具有垂直各向异性(Pt/Co)n/FeMn多层膜的交换偏置

采用磁控溅射方法制备了以Pt为缓冲层和保护层的具有垂直各向异性(Pt/Co)n/FeMn多层膜.研究结果表明,多层膜的垂直交换偏置场Hex和反铁磁层厚度的关系与其具有平面各向异性的交换偏置场随反铁磁层厚度变化趋势相近.随着铁磁层调制周期数的增加,垂直交换偏置场Hex相应减小,并且与铁磁层的调制周期数近似成反比关系.(Pt/Co)3/FeMn的垂直交换偏置场Hex已经达到22.3kA/m.为了进一步提高Hex,在Co/FeMn的界面插入Pt层,当Pt层厚度为0.4nm时,Hex达到最大值39.8kA/m.     

Perpendicular exchange bias of Co/Pt multilayers

Exchange bias measurements of ferromagnetic/antiferromagnetic (F/AF) bilayers are typically performed with the magnetization of the F layer parallel to the AF interface. We describe measurements of Co/Pt multilayers with out-of-plane magnetic easy axis that are exchange biased with CoO. Field-cooling experiments with the applied field perpendicular and parallel to the sample plane exhibit loop shifts and enhanced coercivities. Modeling and comparison to biasing of samples with planar easy axis suggests such measurements provide a way to probe the spin projections at F/AF interfaces.     

Size dependence of exchange bias in Co/CoO nanostructures

In Co/CoO nanostructures, of dimensions l×3l, at small Co thickness (≈6,10 nm), a strong increase in the bias field and the associated coercive field are found as the nanostructure size is reduced from l=120 nm to l=30 nm. This property indicates that the characteristic length D(AF) within the antiferromagnet which governs exchange-bias effects is the nanostructure size. By contrast, at larger Co thickness (≈23 nm), the exchange-bias field does not depend on the nanostructure size, implying that D(AF) is smaller than the nanostructure size. The results are discussed in the framework of the Malozemoff model, taking into account that the coupling between CoO grains is weak. Exchange bias is dominated either by coupling within the antiferromagnetic layer (6- and 10-nm-thick Co samples) or by ferromagnetic-antiferromagnetic interfacial coupling (23-nm-thick Co sample).     

具有垂直各向异性(Pt/Co)n/FeMn多层膜的交换偏置

采用磁控溅射方法制备了以Pt为缓冲层和保护层的具有垂直各向异性(Pt/Co)_n/ FeMn多层膜.研究结果表明，多层膜的垂直交换偏置场H_ex和反铁磁层厚度的关 系与其具有平面各向异性的交换偏置场随反铁磁层厚度变化趋势相近.随着铁磁层调制周期 数的增加，垂直交换偏置场H_ex相应减小，并且与铁磁层的调制周期数近似成反 比关系.(Pt/Co)₃/FeMn的垂直交换偏置场H_ex已经达到22.3kA/m.为 关键词： 交换偏置 垂直各向异性 多层膜     

Magnetic properties of Co films and Co/Pt multilayers deposited on PDMS nanostructures

The magnetic properties of magnetron sputtered Co films (with in-plane anisotropy) and Co/Pt multilayers (with perpendicular anisotropy) deposited on elastomeric poly-dimethylsiloxane (PDMS) films and nanostructured templates are presented. Apart from the etched nanosize features, maze-like submicron features develop after the sputter deposition as a result of film buckling due to thermal contraction of the underlying PDMS layer. The nanostructured templates can physically isolate the magnetic entities but magnetic correlations within the range of the buckling features remain. By using oblique deposition geometries in-plane anisotropy develops and it is transferred to the PDMS buckling patterns. In the case of Co/Pt multilayers deposition on PDMS nanostructured templates results in a loss of the perpendicular anisotropy, attributed to the acuteness of the PDMS nanostructures.     

Asymmetric magnetoresistance in an exchange bias Co/CoO bilayer

Magnetoresistance (MR) measurements are carried out on a Co(8 nm)/CoO(3.5 nm) bilayer in the exchange bias (EB) state prepared by molecular beam epitaxy. With the applied magnetic field parallel to the current, the EB MR curves show an asymmetric behavior about the minimum, in contrast to the symmetric one for non-EB systems. We generalize a well-known analytical expression used for the field dependence of the MR of paramagnets. Our generalization incorporates coercivity and EB in a new phenomenological MR expression. Excellent fits of the latter to the experimental MR data are achieved, showing the way to use MR techniques for the quantitative characterization of EB systems. Furthermore, the temperature dependence of the EB field obtained from MR loops can be described with a power law, which yields a value of 96.6 K for the EB blocking temperature, which is significantly below the Néel temperature of 293 K for bulk CoO.     

