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Magnetic ZnFe2O4 nanoferrites studied by X-ray magnetic circular dichroism and Mössbauer spectroscopy
ARTICLE IN PRESS
Physica B 389 (2007) 155–158 www.elsevier.com/locate/physb
Magnetic ZnFe2O4 nanoferrites studied by X-ray magnetic circular dichroism and Mo¨ssbauer spectroscopy S.J. Stewarta,, S.J.A. Figueroaa,b, M.B. Sturlaa, R.B. Scorzellic, F. Garcı´ ad, F.G. Requejoa,b a
IFLP, CONICET, Dto. de Fı´sica, C.C. 67, Fac. Cs. Exactas, Universidad Nacional de La Plata, 1900 La Plata, Argentina b INIFTA, Fac. Cs. Exactas, Universidad Nacional de La Plata, Argentina c Centro Brasileiro de Pesquisas Fı´sicas, Rua X. Sigaud 150, Rio de Janeiro, Brazil d Laborato´rio Nacional de Luz Sı´ncrotron, Campinas, SP, Brazil
Abstract ZnFe2O4 nanoparticles (6 nm) were synthesized by hydrothermal methods. Subsequent mechanical treatment applied to the nanocrystalline material caused an increment of the grain size up to 13 nm. The samples were characterized by X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) at the Fe–K edge and 57Fe Mo¨ssbauer spectroscopy. The absorption pre-edge features indicate that Fe3+ ions occupy non-centrosymmetric sites. XAS data evidence the presence of Fe3+ tetrahedrally coordinated while XMCD spectra reflect the magnetic character of the compound The Mo¨ssbauer results show a superparamagnetic behaviour with blocking temperatures at 40 and 250 K for 6 and 13 nm samples, respectively. The 4.2 K Mo¨ssbauer spectra reflect that Fe3+ ions occupy both octahedral and tetrahedral spinel sites. All these results provide consistent evidence of the high degree of inversion achieved by these combined methods, which modifies the long-range ordering. We also demonstrated that, starting from a non-equilibrium state, an increment of the inversion activated by the milling occurs in spite of the augment of the grain size. r 2006 Elsevier B.V. All rights reserved. PACS: 75.50.Tt; 81.20.Wk; 76.80.+y; 61.10.Ht Keywords: Zinc ferrite; ZnFe2O4; Cation redistribution; Mo¨ssbauer; Mechanical milling; Fe–K XMCD; Spinel
Zinc ferrite ZnFe2O4 is an antiferromagnetic normal spinel with Zn2+ tetrahedrally coordinated by oxygens (sites A) and Fe3+ having an octahedral surrounding (sites B). When this ferrite is in a nanostructured state, Fe3+ and Zn2+ distribute amongst both sites, conferring a ferrimagnetic long-range ordering [1–5]. To consider this non-equilibrium distribution the site occupancy is usually expressed as (Zn1cFec)[ZncFe2c]O4, c being the inversion parameter (0pcp1). An increment of c has always been associated to the grain size reduction, independently whether the sample was synthesized by chemical or mechanical treatments [1–5]. Fe–K X-ray absorption spectra contain information on the electronic transition 1s-4p and the Fe local structure. The X-ray circular dichroism (XMCD) signal is the difference between absorption coefficients m+ and m, in Corresponding author. Fax: +54 221 4252006.
E-mail address: stewart@fisica.unlp.edu.ar (S.J. Stewart). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.07.045
the vicinity of an absorption edge (XANES region), for opposite X-rays circular polarization or alternatively, opposite saturation magnetization parallel (+) and antiparallel () to the direction of X-rays propagation. The analysis of this spin-dependent absorption is used to determine the ferro- or ferrimagnetic properties in the unoccupied state. In the case of Fe–K edge, since the magnetic features are probing by promoting transitions to the 4p bands, the magnetism will be only due to the spin–orbit coupling in the final state. XMCD on spinels allows also identifying site symmetries from the fine structure nearby the Fe–K edge [6]. To probe the local magnetic environment of Fe atoms in nanosized ZnFe2O4, we report here XMCD and Mo¨ssbauer studies on nanoferrites chemically prepared that were afterwards mechanically treated. These experiments will allow us to identify the Fe local coordination to be related with their magnetic properties and microstructure.
