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Magnetic glass ceramics — preparation and properties
•1• ELSEVIER
Journal of Magnetism and Magnetic Materials 196-197 (1999) 264-265
Journal of magnetism and magnetic materials
Magnetic glass ceramics - preparation and properties Th. Klupsch a'*, E. Steinbeiss a, R. Miiller a, C. Ulbrich a, W. SchtippeP, H. Steinmetz ~, Th. H6che b alnstitut fiir Physikalische Hochtechnologie Jena, PF 100239. D-07702 Jena. Germat O" hOtto-Schott-lnstitut fiir Glaschemie, Friedrich-Schiller-Univelwitiit Jena, German.v
Abstract A novel Ba-ferrite powder preparation technique is presented where the magnetic particles become well separated from each other up to at least 70 mass% Ba-ferrite content. Starting from this, a compact glass ceramics can be fabricated where the separation of the Ba-ferrite crystallites is maintained. The magnetic properties of both the powder and the glass ceramics are well described by a modified Stoner-Wohlfarth behaviour. () 1999 Elsevier Science B.V. All rights reserved. Kevwords: Magnetic single-domain ensemble: Magnetic glass ceramics
In this contribution, first results regarding the preparation and magnetic properties of a novel, compact glass ceramics will be given where for the first time, to our knowledge, the idealized state of a system of magnetic, single-domain particles which are well separated from each other may be approached up to large packing densities of the magnetic constituents. This ceramics consists of non-doped Ba-ferrite particles BaFe12019 with a size distribution typical for the well-known single-domain Ba-ferrite particles (preferential diameter range from 50 to 500 nm with increasing aspect ratio from about 6 up to 9 for increasing diameter), where each particle is embedded in a SiO2-matrix. So we have a particle ensemble, the magnetic properties of which are not limited by exchange-coupled particle agglomerates showing a preferential domain-wall switching. Furthermore, a material is found the near ordering of which can be assumed to be different from that of a conventional powder material, which may result in a different dipole-dipole interaction contribution to the magnetic ensemble properties. The preparation of the magnetic glass ceramics widely follows the route of the well-known glass crystallization
* Corresponding author. E-mail: [email protected].
method [1,2] for precipitation of Ba-ferrite powders with a definite crystallite size in a quenched melt, starting from a Fe203-BaO-B203-SiO 2 mixture. In particular. the growth and the size distribution of the Ba-ferrite crystallites can be controlled by annealing of the quenched melt while, at the same time, the matrix segregates into SiOz and Ba-borate. Thus, after dissolution of Baborate, a homogeneous mixture of Ba-ferrite platelets and much finer SiO2 particles (diameter about 5 nm) with more than 80 mass% Ba-ferrite can be got avoiding any additional, mechanical homogenization treatment. To reach smaller Ba-ferrite concentrations, SiO2 powder must be added, followed by a homogenization by milling. So far, we optimized the annealing conditions (820'~C, 24 h) to get a maximum coercitivity Hc of the powder, which is about 5480 Oe. Finally, the glass ceramics is formed by sintering the pressed powder bodies at 980-1000"C. Further details are given in Ref. [3]. During sintering, the sample volume extremely shrinks to about 30% of the powder body volume so that the packing density of the Ba-ferrite particles increases by a factor of about 3 (see Table 1). The structure of the glass ceramics may be understood as a close packing of a mixture of (deformed and agglomerated) SiOz-spheres and Ba-ferrite platelets with a space filling of about 95%.
0304-8853/99/$ - see front matter ,i~ 1999 Elsevier Science B.V. All rights reserved. Pll: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 8 0 9 - 9
265
T. Klupsch et al. / Journal of Magnetism and Magnetic Materials 196-197 (1999) 264-265 Table 1 Magnetic properties of the starting powder and the glass ceramics Ba-ferrite (mass%)
Material
30
Powder Ceramics Powder Ceramics Powder Ceramics Powder Ceramics
50 70 85
Ba-ferrite (vol%) ~ 5 ~- 15 ~ 10 ~ 30 ~ 17 ,~ 50 ~ 25 ~ 70
Specific saturation magnetization (emu/g)
H~ (Oe)
Relative remanence MJM~,
21.0 20.8 35.4 34.8 50.1 49.2 61.5 61.5
5419 4707 5421 4796 5419 4810 5484 4981
0.509 0.514 0.500 0.501 0.509 0.501 0.515 0.505
Fig. 1. TEM micrograph of covered Ba-ferrite crystallites of the starting powder mixture with 80 mass% Ba-ferrite showing SiO2-particles and agglomerated SiO2-particles.
