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Crystal field effects on the magnetic structures of the rare earth compounds with the FeB structure
Solid State Coimnunications, Vol. 25, pp. 735—737, 1978. Perganion Press. Printed in Great Britain
CRYSTAL FIELD EFFECTS ON THE MAGNETIC STRUCTURES OF THE RARE EARTH COMPOUNDS WITH THE FeB STRUCTURE N. Careirom and D. Gignoux Laboratoire Louis Ned, C.N.R.S., 166X, 38042 Grenoble—Cedex, France. (Received 19 December by E.F. Bertaut) In order to confirm the role of the crystalline, electric potential on the stability of non collinear magnetic structures of the rare earth compounds with the FeB—type structure, the magnetic properties of the (Gd0 5Y0 5)Ni compound, where the rare earth orbital moment is nul, are studied. Below its Curie temperature (57 K) the compound is ferromagnetic. The spontaneous magnetization at 0 K reaches 7.05 ~IB per gadolinium atom. Yttrium and nickel atoms being not magnetic the gadolinium moments are parallel and the exchange interactions are positive. Then the non collinear magnetic structures observed when the alloyed rare earths have an orbital moment result from the competition between a multiaxial anisotropy due to the crystal field effects and isotropic exchange interactions of the Heisenberg type.
Introduction
with R Dy, Ho, Er, Tm and Lu crystallise in the orthorhombic FeB—type structure (space group Pnma)’. TbNi crystallises into two different structures depending if the sample has The equiatomic rare earth compounds RNI 2. ture by replacing 10 % of the atoms by However been obtained one can by stabilise quenching or Tb FeB—type after annealing struc— smaller Y atoms. The FeB—type structure can be built from trigonal prisms where the corners are occupied by rare earth atoms and the center by a nickel one (figure 1). Nickel does not contribute to the magnetism of the compounds, because the nickel—nickel distances are too large. The magnetic ordering is essentially due to indir~t interactions between rare earth atoms, occuring through the conduction electrons. Although these compounds exhibit a large spontaneous magnetization at low temperatures, magnetic structures are non collinear. Rare earth atoms lying in a low symmetry site, the second order terms of the crystal potential are preponderant and rare earth atoms are divided into two sub— lattices with different easy magnetization directions. The magnetic structures result from the compromise between positive and isotropic exchange interactions (Heisenberg ty~e) and the strong magnetocrystalline anisotropy ,4. As an example, th~emagnetic structure of DyNi is
— —
:
-
_________
-s
0
S.
Fig. 1 Crystallographic structure of the RNi compounds which crystallise in the FeB—type structure. As an example the magnetic structureis ofpresented DyNi. existence of other exchange interactions than the Heisenberg ones. In order to confirm the role of the crystal field on the non collinear magnetic structures of the FeB—type rare earth compounds we have studied the magnetic properties of a Gd compound with the same crystallographic structure (Gd 0~5Y0•5)Ni. GdNi crystallises in the orthorhombic 1 but replacing CrB—type 50 crystallographic % of the Gd atoms struc— by smaller Y atoms stabilizes the FeB structure. ture
drawn on figure I. An estimation of the second model. The has directions and the in magnitudes of order term been obtained a point charge the R3+ ion magnetic moments thus deduced
interpret correctly the observed magnetic structures. However a recent neutron diffraction study of the cubic compound Gd~ghas shown that the magnetic structure of Gd moments is non collinear5. For an ion in a S state the crystal field effects cannot be invoked to interpret such a canted structure. This indicates the
Experimental and results We have prepared single crystals by slowly cooling down the nelted alloy. The single crystals obtained have been spark cut into 3 = diameter spheres. Magnetization measurements have been performed at the “Service National des Champs Intenses” of Grenoble in fields up
735
MAGNETIC STRUCTURES OF RARE EARTH C0MP~UND~
736
