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Influence of Si substitution on the structure and magnetic properties of YFe11−xSixTi (x⩽2)
Journal of Magnetism and Magnetic Materials 242–245 (2002) 823–825
Influence of Si substitution on the structure and magnetic properties of YFe11xSixTi (xp2) C.B. Cizmasa, C. Djega-Mariadassoub,*, L. Bessaisb a
Department of Physics, Transilvania University, B-dul Eroilor 29, Brasov-2200, Romania b LCMTR, UPR-209 CNRS, 2/8 rue Henri Dunant, B. P. 28, F-94320 Thiais, France
Abstract The unit cell parameters of induction melted (I4/mmm) YFe11xSixTi (xp2) show a small reduction with x increasing. The decrease of the Curie temperature with x results from the competition between the reduction of the Fe magnetic moment and the increase of the Fe–Fe exchange interaction. Both effects are partially explained by the 3dband filling with the 3p-Si electron transfer. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Rare earth-transition metal compounds; Curie temperature; Exchange interactions; Magnetic moments
The Y–Fe intermetallics play an important role in the determination of the Fe sub-lattice contribution to the magnetism of isomorphous R–Fe alloys [1]. The R(FeM)12 compounds, where R symbolizes a rare earth or Y, M the structure stabilizing metal either d (Ti, V, Cr, Mo, Wy) or p (Si, Al), crystallize in the ThMn12type structure with I4/mmm space group. Previous studies established that the solid solution RFe12xMx range is limited around x ¼ 1 for M=Ti and x ¼ 2 for M=Si [2]. Neutron structure determination has revealed that Ti occupies the 8i sites, but Si preferentially occupies the 8f and 8j sites [3], due to the enthalpy value associated with the formation of the R–Ti or R–Si bonds [4]. The pseudobinary intermetallics YFe11Ti and YFe10Si2 are uniaxial ferromagnets with Curie temperature between 520–524 K, for YFe11Ti [5, 6], and 530– 540 K, for YFe10Si [4,7]. In both cases, the magnetic moment per Fe atom (p1.7 mB) is lower than the value of pure iron. This paper is devoted to the effect of the Si substitution for Fe on the structure, magnetic properties and also on the Fe–Fe exchange interaction in YFe11xSixTi compounds (0pxp2). Alloys, with composition x ¼ 0; 0.5, 1, 1.5, 2, were prepared by induction melting of high purity starting elements (99.9% purity). The ingots were annealed at *Corresponding author. Tel.: +01-49-78-11-96; fax: +01-4978-12-03. E-mail address: [email protected] (C. Djega-Mariadassou).
1273 K in evacuated silica tube (106 mbar) for one week and quenched into water. The chemical composition of the alloys was checked by inductively coupled plasmaatomic emission spectroscopy. X-ray diffraction analysis was carried out operating with Cu-Ka radiation for randomly powder samples by means of a Bruker diffractometer. The thermomagnetic analysis (TMA) was performed with a differential sample Manics magnetometer, in a weak external magnetic field (100 mT), in the temperature range 300–850 K. Isothermal magnetization data on magnetically aligned powders were obtained with a SQUID magnetometer, in an external magnetic field up to 5.5 T and temperature range 4.5–300 K. The powder samples, with grain size o36 mm, were mixed with epoxy resin and aligned at room temperature in a standard field of 1 T. All alloys show the tetragonal ThMn12-type structure with around 1% vol. a-(Fe–Si,Ti) and 3% vol. Fe2Ti. The unit cell parameter a decreases with Si content from 0.8503(3)nm down to 0.8442(3)nm, c remains quasiconstant and equal to 0.4784(3)nm. It results that the unit cell volume V0 decreases linearly with x; within a good approximation with the ratio DV0 =Dx equal to – 2.61 103 nm3 per Si atom. Between x ¼ 0 and 2, the observed total reduction of V0 value is 1.56%. By TMA, a single ferromagnetic 1:12-type phase was detected in the investigated temperature range. The Curie temperature values Tc of the YFe11xSixTi series decrease with x from 528 K (for x ¼ 0) to 486 K (for
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 1 0 9 8 - 8
C.B. Cizmas et al. / Journal of Magnetism and Magnetic Materials 242–245 (2002) 823–825
and 8j sites) for stoichiometric pure RFe12 compound [10]. The occupancy for each ai Fe-sites is 1 X M M nai ¼ 1 0 N pai ; ð4Þ Nai M
(a)
480 2.0 1.5 1.0
(b)
0.5 10 9
(c)
8 0.0
0.5
1.0 x
1.5
2.0
Fig. 1. Influence of Si content in YFe11xSixTi compounds (a) on Curie temperature, (b) on Fe magnetic moment, and (c) on Fe coordination number.
