Quantum Theory for Interfacial Roughness and Angle Dependence of Giant Magnetoresistance in Magnetic Multilayers

The giant magnetoresistance (GMR) in magnetic multilayers with current in the plane of the layers is studied by using the quantum-statistical Green's function approach, in which the effects of the interfacial roughness and magnetization configuration on the GMR are included. It is shown that the maximal GMR first increases and then decreases with increasing interfacial roughness, exhibiting a peak at an optimum value of interfacial roughness. An approximately linear dependence of GMR on is obtained, where is the angle between magnetizations of the two successive ferromagnetic layers. Furthermore, the maximal GMR is found to increase with increasing the number of bilayers.

Magnetic Multilayers

Magnetic multilayers are artificially structured materials made of ultrathin films of two different mate- rials,alternately ferromagnetic and non-ferromagnetic.They exhibit new magnetic properties differ- ent from those of bulk materials.We review the magnetic properties of metallic magnetic multilayers and focus on interface magnetic anisotropy,giant magnetoresistance and interlayer couplings,which are of great promise for applications to high density magnetic storage.

Ultrafast magnetization enhancement and spin current injection in magnetic multilayers by exciting the nonmagnetic metal

A systematic investigation of spin injection behavior in Au/FM(FM=Fe and Ni)multilayers is performed using the superdiffusive spin transport theory.By exciting the nonmagnetic layer,the laser-induced hot electrons may transfer spin angular momentum into the adjacent ferromagnetic(FM)metals resulting in ultrafast demagnetization or enhancement.We find that these experimental phenomena sensitively depend on the particular interface reflectivity of hot electrons and may reconcile the different observations in the experiment.Stimulated by the ultrafast spin currents carried by the hot electrons,we propose the multilayer structures to generate highly spin-polarized currents for the development of future ultrafast spintronics devices.The spin polarization of the electric currents carried by the hot electrons can be significantly enhanced by the joint effects of bulk and interfacial spin filtering.Meanwhile,the intensity of the generated spin current can be optimized by varying the number of repeated stacking units and the thickness of each metallic layer.

Effects of MgO Thickness and Roughness on Perpendicular Magnetic Anisotropy in MgO/CoFeB/Ta Multilayers

The dependence of perpendicular magnetic anisotropy （PMA） on the barrier layer MgO thickness in MgO/CoFeB /Ta multilayers is investigated. The results show that the strongest PMA occurs in a small window of about 2 4nm with the increase of MgO thickness from 1-1Onto. The crystalline degree of MgO and the change of interatomic distance along the out-of-plane direction may be the main reasons for the change of PMA in these multilayers. Moreover, the roughnesses of 2- and 4-nm-thick MgO samples are 3.163 and 1.8 nm, respectively, and both the samples show PMA. These results could be used to tune the magnetic characteristic of the ultra thin CoFeB film for future applications in perpendicular magnetic devices.

MICROSTRUCTURE AND MAGNETISM OF Co/Ti and Co/Cu(Ni) MULTILAYERS

The multilayers in the forms of glass/Cu(Ni)(5.0 nm)/[Co(2.0 nm)/Cu(Ni)(0.5～3.7 nm)] 30 and glass/Ti(5.0 nm)/[Co(2.0 nm)/Ti(0.4～3.5 nm)] 30 ,prepared by dual facing target sputtering at room temperature,exhibit a soft magnetic property.The structural and magnetic properties of Co/Cu(Ni) and Co/Ti multilayers were examined as a function of the spacer layer thickness (d Ti and d Cu(Ni) ) by low angle X ray diffraction (LAXRD) and VSM measurements.The saturation magnetization M s of the Co/Ti multilayers was found to decrease with d Ti and approached to a constant value when d Ti was thick enough.But in the Co/Cu(Ni) multilayers,the M s was found to oscillate with d Cu(Ni) when d Cu(Ni) was less than 3.0 nm,and the oscillation period was about 1.0 nm.This arose from the different interlayer magnetic coupling effects.We interpret these two different kinds of interlayer magnetic couplings as the consequence of the competition between the RKKY like and superexchange couplings.

