Effect of Ar–N2 Sputtering Gas on Structure and Tunneling Magnetodielectric Effect in Co–(Si–N) Nanogranular Films

We have investigated the effect of N2 fraction <inline-formula> <tex-math notation="LaTeX">$x$ </tex-math></inline-formula> in Ar–N2 sputtering gas on the tunneling magnetodielectric (TMD) effect in Co–(Si–N) nanogranular films. Co–(Si–N) films were deposited by co-sputtering Co and Si3N4 targets in Ar-<inline-formula> <tex-math notation="LaTeX">$x$ </tex-math></inline-formula> vol.%N2 mixture gas with different N2 gas fractions <inline-formula> <tex-math notation="LaTeX">$x$ </tex-math></inline-formula> of 0–30. All deposited films had a nanogranular structure composed of Co nanogranules with a diameter of 1–3 nm embedded in a Si–N matrix. We realized the TMD effect in the films for <inline-formula> <tex-math notation="LaTeX">$x \ge3.3$ </tex-math></inline-formula>, and the film deposited in Ar-6.6 vol.%N2 gas showed the highest dielectric variations in a magnetic field. For <inline-formula> <tex-math notation="LaTeX">$3.3\le x \le10$ </tex-math></inline-formula>, TMD peak frequency <inline-formula> <tex-math notation="LaTeX">$f_{\mathrm {TMD}}$ </tex-math></inline-formula> decreased from 17 to 40 kHz with increasing <inline-formula> <tex-math notation="LaTeX">$x$ </tex-math></inline-formula> because of the increase in intergranular spacing <inline-formula> <tex-math notation="LaTeX">$s$ </tex-math></inline-formula>. On the other hand, for <inline-formula> <tex-math notation="LaTeX">$10 < x \le30$ </tex-math></inline-formula>, <inline-formula> <tex-math notation="LaTeX">$f_{\mathrm {TMD}}$ </tex-math></inline-formula> increased from 40 kHz to 3.3 MHz as <inline-formula> <tex-math notation="LaTeX">$x$ </tex-math></inline-formula> increased since both <inline-formula> <tex-math notation="LaTeX">$s$ </tex-math></inline-formula> in the out-of-plane direction and <inline-formula> <tex-math notation="LaTeX">$\beta $ </tex-math></inline-formula>, which indirectly represents the measure of the distribution of <inline-formula> <tex-math notation="LaTeX">$s$ </tex-math></inline-formula>, decreased. This study provides a new way to tailor the frequency response of the TMD effect.


I. INTRODUCTION
M AGNETOELECTRIC (ME) materials, which possess both magnetic and electric properties, have attracted much attention for their applications, such as magnetic memory and magnetic sensors.Magnetic metal-insulator nanogranular films, which consist of nanometer-sized magnetic metal (e.g., Co, Fe) granules embedded in an insulating (e.g., MgF 2 , Al 2 O 3 ) matrix, belong to ME materials and exhibit a variety of intriguing functionalities: high-frequency soft magnetic properties [1], [2], [3], Hall effect [4], [5], [6], tunnel magnetoresistance effect [7], [8], [9], tunnel magneto-optic effect [10], and giant Faraday effect [11].Additionally, our group discovered a new ME effect, tunneling magnetodielectric (TMD) effect, which causes a magnetic field-induced increase in the dielectric permittivity of nanogranular films [12].The TMD effect exhibits the frequency dependence.Upon the application of the magnetic field, there exists a maximum change in the dielectric permittivity at certain frequency of ac electric field.So, to control its frequency response is central to the regulation of the TMD effect toward real-world high-frequency device applications.
In addition, a Si 3 N 4 matrix would function as the tunneling barrier owing to its high ρ.Another reason for choosing Co-(Si-N) films is that their intergranular spacing, which has a strong influence on the frequency response of TMD effect [14], [15], can potentially be controlled using the reactive sputtering method, as the structure of sputter-deposited films is highly sensitive to the composition of the sputtering gas.This can be achieved by employing a mixture of Ar-N 2 gas with varying N 2 gas fraction during the deposition process.In this study, to realize the first TMD effect in nitride-based nanogranular films and to control the frequency dependence of TMD effect, we have deposited Co-(Si-N) nanogranular films and investigated the effect of N 2 fraction in Ar-N 2 sputtering gas on the structure and TMD effect of the films.and Si 3 N 4 targets at 0.2 Pa of Ar-x vol.%N 2 (0 ≤ x ≤ 30, N 2 gas fraction in Ar-N 2 sputtering gas) mixture gas atmosphere.The substrate temperature was kept at 400 • C during deposition.The film thickness was 650 ± 50 nm.

