Structurally driven metamagnetism in MnP and related $Pnma$ compounds

We investigate the structural conditions for metamagnetism in MnP and related materials using density-functional theory. A magnetic stability plot is constructed taking into account the two shortest Mn-Mn distances. We find that a particular Mn-Mn separation plays the dominant role in determining the change from antiferromagnetic to ferromagnetic order in such systems. We establish a good correlation between our calculations and structural and magnetic data from the literature. Based on our approach it should be possible to find Mn-containing alloys that possess field-induced metamagnetism and associated magnetocaloric effects.

Figure 1

(Color online) (X)MnP-type ($Pnma$, 62) orthorhombic structure [the diagram was visualized using VESTA (Ref. 19)]. The dominant $d_1$ and $d_2$ Mn-interatomic distances are constructed with dashed and solid lines, respectively.

Figure 2

(Color online) Total energy of nonmagnetic (NM), FM, and AFM solutions as a function of isotropic lattice expansion and compression in MnP.

Figure 3

(Color online) Difference between the total energies of AFM and FM solutions in MnP as a function of $d_1$ and $d_2$, the two shortest Mn-Mn separations. AFM configurations become stable where $\Delta E_{\text{AFM-FM}}<0$. The vertical dashed line represents the experimental (strain-free, $\epsilon =0$) Mn-separation values.

Figure 4

(Color online) Mn magnetic moment vs strain for different magnetic states in MnP (see Table ).

Figure 5

(Color online) Density of states for binary MnP alloy. The total density of states at the Fermi level $\left[N_{Tot}\left(E_{\mathbb{F}}\right)\right]$ decreases for antiferromagnetic (AFM1) configurations (a) with lattice expansion from $\epsilon =0$ (solid line) to $\epsilon =0.055$ (dashed line) while an opposite tendency is observed in (b) for FM alignment.

Figure 6

(Color online) Experimentally observed Mn-Mn distances $\left(d_1,d_2\right)$ in $\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)\text{MnP}$ pseudoternary alloy at 100 K [after (Ref. 30)]. Antiferromagnetism becomes stable as $d_1$ increases above $\sim 2.95\text{Å}$, in line with our calculations on MnP in Fig. 3. Temperature-induced metamagnetism is seen for $0.5\lesssim x\lesssim 0.8$.

Figure 7

(Color online) Temperature evolution of the two shortest Mn-Mn distances $\left(d_1,d_2\right)$ in ternary ${\text{MnAs}}_{0.88}{\text{P}}_{0.12}$ pseudoternary alloy as found experimentally (Ref. 14). In accordance with Fig. 3 the AFM state is stabilized when $d_1$ is greater than $\sim 2.95–2.98\text{Å}$.

Figure 8

(Color online) Mn-Mn separation $\left(d_1,d_2\right)$ observed in XMnSi ternary alloys, as a function of effective X atomic radius (low-temperature data) (Refs. 17, 36, 37, 38, 39). Dashed lines are fitted linearly to data and are only a guide to the eye.

Figure 9

(Color online) Evolution of the two shortest Mn-Mn distances $\left(d_1,d_2\right)$ in pseudoternary ${\text{NiMnGe}}_{1-x}{\text{Si}}_x$ as a function of composition at 80 K. Field-induced metamagnetism is reported within the range $0.25\lesssim x\lesssim 0.55$ (Ref. 37).