High field magnetization of ${\mathrm{FePS}}_3$

High field magnetization measurements in pulsed fields up to 65 T have been performed on ${\mathrm{FePS}}_3$, which is nominally a good example of a two-dimensional Ising-like antiferromagnet on a honeycomb lattice. Measurements with the field parallel to the moment direction confirm the presence of two first-order transitions above 35 T, to $M/M_{\mathrm{sat}}=1/2$ and $M/M_{\mathrm{sat}}=1$, respectively, at 4 K. The measurements are in contradiction with published estimates for the magnetic exchange parameters, but the contradiction can be resolved by allowing for anisotropic exchange parameters in the Hamiltonian. The magnetization with the field perpendicular to the moment direction is anisotropic, with no transitions observed for fields along the a axis while a cascade of first-order transitions is observed for fields above 50 T along the b axis, the latter case also showing a strong degradation of the sample after repeated pulses. The results indicate a strong magnetolattice coupling in ${\mathrm{FePS}}_3$. Temperature-dependent measurements hint at a possible tricritical point.

Figure 1

(a) The magnetic structure of ${\mathrm{FePS}}_3$ in zero field, showing the magnetic moments on the ${\mathrm{Fe}}^{2+}$ ions. The moments point normal to the $ab$ planes. The figure was created using the VESTA program [13]. (b) The magnetic structure in the $ab$ plane, with examples of the exchange interactions between first, second, and third nearest neighbors being labeled with $J_1,J_2$, and $J_3$, respectively.

Figure 2

Calculated phase diagram for the Ising Hamiltonian [equation (2)] on a honeycomb lattice, along with the number and magnitude of plateaus present in the magnetization for H$\left|\right|$c${}^{*}$. The solid lines between the colored regions mark the boundaries between the stable structures in zero field. The stable structures evolve into either partially or fully magnetized states with an increasing magnetic field, and the dashed lines demarcate the regions with different field evolutions. The establishment of each structure will create a plateau in the magnetization as the field is increased, and the magnitudes of the magnetization for each plateau are noted as the fraction of the saturation moment, $M/M_{\mathrm{sat}}$, in the corresponding region of the phase diagram. Schematics for the different magnetic structures are shown, with open and filled circles representing the two directions for the spins, along with a representation of the unit cell for each bound by the red dotted lines. Note that there are two possible structures with $M/M_{\mathrm{sat}}=1/2$, labeled (A) and (B), respectively. The energies for all the states are given in Table 2. Data points for the values in Table 1 are plotted on the figure as square symbols, with the values from Okuda et al. [19] shown as a white-filled square and the values from Lançon et al. [11] shown as a black-filled square.

Figure 3

The time dependence of a typical pulse to 65 T.

Figure 4

Post-measurement photographs of the samples used for the pulsed field experiments for (a) H$\left|\right|$c${}^{*}$, (b) H$\left|\right|$a, and (c) H$\left|\right|$b. The ${\mathrm{FePS}}_3$ samples are black and have been inserted into plastic capsules. Samples (a) and (b) have been fixed to wooden mounts, whereas sample (c) was fixed directly to the wall of the capsule. A small amount of vacuum grease was used to fix the samples.

Figure 5

SQUID and pulsed field magnetization measurements of ${\mathrm{FePS}}_3$ in gaseous helium, performed at 4 K for a field applied along different crystallographic axes. The arrows on the figure with the pulsed data show whether the field was increasing or decreasing for the corresponding data.

Figure 6

High field magnetization for the field applied along different crystallographic directions and as a function of whether the sample was in a liquid or gaseous helium environment.

Figure 7

High field magnetization for H$\left|\right|\left[110\right]$ as a function of sample temperature. The data have been successively shifted up the vertical axis by 0.$5{\mu }_B/$ Fe for clarity. The sample was in a gaseous helium environment for all the measurements.

Figure 8

High field magnetization for H$\left|\right|\left[110\right]$ as a function of the maximum field for the pulse, $H_{\max }$. The data have been successively shifted up the vertical axis by 0.$7{\mu }_B/$ Fe for clarity. The sample was at 4.2 K in a liquid helium environment.