to 150 kOe and for temperatures between 1.5 and 300 K. At 4.2 K all the crystals studied exhibit a strong spontaneous magnetization and a weak superimposed susceptibility. Within the experi— mental accuracy no anisotropy has been observed ; however from one single crystal to the other a large discrepancy has been measured on the absolute value of the magnetization. This behaviour has been attributed to changes in the yttrium concentration from one sample to another. The anisotropy being negligible we have studied polycrystalline samples of (Gd0~5Y0~5)Ni obtained in a levitation furnace in order to have a well homogeneized sample with a well known composition. Above 80 K, the thermal variation of the reciprocal susceptibility is linear with a paramagnetic Curie temperature 9 70 K (figure 2) ; the effective moment per gadolinium atom (7.95 ± 0.1 MB) is very close to the free ion one (7.94 PB). At 1.5 K, the magnetization variation (figure 3) up to 3 kOe is linear with the applied field ; the corresponding slope is equal to the inverse of the demagnetizing field coefficient. Under 10 k0e the spontaneous magnetization is reached. At 1.5 K its value is e.m.u./mole) is weak compared 7.05 MB per Gd4 atom. The superimposed suscepti— to that (24x10 observed in (Th 3 bility e.m.u./mole) when the field 0.5Y0.5)Ni is applied (22x10 along the ferromagnetic component. The thermal vane— tion of the spontaneous magnetization (figure 2) is in agreement with that calculated for a Brillouin function B7/2, however, below 10 K the experimental values fits better with a T3/2 law characteristic of spin waves,
Vol. 25, No. 10
__
CD
8
a
~6
z
2 ~
Gd0~Y05Ni 1.5 K
4
0
1&~
50 APPLIED MAGNETIC
FIELD
150 (KOe)
Fig. 3 Magnetization versus applied field at 1.5 K in ~d05Y05Ni. be attributed only to the Gd atoms and an eventual small polarization of the conduction band. At 4.2 K this spontaneous magnetization reaches 7.05 MB, value which is very close to the free ion value. Hence in this compound the structure is ferromagnetic collinear and the contribution MB) is The due small to the polarization anisotropy is (0.05 very weak. positive extra of the conduction band by the interactions due to Gd atom. This polarization has the same origin in Gd as in Gd metal but it is smaller because 0 5Y0 5Ni interactions are five times smaller. As we have noticed previously the total orbital moment L being zero in Gd, crystal field
30 o S.’
_____
Gd0 5Y0.5Ni]
z8
E 20>I-
0
W6 z 0 45
aw U
“I
10 -J
4 U
z
0
z
0~
aIn
Ui
01
0
50
i6o
1~0 TEMPERATURE
260
250
300
(K.lvin)
Fig. 2 Thermal variation of the reciprocal susceptibility of Gd0,5Y05Ni and thermal variation of the spontaneous magnetization. Discussion In Gd0~5Y0,5Ni,yttrium and nickel being not magnetic, the spontaneous magnetization can
effects are negligible. Thus the collinear magnetic structure observed in Gd0 5Y05Ni confirms that, in the RNi compounds with the FeB—type crystallographic structure, the non
Vol. 25, No. 10
MAGNETIC STRUCTURES OF RARE EARTH COMPOUNDS
collinear result from a compromise magnetic between structures positive and isotropic exchange interactions (Heisenberg type) and a strong magnetocrystalline anisotropy (crystal field effects) which divides the rare earth atoms into two sublattices with different easy magnetization directions. Such a competition is responsible of the non collinear and also non coplanar magnetic structures observed in the compounds with low symmetry (Er3Co6, RA17’8,
9) because the second order terms of the Dy3.Al crystal 2 field potential are preponderant bilinear anisotropic exchanges and exchange terms of higher order are negligible compared to the crystal field effects. Acknowledgements tie wish to thank R. Lemaire for fruitful discussions. REFERENCES
I. 2. 3. 4. 5. 6. 7. 8. 9.
WALLINE, R.E. and WALLACE, W.E., J. Chem. Phys. 41, 1587 (1964). LEMAIRE, R. and PACCARD, D., J. Less Comm. Metals 21, 403 (1970). BARBARA, B., GIGNOUX, D., GIVORD, D., GIVORD, F. and LENAIRE, R., mt. J. Magnetism 4, 77 (1973). ~IGNOUX, D., PACCARD, D., ROSSAT—MIGNOD, J. and TCHEOU, F., 10th Rare Earth Res. Conf. II, 596 (1973). MORIN, P., PIERRE, J., SCHMITT, D. and GIVORD, D., submitted to Phys. Letters. GIGNOUX, D., LEMAIRE, R. and PACCARD, D., Sol. Stat. Commun. !. 391 (1970). BECLE, C., LEMAIRE, R. and PACCARD, D., J. Appi. Phys. 4!, 855 (1970). ASMAT, H., BARBARA, B. and GIGNOUX, D., J. Sol. Stat. Chem. 22, 179 (1977). BARBARA, B., BECLE, C., LEMAIRE, R. and PACCARD, D., J. Physique 32—Cl, 299 (1971).
