简介概要

Fabrication and magnetic properties of NiFe-ZnO nano-granular films

来源期刊：Rare Metals2013年第3期

论文作者：Xue-Yun Zhou Dong-Sheng Yao

文章页码：269 - 272

摘要：Excellent soft magnetic and high frequency properties were obtained successfully in the （Ni75Fe25 ）x（ZnO）1-x granular films fabricated on the glass substrate by RF magnetron oblique sputtering. The microstructure, mag- netic and high frequency properties were investigated systematically. High resolution transmission electron micrographs show that the film consists of fcc Ni75Fe25 particles uniformly embedded in an amorphous insulating matrix ZnO with particle size a few nanometers. The （Ni75Fe25 ） x（ZnO）1-x films exhibit excellent soft magnetic properties in a wide x range from 0.50 to 0.80 with coercivity not exceeding 5×10-4T, which is ascribed to the exchange coupling between magnetic particles. Especially for the sample with x = 0.64, coercivities in hard and easy axes are 5.0×10-5 and 3.6×10-4 T, respectively, and the electric resistivity ρ reaches 1,790 μΩ·cm. The dependence of complex permeability u = u’ - ju" on frequency f shows that the real part u’ is more than 130 below 500 MHz, and the ferromagnetic resonance frequency fr reaches 1.32 GHz, implying the promising for high frequency application.

Rare Metals 2013,32(03),269-272

Fabrication and magnetic properties of NiFe-ZnO nano-granular films

Xue-Yun Zhou Dong-Sheng Yao

Faculty of Science, Jiujiang University

Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparing Technology, Tianjin University

作者简介：Dong-Sheng Yao e-mail:yaodsh@tju.edu.cn;

收稿日期：30 August 2012

基金：financially supported by the National Natural Science Foundation of China (Nos. 50901050 and 60876035);the Science and Technology Plan Projects of Jiangxi Provincial Education Department (No. GJJ11239);

Fabrication and magnetic properties of NiFe–ZnO nano-granular films

Abstract：

Excellent soft magnetic and high frequency properties were obtained successfully in the (Ni75Fe25 )x(ZnO)1-x granular films fabricated on the glass substrate by RF magnetron oblique sputtering. The microstructure, mag- netic and high frequency properties were investigated systematically. High resolution transmission electron micrographs show that the film consists of fcc Ni75Fe25 particles uniformly embedded in an amorphous insulating matrix ZnO with particle size a few nanometers. The (Ni75Fe25 ) x(ZnO)1-x films exhibit excellent soft magnetic properties in a wide x range from 0.50 to 0.80 with coercivity not exceeding 5×10-4T, which is ascribed to the exchange coupling between magnetic particles. Especially for the sample with x = 0.64, coercivities in hard and easy axes are 5.0×10-5 and 3.6×10-4 T, respectively, and the electric resistivity ρ reaches 1,790 μΩ·cm. The dependence of complex permeability u = u' - ju" on frequency f shows that the real part u' is more than 130 below 500 MHz, and the ferromagnetic resonance frequency fr reaches 1.32 GHz, implying the promising for high frequency application.

Keyword：

Soft magnetic property; Nano-granular thin films; High frequency application;

Received： 30 August 2012

1 Introduction

Magnetic thin films used in magnetic components,such as inductive devices and magnetic heads,have attracted more and more attention because of the increasing demand for improvement and miniaturization of electromagnetic devices[1–3].The basic demands for soft magnetic films operated in the GHz range include high resistivity q,high permeability(l),high saturation magnetization M_s,and appropriate anisotropy field H_kso as to effectively suppress eddy current loss and to possess high ferromagnetic resonance(FMR)frequency f_r(which determines the cut-off frequency for high frequency application).Metal insulator granular films(MIGFs)consisting of magnetic metal nanogranules uniformly distributed in an insulator matrix are one of the best candidates for satisfying above demands.In the past decade,soft magnetic properties of K–M–N,K–M–O and K-fluoride films have been reported,where K are magnetic metals(Fe,Co,Ni,and their alloys)and M are nonmagnetic elements,such as Hf,Al,Si,Zr,etc[4–8].Compared with a lot of research results obtained for the MIGFs[4–11],little work has been conducted to optimize the soft-magnetic properties and the high frequency characteristics by making of metallic magnetic particles uniformly distributed in a semiconductor matrix,such as Zn O,Sn O₂,Ti O₂.However,the previous attention on K-semiconductor systems was focused on the study of diluted magnetic semiconductor,where a few of ferromagnetic metal atoms were doped into the semiconductor,and few work concerning the soft magnetic properties and high frequency properties of ferromagnetic metal–semiconductor granular films(MSCGF)was reported.In order to test whether the soft magnetic and good high frequency properties can be realized in the MSCGF systems,the microstructure and magnetic properties of(Ni₇₅Fe₂₅)_x(Zn O)_1-xwere systematically studied in this article,which might facilitate search for new materials with high permeability at high frequency range.

