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Electro-nuclear effects in the low field magnetic susceptibility of Er impurities in CePd3
E L E C T R O - N U C L E A R E F F E C T S IN T H E L O W F I E L D M A G N E T I C S U S C E P T I B I L I T Y OF Er I M P U R I T I E S IN CePd~ A. C. MOTA, D. W O H L L E B E N , R.F. H O Y T and H. B O R C H A R D T I1. Phys. Inst. Univ. zu Ki~ln, 5000 Klein 41, Fed. Rep. Germany
The susceptibility of CePd 3 with < 2000 ppm 166Er shows Curie behavior from 3 K to 10 m K (0 = 0 + 0.5 mK). By replacing Z66Er ( I = 0) with natural Er, X falls below the Curie law of 166Er CePd 3. This is interpreted as the freezing of the nuclear to the electronic moment at thermal energies below the liyperfine splitting within 167Er(4A = 140 inK).
The REPd 3 system is particularly well suited for the study of single ion effects because of its very low magnetic interaction temperatures. For instance, for G d impurities in YPd 3 and CePd 3 one observes 7 m K per 1000 p p m and 2.5 m K per 1000 p p m respectively [1]. Due to the large crystal field splitting of most RE ions, one can expect a pure Curie law below 1 K for Kramers ions. The susceptibility of such ions in low interaction systems could be used for thermometry in the miUikelvin range. We have measured the low field (50 m G ) ac magnetic susceptibility of Er impurities up to 2000 p p m in CePd 3 between 4 K and 7 mK. Fig. 1 shows the susceptibility per impurity versus 1 / T 4.5
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• CePd 3 (2000ppm Er)
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for 2000 p p m of natural Er and for 1000 p p m and 2000 p p m of the isotope 166Er. For natural Er one observes a Curie law between 3 K and about 200 m K and a strong reduction with respect to this behavior below 200 mK. For the two isotopic samples the Curie law is observed over the entire temperature range, with a small deviation below 20 inK, independent of concentration. In fig. 2 the inverse susceptibility of the 166Er samples is plotted as function of temperature below 70 mK. The Curie-Weiss temperature is 0 _ 0.5 mK. The low temperature anomaly in the susceptibility of natural Er is due to hyperfine effects, similar to the one observed previously in dilute AuYb alloys [2]. Natural Er contains 22.9% of 167Er, which is the only natural isotope with I ~ 0 ( I = 7). In our temperature range the nuclear magnetic moment of this isotope couples noticeably to the electronic magnetic m o m e n t of the cubic crystal field ground state. An estimate of the coupling energy can be obtained from the work of Sj6rstrand and Seidel [3] on 167Er impurities in Au. F r o m their microwave EPR data in the millikelvin region which show a resolved hyperfine spectrum,
2,0
125 1.5 1.00 1.0
I
c D
>.0.75
0.5
~050 0
I
I
I
I
I
10
20
30
40
50
I
coo
I
70 lIT [K 1 ]
I
80
V
x
0.25
Fig. 1. Normalized susceptibility of natural Er and 166Er impurities in CePd 3 [4]. +Work supported by Deutsche Forschungsgemeinschaft SFB 125.
t.-
i
i
L
L
~
10
20
30
40
50
i
60 T[mK]
Fig. 2. Reciprocal low field susceptibility of 1000 p p m lSSEr in CePd 3.
Journal of Magnetism and Magnetic Materials 15-18 (1980) 95-96 ©North Holland
95
96
A. C. Mota et al./ Electro-nuclear effects o f Er in CePd 3
they derive a h y p e r f i n e c o u p l i n g c o n s t a n t ]A I = 35 inK. A s s u m i n g a d o u b l e t crystal field g r o u n d state for Er in CePd3, as in the case of Er in Au, the g r o u n d state of the c o u p l e d e l e c t r o n u c l e a r system is then F = 3 or F = 4 d e p e n d i n g on the sign of A. Both states h a v e a Z e e m a n m u l t i p l e t a n d will therefore show a Curie law at sufficiently low temperatures, h o w e v e r with a C u r i e c o n s t a n t which is r e d u c e d with respect to that of the d o u b l e t crystal field state. The zero field h y p e r f i n e splitting between the F = 3 a n d F = 4 multiplets is 4A a c c o r d i n g to the B r e i t - R a b i f o r m u l a , i.e. 140 m K for Er in A u [3] a n d p r o b a b l y very close to that value in o u r case. I n d e e d we o b s e r v e a c h a n g e over f r o m one Curie c o n s t a n t to a n o t h e r one with a r e d u c e d value in the e x p e c t e d t e m p e r a t u r e r a n g e for n a t u r a l erbium. H o w e v e r , a p u r e Z e e m a n split-
ting of F = 3 or F = 4 does n o t a c c o u n t for the observed low-temperature deviation. A s o n e c a n see in fig. 2, the s y s t e m 166ErxCe~_xPd3 is suitable as a t h e r m o m e t e r in the millikelvin range. F o r x = 2000 p p m the Curie c o n s t a n t p e r unit v o l u m e is o n l y a b o u t three times s m a l l e r t h a n that of c e r i u m m a g n e s i u m nitrate (CMN).
References [11 A. C. Mota, R. F. Hoyt and D. K. Wohlleben, J. de Phys. 39 (1978) C6-884. 12] G. Frossati, J. M. Mignot, D. Thoulouze and R. Tournier, Phys. Rev. Lett. 36 (1976) 203. I3] M. E. Sj~strand and G. Seidel, Phys. Rev. Bll (1975) 3292. [4] The samples with 16~Er still contain 3.3% of 167Er.
