Dynamical Negative Differential Resistance in Antiferromagnetically Coupled Few-Atom Spin Chains

We present the appearance of negative differential resistance (NDR) in spin-dependent electron transport through a few-atom spin chain. A chain of three antiferromagnetically coupled Fe atoms (Fe trimer) was positioned on a ${\mathrm{Cu}}_2\mathrm{N}/\mathrm{Cu}(100)$ surface and contacted with the spin-polarized tip of a scanning tunneling microscope, thus coupling the Fe trimer to one nonmagnetic and one magnetic lead. Pronounced NDR appears at the low bias of 7 mV, where inelastic electron tunneling dynamically locks the atomic spin in a long-lived excited state. This causes a rapid increase of the magnetoresistance between the spin-polarized tip and Fe trimer and quenches elastic tunneling. By varying the coupling strength between the tip and Fe trimer, we find that in this transport regime the dynamic locking of the Fe trimer competes with magnetic exchange interaction, which statically forces the Fe trimer into its high-magnetoresistance state and removes the NDR.

Figure 1

(a) Schematic of the experimental setup. (b) The ${\mathrm{Cu}}_2\mathrm{N}$ layer and the vacuum form a two-barrier structure around the Fe trimer. (c), (d) STM constant-current topographies of a few-atom spin chain measured with a spin-polarized tip (c) and a non-spin-polarized tip (d) at a tunnel junction set point of 2 nS with 5 mV. (e), (f) $I(V)$ curves recorded with a spin-polarized tip (e) and with a non-spin-polarized tip (f) on the side atom of the Fe trimer [indicated as small white arrows in (c) and (d)]. The tunnel junction set point is 54 nS at $+15\mathrm{mV}$, and the external field is 1 T. (g), (h) $dI/dV$ spectra recorded with a spin-polarized tip (g) and a non-spin-polarized tip (h) recorded simultaneously with the $I(V)$ curves shown in (e) and (f).

Figure 2

(a) Conductance-dependent $I(V)$ curves recorded with a spin-polarized tip on the side atom of the Fe trimer (tunnel junction conductance changes from 0.01 to $1.06\mu \mathrm{S}$ with about $0.105\mu \mathrm{S}$ per interval and a 1 T magnetic field). The inset shows the location of the valley and the peak current ($I_v$, $I_p$). The position of the NDR is defined as the voltage of highest negative slope of the $I(V)$. (b) Peak-to-valley ratio ($\mathrm{PVR}=I_p/I_v$) as a function of set point conductance (colored points, experimental data; solid line, calculation). (c) Position of the NDR as a function of the set point conductance (colored points, experimental data; solid line, calculation).

Figure 3

(a) Calculated (black line) and measured (gray line) spin-polarized $I(V)$ curves on the side atom (set point of $0.054\mu \mathrm{S}$ at 1 T). The elastic (green line) and the inelastic (red line, $10\times$ magnified) current contributions are shown separately. (b) Calculated voltage-dependent occupation $n_i(V)$ of the two low-energy states ($|1⟩$ and $|2⟩$) for a 1 T (solid lines) and 2 T (dashed lines) magnetic field. (c) The $dI/dV$ spectra measured at 1 T (black line) and 2 T (green line) with a set point of $0.054\mu \mathrm{S}$. These two spectra are measured with the same spin-polarized tip on the same side atom of the Fe trimer. (d) The correspondingly calculated $dI/dV$ spectra at 1 T (black line) and 2 T (green line) with a set point of $0.054\mu \mathrm{S}$.

Figure 4

(a), (b) The measured conductance-dependent $dI/dV$ spectra (gray lines) with a 1 and 2 T magnetic field applied, respectively. The calculated $dI/dV$ spectra are shown as colored lines. For visibility, the spectra are normalized to the set point conductance and are offset. (b) The strength of the tip interaction as a function of the conductance set point as extracted by fitting the measured conductance-dependent $dI/dV$ spectra. Colored markers indicate the junction conductances used for the fit (dots, 1 T; crosses, 2 T).