Symmetry-breaking induced exchange bias in ferromagnetic Ni-Cu-Co and Ni-Fe-Co sandwiches grown on a vicinal Cu(001) surface

Ferromagnetic Ni-Cu-Co and Ni-Fe-Co sandwiches were grown epitaxially onto a vicinal Cu(001) substrate and investigated using magneto-optical Kerr effect and x-ray magnetic circular dichroism techniques. We find that the atomic steps of the vicinal surface break the magnetic reversal symmetry to induce an exchange bias in the Ni perpendicular magnetic hysteresis loop. The Ni exchange bias direction can be switched by changing the direction of the in-plane Co magnetization. In addition, the exchange bias can be tailored by changing the Cu or Fe spacer layer thickness.     

Scanning tunneling microscopy investigation of CoO/Fe(001) and Fe/CoO/Fe(001) layered structures

We report on the nanometer scale morphology of CoO thin films grown on top of Fe(001) substrates from the early stages of interface formation (few atomic layers), and on the surface topography of Fe/CoO/Fe(001) layered structures. The growth of the CoO films is dominated by formation of islands up to about 5 nominal atomic layers, then it proceeds in the layer-plus-island regime. The surface topography of thin Fe films grown on top of the CoO/Fe systems is strongly influenced by the morphology of the latter. Moreover, we observe a strong relationship between the growth mode and the chemical interactions at the CoO/Fe interface, since thick layers of iron oxides develop only below the CoO islands, as an effect of the proximity between Fe and Co atoms. We finally discuss possible implications of our observations on the magnetic properties of these layered magnetic structures.     

Spacer induced magnetism and its effect on interlayer exchange coupling in Fe/(Pd,Cu, Au,Ag) multilayered nanowires

We have investigated the one dimensional Fe/Pd, Fe/Cu, Fe/Ag, and Fe/Au multilayered nanowire systems by using first principles density functional theory. Our study reveals a gain in the binding energies of these heterostructures regardless of nature of the spacer. We have identified the electronic structure dependent enhancement of magnetic properties, and a switching behavior of the interlayer exchange coupling, with respect to the nature and dimension of the nonmagnetic spacer layer. We observe a down-spin (minority) d-charge depletion at the Fe site and a up-spin d-charge gain at Pd site in Fe/Pd nanowire which accounts for the enhanced magnetic moment of the Fe atoms and the ferromagnetic behavior of the Pd, in contrast to the paramagnetism appearing in their bulk state. We find the interlayer exchange coupling, I _ex , in Fe/Pd nanowire to be very strong, and it shows a change of sign and a decrease in magnitude with increase in Pd spacer width. On the other hand, I _ex shows an unusual increasing trend with increase in the Cu spacer layer thickness.     

Mössbauer study of magnetic Fe/FeSi multilayers

Many useful properties of magnetic multilayers depend on the coupling between the ferromagnetic layers. The coupling often oscillates with the thickness of non-magnetic spacer layers: it is ferro- or antiferromagnetic or even non-collinear near a critical thickness. We investigated the magnetron-sputtered Fe/FeSi multilayers with spacer thickness around 1.7 nm by means of Conversion Electron Mössbauer Spectroscopy with oblique incidence of the γ beam in order to gain information on the orientation of the local magnetic moments in the multilayer plane. The results show that the local moments make an angle of 45°–50° with the direction of the remanent magnetization. This is consistent with strong biquadratic coupling which in turn is expected at this spacer thickness from our magnetic measurements. An analysis of the distribution ofB _hf corresponding to different numbers of n.n. Si atoms in the bcc Fe structure points to weak diffusion of Si through the Fe/FeSi interface characterized by a diffusion length of about twice the substrate roughness.     

