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Low temperature specific heat of ferromagnetic CePt and isostructural LaPt
Physica I07B (1981) 255-256 North-Holland Publishing Company
EB 4
LOW TEMPERATURE
SPECIFIC HEAT OF FERROMAGNETIC
B.J. Holt #, J.D. Ramsden*,
CePt AND ISOSTRUCTURAL
LaPt #
H.H. Sample and J.G. Huber
Physics Department Tufts University Medford, Massachusetts 02155 The intermetallic compound CePt orders ferromagnetically at heats of CePt and LaPt (which also has the orthorhombic CrB measured between 1.5 K and 16 K. The difference of the two entropy of ordering per Ce 4f I moment, kBln2. The specific observed to have an AT + BT3 temperature dependence below 5 i.
INTRODUCTION
Magnetic ordering of Ce ions in intermetallic compuunds is uncommon; that such ordering is ferromagnetic is quite rare. Magnetization measurements have shown the equiatomic compound CePt (orthorhombic CrB structure) to order ferromagnetlcally at 5.8 K I. The infrequency of this occurrance and the existence of the compound UPt, isostructural with CePt and ordering ferromagnetically at 30 K 2, invite detailed study of CePt, especially the properties associated with the Ce 4f electrons. Huber and Luengo have measured the paramagnetie susceptibilities of both CePt and UPt to temperatures well above the ferromagnetic ordering temperatures S. They argue that these data in conjunction with specific heat data give evidence for 4f I and 5f I configurations, modified by crystalline electric fields, for the Ce and U ions, respectively, in CePt and UPt. Presented here are the measurements of low temperature specific heat from which was extracted a magnetic entropy of kBln2 per Ce ion in CePt. In order to isolate just the magnetic contribution, the specific heat of the isostructural compound LaPt was measured as well as that of CePt. The compounds differ only by a localized 4f electron and an extra nucleon or so per Ce ion, so they should have very similar conduction electron and lattice specific heats at low temperatures. 2.
EXPERIMENTAL
5.8 K. The specific structure) have been is taken to determine the heat of ordered CePt is K.
measurements is periodically checked by measuring the specific heat of high purity Copper. Systematic errors are thereby reduced to about i%. 3.
RESULTS AND DISCUSSION
Shown in Figure I are the specific heats, C, of CePt and LaPt between 1.5 K and 16 K. The data (only representative points are shown for LaPt) a r e p l o t t e d a s C/T v e r s u s T a n d a r e f i t t e d w i t h smooth curves. We define a mole with a formula unit; i.e., Avogadro's number of atoms of both
m
m
t-
_ _ _
. . . . . . .
= = = = =
=
-
~
,
T (KI
DETAILS
The compounds CePt and LaPt were prepared by arc melting under Argon stoichiometric amounts of the high purity constituent elements. Losses in melting were negligible. Each sample was annealed in a Helium atmosphere sealed in a quartz tube for about i0 days at 700°C. X-ray powder diffraction patterns of both CePt and LaPt showed only CrB lines. Heat capacity measurements were made In a He 4, adiabatic, heat-pulse calorimeter ~ on 1.58 gm. and 1.99 gm. masses of CePt and LaPt, respectively. The mass of the addenda (mostly Copper) was 4.15 gm. As described in Ref. 4, the Germanium resistance thermometer used for the
03784363/81/0000-00(~/$02.50
© North-Holland Publishing Company
O4
O,
0 4
r IK)
Figure Figure
1 (above): 2 (below):
S p e c i f i c h e a t s o f CePt a n d L a P t M a g n e t i c e n t r o p y p e r Ce i o n i n CePt 255
256
elements. It is readily apparent that the specific heat of CePt has a well-defined peak at 5.8 K and that C/T for CePt and LaPt are merging at higher temperatures. With the assumption that the conduction electron and lattice specific heats of the two compounds should he essentially identical, we have taken the area between the two curves in Figure 1 to be the magnetic entropy of CePt (extrapolations to T=0 were made wlth the aid of Figure 3). We
1
1m
4.
II E
T 2 ( K 2)
i
,
i
i
• C@Pt
• Lo~
O%co
°•
t
L6~
The AT + BT 3 temperature dependence below 5 K of the specific heat of CePt in the ferromagnetic state we offer as an observation without explanation.
! E
S
5.
REFERENCES
t
Supported by the National Science Foundation. Present Address: Congressional Budget Office, Natural Resources and Commerce Division, U.S. Congress, Washington, DC. Present Address: Janls Research, Inc., 22 Spencer St., Stoneham, MA 02180. J.G. Huber, to be published. B.T. Matthias, C.W. Chu, E. Corenzwltt and D. Wohlleben, Proc. Natl. Acad. Sci. 64, 459 (1969). J.G. Nuber and C.A. Luengo, J. Physique Col. C6, 781 (1978). M. Jlrmanus, H.H. Sample and L.J. Neuringer, J. Low Temp. Phys. 20, 229(1975).