x ¼ 2) as also the absolute value of the ratio DTc =Dx (Fig. 1a). The magnetization isotherms of the magnetically aligned samples, measured in the magnetic field applied parallel Mjj ðTÞ to the easy magnetization direction, were used to define the saturation magnetization value Ms ðTÞ: The Ms ðTÞ value was determined by fitting the Mjj ðTÞ curves with the usual approach to saturation law MðT; HÞ ¼ Ms ðTÞ aðTÞ=H 2 þ wðTÞ H
ð1Þ
Further on, the saturation magnetization at 0 K, Ms ð0Þ was obtained by fitting the Ms ¼ Ms ðTÞ curves with the following law [8]: Ms ðTÞ ¼ Ms ð0Þf1 bðT=Tc Þn g;
ð2Þ
where b and n were found equal respectively to 0.40(70.07) and 2.5(70.6). The average magnetic moment per atom (ms;at ) was deduced from the Ms ð0Þ values. The average magnetic moment per Fe atom (ms;Fe ) was also calculated with the simple dilution law, assuming a magnetic moment equal to zero for Y, Si and Ti (Fig. 1b). Both values, ms;at and ms;Fe drastically decrease with Si content, unlike the Tc variation, the absolute value of the ratio Dms;Fe =Dx increases with Si content (Fig. 1b). Due to the substitution, a change of Fe coordination number is expected. The mean number of Fe atom nearest-neighbors to one Fe atom (ZFeFe ) can be derived explicitly on the basis of Ref. [9]. For the R(Fe,M)12 intermetallics, the ZFeFe value can be calculated in function of the Fe occupancy, for each Fe site, with the following equation P Fe ai ;aj Nai ;aj nai naj P ZFeFe ¼ : ð3Þ ai nai In Eq. (3), NaFei ;aj is the number of Fe-atoms in aj sites, nearest neighbors of one Fe-atom in site ai (ai ; aj ¼ 8f; 8i
where Na0i is the number of ai sites per unit formula, N M the number of M atoms (M=Si, Ti) per unit formula and pM ai is the substitution probability with the M atoms Si Si of ai site. We have used pTi 8i ¼ 1; p8j ¼ p8f ¼ 0:5 and Ti Ti Si p8j ¼ p8f ¼ p8i ¼ 0 [3]. It appears that ZFeFe decreases linearly from 9.75 to 8.25 with x increasing (Fig. 1c). The 3d magnetism in intermetallics between rare earth and ferromagnetic 3d-elements (Fe, Co, Ni) can be understood within the simple concept of magnetic valence Zm [11,8]. In the assumption of strong ferromagnetism, the expression of the average magnetic moment per atom can be written as: m ; m ¼ Zm þ 2Nsp
ð5Þ
m is the number of s and p electrons in the spin where Nsp up state. For the substituted YFe11xSixTi we have used m Nsp ¼ 0:3 [8] and Zm ¼ 2Ndm ZFe xFe ZY xY ZSi xSi ZTi xTi ; where Nd ¼ 5 and xk (k=Fe, Y, Si or Ti) is the atomic fraction of the k element, Zk is its chemical valence respectively equal to 8, 3, 4, 4 for Fe, Y, Si and Ti. The experimental and calculated (Eq. (5)) values of the magnetic moment per atom are reported on Fig. 2a. The comparison between the experimental and the calculated values shows that the Fe–Fe exchange interaction does not obey a strong ferromagnetism but approaches it, as the Si content increases. Such behavior has yet been mentioned for YFe12x Mox [8]. In such conditions, the Tc behavior induced by the Si substitution (Fig. 1a) can be explained only by an increase of the exchange Fe–Fe interaction which compensates partially the reduction of ms;Fe and ZFeFe :
2.0
µs, at. ( µB / at. )
500
Y Fe11-x Six Ti
1.6 1.2 0.8 0.4
250
-23
Y Fe 11-x Si x Ti
520
ZFeFe
µ s, Fe ( µ B )
Tc ( K )
540
JFeFe ( 10 J )
824
calc(Eq.3) exp.
