Specific Heat And Magnetic Properties Of Nd0.5sr 0.5mno3 And R0.5ca0.5mno 3 (r=nd, Sm, Dy, And Ho)

Magnetization and specific heat measurements of Nd0.5Sr 0.5Mn3 and R0.5Ca0.5MnO3 were performed. The entropy variation corresponding to the CO transition was higher than the corresponding to the ferromagnetic transition. The specific heat curves showed an abrupt change in slope for the two compounds with Nd 3+ ions, which were correlated with the corresponding antiferromagnetic transition.94743954399Radaelli, P.G., Cox, D.E., Marezio, M., Cheong, S.-W., (1997) Phys. Rev. B, 55, p. 3015Zhao, G., Ghosh, K., Greene, R.L., (1998) J. Phys. Condens. Matter, 10, pp. L737Moritomo, Y., (1999) Phys. Rev. B, 60, p. 10374Xiao, G., Gong, G.Q., Canedy, C.L., McNiff E.J., Jr., Gupta, A., (1997) J. Appl. Phys., 81, p. 5324Mori, S., Chen, C.H., Cheong, S.-W., (1998) Nature (London), 392, p. 473López, J., Lisboa-Filho, P.N., Passos, W.A.C., Ortiz, W.A., Araujo-Moreira, F.M., (2001) J. Magn. Magn. Mater., 226, p. 507http://arxiv.org/abs/cond-mat/0004460López, J., Lisboa Filho, P.N., Passos, W.A.C., Ortiz, W.A., Araujo-Moreira, F.M., Ghosh, K., De Lima, O.F., Schaniel, D., (2001) Phys. Rev. B, 63, p. 224422http://arxiv.org/abs/cond-mat/0103305Salamon, M.B., Jaime, M., (2001) Rev. Mod. Phys., 73, p. 583Ghivelder, L., Abrego Castillo, I., Gusmão, M.A., Alonso, J.A., Cohen, L.F., (1999) Phys. Rev. B, 60, p. 12184Schnelle, W., Poddar, A., Murugaraj, P., Gmelin, E., Kremer, R.K., Sasaki, K., Maier, J., (2000) J. Phys.: Condens. Matter, 12, p. 4001Gordon, J.E., Fisher, R.A., Jia, Y.X., Phillips, N.E., Reklis, S.F., Wright, D.A., Zettl, A., (1999) Phys. Rev. B, 59, p. 127Bartolomé, F., Bartolomé, J., Castro, M., Melero, J.J., (2000) Phys. Rev. B, 62, p. 1058Smolyaninova, V.N., Ghosh, K., Greene, R.L., (1998) Phys. Rev. B, 58, pp. R14725Smolyaninova, V.N., Biswas, A., Zhang, X., Kim, K.H., Kim, B.-G., Cheong, S.-W., Greene, R.L., (2000) Phys. Rev. B, 62, pp. R6093López, J., Lisboa-Filho, P.N., De Lima, O.F., Araujo-Moreira, F.M., (2002) J. Magn. Magn. Mater., 242, p. 683http://arxiv.org/abs/cond-mat/0105571López, J., Lisboa-Filho, P.N., De Lima, O.F., Araujo-Moreira, F.M., (2002) Phys. Rev. B, 66, p. 214402http://arxiv.org/abs/cond-mat/0208381Filho, L., Zanetti, P.N.S.M., Leite, E.R., Ortiz, W.A., (1999) Mater. Lett., 38, p. 289Kajimoto, R., Yoshizawa, H., Kawano, H., Kuwahara, H., Tokura, Y., Ohoyama, K., Ohashi, M., (1999) Phys. Rev. B, 60, p. 9506Millange, F., De Brion, S., Chouteau, G., (2000) Phys. Rev. B, 62, p. 5619Tokura, Y., Tomioka, Y., (1999) J. Magn. Magn. Mater., 200, p. 1Terai, T., Sasaki, T., Kakeshita, T., Fukuda, T., Saburi, T., Kitagawa, H., Kindo, K., Honda, M., (2001) Phys. Rev. B, 61, p. 3488Mathieu, R., Nordblad, P., Raju, A.R., Rao, C.N.R., (2001), http://arxiv.org/abs/cond-mat/0106606Ramirez, A.P., Schiffer, P., Cheong, S.-W., Chen, C.H., Bao, W., Palstra, T.T.M., Gammel, P.L., Zegarski, B., (1996) Phys. Rev. Lett., 76, p. 3188Raychaudhuri, A.K., Guha, A., Das, I., Rawat, R., Rao, C.N.R., (2001) Phys. Rev. B, 64, p. 165111Shimomura, S., Tajima, K., Wakabayashi, N., Kobayashi, S., Kuwahara, H., Tokura, Y., (1999) J. Phys. Soc. Jpn., 68, p. 194

