IEEE TRANSACTIONS ON MAGNETICS, VOL. 49, NO. 7, JULY 2013 4291

Magnetic Properties of Sr Substituted Y-Type HexaferriteKwang Lae Cho and Chul Sung Kim

Department of Physics Kookmin University, Seoul 136-702, Korea

With the recent advances in high frequency antennas and RF devices requiring miniaturization, broad bandwidth, and impedancematching, magnetic material such as hexaferrites with high resonance frequency have been studied extensively. Here, we synthesized theY-type hexaferrites (Ba Sr Co Zn Fe O ) by solid state reaction method and investigated their crystalline and magnetic prop-erties by XRD, VSM, Mössbauer spectrometer and network analyzer. XRD patterns show that Ba Sr Co Zn Fe O sampleshave rhombohedral structures with space group - . The VSM measurements show that the Zn substitution decreases the Néel tem-perature significantly while the Sr substitution does not. However, the spin reorientation temperature, corresponding to non-collinear he-lical to collinear ferrimagnetic transition, of Ba Sr Co Zn Fe O increased with Sr concentration. Complex permeability andpermittivity of Ba Sr Co Zn Fe O samples weremeasured by network analyzer between 100MHz to 4 GHz. Our study showsthat the permeability of Ba Sr Co Zn Fe O sintered at 1100 C is 2.38 at 2.45 GHz with .Our experimental observation shows the potential application of Ba Sr Co Zn Fe O in RF devices in UHF band.

Index Terms—Cation occupancy, Mössbauer spectroscopy, Y-type hexaferrite.

I. INTRODUCTION

R ECENTLY, for the development of information andcommunication technology such as multilayer chip

inductors (MLCIs) and Bluetooth devices, the miniaturizationand broader bandwidth of the wireless communication devicesare becoming issues of interests [1], [2]. Previously patch an-tennas with high dielectric constant substrates were commonlyused. However, their low radiation and narrow bandwidthlimit their applications in radio frequency (RF) devices. Therecent advances in high frequency antennas and RF devicesrequire miniaturization and broader bandwidth, which can becharacterized by and BW as wellas impedance matching. For such purposes, magnetic materialssuch as hexaferrites with high resonance frequency have beenstudied extensively [3]. Among the various hexaferrites, CoY-type hexaferrite can be easily applied to RF devices at ultrahigh frequency (UHF) band due to high magnetic anisotropy.The unit cell of Y-type hexaferrite consists of three S-blocksand T-blocks. The Fe ions located at four octahedral sites( and ) and two tetrahedral sites (and ) [4]. However there is few studies regarding the siteoccupancy with transition metal. Here we studied the relativesite occupancy in Y-type hexaferrite by comparing the effectof Zn and Sr substitution. Since Zn ion prefers tetrahedralsites [5] and Sr is typically substituted for Ba in M-typehexaferrite, such difference is expected to lead to the change inthe super-exchange interaction [6], and the resulting magneticproperties of Zn-substituted and Sr-substituted Y-type hexafer-rites are investigated by Mossbauer spectroscopy.

Manuscript received November 05, 2012; revised January 08, 2013; acceptedJanuary 22, 2013. Date of current version July 15, 2013. Corresponding author:C. S. Kim (e-mail: cskim@kookmin.ac.kr).Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TMAG.2013.2243125

II. EXPERIMENT PROCEDURE

A. Synthesis of Ba Sr Co Zn Fe O Series

The polycrystalline Ba Sr Co Zn Fe O sampleswere prepared by the solid-state reaction method. The startingmaterials of Fe O (99.42%), BaCO (98%), SrCO (98.96%),ZnO (99.8%) and Co O (99%) were mixed in a ball mill for24 h in distilled water with dispersing agent. After drying, themixed samples were calcined at 1000 C for 3 h in air. Later thesample was ball-milled again with polyvinyl alcohol (PVA).Finally, the samples were pressed into a toroidal shape withoutside diameter of 7 mm, inside diameter of 3.04 mm andthickness of 3 mm and sintered at 1100 C for 3 h in air.

B. Measurements

The phase structure of Ba Sr Co Zn Fe O wasanalyzed by X-ray diffractometer (XRD) using Cu radi-ation . The spin transition temperature ,Curie temperature , saturation magnetization andcoercivity were measured by using vibrating samplemagnetometer (VSM). Also, the hyperfine interaction and sitepreference of cation, we were measured with Mössbauer spec-trometer. Network analyzer was used to measure frequencydependence of the complex permeability and permittivity from100 MHz to 4 GHz.

III. RESULTS AND DISCUSSION

Fig. 1 shows the XRD pattern of Ba Sr CoZn Fe O sintered at 1100 C. All samples were con-firmed to be rhombohedral with space group - . Table I isresult of refined lattice constants and porosity as wellas the calculated and measured diffraction densities.With Zn substitution, lattice constants and were slightlyincreased due to differences in the ion radius of Zn (0.60in tetrahedral sites), Co (0.74 in octahedral sites) and

Fe (0.49 and 0.64 in tetrahedral and octahedral sites,respectively) [7]. With Sr substitution, the unit cell of volume

decreased from 1298 to 1295 with decreasing and. Also, the decreased from 5.44 to 5.35 g/cm , while

was increased from 4.42 to 4.65 g/cm . The decrease in is

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4292 IEEE TRANSACTIONS ON MAGNETICS, VOL. 49, NO. 7, JULY 2013

Fig. 1. Refined X-ray diffraction patterns of Ba Sr Co Zn Fe O at 295 K.

