NiO/FeCo磁性多层膜的高频磁性

制备了一系列具有铁磁/反铁磁交换偏置作用的[NiO/Fe65Co35]10多层膜,使用振动样品磁强计(VSM)测量了样品的静磁参数,利用微带线法测量了样品4GHz-10GHz的磁谱,首次制备并测得了自然共振频率(fr)在6GHz以上,最高到fr=9.6GHz的薄膜样品。结果表明交换偏置场(Hex)、各向异性场(Hua)、以及矫顽力(Hc)随铁磁层厚度(tFM)增大而减小;基于Landau-Lifishitz(L-L)方程对静磁参量和磁谱进行了比较,发现样品自然共振频率较L-L方程计算值偏大30%以上。     

Broadband microwave absorption in [NiFe/FeMn]n exchange-coupled multilayer films

[NiFe/FeMn]_n exchange-coupled multilayer films have been fabricated on the silicon substrate by magnetron sputtering deposition. The static and dynamic magnetic properties of multilayer films have been investigated with varying numbers of layers. The results show that the linewidth and permeability of imaginary resonance peak are increased with increasing numbers of layers. For the NiFe/FeMn/NiFe sample, the resonance frequency shows a different shift with applying external magnetic field along the direction of easy and hard magnetization axis of the sample, respectively, indicating a different magnetic reversal process in two ferromagnetic layers. It proved that the increase of linewidth was originated from the different interface exchange coupling.     

Ferromagnetic Resonance Studies of Exchange Biased CoO/Fe Bilayer Grown on MgO Substrate

Dynamic and static magnetizations of an exchange biased bilayer system which is constructed as a proximity of a CoO layer on an Fe-layer grown on the (100) oriented MgO substrate by ion beam sputtering technique have been investigated by ferromagnetic resonance (FMR) and vibrating sample magnetometry (VSM) techniques. The room-temperature FMR measurements reveal that the Fe layer is epitaxially grown on MgO substrate with four-fold magnetocrystalline anisotropy and the hard magnetization axis of the sample is the [100] crystallographic directions of MgO substrate. We have determined the g-value, effective magnetization, magnetocrystalline anisotropy constants and contributions to FMR linewidth due to the intrinsic Gilbert damping and inhomogeneity of magnetization by using Landau–Lifshitz–Gilbert (LLG) equation. We observed an unusual FMR line shape attributed to impedance switching of resonance cavity and complex component of conductivity of sample system. The low-temperature FMR measurement shows asymmetric hysteretic behavior of resonance field related to magnetic coupling of ferromagnetic and antiferromagnetic layers. From both FMR and VSM measurements between 10–300 K, the magnetocrystalline anisotropy is observed to dominate above blocking temperature, while unidirectional anisotropy is observed to dominate below blocking temperature over internal magnetic anisotropy. FMR spectra have a comparatively small linewidth between 40–100 Oe, which indicates to a high crystallinity of the Fe film. Gilbert constant was calculated as 0.007 from the linewidth fitting of FMR spectra. This small value is a suitable for reducing the critical switching current used in magnetic tunneling junction. Detailed exchange bias studies were carried out for hard and easy axis of the sample in the temperature range of 10–300 K. From both low-temperature FMR and VSM measurements, the blocking temperature of the system was determined as ～60 K.     

Exchange Bias Effect and Ferromagnetic Resonance Study of NiO/NiFe/NiO Trilayers with Different Thicknesses of NiO Layers

Antiferromagnetic (AFM₁)-ferromagnetic (FM)-antiferromagnetic (AFM₂) multilayer films are fabricated using a magnetron sputtering technique. The AFM₁, FM, and AFM₂ layers are NiO, NiFe, and NiO films, respectively, where AFM₁ and AFM₂ are of the same thickness ranging from 10 to 60 nm. The exchange bias effect and ferromagnetic resonance of NiO/NiFe (50 nm)/NiO have been investigated using the vibrating sample magnetometer (VSM) and ferromagnetic resonance (FMR) meter. Both saturation magnetization (M _s) and coercivity (H _c) increase first and then decrease with the increasing thickness of NiO layers. The maximum values (M _s = 457 kA/m, H _c = 1.86 kA/m) are achieved when the NiO thickness is 30 nm. The in-plane resonance field (H _r∥) and ferromagnetic resonance linewidth (ΔH _∥) show the lowest values (H _r = 77.4 kA/m, ΔH _∥ = 7.2 kA/m) which occur when the thickness of NiO layers is 30 nm. The dependence between exchange bias and NiO film thickness in NiO/NiFe/NiO trilayers has also been discussed.     

