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Observation of superconductivity in TDAE-C60
Pergamon
PII: soo38-10!38(96)oo6o4-7
OBSERVATION OF SUPERCONDUCIIVITY IN TDAE-cm M. Ric&,” M. Bisbiglia,” R. De Renzi” and F. Bolzonib ‘Dipartimento di Fisica and Istituto Nazionale di Fisica della Materia, Universita di Parma, I-43100 Parma, Italy bIstituto MASPEC, CNR, Parma, Italy (Received 3 August 1996; accepted 30 September 1996 by E. Mohaari) SQUID magnetometry performed on the fullerene based charge transfer salt TDAE&, shows the presence of a superconducting phase below T, = 17.4K, coexisting with the already known magnetic phase. A large fraction Meissner effect (f w 0.3) appears only when the compound is slowly cooled across the temperatures corresponding to (partial) freezing of the molecular rotations. The ferromagnetic response prevails over the Meissner effect for Zf > 6 Oe. A strong diamagnetic contribution to the RF susceptibility, appearing below T,, c&inns these findings. This phenomenon sheds new light on the complementary nature of superconductivity and magnetism in low dimensional Mot&Hubbard systems. Copyright 8 1997 Elsevier Science Ltd Keywords: A. fullerenes, magnetically ordered materials, superconductors.
TDAE-CW is a fullerene based molecular magnet, which was first reported by Allemand [l] to exhibit a magnetic ordering transition at T, = 16.1 K. The ordered state possesses a sizeable macroscopic magnetization, although its exact nature and origin are still a subject of debate. It has attracted great interest for two main reasons: (i) 16K is a large critical temperature for organic ferromagnetism and an absolute record for non-polymeric organic materials; (ii) the proximity of magnetism and superconductivity (present in the related alkali-metal fullerides A&,,) is a common feature of a much larger class of highly correlated electron sys’tems, including heavy fermions, nickel borocarbides and copper perovskites. TDAE stands for tetrakis-dimethyl-amino-ethylene, one of the strongest organic donors. In the 1: 1 stoichiometry each TDAE molecule donates one electron to the band formed by the lowest unoccupied tlu molecular orbital (LIMO) of the Cm units. The crystal is monoclinic [2], with shorter C,-C, distances along the c axis, and it displays the activated d.c. transport properties characteristic of a large gap semiconductor [3], or rather of a Mot&Hubbard insulator [4]. The magnetic behavior is still controversial. Experiments on earlier samples identified signatures of weak and/or disordered magnetism. Spin glass features were seen in the a.c. susceptibility [S], cusps in x’ in the heat
capacity [6], a broad peak around T, in the electron spin resonance (ESR) [7] and muon spin rotation @SR) signals [8], as a broad distribution of static local fields. More recently long range order was inferred from the rather narrow local magnetic field distribution probed by implanted muons [9, lo] and ESR on single crystals [ll]. The strongly anisotropic ESR line intensity was interpreted in terms of an antiferromagnetic exchange along the c axis, giving rise to weak ferromagnetism of the Dzyaloshinsky-Moriya type [12]. Growing experimental evidence points towards a fundamental role played by the orientational (merohedral) order of the Jahn-Teller distorted fullerene molecules on the magnetic properties of this compound. It suggests that, when the C, orientational degrees of freedom become frozen, at the so-called phstic phase transitions, the degree of merohedral order controls the nature of the exchange paths among neighbor molecules. In particular recent a.c. susceptibility and ESR measurements [ 131show large variations of the magnetic response upon different cooling protocols, from the room temperature plastic phase across the first freezing temperature of the rotational dynamics of C!,, T,, - 170 K. A ten-fold increase of x” is observed below T, for slowly cooled with respect to quenched samples. Here we report for the first time the detection of a superconducting phase in this compound, observed from
413
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OBSERVATION OF SUPERCONDUCTIVITY IN TDAE-Cm
