High-Frequency Magnetic Loss in Nanogranular FeCoTiO Films With Different Histories of Induced Uniaxial Anisotropy

Abstract

Soft magnetic films have attracted considerable attentions for high frequency applications, such as on-chip power inductors, transformers and voltage regulaters. Loss control is the most critical tasks for these applications.[1] The high frequency loss of magnetic thin films typically includes hysteresis loss, eddy current loss, and ferromagnetic resonance (FMR) damping loss. Nanogranular FeCo-X-O (N) (X denotes Si, Hf, Zr, Ti, Zn etc.) have higher resistivity and saturation magnetization than those of permalloy or amorphous alloys. Meanwhile, the critical thickness can be 500 nm or above due to the suppressed growth of columnar structures by insulating layers.[2] All these make the nanogranular films very attractive for on-chip passive devices. So far, nanogranular films are mainly grown by sputtering. The in-plane uniaxial anisotropy $( H_{\mathrm {k}})$ required for hard-axis exited inductors and transformers can be induced through in-situ magnetic biasing field $( H_{\mathrm {bias}})$ during growth or oblique sputtering, both of which are widely adopted in the fabrication of CMOS compatible magnetic devices.[4] However, the research about high frequency magnetic loss related to these two processes is still scarce. We here report our efforts to reducing the effective damping factor of nanogranular FeCoTiO films. Fig. 1(a-c) show the imaginary part of the permeability spectra of FeCoTiO films deposited at different oblique angles (β) upon applying magnetic field along the easy axis (called low-field FMR). It can be seen that the FMR frequency $( f_{\mathrm {r}})$ increases from 3 GHz to 5 GHz upon increasing β from 17° to 32° at zero field. Therefore, the working frequency of nanogranular films can be simply tuned by adjusting β angle. Fig. 1(d) shows that full width at half maximum (Δf) of the imaginary permeability spectra. The lowest Δf about 0.76 GHz was obtained in the film with β of 17°, corresponding to a low $\alpha _{\mathrm {eff}}$ of 0.013. Further increasing β angle cause significant structural inhomogeneities and larger $\alpha _{{\mathrm {eff}}}$. Fig. 2(a-c) show the permeability spectra of FeCoTiO films deposited with in-situ $H_{\mathrm {bias}}$ of 15, 70 and 115 Oe, respectively. Regardless of the magnitude of $H_{\mathrm {bias}}$, it is found that $H_{\mathrm {k}}$ varies in a small range between 70 Oe and 80 Oe. Our results clearly reveal that the rearrangement of Fe-Fe(Co) atom pairs almost fully completed at an external field as small as 15 Oe during sputtering. According to the two magnon theory, the extrinsic $\alpha _{\mathrm {eff}}$ is generally related to the magnetic inhomogenieties.[4] The low field FMR results in Fig. 2 indicate that (i) further increasing biasing field cannot fully suppress magnetic inhomogenieties, which is true for even for $H_{\mathrm {bias}}$ up to 900 Oe (not shown); and (ii) Δf decreases with the increase of the external field and the minimal Δf is 0.55 at an external field of 200 Oe, as shown in Fig. 2 (d). Finally, we calculated the high frequency loss μ”/μ’ of FeCoTiO films. The two sets of data are from the film deposited under in-situ $H_{\mathrm {bias}}$ of 115 Oe and the film by oblique sputtering at β = 17°. The μ”/μ’ of oblique sputtered FeCoTiO film dominantly comes from FMR damping above 1.6 GHz, below which only hysteresis loss can be observed. However, for the FeCoTiO film deposited under $H_{\mathrm {bias}}$ of 115 Oe, the FMR damping can be observed down to 200 MHz due to a large $\alpha _{ {eff}}$ of 0.026. Furthermore, the overall high frequency loss of oblique sputtered FeCoTiO film is much smaller than that of the FeCoTiO film deposited under in-situ Hbias, although the coercivity along the hard axis $( H_{\mathrm {ch}})$ of the former (11 Oe) is much larger than the latter, 3 Oe. The large $H_{\mathrm {ch}}$ of oblique sputtered FeCoTiO films may come from the pinning effect of columnar structures. On the other hand, such columnar structure also brings the advantages of well aligned magnetic moments and reduced magnetic inhomogeneities in the FeCoTiO film.