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Local magnetic properties of the Fe2Ti Laves phase
Journal of Magnetism and Magnetic Materials MO-144 (1995) 59-60
L4xal magneticpropertiesof the FeJi -Lawsphase J. Pelloth,* R.A. Brand,W. Keune Laboratorium fir Angewandte Physik, Uniuersitiit Duisburg, D 47048 Duisburg, Germany
Abstract We have investigated Fe,+,Ti, -x alloys ( -0.05 < x < 0,lO) by “Fe-MGssbauer spectroscopy (4.2-310 K). The spectra are well described on the basis of local magnetic effects associated with 6h and 2a sites. For T > 200 K, stoichiometric Fe,Ti shows a complicated magnetic behavior.
Recent magnetic measurements on the hexagonal Fe;e2+,Til-, Laves phase (Cl4 structure) have demonstrated the complicated magnetic behaviour of these alloys [l]. Wassermann et al. [l] found that for x C 0, Fe,+,Ti,-, alloys are mainly antiferromagnetic. The planes of Fe atoms in the 6h sites have ferromagnetic interactions within each plane, but each 6h plane is coupled antiferromagnetitally to the next 6h plane. The second Fe site &I site) is located between the 6h planes and is paramagnetic. For iron rich alloys (X > 0) the authors in [1,2] assumed that a surplus of iron leads to ferromagnetic clusters in an antiferromagnetic matrix. Magnetovolume effects are also well known in Fe,Ti alloys and calculations of these effects have been carried out [1,4]. Since Massbauer spectroscopy provides insight into the local magnetic properties, we have performed 57Fe measurements for 4.2 < T < 310 K in zero and applied magnetic fields (B,,, = 5 T) on Fe,+,Ti, --L powder samples (- 0.05 I; x 5 0.1) in a conventional transmission geometry using a 20 mCi 57Co in Rh source. Experiments were carried out on the same samples studier! by Wassermann et al. [I]. Because of the strong quadrupole interaction relative tc, the magnetic hyperfine interaction, and the overlap of ihe subspectra the full Hamiltonian and the transmission integral have been used in the fitting procedure. The 4.2 K spectra of a series of Fe,+XTil-X alloys of different compositions are shown in Fig. 1 with the hyperfine parameters listed in Table 1. The spectraof stoichiometric Fe,Ti and for samples with x < 0 show two sites the antiferromagnetically coupled 6h site, and the paramagnetic 2a site. The present results agree with those of earlier workers [2,3]. There is a significant difference in the isomer shift of A6 = 0.027 mm/s between the two
* Corresponding author. Fax: f 49-203-379-3163; hm311he@duc220,uni-duisburg.de. 0304-8853/95/$09.50
cmail:
(Tz
sites at 4.2 K, by comparison at high temperature 310 K), only one quadnspole doublet is observed for both sites. Moreover, Table 1 shows that the ebctric field gradient V,, of the 6h sites chaages in sign and value with temperature (further details will be published elsewhere). Both of thesechangessuggesta changein the local electronic band structure. Although the change of the isomer shift between low and high temperatures could afso be linked with a simple volume effect, the changes expected on this basis would be one order of magnitude smaller (assuming only a typical vofume dependence of the isomer shift for hcp
Fig. 1. Fe,, ,Ti, -x Miissbauer spectra at T = 4.2 K. The fits to the spectra are described in the text. In addition the spectita For the sample with x = 0.10 was fitted using a distribution of hyperfme fields.
Q 1995 Elsevier Science B.V. All rights reserved
SSDI 0304-8853(94)00836-l
60
J. Pelloth et al. /Journal
of Magnetism and Magnetic Materials 140-144 (1995) 59-60
metals [51X With a small surplus of iron (x = 0.025, Fig.
