Demonstrating multibit magnetic memory in the ${\mathrm{Fe}}_8$ high-spin molecule by muon spin rotation

We develop a method to detect the quantum nature of high-spin molecules using muon spin rotation and a three-step field cycle ending always with the same field. We use this method to demonstrate that the ${\mathrm{Fe}}_8$ molecule can remember six (possibly eight) different histories (bits). A wide range of fields can be used to write a particular bit, and the information is stored in discrete states. Therefore, ${\mathrm{Fe}}_8$ can be used as a model compound for multibit magnetic memory. Our experiment also paves the way for magnetic quantum tunneling detection in films.

Figure 1

(Color online) An indirect $\mu \mathrm{SR}$ experimental setup showing the mosaic of single crystals and the external field $\left(H\right)$ and the beam directions, which are both in the molecules $\widehat{\mathbf{z}}$ direction. $\mathbf{M}$ is a possible field at the muon site due to the molecules, and $\mathbf{B}$ is the total field around which the muon spin rotates. The positron detectors are also presented.

Figure 2

(Color online) The muon asymmetry as a function of time from a sample of ${\mathrm{Fe}}_8$ or empty silver sample holder and different intermediate fields $H_i$ for both an indirect configuration where muons hit the silver sample holder only (a1)–(a4) and a direct configuration where muons hit mostly the ${\mathrm{Fe}}_8$ (b1)–(b6). The different intermediate fields $\left(H_i\right)$ lead to different muon rotation frequencies when ${\mathrm{Fe}}_8$ is included [(a1)–(a2) and (b1)–(b3)], but to similar frequencies in an empty holder [(a3)–(a4) and (b4)–(b6)].

Figure 3

(Color online) The shift in the muon low rotation frequency ${\omega }_l$ as a function of the intermediate field $H_i$ between an empty sample holder and a holder with ${\mathrm{Fe}}_8$ in the case of indirect measurement. The high-frequency component ${\omega }_h$ versus $H_i$ is shown in the inset. The solid lines are guides to the eye.

Figure 4

(Color online) The shift of the muon rotation frequency as a function of the intermediate field $H_i$ between the measurement of the empty silver sample holder and that of a holder with ${\mathrm{Fe}}_8$ in a direct measurement. The solid line is a guide to the eye. Raw data of $\omega$ versus $H_i$ with and without ${\mathrm{Fe}}_8$ are shown in the insert.

Figure 5

(Color online) A demonstration that ${\mathrm{Fe}}_8$ depolarize muons immediately. The asymmetry of a hematite and glue mask (a) is very similar to mask and ${\mathrm{Fe}}_8$ (b), but different from mask and silver (c). Therefore, muons in ${\mathrm{Fe}}_8$ do not contribute asymmetry (see text). In (d) we demonstrate that unpolarized ${\mathrm{Fe}}_8$ sample has very small muon relaxation. Therefore the signal is from muons that experience the averaged field which is zero only outside the sample.