Surface plasmon resonance (SPR) sensors, which utilize the collective oscillation of electrons at the interface between a metal and a dielectric thin film, are promising candidates for ultrasensitive chemical and biomedical sensing applications. Because of the high optical loss of noble metals used in SPR sensors, the limits of detection (LODs) of SPR devices based on intensity modulation are usually in the range of ${10}^{-5}-{10}^{-6}$ refractive index unit (RIU) ^[1,2]. To further improve device sensitivity, magneto-optical SPR (MOSPR) devices are developed. Such a device introduces a ferromagnetic thin film such as cobalt into the SPR structure. The highly confined electromagnetic field caused by SPR can also lead to a significantly enhanced transverse magneto-optical Kerr effect (TMOKE) in the Co layer, which can be utilized to modulate the SPR resonance condition with applied magnetic fields. Because of the high absorption loss of the ferromagnetic metal thin films, a noble metal/ferromagnetic metal multilayer structure is usually applied in MOSPR sensors. For example, Au/Co multilayer MOSPR transducers have been demonstrated to show 3 times higher sensitivity experimentally compared to conventional SPR sensors ^[3], whereas a potential 1 order of magnitude improvement of the device sensitivity has been predicted theoretically ^[3].

Several methods that consider the fundamental trade-off between the high magneto-optical response and low loss of an MOSPR transducer have been proposed to improve the sensitivity of MOSPR devices. One way to achieve this is to optimize the noble metal and ferromagnetic metal layers’ thicknesses and positions, which influence the modal distribution in the ferromagnetic metal layer of the MOSPR device ^[4–7]. Another way is to use noble metal materials with lower optical loss, such as using Ag instead of Au, to introduce lower optical absorption in the MOSPR device ^[8]. On the other hand, several different magnetic metal thin films are applied for MOSPR devices, such as Fe, Co, NiFe (Permalloy), and IrMn ^[8–14]. However, there are very few reports that study the influence of different magnetic metals on device performance. For example, a much higher magneto-optical coefficient has been observed in Fe compared to Co at 650 nm wavelength ^[15], while the optical loss of Fe is much higher. How the optical and magneto-optical properties of the magnetic metal thin films would influence device performance is unclear. Therefore, exploring novel ferromagnetic metal materials with different optical and magneto-optical properties may also improve the performance of MOSPR sensors.

In this work, we report a study on the optical and magneto-optical properties of ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ ($x=0$, 0.29, 0.47, 0.65, and 1) thin films in an $\mathrm{Au}/{\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ bilayer structure for MOSPR sensor applications. We observed that when doping Fe with Co, the magneto-optical effect of ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ first increases and then decreases with further increase in Co concentrations ^[15]. Meanwhile, the optical constants of ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ vary with the compositions, leading to lower optical loss with higher Co doping concentration. The highest sensitivity for all ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ concentrations studied is observed in $\mathrm{Au}/{\mathrm{Fe}}_{0.7}{\mathrm{Co}}_{0.3}$ MOSPR transducers, which is mostly due to the highest magneto-optical effect of ${\mathrm{Fe}}_{0.7}{\mathrm{Co}}_{0.3}$, indicating a promising potential for using these devices for chemical and biomedical sensing applications.

The paper is arranged as follows. In Section 2, we introduce the simulation and experimental methods. In Section 3, we report the structure and optical and magneto-optical properties of sputter deposited ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ thin films in an $\mathrm{Au}/{\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ bilayer structure deposited on glass substrates. The full permittivity tensors of ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ thin films are characterized at 650 nm wavelength. Then, based on the characterized material properties, the performance of $\mathrm{Au}/{\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ plasmonic sensors as a function of the device geometry is presented. Finally, devices with the optimum geometries for Au/Fe, $\mathrm{Au}/{\mathrm{Fe}}_{0.7}{\mathrm{Co}}_{0.3}$, and Au/Co bilayer structures are fabricated for TMOKE spectrum characterizations and index sensing experiments in water-based solutions. The experimental results are then compared with theoretical calculations.

