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Magnetic superconducting properties of LiTi2O4 single crystal
Solid State Communications, Vol. 74, No. 7, pp. 621-624, 1990. Printed in Great Britain.
0038-1098/90 $3.00 + .00 Pergamon Press plc
MAGNETIC S U P E R C O N D U C T I N G PROPERTIES OF LiTi204 SINGLE CRYSTAL O. Durmeyer 1, J.P. Kappler ~, A. Derory 2, M. Drillon 2 and J.J. Capponi 3 ~IPCMS-GEMME (UMR 46 CNRS), Universit6 Louis Pasteur, 4 rue Blaise Pascal, 67070 Strasbourg, France 2IPCMS-GMI, EHICS, 1 rue Blaise Pascal, 67008 Strasbourg, France 3Laboratoire de Cristallographie, CNRS, 38042, Grenoble, France
(Received 22 September 1989 by M. Balkanski) Magnetic superconducting properties of polycrystalline (T, = 12.7 K) and single crystal (T, = 10.7 K) samples of LiTi204 are compared. A perfect diamagnetic shielding effect is observed in the single crystals, while only 20% of the effect occurs in polycrystalline samples. Magnetization of the LiTi204 single crystal exhibits magnetic jumps at low temperature. The different characteristic parameters of the superconducting state are discussed. THE DISCOVERY of new high T, superconductors raised the possibility of novel mechanisms for explaining superconductivity. For a better understanding of the relative role of the different factors leading to high T~ superconductors, the study of spinel compounds and especially of LiTi204, as was previously pointed out [1] seems to be attractive [4, 10-12]. Specific heat data and transport properties establish the stoichiometric polycrystalline sample as a weak coupling d-band superconductor with superconducting state properties well described by the BCS theory. Recently, the growth possibility of large single crystal has increased the interest of the superconducting properties of the LiTi204 spinel oxide. In this letter, we propose a comparative study of different kinds of LiTi204 samples. Particular attention is paid to the magnetic properties in the superconducting state (T < T, = 12.4K). The sample denoted A is a polycrystalline sample prepared in the manner previously described [1]. The samples denoted B and C are single crystals of mass 30 mg and 8 mg respectively. A picture of crystal C is displayed in Fig. 1. They were prepared by electrolysing at 780°C a molten bath containing NaBO2, LiBO2, NaF and TiO2 [2]. They grew as millimeter size octahedra along the (1 00) direction on a titanium wire cathode. Refinements of X-ray single crystal data taken on a four circle diffractometer give a lattice parameter of 8.4090 (5) and a full site occupation for Li (tetrahedral site) and Ti (octahedral site), as represented by the nominal formula Li[Ti3÷ Ti 4+ ]04. The magnetic measurements have been performed in magnetic field up to 2T in the temperature range 2-10K. The magnetic field resolution is about
10 2mT for H < 20mT. The different magnetic behaviour of the three samples are illustrated in Fig. 2 where the magnetization M (H, 4.2 K) is plotte d as a function of field up to ! T. All the samples exhibit the characteristic type II superconducting magnetic behaviour, but with different characteristic parameters, namely flux exclusion, low and high critical fields (He,, He2), Ginzburg-Landau parameter to. The flux exclusion due to the diamagnetic shielding in a layer depth 2(T) (the field penetration depth at the surface of the sample), given by the initial slope M (H), is for the two single crystals B and C of the order of 100% of the theoretical value ( - 1/(4g) emucm 3). Contrastingly only 20% of this perfect diamagnetic shielding effect is observed for the polycrystalline sample. The thermal variation of the magnetization in very low field (field cooled, H < 5mT), i.e. the Meissner effect, is reduced by a factor 2.5 with respect to the diamagnetic shielding effect for the polycrystalline sample and by a factor 6.5 for the single crystals. The lower critical field H,,, defined by the departure from the linearity of the first magnetization curve, corresponds to the field for which the first flux enters in a type II superconductors (i.e. ~c >> 1). Correcting all the data by an appropriate demagnetizing factor, we found a nearly independent sample value, H,. (4.2K) = 15mT. The temperature dependence of H,,, for the LiTi204 single crystal B follows near T~ and up to 0.4Tc a linear behaviour (Fig. 3) Hc,(t) = H,,(0) ( 1 - t) (1) with t = T/T, and H,,(0) = 23 + 2mT. In order to estimate the temperature dependence of 2(0, namely the field penetration depth, we used the approximate formula: Hc,(t) = (~o/4FIZz(t)) log• (2) where K value is taken
621
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MAGNETIC SUPERCONDUCTING PROPERTIES
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LiTi204 single crystal
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Fig. 3. H,, versus temperature plot for LiTi204 single crystal B. Fig. 1. Picture of a crystal of LiTi204. Crystal C with mass: 8 mg.
