Local magnetism in the molecule-based metamagnet [Ru${}_2$(O${}_2$CMe)${}_4$]${}_3$[Cr(CN)${}_6$] probed with implanted muons

We present a muon-spin relaxation study of local magnetism in the molecule-based metamagnet [Ru${}_2$(O${}_2$CMe)${}_4$]${}_3$[Cr(CN)${}_6$]. We observe magnetic order with $T_{\mathrm{N}}=33$ K, although above 25 K the sublattice spins become less rigid and a degree of static magnetic disorder is observed. The comparison of measurements in applied magnetic field with simulations allows us to understand the origin of the muon response across the metamagnetic transition and to map out the phase diagram of the material. Applied hydrostatic pressures of up to 6 kbar lead to an increase in the local magnetic field along with a complex change in the internal magnetic-field distribution.

Figure 1

(a) ZF ${\mu }^{+}$ SR showing the temperature evolution of the signal for temperatures $2⩽T⩽35$ K. (Spectra have been offset for clarity.) (b) Proposed ground state (Ref. 4) of a single sublattice. (c) Temperature evolution of the muon-spin precession frequency, which ceases to be resolvable above $T\approx 25$ K, and the relaxation rate $\sigma$ of the fast relaxing component.

Figure 2

(a) The evolution of the initial asymmetry $A\left(0\right)$ with temperature at applied LFs. (b) Evolution of $\Delta A(\propto A_{\mathrm{osc}})$ with LF at several temperatures. Lines are guides to the eye showing the positions of changes in slope we identify with magnetic transitions. Inset: Detail of measurement of $A\left(0\right)$ made at 30 K showing a typical transition.

Figure 3

(a) Zero pressure behavior for $\Delta A$ compared with simulations. Inset: $\Delta A$ normalized by the prediction of Eq. (1) for a single site with $B_0=39$ mT. (b) Pressure dependence of $\Delta A$ as a function of applied magnetic field. The lines represent fits described in the text.

Figure 4

Notional phase diagram at ambient pressure derived from our measurements (see main text).