Structural and magnetic characterization of ( Ba , Sr )-hexaferrite powders ( , )

Results on magnetic and structural characterization of ferrimagnetic compounds of BaxSrj.xFei20i9 (x=0.0, 0.25, 0.50, 0.75 and 1.0) prepared by the conventional ceramic method are reported. The samples were systematically examined using atomic forcé microscopy (AFM), X-ray diffraction and vibrating sample magnetometer. Structural and magnetic differences among the specimens were observed. The relations between structural features and magnetic properties are discussed. The presintering temperatures of the samples were 800°C and 1,000°C. The specimens were sintered at 1,200°C for one hour.


INTRODUCTION
A group of iron oxide materials named Ferrites with the formula MO6Fe 2 0 3 , where M is one or more of the divalent metáis Ba, Sr and Pb, is one of the most commercial magnetic materials produced.Substitution ferrites, where Co-Ti or Co-Sn substitutes some iron, have received much attention due to their potential application as high-density magnetic storage systems (1).Also they have been extensively used as permanent magnets and microwave devices because of their large intrinsic uniaxial anisotropy and their high coercivity (2).This kind of materials can be prepared using various techniques (3)(4)(5)(6)(7)(8)(9)(10). The technique of preparation determines the magnetic properties of the samples and therefore discrepancies have been observed in the valúes of the magnetic parameters when different preparation techniques are used (11).Also, several studies were made on samples with the composition (BaO) 075 (SrO) 0256 Fe 2 O 3 with addition of group III oxides (12), on single crystal of mixed Ba-Sr ferrites as single crystal (13) and on samples prepared with the self propagating high temperature synthesis method (SHS) (14).To optimize hard ferrites for permanent magnet applications, a compromise is usually assumed between a high valué of remanence or a high coercivity.The remanence is a strong function of density, chemistry, and orientation, while the coercive forcé is dependent on microstructural features, and thus related with the grain size (15).In this work the influence of the microstructural features of the grain for different concentration of Ba and Sr in samples of Ba x Sr," x Fe 12 0 19 over the coercive forcé is studied.Results of surface characterization in samples prepared by the conventional ceramic method with different concentrations of Ba and Sr using an atomic forcé microscope, AFM (16) are presented.AFM is able to provide information about microstructural characteristics of the surface that might be correlated with the formation or growing parameters of the researched materials.These results are complemented with X-ray patterns of the samples.Magnetic characterization was performed using a vibrating sample magnetometer (VSM) in order to obtain coercivity (iHc) and saturation magnetization (o s ).

EXPERIMENTAL PROCEDURE
(Ba,Sr)-M type ferrites were prepared by the ceramic method.In order to produce ferrites by this method the basic powders BaC0 3 , SrC0 3 and Fe 2 C0 3 were mixed stoiquiometrically to obtain Ba x Sr!_ x Fe 12 0 19 , (x=0.0, 0.25, 0.50, 0.75 and 1.0).High purity powders from Strem Chemicals were used.In the preparation of the specimens, all the constituent oxide powders were mixed homogeneously by ball milling in ethanol for two hours.After this, the resulting mixture was ground in air using also a silicon nitride mortar in order to reduce the particle size.Powders were presintered at 800 °C or 1,000 °C during 2.75 h.A heating rate of 550 °C /h was used.Polyvinyl alcohol was used as a binder and samples of different sizes were pressed as discs by applying a pressure of 35 MPa.For the sintering process the samples were heated at 300 °C /h, hold at 600 °C during one hour and finally heated at 300 °C /h up to 1,200°C.A sintering time of one hour was used.The topographic surface images were obtained using an atomic forcé microscope (Nanoscope Illa from Digital Instruments).AFM pro vides high resolution surface profiles.By AFM we estímate the root mean square (RMS) roughness, from a representative grain; the mean grain size, MGS, and the shape topography.The grain size was estimated from a representative área of 10 X 10 mm 2 using a specialized software that takes as height threshold the height of larger frequency in the height spectral distribution.These spectra have a Gaussian shape in all cases.Thus, we selected the médium height to measure the grain size and we use boundary grain criteria to define the grains.In this case, grains are the zones closed and disconnected by boundaries from other regions.
X-ray analysis of the samples was carried out with a diffractometer Rigaku 2200.Magnetic characterization was performed with a model 3001 Oxford vibrating sample magnetometer, at room temperature, in spherical samples.Corrections due to the demagnetizing field were done according to the shape of the samples.The weight percentage of Ba and Sr in several grains was estimated using an energy dispersive X-ray spectrometer, EDX, in a JEOL scanning electrón microscope, SEM.two kinds of grain size distribution, one narrow for the samples with x = 0.25, 0.50, and 0.75, and one wide for the samples with just one component, barium (x = 1) or strontium (x = 0).On the other hand, only small differences in the main grain size was measured among the different samples.These differences consists of a bit larger main grain size for the samples with x = 0.0, 0.50, and 1.0.Thus, the x = 0.5 and x = 0 have a RMS roughness greater than samples with MGS smaller; i.e. those for x = 1, x = 0.25 and x = 0.75.Figure 2 shows for this set of samples the coercivity and saturation magnetization valúes as a ñmction of concentration.This figure shows that the magnetic properties are improved in the samples with two components.Therefore, for samples with x = 0 and x = 1, we found poorest magnetic properties than samples with x = 0.25, x = 0.50 and x = 0.75.Better valúes of magnetic properties exhibited by samples with x = 0.25 and x = 0.5 were related with a more regular grain shape and a narrow grain size distribution.For x = 0.75 the valúes of magnetic properties diminish although their valúes are greater than those of puré samples.We can observe in AFM pictures that samples with x = 0.75 show an irregular grain shape and a larger grain roughness than the samples wiht x = 0.25 and 0.50.This fact might be related with the lower coercivity and saturation magnetization observed.The irregular shape and grain roughness is related with the presintered temperature.