of the samples, and especially to the presence of impurities becoming in some manner pinning centers at high field, as described below. The value of, from [3] for polycrystalline samples. Using ~c = 2/~, the normal-superconducting transition temperature we found for t = 0 2(0) = 2000/k and ~(0) = 19/k T~, extracted from the ZFC and FC data, is 12.7K for which correspond to reported values for powdered the sample A, while T, (i.e. at low field H < 5 mT) is samples [4, 5]. The value of ~cis supposed to be nearly only 10.7 K for the two single crystals. These charactemperature independent. The expressions (1) and (2) teristic temperatures are in agreement with those ofH~,, permit us to write 2 as a function o f ( l - t) 1/2 observed in resistivity measurements [1, 9]. This This experimental variation is in good agreement, in observed discrepancy between the powder and the the weak coupling local limit with sufficient short single crystal cannot be attributed to an off stoichiovalue of the coherence length ~(t), with the conven- metric effect of oxygen content that is unlikely in a tional theory [3] where H~, ~ 2 2 ~ ( l - - t ) , near T~. spinel compound. Hence, we conclude that the observation of H~'s The nonstoichiometries most commonly conlinear variation near T~, as in Ginzburg-Landau sidered are substitution of Li to Ti, Li[Li,Ti 2 ~]O4 and theory, is consistent with a clean local-limit BCS form. occupation of some tetrahedral sites by Ti, Li~ ~Ti~ Concerning the higher critical field H~,, we observe (LixTi2 x)O4 [10]. These types would affect the Li/Ti very different values at 4.2 K. These large discrepancies ratio, and are not expected in single crystals as can be attributed to the preparation and to the quality checked by X-ray data refinements. Furthermore they are excluded by the lattice parameter values in powder as in single crystal. Also there is a third type of cation non stoichiometry, namely the variability of x in Li~ ,Ti204, as E o ° - - - shown by Murphy et al. [6]. Small variations o f x yield E ® ~ \'%% changes in T~ from T~ = ll K (x = 0) to T( = 13K • -o \ o (x = 0.2) [4]. As the nominal stoichiometry has been -2.~ -~. ~ -, ~,.~. o~.~ _ found in the crystals, we can imagine that the powder •~ N are slightly off stoichiometric in Li. Further studies are .N ,, " I in progress on the link between Li stoichiometry and "~, • I "N T~ on single crystal. g c '- . . . . '0 The Fig. 4 illustrates the magnetic hysteresis curve -5 I I ; "~ ~ on the single crystal C at 4.2 K. One notices the pres0 --0.2 0.4 0.6 0.8 ence of large magnetization jumps, which have already H (T) been observed in classical superconductors [7] and Fig. 2. First magnetization curves at 4.2 K for the three samples: A Polycrystal. B Single crystal. C Single also, recently, on the high T~ Y - B a - C u - O system [8]. crystal: H It (a-b) plane (basis of the octahedron). At 4.2 K, no flux jump is observed on the initial magnetization (first quadrant). In turn, instabilities occur Inset: Initial slope of the magnetization curves at 4.2 K for H ~ 50mT. See text about the definition of H,]. in the low field region for decreasing fields (second and - 2
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Fig. 4. Magnetization versus field for LiTi204 single crystal C at 4.2 K up to l T. H II (a-b) plane: basis of the octahedron.
