Tunnel-Type Magneto-Dielectric Effect and Its Annealing Study in Co–SiO2 Granular Films
2018 Volume 59 Issue 4 Pages 585-589
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2018 Volume 59 Issue 4 Pages 585-589
Tunnel-type magneto-dielectric (TMD) effect arising from the spin-dependent quantum tunneling between nano-sized granular pairs, has opened up new route for room temperature magnetoelectric fields. We first investigated the TMD properties in metal-oxide (Co–SiO2) granular films and their annealing effect in this work. Results show that the films exhibit a TMD ratio ($\Delta \varepsilon '/\varepsilon '_{0}$) of 1% with high electrical resistivity of >108 µΩ·m and intermediate optical transmittance in Co0.24–(SiO2)0.76 films. Annealing investigations suggest that the samples remain TMD response up to 573 K, and further increment in annealing temperature leads to the inter-diffusion between Co and SiO2 interfaces, thus producing the increasing oxidation of metallic Co. This study demonstrates the possibility of TMD effect in metal-oxide composite materials, and may be desirable for a variety of other wide oxide-based candidates for magnetoelectric device applications.
Fig. 3 (a) Frequency dependence of the real part of dielectric constants for x = 0.24, with and without the application of magnetic field H = 800 kA/m; inset presents the detail of dielectric difference $\Delta \varepsilon ' = \varepsilon '_{\text{H}} - \varepsilon '_{0}$ between dielectric constants ε′ with and without magnetic field. (b) Frequency dependence of the magneto-dielectric response $\Delta \varepsilon '/\varepsilon '_{0}$ for various x values from 0.12 to 0.24; insets show the pictures of films deposited on quartz (SiO2) substrate, which may reflect their optical transmittance of 450-nm-thick films with different Co concentration from 0.12 to 0.27.
Over the last few decades, the magnetoelectric materials has attracted considerable attention due to their correlation among the polarization, magnetization and stress, which may lead to a broad range of multi-functional magnetoelectric applications.1–5) On the other hand, the ease of adjusting functionality of granular nanostructure including electronic, magnetic, optical properties, has assured them an important role for practical applications and makes them most suitable for fundamental studies of disordered solids.6) As the metallic magnetic granule dimension falls to several nanometers, quantum tunneling effect among granules appears possible and it may be spin-dependent. In this regards, one of the typical examples is the widely studied tunnel-type magnetoresistance (TMR) effect in this structure.7–9) Generally, the TMR effect in magnetic granular materials is observed in the granule content fractions of 0.3–0.5 due to its super-paramagnetism, wherein each nanometer-sized granule remains single domain structure.
As further reducing the granule content to 0–0.3, continuous quantum tunneling becomes impossible due to the increase in inter-granular distance; within this content range, nevertheless, the change in dielectric properties upon the application of magnetic field, named as tunnel-type magneto-dielectric (TMD) effect, has been discovered, offering new possibility for the room temperature magnetoelectric fields.10,11) Compared with other MD composites,4,5) the nano-granular films are chemically stable and easily fabricated, and the TMD properties may be theoretically realized in several granular pairs, indicating that the device structure may be greatly simplified to nanoscale size, which may represent an advantage from the viewpoint of application in devices including tunable filters, antennas and magnetic sensors, etc. The underlying physics has been explained by the spin-dependent charge oscillation between two neighboring magnetic granules through insulator matrix, wherein significant numbers of granular pairs served as electric dipoles, are responsible for the magneto-capacitance (magneto-dielectric, MD) effect with the application of ac bias voltage and magnetic fields. Typically, a peak dielectric change $\Delta \varepsilon '/\varepsilon '_{0}$ is observed at a specific frequency, denoted as fTMD, as frequency f increases.10) Further, the granule concentration dependence on the position variation of fTMD in Co–MgF2 films has been both experimentally and theoretically interpreted in terms of the structure change including the intergranular distance and its distribution with increasing granule concentration.12) To enhance the TMD response, many efforts have been devoted by means of improvement in either films structure or experimental design.13,14) Structurally, by artificially constructing a two-dimensional (2D) Co/AlF granular structure, the balanced control of ferromagnetism and super-paramagnetism has been achieved, which is of key importance in producing the low-field TMD enhancement.13) Experimentally, we addressed this issues by designing a novel co-separate sputtering method, which is pivotal to deposit the granular structure with clear granule-ceramic boundary to achieve improved TMD effect.14) Whereas these film preparations have so far been limited in metal-fluoride material systems and choices of the fluorides, e.g. AlF3, MgF2 and CaF2, are so few and they are often toxic substances, which has thus far redistricted their environmental-friendly use of TMD films in device application. In contrast to the metal-fluoride systems, however, search for common and as-yet-unrealized metal-oxide systems with TMD effect is pressing and has not been reported yet. In addition, thermal stability will be crucial for understanding the TMD films failure and mechanism operated at relative high temperatures.
