Ya-Peng Fang

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 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.

Uniaxial magnetic anisotropy of quasi-one-dimensional Fe chains on Pb∕Si

We fabricated quasi-one-dimensional Fe chains on a 4° miscut Si (111) substrate with a Pb film as a buffer layer. The magnetic properties and morphology of Fe chains were investigated by means of scanning tunneling microscope (STM) and surface magneto-optical Kerr effect (SMOKE). STM images show that Fe chains are formed by Fe random islands along the steps of the Pb film due to step decoration. SMOKE data indicate that the Fe chains exhibit in-plane uniaxial magnetic anisotropy along the step direction. The effective in-plane uniaxial anisotropy constant at room temperature was determined by means of electron spin resonance.

Tb nanocrystalline array assembled directly from alloy melt

A two-dimensional (2D) thin film composed of Tb nanocrystals with uniform orientations is fabricated directly from Tb65Fe25Al10 alloy melt upon quenching. The Tb nanocrystals with vertical height of 15–30 nm and lateral width of 10–20 nm are assembled in the nanocrystalline array on an amorphous substrate. The formation mechanism for the aligned Tb nanocrystals is discussed. The single-step formation method may provide a new and flexible alternative to fabricate nanostructured films or arrays used for submicron devices.

Magnetic aftereffect and magnetic force microscopy studies of Fe–B∕FePt-type nanocomposite ribbons

Magnetic aftereffect and surface magnetic domain structure of (Fe0.675Pt0.325)100−xBx(x=12–20) nanocomposites have been investigated for correlating with their corresponding phases and magnetic properties. Exchange-coupling effect between grains is present for all the studied ribbons as evidenced by Henkel plot. The volume fraction of magnetically soft phases decreases with increasing B concentration from x=12 to x=18. It leads to the reduction in the activation volume of the reverse domain. Among all samples, the (Fe0.675Pt0.325)82B18 ribbon having the highest intrinsic coercivity (Hci) exhibits minimum value in activation volume V=1.1×10−18cm3, because it possesses the least volume fraction of the magnetic soft phases, i.e., Fe2B or Fe3B phases. Magnetic force microscopy studies reveal the magnetic domain structures of the ribbons, confirming the existence of strong exchange coupling between magnetic grains.

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.

Monte Carlo simulation of magnetic properties of irregular Fe islands on Pb/Si(111) substrate based on the scanning tunneling microscopy image

Magnetic properties of irregular Fe islands grown on Si(111) substrate with Pb buffer layer by molecular beam epitaxy have been investigated by means of Monte Carlo simulation. During the simulation, the dipolar interaction energy among islands and the demagnetization energy of each island were accurately calculated in the Fourier space with the aid of cluster multiple labeling technique. The simulated results, i.e., the magnetic hysteresis loops and the temperature dependence of remanent magnetization, are in good agreement with the experimental ones measured by in situ surface magneto-optical Kerr effect (SMOKE).

Improvement of the uniformity and dipole ferromagnetism in Co nanodots assemblies on Pb/Si(111) via step tuned dimensionality variation

We fabricated quasi-one-dimensional Co nanochain assemblies and two-dimensional Co nanodot assemblies on Pb/Si(111) substrates by step decoration. The morphology and magnetic properties of these two kinds of Co nanodot assemblies were investigated by in situ scanning tunneling microscopy and magneto-optical Kerr effect measurements. It was found that the steps cannot only improve the uniformity of the Co nanodots, but also increase the critical temperature T(c). Monte Carlo simulation indicates that the ferromagnetism mainly originates from the dipolar interactions and the critical temperature T(c) can be enhanced by introducing an in-plane uniaxial magnetic anisotropy via the step tuned dimensionality variation of the nanodot assemblies.

Uniaxial magnetic anisotropy of quasi-one-dimensional Fe chains on Pb/Si: A Monte Carlo simulation

Magnetic behaviors of Fe nanowires grown on 4° miscut Si(111) substrate with Pb buffer layers have been investigated by means of Monte Carlo method. A simple model is constructed, in which the Fe chains are assumed to be assemblies of single domain Fe nanoclusters with magnetostatic energy and exchange coupling energy. The coverage dependence of the magnetic ordering temperature TC of the system is discussed. By accurately calculating the magnetostatic energy of the Fe chains, the simulated results are in agreement with the experimental ones measured by in situ surface magneto-optical Kerr effect. In addition to the magnetostatic energy, the exchange coupling between the overlapping islands is also responsible for the ferromagnetic ordering of high coverage Fe chains at room temperature. Our model was able to predict the essential features of the system.