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Electrical resistivity of UMn2
Solid State Communications, Vol. 36, pp. 245—247. Pergamon Press Ltd. 1980. Printed in Great Britain. ELECTRICAL RESISTIVITY OF UMn2 LM. Foumier Centre d’Etudes Nucléaires de Grenoble, Département de Recherche Fondamentale, Section de Physique du Solide, 85X—38041 Grenoble Cedex, France (Received 28 March 1980 by E.F. Bertaut) The electrical resistivity of polycristalline UMn2 has been measured between 4.2 and 300 K. There is a very small anomaly 2 dependence around the is tentativedistorsion (TD =to212K). low temperature the T ly attributed 3d spinAtfluctuations. 1. INTRODUCTION URANIUM forms Laves phase alloys with the 3d transition metals Mn through Ni [1]. While UFe 2 is ferromagnetic [21,it was suggested from magnetic susceptibility measurements that UMn2 is antiferromag. netic [3,4]. However X-ray and neutron diffraction study on UMn2 [5] show no evidence for antiferromagnetic ordering but an unusually large structural distorsion from cubic MgCu2 to monoclinic. We have investigated the influence of this large distorsion on electrical transport properties, expecting an important effect around the transition temperature.
A bar shaped sample was cut from the original pellet and contacts were made using silver paste. The electrical resistivity measurements was carried out by the d.c. 4.point method using a Solartron A210 digital voltmeter. The temperature control of the sample was made using a special helium flow cryostat. Automatic scanning of the data was performed between 4.2 and 300 K. The resistance was deduced from the measurement of the current through and the voltage across the sample which were reversed for each point —
to compensate thermal e.m.f.’s. More details of the experimental method are found in [6]. 3. EXPERIMENTAL RESULTS
2. EXPERIMENTAL
The temperature dependence of the resistivity p is A polycristalline sample of about 10 g of UMn2 was shown in Fig. 1. After a strongly curved region between obtained by arc melting stoichiometric amounts of pure 100 and 200 K the temperature dependence becomes U and Mn metals in argon atmosphere. X-ray analysis almost linear above 230 K. There is barely an anomaly was made to check the sample quality. in the p(T) curve around the transition and it is 100 __________________________________________________________________ I
—.
75
F
.
~50~ UMn2 w
0
50
100
150 TEMPERATURE
Fig. 1. Temperature dependence of the electrical resistivity p.
245
200 (K)
250
300
246
ELECTRICAL RESISTIVITY OF UMn2
Vol. 36, No. 3
UMn2
Q05
-
-
300
200 TEMPERAIURE (K)
Fig. 2. Temperature dependence of the derivative of the electrical resistivity ap/aT. necessary to plot the derivative (ap/aT)(T) to put it clearly in evidence (Fig. 2). Figure 2 shows a clear change at 225 K. At of lowbehaviour temperature, the temperature dependence of the resistivity is also interesting since it is dominated by a T2 term up to 40K, as shown in Fig. 3. From this low temperature analysis in p = Po + aT we obtain the
F I
/ /
20 U
Mn2
10
I
/
2
/
‘~
-
05
/
/
/
-
/ 02
0
/‘
10I
I 20
.
In [5] it is proposed that above the transition, UMn
.
.
-
/ 1
=
~.3pf~-cm, 3~z&2-cmK~. 8 x l0 4. DISCUSSION
Weiss law at high temperature. (2) The U—U distance is too small (3.1 A) for the existence of localized moment according to Hill criteria based on direct Sf—Sf overlap. (3) It predicts a change in electrical resistivity at the transition, which is not observed.
2 I
-
a
— —
2 is a localized 51 electron system exhibiting a temperature dependent susceptibility and that the transition is a band Jahn—Teller effect. Such a proposition has to be rejected for the following reasons: (1) The susceptibility does not follow at Curie—
parameters I
Po
so
ioo
TEMPERATURE I K] Fig. 3. Bilogarithmic plot of ~p [isp = p(T) —p(0)] vs temperature.
In fact it is possible to use information accumulated for the rare earths Laves phases and for UFe2. It is known [7] that in the RE 3d2 compounds there is a charge transfer towards the 3d metal which tends to fill the 3d band (Ni is never magnetic in these compounds). In uranium Laves phases we expect this transfer to be even stronger due to possible additional transfer from the Sf band. In fact, in the case of UFe2 [2] no moment was observed on the U site and the Fe moment1Bwas in greatly reduced from 1.4 p~in RE Fe2 to 0.4 I U Fe 2. We thus expect that in UMn2 the Sf band has a small occupation number and a small density of states2 at the It is thentotempting to attribute the T termFermi at lowlevel. temperature spin fluctuations coming from the 3d band of manganese. The small electronic response at the distorsion is in favour of a phonon driven transition.
Vol. 36, No.3
ELECTRICAL RESISTIViTY OF UMn2
Specific heat measurements are under way and single crystals have been grown in order to further study this interesting intermetallic compound. Acknowledgement It is a pleasure to thank Dr J.C. Spirlet for the kind preparation of the sample.
2. 3. 4.
—
REFERENCES 1.
A.J. Freeman & J.B. Darby (eds.), The Actinides, Electronic Structure and Related Properties, Vol.
