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Magnetic properties of GdMg
Solid State Communications, Vol. 18, pp. 609—6 12, 1976.
Pergamon Press.
Printed in Great Britain
MAGNETIC PROPERTIES OF GdMg K.H.J. Buschow Philips Research Laboratories Eindhoven, The Netherlands and C.J. Schinkel Natuurkundig Laboratorium, Universiteit van Amsterdam, Amsterdam, The Netherlands (Received 19 September 1975 by A.R. Miedema) The lattice constants at room temperature and the magnetic properties of several members of the series Gd~La1_~Mg were studied in the temperature range 4.2—300 K with applied fields up to 340 kG. Several of the pseudobinary compounds were found to give rise to antiferromagnetic ordering in low applied fields. In their magnetic isotherms at 4.2 K critical fields occur which are associated with the destruction of the antiferromagnetic ground state. These critical fields were found to increase with La concentration. It is Shown that in Gd~La1_~Mg the antiferromagnetic interaction between the Gd spins decreases with increasing x whereas the ferromagnetic interaction increases in the same sense. In GdMg the antiferromagnetic interaction has almost completely vanished, so that superficially this compound looks like a normal ferromagnet. THE RECENTLY REPORTED magnetic properties of the cubic compound GdMg (CsC1-type) are remarkable in so far that the temperature dependence of the magnetization indicates ferromagnetism whereas the saturation moment is appreciably lower than the value expected for ferromagnetic alignment of the Gd spins.’ In the same investigation further indications that the magnetic ordering in GdMg is of a more complex nature than simply ferromagnetic ordering were also reached from the NMR spin echo spectra observed for this compound. In order to obtain more information on the magnetically ordered state of GdMg we have studied the magnetization of this compound in much higher field strengths than those applied previously. In addition we have investigated the effect of increasing the interatomic separation of the Gd atoms by means of the series Gd~La1_~Mg. The samples of the compounds Gd~_~La~Mg were prepared from Gd and La of 99.9% purity and Mg of 99.99% punty. Stoichiometric proportions of the constituent metals were sealed into a molybdenum container. This container was heated in vacuum for several hours at temperatures between 1000 and 1100°Cand subsequently quenched in water. X-ray diffraction showed that the samples were of a single phase after this procedure. The X-ray diagrams were ins.lexed according to the cubic CsC1 structure type. The vanation of the lattice constant with composition is shown in the inset of Fig. 1. 609
2oo[~—
-~
Gd~Lo1~Mg
o.
ow
150 390
-
~
100
02
08 —
~ ,
‘/
/
- - -
50
,
-
,‘
0 0
02
0/.
05
08
0
Fig. 1. Variation with composition of the asymptotic Curie temperature (Or) in the series of compounds Gd~La1_~Mg. The inset shows the composition dependence of the lattice constant. The magnetic measurements were performed on lumps of polycrystaffine material. The temperature dependence of the magnetization (a) and susceptibility (x) were studied at field strengths up to 18 kOe by means of the (modified) Faraday method. The field dependence at 4.2 K in very high applied fields (up to about 34 T) was determined with the equipment des2 cribed by Roeland, Muller and Gersdorf.
610
MAGNETIC PROPERTIES OF GdMg
Vol. 18, No. 5
Table 1. Magnetic data ofseveral compounds of the series Gd~La1_~Mg Gd~La1.~Mg x=1.Or x=0.9 x = 0.8 x = 06 x=0.4 x=0.3
120
PB/Gd
p(340kG) PB/Gd
(K)
8.1 8.4 8.5 8.3 8.7 8.7
7.55 7.10 6.85 6.85 7.20 6.95
118 82 61 27 7 7
I.Leff
Gd09Lo01Mg
2750
T~,TN (K) T~
—
TN= TN = TN
(kOe) 121 88 88 60
—
7 35 60
— —
160 120
5
100
40 ~~60~gx~
E b
001Mg
80 0
-40 -80 -120
7
________________
-160
_____________________
-30
Gd08La02Mg
20
-20
-10
0
10
20
____
30
isotherms at 4.2 K of the compound I.
