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Mat. Res. Soc. Symp. Proc. Vol. 488 01998 Materials Research Society

EXPERIMENTAL Single crystals are prepared by the diffusion method with dry toluene as solvent at temperatures 15'C ,20*C, 25°C and 30'C. Pure C6 0 and TDAE were placed in opposite sides of the H-shaped cell separated by a glass filter, and placed in an inert atmosphere. The crystals were observed to grow on the C60 side of the filter after about one month. Single crystals used for magnetic measurements were oriented by the Xray diffraction method. Typical dimension of plate shaped crystals was 1.OxO.5x0.2 mm 3 with c-axis perpendicular to the extended a-b plane. Oriented single crystal was mounted in a quartz tube under inert atmosphere and sealed with He gas. The DC magnetization was measured

by Quantum Design SQUID magnetometer. Thin films were prepared in two steps. First, pristine C6 0 thin films were prepared by vacuum deposition of pure C 6 0 onto quartz substrates. Then, the pristine C60 film was immersed into liquid TDAE in inert atmosphere to obtain a film of the charge transfer complex, TDAE-C60. RESULTS AND DISCUSSION Temperature and Field Dependence of Magnetization

6.0 5.0

\4

4q. U

£0)

E 3.0

V V * 0 A A 0 0 *

100G FC 100G ZFC 10G FC 1OG ZFC 5G FC 5G ZFC 2G FC 2G ZFC 1G FC

E)

E

o 1G ZFC

22.0 1.0 0.0

0

5

10 15 T (K)

-100

20

0 H (G)

100

Fig. 1 Temperature dependence M-T (a) and field dependence M-H (b) of magnetization of TDAE-C 60 single crystal #8 (H1Lc).

478

Fig.l(a) shows the temperature dependence of d.c. magnetization M of our single crystal sample #8. The sample was first cooled at zero-field, then the magnetic field perpendicular to the c-axis was applied. Although both zero-field-cooled (ZFC) and field-cooled (FC) magnetization are shown in the figure, the difference between the two is not clear. Note that the magnetization is almost independent of temperature at low fields. Fig.l(b) shows the magnetization curves of the same sample at different temperatures. The magnetic field was applied along the a-b plane, i.e. Hic, and correction due to demagnetization field is not taken into account. It is to be noted that, at low fields below about 10 Gauss, magnetization is proportional to the applied field, and the slope i.e. magnetic susceptibility is independent of temperature as shown in Fig.l(b). This temperature independent magnetic susceptibility, commonly observed in ferromagnet, is due to the demagnetization effect. This also explains why the magnetization at low fields is temperature independent as shown in Fig.l(a). However, the correction of demagnetization effect is not straightforward because our single crystals are not well shaped. Almost no hysteresis and zero coercivity were observed in Fig. l(b). The saturation magnetization at low temperature amounts to almost 6 emu/g corresponding to 1.0 LBIC60 in this sample. Other samples showed smaller values. In the temperature dependence, the remanence or the irreversibility, i.e. the difference between the zero-field-cooled (ZFC) and field-cooled (FC) ma