Force-Based Reading and Writing of Individual Single-Atom Magnets

Abstract

The integration of single-atom bits enables the realization of the highest data-density memory. Reading and writing information to these bits through mechanical interactions opens the possibility of operating the magnetic devices with low heat generation and high density recording. To achieve this visionary goal, we demonstrate the use of magnetic exchange force microscopy to read and write the spin orientation of individual holmium adatoms on MgO thin films. The spin orientation of the holmium adatom is stabilized by the strong uniaxial anisotropy of the adsorption site and can be read out by measuring the exchange forces between the magnetic tip and the atom. The spin orientation can be written by approaching the tip closer to the holmium adatom.We explain this writing mechanism by the symmetry reduction of the adsorption site of the Ho adatom. These findings demonstrate the potential for information storage with minimal energy loss and pave the way for a new field of atomic-scale mechano-spintronics.

Figures (5)

• Figure 1: Energy diagram of holmium adatom on MgO and experimental set-up to read and write the magnetic states of Ho adatoms. (a) Three-dimensional views of the adsorption configuration of a Ho adatom at the top site in the high-symmetry $C_{4v}$ position on MgO/Ag(100), together with top and side views of the same configuration on MgO. Green ball: Ho atom, orange ball: Mg atom, red ball: O atom, gray ball: Ag atom. (b) Calculated eigenvalues of top-site Ho in high-symmetry $C_{4v}$ on MgO/Ag(100) at $B = 3.0T$. The large uniaxial crystal field, with only minor transverse components, suppresses efficient direct transitions between the ground and metastable states (Ho$\uparrow$ and Ho$\downarrow$). The red and blue arrows in (b) indicate the Ho$\uparrow$ and Ho$\downarrow$. (c) Schematic of the force-based reading and writing of single atom magnets. The Co tip mounted on a LER at a resonance frequency $f$$_{\rm{0}}$ of $\sim 1$ MHz with amplitude $A$ = 65pm.
• Figure 2: Reading and writing of Ho spin on MgO using MExFM. (a--c) $\Delta f(t)$ spectra measured on top of Ho$_{\rm{top}}$ while varying the bias voltage and the tip height. (a) Time evolution of $\Delta f$, (b) applied bias voltage, and (c) tip–sample distance. The blue and red in (a) indicate the Ho$\downarrow$ and Ho$\uparrow$. At 33s $\leq t \leq$ 43s, the transition from the Ho$\downarrow$ to Ho$\uparrow$ state can be detected by a sudden jump in $\Delta f(t)$, marked by the black arrow in (a).
• Figure 3: Probing Ho$\uparrow$ and Ho$\downarrow$ using MExFM. (a) Frequency shift obtained on top of the Ho$\uparrow$ ($\Delta$$f$$_{\rm{Ho\uparrow}}$($z$), red solid curve), Ho$\downarrow$ ($\Delta$$f$$_{\rm{Ho\downarrow}}$($z$), blue solid curve) and MgO ($\Delta f_{\mathrm{MgO}}(z)$, orange solid curve). Measurement conditions: $V = + 200µV$. (b) Short-range forces obtained on top of Ho$\uparrow$ ($F$$_{\rm{Ho\uparrow}}$($z$), red solid curve) and Ho$\downarrow$ ($F$$_{\rm{Ho\downarrow}}$($z$), blue solid curve). Inset in (b) shows magnetic exchange force $F$$_{\rm{MExFM}}$($z$) obtained on top of the Ho adatom. The gray dotted line is a guide for the eye, indicating $F$$_{\rm{MExFM}}$($z$) = 0.
• Figure 4: Switching between Ho$\uparrow$ and Ho$\downarrow$. (a,b) Telegraph signal due to the force-induced magnetic switching between Ho$\uparrow$ and Ho$\downarrow$ by changing the tip height. The blue and red indicate the Ho$\downarrow$ and Ho$\uparrow$. Measurement conditions: constant-height mode, $V$$=$ + 1.0mV, (a) $z$ = -6.0±; (b) $z$ = -16.0±. (c) Spin switching rate as a function of tip--sample distances. The exponential fits are represented by the solid, dashed and dotted lines. Measurement conditions: constant height mode, $V$ = 1.0 for force induced spin switching and $V$ = 150 for current induced spin switching.
• Figure 5: Imaging Ho$\uparrow$ and Ho$\downarrow$ using MExFM. (a) $\Delta f$ image of two Ho adatoms, both Ho adatom in the Ho$\uparrow$. Imaging parameters: constant height mode, $V$$=$ + 200µV. (b) $\Delta f$ image of the same area in (a), after the left Ho adatom was manipulated from Ho$\uparrow$ to Ho$\downarrow$. Imaging parameters: constant height mode, $V$$=$ + 200µV. (a) and (b) were obtained at the same tip height. (c) Line profiles obtained above the Ho adatoms by the dotted curve for (a) and the solid curve for (b). The blue and red indicate the Ho$\downarrow$ and Ho$\uparrow$. The positions of the line profiles are indicated by the dotted lines in (a) and (b).