II. EXPERIMENTAL PROCEDURE
The film composition and the chemical states of the elements in the films were detected with X-ray fluorescence and X-ray photoelectron spectroscopy (XPS), respectively.The crystal structure and morphology of the films were observed with transmission electron microscopy (TEM).The electrical resistivity in the in-plane direction was measured using the four-probe method.The magnetization curve in the in-plane direction was measured with a vibrating sample magnetometer.Permittivity ε in the out-of-plane direction was evaluated with an impedance analyzer from 0.01 to 100 MHz with and without an external magnetic field H up to 800 kA/m.Herein, H was applied to the films in the in-plane direction.TMD response was calculated using where ε ′ H and ε ′ 0 are the real parts of ε with and without H .All measurements were conducted at room temperature.

A. Composition and Structure
Fig. 1 shows the N 2 gas fraction, x, dependence of (a) Co content and (b) N/Si ratio of Co-(Si-N) films.Co contents of all films were confirmed to retain a very small composition deviation of 19 ± 0.7 at.%. N/Si ratio increased dramatically with increasing x from 0 (N/Si = 1.03) to 6.6 vol.% (N/Si = 1.72) and then increased slightly with increasing x from 6.6 to 30 vol.% (N/Si = 1.92).Fig. 2 shows the XPS depth profile of Si in Co 19 -(Si-N) 81 films deposited in Ar-0, 6.6, and 30 vol.%N 2 gas.According to the XPS analysis, Si in the films existed in three states: metal Si, Si-N, and Si-O.The atomic fraction of metal Si decreased with increasing x from 0 to 6.6 vol.%.On the other hand, the atomic fraction of Si-N decreased with increasing x from 0 to 6.6 vol.%.For x ≥ 6.6, the atomic fractions of both metal Si and Si-N were independent of x.These results indicate the progress of metal Si nitriding for 0 ≤ x < 6.6.The rapid increase in the N/Si ratio in Fig. 1(b) is considered to originate from the nitriding of metal Si.The effect of Si-O seems to be small since the atomic fraction of Si-O is much smaller than that of Si-N in all films.
Fig. 3(a) shows the diffraction image of Co 19 -(Si-N) 81 films deposited in Ar-10 vol.%N 2 gas.The diffraction patterns showed the presence of hcp Co, but there were no observable patterns corresponding to Si 3 N 4 .Fig. 3 shows the TEM images of the films deposited in Ar-(b) 0, (c) 10, and (d) 30 vol.%N 2 gas.Insets display the high-resolution TEM images of the films.These diffraction images and TEM images revealed that Co 19 -(Si-N) 81 films had a nanogranular structure composed of crystalline Co nanogranules embedded in an amorphous Si-N matrix.Fig. 3(e) shows the distribution of granule diameter, d, measured from TEM images of the films deposited in Ar-x vol.%N 2 gas (0 ≤ x ≤ 30).According to Fig. 3(e), the average diameter, d Ave , of Co granules increased with increasing x from 0 (d Ave = 1.6 nm) to 10 vol.% (d Ave = 2.5 nm).In contrast, d Ave slightly decreased for x > 10.Moreover, the films deposited in Ar-20 and 30 vol.%N 2 gas had a wider distribution of d than the films deposited in Ar-0 to 10 vol.%N 2 gas.Remarkably, Co granules in the film deposited in Ar-30 vol.%N 2 gas were aligned in the out-of-plane direction, which is different from those in the films deposited in Ar-0 and 10 vol.%N 2 gas, where Co granules homogeneously dispersed.This nanostructural evolution, varying with x, is schematically illustrated in Fig. 3(f).

