Very strong interlayer exchange coupling in epitaxial Fe/Fe1−xSix/Fe trilayers (x=0.4–1.0)

Fe/Fe_1−xSi_x/Fe (x=0.4–1.0) wedge-type epitaxial trilayers with improved homogeneity are grown by co-evaporation from two electron-beam sources. The coupling strengths of the bilinear (J₁) and biquadratic (J₂) coupling terms are derived from Brillouin light scattering (BLS) spectra and longitudinal MOKE hysteresis loops. The total coupling strength J=J₁+J₂ increases dramatically with increasing x and reaches values in excess of 6 mJ/m².     

Angular dependence of the ferromagnetic resonance spectrum in continuous/heterogeneous multilayers

We present here a study of the angular dependence of the ferromagnetic resonance (FMR) spectra in trilayers formed by two continuous ferromagnetic layers, Fe and Ni₈₀Fe₂₀ (permalloy), separated by a granular film of Fe(x)–SiO₂(1–x). The study of the Fe/Fe-SiO₂/Ni₈₀Fe₂₀ trilayer was made for an Fe volume concentration x=0.75 and two thicknesses (t=1 and 18 nm) of the granular layer. One microwave absorption line is in general found close to the field expected for Fe, while the other is coincident with the resonance field of permalloy. However, the Fe-like absorption is considerably wider than what is usually observed in pure Fe films, which suggests the presence of a strong exchange interaction between this layer and the granular spacer. The angular dependence of the resonance field and the line width could be very well fitted with a model that assumes an effective in-plane anisotropy for each layer, indicating that the shape anisotropy dominates the angular response of both modes. When the excitation frequency is increased, the line width of the permalloy-like mode increases by a similar factor. The width of the Fe-like mode is very similar at different frequencies because of the effect of the granular layer.     

Heat-induced effective exchange coupling in magnetic multilayers with semiconductors

Two ferromagnetic films separated by an amorphous semiconducting spacer layer are exchange coupled across the spacer. The coupling is reversibly temperature dependent with a positive temperature coefficient making such layered systems a 2-D realization of the concept of heat-induced magnetism. By studying ferromagentic Fe layers separated by amorphous Si, Ge, or ZnSe layers we explore the possibilities to generate such an effective exchange coupling and address the question of the mechanism responsible for it.     

Dependence of the ferromagnetic resonance modes on the coupling strength in exchange-coupled trilayer structures

The dependence of the dispersion relation and the magnetization on the exchange coupling strength was calculated for a system consisting of two ferromagnetic layers exchange-coupled through a nonmagnetic spacer layer. The magnetic layers are characterized by both uniaxial and cubic magnetocrystalline anisotropies. A minimization procedure was developed which allows the resonance modes to be obtained for any magnetic field orientation and strength, as well as for any exchange coupling strength. If the antiparallel coupled film is unsaturated at resonance, the dispersion relation for both acoustic and optic modes could be rather complex, especially when the field is applied in the plane of the film.     

Thin Co films with tunable ferromagnetic resonance frequency

The tailored production of thin Co films of 50 nm thick with ferromagnetic resonance frequency in a range from 2.9 to 7.3 GHz using the DC magnetron sputtering is reported. The ferromagnetic resonance frequency, coercivity, effective magnetic field and nanocrystalline structure parameters are shown to be governed by the Co deposition rate. For this investigation, FMR, VSM and TEM techniques were used.     

Comparison of parallel and perpendicular ferromagnetic resonance in an exchange-biased bilayer

We have investigated the ferromagnetic resonance spectra of an exchange-biased Ni₈₀Fe₂₀/CoO bilayer between room temperature and 4 K. Primary attention has been paid to the effect of the antiferromagnetic CoO film on the temperature-dependent resonance field shift of the ferromagnetic Ni₈₀Fe₂₀ film with respect to that of an unbiased film. At low temperatures, the field shift with the magnetic field applied perpendicular to the plane was determined to be more than twice the magnitude of the parallel field shift, and of the same sign, while an unoxidized single ferromagnetic film has much smaller parallel and perpendicular low-temperature shifts (here defined with respect to room temperature) of opposite sign. This observation implies that the anisotropy axis can rotate with the applied field, provided that the primary cause of the anisotropy is the interaction between the adjacent ferromagnetic and antiferromagnetic films. Since the perpendicular shift is more than a factor of two larger than the parallel field shift, the rotatable anisotropy is, in fact, anisotropic in this bilayer.     

