Microscopic Understanding of Negative Magnetization in Cu, Mn, and Fe Based Prussian Blue Analogues

A crossover of the field-cooled magnetization from positive to negative has been observed below the magnetic ordering temperature (17.9 K) in a multimetal Prussian Blue analogue (PBA), ${\mathrm{Cu}}_{0.73}{\mathrm{Mn}}_{0.77}[\mathrm{Fe}(\mathrm{CN}{)}_6]·z{\mathrm{H}}_2\mathrm{O}$. The reverse Monte Carlo (RMC) modeling (using the program RMCPOW) has been used to derive the various scattering contributions (e.g., nuclear diffuse, nuclear Bragg, magnetic diffuse, and magnetic Bragg) from the observed neutron diffraction patterns. The RMC analysis combined with the Rietveld refinement technique show an antiferromagnetic ordering of Mn moments with respect to the Cu as well as the Fe moments. Our study gives the first neutron magnetic structure evidence towards the microscopic understanding of the negative magnetization in the PBAs. This information can be effectively utilized to design suitable PBAs for making multifunctional devices.

Figure 1

(a) Rietveld refined neutron diffraction pattern for the $x=0.486$ compound at room temperature. The open circle shows the observed data, and the Rietveld calculated pattern is shown by the solid line. The solid line at the bottom is the difference curve between the observed and the Rietveld calculated curves. The short vertical lines show the position of nuclear Bragg peaks. (b) The observed (open circle) and derived (using RMCPOW) structure factors F(Q) for the 80 K pattern. The thin (red) and thick (blue) solid lines show the calculated nuclear Bragg and nuclear diffuse scattering contributions, respectively.

Figure 2

FC magnetization curves at 200 Oe for the two end compounds (inset) and the $x=0.486$ compound of the $\{{\mathrm{Cu}}_x{\mathrm{Mn}}_{1-x}{\}}_{1.5}\mathrm{Fe}(\mathrm{CN}{)}_6·z{\mathrm{H}}_2\mathrm{O}$ series.

Figure 3

The $Q$ dependence of various intensities at 1.7 K: (a) Observed diffraction pattern (open circle) and the total calculated intensity (solid line) using the RMCPOW, (b) derived nuclear diffuse, (c) derived magnetic diffuse (open square) and total magnetic scattering, i.e., sum of magnetic diffuse and magnetic Bragg (solid line), and (d) the derived magnetic Bragg scattering pattern (open circle), calculated magnetic Bragg (solid line) with the Rietveld refinement. The difference pattern between the derived and calculated magnetic Bragg patterns is shown by a solid line at the bottom.

Figure 4

Schematic representation of the magnetic structure for the ${\mathrm{Cu}}_{0.73}{\mathrm{Mn}}_{0.77}\mathrm{Fe}(\mathrm{CN}{)}_6.z{\mathrm{H}}_2\mathrm{O}$ compound at 1.7 K where 4a Wyckoff site is randomly occupied (maintaining the stoichiometry) by the Cu and Mn ions.