Study On Magnetism And Electrical Transport Of FeCoGe Amorphous Magnetic Semiconductors

When conventional microelectronics technology gradually approaches the limit of the quantum coherence effect, it is a great challenge to design the electronic devices due to the quantum effect. In order to break through the bottleneck of size effect, many researchers try to find a new preparation technology, new materials and new configuration designs. It is a good idea for using electronic charge degree of freedom and spin degree of freedom together. Namely, the memory and logic are integrated on a device or a chip, which will result in a large scale of information processing and storage at the same time. Thus it will improve the performance of devices further, such as improving the processing speed, lowering power consumption, improving integrated capacity and possessing non-volatile storage, etc. In this context, spintronics emerges. Spintronics mainly includes the spin current source, the spin injection and spin transport, spin detection and spin control, etc.At present, the research for spintronic devices mainly includes the following two aspects:1) The research based on ferromagnetic materials mainly focuses on the spin related GMR, TMR effect, such as commercial magnetic sensors, magnetic random access memory, hard disk read head, electrical isolator, and so on.2) The research based on semiconductor materials concentrates on spin polarized current injected into semiconductor materials and preparing spin transistor devices for replacing the conventional semiconductor transistors. Up to now, many schemes have been put forward to realize the application of this kind of device, and it has been made some advances in spin-polarized current injection, transport and detection etc. However, to really achieve the wide use of spintronics devices based on semiconductor, people are facing many problems, and one of which is to find the spin polarized semiconductor materials with Curie temperature higher than the room temperature, so as to make spin polarization inject into non-magnetic semiconductor efficiently, and be compatible with the integrated circuit of microelectronics industry.In the last century 60s and 70s, people discovered a type of natural magnetic semiconductor materials, which were the europium chalcogenides and the chalcogenide spinels. However, this kind of materials was not applied due to lower Curie temperature, complicated crystal growth process and severe lattice mismatch with Si and GaAs semiconductors. Later, inspired by the intrinsic semiconductors can be doped into p-type or n-type semiconductors, people tried to introduce the magnetic elements into the non-magnetic semiconductor and make it a magnetic semiconductor materials.In the 1980s, Leroy L. Chang et al. first prepared Mn doped CdMnTe and ZnMnSe Ⅱ-Ⅳ magnetic semiconductors by molecular beam epitaxy (MBE). Then, other research groups also carried out the related research. However, it was difficult to be doped into p-type or n-type for Ⅱ-Ⅵ magnetic semiconductor materials, which usually showed the anti-ferromagnetic exchange coupling of the local magnetic moment of Mn-Mn. Moreover, as the change of temperature and magnetic ion concentration, the group Ⅱ-Ⅵ magnetic semiconductor materials presented a paramagnetic, spin glass and anti-ferromagnetic behaviors, and many unique magneto-optic properties presented at low temperature disappeared at room temperature, so it was useless.In the 1990s, Ohno et al. prepared Mn doped InMnAs and GaMnAs Ⅲ-Ⅴ magnetic semiconductors, which reactivated the new round research interest for magnetic semiconductors, including the research for Ⅱ-Ⅵ,Ⅲ-Ⅴ and group-Ⅳ magnetic semiconductors, although the reported Curie temperature of GaMnAs is only 200 K so far. In 2002, Park et al. for the first time prepared epitaxial single crystal group Ⅳ MnxGe1-x magnetic semiconductor by using low temperature molecular beam epitaxy technique and the Curie temperature linearly increased with Mn concentration from 25 K to 116 K. The gate voltage controlling carriers concentration and ferromagnetic order suggested hole carriers were spin polarized in MnxGe1-x magnetic semiconductor and the sample was intrinsic magnetic semiconductor. Theoretically, it is predicted to reach more than 400 K for Curie temperature of Ge-based magnetic semiconductor. More importantly, Ge-based magnetic semiconductor can be well compatible with the current mainstream of Si base processing technology, and the high electron and hole mobility of Ge make it become preferred choice for high performance and low power consumption device, so people remain enthusiastic for the current research of Ge-based magnetic semiconductors.In order to realize the application of spintronics devices at room temperature, people not only use the different growth methods (MBE, PLD, ion beam injection, sol gel method, etc.) for obtaining high saturation magnetization and intrinsic magnetic semiconductor materials, but also try to use hydrogen treatment methods in the process of sample growth or post-growth, such as using hydrogen to modulate GaMnAs, GaMnN, ZnCoO and SiMn magnetic semiconductors performance. However, the effects of hydrogen on magnetic properties are quite different for different samples. In some cases, the ferromagnetism