Electronic and magnetic properties of phosphorus across the first-order ferromagnetic transition of $(\text{Mn},\text{Fe}){}_2(\text{P},\text{Si},\text{B})$ giant magnetocaloric materials

To study the evolution of the electronic states and magnetism of the metallic and metalloid atoms across the first-order ferromagnetic transition of (Mn,${\mathrm{Fe})}_2$(P,Si,B) compounds, x-ray absorption (XAS) and magnetic circular dichroism (XMCD) spectra were recorded at the $K$ edges of P, Mn, and Fe. Surprisingly, while the Fe and Mn XAS spectra do not show pronounced changes, the $K$-edge XAS of P is strongly modified across the ferromagnetic transition and is a fingerprint of electronic structure changes at the phase transition. The evolution of the $3p$ electronic states of phosphorus is linked to changes in metal-metalloid bonding. The amplitude of this effect is correlated to the discontinuity in interatomic distances at the transition and progressively disappears with boron substitution. Moreover, it is found that the P atoms carry a finite magnetic moment in the ferromagnetic state in accordance with the results of band-structure calculations.

Figure 1

Schematic representation of the MnFe(P,Si) unit cell in the ferromagnetic state. The arrows mark the evolution of the metal-metalloids distances across the ferromagnetic transition (upon warming).

Figure 2

Magnetization as a function of the temperature measured upon cooling (open symbols) and heating (filled symbols) at $B=1$ T for ${\mathrm{MnFe}}_{0.95}{\mathrm{P}}_{0.66-x}{\mathrm{Si}}_{0.34}{\mathrm{B}}_x$ compounds with $x=0$ (squares), 0.040 (circles), and 0.075 (triangles).

Figure 3

(a),(b) XAS spectra for ${\mathrm{MnFe}}_{0.95}{\mathrm{P}}_{0.66}{\mathrm{Si}}_{0.34}$ at $T=30$ K (black) and 310 K (red) for the $K$-edge of Mn and Fe, respectively. (c),(d) XMCD spectra for ${\mathrm{MnFe}}_{0.95}{\mathrm{P}}_{0.66}{\mathrm{Si}}_{0.34}$ at $T=30$ K and $B=0.9$ T for the $K$-edge of Mn and Fe, respectively. The XMCD signal is normalized to the edge jump.

Figure 4

$K\text{−}\mathrm{edge}$ of P for ${\mathrm{MnFe}}_{0.95}{\mathrm{P}}_{0.66}{\mathrm{Si}}_{0.34}$: (a) XAS spectra at $T$ = 30 K and 310 K. (b) XMCD spectrum at $T=30$ K and $B=0.9$ T, normalized to the edge jump.

Figure 5

(a) XAS spectra at the $K\mathrm{edge}$ of P for ${\mathrm{MnFe}}_{0.95}{\mathrm{P}}_{0.66-x}{\mathrm{Si}}_{0.34}{\mathrm{B}}_x$ with $x=0$, 0.040, and 0.075 at $T=30$ and 310 K. (b) Evolution of the XAS spectra across the ferromagnetic transition as a function of the boron content: difference in the energy position of the peak A between 30 and 310 K (squares, left axis) and difference in the integral of XAS spectra recorded at $T=30$ K and 310 K (circles, right axis). The range of integration is chosen between 2140 and 2152 eV.

Figure 6

Partial densities of state for ${\mathrm{MnFeP}}_{2/3}{\mathrm{Si}}_{1/3}$ (i) in the ferromagnetic state (a),(b) and in a nonmagnetic state (c),(d).

Figure 7

(a) Comparison between the calculated ${\mathrm{MnFeP}}_{2/3}{\mathrm{Si}}_{1/3}$(i) and experimental $K$-edge XMCD spectra of P for ${\mathrm{MnFe}}_{0.95}{\mathrm{P}}_{0.66}{\mathrm{Si}}_{0.34}$ at $T=30$ K and $B=0.9$ T. (b) Calculated $K$-edge XAS spectra of P for the low-temperature and room-temperature states. The top curves correspond to a weighted average of $2c$ and $1b$ sites contributions (with an energy-dependent broadening). The two lower curves represent the contribution of each site (fixed Lorentzian broadening).