Tunable Magnetic and Optical Anisotropy in ZrO2‐Co Vertically Aligned Nanocomposites

Metamaterials have gained great research interest in recent years owing to their potential for property tunability, multifunctionality, and property coupling. As a new group of self‐assembled thin films, vertically aligned nanocomposite (VAN)‐based hybrid metamaterials have been demonstrated with significant anisotropic physical properties and a broad range of property tailorability, such as optical anisotropy, magnetic anisotropy, hyperbolic dispersion, and enhanced second harmonic generation properties. Herein, self‐assembled ZrO2‐Co nanocomposite films, with high epitaxial quality and ultra‐fine vertically aligned Co nanopillars (with an average diameter of ≈2 nm) embedded in a ZrO2 matrix, are fabricated using a pulsed laser deposition (PLD) method. The Co pillar density can be effectively tuned by varying the Co concentration in the target, which results in tunable optical properties and magnetic properties. Specifically, a high saturation magnetization of 100 emu cm−3, strong out‐of‐plane magnetic anisotropy and tailorable magnetization properties are achieved via tuning the Co nanopillar density. Coupled with hyperbolic dispersion of dielectric constant from 950 to 1500 nm in wavelength, plasmonic Co metal nanopillars, and the unique dielectric ZrO2 matrix, this new nanoscale hybrid metamaterial shows great potential for future integrated optical and magnetic device designs.


Introduction
Functional materials have attracted enormous interests in the past decades with the focuses on discovering new material systems, tunable materials physical properties, and well coupled multifunctionalities. These functional materials find various applications in different fields, such as electronic devices, magnetic data storage media, spintronics, multiferroics, and optical device designs. [1,2,3,4] Since the number of naturally existing multifunctional materials is limited, engineered metamaterials are considered to be a viable material group to achieve enhanced physical properties, strong coupled functionalities, and unique device functions, including multiferroic properties, e.g., combined ferroelectricity [5] and ferromagnetism, [6] hyperbolic optical dispersion, [7] magneto-optical coupling, [8] and optical magnetism. [9] Various methods have been demonstrated for the fabrication of functional metamaterials, including e-beam lithography, [10] membrane projection lithography, [11] and physical vapor deposition (PVD). [12] As an alternative group of metamaterials processing methods beyond nanolithography and patterning methods, direct selfassembled two-phase or three-phase nanocomposites, e.g., vertically aligned nanocomposites (VAN) structures, have been demonstrated with various materials integrated to achieve unique properties, including oxide-oxide, oxide-metal, and nitride-metal systems. [13,14,15,16] Pulsed laser deposition (PLD) has been used in fabricating many of these epitaxial hybrid nanocomposite thin films. [17,18,19,20,21] Several metals, such as gold (Au), nickel (Ni), iron (Fe), and copper (Cu), have been explored for their unique properties when coupled with nitrides or oxides in the VAN thin film form. [7,22,23,24] Co is a well-known ferromagnetic material with a large coercivity (H c ) and a high Curie temperature (T c = 1388 K), [25,26] and is a plasmonic metal. [27] Considering the unique ferromagnetic, plasmonic and coupled properties with great device potentials, [28,29,30,31] vertically aligned Co-based nanocomposites could be ideal for exploration. Despite its interesting multifunctionalities and device potential, Co is one of the least studied metals in the VAN thin film form with limited success, e.g., ferromagnetic BaZrO 3 -Co VANs, [32] multifunctional Co-CeO 2 -BaTiO 3 systems. [33] This is largely due to the challenges in its chemical instability and co-growth with other matrix materials.
