High-frequency properties of two-layer exchange-coupled ferromagnetic structures

A numerical study of peculiarities of ferromagnetic resonance (FMR) in a two-layer exchange-coupled ferromagnetic structure has been performed at various magnitudes and directions of an external magnetic field. The layers have a finite thickness and are characterized by an anisotropy of the easy-plane or easy-axis type. The FMR frequencies have been found to decrease relative to the frequencies calculated in the model of an infinitely thin film. It has been demonstrated that with an increase in the layer thickness the direction of the displacement of the lower FMR mode depends on the magnitude and direction of the external magnetic field and that the frequency of the higher mode decreases at any direction of the external field. The amplitudes of the resonance curves decrease with increasing magnetic field, with the ratio of the half-width of the resonance curve to the resonance frequency decreasing up to the saturation point, after which this parameter varies only slightly. The FMR frequencies have been studied as a function of the constant of the interlayer exchange interaction for fields located in the saturation region.     

Magnetic anisotropy of Co/Cu/Co nanocrystalline films upon magnetic annealing

The magnetic anisotropy of Co/Cu/Co films with the thickness of the copper spacer corresponding to the antiferromagnetic and ferromagnetic indirect exchange coupling between Co layers has been studied. The films deposited on naturally oxidized (111) Si single crystals were produced by magnetron sputtering. The films were annealed at 240°C. Such an annealing virtually neither changes the grain size nor leads to the mixing of layers; i.e., no disturbance of the coupling type between the cobalt layers takes place. Changes in the surface and induced magnetic anisotropy and in the effective energy of indirect exchange coupling have been studied upon annealing in the presence and absence of a magnetic field. It has been found that the shape of surface inhomogeneities in the films changes upon annealing in the magnetic field applied along the film plane, which substantially affects, the surface anisotropy. In the films characterized by antiferromagnetic coupling, the easy axis of magnetization is induced only in the magnetic fields exceeding the saturation field. The induced-anisotropy constant estimated theoretically agrees well with those determined experimentally.     

Autoresonance excitation of oscillations of domain walls in a ferromagnetic film

Nonlinear oscillations of domain walls in a ferromagnetic film with a plane-parallel periodic domain structure have been studied analytically and numerically in an external field with slowly changing amplitude and frequency. The conditions for the controlled autoresonance excitation of nonlinear oscillations of a domain wall in the presence of dissipation have been determined. The threshold values of the amplitude and rate of amplitude growth of the field of pumping have been found depending on the geometrical characteristics of the domain structure of the films whose exceeding generates an effect of autophasing in the system. It is shown that the magnetic saturation of a ferromagnetic film in the autoresonance regime can occur in fields that are substantially less than the fields of dynamic stationary magnetization.     

Low-frequency ferromagnetic resonance in a uniaxial magnetic film

The equations of the precession dynamics of the magnetic moment have been analyzed to study the specific features of the low-frequency ferromagnetic resonance corresponding to magnetic biasing along the hard magnetic axis of a film with an in-plane uniaxial anisotropy. It has been found that the precession trajectories exhibited a complicated hopping behavior and the resonance curve was asymmetric. This behavior and the asymmetry were due to an angular bistability arising from the presence of two symmetric angular positions of equilibrium near the anisotropy field.     

Theoretical prediction and experimental realization of transition metal doped rutiles as diluted magnetic semiconductors

1. Introduction The new research area of ‘spintronics’ seeks to extend the properties and applications of electronic devices by making use of the spin of electrons in ad- dition to their charge. The development of magnetic semiconductors would be compatible with standard semiconductor technology and open new opportuni- ties. The so-called diluted magnetic semiconductor (DMS) is used to denote the no magnetic semicon- ductor doped with magnetic ions, and there have been considerable progress…     

Ni50Mn37In13磁驱动形状记忆合金薄膜中的交换偏置

研究了磁场冷却前后Ni50Mn37In13磁驱动形状记忆合金薄膜在低温下的磁学行为。结果表明:磁场冷却后的Ni50Mn37In13薄膜在10 K温度下表现出一定的交换偏置效应。在特征温度Tf以下,零场冷却状态下的Ni50Mn37In13合金处于超自旋玻璃态,而经磁场冷却后发生了从自旋玻璃态向磁有序的超铁磁结构的变化。超铁磁结构中的超铁磁团簇与反铁磁基体构成了铁磁/反铁磁耦合,从而导致了交换偏置的产生。     

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.     

Ferromagnetic nanoparticles: Synthesis,processing, and characterization

Ferromagnetic nanoparticles can be used as building blocks for advanced bulk and thin film magnets, and can also be applied in data storage and biomedical technologies. However, most ferromagnetic particles lose their hysteresis at room temperature when their size is reduced to nanoscale. The exceptions are a few materials such as FePt and SmCo compounds with extremely high magnetocrystalline anisotropy that can hold a permanent magnetic moment at room temperature in particles of several nanometers. By applying newly developed “salt-matrix annealing” and “surfactant-assisted milling” techniques, monodisperse ferromagnetic FePt, SmCo, and NdFeB based nanoparticles have been successfully synthesized. These first-ever-available nanoparticles display various ferromagnetic properties at room temperature, which are found to be strongly size dependent.     