—-
2
_______________________________
0 1
:
The effective moment per Gd atom O~8eff)and the Fig. 3 Magnetic asymptotic Curie temperature Gd0 9La0(Or) 1Mg.were derived from plots of x~ vs T. The values obtained for the various compounds are listed in Table 1. In all cases the effective moments are in excess of to conduction electron polarization by the 7.94pB per Gd atom). ThisGdis spin ascribed the free ionThe value ( moments. values of O~have also been plotted as a
Gd~La of Gd function dependence of the concentration magnetization in Fig. of the 1. The compounds temperature 1_~Mg =various 0.9,room 0.8, 0.6, 0.4K.and 0.3 reare shown corded the samples in during Fig.inwithx 2. heating zero Theapplied to field a vstemperature toT 4.2 curves Magnetization were after cooling
16
vs T curves corresponding to three different field
03
100
00
200 TIKI
~
300
Fig. 2. Temperature dependence of the reciprocal susceptibility and the magnetization (a) of various compounds Gd~La1~Mg.
(xv)
strengths (3,9 and 18 kOe) were obtained during the same run. For the compounds with x = 06, 0.4 and 0.3 only the curves corresponding to 18 kOe are shown (bottom part of Fig. 2). It is seen in the top part of Fig. 2 that Gd0.9La01Mg has an antiferromagnetic— paramagnetic transition (TN = 35 K) at low field strengths (3 kOe). At higher applied field strengths (9 and 18 kOe) this transition is no longer present. The
Vol. 18, No. 5
MAGNETIC PROPERTIES OF GdMg
14 13 12
x
5
10
15
20
10
611
(x ~ 0.6). Saturation does not seem to be reached, even at 340 kOe, in either of the compounds investigated although all the compounds have a magnetic moment (listed in Table 1) rather close to 7PB per Gd atom at the highest field applied. This slow tendency to saturate above the critical field indicates that in this region the compounds can be characterized as saturated paramagnets rather than ferromagnets. In a simple molecular field approach with two magnetic sublattices A and B, the molecular field constants of the intra-sublattice interaction (NAA) and intersublattice interaction (NAB) can be expressed in terms of U~and TN:
4 5K
~
H TESLA)
NAA
=
NAB
=
—~TN+Op)
(1)
~TN
(2)
—~
4. Field dependence of the magnetization at 4.2 K for several of the compounds Gd~La1_~Mg. The isotherms were recorded by decreasing the applied field strength from about 34 T. To avoid overlapping of the various curves the origin for several curves has been shifted in upward direction, Fig.
curve obtained in a separate run at a still higher field strength (27 kOe) is at low temperatures indicative of ferromagnetic ordering or paramagnetic saturation. The field dependence at 4.2 K is shown for this compound in Fig. 3. The curve of initial magnetization has a pronounced jump near 7 kOe. Such a curve would indeed be expected for an antiferromagnetic and a ferromagnetic state (or saturated paramagnetic state), the first being stable at low strengths, other at high field strengths. Thefield presence of anthe appreciable hysteresis suggests that this antiferromagnetic—ferromagnetic transition is of first order. The magnetization process seems to be rather complex, however, since the descending branch of the loop consists of two steps rather than one. The same type of hysteresis loop was obtained when the measurements were repeated with samples of the same material coming from different parts of the melt. The results presented in the middle part of Fig. 2 show that at higher La concentrations the antiferromagnetic state becomes relatively more stable, since all the curves shown for Gd0 8La0 2Mg have well developed Néel ternperatures. At still higher La concentrations the maxima in the a, T curves become less sharp and finally disappear. This is seen in the bottom part of Fig. 2. For GdMg as well as for several of the pseudobinary compounds the field dependence of the magnetization at 4.2 K was studied with very high applied field strengths up to 340 kOe. The results of the magnetization measurement obtained with decreasing field strength are shown in Fig. 4. They reveal that in addition to Gd0 9[2~ 1Mg a critical field in the isotherm exists also for compounds of still higher La concentrations
—
Up).