ARTICLE IN PRESS S.J. Stewart et al. / Physica B 389 (2007) 155–158
156
ZnFe2O4 nanoferrite (Sample 2ZF) was synthesized by a hydrothermal process. Sample 2ZF_10H was obtained by high-energy ball milling the nanocrystalline 2ZF sample for 10 h. Details of sample preparation will be published elsewhere [7]. The XRD patterns only show the Bragg peaks of ZnFe2O4 spinel (Fig. 1). The broad lines evidence the nanocrystalline size dimension achieved with the
Fig. 1. X-ray patterns of nanoferrites samples 2ZF and 2ZF_10 H. A diffractogram of bulk ZnFe2O4 is also included for comparison.
Fig. 2. Room temperature Mo¨ssbauer spectra as a function of the milling time (left). The solid lines correspond to the fitting by assuming a distribution of hyperfine fields Bhf (right).
hydrothermal procedure. As nanocrystalline 2ZF sample is milled, the lines become narrower. The analysis of these data showed that the mechanical treatment causes an increment of the average grain size, D, from 6 (1) to 13 (2) nm, and, at the same time, the micro-strain level decreases. Similar results were obtained by milling CuFe2O4 nanoparticles [8]. The RT Mo¨ssbauer spectra show a broad Fe3+ central doublet (Fig. 2), whose lines broaden after milling. No evidence of a Fe2+ signal was found. We fitted these spectra by considering two Lorentzians doublets DI and DII (Table 1). The DI component, with a lower quadrupole splitting, QS, can be assigned to Fe at B sites, while DII would represent contribution from 57Fe nucleus sensing a more distorted surrounding. However, the similar d values and the lack of resolution do not allow us to clearly distinguish sites A and B. The increment of the QS in nanocrystalline ZnFe2O4 as the crystallite size decreases has several origins that make the different contributions difficult to be resolved [3]. Indeed, (i) Fe atoms locate at the surface grain or interfaces regions with a more distorted environment, (ii) the nonequilibrium cation distribution that also distorts the interstitial sites modifying bond lengths and bond angles on both A and B sites, (iii) the magnetic character of the nanocrystalline ferrite due to the cation redistribution and (iv) relaxation effects, all contribute to the broad Mo¨ssbauer linewidths, G, observed. These contributions were all included in our fitting model by considering a distribution of QS sites. In such case, we observe that the average QS increases and the width of the distribution, s, broadens with D. Indeed, for 2ZF, d ¼ 0:34 mm=s, QS ¼ 0.47 mm/s and s ¼ 0:39 mm=s. While for 2ZF_10 H, d ¼ 0:34 mm=s, QS ¼ 0.81 mm/s and s ¼ 0:70 mm=s. We observed that as the temperature decreases the relative area of the doublet decreases in favor of a magnetic component through a relaxation process (not shown). This behaviour is typically displayed for nanomagnetic compounds due to the superparamagnetic relaxation of particle moments amongst the easy magnetization directions. Considering the blocking temperature, TB, as the temperature at which the relative areas of the non-magnetic and magnetic signals equal, a TB near 40 K and 250 K result for 2ZF and 2ZF_10 H, respectively. The 4.2 K Mo¨ssbauer spectra are composed by resolved sextets with broad absorption lines, being the relaxation effect negligible (Fig. 3). The shape of these spectra reflects
Table 1 Mo¨ssbauer parameters as described in the text from fitting the RT spectra assuming two Lorentzians doublets DI and DII Sample
dI (mm/s)
QSI (mm/s)
GI (mm/s)
RI%
dII (mm/s)
QSII (mm/s)
GII (mm/s
RII %
ZnFe2O4 (bulk) 2ZF 2ZF_10H
0.351 0.341 0.351
0.351 0.401 0.471
0.301 0.321 0.322
100 705 343
0.341 0.331
0.711 0.541
0.381 1.1310
304 665
Errors are quoted as subscripts.