${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ thin films were prepared by magnetron co-sputtering of Fe ($\mathrm{\varphi }=2^{\prime \prime }$, Alfa Aesa, 99.9%) and Co ($\mathrm{\varphi }=2^{\prime \prime }$, Alfa Aesa, 99.9%) targets on BK7 glass substrates at room temperature. Because of the high magnetization of Fe, to make sure Fe target and Co target can ignite under the same Ar pressure, the thickness of Fe target is 0.67 mm which is about one third of Co target’s 2 mm. Before the deposition process, the sputtering chamber was pumped down to a base pressure of $1\times {10}^{-4}\mathrm{Pa}$. For the best film quality, we controlled the Ar pressure to be 0.5 Pa for all depositions. The chemical composition of the thin films (thickness around 18 nm) were controlled by varying the sputtering power of the Fe and Co targets from 0 to 50 W, which yields ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ thin film compositions of $x=1.0$, 0.65, 0.47, 0.29, and 0 as determined by energy dispersive spectroscopy (EDS) and also calibrated by a standard ${\mathrm{Fe}}_{0.5}{\mathrm{Co}}_{0.5}$ sample with known stoichiometry. The crystal structures of all films were characterized by X-ray diffraction (XRD) on a Shimadzu XRD7000 X-ray diffractometer. For optical constant characterization, 12–18 nm ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ thin films were first deposited on BK7 glass substrates. Then an Au thin-film layer with thickness of 18 nm was deposited on top of ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ thin films by direct current (DC) sputtering at room temperature to form the $\mathrm{Au}/{\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ bilayer structure. The reflectivity spectrum as a function of the incident angle of a CW laser operating at 650 nm wavelength is collected with 0 applied magnetic fields. By applying the transfer matrix method and adopting a least square fitting procedure to the reflectivity spectrum, the optical constants of ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ thin films are obtained. In order to avoid the interference effect in thin films ($<100\mathrm{nm}$), optically opaque thin films (200 nm) of ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ are deposited at the same conditions as the bilayer structure for longitudinal magneto-optical Kerr effect (LMOKE) characterizations. The Kerr rotation angle and Kerr ellipticity for all thin films were characterized on a microscopic magneto-optical Kerr effect (MOKE) characterization system with incident laser wavelength of 650 nm at a fixed incident angle of 45°. The off-diagonal components and full dielectric tensor of ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ thin films at 650 nm wavelength were characterized by LMOKE and reflectivity characterizations.

The MOSPR sensor structure studied in this paper is shown in Fig. 1. A prism coupling setup with Kretschmann configuration is used to excite the SPR using the attenuated total internal reflection (ATR) method. An index matching refractive index liquid ($n=1.52$) is applied between the substrates and a BK7 glass prism. The prism and MOSPR sensor device is placed on a $\theta -2\theta$ rotation stage driven by step motors with an angle resolution of 0.001°. A semiconductor laser operating at 650 nm wavelength is incident from the prism side on the substrate/metal-thin-film interface. SPR is excited at a ${\theta }_{\mathrm{SPR}}$ angle when the phase matching condition is met. The magnetic field (2000 Oe) is applied perpendicular to the incident plane, i.e., along the $y$ axis in Fig. 1. The propagation constant of the SPR mode is modulated by the applied magnetic field, which leads to an enhanced TMOKE signal. The sensitivity of the MOSPR sensor is defined as $$S=\frac{\partial (R(+H)-R(-H))}{\partial \theta }\times \frac{\partial \theta }{\partial n}=\frac{\partial \mathrm{\Delta }R}{\partial n},$$(1)where $R(+H)$ and $R(-H)$ stand for the reflectivity of light at positive or negative applied magnetic fields, respectively, $\theta$ is the incident angle, and $n$ is the refractive index of the dielectric sensing medium. To evaluate the device sensitivity, the $4\times 4$ transfer matrix method was applied to study the reflectivity and TMOKE spectrum of $\mathrm{Au}/{\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ devices under in-plane applied magnetic fields ^[16], where the prism coupled $\mathrm{Au}/{\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ system is treated as a four layer system with the first layer to the fourth layer as prism (substrate), ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$, Au, and the dielectric sensing medium, respectively. The optimum sensor geometry with maximum sensitivity $S$ is determined by simulating Eq. (1) using the method described above and exploring the parameter space of Au and ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ thicknesses. This optimum device geometry is then fabricated for index sensing experiments and compared with the theoretical results.

Figure 2(a) shows the XRD patterns of ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ thin films ($\sim 18\mathrm{nm}$) deposited on BK-7 glass substrates in the $2\theta$ range of 40°–50°. A full range XRD scan for the ${\mathrm{Fe}}_{0.7}{\mathrm{Co}}_{0.3}$ thin film is also shown in the inset All ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ thin films are well crystallized. Because we deposit all samples at room temperature without any annealing process, the pure Co thin film shows the hexagonal close-packed (HCP) phase ^[17], whereas all ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ thin films with $x>0$ show the body-centered cubic (BCC) phase ^[18]. The ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ thin films with $0<x<1$ form a BCC phase, which is similar to reports in previous studies ^[19,20]. With increasing $x$, the characteristic (110) diffraction peak of the ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ thin films shifts to lower $2\theta$ angles, indicating a solid solution behavior.

