Nowadays a tapping/lift technique is usually employed for magnetic force microscopy.
This technique allows us to take topographical and magnetic data simultaneously by scanning twice on the same scan line. The first scanning, which is very close to the sample surface, is a normal tapping mode for taking the topographical data. When performing the second scanning, the tip follows the same trajectory of the first scanning and is lifted with a distance, typically 10 to 100 nm, to eliminate the contribution of Van der Waals force. A MFM image usually records the phase of tip oscillation. As the oscillation is subjected to a force, the phase difference Δδ obeys the relation where Q is the quality factor, ke is the spring constant, and F′ is the force gradient.
A method was proposed for analyzing MFM data which modeled the tip and sample as two point magnetic dipoles [141, 143]. Fig. 4.3 shows the schematics of this model. Consider a tip with a dipole moment mtip is magnetized along the z axis, there will be a force gradient acting on the tip
2( 2 ˆ)
⎥⎦
Fitting the MFM data with Eq. (4.4), we can obtain the orientation θ and strength mS
of sample dipole moment.
4.3 Experiment
Cylindrical ZnO NWs were grown by using a vapor-phase transport process. A quartz tube treated as a growth chamber was inserted in a furnace. ZnO powders were placed in a crucible in the growth chamber and heated to 950°C. The chamber was maintained at 200 Pa with a constant flow of argon and a pumping system. For a purpose of controlling NW diameter, gold nanoparticles as catalysts with specified average diameters of 10, 20, 40, 70, and 100 nm were dispersed on quartz substrates.
The substrates were positioned at the downstream end of the growth chamber and were maintained at 500-600°C. Cylindrical ZnO NWs with a controllable diameter were formed on substrates after a growth period of 8 h. The crystalline structure and morphology of ZnO NWs were analyzed by using field-emission scanning electron microscope (SEM, JEOL JSM 7000F) and transmission electron microscope (TEM, JEOL JEM-2010F).
The as-grown ZnO NWs were implanted by Co ions with doses of (1-6) × 1016 cm-2. By using a tandem accelerator (NEC 9SDH-2), the implantation was performed at room temperature. An accelerating energy of 72 keV was used for NWs with average diameters larger than ~70 nm. Thinner NWs were implanted by Co ions with
used to make Zn1-xCoxO NWs. The high bean current of 600 nA/cm2 could somewhat turn out to be thermal treatment due to the high energy ion bombardment in a high vacuum. The chemical composition as well as Co element distribution in Zn1-xCoxO NWs was inspected through energy dispersive x-ray (EDX) and electron energy loss spectroscopy (EELS) mapping. In order to study the origins of ferromagnetism in Zn1-xCoxO NWs, some specimens were post-annealed in argon, in a high vacuum of 5
× 10-5 torr, or in oxygen at 600°C (or 450°C) for several hours. In particular, multiple steps of thermal annealing in a high vacuum were carried out to produce a gradual transition of magnetic states of this DMS material.
In addition to DMS Zn1-xCoxO NWs, ZnO NWs sheathed in amorphous carbon with Co clusters were produced for comparison. These purposely fabricated samples were treated with the same thermal annealing process as that for DMS NWs. Co metal clusters in carbon coated ZnO NWs were intriguingly formed after a high-vacuum annealing. The morphology, crystalline structure and chemical composition of these comparative samples were analyzed in a similar way.
Magnetic properties of DMS Zn1-xCoxO NWs and comparative samples (Co clusters on ZnO NWs) were measured by employing a SQUID magnetometer (Quantum Design MPMS-XL7) with the reciprocating sample option mode. Field cool (FC) and zero-field cool (ZFC) processes were conducted to obtain temperature dependent magnetization during the rising temperature sequence under an external magnetic field of 500 Oe. That is, the samples were subjected to oscillating with decreasing fields and were cooled from 300 K down to 2 K in a zero field. The samples were then warmed up to obtain ZFC magnetization as a function of temperatures in 500 Oe. They were cooled down in the same field and warmed up again to record the FC magnetization. Before NW growth, the magnetic susceptibility of a quartz substrate was estimated to be ~-1.1 × 10-6 emu/cm3 so that diamagnetic
contribution of the substrate can be subtracted from the total magnetization. Magnetic data were presented in unit of μB per Co where the amount of Co ions was evaluated by multiplying the ion dose per cm2 with the substrate area.
After measurements of SQUID, the Zn1-xCoxO NWs on quartz substrates were transferred to flat HOPG substrates by rubbing the two substrates with each other. The Zn1-xCoxO NWs on HOPG substrate were surveyed by a field-emission SEM (JEOL JSM-7000F) to identify the location of individual NWs for taking MFM measurements. The MFM measurements and simultaneously topographic images were taken by using a scanning probe microscope (Seiko Instruments Inc., SPA 300HV) in the tapping/lift mode. A commercial magnetic tip (NanoWorld MFMR) with 40-nm thick cobalt alloy coating was used. The tip has a quality of Q ~ 137, a resonance frequency of ~ 75 kHz, a spring constant of k ~ 2.8 N/m, and a magnetic moment mtip ~ 10-13 emu. Before carrying out the MFM measurements, the tip was magnetized by placing a permanent magnet perpendicular to the cantilever at a distance of ~ 1 cm, where the stray field from the magnet was ~ 7000 Oe. The
Zn-1-xCoxO NWs on the HOPG substrate were magnetized by the same approach in a direction perpendicular or parallel to the substrate. After the MFM measurements, the samples were in situ heated up above room temperature in high vacuum and cooled down to carry out MFM measurements again. All the MFM measurements were performed in atmosphere at room temperature.
4.4 Results and discussion
At first, we demonstrate the growth behavior, crystalline structure, and chemical
composition of ZnO NWs. A SQUID magnetometer is then employed to study annealing effect on the ferromagnetism of Zn1-xCoxO NWs. Finally, we utilized MFM to verify the room-temperature ferromagnetism on an individual DMS NW.