d.c. magnetization and radio frequency (RF) susceptibility experiments. While rapidly cooled samples display ferromagnetic properties only, clear evidence is provided for a superconductive behaviour below 17.4 K. Superconductivity and ferromagnetism coexist at low temperatures. The powders were prepared following the standard procedure [ 11.The reaction between TDAE and a toluene solution of CW was carried out at 0°C in order to slow down its velocity and enhance the crystallinity of the product. The precipitation was completed after 5 h, during which period the formation of black glittering crystals was observed. All the operations were performed under monitored absence of oxygen and moisture. The powder samples were sealed in glass (or quartz) ampoules under 1 mbar He atmosphere in order to ensure thermal contact with the container at low temperature. The glass or quartz contribution to the magnetic measurements was accurately subtracted. In some cases the powders were made into pellets; d.c. and RF magnetic measurements yield the same results in both pellets and powders. The magnetization measurements were performed with a superconducting quantum interference device (SQUID) by Quantum Design Instruments. The RF susceptibility was measured with a Hewlett-Packard HP4191 impedance analyzer connected via a X/2 coaxial cable to a coil containing the sample, within a helium flow cryostat. The RF experiment was performed at fixed frequency (v = 44 MHz), with an a.c. field amplitude of lo-’ Oe. SQUID magnetometry on samples which were rapidly cooled (20 Kmin-‘) from room temperature (RT) down to 20 K reveals the presence of the magnetic phase below 16.5 K, with a hysteresis loop at 5 K. The hysteresis loop closes at H = *lOOOe with magnetization M = 0.92emug-‘; for H > 100 Oe, M(H) slowly increases without reaching full saturation (M = 1.6 emu g -’ at 20000 Oe). This behavior has already been reported [14, 151 and is consistent with either spin glass or weak ferromagnetism (i.e. a canted spin structure). The measured magnetization corresponds to a ferromagnetic component of the local moment, Pi, of 0.15 and 0.26 &Cso, respectively, at 100 and 20 000 Oe, somewhat higher than any previously reported values [14, 151. pSR experiments in zero applied magnetic field, to be reported in detail elsewhere [lo], were performed on the same sample and show the presence of two muon precession frequencies. A total 80(20)% fraction of the implanted muons at 3.1 K experience a local magnetic field of 69(2) and 38(l) Gauss and a static field distribution width (AL*)‘” of 20(3) and 15(3) Gauss, respectively. The local field distribution is similar to (albeit
Vol. 101, No. 6
narrower than) that of previous @R experiments [9, lo] and that observed by ESR on single crystals [ll]. Therefore, when quenched from high temperatures, our samples show identical magnetic properties to the best TDAE-Cso samples reported so far in the literature. If the sample is slowly cooled from room temperature a large diamagnetic response appears below T, = 17.4 K. Figure 1 shows the magnetization (solid circles) measured after the application of an external field of 3 Oe at 20 K, in a field cooling (FC) procedure. The sample was cooled in zero magnetic field from room temperature (RT) to 20K at the rate of 3 Kmin-‘, except for the ranges 180-160 K and 50-30 K, which correspond to the two transitions in molecular reorientations detected by 13C NMR [8, 111, where the cooling rate was further slowed down to 0.3 Kmin-‘. The direction of the sample magnetization below T, is opposite to that of the applied field H,. The diamagnetic response was checked by following the same protocol in a reversed external field (solid triangles in Fig. 1): M always displays the opposite direction with respect to H,. This check was performed to ascertain the inthrence of a small magnetic flux trapped in the superconducting coil of the magnetometer, which is responsible for the slight asymmetry between the *3 Oe experiments. A simple estimate of the residual field is the SQUID yields Hres=s,sOe. This value represents the component parallel to H, of a possibly non-uniform trapped field. It remained constant throughout the whole series of SQUID experiments, but it prevented a true zero field cooling measurement.
4
y
1 H,=+3.0 Oe
I
.
