Behavior of magnetic and magnetoresistive properties of nanocrystalline Co/Cu/Co films during step-by-step annealing

Three-layered Co/Cu/Co films with d _Cu = 1.0 and 2.1 nm (corresponding to the first and second antiferromagnetic maxima, respectively) and single-layer Co films produced during the same manufacturing cycle of magnetron sputtering in an argon atmosphere have been studied. The behavior of the coercive force and magnetoresistance ratio during step-by-step annealing in a temperature range of 250–400°C has been investigated. The correlation factor between the magnetoresistance ratio and saturation field (or the energy of indirect exchange coupling between Co layers) upon step-by-step annealing is 0.97 and 0.95 for the first and second antiferromagnetic maxima, respectively. In a temperature range of 20–300°C, the correlation factor between the coercive force and saturation field is 0.98–0.99. At T _an > 350°C corresponding to intense grain growth, the violation of the correlation between H _c and H _s takes place. Limits of the time and temperature stability of the Δρ/ρ ratio and H _c during the step-by-step annealing of the films have been determined. The behavior of Δρ/ρ, H _c, and magnetic anisotropy of the three-layered films is shown to be due to structural changes (grain size, amplitude of roughness). The behavior of the magnetic anisotropy constants K _u ¹ and K _u ² depends on the annealing temperature and thickness of the nonmagnetic spacer.     

Co/Pt multilayer-based pseudo spin valves with perpendicular magnetic anisotropy

Pseudo spin valves(SVs) exhibiting perpendicular magnetic anisotropy were prepared by magnetron sputtering. Magnetization measurements of the Co/Pt multilayers were performed to select the reference and free layers. The selection criteria are square magnetic hysteresis loops, weaker current shunting effect, and proper coercivity. The optimal reference layer and free layer are Pt(5.0 nm)/[Co(0.4 nm)/Pt(0.6 nm)]3/Co(0.4 nm)/Cu(3.0 nm)and Cu(3.0 nm)/[Co(0.4 nm)/Pt(1.5 nm)]4, respectively.The resulting pseudo SV exhibits two well-separated hysteresis loops when the field is applied perpendicular to the film plane. The minor hysteresis loop corresponding to the free layer shifts toward negative direction of the magnetic field axis, indicating ferromagnetic interlayer exchange coupling between the two magnetic layers. The coupling also enhances the coercivity(HC) of both layers. The perpendicular giant magnetoresistance(GMR) of 2.7 % is achieved with current in plane measurement. The GMR first increases when Pt seed layer is thickened, reaches a maximum of 3.0 % at 4 nm and then decreases with the further increase of thickness. But thicker Cu spacer layer always lowers the GMR of the SV.     

Tb／Co成分调制膜的磁,磁光性能和热稳定性

用直流磁控溅射制备了Ｔｂ／Ｃｏ成分调制膜，膜调制结构的形成与靶的溅射速率和衬底转速密切有关。对不同周期的Ｔｂ／Ｃｏ膜的极向Ｋｅｒｒ膜滞间回线进行了测量，结果表明：当Ｔｂ／Ｃｏ膜的周期较小时，薄膜具有垂直方向的磁各向异性，较大的矫顽力和较大的磁光效应；当Ｔｂ／Ｃｏ膜的周期较大时，上述性能变差。     

Ferromagnetic resonance in a two-layer exchange-coupled ferromagnetic film with a combined uniaxial and cubic anisotropy in the layers

Dynamic properties of ferromagnetic two-layer exchange-coupled (100) films with a combined cubic and uniaxial magnetic anisotropy of layers have been studied numerously upon the magnetization along the [100], [010], and [011] directions. The allowance for cubic anisotropy substantially affects the dependence of the frequencies of the ferromagnetic resonance on the field strength. Repeated changes in the localization of the ferromagnetic-resonance modes between the layers of the film have been found to occur with an increase in the strength of the magnetic field. At a certain relationship between the constants of the combined anisotropy for the directions [010] and [011], an increase in the field leads to a shift of the maximum of the dynamic-susceptibility distribution toward the interlayer boundary without a change in the localization of the modes.     