Exchange couplings in magnetic films

Recent advances in the study of exchange couplings in magnetic films are introduced.To provide a comprehensive understanding of exchange coupling,we have designed different bilayers,trilayers and multilayers,such as anisotropic hard/soft-magnetic multilayer films,ferromagnetic/antiferromagnetic/ferromagnetic trilayers,[Pt/Co]/NiFe/NiO heterostructures,Co/NiO and Co/NiO/Fe trilayers on an anodic aluminum oxide（AAO） template.The exchange-coupling interaction between soft-and hard-magnetic phases,interlayer and interfacial exchange couplings and magnetic and magnetotransport properties in these magnetic films have been investigated in detail by adjusting the magnetic anisotropy of ferromagnetic layers and by changing the thickness of the spacer layer,ferromagnetic layer,and antiferromagnetic layer.Some particular physical phenomena have been observed and explained.

Effect of sub-layer thickness on magnetic and giant magnetoresistance properties of Ni–Fe/Cu/Co/Cu multilayered nanowire arrays

Ni-Fe/Cu/Co/Cu multilayered nanowire arrays were electrodeposited into anodic aluminum oxide template by using dual-bath method at room temperature. Scanning electron microscopy and transmission electron microscopy were used to characterize the morphology and structure of the multilayered nanowire arrays. Vibrating sample magnetometer and physical property measurement system were used to measure their magnetic and giant magnetoresistance （GMR） properties. The effect of sub-layer thickness on the magnetic and GMR properties was investigated. The results indicate that magnetic properties of electmdeposited nanowires are not affected obviously by Cu layer thickness, while magnetic layers （Ni-Fe and Co layers） have significant influence. In addition, GMR ratio presents an oscillatory behavior as Cu layer thickness changes. The magnetic and GMR properties of the multilayered nanowire arrays are optimum at room temperature for the material structure of Ni-Fe （25 nm）/Cu （15 nm）/Co （25 nm）/Cu （15 nm） with 30 deposition cycles.

The phenomenon of even bulk mode variance in a ferromagnetic A-A bilayer system

The eigenprobtems of spin waves in a symmetrical ferromagnetic bilayered system with periodic boundary conditions are solved using the interface-rescaling approach （IRA）. The results show that interface coupling between two sublayers would not change the excitation energy of odd bulk modes, but change excitation energy of even bulk modes. We call this peculiar phenomenon the phenomenon of even bulk mode variance （PEBMV）. There are two kinds of mechanisms which cause PEBMV： phase reversal and phase translation of the magnon at the interface, corresponding, respectively, to the antiferromagnetic and ferromagnetic interface coupling cases. PEBMV embodies the selective effect of the interface on different bulk magnons.

Temperature Dependence of Magnetic Properties of SmCo/FeCo Multilayer Films

Sm22Co78/Fe65Co35/Sm22Co78/Fe65Co35 multilayer films were prepared by magnetron sputtering. The temperature dependence of coercivity (Hc), remanence (Mr) and reduced remanence (Mr/Ms) has been measured. The coercivity decreases with increasing of temperature. The remanence decreases with increasing the temperature from 26 to 100℃, and then increases with continuously increasing the temperature from 100 to 150℃. The reduced remanence increases with increasing the temperature.

Phenomenological Theory for Giant Magnetoresistance Oscillation in Magnetic Multiplayer

A phenomenological theoretical model for magnetic multilayer based on Boltzmann equation and Fuchs-Sondheimer theory is studied. An oscillatory transmission coefficient (Ts) is introduced into the boundary conditions to simulate the alternate ferromagnetic and anti-ferromagnetic coupling between the magnetic layers. The transmission coefficient has an oscillatory factor relating to the thickness of space layer. Other effects such as interface roughness on Ts are also taken into account. The numerical results for [Fe/Cr]n multilayer agree with the experimental data very well, both in the period and range of osciallation, which is leaded by the dependence of giant magnetoresistance(GMR) on layer thickness of space layer.

