ELSEVIER Journal of Magnetism and Magnetic Materials 196-197 (1999) 309- 311

~i~ Journal of magnetism

~ l~ and magnetic

~ l~ materials

Ultrasonic attenuation in a BaTiFelaO19 single crystal

Y. Kawai a, V.A.M. Brabers b'*, Z. Sim~a c, J.H.J. Dalderop b

aFaculty of Science, Gakushuin University, Mejiro, Toshima-ku, Tokyo 171, Japan bDepartment of Applied Physics, Eindhoven University of Technology, P.O. Box 513, NL-5600 MB Eindhoven, The Netherlands

Clnstitute of Physics ASCR, Cukrovarnickh 10, 16253 Praha 6, Czech Republic

Abstract

Acoustical losses, Young's modulus and electrical conductivity of the M-type hexaferrite BaTiFe11019 are reported. The acoustical relaxations at 100 and 400 K are related to electron transitions between the Fe ions. ~© 1999 Elsevier Science B.V. All rights reserved.

Keywords: Hexaferrites; Ultrasonic attenuation; Elastic properties; Electrical conductivity; Single crystal

Besides the simple M-type barium hexaferrites MFe12019 (with M = Ba, Sr or Pb) used for permanent magnets, there has been an increasing interest in chem- ical substituted M-ferrites, which make these materials suited for magneto-optical recording or applicable as particulated recording media. Because of the substitu- tions the material becomes magnetically softer and the magnetic relaxation effects can interfere with the magnet- ization process. From studies on soft-magnetic spinel ferrites it is well-documented that identical ionic or elec- tronic relaxation mechanisms are very often responsible for the magnetic as well as the mechanical relaxations I-1-4]. Moreover, the elastic properties of these hexafer- rites are becoming of more importance, because of the thin film technology for these materials and the mechan- ical stability of these films.

This paper presents the investigation of the elastic properties of BaTiFe 11019 single crystals. Single crystals of this compound were prepared by a recrystallization process in air by means of a floating zone technique in an arc-image furnace. The polycrystalline bars used for this recrystallization process were prepared by a ceramic technique, which includes isostatically pressing, followed by sintering at 1350°C in 10 - 2 atm oxygen and cooling

*Corresponding author. Fax: +31-40-2475724; e-mail: v.a.m.brabers@phys.tue.nl.

0304-8853/99/$ - see front matter © 1999 Elsevier Science B.V. All PII: S0304-8 85 3(9 8)00729-X

down for 1 h to room temperature in 10-6 atm oxygen. The crystal structure turned out to be the magneto- plumbite structure with lattice parameters a = 5.8841 and c = 23.2814 A. From the as-grown single-crystal in- gots (qb6 mm), a rectangular sample (2.5 x 2.5 x 20 mm a) was cut for the ultrasonic measurements. One of the smallest dimensions was along the c-axis and the longest dimension of the sample was perpendicular to the c-axis. The Young's modulus (E) and acoustic losses (Q 1) were measured using a composite-bar resonator system [5] at frequencies between 100 and 200 kHz in the longitudinal mode with strain amplitudes of 10-6 and at temperatures between 85 and 600 K. The external mechanical stress, as well as a magnetic field of 600 Oe were applied parallel to the longest direction of the single crystalline specimen.

In Fig. 1 the acoustic losses and Young's modulus measured at 150 kHz are plotted as a function of temper- ature. In the acoustic losses three peaks accompanied by a softening of the crystal can be distinguished. The Young's modulus decreases with increasing temperature from 20.2x 1011 to 18.8 x 1011 dyncm -2. Data in the literature on the elastic properties of M-type hexaferrites single crystals are rare. Sorokina et al. [6] reported the elastic coefficients cij for single crystalline PbFe12019 as well as for scandium-substituted BaFe12019 [7,8]. Due to the crystal geometry we used, our Young's moduli values are in fact the values for c11, which are smaller compared with Sorokina's data, which are in the range of

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310 Y. Kawai el al. .; Journal (?1' Magnetism and Magnetic" MateriaL~ 196-197 (1999) 3t19 ,71 l

BaFeliTi 019

I0~ ~ ~19.5

Q

0 ~ 4~)0 I ~100 ~ 600 T (K)

0.40

0.30

0,20

g Q- 0.10

0.08

0.06

0.04 I 1 0 1!4 118 2!2 2 . ' 6 - -

T -i( 10 4 K-i)

!

! !

30 34 38

Fig. 1. The temperature dependence of the acoustical losses (Q ~) and Young's modulus (E) for BaFel 1TiOt<) M-type hexa- ferrite, measured in the basal plane.