Fig. 5. Magnetization versus field for LiTi204 single crystal C at 2 K up to 1 T. H 1[(a-b) plane: basis of the octahedron.
fourth quadrant), and become closely spaced over a wide field range in the third quadrant, namely reversed field. On traversing the second cycle, it could be shown that the number of flux jumps in the first and third quadrant are equal; the positions of these instabilities are quite reproducible from cycle to cycle. At 2 K (see Fig. 5) more instabilities with small values are observed, but as well for the flux penetration as for the flux expulsion. They are observed all along the hysteresis cycle between an outer envelope where the magnetization is unstable and an inner state envelope. These erratic magnetization jumps disappear at higher temperature (T = 7K). Note that these effects depend strongly on the field sweep rate plus waiting time between two measurements. They can be attributed to an avalanche effect in the vortex line movement and related to the specific heat and thermal-:onductivity of the samples. When the applied field is changed, moving flux lines produce local heating in the immediate neighbourhood of pinning centers which cannot be released in the experiment time. Then a cascade process is initiated from the small initial movements of flux. The jumps stop when the associated energy flow can be absorbed by the sample and its surrounding [8]. Using the Bean formula, J, = 30AM/d (AM in emu cm 3 and d is an average sample section value in cm) in a given field, the critical current can be estimated from the magnetic hysteresis curve or its envelope when magnetization jumps are seen. In fact, the existence of flux jumps strongly decreases the critical current at low temperature as shown by the comparison of curves without such magnetization jumps. For the single crystal C, an average value of d = 0.1 cm gives a critical current density J, (H ~ 0) about 4 x 103Acre 2 for T < 4.2K. the same calculation performed on the LiTi,O4 polycrystal leads to J, (H ~ 0) ~ 2 0 A c m 2, in agreement with the
previous value of Jc ~ 1 0 A c m 2 extracted from resistivity experiments [1, 9]. The existence of jumps in the magnetic hysteresis is obviously a limitation of the critical current, at a given field and temperature. The systematic study of such a hysteresis as a function of the composition, preparation and sample size will permit to increase the values ofJ~. In fact we noted that the current densities vary by a factor of about 200 between polycrystalline samples and the single crystal C. It is suspected that either the low density or the grain boundary effects are important in limiting J~ in some manner. Moreover, the formation of large pinning centers at sufficient high field (H > 0.3 T) due to superconducting defects in the normal state or the presence of other pinning impurity, can enhance the value of Jc (i.e. for the single crystal C can be explained by the presence of such large superconducting pinning centers). Experimental results demonstrate that the superconducting magnetic properties of different LiTi204 superconductor samples are strongly dependent on the preparation. Large discrepancies in the low temperature magnetization curves and superconducting temperatures are observed between polycrystalline and single crystal samples. But further experiments (microcalorimetry, magnetoresistance and magnetization at very low fields) are in progress to draw definite conclusions. REFERENCES 1. 2. 3. 4.
J.M. Heintz, M. Drillon, R. Kuentzler, Y. Doseman, J.P. Kappler & F. Gautier, Z. fur Physik, 76, 303 (1989). J.J. Capponi, to be published. Tinkham, Introduction to Superconductivity. M.R. Harrison, P.P. Edwards & J.B. Goodenough, Philo. Meg. 52, 679 (1985).
624 5. 6.
.
8.
MAGNETIC SUPERCONDUCTING PROPERTIES M. Itoh, Y. Hasegawa, H. Yasuoka, Y. Ueda & K. Kosuge, Physica 157, 65 (1989). D.W. Murphy, M. Greenblatt, S.M. Zahhurak, R.J. Cava, J.V. Waszczak, G.W. Hull, Jr & R.S. Hutton, Rev. Chim. Min. 19, 441 (1982). J.H.P. Watson, J. Appl. Phys. 3"7, 516 (1966). J.L. Tholence, H. Noel, J.C. Lever, M. Potel & P. Gougeon, Solid State Commun. 65, 1131 (1988).
9. 10. 11. 12.
Vol. 74, No. 7
O. Durmeyer, to be published. D.C. Johnston, J. Low Temp. Phys. 25, 145 (1976). S. Sapathy & R.M. Martin, Phys. Rev. B 36, 7269 (1987). S. Massida, J. Yu & A.J. Freeman, Phys. Rev. B 38, 11352 (1988).