Ferromagnetic Resonance Study of Fe/Cu Multilayer Thin Film

Fe/Cu/Fe multilayer thin film was grown on Si (100) substrate by using magnetron sputtering technique at room temperature. Dynamic and static magnetisations of the film have been investigated using ferromagnetic resonance (FMR) and vibrating sample magnetometer (VSM) techniques in the temperature range of 10–300 K. From the room-temperature in-plane FMR measurements, a growth-induced uniaxial magnetic anisotropy was observed. Out-of-plane FMR measurements exhibited a large magnetic anisotropy due to a large saturation magnetisation of Fe. A computer code was written to simulate the experimental FMR data and to obtain the magnetic parameters of the Fe/Cu/Fe multilayer thin film. g value, effective magnetisation, uniaxial anisotropy field and perpendicular anisotropy constant from the fitting of the angular dependence of the resonance field at both the in-plane and out-of plane geometries were determined. The exchange bias effect was observed from the low-temperature VSM measurements. The saturation magnetisation and coercive field decrease with increasing temperature due to the increase of the thermal fluctuations.     

Thickness and Temperature Dependence of Exchange Bias in Co/CoO Bilayers

The effects of the ferromagnetic (FM) Co layer thickness on magnetic anisotropies and exchange bias (EB) properties of Co/CoO bilayers have been investigated by using ferromagnetic resonance (FMR) and vibrating sample magnetometer (VSM) techniques. The temperature and thickness dependence of EB were studied in temperature range of 4?C320?K. FMR and VSM measurements show that Co/CoO bilayers exhibit negative exchange bias just below the blocking temperature (T _B). Room-temperature in-plane FMR measurements reveal that the Co layer was epitaxially grown on MgO substrate with four-fold magneto-crystalline symmetry. The data also show that the easy axis of magnetization is in the film plane and parallel to the ??110?? crystallographic directions of MgO substrate in all samples.     

Perpendicular exchange bias in IrMn/Pt/[Co/Pt]3 multilayers with cone magnetization

The perpendicular exchange bias and magnetic anisotropy were investigated in IrMn/Pt/[Co/Pt]₃ multilayers through the analysis of in-plane and out-of-plane magnetization hysteresis loops. A phenomenological model was used to simulate the in-plane curves and the effective perpendicular anisotropies were obtained employing the area method. The canted state anisotropy was introduced by taking into account the first and second uniaxial anisotropy terms of the ferromagnet with the corresponding uniaxial anisotropy direction allowed to make a nonzero angle with the film's normal. This angle, obtained from the fittings, was of approximately 15° for IrMn/[Co/Pt]₃ film and decreases with the introduction of Pt in the IrMn/Pt/ [Co/Pt]₃ system, indicating that the Pt interlayer leads to a predominant perpendicular anisotropy. A maximum of the out-of-plane anisotropy was found between 0.5 and 0.6 nm of Pt, whereas a maximum of the perpendicular exchange bias was found at 0.3 nm. These results are very similar to those obtained for IrMn/Cu/[Co/Pt]₃ system; however, the decrease of the exchange bias with the spacer thickness is more abrupt and the enhacement of the perpendicular anisotropy is higher for the case of Cu spacer as compared with that of Pt spacer. The existence of a maximum in the perpendicular exchange bias as a function of the Pt layer thickness was attributed to the predominance of the enhancement of exchange bias due to more perpendicular Co moment orientation over the exponential decrease of the ferromagnetic/antiferromagnetic exchange coupling and, consequently, of the exchange-bias field.     