B. Electric, Magnetic, and Magnetodielectric Properties
Fig. 4 shows the x dependence of (a) ρ and (b) magnetization with H = 800 kA/m, 4π M 800 kA/m .ρ increased with Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.increasing x from 0 (10 4 µ • m) to 6.6 vol.% (10 7 µ • m) and became constant for x ≥ 6.6.4π M 800 kA/m also increased dramatically with increasing x from 0 (4π M 800 kA/m = 1.4 kA/m) to 6.6 vol.% (4π M 800 kA/m = 208 ± 15 kA/m) and was saturated for x ≥ 6.6.Herein, all Co 19 -(Si-N) 81 nanogranular films exhibited superparamagnetism with almost zero coercivity, as shown in Fig. 4(c), which exhibits the magnetization curves of the films deposited in Ar-0, 6.6, and 30 vol.%N 2 gas.These rapid increases in ρ and 4π M 800 kA/m for 0 ≤ x < 6.6 are likely due to the nitriding of metal Si present in the films since metal Si in the Si-N matrix decreases the insulation of the matrix, and there are reports suggesting that the saturation magnetization of Co-Si alloys decreases as the Si content increases [23], [24].
Fig. 5(a) shows the frequency dependence of the real permittivity ε ′ with H = 0 (ε ′ 0 ) and 800 kA/m (ε ′ 800 kA/m ).The ε ′ of the films before the relaxation frequency was much larger than that of amorphous Si 3 N 4 thin films (6.5 [21]), since electrons oscillate between Co granule pairs by tunneling, and these granule pairs function like electric dipoles [12].Thus, the relaxation frequency of ε ′ corresponds to the relaxation of the electron tunneling oscillation.The application of H leads to an increase in relaxation frequency, resulting in enhanced ε ′ near the relaxation frequency, as shown in the inserted figure in Fig. 5(a).Fig. 5(b) shows the H dependence of the TMD ratio in the film deposited in Ar-6.6 vol.%N 2 gas at a frequency of 800 kHz.ε ′ increases by applying H , and this behavior agrees well with −(M/M 800 kA/m ) 2 derived from the magnetization curve in Fig. 4(c).This result indicates that the enhancement of ε ′ is a consequence of the change in magnetization state of Co granules.Fig. 5(c) summarizes the frequency dependence of ε ′ 0 of the films deposited in Ar-3.3, 6.6, 10, 20, and 30 vol.%N 2 gas.The relaxation frequency decreased as x increased from 3.3 to 10 vol.%.On the other hand, it increased with increasing x from 10 to 30 vol.%.Fig. 5(d) shows the TMD response of the films against the frequency.The film deposited in pure Ar gas did not exhibit a TMD effect because of its low ρ and 4π M 800 kA/m .TMD effect appeared for x ≥ 3.3, and maximum TMD response was obtained in the film deposited in Ar-6.6 2 gas.Fig. 5(e) shows the x dependence of f TMD , the frequency at which the peak of the TMD effect According to Fig. 5(d) and (e), f TMD decreased from MHz to 40 kHz with increasing x for 3.3 ≤ x ≤ 10.In contrast, 10 < x ≤ 30, f TMD increased from 40 kHz to 3.3 MHz (calc.) and became broader with increasing x.Solid lines in Fig. 5(c) and (d) are the theoretical fitting described as follows [12]: where ε ∞ is the high-frequency dielectric constant; ε is the dielectric strength; τ r is the dielectric relaxation time, which is proportional to 1/(1 , where P T is spin polarization and M s is saturation magnetization; and β is an exponent (0 < β ≤ 1) representing a measure of the distribution of τ r [25].β also represents the distribution of intergranular spacing s [see the inset in Fig. 3(c)] since τ r is determined by s.Fig. 5(f) shows the x dependence of β. β decreased with increasing x for x > 10, whereas it was almost constant for x ≤ 10.This fitting result indicates that the distribution of s became broader with increasing x for x ≤ 10.
We would like to discuss the origins of the shift in f TMD .Our previous reports revealed f TMD shifts to higher frequencies as s decreases and f TMD shifts to lower frequencies as s increases [14], [15].In Fig. 5(e), f TMD shifted to lower frequencies with increasing x for 3.3 ≤ x ≤ 10.This is attributed to the increase in s due to the increase in d under a constant Co composition [26], [27], as shown in Fig. 3(e) and (f).On the other hand, for 10 < x ≤ 30, f TMD shifted to higher frequencies with increasing x, possibly owing to two reasons.One is the decrease in s in the out-of-plane direction (i.e., the direction of TMD measurement).As shown in Fig. 3(d) and (f), Co granules in the film deposited in Ar-30 vol.%N 2 gas were closely aligned in the out-of-plane direction, and s in that direction decreased.Another reason is the decrease in β for 10 < x ≤ 30, as shown in Fig. 5(f).In a previous study, we reported that f TMD shifted to higher frequencies as β decreased [14].For these two reasons, f TMD would be high frequency for x > 10.
The experimental results combined with theoretical considerations reveal the previously overlooked importance of the sputtering atmosphere in the fine regulation of structural Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.
evolution and the TMD effect, paving a new path to control the frequency characteristics of the TMD effect.

