Designed metamagnetism in CoMnGe${}_{1-x}$P${}_x$

We extend our previous theoretical study of Mn-based orthorhombic metamagnets to those that possess large nearest neighbor Mn-Mn separations ($d{}_1\gtrsim 3.22$ Å). Based on our calculations, we design and synthesize a series of alloys, CoMnGe${}_{1-x}$P${}_x$, to experimentally demonstrate the validity of the model. Unusually, we predict and prepare several metamagnets from two ferromagnetic end-members, thus demonstrating a new example of how to vary crystal structure, within the Pnma symmetry group, to provide highly tunable metamagnetism.

Figure 1

Stability of possible collinear magnetic structures, relative to ferromagnetism, within a single unit cell of MnP as a function of $d{}_1$ Mn-Mn separation. AFM configurations become stable where $▵E_{\mathrm{AFM}-\mathrm{FM}}<0$. The vertical dashed line represents the experimental (strain free, $\varepsilon =0$) lattice of MnP. We see that the FM state is first destabilised by lattice expansion, and then becomes stable again at large $d{}_1$values: FM(2).

Figure 2

Collinear FM total density of states for CoMnGe${}_{1-x}$P${}_x$ with $x=0$, 0.5, and 1. The Fermi energy falls into the hybridization gap for both CoMnP and CoMnGe, but shifted for CoMnP${}_{0.5}$Ge${}_{0.5}$ resulting in a large $N_{\mathrm{Tot}}(E_{\mathbb{F}})$.

Figure 3

Representative SEM micrograph and corresponding elemental mapping from EDX of CoMnGe${}_{0.75}$P${}_{0.25}$.

Figure 4

Volume fraction of the orthorhombic CoMnGe${}_{1-x}$P${}_x$ and hexagonal CoMnGe phases as a function of nominal composition (top) and lattice parameters of the main orthorhombic phase as a function of corrected phosphor concentration (bottom). The lattice parameters of CoMnSi, a metamagnet, are added (star symbols) for comparison.

Figure 5

Magnetization loops of CoMnGe${}_{1-x}$P${}_x$ at 10  K.

Figure 6

Isofield magnetization of CoMnGe${}_{1-x}$P${}_x$ as a function of temperature, measured in a 1 Tesla applied field. We observe metamagnetism in almost all samples, and a sharp change in the metamagnetic critical temperature with composition in the range $\mathit{x}=0.4$ to 0.6.