Pt插层对Co/FeMn界面的影响

采用磁控溅射的方法制备了Co/FeMn/Co多层膜,研究了Co（底部）/FeMn和FeMn/Co（顶部）界面插入Pt层后磁矩的变化情况．通过测量磁滞回线可知,Co（底部）/FeMn界面的Pt插层改变了体系的饱和磁化强度Ｍ_s,随着Co层厚度（t_Co）的增加Ｍ_s不断趋近于Co块体结构理论值1440 kA/m．这是因为Co（底部）/FeMn界面产生了净磁矩,而界面处的Pt插层可以减少这种净磁矩的产生．但是 关键词： 磁性多层膜 垂直磁各向异性 交换耦合     

Mössbauer investigation of the Fe/Cu interfaces in BCC Fe/Cu/Fe(0 0 1) structures

A RHEED study shows quasi layer-by-layer growth in BCC Fe/Cu/Fe(0 0 1) structures. The BCC stacking of Cu layers is maintained up to a critical thickness of 11 layers. The different iron sites at the Fe/Cu interfaces can be identified by Mössbauer spectroscopy from the distinct values measured for the magnetic hyperfine fields H_hf and isomer shifts at the ⁵⁷Fe nuclei. This makes it possible to determine the concentration of ⁵⁷Fe atoms in the different iron sites. The roughness of the Cu on Fe interface estimated from the Mössbauer study is more pronounced than that estimated from a RHEED study of the structure. The growth of Fe on Cu produces CuFe alloy layers at the Cu/Fe interface.     

Enhanced asymmetric magnetization reversal in nanoscale Co/CoO arrays: competition between exchange bias and magnetostatic coupling

Magnetization reversal was studied in square arrays of square Co/CoO dots with lateral size varying between 200 and 900 nm. While reference nonpatterned Co/CoO films show the typical shift and increased width of the hysteresis loop due to exchange bias, the patterned samples reveal a pronounced size dependence. In particular, an anomaly appears in the upper branch of the magnetization cycle and becomes stronger as the dot size decreases. This anomaly, which is absent at room temperature in the patterned samples, can be understood in terms of a competition between magnetostatic interdot interaction and exchange anisotropy during the magnetic switching process.     

Magnetic and electronic properties of Fe/Cu multilayered nanowires: A first-principles investigation

By using the first-principles calculations, we have systematically investigated the equilibrium structure, magnetic and electronic properties of one-dimensional Fe/Cu multilayered nanowires. We find that the stability of the Fe/Cu multilayered nanowires decreases with increasing concentration of nonmagnetic Cu layers, suggesting that rich Fe nanowires are more stable. Analysis of the average magnetic moment (μ_av) per Fe atom in the Fe/Cu multilayered nanowire suggests that there is a slight increase in μ_av with the increase in the number of nonmagnetic Cu layers, which was attributed to the increased Fe–Cu distance with increase in the Cu layers at interfacial layers. Furthermore, analysis of the band structures of these nanowires suggests strong dependence of conductance on the nonmagnetic Cu spacer layer thickness and a half-metallic character is observed for moderate Cu atoms substitutions, opening up the possibility for their application in magnetoelectronics or spintronics.     

Exchange bias with Fe substitution in LaMnO<Subscript>3</Subscript>

We observe the negative shift of the magnetic hysteresis loop at 5 K, while the sample is cooled in external magnetic field in case of 30% of Fe substitution in LaMnO₃. The negative shift and training effect of the hysteresis loops indicate the phenomenon of exchange bias. The cooling field dependence of the negative shift increases with the cooling field below 7.0 kOe and then, decreases with further increase of cooling field. The temperature dependence of the negative shift of the hysteresis loops exhibits that the negative shift decreases sharply with increasing temperature and vanishes above 20 K. Temperature dependence of dc magnetization and ac susceptibility measurements show a sharp peak (T_p) at 51 K and a shoulder (T_f) around 20 K. The relaxation of magnetization shows the ferromagnetic and glassy magnetic components in the relaxation process, which is in consistent with the cluster-glass compound.     

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.     

Spin-glass-like behavior in CoO nanoparticles and the origin of exchange bias in layered CoO/ferromagnet structures

The magnetic behavior of CoO nanoparticles and layered CoO/ferromagnetic (FM) structures has been investigated by magnetization and hysteresis loop measurements. In the amorphous CoO, a large uncompensation of spins is found that is closely related to spin-glass-like behavior below a freezing temperature T(F) approximately 215-220 K. The spin-glass-like phase may be described by the de Almeida-Thouless line for Ising spin systems. The exchange bias in the layered CoO/FM structures is explained by the spin-glass-like state in the nanoparticles constituting the CoO film.     

