Nanoscale Res Lett (2007) 2:81–86 Fig. 5 Magnetization of the b-MnO2 nanorods as a function of temperature in magnetic ﬁelds (a) 5 T, (b) 1 T, (c) 1000 Oe, (d) 100 Oe and (e) 50 Oe. Both zero-ﬁeld-cooled (zfc) and ﬁeld-cooled (fc) runs are shown. Note that the spans of the vertical scale in panels are different. The temperatures of the magnetization maxima expected for the bulk phases of b-MnO2 (TN ~ 92.5 K) and a-Mn2O3 (TN ~ 80 K) are indicated in panel (a) by dashed lines. (f) ac susceptibility v’ in the ﬁeld of amplitude 6.5 Oe as a function of temperature at frequencies 1, 10, 100 and 1000 Hz 85 nanoparticles of varying diameters between 6 and 15 nm showed size-dependent TC between 36 and 41 K. These FM transition temperatures are remark- ably close to that observed in our b-MnO2 nanorods. However, all the peaks in the x-ray spectrum of the nanorods in Fig. 2 can be indexed to the b-MnO2 and a-Mn2O3 phases and do not reveal the presence of any additional phase within the sensitivity of the XRD experiment (about 2% volume). Careful comparison with the x-ray spectra of the c-Mn2O3 nanoparticles (see Fig. 2 of [16]) and the Mn3O4 nanowires (see Fig. 2 of [8]) shows that there are no traces of these two phases in the x-ray spectrum of Fig. 2. The most probable origin of the FM component in the magne- tization of the b-MnO2 nanorods is then magnetic ordering within the mismatch layer at the interface between the b-MnO2 interior phase and the a-Mn2O3 surface layer that can be identiﬁed in Fig. 3a unambiguously as two columns of white spots (located at the position of the right arrow). The physics and chemistry of this thin mismatch layer (of thickness about two atomic monolayers) cannot be discussed easily, but there exists reasonable possibility that FM correlations, analogous to those in Mn3O4 and c-Mn2O3 phases, could develop between the Mn spins within the interface layer. The small magnetization coercivitiy of 0.9 kOe at 5 K of the b-MnO2 nanorods (Fig. 4b) does not, however, support direct analogy with neither the c-Mn2O3 nanoparticle-material nor the Mn3O4, which are both characterized by high coercivities of the order 8 kOe [16, 17, 19]. The FM spin structure within the investigated b-MnO2 nano- rods is obviously much softer. The relatively complicated temperature-dependent magnetism of the investigated b-MnO2 nanorods can, therefore, be explained in terms of a superposition of