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Magnetic anomalies in ErFe3
71 MAGNETIC ANOMALIES IN ErFe3 R.K. DAY National Measurement Laboratory, CSIRO, Sydney, Australia
and G.J. B O W D E N School of Physics, University o[ New South Wales, Kensington, Australia
I. Introduction The rare earth compound ErFe3 crystallizes in the hexagonal PuNi3-type structure which belongs to the R3m space group. Thus there are two crystallographically inequivalent Er atoms a and c in the ratio of 3:6, and three inequivalent Fe atoms b, c and h in the ratio of 3:6: 18, respectively. In general, it is believed that this compound orders in a simple anti-ferromagnetic fashion with a N6el temperature of 555 K [1]. In this paper we present both M6ssbauer and a.c. susceptibility evidence for two magnetic anomalies in ErFe3 at 47 and 131 K, respectively. Thie data is used, in conjunction with point charge crystal field calculations, to suggest that (i) the anomaly at 47 K is characterized by small displacements of the c and h Fe atoms within the PuNi3 type structure, and (ii) that these movements result from a competition between the two Er sites in determining the direction of preferred magnetization. In addition, it is also suggested that the " a n o m a l y " at 131 K is probably due to a spin reorientation within the b - c plane. 2. Experimental results The low field a.c. susceptibility X of ErFe3 between 4.2 and 300 k reveals two anomalies at 47 and 131 K. Moreover the anomaly at 131 K is very similar in character to that in HoFe3 at 97 K, which is known to be due to a spin reorientation of 30° in the basal plane [2]. We are led to suggest therefore that the anomaly in ErFe3 at 131 K may also be due to a spin-reorientation of some kind. Rather surprisingly, however, the anomaly in ErFe3 at 47 K is only just observable in the a.c. susceptibility data, in sharp contrast to the M6ssbauer results which reveal a marked change in the character of the 57Fe spectrum. Subsequent analysis of the latter data, using the methods of Physica 86-88B (1977) 71-72 © North-Holland
Arif et al. [3], reveal (i) an abrupt spin reorientation of approximately 15° which leaves the Fe moments directed along the c-axis of the crystal, and (ii) a sudden increase in the hyperfine field at the c Fe sites of approximately 4.4 T (compare both Van der Kraan et al. [4] and Arif et al. [3]). Rather surprisingly, however, very little change is found in the spectral parameters of the h sites on passing through the transition. 3. Interpretation of the M6ssbauer data In has already been remarked by Arif et al. [3] that an increase of 4 . 4 T in the magnetic hyperfine field at the c sites cannot be explained in terms of the local dipolar fields within the ErFe3 lattice. We are forced therefore to look for an alternative explanation and in this connection we note that significant variations in the strength of the magnetic hyperfine fields can be expected if the distances between neighbouring iron atoms are changed. This suggests therefore that the c atoms may have shifted their positions within the PuNi 3 structure and one such movement, which does not conflict with symmetry requirements, is shown in fig. 1. Moreover, if this model is correct then we may also expect small concomitant movements of the h atoms, as they relax in the face of advancing c-atoms. Both the displacements shown in fig. 1 are allowed within the PuNi3 structure. 4. Point charge crtstal field calculations First of all we note the proposed rearrangements of c and h iron atoms does not occur in the isostructural compounds YFe 3 and TbFe3, which reveal no such anomalies [3]. It would appear, therefore, that the effect is controlled primarily by the particular individual crystal field anisotropies at the two Er sites. Clearly atomic displacements of the kind described above will
72
ID RE
Fe b
[]
c
Fe h
Fe h
@ RE
Fe c
Fe C
o
Fe h
Fe h
@
T c
(2)-* Fe b
b
[]
REc
t)
