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Magnetic interaction in GdN and GdN1−xOx
MAGNETIC INTERACTION IN G d N AND G d N I _xOx P. W A C H T E R and E. K A L D I S Laboratorium fiJr Festkorperphysik, ETHZ, 8093 Zi~rich, Switzerland
Large single crystals of GdNt_xO x, O < x < 0.25, have been prepared and the magnetic, electrical transport and optical properties have been investigated. It is shown that GdN is a rnetamagnet. Oxygen substitution leads to the incorporation of additional electrons, resulting for low concentrations in nearly ferromagnetic behavior and for large concentrations again in antiferromagnetism.
1. Introduction Since about 1970 [1-6] a controversy exists as to the magnetic properties of GdN, claimed to be a ferromagnet [1-3] and an antiferromagnet [4-6] in low fields. In ref. [4], various preparation methods of polycrystalline material seemed to have given evidence that G d N could exist as ferro- or antiferromagnet. In the mean time the derivation of the electronic structure [7, 8] with a magnetic ground state SS7 lying about 7 eV below the Fermi level made it highly improbable that G d N could be a ferromagnet due to c a t i o n - c a t i o n s u p e r e x c h a n g e mechanisms [8], thus the concentration of free carriers with an R K K Y interaction will play a decisive role in determining the magnetic behavior of GdN. A similar result has been obtained for GdP [9]. A simple way to vary the carrier concentration is alloying with oxygen since it is expected that each oxygen ion replacing nitrogen will result in an additional carrier. For 4 compositions GdN~ _xOx, 0 < x < 0.25, the initial susceptibility, the magnetization in fields up to 155 kOe, the carrier concentration and the plasma resonance has been determined (table 1).
2. Experimental results and discussion Single crystal of G d N I_xO~ have been prepared by mixing appropriate amounts of Gd203 to prereacted stoichiometric G d N in tungsten crucibles and heating them in a nitrogen atmosphere in an rf furnace to 2200 ° C. x has been determined from the admixed mole percentage of Gd203. According to Lorenzelli et al. [10] the solubility limit of GdO in G d N depends on the preparation temperature, but can be up to GdNo.6500.35.
The magnetization versus magnetic field up to 150 kOe is shown in fig. 1 for GdNm_xOx. At 4.2 K the saturation moment of 7/~B/Gd ion is achieved in all cases, however, the shape of the curves indicates drastic changes in the magnetic order. The single crystals were oriented with their [100] axes parallel with the field direction. It is evident that G d N is a metamagnet with a critical field for ferromagnetism of about 3.5 kOe. The Nrel temperature could be determined from the point of inflexion of the initial susceptibility, depicted in the insert of fig. 2, which is similar for the [100], [110] and [111] axes. (Data taken on identical crystals are collected in table 1.) Moderate fields of less than 20 kOe suffice to saturate this material. The determination of the free carrier plasma resonance and independently the measurement of the Hall effect both gave carrier concentrations of n/Gd = 0.06. This number is by orders of magnitude less than the predicted value of n/Gd = 1.4 by Darby and Taylor [11]. A magnetization curve as shown in fig. 1 for GdN0.9600.04 is similar to the one claimed to represent ferromagnetic G d N [1-3]. The magnetization reaches 7 # B / G d only at fields near 50 kOe and seems to represent rather a complex kind of spin arrangement, possibly canted antiferromagnetism, with a ferromagnetic moment being present already at fields, > 0.2 kOe as is shown in fig. 2. The demagnetizating field keeps the magnetization constant below the ordering temperature. The initial susceptibility is an odd shaped curve and is depicted also in the insert of fig. 2. It indicates, however, that also this sample basically is a metamagnet with 10 < H c < 190 Oe. For larger oxygen concentrations the curves represent again antiferromagnets, their initial susceptibility being similar to pure GdN, but with different Nrel temperatures (table 1).
