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Field dependence of colossal magnetoresistance in magnetic fields up to 50 T
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 290–291 (2005) 416–419 www.elsevier.com/locate/jmmm
Field dependence of colossal magnetoresistance in magnetic fields up to 50 T K. Do¨rr, K.-H. Mu¨ller, N. Kozlova, P. Reutler, R. Klingeler, B. Bu¨chner, L. Schultz IFW Dresden, Postfach 270116, Dresden 01171, Germany Available online 14 December 2004
Abstract The magnetoresistance (MR) of La0.7Sr0.3MnO3 and La0.7Ca0.3MnO3 epitaxial films and of a La0.75Sr0.25MnO3 single crystal were measured in pulsed magnetic fields up to 50 T over a wide temperature range. MR data have been analysed utilizing the recently suggested Brillouin scaling model. The model applies well in the paramagnetic state, where results prove the presence of superparamagnetic clusters. Near the ferromagnetic ordering temperature, systematic deviations in particular in larger fields indicate that the simple model fails there. The temperature-dependent cluster size parameter is similar for our samples, but differs essentially from that obtained earlier for half-doped manganites. r 2004 Elsevier B.V. All rights reserved. PACS: 75.47.Lx; 73.50.Fq; 75.70.–i; 75.47.Gk Keywords: Magnetoresistance; Pulsed magnetic fields; Ferromagnetic manganites; Epitaxial films
1. Introduction Earlier work by Wagner et al. [1,2] and Mandal et al. [3] had discovered the experimental fact that the field dependence of the colossal magnetoresistance (CMR) of some ferromagnetic manganites (Pr0.5Sr0.5MnO3, Nd0.52Sr0.48MnO3 and La2/3Sr1/3MnO3 thin films) can be fitted by a Brillouin function, Dr/rBJ(g JmBH/kB T), in the ferromagnetic state. JgmB denotes the average magnetic moment of certain magnetic clusters involved in the aligning process of the external magnetic field, with the Bohr magneton mB and g ¼ 2 for J consisting of electron spins. H is the external magnetic field, kB the Boltzmann constant and T the temperature. In order to Corresponding author. Tel.: +49 351 4659 403; fax: +49 351 4659 537. E-mail address: [email protected] (N. Kozlova).
avoid extrinsic magnetoresistance (MR) from grain boundaries [4], single crystals or epitaxial thin films were investigated. The scaling behaviour was found to hold up to large fields (50 T for the Nd manganite), though there were no data shown for T close to TC. Above the ferromagnetic ordering temperature TC, the scaling modifies to Dr=r M 2 B2J ; as expected for a para or superparamagnet (with the magnetization M). This astonishingly simple scaling behaviuor of the CMR in some manganites was attributed to a Mott hopping mechanism with magnetization-dependent energy barriers for the charge carrier transport [1]. Further, the assumption was made that the field-induced magnetization dM in the ferromagnetic state is small in comparison to the spontaneous magnetization MS, and follows dMBJ(gJmBH/kBT). Note that J is dependent on temperature, i.e. only the field dependence at constant temperature is described by BJ.
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.11.488
ARTICLE IN PRESS K. Do¨rr et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 416–419
417
In the present work, the MR of La0.7Sr0.3MnO3 and La0.7Ca0.3MnO3 (thin epitaxial films) and of a single crystal of La0.75Sr0.25MnO3 has been investigated in magnetic fields up to 50 T to answer the following questions: Does the Brillouin scaling apply to (i) further manganites like La0.7Ca0.3MnO3 with its different nature of the ferromagnetic transition, (ii) to single crystals without the possible influence of strain states typically found in thin films and (iii) more close to TC? What is the meaning of the observed temperature dependence of the cluster size?