RESULTS AND DISCUSSION
Figure 3 shows three-dimensional AFM images for the samples with x = 0.75 for presintered temperature of 800°C and 1,000°C.From the figure, we can observe that the grain roughness for the sample presintered at 1,000 °C is smoother than the grain roughness for the sample presintered at 800 °C.The measured RMS roughness valúes are 17.769 nm and 11.398 nm for the samples presintered at 800 and 1,000 °C, respectively.This fact has a correspondence with the coercivity valúes measured in both samples of 3,250 and 3,300 Oe, respectively.The difference in these valúes is whitin the resolution of the magnetometer, which it is about 0.5 %.On the other hand, the sample with x = 0.25 (sample with the best magnetic valúes) has a RMS roughness of 9.219 nm.From this we can conclude that it is possible a qualitative estimation of the growing grain stage using as parameters the roughness and the shape of the grain.
Grain size and homogeneity have been reported as important parameters for magnetic properties in hexaferrites (13 and 15).However, we assumme that there is a strong relationship of magnetic properties with other parameter like the dispersión of the grain size distribution and the grain roughness.It has been reported that the M phase in barium ferrites is reached for smaller temperatures (8) than the presintering and sintering temperatures we used.The samples have a quite homogeneus chemical composition.Little deviation from the expected stoiquiometry was determined by microanalysis with EDX.In five ramdom selected grains these diferences in strontium and barium content consisted of about 0.5 and 0.2 %wt for the samples with x = 0.25 and 0.6 and 0.5 % wt for the sample with x = 0.75.the BaFe 12 0 19 phase (17).Although we do not show XRD patterns obtained at each step of samples preparation, it was found that the main phase of the Mtype ferrites is obtained even at the presintering stage.
From the result we can conclude, that the principal factors determining the magnetic properties for this set of samples are the grain size distribution, the shape of the grains and the grain roughness, which are controlled by the grade mixture of Ba-Sr and the presintered temperature.Other aspects as mean grain size and homogeneity have small changes in this case and his contribution to the magnetic valúes are minimum.dimensional images of the surface microtopography of the samples allow to obtain information about the structural detail of the grain, which may be an important parameter related with the magnetic properties.It was observed that there is a strong relationship between the uniformity of the grain size distribution and the magnetic properties, besides of the grain size, as it has been reported.The structural characterization of hexaferrites of Ba-Sr by means of AFM technique have shown that the shape and the grain size distribution of the samples have a strong dependence on the concentration of Ba-Sr.Furthermore, the presintering temperature regulates the shape and the structure of the grains.We conclude that the mixture (Ba-Sr) diminishes the range of particles size and the irregularity in the particles shapes of mechanically alloyed Ba-and Sr-hexaferrite powders (3).Therefore the magnetic parameter valúes were improved in Ba^r^Fe^O^ ferrites.Samples with x = 0.50 have good magnetic properties related with a narrow size distribution of large grains with uniform shape.Samples with x = 0.25 have the best magnetical properties for application as permanent magnet.These samples present regular shape grains with the smallest size.The grain size distribution is narrow and uniform.Samples with x = 0.75 present poorer magnetic properties due to a more irregular grain shape.
Another important conclusión we found was that the roughness of the grain can indícate, at least in qualitative form, the grain growing stage.This fact permits us to relate the grain shape with concentration and presintering temperature.
Furthermore an appropriate presintered temperature (about 1,000°C) and a low concentration of Ba, improve the magnetic properties of the Ba-Sr hexaferrites prepared by the ceramic method.The resulting ferrites have better magnetic parameters than commercial ferrites prepared by conventional ceramic routes or mixed ferrites prepared by alternative methods, as the SHS method.