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Resistivity of ferromagnetic CePt
~o~rn~lof the Less-Common Metals, 161(1990) 223-226
223
RESISTIVITY OF FERROMAGNETIC CePt J. D. RAMSDEN*
and J. G. HUBERT
PhysicsDepartment, Tujis University,Medford, MA 02155 (U.S.A.) (Received September
11,1989)
Summa~ The resistivity of CePt has been measured between 1.9 and 300 K. CePt orders ferromagnetically at 5.8 K. Its ferromagnetic state resistivity increases as T 3 above a constant residual term. This correlates with a T 3 term in the magnetic specific heat.
1. Introduction The equiatomic intermet~ic compound CePt (o~horhombic CrB st~ct~e) orders fe~oma~etica~y at a Curie temperature of 5.8 K. Susceptibi~~ [l] and low temperature specific heat [2] measurements indicate that the cerium ions have a 4f’ electronic configuration. As predicted by Hund’s rules, the total angular momentum for this configuration is 5/2; but crystalline electric fields lift the sixfold degeneracy, and the ground state is a doublet. The observation, however, of importance to the present investigation is that the magnetic specific heat of CePt can be fit by the expression AT-t- BT” (A and B are constants; T is temperature) from 1.5 K (the liit of measurement) up to 5 K [2]. We have measured the resistivity of CePt between 1.9 and 300 K, and we will discuss, in particular, our ferromagnetic state results.
2. Experimental details The sample was prepared by arc melting together stoichiometric amounts of the constituent elements. A rectangular bar was cut from the as-cast button; and this, along with additional pieces for X-ray analysis, was annealed for 1 week at 700°C. Diffractometer measurements showed only the CrB lines. Platinum leads
*Present address: Loral Infrared and Imaging Systems, Two Forbes Road, Lexington, MA 02173, U.S.A. *Present address: Physics Department, Virginia ~o~onwe~th University, Richmond, VA 23284. U.S.A. 0022-5088,‘90/$3.50
0 Ekevier Sequoia/Printed
in The Netherlands
224
were spot welded to the bar, and its dimensions were determined using a travelling microscope. The resistance was measured using a four-lead a.c. bridge as the temperature was varied between 1.9 and 300 K in a liquid helium cryostat. A germanium resistance thermometer was used below 30 K; a platinum resistance thermometer was used above this temperature.
3. Results The resistivity p of CePt as a function of temperature T ( 1.9 < T < 300 K) is shown in Fig. 1. The gap occurred as the thermometry was changed from the germanium to the platinum resistor while the sample was warming. The open points in Fig. 1 are representative of the full set of data plotted in Fig. 2(a), which shows p vs. T for T < 22 K with expanded scales. Figure 2(b) shows p vs. T 3 for T<7K.
4. Discussion The ferromagnetic ordering of CePt is clearly visible in p vs. T, marked by a pronounced “knee” at about 6 K. At temperatures well above this, more subtle changes in slope are apparent (see Fig. 1). These were taken by Ramsden et al. [31
I
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I
I
cm 40.* Jo.*
.
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.*
.
.
.’
I
.*
l
l
.*
t x
20-
& t Q f
IO-
Z
:
.*
f
.’
.*
.*
f 00 0I*
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I 30
I 100
1 Is0
I 200
I 230
300
TM 1
Fig. 1. The resistivityof CePt vs.temperature (the open points are representativeof the full set of data shownin Fig. 2).
225
capt (01 pv8.T
.
.
-3
E
1 c ,i P
-2
0
I 50
I loo
I 150
I 200
, 250
1 3co
I xii
T3(K 5
Fig. 2. The resistivity of CePt vs. (a) temperature and (b) temperature cubed.
