Self-biased microwave ferromagnetic performance of patterned Ni 80 Fe 20 thin films

Patterned Ni80Fe20 permalloy films with strip length of 5 mm, strip distance of 40μm and various ferromagnetic (FM) strip widths from 20 to 40μm were prepared by pulse laser deposition and photolithography for study the microwave performance of permalloy films. Comparing with the continuous magnetically isotropic permalloy film, the anisotropy fields HK increase with the decrease of the FM strips’ width. As a result, the ferromagnetic resonance frequency fr is enhanced from 1.71 GHz of continuous film to 2.1 GHz of patterned film with FM strip width of 20μm. The comparison between fr and HK demonstrates that the enhancement of fr can be attributed to the increase of HK due to the shape anisotropy of permalloy strips.

Patterned Ni 80 Fe 20 permalloy films with strip length of 5 mm, strip distance of 40 µm and various ferromagnetic (FM) strip widths from 20 to 40 µm were prepared by pulse laser deposition and photolithography for study the microwave performance of permalloy films.Comparing with the continuous magnetically isotropic permalloy film, the anisotropy fields H K increase with the decrease of the FM strips' width.As a result, the ferromagnetic resonance frequency f r is enhanced from 1.71 GHz of continuous film to 2.1 GHz of patterned film with FM strip width of 20 µm.The comparison between f r and H K demonstrates that the enhancement of f r can be attributed to the increase of H K due to the shape anisotropy of permalloy strips.© 2016 Author(s).All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).[http://dx.doi.org/10.1063/1.4972799]

I. INTRODUCTION
With the electronic information technology developing towards high integration, high complexity, miniaturization, light weight, high-performances, multi-functions and high frequency, more and more components are required to be integrated in a smaller substrate. 1In order to reduce the overall size and weight of the electric system, developing thin film components undoubtedly a practical way to meet the requirement. 2The miniaturization of some passive components, such as inductors, capacitors, is key factor to enhance the integration density. 3Microwave soft magnetic films (SMFs) exhibit good magnetic performances, such as high saturation magnetization 4πM S , high permeability µ, and low damping constant α.The inductance of SMFs-covered inductors will be dramatically enhanced, and then their occupying area in monolithic microwave integrated circuits (MMICs) is drastically reduced.On the other hand, the magnetic field generators, which usually have a heavy weight and/ or large power consumption, should be removed from MMICs for satisfying the miniaturization, light weight and low power loss. 4Therefore, metal SMFs with endogenous magnetic anisotropy draw an increasing attention because, on the one hand, the endogenous magnetic anisotropy results in a good microwave magnetic properties in zero-bias magnetic field, on the other hand, metal films have good compatibility with MMIC process. 5,6Various methods are developed to obtain large endogenous magnetic anisotropy field H K , such as in-situ magnetic field deposition, [7][8][9] post magnetic field annealing, 10,11 oblique sputtering, [12][13][14] composition gradient sputtering, 15,16 Interface coupling, 17,18 magnetoelectric coupling, 19,20 Stress induced, 21 and pattern processing, 22,23 etc.
In this work, we successfully prepared patterned Ni 80 Fe 20 permalloy films with self-biased ferromagnetic performances by photolithography technology and pulsed laser deposition method.The shape anisotropy induced the change H K increased with the FM strip width narrowing down from 40 to 20 µm, which results in H K up to 60 Oe and an enhance up to 2.1GHz in permalloy thin films with FM strip width of 20 µm.

II. EXPERIMENTAL DETAILS
Experiment process consists of three steps: lithographic coating, thin films deposition and stripping.Firstly, a thick photoresist layer was spin coated onto a (100) silicon wafer with rotation speed/time as 800 rpm/25 s and 5000 rpm/60s.Subsequently it was soft baked on hot plate at 383 K for 90 s and dried in oven at 383 K for 20 min.After that, the photoresist layer was patterned into various strip widths from 20 µm to 40 µm and, the same strip length of 5 mm, and strip distance of 40 µm by exposure using 350 nm light for 20 s with different masks.Immediately develop for 90 s, rinsing for 70 s, post exposure baking on hot plate at 403 K for 3 min, and striping at 353 K for 80 s were carried out.Secondly, the Ni 80 Fe 20 layer deposited on the patterned substrate by pulsed laser deposition system using a KrF excimer laser (λ = 248 nm) in a vacuum chamber with a base pressure of 8 × 10 -5 Pa at room temperature, using energy density of laser 3 Jcm -2 and the repeating frequency 5 Hz.The thickness of the Ni 80 Fe 20 thin films was controlled as 50 nm by setting pulse number.Finally, the patterned Ni 80 Fe 20 thin films were obtained via stripping out the photoresist in specific stripping liquid at 353 K for 3 min.
The phase structure of the films was characterized by X-ray diffraction (XRD).The surface morphology and magnetic domain of the films was observed using atomic force microscope/magnetic force microscope (AFM/MFM, Park system, XE7).Magnetic properties were detected by a physical properties measurement system with vibrating sample magnetometer accessory (PPMS-VSM, Quantum Design Co., Evercool ).The microwave ferromagnetic performance of Ni 80 Fe 20 thin films were measured with a vector network analyzer (Agilent N5224A).