Journal of Magnetism and Magnetic Materials 15-18 (1980) 305-306 ©North Holland
305
3O6
P. Wachter, E. Kaldis/ Magnetic interaction in GdN and (3dN 1_ xOx TABLE 1 Some physical parameters of the GdN I _~Ox systems
GdNo.96Oo.o4
GdN TN (K) 0 (K)
40 79.14 9.05 7.26 0.06
~EFF(~B)
Ms(#a) n/Gd
i OdN j GdNOoeO004 ~
GdN° 86 j
-
.
40 74.15 9.28 7. l I 0.073
--
--
!
I
-
i i
0
i 100
5()
150
Mogneflc Field (kOe)
Fig. 1. Magnetization versus applied field for GdNl_:,O x at 4.2 K. Field direction parallel to [100I.
8
GdNo.75Oo,25 23 31.4 8.70 7.1 (155 kOe) 0.22
all three A F structures. I n a b s e n c e of useful n e u t r o n s c a t t e r i n g d a t a [3], we r e f r a i n so far f r o m a m o l e c u l a r field analysis. T h e n u m b e r of carriers g i v e n in t a b l e 1 r o u g h l y scales with the c h e m i c a l f o r m u l a , c o n s i d e r i n g t h a t 6% carriers are a l r e a d y p r e s e n t in G d N . T h e c o m plex m a g n e t i c b e h a v i o r of G d N l _ x O x m u s t b e u n d e r s t o o d o n the basis of a n t i f e r r o m a g n e t i c sup e r e x c h a n g e c o m p e t i n g with a n o s c i l l a t o r y R K K Y i n t e r a c t i o n , t e n d i n g to b e f e r r o m a g n e t i c for n / G d between 0.07-0.1.
~
I
GdNo.s6Ooa 4 25 25.37 8.76 6.3 (94 kOe) 0.195
T h e a u t h o r s are g r a t e f u l to H. B o p p a r t for m e a s u r i n g the H a l l effect a n d to J. Miiller for m e a s u r i n g the reflectivity a n d the m a g n e t i z a t i o n . T h e a u t h o r s are g r a t e f u l to Dr. O. V o g t for p e r m i t ting the use of the high field e q u i p m e n t .
!
References
GdNo[60~o ....
,"
10o~
" . ~
~'~ ........- 5~0~i ............... "~U-'""~ 5o '~ .... - ~ ~,~ Temperature ( K ) 2
0
'
i
T
25
50
75
/
~00 Temperolure ( K )
Fig. 2. Magnetization versus temperature for GdNe.96Oo.o4. The insert shows the initial susceptibility of GdN and GdN0.96Oo.o4.
S i m p l e m o l e c u l a r field a n a l y s i s has b e e n a p p l i e d to the G d N ~ _ x O x system, b u t the c o n s i s t e n c y c h e c k w i t h S m a r t ' s p h a s e d i a g r a m was n e g a t i v e for
[1] P. Junod, A. Menth and O. Vogt, Phys. Kondens. Materie 8 (1969) 323. [2] T. R. McGuire, R. J. Gambino, S. J. Pickart and H. A. Alperin, J. Appl. Phys. 40 (1969) 1009. [3] R. J. Gambino, T. R. McGuire, H. A. Alperin and S. J. Pickart, J. Appl. Phys. 41 (1970) 933. [4] G. Busch, E. Kaldis, E. Schaufelberger-Teker and P. Wachter, Colloque Intern. du CNRS 180 (1970) 359. [5] R. A. Cutler and A. W. Lawson, J. Appl. Phys. 46 (1975) 2739. [6] Ch. Ziircher, Thesis ETH 0978) unpublished. [7] E. Kaldis, A. Schlegel, P. Wachter and Ch. Z/Jrcher, J. Magn. Magn. 3 (1976) 1. [8l P. Wachter, Phys. Rep. 44 (1978) 160. [9] P. Wachter, E. Kaldis and R. Hauger, Phys. Rev. Lett. 40 (1978) 1404. [10] N. Lorenzelli, J. Melamed, J. P. Marcon, Colloque Intern. du CNRS 180 (1970) 375. [ll] M. I. Darby and K. N. R. Taylor, Phys. Lett. 14 (1965) 179.
