Giant Magnetoelastic Coupling in a Metallic Helical Metamagnet

Using high resolution neutron diffraction and capacitance dilatometry we show that the thermal evolution of the helimagnetic state in CoMnSi is accompanied by a change in interatomic distances of up to 2%, the largest ever found in a metallic magnet. Our results and the picture of competing exchange and strongly anisotropic thermal expansion that we use to understand them sheds light on a new mechanism for large magnetoelastic effects that does not require large spin-orbit coupling.

Figure 1

Thermal expansion ${ϵ}^i$ of CoMnSi and ${\mathrm{Co}}_{0.95}{\mathrm{Ni}}_{0.05}\mathrm{MnSi}$ along each crystal axis measured on powders using HRPD. Lines are a guide to the eye. The largest change in cell parameters, including an $a$ axis NTE, occurs well below the metamagnetic transition temperature $T_t$. Bulk thermal expansion of ${\mathrm{Co}}_{0.95}{\mathrm{Ni}}_{0.05}\mathrm{MnSi}$ measured using a dilatometer indicates that our as-prepared ingots are textured along the $a$ axis.

Figure 2

Temperature variation of Mn-Mn nearest-neighbor distances $d_1$ and $d_2$ in CoMnSi (a) and ${\mathrm{Co}}_{0.95}{\mathrm{Ni}}_{0.05}\mathrm{MnSi}$, (b) from HRPD. As with $a(T)$, the fastest variation of $d_1$ and $d_2$ occurs well below $T_t$. The crystallographic positions of $d_1$ and $d_2$ are shown in (c), together with the closest commensurate version (for clarity, as modeled later) of the helical spin structure adopted by Co and Mn [9]. Three unit cells are shown in the $c$ direction, drawn using VESTA [10].

Figure 3

(a) Isothermal magnetization of bulk ${\mathrm{Co}}_{0.95}{\mathrm{Ni}}_{0.05}\mathrm{MnSi}$. (b) Transverse magnetostriction of a textured polycrystal of the same material. Magnetostriction is dominated by the $a$-axis texture of the sample (see Fig. 1). The onset of tricriticality is marked by an increase in both magnetostriction and the peak MCE (c).

Figure 4

A comparison of CoMnX ($X=\mathrm{P}$, Si, Ge). (a) and (b) show literature-derived lattice parameters and $d_1$, $d_2$ at 300 K as a function of average $p$ atom size [4, 12, 13, 14]. In (c) we show the electronic densities of states (DOS) majority (upper figure) and minority (lower figure) spins near the Fermi energy, $E_F$ calculated for a ferromagnetic (FM) ground state. In (d) we see that the calculated DOS at $E_F$ of noncollinear AFM CoMnSi is lower than that of a fictitious FM ground state.