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Magnetic ordering in Ce2RhSi3
ELSEVIER
Journal of Magnetism and Magnetic Materials 137 (1994) L239-L242
~ 4 Journal of magnetism and magnetic , ~ i materials
Letter to the Editor
Magnetic ordering in Ce2RhSi 3 I. Das, E.V. Sampatb.kumaran * Tata Institute of Fundamental Research, Homi Bhabha Road, Bombay 400005, India Received 12 August 1994
Abstract
The results of magnetic susceptibility, heat-capacity, electrical resistivity and magnetoresistance measurements on the compound Ce2RhSi3 prove that this alloy orders antiferromagneticallyat 6 K in support of an original report by Chevalier et al. [Solid State Commun. 49 (1984) 753] but in contrast to the conclusions from neutron diffraction results [Szytula et al., J. Magn. Magn. Mater. 118 (1993) 302].
The new ternary compounds of the type R2RhSi 3 (R = rare earths), were reported to crystallize in a hexagonal structure which is derived from A1B2 type [1]. Among these, the Ce compound was reported to order antiferromagnetically at (TN = ) 6 K on the basis of magnetic susceptibility results. Subsequent investigation by neutron diffraction does not show any evidence for the existence of magnetic ordering in this alloy [2]. In this article, we present the results of magnetic susceptibility (X), heat-capacity (C), electrical resistivity ( p ) and magnetoresistance (Ap/O) measurements on Ce2RhSi 3 in order to characterize this compound better. Surprisingly, our results contradict the conclusion from neutron diffraction data, thereby supporting the report of Chevalier et al. [1]. The sample, Ce2RhSi3, was prepared by arc melting followed by homogenization at 800°C for 5 days in an evacuated sealed quartz tube. We could index the lines in the X-ray diffraction pattern (Cu K~) to the proper structure and a weak unidentified line at
* Corresponding author. Fax: + 91-22-215 2110.
2 0 - - 3 4 °, possibly suggesting the existence of an impurity phase ( < 10%), also could be seen in the diffraction pattern. The X (2-300 K) and isothermal magnetization (M, at 2 K) measurements were performed employing a superconducting quantum interference device (SQUID), Heat-capacity measurements were performed (2-60 K) by a semi-adiabatic heat-pulse method employing a set-up fabricated by us [3]. The p data (2-300 K) were taken by a conventional four-probe method. Magnetoresistance ( A p / p = p ( H ) - p(0)) behaviour at 5 K was investigated up to a magnetic field ( H ) of 70 kOe. The plot of X-1 versus T (Fig. 1) is linear in the temperature interval 100-300 K with the value of effective magnetic moment (2.48~B) close to that expected for trivalent Ce ions; the paramagnetic Curie temperature (0p) in this temperature range is about - 6 5 K, close to the value ( - 8 3 K) reported earlier [1]. There is a deviation from linearity below 100 K due to crystal-field effects and the value of 0p is about - 10 K in the temperature interval 10-40 K. The high 0p value is quite large compared to the magnetic ordering temperature (see below), thereby indicating the existence of Kondo effect above 100
0304-8853/94/$07.00 © 1994 Elsevier Science B.V. All fights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 4 ) 0 0 4 8 4 - 6
L240
L Das, E.V. Sampathkumaran/Journal of Magnetism and Magnetic Materials 137 (1994) L239-L242
K; but it appears that the Kondo effect is negligible below 40 K considering that the value of the low temperature 0p is comparable to that of TN (see below). It is to be noted that there is a distinct peak at 6 K in the plot of X versus T (see the top inset of Fig. 1) characteristic of antiferromagnetic ordering. Isothermal magnetization, though increasing monotonically with H, appears to deviate from linear dependence above 30 kOe (see the bottom inset of Fig. 1) as if there is a tendency for metamagnetic transition. These findings are in agreement with those of Chevalier et al. [1]. The results of heat-capacity measurements are shown only below 20 K in various ways in Figs. 2 and 3, since we have not noted any other anomaly in the C data above 20 K. The main finding is that the plot of C versus T exhibits a peak below 8 K,
240
I
proving thereby the existence of a magnetic transition. Clearly the transition is not from any impurity phase, considering that the value of C at the peak is comparable to that observed in any other Ce material exhibiting bulk magnetism. It is to be noted that there is a structure at 6.6 K just above the peak (5.7 K), thereby suggesting the existence of more than one magnetic transition in this material. The plot of C/T versus T 2 is linear over a wide temperature range (shown upto 20 K in Fig. 3) and the value of C/T extrapolated to absolute zero is about 100 m J / m o l K 2, presumably arising from heavy-fermion behaviour. Though the p behaviour was investigated up to 300 K, we show the data only below 20 K (Fig. 4) as the present interest is mainly with respect to low temperature properties of this compound. It may be
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TEMPERATURE (K) Fig. 1. Inverse susceptibility (.I"-1 ) as a function of temperature for Ce2RhSi3, obtained in a magnetic field of 4 kOe. The top inset shows X versus T at low temperatures in an expanded form in order to highlight the peak due to magnetic ordering. Isothermal magnetization ( M ) behaviour at 2 K is plotted in the bottom inset and a line is drawn through the data points.