Discrepancy of the magnetic behaviors and crystalline structure on the Co/FeMn and FeMn/Co interfaces with ultrathin Pt spacer

The exchange coupling at the ferromagnetic/antiferromagnetic (FM/AFM) interface is influenced by both the magnetic structure and the crystalline micro-structure. Co/FeMn/Co thin films with 0.4 nm Pt spacer layer inserted into the Co/FeMn and FeMn/Co interface respectively were deposited by means of magnetron sputtering. The two interfaces upon and beneath the FeMn layer show distinct behaviors before and after the Pt spacer inserted. There is a remarkable shrink of the interfacial uncompensated spins within the FeMn bottom interfacial monolayers, whereas a relaxation of the pinning strength of the FeMn interfacial spins along the out-of-plane direction occurs at the top interface. XRD analysis indicates the Pt layer upon the FeMn layer forms an fcc (002) texture, implying the magnetic discrepancy between the top and bottom FeMn interfaces has crystalline structural origins.     

Resonance modes of two-layer exchange-coupled ferromagnetic film

Ferromagnetic and spin-wave resonances in two-layer exchange-coupled ferromagnetic films have been investigated numerically at different intensities of a magnetic field when it is directed in parallel or perpendicular to the film plane. Layers of the film have finite thicknesses and possess anisotropy of the easy-plane and easyaxis types. It has been shown that at a nonzero parameter of interlayer exchange coupling the dynamic component of magnetization upon ferromagnetic resonance is distributed nonuniformly across the film thickness. Its change has been described when the external magnetic field decreases from the saturating field to zero.     

FERROMAGNETIC RESONANCE STUDIES OF Co/CoFe_2O_4 MULTILAYERS

1.IntroductionMultilayershaveattractedconsiderableattentionfromfundamentalandtechnologicalinterests.Manynewphenomenainmetal-metalmultilayers,suchasthereductionorenhancementofmagnetizationatinterfaces[1],themagneticanisotropyl'1,theoscillatoryinterlay...     

Formation of ordered NiFeMn antiferromagnetic phase in permalloy/manganese bilayers in the course of thermomagnetic treatment

The formation of the ordered NiFeMn antiferromagnetic phase in the course of thermomagnetic treatment of manganese-permalloy bilayers has been investigated. The influence of the type of substrate, number of layers, and modes of thermomagnetic treatment on the magnetic properties of the films is studied. It has been shown that the maximal effect of unidirectional anisotropy associated with the presence of the ordered NiFeMn ferromagnetic phase in ferromagnetic layers is attained at an annealing temperature of 260°C for 4 h. The shift in the magnetic hysteresis loop with a thickness of the switched layer of about 40 nm is 380 Oe, while the blocking temperature is ≌270°C.     

L1o-CoPt/Co bilayer ferromagnetic films: interdiffusion,structure and microstructure

Detailed understanding of the connections between nanostructure and magnetic properties is key to the realization of optimal exchange-spring magnets. To this end, x-ray and electron diffraction and transmission electron microscopy were used to study the evolution of phases, microstructure and texture of model exchange-spring bilayers of L1₀ - CoPt/Co upon annealing at temperatures ranging from 300–550 °C. The work was motivated by previously—detailed changes in the magnetic exchange coupling of the system induced by the annealing treatments [1]. Unannealed CoPt/Co bilayers comprised a <111> fiber-textured L1₀ CoPt layer and a <0001> fiber-textured Co layer. The Co layer predominantly consisted of the hcp (A3) form, but contained a minority fcc (A1) phase. Annealing the bilayers at low temperatures (300–450 °C) or for short times resulted in a strengthening of the hcp Co <0001> fiber texture and disappearance of the minor fcc phase. By contrast, annealing at higher temperatures (450–550 °C) or for longer times resulted in the interdiffusion of the Co and CoPt layers and the formation of new Co-Pt solid solutions. One of these solid solutions was found to be hcp in structure, while the other was fcc. The amount of the fcc solid solution increased at the expense of the L1₀ CoPt, hcp Co and hcp Co-Pt solid solution phases as annealing time and temperature were increased.     

