Anomalous Hall Effect and Magneto-Optic Kerr Effect in Pt/Co/Pt Heterostructure

Abstract

Magnetic multilayer with large perpendicular magnetic anisotropy (PMA) has attracted sustained interest owing to its importance to fundamental physics and applications. In this work, the high quality of Pt/Co/Pt heterostructures with large PMA was successfully achieved to exhibit a large anomalous Hall effect (AHE) with squared Hall loops. By calculating the proportional relationship between the longitudinal resistivity $({\rho }_{xx})$ and the abnormal Hall coefficient (R_s), it is confirmed that the basic mechanism of AHE comes from the external skew scattering (SS) and side jump (SJ), while SS contribution, related to asymmetric scattering from impurities, is dominant in the AHE. Furthermore, the obvious magneto-optical Kerr effect (MOKE) was also observed using the polar MOKE microscopy. The obviously circular magnetic domain can form and propagate in response to the applied out-of-plane magnetic field, resulting in the magnetization reversal of the entire film. This work offers important information in terms of both AHE and MOKE in the ultrathin ferromagnetic films with perpendicular anisotropy, establishing the application foundation for the nonvolatile memories and spintronics.

1. Introduction

The anomalous Hall effect (AHE) of ferromagnets has attracted much attention owing to its potential physical principles and wide applications [1,2,3,4,5]. For example, Trukhanova E.L. et al. investigated the effectiveness of Ni₈₀Fe₂₀/Cu films of different thicknesses for shielding direct current and alternating current electromagnetic signals. In general, the AHE of magnetic films or oxide magnetic materials is attributed to internal and external mechanisms [6,7,8]. The intrinsic mechanism is not related with the impurity scattering and associated with the Berry curvature of the band structures. While the extrinsic mechanisms, including the skew scattering (SS) and side jump (SJ) contributions, are related to spin–orbit coupling-dependent scattering from impurities or interfaces. SS is derived by asymmetric electron scattering and SJ comes from the transverse displacement of the electrons wave function by impurity scattering [9,10]. So far, various kinds of strategies have been developed to modulate AHE for spintronic applications based on these mechanisms [11,12].

As the typical magnetic metal layers that have strong perpendicular magnetic anisotropy (PMA), the Co/Pt layers possess great application potential in high-density nonvolatile magnetic memories and spintronic devices [13,14]. Aboaf et al. researchers controlled different Pt components in Pt/Co alloy magnetic films to achieve different magnetization and coercivity [15]. A.V. Svalov et al. studied the influence of magnetic fields of different directions and strengths on the anomalous Hall effect in Pt/Co films with different thicknesses [16]. Through the first principal calculation and related experiments, it is confirmed that the origin of PMA in Co/heavy metals is the hybridization 3d electron orbitals of Co and 5d electron orbitals of heavy metal in a multilayer membrane system [17,18,19]. In order to understand and observe the magnetization reversal process of the Pt/Co/Pt magnetic film, the magnetic domain of the film was observed using the magneto-optical Kerr microscope with the application of magnetic field outside the surface. The complete and clear magnetic domain expansion image means a large PMA in the heterostructure [13,20,21,22,23].

In this work, the mechanisms of AHE and magneto-optic Kerr effect in Pt/Co/Pt heterostructure have been studied. Our experimental results and data scaling relationships show that both SJ and SS contribute to the AHE in the Pt/Co/Pt heterostructured films and SS plays a dominant role in the AHE. In addition, the clear magnetic domain image was observed by magneto-optic Kerr microscope. The ferromagnetic heterostructures with large AHE and PMA hold great promise for magnetic sensors and nonvolatile memory.

2. Experimental Section

Magnetic thin films of Pt/Co/Pt heterostructures were prepared by magnetron sputtering with 4 mTorr pressure under Ar conditions. Lower partial pressure of nitrogen means better quality of thin film crystals [24]. The metal targets were high purity Pt (99.9%) and Co (99.9%), respectively. Firstly, 120 å Pt layer was grown on Si/SiO₂ substrate as buffer layer and the Co layer was deposited on the Pt layer. Then, an additional 20 å Pt layer was deposited on top of the Co layer to avoid oxidation. Figure 1 shows the X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements for Pt/Co/Pt heterostructures. The magnetic properties of the heterostructures at the temperatures from 100 K to 300 K are measured by PPMS. The magnetic resistance was also measured in the same temperature range by the standard four-probe technique. Magneto-optical images are observed at room temperature with a homemade magneto-optical Kerr effect (MOKE) microscope, using a LED with wavelength of 650 nm as the light source.

3. Results and Discussion

The architecture of the Pt/Co/Pt heterostructures is indicated in Figure 1a.

In Figure 1b, the XRD and XPS were used to further characterize the crystal quality of the magnetic film. The Pt signals with the binding energy of 72.1 eV and 75.1 eV were observed. The peak at 75.1 eV indicates that the surface of the Pt layer adsorbs the H₂O and O₂ in air [7,25]. The XRD result in the inset of Figure 1b shows the diffraction peak of Pt is located at around 39° (Green) indicating a well (111) phase of Pt layer. The peak of 44° (Red) is corresponding to the Co (111) layer [26].

