In a magnetic field of 500 Oe, temperature dependent magnetizations of various average diameters of Zn0.92Co0.08O NWs are shown in Fig. 4.7(a). The magnetization per Co ion of as-implanted Zn0.92Co0.08O NWs depends strongly on the NW diameter.
Thicker NWs exhibit higher magnetization. In addition, magnetizations under field cooled (FC) and zero-field cooled (ZFC) procedures show a division into two separate curves with decreasing temperature. In a similar way, the transition temperature, at which the FC and ZFC magnetization curves bifurcate, is higher for the thicker NWs.
The difference in FC and ZFC magnetization suggests an existence of small magnetic single-domains in Zn0.92Co0.08O NWs. In addition, the high transition temperature implies larger magnetic single-domains existing in thicker NWs. Fig. 4.7(a) displays a non-vanishing and non-decreasing magnetization up to a room temperature, signifying a ferromagnetic ordering as well as RTFM. After annealing in a high vacuum, the temperature behavior of Zn0.92Co0.08O NWs with an average diameter of 38 nm is displayed in Fig. 4.7(b), including reproduced data of as-implanted NWs for comparison. The temperature behavior demonstrates a much higher magnetization (a strong ferromagnetic state) and a coincidence and overlapping of FC and ZFC magnetization. This result indicates a growth and development of large magnetic domains, formed by nonaggregated Co ions in high-vacuum annealed Zn0.92Co0.08O NWs.
This phenomena implies that a high-vacuum annealing produces oxygen vacancies (zinc interstitials), which could result in a ferromagnetic coupling between the Co ions, to enhance a ferromagnetic interaction between Co ions and to intensify a magnetic state. Since the as-implanted Zn0.92Co0.08O NWs consist of the same concentration of oxygen vacancies (zinc interstitials, the size of magnetic domains of
Figure 4.7: (a) FC and ZFC behaviors of temperature dependent magnetization of as-implanted Zn0.92Co0.08O NWs with average diameters of 10, 20, and 40 nm. (b) FC and ZFC magnetizations of as-implanted and high-vacuum annealed Zn0.92Co0.08O
non-aggregated Co ions may be larger in thicker NWs. On the other hand, the size dependent magnetization could be owing to the generation of planar defects, stacking faults and streaking, during the ion bombardment process [38]. Moreover, planar defects could hinder an oxygen-vacancy mediated ferromagnetic ordering so as to abate magnetization and coercivity of thinner, as-implanted Zn1-xCoxO NWs.
In addition to a temperature dependent behavior, data of field dependent magnetizations as well as hysteresis loops were taken at several different temperatures.
Fig. 4.8 exhibits hysteresis loops of as-implanted Zn1-xCoxO NWs. With an equal Co-concentration of 8%, Fig. 4.8(a) shows that thick NWs reveal a high magnetization and a larger hysteresis loop. Fig. 4.8(b) presents a similar manner of a size dependence to convince us of this general phenomena observed in as-implanted Zn1-xCoxO NWs. The consequence of a high magnetization in thick NWs agrees with the temperature dependent magnetization delineated in Fig. 4.7(a).
We have argued that the implantation of a high beam current of 600 nA/cm2 could somewhat introduce a high-vacuum annealing and create oxygen vacancies (zinc interstitials) in ZnO NWs so as to turn on an exchange interaction between non-aggregated Co ions. The Co ions occupying in a certain volume of a ZnO form a magnetic domain. If the ZnO is cut into smaller pieces such as NWs, the magnetic domain and magnetization (moment) will be abated and reduced. This splitting and diminishing of magnetic domains lead to the size effect observed in as-implanted Zn1-xCoxO NWs. Moreover, the small hysteresis loop indicating a low coercive field (force) in thin Zn1-xCoxO NWs may be due to a weak interaction between size-reduced magnetic domains or to planar defects (stacking faults and streaking) induced a reduction of ferromagnetic interactions.
We have observed an increase in magnetization from temperature dependent studies after a high-vacuum annealing, as shown in Fig. 4.7(b). To learn the annealing
studies
Figure 4.8: (a) Hysteresis loops of as-implanted Zn0.92Co0.08O NWs with three different average diameters marked on graph. The data were taken at 5 K. (b) Hysteresis loops of as-implanted Zn0.96Co0.04O NWs with two different average diameters marked on graph. The data were taken at 2 K.
effect, multiple steps of high-vacuum annealing for hours are employed and the field dependent magnetizations are investigated after each step of annealing. Fig. 4.9(a) demonstrates a change in hysteresis loops of Zn0.92Co0.08O NWs with average diameters of 38 after each step of a high-vacuum annealing. The magnetization as well as the loop becomes higher and larger after several steps of high-vacuum annealing. In addition to the dependence of annealing time, different surface ratios of thin and thick NWs may give rise to dissimilar responses to annealing time. Fig. 4.9(b) reveals a larger increase and expansion in magnetization and field-dependent loops for thinner (19-nm average diameter) NWs. A decrease of annealing time and steps for thinner Zn1-xCoxO NWs is due to a large surface-to-volume ratio for oxygen diffusion and a large increase in magnetization could be related to the above-mentioned reduction of magnetization in thinner NWs.
The results of multiple-step annealing imply a diffusion of composing elements in the Zn1-xCoxO material. It has been confirmed from EDX, EELS mapping, and high-resolution TEM inspections that the annealing will not induce detectable diffusion and clustering of Co ions in the DMS NWs. We argued, therefore, that the annealing effect produces oxygen vacancies (zinc interstitials) to enhance an exchange interaction between Co ions. To confirm the creation of oxygen vacancies during the high-vacuum annealing process, the sample is annealed in oxygen to exhibit a weak magnetic state of a low magnetization and small a hysteresis loop and they are subsequently annealed in a high vacuum to recover a strong magnetic state in high-vacuum annealed Zn1-xCoxO NWs. Fig. 4.9(c) shows the results of 100-nm Zn0.92Co0.08O NWs at room temperature.