-6
E
.
t
0
5
10 15 20 Temperature
FC
25 30 [K]
1 35
Fig. 1. SQUID magnetization as a function of temperature in H, = 3 Oe (circles) and Z& = -3 Oe (triangles) after slow cooling (see text). Inset: Field cooling magnetization at 5 K as a function of H = H,, + 0.5 Oe (the vertical scale is the same as for the main panel).
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OBSERVATION OF SUPERCONDU(;TIVITY IN TDAE-C,
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The temperature dependence of M is that expected of a superconducting sample. Similar diamagnetic curves are observed whenever the same protocol is followed with Hdr6,5 Oe (where H = Ho + H,,). The maximum diamagnetic response in a pellet sample is obtained at H = 1.5 Oe, the Meissner fraction being 30 * 10%. The inset of Fig. 1 represents the value of the magnetization at 5 K obtained by the same thermal protocol, as a function of H. The observed behavior can be explained as the coexistence of a superconducting and a ferromagnetic response, almost exactly cancelling each other for H = 6.5 Oe. Interestingly the ferromagnetic response itself is enhanced by the slow cooling protocol, in qualitative agreement with [13]: Mat 100 Oe and 5 K is a factor 1.7 larger than in the quenched sample experiment. Further evidence of a superconducting fraction is provided by the RF susceptibility powder data of Fig. 2. When following the cooling protocol described above the real part of the susceptibility, shown in panel (b), displays a diamagnetic response which prevails between 15 K and 17.4 K. We must emphasize that the value x0 of the flat response above T, = 17.4K represents the true zero of x’ on the plotted scale. The same value is obtained in high external fields, when both ferromagnetic and superconducting contributions are quenched. This proves that, between 15 K and T, , x’ is negative. Indications of a similar behavior are present in earlier data [13,16], although a different explanation was given there, in terms of a spin glass dispersive mode at 500 Hz. The appearance of a large negative response both in the steady state and at 44MHz rules out such interpretation.
The inset (c) shows a tentative separation of the superconducting and ferromagnetic contributions, xs and u, respectively. Following [17] the temperature dependence of xs for small grains is modeled as xsO’-I=
xs~O)[~]z=xs(o)[l - (;)4], (1)
where the simple two-fluid model [ 181 is assumed for the temperature dependence of the penetration depth h. Similar assumptions lead also to a satisfactory fit of the SQUID superconducting response. The quantity u is the difference x’ - xs, obtained by choosing the lowest value of ~~(0) such that a(T) > 0 at all temperatures. The absolute value of x,r is comparable to that of xF (from an order of magnitude estimate x_r= 0.1). The only possible origin of such a large negative term is superconductivity. Figure 2(b) shows the dissipative response, x”, which cannot have contributions from a Meissner regime superconductor, for v << kBTc/h. Hence it may be predominantly ascribed to the ferromagnetic component. It is not surprising that x” resembles xF closely, as in a ferromagnet both the real and the imaginary part of the susceptibility are proportional to the ordered moment. In particular both x” and ti show an abrupt change of slope at T * = 14 K (the vertical dotted line in the figure), which is therefore due to the spin part of the response. The application of a static magnetic field H, parallel to the RF field, increases xF and thus modifies the RF response, as shown in Fig. 3 by the dependence of x’ upon H. The horizontal solid line coincides with the value of x0 of Fig. 2(b). The dashed curve in the figure shows the numerical integral of the response, i.e. Jll (x’(h) - XcJdk which is proportional to the total static magnetization. It consists of the sum of a diamagnetic term, with constant negative slope, dominant at I # l
-a
s __.*
.
--
3
(d
I
.
I
cd..
____-----_ __--__-. .
x0 -d
I’,, r
U
x
1 I
10.0
12.5
I
I
15.0
Temperature
L
I
I
17.5
I 20.0
[K]
Fig. 2. A.c. response as a function of temperature: (a) x’ (T); (b) x’(T); (c) separation of superconductive and magnetic contributions to x’ (see text).
T=15.7K
-a
0
I
I
50 H
_
100
150
[OeI
Fig. 3. x’ at T = 15.7K as a function of H. The dashed curve represents jz(x’(h) - x,&k. The horizontal line indicates the x0 value.