Exchange coupling in CoO–Co bilayer

In this work, exchange bias and coercivity enhancement in ferromagnet (FM)–antiferromagnet (AFM) bilayer have been investigated. CoO film (50 nm) was deposited by sputtering with a relatively high oxygen partial pressure. The deposited films were subsequently annealed at varied temperature up to 973 K in the air atmosphere. The CoO film shows a disordered structure in the as-deposited state and an increase of crystallinity after annealing characterized by XRD and Raman spectra. A 40-nm Co film was deposited on the as-deposited CoO and annealed films. The Co–CoO bilayer shows a large exchange bias up to 1600 Oe and relatively high coercivity up to 3200 Oe (H_C−) at 5 K, which is much larger than that of crystalline Co–CoO bilayer films without any treatment. The spin glass behavior combined with increasing crystallinity, surface roughness of CoO after annealing may be attributed to the large exchange bias and high coercivity.     

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.     

Effect of substrate bias voltage on corrosion of TiN/Ti multilayers deposited by magnetron sputtering

Austenitic stainless steel can be attack by localized corrosion in saline environments, such as seawater. TiN/Ti multilayers can improve the corrosion resistance of the stainless steel better than TiN monolayers, because the titanium layers improve the impermeability of TiN/Ti multilayers. In this work, 1.75-4.55 μm thick layers were deposited on to grounded or −100 V biased substrates of 304 stainless steel substrates by reactive magnetron sputtering. The corrosion resistance of the layers was studied by means of potentiodynamic polarization in 0.5 M NaCl solutions. It was found that the pitting corrosion resistance was dependent on the bias and period number of the multilayers.     

Exchange bias in thin-film (Co/Pt)3/Cr2O3 multilayers

We have fabricated exchange-biased Co/Pt layers ((0.3 nm/1.5 nm)×3) on (0 0 1)-oriented Cr₂O₃ thin films. The multilayered films showed extremely smooth surfaces and interfaces with root mean square roughness of ≈0.3 nm for 10 μm×10 μm area. The Cr₂O₃ films display sufficient insulation with a relative low leakage current (1.17×10⁻² A/cm² at 380 MV/m) at room temperature which allowed us to apply electric field as high as 77 MV/m. We find that the sign of the exchange bias and the shape of the hysteresis loops of the out-of-plane magnetized Co/Pt layers can be delicately controlled by adjusting the magnetic field cooling process through the Néel temperature of Cr₂O₃. No clear evidence of the effect of electric field and the electric field cooling was detected on the exchange bias for fields as high as 77 MV/m. We place the upper bound of the shift in exchange bias field due to electric field cooling to be 5 Oe at 250 K.     

Influence of magnetocrystalline anisotropy on the magnetization reversal mechanism in exchange bias Co/CoO bilayers

We investigated the reversal mechanism in a Co/CoO exchange bias bilayer with a pronounced magnetocrystalline anisotropy in the ferromagnet. The anisotropy, which is induced by the growth of a highly textured Co layer, imposes a distinct reversal mechanism along the magnetically easy and hard direction. It is shown that exchange bias can be induced along both directions, despite the magnetocrystalline anisotropy. The interplay between the magnetocrystalline anisotropy and exchange bias induces a different reversal mechanism for the subsequent reversals in the two crystallographic directions. Along the hard axis, the magnetization reverses according to the reversal mechanism observed before in polycrystalline exchange bias bilayers, i.e. domain wall nucleation and motion for the first reversal and coherent rotation for the subsequent ones. Along the easy axis, domain wall motion remains the dominant reversal mechanism and magnetization rotation has only a minor contribution.     

Exchange bias and blocking temperature in Co/FeMn/CuNi trilayers

Aspects of exchange bias between antiferromagnets and ferromagnets remain unclear despite recent research. An outstanding issue is the relationship between exchange bias and enhanced coercivity in the ferromagnetic layer. This Letter reports the unexpected finding that a substantial exchange bias can be generated between an antiferromagnet (FeMn) with a higher ordering temperature than that of the ferromagnet (CuNi). We interpret the result in terms of a temperature-dependent competition between interfacial exchange and antiferromagnet anisotropy energies. Crossover of these energies during cooling is responsible for the onset of exchange bias at the blocking temperature.     

Bistable coupling states measured on single Co nanoclusters deposited on CoO(111)

We describe novel features of the induced magnetic anisotropy in Co nanoclusters coupled with a CoO(111) layer. Individual cluster magnetism was studied using new microbridge superconducting quantum interference devices. Intrinsically, the Co clusters are single domains with an effective anisotropy constant K(F)≈1.5×10(6) erg·cm(-3). A bistable state of the ferromagnetic-antiferromagnetic coupling is revealed, with a maximum bias systematically observed along CoO[10 ?1] and an interfacial coupling energy of 0.9 erg·cm(-2). The small bias observed in cluster assembly results from an averaging over the two opposite stable states.     