Film thickness dependence of the magnetic resonance in Fe–SiO2 nanocomposites

The magnetic properties of Fe–SiO₂ nanogranular composite thin films were studied as a function of film thickness and Fe concentration, f, using ferromagnetic resonance at X-band (9.4 GHz) and Q-band (35 GHz). Films with an Fe volume percent ranging from 17% to 70% were fabricated from a mosaic target using RF sputtering techniques. Film thickness was varied between 10 and 200 nm. From measurements made at room temperature with the external field applied parallel and perpendicular to the film plane, it was possible to determine an almost linear dependence of the effective anisotropy field with Fe concentration. Small differences observed between X- and Q-band, specially at low f, were attributed to the effects that the different fields applied during the experiment cause on the magnetic state of the sample. No systematic change of the effective field or the g value was observed in films of different thickness. The absorption line width, on the other hand, was found to depend on film thickness indicating a larger distribution of particle shape and size with increasing film thickness. A maximum in the line width was observed around f30–35% and is probably caused by the transition from single domain ferromagnetic clusters to superparamagnetic particles.     

Stripe domains in Permalloy films as observed by ferromagnetic resonance and magnetic force microscopy

We have studied the static and dynamic responses of three (62, 115 and 308 nm thick) Permalloy (Ni_0.81Fe_0.19) films by DC magnetization, ferromagnetic resonance and magnetic force microscopy. The thickest film presents very regular stripe domains with widths slightly smaller than the film thickness. Ferromagnetic resonance performed on the thinnest film shows one absorption peak when the field is applied along the film plane, and more than one in the other samples. These multiple absorptions are analogous to the high frequency susceptibility peaks observed at zero field in Permalloy films above the critical thickness reported previously by other authors.     

Spin-flop coupling and exchange anisotropy in ferromagnetic/antiferromagnetic bilayers

By investigating the antiferromagnetic spin configuration, the exchange anisotropy and the interfacial spin-flop coupling in ferromagnetic/antiferromagnetic (FM/AF) bilayers have been discussed in detail. The results show that there are four possible cases for the AF spins, namely the reversible recovering case, irreversible half-rotating case, irreversible reversing and irreversible half-reversing cases. Moreover, the realization of the cases strongly depends on interface quadratic coupling, interface spin-flop (biquadratic) coupling and AF thickness. The magnetic phase diagram in terms of the AF thickness t_AF, the interfacial bilinear coupling J₁ and the spin-flop coupling J₂ has been constructed. The corresponding critical parameters in which the exchange bias will occur or approach saturation have been also presented. Specially, the small spin-flop exchange coupling may result in an exchange bias without the interfacial bilinear exchange coupling. However, in general, the spin-flop exchange coupling can weaken or eliminate the exchange bias, but always enhances the coercivity greatly.     

Dynamic response of permalloy/Cu/Co magnetoresistive sandwiches

The dynamic response of trilayer magnetoresistive permalloy/Cu/Co films was studied by high-frequency permeability spectra measurements. The resonance frequency is shown to depend on the interlayer copper thickness. This dependence is related to exchange coupling between permalloy and cobalt and the interaction field is estimated using the Landau–Lifschitz–Gilbert model.     

Comparison of frequency,field, and time domain ferromagnetic resonance methods

We present vector network analyzer ferromagnetic resonance measurements of epitaxial Fe films having a thickness of 16 monolayers. Our objective is to test the reliability of this novel frequency domain technique with respect to frequency and damping. For this purpose we compare vector network analyzer ferromagnetic resonance to pulsed inductive microwave magnetometry, time resolved magneto-optic Kerr effect (both methods in the time domain), and conventional ferromagnetic resonance (measured in the field domain) in terms of position and width of the ferromagnetic resonance. In addition, we compare the various techniques with respect to the signal to noise ratio of the raw data. All data is obtained using the same well characterized ultrathin magnetic Fe/GaAs (0 0 1) film. Finally, we demonstrate the potential of the vector network analyzer ferromagnetic resonance technique for the investigation of nano-structured magnetic elements having nonuniform magnetization configuration. The absorption spectrum of Permalloy disks with a diameter of 200 nm and a thickness of 15 nm shows up to eight distinct resonance peaks. The spatial structure of the corresponding modes was derived from numerical calculations and reveals that azimuthal modes up to the fifth order have been observed inductively.     