737
CRYSTAL FIELD EFFECTS ON THE MAGNETIC STRUCTURES OF THE RARE EARTH COMPOUNDS WITH THE FeB STRUCTURE N. Careirom and D. Gignoux Laboratoire Louis Ned, C.N.R.S., 166X, 38042 Grenoble—Cedex, France. (Received 19 December by E.F. Bertaut) In order to confirm the role of the crystalline, electric potential on the stability of non collinear magnetic structures of the rare earth compounds with the FeB—type structure, the magnetic properties of the (Gd0 5Y0 5)Ni compound, where the rare earth orbital moment is nul, are studied. Below its Curie temperature (57 K) the compound is ferromagnetic. The spontaneous magnetization at 0 K reaches 7.05 ~IB per gadolinium atom. Yttrium and nickel atoms being not magnetic the gadolinium moments are parallel and the exchange interactions are positive. Then the non collinear magnetic structures observed when the alloyed rare earths have an orbital moment result from the competition between a multiaxial anisotropy due to the crystal field effects and isotropic exchange interactions of the Heisenberg type.
Introduction
with R Dy, Ho, Er, Tm and Lu crystallise in the orthorhombic FeB—type structure (space group Pnma)’. TbNi crystallises into two different structures depending if the sample has The equiatomic rare earth compounds RNI 2. ture by replacing 10 % of the atoms by However been obtained one can by stabilise quenching or Tb FeB—type after annealing struc— smaller Y atoms. The FeB—type structure can be built from trigonal prisms where the corners are occupied by rare earth atoms and the center by a nickel one (figure 1). Nickel does not contribute to the magnetism of the compounds, because the nickel—nickel distances are too large. The magnetic ordering is essentially due to indir~t interactions between rare earth atoms, occuring through the conduction electrons. Although these compounds exhibit a large spontaneous magnetization at low temperatures, magnetic structures are non collinear. Rare earth atoms lying in a low symmetry site, the second order terms of the crystal potential are preponderant and rare earth atoms are divided into two sub— lattices with different easy magnetization directions. The magnetic structures result from the compromise between positive and isotropic exchange interactions (Heisenberg ty~e) and the strong magnetocrystalline anisotropy ,4. As an example, th~emagnetic structure of DyNi is
— —
:
-
_________
-s
0
S.
Fig. 1 Crystallographic structure of the RNi compounds which crystallise in the FeB—type structure. As an example the magnetic structureis ofpresented DyNi. existence of other exchange interactions than the Heisenberg ones. In order to confirm the role of the crystal field on the non collinear magnetic structures of the FeB—type rare earth compounds we have studied the magnetic properties of a Gd compound with the same crystallographic structure (Gd 0~5Y0•5)Ni. GdNi crystallises in the orthorhombic 1 but replacing CrB—type 50 crystallographic % of the Gd atoms struc— by smaller Y atoms stabilizes the FeB structure. ture
drawn on figure I. An estimation of the second model. The has directions and the in magnitudes of order term been obtained a point charge the R3+ ion magnetic moments thus deduced
interpret correctly the observed magnetic structures. However a recent neutron diffraction study of the cubic compound Gd~ghas shown that the magnetic structure of Gd moments is non collinear5. For an ion in a S state the crystal field effects cannot be invoked to interpret such a canted structure. This indicates the
Experimental and results We have prepared single crystals by slowly cooling down the nelted alloy. The single crystals obtained have been spark cut into 3 = diameter spheres. Magnetization measurements have been performed at the “Service National des Champs Intenses” of Grenoble in fields up
735
MAGNETIC STRUCTURES OF RARE EARTH C0MP~UND~
736
