NiFe/FeMn双层膜交换偏置的形成及热稳定性研究

研究了在铁磁(NiFe)/反铁磁(FeMn)双层膜之间，交换偏置的形成过程和热稳定性，特别是NiFe/FeMn的交换偏置作用与FeMn层晶粒尺寸的关系.和以前作者不同的是，本文方法采用非磁性Ni-Fe-Cr合金作缓冲层材料，改变Cr的含量就可以获得不同晶粒尺寸的反铁磁FeMn层.实验表明，晶粒尺寸较小的FeMn产生较强的铁磁/反铁磁交换偏置场；但是，对于较大晶粒的FeMn层，出现交换偏置作用所要的临界厚度较小.这符合Mauri提出的理论模型.交换偏置场的热稳定性实验表明，具有较大晶粒尺寸的FeMn层给出较 关键词： 交换偏置 热稳定性 反铁磁 晶粒尺寸     

Relative-thickness dependence of exchange bias in bilayers and trilayers

We consider the models of ferromagnetic (FM)/antiferromagnetic (AFM) bilayers and trilayers and perform a modified Monte Carlo method to study their exchange bias (EB) properties at low temperature after field cooling on increasing one component thickness at the expense of the other one. The results indicate that EB is insensitive to the thickness variations as the FM layer is thicker than the AFM one. Otherwise, it has a steep increase with the decrease of FM thickness, but the purely inverse proportion is no longer valid due to the dual influences of FM and AFM thicknesses. EB in trilayers should be approximately twice larger than that in bilayers because there is a double interfacial area in the trilayers compared with the bilayers, but the dispersed FM/AFM distributions may break this relation as a result of thermal destabilization. Moreover, EB is independent of FM/AFM stacking sequences probably because of the ideal interface between them. It has been clarified unambiguously that such control of EB through varying the FM/AFM dimensions in heterostructures is attractive for spintronics applications.     

Bottom and top AF/FM interfaces of NiFe/FeMn/NiFe trilayers

X-ray reflectivity analyses were performed in the Si/WTi (7 nm)/NiFe (30 nm)/FeMn (13 nm)/NiFe (10 nm)/WTi (7 nm) exchange-biased system prepared by magnetron sputtering under three different argon working pressures. Layer-by-layer analyses were realized in order to obtain the interfacial roughness parameters quantitatively. For a fixed argon pressure, the root-mean-square roughness (including the atomic grading) of the upper (FeMn/NiFe) interface are greater than that for the lower one in all studied samples. Argon working pressure also has severe influence over the NiFe/FeMn interfaces, being more pronounced at the upper interfaces.     

An exchange bias observed in Tb/Cr/FeCo trilayers with ultrathin Cr layer at low temperature

Positive exchange bias field (H_e) is observed in Tb/Cr (t_Cr)/FeCo trilayers at 5 K without cooling field, and negative H_e for Tb/FeCo bilayer. The negative H_e of Tb/FeCo implies the FM coupling at the interface due to Co and Fe dominate in the magnetization of the ferrimagnetic interlayer alloy of FeCo and Tb. With the inserting of Cr layer, this situation is broken, and the positive H_e implies the antiferromagnetic interlayer coupling. A peak of H_e = 6.0 mT for trilayers with t_Cr = 1.5 nm is corresponding to the minimum value of coercivity as a function of t_Cr at 5 K, which is used to study the effect of the cooling field (H_fc) on H_e as a function of temperature. It is found that H_fc of 100 mT triggers H_e from positive to negative at T ≤ 15 K. The magnetoresistance results also confirm the coexistence of multiple MR mechanisms in these trilayers.     

Temperature dependence of the exchange coupling in CO/SI(or Ge)/Fe trilayers

The temperature dependences of interfacial exchange coupling in Co/semiconductor (SM)/Fe trilayers (SM≡Si or Ge) with different spacer thicknesses are investigated. Only one step is found in the third (not in the first) quadrant of the hysteresis loop of the trilayers with different SM thicknesses, which is ascribed to a larger interfacial coupling strength of Co/CoGe (or Co/CoSi) than of Fe/FeGe (or Fe/FeSi). Furthermore, in comparison with Co/Ge/Fe, a smaller exchange bias field H_E and no clear step are observed in Co/Si/Fe, which may originate from the weaker interfacial coupling in this trilayer. The variation of coercivity H_C with spacer thickness at low temperatures in Co/Ge/Fe is different from that in Co/Si/Fe, indicating again the important effect of the SM layer in the trilayers.     

