Preparation and Magnetic Properties of Low Temperature Sintering Bi-Substituted Yttrium Iron Garnet

Hongjie Zhao, Ji Zhou∗, Zhilun Gui and Longtu Li

Department of Materials Science and Engineering, State Key Lab of New Ceramics and Fine Processing, Tsinghua University, Beijing 100084, People’s Republic of China

Keywords: Bi-substitution, Yttrium iron garnet, Low temperature sintering, Magnetic properties

Abstract: The low temperature sinterability and magnetic properties of Bi-substituted yttrium iron

garnet (YIG) polycrystalline samples were studied in this paper. The results showed that

Bi-substitution can lower garnet phase formation temperature from about 1200oC to 900

oC and

ceramic sintering temperature from over 1300oC to about 1050

oC respectively. The Bi-substitution

leads to a decrease in initial permeability and an increase in ferromagnetic resonance linewidth (ΔH)

of sintered specimens. Saturation magnetization decreases and coercive force increases a few with an

increase of Bi content. The mechanism of Bi-substitution on initial permeability and ΔH is discussed.

The change in initial permeability of Bi-substituted specimens relative to unsubstituted YIG results

from Bismuth volatilization and weakening of superexchange interaction. The lattice dilatation in

Bi-substituted YIG gives cause for a decrease of saturation magnetization and an increase of

coactivity force.

Introduction

As a new type chip electronic component, multilayer ceramic microwave devices used to Surface

Mount Technology (SMT) have attracted extensive attention due to the necessity for the miniaturi-

zation of microwave communication equipment such as mobile phone, etc. Yttrium Iron Garnet (YIG)

is appropriate magnetic material such as circulators, oscillators and phase shifters for microwave

region, because it possesses controllable saturation magnetization, high resistivity, low magnetic and

dielectric loss tangent (tanδ) in microwave region and small linewidth (ΔH) in ferromagnetic reso-

nance [1]. However, YIG garnet is hardly used in multilayer microwave components such as isolators,

circulators and phase shifters because sintering temperature (>1350oC) of YIG are generally higher

than the melting points of highly conductive metals like Ag-Pd alloy (1145oC)[2]. Obtaining

high-density microwave garnet ceramics at low sintering temperature becomes a key technology in

fabrication process of Surface Mount Device (SMD) for microwave application.

Experimental process

Materials of Bi-substituted YIG having composition Y3-xBixFe5O12, x = 0, 0.4, 0.6, 0.8, 0.9, 1.0, 1.1

and 1.2, were prepared by solid-state reaction method. The raw materials, Fe2O3(A.R.), Bi2O3 (A.R.)

and Y2O3 (99.99% purity), were mixed in a ball mill with ethanol for 24 h. The mixed powders were

calcined in the air at 900oC (0.4 ≤ x ≤ 1.2) and 1200

oC (x = 0), respectively, and then again milled for

24 h. The resulting powders were pressed in a stainless-steel die under a pressure of 40,000 N/m2 with

5wt% polyvinylalcohol as a lubricant. The pressed toroidal samples (20mm outside diameter, 10mm

inside diameters, about 2.5mm thickness) were sintered in the air at 1050oC for YIG: Bi specimens

and 1340oC for YIG specimen, respectively, and then cooled in the furnace.

Phase formation of Bi-substituted garnet ferrite was characterized by X-Ray Diffraction (XRD)

patterns using Cu Kα radiation. Sintering densification was performed by a measurement of density of

∗ Corresponding author. Fax: +86-10-62771160; email address: zhouji@mail.tsinghua.edu.cn.

Key Engineering Materials Vols. 280-283 (2005) pp 481-484Online available since 2007/Feb/15 at www.scientific.net© (2005) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/KEM.280-283.481

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 128.42.202.150, Rice University, Fondren Library, Houston, USA-12/11/14,14:45:54)

sintered specimens and by an observation of Scanning

Electron Microscopy (SEM) photographs of the fracture

surface of sintered specimens. Magnetic properties were

characterized by frequency dependence of complex

permeability using HP4291A impedance ana- lyzer and by

hysteresis loops of calcined powders using Vibrating

Sample Magnetometer (VSM). In addition, the measured

linewidth of ferromagnetic resonance (ΔH) by Electronic

Paramagnetic Resonance Spectrometer (EPRS) are used

to study influence of Bi-substitution on the microwave

magnetic properties of powders from sintered specimens.

Result and discussion

Phase formation and microstructure. Fig. 1 shows the

XRD patterns of samples for different Bi contents. The

Bi-substituted samples (0.6 ≤ x ≤ 1.2) can form a single

garnet phase at lower temperature (900oC) than formation temperature of YIG.

The microstructures of YIG and Bi-substituted YIG specimens are shown in Fig. 2. It can be seen

that the substitution of Bi for Y in YIG can markedly enhances densification of the materials and

promotes grain growth. The grain size is about 1µm for YIG specimen, while for Bi-substituted YIG it

is about 3µm. High density ( 97%≥ theoretical density, T.D.) can be obtained at 1050oC for 6 hours

from Bi-substituted YIG powders as shown in Table. 1.

