FM/FE IrMn/Co/PZT multiferroic multilayers, where the near 180 • deterministic-type magnetization switching is investigated using the inverse piezoelectric effect.

Experimental
Before fabricating Pb(Zr, Ti)O 3 (PZT) films, a 40 nm-thick La 0 . 67 Sr 033 MnO 3 (LSMO) layer was firstly epitaxially grown on 500 µm-thick SrTiO 3 (STO)/Si(001) substrate by pulsed laser deposition (PLD). The 220 nm-thick PZT films were then epitaxially grown at 600 • C on the LSMO/STO substrate. In this case, a KrF excimer laser of 248 nm wavelength was used with 2 Hz repetition and about 2.2 mJ cm −2 energy density in an O 2 gas pressure of 120 mTorr for LSMO deposition and in a N 2 O ambient of 260 mTorr for PZT deposition, followed by a cooling-down procedure under 300 Torr of pure oxygen atmosphere. The exchange coupled to the strong AFM/FM IrMn/Co magnetic bilayer system is realized in the structure of Ta/Co/IrMn/Ta by magnetron sputtering with low Ar pressure of 5.8 × 10 −4 mTorr at room temperature on top of the PZT layer. During the deposition, an external magnetic field of 1000 Oe was applied along [001] direction of single crystal PZT to induce an in-plane magnetic easy axis and exchange coupling. The patterned films are fabricated using micro-patterning techniques processes as illustrated in figure 1. In this configuration, Ta/Co/IrMn/Ta structures act as a cap as well as electrode layers.
The x-ray diffraction (XRD) system (Rigaku 3272) with Cu-Kα radiation was used to examine the crystal orientation of the PZT films. The surface morphology of the PZT films was characterized by atomic force microscopy (AFM) measurements. A ferroelectric test system (Precision LC Radiant Technology) was used to measure their electrical properties. Two types of measurements were applied in this work: firstly, the bottom electrode Ta/Co/IrMn/Ta/LSMO is connected to the drive of the precision LC (denoted as the positive branch), and secondly, the Ta/Co/IrMn/Ta top electrode is connected to the drive (i.e. negative branch). Magnetic properties are investigated by means of the magneto-optical Kerr effect (MOKE). All experimental measurements are performed at room temperature.