0038-1098/90 $3.00 + .00 Pergamon Press plc
MAGNETIC S U P E R C O N D U C T I N G PROPERTIES OF LiTi204 SINGLE CRYSTAL O. Durmeyer 1, J.P. Kappler ~, A. Derory 2, M. Drillon 2 and J.J. Capponi 3 ~IPCMS-GEMME (UMR 46 CNRS), Universit6 Louis Pasteur, 4 rue Blaise Pascal, 67070 Strasbourg, France 2IPCMS-GMI, EHICS, 1 rue Blaise Pascal, 67008 Strasbourg, France 3Laboratoire de Cristallographie, CNRS, 38042, Grenoble, France
(Received 22 September 1989 by M. Balkanski) Magnetic superconducting properties of polycrystalline (T, = 12.7 K) and single crystal (T, = 10.7 K) samples of LiTi204 are compared. A perfect diamagnetic shielding effect is observed in the single crystals, while only 20% of the effect occurs in polycrystalline samples. Magnetization of the LiTi204 single crystal exhibits magnetic jumps at low temperature. The different characteristic parameters of the superconducting state are discussed. THE DISCOVERY of new high T, superconductors raised the possibility of novel mechanisms for explaining superconductivity. For a better understanding of the relative role of the different factors leading to high T~ superconductors, the study of spinel compounds and especially of LiTi204, as was previously pointed out [1] seems to be attractive [4, 10-12]. Specific heat data and transport properties establish the stoichiometric polycrystalline sample as a weak coupling d-band superconductor with superconducting state properties well described by the BCS theory. Recently, the growth possibility of large single crystal has increased the interest of the superconducting properties of the LiTi204 spinel oxide. In this letter, we propose a comparative study of different kinds of LiTi204 samples. Particular attention is paid to the magnetic properties in the superconducting state (T < T, = 12.4K). The sample denoted A is a polycrystalline sample prepared in the manner previously described [1]. The samples denoted B and C are single crystals of mass 30 mg and 8 mg respectively. A picture of crystal C is displayed in Fig. 1. They were prepared by electrolysing at 780°C a molten bath containing NaBO2, LiBO2, NaF and TiO2 [2]. They grew as millimeter size octahedra along the (1 00) direction on a titanium wire cathode. Refinements of X-ray single crystal data taken on a four circle diffractometer give a lattice parameter of 8.4090 (5) and a full site occupation for Li (tetrahedral site) and Ti (octahedral site), as represented by the nominal formula Li[Ti3÷ Ti 4+ ]04. The magnetic measurements have been performed in magnetic field up to 2T in the temperature range 2-10K. The magnetic field resolution is about
10 2mT for H < 20mT. The different magnetic behaviour of the three samples are illustrated in Fig. 2 where the magnetization M (H, 4.2 K) is plotte d as a function of field up to ! T. All the samples exhibit the characteristic type II superconducting magnetic behaviour, but with different characteristic parameters, namely flux exclusion, low and high critical fields (He,, He2), Ginzburg-Landau parameter to. The flux exclusion due to the diamagnetic shielding in a layer depth 2(T) (the field penetration depth at the surface of the sample), given by the initial slope M (H), is for the two single crystals B and C of the order of 100% of the theoretical value ( - 1/(4g) emucm 3). Contrastingly only 20% of this perfect diamagnetic shielding effect is observed for the polycrystalline sample. The thermal variation of the magnetization in very low field (field cooled, H < 5mT), i.e. the Meissner effect, is reduced by a factor 2.5 with respect to the diamagnetic shielding effect for the polycrystalline sample and by a factor 6.5 for the single crystals. The lower critical field H,,, defined by the departure from the linearity of the first magnetization curve, corresponds to the field for which the first flux enters in a type II superconductors (i.e. ~c >> 1). Correcting all the data by an appropriate demagnetizing factor, we found a nearly independent sample value, H,. (4.2K) = 15mT. The temperature dependence of H,,, for the LiTi204 single crystal B follows near T~ and up to 0.4Tc a linear behaviour (Fig. 3) Hc,(t) = H,,(0) ( 1 - t) (1) with t = T/T, and H,,(0) = 23 + 2mT. In order to estimate the temperature dependence of 2(0, namely the field penetration depth, we used the approximate formula: Hc,(t) = (~o/4FIZz(t)) log• (2) where K value is taken
