Anomalous paramagnetism in graphene on hexagonal boron nitride substrates

We report the static magnetization measurements of a few graphene layers on hexagonal boron nitride substrates (gra/h-BN) prepared by chemical vapor deposition. No ferromagnetic signal is detected for the gra/h-BN structures at temperatures between 5 and 300 K. At high temperatures ($T>100$ K), the gra/h-BN structures are diamagnetic with mass magnetic susceptibilities independent of temperature and close to, or slightly larger than, that of h-BN substrates. At low temperatures ($T<20$ K), the gra/h-BN structures exhibit paramagnetic responses, which depend sensitively on the growth time of the graphene layers. The origin of this paramagnetism is attributed to the localized spins of the point defects in the graphene layers. The angular momentum number of these spins is $J=1/2$ and is derived by fitting the magnetization data after subtraction of the diamagnetic background of the h-BN substrates to the standard Brillouin functions. A saturated mass magnetization of about 0.13 to 0.25 emu/g at $T=5$ K is estimated for our gra/h-BN samples. At temperatures between 20 and 50 K, the measured magnetic susceptibilities of the gra/h-BN samples deviate significantly from that of a Curie-type paramagnetic material. A local maximum is found in the temperature dependence of the magnetic susceptibilities of our gra/h-BN samples. Several possible explanations, including the Ruderman-Kittle-Kasuya-Yosida interactions among local magnetic moments and the itinerant electrons in the graphene/h-BN interfaces in our gra/h-BN samples, are discussed and compared to the experimental results.

Figure 1

(a) Typical SEM micrograph of h-BN substrates. (b) Histogram plots of the size distribution of crystallites in h-BN substrates (data from the supplier).

Figure 2

The measured total mass magnetization curve of HOPG at $T=5$ K (solid line), the diamagnetic background (dash-dotted line), and the hysteresis loop (dashed line) of the total mass magnetization after subtraction of the diamagnetic background.

Figure 3

The measured mass magnetization $M$ as functions of magnetic fields $H$ for (a) gra/h-BN sample 1, (b) gra/h-BN sample 2, and (c) h-BN substrate, at different temperatures $T=$ 5, 10, 20, 30, 40, 50, 60, 100, 200, and 300 K.

Figure 4

The temperature dependence of the mass magnetic susceptibility of the gra/h-BN sample 1, sample 2, and h-BN substrate. Inset: The temperature dependence of the inverse of the mass magnetic susceptibility of the gra/h-BN samples at low temperatures.

Figure 5

The corrected mass magnetization ($\Delta M$) isothermal curves of the gra/h-BN samples after subtraction of the diamagnetic background ($M_{h-BN}$) of the h-BN substrates at $T=5$ K fitted to the Brillouin function $B_J\left(x\right)$ with different $J$ ($J=1/2$, 1, $3/2$) for (a) sample 1 and (b) sample 2.