The quantities C and U~are defined by means of the Curie—Weiss law x = C/(T Up) whereas N~and N~ are defined in a way that a negative (positive) sign —
corresponds to a ferromagnetic (antiferromagnetic) interaction. By using the experimental values listed for U~and TN in Table 1 one finds that with increasing x in Gd~La1_~Mg there is a decreasing antiferromagnetic interaction (NAB decreases) and an increasing ferromagnetic interaction (FNAAI increases). This picture is in keeping with the concentration dependence of the critical field since in the same simple molecular field model the critical fields are given by 2MNAB (3) H0 = where M is the magnetization per sublattice. With the values given for H 0 in Table 1 one again finds a decreasing antiferromagnetic interaction with increasing x. The results obtained by using equation (2) as well as those derived by applying equation (3) indicate that GdMg can be regarded as a borderline case where the antiferromagnetic interaction has become extremely small. The above model very likely is an oversimplification. Nevertheless the results of this analysis are in agreement with the notion made before, that GdMg is not a true ferromagnet but has a magnetic moment arrangement in which not all the Gd moments are aligned ferromagnetically’ The increasing antiferromagnetic interaction with increasing La concentration suggest that the origin of the antiferrornagnetic interaction is not due to crystal imperfections in which rare earth atoms occupy Mg sites and vice versa. Since La is larger than Gd one expects that the number of such imperfections decreases with La concentration which would imply a decrease rather than an increase of the antiferrornagnetic interaction in this direction. No indications for the above mentioned atomic disordering
612
MAGNETIC PROPERTIES OF GdMg
were furthermore obtained by neutron diffraction studies on the isomorphous compound NdMg.3 It is seen in Fig 1 that the substitution of La for Gd leads to an increase in the lattice constant. It is likely therefore that the ratio between the strength of the ferromagnetic and antiferromagnetic couplings is sensitive to changes in the interatomic distances. In that case a larger lattice constant corresponds to a relative decrease of the ferromagnetic coupling strength. This is in accordance with the results obtained for the series
Vol. 18, No. 5 x and where U~(°°T0) increases continuously.1 Recently it has been shown that the relatively strong ferromagnetic interaction in GdZn is predominantly due to a coupling of the Gd moments proceeding through interatomic interaction of the polarized Sd electrons of the Gd atoms.4 The expected distance dependence of the interaction between the d electrons could then very well be the reason for the variation in ferromagnetic exchange interaction in the series GdMg 1_~Zn~ as well as in Gd~La1~Mg.
GdMg1_~Zn~ where the lattice constant decreases with
1.
REFERE ICES BUSCHOW K.H.J. & OPPELT A., J. Phys. F.: Metal Phys. 4, 1246 (1974).
2.
ROELAND L.W., MULLER F.A. & GERSDORF R., Coil,
3.
BUSCHOW K.H.J., VAN LAAR B. & ELEMANS J.B.A.A.,J. Phys. F..’ Metal Phys. 4, 1517 (1974).
4.
DORMANN E. & BUSCHOW K.H.J., J. Appl. Phys. (to be published).
5.
ALEONARD R., MORIN P., PIERRE J. & SCHMITT D., Solid State Commun. 17, 599 (1975).
mt.
CNRS 66,75 (1967).
Note added in proof: Experimental evidence that the compound GdMg is not 5 simply ferromagnetic was recently also obtained from a study of the temperature dependence of its magnetization.
Pergamon Press.
Printed in Great Britain
MAGNETIC PROPERTIES OF GdMg K.H.J. Buschow Philips Research Laboratories Eindhoven, The Netherlands and C.J. Schinkel Natuurkundig Laboratorium, Universiteit van Amsterdam, Amsterdam, The Netherlands (Received 19 September 1975 by A.R. Miedema) The lattice constants at room temperature and the magnetic properties of several members of the series Gd~La1_~Mg were studied in the temperature range 4.2—300 K with applied fields up to 340 kG. Several of the pseudobinary compounds were found to give rise to antiferromagnetic ordering in low applied fields. In their magnetic isotherms at 4.2 K critical fields occur which are associated with the destruction of the antiferromagnetic ground state. These critical fields were found to increase with La concentration. It is Shown that in Gd~La1_~Mg the antiferromagnetic interaction between the Gd spins decreases with increasing x whereas the ferromagnetic interaction increases in the same sense. In GdMg the antiferromagnetic interaction has almost completely vanished, so that superficially this compound looks like a normal ferromagnet. THE RECENTLY REPORTED magnetic properties of the cubic compound GdMg (CsC1-type) are remarkable in so far that the temperature dependence of the magnetization indicates ferromagnetism whereas the saturation moment is appreciably lower than the value expected for ferromagnetic alignment of the Gd spins.’ In the same investigation further indications that the magnetic ordering in GdMg is of a more complex nature than simply ferromagnetic ordering were also reached from the NMR spin echo spectra observed for this compound. In order to obtain more information on the magnetically ordered state of GdMg we have studied the magnetization of this compound in much higher field strengths than those applied previously. In addition we have investigated the effect of increasing the interatomic separation of the Gd atoms by means of the series Gd~La1_~Mg. The samples of the compounds Gd~_~La~Mg were prepared from Gd and La of 99.9% purity and Mg of 99.99% punty. Stoichiometric proportions of the constituent metals were sealed into a molybdenum container. This container was heated in vacuum for several hours at temperatures between 1000 and 1100°Cand subsequently quenched in water. X-ray diffraction showed that the samples were of a single phase after this procedure. The X-ray diagrams were ins.lexed according to the cubic CsC1 structure type. The vanation of the lattice constant with composition is shown in the inset of Fig. 1. 609
2oo[~—
-~
Gd~Lo1~Mg
o.
ow
150 390
-
~
100
02
08 —
~ ,
‘/
/
- - -
50
,
-
,‘
0 0
02
0/.
