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Composition dependence of the spin wave stiffness parameter in La1−xCaxMnO3 CMR materials
Journal of Magnetism and Magnetic Materials 226}230 (2001) 775}776
Composition dependence of the spin wave sti!ness parameter in La Ca MnO CMR materials \V V J.J. Rhyne *, H. Kaiser , L. Stumpe , J.F. Mitchell, T. McCloskey, A.R. Chourasia Department of Physics, University of Missouri, Columbia, MO 65211, USA Materials Science Division, Argonne National Laboratory, Argonne, IL, USA Department of Physics, Texas A&M University of Commerce, Commerce, TX 75429, USA
Abstract Long-wavelength spin waves in La Ca MnO CMR materials exhibit conventional Heisenberg dispersion \V V (E"Dq#), but with an anomalous temperature dependence for the spin dispersion parameter D. For x+0.33, near the optimum CMR composition, D remains approximately 50% of its ¹"0 K value before exhibiting a near-"rst-order transition at ¹ , instead of following the expected power law decrease as ¹ approaches ¹ . No temperature hysteresis was observed in either D or in the magnetization. Compositions with x"0.40 and 0.45 show more conventional temperature renormalization. 2001 Elsevier Science B.V. All rights reserved. Keywords: Manganites; CMR; Spin waves
1. Introduction Inelastic neutron scattering studies on polycrystalline samples of the perovskite-based colossal magnetoresistance manganites La Ca MnO with x near the opti\V V mal CMR composition x"0.33 show conventional spin wave dispersion (E"Dq#) at long wavelengths [1,2]. D is the spin wave sti!ness constant and is a q"0 energy gap that may arise from anisotropy and is near 0 in these systems. The temperature dependence of D in an Heisenberg system is given by D(¹)" D(¹"0)[1!(¹/¹ ) ] at low ¹/¹ , with small high er-order correction terms, and at temperatures approaching ¹ , D varies as [1!(¹/¹ )]J\@, where !+0.34. Previous results on polycrystalline La Ca MnO \V V with x"0.33 by Lynn et al. [1] gave the unexpected result that D did not show the normal decrease on ap-
* Corresponding author. Tel.: #1-573-882-6506; fax: #1573-882-4195. E-mail address: [email protected] (J.J. Rhyne).
proach to ¹ , rather it retained close to 50% of its ¹"0 K magnitude at ¹ . Our results taken with "ner temperature increments [2] show that D drops abruptly above 0.98¹ and exhibits a near-"rst-order transition at 262 K that is also re#ected in the magnetization determined with neutrons. No measurable hysteresis was found on warming and cooling in either quantity. Lynn et al. [1] also found that spin wave spectral weight was exchanged with an anomalous E"0 central mode, with an onset near 60% ¹ , that increased rapidly in intensity at ¹ and whose q-width corresponded to a 13 As correla tion range.
2. Spin wave behavior In an attempt to con"rm if the anomalous ¹ dependence of D was present only near the optimal CMR compositions and thus might be associated with magnetic polaron formation [1], we have investigated the spin waves in compositions ranging from x"0.25 to 0.45 all in the ferromagnetic-metal range. Small q inelastic scattering techniques were employed on a triple-axis
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 7 4 6 - 0
776
J.J. Rhyne et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 775}776
Fig. 1. Spin wave intensity versus energy at a constant q"0.07 As \ in La Ca MnO for several temperatures below ¹ . No evidence of a central mode anomaly is seen below 0.98¹ , and the spin wave integrated intensity rises with temper ature in accordance with Bose}Einstein statistics.