Here, we have selected ferromagnetic Co as granules and common SiO2 ceramic as insulating matrix because of its good chemical stability. Noted that the Co–SiO2 granular nanomaterials have been used for investigating various phenomena, including magnetooptical Kerr effect,15) TMR effect,16) and Hall effect,17) nevertheless, studies about the TMD effect in this composite are none. In this study, we report the first use of Co–SiO2 granular nanomaterials with maximum TMD ratio of 1% at room temperature and its annealing study was conducted.
The Co–SiO2 films were deposited on Si (100), quartz (SiO2) and Pt/Ti/SiO2/Si substrates at an Ar gas pressure of 0.65 Pa by co-sputtering both Co and SiO2 targets. The substrate was rotated at a speed of 11 rpm to achieve a homogeneous granular state. A small fraction of the Si/SiO2/Ti/Pt substrate was covered during deposition for allowing access to the bottom electrode for TMD measurement. After deposition, the films were transferred to a vacuum chamber for annealing at different temperatures (373–773 K) for 1 h at Ar protective atmosphere.
The Co granule atomic concentration (x) were determined by an X-ray fluorescence spectrometer (XRF), revealing that the Si-oxide matrix possesses a stoichiometric SiO2 composition. The dc electrical resistivity (ρ) of all the as-deposited samples were measured by a conventional four-point probe method. The structure was examined using field-emission transmission electron microscopy (FE–TEM). The composition and chemical state of the selective films was confirmed by X-ray photoelectron spectroscopy (XPS). Dielectric and TMD properties were measured by an inductance, capacitance, and resistance (LCR) meter in the frequency range of 1–1000 kHz, with a dc magnetic field up to 800 kA/m. All the measurements were performed at room temperature.
For sustaining the applied ac electric field in the TMD measurement, high electrical resistivity (ρ) is necessary. First, we have summarized the ρ of Co–SiO2 films with increasing Co granule concentration using different matrix systems, including Al-fluoride (AlF)14) and Al-nitride (AlN)18) as compared in Fig. 1. A gradual decrease of ρ is observed as x increases over the measured x range. The ρ of Co–SiO2 films for x < 0.5 reached up to 107 µΩ·m, three orders of magnitude greater than that of the Co–AlF films, and likewise, ρ of the latter is also much larger than that of Co–AlN films; this indicated that the selection of matrix are of key importance in determining the ρ of as-deposited films. Our results have manifested that the Co–AlN films has no TMD response due to its low ρ and unclear granular interface between granules and matrix. Therefore, choosing oxide as matrix represents a potential advantage compared with fluoride and nitride material systems.
The cross-section images of the Co–SiO2 films for x = 0.24 are shown in Fig. 2. The dark dots and bright regions in Fig. 2(a) correspond to the Co granules and Si–O matrix, respectively, indicating that the films possess a homogeneous granular structure with no pores. The magnified image of selected areas suggests that the films appears to show a hetero-amorphous structure in Fig. 2(b), that is, amorphous metallic Co granules with a diameter of around 2 nm are dispersed in amorphous Si–O matrix, as indicated by the arrows. The amorphous state has been confirmed by the selected area electron diffraction pattern in the inset of Fig. 2(b).