The magnetic properties of the Pt/Co/Pt film were further measured using the PPMS system. As shown in Figure 2, the film exhibits the obvious out-of-plane (OOP) magnetic hysteresis loops of Pt/Co/Pt thin films as a function of temperature, presenting squared OOP hysteresis loops indicating strong perpendicular magnetic anisotropy [27,28]. The saturation magnetization obviously decreases with decreasing temperature. On the contrary, the coercivity increases with the decrease in temperature. The large temperature dependence of the hysteresis loop is probably attributed to the 20 å Pt with a better (111) phase, which improves the crystal and interface quality between Pt/Co. The effective anisotropy $K_{eff}$ can be calculated by: $K_{eff}=M_s{H}_k/2$, where $M_s$ is the saturation magnetization and $H_k$ is the anisotropy field in VSM, which measuring the area between the hard and easy axes of the magnetization curves [29,30]. When the temperature ranges from 300 to 100 K, the $K_{eff}$ of heterostructure is calculated to be from 3.5 × 10⁵ erg/cc to 7.5 × 10⁵ erg/cc, showing a strong PMA and great application promise in magnetic tunnel junctions. There is a strong orbital hybridization between the Pt (heavy metals) 5d and Co 3d orbitals at the Co/Pt interface, which increases the magnetic moment of the vertical orbital in the Co layer.

To further study the AHE of thin films, the heterostructure devices with four electrodes Hall bar geometry are fabricated by lithography technology. Figure 3a indicates the optical microscope image of the Hall bar device. The measurement schematic is presented in Figure 3b. Direct current is applied with 100 μA in the X-axis. The voltages in X (V_xx) and Y (V_xy) directions are measured with a magnetic field applied along the Z-direction, respectively. Figure 4a displays the dependence of the transverse Hall resistivity (${\rho }_{AH}$) on the magnetic field at different temperatures. The M-H and ${\rho }_{AH}$-H curves show the obvious PMA in the heterostructure with the high squareness ratio of the hysteresis loop [31]. We observed the obvious anomalous Hall effect, where the ${\rho }_{AH}$ exhibits squared hysteresis loop. The coercivity increases with the decrease in temperature, which is consistent with the results from the vibrating sample magnetometer measurement in Figure 2. At the field higher than coercivity, the Hall resistivity ${\rho }_{AH}$ shows a linear and independence on external magnetic field because of the highly alignment of the spins. Figure 4b displays the temperature dependence of the ${\rho }_{AH}$ and ${\rho }_{xx}$, respectively. ${\rho }_{xx}$ is the longitudinal resistivity, which is the resistivity at the magnetic field of 0.15 T with the magnetization reaching saturation. Both ${\rho }_{xx}$ and ${\rho }_{AH}$ increase significantly with increasing temperature, demonstrating the metallic behavior of the film. The metallic nature makes the interaction of spin and charge degrees of freedom form the AHE well. Figure 4c,d display the curves of ${\rho }_{AH}$-${\rho }_{xx}$ and Rs-${\rho }_{xx}$, respectively. There is a positive signal with Hall resistivity indicating that the Fermi level is on the side of the spin-down band, implying the majority of electron characteristics. Fermi level is on the side of the spin-down band [32]. The reason is that the heavy metal Pt layer increases the electron scattering at the Co/Pt interface, inducing the spin–orbit torque on the heterostructures [3,33,34,35]. To further understand the origin of the AHE, we fitted the $R_s$-${\rho }_{xx}$ curves using a simple formula:

$$R_s=a{\rho }_{xx}+b{\rho }_{xx}^2$$

(1)

where $R_s$ is the anomalous Hall coefficient, which can be calculated by $R_s={\rho }_{AH}/\left(M_s\times 4\pi \right)$, for the AHE of the heterostructure shown in Figure 4a; the fitting constants of $a$ and $b$ indicate the contribution of SS and SJ for AHE, respectively. From the fitting curves, the a and b are 2.09 × 10⁻⁵ T⁻¹ and 2.45 × 10⁻⁶ μΩ · cm/T, respectively. The absolute magnitude of SS is an order of magnitude larger than that of the SJ, which reveals that the AHE is attributed to both SS and SJ but the contribution of SS is dominant in the AHE of the Pt/Co/Pt heterostructure.

We further used the magneto-optic Kerr microscope to characterize the magnetic domain of Pt/Co/Pt films. The polarization can be changed upon the reflection of polarized light from the surface of magnetic film, which is called magneto-optic Kerr effect (MOKE), enabling the easy observation of coercivity, magnetization reversal and domain structure. Figure 5a shows the hysteresis loops measured by MOKE. A wide-field MOKE microscope in configured polarized light is used to capture images of magnetic domains which are sensitive to out-of-plane magnetization in magnetic films. The evolution of the magnetic domain as a function of applied field by subtracting the background has been shown in Figure 5b–i. The dark and bright regions are corresponding to magnetic domains with downward and upward magnetization, respectively. Because of rectangularity of the hysteresis loop in Pt/Co/Pt film, circular magnetic domains nucleate at random positions, then diffuse and propagate in the entire film (see Supplementary Material). The magnetic domain wall is the boundary between the two regions of light and dark. The complete and clear magnetic domain expansion image because of the high-quality interface in Pt/Co/Pt can enable a stronger hybridization and a large increase in PMA.

4. Conclusions

In this work, we have successfully fabricated the Pt/Co/Pt heterostructure with superior anomalous Hall effect and magneto-optic Kerr effect. We found that the underlying mechanism of AHE is ascribed to both SS and SJ. Circular magnetic domain of the heterostructure has been clearly observed through a magneto-optic Kerr microscope. The square hysteresis loop and clear magnetic domain indicate the high quality of the ferromagnetic film with large PMA. This work offers key information for fundamental physics and potential applications of ultrathin ferromagnetic film.