of magnetic semiconductor is enhanced due to hydrogen modulation, while in some cases, it is weakened. There is not unified conclusion in internal mechanism for hydrogen modulated magnetic semiconductor so far.Although the sample preparation technology and treatment method have been improved, and resulted in the enhancement of the Curie temperature and the magnetization in some magnetic semiconductors, the Curie temperature is still far below room temperature. Although some reports found the higher Curie temperature than room temperature and higher intensity of magnetization in some magnetic semiconductor materials, which are often caused by the second phase precipitation and/or magnetic clusters due to thermal equilibrium growth, rather than from the intrinsic magnetic semiconductor materials.Based on the above research results, we prepared amorphous Ge-based magnetic semiconductor films (FeCo)xGe1-x and (FeCo)xGe1-x-H with high FeCo concentration in the pure Ar and Ar: H mixed gas by using magnetron co-sputtering technology under thermal non-equilibrium condition. We carried out static and dynamic magnetization measurements on magnetization and exchange interaction of (FeCo)xGe1-x and (FeCo)xGe1-x-H samples to understand the effects of hydrogen on magnetic properties. Furthermore, we also focused on the electrical transport properties of (FeCo)xGe1-x films. The details of our work mainly include the following three aspects:First, we successfully prepared amorphous FeCoGe and FeCoGe-H magnetic semiconductor films with high FeCo concentration by magnetron co-sputtering technology on glass substrates. It is known that under the thermal equilibrium conduction, the low solubility, the second phase precipitation and/or clusters for the magnetic elements are easily formed. In order to overcome those problems, we prepared samples under the thermal non-equilibrium condition. Considering the two magnetic elements co-doping is helpful for the stability of magnetic semiconductor films and the enhancement of magnetism, we choose transition elements FeCo which is characteristics of the high spin polarization and high Curie temperature to co-dope with group IV Ge-based semiconductors. The results from X-ray diffraction measurements, and the fitting of temperature dependence of the resistivity show that all of our samples are amorphous, which is related to the growth of the thermal non-equilibrium condition.Second, we find that hydrogen co-doping can greatly enhance magnetization and exchange interaction in amorphous FeCoGe-H thin films. As we know the effects of hydrogen on magnetic properties are quite different for different magnetic semiconductors and the inner mechanisms are not the same. Therefore, it is necessary to carry out static and dynamic magnetization measurements on magnetization and exchange interaction of Ge-based magnetic semiconductor samples (FeCo)xGe1-x and (FeCo)xGe1-x-H to understand the effects of hydrogen on magnetic properties. The experiment results measured by superconducting quantum interferometer (SQUID) and ferromagnetic resonance (FMR) indicated that the magnetization and exchange interaction in (FeCo)xGe1-x-H film were enhanced markedly as compared with that in (FeCo)xGe1-x films. According to SQUID measurement results, the saturation magnetization of (FeCo)0.70Ge0.30-H and (FeCo)0.70Ge0.30 films is 567 emu/cm3 and 330 emu/cm3 at room temperature, respectively, and the former is 1.72 times of the latter. According to FMR measurement results, spin-wave stiffness constant D in (FeCo)0.70Ge0.30-H films is 176.2 meV-A and 156% larger than that in (FeCo)0.70Ge0.30 films. We also analyzed the possible mechanisms of hydrogen enhanced magnetization and exchange interaction in (FeCo)xGe1-x-H. The Fe(Co) atoms in Ge matrix supplies not only the local magnetic moments (spins), but also the weakly localized carriers (s, p-like holes). H interstitial atoms in Ge matrix supply local ls electrons, which almost do not supply conducting carriers. Both holes and H1s electrons strongly hybridize with the 3d electrons of Fe(Co), so stronger spin-spin exchange interaction between Fe(Co) atoms can be established through s, p-d hybridization. Thus, the intrinsic ferromagnetism in (FeCo)xGe1-x-H is enhanced. This may open an alternative way to design new spintronic materials by hydrogen enhancing ferromagnetic magnetization and exchange interaction.Third, we find that the anomalous hall effect of amorphous (FeCo)xGe1-x films does not meet the usual scaling relations. At present, in some magnetic inhomgeneous systems, the magnetotransport properties are sophisticated and physical mechanism is still controversial. So, it is necessary to study Hall effect in amorphous (FeCo)xGe1-x films. Hall effect measurement results show that Hall sensitivity is about 13.8 μΩ·cmT-1 in Fe0.67Ge0.33 films with a thickness of 7.6 nm. It is found that the Hall sensitivity does not depend on the temperature in the temperature range of 5-300K, and the Hall resistance depends linearly on the magnetic field within the range of-3500- 3500Oe. Especially, it is similar to magnetic particle films, multilayer films and other amorphous thin films, the usual scaling relations (pxys∝Pxxn) between the Hall and the longitudinal resistivities is unsuitable in amorphous (FeCo)xGe1-x films.