Tuning the metamagnetism of an antiferromagnetic metal

We describe a “disordered local moment” first-principles electronic structure theory which demonstrates that tricritical metamagnetism can arise in an antiferromagnetic metal due to the dependence of local moment interactions on the magnetization state. Itinerant electrons can therefore play a defining role in metamagnetism in the absence of large magnetic anisotropy. Our model is used to accurately predict the temperature dependence of the metamagnetic critical fields in CoMnSi-based alloys, explaining the sensitivity of metamagnetism to Mn-Mn separations and compositional variations found previously. We thus provide a finite-temperature framework for modeling and predicting different metamagnets of interest in applications such as magnetic cooling.

Figure 1

(a) The Mn local moment interactions $S^{\left(2\right)}(\vec{q},\vec{m}=0)$ for CoMnSi with structures measured in neutron diffraction experiments labeled by the Mn-Mn spacing $d_1$ in Å. The inset shows $S^{\left(2\right)}({\vec{q}}_{\text{max}},\vec{m})$ vs order parameter $m$ for $d_1=3.07$ Å. (b) $m$ vs applied field for several temperatures for a fixed lattice structure, in which $d_1=$ 3.07 Å. A tricritical point is indicated at 372 K, 2 T.

Figure 2

(a) The critical field $B_c$ for a transition between a helical AFM and a high magnetization state vs $T$ for CoMnSi for the four structures used in Fig. 1 and (b) for Co${}_{0.95}$Ni${}_{0.05}$MnSi and CoMn${}_{0.95}$Cr${}_{0.05}$Si, with $d_1=$ 3.057 Å  (tricritical points denoted by asterisks).

Figure 3

(a) Experimental $B_c\left(T\right)$ for CoMnSi, Co${}_{0.95}$Ni${}_{0.05}$MnSi, and CoMn${}_{0.98}$Cr${}_{0.02}$Si (Ref. 30). (b) Theoretical $B_c\left(T\right)$ for the same three systems, modeled using the lattice parameters found from neutron diffraction (Ref. 30) in zero field. The $B_c\left(T\right)$ of the Ni- and Cr-doped systems modeled using the CoMnSi structural data is also shown (dashed lines) to highlight the relative importance of accurate structural data.