Melting Line With Quantum Correction In A Melt-textured Yba2cu3o7-δ Sample

The irreversibility line for a high quality melt-textured YBa2Cu3O7-δ sample was determined in two orientations of the field H relative to the sample c axis. In both orientations we obtain good fits using a melting equation with quantum correction to describe the Abrikosov-lattice melting. The angular-scaling rule for uniaxially anisotropic materials is verified, giving additional support for the melting hypothesis. Our results are similar to those found in the literature for very clean untwinned samples. We suggest that the Abrikosov-lattice melting is weakly affected by quenched disorder, at least in the relatively low fields (H≤50 kOe) and high temperatures (T/Tc>0.8) probed in this work. © 1995 The American Physical Society.51149383938

Exponential Critical State Model Applied To Ac Susceptibility Data For The Superconductor Yba2cu3o7-δ

We derive new expressions for the average magnetization loops, M(H), based on the exponential critical state model. The components χ′ and χ″ of the complex susceptibility are calculated and an algorithm to fit ac susceptibility data is discussed. This algorithm is employed to study the intergranular response χ′(Hm) and χ″(Hm) measured for two samples of YBa2Cu3O7-δ as a function of the ac field amplitude Hm. One sample is a porous sintered cylinder and the other is a very dense melt-textured bar. In both cases good fits of the calculated components χ′ and χ″ are obtained using an algorithm that involves two free parameters, the full penetration field. Hp, and the sample quality factor, p. An interesting result for the melt-textured sample is the observation of a step in χ′(Hm) curves at very low Hm. possibly associated with grain clustering. © 1996 American Institute of Physics.80633903395Goldfarb, R.B., Lelenthal, M., Thomson, C.A., (1991) Susceptibility of Superconductors and Others Spin Systems, p. 204. , edited by R. A. Hein, T. L. Francavilla, and D. H. Lieberger Plenum Press, New YorkCouach, M., Khoder, A.F., Monnier, F., (1989) Cryogenics, 25, p. 695Goldfarb, R.B., Clark, A.F., (1988) Cryogenics, 27, p. 475Chen, D.X., Goldfarb, R.B., (1988) J. Appl. Phys., 63, p. 980Larbalestier, D.C., Babcock, S.E., Cai, X., Daeumling, M., Hampshire, D.P., Kelly, T.F., Lavanier, L.A., Seuntjens, J., (1988) Physica C, 153-155, p. 1580Worthington, T.K., Gallagher, W.J., Dingerm, T.R., (1987) Phys. Rev. Lett., 59, p. 1160Tanaka, S., Itozaki, H., (1988) J. Appl. Phys., 27, pp. L622Dersch, H., Blatter, G., (1988) Phys. Rev. B, 38, p. 11391Moreira, F.M.A., Ortiz, W.A., De Lima, O.F., (1994) Physica C, 245, p. 3205Bean, C.P., (1962) Phys. Rev. Lett., 8, p. 250Bean, C.P., (1964) Rev. Mod. Phys., 36, p. 31London, H., (1963) Phys. Lett., 6, p. 162Kim, Y.B., Hempstead, C.F., Strnad, A.R., (1963) Phys. Rev., 129, p. 528Fietz, W.A., Beasley, M.R., Silcox, J., Webb, W.W., (1964) Phys. Rev., 136, pp. A335Xu, M., Shi, D., Fox, R.F., (1990) Phys. Rev. B, 42, p. 10773Chen, D.X., Goldfarb, R.B., (1989) J. Appl. Phys., 66, p. 2489Chen, D.X., Sanchez, A., Muñoz, J.S., (1990) J. Appl. Phy., 67, p. 3430Chen, J.L., Yang, T.J., (1994) Physica C, 224, p. 345Chen, D.X., Sanchez, A., (1991) J. Appl. Phys., 70, p. 5463Ishida, T., Goldfarb, R.B., (1990) Phys. Rev. B, 41, p. 8937Sanchez, A., (1994) Physica C, 225, p. 136Shi, D., Chen, J.G., Salem-Sugui Jr., S., Goretta, K.C., (1991) Physica C, 185-189, p. 2329Sanchez, A., Chen, D.X., (1991) Susceptibility of Superconductors and Others Spin Systems, p. 251. , edited by R. A. Hein, T. L. Francavilla, and D. H. Lieberger Plenum, New YorkGelfuso, V.C., Ortiz, W.A., unpublishe