2 Experimental

(Ni₇₅Fe₂₅)_x(Zn O)_1-xgranular thin films were deposited on water-cooled glass substrates by co-sputtering in a RF magnetron sputtering system.The x is defined as the ratio:the volume of alloy Ni₇₅Fe₂₅versus the total volume of Ni₇₅Fe₂₅–Zn O film.The sputtering chamber was evacuated to a base pressure lower than 1.2 9 10^-4Pa,and the ambient gas pressure was maintained at 0.12 Pa during sputtering.The sputtering power of Ni₇₅Fe₂₅target was kept at 100 W,and the composition of sample was changed by changing the sputtering power of Zn O target.The thickness of the films was kept at around 280 nm and was determined by the surface profile meter(Dektak 6M).There was no external magnetic field applied to the substrate during deposition.An in-plane uniaxial anisotropy field was achieved by the incline of the sputtering beams and the oblique angle was fixed at 30°.Saturation magnetization,coercivity and in-plane magnetic anisotropy field H_kwere measured by vibrating sample magnetometer(Lake Shore model 7304).The composition of samples was determined by energy dispersive X-ray spectroscopy(EDX).The resistivity at room temperature was measured by standard dc four-probe method.The structure and microstructure were analyzed by X-ray diffraction(XRD)with Cu Ka radiation and high resolution transmission electronic microscope(HRTEM)using a Tecnai-G2-F30system,which is also equipped with an energy dispersive X-ray spectrometer,respectively.The permeability spectra were carried out with a PNA E8363B vector network analyzer using the shorted microstrip transmission-line perturbation method,which works from 100 MHz to5 GHz.The measurement was performed along the hard axis in-plane of the film.

3 Results and discussion

Figure 1 shows the bright-field TEM image(Fig.1a)with the corresponding electronic diffraction(ED)pattern inserted,as well as the XRD patterns(Fig.1b)for a typical sample with x=0.64.It can be seen that the sample consists of fcc Ni₇₅Fe₂₅particles embedded uniformly in insulating Zn O matrix and the average particle size is4.7 nm determined by the TEM.The ED pattern exhibits the diffraction peaks from(111),(200),and(220)planes of fcc Ni₇₅Fe₂₅,which is totally consistent with the measurement of XRD(Fig.1b).There are no diffraction peaks corresponding to crystalline Zn O detected by both XRD and TEM,indicating that the Zn O is amorphous.Based on the detection of the HRTEM and previous reports,a small amount of Ni₇₅Fe₂₅atoms exist in the Zn O matrix among Ni₇₅Fe₂₅granules[12].The Ni₇₅Fe₂₅atoms embedded in the Zn O matrix contribute to the soft magnetic properties because the electrons which are responsible for the intergranular exchange coupling interaction may transit through such Ni₇₅Fe₂₅atoms[13,14].And the intergranular exchange coupling interaction is one of the important factors to realize soft magnetic properties.As a result,the(Ni₇₅Fe₂₅)_x(Zn O)_1-xgranular films may have excellent soft magnetic and good high frequency properties because of such structure.

Fig.1 Bright-field TEM image a and the corresponding ED pattern inserted and XRD spectrum b for the sample with x=0.64

Figure 2 shows the x dependence of electrical resistivity q of samples.It is seen that q increases slowly with decreasing x,then increases rapidly after x\0.6.It is suggested that the percolation threshold is around 0.50 for these films.The q of the typical sample with x=0.64 reaches 1,790 l X?cm,3orders than that of pure Ni₇₅Fe₂₅alloy.This high q can depress eddy current loss as applied in the high frequency range.Figure 3 shows the hysteresis loops of samples with0.55\x\0.72.The typical sample with x=0.64 exhibits a good in-plane uniaxial anisotropy with H_k=4.3 9 10^-3T The coercivity in hard axes and the coercivity in easy axes are 5.10 9 10^-5and 3.62 9 10^-4T,respectively,and 4p M_s=5.4 kG.The sample with x=0.72 or 0.55 also exhibits the good in-plane uniaxial anisotropy and the small coercivity.When the volume fraction reaches=0.87,there is no obvious in-plane uniaxial anisotropy and the coercivity is much bigger than that of the samples with 0.50\x\0.80.The soft magnetic properties are ascribed to the exchange coupling interaction among Ni₇₅Fe₂₅granules.