IV. CONCLUSION
We fabricated Co 19 -(Si-N) 81 nanogranular films to realize the first TMD effect in nitride-based nanogranular films and to investigate the effect of N 2 gas fraction x in Ar-N 2 sputtering gas on the structure and TMD effect.Accordingly, we conclude the following.
1) All Co 19 -(Si-N) 81 films had a nanogranular structure composed of crystallized Co nanogranules embedded in an amorphous Si-N matrix.2) ρ and 4π M 800 kA/m increased with increasing x from 0 to 6.6 vol.%N 2 , probably owing to the nitriding of metal Si in the films.For x ≥ 6.6, ρ and 4π M 800 kA/m were independent of x since the nitriding of metal Si did not occur.3) Co 19 -(Si-N) 81 films showed a TMD effect for x ≥ 3.3 owing to the improvements in ρ and 4π M 800 kA/m .The maximum TMD effect (1.0%) was obtained in the film deposited in Ar-6.6 vol.%N 2 gas.4) In the range of 3.3 ≤ x ≤ 10, f TMD shifted to lower frequencies as x increased, which can be attributed to the increase in granule diameter d, resulting in larger intergranular spacing s.Conversely, for the range of 10 < x ≤ 30, f TMD shifted to higher frequencies, and the TMD peak became broader as x increased.These are due to the decrease in s in the out-of-plane direction and the decrease in β.

Co 19 -
(Si-N) 81 nanogranular films were deposited on a Pt/Ti/Si (100) and quartz glass substrate by co-sputtering Co © 2023 The Authors.This work is licensed under a Creative Commons Attribution 4.0 License.For more information, see https://creativecommons.org/licenses/by/4.0/Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.

Fig. 1 .
Fig. 1.N 2 gas fraction x dependence of (a) Co content and (b) N/Si ratio of Co-(Si-N) films.

Fig. 5 .
Fig. 5. (a) Frequency dependence of real permittivity ε ′ with H = 0 (ε ′ 0 ) and 800 kA/m (ε ′ 800 kA/m ) and (b) magnetic field dependence of TMD effect in the film deposited in Ar-6.6 vol.%N 2 gas.Frequency dependence of (c) ε ′ 0 and (d) TMD response with H = 800 kA/m of the Co 19 -(Si-N) 81 films deposited in Ar-3.3 to 30 vol.%N 2 gas.x dependence of (e) f TMD , the TMD peak frequency, and (f) β, a measure of the distribution of the relaxation time of ε ′ 0 .