Microscopic theory of magnetism in the magnetocaloric material ${\mathrm{Fe}}_2$P${}_{1-x}$T${}_x$ ($\mathrm{T}=\mathrm{B}$ and Si)

Landau phenomenological theory in combination with first-principles calculations was used to reveal the origin of the metamagnetic nature and the unusually strong dependence of the ordering temperature with doping of the Fe${}_2$P compound. We show that the magnetism of the two sublattices occupied by Fe atoms has an entwined codependency, which is strongly influenced by alloying. We furthermore demonstrate that a constrained disordered local moment approach combined with Monte Carlo simulations can only reproduce the experimental ordering temperatures in these technologically important prototype alloys for magnetocaloric refrigeration.

Figure 1

Magnetic energy surface (in units of mRy) for Fe${}_2$P, Fe${}_2$P${}_{0.9}$Si${}_{0.1}$, and Fe${}_2$P${}_{0.92}$B${}_{0.08}$ calculated as a function of the local magnetic moments at Fe-I ($m_1$) and Fe-II ($m_2$) sites. Energies are shown relative to the nonmagnetic value at $m_1=m_2=0$.

Figure 2

Magnetic energy [$E(m_1,m_2$)] plotted as a function of $m_1$ for $m_2=0$. Solid line, Fe${}_2$P; dashed line, Fe${}_2$P${}_{0.9}$Si${}_{0.1}$; and dashed-dotted line, Fe${}_2$P${}_{0.92}$B${}_{0.08}$.

Figure 3

Theoretical Curie temperatures calculated for FM (squares) and cDLM (circles) states. Experimental data[11, 19] are shown by triangles.