Magnetic skyrmions:Basic properties and potential applications

Magnetic skyrmions are particle-like topological magnetic textures that are potential information carriers in future spintronics.An enormous body of research confirms their existence in a broad range of magnetic materials since their first discovery in 2009.To date,magnetic skyrmions can not only be found in asymmetric systems but also in centrosymmetric ones.Notably,engineered magnetic multilayers are promising structures for skyrmion-based spintronics because they can stabilize small-sized skyrmions at room temperature and facilitate their electric manipulation.In this overview,we introduce the topological nature,their special properties,and nucleation methods of skyrmions,and show their potential for applications.Perspectives on skyrmionic devices and developments toward other,more three-dimensional particle-like magnetic nanostructures,are discussed at the end.

Anisotropic nanocomposite soft/hard multilayer magnets

Experimental and theoretical researches on nanostructured exchange coupled magnets have been carried out since about 1988. Here, we review the structure and magnetic properties of the anisotropic nanocomposite soft/hard multilayer magnets including some new results and phenomena from an experimental point of view. According to the different component of the oriented hard phase in the nanocomposite soft/hard multilayer magnets, three types of magnets will be discussed：1） anisotropic Nd2Fe（14）B based nanocomposite multilayer magnets, 2） anisotropic SmCo5 based nanocomposite multilayer magnets, and 3） anisotropic rare-earth free based nanocomposite multilayer magnets. For each of them, the formation of the oriented hard phase, exchange coupling, coercivity mechanism, and magnetic properties of the corresponding anisotropic nanocomposite multilayer magnets are briefly reviewed, and then the prospect of realization of bulk magnets on new results of anisotropic nanocomposite multilayer magnets will be carried out.

Flexible tuning microwave permeability spectrum in [ferromagnet/antiferromagnet]_n exchange-biased multilayer stack structure

NiFe/[IrMn/NiFe/IrMn] 5 /[NiFe/IrMn] 4 /NiFe structured exchange-biased multilayer films are designed and prepared by magnetron sputtering. The static and the microwave magnetic properties are systematically investigated. The results reveal that adding a partially pinned ferromagnetic layer can effectively broaden the ferromagnetic resonance linewidth toward the low frequency domain. Moreover, a wideband multi-peak permeability spectrum with a 3.1-GHz linewidth is obtained by overlapping the spectra of different partially pinned ferromagnetic layers and [antiferromagnet/ferromagnet/antiferromagnet] n stacks. Our results show that the linewidth of the sample can be feasibly tuned through controlling the proper exchange bias fields of different stacks. The designed multilayered thin films have potential application for a tunable wideband high frequency noise filter.

Improved magnetic anisotropy of Co-based multilayer film with nitrogen dopant

Modulating the magnetic anisotropy of ferromagnetic thin films is crucial for constructing high-density and energy efficient magnetic memory devices. Ta/W(N)/Co/Pt multilayers were deposited on silicon substrates by magnetron sputtering at room temperature. The influences of N dopant on the magnetic anisotropy of the multilayers were investigated by preparing the sample with N incorporation. The results indicate that when sputtering W target with only argon gas(Ar), Ta/W/Co/Pt sample shows inplane magnetic anisotropy(IMA). When sputtering W target at a different amount of N_(2) and Ar atmosphere, it can induce perpendicular magnetic anisotropy(PMA) for proper N-doped Ta/W(N)/Co/Pt sample. When the gas flow ratio of Ar:N_(2) is 16:6, the effective magnetic anisotropy constant reach its maximum value of 1.68×10^(5) J·m^(-3),which enhanced by about 400% than our past works(annealing treatment is necessary to induce PMA in Pt/Co/MgO system). X-ray diffraction(XRD) and X-ray reflection(XRR) results demonstrate that N dopants can effectively promote the formation of b-W phase and reduce the roughness of W(N)/Co interface, which are beneficial for PMA. X-ray electron spectroscopy(XPS) analysis reveals that N doping redistributes Co charges, nitrogen ions participate in electron allocation of Co and attract some electrons of Co to form orbital hybridization between Co3 d and N 2 p. This may be another important reason for the PMA formation.