In this work, we demonstrate the growth of self-assembled ZrO 2 -Co VAN metamaterials using PLD. Co is a reactive metal, which can be easily oxidized to form cobalt oxide. To stabilize the Co metal phase during growth, the deposition condition and matrix material selection are crucial. Herein, ZrN is selected as the target material to form the ZrO 2 matrix and to absorb oxygen in the background to maintain metallic Co phase. Previously, Tung et al. has demonstrated the oxidation phenomenon of ZrN oxidized to be ZrO 2 even annealing under vacuum due to the oxygen residue. [34] It is also known from Ellingham diagram that the Gibbs free energy of formation of zirconium oxide (ZrO 2 ) is significantly lower than that of cobalt oxide (either CoO or Co 3 O 4 ). [35] The higher oxygen affinity of Zr may cause the preferential oxidation of ZrN compared to Co. ZrO 2 is selected considering that it is a high-k dielectric material, which can be widely applied in microelectronic device design, such as dynamic random access memories (DRAM) and complementary metal-oxide semiconductor (CMOS). [36,37] As shown in Figure 1, we propose to optimize the growth condition to achieve epitaxial Co nanopillars in the ZrO 2 matrix, and tune the Co pillars planar density from low to high by adjusting the Co concentration in the targets. It is noted that a thin TiN buffer is applied to effectively establish the epitaxial growth of ZrO 2 and Co and also act as an effective diffusion barrier for the Co phase during high temperature growth. The crystallinity and physical properties of the ZrO 2 -Co VAN films with different Co concentrations were explored and correlated to demonstrate the potential of tunable magnetic and optical properties in the new magnetic and plasmonic hybrid metamaterial.

Structure Analysis by X-ray Diffraction (XRD)
First, the crystallinity of the thin films was characterized using -2 XRD scans. Figure 2 shows the -2 XRD scans of the films with different Co concentrations all deposited under 700°C. It is clear that all the films show obvious (00l) texture for the ZrO 2 film and the TiN buffer layer on SrTiO 3 (STO) substrates. The results confirm that the deposited film was oxidized to be ZrO 2 during the deposition process. Further high-resolution scanning transmission electron microscopy (HRSTEM) image analysis proves the matrix became largely monoclinic ZrO 2 . A similar nitride-tooxide conversion phenomenon was reported in a previous work of HfO 2 -Au VAN system. [38] It is noted that ZrO 2 (002) peak (d(002) = 0.258 nm, 2 = 34.52°) is at a slightly lower angle than the bulk monoclinic ZrO 2 (002) (d(002) = 0.253 nm, 2 = 35.30°), which could result from the compressive strain at the interface of TiN/ZrO 2 . Co peaks were not obvious due to the relatively low concentration in the film. As a comparison, the XRD -2 data for the films deposited under 600 and 800°C are shown in Figure  S1 (Supporting Information) shows the -2 XRD scans of the  film deposited under 600 and 800°C. No obvious ZrO 2 peaks can be identified in the 600°C sample, suggesting poor film crystallinity, while the film deposited under 800°C shows a sharp ZrO 2 peak suggesting very good crystallinity. In short, the XRD data suggests that the ZrO 2 -Co film can be successfully deposited on STO (001) substrate with TiN as a buffer layer. Good film crystallinity can be achieved under the deposition temperature higher than 700°C.

Morphology of the ZrO 2 -Co VANs by TEM/STEM
Since the XRD results are very similar for all the films with different Co concentrations, detailed TEM/STEM analysis was conducted on the films on both plan-view and cross-section samples to better compare their differences. A typical STEM/EDS composite image, a TEM bright-field cross-sectional image and the corresponding selected area electron diffraction (SAED) pattern of the medium concentration sample are shown in Figure 3a-c, respectively. The total film thickness is around 80 nm. Ultra-fine straight Co pillars are well aligned inside the ZrO 2 matrix. The indexed diffraction pattern in Figure 3c shows nearly epitaxial growth quality of monoclinic ZrO 2 and Co with the substrates and buffer layers, which is consistent with the XRD result. As shown in Figure S4 (Supporting Information), as a comparison, the unbuffered ZrO 2 -Co film on STO shows more blurred diffraction pattern, which proves that the existence of TiN buffer can improve the film epitaxial quality. However, the diffraction spot of Co can hardly be identified due to relatively small size of Co pillars. As a comparison, the cross-sectional EDS mappings of films deposited under 600 and 800°C are shown in Figure S2 (Supporting Information). Co pillars can be identified inside the film deposited under 600°C, but the diffraction pattern indicates that the crystallinity of the film is worse than that of the films deposited under 700 and 800°C. However, the Co pillars in the film deposited under 800°C did not grow well in the ZrO 2 matrix, despite having good crystallinity, and most Co dispersed randomly inside the film after a certain thickness. Therefore, 700°C was identified as an optimized temperature for the ZrO 2 -Co VAN growth.
