Magnetic ordering of implanted Mn in HOPG substrates

Currently, significant research efforts are geared towards using carbon-based materials for electronic applications. Here we report the observation of magnetic ordering of implanted Mn in HOPG substrates. Superconducting quantum interference device measurements show higher moments for Mn-doped samples and also the existence of double and inverted hysteresis in both undoped and doped samples. High-resolution transmission electron microscopy shows the presence of nanocrystals in implanted samples. Grazing incidence synchrotron x-ray diffraction studies show the presence of three stable manganese carbides, including antiferromagnetic Mn${}_{23}$C${}_6$. X-ray magnetic circular dichroism (XMCD) measurements show ferromagnetic ordering of Mn moments at temperatures below ∼100 K. However, a very weak XMCD signal indicates that only about 1$\%$ of Mn atoms are ferromagnetically ordered. We conclude that the observed Mn ferromagnetic ordering is caused by uncompensated Mn moments on the surface of antiferromagnetic Mn${}_{23}$C${}_6$ nanocrystals that are aligned by the local magnetic field.

Figure 1

Temperature-dependent in-plane magnetic moment of three HOPG samples.

Figure 2

In-plane magnetic hysteresis of three HOPG samples at (a) 300 K and (b) 5 K. The insets show close-up views near the origin. The arrows show the direction of magnetic field sweep.

Figure 3

High-resolution cross-section TEM images of (a) sample H2 and (b) unimplanted HOPG.

Figure 4

Large area detector synchrotron x-ray diffraction image of sample H2 at an incidence angle of 0.9°.

Figure 5

Grazing-angle synchrotron x-ray diffraction pattern of sample H2.

Figure 6

X-ray absorption spectra of two Mn-doped HOPG samples.

Figure 7

Manganese x-ray absorption spectra (at 14 K) and XMCD at 14 and 300 K of sample H2.

Figure 8

Manganese XMCD major loop hysteresis of sample H2. The inset shows minor loop hysteresis at 14 K.

Figure 9

Temperature dependence of manganese XMCD.