Velocity of motion of domain walls with vortex structure in multilayer ferromagnetic films

A two-dimensional model of the magnetization distribution has been applied to investigating the influence of the layered structure of a ferromagnetic film in which the layers differ in saturation magnetization M _S on the velocity of the motion of domain walls with a vortex distribution of magnetization. Films with uniaxial planar magnetic anisotropy have been analyzed. It is established that, in three-layer films with values of M _S in the near-surface layers that exceed those of the internal layer in a wide range of the external field, the velocity of motion of the domain wall is higher than in a homogeneous film with the same average saturation magnetization.     

Commensurate-incommensurate magnetic phase transition induced in TbNi<Subscript>5</Subscript> by an external magnetic field

Neutron-diffraction studies of a single crystal of TbNi₅ have been performed. Temperature and field dependences of the intensities of the Bragg reflection (001) and magnetic satellites (001)⁻ and (001)⁺ have been determined. In the temperature range of 7–10 K, sharp changes in the intensities of both the Bragg reflection and its satellites have been revealed. A conclusion is made that these changes are due to the existence of a magnetic transition from an incommensurate into a lock-in structure. It has been found that at 11 K an external magnetic field μ₀ H = 0.1 T oriented along the [100] axis induces the same magnetic state of the crystal as its cooling to 7 K in the absence of a field. In an external magnetic field μ₀ H = 0.4 T, there occurs a magnetic transition from the incommensurate phase with a wave vector k = 2π/c (0, 0, 0.18) into a commensurate ferromagnetic phase with k = 0. In the ferromagnetic phase, an increase in the field from 0.4 to 0.6 T is accompanied by an increase in the intensity of the ferromagnetic reflection (001) by a factor of 1.6. This indicates the existence of strong fluctuations of the magnetization at 0.4 T.     

Enhancement of incorporated ferromagnetic nanoparticles content in nanocomposite electrodeposited coatings by gradient magnetic field

Electrodeposited NiCoMnP coatings incorporating ferromagnetic nanoparticles demonstrate excellent magnetic properties compared with plain metal and alloy. However, improving the weight per cent of ferromagnetic nanoparticles incorporated into nanocomposite coatings is still a challenge. In this paper, a gradient magnetic field has been applied to enhance the nanoparticles’ incorporation during composite electrodepositing. Both experimental and theoretical investigations on the effect of magnetic field on the content of nanoparticles incorporated into nanocomposite coatings were carried out. The results show that gradient magnetic field is favourable for incorporating ferromagnetic nanoparticles into coatings, and uniform or static magnetic field has almost no effect for enhancing codeposition of ferromagnetic nanoparticles into coatings. With gradient magnetic field, a maximum nanoparticles incorporation of 8·08 wt-%BaFe₁₂O₁₉ is observed in CoNiMnP–BaFe₁₂O₁₉ nanocomposite coatings deposited with a BaFe₁₂O₁₉ nanoparticles bath concentration of 40 g L^?1 at a current density of 2 A dm^?2.     

Description of the adsorption of magnetic ions on metallic surfaces using the spin-density-functional method

The effect of temperature on the adsorption of magnetic ions of transition metals on a paramagnetic metallic substrate is described for the first time with allowance for the effects of ferromagnetic ordering in a submonolayer film. Conditions have been determined necessary for the formation of magnetic monatomic films stable relative to the island adsorption with changes in the coverage Θ. Four model cases of electron-density distribution depending on the equilibrium distance between the film and the substrate and the equilibrium thickness of the adsorption layer are considered. It is shown that the allowance for the effects of ferromagnetic ordering exerts a substantial influence on the adsorption energy, leading to its noticeable increase.     

Exchange bias behaviour in magnetron sputtered Ni49.8Mn36.1Sn13.9 ferromagnetic shape memory alloy thin film

In the present study we report the observation of exchange bias effect in Ni_49.8Mn_36.1Sn_13.9 ferromagnetic shape memory alloy thin film, grown on Si (1 0 0) substrate at 550 °C by dc magnetron sputtering. The shift in hysteresis loop up to 41 Oe from the origin was observed at 5 K when film was cooled under a magnetic field of 2 T. Above 55 K, the exchange bias field disappears and the coercivity gets significantly reduced due to the fact that the pinning between an antiferromagnet and ferromagnet becomes weak with increase in temperature. The observed exchange bias behaviour in Ni_49.8Mn_36.1Sn_13.9 film is attributed to the presence of AFM-FM interactions that result from the coexistence of antiferromagnetic and ferromagnetic phases in the martensite phase of the film at low temperature. This behaviour is an additional property of the Ni-Mn-Sn thin films apart from various other multifunctional properties.     