Fabrication of Co/CoO Exchange Bias System by Ion Implantation and Its Magnetic Properties

We use ion implantation as a new approach to build an anti-ferromagnetic （AFM） cluster embedded exchange bias （EB） system. Co film with thickness of 130nm is deposited on the Si （111） substrate using magnetron sputtering, 60keV O＋ is chosen to implanted into the Co film to form CoO AFM clusters coupling with Co matrix at the interface. By measuring the hysteresis loop after field-cooling, significant shifts of loop along the applied field are confirmed. When increasing the implantation dose to 2×1017/cm2 and annealed samples in N2 atmosphere, we obtain the highest HEB to 458Oe.     

Magnetic multilayers on nanosphere arrays with various curvature radius

[Co/Pt]_n multilayers with different Co thickness have been deposited on a silicon (Si) substrate to obtain better perpendicular anisotropy. The 0.5 nm thickness of the Co layer was chosen as the optimized thickness of the multilayer. Magnetic nanostructures with cap configuration were fabricated based on the template of polystyrene (PS) colloid sphere arrays with various curvature radius. Compared to the flat multilayer, the cap multilayer showed an oblique average anisotropy axis. When the curvature radius of the colloidal sphere increased, the shape of the multilayers changed from ellipsoidal to spherical, which led to a different dependence of magnetic properties on the field angles. The varying shape anisotropy, the dipole-dipole interaction between small magnetic caps, and the special nucleation mechanism on the spheres larger than 400 nm caused the M_r/M_s ratio and the coercivity to first increase and then decrease with varying curvature radius of the PS spheres.     

Exchange bias and negative magnetoresistance in [Co/Bi/Co]/IrMn thin films]

The artificial control of grain-boundary resistance and its contribution to magnetic and magneto-transport properties in [Co(3 nm)/Bi(2.5 nm)/Co(3 nm)]Ir₂₀Mn₈₀(12 nm) thin films that exhibit exchange bias is studied. Transverse magnetoresistance (MR) loops exhibit a negative MR in thin films grown by magnetron sputtering on Si/SiN_x(100 nm) substrates. This negative MR effect is of the giant-MR (GMR) type, although its magnitude is less than 1%. A considerable exchange bias (EB) effect is observed only at lower temperatures, where both, GMR and isothermal magnetization loops exhibit a shift of −600 Oe at 5 K.     

Investigation of the magnetization reversal of a magnetic dot array of Co/Pt multilayers

The magnetization reversal behavior of a dot array consisting of Co/Pt multilayers with perpendicular magnetic anisotropy was investigated. The size of the dots was varied from 200 nm down to 40 nm, while keeping the filling factor constant at about 0.16. The structural properties were determined by scanning electron microscopy, whereas the magnetic investigation was performed using SQUID and MFM techniques. It was observed that the dot size has a severe impact on the magnetization reversal mechanism where only the smallest dots with a size of 40 nm are found to be in a magnetic single-domain state. Moreover, the patterning process leads to a degradation of the multilayer, leading to a reduction of the switching field and an increase of the switching field distribution with decreasing dot size. In addition, micromagnetic simulations were performed to understand the magnetization reversal mechanism in more detail.     

Co/C and W/Si multilayers deposited by ion-beam sputtering for the soft X-ray range

We deposited Co/C multilayer mirrors for a wavelength of 4.77 nm and W/Si multilayer mirrors for a wavelength of 1.77 nm by use of ion-beam sputtering. The small-angle diffraction spectrum was used to analyze the structure of the multilayers. With a combination of the experimental diffraction spectra and Apeles’ theory for calculation of the interfacial roughnesses of the multilayers, the interfacial roughnesses of Co/C and W/Si are 0.80 nm and 0.60 nm, respectively, which are lower than that of the substrate. The reflectivity of the Co/C multilayer is measured to be about 20% and that of the W/Si multilayer about 1% at the grazing incidence angle of about 12°. Received: 30 May 2000 / Accepted: 1 August 2000 / Published online: 11 February 2002     

NMR spin-echo study on Co/Pd multilayers

Hyperfine field spectra of thin Co/Pd multilayers have been determined by spin echo NMR. The main contribution to the spectra seems to arise from Co ions in a polyerystalline FCC phase. For the layers with thinnest Co sublayer thickness the centre of the spectrum shifts to lower hyperfine fields upon decreasing Co sublayer thickness. This is attributed to a decrease of the magnetic moment caused by stretching of the Co “lattice” due to neighbouring Pd atmos. An analysis of the integrated intensity in the hyperfine field region 19 T to 22 T shows a loss of signal equivalent to about one layer of Co per sublayer.