(a) Cross-section TEM photographs of Co–SiO2 films with granule atomic concentration of 0.24 and (b) the magnified image. The right bottom inset shows the selected area electron diffraction.
Before the investigation of TMD properties, the frequency dependence of real part of dielectric properties for x = 0.24 with and without magnetic field H, is shown in Fig. 3(a). The real part of dielectric constant $\varepsilon '_{0}$ is greatly enhanced over the measured frequency f range compared with that (∼4) of SiO2 ceramic; this dielectric increase is due to the increasing numbers of electric dipoles in the form of granule pairs.10) Meanwhile, $\varepsilon '_{0}$ decreases dramatically by a dielectric relaxation frequency as f increases. Applying H = 800 kA/m to the films produced a dielectric enhancement; the changes in $\varepsilon '_{0}$, denoted as $\Delta \varepsilon '$, are shown in the inset of Fig. 3(a). The calculated TMD ratio $\Delta \varepsilon '/\varepsilon '_{0}$ for x = 0.24 has been shown in Fig. 3(b), reaching a maximum value of 1% at f = 200 kHz. In addition, one can observe that the TMD ratio shows an increase of a peak $\Delta \varepsilon '/\varepsilon '_{0}$ from 0.01% to 1% over the whole frequency range; this TMD enhancement is caused by the improvement in structural homogeneity as x increases, with more single relaxation time of granule pairs approaching the characterized τr0.12) The specific frequency with a peak TMD ratio, denoted as fTMD, shifts toward higher frequency; this is due to the decrease in the intergranular distance as x increases. On the other hand, similar with the amorphous quartz (SiO2) substrate with high transparency, the SiO2 amorphous matrix should be completely transparent as well, thus the granule concentration dependence on the change in the optical transmittance of thin films on quartz (SiO2) substrate with thickness of 450 nm, are shown in the inset of Fig. 3(b). A gradual decrease in optical transparency for visible light is observed as increasing x, indicating that the TMD films with intermediate transmittance may have practical merits in optically transparent magnetoelectric device application.
(a) Frequency dependence of the real part of dielectric constants for x = 0.24, with and without the application of magnetic field H = 800 kA/m; inset presents the detail of dielectric difference $\Delta \varepsilon ' = \varepsilon '_{\text{H}} - \varepsilon '_{0}$ between dielectric constants ε′ with and without magnetic field. (b) Frequency dependence of the magneto-dielectric response $\Delta \varepsilon '/\varepsilon '_{0}$ for various x values from 0.12 to 0.24; insets show the pictures of films deposited on quartz (SiO2) substrate, which may reflect their optical transmittance of 450-nm-thick films with different Co concentration from 0.12 to 0.27.
The magnetic field dependence of $\Delta \varepsilon '/\varepsilon '_{0}$ at f = 100 kHz is shown in Fig. 4. A dielectric increase of 0.8% is observed upon the increase of applied magnetic field to 800 kA/m, indicating that the spin-dependent tunneling probability increases between granule pairs. As the oscillation rate of charge carrier is proportional to the average angle of all pairs of granules, and may be denoted as the square of the normalized magnetization (M/Ms),19) herein, the (M/M5k)2 (solid curve) was plotted to fit the field dependence on TMD response. The agreement between the TMD response and (M/M5k)2 curves suggests that the increase in charge oscillation rate is caused by the change in the magnetization state of Co granules distributed in SiO2 matrix.
The magnetic field dependence of $\Delta \varepsilon '/\varepsilon '_{0}$ at f = 100 kHz for x = 0.24. The solid curve shows the normalized magnetization (M/Ms)2, where the Ms is the magnetization value measured at H = 400 kA/m.