2. Experimental The epitaxial La0.7Sr0.3MnO3 (LSMO) and La0.7Ca0.3MnO3 (LCMO) thin films have been grown on SrTiO3 (1 0 0) and NdGaO3 (1 1 0) substrates, respectively, by pulsed laser deposition. Film thicknesses are 190 nm (LSMO) and 14 nm (LCMO), with very low strain (0.1%) of the LCMO film due to good lattice match with the substrate. Ferromagnetic transition temperatures derived from magnetization measurements are T C ¼ 330 K (LSMO) and 250 K (LCMO). The La0.75Sr0.25MnO3 single crystal has been prepared by the floating zone method and has a T C ¼ 339 K: The MR measurements have been performed in a pulse field facility, where magnetic fields up to 50 T are generated by a solenoid [5]. The field increases to the maximum field within 10 ms and decays within 100 ms. Resistance measurements employ the fourprobe technique. Samples have been mounted on the sample holder with the film plane parallel to the field direction. DC currents of I ¼ 502100 mA (for the film samples) and AC current of I ¼ 100 mA (for the crystal) have been applied parallel to the field direction. The technique of DC transport measurements in pulsed magnetic fields has been described in Ref. [4], the AC MR measurements have been made using digital technique [6]. Particular attention has been paid to verify the absence of Joule and eddy current heating.
3. Results and discussion The measured field dependence of the MR, Dr=rð0Þ ¼ ½rðHÞ2rð0Þ=rð0Þ at several selected temperatures is shown in Figs. 1–3. No hysteresis was observed in high fields. Since the sample temperature is restricted to Tp300 K for the pulse field cryostat, data at higher temperatures have been recorded at static fields (p14 T) in a Quantum Design PPMS. Comparing the MR values of LSMO film and La0.75Sr0.25MnO3 crystal, the smaller MR of the crystal indicates somehow enlarged magnetic
Fig. 1. Field dependence of the MR of an La0.7Sr0.3MnO3 film (open symbols) and La0.75Sr0.25MnO3 single crystal (closed symbols) at temperatures below TC. Lines are fit curves proportional to a Brillouin function, showing systematic deviations at 300 K (T C ¼ 330 and 339 K, respectively).
Fig. 2. Data on field-dependent MR and Brillouin fit curves (BJ at ToT C and B2J at T4T C ) near T C ¼ 339 K measured on the La0.75Sr0.25MnO3 crystal in static magnetic fields.
disorder in the film that might be related to its tensile strain state. Fits to the data according to -Dr=r ¼ CBJ and CB2J in the ferromagnetic and the paramagnetic state, respectively, have been carried out and give the fitting parameters C(T) and J(T). As can be seen in Figs. 1–3, the fits are of good quality for T5T C and TbT C : Systematic deviations appear near TC. These deviations have not been seen in previous experiments [1–3]. The data suggest that approximations of the scaling model are not fulfilled near TC. Moreover, the comparison of 14 and 50 T data show that even though the low field response might be approximated quite well at TTC (Fig. 2), this does not hold for a larger field range.
ARTICLE IN PRESS 418
K. Do¨rr et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 416–419
Fig. 3. Field dependence of the MR of a La0.7Ca0.3MnO3 film above and below T C ¼ 250 K. Solid and dotted lines are proportional to the Brillouin function BJ and B2J ; respectively.
Fig. 4. Temperature dependence of the fitted effective spin moment J for La0.7Sr0.3MnO3, La0.7Ca0.3MnO3 (films) and La0.75Sr0.25MnO3 single crystal. Data from Refs. [1,2] are included. Near TC, fits of 14 T data yield larger J values (due to the deviation of measured data from the model).
Fig. 4 presents the temperature dependence of the magnetic cluster size J(T) for our samples and sketches the data from Refs. [1,2]. Data for the LCMO and LSMO film fall on the same curve if a normalized temperature scale T/TC is used. The 50 T data of the La0.75Sr0.25MnO3 crystal give somewhat lower J, while 14 T data lie again on the curve obtained for the films. For the paramagnetic phase, the validity of the Brillouin scaling model seems likely. The phenomenological relation of resistance and magnetization, rM2 is well established. Thus, our results in the paramagnetic temperature range clearly prove the presence of superparamagnetic clusters considerably above TC, in agreement with other experiments (e.g. Ref. [7]).