Large single crystals of GdNt_xO x, O < x < 0.25, have been prepared and the magnetic, electrical transport and optical properties have been investigated. It is shown that GdN is a rnetamagnet. Oxygen substitution leads to the incorporation of additional electrons, resulting for low concentrations in nearly ferromagnetic behavior and for large concentrations again in antiferromagnetism.
1. Introduction Since about 1970 [1-6] a controversy exists as to the magnetic properties of GdN, claimed to be a ferromagnet [1-3] and an antiferromagnet [4-6] in low fields. In ref. [4], various preparation methods of polycrystalline material seemed to have given evidence that G d N could exist as ferro- or antiferromagnet. In the mean time the derivation of the electronic structure [7, 8] with a magnetic ground state SS7 lying about 7 eV below the Fermi level made it highly improbable that G d N could be a ferromagnet due to c a t i o n - c a t i o n s u p e r e x c h a n g e mechanisms [8], thus the concentration of free carriers with an R K K Y interaction will play a decisive role in determining the magnetic behavior of GdN. A similar result has been obtained for GdP [9]. A simple way to vary the carrier concentration is alloying with oxygen since it is expected that each oxygen ion replacing nitrogen will result in an additional carrier. For 4 compositions GdN~ _xOx, 0 < x < 0.25, the initial susceptibility, the magnetization in fields up to 155 kOe, the carrier concentration and the plasma resonance has been determined (table 1).
2. Experimental results and discussion Single crystal of G d N I_xO~ have been prepared by mixing appropriate amounts of Gd203 to prereacted stoichiometric G d N in tungsten crucibles and heating them in a nitrogen atmosphere in an rf furnace to 2200 ° C. x has been determined from the admixed mole percentage of Gd203. According to Lorenzelli et al. [10] the solubility limit of GdO in G d N depends on the preparation temperature, but can be up to GdNo.6500.35.
The magnetization versus magnetic field up to 150 kOe is shown in fig. 1 for GdNm_xOx. At 4.2 K the saturation moment of 7/~B/Gd ion is achieved in all cases, however, the shape of the curves indicates drastic changes in the magnetic order. The single crystals were oriented with their [100] axes parallel with the field direction. It is evident that G d N is a metamagnet with a critical field for ferromagnetism of about 3.5 kOe. The Nrel temperature could be determined from the point of inflexion of the initial susceptibility, depicted in the insert of fig. 2, which is similar for the [100], [110] and [111] axes. (Data taken on identical crystals are collected in table 1.) Moderate fields of less than 20 kOe suffice to saturate this material. The determination of the free carrier plasma resonance and independently the measurement of the Hall effect both gave carrier concentrations of n/Gd = 0.06. This number is by orders of magnitude less than the predicted value of n/Gd = 1.4 by Darby and Taylor [11]. A magnetization curve as shown in fig. 1 for GdN0.9600.04 is similar to the one claimed to represent ferromagnetic G d N [1-3]. The magnetization reaches 7 # B / G d only at fields near 50 kOe and seems to represent rather a complex kind of spin arrangement, possibly canted antiferromagnetism, with a ferromagnetic moment being present already at fields, > 0.2 kOe as is shown in fig. 2. The demagnetizating field keeps the magnetization constant below the ordering temperature. The initial susceptibility is an odd shaped curve and is depicted also in the insert of fig. 2. It indicates, however, that also this sample basically is a metamagnet with 10 < H c < 190 Oe. For larger oxygen concentrations the curves represent again antiferromagnets, their initial susceptibility being similar to pure GdN, but with different Nrel temperatures (table 1).
