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Magnetic properties of aluminium substituted Zn3W hexagonal ferrites
941
MAGNETIC PROPERTIES OF ALUMINIUM SUBSTITUTED Zn2-W HEXAGONAL FERRITES G. A L B A N E S E , M. C A R B U C I C C H I O lstituto di Fisica, Universit~ di Parma, Parma, Italy
F. B O L Z O N I , S. R I N A L D I Lab. M A S P E C del CNR, Parma, Italy
and G. S L O C C A R I and E. L U C C H I N I lstituto di Chimica Applicata, Unioersith di Trieste, Trieste, Italy
The temperature dependence of the saturation magnetization and of the anisotropy field in BaZn2Fe,6_~ALO27 compounds for x = !, 2, 3, 4 has been measured. M6ssbauer absorption spectra of ~TFey-rays have been also measured for all the samples in the temperature range from 78 to 700°C. These data indicate that up to x = 4 aluminium preferentially substitutes the iron in the octahedral sites of the spinel block. No remarkable effects on the equilibrium of superexchange interactions have been detected.
The crystalline structure of W compounds is built up by the superposition of the so called R-block and two S blocks [1]. The cations are distributed among different sublattices denoted by the same symbols as in Zn2-W ferrite [2]. The BaZn2Fe16027(Zn2-W) ferrite presents the largest saturation magnetization among the known hexagonal ferrimagnetic oxides. A detailed study of the intrinsic properties of this c o m p o u n d has been reported recently [2]. In this paper we present and discuss M6ssbauer results and magnetic measurements on the Al substituted c o m p o u n d BaZnEFe16_,AlxOz7 for x = l, 2, 3, 4. The polycrystalline samples have been prepared by standard ceramic techniques and controlled by means of micrographic and X-rays tests. The M6ssbauer, magnetization and anisotropy measurements have been done by means of previously described techniques [2, 3]. The M6ssbauer absorption spectra of the aluminium substituted Zn2-W strongly resemble the spectrum of the pure c o m p o u n d previously studied [2]. In fig. 1 the spectra for the compounds with x = 1 and 3 at room temperature are reported. These spectra can be interpreted as the superposition of three Zeeman sextets. We attribute sextet I to iron ions in the K sublattice and to part of the iron ions in the a sublattice. The sextet II is due to the [iv and to the rem~/ining part of a sublattice while to the sextet III contributes the /vt sublattice. The Physica 86-88B (1977)941-942 © North-Holland
i
.......•.=.
,,. ~
~ o~5 r~ ~09~ 09; 09~ I
t
LI
t j
velocity (mrn/$ec)
Fig.
i. M6ssbauer
spectra
at
room
temperature
for
BaZn=Fe~6_,AI,O27 compounds for x = l (a) and x = 3 (b); I, sublattice K and part of sublattice a; II, sublattice/iv and part of sublattice a; III, sublattice/v,. sextet originated by b sublattice has not been detected because of its low intensity. The obtained spectra put in evidence a systematic decrease of the relative intensity of sextet II with increasing x indicating the preferential entrance of Al in the cationic sublattices inside the spinel b|ocks. From the M6ssbauer spectra the temperature
942 d e p e n d e n c e of the magnetic hyperfine fields (Hhf) relative to the various sextets has been determined. We may notice that the d e p e n d e n c e of Hhf(T)/Hhf(O) vs. the reduced t e m p e r a t u r e T/Tc looks very similar to the c o m p o u n d s with different aluminium content. By measuring the temperature d e p e n d e n c e of the line width, we obtained the Curie temperatures that turned out to be Tc = 648, 613, 588, 558 and 533+-5 K for x = 0, 1, 2, 3 and 4, respectively; these data agree with those obtained f r o m magnetization measurements. In fig. 2 the t e m p e r a t u r e d e p e n d e n c e of the saturation magnetization for the various compounds is reported. The values at 0 K turns out to be o'(0)= 123, 108, 94, 80 and 66 emu/g for x = 0, 1, 2, 3 and 4, respectively. We can notice that both the saturation magnetizations at 0 K and the Curie temperatures vary linearly with the AI content. The anisotropy constant Kx is reported in fig. 3 for the various x as a function of the reduced t e m p e r a t u r e T[Tc. We can notice that the behaviour for x = 0, 1, 2 and 3 is the same while the data for x = 4 deviate slightly f r o m the others. As regards the order of substitution of Fe 3÷ b y AI 3÷ in the various sublattices, the M6ssbauer data, as we noticed before clearly indicate that A! 3÷ enter only the lattice sites of the spinel block. We can now distinguish between the octahedral a and tetrahedral f~v sublattices taking into account the magnetic order of Zn2-W ferrite and the d e p e n d e n c e of the saturation magnetization at 0 K on the order of substitution. As already k n o w n [2] the magnetic order of Zn2-W 4rrMs(O)
""".,X:o
100 "~" 1
i
r
"""
15000
3
Kl (lo%.g/om3) 3.