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OBSERVATION OF SUPERCONDUCTIVITY IN TDAE-Cm
low fields, plus an S-shaped ferromagnetic term. The latter presumably starts off flat, approximately as a first-magnetization curve. D.c. resistivity was also measured on a sintered pellet, in zero magnetic field (H < lOmOe), by 4- and 2-wire methods, carefully avoiding exposure to air at any time. Our results agree with previous measurements [3] down to 150% the resistance being too large to be measured below that temperature. No zero resistance transition was detected. This may be due to the absence of percolation among superconducting domains. However similar negative results were obtained in an identical measurement on a K,C& pellet, which shows a full Meissner effect (actually to our knowledge resistivity transitions were never observed in A3Cm pellets). We take the a.c. susceptibility and d.c. magnetization data as conclusive evidence for the presence of coexisting superconducting (SC) and ferromagnetic (FM) properties. The sizeable Meissner fraction, 0.3(l), together with the large proportion, 0.8(2), of implanted muons experiencing a net spontaneous local field indicate that both SC and FM are intrinsic properties of TDAE+,. A complete picture is clearly still lacking. It must further take into account the antiferromagnetic exchange interactions invoked by Blinc et al. [ll], which, incidentally, would not be antithetical to superconductivity. Our results prove that three main ingredients should be included: merohedral order, magnetic interactions and pairing interactions. As a last remark it is tempting to connect the observed coexistence of superconductivity and magnetism to the inherent instabilities of quasi-one-dimensional systems, which may lead to the formation of charge and spin density waves. A closely related subject is that of intrinsic phase separation, invoked by Emery and Kivelson [ 191 in the context of high temperature superconductivity in copper perovskites: low dimensional strongly correlated electron systems show a tendency (see, e.g. [ZO]) to phase separate spontaneously into carrier rich and carrier depleted regions, and remarkable examples of a magnetic and a superconducting behavior coexisting in chemically homogeneous samples are documented [21]. The two competing magnetic responses in TDAE-Cm could be due to a mechanism of this kind.
4. 5. 6. 7. 8.
9. 10.
11. 12. 13. 14. 15. 16.
17. 18. 19. 20.
REFERENCES 1. 2. _ 3.
Allemand, P.M. et al., Science, 253, 1991, 301. Stephens, P.W. et al., Nature, 355, 1992,331. _ __.. Schlider, A., Koos, H., Rystau, I., Schutz, W. and
21.