To characterize the optical constants of ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ thin films in an $\mathrm{Au}/{\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ MOSPR device structure, we measured the reflectivity spectrum of the MOSPR transducers using prism coupling via the angular interrogation method. The Au and ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ thicknesses are both fixed at 18 nm for different samples. Figure 2(b) shows the reflectivity spectrum for Au/Fe, $\mathrm{Au}/{\mathrm{Fe}}_{0.7}{\mathrm{Co}}_{0.3}$, and Au/Co samples. Surface plasmon polaritons are excited in all three structures, as indicated by the broad SPR peaks at around 45°. We used the least square method to fit the experimental spectrum with the calculated spectrum by using the transfer matrix method by fixing the Au and ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ layer thicknesses and varying the ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ thin films’ optical constants, therefore obtaining the fitted $n$, $k$ values of ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ thin films at a 650 nm wavelength. The experimental and fitted reflection spectrum of the $\mathrm{Au}(18\mathrm{nm})/{\mathrm{Fe}}_{0.7}{\mathrm{Co}}_{0.3}$(18 nm) sample is shown in the inset of Fig. 2(b). Similarly, the diagonal component of the permittivity tensor of ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ and Fe are obtained as shown in Table 1. We noticed that Fe and ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ thin films showed higher optical loss than Co, which resulted in larger FWHM in the reflectivity spectrum. This trend is consistent with the optical constants of ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ thin films from literature reports ^[21,22], despite of a much thinner film thickness in our samples compared to previous studies.

To characterize the off-diagonal component of the ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ thin films, the LMOKE spectrum was measured on optically opaque (200 nm) ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ thin films deposited on glass substrates at 650 nm wavelength. Figure 3(a) shows room temperature LMOKE hysteresis of Co, ${\mathrm{Fe}}_{0.7}{\mathrm{Co}}_{0.3}$ and Fe thin films. The Kerr rotation and Kerr ellipticity for all samples are plotted in Fig. 3(b). The error bars are generated by accounting for the standard deviation of five measurements for each ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ sample. The Kerr rotation angle of ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ thin films first increases with $x$, peaks at ${\mathrm{Fe}}_{0.7}{\mathrm{Co}}_{0.3}$, and then decreases for the pure Fe. The Kerr ellipticity, on the other hand, reaches the maximum at the composition of ${\mathrm{Fe}}_{0.3}{\mathrm{Co}}_{0.7}$. The off-diagonal elements of the permittivity tensor can be calculated from the longitude Kerr rotation and ellipticity using the following equations ^[23]: $${\mathrm{\Theta }}_K=\mathrm{Re}\frac{(\pm r_{sp})}{r_{ss}(r_{pp})}=\mathrm{Im}\frac{\sin \phi {\eta }^2[\sin \phi \tan \phi \pm {({\eta }^2-{\sin }^2\phi )}^{1/2}]}{({\eta }^2-1)({\eta }^2-{\tan }^2\phi ){({\eta }^2-{\sin }^2\phi )}^{1/2}}Q,$$(2)$${\psi }_K=\mathrm{Im}\frac{(\pm r_{sp})}{r_{ss}(r_{pp})}=\mathrm{Re}\frac{\sin \phi {\eta }^2[\sin \phi \tan \phi \pm {({\eta }^2-{\sin }^2\phi )}^{1/2}]}{({\eta }^2-1)({\eta }^2-{\tan }^2\phi ){({\eta }^2-{\sin }^2\phi )}^{1/2}}Q,$$(3)where ${\mathrm{\Theta }}_K$ is the magneto-optical Kerr rotation angle, ${\psi }_K$ is the magneto-optical Kerr ellipticity, and $\eta =n_2/n_1$ represents the complex refraction index ratio between air ($n_1$) and ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ ($n_2$). The $r_{sp}$, $r_{ss}$, and $r_{pp}$ terms are reflectivity of $s$-polarized, $s$-polarized, and $p$-polarized light, with $p$-polarized, $s$-polarized and $p$-polarized incident light, respectively. $\phi$ is the incidence angle with respect to the film surface normal, and $Q$ is the Voigt constant, which was solved from the equations. Using the measured MOKE angle, ellipticity and the thin-film optical constants, the off-diagonal elements can be calculated as $${ϵ}_{xz}=i*{ϵ}_{xx}*Q.$$(4)

The result is also shown in Table 1. For both the real and imaginary part of the off-diagonal component of the permittivity tensor, they first increase with Fe concentration, maximize at ${\mathrm{Fe}}_{0.7}{\mathrm{Co}}_{0.3}$, and then decrease for pure Fe. This trend agrees with the literature reports on epitaxial ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ thin films ^[15].