as evidence for the excited states, separated in energy from the ground state doublet owing to the presence of crystalline electric fields. Gignoux and GomezSal [4] have estimated the excitation energies from their CePt resistivity data. Using point charge model calculations, they place the excited state doublets at 118.5 and 158.2 K above the ground state. Itoh et al. [5] later measured the resistivity of CePt under pressures up to 30 kbar. They subtracted off the resistivity of LaPt as representing the lattice contribution, and considered the magnetic resistivity pm of CePt. Above the Curie temperature, for all pressures, pm decreases slightly to at least 20 K, increases sharply to a broad maximum around 200 K, then falls gradually with a slope linear in In T [5]. These latter features, also seen by Gignoux and Gomez-Sal [4] at zero pressure, were taken by both groups as evidence for Kondo behavior. It is not the nature of CePt at elevated temperatures and in the normal state, however, with which we wish to concern ourselves here. What merits attention, we feel, is the temperature dependence of p vs. T for CePt in the ferromagnetic state. As may be seen in Fig. 2(b) the resistivity between 1.9 and 6.0 K can be described by p = p,, + CT 3 where p. is a residual value and C is a constant (p. = 0.9 ,usZ cm and C= 0.015 ,uQ cm Kp3 for the sample measured). The constant term is, of course, the result of impurities and defects. We note that since the increase in resistivity with increasing temperature above 6 K is very gradual (see Fig. 2), the scattering by phonons below 6 K must be negligible. We consider now the results of Holt et al. [2] for the magnetic contribution to the low temperature specific heat of CePt. The conduction electron and phonon contributions, small in comparison, may be subtracted from the total specific heat as being comparable with those of the isostructural compound LaPt. The resultant
226
y’TandB’T3termsfor T<5K(y’=95mJmol11K-2and~‘=56mJmol11K-4 where a mole is that of a formula unit) [2] would seem to be due, respectively, to a large 4f electron density of states at the Fermi level and the generation of magnetic excitations. In the sense that it may be said for phonons, the density of these excitations varies as T 3 up to nearly the Curie temperature. We suggest that the scattering of conduction electrons by the magnetic excitations is responsible for the temperature dependence of the resistivity of CePt in its ferromagnetic state. Sparn et al. [6] have made such a correlation between T 3 terms in the specific heat and resistivity of another ordered cerium system, i.e. CeAl,. This “Kondo lattice” becomes anti-ferromagnetic at a NCel temperature of 3.8 K, and a specific heat T3 term below this temperature has been attributed to long wavelength acoustic magnons [7]. Sparn et al. [6] pointed out that in the absence of small-angle magnon scattering, an expected T 5 resistivity term (drawn from analogy with phonon scattering) is reduced to T3; and they argued that the presence of an incommensurate wave vector in the modulated anti-ferromagnetic structure of CeAl, [8] ensures finite scattering angles. The ferromagnetic structure of CePt is not known; hence, we offer no reason why the magnetic excitation scattering at these low temperatures is evidently not limited to the forward direction. Certainly, low temperature neutron scattering measurements are in order. The range of the T 3 correlation for ferromagnetic CePt especially invites further study. In contrast to CeAl, for which a T3 dependence of its resistivity is valid only up to half the Neel temperature [6], for CePt this holds right up to the Curie temperature. 5. Conclusions The resistivity of CePt in the ferromagnetic state varies as the sum of a constant plus a T 3 term. The latter term is attributed to scattering by magnetic excitations, evidence for the generation of which is found in a T 3 term in the magnetic specific heat. The exact nature of these excitations in ferromagnetic CePt is not yet understood; detailed characterization awaits further measurements. Acknowledgment Supported by the National Science Foundation under grant DMR 80-10530. References J. G. Huber and C. A. Luengo, J. Phys. (Paris), C6 (1978) 781. B. J. Holt, J. D. Ramsden, H. H. Sample and J. G. Huber, Physica, 1078 (1981) 255. J. D. Ramsden, B. J. Holt, H. H. Sample and J. G. Huber, Bull. Am. Phys. Sot., 26 (1981) 409. D. Gignoux and J. C. Gomez-Sal, Solid State Commun., 45 (1983) 779. Y. Itoh, H. Kadomatsu, M. Kurisu and H. Fujiwana,J. Phys. Sot. Jpn., 56 ( 1987) 1159. G. Spam, W. Lieke, U. Gottwick, F. Steglich and N. Grewe, J. Mugrz.Mugn. Mater., 47, 48 (1985) 521. C. D. Bredl, F. Steglich and K. D. Schotte, Z. Phys., B29( 1978) 327. B. Barbara, J. X. Boucherle, J. L. Buevoz, M. F. Rossignol and J. Schweizer, Solid State Commun., 24(1977)481.