Demonstration of sixth-order coulomb correlations in a semiconductor single quantum well

Spectroscopic magnetization-induced optical second harmonic generation (MSHG) measurements from a clean Ni(110) surface reveal strong resonance effects near 2.7 eV that can be attributed to the presence of an empty surface state. The good agreement with model calculations shows the potential of MSHG to probe spin-polarized interface band structures.     

A study of the induced magnetism in the Au spacer layer of Co/Au/CoO exchange-bias trilayers and related systems

We report the first observation of the effects of exchange bias on the nuclear spin polarization and induced magnetic moments at a magnetic/non-magnetic interface, applying low temperature nuclear orientation (LTNO) to Co/Au(x)/CoO trilayer systems. This technique allows us to determine simultaneously the average alignment of the nuclear moments for the two radioactive probe isotopes 198Au and 60Co with respect to an external magnetic field axis. The total average Au γ-ray anisotropy measured was found (i) to decrease with increasing Au thickness, indicating that large hyperfine fields are restricted to the interfacial Au layers and (ii) to be canted away from the applied field axis even when the Co layers are magnetically saturated. This canting was found to originate at the CoO/Au interface as could be shown from comparative measurements on CoO/Au/CoO trilayers containing two AFM CoO/Au interfaces and on a Co/Au/Co trilayer with two FM Co/Au interfaces. In the case of CoO/Au/CoO, the observed canting was found to be dependent on the Au layer thickness.     

Field-and Current-Driven Magnetization Reversal and Dynamic Properties of CoFeB-MgO-Based Perpendicular Magnetic Tunnel Junctions

We report a perpendicular magnetic tunnel junction(p MTJ) cell with a tunnel magnetoresistance(TMR) ratio of nearly 200% at room temperature based on Co Fe B/Ta/Co Fe B as the free layer(FL) and a synthetic antiferromagnetic(SAF) multilayer [Pt/Co]/Ru/[Pt/Co]/Ta/Co Fe B as the reference layer(RL). The field-driven magnetization switching measurements show that the p MTJs exhibit an anomalous TMR hysteresis loop. The spin-polarized layer Co Fe B of SAF-RL has a lower critical switching field than that of FL. The reason is related to the interlayer exchange coupling(IEC) through a moderately thick Ta spacer layer among SAF-RLs, which generates a moderate and negative bias magnetic field on Co Fe B of RL. However, the IEC among RLs has a negligible influence on the current-driven magnetization switching of FL and its magnetization dynamics.     

Magnetization reversal process at atomic scale in systems with itinerant electrons

The magnetic response of itinerant electrons systems to an external magnetic field is investigated on the basis of a microscopic Hamiltonian from which the spin-polarized electronic structure is determined. The magnetic moment and grand thermodynamic potential of the d-electronic subsystem on a particular atomic site in the presence of the external field are calculated as a function of the moment's orientation for fixed electron configuration of its local environment. Self-consistent magnetic solutions strongly depend on the d-electron number, determined by the position of the d level relative to the Fermi energy. For parameters corresponding to α-Fe, two branches of self-consistent solutions with high and low magnetic moments are found. For parameters corresponding to bulk Cr, a Fe impurity in the Cr matrix and a Cr impurity in the Fe matrix, there are only low-spin solutions. The theory is also applied for describing magnetization reversal processes in exchange spring magnets. A slab of Fe was considered as a soft magnetic layer. The influence of the hard magnet is modeled by the inclusion of an external magnetic field applied to the interface Fe layers. The dependence of the hysteresis loop on the thickness of the Fe slab and on the value of the interface field is investigated.     

MOCVD growth, structure and magnetic properties of Fe films grown on GaAs (001) substrates

Thin iron films have been grown on (001) GaAs substrates by low pressure metal organic chemical vapor deposition (LP-MOCVD) at different temperatures with the pressure of 150 Torr. X-ray diffraction (XRD) analysis showed that all films have only one strong diffraction peak (110). The surface of Fe film became smooth with increasing the growth temperature. Magnetization measurements showed that the Fe films grown at different temperatures were ferromagnetic with easy axis parallel to the film surface and hard axis perpendicular to the substrates. The field dependence of magnetization along two axes showed a remarkable difference, implying that the samples have strong magnetic anisotropy. Furthermore, when the applied magnetic field is perpendicular to the Fe surface, a sharp jump in the hysteresis loop could be observed, followed by a broad shoulder, which is related to the interface effect, the existence of carbon and the formation of 180^°/90^° magnetic domains.     

