Z. C. Huang

Anisotropy of ultrathin epitaxial Fe3O4 films on GaAs(100)

Magnetic anisotropies of single crystal ultrathin Fe3O4 films on GaAs(100) have been studied by ferromagnetic resonance (FMR). The dependence of the FMR fields on the field orientation was measured both in plane and out of plane. The ultrathin films show predominantly an in-plane uniaxial magnetic anisotropy with the easy axis along the [0−11] direction of the GaAs substrate. The in-plane uniaxial anisotropy constant decreases with increasing thickness and changes from 7.1×104to2.1×104ergs∕cm3 when the thickness tFe varies from 4to8nm. An in-plane fourfold anisotropy due to cubic magnetocrystalline anisotropy coexists with the uniaxial magnetic anisotropy and increases with increasing film thickness. For tFe=8nm, the cubic anisotropy constant K1 reaches −8.4×104erg∕cm3, which is 76% of the value of bulk spinel Fe3O4. The out-of-plane measurements indicate a negative perpendicular anisotropy in these ultrathin Fe3O4 films.

The interface effect of the magnetic anisotropy in ultrathin epitaxial Fe3O4 film

The magnetic anisotropy of epitaxial layered structures of Fe3O4(tFe=4nm)∕GaAs(100), MgO(3nm)∕Fe3O4(tFe=4nm)∕GaAs(100), and Fe3O4(tFe=4nm)∕MgO(3nm)∕Fe3O4(tFe=4nm)∕GaAs(100) was studied by ferromagnetic resonance. It was shown that a predominant in-plane uniaxial magnetic anisotropy and a small fourfold cubic magnetocrystalline anisotropy existed. The in-plane uniaxial anisotropy constant decreased when the MgO layer was covered on Fe3O4∕GaAs, while the cubic anisotropy increased. In the sandwich structures, two resonance peaks were observed. One is similar to that in Fe3O4∕GaAs layer, while another corresponding to Fe3O4 on MgO showed more remarkable fourfold anisotropy and lower uniaxial anisotropy due to smaller mismatch between Fe3O4 and MgO. The interface the Fe3O4 layer is deposited on has dominant effect.

Influence of Capping Layers on Magnetic Anisotropy in Fe/MgO/GaAs(100) Ultrathin Films

Magnetic anisotropies of Fe/MgO/GaAs(100) hybrid structure with two different nonmagnetic capping materials, Au and MgO have been studied by ferromagnetic resonance (FMR). A uniaxial anisotropy, unexpected from the crystal structure was observed in the ultrathin films for both capping materials. Its global easy axis is along [0-11] direction while two (010) directions are equally magnetic hard regardless of the overlayer material. The in-plane uniaxial anisotropy (in-plane cubic anisotropy) of the Au capped samples is stronger (weaker) than that of the MgO capped ones within the range of t Fe = 7.1 to 28 ML. This suggests that the MgO overlayer suppresses the uniaxial anisotropy faster than the Au overlayer.

Thickness dependence of the molecular magnetic moment of single crystal Fe3O4 films on GaAs (100)

The hysteresis loops and the saturation magnetization of the epitaxial Fe3O4 ultrathin films with different thicknesses grown on the GaAs (100) surface have been studied. The molecular magnetic moment of the ultrathin films was obtained from the data of the measured saturation magnetic moment of the films and the number of Fe ions in each film calculated from the measured thickness of Fe films before oxidation and film area. It was found that the values of the molecular magnetic moment of films with thicknesses of 4 and 6 nm were close to the theoretical bulk value of magnetite. For the film thickness of 8 nm, larger magnetization was observed probably caused by less oxidation of the deeper Fe layer. The maximum magnetic moment of the film of 2 nm thick was lower due to the structure imperfection. The low coercivity of the films indicated that the quality of the films was reasonably good.

Discontinuous properties of current-induced magnetic domain wall depinning

The current-induced motion of magnetic domain walls (DWs) confined to nanostructures is of great interest for fundamental studies as well as for technological applications in spintronic devices. Here, we present magnetic images showing the depinning properties of pulse-current-driven domain walls in well-shaped Permalloy nanowires obtained using photoemission electron microscopy combined with x-ray magnetic circular dichroism. In the vicinity of the threshold current density (Jth = 4.2 × 1011u2005A.m−2) for the DW motion, discontinuous DW depinning and motion have been observed as a sequence of “Barkhausen jumps”. A one-dimensional analytical model with a piecewise parabolic pinning potential has been introduced to reproduce the DW hopping between two nearest neighbour sites, which reveals the dynamical nature of the current-driven DW motion in the depinning regime.