Experimental results and discussion
A typical θ-2θ XRD patterns spectrum of a 220 nm-thick PZT film grown on LSMO/STO/Si(001) is shown in figure 2(a), indicating a purely (00l) orientated perovskite structure. It indicates that the LSMO and PZT layers are preferentially oriented along the c-axis perpendicular to the surface of STO substrate. The AFM images with scanning 1 × 1 µm 2 area are shown in figure 2(b). The two-dimensional AFM picture manifests that most of the PZT grains are   equiaxed. The three-dimensional photograph, however, shows conical-shaped grains with a root-mean-square roughness of about 3 nm, which promotes a sufficiently flat surface for the subsequent growth of the magnetic layers (figure 2(c)). Shown in figure 3(a) is the C-V (and the corresponding conversional C-E) characteristics performed at 10 kHz for the investigated PZT films. The drive is connected to the bottom electrode (i.e. in the positive branch) and the dc voltage was swept from 7 to −7 V and then reversely swept again. In this case, the C-V characteristic exhibits the typical butterfly shape with asymmetry. The coercivity is shifted to the positive applied voltage and an enhancement of the capacitance is accompanied. Indeed, the coercive fields of the film are of +63.6 and −4.5 kV cm −1 , which yield an absolute coercive field of 34.05 kV cm −1 . This asymmetry can be related to the dissimilar electrodes with different mobile and interface charge traps [20] and different work function [21]. Presented in figure 3(b) is the leakage current density data, indicating a more pronounced asymmetry. For the negative branch, the leakage current strongly increases at the low applied voltages (between 1.0 and 1.5 V), and reaches a value as large as 10 −1 A cm −2 at E = 60 kV cm −1 . This large current density increases with increasing voltage and finally almost saturates with a value of 10 A cm −2 at E > 875 kV cm −1 . For the positive branch, leakage currents as low as 10 −3 A cm −2 remain in the electrical fields up to -800 kV cm −1 , indicating a high structural quality of the ferroelectric layer. The leakage current jumps up at E = −810 kV cm −1 and the saturation follows after. Note that between +17 and -17 V applied voltage, the corresponding leakage current is always much higher for the negative branch than for the positive one, e.g. -5 V;10 −1 A cm −2 and 5 V;10 −5 A cm −2 . This may relate to the fact that the Schottky barrier between magnetic and PZT layers induced a diode behavior [6,22].
Large electrical currents flowing into the structure under electric field may give a certain contribution of the Joule effect and may affect the magnetic properties of the magnetic layers. In order to avoid this factor, the positive branch configuration was firstly considered, where the low leakage current almost exists in the whole applied voltage range. The normalized magnetization loops measured under various bias voltages (V) are illustrated in figure 4(a). It is clearly seen that, for the unbiased sample (V = 0), the existence of a clear shift of the loop reflects the existence of an exchange bias field H ex (≈79.5 Oe). When applying a bias-voltage across the PZT layer, the negative magnetic coercivity decreases, while the positive field increases, indicating the suppression of H ex ( figure 4(b)). Inspection of the magnetic loops in figure 4(a) suggests that it should be possible to reverse the magnetization of the magnetic layer upon suitable electric and magnetic field biasing. A modification of the magnetization can be indeed processed in fixing a external magnetic field of H a = −132 Oe as shown by the arrow in figure 4(a). At this bias magnetic field, the normalized magnetization M/M s already lowers to the value of 0.32 at V = 0. When increasing applied voltage V, the magnetization continues to lower, switches its sign at V ≈ 20 V and reaches the value of -0.32 at about V ≈ 25 V. This electric-field bias dependence of the magnetization is described in figure 4(c). Electric-field tuning of exchange bias and deterministic magnetization reversal observed in AFM/FM/FE multilayers can be understood as an intrinsic magnetization phenomenon resulting from the competition between the effective uniaxial anisotropy K eff (consists of the uniaxial anisotropy constant K u and the E-field induced elastic anisotropy term of (3/2)λσ ) and the anisotropy energy K ex associated with the unidirectional exchange coupling [23,24].
Although the leakage current exhibits the abrupt change between the applied voltage of 15 and 20 V, the variation of the magnetic coercivity and exchange bias field reflects almost a unique tendency for the whole applied voltage range (see figure 4(b)). This may imply that the magnetic change depends mainly on the amplitude of the electric field rather than the applied current. To tackle this point, the magnetic loops measured under the bias voltages of 15 V (with the current of 0.052 mA) and of −15 V (with the current of 2.2 mA) are illustrated in figure 5. Surprisingly, although the current is 40 times larger for the latter case, the difference in coercivity field is a few per cent only. Thus, this encourages measurement of the magnetic loops under the bias voltage up to −40 V to study the piezoelectric effect on the magnetization switching. The results are presented in figure 6(a). It can be seen from this figure that when a voltage of 40 V is applied, H ex significantly decreases from 60 to 10 Oe with H ex /H ex (40 V) = 500%. As shown by the arrow in this figure, at the external bias magnetic field of -100 Oe, the normalized magnetization M/M s already lowers down to the value of 0.65. When increasing applied voltage V, the magnetization continues to lower, switches its sign between −25 and -30 V and finally reaches the value of -0.8 at about V = −40 V. The observed electric-field bias dependence of the magnetization is described in figure 6(b). Clearly, the near 180 • deterministic magnetization switching can be electrically realized in the investigated AFM/FM/FE multiferroic multilayers.

Conclusion
To summarize, high-quality piezoelectric films have been fabicated and used for electrically controlling the exchange bias in AFM/FM/FE IrMn/Co/PZT multiferroic multilayers. A rather large exchange bias field shift up to H ex /H ex = 500% was obtained. The asymmetry is observed in both C-V and J-V characteristics, however, the exchange bias field change depends mainly on the electric-field strength rather than the applied current. The observed deterministic magnetization switching near 180 • shows great potential in E-field writing of novel spintronics and memory devices.