TABLE IREFINEMENT PARAMETER FOR X-RAY DIFFRACTION AND MAGNETIC PARAMETERS OF Ba Sr Co Zn Fe O at 295 K

Fig. 2. Temperature dependence of the magnetization curve for Ba SrCo Zn Fe O .

due to the facts that ion radius of Sr is smaller than that ofBa and atom mass of Sr is lighter than that of Ba. Since thePorosity can be determined as , with substitutionof Sr, the corresponding porosity becomes lower leading to theincrease of permeability.Fig. 2 shows M-T curve of Ba Sr Co Zn Fe O sin-

tered at 1100 C with applied field of 100 Oe. The substitutionof Zn for Y-type hexaferrites tends to decrease and [8],[9] while the substitution of Sr tends to increase and decrease[10]. The decreased from 215 to 183 K with increasing

Zn and increased from 183 to 192 K with increasing Sr, whiledecreased with both Zn and Sr substitutions. These suggest

the change in the spin structure with Zn and Sr substitutions.

Fig. 3. External applied field dependence of the hysteresis loops forBa Sr Co Zn Fe O at 295 K.

Fig. 3 shows hysteresis loops of Ba Sr CoZn Fe O sintered at 1100 C at 295 K. The increasingSr contents lead to the increase in and as shown in theTable I. The increase in can be attributed to the fact that thesubstituted non-magnetic Zn ion prefers tetrahedral sites onlywith down spin state and affects the super-exchange interactionwith smaller ion radius of Sr than that of Ba . We observedmagnetization was continuously increased with external field.It is due to the presence of the easy axis oriented in variousdirections in our polycrystalline sample [11].Fig. 4 shows the analysis of Mössbauer spectra of

Ba Sr Co Zn Fe O measured at 295 K. The Fe ionsare expected to locate at four octahedral sites (

CHO AND KIM: MAGNETIC PROPERTIES OF SR SUBSTITUTED Y-TYPE HEXAFERRITE 4293

TABLE IIMÖSSBAUER PARAMETER FOR Ba Sr Co Zn Fe O : (a) Ba Co Fe O , (b) Ba Co Zn Fe O , (c)

Ba Sr Co Zn Fe O

Fig. 4. Mössbauer spectra and hyperfine field to the relative site offor Ba Sr Co Zn Fe O at 295 K.

and ) and two tetrahedral sites ( and ). Table IIshows corresponding Mössbauer parameters. Since Zn prefersthe tetrahedral sites, the relative area ratios of and aresmaller compared to the value in the pure Co Y hexaferrite.The relative area ratios of Fe ion were maintained constantregardless of the Sr concentration, indicating the complete Srsubstitution for Ba in Y-type hexaferrite. Additionally theisomer shifts below 0.4 mm/s at six sites suggest the Fe ionstate [12].Fig. 5 shows the frequency dependence of complex perme-

ability of Ba Sr Co Zn Fe O sintered at 1100 C. Thepermeability at 100 MHz was increased from 2.51 to 3.17 bysubstitution Sr. However magnetic loss was dramati-cally increased from 0.1 to 0.28. These experimental observa-tions agree well with the higher density and and lowerporosity in Ba Sr Co Zn Fe Othan in Ba Sr Co Zn Fe O [2].For RF device applications, low is preferable to high

Fig. 5. Frequency dependence of complex permeability for Ba Sr CoZn Fe O at 295 K.

Fig. 6. Frequency dependence of complex permittivity for Ba SrCo Zn Fe O at 295 K.

permeability. The permeability of Ba Sr Co Zn Fe Osintered at 1100 C was 2.38 at 2.45 GHz

with . Fig. 6 shows the frequency dependence ofcomplex permittivity of Ba Sr Co Zn Fe O sinteredat 1100 C. In Ba Sr Co Zn Fe O andBa Sr Co Zn Fe O , real and imagi-nary permittivities were remained unchanged between 100MHzand 4 GHz. The low dielectric loss below 0.05makes themmoresuitable for RF applications than Ba Sr Co Zn Fe O

4294 IEEE TRANSACTIONS ON MAGNETICS, VOL. 49, NO. 7, JULY 2013

. Our experimental investigation clearly indi-cates the potential application of Ba Sr Co Zn Fe Oin RF devices in UHF band.

IV. CONCLUSION

The polycrystalline Ba Sr Co Zn Fe O sampleswere prepared by the solid-state reaction method. With Srsubstitution, the from non-collinear helical state to collinearferrimagnetic state was increased due to the changed in thespin structure. Also, Zn and Sr substitution result in larger

and with the change in super-exchange interaction.We confirm that Zn ion prefers tetrahedral sites and Sr issubstituted for Ba completely in Y-type hexaferrite fromthe analysis of the Mössbauer spectroscopy. The perme-ability of Ba Sr Co Zn Fe Osintered at 1100 C was 2.38 at 2.45 GHz withbelow 0.1. Our study suggests the potential application ofBa Sr Co Zn Fe O in RF devices in UHF band.

ACKNOWLEDGMENT

This work was supported by Mid-career Researcher Programthrough the National Research Foundation of Korea (NRF)grant funded by the Ministry of Education, Science and Tech-nology (MEST)(2012-0000169).

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