L Das, E.V. Sampathkumaran/JournaloJ:Magnetism and MagneticMaterials 137 (1994) L239-L242 8
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TEMPERATURE (K)
TEMPERATURE (X) Fig. 2. Heat-capacity as a function of temperature for Ce2RhSi 3.
Fig. 4. Electrical resistivity as a function of temperature below 20 K for Ce2RhSi 3.
stated that p decreases smoothly with decreasing temperature, by about 10% from 300 to 100 K and by about 25% from 100 to 20 K, attributable to crystal-field effects. A point of note is that there is a drastic fall in p below 7 K, the temperature at which X and C exhibit anomalies due to magnetic ordering. There is no evidence for logarithmic increase of /9
with decreasing temperature below 30 K; this finding may imply that the Kondo effect is insignificant at low temperatures consistent with the conclusion from the 0p data. We have also investigated the magnet•resistance behaviour at 5 K in order to characterize this com-
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Tz (K~) Fig. 3. Heat-capacity divided by temperature (T) as a function of T 2 for CeRhzSi 3.
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Fig. 5. Magnet•resistance as a function of magnetic field at 5 K for Ce2RhSi 3.
L242
I. Das, E. V. Sampathkumaran/Journal of Magnetism and MagneticMaterials 137 (1994)L239-L242
pound further. The A p i p is positive (Fig. 5), typical of simple antiferromagnets [4]. Clearly, the positive values of A p / p rule out the formation of an antiferromagnetic energy gap [5]. The shape of the plot of A p / p suggests the existence of a metamagnetic transition around 30 kOe with the application of magnetic field [6], consistent with the inference from isothermal magnetization data. To conclude, X, P, C and A p / p data establish that the compound Ce 2 RhSi 3 orders antiferromagnetically below 6 K, in support of the original report by Chevalier et al. The origin of the discrepancy with the conclusion from the neutron diffraction data is baffling. It is possible that the ground state of this alloy is highly dependent on stoichiometry. It is also worth probing whether small traces of magnetic impurities (polarising the d-band of Rh) modify the ground state. We call for all types of investigation
including neutron diffraction on the same specimen in order to resolve this issue. We thank K.V. Gopalakrishnan for SQUID measurements and R. Vijayaraghavan for his support.
References [1] B. Chevalier, P. Lejay, J. Etourneau and P. Hagenmuller, Solid State Commun. 49 (1984) 753. [2] A. Szytula, J. Leceijewicz and K. Maletka, J. Magn. Magn. Mater. 118 (1993) 302. [3] I. Das and E.V. Sampathkumaran, Pramana - J. Phys. 42 (1994) 251. [4] K. Yamada and S. Takoda, Prog. Theor. Phys. 71 (1973) 1401. [5] I. Das, E.V. Sampathkumaran and R. Vijayaraghavan, Phys. Rev. B 44 (1991) 159 and references therein. [6] I. Das and E.V. Sampathkumaran, Phys. Rev. B 49 (1994) 3972.