Fig.2 x dependence of the electric resistivity q

Fig.3 Magnetic hysteresis loops for the samples with x from 0.87 to0.5 5

For metal–insulator nano-granular films,the exchange interaction which leads to magnetic ordering within a particle also extends out to the neighboring environments(through spin polarization or the superexchange interaction mechanism)within the exchange length(L_ex)which is expressed as:[15]

where the A is exchange stiffness coefficient,K₁is magnetic crystalline anisotropy constant of inpidual magnetic particle.When the average size D of magnetic particles and the distance S between particles are reduced to smaller than L_ex,the exchange interaction between particles may take place,which tends to align the magnetic moments of neighboring particles and thus overcomes the magneto-crystalline anisotropy of inpidual particle.As a result,the effective magnetic anisotropy of materials is reduced significantly,as shown

where n is the number of the particles within L³_ex.The reduced effective anisotropy leads to the decrease of coercivity,and hence the increase of permeability significantly,namely,good soft magnetic properties are obtained.Substituting n?x?L_ex=D?³into Eq.(2),gives

It is clear from Eq.(3)that for certain D both large L_exand small K₁is in favor of getting good soft magnetic properties.

For our samples,with decreasing x from 0.72 to 0.55,the average particle size changes below 4.7 nm,and the average distance between particles is about 1 nm estimated by TEM,while the L_exof Ni₇₅Fe₂₅is about 153 nm estimated by Eq.(1).Thus,D(L_exholds in this wide range,which leads to the realization of the exchange coupling among Ni₇₅Fe₂₅particles.Combining with the small D of Ni₇₅Fe₂₅particles themselves,good soft magnetic properties in a wide range are obtained.

Figure 4 shows the dependence of the complex permeability l=l⁰-jl⁰⁰on the frequency for the films with x changed from 0.55 to 0.72,where l⁰and l⁰⁰represent the real and imaginary parts of complex permeability,respectively.For the typical sample with x=0.64,it can be seen that l⁰is more than 120 below 1 GHz and then gradually decreases with frequency,while the imaginary part l⁰⁰gradually increases to a maximum at f=1.1 GHz,which can be ascribed to the ferromagnetic resonance(FMR).The measured l–f curves were fitted well with the solution of Landau–Lifshitz equations taking account of the coherent spin procession,as shown by the solid lines in Fig.4.The FMR frequency obtained by this fitting is around 1.1 GHz,which is consistent with the experimental result.The high FMR frequency also implies that the granular films are promising for applications in the high frequency range.The sample with x=0.72 or 0.55 also exhibits good high frequency properties:at the frequency lower than 1.0 GHz,the real part l⁰of complex permeability of this sample is more than 130 and the FMR frequency also reaches1.05 GHz,while the FMR peak becomes much wider,which is resulted from the decrease of 4p M_sand the dispersion of uniaxial in-plane anisotropy field H_k.

Fig.4 Dependence of complex permeability l=l0-jl00on frequency f for the films with x=0.72,0.64,and 0.55.The green and blue lines being the fitting results according to the Landau–Lifshitz equation

4 Conclusion

(Ni₇₅Fe₂₅)_x(Zn O)_1-xgranular films were fabricated by magnetron sputtering,and good soft magnetic properties and appropriate large H_khave been obtained in a wide x range.For the typical sample with x=0.64,the coer civity of 5 9 10^-5T in the hard axis and 3.6 9 10^-4T in the easy axis are obtained,and the resistivity reaches1,790 l X?cm.At a frequency lower than 1.0 GHz,the rea part of the complex permeability of this sample is more than 120,and the FMR frequency reaches 1.1 GHz,which implies that the films is promising for high frequency applications.Such excellent soft magnetic properties are ascribed to the exchange coupling among magnetic particles.

Acknowledgments This study was financially supported by the National Natural Science Foundation of China(Nos.50901050 and60876035),and the Science and Technology Plan Projects of Jiangxi Provincial Education Department(No.GJJ11239).