Hao-Liang Liu

Exceeding natural resonance frequency limit of monodisperse Fe3O4 nanoparticles via superparamagnetic relaxation

Magnetic nanoparticles have attracted much research interest in the past decades due to their potential applications in microwave devices. Here, we adopted a novel technique to tune cut-off frequency exceeding the natural resonance frequency limit of monodisperse Fe3O4 nanoparticles via superparamagnetic relaxation. We observed that the cut-off frequency can be enhanced from 5.3 GHz for Fe3O4 to 6.9 GHz forFe3O4@SiO2 core-shell structure superparamagnetic nanoparticles, which are much higher than the natural resonance frequency of 1.3 GHz for Fe3O4 bulk material. This finding not only provides us a new approach to enhance the resonance frequency beyond the Snoeks limit, but also extend the application for superparamagnetic nanoparticles to microwave devices.

Determination of magnetic anisotropy constants in Fe ultrathin film on vicinal Si(111) by anisotropic magnetoresistance

The epitaxial growth of ultrathin Fe film on Si(111) surface provides an excellent opportunity to investigate the contribution of magnetic anisotropy to magnetic behavior. Here, we present the anisotropic magnetoresistance (AMR) effect of Fe single crystal film on vicinal Si(111) substrate with atomically flat ultrathin p(2 × 2) iron silicide as buffer layer. Owing to the tiny misorientation from Fe(111) plane, the symmetry of magnetocrystalline anisotropy energy changes from the six-fold to a superposition of six-fold, four-fold and a weakly uniaxial contribution. Furthermore, the magnitudes of various magnetic anisotropy constants were derived from torque curves on the basis of AMR results. Our work suggests that AMR measurements can be employed to figure out precisely the contributions of various magnetic anisotropy constants.

Surface morphology and magnetic anisotropy of obliquely deposited Co/Si(111) films

We report an investigation on magnetic anisotropy of Co/Si(111) films deposited at oblique incidence. An in-plane uniaxial magnetic anisotropy (UMA) with the easy axis perpendicular to the incident flux plane was observed to superimpose on sixfold magnetocrystalline anisotropy of Co films. We built a total energy model to investigate the magnetization reversal mechanism around hard axis. The simulated value of UMA is Ku=1.7×105 erg/cm3, which is consistent with Kshape=1.1×105 erg/cm3 calculated from scanning tunneling microscope image. This good agreement suggests the in-plane UMA is mainly originated from the shape of the oblique deposited Co stripes.

Determination of the critical interspacing for the noninteracting magnetic nanoparticle system

The dipole–dipole interactions of monodisperse Fe3O4 nanoparticles (NPs) can be directly controlled by a uniform SiO2 shell with different thickness, i.e., different interspacings. Thus, the interacting strength of a serial of Fe3O4–SiO2 NPs system can be revealed by fitting the blocking temperature TB measured at ac fields to the Vogel–Fulcher law. The interspacing over five times of diameter for less than 8.0 nm Fe3O4 NPs is the critical value to achieve a noninteracting system. Furthermore, a general equation to evaluate critical interspacing for noninteracting magnetic NPs systems with different sizes and saturation magnetizations was calculated by Monte Carlo method.

Tuning magnetic anisotropies of Fe films on Si(111) substrate via direction variation of heating current

We adopted a novel method to tune the terrace width of Si(111) substrate by varying the direction of heating current. It was observed that the uniaxial magnetic anisotropy (UMA) of Fe films grown on the Si(111) substrate enhanced with decreasing the terrace width and superimposed on the weak six-fold magnetocrystalline anisotropy. Furthermore, on the basis of the scanning tunneling microscopy (STM) images, self-correlation function calculations confirmed that the UMA was attributed mainly from the long-range dipolar interaction between the spins on the surface. Our work opens a new avenue to manipulate the magnetic anisotropy of magnetic structures on the stepped substrate by the decoration of its atomic steps.