Fig. 2. TEM micrograph of dense packed, separated Ba-ferrite crystallites in the glass ceramics.
Although SiO2 and Ba-ferrite form an immiscible system, it can be shown by T E M observations that after dissolution of the Ba-borate, the Ba-ferrite crystallites are covered by the much finer SiO2-particles, probably due to adhesive forces (see Fig. 1). This effect acts to maintain the particle-particle separation of the Ba-ferrite crystallites in the glass ceramics, if at all the SiO2 content is large enough to guarantee the covering (see Fig. 2). Assuming for a crude estimation that this occurs if at least each of the surfaces of a Ba-ferrite platelet is covered by a surface layer of SiO2-spheres with one half of the maximum 2D-packing density we arrive at an upper limiting value of about 66 v o l % and 80 mass% Ba-ferrite content, respectively. The magnetic behaviour of both the Ba-ferrite SiO2powder mixtures and glass ceramics investigated so far (see Table 1) can be described by a modified StonerWohlfarth model where the reduction of the coercitivity Hc is explained, first of all, by the shape anisotropy and the particle size distribution [4,5]. The additional decrease of Hc observed for the glass ceramics compared
with the powder mixture, which is found for all Ba-ferrite concentrations, indicates that a continued, lateral growth of some large Ba-ferrite crystallites during sintering takes place so that their size becomes too large for singledomain switching. To prevent this further effort to optimize the technology is required. This contribution was supported by the D F G through grant INK6/A1.
References [1] B.T. Shirk, W.R. Buessem, J. Am. Ceram. Soc. 53 (1970) 192. [2] P. G6rnert et al., IEEE Trans. Magn. 26 (1990) 12. [3] C. Ulbrich, R. Miiller, W. Schiippel, H. Steinmetz, , E. Steinbeiss, in: K. Friedrich (Ed.), Verbundwerkstoffe und Werkstoffverbunde, DGM Informationsgesellschaft, 1997, p. 265 (in German). [4] H. Pfeiffer, Phys. Stat. Sol. A 118 (1990) 295. [5] H. Pfeiffer, W. Schiippel, Phys. Stat. Sol. A 119 (1990) 259.
Journal of Magnetism and Magnetic Materials 196-197 (1999) 264-265
Journal of magnetism and magnetic materials
Magnetic glass ceramics - preparation and properties Th. Klupsch a'*, E. Steinbeiss a, R. Miiller a, C. Ulbrich a, W. SchtippeP, H. Steinmetz ~, Th. H6che b alnstitut fiir Physikalische Hochtechnologie Jena, PF 100239. D-07702 Jena. Germat O" hOtto-Schott-lnstitut fiir Glaschemie, Friedrich-Schiller-Univelwitiit Jena, German.v
Abstract A novel Ba-ferrite powder preparation technique is presented where the magnetic particles become well separated from each other up to at least 70 mass% Ba-ferrite content. Starting from this, a compact glass ceramics can be fabricated where the separation of the Ba-ferrite crystallites is maintained. The magnetic properties of both the powder and the glass ceramics are well described by a modified Stoner-Wohlfarth behaviour. () 1999 Elsevier Science B.V. All rights reserved. Kevwords: Magnetic single-domain ensemble: Magnetic glass ceramics
In this contribution, first results regarding the preparation and magnetic properties of a novel, compact glass ceramics will be given where for the first time, to our knowledge, the idealized state of a system of magnetic, single-domain particles which are well separated from each other may be approached up to large packing densities of the magnetic constituents. This ceramics consists of non-doped Ba-ferrite particles BaFe12019 with a size distribution typical for the well-known single-domain Ba-ferrite particles (preferential diameter range from 50 to 500 nm with increasing aspect ratio from about 6 up to 9 for increasing diameter), where each particle is embedded in a SiO2-matrix. So we have a particle ensemble, the magnetic properties of which are not limited by exchange-coupled particle agglomerates showing a preferential domain-wall switching. Furthermore, a material is found the near ordering of which can be assumed to be different from that of a conventional powder material, which may result in a different dipole-dipole interaction contribution to the magnetic ensemble properties. The preparation of the magnetic glass ceramics widely follows the route of the well-known glass crystallization
* Corresponding author. E-mail: [email protected].