Mandal et al. have pointed out an exponential increase of J with temperature in a wide temperature range below TC for La2/3Sr1/3MnO3 (film) [3]. This also holds for our data for the linear part of the J(T) curve in Fig. 4 (plotted with a logarithmic J scale). It is valid at temperatures not too near to TC and ceases to be correct for low temperatures where unphysically small Jo2 have been calculated. An average Mn ion has a magnetic moment of 3.7 mB, giving an average number of Mn ions per cluster of J/2. The failure of the model at low temperatures might be related to the metallic (and not hopping) nature of charge transport there. There is a very strong rise of J near TC for our three samples (that is not an artefact of the non-ideal fits). Note that the assumption dM5M (see introduction) might be incorrect near TC where the spontaneous magnetization drops. Since a ferromagnet obeys the relation MBJ(gJmB[H+lM]/kBT) with the Weiss parameter l in mean-field approximation, both contributions from the external field and the Weiss field lM(H) change at TTC and must be taken into account for a better description. Thus, the meaning of J(T) is not really a cluster size but has a contribution related to the lM(T) term not yet clarified. On the other hand, the mean-field description for ferromagnets which underlies the Brillouin scaling applies only for temperatures not too close to TC, i.e. near TC the effect of fluctuations cannot be neglected. Interestingly, for the half-doped Nd0.52Sr0.48MnO3 and Pr0.5Sr0.5MnO3 samples [1,2] the cluster size has a temperature dependence of different character (Fig. 4). In particular, the Nd0.52Sr0.48MnO3 sample has been investigated below and above TC and no peak of J(T) near TC was detected. Here, the presence of antiferromagnetic short-range order might be important, since in these compounds either an antiferromagnetic orbital and charge ordered ground state is realized or does exist for only slightly deviating composition. We emphasize that nanoscale phase separation which is well known for the half-doped compounds might strongly affect the MR. Therefore, the present work has been devoted to compounds with a clear ferromagnetic ground state and, for LSMO, with the highest possible ordering temperature. The different behaviour of our samples and the half-doped ones suggests that the model parameter J(T) is sensitive to details like the balance of ferro and antiferromagnetic interactions and short-range fluctuations.
Acknowledgements We gratefully acknowledge support of this work by DFG, FOR 520 and BMBF project FKZ 03SC5DRE.
ARTICLE IN PRESS K. Do¨rr et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 416–419
References [1] P.H. Wagner, V. Metlushko, L. Trappeniers, A. Vantomme, J. Vanacken, G. Kido, V.V. Moshchalkov, Y. Bruynseraede, Phys. Rev. B 55 (1997) 3699. [2] P. Wagner, I. Gordon, L. Trappeniers, J. Vanacken, F. Herlach, V.V. Moshchalkov, Y. Bruynseraede, Phys. Rev. Lett. 81 (1998) 3980. [3] P. Mandal, P. Choudhury, K. Ba¨rner, R. von Helmolt, A.G.M. Jansen, J. Appl. Phys. 91 (2002) 5940.
419
[4] N. Kozlova, K. Do¨rr, D. Eckert, T. Walter, K.-H. Mu¨ller, J. Magn. Magn. Mater. 261 (2003) 48. [5] H. Krug, M. Doerr, D. Eckert, H. Eschrig, F. Fischer, P. Fulde, R. Groessinger, A. Handstein, F. Herlach, D. Hinz, et al., Physica B 294–295 (2001) 605. [6] N. Kozlova, M. Kozlov, D. Eckert, K.-H. Mu¨ller, L. Schultz, J. Magn. Magn. Mater. 272–276 (2004) 1679. [7] J.M. de Teresa, M.R. Ibarra, P.A. Algarabel, C. Ritter, C. Marquina, J. Blasco, J. Garcia, A. del Moral, Z. Arnold, Nature 386 (1997) 256.
