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High field Mössbauer investigations of the magnetic phase transition in Sc0.25Ti0.75Fe2 at 4.2 K
19
Journal of Magnetism and Magnetic Materials 98 (1991) 19-24 North-Holland
High field Miissbauer investigations of the magnetic phase transition in Sc,.,,Ti,,,Fe, at 4.2 K A. Piisinger,
W. Steiner
Instiiut fir Angewandte
und Technische Physik, TU Wien, Vienna, Austria
and Y. Nishihara Electrotechnical
Laboratory,
Tsukuba, Ibaraki 305, Japan
Received 19 January 1991; in revised form 4 March 1991
Mijssbauer measurements in applied fields up to 13.5 T on !Gq,,,Ti,,,, Fe, are reported. The hysteretic behaviour observed in the field dependence of the bulk magnetization is also visible in the Miissbauer spectra, but hysteresis in the hyperfine fields is only found for Fe on 6h sites. In low external fields the hyperfine fields on these sites form a cone (opening angle = 140 o ) with the resulting field parallel to those of 2a sites. At high fields the cone angle drops to zero and all hyperfine fields are antiparallel to the applied field. The field induced phase transition can be described as a transition from canted ferromagnetism to ferromagnetism of the Fe atoms occupying the 6h positions.
1. Introduction In the Laves phases Sc,_,Ti,Fe,.,, two ferromagnetic states with different degrees of localization are found and for x 2 0.7 a coexistent state of ferromagnetism and antiferromagnetism exists [l]. A transition between these two ferromagnetic states takes place at = 70 K in Sc,,,,Ti,,,Fe,,s,. At 4.2 K the magnetization of Sc,.,,Ti,,,,Fe, shows a field induced phase transition at an applied field (B,) of = 9 T with a pronounced hysteresis [2]. To obtain additional information about the magnetic order of the Fe atoms and to decide if this field induced transition is due to a ferro- to ferromagnetic transitions [3] or due to a reorientation of the Fe moments, this compound was investigated in the present work at 4.2 K by Mijssbauer transmission spectroscopy in applied fields up to 13.5 T. The series SC, _,Ti,Fez (0 I x I 1) crystallizes with the hexagonal MgZn, structure type. The SC 0304-8853/91/$03.50
and Ti atoms occupy the 6f sites and the Fe atoms are positioned on the 6h and 2a sites [4]. In TiFe, for the Fe atoms on the 2a positions no magnetic hyperfine splitting was observed whereas for the ones on the 6h-positions a hyperfine field of 9.7 T was measured at 20.4 K [5]. These atoms, arranged in the layers formed by the bases of the Fe tetrahedra, are coupled ferromagnetically, the layers themselves antiferromagnetically. The Fe atoms on the 2a positions which are halfway between the layers are paramagnetic. ScFe, is ferromagnetic with hyperfine fields of 17.3 T for the 2a sites and 17.8 T for the 6h sites [6]. From calculations by means of the local spin density approximation [6] magnetic moments were obtained of 1.54(1.6)pr, for Fe in ScFe, on the 2a (6h) sites and O(1.14)pa for Fe at these positions in TiFe,. Ti has no magnetic moment, but SC has a moment of 0.52~~ in the opposite direction to Fe. Whereas the magnetic properties of Ti, _yFe2+y (-0.05
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
20
A. Piisinger et al. /
Magneticphase transition in S+,,Ti,
homogeneity range [7], those of Sc,_,Fe,+, less influenced by a change of y [8].