ARTICLE IN PRESS S.J. Stewart et al. / Physica B 389 (2007) 155–158
the existence of at least two magnetic components, I and II (Table 2). We assign the subspectrum II with the highest hyperfine field Bhf to Fe3+ at B sites, while the subspectra I correspond to Fe3+ ions at A sites. The significant increment of the RI/RII area ratio after milling 2ZF shows that the Fe3+ ions redistributes amongst the interstitial sites, giving rise to an increase of the degree of inversion, c, from E0.5 to 1. As seen previously [4], when the Zn ferrite is a nanostructured state, no matter the preparation method employed, it shows a long-range magnetic ordering even at RT. This takes place as a consequence of the cation redistribution that modifies the equilibrium site occupancy. Thus, superexchange interactions Fe3+(A)–O2–Fe3+[B] become operative and contribute to a magnetic ordering at temperatures higher than the Ne´el temperature of normal spinel TN ¼ 10 K. Recently, neutron diffraction results on ZnFe2O4 (DE8 nm), obtained by milling bulk ZnFe2O4 indicate that the material orders ferrimagnetically at
Fig. 3. Mo¨ssbauer spectra taken at 4.2 K for the as-prepared zinc nanoferrite (2ZF) and for the sample milled 10 h (2ZF_10H). The spectra were fitted (solid lines) assuming two sextets with Lorentzian line-shapes.
157
TNE490 K [4]. In our case, the 4.2 K Mo¨ssbauer spectra indicate a ferrimagnetic type ordering, while the increment of the RI/RII ratio reveals the occurrence of a millinginduced cationic inversion. The pre-edge structure of the XANES spectra reflects the Fe3+ oxidation state and moreover a local coordination geometry different from a non-centrosymmetric site for both samples (Fig. 4). To our knowledge, we are reporting the first evidence of a Fe–K XMCD signal in zinc ferrites. Indeed, previous reports on bulk ZnFe2O4 revealed the absence of XMCD signals due to its antiferromagnetic behaviour [6]. The pre-edge region consists of a positive to negative signal and originates from the tetrahedrally coordinated Fe atoms. The main-edge region shows a negative to positive signal, as observed for other spinels compounds [6]. It is worth mentioning that XMCD signals were also detected at RT for both nanoferrites. This analysis indicates, in agreement with the Mo¨ssbauer results, the ferro- or ferrimagnetic type ordering of the nanosized compound. All these results provide evidence of the high degree of inversion of both chemically and mechanically obtained samples. As a general rule the increment of the inversion in nanoferrites has been associated to the particle (or grain) size reduction [1–5]. However, we observe the opposite effect, i.e., an increment of D and, at the same time, an enhancement of the inversion. Thus, the introduction of
Fig. 4. XANES spectra and XMCD signals at 10 K for 2ZF and 2ZF_10H samples.
Table 2 Mo¨ssbauer parameters from fitting the 4.2 K spectra assuming two Lorentzians magnetic sites Sample
BI (T)
dI (mm/s)
2eI (mm/s)
GI (mm/s
BII (T)
dII (mm/s)
2eII (mm/s)
GII (mm/s
RI/RII
2ZF 2ZF_10H
47.82 50.12
0.231 0.191
0.051 0.081
0.622 0.492
50.72 51.82
0.211 0.211
0.011 0.041
0.542 0.452
0.45 1.07
2e is the quadrupolar shift.
ARTICLE IN PRESS 158
S.J. Stewart et al. / Physica B 389 (2007) 155–158
mechanical energy into a system that is in a nonequilibrium or metastable state, as the nanocrystalline state can be considered, leads to another non-equilibrium situation with a higher degree of inversion. Probably, the inversion due to the nanosized character of the ferrite is mainly associated to surface effects, i.e., involves those cations allocated at the surface layer. Instead, the milling activates a cation redistribution involving the whole particle, as can be inferred from the c increment with D. We thank S.G. Marchetti and J.F. Bengoa for sample preparation. SJS, SJAF, MBS, FGR thank CONICET, Argentina. SJS also thanks CBPF/MCT, Brazil. This work
was partially supported by LNLS, Campinas, SP, Brazil (Proposal # D06A-DXAS # 4349/01), ANPCYT (under PICT # 06-17492) and Fundacio´n Antorchas (Argentina). References [1] [2] [3] [4] [5] [6] [7] [8]
C.N. Chinnasamy, et al., J. Phys.: Condens. Matter 12 (2000) 7795. H. Ehrhardt, et al., J. Alloys Compd. 339 (2002) 255. H. Ehrhardt, et al., Scr. Mater. 48 (2003) 11431. M. Hoffman, et al., J. Mater. Sci. 39 (2004) 5057. S. Ammar, et al., J. Non-Cryst. Solids 345 and 346 (2004) 658. K. Matsumoto, et al., Jpn. J. Appl. Phys. 39 (2000) 6089. S.J. Stewart, et al., to be published. S.J. Stewart, et al., Solid State Commun. 129 (2004) 347.