Fig. 1. Vertical cross section in a b - c plane showing movement of the c and h atoms. give rise to c h a n g e s in the e l e c t r i c field g r a d i e n t s within the lattice, a n d the p o s s i b i l i t y e x i s t s of a c o m p e t i t i o n b e t w e e n the t w o E r sites in the d e t e r m i n a t i o n o f n o t o n l y the p r e f e r r e d d i r e c t i o n of m a g n e t i z a t i o n b u t also the p o s i t i o n s o f the c a n d h F e a t o m s . T o c h e c k this s u g g e s t i o n w e h a v e c o m p u t e d the c r y s t a l field coefficients A ~,,, d u e to b o t h the E r a n d F e s u b l a t t i c e s , as a f u n c t i o n of p o s i t i o n of the c a n d h F e a t o m s within the PuNi3 s t r u c t u r e . T h e s e r e s u l t s r e v e a l
t h a t c e r t a i n o f the A20 a n d A~3 c r y s t a l field coefficients g e n e r a t e d b y ~ the F e s u b l a t t i c e , a r e s t r o n g l y d e p e n d e n t on the p r e c i s e p o s i t i o n s o f the c a n d h a t o m s . In c o n c l u s i o n , we a r e led to s u g g e s t : (i) F o r T > 131 K the c(6) E r a t o m s are primarily responsible for a preferred direction of m a g n e t i z a t i o n c l o s e to the b a s a l p l a n e . (ii) A t T - 131 K a s p i n - r e o r i e n t a t i o n o c c u r s within the b - c p l a n e w h i c h m o v e s the F e mom e n t s c l o s e r to the c - a x i s . (iii) A t T = 47 K a f u r t h e r s p i n - r e o r i e n t a t i o n t a k e s p l a c e w h i c h l e a v e s the F e m o m e n t s dir e c t e d a l o n g the c - a x i s . (iv) B e l o w 47 K the a (3) E r a t o m s are r e s p o n sible f o r the c - a x i s d i r e c t i o n of m a g n e t i z a t i o n . (v) T h e d i s p l a c e m e n t of c a n d h F e a t o m s b e l o w 47 K p r o d u c e s a m o r e f a v o u r a b l e c r y s t a l field i n t e r a c t i o n at the c(6) R E sites.
References [1] K.H.J. Buschow, Phys. Stat. Solidi (a) 7 (1971) 199. [2] S.K. Arif, G.J. Bowden and D. St P. Bunbury, Proceedings 18th Ampere Congress (1974) 85-6 (North-Holland, Amsterdam). [31 S.K. Arif, D. St P. Bunbury, G.J. Bowden and R.K. Day, J. Phys. F. Metal Phys. 5 (1975) 1785. [4] A.M. van der Kraan, P.C.M. Gubbens and K.H.J. Buschow, Phys. Stat. Solidi (a) 31 (1975) 495.
and G.J. B O W D E N School of Physics, University o[ New South Wales, Kensington, Australia
I. Introduction The rare earth compound ErFe3 crystallizes in the hexagonal PuNi3-type structure which belongs to the R3m space group. Thus there are two crystallographically inequivalent Er atoms a and c in the ratio of 3:6, and three inequivalent Fe atoms b, c and h in the ratio of 3:6: 18, respectively. In general, it is believed that this compound orders in a simple anti-ferromagnetic fashion with a N6el temperature of 555 K [1]. In this paper we present both M6ssbauer and a.c. susceptibility evidence for two magnetic anomalies in ErFe3 at 47 and 131 K, respectively. Thie data is used, in conjunction with point charge crystal field calculations, to suggest that (i) the anomaly at 47 K is characterized by small displacements of the c and h Fe atoms within the PuNi3 type structure, and (ii) that these movements result from a competition between the two Er sites in determining the direction of preferred magnetization. In addition, it is also suggested that the " a n o m a l y " at 131 K is probably due to a spin reorientation within the b - c plane. 2. Experimental results The low field a.c. susceptibility X of ErFe3 between 4.2 and 300 k reveals two anomalies at 47 and 131 K. Moreover the anomaly at 131 K is very similar in character to that in HoFe3 at 97 K, which is known to be due to a spin reorientation of 30° in the basal plane [2]. We are led to suggest therefore that the anomaly in ErFe3 at 131 K may also be due to a spin-reorientation of some kind. Rather surprisingly, however, the anomaly in ErFe3 at 47 K is only just observable in the a.c. susceptibility data, in sharp contrast to the M6ssbauer results which reveal a marked change in the character of the 57Fe spectrum. Subsequent analysis of the latter data, using the methods of Physica 86-88B (1977) 71-72 © North-Holland