To learn more about the high-vacuum annealing enhancement of ferromagnetic ordering, temperature dependence of hysteresis loops of the 38-nm average diameter Zn0.92Co0.08O NWs annealed for 6 hours are displayed in Fig. 4.10(a). An appreciable
Z
Figure 4.9: (a) Hysteresis loops, taken at 10 K, of as-implanted, 6-h vacuum annealed, and 12-h vacuum annealed Zn0.92Co0.08O NWs with a 38-nm average diameter. (b) Hysteresis loops, taken at 2 K, of as-implanted, 3-h vacuum annealed, and 6-h vacuum annealed Zn0.92Co0.08O NWs with a 19-nm average diameter. (c) Hysteresis loops, taken at 300 K, of as-implanted, 12-h vacuum annealed, 6-h oxygen annealed,
hysteresis loop as observed at 300 K clearly points to stabilized RTFM in these NWs.
The field-dependent magnetization reveals only a minor temperature dependence of the saturated magnetization. On the other hand, a large increase in the coercive field with decreasing temperature is evident. The measured coercive field in Fig. 4.10(b) obeys a square-root temperature dependence. Such temperature behavior, which is in line with a superparamagnetic feature, can be described by Eq. (4.1). From the linear fit indicated in Fig. 4.10(b), a blocking temperature of ~700 K was deduced. Fig.
4.10(c) shows the temperature dependence of hysteresis loops and coercive fields for 19-nm average diameter Zn0.92Co0.08O NWs. These thinner DMS NWs, which were annealed only for 3 h, already display a prominent hysteresis loop at room temperature. Thus, the stabilized RTFM in the Zn0.92Co0.08O nanowires is again confirmed. It should be noted that the room-temperature hysteresis loop in these 19-nm average diameter NWs is considerably larger than that in the 38-nm ones. This observation points to a strong magnetic state in the former, the thin nanowires. The variation of the coercive field with the square root of temperature is shown in Fig.
4.10(d). The linear behavior suggests that our sample exhibits a characteristic of temperature-dependent coercivity coincident with a superparamagnetic feature. The blocking temperature inferred from the linear fit is ~2300 K.
If we assume that the temperature dependent coercivity is originated from Co clusters, we may estimate the cluster diameter according to the equation [136]:
Co metal, kB is the Boltzmann constant, and <V> is the average volume of Co clusters.
Assume a spherical geometry for Co clusters, average diameters of ~9 and ~15 nm are derived for TB ~700 and 2300 K, respectively. Such large clusters of ~9 nm in
diameter,
Figure 4.10: (a) Hysteresis loops of Zn0.92Co0.08O NWs with (a) 38-nm and (c) 19-nm average diameter at several temperatures after annealing in a high vacuum for 6 h and 3 h, respectively. (b) and (d) are the coercive field, estimated from Fig. 13(a) and Fig.
13(c) respectively, as a function of square root of temperature.
diameter, if any exist, should be readily detectable. We had implemented HRTEM to carefully inspect our high-vacuum annealed nanowires. However, no microstructure was detected. Moreover, we have performed EDX and EELS mapping studies, which again confirmed a uniform distribution of Co ions in all of our DMS NWs. Therefore, we conclude that the observed square root temperature behavior of coercivity reflects an intrinsic characteristic of our DMS NWs. Moreover, Fig. 4.10(a) demonstrates a temperature independence of magnetization saturation that is consistent with the result shown in Fig. 4.7(b). The ferromagnetic ordering remains up to room temperature so the RTFM in the high-vacuum annealed Zn1-xCoxO NWs is confirmed.
The temperature and field dependent magnetization of ZnO sheathed in amorphous carbon with Co clusters sample is displayed in Fig. 4.11 for a comparative study. As shown in Fig. 4.11(a), FC and ZFC magnetizations are separated into two parts with a decrease of temperature. The undeniable bifurcation of temperature dependent magnetization in FC and ZFC procedures stands for a superparamagnetic feature of Co clusters [135]. This feature will be evident if the Co clusters are mono-dispersed and uniform in size. As we have shown in Fig. 4.5(d), the Co clusters have a wide distribution and a standard deviation of ~6.0 nm in diameter that causes a relatively small deviation in FC and ZFC magnetization at low temperatures in comparison with ideal ferromagnetic colloids. Fig. 4.11(b) shows shrinkage of hysteresis loops as well as a decrease in coercive fields with increasing temperature indicating the superparamagnetism of the Co clusters. The blocking temperature is determined to be ~420 K via a least square fitting, shown as a red line in the inset of Fig. 4.11(b). The average diameter of ~9 nm can be estimated by using Eq. (4.6) with TB = 420 K. The average diameter of ~9 nm agrees very well with that calculated from a statistical distribution of cluster diameters from TEM measurements (9.4 nm in Fig. 4.5(f)). This result sustains the analyses and deductions used in this work. It is
noted
Figure 4.11: (a) FC and ZFC magnetization of ZnO sheathed in amorphous carbon with Co clusters. The Co ion dose and average diameter of ZnO are 4 × 1016 cm-2 and 38 nm, respectively, for this sample. (b) Hysteresis loops of ZnO sheathed in amorphous carbon with Co clusters. The inset shows the coercive field as a function of square root of temperature.
noted that all of the three characteristics of superparamagnetic Co clusters as well as ferromagnetic colloids have been observed. These features are a bifurcation of FC and ZFC magnetication, a temperature dependent coercive field, and the same average diameter evaluated from both TEM measurements and TB estimations.