1) we can identify an additional magnetically split sextet and a weak paramagnetic doublet with a more positive isomer shift (see Table 1). The relative spectra1 areas are similiar to the values expected from a binomial distribution with the first neighbour shell of each iron site having one or more anti-structure Fe atoms on a Ti site. The hyperfine fields indicate that this new sextet is due to disturbed iron 6h sites, and that the additional doublet is due to disturbed 2a sites. This is expected as an extra Fe atom on a Ti site sl.su!d Iead to a more positive isomer shift as observed and as listed in Table 1. The appearance of the additional paramagnetic doublet indicates that a small anti-structure atom concentration will not result in ferromagnetic clusters at 4.2 K. With increasing iron concentration (x r 0.055) the subspectra for the disturbed 2a sites develop a small hyperfine field of 3.5 T. Also the undisturbed Za spectra show a hyperfine field of about 2 T, this may be an induced effect due to the decrease of the lattice constant [l] with increasing Fe concentration. This effect also induces a small magnetic moment at the 2a sites (as is
known for the case of a small amount of SC in F$Ti alloys [6,7]). On the basis of our fitting procedure two additional sextets are found to be present at high Fe concentration (x = 0.10). The first sextet with B,, = 22 T is considered to be due to Fe antisite atoms with the second one (B,,= 11.2 T and 6 = 0.048 mm/s) being due to the disturbed 2a sites. From these concentration dependent spectra at 4.2 K we can conclude that for a small surph.~s of iron (X 5 0.025) the antistructure atoms will not form a magnetic cluster with their nearest neighbour 2a atoms at 4.2 K, for this concentration the anti-structure atoms themselves may have a magnetic moment, however their concentration is too low for the related hyperfine effect to be detected in the MGssbauer spectra. By increasing the Fe content (x> 0.041, as the increase of the byperfine field at the disturbed 2a sites shows, a magnetic cluster is created around a Fe atom on a Ti site. The temperature dependent spectra of stoichiometric Fe,Ti (not shown) for T 2 200 K indicate a decrease of the relative spectral area of the sextet and an increase of the area of the doublet with increasing temperature. Simultaneously, the doublet develops asymmetric line intensities, and the linewidths of both subspectra (sextet and doublet) increase until finally the spectrum cdlapses at TN * 300 K into a single doublet. The %el temperature, TN, was found to be about 20 K higher than that given in Ref. [l]. Further investigations on single crystals are in progress to explain the magnetic behavior, especially for the temperature range 200 < T < 300 K, where a magnetic phase transition at T,* - 200 K from the coexisting antiferromagnetic and ferromagnetic phases to an antiferromag netic phase was observed [l]. Acknowledgemt+fint: This work was supported by the DFG (SFB 166). References [l] E.F. Wassermaaa ~1 al.. Phys. Rev. B, submitted. [Z] G.K. Wertheim et al., Sol. Slate Commun. 7 (1969) 1399. [3] P.J. Brown, J. Deportes and B. Ouladdiaf, J. Phys: Condens.
Matter 4 (19!J2) 1015. [4] S. Asano and S. Ishid:, J. Magn. Magn. Mater. 70 (1987) 39. [s] D.L. Williamson, in: Mijssbauer Isomer Shifts, ads. G.K. Shcnoy and F.E. Wagner (North-Holland, Amsterdam. 1978)
p* 335. [6] Y. Nishihara et al., J. Phys, Sot. Jpn. 54 (1985) 1122. [7] Y. Nishihara et al., J. Phys, Sot. Jpn. 55 (1986) 920. ISI N. Blaes et al.. Nuct. Instr. Methods B 9 (lY8S) 201.
L4xal magneticpropertiesof the FeJi -Lawsphase J. Pelloth,* R.A. Brand,W. Keune Laboratorium fir Angewandte Physik, Uniuersitiit Duisburg, D 47048 Duisburg, Germany
Abstract We have investigated Fe,+,Ti, -x alloys ( -0.05 < x < 0,lO) by “Fe-MGssbauer spectroscopy (4.2-310 K). The spectra are well described on the basis of local magnetic effects associated with 6h and 2a sites. For T > 200 K, stoichiometric Fe,Ti shows a complicated magnetic behavior.
Recent magnetic measurements on the hexagonal Fe;e2+,Til-, Laves phase (Cl4 structure) have demonstrated the complicated magnetic behaviour of these alloys [l]. Wassermann et al. [l] found that for x C 0, Fe,+,Ti,-, alloys are mainly antiferromagnetic. The planes of Fe atoms in the 6h sites have ferromagnetic interactions within each plane, but each 6h plane is coupled antiferromagnetitally to the next 6h plane. The second Fe site &I site) is located between the 6h planes and is paramagnetic. For iron rich alloys (X > 0) the authors in [1,2] assumed that a surplus of iron leads to ferromagnetic clusters in an antiferromagnetic matrix. Magnetovolume effects are also well known in Fe,Ti alloys and calculations of these effects have been carried out [1,4]. Since Massbauer spectroscopy provides insight into the local magnetic properties, we have performed 57Fe measurements for 4.2 < T < 310 K in zero and applied magnetic fields (B,,, = 5 T) on Fe,+,Ti, --L powder samples (- 0.05 I; x 5 0.1) in a conventional transmission geometry using a 20 mCi 57Co in Rh source. Experiments were carried out on the same samples studier! by Wassermann et al. [I]. Because of the strong quadrupole interaction relative tc, the magnetic hyperfine interaction, and the overlap of ihe subspectra the full Hamiltonian and the transmission integral have been used in the fitting procedure. The 4.2 K spectra of a series of Fe,+XTil-X alloys of different compositions are shown in Fig. 1 with the hyperfine parameters listed in Table 1. The spectraof stoichiometric Fe,Ti and for samples with x < 0 show two sites the antiferromagnetically coupled 6h site, and the paramagnetic 2a site. The present results agree with those of earlier workers [2,3]. There is a significant difference in the isomer shift of A6 = 0.027 mm/s between the two
* Corresponding author. Fax: f 49-203-379-3163; hm311he@duc220,uni-duisburg.de. 0304-8853/95/$09.50
cmail:
(Tz
sites at 4.2 K, by comparison at high temperature 310 K), only one quadnspole doublet is observed for both sites. Moreover, Table 1 shows that the ebctric field gradient V,, of the 6h sites chaages in sign and value with temperature (further details will be published elsewhere). Both of thesechangessuggesta changein the local electronic band structure. Although the change of the isomer shift between low and high temperatures could afso be linked with a simple volume effect, the changes expected on this basis would be one order of magnitude smaller (assuming only a typical vofume dependence of the isomer shift for hcp
Fig. 1. Fe,, ,Ti, -x Miissbauer spectra at T = 4.2 K. The fits to the spectra are described in the text. In addition the spectita For the sample with x = 0.10 was fitted using a distribution of hyperfme fields.
