Nanoscale Res Lett (2007) 2:81–86 83 Fig. 2 X-ray diffraction pattern of the b-MnO2 nanorods. Two sets of peaks are identiﬁed, one indexed to the b-MnO2 pyrolusite phase and the other to the a-Mn2O3 bixbyte phase the manganese in the last step of the synthesis procedure. In Fig. 3b, atomic model of the b-MnO2 phase (viewed along [1 1 0]) is superimposed on the experimental HRTEM image, showing good matching and conﬁrming single-crystalline form of the nanorod. The b-MnO2 pyrolusite unit cell is also shown. Here we mention that, despite the two-phase structure of the nanorods, we shall keep labeling them by the generic name ‘‘b-MnO2 nanorods’’. Magnetic properties Magnetic measurements were performed by a Quan- tum Design SQUID magnetometer equipped with a 5 T magnet and the measurements were conducted between room temperature and 2 K. The M(H) curves at 5, 55 and 110 K are displayed in Fig. 4a, showing linear dependence of the magnetization on the mag- netic ﬁeld with a positive slope up to the highest ﬁeld 5 T. The M(H) curve at 5 K exhibits small hysteresis around H = 0 with a coercivity of 0.9 kOe (shown on an expanded scale in Fig. 4b), demonstrating that the magnetization at this temperature contains a FM component, which is not observed at 55 K and higher. The magnetization as a function of temperature, M(T), in magnetic ﬁelds 50 Oe, 100 Oe, 1000 Oe, 1 T and 5 T is displayed in Fig. 5a–e. Both zero-ﬁeld-cooled (zfc) and ﬁeld-cooled (fc) runs are shown. Upon cooling from room temperature, the M(T) curves ﬁrst exhibit two close maxima at temperatures 93 K and 80 K (best observed on the 5 T and 1 T curves) that are indepen- dent of the magnetic ﬁeld, followed by a decrease of Fig. 3 (Color online) (a) Atomic-resolved HRTEM image of a b-MnO2 nanorod, showing that the b-MnO2 single-crystalline material in the interior of the rod is covered by a 2.5-nm thin surface layer of the a-Mn2O3 phase (located between the two arrows). The electron diffraction patterns of the two phases are also shown. The right arrow points at the interface layer between the b-MnO2 and the a-Mn2O3 phases. In (b), atomic model of the b-MnO2 phase (viewed along [1 1 0]) is superimposed on the experimental HRTEM image of thickness 44 nm and defocus – 44 nm (Mn, green (large) circles; O, orange (small) circles). The b-MnO2 pyrolusite unit cell is also shown the magnetization. Below the temperature of about 45 K the zfc–fc splitting starts to be observed. The zfc–fc splitting is quite signiﬁcantly affected by the external magnetic ﬁeld. In the low-ﬁeld regime