spectrometer using an incident neutron energy of 14.2 meV. Focusing e!ects present at small angle yielded an e!ective resolution for spin waves of about 0.12 meV, which is approximately of the elastic resolution. Useful spin wave data were obtained down to q"0.04 As \ with an upper q limit imposed by the spin wave energies and kinematic considerations. These restrictions also precluded obtaining values of the spin wave sti!ness parameter at low ¹. Fig. 1 illustrates the spin wave intensity versus energy transfer for the x"0.36 composition at several temperatures below the ¹ "269 K. Above ¹ , the energy-gain and energy-loss spin wave excitation peaks collapse into a single Lorentzian scattering peak at E"0. Conventional Heisenberg dispersion was observed for the data of Fig. 1, as well as that for all other compositions and temperatures. No anomalous central peak phenomenon was found up to 0.98 ¹ for any composition except for x"0.25. This compound exhibited a 4% di!erence in the peak temperature of the resistivity and the susceptibility peak, unlike others that show no measurable di!erence in these temperatures. One is tempted to associate this temperature di!erence and the occurrence of a central peak to a common physical origin. The results for the temperature dependence of D for the six compositions derived from the dispersion curves are plotted in Fig. 2. The solid lines are the result of a "t to above-mentioned power law at higher temperatures and to the ¹ dependence at low ¹. It is noted that all compositions up through x"0.36 do exhibit the abrupt temperature renormalization of D near ¹ , and conse quently the "ts produce unphysical values of the expo-
Fig. 2. Temperature dependence of the spin wave sti!ness parameter D in La Ca MnO for six compositions in the fer\V V romagnetic metal regime. The solid lines are the result of a "t to a power law at high ¹ and to a ¹ dependence at low ¹ (see text). The curve for the theoretically expected power-law exponents !"0.34 is shown for comparison in the x"0.36 frame.
nents !. The x"0.40 and 0.45 compositions show a more normal ¹ dependence and yielded values of !"0.36 and 0.38, respectively. These higher compositions show a reduced CMR e!ect and are approaching the phase boundary at x"0.5 between the ferromagnetic metal and the antiferromagnetic insulator states. The ¹"0 K values of D given in the "gure are extrapolated, due to the kinematic restrictions stated above. The observed progression to a conventional second order transition at the higher Ca doping levels is consistent with the Monte Carlo Random Field Ising Model calculations of Moreo et al. [3].
Acknowledgements The authors acknowledge the support from the UM Research Board for this research.
References [1] J.W. Lynn, R.W. Erwin, J.A. Borchers, Q. Huang, A. Santoro, J.-L. Peng, Z.Y. Li, Phys. Rev. Lett. 76 (1996) 4046. [2] J.J. Rhyne, H. Kaiser, L. Stumpe, J.F. Mitchell, T. McCloskey, A.R. Chourasia, J. Appl. Phys. 87 (2000) 5815.
Composition dependence of the spin wave sti!ness parameter in La Ca MnO CMR materials \V V J.J. Rhyne *, H. Kaiser , L. Stumpe , J.F. Mitchell, T. McCloskey, A.R. Chourasia Department of Physics, University of Missouri, Columbia, MO 65211, USA Materials Science Division, Argonne National Laboratory, Argonne, IL, USA Department of Physics, Texas A&M University of Commerce, Commerce, TX 75429, USA
Abstract Long-wavelength spin waves in La Ca MnO CMR materials exhibit conventional Heisenberg dispersion \V V (E"Dq#), but with an anomalous temperature dependence for the spin dispersion parameter D. For x+0.33, near the optimum CMR composition, D remains approximately 50% of its ¹"0 K value before exhibiting a near-"rst-order transition at ¹ , instead of following the expected power law decrease as ¹ approaches ¹ . No temperature hysteresis was observed in either D or in the magnetization. Compositions with x"0.40 and 0.45 show more conventional temperature renormalization. 2001 Elsevier Science B.V. All rights reserved. Keywords: Manganites; CMR; Spin waves
1. Introduction Inelastic neutron scattering studies on polycrystalline samples of the perovskite-based colossal magnetoresistance manganites La Ca MnO with x near the opti\V V mal CMR composition x"0.33 show conventional spin wave dispersion (E"Dq#) at long wavelengths [1,2]. D is the spin wave sti!ness constant and is a q"0 energy gap that may arise from anisotropy and is near 0 in these systems. The temperature dependence of D in an Heisenberg system is given by D(¹)" D(¹"0)[1!(¹/¹ ) ] at low ¹/¹ , with small high er-order correction terms, and at temperatures approaching ¹ , D varies as [1!(¹/¹ )]J\@, where !+0.34. Previous results on polycrystalline La Ca MnO \V V with x"0.33 by Lynn et al. [1] gave the unexpected result that D did not show the normal decrease on ap-
* Corresponding author. Tel.: #1-573-882-6506; fax: #1573-882-4195. E-mail address: [email protected] (J.J. Rhyne).