Journal of Magnetism and Magnetic Materials 137 (1994) L239-L242
~ 4 Journal of magnetism and magnetic , ~ i materials
Letter to the Editor
Magnetic ordering in Ce2RhSi 3 I. Das, E.V. Sampatb.kumaran * Tata Institute of Fundamental Research, Homi Bhabha Road, Bombay 400005, India Received 12 August 1994
Abstract
The results of magnetic susceptibility, heat-capacity, electrical resistivity and magnetoresistance measurements on the compound Ce2RhSi3 prove that this alloy orders antiferromagneticallyat 6 K in support of an original report by Chevalier et al. [Solid State Commun. 49 (1984) 753] but in contrast to the conclusions from neutron diffraction results [Szytula et al., J. Magn. Magn. Mater. 118 (1993) 302].
The new ternary compounds of the type R2RhSi 3 (R = rare earths), were reported to crystallize in a hexagonal structure which is derived from A1B2 type [1]. Among these, the Ce compound was reported to order antiferromagnetically at (TN = ) 6 K on the basis of magnetic susceptibility results. Subsequent investigation by neutron diffraction does not show any evidence for the existence of magnetic ordering in this alloy [2]. In this article, we present the results of magnetic susceptibility (X), heat-capacity (C), electrical resistivity ( p ) and magnetoresistance (Ap/O) measurements on Ce2RhSi 3 in order to characterize this compound better. Surprisingly, our results contradict the conclusion from neutron diffraction data, thereby supporting the report of Chevalier et al. [1]. The sample, Ce2RhSi3, was prepared by arc melting followed by homogenization at 800°C for 5 days in an evacuated sealed quartz tube. We could index the lines in the X-ray diffraction pattern (Cu K~) to the proper structure and a weak unidentified line at
* Corresponding author. Fax: + 91-22-215 2110.
2 0 - - 3 4 °, possibly suggesting the existence of an impurity phase ( < 10%), also could be seen in the diffraction pattern. The X (2-300 K) and isothermal magnetization (M, at 2 K) measurements were performed employing a superconducting quantum interference device (SQUID), Heat-capacity measurements were performed (2-60 K) by a semi-adiabatic heat-pulse method employing a set-up fabricated by us [3]. The p data (2-300 K) were taken by a conventional four-probe method. Magnetoresistance ( A p / p = p ( H ) - p(0)) behaviour at 5 K was investigated up to a magnetic field ( H ) of 70 kOe. The plot of X-1 versus T (Fig. 1) is linear in the temperature interval 100-300 K with the value of effective magnetic moment (2.48~B) close to that expected for trivalent Ce ions; the paramagnetic Curie temperature (0p) in this temperature range is about - 6 5 K, close to the value ( - 8 3 K) reported earlier [1]. There is a deviation from linearity below 100 K due to crystal-field effects and the value of 0p is about - 10 K in the temperature interval 10-40 K. The high 0p value is quite large compared to the magnetic ordering temperature (see below), thereby indicating the existence of Kondo effect above 100
0304-8853/94/$07.00 © 1994 Elsevier Science B.V. All fights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 4 ) 0 0 4 8 4 - 6
L240
L Das, E.V. Sampathkumaran/Journal of Magnetism and Magnetic Materials 137 (1994) L239-L242
K; but it appears that the Kondo effect is negligible below 40 K considering that the value of the low temperature 0p is comparable to that of TN (see below). It is to be noted that there is a distinct peak at 6 K in the plot of X versus T (see the top inset of Fig. 1) characteristic of antiferromagnetic ordering. Isothermal magnetization, though increasing monotonically with H, appears to deviate from linear dependence above 30 kOe (see the bottom inset of Fig. 1) as if there is a tendency for metamagnetic transition. These findings are in agreement with those of Chevalier et al. [1]. The results of heat-capacity measurements are shown only below 20 K in various ways in Figs. 2 and 3, since we have not noted any other anomaly in the C data above 20 K. The main finding is that the plot of C versus T exhibits a peak below 8 K,
240
I
proving thereby the existence of a magnetic transition. Clearly the transition is not from any impurity phase, considering that the value of C at the peak is comparable to that observed in any other Ce material exhibiting bulk magnetism. It is to be noted that there is a structure at 6.6 K just above the peak (5.7 K), thereby suggesting the existence of more than one magnetic transition in this material. The plot of C/T versus T 2 is linear over a wide temperature range (shown upto 20 K in Fig. 3) and the value of C/T extrapolated to absolute zero is about 100 m J / m o l K 2, presumably arising from heavy-fermion behaviour. Though the p behaviour was investigated up to 300 K, we show the data only below 20 K (Fig. 4) as the present interest is mainly with respect to low temperature properties of this compound. It may be
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250
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TEMPERATURE (K) Fig. 1. Inverse susceptibility (.I"-1 ) as a function of temperature for Ce2RhSi3, obtained in a magnetic field of 4 kOe. The top inset shows X versus T at low temperatures in an expanded form in order to highlight the peak due to magnetic ordering. Isothermal magnetization ( M ) behaviour at 2 K is plotted in the bottom inset and a line is drawn through the data points.