III. RESULTS AND DISCUSSION
Figure 1 showed XRD patterns of continuous and patterned Ni 80 Fe 20 thin films.Only the strongest diffraction peak at 44.4 o was observed for the two kinds of permalloy films, which is consistent with the (111) peak of standard XRD data of permalloy.The intensity of Si (100) in continuous permalloy film is weaker than that in patterned film, which is consistent with the fact that the patterned film covers small area of Si substrate.
Figure 2 showed the hysteresis loops of continuous and patterned Ni 80 Fe 20 thin films.As illustrated, the magnetic properties of continuous film is nearly isotropic [Fig.2(a)], and tiny anisotropy can be attributed to the randomly dispersed residual stress.As comparison, the patterned films show obvious magnetic anisotropy, which is sensitive to the ferromagnetic strip width [Fig.2(b-d The magnetic anisotropy of patterned permalloy films were further verified by MFM observation.Figure 3 shows the comparison of the magnetic domains between continuous and patterned films.As illustrated, the domain in continuous film shows a random dispersed structure, indicating a random anisotropy of the continuous films.However, for the patterned film (w=20 µm), some strip domains are nearly parallel to the length direction of the permalloy strips, demonstrating that the shape anisotropy drives the magnetic moments along the length (L) direction.This fact is consistent with the results of hysteresis loops and magnetic spectra measurement.An obvious magnetic anisotropy was observed in patterned permalloy films with the easy axis along the parallel (//) direction (L direction) and the hard axis along the perpendicular (⊥) direction (width direction) respectively [Fig.2(b-d)].According to the Kittel equation, 25 the ferromagnetic resonance frequency of the patterned thin films with shape-anisotropy can be expressed as Where γ is the gyromagnetic ratio (= 2.8 MHz/Oe), α is the damp constant, H K (= H 0 + 4πεM s ) is the effective anisotropy field, H 0 is the measured uniaxial anisotropy field of continuous film, ε (= t/w) is the effective demagnetization factor, t is film thickness, w is film width.According to Eq. ( 1 As can be seen in Fig. 2(b-d), for the patterned Ni 80 Fe 20 thin films, the slopes of the hard-axis loops as gradually decreased as the strips' width become narrower, while the easy axis shows little changes.Meanwhile, the smaller the width of the strip, the bigger the effective anisotropy field.Therefore, it can conclude that it is an important way to control the shape anisotropy by tuning the demagnetization factors of the patterned thin films. 26he comparison of frequency dependence of permeability for continuous and patterned (w = 20 µm) was shown in Fig. 4(a).As illustrated, the ferromagnetic resonance frequency f r is enhanced from 1.7 GHz in continuous film to 2.1 GHz in patterned film with w = 20 µm. Figure 4(b) summarized the effect of permalloy strip width on the ferromagnetic resonance frequency f r and magnetic anisotropy field H K .The relationship of f r and H K demonstrates that the enhancement of f r can be attributed to the increase of H K due to the shape anisotropy of permalloy strips.Therefore, it is effective to tuning the uniaxial magnetic anisotropy by controlling the shape anisotropy in high-frequency applied field.

IV. CONCLUSIONS
Self-biased patterned Ni 80 Fe 20 thin films with various ferromagnetic (FM) strip widths were prepared by photolithography technology and pulsed laser deposition method.As ferromagnetic (FM) strip widths decrease, the uniaxial magnetic anisotropy field H K increase from 40 Oe of continuous film to 62 Oe of patterned film with FM strip width of 20µm, leading to a considerable increase of resonance frequency f r from 1.71 GHz to 2.1 GHz.From the results of analysis, the increasing of the uniaxial magnetic anisotropy field H K can be attributed to a change in the shape anisotropy.From an application perspective, it is a viable way for tuning the uniaxial magnetic anisotropy of the SMFs by the strips' width in high-frequency microwave devices.

FIG. 2 .
Figure1showed XRD patterns of continuous and patterned Ni 80 Fe 20 thin films.Only the strongest diffraction peak at 44.4 o was observed for the two kinds of permalloy films, which is consistent with the (111) peak of standard XRD data of permalloy.The intensity of Si (100) in continuous permalloy film is weaker than that in patterned film, which is consistent with the fact that the patterned film covers small area of Si substrate.Figure2showed the hysteresis loops of continuous and patterned Ni 80 Fe 20 thin films.As illustrated, the magnetic properties of continuous film is nearly isotropic [Fig.2(a)], and tiny anisotropy can be attributed to the randomly dispersed residual stress.As comparison, the patterned films show obvious magnetic anisotropy, which is sensitive to the ferromagnetic strip width [Fig.2(b-d)].The easy axis (EA) and hard axis (HA) of patterned films are parallel and perpendicular to the length