Fig. 2. The electrical resistivity of BaFe~ 1TiO~ M-type hexa- ferrite determined in the direction perpendicular to the c-axis as a function of the reciprocal temperature.

22-32× l0 t l d y n cm 2. With increasing temperature a softening of c~l was also reported for the scandium- substituted barium ferrite near 135 and 220 K but no analysis in tin'ms of the relaxation mechanism was per- formed [7].

The first peak (see Fig. 1t at 580 K, which coincides with the Curie temperature of the hexaferrite, does not shift with the measuring frequency and its magnitude is dependent on the applied magnetic field. Such a behav- iour is common for other ferrites and is due to the magneto-acoustic coupling. The second peak, positioned around 400 K, is independent of the applied field and the temperature of its maximum shifts with the resonance frequency according to an Arrhenius law with an activa- tion energy of 0.47 eV and a pre-exponential frequency of 10 ~' Hz. The third peak, positioned around 100 K, is also thermally activated with a small activation energy of about 0.025 eV and a pre-exponential frequency of l0 T Hz. The magnitude of the pre-exponential frequen- cies of the two relaxations does not point to an ionic diffusion mechanism because these frequencies are far below the lattice frequencies. In spinel ferrites, similar frequencies are found for electronic transitions between mix-valent transition metal ions [5], which suggests that both relaxations are due to electron transitions between Fe z+ and Fe ~ + ions. Moreover, the derived activation energies are too low for ionic diffusion processes, which are usually of the order of I eV and higher. Since the electrical conduction in ferrites is also caused by the electron transfer between Fe-" + and Fe 3 + ions, measure- ments of the electrical resistivity were performed in the range of 300-950 K on a single crystal perpendicularly to the c-axis using a four-probe-technique: this means that the resistivity is measured in the basal plane of the hexag- onal structure. In Fig. 2 the resistivity is plotted as a function of the reciprocal temperature. At 400 K the activation energy changes from 0.09 to 0.06 eV and near

580 K an anomaly is observed, which is quite commonly found in ferro-ferrites at the Curie temperature. Unfortu- nately, the activation energies for the electrical resistivity do not correspond exactly with those for the acousti- cal losses, which indicates that, unlike in the spinel ferrites, the electron transitions dominant in tile conductivity process are not identical with those causing the acoustical losses. This phenomenon is quite plausible if we take the particular crystal structure of the hexaferrites into account, in which five crystallo- graphic sublattices exist for the Fe ions [9]. As pointed out by Zaveta [10], the variation in distances between the cations in these sublattices and in between these sublattices must result in diverse values for the activation energy for the electron transfer between the Fe ions. It is now realistic to suppose that the observed two acoustic relaxations are related to two different electron transitions between Fe ions, given the range of activation energies, 0.04-0.5 eV, which have been reported in sev- eral papers for the electrical conductivity of the hexafer- rites [10-12].

R e f e r e n c e s

[1] Y. Kawai, V.A.M. Brabers, Z. Sim~a. Proc. Int. Conf. on Ferrites (ICF6), Tokyo, 1992. p. 687.

[2] Y. Kawai, V.A.M. Brabers, Z. Sim~a, J. Magn. Magn. Mater. 104-107 (19921 407.

[3] Y. Kawai, V.A.M. Brabers, Z. Sim~a, J. Magn. Magn. Mater. 157-158 {1996) 537.

[4] Y. Kawai, V.A.M. Brabers, Z. Sim~a, IEEE Trans. Magn. 30 (1994) 966.

[5] Y. Kawai, T. Ogawa, Phys. Stat. Sol. A 76 (19831 375. [6] T.P. Sorokina, S.P. Kuntsevich. A.A. Bezlepkin. G.M.

Kvashnm, Sov. Phys. Solid State 34 (19921 215. [7] T.P. Sorokina, A.M. Kapitonov, L.N. Bexmaternykh, Ukr.

Fiz. Zhmnal 35 11990) 1042.

E Kawai et al. /Journal of Magnetism and Magnetic Materials 196-197 (1999) 309-311 311

[8] T.P. Sorokina, S.I. Burkov, B.P. Sorokin, Sov. Phys. Solid State 31 (1989) 2113.

[9] R. Gerber, Z. Sim~a, L. Jen~ovsky, Czechoslovak, J. Phys. 44 (1994) 937.

[10] K. Zavet~,, Phys. Stat. Sol. 3 (1963) 2111.

[11] R. Satyanarayana, S. Ramana Murthy, Phys. Stat. Sol. A 84 (1984) 655.

[-12] J.J. Went, G. Rathenau, E. Gorter, G. van Oosterhout, Philos. Tecb. Rev. 13 (1952) 361.