05
08
0
Fig. 1. Variation with composition of the asymptotic Curie temperature (Or) in the series of compounds Gd~La1_~Mg. The inset shows the composition dependence of the lattice constant. The magnetic measurements were performed on lumps of polycrystaffine material. The temperature dependence of the magnetization (a) and susceptibility (x) were studied at field strengths up to 18 kOe by means of the (modified) Faraday method. The field dependence at 4.2 K in very high applied fields (up to about 34 T) was determined with the equipment des2 cribed by Roeland, Muller and Gersdorf.
610
MAGNETIC PROPERTIES OF GdMg
Vol. 18, No. 5
Table 1. Magnetic data ofseveral compounds of the series Gd~La1_~Mg Gd~La1.~Mg x=1.Or x=0.9 x = 0.8 x = 06 x=0.4 x=0.3
120
PB/Gd
p(340kG) PB/Gd
(K)
8.1 8.4 8.5 8.3 8.7 8.7
7.55 7.10 6.85 6.85 7.20 6.95
118 82 61 27 7 7
I.Leff
Gd09Lo01Mg
2750
T~,TN (K) T~
—
TN= TN = TN
(kOe) 121 88 88 60
—
7 35 60
— —
160 120
5
100
40 ~~60~gx~
E b
001Mg
80 0
-40 -80 -120
7
________________
-160
_____________________
-30
Gd08La02Mg
20
-20
-10
0
10
20
____
30
isotherms at 4.2 K of the compound I.
—-
2
_______________________________
0 1
:
The effective moment per Gd atom O~8eff)and the Fig. 3 Magnetic asymptotic Curie temperature Gd0 9La0(Or) 1Mg.were derived from plots of x~ vs T. The values obtained for the various compounds are listed in Table 1. In all cases the effective moments are in excess of to conduction electron polarization by the 7.94pB per Gd atom). ThisGdis spin ascribed the free ionThe value ( moments. values of O~have also been plotted as a
Gd~La of Gd function dependence of the concentration magnetization in Fig. of the 1. The compounds temperature 1_~Mg =various 0.9,room 0.8, 0.6, 0.4K.and 0.3 reare shown corded the samples in during Fig.inwithx 2. heating zero Theapplied to field a vstemperature toT 4.2 curves Magnetization were after cooling
16
vs T curves corresponding to three different field
03
100
00
200 TIKI
~
300
Fig. 2. Temperature dependence of the reciprocal susceptibility and the magnetization (a) of various compounds Gd~La1~Mg.
(xv)
strengths (3,9 and 18 kOe) were obtained during the same run. For the compounds with x = 06, 0.4 and 0.3 only the curves corresponding to 18 kOe are shown (bottom part of Fig. 2). It is seen in the top part of Fig. 2 that Gd0.9La01Mg has an antiferromagnetic— paramagnetic transition (TN = 35 K) at low field strengths (3 kOe). At higher applied field strengths (9 and 18 kOe) this transition is no longer present. The
Vol. 18, No. 5
MAGNETIC PROPERTIES OF GdMg
14 13 12
x
5
10
15
20
10
611
(x ~ 0.6). Saturation does not seem to be reached, even at 340 kOe, in either of the compounds investigated although all the compounds have a magnetic moment (listed in Table 1) rather close to 7PB per Gd atom at the highest field applied. This slow tendency to saturate above the critical field indicates that in this region the compounds can be characterized as saturated paramagnets rather than ferromagnets. In a simple molecular field approach with two magnetic sublattices A and B, the molecular field constants of the intra-sublattice interaction (NAA) and intersublattice interaction (NAB) can be expressed in terms of U~and TN:
4 5K
~
H TESLA)
NAA
=
NAB
=
—~TN+Op)
(1)
~TN
(2)
—~
4. Field dependence of the magnetization at 4.2 K for several of the compounds Gd~La1_~Mg. The isotherms were recorded by decreasing the applied field strength from about 34 T. To avoid overlapping of the various curves the origin for several curves has been shifted in upward direction, Fig.
