Physica B 327 (2003) 266–269 Influence of La doping on the properties of SrBa hexagonal ferrites Pham Quang Niema, Nguyen Chaua,*, Nguyen Hoang Luonga, Dang Le Minhb b a Center for Materials Science, National University of Hanoi, 334 Nguyen Trai, Hanoi, Viet Nam Department of Solid State Physics, National University of Hanoi, 334 Nguyen Trai, Hanoi, Viet Nam Abstract A series of SrBa ferrites with composition (Sr0.75Ba0.25)1Àx(La2O3)x/2 Á 5.3Fe2O3 has been prepared by the conventional ceramic technology The size of the crystallites corresponds to that of a single magnetic domain Doping of La not only enhances the coercivity of these materials but also improves the remanence The reasons for improving the hard magnetic properties of the investigated ferrites are discussed r 2002 Elsevier Science B.V All rights reserved Keywords: Magnetic oxides; Hard magnetic materials; Fine particles; Hexagonal ferrites The high quality of hard magnetic materials is determined by three main parameters of the demagnetization curve in the second quadrant: the coercive field HC ; the remanence Br and the convex coefcient Z ẳ Bd Hd =BHịmax : The magnetization process of hard ferrites consisting of single magnetic domain particles is governed by the rotation process of the domain moments In that case the coercive field can be expressed as follows: HC ẳ a K1 ls t ỵ bN1 N2 ịIs ỵ c ; Is Is 1ị where a; b; c are numerical constants, N1 and N2 are demagnetization factors along two perpendicular directions, lS is the magnetostriction and t the mechanical strain The first term in expression (1) corresponds to the contribution of the magnetocrystalline aniso*Corresponding author Tel./fax: +81-4-858-9496 E-mail address: chau@cms.edu.vn (N Chau) tropy of the material, the second one is given by the shape anisotropy of the crystalline particles (single magnetic domains) and the third one originates from the action of the elastic mechanical deformation In fact, the first term plays the decisive role for creating a high coercivity, the second term can take part of several tens of a percent and the last one can only contribute several hundreds of Gauss to H C For that reason, almost all authors have concentrated their investigations to enhance the magnetic anisotropy by partly substituting Me or Fe in the hexagonal ferrite MeO Á 6Fe2O3 by various elements including rare-earth elements Recently, the substitution of small amounts of SrO by La2O3 in the hexagonal Sr ferrites has been found to lead to an evident improvement of the magnetic parameters [1–5] In this paper we present our study of the influence of La doping on the structure and properties of a SrBa hexagonal ferrite 0921-4526/03/$ - see front matter r 2002 Elsevier Science B.V All rights reserved PII: S - ( ) - P.Q Niem et al / Physica B 327 (2003) 266–269 Intensity (a.u) Raw materials of Fe2O3, SrCO3, BaCO3, La2O3 and SiO2 with high purity have been used as starting materials The investigated ferrite system has the following composition: (Sr0.75Ba0.25)1Àx (La2O3)x/2 Á 5.3Fe2O3 where x ¼ 0:00; 0.02, 0.04, 0.06 The ratio (SrO, BaO): Fe2O3=1:5.3 was chosen according to Refs [6,7] Powders were mixed and milled in a vibrating ball mill, pressed in pellets and presintered at 12501–12601C for 1–2 h and then milled in the vibrating-ball mill with milling times varying from 24 to 48 h Powder samples were isotropically and anisotropically pressed at pressures of and 0.3 t/cm2, respectively, and then sintered at 1250–12601C for 1– 1.5 h in air The experimental techniques employed in our investigations include: thermal analysis (by SDT 2960, TA-Instruments), crystallographic-structure