| Popis: |
This paper describes an experimental and theoretical examination of a 10-\ensuremath{\mu}g $c$-axis-oriented single crystal of MnAs. The critical magnetic field for the first-order transition to the ferromagnetic phase has been measured as a function of temperature (15 to 65\ifmmode^\circ\else\textdegree\fi{}C) and pressure (0 to 1000 bars gauge), using miniature-coil pulsed fields to 110 kOe. Data analysis substantiates the thesis of the recent Bean-Rodbell theory on magnetic first-order phase transitions that the transition in MnAs near 45\ifmmode^\circ\else\textdegree\fi{}C is between ferromagnetic and paramagnetic phases and arises from a sufficiently sensitive dependence of exchange energy on lattice strain and a sufficiently high compressibility. The match between theory and experiment yields a compressibility $K$ of 4.55\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}12}$ ${\mathrm{dyn}}^{\ensuremath{-}1}$ ${\mathrm{cm}}^{2}$, a lattice thermal expansion coefficient $\ensuremath{\alpha}$ of 5.71\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}5}$/\ifmmode^\circ\else\textdegree\fi{}C, an apparent paramagnetic Curie temperature ${{T}_{0}}^{*}$ of 277.1\ifmmode^\circ\else\textdegree\fi{}K, and a Curie-temperature dependence on volume strain, $\ensuremath{\beta}[\ensuremath{\equiv}(\frac{d{T}_{c}}{{T}_{0}}){(\frac{\mathrm{dV}}{{V}_{0}})}^{\ensuremath{-}1}]$, of 18.9, where ${T}_{0}$ is the Curie temperature of the unstrained specimen at 0\ifmmode^\circ\else\textdegree\fi{}K. Discrepancies in the match suggest a primary need to include short-range order in the theory. Auxiliary experiments show the magnetocrystalline anisotropy sum, ${K}_{1}+2{K}_{2}+3{K}_{3}$, to be about -7.6 and -12.0\ifmmode\times\else\texttimes\fi{}${10}^{6}$ ergs/${\mathrm{cm}}^{3}$ at 299 and 77\ifmmode^\circ\else\textdegree\fi{}K, respectively. |