Fishtail Effect Studied By Ac Susceptibility In Erba2cu3o7-δ Single Crystal

A study of the second magnetization peak (SMP) using a scaling procedure for ac susceptibility measurements, in an ErBa2Cu3O7-δ single crystal is presented. In addition, its sample size dependence, ac susceptibility (χ) is a very versatile technique, widely used in the study and characterization of superconducting materials. Its major advantages are the experimental simplicity, low cost, and great sensitivity.95313011306Berlincourt, T.G., Hake, R.R., Leslie, D.H., (1961) Phys. Rev. Lett., 6, p. 671Däumling, M., Seuntjens, J.M., Larbalestier, D.C., (1990) Nature (London), 346, p. 332Bean, C.P., (1962) Phys. Rev. Lett., 8, p. 250Bean, C.P., (1964) Rev. Mod. Phys., 36, p. 31Vargas, J.L., Larbalestier, D.C., (1992) Appl. Phys. Lett., 60, p. 1741Osofsky, M.S., Cohn, J.L., Skelton, E.F., Miller, M.M., Soulen Jr., R.J., Wolf, S.A., Vanderah, T.A., (1992) Phys. Rev. B, 45, p. 4916Harneit, W., (1996) Physica C, 267, p. 270Yeshurun, Y., Malozemoff, A.P., Shaulov, A., (1996) Rev. Mod. Phys., 68, p. 911Blatter, G., Feigel'man, M.V., Geshkenbein, V.B., Larkin, A.I., Vinokur, V.M., (1994) Rev. Mod. Phys., 66, p. 1125Krusin-Elbaum, L., Civale, L., Vinokur, V.M., Holtzberg, F., (1992) Phys. Rev. Lett., 69, p. 2280Yeshurun, Y., Bontemps, N., Burlachkov, L., Kapitulnik, A., (1994) Phys. Rev. B, 49, p. 1548Abulafia, Y., (1996) Phys. Rev. Lett., 77, p. 1596Farmer, J.W., Kornecki, M., Cowan, D.L., (2001) Physica C, 364, p. 334Mikitik, G.P., Brandt, E.H., (2001) Phys. Rev. B, 64, p. 184514Yamaguchi, Y., Shirakawa, N., Rajaram, G., Oka, K., Mumtaz, A., Obara, H., Nakagawa, T., Bando, H., (2001) Physica C, 361, p. 244Kopelevich, Y., Esquinazi, P., (1998) J. Low Temp. Phys., 113, p. 1Esquinazi, P., Setzer, A., Fuchs, D., Kopelevich, Y., Zeldov, E., Assmann, C., (1999) Phys. Rev. B, 60, p. 12454Gurevich, A., Vinokur, V.M., (1999) Phys. Rev. Lett., 83, p. 3037Cai, X.Y., Gurevich, A., Larbalestier, D.C., Kelley, R.J., Onellion, M., Berger, H., Margaritondo, G., (1994) Phys. Rev. B, 50, p. 16774Passos, W.A.C., Lisboa-Filho, P.N., Caparroz, R., De Faria, C.C., Venturini, P.C., Araujo-Moreira, F.M., Sergeenkov, S., Ortiz, W.A., (2001) Physica C, 354, p. 189(1991) Magnetic Susceptibility of Superconductors and Other Spin Systems, , edited by R. A. Hein, T. L. Francavilla, D. H. Liebenberg (Plenum, New York)Clem, J.R., Kerchner, H.R., Sekula, S.T., (1976) Phys. Rev. B, 14, p. 1893Ji, L., Sohn, H.R., Spalding, G.C., Lobb, C.J., Tinkham, M., (1989) Phys. Rev. B, 40, p. 10936Brandt, E.H., (1998) Phys. Rev. B, 58, p. 6506Brandt, E.H., (1998) Phys. Rev. B, 58, p. 6523De Lima, O.F., Cardoso, C.A., (2000) Phys. Rev. B, 61, p. 11722Cardoso, C.A., De Lima, O.F., (2000) Physica C, 334, p. 185Cardoso, C.A., Avila, M.A., Ribeiro, R.A., De Lima, O.F., (2001) Physica C, 354, p. 165Worthington, T.K., Gallagher, W.J., Dinger, T.R., (1987) Phys. Rev. Lett., 59, p. 1160Welp, U., Fleshler, S., Kwok, W.K., Klemm, R.A., Vinokur, V.M., Downey, J., Veal, B., Crabtree, G.W., (1991) Phys. Rev. Lett., 67, p. 3180Klein, L., Yacoby, E.R., Yeshurun, Y., Erb, A., Müller-Vogt, G., Breit, V., Wühl, H., (1994) Phys. Rev. B, 49, p. 4403Mochida, T., Murakami, M., (1997) Physica C, 290, p. 311Fàbrega, L., Fontcuberta, J., Civale, L., Piñol, S., (1994) Phys. Rev. B, 50, p. 1199Jönsson, B.J., Rao, K.V., Yun, S.H., Karlsson, U.O., (1998) Phys. Rev. B, 58, p. 5862Fàbrega, L., Fontcuberta, J., Piñol, S., Van Der Beek, C.J., Kes, P.H., (1993) Phys. Rev. B, 47, p. 15250Zeldov, E., Amer, N.M., Koren, G., Gupta, A., McElfresh, M.W., Gambino, R.J., (1990) Appl. Phys. Lett., 56, p. 680Hardy, V., Wahl, A., Ruyter, A., Maignan, A., Martin, C., Coudrier, L., Provost, J., Simon, Ch., (1994) Physica C, 232, p. 347Pashitski, A.E., (1997) Science, 275, p. 36