are
2. Experimental Fe, sample was prepared by arc The %.25%.75 melting of 99.9% SC, 99.99% Ti and 99.99% Fe. After the melting, it was annealed at 1000 o C for a week and confirmed to have the Cl4 structure by X-ray diffraction. The temperature dependence of the magnetization for this sample is given in the previous paper of Nishihara and Yamaguchi [l]. The spectra were recorded at 4.2 K in transmission geometry on a powdered sample in applied fields produced by a superconducting coil. The direction of B, was parallel to the y-ray direction. The 57CoRh source was situated in a field compensated area (B < 0.1 T). For the velocity calibration an ar-Fe reference spectrum was collected simultaneously at room temperature on the upper side of the drive. Linewidths of 0.26 mm/s were obtained. For the fitting procedure of the spectra nine subspectra were used. According to the occupation of the 6h- and 2a sites the subspectra were divided into two groups with intensity ratio of 3 : 1. Within each group the intensity ratios were calculated by means of a binomial distribution function assuming that the Ti and SC atoms are distributed randomly at the nearest 4f neighbour sites (table 1). For 2a sites with two and three nearest Ti neighbours only one subspectrum was calculated because even the sum of the intensity
Table 1 Intensity ratios of the nine subspectra the MBssbauer spectra
used in the analyses
Number of nearest Ti neighbours
6 h sites
2a sites
2
0.025 0.099 >
0.041
4 5 6
0.224 0.268 0.134
0.074 0.089 0.045
of
,jFe,
at 4.2 K
ratios for these two sites contributes only with 4.1% to the whole spectrum. The linewidth (r/2) for this subspectrum was taken somewhat larger than the other ones which all had the same value (0.17 mm/s). Since no information about the influence of different numbers of SC (Ti) nearest neighbours on the quadrupole splitting (eQT/,,/2) is available and approximately the same values were measured for eQV,,/2 for ScFe, and TiFe,, for all subspectra eQV,,/2 = -0.44 mm/s were taken. The center shift was assumed to decrease with increasing number of Ti neighbours according to the analyses for different x at B, = 0 of (SC, Ti)Fe,.,, [l]. The asymmetry parameter was set to zero for all subspectra. The complete Hamiltonian was solved for analysing the spectra. The polycrystalline character of the absorber was taken into account for the spectra recorded in applied fields by averaging the different directions of the electric field gradient with respect to the y-ray direction assuming a random distribution of the crystallites in the absorber.
3. Results and discussion Identical spectra were recorded before and after the field sweep up to 13.5 T. These spectra (with B, = 0) were calculated with an angle of 54.7 o between y-ray direction and direction of the hyperfine field to simulate the uniform distribution over all directions of hyperfine fields (B,,). Although the arrangement of the used nine subspectra in the fitting procedure always contains some arbitrariness, only one set of parameters was found which was usable for all recorded spectra (with and without external fields) and leads to results in agreement with bulk magnetic measurements. An angle between B,, and eQV,, of = 70” was found for all subspectra of Fe on 6h and = 0” for all on 2a positions. Values for B,, between 17.5 and 9.2 T for the different surroundings of the 6h positions and between 11.6 and 3.6 T for those of the 2a positions were obtained indicating that the different environments lead to differences of the hyperfine fields which are nearly the same for both Fe sites. For the center shift values between
A. Piisinger et al. / Magnetic phase transition in Sco,,,Ti,,,5Fe,
-3.0
-5.0
-1.0
1.0
Velocity
“0
5
10
B,
15
[T] -->
Fig. 2. Field dependence of the magnetization for Sq,25Tio.,5Fe, at 4.2 K. The arrows mark the fields at which the Miissbauer spectra were taken.
3.0
5.0
(mm/s>
Fig. 1. Mijssbauer transmission spectrum of Sc,,,,Ti,,,
-0.24 to -0.31 mm/s (relative to 57Fe in Rh at 295 K) were calculated (fig. 1). The hysteretic behaviour observed in the magnetization (fig. 2) is also visible in the Mossbatter spectra (see e.g. the spectra recorded in B, = 8 T for increasing and decreasing fields, fig. 3). At B, = 4 T the magnetic splitting of the spectrum measured with decreasing field is broader than the one measured with increasing field, although in this field range the hysteresis of the bulk magnetization vanishes. For the center shift nearely the same values were found as at B, = 0, but the accuracy of the data is not high enough to observe
21
at 4.2 K
F e, at 4.2 K without external field.