Journal of Magnetism and Magnetic Materials 290–291 (2005) 416–419 www.elsevier.com/locate/jmmm
Field dependence of colossal magnetoresistance in magnetic fields up to 50 T K. Do¨rr, K.-H. Mu¨ller, N. Kozlova, P. Reutler, R. Klingeler, B. Bu¨chner, L. Schultz IFW Dresden, Postfach 270116, Dresden 01171, Germany Available online 14 December 2004
Abstract The magnetoresistance (MR) of La0.7Sr0.3MnO3 and La0.7Ca0.3MnO3 epitaxial films and of a La0.75Sr0.25MnO3 single crystal were measured in pulsed magnetic fields up to 50 T over a wide temperature range. MR data have been analysed utilizing the recently suggested Brillouin scaling model. The model applies well in the paramagnetic state, where results prove the presence of superparamagnetic clusters. Near the ferromagnetic ordering temperature, systematic deviations in particular in larger fields indicate that the simple model fails there. The temperature-dependent cluster size parameter is similar for our samples, but differs essentially from that obtained earlier for half-doped manganites. r 2004 Elsevier B.V. All rights reserved. PACS: 75.47.Lx; 73.50.Fq; 75.70.–i; 75.47.Gk Keywords: Magnetoresistance; Pulsed magnetic fields; Ferromagnetic manganites; Epitaxial films
1. Introduction Earlier work by Wagner et al. [1,2] and Mandal et al. [3] had discovered the experimental fact that the field dependence of the colossal magnetoresistance (CMR) of some ferromagnetic manganites (Pr0.5Sr0.5MnO3, Nd0.52Sr0.48MnO3 and La2/3Sr1/3MnO3 thin films) can be fitted by a Brillouin function, Dr/rBJ(g JmBH/kB T), in the ferromagnetic state. JgmB denotes the average magnetic moment of certain magnetic clusters involved in the aligning process of the external magnetic field, with the Bohr magneton mB and g ¼ 2 for J consisting of electron spins. H is the external magnetic field, kB the Boltzmann constant and T the temperature. In order to Corresponding author. Tel.: +49 351 4659 403; fax: +49 351 4659 537. E-mail address: [email protected] (N. Kozlova).
avoid extrinsic magnetoresistance (MR) from grain boundaries [4], single crystals or epitaxial thin films were investigated. The scaling behaviour was found to hold up to large fields (50 T for the Nd manganite), though there were no data shown for T close to TC. Above the ferromagnetic ordering temperature TC, the scaling modifies to Dr=r M 2 B2J ; as expected for a para or superparamagnet (with the magnetization M). This astonishingly simple scaling behaviuor of the CMR in some manganites was attributed to a Mott hopping mechanism with magnetization-dependent energy barriers for the charge carrier transport [1]. Further, the assumption was made that the field-induced magnetization dM in the ferromagnetic state is small in comparison to the spontaneous magnetization MS, and follows dMBJ(gJmBH/kBT). Note that J is dependent on temperature, i.e. only the field dependence at constant temperature is described by BJ.
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.11.488
ARTICLE IN PRESS K. Do¨rr et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 416–419
417
In the present work, the MR of La0.7Sr0.3MnO3 and La0.7Ca0.3MnO3 (thin epitaxial films) and of a single crystal of La0.75Sr0.25MnO3 has been investigated in magnetic fields up to 50 T to answer the following questions: Does the Brillouin scaling apply to (i) further manganites like La0.7Ca0.3MnO3 with its different nature of the ferromagnetic transition, (ii) to single crystals without the possible influence of strain states typically found in thin films and (iii) more close to TC? What is the meaning of the observed temperature dependence of the cluster size?