Study of magnetic and specific heat measurements at low temperatures in Nd0.5Sr0.5MnO3 and Nd0.5Ca0.5MnO3

The magnetization at low temperatures for Nd0.5Sr0.5MnO3 and Nd0.5Ca0.5MnO3 samples showed a rapid increase with decreasing temperatures, contrary to a La0.5Ca0.5MnO3 sample. Specific heat measurement at low temperatures showed a Schottky-like anomaly for the first two samples. However, there is not a straight forward correlation between the intrinsic magnetic moment of the Nd3+ ions and the Schottky-like anomaly.Comment: To be presented in the 1 Joint European Magnetic Symposia, Grenoble,France (2001). Also submitted to Journal of Magnetism and Magnetic Material

Magnetic Field Dependence Of The Curie-weiss Paramagnetism In Crv Alloys

The determination of the magnetic properties of antiferromagnetic Cr alloys requires careful consideration of the influence of the applied magnetic field. In this work we show that alloys of Cr-x at. % V present a Curie-Weiss paramagnetism above the Néel temperature, which is suppressed by a characteristic field HL. Samples with x=0.1, 0.2, and 0.4 were investigated through measurements of the magnetic susceptibility as a function of temperature, for different values of the magnetic field. A magnetic phase diagram showing the characteristic line HL vs x at. % V is proposed. © 1997 American Institute of Physics.818 PART 2A42094211Suzuki, T., Takaki, H., (1964) J. Phys. Soc. Jpn., 19, p. 1241Suzuki, T., (1966) J. Phys. Soc. Jpn., 21, p. 442Aidun, R., Arajs, S., Moyer, C.A., (1985) Phys. Status Solidi B, 128, p. 133Hill, P., Ali, N., De Oliviera, A.J.A., Ortiz, W.A., De Camargo, P.C., Fawcett, E., (1994) J. Phys. Condens. Matter., 6, p. 1761De Oliveira, A.J.A., Otriz, W.A., De Camargo, P.C., Galkin, V.Yu., (1996) J. Magn. Magn. Mater., 152, p. 86De Oliveira, A.J.A., De Lima, O.F., De Camargo, P.C., Ortiz, W.A., Fawcett, E., (1996) J. Phys. Condens. Matter., 8, pp. L403Benediktsson, G., Hedman, L., Aström, H.U., Rao, K.V., (1982) J. Phys. F, 12, p. 1439Booth, J.G., (1964) Phys. Status Solidi, 7 K, p. 157Bender, D., Müller, J., (1970) Phys. Kondens. Mater., 10, p. 342Buzdin, A.I., Men'shov, V.N., Tugushev, V.V., (1986) Zh. Eksp. Teor. Fiz., 91, p. 2204(1986) Sov. Phys. JETP, 64. , Engl. TranslTugushev, V.V., (1992) Modulated and Localized Structures of the Spin-Density Wave in Itinerant Antiferromagnets in Electronic Phase Transitions, p. 237. , edited by W. Hanke and Yu. Koapev Elsevier, AmsterdamDe Oliveira, A.J.A., De Lima, O.F., Ortiz, W.A., De Camargo, P.C., (1995) Solid State Commun., 96, p. 383De Oliveira, A.J.A., (1996), PhD. thesis, Universidade Federal de São Carlo

