Isolating the step contribution to the uniaxial magnetic anisotropy in nanostructured Fe/Ag(001) films

We have investigated the possibility of isolating the step-induced in-plane uniaxial magnetic anisotropy in Fe/Ag(001) films on which nanoscale surface ripples were fabricated by the ion sculpting technique. For rippled Fe films deposited on flat Ag(001), the steps created along the ripple sidewalls are shown to be the only source of uniaxial anisotropy. Ion sculpting of ultrathin magnetic films allows one to selectively study the step-induced anisotropy and to investigate the correlation between local atomic environment and magnetic properties.     

Nano-patterning of perpendicular magnetic recording media by low-energy implantation of chemically reactive ions

Magnetic nano-patterning of perpendicular hard disk media with perpendicular anisotropy, but preserving disk surface planarity, is presented here. Reactive ion implantation is used to locally modify the chemical composition (hence the magnetization and magnetic anisotropy) of the Co/Pd multilayer in irradiated areas. The procedure involves low energy, chemically reactive ion irradiation through a resist mask. Among N, P and As ions, P are shown to be most adequate to obtain optimum bit density and topography flatness for industrial Co/Pd multilayer media. The effect of this ion contributes to isolate perpendicular bits by destroying both anisotropy and magnetic exchange in the irradiated areas. Low ion fluences are effective due to the stabilization of atomic displacement levels by the chemical effect of covalent impurities.     

Magnetic anisotropy and spin reorientation in Co/Pt multilayers: Influence of preparation

The magnetic properties of Co/Pt multilayers grown by ion beam and DC magnetron sputtering techniques are compared. For Pt/Co/Pt films the spin reorientation as a function of Co thickness is investigated in second-order uniaxial anisotropy approximation. The influence of the growth technique on the interface and volume contribution to the first-order anisotropy is investigated. The correlations between structure and magnetic properties are discussed.     

α-Fe_2O_3纳米粒子Morin相变的Raman光谱研究

本文首次用Raman光谱法研究了粒子尺寸和Co2 +包附对α-Fe2 O3纳米粒子Morin相变温度TM的影响机制。测量结果显示纳米粒子α -Fe2 O3的振动模相对大块样品发生了红移和宽化 ,粒子愈小红移和宽化愈显著。表明表面Fe3+离子与配位氧离子的键距增大 ,键长呈从体相Fe-O键长过渡到表面Fe -O键长的某种分布。这种小尺寸引起的键长改变导致α -Fe2 O3粒子的单离子各向异性能减小 ,因此降低了样品的TM。分析显示Co2 +包附导致α -Fe2 O3粒子TM 的大幅下降可能是由单离子各向异性的减小和Co2 +与Fe3+之间的磁偶极子相互作用各向异性共同作用的结果     

Magnetic nanoparticles as building blocks for high-performance magnetic materials

We report on the magnetic behaviour of films of Fe nanoparticles deposited from the gas phase with sizes in the range 2–3 nm embedded in Ag and Co matrices and Co nanoparticles of the same size embedded in Ag matrices. Magnetometry measurements, using a VSM, of very low volume fraction (1–2%) assemblies of Fe and Co in Ag show perfect superparamagnetism and enable us to confirm that the size distribution of the particles in the matrix is the same as that of the free particles prior to deposition. The hysteresis loops at 2 K, which is below the blocking temperature, show that the particles have a uniaxial anisotropy that is randomly oriented in three dimensions with the Co nanoparticles having a much higher anisotropy than the Fe particles. The soft magnetic behaviour of pure Fe and Co nanoparticle films with no matrix is well described by a random anisotropy model and is consistent with the formation of a correlated super-spin glass (CSSG) characteristic of amorphous materials. The Co nanoparticle films appear to have a lower random anisotropy than the Fe ones in contrast to the behaviour observed for the isolated particles. Films of Fe nanoparticles embedded in Co matrices, whose saturation magnetization exceeds the Slater–Pauling curve, also show magnetic behaviour consistent with a CSSG. At high volume fractions, the films of Fe nanoparticles embedded in Co matrices behave almost identically to films of pure Co nanoparticles.     

