Specific heat of the Europium-graphite intercaltion compound C6Eu

Synthetic Metals, 6 (1983) 135 - 140

135

SPECIFIC HEAT OF THE EUROPIUM-GRAPHITE INTERCALATION COMPOUND C6Eu

KAZUYA OHMATSU and HIROYOSHI SUEMATSU

Institute of Materials Science, University of Tsukuba, Sakura-mura, Ibaraki 305 (Japan) MASATSUGU SUZUKI

Department of Physics, Ochanomizu University, Bunkyo-ku, Tokyo 112 (Japan) (Received April 15, 1983)

Summary The specific heat of the first-stage graphite intercalation c o m p o u n d C6Eu has been studied with the use of a.c. calorimetry. The specific heat anomaly associated with the magnetic phase transition has been observed at T n = 40.0 + 1.0 K. Another weak anomaly was observed a few degrees below TN, and this is discussed in relation to the metamagnetic phase transitions of the compound.

1. Introduction The graphite intercalation compounds of rare-earth elements are new magnetic materials consisting of conduction 7r electrons of the graphite layers and 4f electrons of the rare earths. They have also a characteristic crystal structure in which a magnetic ion forms a planar triangular lattice between the graphite layers. These systems are expected to provide us with interesting aspects of magnetism having two-dimensional or highly anisotropic nature [1 - 6]. The magnetic properties of C6Eu have been studied previously by the present authors [1, 2]. The magnetic phase transition at T N = 40 K and the remarkable 1/3-metamagnetism at T ~ TN were revealed by the studies of the high and very high field magnetizations. Recently Date e t al. [3, 4] proposed a magnetic structure model in which the metamagnetic magnetization is well described in terms of the successive phase transitions of the antiferromagnetic (AF), ferrimagnetic (FI), canted spin (CAN) and high field ferromagnetic (FO) states. They also pointed out the important contribution of the four-spin exchange interaction to the stability of the FI phase. According to this model we can expect another phase transition at a temperature immediately below TN, at which the effect of the four-spin exchange interaction may disappear. 0379-6779/83/$3.00

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136 We study here the specific heat anomalies associated with these magnetic phase transitions and discuss the possibility of the lower temperature transition.

2. Experimental procedures C6Eu samples were prepared by vapor-phase reaction [7] in a clamped stainless steel container which was vacuum-sealed in a Pyrex glass ampoule. Two types of graphite crystals were used, one being synthetic single crystals (Kish graphite, typically 2 mm × 2 mm × 0.2 mm) and the other highly oriented pyrolytic graphite (HOPG). Europium metal was 99.9% pure. The reaction was carried out at Tu = 470 - 540 °C for 20 - 60 days. X-ray diffraction analyses and weight uptake measurements indicated that the samples were a mixture of C6Eu and pristine graphite. The C6Eu content was between 66 wt.% (#SB) and 74 wt.% (#SP) for the Kish graphite samples, and 86 wt.% for the HOPG sample (#HB). The content depends strongly upon the crystal size since only the peripheral region (0.5 mm) of a sample is C6Eu, though it depends on the nature of the pristine graphite as well as the condition and the period of reaction. The sample prepared at 600 °C contained some impurities such as EuC: in addition to pristine graphite. The specific heat of the c o m p o u n d was measured by means of an a.c. calorimeter [8, 9]. This m e t h o d is better for the measurement of the relative value of Cp rather than its absolute value, since there is some ambiguity in the determination of the absolute value. The absolute value was calibrated at room temperature by using a thermal relaxation m e t h o d and comparing the values for standard Cu and A1 samples [10]. A correction for the contribution of the remaining graphite was made to the measured value.

3. Results and discussion The specific heat of C6Eu was found to be (1.3 + 0.1) × 102 J mo1-1 K -1 at 300 K for several samples. The value is comparable with that for C6Li (~8 × 10 J mol 1 K-I at 300 K) [11]. This is reasonable because these compounds have a similar crystal structure and the magnetic contribution to Cp is negligible at room temperature. The temperature dependence of specific heat C~ for C6Eu is shown in Fig. 1. We can see a distinct anomaly at 40.0 K. This anomaly corresponds to the magnetic phase transition of the paramagnetic to the AF state. In contrast to the previous investigations [1, 2, 6] in which we did not obtain a clear anomaly just at TN, the Cp anomaly was very remarkable and ascertains the presence of the phase transition at TN. The profile of the anomaly, however, is highly asymmetric with respect to TN, and Cp does not show a divergent nature at T N which is usual for the second-order phase transition. In this study we determined the low-temperature edge of the sharp decrease