Journal of Magnetism and Magnetic Materials 15-18 (1980) 305-306 ©North Holland
305
3O6
P. Wachter, E. Kaldis/ Magnetic interaction in GdN and (3dN 1_ xOx TABLE 1 Some physical parameters of the GdN I _~Ox systems
GdNo.96Oo.o4
GdN TN (K) 0 (K)
40 79.14 9.05 7.26 0.06
~EFF(~B)
Ms(#a) n/Gd
i OdN j GdNOoeO004 ~
GdN° 86 j
-
.
40 74.15 9.28 7. l I 0.073
--
--
!
I
-
i i
0
i 100
5()
150
Mogneflc Field (kOe)
Fig. 1. Magnetization versus applied field for GdNl_:,O x at 4.2 K. Field direction parallel to [100I.
8
GdNo.75Oo,25 23 31.4 8.70 7.1 (155 kOe) 0.22
all three A F structures. I n a b s e n c e of useful n e u t r o n s c a t t e r i n g d a t a [3], we r e f r a i n so far f r o m a m o l e c u l a r field analysis. T h e n u m b e r of carriers g i v e n in t a b l e 1 r o u g h l y scales with the c h e m i c a l f o r m u l a , c o n s i d e r i n g t h a t 6% carriers are a l r e a d y p r e s e n t in G d N . T h e c o m plex m a g n e t i c b e h a v i o r of G d N l _ x O x m u s t b e u n d e r s t o o d o n the basis of a n t i f e r r o m a g n e t i c sup e r e x c h a n g e c o m p e t i n g with a n o s c i l l a t o r y R K K Y i n t e r a c t i o n , t e n d i n g to b e f e r r o m a g n e t i c for n / G d between 0.07-0.1.
~
I
GdNo.s6Ooa 4 25 25.37 8.76 6.3 (94 kOe) 0.195
T h e a u t h o r s are g r a t e f u l to H. B o p p a r t for m e a s u r i n g the H a l l effect a n d to J. Miiller for m e a s u r i n g the reflectivity a n d the m a g n e t i z a t i o n . T h e a u t h o r s are g r a t e f u l to Dr. O. V o g t for p e r m i t ting the use of the high field e q u i p m e n t .
!
References
GdNo[60~o ....
,"
10o~
" . ~
~'~ ........- 5~0~i ............... "~U-'""~ 5o '~ .... - ~ ~,~ Temperature ( K ) 2
0
'
i
T
25
50
75
/
~00 Temperolure ( K )
Fig. 2. Magnetization versus temperature for GdNe.96Oo.o4. The insert shows the initial susceptibility of GdN and GdN0.96Oo.o4.
S i m p l e m o l e c u l a r field a n a l y s i s has b e e n a p p l i e d to the G d N ~ _ x O x system, b u t the c o n s i s t e n c y c h e c k w i t h S m a r t ' s p h a s e d i a g r a m was n e g a t i v e for
[1] P. Junod, A. Menth and O. Vogt, Phys. Kondens. Materie 8 (1969) 323. [2] T. R. McGuire, R. J. Gambino, S. J. Pickart and H. A. Alperin, J. Appl. Phys. 40 (1969) 1009. [3] R. J. Gambino, T. R. McGuire, H. A. Alperin and S. J. Pickart, J. Appl. Phys. 41 (1970) 933. [4] G. Busch, E. Kaldis, E. Schaufelberger-Teker and P. Wachter, Colloque Intern. du CNRS 180 (1970) 359. [5] R. A. Cutler and A. W. Lawson, J. Appl. Phys. 46 (1975) 2739. [6] Ch. Ziircher, Thesis ETH 0978) unpublished. [7] E. Kaldis, A. Schlegel, P. Wachter and Ch. Z/Jrcher, J. Magn. Magn. 3 (1976) 1. [8l P. Wachter, Phys. Rep. 44 (1978) 160. [9] P. Wachter, E. Kaldis and R. Hauger, Phys. Rev. Lett. 40 (1978) 1404. [10] N. Lorenzelli, J. Melamed, J. P. Marcon, Colloque Intern. du CNRS 180 (1970) 375. [ll] M. I. Darby and K. N. R. Taylor, Phys. Lett. 14 (1965) 179.