to 150 kOe and for temperatures between 1.5 and 300 K. At 4.2 K all the crystals studied exhibit a strong spontaneous magnetization and a weak superimposed susceptibility. Within the experi— mental accuracy no anisotropy has been observed ; however from one single crystal to the other a large discrepancy has been measured on the absolute value of the magnetization. This behaviour has been attributed to changes in the yttrium concentration from one sample to another. The anisotropy being negligible we have studied polycrystalline samples of (Gd0~5Y0~5)Ni obtained in a levitation furnace in order to have a well homogeneized sample with a well known composition. Above 80 K, the thermal variation of the reciprocal susceptibility is linear with a paramagnetic Curie temperature 9 70 K (figure 2) ; the effective moment per gadolinium atom (7.95 ± 0.1 MB) is very close to the free ion one (7.94 PB). At 1.5 K, the magnetization variation (figure 3) up to 3 kOe is linear with the applied field ; the corresponding slope is equal to the inverse of the demagnetizing field coefficient. Under 10 k0e the spontaneous magnetization is reached. At 1.5 K its value is e.m.u./mole) is weak compared 7.05 MB per Gd4 atom. The superimposed suscepti— to that (24x10 observed in (Th 3 bility e.m.u./mole) when the field 0.5Y0.5)Ni is applied (22x10 along the ferromagnetic component. The thermal vane— tion of the spontaneous magnetization (figure 2) is in agreement with that calculated for a Brillouin function B7/2, however, below 10 K the experimental values fits better with a T3/2 law characteristic of spin waves,
Vol. 25, No. 10
__
CD
8
a
~6
z
2 ~
Gd0~Y05Ni 1.5 K
4
0
1&~
50 APPLIED MAGNETIC
FIELD
150 (KOe)
Fig. 3 Magnetization versus applied field at 1.5 K in ~d05Y05Ni. be attributed only to the Gd atoms and an eventual small polarization of the conduction band. At 4.2 K this spontaneous magnetization reaches 7.05 MB, value which is very close to the free ion value. Hence in this compound the structure is ferromagnetic collinear and the contribution MB) is The due small to the polarization anisotropy is (0.05 very weak. positive extra of the conduction band by the interactions due to Gd atom. This polarization has the same origin in Gd as in Gd metal but it is smaller because 0 5Y0 5Ni interactions are five times smaller. As we have noticed previously the total orbital moment L being zero in Gd, crystal field
30 o S.’
_____
Gd0 5Y0.5Ni]
z8
E 20>I-
0
W6 z 0 45
aw U
“I
10 -J
4 U
z
0
z
0~
aIn
Ui
01
0
50
i6o
1~0 TEMPERATURE
260
250
300
(K.lvin)
Fig. 2 Thermal variation of the reciprocal susceptibility of Gd0,5Y05Ni and thermal variation of the spontaneous magnetization. Discussion In Gd0~5Y0,5Ni,yttrium and nickel being not magnetic, the spontaneous magnetization can
effects are negligible. Thus the collinear magnetic structure observed in Gd0 5Y05Ni confirms that, in the RNi compounds with the FeB—type crystallographic structure, the non
Vol. 25, No. 10
MAGNETIC STRUCTURES OF RARE EARTH COMPOUNDS
collinear result from a compromise magnetic between structures positive and isotropic exchange interactions (Heisenberg type) and a strong magnetocrystalline anisotropy (crystal field effects) which divides the rare earth atoms into two sublattices with different easy magnetization directions. Such a competition is responsible of the non collinear and also non coplanar magnetic structures observed in the compounds with low symmetry (Er3Co6, RA17’8,
9) because the second order terms of the Dy3.Al crystal 2 field potential are preponderant bilinear anisotropic exchanges and exchange terms of higher order are negligible compared to the crystal field effects. Acknowledgements tie wish to thank R. Lemaire for fruitful discussions. REFERENCES
I. 2. 3. 4. 5. 6. 7. 8. 9.
WALLINE, R.E. and WALLACE, W.E., J. Chem. Phys. 41, 1587 (1964). LEMAIRE, R. and PACCARD, D., J. Less Comm. Metals 21, 403 (1970). BARBARA, B., GIGNOUX, D., GIVORD, D., GIVORD, F. and LENAIRE, R., mt. J. Magnetism 4, 77 (1973). ~IGNOUX, D., PACCARD, D., ROSSAT—MIGNOD, J. and TCHEOU, F., 10th Rare Earth Res. Conf. II, 596 (1973). MORIN, P., PIERRE, J., SCHMITT, D. and GIVORD, D., submitted to Phys. Letters. GIGNOUX, D., LEMAIRE, R. and PACCARD, D., Sol. Stat. Commun. !. 391 (1970). BECLE, C., LEMAIRE, R. and PACCARD, D., J. Appi. Phys. 4!, 855 (1970). ASMAT, H., BARBARA, B. and GIGNOUX, D., J. Sol. Stat. Chem. 22, 179 (1977). BARBARA, B., BECLE, C., LEMAIRE, R. and PACCARD, D., J. Physique 32—Cl, 299 (1971).
737