5. 6. 7.
247
II, Chap. 4. Academic Press, New York (1974). M. Yessik,J. Appi. Phys. 40, 1133 (1969). S.T. [in, A.R. Kaufmann,Phys. Rev. 108, 1171 (1957). D.J. Lam & A.T. Aldred, AlP Conf Proc. 24, 349 (1975). G.R. Marpoe, G.H. Lander, Solid State Commun. 26, 599 (1978). J.M. Constantini, Thesis, University of Paris (1980). M. Cyrot, D. Gignoux, F. Givord, M. L.avagna, J. de Phys. CS, 171(1979) and references therein.
A bar shaped sample was cut from the original pellet and contacts were made using silver paste. The electrical resistivity measurements was carried out by the d.c. 4.point method using a Solartron A210 digital voltmeter. The temperature control of the sample was made using a special helium flow cryostat. Automatic scanning of the data was performed between 4.2 and 300 K. The resistance was deduced from the measurement of the current through and the voltage across the sample which were reversed for each point —
to compensate thermal e.m.f.’s. More details of the experimental method are found in [6]. 3. EXPERIMENTAL RESULTS
2. EXPERIMENTAL
The temperature dependence of the resistivity p is A polycristalline sample of about 10 g of UMn2 was shown in Fig. 1. After a strongly curved region between obtained by arc melting stoichiometric amounts of pure 100 and 200 K the temperature dependence becomes U and Mn metals in argon atmosphere. X-ray analysis almost linear above 230 K. There is barely an anomaly was made to check the sample quality. in the p(T) curve around the transition and it is 100 __________________________________________________________________ I
—.
75
F
.
~50~ UMn2 w
0
50
100
150 TEMPERATURE
Fig. 1. Temperature dependence of the electrical resistivity p.
245
200 (K)
250
300
246
ELECTRICAL RESISTIVITY OF UMn2
Vol. 36, No. 3
UMn2
Q05
-
-
300
200 TEMPERAIURE (K)
Fig. 2. Temperature dependence of the derivative of the electrical resistivity ap/aT. necessary to plot the derivative (ap/aT)(T) to put it clearly in evidence (Fig. 2). Figure 2 shows a clear change at 225 K. At of lowbehaviour temperature, the temperature dependence of the resistivity is also interesting since it is dominated by a T2 term up to 40K, as shown in Fig. 3. From this low temperature analysis in p = Po + aT we obtain the
F I
/ /
20 U
Mn2
10
I
/
2
/
‘~
-
05
/
/
/
-
/ 02
0
/‘
10I
I 20
.
In [5] it is proposed that above the transition, UMn
.
.
-
/ 1
=
~.3pf~-cm, 3~z&2-cmK~. 8 x l0 4. DISCUSSION
Weiss law at high temperature. (2) The U—U distance is too small (3.1 A) for the existence of localized moment according to Hill criteria based on direct Sf—Sf overlap. (3) It predicts a change in electrical resistivity at the transition, which is not observed.
2 I
-
a
— —
2 is a localized 51 electron system exhibiting a temperature dependent susceptibility and that the transition is a band Jahn—Teller effect. Such a proposition has to be rejected for the following reasons: (1) The susceptibility does not follow at Curie—
parameters I
Po
so
ioo
TEMPERATURE I K] Fig. 3. Bilogarithmic plot of ~p [isp = p(T) —p(0)] vs temperature.
In fact it is possible to use information accumulated for the rare earths Laves phases and for UFe2. It is known [7] that in the RE 3d2 compounds there is a charge transfer towards the 3d metal which tends to fill the 3d band (Ni is never magnetic in these compounds). In uranium Laves phases we expect this transfer to be even stronger due to possible additional transfer from the Sf band. In fact, in the case of UFe2 [2] no moment was observed on the U site and the Fe moment1Bwas in greatly reduced from 1.4 p~in RE Fe2 to 0.4 I U Fe 2. We thus expect that in UMn2 the Sf band has a small occupation number and a small density of states2 at the It is thentotempting to attribute the T termFermi at lowlevel. temperature spin fluctuations coming from the 3d band of manganese. The small electronic response at the distorsion is in favour of a phonon driven transition.
Vol. 36, No.3
ELECTRICAL RESISTIViTY OF UMn2
Specific heat measurements are under way and single crystals have been grown in order to further study this interesting intermetallic compound. Acknowledgement It is a pleasure to thank Dr J.C. Spirlet for the kind preparation of the sample.
2. 3. 4.
—
REFERENCES 1.
A.J. Freeman & J.B. Darby (eds.), The Actinides, Electronic Structure and Related Properties, Vol.
5. 6. 7.
247
II, Chap. 4. Academic Press, New York (1974). M. Yessik,J. Appi. Phys. 40, 1133 (1969). S.T. [in, A.R. Kaufmann,Phys. Rev. 108, 1171 (1957). D.J. Lam & A.T. Aldred, AlP Conf Proc. 24, 349 (1975). G.R. Marpoe, G.H. Lander, Solid State Commun. 26, 599 (1978). J.M. Constantini, Thesis, University of Paris (1980). M. Cyrot, D. Gignoux, F. Givord, M. L.avagna, J. de Phys. CS, 171(1979) and references therein.