the influence of the measured volume magnetostriction [9]. The internal fields ( Bi,,) were calculated from the measured B,, and the fitted angles between B,, and B, (0) (fig. 4) according to Bin,= B,,- B,‘, with B,‘= B, - B,, where B, represents the demagnetizing field. The demagnetization factor was determined from measurements of a-Fe assuming that the shape of the grains for both materials is similar. Bint for the different 6h sites decreases with increasing number of nearest Ti neighbours (fig. 4a) in agreement with the concentration dependence of B,, determined at B, = 0 which also decreases with increasing x [l]. The same is valid for the 2a sites. For all different surroundings Bint depends on B, (fig. 4a) even in the field ranges where for the bulk magnetization a nearly field independent behaviour is observed (fig. 2). Since this dependence is the same for all different environments Of Bint and 0 is nearly independent of the individual surrounding (fig. 4), the weighted (by means of the normalized intensity ratio) mean values of the measured hyperfine fields (B,,r) and of the internal fields (Bint) are shown for the 6h and 2a sites in fig. 5. The hysteretic behaviour of the magnetization is only visible for the 6h sites and is more pronounced for the weighted mean angle between B,, and B, (8) than for the value of Bhf. Con-
22
A. Piisinger et al. /
Magneticphase transition in SC~.,,T~,,~F~~ at 4.2 K
Fig. 3. Recorded and calculated (full line) spectra of Sea 25Ti a.,sFq
at 4.2 K. Left (right) side increasing (decreasing) fields. A 1
100
100'
80
a0 Y
2
60
: 6o M
z
40
40
z+! u ‘:
20
20
T
,
,200
1001 ao- a,
ao 60
60
:
F
40
40
20
zo-
t? a
0 0
Be
[T]
0
T :
e
[T] -->
$
Y2oA
100
z
Ba
A 120,
:b
120
-->
4
a
Ba [T] -->
Fig. 4. (a) Measured hyperfine fields (right side) and calculated internal fields (left side) on the neighbour environments for increasing (upper part) and decreasing (lower part) applied field. direction and hyperfine field (right side) and calculated angles between internal and applied field different nearest neighbour environments for increasing (upper part) and decreasing (lower part) guides to the eyes.
0
120
Ba
[T]
0
-->
6h positions for different nearest (b) Measured angles between y (left side) on the 6h positions for applied field. The lines are only
A. PGsinger et al. / Magnetic phase transition in Sco,,,Tio,,5Fe,
20
-
20
';
o IQ 16 12
A 20 : I I ‘6
p
12
-
a
0
20 *
W a-sites
16
2’ _.-
24 leio
12
dYm
,
,
,
a
12
Ba
[T]
-->
‘W‘&.-dl_ I,, 0
I -
SE
II
4
' 7 -
4
,
,
a
12
BII [T]
-
-->
Fig. 5. Mean measured hyperfine (right side) and internal fields (left side) of Fe at 6h (0, 0) and 2a (CI, H) positions as function of B, (see text). Upper part: Mean angles Z between B, and Bint (left side) and 8 between y direction and B,, (right side) on the 6h positions as a function of Ea. Open (full) symbols increasing (decreasing) applied field. The lines are guides to the eyes.
trary to the behaviour of a simple ferromagnet, where Bin, is nearly independent of B, for B, > Bint of the 2a sites increases with increasing B B,9’
The angle (Y between B, and Bin,can be calculated from the values of B,‘, the fitted B,, and the fitted angle between applied and hyperfine field, since from the smaller values measured for B,, at B, = 13.5 T, compared to those determined at B, = 0, it can be concluded, that Bint points in the opposite direction of the one of B,. The differences of the a! values determined for different numbers of Sc/Ti neighbours are small for the 6h sites (fig. 4b) and (Y= 0 is obtained for the 2a sites. The weighted (in the same way as Bin,) mean values of CY(Z) as a function of B, are shown for the 6h sites in the upper part of fig. 5. In increasing fields Z stays roughly constant at 70” up to B, = 8 T. This is the field region where the bulk magnetization exhibits the first plateau. The result is consistent with the magnetic structure estimated from magnetization and zero field Mossbauer measurements by Nishihara and Yamaguchi [lo], where an angle of approximately 140” between
at 4.2 K
23
the moments on the 6h and 0” between those on the 2a sites was estimated. For the fields where the sudden increase of the magnetization appeares, Z determined for the 6h sites drops to zero, indicating that all internal fields are aligned parallel to B,. Since furthermore B,, decreases with increasing B,, Bin, points to the opposite direction of the one of B, and must now be parallel to the one of the 2a sites. In decreasing applied fields E remains low down to B, = 6 T, again resembling the field dependence of the magnetization. The Mijssbauer investigations reveal that the phase transition observed in the high field magnetization measurement [2] is a transition from a canted ferromagnetic state to a ferromagnetic one, where with increasing magnetic field the cone angle for the 6h sites becomes zero at a critical field. A transition from a canted state to a ferromagnetic state might be described by the itinerant electron model of Moriya and Usami [ll], extended for anisotropic systems by Isoda [12], taking into account the effects of correlated spin fluctuations. However for the present system in the field range where the bulk magnetization exhibits only a slight increase increasing hyperfine fields for both Fe sites are obtained and the sudden change of Bint at the phase transition points to a discontinuous change of the moments at the 6h sites. Both results should be included in the itinerant electron model to permit a complete description of the mechanism of this phase transition.
