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Unstable magnetic ordering in Ce(Fe1−yXy)2, X = Al, Ga, Si, Ru, Co
Journal of Magnetism and Magnetic Materials 90 & 91 (1990) 471-473 North- Holland
471
Unstable magnetic ordering in Ce(Fe 1 _ yX y ) 2' X Ru,Co
=
AI, Ga, Si,
M. Forsthuber, F. Lehner; G. Wiesinger, G. Hilscher, T. Huber, E. Gratz and G. Wortmann a Institut fur Experimentalphysik, Technical University of Vienna, Wiedner Hauptstrasse 8 -10, A-I040 Vienna, Austria Fachbereicb Physik, Univ. Paderborn, Fed. Rep. Germany
a
We performed X-ray diffraction, L 3 edge X-ray absorption, and Mossbauer measurements on CeFe2 and pseudobinaries Ce(Fel_ yX y h as well as on their hydrides to investigate the magnetic ordering in these materials. While the hydrides order ferromagnetically in the whole temperature range below Tc, the magnetic ordering type of the pseudobinaries in the low-temperature anti ferromagnetic phase is found to be much more complex.
Within the series of cubic Laves phase compounds (R stands for a rare earth), CeF~ shows outstanding features of magnetic, transport and specific heat properties [1], pointing towards the formation of a 4f band and a hybridization of it with the 3d band. The ordering type in CeF~ (Tc = 233 K, 2.58J.tB/Lu. at 4.2 K) is now believed to be ferrimagnetic, the magnetic moments of Fe and Ce being 1.6 and -0.6J.tB' respectively [2,3]. The direction of the Fe moments was found to point into the [001] direction only below a spin reorientation temperature T R = 132 K, above which the Fe moments show a deviation from the [001] axis of about 20° [4]. In order to examine the temperature dependence of the lattice constant we recorded the (222), (422) and (440) line profiles by X-ray diffraction step-scanning. The lattice constant of CeFe2' G, and the line width of RF~
the (440) line profile, .1, as function of the temperature are given in fig. 1. Within the resolution of our experimental equipment no splitting of the X-ray line profiles investigated was found. Therefore we conclude that no substantial structural distortion of CeFe2 at the Curie temperature or the spin reorientation temperature is present. However, a weak broadening of .1 occurs below T.:, which may be an indication for an unresolved splitting of the lines. In the pseudobinary compounds Ce(Fel_yXyh, X = AI, Si, Ga, Ru, Co, the spontaneous magnetization was found to vanish abruptly below a second critical temperature T.:2 < 100 K for a certain concentration range of the substitute [5,6). We hydrided Ce(Fe, Coh samples (y = 0-0.4) and found a reconstitution of the ferromagnetic behaviour after hydrogen uptake in the whole temperature range down to 4.2 K, together with a
olA] 7300 structure-cubic CI5
[;.
(deg)
7.295
0.3~
030 0.26
7.290
f 022 TlKl 100
150
200
250
300
Fig. 1. Temperature dependence of the lattice parameter of CeFe2 and the line width of the (440) line profile. Curie temperature and the spin reorientation temperature.