2. Experimental The epitaxial La0.7Sr0.3MnO3 (LSMO) and La0.7Ca0.3MnO3 (LCMO) thin films have been grown on SrTiO3 (1 0 0) and NdGaO3 (1 1 0) substrates, respectively, by pulsed laser deposition. Film thicknesses are 190 nm (LSMO) and 14 nm (LCMO), with very low strain (0.1%) of the LCMO film due to good lattice match with the substrate. Ferromagnetic transition temperatures derived from magnetization measurements are T C ¼ 330 K (LSMO) and 250 K (LCMO). The La0.75Sr0.25MnO3 single crystal has been prepared by the floating zone method and has a T C ¼ 339 K: The MR measurements have been performed in a pulse field facility, where magnetic fields up to 50 T are generated by a solenoid [5]. The field increases to the maximum field within 10 ms and decays within 100 ms. Resistance measurements employ the fourprobe technique. Samples have been mounted on the sample holder with the film plane parallel to the field direction. DC currents of I ¼ 502100 mA (for the film samples) and AC current of I ¼ 100 mA (for the crystal) have been applied parallel to the field direction. The technique of DC transport measurements in pulsed magnetic fields has been described in Ref. [4], the AC MR measurements have been made using digital technique [6]. Particular attention has been paid to verify the absence of Joule and eddy current heating.
3. Results and discussion The measured field dependence of the MR, Dr=rð0Þ ¼ ½rðHÞ2rð0Þ=rð0Þ at several selected temperatures is shown in Figs. 1–3. No hysteresis was observed in high fields. Since the sample temperature is restricted to Tp300 K for the pulse field cryostat, data at higher temperatures have been recorded at static fields (p14 T) in a Quantum Design PPMS. Comparing the MR values of LSMO film and La0.75Sr0.25MnO3 crystal, the smaller MR of the crystal indicates somehow enlarged magnetic
Fig. 1. Field dependence of the MR of an La0.7Sr0.3MnO3 film (open symbols) and La0.75Sr0.25MnO3 single crystal (closed symbols) at temperatures below TC. Lines are fit curves proportional to a Brillouin function, showing systematic deviations at 300 K (T C ¼ 330 and 339 K, respectively).
Fig. 2. Data on field-dependent MR and Brillouin fit curves (BJ at ToT C and B2J at T4T C ) near T C ¼ 339 K measured on the La0.75Sr0.25MnO3 crystal in static magnetic fields.
disorder in the film that might be related to its tensile strain state. Fits to the data according to -Dr=r ¼ CBJ and CB2J in the ferromagnetic and the paramagnetic state, respectively, have been carried out and give the fitting parameters C(T) and J(T). As can be seen in Figs. 1–3, the fits are of good quality for T5T C and TbT C : Systematic deviations appear near TC. These deviations have not been seen in previous experiments [1–3]. The data suggest that approximations of the scaling model are not fulfilled near TC. Moreover, the comparison of 14 and 50 T data show that even though the low field response might be approximated quite well at TTC (Fig. 2), this does not hold for a larger field range.
ARTICLE IN PRESS 418
K. Do¨rr et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 416–419
Fig. 3. Field dependence of the MR of a La0.7Ca0.3MnO3 film above and below T C ¼ 250 K. Solid and dotted lines are proportional to the Brillouin function BJ and B2J ; respectively.
Fig. 4. Temperature dependence of the fitted effective spin moment J for La0.7Sr0.3MnO3, La0.7Ca0.3MnO3 (films) and La0.75Sr0.25MnO3 single crystal. Data from Refs. [1,2] are included. Near TC, fits of 14 T data yield larger J values (due to the deviation of measured data from the model).
Fig. 4 presents the temperature dependence of the magnetic cluster size J(T) for our samples and sketches the data from Refs. [1,2]. Data for the LCMO and LSMO film fall on the same curve if a normalized temperature scale T/TC is used. The 50 T data of the La0.75Sr0.25MnO3 crystal give somewhat lower J, while 14 T data lie again on the curve obtained for the films. For the paramagnetic phase, the validity of the Brillouin scaling model seems likely. The phenomenological relation of resistance and magnetization, rM2 is well established. Thus, our results in the paramagnetic temperature range clearly prove the presence of superparamagnetic clusters considerably above TC, in agreement with other experiments (e.g. Ref. [7]).