Evolution of modulated structure in Fe–Cr–Co alloy during isothermal ageing with different external magnetic field conditions

The evolution of modulated structure in Fe–Cr–Co alloy during isothermal ageing with and without external magnetic field was investigated by using transmission electron microscopy (TEM) and phase-field simulation. The isotropic modulated structure in Fe–Cr–Co alloy formed during isothermal ageing without external magnetic field could be converted to be anisotropy during further isothermal ageing under an 8 kOe external magnetic (thermo-magnetic treatment), and the formation of anisotropy was slower than the direct formation from spinodal decomposition under external magnetic field and is time dependent. The anisotropy characteristic of modulated structure in Fe–Cr–Co alloy subjected to thermo-magnetic treatment for 1 h remained during further isothermal ageing without external magnetic field for up to 20 h. Novel modulated structure could be obtained through specific thermo-magnetic treatment processes, which was confirmed by phase-field simulation.     

Magnetic anisotropy and domain patterns in electrodeposited cobalt nanowires

The magnetic anisotropy and domain structure of electrodeposited cylindrical Co nanowires with length of 10 or 20 μm and diameters ranging from 30 to 450 nm are studied by means of magnetization and magnetic torque measurements, as well as magnetic force microscopy. Experimental results reveal that crystal anisotropy either concurs with shape anisotropy in maintaining the Co magnetization aligned along the wire or favours an orientation of the magnetization perpendicular to the wire, hence competing with shape anisotropy, depending on whether the diameter of the wires is smaller or larger than a critical diameter of 50 nm. This change of crystal anisotropy, originating in changes in the crystallographic structure of Co, is naturally found to strongly modify the zero (or small) field magnetic domain structure in the nanowires. Except for nanowires with parallel-to-wire crystal anisotropy (very small diameters) where single-domain behaviour may occur, the formation of magnetic domains is required to explain the experimental observations. The geometrical restriction imposed on the magnetization by the small lateral size of the wires proves to play an important role in the domain structures formed. Received 14 September 2000     

Influence of layer thickness on the structure and the magnetic properties of Co/Pd epitaxial multilayer films

Co/Pd epitaxial multilayer films were prepared on Pd(111)_fcc underlayers hetero-epitaxially grown on MgO(111)_B1 single-crystal substrates at room temperature by ultra-high vacuum RF magnetron sputtering. In-situ reflection high energy electron diffraction shows that the in-plane lattice spacing of Co on Pd layer gradually decreases with increasing the Co layer thickness, whereas that of Pd on Co layer remains unchanged during the Pd layer formation. The CoPd alloy phase formation is observed around the Co/Pd interface. The atomic mixing is enhanced for thinner Co and Pd layers in multilayer structure. With decreasing the Co and the Pd layer thicknesses and increasing the repetition number of Co/Pd multilayer film, stronger perpendicular magnetic anisotropy is observed. The relationships between the film structure and the magnetic properties are discussed.     

Domain-wall pinning by local control of anisotropy in Pt/Co/Pt strips

We theoretically and experimentally analyze the pinning of a magnetic domain wall (DW) at engineered anisotropy variations in Pt/Co/Pt strips with perpendicular magnetic anisotropy. An analytical model is derived showing that a step in the anisotropy acts as an energy barrier for the DW. Quantitative measurements are performed showing that the anisotropy can be controlled by focused ion beam irradiation with Ga ions. This tool is used to experimentally study the field-induced switching of nanostrips which are locally irradiated. The boundary of the irradiated area indeed acts as a pinning barrier for the domain wall and the pinning strength increases with the anisotropy difference. Varying the thickness of the Co layer provides an additional way to tune the anisotropy, and it is shown that a thinner Co layer gives a higher starting anisotropy thereby allowing tunable DW pinning in a wider range of fields. Finally, we demonstrate that not only the anisotropy itself, but also the width of the anisotropy barrier can be tuned on the length scale of the domain wall.     

Self-organization of ripples on Ti irradiated with focused ion beam

30 keV focused Ga⁺ ions were used to raster the metallographically polished surface of commercially pure Ti (CP Ti) at various FIB incidence angles over a wide range of doses (10¹⁶-10¹⁸ ions/cm²) at room temperature. The sputtered surfaces were observed in situ using FIB imaging and later carefully characterized ex situ under scanning electron microscope (SEM) and atomic force microscope (AFM). Ripples were observed on the irradiated surfaces even at the normal FIB incidence angle. The ripple evolution is analyzed as functions of surface diffusion, surface crystallographic orientation, ion dose and incidence angle. It is found that the ripple orientation was progressively influenced by the ion beam direction with incidence angle increasing and in some cases curved ripples or fragmented rods viewed from different angles occurred at high ion doses. The morphological evolution from the well-developed straight ripples to the curved ones is never observed. The formation of ripples is attributed to the competition between the formation of ripples due to anisotropic surface diffusion and the formation of incidence-angle dependent ripples determined by Bradley-Harper (BH) model.     