621
622
MAGNETIC SUPERCONDUCTING PROPERTIES
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of the samples, and especially to the presence of impurities becoming in some manner pinning centers at high field, as described below. The value of, from [3] for polycrystalline samples. Using ~c = 2/~, the normal-superconducting transition temperature we found for t = 0 2(0) = 2000/k and ~(0) = 19/k T~, extracted from the ZFC and FC data, is 12.7K for which correspond to reported values for powdered the sample A, while T, (i.e. at low field H < 5 mT) is samples [4, 5]. The value of ~cis supposed to be nearly only 10.7 K for the two single crystals. These charactemperature independent. The expressions (1) and (2) teristic temperatures are in agreement with those ofH~,, permit us to write 2 as a function o f ( l - t) 1/2 observed in resistivity measurements [1, 9]. This This experimental variation is in good agreement, in observed discrepancy between the powder and the the weak coupling local limit with sufficient short single crystal cannot be attributed to an off stoichiovalue of the coherence length ~(t), with the conven- metric effect of oxygen content that is unlikely in a tional theory [3] where H~, ~ 2 2 ~ ( l - - t ) , near T~. spinel compound. Hence, we conclude that the observation of H~'s The nonstoichiometries most commonly conlinear variation near T~, as in Ginzburg-Landau sidered are substitution of Li to Ti, Li[Li,Ti 2 ~]O4 and theory, is consistent with a clean local-limit BCS form. occupation of some tetrahedral sites by Ti, Li~ ~Ti~ Concerning the higher critical field H~,, we observe (LixTi2 x)O4 [10]. These types would affect the Li/Ti very different values at 4.2 K. These large discrepancies ratio, and are not expected in single crystals as can be attributed to the preparation and to the quality checked by X-ray data refinements. Furthermore they are excluded by the lattice parameter values in powder as in single crystal. Also there is a third type of cation non stoichiometry, namely the variability of x in Li~ ,Ti204, as E o ° - - - shown by Murphy et al. [6]. Small variations o f x yield E ® ~ \'%% changes in T~ from T~ = ll K (x = 0) to T( = 13K • -o \ o (x = 0.2) [4]. As the nominal stoichiometry has been -2.~ -~. ~ -, ~,.~. o~.~ _ found in the crystals, we can imagine that the powder •~ N are slightly off stoichiometric in Li. Further studies are .N ,, " I in progress on the link between Li stoichiometry and "~, • I "N T~ on single crystal. g c '- . . . . '0 The Fig. 4 illustrates the magnetic hysteresis curve -5 I I ; "~ ~ on the single crystal C at 4.2 K. One notices the pres0 --0.2 0.4 0.6 0.8 ence of large magnetization jumps, which have already H (T) been observed in classical superconductors [7] and Fig. 2. First magnetization curves at 4.2 K for the three samples: A Polycrystal. B Single crystal. C Single also, recently, on the high T~ Y - B a - C u - O system [8]. crystal: H It (a-b) plane (basis of the octahedron). At 4.2 K, no flux jump is observed on the initial magnetization (first quadrant). In turn, instabilities occur Inset: Initial slope of the magnetization curves at 4.2 K for H ~ 50mT. See text about the definition of H,]. in the low field region for decreasing fields (second and - 2
f
MAGNETIC SUPERCONDUCTING PROPERTIES
Vol~ 74, No. 7 6
623
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T=2K 0
•
4
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,
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Fig. 4. Magnetization versus field for LiTi204 single crystal C at 4.2 K up to l T. H II (a-b) plane: basis of the octahedron.
