Realization of a Magnet Using an Antiferromagnetic Phase of Single-Chain Magnets

A system presenting an antiferromagnetic (AF) phase of single-chain magnets (SCMs) is studied at low temperatures. The phase diagram is first discussed to specify the magnitude of the interchain couplings. Then, we show experimentally and theoretically that slow relaxation of the magnetization can still be observed in the AF phase, with a maximum of the relaxation time close to the AF-paramagnetic phase transition. This counterintuitive result shows that materials presenting such an ordered state can be used to design high temperature magnets using SCM-based materials.

Figure 1

(a) Field dependence of the magnetization below 2.9 K (field sweeping rate: $280\mathrm{mT}/\mathrm{s}$) measured in the easy magnetic direction ($a+c$) for a single crystal; (b) Field dependence of the single crystal susceptibility from dc measurements at 2.9 K (dots) with a field sweeping rate of $2.2\mathrm{mT}/\mathrm{s}$. The susceptibility deduced from relaxation data after normalization is also shown (squares). Powder magnetic susceptibility data at three different applied fields are shown in inset. The straight line gives the activation energy, here ${\Delta }_{\xi }/k_B=18\mathrm{K}$.

Figure 2

Location of the maximum of susceptibility from $M(H)$: (◆) single crystal, (▾) powder measurements, or (▴) from the $T$ dependence of the powder ac susceptibility at a given dc field. Experimental points deduced from the dynamic measurements: (▪) location on the main maximum of the relaxation time and (●) location of the second maximum (inversion point). The continuous line is the theoretical estimation of the AF-paramagnetic phase transition, and the dashed line gives the line of inversion points ($4J_{\parallel }{S}^2/k_B=20.2\mathrm{K}$ and $r=0.3$). The arrows are schematizing the orientation and magnitude of the two order parameters ($m_1$ and $m_2$).

Figure 3

Time dependence of the normalized magnetization for different values of the applied magnetic field at 2.9 K. Inset: relaxation time versus $1/T$ in a semilog plot in zero-dc field (data on a single crystal and powder sample are shown in squares and dots, respectively). The continuous line emphasizes the activated behavior of the relaxation time with ${\Delta }_{\tau }/k_B=54\mathrm{K}$ and ${\tau }_0=3.1\times {10}^{-10}\mathrm{s}$.

Figure 4

Deduced relaxation time (normalized at zero field): dots (a) at $T=2.9\mathrm{K}$, (b) at $T=3.3\mathrm{K}$. The dotted line shows the expected relaxation time when the magnetization $m_1$ of the sublattice $\#1$ is saturated. The continuous line gives the calculated relaxation time.