(a) (b)
200 150 100 50 0 0.0
0.5
1.0
1.5
2.0
x
Fig. 2. (a) Average magnetic moment per atom, and (b) Exchange-coupling JFeFe parameter of YFe11xSixTi compounds.
C.B. Cizmas et al. / Journal of Magnetism and Magnetic Materials 242–245 (2002) 823–825
Consequently, we have calculated the exchange-coupling parameter JFeFe in the assumption of the molecular field approximation theory [12] with the following relation: 3kB Tc JFeFe ¼ ; ð6Þ 2ZFeFe GFe where kB is the Boltzman constant, Tc the Curie temperature of the compound, which can be considered as the Curie temperature of the Fe sub-lattice and ZFeFe is the number of Fe atoms nearest-neighbors to one Fe atom (Eq. (3)). GFe ¼ SFe ðSFe þ 1Þ ¼ p2eff =4 is the de Gennes factor of Fe, peff is the effective paramagnetic moment per Fe atom , given in the assumption of the ratio between peff and the spontaneous moment mFe of Fe equal to 2. One observes from Fig. 2b a significant increase of JFeFe with increasing Si content. The influence of Si can be analyzed in terms of the 3d3p hybridization. The Curie temperature behavior of YFe11xSixTi corresponds to the decrease of the ordering temperature of the Fe sub-lattice. It might be assumed that it results from the competition between several effects: the lattice contraction, the decrease of the Fe magnetic moment, the Fe coordination number reduction of Fe atoms, and the increase of the direct exchange interaction parameters JFeFe : The strong decrease of the magnetic moment ms ;Fe in the YFe11xSixTi series is ruled by the modification of the density of state at the Fermi level [1,2], altered by the 3pSi electron transfer into the Fe 3d-band as it was observed yet in the substitution by V [2] or Mo [8] . We
825
might consider also that the increase of the exchange interaction parameter JFeFe is favored by the Si substitution which induces an additional filling of the Fe 3d-band with the 3p-Si electrons and approaches strong ferromagnetism.
References [1] D. Givord, D. Courtois, J. Magn. Magn. Mater. 196–197 (1999) 684. [2] R. Coehoorn, Phys. Rev. B 41 (1990) 11790. [3] K.H.J. Buschow, J. Appl. Phys. 63 (1988) 3130. [4] D.B. Mooij, K.H.J. Buschow, J. Less-Common Met. 136 (1988) 207. [5] L.Y. Zhang, W.E. Wallace, J. Less-Common Met. 149 (1989) 371. [6] P.P. Hu, H.S. Li, J.P. Gavigan, J.M.D. Coey, J. Phys.: Condens. Matter 1 (1989) 775. [7] H.S. Li, M. Coey, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Vol. 6, Elsevier, Amsterdam, 1991. [8] B. Garcia-Landa, D. Fruchard, D. Ginoux, J.L. Soubeyroux, R. Vert, J. Magn. Magn. Mater. 182 (1998) 207. [9] P.E. Brommer, Physica B 173 (1991) 277. [10] L. Gelato, J. Appl. Crystallogr. 14 (1981) 141. [11] J.P. Gavigan, D. Givord, H.S. LI, J. Voiron, Physica B 149 (1988) 345. [12] N.H. Duc, in: K. A. Gschneider Jr., L. Eyring (Eds.), Handbook on the Physics and Chemistry of Rare Earths, Vol. 24, Elsevier, Amsterdam, 1997.
