Sensing single molecule magnets with nitrogen vacancy centers

Abstract

Single-molecule magnets (SMMs) are molecules that can function as nanoscale magnets with potential use as magnetic memory bits. While SMMs can retain magnetization at low temperatures, characterizing them on surface and at room temperature remains challenging and requires specialized nanoscale techniques. Here, we use single nitrogen-vacancy (NV) centers in diamond as highly sensitive, broadband magnetic field sensors to detect the magnetic noise of cobalt-based SMMs deposited on a diamond surface. We measure the NV relaxation and decoherence times at 296 K and at 5-8 K, observing a significant influence of the SMMs on them. From this, we can infer the SMMs' magnetic noise spectral density (NSD) and underlying magnetic properties. Moreover, we observe the effect of an applied magnetic field on the SMMs' NSD at low temperatures. The method provides nanoscale sensitivity for characterizing SMMs under realistic conditions relevant to their use as surface-bound memory units.

Figures (5)

• Figure 1: General scheme of the low-temperature NV setup (not to scale). Single-molecule magnets are deposited on top of a diamond nanopillar ($\sim$450 nm diameter). A confocal microscope setup is used to excite the NV center with a 520 nm laser, emitted photons are collected back, and measured. A DC magnetic field is applied with three superconducting coils. The LT system can reach 5 K and resides in ultra-high vacuum (UHV) conditions. A microwave (MW) antenna is used for spin state manipulation.
• Figure 2: The effect of the cobalt-based SMMs at RT and 5 K. The NV center (NV17) $T_2$ coherence curve, in the presence of SMMs, at 296 K (red) and 5 K (blue), with $T_2=5.9\pm0.3~\text{\textmu s}$ and $T_2=0.62\pm0.09~\text{\textmu s}$, respectively. The data were fitted based on Sec. S3 in the SI. Inset: the $T_2$ pulse sequence.
• Figure 3: The effect of the cobalt-based SMMs at $\sim$5 K. (a) The NV center (NV24) $T_2$ coherence curves in the presence (purple) and absence (gray) of SMMs, with $T_2=0.80\pm0.04\,\text{\textmu s}$ and $T_2=4.0\pm0.2\,\text{\textmu s}$, respectively. (b)$T_1$ relaxation curves (NV24) in the presence (purple) and absence (gray) of SMMs with $T_1=0.35\pm0.05$ ms and $T_1=22\pm4$ ms, respectively. The data were fitted based on Sec. S3 in the SI.
• Figure 4: NSDs and filter functions. The upper part depicts the normalized NSD of the cobalt-based SMM at 5 K (purple) and 296 K (red) based on reference data Rechkemmer2016. In blue is the fitted NSD at 5 K (for NV22, based on Eq. S3 and Eq. S4 in the SI, Sec. S6). The lower part depicts the $T_2$ (green) and the $T_1$ (brown) filter functions.
• Figure 5: NV coherence $T_2$ vs. temperature at different magnetic fields. Observing the effect of applied magnetic fields on the spin dynamics of cobalt-based SMMs. (a) Data with SMMs. $T_2$ values of NV22 as a function of temperature and magnetic field in the presence of SMMs. (b) Reference data, without SMMs. $T_2$ values of NV22 as a function of temperature and magnetic field in the absence of SMMs. All data were fitted based on Sec. S3 in the SI.