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Nonlinear electric field effect on perpendicular magnetic anisotropy in Fe/MgO interfaces

The electric field effect on magnetic anisotropy was studied in an ultrathin Fe(001) monocrystalline layer sandwiched between Cr buffer and MgO tunnel barrier layers, mainly through post-annealing temperature and measurement temperature dependences. A large coefficient of the electric field effect of more than 200 fJ/Vm was observed in the negative range of electric field, as well as an areal energy density of perpendicular magnetic anisotropy (PMA) of around 600 uJ/m2. More interestingly, nonlinear behavior, giving rise to a local minimum around +100 mV/nm, was observed in the electric field dependence of magnetic anisotropy, being independent of the post-annealing and measurement temperatures. The insensitivity to both the interface conditions and the temperature of the system suggests that the nonlinear behavior is attributed to an intrinsic origin such as an inherent electronic structure in the Fe/MgO interface. The present study can contribute to the progress in theoretical studies, such as ab initio calculations, on the mechanism of the electric field effect on PMA.


Introduction
The electric field effect on magnetic anisotropy, which is often called "voltage-control of magnetic anisotropy (VCMA)", in ferromagnetic metal layers attracts much interest in recent years . This technology can help to realize low-power magnetization switching in devices, which is a key to next generation magnetic random access memories (MRAMs). In fact, by using the VCMA, direct manipulation of magnetization by voltage [25][26][27] and assistance to spin transfer torque switching [28,29] have been demonstrated.
Cr-buffered Fe/MgO heterostructures are of particular importance in perpendicular magnetic anisotropy (PMA) and relevant studies, and Nozaki et al. have recently achieved a remarkable progress in the VCMA study using magnetic tunnel junctions (MTJs) with the Cr/Fe/MgO structure [23]. First, a very large interface PMA was predicted for the Cr/Fe/MgO structure by ab initio calculations [30,31], followed by an experimental demonstration of PMA of ~1.4 MJ/m 3 (~1000 J/m 2 for areal energy density) in previous studies [32,33]. Then, Nozaki et al. successfully reproduced such a large PMA in their MTJs including the Cr/Fe/MgO structure, so that its VCMA was examined. The VCMA coefficient obtained for the PMA reached 290 fJ/Vm [23].
The mechanism of the large VCMA in Cr/Fe/MgO has not been well understood, although ab initio studies have described the effect of charge accumulation and depletion at the interface that affects the spin-dependent screening and electron's occupancy of 3d orbitals [32,34]. Furthermore, an open question is the nonlinear behavior observed unexpectedly in the electric field dependence of PMA energy density [23]. Since it appears to be a unique feature for the VCMA coefficient more than 200 fJ/Vm, the nonlinear behavior could be a key to develop further large VCMA. At the same time, one may wonder if the nonlinearity can be associated with possible sample-to-sample variation, since no theoretical explanation has been provided. As Cr diffusion into the Fe layer (also into the Fe/MgO interface region) and possible interface contamination with carbon and/or oxygen are somehow discussed in Ref. 23, it may be difficult to obtain the well-defined Fe/MgO interface repeatedly and systematically.
In this work, we examined the VCMA in Cr/Fe/MgO under different conditions, i.e., temperature for annealing the structure and temperature of the VCMA measurement, to confirm the presence of the nonlinear VCMA behavior occurring characteristically in the Cr/Fe/MgO with a large VCMA coefficient. The nonlinear VCMA that was obtained together with a large areal PMA energy of ~600 J/m 2 and a large VCMA coefficient of more than 200 fJ/Vm gave rise to a local minimum at around 100 mV/nm in the VCMA curve, being independent of both the post-annealing temperature (i.e., interface quality) and the measurement temperatures. The results insensitive to the interface quality and the measurement conditions suggest that the nonlinear VCMA has an intrinsic origin such as a basic feature of the interface electronic structure. The bottom 0.7 nm Fe layer is the free layer to respond the voltage effect with different PMA energy density, while the 2 nm top Fe layer with in-plane magnetization is the reference layer to quantitatively evaluate the VCMA effect. Here, the thickness of the bottom (free) Fe layer tFe corresponds to five monolayers of bcc-Fe(001). At the beginning, the substrate was annealed at 800°C to clean its surface, followed by the deposition of a 5 nm MgO seed layer at 450°C. A Cr buffer layer was deposited at 150°C, and then was post-annealed at 800°C to obtain a flat Cr(001) surface. This crucial post-annealing temperature is critical to obtain large PMA for an ultrathin Fe layer deposited on the Cr buffer. The conditions used for the ultrathin Fe were 150°C for growth and 250°C for post-annealing to improve the surface flatness. Then, the MgO barrier layer was deposited at 150°C, followed with post-annealing at different temperatures (325°C, 350°C, 375°C, and 400°C) to modify the Fe/MgO interface conditions. The reference Fe layer was deposited at 150°C without post-annealing. Finally, a Ru capping layer was sputter-deposited at room temperature (RT). Through the growth process, surface structures and epitaxial growth were monitored by reflection high-energy electron diffraction (RHEED). The MTJ multilayer stacks prepared were patterned into a 5×10 μm 2 elliptical shape by using photo-lithography, Ar ion-beam milling and lift-off processes. The tunnel magnetoresistance (TMR) ratio was measured using the physical property measurement system (PPMS) under an in-plane external magnetic field at RT and low temperatures. The positive sign of applied voltage was defined as the polarity corresponding to the current flow from the top to bottom Fe electrode. The typical resistance-area product (RA) of the MTJs was the order of 10 4 Ωμm 2 . Separately, the magnetization curve of the ultrathin Fe layer was characterized by using a vibrating sample magnetometer to determine the saturation magnetization (Ms).    the easy axis of the ultrathin Fe layer is aligned perpendicular to the film plane; meanwhile, the top Fe layer has an easy axis parallel to the film plane. This orthogonal-easy-axis design enables us to electrically detect the rotation of the magnetization in the ultrathin Fe layer by applying an Hex [11]. As described in Fig. 2(a), the two Fe layers take the orthogonal magnetization configuration at Hex = 0, which brings about a high tunnel resistance state of the MTJ. By applying an Hex, the magnetization of the top Fe layer (reference layer) immediately turns parallel to the Hex due to its in-plane magnetic anisotropy and small in-plane coercivity (<a few Oe), i.e., the magnetization of the top layer is almost always parallel to the Hex direction. On the other hand, the magnetization of the bottom Fe layer (free layer) gradually tilts and finally becomes parallel to the Hex direction when the Hex reaches the anisotropy field of the bottom Fe layer (Hk). Therefore, the TMR curve reflects the magnetization process of the free Fe layer, i.e., the tunnel resistance takes the maximum at Hex = 0 (orthogonal magnetization configuration) and gradually decreases down to the minimum with the increase of |Hex| (parallel magnetization configuration). The TMR ratio in Fig. 2(a) corresponds to a half of the whole TMR change between parallel and antiparallel magnetization configurations. From the normalized TMR curves, we can evaluate the effective PMA energy density (Keff), including the contribution of the shape magnetic anisotropy, as follows: the tunnel resistance is given by the relative angle between the magnetizations of the free and reference magnetic layers. In the sample design, the maximum resistance occurs at the 90° configurations (R90 at Hex = 0), while the minimum resistance does in the parallel configuration (Rp at Hex = Hk). The resistance at a given relative angle  is expressed as [23]:

