Recently, there has been intense searching for ferromagnetic ordering in doped, diluted magnetic semiconductors (DMS) focusing on possible spin-transport properties, which has many potentially interesting device applications. Spintronic devices such as spin-valve transistor, spin light-emitting diodes, non-volatile memory, logic devices, optical isolators and ultrafast optical switches are some of the areas of interest for introducing the ferromagnetic properties at room temperature in a semiconductor. Among the materials reported so far, Mn-doped GaAs has been found to be ferromagnetic with the highest reported Curie temperature, $T_c$ ~ 172 K. ZnO and GaN were predicted on a theoretical basis to exhibited ferromagnetism above room temperature on doping with Mn. This prediction initiated intensive experimental work on a variety of doped DMS. Despite the extensive research devoted to high $T_c$ DMS resulted in some reports about the room temperature ferromagnetism such as Mn-doped GaN, Co- and Mn-doped ZnO, however, there have been few reports on the magnetotransport properties of high $T_c$ DMS to confirm that the carriers are indeed coupled to the magnetism, and the origin of ferromagnetism remains an issue of debate. For device applications we should prove intrinsic ferromagnetism in DMS with magnetotransport (spin-transport) properties such as anomalous Hall effect well-known ferromagnetic response of spin polarized charge carriers in ferromagnetic semiconductors.
In this study, p-type spinel $ZnCo_2O_4$, n-type $Zn_{1-x}Cr_xO$, and p-type $Zn_{1-x}Mn_xO$ thin films are fabricated by reactive co-sputtering method to examine their potential possibility of room temperature ferromagnetism with magnetotransport properties. In particular, the hole-induced ferromagnetism, the spin polarized charge carriers-mediated ferromagnetic exchange interaction between distant localized spins of magnetic impurities, in p-type $Zn_{1-x}Mn_xO$ is closely investigated for verification the theoretical prediction about room temperature ferromagnetism in Mn-doped ZnO.
We have successfully grown p-type cubic spinel $ZnCo_2O_4$ film with ferromagnetic ordering at 5 K using oxygen rich condition. The sputtering power ratio between Co and Zn metal targets was found to be a critical factor for the synthesis of the spinel $ZnCo_2O_4$. The conductivity and the carrier concentration varied depending on the oxygen partial pressure ratio in the sputtering gas mixture. It is demonstrated that sputtered spinel $ZnCo_2O_4$ films exhibit both n-type and p-type conduction by tuning the oxygen partial pressure ratio, which make them a promising material system for oxide based electronic devices. A ferromagnetic coupling was observable in p-type $ZnCo_2O_4$ at 5 K, whereas an antiferromagnetic interaction was found for n-type and insulating $ZnCo_2O_4$, revealing hole-induced ferromagnetic transition in $ZnCo_2O_4$.
We have obtained ferromagnetic ordering at 5 K in $Zn_{1-x}Cr_xO:Al$ film with electron concentration of $8.4×10^20/cm^3$. The x-ray diffraction and wavelength dispersive spectroscopy reveal the solubility limit of Cr in ZnO is 1 at.%. The observed result suggests that the electron dopant by Al doping in $Zn_{1-x}Cr_xO$ induces the ferromagnetic ordering considering the paramagnetic behavior of $Zn_{1-x}Cr_xO$ with electron concentration of $7.29×10^{18}/cm^3$.
We have found hole-induced ferromagnetism in $Zn_{1-x}Mn_xO$ (x = 0.01) films. The p-type conduction with the highest concentration of $6.7× 10^{18}/cm^3$ in $Zn_{0.99}Mn_0.01O:P$ film is accomplished by annealing at 800℃ under $N_2$ ambient, which is come from the phosphorus as an acceptor dissociated from $P_2O_5$ at high temperature. The p-type $Zn_{0.99}Mn_{0.01}O:P$ exhibits room temperature ferromagnetism with saturation magnetization of $0.301 emu/cm^3$ and coercive field of 60 Oe in contrast with paramagnetism in n-type $Zn_{0.99}Mn_{0.01}O:P$ film. The cross-sectional TEM and HRTEM analysis provided no nanosized impurity phase was observed in p-type $Zn_0.99Mn_0.01O:P$ film. The anomalous Hall effect was observed at room temperature in p-type film. This result manifests the intrinsic nature of ferromagnetism in this compound, and represents the possible realization of DMS spintronics devices operable at room temperature.