Investigations on the interlayer coupling in Co/Pt multilayers with perpendicular anisotropy via the extraordinary Hall effect

The interlayer coupling in Co/Pt multilayers has been investigated via the measurements of extraordinary Hall effect. In the conventional [Co/Pt]_n multilayer, the coercivity H_C has been observed to increase exponentially for n < 6 and almost linearly for n > 6 with the decrease of temperature. Surprisingly, H_C for n = 1 shows the fastest increase at low temperatures, and becomes the largest one at T < 40 K. As a function of the repetition number n, the almost temperature-independent oscillation of H_C has been observed, being indicative of the Ruderman-Kittel-Kasuya-Yosida type ferromagnetic interlayer coupling in the [Co/Pt]_n multilayer. However, the antiferromagnetic interlayer coupling can be realized via the spin-valve configuration of the Co/Pt multilayer. In the Co/Pt/[Co/Pt]_n multilayer, the antiferromagnetic-to-ferromagnetic transition of the interlayer coupling has been observed at low temperatures. These observed phenomena are strongly related to the temperature-dependent polarization in the Co/Pt multilayers.     

Spin-Wave Excitations in Evaporated Co/Pt Multilayers

The magnetic properties of evaporated Co/Pt multilayers have been studied by magnetic measurements and ferromagnetic resonance (FMR). The spin-wave resonances were observed in some multilayers in FMR experiments, which implied that spin waves were sustained by the whole and propagated through Pt layers. The relation of the resonance field H _res with the mode number n obeys the so-called n ² law and the interlayer coupling strength J _I has been determined. The temperature dependence of the spontaneous magnetization can be well described by Bloch’s law, in all multilayers. The increase of the spin-wave parameter B with decreasing cobalt thickness has been discussed. A spin-wave theory has been used to explain the temperature dependence of the magnetization and the approximate values for the bulk exchange interaction J _b and surface exchange interaction J _S for various Co/Pt multilayers have been obtained.     

From Local Moment EPR in Superconductors to Nanoscale Ferromagnets

Electron paramagnetic resonance (EPR) in metals has contributed a lot to the understanding of the electronic structure and magnetic properties in dilute alloys as well as in concentrated ferromagnets. We recall some pioneering work of the Kazan group and others, studying local moment EPR in superconductors. An SNS Josephson junction has been used as a microwave generator and as an EPR detector at once. EPR was also used to study the Kondo effect in the EPR g-shift and linewidth. Moreover, the high sensitivity of EPR (down to 10¹⁰ spins) allows to study single atomic layers of ferromagnets below and above the Curie temperature T _C as well as the spin fluctuations at T _C. The in situ ferromagnetic resonance (FMR) in ultrahigh vacuum (UHV) offers a unique possibility to study the interlayer exchange coupling (IEC) and spin dynamics of coupled ferromagnetic films. Furthermore, the magnetic resonance enables us to measure basic parameters of nanoscale magnets in absolute energy units (i.e., μeV/spin). The current status of the UHV-FMR in nanoscale ferromagnets will be discussed.     

Effect of Exchange Bias on Magnetic Anisotropies in Fe/CoO Bilayers

We report on the structural and magnetic properties of exchange-biased Fe/CoO bilayers grown on MgO (001) substrates by using rf-sputtering. For varying Fe thicknesses (4 nm, 10 nm, and 20 nm) the ferromagnetic resonance (FMR) spectra of bilayers have been studied as a function of temperature at X-band frequency. The resonance lines of FMR spectra have a relatively small linewidth indicating a high crystallinity of the Fe films. The room-temperature FMR data also show that the easy axis of Fe is in the film plane and parallel to the [110] crystallographic direction of MgO substrate. In addition, M versus H loops were recorded at selected temperatures by using VSM magnetometry. The VSM measurements indicate that the Fe thickness and temperature dependence of exchange-bias properties are in good agreement with the previous results on similar systems. However, the blocking temperature of the exchange-biased system is strongly reduced compared to the bulk values. This reduction in the blocking temperature is explained by both the thickness and superstoichiometric structure of antiferromagnetic CoO layer.     

Magnetic Skyrmions in a Hall Balance with Interfacial Canted Magnetizations

Magnetic skyrmions are attracting interest as efficient information-storage devices with low energy consumption, and have been experimentally and theoretically investigated in multilayers including ferromagnets, ferrimagnets, and antiferromagnets. The 3D spin texture of skyrmions demonstrated in ferromagnetic multilayers provides a powerful pathway for understanding the stabilization of ferromagnetic skyrmions. However, the manipulation mechanism of skyrmions in antiferromagnets is still lacking. A Hall balance with a ferromagnet/insulating spacer/ferromagnet structure is considered to be a promising candidate to study skyrmions in synthetic antiferromagnets. Here, high-density Néel-type skyrmions are experimentally observed at zero field and room temperature by Lorentz transmission electron microscopy in a Hall balance (core structure [Co/Pt]_n/NiO/[Co/Pt]_n) with interfacial canted magnetizations because of interlayer ferromagnetic/antiferromagnetic coupling between top and bottom [Co/Pt]_n multilayers, where the Co layers in [Co/Pt]_n are always ferromagnetically coupled. Micromagnetic simulations show that the generation and density of skyrmions are strongly dependent on interlayer exchange coupling (IEC) and easy-axis orientation. Direct experimental evidence of skyrmions in synthetic antiferromagnets is provided, suggesting that the proposed approach offers a promising alternative mechanism for room-temperature spintronics.     