Tc
and T R indicate the
0304-8853/90/S03.50 © 1990 - Elsevier Science Publishers B.V. (North-Holland) and Yamada Science Foundation
472
M. Forsthuber et al. / Unstable magnetic ordering in Ce(Fe l
_
,.XyJz
Cefel.9RuO.1 T=005K
strongly increased magnetic moment (5.1 Iln/Lu. at 4.2 K for CeFe2H3.s)' In order to determine the Ce valence we performed L3 edge X-ray absorption measurements and obtained a Ce valence of 3.3 for the parent compounds, but 3.0 for the hydrides. Thus we have strong evidence that in the hydrides of CeF~ and its pseudobinaries, the Ce 4f levels retain their localized status producing a Ce moment comparable to an isolated Ce3+ moment (2.IfLn)' As expected for the localized moment of a light rare earth, ferromagnetic ordering is consolidated between the 4f moment and the 3d moment (1.6fLn/Fe) yielding a total moment of roughly 5.3Iln/f.u. On the other hand, a Ce valence of 3.3 in the parent compound Ce(Fe, Coh corroborates the hybridization of the 4f band with the 3d bands predicted from theory [2]; a small Ce moment which couples Ierrimagnetically to the Fe moments reduces further the spontaneous magnetization leading to a rather small value, as e.g. 2.6Iln/Lu. obtained for CeF~ . To investigate the transition from ferrimagnetism to anti ferromagnetism in Ce(Fet_yXyh more in detail, we used Mossbauer spectroscopy in order to obtain information on the direction of the Fe moments . Since the quadrupole interaction was found to be too large to be treated as a smaIl perturbation to the magnetic hyperfine interaction, the spectra were analyzed by using a program to solve the complete Hamiltonian for simultaneous electric and magnetic hyperfine interactions 17]. From the similar thermal behaviour of susceptibility and magnetization for different substituents X [5,6], an almost identical mechanism of magnetic ordering is assumed. We therefore restricted the Mossbauer measurements to X = Ru and Ga. In fig. 2 the pattern of CeFet.9Ruo.t for T= 4.2 K is presented as a typical example. In the spectra recorded above the spin reorientation temperature can sufficiently well be interpreted assuming a [001] easy axis of magnetization. The line broadening is due to the influence of the varying local environment and has been included in the fitting proce-
:z C)
U"'.l U"'.l
I.
0.92
-6
-4
-2
VELOCITY
2
(mm/s)
Fig. 2. Mossbauer spectrum of CeFe1.9Ruo.1 at 4.2 K. Symbols represent experimental points. solid lines the computer fit described in the tex t.
dure by taking the first two neighbour shells into account. On reducing the temperature below T R' particularly the outer lines of the spectra change their shape. Obviously, the easy axis of magnetization is no longer [001], and the Fe sites are no longer magnetically equivalent. Considering Fe sites with six Fe nearest neighbours, the best fit was obtained by superposing three multiplets in the intensity ratio of 2: 1 : 1, which are referred in table 1 as a, hI, and b2, respectively. (A fourth multiplet, not shown in table 1, was used in addition to represent the Fe atoms with five Fe nearest neighbours, as the remaining nearest neighbour configurations produce patterns with too small intensity to be of significant influence on the quality of the fit.) Table 1 shows data of these Mossbauer spectra for CeFet.9GaO.t and
Table 1 Effective hyperfine field (Bd t ) , isomer shift (IS, rel. a-Fe at room temperature). and the angle between the hyperfine field and the main axis of the EFG (t'}), in CeFel.9Gao.t and CeFel.9Ruo.1 for 110 and 4.2 K, respectively
T, sublattice
110K 4.2K
CeFe\.9 R uO.1
CeFel.9Gao.1
{
~1b2
Bert (T)
IS (rnmys)
14.6 14.6 16.1 17.4
-0.10 +0.10
-0.02 0.00
t'}
Bert (T)
IS (rnmys)
71.3 0 70.3 0 61.0 0
14.2 14.7 15.8 17.4
-0.14 +0.03 -0.01 ~0.04
t'}
74.0 0 72.0 0 69.0 0
M. Forsthuber et al. / Unstable magnetic ordering in Ce(Fel _ )'X)'h
