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L. Guo et al. / Journal of Alloys and Compounds 534 (2012) 6–8
Fig. 4. M-H curves of the TiO2/Ni hybrid NT arrays. (a): H||, H\ denote the applied field parallel and perpendicular to NT’s axis respectively, (b): Measured at 100, 200 and
300 K when the applied field parallels the NT’s axis.
3 h (Fig. 2a). This agrees well to the SEM results. SAED pattern of
the isolated Ni NT is inserted. The weak diffuse rings doped with
bright spots suggest that these Ni NTs are polycrystalline. HRTEM
observation is also performed on the particles consisted in the Ni
NTs. The clear lattice fringes (Fig. 2b) demonstrate that the Ni par-
ticles are well-crystallized. For further characterizing the structure
of the hybrid NTs, XRD pattern of the as-prepared samples after
electro-deposition is displayed in Fig. 3. Besides the Ti substrate
(the anodized TiO2 NTs are amorphous without diffraction peaks),
all the other peaks agree well with those of standard face centered
cubic (FCC) Ni (PDF, Card No. 4-850). The relative intensity of these
peaks has no obvious difference with the criteria card, which fur-
ther confirms that the deposited Ni NTs are mainly polycrystalline
structure with no preferred orientation.
Magnetic hysteresis loops (M–H, Fig. 4a) of the Ni NTs embedded
in the TiO2 templates are measured at a maximum field of 10 kOe at
room temperature with the external applied field parallel or per-
pendicular to the NTs axis. As the applied field is up to 8 kOe, the
samples are saturated in both of the applied field directions and
the saturated magnetization (Ms) value is about 6 emu/g. Moreover,
the Ni NT arrays exhibit uniaxial magnetic anisotropy and the easy
axis is parallel to the NTs axis. Typical coercivities are Hc// ꢀ 47.5
and Hc\ ꢀ 60.7 Oe (inset of Fig. 4a), which is higher comparing to
that of bulk counterparts (around 0.7 Oe for Ni) [15]. Due to the
intrinsic shape anisotropy of the NTs, the parallel hysteresis yields
even higher values of remanent magnetization (Mr, 2 emu/g). This
is consistent with previous reports on Ni NT arrays [6,15]. Under
a field applied parallel the NTs axis, the M–H curves of sample mea-
sured at 100, 200 and 300 K are shown in Fig. 4b respectively. It is
observed that the shape of the curves, values of Ms or Mr have no
distinct changes for the sample at different temperatures. But the
Hc increases evidently with the reducing of temperatures because
of the decrease of thermal energy at a lower temperature which
weakens the shape anisotropy energy. Qualitatively speaking, the
shape anisotropy is weakened by the preferential increase of the
effective magnetocrystalline anisotropy as well as the dipolar inter-
actions between the NTs at lower temperatures [11]. Further works
for deeper processing of the TiO2/Ni NT arrays, investigation of their
electric or magnetoelectric coupling properties are underway.
NT arrays. All the Ni NTs show face-centered cubic polycrystalline
structure. Room temperature M–H curves reveal that the easy axis
of the sample parallel to the NT’s channel axis attributing to the
large shape anisotropy. Temperature dependent magnetization
show that the coercivity of Ni NTs increases with the decreasing
of temperatures because of the enhanced magnetocrystalline
anisotropy as well as the dipolar interactions between the NTs un-
der lower thermal energy.
Acknowledgments
The work was supported by Ministry of Sciences and Technol-
ogy of China through National Basic Research Program of China
(973 Program 2009CB623301), National Science Fund for distin-
guished young scholars (Grant No. 50625204), Science Fund for
Creative Research Groups (Grant No. 50921061), outstanding tu-
tors for doctoral dissertations of S and T Project in Beijing (No.
YB20081000302), and Tsinghua University Initiative Scientific Re-
search Program.
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4. Conclusions
In summary, highly ordered Ni NTs with uniform wall thickness
and tube diameter have been electrodeposited into anodized TiO2