Magnetic properties of ultrathin single crystal Fe3O4 film on InAs(100) by ferromagnetic resonance

The evolution of both in-plane and out-of-plane magnetic anisotropies has been studied on different thicknesses of Fe3O4/InAs(100) hybrid spintronic structures by ferromagnetic resonance. The uniaxial magnetic anisotropy with easy axis along InAs [011] direction and hard axis along [0-11] direction and the cubic magnetic anisotropy are determined by fitting the resonance field. The cubic magnetic anisotropy constant K1 is negative, as that of bulk magnetite, and its absolute value increases rapidly from 0.2u2009×u2009104u2009erg/cm3 to 10.8u2009×u2009104u2009erg/cm3 with increasing thickness of Fe3O4 film. When the thickness of Fe3O4 film is only several monolayers (tFeu2009=u20096u2009nm), a bulk-like cubic magneto-crystalline anisotropy is built up. The uniaxial anisotropy constant is much smaller than that in Fe3O4/GaAs film with the same thickness of Fe3O4 film, as expected from the less lattice mismatch at the interface.

Magnetic anisotropies in epitaxial Fe3O4/GaAs(100) patterned structures

Previous studies on epitaxial Fe3O4 rings in the context of spin-transfer torque effect have revealed complicated and undesirable domain structures, attributed to the intrinsic fourfold magnetocrystalline anisotropy in the ferrite. In this Letter, we report a viable solution to this problem, utilizing a 6-nm-thick epitaxial Fe3O4 thin film on GaAs(100), where the fourfold magnetocrystalline anisotropy is negligible. We demonstrate that in the Fe3O4 planar wires patterned from our thin film, such a unique magnetic anisotropy system has been preserved, and relatively simple magnetic domain configurations compared to those previous reports can be obtained.

Identification of nasopharyngeal carcinoma from photoluminescence spectra of 3C-SiC nanocrystals

The identification of intracellular pH (pHi) during carcinogenesis progression plays a crucial role in the studies of biochemistry, cytology, and clinical medicine. In this work, 3C-SiC nanocrystals (NCs), which can effectively monitor the pH environment by using the linear relation between photoluminescence intensity and surface OH– and H+ concentration, are adapted as fluorescent probes for monitoring carcinogenesis progression of nasopharyngeal carcinoma. Our results demonstrated that 3C-SiC NCs are compatible with living cells and have low cytotoxicity. The pHi measurements in different carcinogenesis environments indicate the validity and sensitivity of this technology in identifying nasopharyngeal carcinoma in application.

Investigation of Saturation Magnetization and Damping in Tb/Cr/Fe Trilayers

The Tb(4 nm)/Cr(t nm)/Fe(5 nm) trilayers with different thicknesses of Cr layer have been investigated by X-ray diffraction, vibrating sample magnetometer, and ferromagnetic resonance. When Cr is thinner than 1.8 nm, the ferromagnetic coupling of Tb and Fe leads to higher saturation magnetization (Ms) than that of Fe(5 nm) film. On the other hand, the Gilbert damping constant α in all the trilayers have been found to exceed the magnitude of the Fe. In particular, when Cr layer is 1 nm thick, α increases by 38% and Ms is increased by 6%; when Cr layer is 4 nm thick, α is increased by 46%, while Ms is only decreased by 5%. Accordingly, we have demonstrated a way for enhancing damping and magnetic moment simultaneously in the Tb(4 nm)/Cr(t nm)/Fe(5 nm) trilayers.

Magnetocrystalline anisotropy in textured Fe 3 O 4 film

The magnetic anisotropies are one of the dominating factors affecting the behavior of magnetic materials, which is profoundly evident in magnetic ultrathin films, and significant in the application of information storage and magnetic sensor field. Extensive studies of magnetic anisotropies in various films have been carried out. It is well known that the contribution of magnetocrystalline anisotropy is negligible in isotropic polycrystalline films while essential in single-crystal films. However, in textured film, a polycrystalline film composed of nanograins with the near same lattice orientation, the competing interactions between magnetocrystalline and other anisotropy is still an open issue. The quantitative investigations on the contribution of magnetocrystalline anisotropy in the textured film are rarely reported. In this paper, the expression of magnetocrystalline anisotropy energy in textured film (texture anisotropy energy) is given, and angle-resolved FMR studies on the textured Fe3O4 film is also presented.