Mandal et al. have pointed out an exponential increase of J with temperature in a wide temperature range below TC for La2/3Sr1/3MnO3 (film) [3]. This also holds for our data for the linear part of the J(T) curve in Fig. 4 (plotted with a logarithmic J scale). It is valid at temperatures not too near to TC and ceases to be correct for low temperatures where unphysically small Jo2 have been calculated. An average Mn ion has a magnetic moment of 3.7 mB, giving an average number of Mn ions per cluster of J/2. The failure of the model at low temperatures might be related to the metallic (and not hopping) nature of charge transport there. There is a very strong rise of J near TC for our three samples (that is not an artefact of the non-ideal fits). Note that the assumption dM5M (see introduction) might be incorrect near TC where the spontaneous magnetization drops. Since a ferromagnet obeys the relation MBJ(gJmB[H+lM]/kBT) with the Weiss parameter l in mean-field approximation, both contributions from the external field and the Weiss field lM(H) change at TTC and must be taken into account for a better description. Thus, the meaning of J(T) is not really a cluster size but has a contribution related to the lM(T) term not yet clarified. On the other hand, the mean-field description for ferromagnets which underlies the Brillouin scaling applies only for temperatures not too close to TC, i.e. near TC the effect of fluctuations cannot be neglected. Interestingly, for the half-doped Nd0.52Sr0.48MnO3 and Pr0.5Sr0.5MnO3 samples [1,2] the cluster size has a temperature dependence of different character (Fig. 4). In particular, the Nd0.52Sr0.48MnO3 sample has been investigated below and above TC and no peak of J(T) near TC was detected. Here, the presence of antiferromagnetic short-range order might be important, since in these compounds either an antiferromagnetic orbital and charge ordered ground state is realized or does exist for only slightly deviating composition. We emphasize that nanoscale phase separation which is well known for the half-doped compounds might strongly affect the MR. Therefore, the present work has been devoted to compounds with a clear ferromagnetic ground state and, for LSMO, with the highest possible ordering temperature. The different behaviour of our samples and the half-doped ones suggests that the model parameter J(T) is sensitive to details like the balance of ferro and antiferromagnetic interactions and short-range fluctuations.
Acknowledgements We gratefully acknowledge support of this work by DFG, FOR 520 and BMBF project FKZ 03SC5DRE.
ARTICLE IN PRESS K. Do¨rr et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 416–419
References [1] P.H. Wagner, V. Metlushko, L. Trappeniers, A. Vantomme, J. Vanacken, G. Kido, V.V. Moshchalkov, Y. Bruynseraede, Phys. Rev. B 55 (1997) 3699. [2] P. Wagner, I. Gordon, L. Trappeniers, J. Vanacken, F. Herlach, V.V. Moshchalkov, Y. Bruynseraede, Phys. Rev. Lett. 81 (1998) 3980. [3] P. Mandal, P. Choudhury, K. Ba¨rner, R. von Helmolt, A.G.M. Jansen, J. Appl. Phys. 91 (2002) 5940.
419
[4] N. Kozlova, K. Do¨rr, D. Eckert, T. Walter, K.-H. Mu¨ller, J. Magn. Magn. Mater. 261 (2003) 48. [5] H. Krug, M. Doerr, D. Eckert, H. Eschrig, F. Fischer, P. Fulde, R. Groessinger, A. Handstein, F. Herlach, D. Hinz, et al., Physica B 294–295 (2001) 605. [6] N. Kozlova, M. Kozlov, D. Eckert, K.-H. Mu¨ller, L. Schultz, J. Magn. Magn. Mater. 272–276 (2004) 1679. [7] J.M. de Teresa, M.R. Ibarra, P.A. Algarabel, C. Ritter, C. Marquina, J. Blasco, J. Garcia, A. del Moral, Z. Arnold, Nature 386 (1997) 256.