Arif et al. [3], reveal (i) an abrupt spin reorientation of approximately 15° which leaves the Fe moments directed along the c-axis of the crystal, and (ii) a sudden increase in the hyperfine field at the c Fe sites of approximately 4.4 T (compare both Van der Kraan et al. [4] and Arif et al. [3]). Rather surprisingly, however, very little change is found in the spectral parameters of the h sites on passing through the transition. 3. Interpretation of the M6ssbauer data In has already been remarked by Arif et al. [3] that an increase of 4 . 4 T in the magnetic hyperfine field at the c sites cannot be explained in terms of the local dipolar fields within the ErFe3 lattice. We are forced therefore to look for an alternative explanation and in this connection we note that significant variations in the strength of the magnetic hyperfine fields can be expected if the distances between neighbouring iron atoms are changed. This suggests therefore that the c atoms may have shifted their positions within the PuNi 3 structure and one such movement, which does not conflict with symmetry requirements, is shown in fig. 1. Moreover, if this model is correct then we may also expect small concomitant movements of the h atoms, as they relax in the face of advancing c-atoms. Both the displacements shown in fig. 1 are allowed within the PuNi3 structure. 4. Point charge crtstal field calculations First of all we note the proposed rearrangements of c and h iron atoms does not occur in the isostructural compounds YFe 3 and TbFe3, which reveal no such anomalies [3]. It would appear, therefore, that the effect is controlled primarily by the particular individual crystal field anisotropies at the two Er sites. Clearly atomic displacements of the kind described above will
72
ID RE
Fe b
[]
c
Fe h
Fe h
@ RE
Fe c
Fe C
o
Fe h
Fe h
@
T c
(2)-* Fe b
b
[]
REc
t)
Fig. 1. Vertical cross section in a b - c plane showing movement of the c and h atoms. give rise to c h a n g e s in the e l e c t r i c field g r a d i e n t s within the lattice, a n d the p o s s i b i l i t y e x i s t s of a c o m p e t i t i o n b e t w e e n the t w o E r sites in the d e t e r m i n a t i o n o f n o t o n l y the p r e f e r r e d d i r e c t i o n of m a g n e t i z a t i o n b u t also the p o s i t i o n s o f the c a n d h F e a t o m s . T o c h e c k this s u g g e s t i o n w e h a v e c o m p u t e d the c r y s t a l field coefficients A ~,,, d u e to b o t h the E r a n d F e s u b l a t t i c e s , as a f u n c t i o n of p o s i t i o n of the c a n d h F e a t o m s within the PuNi3 s t r u c t u r e . T h e s e r e s u l t s r e v e a l
t h a t c e r t a i n o f the A20 a n d A~3 c r y s t a l field coefficients g e n e r a t e d b y ~ the F e s u b l a t t i c e , a r e s t r o n g l y d e p e n d e n t on the p r e c i s e p o s i t i o n s o f the c a n d h a t o m s . In c o n c l u s i o n , we a r e led to s u g g e s t : (i) F o r T > 131 K the c(6) E r a t o m s are primarily responsible for a preferred direction of m a g n e t i z a t i o n c l o s e to the b a s a l p l a n e . (ii) A t T - 131 K a s p i n - r e o r i e n t a t i o n o c c u r s within the b - c p l a n e w h i c h m o v e s the F e mom e n t s c l o s e r to the c - a x i s . (iii) A t T = 47 K a f u r t h e r s p i n - r e o r i e n t a t i o n t a k e s p l a c e w h i c h l e a v e s the F e m o m e n t s dir e c t e d a l o n g the c - a x i s . (iv) B e l o w 47 K the a (3) E r a t o m s are r e s p o n sible f o r the c - a x i s d i r e c t i o n of m a g n e t i z a t i o n . (v) T h e d i s p l a c e m e n t of c a n d h F e a t o m s b e l o w 47 K p r o d u c e s a m o r e f a v o u r a b l e c r y s t a l field i n t e r a c t i o n at the c(6) R E sites.
References [1] K.H.J. Buschow, Phys. Stat. Solidi (a) 7 (1971) 199. [2] S.K. Arif, G.J. Bowden and D. St P. Bunbury, Proceedings 18th Ampere Congress (1974) 85-6 (North-Holland, Amsterdam). [31 S.K. Arif, D. St P. Bunbury, G.J. Bowden and R.K. Day, J. Phys. F. Metal Phys. 5 (1975) 1785. [4] A.M. van der Kraan, P.C.M. Gubbens and K.H.J. Buschow, Phys. Stat. Solidi (a) 31 (1975) 495.