~
O 0
A
.2
X=O X=l X=2 X'3 X=4
x 0 x
'
.~,
'
,6
'--.8-
'
• =
• o x
i
T/-~l
Fig. 3. Anisotropy constant K, vs. reduced temperature for the compounds BaZn2Fe,6_~A1,O27.
c o m p o u n d deviates f r o m the Gorter scheme [1]: in fact, the value of 35/~B m e a s u r e d at 0 K for Zn2-W can be explained only by admitting that 10% of the Fe 3+ ions in the a sublattice reverse their spin due to the weakening of the tetrahedral-octahedral superexchange interactions caused by the presence of Zn-ions in tetrahedral sublattice. The d e p e n d e n c e of the saturation magnetization at 0 K for the various x agrees very well with the values calculated assuming that all the AI 3+ ions enter statistically the a sites. The t e m p e r a t u r e d e p e n d e n c e of the anisotropy constant also agrees with the assumed order of substitution. In fact, it is known [4] that the main contribution to the anisotropy of these c o m p o u n d s c o m e s f r o m the b and the K sublattices which are not interested to the substitution. All the obtained data indicate that up to x = 4 the substitution of Fe 3+ by A13÷ in Zn2-W compound does not affect the equilibrium of the superexchange interactions. In fact for various x no differences in the curve of the sublattice magnetization vs. reduced t e m p e r a t u r e have been observed. Moreover, also the tr(T)lo'(O) vs. T/T¢ curves are the same for all x.
k References
50 1000 0
100
300
500
T(°K)
Fig. 2. Temperature dependence of the saturation magnetization for the compounds BaZn2Fe,6 ~A1=O2~.
[1] J. Smith and H.P.J. Wijn: Ferrites (Philips Technical Library, Eindhoven, 1959). [2] G. Albanese, M. Carbucicchio and G. Asti, Appl. Phys. 11 (1976) 81. [3] G. Asti and S. Rinaldi, J. Appl. Phys. 45 (1974) 3400. [4] G. Asti and S. Rinaldi presented to Joint MMM-Intermag, Conf., Pittsburg (1976).
MAGNETIC PROPERTIES OF ALUMINIUM SUBSTITUTED Zn2-W HEXAGONAL FERRITES G. A L B A N E S E , M. C A R B U C I C C H I O lstituto di Fisica, Universit~ di Parma, Parma, Italy
F. B O L Z O N I , S. R I N A L D I Lab. M A S P E C del CNR, Parma, Italy
and G. S L O C C A R I and E. L U C C H I N I lstituto di Chimica Applicata, Unioersith di Trieste, Trieste, Italy
The temperature dependence of the saturation magnetization and of the anisotropy field in BaZn2Fe,6_~ALO27 compounds for x = !, 2, 3, 4 has been measured. M6ssbauer absorption spectra of ~TFey-rays have been also measured for all the samples in the temperature range from 78 to 700°C. These data indicate that up to x = 4 aluminium preferentially substitutes the iron in the octahedral sites of the spinel block. No remarkable effects on the equilibrium of superexchange interactions have been detected.
The crystalline structure of W compounds is built up by the superposition of the so called R-block and two S blocks [1]. The cations are distributed among different sublattices denoted by the same symbols as in Zn2-W ferrite [2]. The BaZn2Fe16027(Zn2-W) ferrite presents the largest saturation magnetization among the known hexagonal ferrimagnetic oxides. A detailed study of the intrinsic properties of this c o m p o u n d has been reported recently [2]. In this paper we present and discuss M6ssbauer results and magnetic measurements on the Al substituted c o m p o u n d BaZnEFe16_,AlxOz7 for x = l, 2, 3, 4. The polycrystalline samples have been prepared by standard ceramic techniques and controlled by means of micrographic and X-rays tests. The M6ssbauer, magnetization and anisotropy measurements have been done by means of previously described techniques [2, 3]. The M6ssbauer absorption spectra of the aluminium substituted Zn2-W strongly resemble the spectrum of the pure c o m p o u n d previously studied [2]. In fig. 1 the spectra for the compounds with x = 1 and 3 at room temperature are reported. These spectra can be interpreted as the superposition of three Zeeman sextets. We attribute sextet I to iron ions in the K sublattice and to part of the iron ions in the a sublattice. The sextet II is due to the [iv and to the rem~/ining part of a sublattice while to the sextet III contributes the /vt sublattice. The Physica 86-88B (1977)941-942 © North-Holland
i
.......•.=.