Interfacial roughness and temperature effects on exchange bias properties in coupled ferromagnetic/antiferromagnetic bilayers

Monte Carlo simulations have been used to study the relationship between the exchange bias properties and the interface roughness in coupled ferromagnetic/antiferromagnetic (FM/AFM) films of classical Heisenberg spins. It is shown that the variation of the exchange bias field versus the AFM anisotropy strongly depends on the FM/AFM interface. Unlike the flat interface, a non-monotonic dependence is observed for the roughest FM/AFM interface. This is explained by canted magnetic configurations at the FM/AFM interface, which appear after the first reversal due to the magnetic frustration. The temperature dependence of the exchange field is also dependent on the roughness. While the exchange field is roughly constant for the flat interface, a decrease is observed for the roughest interface as the temperature increases. This has been interpreted as a significant decrease of the effective coupling between the FM and the AFM due to the disordering of the moments at the FM/AFM interface because of the combination of magnetic frustration and temperature activation.     

Structural analysis of ion irradiated polycrystalline NiFe/FeMn exchange bias systems

Structural investigations of the ion irradiated polycrystalline NiFe/FeMn exchange bias bilayer system were carried out by means of transmission electron microscopy. Key structural parameters like average grain size, lattice constant, and texture, as well as their dependence upon ion irradiation were determined. This information was extracted from a detailed analysis of a series of bright field images, dark field images, and diffraction patterns. Furthermore a previously established model was tested which ascribes changes in the magnetic properties upon irradiation with 5 keV He+ ions to the creation of point defects within the antiferromagnetic layer and to the intermixing between the ferromagnetic and antiferromagnetic layers. The obtained results indirectly support this model by excluding a change of the aforementioned structural parameters as a possible source of the observed modifications of the magnetic properties.     

(Ni0.81Fe0.19)1-xCrx作为种子层对NiFe/FeMn交换偏置的影响

采用一种新的种子层材料:(Ni081Fe019)1-xCrx,通过改变种子层中Cr原子的含量,使得在其上生长的NiFeFeMn双层膜的织构和晶粒尺寸产生极大的差异,系统研究了NiFeFeMn双层膜中FeMn晶粒尺寸和织构对交换偏置的影响.实验结果表明,在FeMn的γ相(111)织构较好的前提下,交换偏置场的大小与织构的差异没有关系;FeMn的晶粒尺寸对交换偏置场有很大影响,较小的反铁磁层晶粒对交换偏置场有利,过大的反铁磁层晶粒不利于交换偏置场.将(Ni081Fe019)05Cr05与传统的种子层材料Ta进 关键词： 交换偏置 晶粒尺寸 织构 种子层     

Rotation of the pinning direction in the exchange bias training effect in polycrystalline NiFe/FeMn bilayers

For polycrystalline NiFe/FeMn bilayers, we have observed and quantified the rotation of the pinning direction in the exchange bias training and recovery effects. During consecutive hysteresis loops, the rotation of the pinning direction strongly depends on the magnetization reversal mechanism of the ferromagnet layer. The interfacial uncompensated magnetic moment of antiferromagnetic grains may be irreversibly switched and rotated when the magnetization reversal process of the ferromagnet layer is accompanied by domain wall motion and domain rotation, respectively.     

Influence of ferromagnetic layer on the exchange coupling of antiferromagnetic NiO-based films

Strong effects of ferromagnetic layer (FMCo, and Ni₈₀Fe₂₀) on the magnitude and blocking temperature of exchange coupling are observed in antiferromagnetic NiO-based films NiO (5 nm)/FM₁ (t nm)/FM₂ (6-t nm). The existence of interfacial spins configuration in glass-like state and FM anisotropy are proposed to interpret a minimum shown in thermal magnetization curves for films with strong exchange coupling effect. The microstructural change of FM layer and the long-range interaction of exchange bias are taken into account to explain a strong dependence of exchange coupling energy density on the thickness t_F of FM layer when t_F<5 nm.     