Influence of Si substitution on the structure and magnetic properties of YFe11xSixTi (xp2) C.B. Cizmasa, C. Djega-Mariadassoub,*, L. Bessaisb a
Department of Physics, Transilvania University, B-dul Eroilor 29, Brasov-2200, Romania b LCMTR, UPR-209 CNRS, 2/8 rue Henri Dunant, B. P. 28, F-94320 Thiais, France
Abstract The unit cell parameters of induction melted (I4/mmm) YFe11xSixTi (xp2) show a small reduction with x increasing. The decrease of the Curie temperature with x results from the competition between the reduction of the Fe magnetic moment and the increase of the Fe–Fe exchange interaction. Both effects are partially explained by the 3dband filling with the 3p-Si electron transfer. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Rare earth-transition metal compounds; Curie temperature; Exchange interactions; Magnetic moments
The Y–Fe intermetallics play an important role in the determination of the Fe sub-lattice contribution to the magnetism of isomorphous R–Fe alloys [1]. The R(FeM)12 compounds, where R symbolizes a rare earth or Y, M the structure stabilizing metal either d (Ti, V, Cr, Mo, Wy) or p (Si, Al), crystallize in the ThMn12type structure with I4/mmm space group. Previous studies established that the solid solution RFe12xMx range is limited around x ¼ 1 for M=Ti and x ¼ 2 for M=Si [2]. Neutron structure determination has revealed that Ti occupies the 8i sites, but Si preferentially occupies the 8f and 8j sites [3], due to the enthalpy value associated with the formation of the R–Ti or R–Si bonds [4]. The pseudobinary intermetallics YFe11Ti and YFe10Si2 are uniaxial ferromagnets with Curie temperature between 520–524 K, for YFe11Ti [5, 6], and 530– 540 K, for YFe10Si [4,7]. In both cases, the magnetic moment per Fe atom (p1.7 mB) is lower than the value of pure iron. This paper is devoted to the effect of the Si substitution for Fe on the structure, magnetic properties and also on the Fe–Fe exchange interaction in YFe11xSixTi compounds (0pxp2). Alloys, with composition x ¼ 0; 0.5, 1, 1.5, 2, were prepared by induction melting of high purity starting elements (99.9% purity). The ingots were annealed at *Corresponding author. Tel.: +01-49-78-11-96; fax: +01-4978-12-03. E-mail address: [email protected] (C. Djega-Mariadassou).
1273 K in evacuated silica tube (106 mbar) for one week and quenched into water. The chemical composition of the alloys was checked by inductively coupled plasmaatomic emission spectroscopy. X-ray diffraction analysis was carried out operating with Cu-Ka radiation for randomly powder samples by means of a Bruker diffractometer. The thermomagnetic analysis (TMA) was performed with a differential sample Manics magnetometer, in a weak external magnetic field (100 mT), in the temperature range 300–850 K. Isothermal magnetization data on magnetically aligned powders were obtained with a SQUID magnetometer, in an external magnetic field up to 5.5 T and temperature range 4.5–300 K. The powder samples, with grain size o36 mm, were mixed with epoxy resin and aligned at room temperature in a standard field of 1 T. All alloys show the tetragonal ThMn12-type structure with around 1% vol. a-(Fe–Si,Ti) and 3% vol. Fe2Ti. The unit cell parameter a decreases with Si content from 0.8503(3)nm down to 0.8442(3)nm, c remains quasiconstant and equal to 0.4784(3)nm. It results that the unit cell volume V0 decreases linearly with x; within a good approximation with the ratio DV0 =Dx equal to – 2.61 103 nm3 per Si atom. Between x ¼ 0 and 2, the observed total reduction of V0 value is 1.56%. By TMA, a single ferromagnetic 1:12-type phase was detected in the investigated temperature range. The Curie temperature values Tc of the YFe11xSixTi series decrease with x from 528 K (for x ¼ 0) to 486 K (for
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 1 0 9 8 - 8
C.B. Cizmas et al. / Journal of Magnetism and Magnetic Materials 242–245 (2002) 823–825
and 8j sites) for stoichiometric pure RFe12 compound [10]. The occupancy for each ai Fe-sites is 1 X M M nai ¼ 1 0 N pai ; ð4Þ Nai M
(a)
480 2.0 1.5 1.0
(b)
0.5 10 9
(c)
8 0.0
0.5
1.0 x
1.5
2.0
Fig. 1. Influence of Si content in YFe11xSixTi compounds (a) on Curie temperature, (b) on Fe magnetic moment, and (c) on Fe coordination number.