223
RESISTIVITY OF FERROMAGNETIC CePt J. D. RAMSDEN*
and J. G. HUBERT
PhysicsDepartment, Tujis University,Medford, MA 02155 (U.S.A.) (Received September
11,1989)
Summa~ The resistivity of CePt has been measured between 1.9 and 300 K. CePt orders ferromagnetically at 5.8 K. Its ferromagnetic state resistivity increases as T 3 above a constant residual term. This correlates with a T 3 term in the magnetic specific heat.
1. Introduction The equiatomic intermet~ic compound CePt (o~horhombic CrB st~ct~e) orders fe~oma~etica~y at a Curie temperature of 5.8 K. Susceptibi~~ [l] and low temperature specific heat [2] measurements indicate that the cerium ions have a 4f’ electronic configuration. As predicted by Hund’s rules, the total angular momentum for this configuration is 5/2; but crystalline electric fields lift the sixfold degeneracy, and the ground state is a doublet. The observation, however, of importance to the present investigation is that the magnetic specific heat of CePt can be fit by the expression AT-t- BT” (A and B are constants; T is temperature) from 1.5 K (the liit of measurement) up to 5 K [2]. We have measured the resistivity of CePt between 1.9 and 300 K, and we will discuss, in particular, our ferromagnetic state results.
2. Experimental details The sample was prepared by arc melting together stoichiometric amounts of the constituent elements. A rectangular bar was cut from the as-cast button; and this, along with additional pieces for X-ray analysis, was annealed for 1 week at 700°C. Diffractometer measurements showed only the CrB lines. Platinum leads
*Present address: Loral Infrared and Imaging Systems, Two Forbes Road, Lexington, MA 02173, U.S.A. *Present address: Physics Department, Virginia ~o~onwe~th University, Richmond, VA 23284. U.S.A. 0022-5088,‘90/$3.50
0 Ekevier Sequoia/Printed
in The Netherlands
224
were spot welded to the bar, and its dimensions were determined using a travelling microscope. The resistance was measured using a four-lead a.c. bridge as the temperature was varied between 1.9 and 300 K in a liquid helium cryostat. A germanium resistance thermometer was used below 30 K; a platinum resistance thermometer was used above this temperature.
3. Results The resistivity p of CePt as a function of temperature T ( 1.9 < T < 300 K) is shown in Fig. 1. The gap occurred as the thermometry was changed from the germanium to the platinum resistor while the sample was warming. The open points in Fig. 1 are representative of the full set of data plotted in Fig. 2(a), which shows p vs. T for T < 22 K with expanded scales. Figure 2(b) shows p vs. T 3 for T<7K.
4. Discussion The ferromagnetic ordering of CePt is clearly visible in p vs. T, marked by a pronounced “knee” at about 6 K. At temperatures well above this, more subtle changes in slope are apparent (see Fig. 1). These were taken by Ramsden et al. [31
I
I
I
I
I
cm 40.* Jo.*
.
.-
.’
.*
.
.
.’
I
.*
l
l
.*
t x
20-
& t Q f
IO-
Z
:
.*
f
.’
.*
.*
f 00 0I*
OO
I 30
I 100
1 Is0
I 200
I 230
300
TM 1
Fig. 1. The resistivityof CePt vs.temperature (the open points are representativeof the full set of data shownin Fig. 2).
225
capt (01 pv8.T
.
.
-3
E
1 c ,i P
-2
0
I 50
I loo
I 150
I 200
, 250
1 3co
I xii
T3(K 5
Fig. 2. The resistivity of CePt vs. (a) temperature and (b) temperature cubed.
