recently , 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 requirements for these films operated in the ghz range are high resistivity  , high permeability  , high saturation magnetization ms and appropriate large anisotropy field
hk so as to effectively suppress eddy current loss and to possess high ferromagnetic resonance ( fmr ) frequency fr ( which determines the cut - off frequency for high frequency application ) .
insulator granular films ( migf ) consisting of magnetic metal nano - granules uniformly distributed in an insulator matrix are one of the best candidates for satisfying above demands , because these films have the advantages of high ms and high  of magnetic metals and high  of insulator .
up to now , soft magnetic properties of granular films composed of magnetic metals ( and their alloys ) and various x-(oxide , nitride and fluoride ) where x is nonmagnetic elements such as hf , al , si , zr , rare earth , etc . have been investigated [ 4 - 8 ] .
the mechanism of the soft magnetic properties of migfs has been confirmed to be the exchange coupling between granules through the intergranular regions .
however , the microscopic origin of exchange coupling has not been totally understood , such as the medium of the exchange coupling for different constitution nano - granular films . by considering the high resistivity of semiconductor oxides , metal  semiconductor granular films ( msgf ) consisting of magnetic metal nano - granules uniformly distributed in semiconductor matrix , such as the feco  zno systems , may possess good high frequency properties .
as one kind of wide used semiconductor oxides due to the excellent electrical and optical properties , the current carriers of zno may mediate the exchange coupling between magnetic granules and hence favor good soft magnetic properties .
so far , there is no work studying systematically the soft magnetic properties of the feco 
zno granule films reported . in order to test whether zno can mediate the exchange coupling , the structure , magnetic properties and electronic transport properties of feco  zno msgfs were systematically studied in this work .
( fe65co35)x(zno)1x granular thin films were deposited on water - cooled glass substrates by radio frequency ( rf ) magnetron sputtering at room temperature with a background pressure lower than 4  10 pa . a three - inch fe65co35 alloy disk covered with zno chips ( 5  5  1 mm )
the sputtering power was 100 w , and sputtering ar gas was kept at 0.40 pa .
an external magnetic field obtained by a pair of permanent magnets located on both sides of the substrates was applied during deposition to induce an in - plane uniaxial anisotropy for the samples , and the intensity of external magnetic field is 500 oe . the saturation magnetization 4ms , coercivity hc and in - plane magnetic anisotropy field hk were determined by a vibrating sample magnetometer ( vsm , lakeshore 7304 model ) .
the composition of the samples was measured by energy dispersive x - ray spectroscopy ( eds ) .
the structure and microstructure were analyzed by x - ray diffraction ( xrd ) ( philips xpert model ) with cu - k radiation and high resolution transmission electronic microscopy ( hrtem ) , 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 to 5 ghz .
all the above measurements were performed for as - deposited samples without any post - heat treatment .
figure 1 shows the bright - field hrtem image ( a ) and the corresponding electron diffraction ( ed ) pattern ( b ) , as well as the xrd spectrum ( c ) for a typical sample of x = 0.67 .
it can be seen that the sample consists of bcc fe65co35 particles embedded uniformly in the insulating zno matrix from the image ( a ) .
the average size of fe65co35 particles and the distance between them are about 2 and 4.5 nm , respectively .
figure 1b exhibits the diffraction peaks from ( 110 ) and ( 200 ) planes of bcc fe65co35 alloys , ( 111 ) and ( 103 ) planes of zno , respectively , which are totally consistent with those of xrd shown in fig .
the average diameter of fe65co35 grains calculated by the full width at half maximum ( fwhm ) of the diffraction peak in xrd spectrum through scherrer s equation is about 2.1 nm , which is close to that of fe65co35 granules .
hrtem image ( a ) and the corresponding ed pattern ( b ) as well as the xrd spectrum ( c ) for a typical sample of x= 0.67 figure 2 expresses the x dependence of hc , hk and . it is seen that  increases slowly with decreasing x and then increases rapidly after x < 0.64 .
hc is less than 15 oe , and hkis over 43 oe in the range 0.63 < x < 0.72 , which means good soft magnetic properties are obtained in this range .
especially , for the sample with x = 0.67 , its in - plane hysteresis loop shown in fig .