The plan-view energy-dispersive X-ray spectroscopy (EDS) mappings of ZrO 2 -Co films with low, medium, and high Co concentrations were shown in Figure 3d-f, respectively. It can be observed that the diameters of the Co pillars hardly change as the Co concentration increases. However, the planar density of the Co pillars increases as the Co concentrations increase, which can lead to tunable physical properties. Specifically, the average diameter of the Co pillars is estimated to 1.9, 2.0, and 2.0 nm, and the pillar density increases from 5600/μm 2 , 7000/μm 2 , to 10 400/μm 2 , for the low, medium, and high concentration samples, respectively.
To further understand the interfacial structure of the film stack and their epitaxial relationship, high-resolution STEM analysis was performed on the ZrO 2 -Co films deposited under 700°C with the medium Co concentration. Cross-sectional HRSTEM images of both the ZrO 2 /TiN interface and the TiN/STO interface were shown in Figure 4. Based on the HRSTEM images, the interface epitaxy was analyzed with fast Fourier transfer (FFT) process to reveal the interfacial plane matching relationship. As shown in Figure S8 (Supporting Information), the dspacing of the film was calculated to be 5.09 Å based on the HRSTEM images, which is close to the bulk monoclinic ZrO 2 lattice parameter 5.13 Å. This result also indicates the ZrN is mostly oxidized to be monoclinic ZrO 2 . It is also noted that, later in the optical measurements, there is a metallic nature at near-IR region which suggests that some ZrN phase remains inside the ZrO 2 phase. As shown in Figure 4a,b, there is an obvious domainmatching epitaxy relationship between the ZrO 2 layer and TiN layer (i.e., ZrO 2 (020): TiN (020) = 5:6) evidenced by the ordered misfit dislocation array identified at the interface as red T signs). Such domain matching relationship can significantly reduce the lattice mismatch between two layers. Since the lattice constant of ZrO 2 and TiN is 5.13 and 4.24 Å respectively, the lattice mismatch after the domain-matching epitaxy of ZrO 2 (020) and TiN (020) can be calculated as: which is much smaller than the estimated bulk lattice mismatch of 19.0% before domain matching. Meanwhile, this positive number also indicates the compressive strain at the interface of TiN/ZrO 2 , which agrees with the previous XRD results.
The compressive strain at the interface of TiN/ZrO 2 is based on the domain machine epitaxy relationship, i.e., the lattice parameter for TiN and ZrO 2 is 4.24 and 5.13 Å, and 5 of ZrO 2 lattice (5* 5.13 Å = 25.65 Å) is slightly larger than 6 of TiN lattice (6*4.24 Å = 25.44 Å). Interestingly, at the interface between the TiN layer and STO layer, the domain-matching epitaxy was also observed. As shown in Figure 4c,d, the domain-matching epitaxy was identified as TiN (020): STO (020) = 12:13, which can reduce the lattice mismatch from 8.35% to 0.35%. It can be also observed that there is some lattice distortion at the first three layers of the TiN layer showing as dark contrast area at the interface. As shown in Figure S4 (Supporting Information), the ZrO 2 -Co directly deposited on the STO substrate shows much worse crystallinity and randomized Co pillars, which indicates the TiN buffer layer plays an important role to reduce the strain between the ZrO 2 layer and the substrate and facilitate the epitaxial growth of the ZrO 2 -Co VANs.