as evidence for the excited states, separated in energy from the ground state doublet owing to the presence of crystalline electric fields. Gignoux and GomezSal [4] have estimated the excitation energies from their CePt resistivity data. Using point charge model calculations, they place the excited state doublets at 118.5 and 158.2 K above the ground state. Itoh et al. [5] later measured the resistivity of CePt under pressures up to 30 kbar. They subtracted off the resistivity of LaPt as representing the lattice contribution, and considered the magnetic resistivity pm of CePt. Above the Curie temperature, for all pressures, pm decreases slightly to at least 20 K, increases sharply to a broad maximum around 200 K, then falls gradually with a slope linear in In T [5]. These latter features, also seen by Gignoux and Gomez-Sal [4] at zero pressure, were taken by both groups as evidence for Kondo behavior. It is not the nature of CePt at elevated temperatures and in the normal state, however, with which we wish to concern ourselves here. What merits attention, we feel, is the temperature dependence of p vs. T for CePt in the ferromagnetic state. As may be seen in Fig. 2(b) the resistivity between 1.9 and 6.0 K can be described by p = p,, + CT 3 where p. is a residual value and C is a constant (p. = 0.9 ,usZ cm and C= 0.015 ,uQ cm Kp3 for the sample measured). The constant term is, of course, the result of impurities and defects. We note that since the increase in resistivity with increasing temperature above 6 K is very gradual (see Fig. 2), the scattering by phonons below 6 K must be negligible. We consider now the results of Holt et al. [2] for the magnetic contribution to the low temperature specific heat of CePt. The conduction electron and phonon contributions, small in comparison, may be subtracted from the total specific heat as being comparable with those of the isostructural compound LaPt. The resultant
226
y’TandB’T3termsfor T<5K(y’=95mJmol11K-2and~‘=56mJmol11K-4 where a mole is that of a formula unit) [2] would seem to be due, respectively, to a large 4f electron density of states at the Fermi level and the generation of magnetic excitations. In the sense that it may be said for phonons, the density of these excitations varies as T 3 up to nearly the Curie temperature. We suggest that the scattering of conduction electrons by the magnetic excitations is responsible for the temperature dependence of the resistivity of CePt in its ferromagnetic state. Sparn et al. [6] have made such a correlation between T 3 terms in the specific heat and resistivity of another ordered cerium system, i.e. CeAl,. This “Kondo lattice” becomes anti-ferromagnetic at a NCel temperature of 3.8 K, and a specific heat T3 term below this temperature has been attributed to long wavelength acoustic magnons [7]. Sparn et al. [6] pointed out that in the absence of small-angle magnon scattering, an expected T 5 resistivity term (drawn from analogy with phonon scattering) is reduced to T3; and they argued that the presence of an incommensurate wave vector in the modulated anti-ferromagnetic structure of CeAl, [8] ensures finite scattering angles. The ferromagnetic structure of CePt is not known; hence, we offer no reason why the magnetic excitation scattering at these low temperatures is evidently not limited to the forward direction. Certainly, low temperature neutron scattering measurements are in order. The range of the T 3 correlation for ferromagnetic CePt especially invites further study. In contrast to CeAl, for which a T3 dependence of its resistivity is valid only up to half the Neel temperature [6], for CePt this holds right up to the Curie temperature. 5. Conclusions The resistivity of CePt in the ferromagnetic state varies as the sum of a constant plus a T 3 term. The latter term is attributed to scattering by magnetic excitations, evidence for the generation of which is found in a T 3 term in the magnetic specific heat. The exact nature of these excitations in ferromagnetic CePt is not yet understood; detailed characterization awaits further measurements. Acknowledgment Supported by the National Science Foundation under grant DMR 80-10530. References J. G. Huber and C. A. Luengo, J. Phys. (Paris), C6 (1978) 781. B. J. Holt, J. D. Ramsden, H. H. Sample and J. G. Huber, Physica, 1078 (1981) 255. J. D. Ramsden, B. J. Holt, H. H. Sample and J. G. Huber, Bull. Am. Phys. Sot., 26 (1981) 409. D. Gignoux and J. C. Gomez-Sal, Solid State Commun., 45 (1983) 779. Y. Itoh, H. Kadomatsu, M. Kurisu and H. Fujiwana,J. Phys. Sot. Jpn., 56 ( 1987) 1159. G. Spam, W. Lieke, U. Gottwick, F. Steglich and N. Grewe, J. Mugrz.Mugn. Mater., 47, 48 (1985) 521. C. D. Bredl, F. Steglich and K. D. Schotte, Z. Phys., B29( 1978) 327. B. Barbara, J. X. Boucherle, J. L. Buevoz, M. F. Rossignol and J. Schweizer, Solid State Commun., 24(1977)481.