The annealing temperature Ta dependence of $\Delta \varepsilon '/\varepsilon '_{0}$ for x = 0.24 is shown in Fig. 5. It is seen that the film almost remains a constant $\Delta \varepsilon '/\varepsilon '_{0}$ of 0.8%, showing thermal stability up to 573 K. Further increasing the Ta to 673 K led to the absence of TMD response. The picture of post-annealed samples has been shown in the inset of Fig. 5, suggesting that there is no appreciable change in color until Ta = 673 K. The film color annealed at Ta = 673 K changed to green and there are some peeling defects by Ta = 773 K (not shown here). This annealing study provides reference for failure analysis of TMD films operated at relative high temperatures.
Annealing temperature dependence of $\Delta \varepsilon '/\varepsilon '_{0}$ for x = 0.24. Insets present their corresponding pictures of post-annealed samples holding in different temperatures on substrates with bottom electrodes.
To reveal the origin of annealing effect on TMD properties, we examined the film compositions and chemical state of as-deposited and annealed films in Fig. 6. It is confirmed that as-deposited film has Cobalt oxide (CoO) of ca. 5% in Fig. 6(a); this is probably because the films were sputter-deposited from two target sources in single one chamber. After annealing, the primary composition variation is the metallic Co and CoO, thus we may infer that the high annealing temperature of 673 K produces the inter-diffusion between Co granule and SiO2 matrix, leading to an increase in the oxidation of metallic Co to ca. 10%; this is confirmed by the films structure as shown in the inset of Fig. 6(b). The granule-matrix interface appears unclear compared with that of as-deposited films and there is no appreciable change in the crystallinity of both granules and matrix as observed from the selected area diffraction pattern.
XPS depth profiles of (a) as-deposited films for x = 0.24 and (b) annealed films with holding temperature of 673 K for 1 h. Inset shows the corresponding cross-section TEM image and selected area diffraction pattern of annealed sample.
The electrical resistivity (ρ), dielectric and magneto-dielectric results of both metal-oxide and reported metal-fluoride granular films are listed in Table 1. The Co–SiO2 granular films possess very high ρ of >108 µΩ·m, which are good insulators to sustain ac field for TMD effect. Over the measured frequency range (1–1000 kHz), ε′ at fTMD, named as $\varepsilon '_{\text{fTMD}}$, exhibits large enhancement compared with that ($\varepsilon '_{0} = 1\text{--}10$) of their matrix (MgF2, AlF3 or SiO2); this is caused by the formation of electric dipoles (granule pairs) via their intergranular matrix. With the application of magnetic field, maximum $\Delta \varepsilon '/\varepsilon '_{0}$ of 0.6–1% has been achieved in Co–SiO2 thin films, which is almost equivalent to that of Co–AlF3 composites. It is also noted that the as-deposited TMD films possess a small part of oxidation of metallic Co, therefore one might expect that the TMD response may be improved if the oxidation is further suppressed. Based on above analysis, observation of TMD effect in metal-oxide composites offers wide choices for candidate materials.
We have observed the tunnel-type magneto-dielectric (TMD) effect in Co–SiO2 granular thin films with varied granule concentration. We confirmed extremely high electrical resistivity of the films, reaching >108 µΩ·m, a value is larger than that of fluoride and nitride composites. The TMD ratio $\Delta \varepsilon '/\varepsilon '_{0}$ increases as metallic Co concentration increased, especially, the film for x = 0.24 exhibits $\Delta \varepsilon '/\varepsilon '_{0}$ of 1% at fTMD = 200 kHz. Annealing studies indicate that the films may have thermal stability up to 573 K. The present work demonstrate the possibility of TMD effect in metal-oxide thin films, and may enable broad choices of metal-oxide granular nanomaterials for magnetoelectric device application.
This work has been funded by the JSPS KAKENHI Grant-in-Aid No. 17H03385. This work is a cooperative program (Proposal No. 16G0207) of the CRDAM-IMR, Tohoku University. The authors wish to express their thanks to Dr. T. Miyazaki for the TEM observations and Ms. K. Oomura for assistance with XPS measurements.