Physica B 389 (2007) 155–158 www.elsevier.com/locate/physb
Magnetic ZnFe2O4 nanoferrites studied by X-ray magnetic circular dichroism and Mo¨ssbauer spectroscopy S.J. Stewarta,, S.J.A. Figueroaa,b, M.B. Sturlaa, R.B. Scorzellic, F. Garcı´ ad, F.G. Requejoa,b a
IFLP, CONICET, Dto. de Fı´sica, C.C. 67, Fac. Cs. Exactas, Universidad Nacional de La Plata, 1900 La Plata, Argentina b INIFTA, Fac. Cs. Exactas, Universidad Nacional de La Plata, Argentina c Centro Brasileiro de Pesquisas Fı´sicas, Rua X. Sigaud 150, Rio de Janeiro, Brazil d Laborato´rio Nacional de Luz Sı´ncrotron, Campinas, SP, Brazil
Abstract ZnFe2O4 nanoparticles (6 nm) were synthesized by hydrothermal methods. Subsequent mechanical treatment applied to the nanocrystalline material caused an increment of the grain size up to 13 nm. The samples were characterized by X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) at the Fe–K edge and 57Fe Mo¨ssbauer spectroscopy. The absorption pre-edge features indicate that Fe3+ ions occupy non-centrosymmetric sites. XAS data evidence the presence of Fe3+ tetrahedrally coordinated while XMCD spectra reflect the magnetic character of the compound The Mo¨ssbauer results show a superparamagnetic behaviour with blocking temperatures at 40 and 250 K for 6 and 13 nm samples, respectively. The 4.2 K Mo¨ssbauer spectra reflect that Fe3+ ions occupy both octahedral and tetrahedral spinel sites. All these results provide consistent evidence of the high degree of inversion achieved by these combined methods, which modifies the long-range ordering. We also demonstrated that, starting from a non-equilibrium state, an increment of the inversion activated by the milling occurs in spite of the augment of the grain size. r 2006 Elsevier B.V. All rights reserved. PACS: 75.50.Tt; 81.20.Wk; 76.80.+y; 61.10.Ht Keywords: Zinc ferrite; ZnFe2O4; Cation redistribution; Mo¨ssbauer; Mechanical milling; Fe–K XMCD; Spinel
Zinc ferrite ZnFe2O4 is an antiferromagnetic normal spinel with Zn2+ tetrahedrally coordinated by oxygens (sites A) and Fe3+ having an octahedral surrounding (sites B). When this ferrite is in a nanostructured state, Fe3+ and Zn2+ distribute amongst both sites, conferring a ferrimagnetic long-range ordering [1–5]. To consider this non-equilibrium distribution the site occupancy is usually expressed as (Zn1cFec)[ZncFe2c]O4, c being the inversion parameter (0pcp1). An increment of c has always been associated to the grain size reduction, independently whether the sample was synthesized by chemical or mechanical treatments [1–5]. Fe–K X-ray absorption spectra contain information on the electronic transition 1s-4p and the Fe local structure. The X-ray circular dichroism (XMCD) signal is the difference between absorption coefficients m+ and m, in Corresponding author. Fax: +54 221 4252006.
E-mail address: stewart@fisica.unlp.edu.ar (S.J. Stewart). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.07.045
the vicinity of an absorption edge (XANES region), for opposite X-rays circular polarization or alternatively, opposite saturation magnetization parallel (+) and antiparallel () to the direction of X-rays propagation. The analysis of this spin-dependent absorption is used to determine the ferro- or ferrimagnetic properties in the unoccupied state. In the case of Fe–K edge, since the magnetic features are probing by promoting transitions to the 4p bands, the magnetism will be only due to the spin–orbit coupling in the final state. XMCD on spinels allows also identifying site symmetries from the fine structure nearby the Fe–K edge [6]. To probe the local magnetic environment of Fe atoms in nanosized ZnFe2O4, we report here XMCD and Mo¨ssbauer studies on nanoferrites chemically prepared that were afterwards mechanically treated. These experiments will allow us to identify the Fe local coordination to be related with their magnetic properties and microstructure.