Out-of-plane exchange bias and magnetic anisotropy in MnPd/Co multilayers

Magnetic and structural properties in [MnPd/Co]₁₀ multilayers deposited onto Si(1 1 1) substrates have been investigated. The dependences of anisotropy and exchange bias on the thicknesses of both MnPd and Co layers have been studied. In most of the samples, the out-of-plane magnetic anisotropy and both large out-of-plane and in-plane exchange biases have been observed at cryogenic temperature below the blocking temperature T_B≈240 K. With appropriate MnPd and Co thicknesses, we have obtained samples with a large out-of-plane exchange bias along with a large out-of-plane magnetic anisotropy. The origin of the out-of-plane magnetic anisotropy in the samples has been suggested to be due to the formation of CoPd interfacial alloys which have tensile in-plane strains, while the spin structure of the antiferromagnetic layer at the interface which is believed to be responsible for exchange bias may be the same as that of the bulk material. Also, the present study shows that the interplay between the out-of-plane magnetic anisotropy and exchange bias is evident in our multilayers and plays an important role in the out-of-plane exchange-bias mechanism.     

Propulsion of ripples on glass by ion bombardment

The propulsion of surface ripples on SiO(2) by an ion beam was investigated by in situ electron microscopy. The observed propagation of the ripples contradicts existing models for ion-beam-induced ripple development. A new model based on the Navier-Stokes relations for viscous flow in a thin layer is introduced. It includes inhomogeneous viscous flow, driven by spatial variations in the deposition of the energy of the ion beam. The model explains the observed reversed propagation. The hitherto unknown propulsion mechanism is important for understanding nanoscale pattern formation by ion bombardment.     

Regulating the magnetic anisotropy by Hf thickness and heat treatment in Pt/Co/Hf films

Pt/Co/Hf multilayers were prepared by magnetron sputtering, and the magnetic anisotropy was effectively regulated by Hf thickness and heat treatment in Pt/Co/Hf films. The interface microstructures were characterized. The influence of the interface microstructure on magnetic properties was studied. The results show that the magnetic anisotropy in Pt/Co/Hf films is closely related to the interface microstructure, which is influenced by Hf thickness and the heat treatment temperature. Microstructure analysis shows that after the Pt(3)/Co(1.5)/Hf(1) film is heat-treated, the CoO_x content increases, more CoPt(111) forms, the interface is smoother and sharper, and the roughness of the Co/Hf interface decreases. Several factors work together to cause the magnetic anisotropy of the sample to change from in-plane magnetic anisotropy (IMA)to perpendicular magnetic anisotropy (PMA).     

The effect of Fe doping on structural and magnetic properties of nanocrystallite Co2Cr1-xFexAl Heusler alloys prepared by mechanical alloying

Mechanical alloying has been used to produce nanocrystallite Co2Cr1-xFexAl(x=0,0.4,0.6,1) Heusler alloys.The formation of the L21 phase of Co2Cr1-xFexAl by the mechanical alloying method was investigated.The effect of Fe doping on the structural and magnetic properties of the samples was also studied.The results were compared with the Slater Pauling prediction.A comparison between these samples and those prepared by the arc-melting method in the literature was made.An increase of the coercivity H c with the increasing Fe doping level was observed.This phenomenon was explained by the increases of lattice strain and magnetic anisotropy with the increasing Fe content.     

Magnetic remanent states in antiferromagnetically coupled multilayers

In antiferromagnetically coupled multilayers with perpendicular anisotropy unusual multidomain textures can be stabilized due to a close competition between long-range demagnetization fields and short-range interlayer exchange coupling. In particular, the formation and evolution of specific topologically stable planar defects within the antiferromagnetic ground state, i.e. wall-like structures with a ferromagnetic configuration extended over a finite width, explain configurational hysteresis phenomena recently observed in [Co/Pt(Pd)]/Ru and [Co/Pt]/NiO multilayers. Within a phenomenological theory, we have analytically derived the equilibrium sizes of these “ferroband” defects as functions of the antiferromagnetic exchange, a bias magnetic field, and geometrical parameters of the multilayers. In the magnetic phase diagram, the existence region of the ferrobands mediates between the regions of patterns with sharp antiferromagnetic domain walls and regular arrays of ferromagnetic stripes. The theoretical results are supported by magnetic force microscopy images of the remanent states observed in [Co/Pt]/Ru.     