To investigate the orientation relationship between the Co pillars and the ZrO 2 matrix, more detailed analysis on the planview TEM/STEM images was conducted. The high-angle annular dark-field (HAADF) STEM images in Figure 5a,b show the distribution of Co nanopillars in the ZrO 2 matrix. The Co pillars and ZrO 2 matrix can be clearly distinguished by the HAADF image considering the contrast is proportional to Z 1.7 , i.e., the brighter contrast of Co versus the darker contrast in ZrO 2 . The Co nanopillars were uniformly dispersed inside the ZrO 2 matrix. From the high-resolution STEM image in Figure 5b, Co was confirmed to be metallic phase in FCC crystal structure with the out-of-plane growth orientation either in [110] or [100], and the ZrO 2 matrix is in [001] out-of-plane, which matches with the XRD and TEM SAED results. It is hypothesized that, since the Zr ions grabbed oxygen and formed ZrO 2 during deposition, the Co can remain its metallic phase rather than being oxidized. It is worth noting that there are domain structures in the ZrO 2 matrix corresponding to 90°rotation about its [001] axis. The atomic structure drawings of the domain structures are shown in Figure 5b, and one of the domain boundaries was marked with an orange line. The 4-domain variants are expected and are equivalent, due to the symmetry difference in the monoclinic structure of ZrO 2 . This is corresponding to a-axis of the monoclinic phase parallel to xaxis (i.e., +x. and −x, two directions) and y-axis (i.e., +y. and -y, two directions) of the cubic phase, respectively. It agrees well with the indexed SAED pattern in Figure 3c. The strain from the asymmetric monoclinic structure can be compensated by such in-plane rotated domain structure, and the existence of the domain structure also explains the high-quality epitaxial growth of the monoclinic ZrO 2 . Further EDS mapping and line-scan analysis shown in Figures 5c,d, and 4e, further confirm very thin diameter of the Co nanopillars, ≈2 nm, which is smaller than most of the previously reported Co pillars in VAN structures and others structures. [32,33,39,40] Adv. Mater. Interfaces 2023, 10, 2300150

Magnetic Properties of the ZrO 2 -Co VANs
As mentioned above, ferromagnetic Co presents great potential in various applications, such as magnetic data storage and spintronic devices. To investigate the magnetic properties of the ZrO 2 -Co films, magnetization measurements were conducted for all the samples. The M-H measurements with applied magnetic field either in-plane (IP, i.e., field parallel to the substrate) or out-of-plane (OP, i.e., field perpendicular to the substrate) direction under 10 K, as shown in Figure 6a,b, respectively, and the correction factors were applied to the obtained data. The hysteresis loops show strong ferromagnetic behavior and magnetic anisotropy. Specifically, coercive field values of the films deposited with the same Co concentrations in OP are larger than those in IP. This is due to the unique VAN structure with highly OP aligned Co pillars embedded in the dielectric ZrO 2 matrix, which leads to strong OP magnetic anisotropy. It can also be observed that the saturation magnetization values in both in-plane and out-of-plane direction increase as the Co pillars density increases. As the saturation magnetization is correlated to the amount of the Co pillars, tunable magnetic properties of ZrO 2 -Co VAN are achieved effectively by adjusting the Co target concentration and Co pillar density. The room temperature M-H loops are shown as Figure 6c,d. Similarly, the ZrO 2 -Co films show very strong OP magnetic anisotropy with stronger coercive field in OP than those in IP.

Optical Properties of the ZrO 2 -Co VANs
Considering the plasmonic nature of Co, the optical properties of the ZrO 2 -Co films were also measured via optical transmittance and ellipsometry measurements. The dielectric constants were fitted and presented in Figure 7. Because of the anisotropic structure of the ZrO 2 -Co VAN film, the obtained data was modeled as in-plane (ɛ ∥ ) and out-of-plane (ɛ ⊥ ) components using the general oscillator models (Kramers−Kronig model). A hyperbolic region between 950 and 1500 nm could be observed for the high concentration sample, where the out-of-plane permittivity is negative while the in-plane permittivity is positive. Such hyperbolic region is tuned systematically to 1050-1500 nm, and 1100-1500 nm for the medium and low concentration samples, respectively. It is obvious that the out-of-plane (ɛ ⊥ ) components of the dielectric constant becomes more negative as the Co concentration increases, suggesting a Type I hyperbolic behavior. The anisotropic optical property makes the ZrO 2 -Co film as an ideal hyperbolic metamaterial for different optical applications. The transmittance data of the ZrO 2 -Co film with different Co concentrations was plotted in Figure S5 (Supporting Information), and two absorption valleys can be visible around 510 and 480 nm in both samples, which can result from Co nanopillars. [27] It can be observed that the absorption valleys in high concentration sample is more clear due to the higher Co concentration.