4. Conclusion The performed high field Mbssbauer experiments favour the following model for the magnetic structure of Sco,25Tio.,5Fe,: In the state with the lower magnetization the iron moments on the 6h sites form a cone with an opening angle of approximately 140 O, the resultant moment of this cone is parallel to the moment of the 2a sites. The moment of the SC atoms is then aligned antiparallel to fit the magnetization data. For B, > 8 T the cone angle drops to zero and all Fe moments are aligned parallel for B, > 10 T. Thus the field induced transition and the hysteretic behaviour visi-
24
A. Pijsinger et al. /
Magneticphase transition in SC~,,,T~,,~F~,
ble in the Mijssbauer spectra and the magnetization points to the coexistence of a ferro and a canted ferromagnetic state.
References [1] Y. Nishihara and Y. Yamaguchi, J. Phys. Sot. Jpn. 55 (1986) 920. [2] G. Kido, Y. Nakagawa, Y. Nishihara and Y. Yamaguchi, J. Magn. Magn. Mater. 70 (1987) 181. [3] S. Asano and S. Ishida, J. Magn. Magn. Mater. 70 (1987) 39. [4] K. Schubert, Kristallstrukturen zweikomponentiger Phasen (Springer, Berlin, 1964).
at 4.2 K
[5] G.K. Wertheim, J.H. Wernick and R.C. Sherwood, Solid State Commun. 7 (1969) 1399. [6] S. Asano and S. Ishida, J. Magn. Magn. Mater. 70 (1987) 187. [7] T. Nakamichi, J. Phys. Sot. Jpn. 25 (1968) 1189. [8] R. Grossinger, R. Haferl, G. Hilscher, G. Wiesinger, K.H.J. Buschow and P.H. Smit, Inst. Phys. Conf. Ser. No. 55 (1980) 295. [9] G. Kido, Y. Nakagawa, Y. Yamaguchi and Y. Nishihara, J. de Phys. 49 (1988) C8-251. [lo] Y. Nishihara and Y. Yamaguchi, J. Phys. Sot. Jpn. 54 (1985) 1122. [ll] T. Moriya and K. Usami, Solid State Commun. 23 (1977) 935. [12] M. Isoda, J. Phys. Sot. Jpn. 53 (1984) 3587.
Journal of Magnetism and Magnetic Materials 98 (1991) 19-24 North-Holland
High field Miissbauer investigations of the magnetic phase transition in Sc,.,,Ti,,,Fe, at 4.2 K A. Piisinger,
W. Steiner
Instiiut fir Angewandte
und Technische Physik, TU Wien, Vienna, Austria
and Y. Nishihara Electrotechnical
Laboratory,
Tsukuba, Ibaraki 305, Japan
Received 19 January 1991; in revised form 4 March 1991
Mijssbauer measurements in applied fields up to 13.5 T on !Gq,,,Ti,,,, Fe, are reported. The hysteretic behaviour observed in the field dependence of the bulk magnetization is also visible in the Miissbauer spectra, but hysteresis in the hyperfine fields is only found for Fe on 6h sites. In low external fields the hyperfine fields on these sites form a cone (opening angle = 140 o ) with the resulting field parallel to those of 2a sites. At high fields the cone angle drops to zero and all hyperfine fields are antiparallel to the applied field. The field induced phase transition can be described as a transition from canted ferromagnetism to ferromagnetism of the Fe atoms occupying the 6h positions.
