Magnetic Bistability and Nucleation of Magnetic Bubbles in a Layered 2D Organic-Based Magnet $[\mathrm{Fe}(\mathrm{TCNE})(\mathrm{NCMe}{)}_2][{\mathrm{FeCl}}_4]$

The 2D layered organic-based magnet $[\mathrm{Fe}(\mathrm{TCNE})(\mathrm{NCMe}{)}_2][{\mathrm{FeCl}}_4]$ ($\mathrm{TCNE}=\mathrm{\text{tetracyanoethylene}}$) exhibits a unique macroscopic magnetic bistability between the field-cooled and zero-field-cooled states, which cannot be explained by either superparamagnetic behavior or spin freezing due to spin glass order. This magnetic bistability is described through consideration of the ensemble of uncoupled 2D Ising layers and their magnetization reversal initiated by a field-induced nucleation of magnetic bubbles in individual layers. The bubble nucleation rate strongly depends on the external field and temperature resulting in anomalous magnetic relaxation.

Figure 1

(a) Field-cooled (solid symbols) and zero-field-cooled (open symbols) magnetization for different applied magnetic fields ($H=10$, 50, 100, 150, 200, 300, 500, 1000 Oe). (b) Time relaxation of $M_{\mathrm{ZFC}}$, $M_{\mathrm{FC}}$, and $M_{\mathrm{TRM}}$ at $T=70\mathrm{K}$, $H=100\mathrm{Oe}$ for waiting time $t_w={10}^3$ and ${10}^4\mathrm{s}$.

Figure 2

(a) A $T$ dependent crossover field obtained from minor hysteresis loop as described in inset. Inset shows minor hysteresis loop for field scan range of $\pm 1000$ and $\pm 2000\mathrm{Oe}$. (b) $M_{\mathrm{FC}}(T)$ (open symbols) and $M_{\mathrm{ZFC}}(T)+M_{\mathrm{TRM}}(T)$ (solid symbols) for different applied fields ($H=50$, 100, 150, 200, 300, 500, 1000 Oe). Inset shows $M_{\mathrm{ZFC}}(T)+M_{\mathrm{TRM}}(T)-M_{\mathrm{FC}}(T)$ for each applied field.

Figure 3

(a) The characteristic relaxation time ${\tau }_c$ ($T$) for dc applied field $H=0$, 50, and 100 Oe, obtained from complex linear susceptibility. The dashed line indicates a power-law fit. (b) The characteristic relaxation time ${\tau }_c(H)$ for a fixed temperature $T=83.0$ and 83.5 K. Inset displays broadening of width of spin relaxation time distribution as increasing external field.