The susceptibility of CePd 3 with < 2000 ppm 166Er shows Curie behavior from 3 K to 10 m K (0 = 0 + 0.5 mK). By replacing Z66Er ( I = 0) with natural Er, X falls below the Curie law of 166Er CePd 3. This is interpreted as the freezing of the nuclear to the electronic moment at thermal energies below the liyperfine splitting within 167Er(4A = 140 inK).
The REPd 3 system is particularly well suited for the study of single ion effects because of its very low magnetic interaction temperatures. For instance, for G d impurities in YPd 3 and CePd 3 one observes 7 m K per 1000 p p m and 2.5 m K per 1000 p p m respectively [1]. Due to the large crystal field splitting of most RE ions, one can expect a pure Curie law below 1 K for Kramers ions. The susceptibility of such ions in low interaction systems could be used for thermometry in the miUikelvin range. We have measured the low field (50 m G ) ac magnetic susceptibility of Er impurities up to 2000 p p m in CePd 3 between 4 K and 7 mK. Fig. 1 shows the susceptibility per impurity versus 1 / T 4.5
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,
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,
,
,
• CePd 3 (2000ppm Er)
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°~/~'~°
• CePd 3 (2000ppm ~Er) o CePd 3 (1000ppm leSEr) . / ~ /
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for 2000 p p m of natural Er and for 1000 p p m and 2000 p p m of the isotope 166Er. For natural Er one observes a Curie law between 3 K and about 200 m K and a strong reduction with respect to this behavior below 200 mK. For the two isotopic samples the Curie law is observed over the entire temperature range, with a small deviation below 20 inK, independent of concentration. In fig. 2 the inverse susceptibility of the 166Er samples is plotted as function of temperature below 70 mK. The Curie-Weiss temperature is 0 _ 0.5 mK. The low temperature anomaly in the susceptibility of natural Er is due to hyperfine effects, similar to the one observed previously in dilute AuYb alloys [2]. Natural Er contains 22.9% of 167Er, which is the only natural isotope with I ~ 0 ( I = 7). In our temperature range the nuclear magnetic moment of this isotope couples noticeably to the electronic magnetic m o m e n t of the cubic crystal field ground state. An estimate of the coupling energy can be obtained from the work of Sj6rstrand and Seidel [3] on 167Er impurities in Au. F r o m their microwave EPR data in the millikelvin region which show a resolved hyperfine spectrum,
2,0
125 1.5 1.00 1.0
I
c D
>.0.75
0.5
~050 0
I
I
I
I
I
10
20
30
40
50
I
coo
I
70 lIT [K 1 ]
I
80
V
x
0.25
Fig. 1. Normalized susceptibility of natural Er and 166Er impurities in CePd 3 [4]. +Work supported by Deutsche Forschungsgemeinschaft SFB 125.
t.-
i
i
L
L
~
10
20
30
40
50
i
60 T[mK]
Fig. 2. Reciprocal low field susceptibility of 1000 p p m lSSEr in CePd 3.
Journal of Magnetism and Magnetic Materials 15-18 (1980) 95-96 ©North Holland
95
96
A. C. Mota et al./ Electro-nuclear effects o f Er in CePd 3
they derive a h y p e r f i n e c o u p l i n g c o n s t a n t ]A I = 35 inK. A s s u m i n g a d o u b l e t crystal field g r o u n d state for Er in CePd3, as in the case of Er in Au, the g r o u n d state of the c o u p l e d e l e c t r o n u c l e a r system is then F = 3 or F = 4 d e p e n d i n g on the sign of A. Both states h a v e a Z e e m a n m u l t i p l e t a n d will therefore show a Curie law at sufficiently low temperatures, h o w e v e r with a C u r i e c o n s t a n t which is r e d u c e d with respect to that of the d o u b l e t crystal field state. The zero field h y p e r f i n e splitting between the F = 3 a n d F = 4 multiplets is 4A a c c o r d i n g to the B r e i t - R a b i f o r m u l a , i.e. 140 m K for Er in A u [3] a n d p r o b a b l y very close to that value in o u r case. I n d e e d we o b s e r v e a c h a n g e over f r o m one Curie c o n s t a n t to a n o t h e r one with a r e d u c e d value in the e x p e c t e d t e m p e r a t u r e r a n g e for n a t u r a l erbium. H o w e v e r , a p u r e Z e e m a n split-
ting of F = 3 or F = 4 does n o t a c c o u n t for the observed low-temperature deviation. A s o n e c a n see in fig. 2, the s y s t e m 166ErxCe~_xPd3 is suitable as a t h e r m o m e t e r in the millikelvin range. F o r x = 2000 p p m the Curie c o n s t a n t p e r unit v o l u m e is o n l y a b o u t three times s m a l l e r t h a n that of c e r i u m m a g n e s i u m nitrate (CMN).
References [11 A. C. Mota, R. F. Hoyt and D. K. Wohlleben, J. de Phys. 39 (1978) C6-884. 12] G. Frossati, J. M. Mignot, D. Thoulouze and R. Tournier, Phys. Rev. Lett. 36 (1976) 203. I3] M. E. Sj~strand and G. Seidel, Phys. Rev. Bll (1975) 3292. [4] The samples with 16~Er still contain 3.3% of 167Er.