CeFel.9Ruo.l for temperatures below and above the anti ferromagnetic transition. If a unique magnetization direction were present for all four Fe sites, the existence of three multiplets with an intensity ratio of 2: 1 : 1 would indicate a [uuw] easy axis of magnetization [4]. However, since all multiplets produce an angle between the hyperfine field and the main axis of EFG {} "" 70 0 , there cannot exist a uniform magnetization direction. More likely a canted or cone-l ike arrangement of the Fe moments occurs. Assuming Fe moments oriented along a [uuw] direction (even if a unique magnetization direction does not exist any more), we calculated directions to enclose the angle of 70 0 with the [111] direction of the main axis of the EFG. We found a [115] direction to meet this condition; moreover it shows an angle of 39 0 to another [Ill] direction, in agreement with recent neutron scattering results on Ce(Fe, Co), [8]. For the different [111] directions of the EFG the Fe moments will also point into different (115) directions; we found that combining Fe moments of about 1.6/LB arranged along the [1 ± 1 ± 5] and [1 ± 1 =+= 5] directions forms a total magnetic moment which is able to compensate a Ce moment of roughly -0.6/LB aligned along the.ll00] axis. This suggestion of the arrangement of the Fe moments somewhat differs from the one given by Kennedy et a1. [8]. Nonetheless, it is able to explain the vanishing
473
magnetization at low temperature taking the non-zero Ce moment into account, which has not been considered in ref. [8]. A final proof of the correct moment arrangement has to be carried out by polarized neutron scattering experiments. This work was supported by the "JubiHiumsfonds der Osterreichischen Nationalbank" under project 3492. One of us (M .F.) is indebted to the " Osterreichische Forschungsgemeinsch aft" (project no. 06/1036) and to the "Verband der Freunde und Absolventen der TV \Vien" for financial support. References [I) A.K. Rastogi, G. Hilscher, E. Gratz and N. Pittmayr, J. de Phys. 49 (1988) C8-277. [2) O. Eriksson, L. Nordstrom, M.S.S. Brooks and B. Johansson, Phys. Rev. Lett. 60 (1988) 2523. [3) B. Rainford and O. Hilscher, to be published. (4) U. Atzmony and M.P. Dariel, Phys Rev. B 10 (1974) 2060. [51. A.K. Grover, R.O. Pillay, V. Balasubrarnanian and P.N. Tandon, Solid Stale Commun. 67 (1988) 1223. [6) S.B. Roy and n .R. Coles, 1. Phys. Condens. Matter 1 (1989) 419. (7) W. KUndig, Nue\. Instr. and Melh. 48 (1967) 219. [8) S.l . Kennedy, A.P. Murani, 1.K. Cockcroft, S.B. Roy and n .R. Coles, 1. Phys. Condens. Matter 1 (1989) 629.
471
Unstable magnetic ordering in Ce(Fe 1 _ yX y ) 2' X Ru,Co
=
AI, Ga, Si,
M. Forsthuber, F. Lehner; G. Wiesinger, G. Hilscher, T. Huber, E. Gratz and G. Wortmann a Institut fur Experimentalphysik, Technical University of Vienna, Wiedner Hauptstrasse 8 -10, A-I040 Vienna, Austria Fachbereicb Physik, Univ. Paderborn, Fed. Rep. Germany
a
We performed X-ray diffraction, L 3 edge X-ray absorption, and Mossbauer measurements on CeFe2 and pseudobinaries Ce(Fel_ yX y h as well as on their hydrides to investigate the magnetic ordering in these materials. While the hydrides order ferromagnetically in the whole temperature range below Tc, the magnetic ordering type of the pseudobinaries in the low-temperature anti ferromagnetic phase is found to be much more complex.
Within the series of cubic Laves phase compounds (R stands for a rare earth), CeF~ shows outstanding features of magnetic, transport and specific heat properties [1], pointing towards the formation of a 4f band and a hybridization of it with the 3d band. The ordering type in CeF~ (Tc = 233 K, 2.58J.tB/Lu. at 4.2 K) is now believed to be ferrimagnetic, the magnetic moments of Fe and Ce being 1.6 and -0.6J.tB' respectively [2,3]. The direction of the Fe moments was found to point into the [001] direction only below a spin reorientation temperature T R = 132 K, above which the Fe moments show a deviation from the [001] axis of about 20° [4]. In order to examine the temperature dependence of the lattice constant we recorded the (222), (422) and (440) line profiles by X-ray diffraction step-scanning. The lattice constant of CeFe2' G, and the line width of RF~
the (440) line profile, .1, as function of the temperature are given in fig. 1. Within the resolution of our experimental equipment no splitting of the X-ray line profiles investigated was found. Therefore we conclude that no substantial structural distortion of CeFe2 at the Curie temperature or the spin reorientation temperature is present. However, a weak broadening of .1 occurs below T.:, which may be an indication for an unresolved splitting of the lines. In the pseudobinary compounds Ce(Fel_yXyh, X = AI, Si, Ga, Ru, Co, the spontaneous magnetization was found to vanish abruptly below a second critical temperature T.:2 < 100 K for a certain concentration range of the substitute [5,6). We hydrided Ce(Fe, Coh samples (y = 0-0.4) and found a reconstitution of the ferromagnetic behaviour after hydrogen uptake in the whole temperature range down to 4.2 K, together with a
olA] 7300 structure-cubic CI5
[;.