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Gotschy, B., Phys. Rev. Lett., 73, 1994, 1299; Hassanien, A., MuSeviE, I., MilhailoviE, D., Omerzu, A. and Tom%, S., in Physics and Chemistry of Fullerene and Derivatives (Edited by H. Kuzmany, J. Fink, M. Mehring and S. Roth), p. 489, World Scientific, Singapore, 1995. Arovas, D.P. and Auerbach, A, Phys Rev., B52, 1995,10114. Tanaka, K. et al., Chem. Phys. Lett., 237, 1995, 123. Tanaka, K. et al., Chem. Phys. Lett., 230, 1994, 271. Venturini, P. et al., Znt. J. Mod. Phys., 6, 1992, 3947. Cristofolini, L. et al., in Progress in Fullerene Research (Edited by H. Kuzmany, J. Fink, M. Mehring and S. Roth), p. 279, World Scientific, Singapore, 1994; Ric& M., Cristofolini, L., De Renzi, R. and Ruani, P., in Fullerenes (Edited by KM. Kadish and R.S. Ruoff), p. 1187, The Electrochemical Society, Pennington NJ, 1994. Lappas, A. et al., Science, 267, 1995, 1799. Ricco, M., Cristofolini, L., De Rend, R. and Ruani, G., in Physics and Chemistry of Fullerenes and Derivatives (Edited by H. Kuzmany, J. Fink, M. Mehring and S. Roth), World Scientific, Singapore, 1995; I&&, M., De Renzi, R. and Bisbiglia, M. (to be published). Blinc, R. et al., Phys. Rev. Lett., 76, 1996, 523. Dzyaloshinsky, I., J. Phys. Chem. Solids, 4,1958, 241; Moriya, T., Phys. Rev. Lett., 4, 1960,228. Mihailovic, D., A&n, D., Venturini, P., Blinc, R., Omerzu, A. and Cevc, P., Science, 268,1995,400. Suzuki, A., Suzuki, T., Whitehead, R.J. and Maruyama, Y., Chem. Phys. Len., 223,1994,517. Tanaka, K. et al., Phys. Rev., B47, 1993,7554. MihailoviE, D., K&cvec, V., A&n, D., Venturini, P. and Cevc, P., in Prog. in Fullerene Research (Edited by H. Kuzmany, J. Fink, M. Mehring and S. Roth), p. 289, World Scientific Singapore, 1994. Neminsky, A.M., Nikolaev, P.N., Shovkun, D.V., Laukhina, E.E. and Yagubskii, E.B., Phys. Rev. Lett., 72, 1994,3092. Cyrot, M. and Pavuna, D., Introduction to Superconductivity and HTC Materials, World Scientific, Singapore, 1992. Emery, J.J., Kivelson, S.A. and Lin, H.Q., Phys. Rev. Lett., 64, 1990, 475; Emergy, V.J. and Kivelson, S.A., Physica, C209, 1993,597. Tranquada, J.M. et al., Phys. Rev. Lett., 70, 1993, 445; Cho, J.H. et al., Phys. Rev. Lett., 70, 1993, 222; Borsa, F. et al., Phys. Rev., B52,1995,7334; Bucci, C. et al. Hyperfine Int. (to appear). Weidinger, A. et al., Phys. Rev. Lett., 62, 1989, 102; Tomy, C.V. et al., Phys. Rev., B52, 1995, 9186.
PII: soo38-10!38(96)oo6o4-7
OBSERVATION OF SUPERCONDUCIIVITY IN TDAE-cm M. Ric&,” M. Bisbiglia,” R. De Renzi” and F. Bolzonib ‘Dipartimento di Fisica and Istituto Nazionale di Fisica della Materia, Universita di Parma, I-43100 Parma, Italy bIstituto MASPEC, CNR, Parma, Italy (Received 3 August 1996; accepted 30 September 1996 by E. Mohaari) SQUID magnetometry performed on the fullerene based charge transfer salt TDAE&, shows the presence of a superconducting phase below T, = 17.4K, coexisting with the already known magnetic phase. A large fraction Meissner effect (f w 0.3) appears only when the compound is slowly cooled across the temperatures corresponding to (partial) freezing of the molecular rotations. The ferromagnetic response prevails over the Meissner effect for Zf > 6 Oe. A strong diamagnetic contribution to the RF susceptibility, appearing below T,, c&inns these findings. This phenomenon sheds new light on the complementary nature of superconductivity and magnetism in low dimensional Mot&Hubbard systems. Copyright 8 1997 Elsevier Science Ltd Keywords: A. fullerenes, magnetically ordered materials, superconductors.