# 4~
* •e oO
. . . .
~
~0
-
m
1
l 20
1
1
I
30
•
•
4~ i
•
T2(K Z )
Figure 3a (above) and 3b (below): Specific h e a t s o f L a P t and C e P t p r e s e n t e d t o show AT + BT 3 temperature dependence have divided the magnetic entropy per Ce ion, AS, by kBln2 and plotted this quantity versus T in Figure 2. AS/kBIn2 becomes equal to i at around 16 K; we therefore deduce that the ground state for the Ce ions is a magnetic doublet. We note that no beginning of a Schottky peak, indicative of exicted states, is yet visible at 16 K. Efforts
CONCLUSIONS
The sharp peak in C/T at 5.81 K for CePt (see Figures i and 3 ) is in good agreement with ferromagnetic ordering temperature of 5.85 K by Arrott plot analysis of magnetization data . The entropy evidence for a ground state magnetic doublet is consistent with a Ce ion 4f I configuration in CePt. The six-fold degeneracy of the Hund's rules total angular momentum J = 5/2 Is lifted by the crystalline electric fields of the orthorhombic structure. That a Schottky anomaly is not yet apparent in our data indicates that the excited states do not become significantly occupied until much higher temperatures.
• LoPt P
~
dependence of the magnetic specific heat of CePt led evenutally to Figure 3. There we have plotted for both compounds the same C/T of Figure i versus T 2. This, of course, is the usual procedure for LaPt, a normal metal; and from the data below 5 K in Figure 3a we have determined a conduction electron y = 5.0 mJ/ mole-K 2 and a lattice ~ = 0.52 mJ/mole-K4. But, C/T versus T 2 for CePt in the ferromagnetically ordered state is also linear up to about 5 K, as can be seen by the straight line through the data in Figure 3b. Thus, for ordered CePt too we have the low temperature form C = AT + BT 3, where A = i00 mJ/mole-K 2 and B = 56.7 mJ/ mole-K4 (to remove the small conduction electron and lattice contributions, the y and B of LaPt above may be subtracted from A and B, respectively).
to find a power law in the temperature
I 2
s
EB 4
LOW TEMPERATURE
SPECIFIC HEAT OF FERROMAGNETIC
B.J. Holt #, J.D. Ramsden*,
CePt AND ISOSTRUCTURAL
LaPt #
H.H. Sample and J.G. Huber
Physics Department Tufts University Medford, Massachusetts 02155 The intermetallic compound CePt orders ferromagnetically at heats of CePt and LaPt (which also has the orthorhombic CrB measured between 1.5 K and 16 K. The difference of the two entropy of ordering per Ce 4f I moment, kBln2. The specific observed to have an AT + BT3 temperature dependence below 5 i.
INTRODUCTION
Magnetic ordering of Ce ions in intermetallic compuunds is uncommon; that such ordering is ferromagnetic is quite rare. Magnetization measurements have shown the equiatomic compound CePt (orthorhombic CrB structure) to order ferromagnetlcally at 5.8 K I. The infrequency of this occurrance and the existence of the compound UPt, isostructural with CePt and ordering ferromagnetically at 30 K 2, invite detailed study of CePt, especially the properties associated with the Ce 4f electrons. Huber and Luengo have measured the paramagnetie susceptibilities of both CePt and UPt to temperatures well above the ferromagnetic ordering temperatures S. They argue that these data in conjunction with specific heat data give evidence for 4f I and 5f I configurations, modified by crystalline electric fields, for the Ce and U ions, respectively, in CePt and UPt. Presented here are the measurements of low temperature specific heat from which was extracted a magnetic entropy of kBln2 per Ce ion in CePt. In order to isolate just the magnetic contribution, the specific heat of the isostructural compound LaPt was measured as well as that of CePt. The compounds differ only by a localized 4f electron and an extra nucleon or so per Ce ion, so they should have very similar conduction electron and lattice specific heats at low temperatures. 2.
EXPERIMENTAL
5.8 K. The specific structure) have been is taken to determine the heat of ordered CePt is K.
measurements is periodically checked by measuring the specific heat of high purity Copper. Systematic errors are thereby reduced to about i%. 3.