x ¼ 2) as also the absolute value of the ratio DTc =Dx (Fig. 1a). The magnetization isotherms of the magnetically aligned samples, measured in the magnetic field applied parallel Mjj ðTÞ to the easy magnetization direction, were used to define the saturation magnetization value Ms ðTÞ: The Ms ðTÞ value was determined by fitting the Mjj ðTÞ curves with the usual approach to saturation law MðT; HÞ ¼ Ms ðTÞ aðTÞ=H 2 þ wðTÞ H
ð1Þ
Further on, the saturation magnetization at 0 K, Ms ð0Þ was obtained by fitting the Ms ¼ Ms ðTÞ curves with the following law [8]: Ms ðTÞ ¼ Ms ð0Þf1 bðT=Tc Þn g;
ð2Þ
where b and n were found equal respectively to 0.40(70.07) and 2.5(70.6). The average magnetic moment per atom (ms;at ) was deduced from the Ms ð0Þ values. The average magnetic moment per Fe atom (ms;Fe ) was also calculated with the simple dilution law, assuming a magnetic moment equal to zero for Y, Si and Ti (Fig. 1b). Both values, ms;at and ms;Fe drastically decrease with Si content, unlike the Tc variation, the absolute value of the ratio Dms;Fe =Dx increases with Si content (Fig. 1b). Due to the substitution, a change of Fe coordination number is expected. The mean number of Fe atom nearest-neighbors to one Fe atom (ZFeFe ) can be derived explicitly on the basis of Ref. [9]. For the R(Fe,M)12 intermetallics, the ZFeFe value can be calculated in function of the Fe occupancy, for each Fe site, with the following equation P Fe ai ;aj Nai ;aj nai naj P ZFeFe ¼ : ð3Þ ai nai In Eq. (3), NaFei ;aj is the number of Fe-atoms in aj sites, nearest neighbors of one Fe-atom in site ai (ai ; aj ¼ 8f; 8i
where Na0i is the number of ai sites per unit formula, N M the number of M atoms (M=Si, Ti) per unit formula and pM ai is the substitution probability with the M atoms Si Si of ai site. We have used pTi 8i ¼ 1; p8j ¼ p8f ¼ 0:5 and Ti Ti Si p8j ¼ p8f ¼ p8i ¼ 0 [3]. It appears that ZFeFe decreases linearly from 9.75 to 8.25 with x increasing (Fig. 1c). The 3d magnetism in intermetallics between rare earth and ferromagnetic 3d-elements (Fe, Co, Ni) can be understood within the simple concept of magnetic valence Zm [11,8]. In the assumption of strong ferromagnetism, the expression of the average magnetic moment per atom can be written as: m ; m ¼ Zm þ 2Nsp
ð5Þ
m is the number of s and p electrons in the spin where Nsp up state. For the substituted YFe11xSixTi we have used m Nsp ¼ 0:3 [8] and Zm ¼ 2Ndm ZFe xFe ZY xY ZSi xSi ZTi xTi ; where Nd ¼ 5 and xk (k=Fe, Y, Si or Ti) is the atomic fraction of the k element, Zk is its chemical valence respectively equal to 8, 3, 4, 4 for Fe, Y, Si and Ti. The experimental and calculated (Eq. (5)) values of the magnetic moment per atom are reported on Fig. 2a. The comparison between the experimental and the calculated values shows that the Fe–Fe exchange interaction does not obey a strong ferromagnetism but approaches it, as the Si content increases. Such behavior has yet been mentioned for YFe12x Mox [8]. In such conditions, the Tc behavior induced by the Si substitution (Fig. 1a) can be explained only by an increase of the exchange Fe–Fe interaction which compensates partially the reduction of ms;Fe and ZFeFe :
2.0
µs, at. ( µB / at. )
500
Y Fe11-x Six Ti
1.6 1.2 0.8 0.4
250
-23
Y Fe 11-x Si x Ti
520
ZFeFe
µ s, Fe ( µ B )
Tc ( K )
540
JFeFe ( 10 J )
824
calc(Eq.3) exp.
