Magnetoelastic coupling and competing entropy changes in substituted CoMnSi metamagnets

We use neutron diffraction, magnetometry, and low-temperature heat capacity to probe giant magnetoelastic coupling in CoMnSi-based antiferromagnets and to establish the origin of the entropy change that occurs at the metamagnetic transition in such compounds. We find a large difference between the electronic density of states of the antiferromagnetic and high-magnetization states. The magnetic field-induced entropy change is composed of this contribution and a significant counteracting lattice component, deduced from the presence of negative magnetostriction. In calculating the electronic entropy change, we note the importance of using an accurate model of the electronic density of states, which here varies rapidly close to the Fermi energy.

Figure 1

Specific heat capacity as a function of temperature for four CoMnSi-based alloys. The anomaly around 380 K is assigned to the Néel phase transition, $T_N$. Measurements were conducted in zero magnetic field. Despite the small difference in Néel temperature (shown by the arrows), the alloys have quite different magnetic characteristics and strong variations in thermal expansion. Symbols and colors are the same as in Figs. 2 and 3.

Figure 2

Temperature dependence of lattice parameters measured on powders using HPRD. Similar features are seen in all four samples. The large change in cell parameters, including an $a$-axis NTE, occurs well below the Néel transition temperature, $T_N$. Such features are strongest in the Ni- and Fe-substituted samples.

Figure 3

Manganese nearest-neighbor distances as a function of temperature for the same four samples as in Fig. 2. Distance $d_1$ is between different chains of manganese and $d_2$ is between Mn on the same chain.

Figure 4

Top: critical field vs temperature phase diagram for the metamagnetic transition of the four main alloys studied here. Bottom three plots: isothermal magnetization data at 300, 250, and 200 K as examples of the data used to construct the phase diagram. Tricritical points are seen from the narrowing of the hysteresis. Data for the cms40-a sample of CoMnSi are shown.

Figure 5

Magnetic susceptibility of the four main alloys studied here measured at 137 Hz in a 100-A/m field. The susceptibilities of CoMnSi and CoMn${}_{0.98}$Cr${}_{0.02}$Si are lower than that of the other two alloys and are plotted on a separate axis.

Figure 6

Isothermal entropy change of the four main alloys studied in integer field steps up to 5 T (top) or 9 T (bottom). We see that increased proximity to a room-temperature tricritical point in Fe- and Ni-doped samples increases the low-field entropy change. The color scheme used is the same in all four plots, and up- or down-pointing triangles represent the effect of applying the Maxwell relation to increasing-field or decreasing-field $M\left(H\right)$ data.

Figure 7

Magnetic field dependence of lattice parameters of CoMnSi (cms40-a) as a function of temperature. The zero-field values agree well with the data obtained from HRPD on a different sample of CoMnSi (cms38-a). The magnitude of the lattice parameter changes increases with decreasing temperature.

Figure 8

Low-temperature specific heat data for four CoMnSi-based metamagnets: the $C/T$ axis intercept is the Sommerfeld coefficient, $\gamma$, and is tabulated in Table .

Figure 9

A simple V-shaped model DOS for the AFM state near $E_F$ (red solid line, inset) yields a reduced entropy change (red solid line, main figure) relative to that of two energy-independent DOS in both the AFM and FM states.