RESULTS AND DISCUSSION
Shown in Figure I are the specific heats, C, of CePt and LaPt between 1.5 K and 16 K. The data (only representative points are shown for LaPt) a r e p l o t t e d a s C/T v e r s u s T a n d a r e f i t t e d w i t h smooth curves. We define a mole with a formula unit; i.e., Avogadro's number of atoms of both
m
m
t-
_ _ _
. . . . . . .
= = = = =
=
-
~
,
T (KI
DETAILS
The compounds CePt and LaPt were prepared by arc melting under Argon stoichiometric amounts of the high purity constituent elements. Losses in melting were negligible. Each sample was annealed in a Helium atmosphere sealed in a quartz tube for about i0 days at 700°C. X-ray powder diffraction patterns of both CePt and LaPt showed only CrB lines. Heat capacity measurements were made In a He 4, adiabatic, heat-pulse calorimeter ~ on 1.58 gm. and 1.99 gm. masses of CePt and LaPt, respectively. The mass of the addenda (mostly Copper) was 4.15 gm. As described in Ref. 4, the Germanium resistance thermometer used for the
03784363/81/0000-00(~/$02.50
© North-Holland Publishing Company
O4
O,
0 4
r IK)
Figure Figure
1 (above): 2 (below):
S p e c i f i c h e a t s o f CePt a n d L a P t M a g n e t i c e n t r o p y p e r Ce i o n i n CePt 255
256
elements. It is readily apparent that the specific heat of CePt has a well-defined peak at 5.8 K and that C/T for CePt and LaPt are merging at higher temperatures. With the assumption that the conduction electron and lattice specific heats of the two compounds should he essentially identical, we have taken the area between the two curves in Figure 1 to be the magnetic entropy of CePt (extrapolations to T=0 were made wlth the aid of Figure 3). We
1
1m
4.
II E
T 2 ( K 2)
i
,
i
i
• C@Pt
• Lo~
O%co
°•
t
L6~
The AT + BT 3 temperature dependence below 5 K of the specific heat of CePt in the ferromagnetic state we offer as an observation without explanation.
! E
S
5.
REFERENCES
t
Supported by the National Science Foundation. Present Address: Congressional Budget Office, Natural Resources and Commerce Division, U.S. Congress, Washington, DC. Present Address: Janls Research, Inc., 22 Spencer St., Stoneham, MA 02180. J.G. Huber, to be published. B.T. Matthias, C.W. Chu, E. Corenzwltt and D. Wohlleben, Proc. Natl. Acad. Sci. 64, 459 (1969). J.G. Nuber and C.A. Luengo, J. Physique Col. C6, 781 (1978). M. Jlrmanus, H.H. Sample and L.J. Neuringer, J. Low Temp. Phys. 20, 229(1975).
# 4~
* •e oO
. . . .
~
~0
-
m
1
l 20
1
1
I
30
•
•
4~ i
•
T2(K Z )
Figure 3a (above) and 3b (below): Specific h e a t s o f L a P t and C e P t p r e s e n t e d t o show AT + BT 3 temperature dependence have divided the magnetic entropy per Ce ion, AS, by kBln2 and plotted this quantity versus T in Figure 2. AS/kBIn2 becomes equal to i at around 16 K; we therefore deduce that the ground state for the Ce ions is a magnetic doublet. We note that no beginning of a Schottky peak, indicative of exicted states, is yet visible at 16 K. Efforts
CONCLUSIONS
The sharp peak in C/T at 5.81 K for CePt (see Figures i and 3 ) is in good agreement with ferromagnetic ordering temperature of 5.85 K by Arrott plot analysis of magnetization data . The entropy evidence for a ground state magnetic doublet is consistent with a Ce ion 4f I configuration in CePt. The six-fold degeneracy of the Hund's rules total angular momentum J = 5/2 Is lifted by the crystalline electric fields of the orthorhombic structure. That a Schottky anomaly is not yet apparent in our data indicates that the excited states do not become significantly occupied until much higher temperatures.
• LoPt P
~
dependence of the magnetic specific heat of CePt led evenutally to Figure 3. There we have plotted for both compounds the same C/T of Figure i versus T 2. This, of course, is the usual procedure for LaPt, a normal metal; and from the data below 5 K in Figure 3a we have determined a conduction electron y = 5.0 mJ/ mole-K 2 and a lattice ~ = 0.52 mJ/mole-K4. But, C/T versus T 2 for CePt in the ferromagnetically ordered state is also linear up to about 5 K, as can be seen by the straight line through the data in Figure 3b. Thus, for ordered CePt too we have the low temperature form C = AT + BT 3, where A = i00 mJ/mole-K 2 and B = 56.7 mJ/ mole-K4 (to remove the small conduction electron and lattice contributions, the y and B of LaPt above may be subtracted from A and B, respectively).
to find a power law in the temperature
I 2
s