1. Introduction In the Laves phases Sc,_,Ti,Fe,.,, two ferromagnetic states with different degrees of localization are found and for x 2 0.7 a coexistent state of ferromagnetism and antiferromagnetism exists [l]. A transition between these two ferromagnetic states takes place at = 70 K in Sc,,,,Ti,,,Fe,,s,. At 4.2 K the magnetization of Sc,.,,Ti,,,,Fe, shows a field induced phase transition at an applied field (B,) of = 9 T with a pronounced hysteresis [2]. To obtain additional information about the magnetic order of the Fe atoms and to decide if this field induced transition is due to a ferro- to ferromagnetic transitions [3] or due to a reorientation of the Fe moments, this compound was investigated in the present work at 4.2 K by Mijssbauer transmission spectroscopy in applied fields up to 13.5 T. The series SC, _,Ti,Fez (0 I x I 1) crystallizes with the hexagonal MgZn, structure type. The SC 0304-8853/91/$03.50
and Ti atoms occupy the 6f sites and the Fe atoms are positioned on the 6h and 2a sites [4]. In TiFe, for the Fe atoms on the 2a positions no magnetic hyperfine splitting was observed whereas for the ones on the 6h-positions a hyperfine field of 9.7 T was measured at 20.4 K [5]. These atoms, arranged in the layers formed by the bases of the Fe tetrahedra, are coupled ferromagnetically, the layers themselves antiferromagnetically. The Fe atoms on the 2a positions which are halfway between the layers are paramagnetic. ScFe, is ferromagnetic with hyperfine fields of 17.3 T for the 2a sites and 17.8 T for the 6h sites [6]. From calculations by means of the local spin density approximation [6] magnetic moments were obtained of 1.54(1.6)pr, for Fe in ScFe, on the 2a (6h) sites and O(1.14)pa for Fe at these positions in TiFe,. Ti has no magnetic moment, but SC has a moment of 0.52~~ in the opposite direction to Fe. Whereas the magnetic properties of Ti, _yFe2+y (-0.05
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
20
A. Piisinger et al. /
Magneticphase transition in S+,,Ti,
homogeneity range [7], those of Sc,_,Fe,+, less influenced by a change of y [8].
are
2. Experimental Fe, sample was prepared by arc The %.25%.75 melting of 99.9% SC, 99.99% Ti and 99.99% Fe. After the melting, it was annealed at 1000 o C for a week and confirmed to have the Cl4 structure by X-ray diffraction. The temperature dependence of the magnetization for this sample is given in the previous paper of Nishihara and Yamaguchi [l]. The spectra were recorded at 4.2 K in transmission geometry on a powdered sample in applied fields produced by a superconducting coil. The direction of B, was parallel to the y-ray direction. The 57CoRh source was situated in a field compensated area (B < 0.1 T). For the velocity calibration an ar-Fe reference spectrum was collected simultaneously at room temperature on the upper side of the drive. Linewidths of 0.26 mm/s were obtained. For the fitting procedure of the spectra nine subspectra were used. According to the occupation of the 6h- and 2a sites the subspectra were divided into two groups with intensity ratio of 3 : 1. Within each group the intensity ratios were calculated by means of a binomial distribution function assuming that the Ti and SC atoms are distributed randomly at the nearest 4f neighbour sites (table 1). For 2a sites with two and three nearest Ti neighbours only one subspectrum was calculated because even the sum of the intensity
Table 1 Intensity ratios of the nine subspectra the MBssbauer spectra
used in the analyses
Number of nearest Ti neighbours
6 h sites
2a sites
2
0.025 0.099 >
0.041
4 5 6
0.224 0.268 0.134
0.074 0.089 0.045
of
,jFe,
at 4.2 K
ratios for these two sites contributes only with 4.1% to the whole spectrum. The linewidth (r/2) for this subspectrum was taken somewhat larger than the other ones which all had the same value (0.17 mm/s). Since no information about the influence of different numbers of SC (Ti) nearest neighbours on the quadrupole splitting (eQT/,,/2) is available and approximately the same values were measured for eQV,,/2 for ScFe, and TiFe,, for all subspectra eQV,,/2 = -0.44 mm/s were taken. The center shift was assumed to decrease with increasing number of Ti neighbours according to the analyses for different x at B, = 0 of (SC, Ti)Fe,.,, [l]. The asymmetry parameter was set to zero for all subspectra. The complete Hamiltonian was solved for analysing the spectra. The polycrystalline character of the absorber was taken into account for the spectra recorded in applied fields by averaging the different directions of the electric field gradient with respect to the y-ray direction assuming a random distribution of the crystallites in the absorber.