Next we evaluate the sensitivity of $\mathrm{Au}/{\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ MOSPR transducers as index sensors. The device sensitivity is first simulated numerically according to Eq. (1) in the parameter space of Au and ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ thin-film thicknesses. A typical simulation result for the Au/Co transducer is shown in Fig. 4(a). Optimum device geometry is observed with Co and Au layer thicknesses of 13 and 38 nm, respectively. This observation can be understood as a result of the trade-off between the MO effect and the optical absorption in $\mathrm{Au}/{\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ devices ^[6]. In the case of Fe and ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$, the optimum Au layer becomes a little thicker, which is due to the higher loss and higher index in Fe rich ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ thin films. The maximum sensitivity of each $\mathrm{Au}/{\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ MOSPR transducer is shown in Fig. 4(b); the $\mathrm{Au}/{\mathrm{Fe}}_{0.7}{\mathrm{Co}}_{0.3}$ device show the highest sensitivity of $0.385\pm 0.008{\mathrm{RIU}}^{-1}$, mostly due to its highest MO effect. The error bar in the sensitivity value is from the error bar in determination of the ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$’s off-diagonal component.

The optimum device structure for Au/Fe, $\mathrm{Au}/{\mathrm{Fe}}_{0.7}{\mathrm{Co}}_{0.3}$, and Au/Co devices are then fabricated for index sensing experiments. The reflectivity spectra of all three devices are characterized as a function of the light incident angle, as shown in Fig. 5(a) by the ATR method measured in water ($n=1.332$). The spectra feature reflectivity minima at around 74°, which corresponds to SPR excitation in all MOSPR transducers. The simulated reflectivity spectrum agrees with the experimental spectrum very well. By measuring the $R(+H)-R(-H)$ spectrum as a function of incidence angle, clear TMOKE enhancement due to SPR excitation is observed for all devices, as shown in Fig. 5(b). The numerical simulation results using the material optical constants shown in Table 1 also agree well with the experimental spectra. The highest TMOKE is observed in the $\mathrm{Au}/{\mathrm{Fe}}_{0.7}{\mathrm{Co}}_{0.3}$ sample, which is consistent with the strongest magneto-optical effect observed in this material. Index sensing performance of the three devices of Au/Fe, Au/Co, and $\mathrm{Au}/{\mathrm{Fe}}_{0.7}{\mathrm{Co}}_{0.3}$ are characterized, with the example TMOKE spectrum of the $\mathrm{Au}/{\mathrm{Fe}}_{0.7}{\mathrm{Co}}_{0.3}$ transducer as a function of the solution index of refraction, as shown in Fig. 5(c). With increasing the solution index, the TMOKE spectrum gradually shifts to higher angles, which is due to the larger effective index of the excited SPR modes. As shown in Fig. 5(d), all three MOSPR devices show good linearity as a function of the solution index for a large range of refractive indices. The highest sensitivity of $0.306\pm 0.008{\mathrm{RIU}}^{-1}$ is observed in the $\mathrm{Au}/{\mathrm{Fe}}_{0.7}{\mathrm{Co}}_{0.3}$ device, while lower sensitivity values of $0.268\pm 0.007$ and $0.153\pm 0.015{\mathrm{RIU}}^{-1}$, respectively, are observed in the Au/Fe and Au/Co transducers. Compared to Au/Co transducers, the $\mathrm{Au}/{\mathrm{Fe}}_{0.7}{\mathrm{Co}}_{0.3}$ device shows about 2 times higher index sensitivity, therefore demonstrating its promising potential for chemical and biomedical sensing applications.

In summary, in this paper we report the enhanced TMOKE and refractive index sensitivity of $\mathrm{Au}/{\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ bilayer MOSPR transducers. The optical and magneto-optical properties of ${\mathrm{Fe}}_x{\mathrm{Co}}_{1-x}$ thin films are characterized as a function of the Fe concentrations. Both numerical simulations and experimental results show that the device sensitivity is strongly related to the composition of the magneto-optical alloy thin films. For the film compositions studied in this paper, the highest device sensitivity, of $0.306\pm 0.008{\mathrm{RIU}}^{-1}$, is observed in $\mathrm{Au}/{\mathrm{Fe}}_{0.7}{\mathrm{Co}}_{0.3}$ transducers. This is about 2 times higher than the Au/Co transducers, and is strongly related to the enhanced magneto-optical properties of the ${\mathrm{Fe}}_{0.7}{\mathrm{Co}}_{0.3}$ thin films. Our study shows that the sensitivity of the MOSPR device is strongly related to the optical and magneto-optical properties of the magnetic metal thin films. The high sensitivity of the $\mathrm{Au}/{\mathrm{Fe}}_{0.7}{\mathrm{Co}}_{0.3}$ MOSPR transducer makes it a promising candidate for chemical and biomedical sensing applications.