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of C, as T N. The non-divergent nature and asymmetric profile are common to the samples studied and they do not seem to come from the T N variation due to the inhomogeneity in a sample. In addition to the anomaly at TN we observed another weak and broad anomaly around 30 - 36 K, which is superimposed on the low-temperature side of the T N anomaly. Although its magnitude depends on the sample, almost all of the samples show this anomaly. According to the recent studies [2 - 4], the ground spin state is considered to be the AF state with the triangular 120 ° spin configuration, and with increasing field the compound undergoes field-induced phase transitions; the proposed spin structures at T < T N are the AF (H < 22 kOe), FI (22 < H < 82 kOe), CAN (82 ~. H 205 kOe) and the high field F (H > 205 kOe). Below 10 kOe the ground state (AF) is stable up to 36 K, but above 36 K the CAN state appears [12]. The excitation of this high field spin configuration may become significant n e a r TN, even at H = 0. This effect results in the broad anomaly of C, a few degrees below T s. A very similar feature is observed in the temperature dependence of the resistivity; the resistivity shows a broad maximum around 35 K which decreases with applied magnetic field [12]. This anomaly in resistivity is interpreted in terms of the electron scattering due to spin fluctuation. An alternative interpretation for the 30 - 36 K anomaly is that it comes from some magnetic impurity. However, we have not at present found such an impurity having T c at 30 - 36 K, although this interpretation is favorable to the explanation of the dependence of the magnitude of the anomaly on the sample. The effect of magnetic field on C, has been studied in order to elucidate the nature of these phase transitions. In Fig. 2 the magnetic specific

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Fig. 3. The specific heat Cp for the sample ( # S J ) prepared at 600 °C. The effect o f the magnetic field is s h o w n for H < 3 kOe (H _Lc). The anomalies at 12 and 22 K are attributed to the magnetic phase transitions of impurities.

heat C m is shown as a function of field up to 3 kOe (H £ c), where Cm is shown instead of Cp in order to clarify the effect of the field. The anomaly associated with the T s transition shows no appreciable variation with magnetic field. On the other hand, the anomaly around 30 - 36 K becomes weaker with increasing field and almost disappears at H > 0.5 kOe. This fact suggests that the spin fluctuation may be suppressed by the external field. The other type of anomalies in Cp are observed for some samples prepared at high temperatures. Figure 3 shows the temperature dependence of Cp for the sample (#SJ) which was prepared at 600 °C. We can see two anomalies at 12 and 22 K in addition to the anomalies mentioned above. These two anomalies are significantly affected by magnetic field and disappear at H > 3 kOe, as shown in Fig. 3. This effect indicates that the anomalies at 12 and 22 K originate from magnetic phase transitions. Very recently, Kaindl e t al. [6] reported the presence of three types of impurities, viz., EuO (Tc = 6 9 K), EuC2 ( T c ~ 22 K) and EuCx ( T c ~ 13 K) in their measurements of the MSssbauer spectra and magnetization, and they pointed out that these carbides are synthesized at a higher temperature than the usual condition for C6Eu. Thus we conclude that the Cp anomaly at 22 K can be attributed to the phase transition of the ferromagnetic impurity EuC2, while the anomaly at 12 K is due to the impurity EuCx. The content of these carbides is considered to increase when a sample is synthesized above 500 °C.

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Fig. 4. The magnetic specific heat C m and e n t r o p y S m for C6Eu.

We estimated the magnetic specific heat Cm and entropy Sm (Fig. 4) associated with the phase transitions rather crudely by taking the nonmagnetic part of Cp to be a function of Cp = A T + B T 3, and by choosing the parameters A and B so as to reproduce the experimental Cp above TN. Sm was calculated in the temperature range 10 - 60 K. The Sm for the magnetic disorder at T N w a s found to be 7.2 J tool -1. The experimental value is somewhat smaller than the theoretical value of 17.29 J mo1-1 calculated from Sm = R ln(2J + 1) and J = 7/2 for Eu 2+ ion. However, we cannot at present discuss this difference since the experimental value obtained above possibly includes a fairly large amount o f experimental errors in the determination of the absolute values of Cp and S m and also in the estimation of the C6Eu content in the sample. Acknowledgements The authors axe greatly indebted to H. Matsuo, M. Koyama and N. Joshima for preparing high-quality crystals of kish graphite. They are also grateful to Professor A. Ikushima for providing facilities for the calorimetric measurements. This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan. References 1 H. Suematsu, K. Ohmatsu and R. Y o s h i z a k i , Solid State Commun., 38 (1981) 1103. 2 H. Suematsu, K. Ohmatsu, K. Sugiyama, T. Sakakibara, M. M o t o k a w a and M. Date, Solid State Commun., 40 (1981) 241.

140 3 M. Date, T. Sakakibar, K. Sugiyama and H. Suematsu, in M. Date (ed.), High Field Magnetism, North-Holland, Amsterdam, 1983, p. 41. 4 T. Sakakibara, K. Sugiyama, M. Date and H. Suematsu, Synth. Met., 6 (1983) 165. 5 D.M. Hwang and D. Gu~rard, Solid State Commun., 40 (1981) 759. 6 G. Kaindl, J. Feldhaus, U. Ladewig and K. H. Frank, Phys. Rev. Lett., 50 (1983) 123. 7 M. E1 Makrini, D. Gu~rard, P. Lagrange and A. H~rold, Physica B, 99 (1980) 481. 8 P. F. Sullivan and G. Seidel, Phys. Rev., 173 (1968)679. 9 I. Hatta and A. Ikushima, J. Phys. Chem. Solids, 34 (1973) 57. 10 I. Hatta, Rev. Sci. Instrum., 50 (1979) 292. 11 C. Ayache, E. Bonjour, R. Lagnier and J. E. Fischer, Physica B, 99 (1980) 547. 12 H. Suematsu, K. Ohmatsu, T. Sakakibara, M. Date and M. Suzuki, Synth. Met., 8 (1983) 23.