Long-range exchange bias through a metal spacer

We have theoretically analyzed the long-range exchange bias between a ferromagnet and an antiferromagnet separated by a nonmagnetic metal spacer. The Fermi–Dirac distribution was included in the Ruderman–Kittel–Kasuya–Yosida interaction to study the thermal effects of the conduction electrons, and thus to study the temperature effects and thickness dependence in the trilayer structure. The experimentally observed oscillatory exchange bias through the metal spacer is in good agreement with the calculated results.     

Ferromagnetic resonance study of interface anisotropies in exchange-biased Fe(0 0 1)/KNiF3 bilayers

Magnetic anisotropies at epitaxial Fe/KNiF₃ interfaces were probed by ferromagnetic resonance. Fe(0 0 1) films coupled to single crystal KNiF₃ exhibit four-fold in-plane anisotropy and a unidirectional bias upon field-cooling. In Fe(0 0 1) with polycrystalline KNiF₃, the bias direction deviates from the field-cooling direction. Lattice mismatch strain due to polycrystalline KNiF₃ also induces uniaxial anisotropy in Fe.     

Probing antiferromagnetic order with heat capacity in exchange-biased structures

We explore the magnetic heat capacity in exchange-biased ferromagnet/antiferromagnet bilayers theoretically. We show that changes in the antiferromagnetic structure due to the reversal of the ferromagnet layer can be detected by distinct features in the heat capacity. This offers a method for probing antiferromagnetic domains in exchange-biased systems.     

Interlayer exchange coupling: Preasymptotic corrections

In the asymptotic limit, the interlayer exchange coupling decays as D^-2, where D is the spacer thickness. A systematic procedure for calculating the preasymptotic corrections, i.e., the terms of order D^-n with ,is presented. The temperature dependence of the preasymptotic corrections is calculated. The results are used to discuss the preasymptotic corrections for the Co/Cu/Co(001) system. Received 7 January 1999     

外应力场下双层铁磁薄膜中的铁磁共振性质

用铁磁共振方法得到了双层铁磁薄膜的色散关系解析表达式.发现共振场依赖于层间耦合强度和应力场.假定层间为反铁磁性耦合，且铁磁层Ａ有较强的平面内各向异性.随着外磁场的增强，铁磁层Ｂ中的磁化强度突然由最初的反平行转为平行，从而导致色散曲线的阶跃，并且发现光学模阶跃幅度比声学模大.随着应力场的增强，Ｂ层中磁化强度反转所需的外磁场减弱.此外，在不同的交换耦合强度和应力场下，光学模共振场对外磁场方向的依赖性较强. 关键词： 双层铁磁薄膜 界面相互作用 应力各向异性场 铁磁共振     

Micromagnetic simulation of ferromagnetic resonance of perpendicular granular media: Influence of the intergranular exchange on the Landau-Lifshitz-Gilbert damping constant

A micromagnetic approach was used to simulate ferromagnetic resonance frequency (FMR) profiles of perpendicular granular CoCrPt-oxide thin films. From the obtained FMR line-width we computed the effective damping constant. The influence of the intergranular exchange on the effective damping was investigated, showing an increase in the damping constant with increase in intergranular exchange coupling. Moreover, the effective damping constant increases with decrease in mesh size of the model, and eventually saturates for mesh sizes of about 1 nm. These dependencies are explained in terms of different modes that can be excited in the granular medium due to interactions between the individual spins.     

Investigation of ferromagnetic resonance and damping properties of CoFeB

In this study, the influences of thin film thickness and post-annealing process on the magnetic properties of CoFeB thin films were investigated. The angular dependency and linewidth of the ferromagnetic resonance signal were used to explore the magnetic behavior of sputtered single-layer and trilayer thin film stacks of CoFeB. A micromagnetic simulation model was employed based on the metropolis algorithm comprising the demagnetizing field and in-plane induced uniaxial anisotropy terms with all relevant contributions. Our results reveal that the direction of magnetization changes from in-plane to out-of-plane as a result of the annealing process and induces a perpendicular magnetic anisotropy in the 1-nm thick CoFeB thin film. The ferromagnetic resonance (FMR) linewidth can be defined well by the intrinsic Gilbert damping effect and the magnetic inhomogeneity contribution in both as-grown and annealed samples. The difference between the linewidths of the single and trilayer film is mainly caused by the spin pumping effect on damping which is associated with the interface layers.