Magnetic And Superconducting Properties Of Ru Sr2 Gd1.5 Ce0.5 Cu2 O10-δ Samples: Dependence On The Oxygen Content And Aging Effects

The magnetic and superconducting properties of Ru Sr2 Gd1.5 Ce0.5 Cu2 O10-δ polycrystalline samples with different oxygen-doping levels are presented. A strong suppression of the superconducting temperature (Tc), as well as a reduction in the superconducting fraction, occurs as the oxygen content is reduced by annealing the samples in oxygen-deprived atmospheres. Drastic changes in the electrical resistivity are observed above Tc, possibly associated with oxygen removal, mainly from grain boundaries. However, the magnetic ordering is relatively less affected by the changes in oxygen content of the samples. The spin-glass transition is enhanced and shifted to higher temperatures with the reduction in oxygen content. This could be correlated with an increase in the spin disorder and frustration for the oxygen-depleted samples. Also, the same oxygen-vacancy-induced disorder could explain the reduction in the fraction of the sample showing antiferromagnetic order. We also report significant changes in the measured properties of the samples as a function of time. © 2005 The American Physical Society.7113Felner, I., Asaf, U., Levi, Y., Millo, O., (1997) Phys. Rev. B, 55, p. 3374. , PRBMDO 0163-1829 10.1103/PhysRevB.55.R3374Bernhard, C., Tallon, J.L., Niedermayer, Ch., Blasius, Th., Golnik, A., Brücher, E., Kremer, R.K., Ansaldo, E.J., (1999) Phys. Rev. B, 59, p. 14099. , PRBMDO. 0163-1829. 10.1103/PhysRevB.59.14099Awana, V.P.S., Karppinen, M., Yamauchi, H., (2003) Studies of High Tc Superconductors, 46, p. 77. , edited by A. V. Narlikar (Nova Science Publishers, New YorkMatvejeff, M., Awana, V.P.S., Jang, L.-Y., Liu, R.S., Yamauchi, H., Karppinen, M., (2003) Physica C, 392-396, p. 87. , PHYCE6 0921-4534Cardoso, C.A., Araujo-Moreira, F.M., Awana, V.P.S., Kishan, H., Takayama-Muromachi, E., De Lima, O.F., (2004) Physica C, 405, p. 212. , PHYCE6 0921-4534Cardoso, C.A., Araujo-Moreira, F.M., Awana, V.P.S., Takayama-Muromachi, E., De Lima, O.F., Yamauchi, H., Karppinen, M., (2003) Phys. Rev. B, 67, p. 020407. , PRBMDO 0163-1829 10.1103/PhysRevB.67.020407Shi, L., Li, G., Fan, X.J., Feng, S.J., Li, X.-G., (2003) Physica C, 399, p. 69. , PHYCE6 0921-4534Felner, I., Asaf, U., Ritter, F., Klamut, P.W., Dabrowski, B., (2001) Physica C, 364-365, p. 368. , PHYCE6 0921-4534Felner, I., Asaf, U., Galstyan, E., (2002) Phys. Rev. B, 66, p. 024503. , PRBMDO 0163-1829 10.1103/PhysRevB.66.024503Felner, I., Asaf, U., Levi, Y., Millo, O., (2000) Physica C, 334, p. 141. , PHYCE6 0921-4534 10.1016/S0921-4534(00)00250-1Awana, V.P.S., Ansari, M.A., Gupta, A., Saxena, R.B., Kishan, H., Buddhikot, D., Malik, S.K., (2004) Phys. Rev. B, 70, p. 104520. , PRBMDO 0163-1829 10.1103/PhysRevB.70.104520Yoshizawa, H., Mitsuda, S., Aruga, H., Ito, A., (1987) Phys. Rev. Lett., 59, p. 2364. , PRLTAO 0031-9007 10.1103/PhysRevLett.59.2364Binder, K., Young, A.P., (1986) Rev. Mod. Phys., 58, p. 801. , RMPHAT 0034-6861 10.1103/RevModPhys.58.801Mydosh, J.A., (1993) Spin Glasses An Experimental Introduction, , Taylor & Francis, LondonFelner, I., Galstyan, E., Herber, R.H., Nowik, I., (2004) Phys. Rev. B, 70, p. 094504. , PRBMDO 0163-1829 10.1103/PhysRevB.70.094504Shengelaya, A., Khasanov, R., Eschenko, D.G., Felner, I., Asaf, U., Savić, I.M., Keller, H., Müller, K.A., (2004) Phys. Rev. B, 69, p. 024517. , PRBMDO. 0163-1829. 10.1103/PhysRevB.69.024517Xue, Y.Y., Cao, D.H., Lorenz, B., Chu, C.W., (2002) Phys. Rev. B, 65, p. 020511. , PRBMDO 0163-1829 10.1103/PhysRevB.65.02051