Properties of the roughness in NiFe/FeMn exchange-biased system

X-ray reflectivity and atomic force microscopy analyses were performed in the Si/WTi (7 nm)/NiFe (5 nm)/FeMn (13 nm)/WTi (7 nm) exchange-biased system prepared by magnetron sputtering. Layer-by-layer analyses were done in order to have interfacial roughness parameters quantitatively. X-ray reflectivity results indicate that the successive layer deposition gives rise to a cumulative roughness. In addition, the atomic force microscopic images analyses have revealed that the roughness enhancement caused by the successive layer deposition can be associated with an appearance of a longer wavelength roughness induced by the NiFe layer deposition.     

Interface induced exchange bias effect in La0.67Sr0.33MnO3/SrIrO3 multilayer

Epitaxial superlattices of ferromagnetic/paramagnetic La_0.67Sr_0.33MnO₃/SrIrO₃ materials have been prepared on SrTiO₃ (100) substrate using pulse laser deposition technique. An unexpected onset of interface magnetic interaction has been observed around 40 K. Interestingly, magnetic exchange bias effect has been observed in both field cooled and zero field cooled magnetization loops, however, the shifting of loop is opposite in both measurements. Exchange bias field vanishes as temperature increases to interface magnetic ordering temperature. Moreover, exchange bias field is found to decrease with increasing cooling field. We believe that tuning of magnetic exchange at interface during field cooling induces this evolution in nature of exchange bias field.     

Dependence of intermediated noble metals on the optical and electrical properties of ITO/metal/ITO multilayers

Sn doped In₂O₃ (ITO) single layer and a sandwich structure of ITO/metal/ITO (IMI) multilayer films were deposited on a polycarbonate substrate using radio-frequency and direct-current magnetron sputtering process without substrate heating. The intermediated metal films in the IMI structure were Au and Cu films and the thickness of each layer in the IMI films was kept constant at 50 nm/10 nm/40 nm. In this study, the ITO/Au/ITO films show the lowest resistivity of 5.6 × 10⁻⁵ Ω cm.However the films show the lower optical transmission of 71% at 550 nm than that (81%) of as deposited ITO films. The ITO/Cu/ITO films show an optical transmittance of 54% and electrical resistivity of 1.5 × 10⁻⁴ Ω cm. Only the ITO/Au/ITO films showed the diffraction peaks in the XRD pattern. The figure of merit indicated that the ITO/Au/ITO films performed better in a transparent conducting electrode than in ITO single layer films and ITO/Cu/ITO films.     

A new paradigm for exchange bias in polycrystalline thin films

In this paper we provide a review and overview of a series of works generated in our laboratory over the last 5 years. These works have described the development and evolution of a new paradigm for exchange bias in polycrystalline thin films with grain sizes in the range 5-15 nm. We have shown that the individual grains in the antiferromagnetic (AF) layer of exchange bias systems contain a single AF domain and reverse over an energy barrier which is grain volume dependent. We show that the AF grains are not coupled to each other and behave independently. Understanding this process and using designed measurement protocols has enabled us to determine unambiguously the blocking temperature distribution of the AF grains, the anisotropy constant (K_AF) of the AF, understand the AF grain-setting process, and predict its magnetic viscosity. We can explain and predict the grain size and film thickness dependence of the exchange field H_ex. We have also studied interfacial effects and shown that there are processes at the interface, which can occur independently of the bulk of the AF grains. We have seen these effects via studies of trilayers and also via the field dependence of the setting process which does not affect the blocking. From separate experiments we have shown that the disordered interfacial spins exist as spin clusters analogous to a spin glass. These clusters can order spontaneously at low temperatures or can be ordered by the setting field. We believe it is the degree of order of the interfacial spins that gives rise to the coercivity in exchange bias systems. Based on this new understanding of the behaviour of the bulk of the grains in the antiferromagnet and the interfacial spins we believe that we have now a new paradigm for the phenomenon of exchange bias in sputtered polycrystalline thin films. We emphasize that the phenomenological model does not apply to core-shell particles, epitaxial single-crystal films and large grain polycrystalline films.     