Exchange anisotropy of cobalt fine particles

The present paper is devoted to the investigation of exchange anisotropy of fine cobalt particles with oxidized surface. From the value of hysteresis loop shift the value of exchange magnetic anisotropy constant and its dependence of the particle size has been determined. X-ray structure analysis has shown, that cobalt core of the particle is surrounded by a thin layer of the CoO which, in turn, is covered by a layer of Co₃O₄.     

Thickness dependence of X-ray absorption and photoemission in Fe thin films on Si(0 0 1)

To investigate the initial growth of Fe films on Si(0 0 1) and the Fe/Si interface, Fe films at various thicknesses have been systematically studied by soft X-ray absorption spectroscopy (XAS) and X-ray photoemission spectroscopy (XPS). The Fe L edge XAS spectrum shows a strong thickness dependence with broader line-width for thinner films. Detailed analysis of the Fe absorption signal as a function of the thickness shows that the broad linewidth of Fe L edge XAS spectra is mostly contributed by the first Fe layer at the Fe/Si interface. In contrast to XAS, Fe 2p photoemission spectra for these films are identical. However, valence band photoemission also shows a strong thickness dependence. Comparing the valence band photoemission spectra of the thin Fe/Si(0 0 1) films with that of pure Si and the thickest Fe film, the difference spectra at all thicknesses show almost identical shape indicating the same origin: the Fe/Si interface. Thus, it is mainly the first Fe layer at Fe/Si layer that is reactive with the Si substrate changing its electronic structure.     

Microscopic nature of ferro- and antiferromagnetic interface coupling of uncompensated magnetic moments in exchange bias systems

Exchange bias in layered CoO/Fe structures is investigated by x-ray resonant magnetic reflectivity (XRMR) measurements. Element-specific hysteresis loops are obtained from x-ray magnetic circular dichroism effects in the XRMR spectra. Evidence is provided for the existence of different types of uncompensated moments in the antiferromagnetic material. Explanations are given for the microscopic nature of these moments and the complex exchange interactions that determine the magnetization reversal in exchange bias systems.     

金属离子掺杂对CuO基纳米复合材料的交换偏置调控

为了研究反铁磁基体中掺杂的金属离子对交换偏置效应的影响, 本文采用非均相沉淀法制备了纳米复合材料. X射线衍射图(XRD)和场发射扫描电子显微镜(SEM) 照片清晰表明CuO纳米复合样品具有统一的颗粒尺寸, 约为80 nm. 通过体系中掺杂磁性金属离子Ni和Fe, 实现了亚铁磁MFe₂O₄ (M=Cu, Ni)晶粒镶嵌在反铁磁(AFM) CuO 基体中. 在CuO基体中加入少量的Ni能改变两相交界面的磁无序从而生成类自旋玻璃相, 相应提高对铁磁相磁矩的钉扎作用. 同时, 场冷过程中反铁磁相内形成磁畴, 冻结在原始状态或磁场方向上, 畴壁也起到钉扎铁磁自旋的作用, 进而提高交换偏置效应. 随后加入的Ni 会生成各向异性能较大的NiO, 也能够提高交换偏置场. 在带场冷却下, 所有样品均发生垂直交换偏置, 也证明了样品在场冷过程中形成了自旋玻璃相, 正是由于亚铁磁与自旋玻璃相界面上的磁交换耦合, 才导致回线在整个测量范围内发生了向上偏移. 零场冷却和场冷却(ZFC/FC)情况下磁化强度与温度变化曲线(M-T)说明在这些复合材料中的交换偏置效应是由于存在亚铁磁颗粒和类自旋玻璃相界面处的交换耦合作用. 研究发现随着持续掺杂Ni离子, 交换偏置场先缓慢增加后又急剧增加, 生成各向异性能高的反铁磁相NiO 和反铁磁相内的畴态组织是这一结果的原因.