Microstructure,Magnetic and Magnetoresistance Properties of Electrodeposited [Co/Zn]<Subscript>50</Subscript> Multilayers

In this work, we report experimental results concerning the giant magnetoresistance effect and the magnetic properties of [Co/Zn] _n and [Zn/Co] _n electrodeposited multilayers. Two series of multilayers starting with Co and Zn layers, respectively, were grown onto Cu (100) substrate under identical conditions. We investigated the effect of the seed layer (Co or Zn) and Zn layer’s thicknesses on the magnetic and magnetoresistant properties of electrodeposited multilayers. We found out that the magnetic anisotropy and the shape of the hysteresis cycle are strongly influenced by the multilayer features. The magnetic properties of the [Co/Zn] _n and [Zn/Co] _n strongly depend on the Zn thickness and on the seed layer (the first layer deposited onto the Cu substrate). The coercivity fields varied between (145 Oe–208 Oe) as a function of the Zn layer’s thicknesses. [Co/Zn] _n and [Zn/Co] _n electrodeposited multilayers display magnetoresistance (14%) effect which can be explained mainly by the exchange interaction among neighboring layers and by the spatially inhomogeneous magnetic structure of the nanostructured multilayer; in addition, the thickness of the Zn interlayer has an important role on the transportation and diffusion processes.     

Enhanced electric field tunable magnetic properties of lead-free Na0.5Bi0.5TiO3–MnFe2O4 multiferroic composites

Strong magnetoelectric (ME) interaction was exhibited at both dc and microwave frequencies in a lead-free multiferroic particulate composites of Na_0.5Bi_0.5TiO₃ (NBT) and MnFe₂O₄ (MFO) multiferroic, which were prepared by sol–gel route. The room temperature permeability measurements were carried out in the frequency range of 1 MHz–1 GHz. A systematic study of structural, magnetic and ME properties were undertaken. The room temperature ferromagnetic resonance (FMR) was studied. Strong ME coupling is demonstrated in 70NBT–30MFO composite by an electrostatically tunable FMR field shift up to 428 Oe (at E = 4 kV/cm), which increases to a large value of 640 Oe at E = 8 kV/cm. Furthermore, these lead-free multiferroic composites exhibiting electrostatically induced magnetic resonance field at microwave frequencies provide great opportunities for electric field tunable microwave devices.     

Magnetic resonance study of γ-Fe2O3 nanoparticles dressed in oxygen based free radicals

Two composites consisting of γ-Fe₂O₃ (maghemite) nanoparticles covered by two different oxygen-based free radicals derived from a 4-(methylamino)phenol sulphate and 8-hydroxy-1,3,6-trisulfonic trisodium salt acid were prepared and investigated by the magnetic resonance method in the 4–300 K range. Both composites displayed broad and very intense ferromagnetic resonance (FMR) lines originating from γ-Fe₂O₃ agglomerated nanoparticles. The FMR spectrum was fitted satisfactorily at each temperature by two Landau-Lifshitz functions reflecting the existence of magnetic anisotropy in the investigated system. The temperature dependence of the obtained FMR parameters (resonance field, linewidth, integrated intensity) was studied and the results were interpreted in terms of magnetic interactions between free radicals and nanoparticle agglomerates. A comparison with previously studied similar systems containing maghemite nanoparticles was made and conclusions about the role of free radicals were drawn.     