TDAE-CW is a fullerene based molecular magnet, which was first reported by Allemand [l] to exhibit a magnetic ordering transition at T, = 16.1 K. The ordered state possesses a sizeable macroscopic magnetization, although its exact nature and origin are still a subject of debate. It has attracted great interest for two main reasons: (i) 16K is a large critical temperature for organic ferromagnetism and an absolute record for non-polymeric organic materials; (ii) the proximity of magnetism and superconductivity (present in the related alkali-metal fullerides A&,,) is a common feature of a much larger class of highly correlated electron sys’tems, including heavy fermions, nickel borocarbides and copper perovskites. TDAE stands for tetrakis-dimethyl-amino-ethylene, one of the strongest organic donors. In the 1: 1 stoichiometry each TDAE molecule donates one electron to the band formed by the lowest unoccupied tlu molecular orbital (LIMO) of the Cm units. The crystal is monoclinic [2], with shorter C,-C, distances along the c axis, and it displays the activated d.c. transport properties characteristic of a large gap semiconductor [3], or rather of a Mot&Hubbard insulator [4]. The magnetic behavior is still controversial. Experiments on earlier samples identified signatures of weak and/or disordered magnetism. Spin glass features were seen in the a.c. susceptibility [S], cusps in x’ in the heat
capacity [6], a broad peak around T, in the electron spin resonance (ESR) [7] and muon spin rotation @SR) signals [8], as a broad distribution of static local fields. More recently long range order was inferred from the rather narrow local magnetic field distribution probed by implanted muons [9, lo] and ESR on single crystals [ll]. The strongly anisotropic ESR line intensity was interpreted in terms of an antiferromagnetic exchange along the c axis, giving rise to weak ferromagnetism of the Dzyaloshinsky-Moriya type [12]. Growing experimental evidence points towards a fundamental role played by the orientational (merohedral) order of the Jahn-Teller distorted fullerene molecules on the magnetic properties of this compound. It suggests that, when the C, orientational degrees of freedom become frozen, at the so-called phstic phase transitions, the degree of merohedral order controls the nature of the exchange paths among neighbor molecules. In particular recent a.c. susceptibility and ESR measurements [ 131show large variations of the magnetic response upon different cooling protocols, from the room temperature plastic phase across the first freezing temperature of the rotational dynamics of C!,, T,, - 170 K. A ten-fold increase of x” is observed below T, for slowly cooled with respect to quenched samples. Here we report for the first time the detection of a superconducting phase in this compound, observed from
413
414
OBSERVATION OF SUPERCONDUCTIVITY IN TDAE-Cm
d.c. magnetization and radio frequency (RF) susceptibility experiments. While rapidly cooled samples display ferromagnetic properties only, clear evidence is provided for a superconductive behaviour below 17.4 K. Superconductivity and ferromagnetism coexist at low temperatures. The powders were prepared following the standard procedure [ 11.The reaction between TDAE and a toluene solution of CW was carried out at 0°C in order to slow down its velocity and enhance the crystallinity of the product. The precipitation was completed after 5 h, during which period the formation of black glittering crystals was observed. All the operations were performed under monitored absence of oxygen and moisture. The powder samples were sealed in glass (or quartz) ampoules under 1 mbar He atmosphere in order to ensure thermal contact with the container at low temperature. The glass or quartz contribution to the magnetic measurements was accurately subtracted. In some cases the powders were made into pellets; d.c. and RF magnetic measurements yield the same results in both pellets and powders. The magnetization measurements were performed with a superconducting quantum interference device (SQUID) by Quantum Design Instruments. The RF susceptibility was measured with a Hewlett-Packard HP4191 impedance analyzer connected via a X/2 coaxial cable to a coil containing the sample, within a helium flow cryostat. The RF experiment was performed at fixed frequency (v = 44 MHz), with an a.c. field amplitude of lo-’ Oe. SQUID magnetometry on samples which were rapidly cooled (20 Kmin-‘) from room temperature (RT) down to 20 K reveals the presence of the magnetic phase below 16.5 K, with a hysteresis loop at 5 K. The hysteresis loop closes at H = *lOOOe with magnetization M = 0.92emug-‘; for H > 100 