In this work, pronounced oscillations in the time-resolved reflectivity of Heusler alloy Co_2MnAl films which are epitaxially grown on Ga As substrates are observed and investigated as a function of film thickness, probe wavelength,external magnetic field and temperature. Our results suggest that the oscillation response at 24.5 GHz results from the coherent phonon generation in Co_2MnAl film and can be explained by a propagating strain pulse model. From the probe wavelength dependent oscillation frequency, a sound velocity of(3.85±0.1)×10~3m/s at 800 nm for the epitaxial Co_2MnAl film is determined at room temperature. The detected coherent acoustic phonon generation in Co_2MnAl reported in this work provides a valuable reference for exploring the high-speed magnetization manipulation via magnetoelastic coupling for future spintronic devices based on Heusler alloy films.
Spin pumping in yttrium-iron-garnet(YIG)/nonmagnetic-metal(NM) layer systems under ferromagnetic resonance(FMR) conditions is a popular method of generating spin current in the NM layer.A good understanding of the spin current source is essential in extracting spin Hall angle of the NM and in potential spintronics applications.It is widely believed that spin current is pumped from precessing YIG magnetization into NM layer.Here,by combining microwave absorption and DC-voltage measurements on thin YIG/Pt and YIG/NM_1/NM_2(NM_1 =Cu or Al,NM_2 =Pt or Ta),we unambiguously showed that spin current in NM,instead of from the precessing YIG magnetization,came from the magnetized NM surface(in contact with thin YIG),either due to the magnetic proximity effect(MPE) or from the inevitable diffused Fe ions from YIG to NM.This conclusion is reached through analyzing the FMR microwave absorption peaks with the DC-voltage peak from the inverse spin Hall effect(ISHE).The voltage signal is attributed to the magnetized NM surface,hardly observed in the conventional FMR experiments,and was greatly amplified when the electrical detection circuit was switched on.
Amorphous MnxGe1-x :H ferromagnetic semiconductor films prepared in mixed Ar with 20% H2 by magnetron cosputtering show global ferromagnetism with positive coercivity at low temperatures. With increasing temperature, the coercivity of MnxGe1-x :H films first changes from positive to negative, and then back to positive again, which was not found in the corresponding MnxGe1-x and other ferromagnetic semiconductors before. For Mn0.4Ge0.6 :H film, the inverted Hall loop is also observed at 30 K, which is consistent with the negative coercivity. The negative coercivity is explained by the antiferromagnetic exchange coupling between the H-rich ferromagnetic regions separated by the H-poor non-ferromagnetic spacers. Hydrogenation is a useful method to tune the magnetic properties of MnxGe1-x films for the application in spintronics.
The influence of surface polarity on the structural properties of BiFeO3(BFO) thin films is investigated. BFO thin films are epitaxially grown on SrTiO3(STO)(100) and polar(111) surfaces by oxygen plasma-assisted molecular beam epitaxy. It is shown that the crystal structure, surface morphology, and defect states of BFO films grown on STO substrates with nonpolar(001) or polar(111) surfaces perform very differently. BFO/STO(001) is a fully strained tetragonal phase with orientation relationship(001)[100]BFO‖(001)[100]STO, while BFO/STO(111) is a rhombohedral phase. BFO/STO(111) has rougher surface morphology and less defect states, which results in reduced leakage current and lower dielectric loss. Moreover, BFO films on both STO(001) and STO(111) are direct band oxides with similar band gaps of 2.65 eV and2.67 eV, respectively.