Fig. 2 SEM photographs of Y3-xBixFe5O12 specimens sintered at 1050

oC for (a) x=1.0 and at 1340

oC

for (b) x=0

Table. 1 Density of Y3-xBixFe5O12 specimens

x 0 0.8 0.9 1.0 1.1 1.2

Density(g/cm3) 4.98 5.54 5.65 5.72 5.77 5.84

T.D. * (g/cm3) 5.17 5.71** 5.77** 5.83** 5.90** 5.96**

Relative T.D. (%) 96.3 97.2 97.8 98.0 97.8 97.9

* theoretical density. ** These values are calculated from T.D. of Y3Fe5O12 (5.17g/cm3) and

Bi2.88Fe5O12 (7.05g/cm3) according to Ref. [3].

Fig. 3 shows the lattice parameter of powders for different Bi contents. The lattice parameter

increases with an increases of Bi content. The lattice dilatation results from the difference of ionic

radius between Bi3+

and Y3+

, in which the ionic radius of Bi3+

is larger than that of Y3+

.

Ferromagnetic resonance. Fig. 4 shows the ferromagnetic resonance absorption curve of x=0.6

sample. The linewidths of Bi-substituted specimens are larger than that of pure YIG specimen as

shown in Table. 2.

Fig. 1 X-ray diffraction patterns of

Y3-xBixFe5O12 powders calcined at

900oC (0.4 ≤ x ≤ 1.2) and at 1200

oC

(x=0)

482 High-Performance Ceramics III

Fig. 3 Lattice parameter of Y3-xBixFe5O12

powders

Fig. 4 Ferromagnetic resonance absorption of

Bi-substituted YIG for x=0.6

Fig. 5 Ms, Mr, Hc and hysteresis loop for x=1.2 Fig. 6 Frequency dependence of complex

permeability for sintered at 1050oC specimens

Table. 2 Ferromagnetic resonance linewidth of Y3-xBixFe5O12 specimens

X 0 0.6 0.8 1.0 1.2

Linewidth(Oe) 117.2 187.7 246.3 240.5 205.3

On the one hand, substitution of Bi3+

for Y3+

causes the non-uniform dipole field, which occurs

between different cations at the same crystal sides (24c side). Therefore, the intrinsic linewidth

increases. On the other hand, an increase of linewidth for Bi-substituted YIG should result from an

increase of anisotropy due to the lattice dilatation and the Bismuth volatilization in sintering process.

Magnetic hysteresis loop. Fig. 5 shows the hysteresis loop of Bi-substituted powder (x=1.2) and

content dependence of saturation magnetization (Ms), remanent magnetization (Mr) and coercive

force (Hc). As Bi content increases, the Ms, Mr decrease a few and the Hc increases. The weakening

of superexchange interaction due to lattice dilatation in Bi-substituted YIG should give cause for a

decrease of Ms and an increase of Hc as Bi content. For x=0.4 sample, the main phase should be

(Y,Bi)FeO3, which has perovskite type structure and only possesses parasitical ferromagnetism.

Therefore, Y2.6Bi0.4Fe5O12 powder indicates smaller Ms, Mr and larger Hc.

Dependence of complex permeability. Fig. 6 shows the frequency dependence of complex

permeability for sintered at 1050oC specimens. As Bi content increases, the initial permeability (µi)

decreases obviously, while the natural resonance frequency (fr) increases. Product of µi and fr accords

with Snoek law:

Msf rr γµ3

4)1( 00 =− . (1)

On the one hand, as Bi content increases, the superexchange interaction should weaken due to the

Key Engineering Materials Vols. 280-283 483

lattice dilatation. On the other hand, a decrease of permeability with an increase of Bi content should

result from an increase of anisotropy due to the lattice dilatation and due to the Bismuth volatilization

in sintering process.

Acknowledgement

This work was supported by the Ministry of Sciences and Technology of China through 863-project

under grant 2001AA325020 and 973-project under grant 2002CB6130.

References

[1] C. Y. Tsay, C. Y. Liu, K. S. Liu, I. N. Lin, L. J. Hu and T. S. Yeh: J. Magn. Magn. Mater. Vol.

239 (2002), pp. 490.

[2] K. Matsumoto, K. Yamaguchi and T. Fujii: J. Appl. Phys. Vol. 69 (1991), pp. 5918.

[3] International Centre for Diffraction Data, Joint Committee on Powder Diffraction Standard

(JCPDS). PDF card №: 83-1027, 86-0368, 1999.

484 High-Performance Ceramics III

High-Performance Ceramics III 10.4028/www.scientific.net/KEM.280-283 Preparation and Magnetic Properties of Low Temperature Sintering Bi-Substituted Yttrium Iron

Garnet 10.4028/www.scientific.net/KEM.280-283.481

DOI References

[2] K. Matsumoto, K. Yamaguchi and T. Fujii: J. Appl. Phys. Vol. 69 (1991), pp. 5918.

doi:10.1063/1.347815