Room temperature ferromagnetic multilayer thin film based on indium oxide and iron oxide for transparent spintronic applications

Pulsed laser deposition technique is used for fabrication of multilayer thin film of indium oxide (In₂O₃) and iron oxide (Fe₃O₄). X-ray diffraction study shows that In₂O₃ film is highly oriented along (222) direction. The optical band gap of the multilayer is observed to be 3.65 eV. The film shows n-type behavior with resistivity, carrier concentration, and mobility of 5.59 × 10⁻⁴ Ω.cm, 2.33 × 10²⁰ cm⁻³, and 48 cm ²v⁻¹ s⁻¹ respectively. Magnetic measurement shows that the film is ferromagnetic at room temperature. Hysteresis measurements at 5 K after field cooling show a shift and broadening of the hysteresis loop, which is due to exchange bias coupling.     

Transformation of Magnetic State under the Influence of Superconductivity in Epitaxial V/Pd1−x Fe x Bilayers

We have performed ferromagnetic resonance (FMR) studies of epitaxial V/Pd_1−x Fe _x (001) bilayers with a V thickness of the order of 40 nm and with a Pd_1−x Fe _x thickness in the range from 0.8 to 4.4 nm. For a bilayer with a Pd_1−x Fe _x thickness of 1.2 nm and x = 0.03, the FMR measurements revealed a decrease in the effective magnetization 4πM _eff of the ferromagnetic layer below the superconducting transition temperature of vanadium. As a possible explanation for this decrease, we suggest a spatial modulation of the ferromagnetic order in the Pd_1−x Fe _x layer due to modifications of the indirect exchange interaction of magnetic ions via conduction electrons in the superconducting state. A comparison with a recent theoretical investigation supports this possibility.     

Magnetic Resonance in ZnGeP2 and (Zn,Mn)GeP2

Electron paramagnetic (EPR) and ferromagnetic resonance (FMR) have been studied in the system of (Zn,Mn)GeP₂ ferromagnetic layer grown on undoped ZnGeP₂ single crystal. Strong FMR signals are registered in the wide temperature range up to room temperature. EPR and photo-EPR of intrinsic defects are observed in ZnGeP₂ substrate. EPR spectra characteristic of Mn²⁺ ions on Zn²⁺ sites in the bulk appear after the growth of the ferromagnetic layer on ZnGeP₂ crystal indicating the efficient Mn-diffusion into the bulk crystal by the annealing treatments.     

Magnetic characterization of nanocrystalline iron samples with different size distributions

Nanocrystalline iron was obtained by fusing magnetite and promoters. The oxidized form was reduced with hydrogen and passivated (sample P0). The average nanocrystallite size in sample P0 was d(P0) =16 nm and the width of size distribution was σ(P0) = 18 nm. Samples of nanocrystalline iron with narrower diameter ranges and larger and smaller average crystallite sizes were also synthesized. They were: sample P1 (d(P1) = 28 nm, σ(P1) = 5 nm), sample P2 (d(P2) = 22 nm, σ(P1) = 5 nm), sample P3 (d(P3) = 12 nm, σ(P1) = 9 nm). These four samples were studied at room temperature by dc magnetization measurements and ferromagnetic resonance at microwave frequency. Correlations between samples sizes distributions (average size and width of the sizes) and magnetic parameters (effective magnetization, anisotropy field, anisotropy constant, FMR linewidth) were investigated. It was found that the anisotropy field and effective magnetization determined from FMR spectra scale linearly with nanoparticle sizes, while the effective magnetic anisotropy constant determined from the hysteresis loops decreases with nanoparticle size increase.     

Ferromagnetic and spin wave resonances in thin layer of expanded austenite phase

Four samples of austenite coatings deposited by reactive magnetron sputtering on silicon substrate at four different temperatures and pressures were investigated by ferromagnetic resonance (FMR) method at room temperature. The expanded austenite phase S (γ _N ) layers with thickness in the 160–273 nm range and concentration of magnetic atoms: 72 % Fe, 18 % Cr and 10 % Ni, were obtained. The coatings with nanometric size grains were strongly textured and grown mostly in [100] direction, perpendicular to the sample surface. Intense FMR spectra were recorded at various angles between the static magnetic field direction and the sample surface. A strong magnetic anisotropy of the main uniform FMR mode was observed and the effective magnetization 4πM _eff determined. Spin wave resonance (SWR) modes were observed in all investigated samples in out-of-plane geometry of the magnetic field. The resonance fields of SWR modes in our samples varied linearly with the spin wave mode number. The value of the effective magnon stiffness constant was determined assuming a parabolic shape of the magnetization variation across the sample thickness.