The effects of Co-metal clusters on exchange bias for Co-doped NiO/FeNi bilayers

Co-doped NiO inhomogeneous films were synthesized by sputtering metallic Co chips and NiO together and the exchange bias of bilayers Co-doped NiO/FeNi was investigated. When Co content was up to 25.2%, the exchange bias field H_E at the room temperature increased to the maximum which was about three times compared to the undoped-bilayers. With further increase of Co content, the exchange bias field H_E and blocking temperature T_B decreased. Analysis suggests that the configuration of nanometer-sized Co-metal clusters enchased into NiO matrix played an important role in the change of magnetic behavior for the bilayers.     

Cooling-field dependence of exchange bias in Mg-diluted Ni1−xMgxO/Ni granular systems

The exchange-bias (EB) properties of Mg-diluted Ni_1−xMg_xO/Ni (0?x?0.3) granular systems have been investigated. Magnetic dilution with Mg greatly affects the EB field and the coercivity. The temperature dependence of the EB field and the coercivity can be explained in terms of formation of domain states. The value of the EB field increases and shows a maximum value with increasing cooling field, which can be explained by the competition between the field-dependent Zeeman energy and the exchange interaction at the interface.     

Influence of superalloy substrate roughness on adhesion and oxidation behavior of magnetron-sputtered NiCoCrAlY coatings

The present study has been conducted in order to determine the influence of superalloy substrate roughness on adhesion and oxidation behavior of magnetron-sputtered NiCoCrAlY coatings. Six types of coating samples with different substrate roughness were tested. The surface roughness and real surface area of both the substrates and coatings were studied by atomic force microscopy (AFM) techniques. The scratch tests performed at progressive loads were employed to evaluate the adhesion of the coatings. Cyclic oxidation tests were performed at 1100 °C in air for 50 cycles, each cycle consisting of 1 h heating in the tube furnace followed by 15 min cooling in the open air. The AFM measurements exhibit that the surface roughness of the sputtered NiCoCrAlY coating increases with the increasing of the superalloy substrate roughness. The NiCoCrAlY coatings present slightly lower roughness than the corresponding superalloy substrate. The scratch adhesion tests indicate that the coatings on substrates with a smoother surface possess better adhesion than on those with a rougher surface. Both the real surface area and oxidation weight gain of the coatings decrease with the decreasing of the superalloy substrate roughness. The NiCoCrAlY coating sputtered on the superalloy substrate with lower roughness provides relatively higher antioxidant protection than that provided by the coating with rougher substrate.     

PtMn层厚度对NiFe/PtMn双层膜交换偏置形成及热稳定性的影响

采用磁控溅射的方法制备了一组以(Ni_0.81Fe_0.19)_1-xCr_x作为缓冲层的NiFe/PtMn双层膜样品，研究了NiFe/PtMn双层膜的形成过程和热稳定性.实验表明，Cr成分的不同会引起NiFe/PtMn双层膜中PtMn层晶粒尺寸的不同，使NiFe/PtMn双层膜的交换偏置场与PtMn层厚度之间呈现不同的变化关系.热稳定性实验表明，PtMn晶粒尺寸较大的样品，出现交换偏置现象所需要的临界厚度较小，热稳定性好，这与Mauri的理论模型一致. 关键词： NiFe/PtMn双层膜 交换偏置场 热稳定性     

Mo/Si软X射线多层膜的界面粗糙度研究

 用磁控溅射法分别制备了以Mo膜层和Si膜层为顶层的Mo/Si多层膜系列, 利用小角X射线衍射确定了各多层膜的周期厚度。以不同周期数的Mo/Si多层膜的新鲜表面近似等同于同一多层膜的内界面,通过原子力显微镜研究了多层膜界面粗糙度随膜层数的变化规律。并在国家同步辐射实验室测量了各多层膜的软X射线反射率。研究表明：随着膜层数的增加,Mo膜层和Si膜层的界面粗糙度先减小后增加然后再减小,多层膜的峰值反射率先增加后减小。