Interface study of ion irradiated Cu/Ni/Cu(0 0 2)/Si magnetic thin film by X-ray reflectivity

The irradiation effect of 1 MeV C⁺ on the interface and magnetic anisotropy of epitaxial Cu/Ni system with a perpendicular magnetic anisotropy was investigated by using magneto-optical Kerr effects, grazing incident diffraction and X-ray reflectivity. The magnetic easy-axis was altered from the direction along the surface normal to in-plane and the strain in the Ni layer was relaxed after ion irradiation. Though the interface between the top Cu layer and the Ni layer becomes rough, the contrast of electron densities of Cu and Ni layer increases and the grain-growth occurs during ion irradiation. These phenomena arise from thermo-chemical driving force, i.e. heat of formation, which may be a crucial factor in determining the interface shape in the case of indirect energy transfer mechanism. Therefore, the change of the magnetic anisotropy of the Ni/Cu system after ion irradiation is not due to the formation of the intermixed layer at the interface. The ion irradiation effects on the grain-growth and enhancement of the electronic contrast between Ni and Cu are explained by the interfacial atomic movement caused by thermo-chemical driving force.     

Strongly enhanced orbital moment by reduced lattice symmetry and varying composition of Fe1-xCox alloy films

We studied tetragonally distorted Fe(1-x)Co(x) alloy films on Rh(001), which show a strong perpendicular anisotropy in a wide thickness and composition range. Analyzing x-ray magnetic circular dichroism spectra at the L_(3,2) edges we found a dependence of the Co magnetic orbital moment on the chemical composition of the Fe(1-x)Co(x) alloy films, with a maximum at x=0.6. For this composition, we observed an out-of-plane easy axis of magnetization at room temperature for film thickness up to 15 monolayers. Since both the magnetic orbital moment and the anisotropy energy show similar composition dependence, it confirms that both quantities are directly related. Our experiments show that the adjustment of the Fermi level by a proper choice of the alloy composition is decisive for the large magnetic orbital moment and for a large magnetic anisotropy in a tetragonally distorted lattice.     

Interface biquadratic coupling and magnon scattering in exchange-biased ferromagnetic thin films grown on epitaxial FeF2

The magnetic anisotropy of ferromagnetic (FM) Ni, Co, and Fe polycrystalline thin films grown on antiferromagnetic (AF) FeF(2)(110) epitaxial layers was studied, as a function of temperature, using ferromagnetic resonance. In addition to an in-plane anisotropy in the FM induced by fluctuations in the AF short-range order, a perpendicular (biquadratic) magnetic anisotropy, with an out-of-plane component, was found which increased with decreasing temperature above the AF Neél temperature (T(N) = 78.4 K). This is a surprising result given that the AF's uniaxial anisotropy axis was in the plane of the sample, but is consistent with prior experimental and theoretical work. The resonance linewidth had a strong dependence on the direction of the external magnetic field with respect to in-plane FeF(2) crystallographic directions, consistent with interface magnon scattering due to defect-induced demagnetizing fields. Below T(N), the exchange bias field H(E) measured via FMR for the Ni sample was in good agreement with H(E) determined from magnetization measurements if the perpendicular out-of-plane anisotropy was taken into account. A low field resonance line normally observed at H ≈ 0, associated with domain formation during magnetization in ferromagnets, coincided with the exchange bias field for T < T(N), indicating domain formation with the in-plane FM magnetization perpendicular to the AF easy axis. Thus, biquadratic FM-AF coupling is important at temperatures below and above T(N).     

多层膜［Co85Cr15/Pt］20的磁性、垂直磁记录特性和微结构的关系

采用磁控溅射法制备了性能优良的以Pt为缓冲层的［Co85₈₅Cr15_{15/Pt］2020 多层膜，研究了溅射气压对［Co8585Cr1515/Pt］2020多层膜微结构和磁性的 影响.研究结果表明，Ar溅射气压对［Co8585Cr1515/Pt］2020多层膜的微结构 、垂直磁各向异性和矫顽力有重要的影响 关键词： 溅射气压 多层膜 垂直磁各向异性 有效磁各向异性常数}     

Asymmetric domain nucleation and unusual magnetization reversal in ultrathin Co films with perpendicular anisotropy

We report unexpected phenomena during magnetization reversal in ultrathin Co films and Co/Pt multilayers with perpendicular anisotropy. Using magneto-optical Kerr microscopy and magnetic force microscopy we have observed asymmetrical nucleation centers where the reversal begins for one direction of the field only and is characterized by an acute asymmetry of domain-wall mobility. We have also observed magnetic domains with a continuously varying average magnetization, which can be explained in terms of the coexistence of three magnetic phases: up, down, and striped.