(a) (b)
200 150 100 50 0 0.0
0.5
1.0
1.5
2.0
x
Fig. 2. (a) Average magnetic moment per atom, and (b) Exchange-coupling JFeFe parameter of YFe11xSixTi compounds.
C.B. Cizmas et al. / Journal of Magnetism and Magnetic Materials 242–245 (2002) 823–825
Consequently, we have calculated the exchange-coupling parameter JFeFe in the assumption of the molecular field approximation theory [12] with the following relation: 3kB Tc JFeFe ¼ ; ð6Þ 2ZFeFe GFe where kB is the Boltzman constant, Tc the Curie temperature of the compound, which can be considered as the Curie temperature of the Fe sub-lattice and ZFeFe is the number of Fe atoms nearest-neighbors to one Fe atom (Eq. (3)). GFe ¼ SFe ðSFe þ 1Þ ¼ p2eff =4 is the de Gennes factor of Fe, peff is the effective paramagnetic moment per Fe atom , given in the assumption of the ratio between peff and the spontaneous moment mFe of Fe equal to 2. One observes from Fig. 2b a significant increase of JFeFe with increasing Si content. The influence of Si can be analyzed in terms of the 3d3p hybridization. The Curie temperature behavior of YFe11xSixTi corresponds to the decrease of the ordering temperature of the Fe sub-lattice. It might be assumed that it results from the competition between several effects: the lattice contraction, the decrease of the Fe magnetic moment, the Fe coordination number reduction of Fe atoms, and the increase of the direct exchange interaction parameters JFeFe : The strong decrease of the magnetic moment ms ;Fe in the YFe11xSixTi series is ruled by the modification of the density of state at the Fermi level [1,2], altered by the 3pSi electron transfer into the Fe 3d-band as it was observed yet in the substitution by V [2] or Mo [8] . We
825
might consider also that the increase of the exchange interaction parameter JFeFe is favored by the Si substitution which induces an additional filling of the Fe 3d-band with the 3p-Si electrons and approaches strong ferromagnetism.
References [1] D. Givord, D. Courtois, J. Magn. Magn. Mater. 196–197 (1999) 684. [2] R. Coehoorn, Phys. Rev. B 41 (1990) 11790. [3] K.H.J. Buschow, J. Appl. Phys. 63 (1988) 3130. [4] D.B. Mooij, K.H.J. Buschow, J. Less-Common Met. 136 (1988) 207. [5] L.Y. Zhang, W.E. Wallace, J. Less-Common Met. 149 (1989) 371. [6] P.P. Hu, H.S. Li, J.P. Gavigan, J.M.D. Coey, J. Phys.: Condens. Matter 1 (1989) 775. [7] H.S. Li, M. Coey, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Vol. 6, Elsevier, Amsterdam, 1991. [8] B. Garcia-Landa, D. Fruchard, D. Ginoux, J.L. Soubeyroux, R. Vert, J. Magn. Magn. Mater. 182 (1998) 207. [9] P.E. Brommer, Physica B 173 (1991) 277. [10] L. Gelato, J. Appl. Crystallogr. 14 (1981) 141. [11] J.P. Gavigan, D. Givord, H.S. LI, J. Voiron, Physica B 149 (1988) 345. [12] N.H. Duc, in: K. A. Gschneider Jr., L. Eyring (Eds.), Handbook on the Physics and Chemistry of Rare Earths, Vol. 24, Elsevier, Amsterdam, 1997.