Fig. 5. Magnetization versus field for LiTi204 single crystal C at 2 K up to 1 T. H 1[(a-b) plane: basis of the octahedron.
fourth quadrant), and become closely spaced over a wide field range in the third quadrant, namely reversed field. On traversing the second cycle, it could be shown that the number of flux jumps in the first and third quadrant are equal; the positions of these instabilities are quite reproducible from cycle to cycle. At 2 K (see Fig. 5) more instabilities with small values are observed, but as well for the flux penetration as for the flux expulsion. They are observed all along the hysteresis cycle between an outer envelope where the magnetization is unstable and an inner state envelope. These erratic magnetization jumps disappear at higher temperature (T = 7K). Note that these effects depend strongly on the field sweep rate plus waiting time between two measurements. They can be attributed to an avalanche effect in the vortex line movement and related to the specific heat and thermal-:onductivity of the samples. When the applied field is changed, moving flux lines produce local heating in the immediate neighbourhood of pinning centers which cannot be released in the experiment time. Then a cascade process is initiated from the small initial movements of flux. The jumps stop when the associated energy flow can be absorbed by the sample and its surrounding [8]. Using the Bean formula, J, = 30AM/d (AM in emu cm 3 and d is an average sample section value in cm) in a given field, the critical current can be estimated from the magnetic hysteresis curve or its envelope when magnetization jumps are seen. In fact, the existence of flux jumps strongly decreases the critical current at low temperature as shown by the comparison of curves without such magnetization jumps. For the single crystal C, an average value of d = 0.1 cm gives a critical current density J, (H ~ 0) about 4 x 103Acre 2 for T < 4.2K. the same calculation performed on the LiTi,O4 polycrystal leads to J, (H ~ 0) ~ 2 0 A c m 2, in agreement with the
previous value of Jc ~ 1 0 A c m 2 extracted from resistivity experiments [1, 9]. The existence of jumps in the magnetic hysteresis is obviously a limitation of the critical current, at a given field and temperature. The systematic study of such a hysteresis as a function of the composition, preparation and sample size will permit to increase the values ofJ~. In fact we noted that the current densities vary by a factor of about 200 between polycrystalline samples and the single crystal C. It is suspected that either the low density or the grain boundary effects are important in limiting J~ in some manner. Moreover, the formation of large pinning centers at sufficient high field (H > 0.3 T) due to superconducting defects in the normal state or the presence of other pinning impurity, can enhance the value of Jc (i.e. for the single crystal C can be explained by the presence of such large superconducting pinning centers). Experimental results demonstrate that the superconducting magnetic properties of different LiTi204 superconductor samples are strongly dependent on the preparation. Large discrepancies in the low temperature magnetization curves and superconducting temperatures are observed between polycrystalline and single crystal samples. But further experiments (microcalorimetry, magnetoresistance and magnetization at very low fields) are in progress to draw definite conclusions. REFERENCES 1. 2. 3. 4.
J.M. Heintz, M. Drillon, R. Kuentzler, Y. Doseman, J.P. Kappler & F. Gautier, Z. fur Physik, 76, 303 (1989). J.J. Capponi, to be published. Tinkham, Introduction to Superconductivity. M.R. Harrison, P.P. Edwards & J.B. Goodenough, Philo. Meg. 52, 679 (1985).
624 5. 6.
.
8.
MAGNETIC SUPERCONDUCTING PROPERTIES M. Itoh, Y. Hasegawa, H. Yasuoka, Y. Ueda & K. Kosuge, Physica 157, 65 (1989). D.W. Murphy, M. Greenblatt, S.M. Zahhurak, R.J. Cava, J.V. Waszczak, G.W. Hull, Jr & R.S. Hutton, Rev. Chim. Min. 19, 441 (1982). J.H.P. Watson, J. Appl. Phys. 3"7, 516 (1966). J.L. Tholence, H. Noel, J.C. Lever, M. Potel & P. Gougeon, Solid State Commun. 65, 1131 (1988).
9. 10. 11. 12.
Vol. 74, No. 7
O. Durmeyer, to be published. D.C. Johnston, J. Low Temp. Phys. 25, 145 (1976). S. Sapathy & R.M. Martin, Phys. Rev. B 36, 7269 (1987). S. Massida, J. Yu & A.J. Freeman, Phys. Rev. B 38, 11352 (1988).