Structural And Magnetic Study Of Labacocu O5+δ

The structure and magnetic properties of the compound LaBaCuCo O5+δ have been studied for the non-stoichiometric oxygen concentration δ≈0.6. The structure is pseudo-cubic with a tripled perovskite unit cell. The crystal structure was determined by a combined Rietveld fit to neutron and synchrotron x-ray powder diffraction data in the orthorhombic Pmmm space group, with cell parameters a=3.9223 (3) Å, b=3.9360 (3) Å, c=11.7073 (8) Å, and V=180.74 (2) Å3 (room temperature). Antiferromagnetic ordering of Cu and Co magnetic moments is observed below 205 (4) K. The magnetic structure with cell aM =2a, bM =2b, and cM =2c, could be described with the Shubnikov space group Fmm m′. The magnetic moments of both equivalent Cu/Co sites were determined at 50 and 170 K to be 0.83 (3) μB and 0.58 (3) μB, respectively, consistent with one unpaired electron per atom. The fit of the intensities to a simple mean field magnetic model appeared to be insufficient to account for the variation of moments at temperatures close to TN while a three dimensional Heisenberg model could improve the fit. Susceptibility measurements between 4 and 350 K also show irreversibility below 150 K. The local environments of Cu and Co were studied by extended x-ray absorption fine structure spectroscopy at both absorption edges. Cu atoms adopt an elongated octahedral or square-based pyramidal oxygen environment which suggests mainly the presence of Cu(II) in the structure. Co adopts different local environments, depending on the electronic and spin states. © 2005 The American Physical Society.7114Er-Rakho, L., Michel, C., Lacorre, P., Raveau, B., (1988) J. 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Irreversible Magnetization Under Rotating Fields And Lock-in Effect On A Erba2cu3o7-δ Single Crystal With Columnar Defects