Oe, M(H) slowly increases without reaching full saturation (M = 1.6 emu g -’ at 20000 Oe). This behavior has already been reported [14, 151 and is consistent with either spin glass or weak ferromagnetism (i.e. a canted spin structure). The measured magnetization corresponds to a ferromagnetic component of the local moment, Pi, of 0.15 and 0.26 &Cso, respectively, at 100 and 20 000 Oe, somewhat higher than any previously reported values [14, 151. pSR experiments in zero applied magnetic field, to be reported in detail elsewhere [lo], were performed on the same sample and show the presence of two muon precession frequencies. A total 80(20)% fraction of the implanted muons at 3.1 K experience a local magnetic field of 69(2) and 38(l) Gauss and a static field distribution width (AL*)‘” of 20(3) and 15(3) Gauss, respectively. The local field distribution is similar to (albeit
Vol. 101, No. 6
narrower than) that of previous @R experiments [9, lo] and that observed by ESR on single crystals [ll]. Therefore, when quenched from high temperatures, our samples show identical magnetic properties to the best TDAE-Cso samples reported so far in the literature. If the sample is slowly cooled from room temperature a large diamagnetic response appears below T, = 17.4 K. Figure 1 shows the magnetization (solid circles) measured after the application of an external field of 3 Oe at 20 K, in a field cooling (FC) procedure. The sample was cooled in zero magnetic field from room temperature (RT) to 20K at the rate of 3 Kmin-‘, except for the ranges 180-160 K and 50-30 K, which correspond to the two transitions in molecular reorientations detected by 13C NMR [8, 111, where the cooling rate was further slowed down to 0.3 Kmin-‘. The direction of the sample magnetization below T, is opposite to that of the applied field H,. The diamagnetic response was checked by following the same protocol in a reversed external field (solid triangles in Fig. 1): M always displays the opposite direction with respect to H,. This check was performed to ascertain the inthrence of a small magnetic flux trapped in the superconducting coil of the magnetometer, which is responsible for the slight asymmetry between the *3 Oe experiments. A simple estimate of the residual field is the SQUID yields Hres=s,sOe. This value represents the component parallel to H, of a possibly non-uniform trapped field. It remained constant throughout the whole series of SQUID experiments, but it prevented a true zero field cooling measurement.
4
y
1 H,=+3.0 Oe
I
.
-6
E
.
t
0
5
10 15 20 Temperature
FC
25 30 [K]
1 35
Fig. 1. SQUID magnetization as a function of temperature in H, = 3 Oe (circles) and Z& = -3 Oe (triangles) after slow cooling (see text). Inset: Field cooling magnetization at 5 K as a function of H = H,, + 0.5 Oe (the vertical scale is the same as for the main panel).
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OBSERVATION OF SUPERCONDU(;TIVITY IN TDAE-C,
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The temperature dependence of M is that expected of a superconducting sample. Similar diamagnetic curves are observed whenever the same protocol is followed with Hdr6,5 Oe (where H = Ho + H,,). The maximum diamagnetic response in a pellet sample is obtained at H = 1.5 Oe, the Meissner fraction being 30 * 10%. The inset of Fig. 1 represents the value of the magnetization at 5 K obtained by the same thermal protocol, as a function of H. The observed behavior can be explained as the coexistence of a superconducting and a ferromagnetic response, almost exactly cancelling each other for H = 6.5 Oe. Interestingly the ferromagnetic response itself is enhanced by the slow cooling protocol, in qualitative agreement with [13]: Mat 100 Oe and 5 K is a factor 1.7 larger than in the quenched sample experiment. Further evidence of a superconducting fraction is provided by the RF susceptibility powder data of Fig. 2. When following the cooling protocol described above the real part of the susceptibility, shown in panel (b), displays a diamagnetic response which prevails between 15 K and 17.4 K. We must emphasize that the value x0 of the flat response above T, = 17.4K represents the true zero of x’ on the plotted scale. The same value is obtained in high external fields, when both ferromagnetic and superconducting contributions are quenched. This proves that, between 15 K and T, , x’ is negative. Indications of a similar behavior are present in earlier data [13,16], although a different explanation was given there, in terms of a spin glass dispersive mode at 500 Hz. The appearance of a large negative response both in the steady state and at 44MHz rules out such interpretation.
