Mössbauer Spectral Study of the Magnetocaloric FeMnP₁₋ₓAsₓ Compounds
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The magnetic phase transitions in the FeMnP1−xAsx compounds with x= 0.25, 0.35, 0.45, 0.50, and 0.55, have been studied by iron-57 Mössbauer spectroscopy. The ferromagnetic and antiferromagnetic spectra have been analyzed with a model that takes into account the random distribution of the P and as near-neighbor anions of a given iron site. This distribution is a binomial distribution of the contributions to the spectra of each iron with n as near neighbors. A magnetostriction model has been used to characterize the temperature induced paramagnetic to ferromagnetic first-order phase transition and order parameters, ηF=2.6 , 2.3, 2.0, 1.57, and 1.43 have been obtained for x=0.25 , 0.35, 0.45, 0.50, and 0.55, respectively. A detailed phase diagram has been derived from the Mössbauer spectral analysis and reveals a magnetic triple point at x=∼0.35 and ∼210 K . A model that takes into account the random binomial P and as distribution and the contribution from the iron and manganese magnetic sublattices yields excellent fits of the spectral components assigned to the ferromagnetic and incommensurate antiferromagnetic components for the x=0.25 and 0.35 compounds at all temperatures.