(deg)
7.295
0.3~
030 0.26
7.290
f 022 TlKl 100
150
200
250
300
Fig. 1. Temperature dependence of the lattice parameter of CeFe2 and the line width of the (440) line profile. Curie temperature and the spin reorientation temperature.
Tc
and T R indicate the
0304-8853/90/S03.50 © 1990 - Elsevier Science Publishers B.V. (North-Holland) and Yamada Science Foundation
472
M. Forsthuber et al. / Unstable magnetic ordering in Ce(Fe l
_
,.XyJz
Cefel.9RuO.1 T=005K
strongly increased magnetic moment (5.1 Iln/Lu. at 4.2 K for CeFe2H3.s)' In order to determine the Ce valence we performed L3 edge X-ray absorption measurements and obtained a Ce valence of 3.3 for the parent compounds, but 3.0 for the hydrides. Thus we have strong evidence that in the hydrides of CeF~ and its pseudobinaries, the Ce 4f levels retain their localized status producing a Ce moment comparable to an isolated Ce3+ moment (2.IfLn)' As expected for the localized moment of a light rare earth, ferromagnetic ordering is consolidated between the 4f moment and the 3d moment (1.6fLn/Fe) yielding a total moment of roughly 5.3Iln/f.u. On the other hand, a Ce valence of 3.3 in the parent compound Ce(Fe, Coh corroborates the hybridization of the 4f band with the 3d bands predicted from theory [2]; a small Ce moment which couples Ierrimagnetically to the Fe moments reduces further the spontaneous magnetization leading to a rather small value, as e.g. 2.6Iln/Lu. obtained for CeF~ . To investigate the transition from ferrimagnetism to anti ferromagnetism in Ce(Fet_yXyh more in detail, we used Mossbauer spectroscopy in order to obtain information on the direction of the Fe moments . Since the quadrupole interaction was found to be too large to be treated as a smaIl perturbation to the magnetic hyperfine interaction, the spectra were analyzed by using a program to solve the complete Hamiltonian for simultaneous electric and magnetic hyperfine interactions 17]. From the similar thermal behaviour of susceptibility and magnetization for different substituents X [5,6], an almost identical mechanism of magnetic ordering is assumed. We therefore restricted the Mossbauer measurements to X = Ru and Ga. In fig. 2 the pattern of CeFet.9Ruo.t for T= 4.2 K is presented as a typical example. In the spectra recorded above the spin reorientation temperature can sufficiently well be interpreted assuming a [001] easy axis of magnetization. The line broadening is due to the influence of the varying local environment and has been included in the fitting proce-
:z C)
U"'.l U"'.l
I.
0.92
-6
-4
-2
VELOCITY
2
(mm/s)
Fig. 2. Mossbauer spectrum of CeFe1.9Ruo.1 at 4.2 K. Symbols represent experimental points. solid lines the computer fit described in the tex t.