L Das, E.V. Sampathkumaran/JournaloJ:Magnetism and MagneticMaterials 137 (1994) L239-L242 8
•
,
900
• • 6
% °
1,
Ce2RhSi ~
800
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!
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TEMPERATURE (K)
TEMPERATURE (X) Fig. 2. Heat-capacity as a function of temperature for Ce2RhSi 3.
Fig. 4. Electrical resistivity as a function of temperature below 20 K for Ce2RhSi 3.
stated that p decreases smoothly with decreasing temperature, by about 10% from 300 to 100 K and by about 25% from 100 to 20 K, attributable to crystal-field effects. A point of note is that there is a drastic fall in p below 7 K, the temperature at which X and C exhibit anomalies due to magnetic ordering. There is no evidence for logarithmic increase of /9
with decreasing temperature below 30 K; this finding may imply that the Kondo effect is insignificant at low temperatures consistent with the conclusion from the 0p data. We have also investigated the magnet•resistance behaviour at 5 K in order to characterize this com-
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10
12 f" CezRhSi 3 .~
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Tz (K~) Fig. 3. Heat-capacity divided by temperature (T) as a function of T 2 for CeRhzSi 3.
I
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I
20
40
60
80
H (kOe)
Fig. 5. Magnet•resistance as a function of magnetic field at 5 K for Ce2RhSi 3.
L242
I. Das, E. V. Sampathkumaran/Journal of Magnetism and MagneticMaterials 137 (1994)L239-L242
pound further. The A p i p is positive (Fig. 5), typical of simple antiferromagnets [4]. Clearly, the positive values of A p / p rule out the formation of an antiferromagnetic energy gap [5]. The shape of the plot of A p / p suggests the existence of a metamagnetic transition around 30 kOe with the application of magnetic field [6], consistent with the inference from isothermal magnetization data. To conclude, X, P, C and A p / p data establish that the compound Ce 2 RhSi 3 orders antiferromagnetically below 6 K, in support of the original report by Chevalier et al. The origin of the discrepancy with the conclusion from the neutron diffraction data is baffling. It is possible that the ground state of this alloy is highly dependent on stoichiometry. It is also worth probing whether small traces of magnetic impurities (polarising the d-band of Rh) modify the ground state. We call for all types of investigation
including neutron diffraction on the same specimen in order to resolve this issue. We thank K.V. Gopalakrishnan for SQUID measurements and R. Vijayaraghavan for his support.
References [1] B. Chevalier, P. Lejay, J. Etourneau and P. Hagenmuller, Solid State Commun. 49 (1984) 753. [2] A. Szytula, J. Leceijewicz and K. Maletka, J. Magn. Magn. Mater. 118 (1993) 302. [3] I. Das and E.V. Sampathkumaran, Pramana - J. Phys. 42 (1994) 251. [4] K. Yamada and S. Takoda, Prog. Theor. Phys. 71 (1973) 1401. [5] I. Das, E.V. Sampathkumaran and R. Vijayaraghavan, Phys. Rev. B 44 (1991) 159 and references therein. [6] I. Das and E.V. Sampathkumaran, Phys. Rev. B 49 (1994) 3972.