proach to ¹ , rather it retained close to 50% of its ¹"0 K magnitude at ¹ . Our results taken with "ner temperature increments [2] show that D drops abruptly above 0.98¹ and exhibits a near-"rst-order transition at 262 K that is also re#ected in the magnetization determined with neutrons. No measurable hysteresis was found on warming and cooling in either quantity. Lynn et al. [1] also found that spin wave spectral weight was exchanged with an anomalous E"0 central mode, with an onset near 60% ¹ , that increased rapidly in intensity at ¹ and whose q-width corresponded to a 13 As correla tion range.
2. Spin wave behavior In an attempt to con"rm if the anomalous ¹ dependence of D was present only near the optimal CMR compositions and thus might be associated with magnetic polaron formation [1], we have investigated the spin waves in compositions ranging from x"0.25 to 0.45 all in the ferromagnetic-metal range. Small q inelastic scattering techniques were employed on a triple-axis
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 7 4 6 - 0
776
J.J. Rhyne et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 775}776
Fig. 1. Spin wave intensity versus energy at a constant q"0.07 As \ in La Ca MnO for several temperatures below ¹ . No evidence of a central mode anomaly is seen below 0.98¹ , and the spin wave integrated intensity rises with temper ature in accordance with Bose}Einstein statistics.
spectrometer using an incident neutron energy of 14.2 meV. Focusing e!ects present at small angle yielded an e!ective resolution for spin waves of about 0.12 meV, which is approximately of the elastic resolution. Useful spin wave data were obtained down to q"0.04 As \ with an upper q limit imposed by the spin wave energies and kinematic considerations. These restrictions also precluded obtaining values of the spin wave sti!ness parameter at low ¹. Fig. 1 illustrates the spin wave intensity versus energy transfer for the x"0.36 composition at several temperatures below the ¹ "269 K. Above ¹ , the energy-gain and energy-loss spin wave excitation peaks collapse into a single Lorentzian scattering peak at E"0. Conventional Heisenberg dispersion was observed for the data of Fig. 1, as well as that for all other compositions and temperatures. No anomalous central peak phenomenon was found up to 0.98 ¹ for any composition except for x"0.25. This compound exhibited a 4% di!erence in the peak temperature of the resistivity and the susceptibility peak, unlike others that show no measurable di!erence in these temperatures. One is tempted to associate this temperature di!erence and the occurrence of a central peak to a common physical origin. The results for the temperature dependence of D for the six compositions derived from the dispersion curves are plotted in Fig. 2. The solid lines are the result of a "t to above-mentioned power law at higher temperatures and to the ¹ dependence at low ¹. It is noted that all compositions up through x"0.36 do exhibit the abrupt temperature renormalization of D near ¹ , and conse quently the "ts produce unphysical values of the expo-
Fig. 2. Temperature dependence of the spin wave sti!ness parameter D in La Ca MnO for six compositions in the fer\V V romagnetic metal regime. The solid lines are the result of a "t to a power law at high ¹ and to a ¹ dependence at low ¹ (see text). The curve for the theoretically expected power-law exponents !"0.34 is shown for comparison in the x"0.36 frame.
nents !. The x"0.40 and 0.45 compositions show a more normal ¹ dependence and yielded values of !"0.36 and 0.38, respectively. These higher compositions show a reduced CMR e!ect and are approaching the phase boundary at x"0.5 between the ferromagnetic metal and the antiferromagnetic insulator states. The ¹"0 K values of D given in the "gure are extrapolated, due to the kinematic restrictions stated above. The observed progression to a conventional second order transition at the higher Ca doping levels is consistent with the Monte Carlo Random Field Ising Model calculations of Moreo et al. [3].
Acknowledgements The authors acknowledge the support from the UM Research Board for this research.
References [1] J.W. Lynn, R.W. Erwin, J.A. Borchers, Q. Huang, A. Santoro, J.-L. Peng, Z.Y. Li, Phys. Rev. Lett. 76 (1996) 4046. [2] J.J. Rhyne, H. Kaiser, L. Stumpe, J.F. Mitchell, T. McCloskey, A.R. Chourasia, J. Appl. Phys. 87 (2000) 5815.