Results and discussion
The Min-plane/Ms calculated from the observed TMR curve of the MTJ with post-annealing at 350°C is shown in the inset of Fig. 2(b), as an example. The Keff is finally obtained by calculating the product of the area of shadow region Ain-plane and the Ms.  The appearance of the local minimum always observed at around +100 mV/nm despite the variation in the annealing temperature would be specific to the VCMA characteristics of Cr/Fe (0.7 nm)/MgO structures. To explore it further, the VCMA characteristics for the MTJ annealed at 400 °C was evaluated at low measurement temperatures. Figure 4 shows the Keff tFe as a function of E at 10, 100, 200 and 300 K, respectively. It is clearly seen that the Keff tFe increases with decreasing the measurement temperature. Interestingly, the local minimum positions (~ +100 mV/nm) are independent of the temperatures, while the nonlinearity becomes significant at lower temperatures. In addition, there might be a few fine structures, as implied by the faint peak at around -50 mV/nm, in the E dependence of Keff tFe at low temperatures.
As shown in Figs. 3 and 4, the nonlinear behavior, particularly the local minimum at around +100 mV/nm, has been found to be independent of both the annealing and measurement temperatures. The former temperature is likely to influence the interface conditions, and indeed the PMA and the VCMA coefficient vary with the annealing temperature. The latter temperature may be related with possible extrinsic effects such as an epitaxial strain induced in the MTJ and some kinds of artifacts. Thus, the observed insensitivity of the minimum position suggests that the nonlinear behavior is attributed to an intrinsic origin such as a basic feature of the electronic structure at the Fe/MgO interface. In fact, interface resonant states (IRSs) are formed in the minority spin band of the Fe/MgO system, and the IRSs may affect the transport properties in the Fe/MgO-based MTJs, as proposed by Belashchenko et al. [35]. The effect of IRSs on the VCMA at the Fe/MgO interface was also studied by means of ab initio calculations [36]. Thus, the results obtained are expected to contribute to the progress in theoretical studies, particularly in ab initio calculations [36][37][38][39][40][41], on the mechanism of VCMA.

Conclusions
The nonlinear VCMA characteristics in orthogonally-magnetized MTJs with a Cr/Fe(0.7nm)/MgO structure was studied by evaluating post-annealing and measurement temperature dependences. A large VCMA coefficient of more than 200 fJ/Vm and a large areal PMA energy density of around 600 J/m 2 at RT were obtained at the 0.7-nm Fe/MgO interfaces. More interestingly, regardless of the post-annealing and measurement temperatures, a clear local minimum around +100 mV/nm was observed in the electric field dependence of magnetic anisotropy. The present results imply that the origin of the local minimum is attributed to an inherent electronic structure in the Cr/Fe/MgO.