Magnetoelectric nanocomposites have been a topic of intense research due to their profound potential in the applications of electronic devices based on spintronic technology. Nevertheless, in spite of significant progress made in the growth of high-quality nanocomposite thin films, the substrate clamping effect still remains a major hurdle in realizing the ultimate magnetoelectric coupling. To overcome this obstacle, an alternative strategy of fabricating a self-assembled ferroelectric-ferrimagnetic bulk heterojunction on a flexible muscovite via van der Waals epitaxy is adopted. In this study, we investigated the magnetoelectric coupling in a self-assembled BiFeO (BFO)-CoFeO (CFO) bulk heterojunction epitaxially grown on a flexible muscovite substrate. The obtained heterojunction is composed of vertically aligned multiferroic BFO nanopillars embedded in a ferrimagnetic CFO matrix. Moreover, due to the weak interaction between the flexible substrate and bulk heterojunction, the interface is incoherent and, hence, the substrate clamping effect is greatly reduced. The phase-field simulation model also complements our results. The magnetic and electrical characterizations highlight the improvement in magnetoelectric coupling of the BFO-CFO bulk heterojunction. A magnetoelectric coupling coefficient of 74 mV/cm·Oe of this bulk heterojunction is larger than the magnetoelectric coefficient reported earlier on flexible substrates. Therefore, this study delivers a viable route of fabricating a remarkable magnetoelectric heterojunction and yet flexible electronic devices that are robust against extreme conditions with optimized performance.

A bimorph composed of ferrimagnetic cobalt ferrite (CoFeO, CFO) and flexible muscovite was fabricated via van der Waals epitaxy. The combination of X-ray diffraction and transmission electron microscopy was conducted to reveal the heteroepitaxy of the CFO/muscovite system. The robust magnetic behaviors against mechanical bending were characterized by hysteresis measurements and magnetic force microscopy, which maintain a saturation magnetization (M) of ∼120-150 emu/cm under different bending states. The large magnetostrictive response of the CFO film was then determined by digital holographic microscopy, where the difference of magnetostrction coefficient (Δλ) is -104 ppm. The superior performance of this bimorph is attributed to the nature of weak interaction between film and substrate. Such a flexible CFO/muscovite bimorph provides a new platform to develop next-generation flexible magnetic devices.

CoFeB films were deposited on glass substrate by the sputtering method. From x-ray-diffraction and electron-diffraction-ring patterns, the major phase in the as-deposited CoFeB film is amorphous (or nanocrystalline). However, we could also identify a minor CoFe(110) crystalline phase in the film. We have tried to suppress this crystalline phase by changing the Ar partial pressure (PAr) during deposition and found that the optimal condition is PAr=5×10−3Torr. Because the electrical resistivity value (ρ) of the film is in general larger than 100μΩcm, it also indicates that the amorphous phase is dominant. From the temperature coefficient of resistance measurement, we learn that the amorphous phase in the CoFeB film crystallizes in succession at two higher temperatures (Tcr1 and Tcr2) than the room temperature (RT). Besides the electrical properties, the film thickness (tf) dependence of saturation magnetization (Ms), saturation magnetostriction (λs), and coercivity (Hc) has also been discussed. From the Auger-depth profile analysis, it is found that there is one CoOx (with 0.4⩽x<1) oxide layer, about 15 Å in thickness, lying on the top surface of the CoFeB film, and another CoOx oxide layer, about 20 Å, lying near the CoFeB/glass interface. At RT CoOx is supposed to be paramagnetic. However, due to the proximity effect between CoOx and CoFeB, the CoOx layers may become ferromagnetic with the average magnetization Mox. By fitting the Ms data as a function of (1∕tf), we can show that the last conjecture is correct, and Mox is not zero. The CoOx layer plays an important role on Ms,λs, and Hc of the CoFeB films with tf ranging from 50 to 503 Å.

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Hall-effect measurements were carried out on a series of Co100−xPdx alloys from 4.2 to 255 K. The extraordinary Hall coefficient RS is analyzed. The side-jump mechanism is dominant for Co-Pd alloys with x≤65 at. %; however, when x≳65 at. %, both the side-jump and skew scattering mechanisms are equally effective. The Hall conductivity γH changes its sign around xH=77 at. %. xH is shifted to the right-hand side of the band-gap position xG, as implied from the anisotropic magnetoresistance data and the split-band theory.