We have measured the irreversible magnetization (Mi) of an ErBa2Cu3O7-δ single crystal with columnar defects (CD), using a technique based on sample rotation under a fixed magnetic field H. This method is valid for samples whose magnetization vector remains perpendicular to the sample surface over a wide angle range - which is the case for platelets and thin films - and presents several advantages over measurements of ML(H) loops at fixed angles. The resulting Mi(Θ) curves for several temperatures show a peak in the CD direction at high fields. At lower fields, a very well defined plateau indicative of the vortex lock-in to the CD develops. The H dependence of the lock-in angle ΦL follows the H-1 theoretical prediction, while the temperature dependence is in agreement with entropic smearing effects corresponding to long range vortex-defects interactions.641414450211445028Civale, L., Marwick, A.D., Worthington, T.K., Kirk, M.A., Thompson, J.R., Krusin-Elbaum, L., Sun, Y., Holtzberg, F., (1991) Phys. Rev. Lett., 67, p. 648Silhanek, A., Civale, L., Candia, S., Nieva, G., Pasquini, G., Lanza, H., (1999) Phys. Rev. B, 59, p. 13620Nelson, D.R., Vinokur, V.M., (1992) Phys. Rev. Lett., 68, p. 2398(1993) Phys. Rev. B, 48, p. 13060Blatter, G., Feigel'man, M.V., Geshkenbein, V.B., Larkin, A.I., Vinokur, V.M., (1994) Rev. Mod. Phys., 66, p. 1125Silhanek, A., Niebieskikwiat, D., Civale, L., Avila, M.A., Billoni, O., Casa, D., (1999) Phys. Rev. B, 60, p. 13189Avila, M.A., (2001), Ph. D. thesis, University of Campinas (UNICAMP), BrazilCandia, S., Civale, L., (1999) Supercond. Sci. Technol., 12, p. 192Clem, J.R., Sanchez, A., (1994) Phys. Rev. B, 50, p. 9355Prozorov, R., Poddar, A., Sheriff, E., Shaulov, A., Yeshurun, Y., (1996) Physica C, 264, p. 27Zhukov, A.A., Perkins, G.K., Bugoslavsky, Yu.V., Caplin, A.D., (1997) Phys. Rev. B, 56, p. 2809Hasanain, S.K., Ahmad, I., Semerad, R., (1999) Supercond. Sci. Technol., 12, p. 633Clem, J.R., (1982) Phys. Rev. B, 26, p. 2463Clem, J.R., Perez-Gonzalez, A., (1986) Phys. Rev. B, 33, p. 1601Perez-Gonzalez, A., Clem, J.R., (1990) Phys. Rev. B, 42, p. 4100Goeckner, H.P., Kouvel, J.S., (1994) Phys. Rev. B, 50, p. 3435Hasanain, S.K., Manzoor, S., Aftab, M., (1996) Physica C, 272, p. 43Hasan, M.K., Kouvel, J.S., (1997) Physica C, 276, p. 289Obaidat, I.M., Goeckner, H.P., Kouvel, J.S., (1997) Physica C, 291, p. 8Vlasko-Vlasov, V.K., Welp, U., Crabtree, G.W., Gunter, D., Kabanov, V.V., Nikitenko, V.I., Paulius, L.M., (1998) Phys. Rev. B, 58, p. 3446Obaidat, I.M., Park, S.J., Kouvel, J.S., (1998) Physica C, 308, p. 185Hasan, M.K., Shobaki, J., Al-Omari, I.A., Albiss, B.A., Al-Akhras, M.A., Azez, K.A., El-Qisari, A.K., Kouvel, J.S., (1999) Supercond. Sci. Technol., 12, p. 606Niebieskikwiat, D., Silhanek, A., Civale, Nieva, G., Levy, P., Krusin Elbaum, L., (2001) Phys. Rev. B, 63, p. 144504Krusin-Elbaum, L., Civale, L., Thompson, J.R., Feild, C., (1996) Phys. Rev. B, 53, p. 11744Civale, L., Pasquini, G., Levy, P., Nieva, G., Casa, D., Lanza, H., (1996) Physica C, 263, p. 389Zhukov, A.A., Perkins, G.K., Thomas, J.V., Caplin, A.D., Kupfer, H., Wolf, T., (1997) Phys. Rev. B, 56, p. 348

Angular dependence of the bulk nucleation field Hc2 of aligned MgB2 crystallites

Studies on the new MgB2 superconductor, with a critical temperature Tc ~ 39 K, have evidenced its potential for applications although intense magnetic relaxation effects limit the critical current density, Jc, at high magnetic fields. This means that effective pinning centers must be added into the material microstructure, in order to halt dissipative flux movements. Concerning the basic microscopic mechanism to explain the superconductivity in MgB2, several experimental and theoretical works have pointed to the relevance of a phonon-mediated interaction, in the framework of the BCS theory. Questions have been raised about the relevant phonon modes, and the gap and Fermi surface anisotropies, in an effort to interpret spectroscopic and thermal data that give values between 2.4 and 4.5 for the gap energy ratio. Preliminary results on the anisotropy of Hc2 have shown a ratio, between the in-plane and perpendicular directions, around 1.7 for aligned MgB2 crystallites and 1.8 for epitaxial thin films. Here we show a study on the angular dependence of Hc2 pointing to a Fermi velocity anisotropy around 2.5. This anisotropy certainly implies the use of texturization techniques to optimize Jc in MgB2 wires and other polycrystalline components.Comment: 10 pages + 4 Figs.; Revised version accepted in Phys. Rev.