curve obtained in a separate run at a still higher field strength (27 kOe) is at low temperatures indicative of ferromagnetic ordering or paramagnetic saturation. The field dependence at 4.2 K is shown for this compound in Fig. 3. The curve of initial magnetization has a pronounced jump near 7 kOe. Such a curve would indeed be expected for an antiferromagnetic and a ferromagnetic state (or saturated paramagnetic state), the first being stable at low strengths, other at high field strengths. Thefield presence of anthe appreciable hysteresis suggests that this antiferromagnetic—ferromagnetic transition is of first order. The magnetization process seems to be rather complex, however, since the descending branch of the loop consists of two steps rather than one. The same type of hysteresis loop was obtained when the measurements were repeated with samples of the same material coming from different parts of the melt. The results presented in the middle part of Fig. 2 show that at higher La concentrations the antiferromagnetic state becomes relatively more stable, since all the curves shown for Gd0 8La0 2Mg have well developed Néel ternperatures. At still higher La concentrations the maxima in the a, T curves become less sharp and finally disappear. This is seen in the bottom part of Fig. 2. For GdMg as well as for several of the pseudobinary compounds the field dependence of the magnetization at 4.2 K was studied with very high applied field strengths up to 340 kOe. The results of the magnetization measurement obtained with decreasing field strength are shown in Fig. 4. They reveal that in addition to Gd0 9[2~ 1Mg a critical field in the isotherm exists also for compounds of still higher La concentrations
—
Up).
The quantities C and U~are defined by means of the Curie—Weiss law x = C/(T Up) whereas N~and N~ are defined in a way that a negative (positive) sign —
corresponds to a ferromagnetic (antiferromagnetic) interaction. By using the experimental values listed for U~and TN in Table 1 one finds that with increasing x in Gd~La1_~Mg there is a decreasing antiferromagnetic interaction (NAB decreases) and an increasing ferromagnetic interaction (FNAAI increases). This picture is in keeping with the concentration dependence of the critical field since in the same simple molecular field model the critical fields are given by 2MNAB (3) H0 = where M is the magnetization per sublattice. With the values given for H 0 in Table 1 one again finds a decreasing antiferromagnetic interaction with increasing x. The results obtained by using equation (2) as well as those derived by applying equation (3) indicate that GdMg can be regarded as a borderline case where the antiferromagnetic interaction has become extremely small. The above model very likely is an oversimplification. Nevertheless the results of this analysis are in agreement with the notion made before, that GdMg is not a true ferromagnet but has a magnetic moment arrangement in which not all the Gd moments are aligned ferromagnetically’ The increasing antiferromagnetic interaction with increasing La concentration suggest that the origin of the antiferrornagnetic interaction is not due to crystal imperfections in which rare earth atoms occupy Mg sites and vice versa. Since La is larger than Gd one expects that the number of such imperfections decreases with La concentration which would imply a decrease rather than an increase of the antiferrornagnetic interaction in this direction. No indications for the above mentioned atomic disordering
612
MAGNETIC PROPERTIES OF GdMg
were furthermore obtained by neutron diffraction studies on the isomorphous compound NdMg.3 It is seen in Fig 1 that the substitution of La for Gd leads to an increase in the lattice constant. It is likely therefore that the ratio between the strength of the ferromagnetic and antiferromagnetic couplings is sensitive to changes in the interatomic distances. In that case a larger lattice constant corresponds to a relative decrease of the ferromagnetic coupling strength. This is in accordance with the results obtained for the series
Vol. 18, No. 5 x and where U~(°°T0) increases continuously.1 Recently it has been shown that the relatively strong ferromagnetic interaction in GdZn is predominantly due to a coupling of the Gd moments proceeding through interatomic interaction of the polarized Sd electrons of the Gd atoms.4 The expected distance dependence of the interaction between the d electrons could then very well be the reason for the variation in ferromagnetic exchange interaction in the series GdMg 1_~Zn~ as well as in Gd~La1~Mg.
GdMg1_~Zn~ where the lattice constant decreases with
1.
REFERE ICES BUSCHOW K.H.J. & OPPELT A., J. Phys. F.: Metal Phys. 4, 1246 (1974).
2.
ROELAND L.W., MULLER F.A. & GERSDORF R., Coil,
3.
BUSCHOW K.H.J., VAN LAAR B. & ELEMANS J.B.A.A.,J. Phys. F..’ Metal Phys. 4, 1517 (1974).
4.
DORMANN E. & BUSCHOW K.H.J., J. Appl. Phys. (to be published).
5.
ALEONARD R., MORIN P., PIERRE J. & SCHMITT D., Solid State Commun. 17, 599 (1975).
mt.
CNRS 66,75 (1967).
Note added in proof: Experimental evidence that the compound GdMg is not 5 simply ferromagnetic was recently also obtained from a study of the temperature dependence of its magnetization.