analysis (by X-ray diffractometer D 5005, Bruker), particle-size measurements (by Mastersizer Microplus Ver 2–17, Malvern), microstructure analysis (by scanning electron microscope (SEM) 5410 LV, Jeol) and magnetic measurements (by vibratingsample magnetometer (VSM) DMS 880, Digital Measurement Systems) From the DSC analysis of the powder mixtures we observe that at around 8501C there is a thermal decomposition of SrCO3 and BaCO3 In this temperature region, the ferritization reaction is strongly enhanced Moreover, the temperature at which the ferritization reaction stops increases with increase of the La doping, namely 11191C, 11271C, 11431C and 11621C corresponding with x ¼ 0; 0.02, 0.04 and 0.06, respectively The X-ray diffraction patterns (Fig 1) show that the samples are with hexagonal structure Their lattice parameters are collected in Table From this table one can see that the lattice parameter c slightly increases whereas the parameter a is somewhat fluctuating The particle size analysis shows that after milling for 48 h in the vibrating-ball mill, the particles are reduced in dimension with increasing La doping in the sample Fig presents a SEM picture of an anisotropic sample with x ¼ 0:02 taken along the axis perpendicular to the preferred magnetization direction We can see from this figure that almost all the 267 x = 0.06 x = 0.04 x = 0.02 x = 0.00 20 30 40 50 60 70 80 90 2-Theta Scale Fig X-ray diffraction patterns of (Sr0.75Ba0.25)1Àx (La2O3)x/2 Á 5.3Fe2O3 samples for different x-values Table Lattice constants samples x ( a (A) ( c (A) of (Sr0.75Ba0.25)1Àx(La2O3)x=2 Á 5.3Fe2O3 0.00 0.02 0.04 0.06 5.881 23.058 5.875 23.062 5.879 23.064 5.880 23.070 Fig SEM picture of an anisotropic (Sr0.75Ba0.25)1Àx (La2O3)x/2 Á 5.3Fe2O3 sample with x ¼ 0:02 along the plane perpendicular to the preferred direction grains have the size of a single magnetic domain (o1.3 mm) Moreover, there is a high degree of orientation of the particles in the anisotropic pressing The role of SiO2 in our sample is to limit the grain growth Fig shows the hysteresis loop of an anisotropic sample with x ¼ 0:06 measured parallel and perpendicular to the preferred magnetization P.Q Niem et al / Physica B 327 (2003) 266–269 268 isotropic samples For the isotropic ferrite (Sr0.75Ba0.25)0.94(La2O3)0.03 Á 5.3Fe2O3, the energy product (BH)max reaches the rather high value of 1.34 MG Oe Substitution of La for (Sr, Ba) leads to the following reaction: direction We see that along the ‘‘hard’’ axis the magnetization is far from saturation and that the squareness coefficient of the hysteresis loop is much higher for the ‘‘easy’’ axis The hysteresis loop parameters of the anisotropic samples are collected in Table From this table we suggest that the orienting magnetic field in the pressing process was too low (in our experiment it was 10 kOe) to achieve a complete orientation of the ferrite particles From Table it seems that doping of La in our samples not only improves the coercivity and magnetization but the maximum energy product is also enhanced due to the increase of the squareness S (and therefore increase of the convex coefficient) We also can conclude from Table that doping of La leads to an increase of the hysteresis loop parameters of La Sr; Baị2ỵ ỵFe3ỵ - La3ỵ ỵFe2ỵ : 2ị Sr;Ba The formed Fe2+ ions possibly are located at the 4f1, 4f2 positions This leads to an increase of the total magnetization This conclusion agrees with results from the temperature dependence of the saturation magnetization of these samples Fig As we can see from Tables and 3, at using small amounts of La2O3 (o6 mol%) as a substitute for (SrO, BaO) and at applying a traditional ceramic technology, the magnetic properties of hexagonal (Sr, Br) ferrites have been improved As explained in Refs [4,5], the appropriate La2O3 amount and the small SiO2 doping contribute to the creation of needle shape particles with the size of a single magnetic domain, leading to an increase of the magnetization as well as the magnetic anisotropies (both magnetocrystalline anisotropy and shape anisotropy of the particles) All these Table Characteristics of demagnetization curves of isotropic ferrite samples (sintering temperature 12501C) Fig Hysteresis loops of an anisotropic (Sr0.75Ba0.25)1Àx (La2O3)x/2 Á 5.3Fe2O3 sample with x ¼ 0:06 measured parallel and perpendicular to the preferred direction Sample x ¼ 0:00 x ¼ 0:02 x ¼ 0:04 x ¼ 0:06 Br (kG) B HC (kOe) (BH)max (MG Oe) 1.96 1.98 1.03 2.07 2.12 1.22 2.23 2.11 1.34 2.01 2.09 1.09 Table Characteristics of hysteresis loops of anisotropic samples (sintering temperature 12601C) Sample x ¼ 0:00 Direction > > > > j HC (Oe) M13.5 (emu/g)a Mr (emu/g) S 1926 55 22 0.40 1886 55 30 0.54 3513 58 24 0.41 3500 62 24 0.71 3089 57 23 0.39 2987 61 41 0.67 3371 57 24 0.43 3243 61 44 0.70 a x ¼ 0:02 M13:5 is magnetization measured at 13.5 kOe x ¼ 0:04 x ¼ 0:06 P.Q Niem et al / Physica B 327 (2003) 266–269 Â 103 21:8 Â 103 O cm) From this result it could be concluded that substitution of La for (SrBa) leads to a change of valence of Fe3+ ions to Fe2+ ions at the 4f2 position in the octahedral sublattice 10 x = 0.00 x = 0.02 80 M (emu/g) x = 0.04 x = 0.06 60 40 Acknowledgements 20 100 269 200 300 400 500 600 700 800 We express our sincere thanks to the National Program for Fundamental Research for financial support T (K) Fig Thermomagnetic curves of isotropic (Sr0.75Ba0.25)1Àx (La2O3)x/2 Á 5.3Fe2O3 samples for different x-values (sintered at 12501C, maximum applied field 13.5 kOe) factors see Eq (1) contribute to the hard-magnetic properties of the studied ferrites Measurements of the electrical properties of the samples show that the decrease of the specific resistance in the doped sample is related to the enhancement of the conductivity due to the formation of Fe2+ ions in the octahedral sublattice when La3+ ions are substituted for (Sr, Ba)2+ (see Eq (2)) In fact, doping of La leads to a strong decrease of the resistance of the samples (x ¼ 0; r ¼ 3:2 Â 105 O cm; x ¼ 0:0220:06; r ¼ References [1] H Yamamoto, M Nagakura, H Tarada, IEEE Trans Magn 26 (1990) 1144 [2] H Taguchi, T Takeishi, K Suwa, K Masuzawa, Y Minachi, J Phys IV (France) (1997) C1-311 [3] M Sagawa, H Nagate, T Wantanabe, O Itatazi, J Phys IV (France) (1997) C1-307 [4] N.K Dung, N Chau, B.T Cong, D.L Minh, N.X Phuc, J Phys IV (France) (1997) C1-313 [5] N.K Dung, N Chau, B.T Cong, D.L Minh, Proceedings of the Third International Workshop on Materials Science (IWOMS’99) Hanoi, November 2–4, 1999, p 357 [6] N Chau, J Science of the Hanoi University, Phys (1990) 51 [7] C.Z Srig, D Bonnengerg, K.A Hempel, P Karduck, H.J Kloor, Ch Saner, J Phys (France) (1997) C1-315 ... located at the 4f1, 4f2 positions This leads to an increase of the total magnetization This conclusion agrees with results from the temperature dependence of the saturation magnetization of these... that the decrease of the specific resistance in the doped sample is related to the enhancement of the conductivity due to the formation of Fe2+ ions in the octahedral sublattice when La3 + ions... reaches the rather high value of 1.34 MG Oe Substitution of La for (Sr, Ba) leads to the following reaction: direction We see that along the ‘‘hard’’ axis the magnetization is far from saturation