Determination of magnetic anisotropies in ultrathin iron films on vicinal Si(111) substrate by the ferromagnetic resonance

Fe single crystal film with thickness of 45 monolayer was fabricated on vicinal Si(111) substrate using ultrathin p(2×2) iron silicide as buffer layer. Scanning tunneling microscope images show that the Fe nanoclusters form chains on vicinal substrate. The first- and second-order magnetocrystalline anisotropies, uniaxial magnetic anisotropy constants of the films were obtained by fitting the ferromagnetic resonance data. The sixfold symmetry of the in-plane resonance field for Fe(111) film was changed into the superposition of a fourfold and a twofold contribution due to the effect of the vicinal substrate.

Magnetization reversal asymmetry in (Co/Pt)/CoFe/IrMn multilayers with enhanced perpendicular exchange bias

The magnetization reversal of perpendicular exchange biased [Co/Pt]/Co60Fe40/IrMn and [Co/Pt]/Co/IrMn multilayers was investigated by time-resolved surface magneto-optical Kerr effect and Kerr microscopy. Compared with the nearly symmetric reversal of the [Co/Pt]/Co/IrMn with a smaller exchange bias field, significantly asymmetric domain evolution in the [Co/Pt]/Co60Fe40/IrMn with a larger exchange bias field was directly observed by Kerr microscopy. The asymmetric magnetization reversal is discussed in terms of the average thermally activated energy barriers as well as the dispersions of the barriers. The substitution of the interfacial Co60Fe40 layer for Co layer results in an enhancement of the interfacial exchange coupling eint and absolute dispersion of interfacial exchange coupling, σeint, and consequently increases the exchange bias field and reversal asymmetry.

Magnetic anisotropy of ultrathin Fe films grown on vicinal Si (111)

We have investigated magnetic anisotropy of ultrathin Fe films grown on vicinal Si (111) with 4° miscut towards [11-2] direction. Spin reorientation transition (SRT) from out-of-plane to in-plane proceeds in a wider thickness range than on flat substrates. Meanwhile, the easy axis of in-plane uniaxial magnetic anisotropy varies from [11-2] to [-110] with an intermediate state of approximate four-fold symmetry. The evolution of magnetic anisotropy is attributed to competition of surface magnetic anisotropy, first-order magnetocrystalline anisotropy, and step induced magnetic anisotropy from symmetry breaking and dipolar interactions.

Nano-faceting of Cu capping layer grown on Fe/Si (111) and its effect on magnetic anisotropy

We represent a report on the growth and structure of Cu capping layer on ultrathin Fe films on Si (111) substrate as well as its effect on magnetic anisotropy. Cu grows as forming triangular-shaped pyramids with nano-facets and in epitaxial mode with Kurdjumov-Sachs orientation. Spin reorientation transition (SRT) from out-of-plane to in-plane of Fe films is induced by Cu capping, which is believed to be mainly affected by strain change in Fe films. Based on strain relief mechanism, rapid decrease in Cu critical thickness to induce SRT with increasing Fe underlayer thickness can be interpreted quite well.

Magnetic anisotropy and magnetization reversal of ultrathin iron films with in-plane magnetization on Si(111) substrates

The magnetic anisotropy and magnetization reversal of single crystal Fe films with thickness of 45 monolayer (ML) grown on Si(111) have been investigated by ferromagnetic resonance (FMR) and vibrating sample magnetometer (VSM). Owing to the significant modification of the energy surface in remanent state by slight misorientation from (111) plane and a uniaxial magnetic anisotropy, the azimuthal angular dependence of in-plane resonance field shows a six-fold symmetry with a weak uniaxial contribution, while the remanence of hysteresis loops displays a two-fold one. The competition between the first and second magnetocrystalline anisotropies may result in the switching of in-plane easy axis of the system. Combining the FMR and VSM measurements, the magnetization reversal mechanism has also been determined.