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Interpreting and modelling magnetocaloric data and properties: the Landau theory of phase transitions and mean-field theory.

Author(s) : AMARAL J. S., AMARAL V. S.

Summary

The authors have recently shown how the Landau theory of phase transitions can be used to model magnetocaloric properties of ferromagnets, such as the dependence of delta-S(M) in delta-H and T, and studied how the Landau coefficients, related to magnetoelastic coupling and electron spin condensation energy in manganites, and their temperature dependence can be used to interpret the change from 1st- to 2nd-order phase transitions (PT). The insight given by the Landau theory is therefore of extreme value when one wants to understand how the couplings of the studied systems influence a 1st- or 2nd-order PT and the magnetic entropy changes, guiding materials modifications and conditions for optimization. However, the Landau theory is not designed to account for the magnetic behaviour up to high fields/ magnetic saturation, since it depends on a power expansion of the order parameter. This limitation can be overcome by using a generalized mean-field analysis of magnetization data, which is also well suited to estimate magnetocaloric properties of materials. The generalization proposed does not suppose a constant lambda, and instead allows extracting the dependence of lambda in M and/or T to account for additional couplings of the magnetization to other degrees of freedom. The authors show results of this analysis for manganite systems with 1st- and 2nd-order PT, and T(c) ranging from 150 to 340 K. This method allows one to analyze the dependence of lambda in M and/or T, which would respectively indicate adiabatic magnetoelastic couplings or vibronic coupling proposed for manganites. The analysis of the data in a large temperature and field range allows a suitable interpolation scheme and a consistent study of the magnetic entropy.

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Details

  • Original title: Interpreting and modelling magnetocaloric data and properties: the Landau theory of phase transitions and mean-field theory.
  • Record ID : 2007-1559
  • Languages: English
  • Source: 2nd International Conference on Magnetic Refrigeration at Room Temperature
  • Publication date: 2007/04/11

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