3. Results and discussion Identical spectra were recorded before and after the field sweep up to 13.5 T. These spectra (with B, = 0) were calculated with an angle of 54.7 o between y-ray direction and direction of the hyperfine field to simulate the uniform distribution over all directions of hyperfine fields (B,,). Although the arrangement of the used nine subspectra in the fitting procedure always contains some arbitrariness, only one set of parameters was found which was usable for all recorded spectra (with and without external fields) and leads to results in agreement with bulk magnetic measurements. An angle between B,, and eQV,, of = 70” was found for all subspectra of Fe on 6h and = 0” for all on 2a positions. Values for B,, between 17.5 and 9.2 T for the different surroundings of the 6h positions and between 11.6 and 3.6 T for those of the 2a positions were obtained indicating that the different environments lead to differences of the hyperfine fields which are nearly the same for both Fe sites. For the center shift values between
A. Piisinger et al. / Magnetic phase transition in Sco,,,Ti,,,5Fe,
-3.0
-5.0
-1.0
1.0
Velocity
“0
5
10
B,
15
[T] -->
Fig. 2. Field dependence of the magnetization for Sq,25Tio.,5Fe, at 4.2 K. The arrows mark the fields at which the Miissbauer spectra were taken.
3.0
5.0
(mm/s>
Fig. 1. Mijssbauer transmission spectrum of Sc,,,,Ti,,,
-0.24 to -0.31 mm/s (relative to 57Fe in Rh at 295 K) were calculated (fig. 1). The hysteretic behaviour observed in the magnetization (fig. 2) is also visible in the Mossbatter spectra (see e.g. the spectra recorded in B, = 8 T for increasing and decreasing fields, fig. 3). At B, = 4 T the magnetic splitting of the spectrum measured with decreasing field is broader than the one measured with increasing field, although in this field range the hysteresis of the bulk magnetization vanishes. For the center shift nearely the same values were found as at B, = 0, but the accuracy of the data is not high enough to observe
21
at 4.2 K
F e, at 4.2 K without external field.
the influence of the measured volume magnetostriction [9]. The internal fields ( Bi,,) were calculated from the measured B,, and the fitted angles between B,, and B, (0) (fig. 4) according to Bin,= B,,- B,‘, with B,‘= B, - B,, where B, represents the demagnetizing field. The demagnetization factor was determined from measurements of a-Fe assuming that the shape of the grains for both materials is similar. Bint for the different 6h sites decreases with increasing number of nearest Ti neighbours (fig. 4a) in agreement with the concentration dependence of B,, determined at B, = 0 which also decreases with increasing x [l]. The same is valid for the 2a sites. For all different surroundings Bint depends on B, (fig. 4a) even in the field ranges where for the bulk magnetization a nearly field independent behaviour is observed (fig. 2). Since this dependence is the same for all different environments Of Bint and 0 is nearly independent of the individual surrounding (fig. 4), the weighted (by means of the normalized intensity ratio) mean values of the measured hyperfine fields (B,,r) and of the internal fields (Bint) are shown for the 6h and 2a sites in fig. 5. The hysteretic behaviour of the magnetization is only visible for the 6h sites and is more pronounced for the weighted mean angle between B,, and B, (8) than for the value of Bhf. Con-
22
A. Piisinger et al. /
Magneticphase transition in SC~.,,T~,,~F~~ at 4.2 K
Fig. 3. Recorded and calculated (full line) spectra of Sea 25Ti a.,sFq
at 4.2 K. Left (right) side increasing (decreasing) fields. A 1
100
100'
80
a0 Y
2
60
: 6o M
z
40
40
z+! u ‘:
20
20
T
,
,200
1001 ao- a,
ao 60
60
:
F
40
40
20
zo-
t? a
0 0
Be
[T]
0
T :
e
[T] -->
$
Y2oA
100
z
Ba
A 120,
:b
120
-->
4
a
Ba [T] -->
Fig. 4. (a) Measured hyperfine fields (right side) and calculated internal fields (left side) on the neighbour environments for increasing (upper part) and decreasing (lower part) applied field. direction and hyperfine field (right side) and calculated angles between internal and applied field different nearest neighbour environments for increasing (upper part) and decreasing (lower part) guides to the eyes.