Multiferroic BiFeO<Subscript>3</Subscript> thick film fabrication by aerosol deposition

BiFeO₃ (BFO) multiferroic materials in the crystalline phase require very delicate processing conditions. In order to fabricate a high quality BFO thick film, aerosol deposition (AD) was employed and the phase evolution and multiferroic properties of the film were investigated for different annealing temperatures. A BFO thick film annealed at 500 °C had a dielectric constant of 80 at 1 kHz and possessed ferroelectric characteristics. At an applied electric field of ∼900 kV/cm, the remaining polarization and coercive field (E_c) were approximately 7.5 μC/cm² and 370 kV/cm, respectively. In addition, the BFO thick film fabricated via AD and annealed at 500 °C showed weak ferromagnetic behavior between −1250 Oe and +1250 Oe and was saturated at the higher magnetic field strength, showing ferromagnetic behavior.     

Anomalous surface morphology of iron generated after anodic dissolution under magnetic fields

The effects of applied magnetic fields on the anodic dissolution of iron in a sulfuric acid solution were studied by potentiodynamic polarization measurements and electrode morphology observations. Uneven anodic dissolution occurs in the presence of magnetic field and the extent of the electrode surface inhomogeneity increases with magnetic flux density. Severely local dissolution at two edge areas of the iron electrode in the presence of magnetic fields is caused by the inhomogeneous distribution of the magnetic flux density at the ferromagnetic iron electrode and the resultant enhancement of the mass transport rate of interfacial film at local areas.     

Chapter 8 Rotational hysteresis and chirality of the spin spiral structure in exchange-coupled heterostructures

Evolution of the net magnetization M of a nanocomposite magnetic bilayer consisting of exchange-coupled soft ferromagnetic/antiferromagnetic films under a rotating magnetic field H has been investigated using an advanced magnetooptic-indicator-film technique. The magnetization reversal of the ferromagnetic film was found to proceed via an exchange-spring formation due to twisting of the layers of the spin system. Two magnetization-reversal modes were found. At high fields, M rotates synchronously with the field. At low fields, there are stages of M and H rotation in either the same or opposite directions. The revealed features of the magnetization-reversal processes are compared with those for soft/hard ferromagnetic bilayers. The experimental data are discussed in terms of the hybrid spin-spring nucleation and evolution with allowance for the unidirectional anisotropy affected by interface imperfections and crystal-lattice defects.     

Microwave excitation of a domain wall in a two-layered magnetic film with a large uniaxial-anisotropy constant

The excitation spectrum of spin waves localized on a domain wall in a two-layered uniaxial ferromagnetic film has been investigated with allowance for the demagnetizing field of the film. A resonance behavior of the excited standing spin waves has been revealed.     

磁记忆现象与应力场的关系

金属磁记忆检测可以对应力集中早期预报，不同应力状态下铁磁性构件的磁记忆信号分布在发生变化。通过对一批预埋缺陷的钢棒进行静载荷拉伸试验，检测钢棒拉伸装夹前后、拉伸前后、拉伸卸下前后和拉伸过程中的磁记忆信号分布变化，对产生的磁记忆现象与应力场的关系进行了较深入定性的初步探讨。     

On the Rayleigh law of magnetization. Symmetrical and asymmetric hysteresis loops. Experiment

Results of an experimental study of symmetrical and asymmetric loops of magnetic hysteresis in weak ac fields are given (Rayleigh region). The asymmetric loops are observed when a dc magnetizing field is additionally applied to the ferromagnetic material together with a weak ac field. The studies have been carried out on a manganese-zinc ferrite of grade M1000NM-A. It has been shown that the symmetrical hysteresis loops obey the mathematical Rayleigh model by no means strictly. It has been revealed that if the dc magnetizing field is in the Rayleigh region, then the hysteresis loops in the ac field remain symmetrical as before. But if the dc field falls outside the Rayleigh region, then the hysteresis loops become asymmetric. This asymmetry is caused by the different values of the remanences on the ascending and descending branches of the hysteresis loop with the same quadratic dependence of magnetization on the field as in the Rayleigh case.     

纳米Fe-In2O3颗粒膜的结构和磁特性

采用射频溅射法制备了纳米“铁磁金属－半导体基体”Fex(In2O3)1-x颗粒膜，并研究了其结构和磁特性，根据颗粒膜低场膜化率χ（T）温度关系和不同温度下的磁滞回线，证实了在一定的温度范围内，颗粒膜中的纳米铁颗粒表现出磁性弛豫效应：当截止温度TB＝50K时，颗粒膜的磁性由超顺磁性转变为铁磁性，当温度降低到某一临界温度TP时，颗粒膜中结构变化导致磁化状态发生“铁磁态－类自旋玻璃态”转变，探讨分析了在低温下颗粒膜发生结构变化的原因。