the coercivities in hard and easy axes are 1.43 and 7.08 oe , respectively , 4ms = 9.85 kg , and  reaches 2.06 m cm , which is three orders higher than that of pure feco alloy .
these good soft magnetic properties and the high  imply that the films are promising for high frequency applications .
h curves for ( fe65co35)0.67(zno)0.33 film figure 4 shows the dependence of complex permeability  =  
j on frequency f for the film of x = 0.67 , where  and  represent the real and imaginary parts of complex permeability , respectively .
it can be seen that  is more than 100 below 1.83 ghz and then gradually decreases with frequency , while the imaginary part  gradually increases to a maximum at f = 2.31 ghz , which can be ascribed to the ferromagnetic resonance ( fmr ) .
lifshitz equations taking account of the coherent spin procession , as shown by the solid lines in fig .
the fmr frequency obtained by this fitting is around 2.31 ghz , consistent with the experimental result .
the high fmr frequency also implies that the granular films are promising for applications in the high frequency range .
permeability spectra (  ~ f ) for the sample of x = 0.67 considering the mechanism of the good soft magnetic properties , we turn to the exchange coupling model . according to the herzer s statement , if the average size d of magnetic particles and the distance s between particles are reduced to be smaller than a characteristic length , namely , the exchange length lex , the exchange interaction between particles may take place , which tends to align the magnetic moments of neighboring particles parallel and so overcome the magneto - crystalline anisotropy and the demagnetization effect of individual particles . as a result ,
the effective magnetic anisotropy of materials is reduced significantly , which leads to the decrease in coercivity and hence the increase in permeability , and then good soft magnetic properties are obtained . for fe65co35 , the characteristic length lex is about 26 nm , while the d and s are 2.3 and 4.5 nm , respectively , based on the results of hrtem spectrum .
thus , ( d + s )  lex holds , exchange coupling can be realized in this film , which is responsible for the good soft magnetic properties . in order to study the microscopic origin of the exchange coupling between granules , the temperature dependence of resistivity
figure 5a depicts thecurve , which exhibits a negative temperature coefficient of resistivity ( tcr ) .
5b , which indicates that the mechanism of electronic transportation of the sample is weak localized electron  electron interaction .
these weak localized electrons may also be the medium of exchange coupling between magnetic granules .
resistivity as a function of temperature ( a ) and fitting result ( b ) for a typical sample of x = 0.67
( fe65co35)x(zno)1x granular films were fabricated by magnetron sputtering , and good soft magnetic properties have been obtained in x range from 0.63 to 0.72 with small hc and appropriate large hk . for the typical sample of x = 0.67 ,
the coercivity of 1.43 oe in the hard axis and 7.08 oe in the easy axis are obtained , and the resistivity reaches 2.06 m cm . at a frequency lower than 1.83 ghz , the real part of the complex permeability of this sample is more than 100 , and the fmr frequency reaches 2.31 ghz , which implies that the film is promising for high frequency applications .
the good soft magnetic properties origin from the exchange coupling between magnetic granules and the weak localized electrons existing in films may be the medium of this exchange coupling .
this work was supported by national natural science foundation of china ( grant no.50571041 ) .
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this article is distributed under the terms of the creative commons attribution noncommercial license which permits any noncommercial use , distribution , and reproduction in any medium , provided the original author(s ) and source are credited .
 a series of nano - granular films composed of magnetic metal ( fe65co35 ) granules with a few nanometers in size and semiconductor oxide ( zno ) have been fabricated by a magnetron sputtering method , and excellent soft magnetic properties have been achieved in a wide metal volume fraction ( x ) range for as - deposited samples due to the exchange coupling between feco granules ( a ferromagnetic interaction in nano - scale ) . in a wide range ( 0.53 < x < 0.71 ) , the films exhibit coercivity hc not exceeding 15 oe , along with high resistivity . especially for the sample with x = 0.67 , coercivities in hard and easy axes are 1.43 and 7.08 oe , respectively , 4ms = 9.85 kg , and  reaches 2.06  103  cm . the dependence of complex permeability  =   j on frequency shows that the real part  is more than 100 below 1.83 ghz and that the ferromagnetic resonance frequency reaches 2.31 ghz , implying the promising for high frequency application . 
 the measured negative temperature coefficient of resistivity reveals that may be the weak localized electrons existing in samples mediate the exchange coupling . 