Effect of exchange properties of the interface on the reflection of volume spin waves from a biaxial multilayer ferromagnet

The coefficient of reflection of volume spin waves from a multilayer biaxial ferromagnetic structure with periodic modulated parameters of exchange interaction, uniaxial and orthorhombic anisotropy, and the saturation magnetization for the case of a nonideal exchange between layers have been calculated. The effect of the wave frequency, external dc homogeneous magnetic field, and parameter of surface exchange in the interface on the coefficient of reflection of volume spin waves from a multilayer ferromagnet have been studied.     

Effect of annealing on the magnetic anisotropy and hysteretic properties of film structures containing Tb-Co amorphous layers

The effect of heat treatment on the magnetic properties of Tb-Co amorphous films and film structures based on them has been studied. Regularities of variations of the coercive force and induced magnetic anisotropy constants of Tb-Co amorphous films and the coercive force and unidirectional anisotropy field of the permalloy layer in Fe19Ni81/Tb-Co films were determined as functions of the annealing temperature. The regularities found have been interpreted by assuming temperature-induced changes in the shortrange atomic order in the amorphous structure and interlayer diffusion in two-layer films.     

Exchange bias in CrN/Co nanocomposites consisting of CrN-coated Co nanocapsules and CrN nanoparticles

Nanocomposites consisting of ferromagnetic (FM) CrN-coated Co nanocapsules and antiferromagnetic (AFM) CrN nanoparticles are synthesized by the arc-discharge technique. Exchange bias (EB) and magnetic properties of the CrN/Co nanocomposites have been investigated. The shift of the hysteresis loop is 630 Oe at 5 K, which may be related to uncompensated and pinned AFM spins on the interface between the FM and AFM phases in the nanocomposites. The EB field varies with the strength of cooling magnetic field, due to the competition between the Zeeman energy and the interfacial exchange interaction. The order of magnitude of the unidirectional surface energy density is consistent with those reported for AFM oxides.     

Magnetic Property Enhancement of Co0.85W0.15 Magnetic Films by Addition of Vanadium in Ru Buffer Layer

垂直磁记录材料需要高的磁晶各向异性能。尝试了通过改变缓冲层成分来产生晶格畸变以提高磁晶各向异性的方法。300℃下,利用直流磁控溅射设备在RuV缓冲层上沉积了Co0.85W0.15磁性薄膜,其中Ru缓冲层中添加15%V。利用X射线衍射(XRD)分析了薄膜的精细晶体结构,确定了薄膜的结构、取向关系、晶格常数、c轴分散性、fcc/hcp体积分数比和堆垛层错密度。根据实验结果,研究了Co0.85W0.15/Ru0.85V0.85/MgO(111)薄膜的晶体结构和磁晶各向异性能间的关系。实验结果表明,由于缓冲层中V的添加,晶格常数比降低,显著提高了Co0.85W0.15薄膜的磁晶各向异性。因此,可以通过缓冲层的成分设计实现不同磁性材料的磁晶各向异性。     

The structure and magnetic properties of Co films on Si(111) and Si(001) substrates

Epitaxial Co films have been grown on single-crystal Si(111) and Si(001) substrates. The structure and the lattice parameter of Co films have been determined using the method of reflection high-energy electron diffraction. The domain structure of the epitaxial Co films has been studied, and the presence of a magnetocrystalline anisotropy and an anisotropy induced by substrate steps have been established. It has been shown that the coercive force of epitaxial Co films is determined by the roughness of the surface and by misfit dislocations.     

Effect of the structure and thickness of layers on the magnetic and magnetoresistive properties of <Emphasis Type="Italic">Py</Emphasis>/Co/Cu/Co nanocrystalline films