Q 1995 Elsevier Science B.V. All rights reserved
SSDI 0304-8853(94)00836-l
60
J. Pelloth et al. /Journal
of Magnetism and Magnetic Materials 140-144 (1995) 59-60
metals [51X With a small surplus of iron (x = 0.025, Fig.
1) we can identify an additional magnetically split sextet and a weak paramagnetic doublet with a more positive isomer shift (see Table 1). The relative spectra1 areas are similiar to the values expected from a binomial distribution with the first neighbour shell of each iron site having one or more anti-structure Fe atoms on a Ti site. The hyperfine fields indicate that this new sextet is due to disturbed iron 6h sites, and that the additional doublet is due to disturbed 2a sites. This is expected as an extra Fe atom on a Ti site sl.su!d Iead to a more positive isomer shift as observed and as listed in Table 1. The appearance of the additional paramagnetic doublet indicates that a small anti-structure atom concentration will not result in ferromagnetic clusters at 4.2 K. With increasing iron concentration (x r 0.055) the subspectra for the disturbed 2a sites develop a small hyperfine field of 3.5 T. Also the undisturbed Za spectra show a hyperfine field of about 2 T, this may be an induced effect due to the decrease of the lattice constant [l] with increasing Fe concentration. This effect also induces a small magnetic moment at the 2a sites (as is
known for the case of a small amount of SC in F$Ti alloys [6,7]). On the basis of our fitting procedure two additional sextets are found to be present at high Fe concentration (x = 0.10). The first sextet with B,, = 22 T is considered to be due to Fe antisite atoms with the second one (B,,= 11.2 T and 6 = 0.048 mm/s) being due to the disturbed 2a sites. From these concentration dependent spectra at 4.2 K we can conclude that for a small surph.~s of iron (X 5 0.025) the antistructure atoms will not form a magnetic cluster with their nearest neighbour 2a atoms at 4.2 K, for this concentration the anti-structure atoms themselves may have a magnetic moment, however their concentration is too low for the related hyperfine effect to be detected in the MGssbauer spectra. By increasing the Fe content (x> 0.041, as the increase of the byperfine field at the disturbed 2a sites shows, a magnetic cluster is created around a Fe atom on a Ti site. The temperature dependent spectra of stoichiometric Fe,Ti (not shown) for T 2 200 K indicate a decrease of the relative spectral area of the sextet and an increase of the area of the doublet with increasing temperature. Simultaneously, the doublet develops asymmetric line intensities, and the linewidths of both subspectra (sextet and doublet) increase until finally the spectrum cdlapses at TN * 300 K into a single doublet. The %el temperature, TN, was found to be about 20 K higher than that given in Ref. [l]. Further investigations on single crystals are in progress to explain the magnetic behavior, especially for the temperature range 200 < T < 300 K, where a magnetic phase transition at T,* - 200 K from the coexisting antiferromagnetic and ferromagnetic phases to an antiferromag netic phase was observed [l]. Acknowledgemt+fint: This work was supported by the DFG (SFB 166). References [l] E.F. Wassermaaa ~1 al.. Phys. Rev. B, submitted. [Z] G.K. Wertheim et al., Sol. Slate Commun. 7 (1969) 1399. [3] P.J. Brown, J. Deportes and B. Ouladdiaf, J. Phys: Condens.
Matter 4 (19!J2) 1015. [4] S. Asano and S. Ishid:, J. Magn. Magn. Mater. 70 (1987) 39. [s] D.L. Williamson, in: Mijssbauer Isomer Shifts, ads. G.K. Shcnoy and F.E. Wagner (North-Holland, Amsterdam. 1978)
p* 335. [6] Y. Nishihara et al., J. Phys, Sot. Jpn. 54 (1985) 1122. [7] Y. Nishihara et al., J. Phys, Sot. Jpn. 55 (1986) 920. ISI N. Blaes et al.. Nuct. Instr. Methods B 9 (lY8S) 201.