0
120
Ba
[T]
0
-->
6h positions for different nearest (b) Measured angles between y (left side) on the 6h positions for applied field. The lines are only
A. PGsinger et al. / Magnetic phase transition in Sco,,,Tio,,5Fe,
20
-
20
';
o IQ 16 12
A 20 : I I ‘6
p
12
-
a
0
20 *
W a-sites
16
2’ _.-
24 leio
12
dYm
,
,
,
a
12
Ba
[T]
-->
‘W‘&.-dl_ I,, 0
I -
SE
II
4
' 7 -
4
,
,
a
12
BII [T]
-
-->
Fig. 5. Mean measured hyperfine (right side) and internal fields (left side) of Fe at 6h (0, 0) and 2a (CI, H) positions as function of B, (see text). Upper part: Mean angles Z between B, and Bint (left side) and 8 between y direction and B,, (right side) on the 6h positions as a function of Ea. Open (full) symbols increasing (decreasing) applied field. The lines are guides to the eyes.
trary to the behaviour of a simple ferromagnet, where Bin, is nearly independent of B, for B, > Bint of the 2a sites increases with increasing B B,9’
The angle (Y between B, and Bin,can be calculated from the values of B,‘, the fitted B,, and the fitted angle between applied and hyperfine field, since from the smaller values measured for B,, at B, = 13.5 T, compared to those determined at B, = 0, it can be concluded, that Bint points in the opposite direction of the one of B,. The differences of the a! values determined for different numbers of Sc/Ti neighbours are small for the 6h sites (fig. 4b) and (Y= 0 is obtained for the 2a sites. The weighted (in the same way as Bin,) mean values of CY(Z) as a function of B, are shown for the 6h sites in the upper part of fig. 5. In increasing fields Z stays roughly constant at 70” up to B, = 8 T. This is the field region where the bulk magnetization exhibits the first plateau. The result is consistent with the magnetic structure estimated from magnetization and zero field Mossbauer measurements by Nishihara and Yamaguchi [lo], where an angle of approximately 140” between
at 4.2 K
23
the moments on the 6h and 0” between those on the 2a sites was estimated. For the fields where the sudden increase of the magnetization appeares, Z determined for the 6h sites drops to zero, indicating that all internal fields are aligned parallel to B,. Since furthermore B,, decreases with increasing B,, Bin, points to the opposite direction of the one of B, and must now be parallel to the one of the 2a sites. In decreasing applied fields E remains low down to B, = 6 T, again resembling the field dependence of the magnetization. The Mijssbauer investigations reveal that the phase transition observed in the high field magnetization measurement [2] is a transition from a canted ferromagnetic state to a ferromagnetic one, where with increasing magnetic field the cone angle for the 6h sites becomes zero at a critical field. A transition from a canted state to a ferromagnetic state might be described by the itinerant electron model of Moriya and Usami [ll], extended for anisotropic systems by Isoda [12], taking into account the effects of correlated spin fluctuations. However for the present system in the field range where the bulk magnetization exhibits only a slight increase increasing hyperfine fields for both Fe sites are obtained and the sudden change of Bint at the phase transition points to a discontinuous change of the moments at the 6h sites. Both results should be included in the itinerant electron model to permit a complete description of the mechanism of this phase transition.
4. Conclusion The performed high field Mbssbauer experiments favour the following model for the magnetic structure of Sco,25Tio.,5Fe,: In the state with the lower magnetization the iron moments on the 6h sites form a cone with an opening angle of approximately 140 O, the resultant moment of this cone is parallel to the moment of the 2a sites. The moment of the SC atoms is then aligned antiparallel to fit the magnetization data. For B, > 8 T the cone angle drops to zero and all Fe moments are aligned parallel for B, > 10 T. Thus the field induced transition and the hysteretic behaviour visi-
24
A. Pijsinger et al. /
Magneticphase transition in SC~,,,T~,,~F~,
ble in the Mijssbauer spectra and the magnetization points to the coexistence of a ferro and a canted ferromagnetic state.
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at 4.2 K
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