ARTICLE IN PRESS S.J. Stewart et al. / Physica B 389 (2007) 155–158
156
ZnFe2O4 nanoferrite (Sample 2ZF) was synthesized by a hydrothermal process. Sample 2ZF_10H was obtained by high-energy ball milling the nanocrystalline 2ZF sample for 10 h. Details of sample preparation will be published elsewhere [7]. The XRD patterns only show the Bragg peaks of ZnFe2O4 spinel (Fig. 1). The broad lines evidence the nanocrystalline size dimension achieved with the
Fig. 1. X-ray patterns of nanoferrites samples 2ZF and 2ZF_10 H. A diffractogram of bulk ZnFe2O4 is also included for comparison.
Fig. 2. Room temperature Mo¨ssbauer spectra as a function of the milling time (left). The solid lines correspond to the fitting by assuming a distribution of hyperfine fields Bhf (right).
hydrothermal procedure. As nanocrystalline 2ZF sample is milled, the lines become narrower. The analysis of these data showed that the mechanical treatment causes an increment of the average grain size, D, from 6 (1) to 13 (2) nm, and, at the same time, the micro-strain level decreases. Similar results were obtained by milling CuFe2O4 nanoparticles [8]. The RT Mo¨ssbauer spectra show a broad Fe3+ central doublet (Fig. 2), whose lines broaden after milling. No evidence of a Fe2+ signal was found. We fitted these spectra by considering two Lorentzians doublets DI and DII (Table 1). The DI component, with a lower quadrupole splitting, QS, can be assigned to Fe at B sites, while DII would represent contribution from 57Fe nucleus sensing a more distorted surrounding. However, the similar d values and the lack of resolution do not allow us to clearly distinguish sites A and B. The increment of the QS in nanocrystalline ZnFe2O4 as the crystallite size decreases has several origins that make the different contributions difficult to be resolved [3]. Indeed, (i) Fe atoms locate at the surface grain or interfaces regions with a more distorted environment, (ii) the nonequilibrium cation distribution that also distorts the interstitial sites modifying bond lengths and bond angles on both A and B sites, (iii) the magnetic character of the nanocrystalline ferrite due to the cation redistribution and (iv) relaxation effects, all contribute to the broad Mo¨ssbauer linewidths, G, observed. These contributions were all included in our fitting model by considering a distribution of QS sites. In such case, we observe that the average QS increases and the width of the distribution, s, broadens with D. Indeed, for 2ZF, d ¼ 0:34 mm=s, QS ¼ 0.47 mm/s and s ¼ 0:39 mm=s. While for 2ZF_10 H, d ¼ 0:34 mm=s, QS ¼ 0.81 mm/s and s ¼ 0:70 mm=s. We observed that as the temperature decreases the relative area of the doublet decreases in favor of a magnetic component through a relaxation process (not shown). This behaviour is typically displayed for nanomagnetic compounds due to the superparamagnetic relaxation of particle moments amongst the easy magnetization directions. Considering the blocking temperature, TB, as the temperature at which the relative areas of the non-magnetic and magnetic signals equal, a TB near 40 K and 250 K result for 2ZF and 2ZF_10 H, respectively. The 4.2 K Mo¨ssbauer spectra are composed by resolved sextets with broad absorption lines, being the relaxation effect negligible (Fig. 3). The shape of these spectra reflects
Table 1 Mo¨ssbauer parameters as described in the text from fitting the RT spectra assuming two Lorentzians doublets DI and DII Sample
dI (mm/s)
QSI (mm/s)
GI (mm/s)
RI%
dII (mm/s)
QSII (mm/s)
GII (mm/s
RII %
ZnFe2O4 (bulk) 2ZF 2ZF_10H
0.351 0.341 0.351
0.351 0.401 0.471
0.301 0.321 0.322
100 705 343
0.341 0.331
0.711 0.541
0.381 1.1310
304 665
Errors are quoted as subscripts.