,,. ~
~ o~5 r~ ~09~ 09; 09~ I
t
LI
t j
velocity (mrn/$ec)
Fig.
i. M6ssbauer
spectra
at
room
temperature
for
BaZn=Fe~6_,AI,O27 compounds for x = l (a) and x = 3 (b); I, sublattice K and part of sublattice a; II, sublattice/iv and part of sublattice a; III, sublattice/v,. sextet originated by b sublattice has not been detected because of its low intensity. The obtained spectra put in evidence a systematic decrease of the relative intensity of sextet II with increasing x indicating the preferential entrance of Al in the cationic sublattices inside the spinel b|ocks. From the M6ssbauer spectra the temperature
942 d e p e n d e n c e of the magnetic hyperfine fields (Hhf) relative to the various sextets has been determined. We may notice that the d e p e n d e n c e of Hhf(T)/Hhf(O) vs. the reduced t e m p e r a t u r e T/Tc looks very similar to the c o m p o u n d s with different aluminium content. By measuring the temperature d e p e n d e n c e of the line width, we obtained the Curie temperatures that turned out to be Tc = 648, 613, 588, 558 and 533+-5 K for x = 0, 1, 2, 3 and 4, respectively; these data agree with those obtained f r o m magnetization measurements. In fig. 2 the t e m p e r a t u r e d e p e n d e n c e of the saturation magnetization for the various compounds is reported. The values at 0 K turns out to be o'(0)= 123, 108, 94, 80 and 66 emu/g for x = 0, 1, 2, 3 and 4, respectively. We can notice that both the saturation magnetizations at 0 K and the Curie temperatures vary linearly with the AI content. The anisotropy constant Kx is reported in fig. 3 for the various x as a function of the reduced t e m p e r a t u r e T[Tc. We can notice that the behaviour for x = 0, 1, 2 and 3 is the same while the data for x = 4 deviate slightly f r o m the others. As regards the order of substitution of Fe 3÷ b y AI 3÷ in the various sublattices, the M6ssbauer data, as we noticed before clearly indicate that A! 3÷ enter only the lattice sites of the spinel block. We can now distinguish between the octahedral a and tetrahedral f~v sublattices taking into account the magnetic order of Zn2-W ferrite and the d e p e n d e n c e of the saturation magnetization at 0 K on the order of substitution. As already k n o w n [2] the magnetic order of Zn2-W 4rrMs(O)
""".,X:o
100 "~" 1
i
r
"""
15000
3
Kl (lo%.g/om3) 3.
~
O 0
A
.2
X=O X=l X=2 X'3 X=4
x 0 x
'
.~,
'
,6
'--.8-
'
• =
• o x
i
T/-~l
Fig. 3. Anisotropy constant K, vs. reduced temperature for the compounds BaZn2Fe,6_~A1,O27.
c o m p o u n d deviates f r o m the Gorter scheme [1]: in fact, the value of 35/~B m e a s u r e d at 0 K for Zn2-W can be explained only by admitting that 10% of the Fe 3+ ions in the a sublattice reverse their spin due to the weakening of the tetrahedral-octahedral superexchange interactions caused by the presence of Zn-ions in tetrahedral sublattice. The d e p e n d e n c e of the saturation magnetization at 0 K for the various x agrees very well with the values calculated assuming that all the AI 3+ ions enter statistically the a sites. The t e m p e r a t u r e d e p e n d e n c e of the anisotropy constant also agrees with the assumed order of substitution. In fact, it is known [4] that the main contribution to the anisotropy of these c o m p o u n d s c o m e s f r o m the b and the K sublattices which are not interested to the substitution. All the obtained data indicate that up to x = 4 the substitution of Fe 3+ by A13÷ in Zn2-W compound does not affect the equilibrium of the superexchange interactions. In fact for various x no differences in the curve of the sublattice magnetization vs. reduced t e m p e r a t u r e have been observed. Moreover, also the tr(T)lo'(O) vs. T/T¢ curves are the same for all x.
k References
50 1000 0
100
300
500
T(°K)
Fig. 2. Temperature dependence of the saturation magnetization for the compounds BaZn2Fe,6 ~A1=O2~.
[1] J. Smith and H.P.J. Wijn: Ferrites (Philips Technical Library, Eindhoven, 1959). [2] G. Albanese, M. Carbucicchio and G. Asti, Appl. Phys. 11 (1976) 81. [3] G. Asti and S. Rinaldi, J. Appl. Phys. 45 (1974) 3400. [4] G. Asti and S. Rinaldi presented to Joint MMM-Intermag, Conf., Pittsburg (1976).