The inset (c) shows a tentative separation of the superconducting and ferromagnetic contributions, xs and u, respectively. Following [17] the temperature dependence of xs for small grains is modeled as xsO’-I=
xs~O)[~]z=xs(o)[l - (;)4], (1)
where the simple two-fluid model [ 181 is assumed for the temperature dependence of the penetration depth h. Similar assumptions lead also to a satisfactory fit of the SQUID superconducting response. The quantity u is the difference x’ - xs, obtained by choosing the lowest value of ~~(0) such that a(T) > 0 at all temperatures. The absolute value of x,r is comparable to that of xF (from an order of magnitude estimate x_r= 0.1). The only possible origin of such a large negative term is superconductivity. Figure 2(b) shows the dissipative response, x”, which cannot have contributions from a Meissner regime superconductor, for v << kBTc/h. Hence it may be predominantly ascribed to the ferromagnetic component. It is not surprising that x” resembles xF closely, as in a ferromagnet both the real and the imaginary part of the susceptibility are proportional to the ordered moment. In particular both x” and ti show an abrupt change of slope at T * = 14 K (the vertical dotted line in the figure), which is therefore due to the spin part of the response. The application of a static magnetic field H, parallel to the RF field, increases xF and thus modifies the RF response, as shown in Fig. 3 by the dependence of x’ upon H. The horizontal solid line coincides with the value of x0 of Fig. 2(b). The dashed curve in the figure shows the numerical integral of the response, i.e. Jll (x’(h) - XcJdk which is proportional to the total static magnetization. It consists of the sum of a diamagnetic term, with constant negative slope, dominant at I # l
-a
s __.*
.
--
3
(d
I
.
I
cd..
____-----_ __--__-. .
x0 -d
I’,, r
U
x
1 I
10.0
12.5
I
I
15.0
Temperature
L
I
I
17.5
I 20.0
[K]
Fig. 2. A.c. response as a function of temperature: (a) x’ (T); (b) x’(T); (c) separation of superconductive and magnetic contributions to x’ (see text).
T=15.7K
-a
0
I
I
50 H
_
100
150
[OeI
Fig. 3. x’ at T = 15.7K as a function of H. The dashed curve represents jz(x’(h) - x,&k. The horizontal line indicates the x0 value.
416
OBSERVATION OF SUPERCONDUCTIVITY IN TDAE-Cm
low fields, plus an S-shaped ferromagnetic term. The latter presumably starts off flat, approximately as a first-magnetization curve. D.c. resistivity was also measured on a sintered pellet, in zero magnetic field (H < lOmOe), by 4- and 2-wire methods, carefully avoiding exposure to air at any time. Our results agree with previous measurements [3] down to 150% the resistance being too large to be measured below that temperature. No zero resistance transition was detected. This may be due to the absence of percolation among superconducting domains. However similar negative results were obtained in an identical measurement on a K,C& pellet, which shows a full Meissner effect (actually to our knowledge resistivity transitions were never observed in A3Cm pellets). We take the a.c. susceptibility and d.c. magnetization data as conclusive evidence for the presence of coexisting superconducting (SC) and ferromagnetic (FM) properties. The sizeable Meissner fraction, 0.3(l), together with the large proportion, 0.8(2), of implanted muons experiencing a net spontaneous local field indicate that both SC and FM are intrinsic properties of TDAE+,. A complete picture is clearly still lacking. It must further take into account the antiferromagnetic exchange interactions invoked by Blinc et al. [ll], which, incidentally, would not be antithetical to superconductivity. Our results prove that three main ingredients should be included: merohedral order, magnetic interactions and pairing interactions. As a last remark it is tempting to connect the observed coexistence of superconductivity and magnetism to the inherent instabilities of quasi-one-dimensional systems, which may lead to the formation of charge and spin density waves. A closely related subject is that of intrinsic phase separation, invoked by Emery and Kivelson [ 191 in the context of high temperature superconductivity in copper perovskites: low dimensional strongly correlated electron systems show a tendency (see, e.g. [ZO]) to phase separate spontaneously into carrier rich and carrier depleted regions, and remarkable examples of a magnetic and a superconducting behavior coexisting in chemically homogeneous samples are documented [21]. The two competing magnetic responses in TDAE-Cm could be due to a mechanism of this kind.
4. 5. 6. 7. 8.
9. 10.
11. 12. 13. 14. 15. 16.
17. 18. 19. 20.
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