Oxide diluted magnetic semiconductors (DMS) have attracted much attention because of their promising potential in spintronics. Even some report their room temperature ferromagnetism. Recently, theoretical and experimental studies suggested that wide bandgap oxide semiconductors with high carrier density are one of the most favorable host materials to the DMS with high Curie temperature. In particular there are some claims that novel oxide DMS materials In₂O₃, SnO₂ and et al possess ferromagnetism near room temperature. The In₂O₃-based DMS show ferromagnetism and anomalous hall effect at room temperature, ascribed to the carrier induced ferromagnetism.
Mn-doped ZnO [(Zn, Mn)O] is one of the most widely DMS studied, since has predicted possibility of room-temperature ferromagnetism in p-type (Zn, Mn)O. Due to the practical difficulties in fabricating the p-type ZnO, however, most experimental studies have been performed instead on the n-type (Zn, Mn)O. Meanwhile, there have been a few recent works reporting realization of p-type ZnO via codoping of Mg and P. The addition of P into ZnO gives holes as acceptors and that of Mg shifts the conduction band edge to higher energy, reducing the electron concentration. In this study, we report on the appearance of room temperature ferromagnetism in In₂O₃codoped by Sn and Cr and (Zn, Mn)O codoped by Mg and P.
In this study of the In₂O₃ codoped by Sn and Cr, crystal structure analysis by XRD,SEM and AFM measurements indicated that the all (100)-oriented films of bixbyite structure were grown without any detectable formation of secondary phases. An Cr ion solubility for $(In_{0.95-x}Cr_{x}Sn_{0.05})_2O_3$ film was limited ~20mol%. An amorphous phase appeared above Cr 20mol% addition. The bandgap of the samples measured by UV-vis-IR spectrometry was all above 3.5eV and the transmittance was rather high over 70% in the visible region (500~1400nm). In the near-IR region (wavelength over 1600nm), the transmittance of the films became higher with Cr addition. This supports that the carrier concentration decreases with the Cr ion doping because the carrier absorption causes transmittance reduction in the near-IR region, and this in turn implies that the Cr ion acts as an acceptor. The electron concentration decreased from $~1.28\times10^{20} cm^{-3}$ (Cr non-doped film) to $~3.46\times10^{19} cm^{-3}$ (Cr 10mol% film), and the p-type conductivity appeared above 15 mol% Cr ion addition. The carrier mobility reduced according Cr substitution due to the alloying effect. The p-type conductivity was verified by the p-n junction (n-type ZnO/ p-type Cr 15mol% ITO) experiment. The Cr doped ITO films showed the stronger semiconductor behavior with the Cr doping. The magnetic measurement carried out on Cr 5mol%, in the n-type conductivity region, 10mol% and 15mol% films in p-type conductivity region by SQUID magnetometer. In the n-type Cr 5mol% film, the room-temperature ferromagnetism (RTM) was observed with the saturated magnetization ~0.7 emu/㎤, remnant magnetization ~0.2 emu/㎤, cohesive field 28 Oe, while others showed paramagnetic behaviors. Also, the M-T curves in magnetic field of 1 kOe for the zero field cooing and field cooling for the Cr 5mol% film displayed the steep increase of magnetization below 30K for decreasing temperature either without any abnormality which may indicate the Neel temperature of antiferromagnetism or noticeable deviation indicating the spin-glass behavior. The measured magnetoresistance (MR) for the Cr 5mol% film showed the positive MR in the region from 5 kOe to 28 kOe, which might be considered to be the s-d exchange coupling between the conducting carriers and the localized spins of Cr ions. The absence of Cr metal clusters and the homogeneous distribution of Cr ion with the film depth were verified by TEM, XPS and secondary ion mass spectrometer (SIMS). However it showed the absence of the anomalous hall effect that is conventionally attributed to the asymmetric scattering process involving exchange interaction between the conduction carriers and the magnetic moments. Furthermore, we examined the effect of Sn co-doping on transport and magnetic properties of Cr doped In₂O₃to explore the origin of ferromagnetism in the Cr-doped ITO. Sn ions have been doped from 0 mol% to 12 mol% while the Cr ion is fixed 5 mol%. The films were grown on SiO₂/Si substrates by pulsed laser deposition at the substrate temperature of 400℃ under oxygen pressure of $5×10^{-5}$ torr, using the sintered targets synthesized by the standard solid-state reaction method. They were carefully examined by x-ray diffraction, transport and magnetic measurements. Structural analysis indicates that all the films of bixbyite structure are grown without any detectable formation of secondary phases. With the Sn doping, the carrier concentration increased from $4.61×10^{20} cm^{-3}$ to $2.78×10^{21} cm^{-3}$ at room temperature. Also, it was found that the magnetization for the films increases with the Sn co-doping, which the anomalous hall effect at 350K for $(In_{0.90}Cr_{0.05}Sn_{0.05})_2O_3$ film was observed. This experimental result supports that the electrons play a significant role in the emergence of ferromagnetism for Cr-doped ITO system without magnetic cluster formation.
In the study for (Zn, Mn)O the (Zn, Mn)O thin films codoped by Mg and P were grown on $SiO_{2}$/Si substrates by pulsed laser deposition (PLD) from the ceramic target at a substrate temperature of 600℃ in an oxygen pressure of $5 X10^{-5}$ torr, followed by an in situ annealing at temperature of 600℃ for 60 min under O₂atmosphere. The $(Zn_{0.97}Mg_{0.01}Mn_{0.02})O:P$ ceramic targets were fabricated by the standard solid solution method with the P doping level of 2 mol%. Structural analysis by x-ray diffraction indicated that the highly (002) oriented films were grown without any detectable of secondary phases. The (Zn, Mn)O films codoped by Mg and P showed a hole concentration of $1.75X10^{18}cm^{-3}$, a mobility of 375 cm $^2V^{-1}s^{-1}$ and a resistivity of $9.93 X 10^{-3}$ Ωcm. The magnetic property measurements revealed a ferromagnetic behavior with a coercive filed of 59 Oe at 5K and 29 Oe at 300 K. Finally, we examined the extrinsic origin of ferromagnetism possibly originated from the MnP impurity phase precipitation, leading to the conclusion of an appearance of an intrinsic ferromagnetism from the presence of spin-polarized free carriers coupled with magnetization in (Zn, Mn)O codoped by Mg and P.
