Order-disorder induced magnetic structures of FeMnP${}_{0.75}$Si${}_{0.25}$

We report on the synthesis and structural characterization of the magnetocaloric FeMnP${}_{0.75}$Si${}_{0.25}$ compound. Two types of samples (quenched and slowly cooled) were synthesized and characterized structurally and magnetically. We have found that minor changes in the degree of crystallographic order causes large changes in the magnetic properties. The slow-cooled sample, with a higher degree of order, is antiferromagnetic. The quenched sample has a net moment of 1.26 ${\mu }_B$ per formula unit and ferrimagnetic behavior. Theoretical calculations give rather large values for the Fe and Mn magnetic moments, both when occupied on the tetrahedral and the pyramidal lattice sites.

Figure 1

Fe${}_2$P structure with iron atom positions Fe(1) (dark gray) and Fe(2) (black), and phosphorus atom positions P(2) (light gray) and P(1) (white).

Figure 2

XRD patterns of (a) a slowly cooled and (b) a quenched FeMnP${}_{0.75}$Si${}_{0.25}$ sample. The blue lines mark the indexed peaks of the FeMnP${}_{0.75}$Si${}_{0.25}$ phase.

Figure 3

Room temperature Mössbauer spectra of FeMnP${}_{0.75}$Si${}_{0.25}$. Spectrum (a) corresponds to the quenched sample and spectrum (b) to the slowly cooled sample. The shaded doublets emanate from the pyramidal coordinated Fe(2) site.

Figure 4

Mössbauer spectra recorded at 77 K for FeMnP${}_{0.75}$Si${}_{0.25}$, quenched and slowly cooled. The light gray subpattern emanates from Fe(2) atoms and the dark gray subpattern from Fe(1) atoms.

Figure 5

The relative Fe magnetic moment population probabilities for FeMnP${}_{0.75}$Si${}_{0.25}$, quenched and slowly cooled samples. The conversion factor 10 T/${\mu }_B$ has been used in converting the magnetic hyperfine fields into magnetic moments. Gray distributions emanate from Fe(1) and hatched distributions from Fe(2) atoms.

Figure 6

Total energy per site for ordered and disordered phases of hexagonal FeMnP${}_{0.75}$Si${}_{0.25}$ as functions of the lattice parameter ($c/a=0.5894$). The ordered phases correspond to the Mn atom occupying the pyramidal (high moment) and tetrahedral (low moment, labeled Fe pyramidal) positions, respectively. In the disordered phase the two positions are randomly occupied by Mn and Fe atoms. The dashed vertical line indicates the experimental $a$ lattice parameter.

Figure 7

Ordered magnetic moments per site for ordered and disordered phases of hexagonal FeMnP${}_{0.75}$Si${}_{0.25}$ as functions of the lattice parameter ($c/a=0.5894$). The ordered phases, correspond to the Mn atom occupying the pyramidal (high moment) and tetrahedral (low moment, labeled Fe pyramidal) positions, respectively. In the disordered phase the two positions are randomly occupied by Mn and Fe atoms. The dashed-dotted vertical line indicates the experimental $a$ lattice parameter.

Figure 8

The site-projected moments of the disordered phases of FeMnP${}_{0.75}$Si${}_{0.25}$ as a function of the lattice parameter ($c/a=0.5894$). The dashed vertical line indicates the experimental $a$ lattice parameter.

Figure 9

The site-projected moments of the ordered phases of FeMnP${}_{0.75}$Si${}_{0.25}$ as a function of the lattice parameter ($c/a=0.5894$). The dashed vertical line indicates the experimental $a$ lattice parameter.

Figure 10

Magnetic susceptibility vs temperature for FeMnP${}_{0.75}$Si${}_{0.25}$ measured on the quenched sample in an applied field of $~$4 kA/m (black circles) and on the annealed sample under an applied field of $~$40 kA/m (red triangles, inset). The open symbols indicate measurements using a zero field cold (ZFC) protocol and the filled symbols using a field cooled (FC) protocol.

Figure 11

Magnetization vs applied field for a quenched sample at 30 K (black circles) and an annealed sample at 30 K (red triangles).

Figure 12

Experimental magnetic moments/f.u. in comparison with calculated moments assuming ferro-, ferri-, and antiferromagnetic ordering.