ARTICLE IN PRESS S.J. Stewart et al. / Physica B 389 (2007) 155–158
the existence of at least two magnetic components, I and II (Table 2). We assign the subspectrum II with the highest hyperfine field Bhf to Fe3+ at B sites, while the subspectra I correspond to Fe3+ ions at A sites. The significant increment of the RI/RII area ratio after milling 2ZF shows that the Fe3+ ions redistributes amongst the interstitial sites, giving rise to an increase of the degree of inversion, c, from E0.5 to 1. As seen previously [4], when the Zn ferrite is a nanostructured state, no matter the preparation method employed, it shows a long-range magnetic ordering even at RT. This takes place as a consequence of the cation redistribution that modifies the equilibrium site occupancy. Thus, superexchange interactions Fe3+(A)–O2–Fe3+[B] become operative and contribute to a magnetic ordering at temperatures higher than the Ne´el temperature of normal spinel TN ¼ 10 K. Recently, neutron diffraction results on ZnFe2O4 (DE8 nm), obtained by milling bulk ZnFe2O4 indicate that the material orders ferrimagnetically at
Fig. 3. Mo¨ssbauer spectra taken at 4.2 K for the as-prepared zinc nanoferrite (2ZF) and for the sample milled 10 h (2ZF_10H). The spectra were fitted (solid lines) assuming two sextets with Lorentzian line-shapes.
157
TNE490 K [4]. In our case, the 4.2 K Mo¨ssbauer spectra indicate a ferrimagnetic type ordering, while the increment of the RI/RII ratio reveals the occurrence of a millinginduced cationic inversion. The pre-edge structure of the XANES spectra reflects the Fe3+ oxidation state and moreover a local coordination geometry different from a non-centrosymmetric site for both samples (Fig. 4). To our knowledge, we are reporting the first evidence of a Fe–K XMCD signal in zinc ferrites. Indeed, previous reports on bulk ZnFe2O4 revealed the absence of XMCD signals due to its antiferromagnetic behaviour [6]. The pre-edge region consists of a positive to negative signal and originates from the tetrahedrally coordinated Fe atoms. The main-edge region shows a negative to positive signal, as observed for other spinels compounds [6]. It is worth mentioning that XMCD signals were also detected at RT for both nanoferrites. This analysis indicates, in agreement with the Mo¨ssbauer results, the ferro- or ferrimagnetic type ordering of the nanosized compound. All these results provide evidence of the high degree of inversion of both chemically and mechanically obtained samples. As a general rule the increment of the inversion in nanoferrites has been associated to the particle (or grain) size reduction [1–5]. However, we observe the opposite effect, i.e., an increment of D and, at the same time, an enhancement of the inversion. Thus, the introduction of
Fig. 4. XANES spectra and XMCD signals at 10 K for 2ZF and 2ZF_10H samples.
Table 2 Mo¨ssbauer parameters from fitting the 4.2 K spectra assuming two Lorentzians magnetic sites Sample
BI (T)
dI (mm/s)
2eI (mm/s)
GI (mm/s
BII (T)
dII (mm/s)
2eII (mm/s)
GII (mm/s
RI/RII
2ZF 2ZF_10H
47.82 50.12
0.231 0.191
0.051 0.081
0.622 0.492
50.72 51.82
0.211 0.211
0.011 0.041
0.542 0.452
0.45 1.07
2e is the quadrupolar shift.
ARTICLE IN PRESS 158
S.J. Stewart et al. / Physica B 389 (2007) 155–158
mechanical energy into a system that is in a nonequilibrium or metastable state, as the nanocrystalline state can be considered, leads to another non-equilibrium situation with a higher degree of inversion. Probably, the inversion due to the nanosized character of the ferrite is mainly associated to surface effects, i.e., involves those cations allocated at the surface layer. Instead, the milling activates a cation redistribution involving the whole particle, as can be inferred from the c increment with D. We thank S.G. Marchetti and J.F. Bengoa for sample preparation. SJS, SJAF, MBS, FGR thank CONICET, Argentina. SJS also thanks CBPF/MCT, Brazil. This work
was partially supported by LNLS, Campinas, SP, Brazil (Proposal # D06A-DXAS # 4349/01), ANPCYT (under PICT # 06-17492) and Fundacio´n Antorchas (Argentina). References [1] [2] [3] [4] [5] [6] [7] [8]
C.N. Chinnasamy, et al., J. Phys.: Condens. Matter 12 (2000) 7795. H. Ehrhardt, et al., J. Alloys Compd. 339 (2002) 255. H. Ehrhardt, et al., Scr. Mater. 48 (2003) 11431. M. Hoffman, et al., J. Mater. Sci. 39 (2004) 5057. S. Ammar, et al., J. Non-Cryst. Solids 345 and 346 (2004) 658. K. Matsumoto, et al., Jpn. J. Appl. Phys. 39 (2000) 6089. S.J. Stewart, et al., to be published. S.J. Stewart, et al., Solid State Commun. 129 (2004) 347.