Crystalline metals and AAs behave quite differently as oxygen is introduced into them, respectively (Fig. 1). The solubility of oxygen in a crystalline metal limits the maximal amount of oxygen that enters its lattice structure, whereas no similar limitation exists for oxygen to be included in an AA. Moreover, amorphous materials could access to lower energy states through structural relaxation[27–31]. Local atomic rearrangement reduces local strains induced by the oxygen addition, thereby permiting more oxygen atoms to be included in amorphous materials. With increasing the oxygen content, the conduction electrons of the AA are gradually localized due to the formation of ionic or/and covalent bonds between the constituents of the AA and the included oxygen atoms. One can expect that the original AA becomes more and more insulating as the oxygen content continuously increases. As a result, a metal–semiconductor transition occurs at a critical value of the oxygen content.
Following the idea of transferring AAs to semiconductors, an alloy system is selected owing to the following three reasons. First, ferromagnetic alloys are preferred in order to obtain MSs with a combination of desirable functionalities. Moreover, the Curie temperatures of the selected alloy systems should be higher than 500 K, which is required for a ferromagnet to work in practical applications. For amorphous ferromagnets, it is thus better to select AA systems containing 3d transition metal elements of Fe and/or Co, contributing to their robust ferromagnetism. Second, the ferromagnetism of the selected AA should be sustained even after it is heavily oxidized. To achieve this, the ferromagnetic metals in the alloy system are better to have smaller affinity for oxygen than the other constituents. Hence, the ferromagnetic metal elements are least oxidized compared with other constitutes in the alloy system. Third, the selected AA system should be a good glass former so that it can maintain its amorphous structure even when a high oxygen content is added. Based on these considerations, a good glass former Co–Fe–Ta–B system was selected[33, 34].
The Co–Fe–Ta–B–O thin films were deposited by radio frequency (RF) magnetron sputtering with an alloy target under a gas mixture of argon and oxygen[24–26]. Varying the oxygen partial pressure led to the formation of amorphous Co–Fe–Ta–B–O thin films with different oxygen contents. Ta and B were firstly oxidized and diffused out to form the surface oxide. The surface metal oxide increased with increasing the oxygen content. They contacted with each other by forming a continuous network (Fig. 2). The size of the inner AA nanoparticles became smaller and smaller. At a critical oxygen content, the AA phase disappeared and a single oxide phase was formed.
The structure of the (Co0.53Fe0.23Ta0.08B0.16)100–xOx (0 ≤ x ≤ 50 at%, abbreviated as CFTBOx hereafter) samples evolved gradually with increasing the oxygen content. High resolution transmission electron microscopy (HRTEM) images and selected area electron diffraction (SAED) patterns were taken for the CFTBOx system, respectively (Fig. 3). The Co53Fe23Ta8B16 AA shows maze-like atomic arrangements typical for amorphous structure (Fig. 3(a)). Its SAED pattern further verifies the formation of a single amorphous phase in the AA (Fig. 3(b)). With increasing the oxygen content above 15 at.%, an amorphous oxide (AO) phase emerges in the CFTBOx thin films. Fig. 3(c) shows the formation of a dual phase nanocomposite in the CFTBO44 thin film, comprising the nanometer-sized AA particles embedded in the AO matrix. Its SAED pattern exhibits two sets of halos (Fig. 3(d)). One arises from the AA nanoparticles (Figs. 3(b) and 3(d)), while the other originates from the AO matrix (Figs. 3(d) and 3(f)). Further increasing the oxygen content to 46 at.% enables the formation of a single AO phase (Fig. 3(e)). The SAED pattern only shows the broad diffraction halo resulting from the single AO phase, which is semiconducting and ferromagnetic.
Fig. 4(a) shows optical transmittance of the CFTBOx samples at thickness of ~100 nm. With increasing the oxygen content, their optical transmittance increases as well. The optical bandgap of the CFTBO46 thin film is estimated to be about 2.4 eV based on the Tauc plot (Fig. 4(b)). In addition, the thin film exhibits 488 nm-peaked photoluminescence specturm, correpsonding to a photon energy of about 2.5 eV for blue light. This value is in consistent with its optical bandgap energy.
As the charge carrier concentration decreases with the oxygen content, the bandgap of the CFTBOx samples is gradually opened. That is, metal-semiconductor-insulator transitions are induced through this simple oxidization of the ferromagnetic AA. As a result, the resistivity of the CFTBOx samples increases with increasing the oxygen content (Fig. 5). The CFTBO46 thin film exhibits a negative temperature dependence of ln(ρ/ρ0)–1/T1/2 (inset of Fig. 5), characteristic of a semiconductor behavior. Further increasing the oxygen content above 60 at.% makes the CFTBOx samples become insulating.
The magnetic properties of the CFTBOx thin films are shown in Fig. 6. All the thin films are ferromagnetic at the oxygen contents ranging from 16 to 46 at.%. Noted that the saturation magnetization (Ms) of these thin films measured at room temperature increases from 728 to 867 emu/cm3 as the oxygen content increases from 16 to 25 at.%. With further increasing in the oxygen content up to 46 at.%, Ms decreases from 867 to 433 emu/cm3 (Figs. 6(a) and 6(b)). Furthermore, the zero-field cooling (ZFC) curve of the thin film with a low oxygen content of 16 at.% coincides with its field cooling (FC) curve, which is similar to that of the Co–Fe–Ta–B AA without containing oxygen[35, 36]. However, the ZFC curves of the CFTBOx thin films with the oxygen contents above 16 at.% deviate from their FC curves at low temperatures, demonstrating spin-glass-like maximum below 100 K. It is suggested that the spin-glass-like behavior is associated with the formation of the magnetic AO phase.
The high-temperature magnetization–temperature (M–T) curve of the CFTBO46 sample shows that its glass transition occurs at about 600 K. The thin film is still ferromagnetic before the glass transition sets in. Therefore, the thin film should have a Curiecurie temperature above 600 K. At about 700 K, the magnetization increases owing to the apparent crystallization, similar to that found in Co16Fe68Hf9B7 alloy[37, 38]. Together with its electrical behavior, it can be concluded that the CFTBO46 thin film is semiconducting and ferromagnetic. A new type of AMSs has been developed with a high Curie temperature above 600 K through simply oxidizing an originally ferromagnetic alloy. The conduction type of this AMS is determined to be p-type.