There are several unique properties presented by this new ZrO 2 -Co VAN metamaterial system. First, the as-deposited ZrO 2 -Co thin films show good epitaxy, and Co remains metallic during the high temperature growth because of the unique matrix material selection. Second, because of the ultrafine Co nanopillars in ZrO 2 matrix, the films show strong anisotropic magnetic and optical properties, which make it as ideal as a hybrid metamaterial. Comparing this Co-VAN system with the prior Co-based ones, much thinner and ordered Co pillars of 2 nm in diameter were achieved compared to previous reported ones with 5 nm diameter. [32,36] Third, the magnetic and optical properties can be tuned by adjusting Co concentrations. Such strong structural and property anisotropy and tunability have not been demonstrated previously. [41,42] It is worth noting that the Co pillars did not directly nucleate on the TiN buffer layer. Instead, a very thin multilayer structure at the initial growth interface forms and then grows into VAN structure. Especially in the film deposited under 600°C, there were more multilayer shown in the beginning of the deposition. It suggests that the deposition temperature and initial interfacial strain play important roles in the nucleation and growth of Co-nanopillars in the system. Further research can be conducted to understand the growth mechanisms which are critical for achieving highly ordered Co-based VANs and other metal-VAN metamaterials.

Conclusion
In summary, ZrO 2 -Co nanocomposite thin films with ultra-thin Co nanopillars of ≈2 nm has been successfully deposited on STO substrates by using a ZrN target. During the PLD process, ZrN was oxidized to be ZrO 2 during the co-growth of ZrN-Co, and Co remain to be metallic nanopillars. The as-deposited films show good epitaxy and crystallinity with well-aligned Co nanopillars. The ZrO 2 -Co VAN films show tunable magnetic properties and strong out-of-plane anisotropy in both room temperature and 10 K, as well as strong optical anisotropy with Type I hyperbolic behavior. This work paves a new way to fabricate Co-based hybrid VAN metamaterials with strong anisotropic structure and integrated optical and magnetic properties for future devices. The findings of structural and property tunability can be extended to many other metal-VAN systems.

Experimental Section
Thin Film Growth: The self-assembled thin films were deposited under vacuum using pulsed laser deposition (with a KrF excimer laser, = 248 nm). The ZrO 2 -Co thin film was fabricated with a two-step growth using a TiN buffer layer to reduce the strain. Firstly, TiN layer was directly deposited on the single crystal STO (001) substrate with a TiN target. Then the ZrO 2 -Co layer was deposited on the top of the TiN buffer layer with a ZrN/Co target. All the films with different Co concentrations were deposited under vacuum at a series of temperatures 600, 700, and 800°C.
Structure and Physical Properties Characterization: The microstructure of the films was characterized using X-ray diffraction (XRD, PANalytical Empyrean), Transmission Electron Microscopy (TEM), and Scanning Transmission Electron Microscopy (STEM) (FEI TALOS 200X operated at 200 kV, and FEI Titan G2 80-200 STEM with a Cs probe corrector and ChemiSTEM technology, operated at 200 kV), and STEM electrondispersive X-ray spectroscopy (EDS). The dielectric permittivity of the films was measured using a spectroscopic ellipsometer (JA Woollam RC2). The obtained data was modeled as in-plane (ɛ ∥ ) and out-of-plane (ɛ ⊥ ) components using the general oscillator models to make them Kramers−Kronig consistent. The magnetic response was measured using Quantum Design MPMS-3 SQUID magnetometer.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.