Py/Co₁/Cu/Co₂ films (Py is the Ni₆₃Co₂₅Fe₁₂ ternary permalloy) were produced by magnetron sputtering on glass substrates. The thicknesses of layers are d _Cu = 0.7–4 nm, \(d_{Co_2 } \) = 2–12 nm, and d _Py = 2–12 nm. The polar diagrams of the films with d _Cu = 1.2nm demonstrate a biaxial anisotropy, whereas the films with 2.0 ≤ d _Cu ≤ 3.0 nm are characterized by uniaxial anisotropy. The first maximum in the Δρ/ρ = f(d _Cu) curve is due to an antiferromagnetic coupling between the Py/Co₁ and Co₂ layers; the second maximum is due to the independent magnetization reversal of the Py/Co₁ soft magnetic bilayer and Co₂ hard magnetic layer. The magnetoresistance measured along the EA exhibits an almost twofold increase with increasing thickness of ferromagnetic layers. The coercive force was shown to decrease with increasing thickness of the Co₂ layer, whereas the magnetic anisotropy field is unchanged. The coercive force and induced magnetic anisotropy were estimated theoretically taking into account changes in the mean-square amplitude and roughness period with increasing thickness of the Co₂ layer. The results obtained agree with experimental data.     

Effect of Ti layer thickness on microstructure and magnetic properties of Ti/Co/Ti films

TiCoTi granular films were prepared by DC facing-target magnetron sputtering system onto glass substrates and subsequently in situ annealing in vacuum. Structural of Ti ( t nm)/Co (40 nm)/Ti ( t nm) films were investigated in detail, which shows that the majority Co nanograins are formed as the hexagonal-close-packed (HCP) structure. Vibrating sample magnetometer (VSM) and scanning probe microscope (SPM) were applied to study the magnetic properties, morphologies and domain structures of these samples. It has been found that the structure and magnetic properties of the Ti/Co/Ti films depend strongly on the Ti layer thickness. The out-of-plane coercivities ( Hc) of the film is maximum about 78.8 kA·m-1 when t =5 nm with annealing at 300 ℃; the distributing of grains of the sample is uniformity; and the average size of particles is about 13 nm. The obtained results suggest that this system is perpendicular anisotropy and might be applicable to perpendicular magnetic recording media.     

Effect of thermomagnetic treatment on the magnetic properties of permalloy/manganese bilayer films

Formation of unidirectional exchange anisotropy has been studied during thermomagnetic treatment of permalloy-manganese bilayers. It has been found that, starting from the annealing temperature of 230°C, there is observed a shift of the hysteresis loop. The maximum value of the exchange bias field is achieved after annealing at 250°C for two hours and amounts to 155 Oe. Using electron microscopy, it has been demonstrated that the effects of the occurrence of unidirectional exchange anisotropy and of increase in the coercive force are due to the appearance of an ordered antiferromagnetic phase NiFeMn, which is formed as a result of diffusive interaction between permalloy and manganese upon annealing.     

Effects of Co addition on microstructure and magnetic properties of ferromagnetic CoFeSiB alloy films

A correlation between composition, microstructure, and magnetic properties of sputter-deposited CoFeSiB alloy films has been studied. Various analytical tools and micromagnetic models were employed to understand the details of microstructural evolution and magnetic reversal processes, respectively. The CoFeSiB alloy film shows significantly different microstructure and magnetic properties depend on the Co concentration. When the Co concentration is below 75 at.%, the alloy has an amorphous phase exhibiting magnetic softness with negative remanence. Meanwhile, beyond 75 at.%, the structure consists of nanocrystals precipitated in the amorphous matrix, which becomes magnetically hard with positive remanence as well as antiferromagnetic exchange coupling.     

Magnetization and coercivity in Co/Pt multilayers with constant total Co layer thickness

Co/Pt multilayers with perpendicular anisotropy were deposited using a dc magnetron sputtering system under high vacuum. Magnetization process was investigated by the measurement of magnetic components parallel and perpendicular to the applied field. A dependence of the coercivity of Co/Pt multilayers on the Co layer thickness was reported, in which the total thickness of Co layers kept constant. It is observed that the coercivity increases with the increment of Co layer thickness. For the samples with the same Co layer thickness while different total Co layer thickness, the coercivity first increases and then decreases with the increase of the total thickness of Co layers. This effect could be attributed to the competition between the reduction of HC related to incoherent reversal and the step-up of HC contributed by the magnetic polarization of Pt atoms at the interface of Co and Pt layers during magnetization reversal. The results show that the change of the coercivity is strongly related to the Co layer thickness, but not the total thickness of Co layers. The dependence of the coercivity on the angle between an applied field and the easy axis shows that the nucleation mode is dominant in magnetization reversal process of the samples.