dure by taking the first two neighbour shells into account. On reducing the temperature below T R' particularly the outer lines of the spectra change their shape. Obviously, the easy axis of magnetization is no longer [001], and the Fe sites are no longer magnetically equivalent. Considering Fe sites with six Fe nearest neighbours, the best fit was obtained by superposing three multiplets in the intensity ratio of 2: 1 : 1, which are referred in table 1 as a, hI, and b2, respectively. (A fourth multiplet, not shown in table 1, was used in addition to represent the Fe atoms with five Fe nearest neighbours, as the remaining nearest neighbour configurations produce patterns with too small intensity to be of significant influence on the quality of the fit.) Table 1 shows data of these Mossbauer spectra for CeFet.9GaO.t and
Table 1 Effective hyperfine field (Bd t ) , isomer shift (IS, rel. a-Fe at room temperature). and the angle between the hyperfine field and the main axis of the EFG (t'}), in CeFel.9Gao.t and CeFel.9Ruo.1 for 110 and 4.2 K, respectively
T, sublattice
110K 4.2K
CeFe\.9 R uO.1
CeFel.9Gao.1
{
~1b2
Bert (T)
IS (rnmys)
14.6 14.6 16.1 17.4
-0.10 +0.10
-0.02 0.00
t'}
Bert (T)
IS (rnmys)
71.3 0 70.3 0 61.0 0
14.2 14.7 15.8 17.4
-0.14 +0.03 -0.01 ~0.04
t'}
74.0 0 72.0 0 69.0 0
M. Forsthuber et al. / Unstable magnetic ordering in Ce(Fel _ )'X)'h
CeFel.9Ruo.l for temperatures below and above the anti ferromagnetic transition. If a unique magnetization direction were present for all four Fe sites, the existence of three multiplets with an intensity ratio of 2: 1 : 1 would indicate a [uuw] easy axis of magnetization [4]. However, since all multiplets produce an angle between the hyperfine field and the main axis of EFG {} "" 70 0 , there cannot exist a uniform magnetization direction. More likely a canted or cone-l ike arrangement of the Fe moments occurs. Assuming Fe moments oriented along a [uuw] direction (even if a unique magnetization direction does not exist any more), we calculated directions to enclose the angle of 70 0 with the [111] direction of the main axis of the EFG. We found a [115] direction to meet this condition; moreover it shows an angle of 39 0 to another [Ill] direction, in agreement with recent neutron scattering results on Ce(Fe, Co), [8]. For the different [111] directions of the EFG the Fe moments will also point into different (115) directions; we found that combining Fe moments of about 1.6/LB arranged along the [1 ± 1 ± 5] and [1 ± 1 =+= 5] directions forms a total magnetic moment which is able to compensate a Ce moment of roughly -0.6/LB aligned along the.ll00] axis. This suggestion of the arrangement of the Fe moments somewhat differs from the one given by Kennedy et a1. [8]. Nonetheless, it is able to explain the vanishing
473
magnetization at low temperature taking the non-zero Ce moment into account, which has not been considered in ref. [8]. A final proof of the correct moment arrangement has to be carried out by polarized neutron scattering experiments. This work was supported by the "JubiHiumsfonds der Osterreichischen Nationalbank" under project 3492. One of us (M .F.) is indebted to the " Osterreichische Forschungsgemeinsch aft" (project no. 06/1036) and to the "Verband der Freunde und Absolventen der TV \Vien" for financial support. References [I) A.K. Rastogi, G. Hilscher, E. Gratz and N. Pittmayr, J. de Phys. 49 (1988) C8-277. [2) O. Eriksson, L. Nordstrom, M.S.S. Brooks and B. Johansson, Phys. Rev. Lett. 60 (1988) 2523. [3) B. Rainford and O. Hilscher, to be published. (4) U. Atzmony and M.P. Dariel, Phys Rev. B 10 (1974) 2060. [51. A.K. Grover, R.O. Pillay, V. Balasubrarnanian and P.N. Tandon, Solid Stale Commun. 67 (1988) 1223. [6) S.B. Roy and n .R. Coles, 1. Phys. Condens. Matter 1 (1989) 419. (7) W. KUndig, Nue\. Instr. and Melh. 48 (1967) 219. [8) S.l . Kennedy, A.P. Murani, 1.K. Cockcroft, S.B. Roy and n .R. Coles, 1. Phys. Condens. Matter 1 (1989) 629.