method [1,2] for precipitation of Ba-ferrite powders with a definite crystallite size in a quenched melt, starting from a Fe203-BaO-B203-SiO 2 mixture. In particular. the growth and the size distribution of the Ba-ferrite crystallites can be controlled by annealing of the quenched melt while, at the same time, the matrix segregates into SiOz and Ba-borate. Thus, after dissolution of Baborate, a homogeneous mixture of Ba-ferrite platelets and much finer SiO2 particles (diameter about 5 nm) with more than 80 mass% Ba-ferrite can be got avoiding any additional, mechanical homogenization treatment. To reach smaller Ba-ferrite concentrations, SiO2 powder must be added, followed by a homogenization by milling. So far, we optimized the annealing conditions (820'~C, 24 h) to get a maximum coercitivity Hc of the powder, which is about 5480 Oe. Finally, the glass ceramics is formed by sintering the pressed powder bodies at 980-1000"C. Further details are given in Ref. [3]. During sintering, the sample volume extremely shrinks to about 30% of the powder body volume so that the packing density of the Ba-ferrite particles increases by a factor of about 3 (see Table 1). The structure of the glass ceramics may be understood as a close packing of a mixture of (deformed and agglomerated) SiOz-spheres and Ba-ferrite platelets with a space filling of about 95%.
0304-8853/99/$ - see front matter ,i~ 1999 Elsevier Science B.V. All rights reserved. Pll: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 8 0 9 - 9
265
T. Klupsch et al. / Journal of Magnetism and Magnetic Materials 196-197 (1999) 264-265 Table 1 Magnetic properties of the starting powder and the glass ceramics Ba-ferrite (mass%)
Material
30
Powder Ceramics Powder Ceramics Powder Ceramics Powder Ceramics
50 70 85
Ba-ferrite (vol%) ~ 5 ~- 15 ~ 10 ~ 30 ~ 17 ,~ 50 ~ 25 ~ 70
Specific saturation magnetization (emu/g)
H~ (Oe)
Relative remanence MJM~,
21.0 20.8 35.4 34.8 50.1 49.2 61.5 61.5
5419 4707 5421 4796 5419 4810 5484 4981
0.509 0.514 0.500 0.501 0.509 0.501 0.515 0.505
Fig. 1. TEM micrograph of covered Ba-ferrite crystallites of the starting powder mixture with 80 mass% Ba-ferrite showing SiO2-particles and agglomerated SiO2-particles.
Fig. 2. TEM micrograph of dense packed, separated Ba-ferrite crystallites in the glass ceramics.
Although SiO2 and Ba-ferrite form an immiscible system, it can be shown by T E M observations that after dissolution of the Ba-borate, the Ba-ferrite crystallites are covered by the much finer SiO2-particles, probably due to adhesive forces (see Fig. 1). This effect acts to maintain the particle-particle separation of the Ba-ferrite crystallites in the glass ceramics, if at all the SiO2 content is large enough to guarantee the covering (see Fig. 2). Assuming for a crude estimation that this occurs if at least each of the surfaces of a Ba-ferrite platelet is covered by a surface layer of SiO2-spheres with one half of the maximum 2D-packing density we arrive at an upper limiting value of about 66 v o l % and 80 mass% Ba-ferrite content, respectively. The magnetic behaviour of both the Ba-ferrite SiO2powder mixtures and glass ceramics investigated so far (see Table 1) can be described by a modified StonerWohlfarth model where the reduction of the coercitivity Hc is explained, first of all, by the shape anisotropy and the particle size distribution [4,5]. The additional decrease of Hc observed for the glass ceramics compared
with the powder mixture, which is found for all Ba-ferrite concentrations, indicates that a continued, lateral growth of some large Ba-ferrite crystallites during sintering takes place so that their size becomes too large for singledomain switching. To prevent this further effort to optimize the technology is required. This contribution was supported by the D F G through grant INK6/A1.
References [1] B.T. Shirk, W.R. Buessem, J. Am. Ceram. Soc. 53 (1970) 192. [2] P. G6rnert et al., IEEE Trans. Magn. 26 (1990) 12. [3] C. Ulbrich, R. Miiller, W. Schiippel, H. Steinmetz, , E. Steinbeiss, in: K. Friedrich (Ed.), Verbundwerkstoffe und Werkstoffverbunde, DGM Informationsgesellschaft, 1997, p. 265 (in German). [4] H. Pfeiffer, Phys. Stat. Sol. A 118 (1990) 295. [5] H. Pfeiffer, W. Schiippel, Phys. Stat. Sol. A 119 (1990) 259.