应力场对多层膜Permalloy/Cu/Co/NiO铁磁共振性质影响的理论研究

采用能量极小原理研究了Permalloy(Py)/Cu/Co/Ni O多层膜结构中层间耦合强度和应力各向异性场对薄膜共振频率的影响,得到共振频率随外磁场强度变化关系式.结果发现外应力场强度和方向对系统共振频率的影响在本文中要强于层间耦合强度和交换各向异性场,外应力场方向对光学模共振频率的影响强于声学模,而外应力场强度对声学模共振频率的影响强于光学模.     

应力场对多层膜Permalloy/Cu/Co/NiO铁磁共振性质影响的理论研究

采用能量极小原理研究了Permalloy(Py)/Cu/Co/NiO多层膜结构中层间耦合强度和应力各向异性场对薄膜共振频率的影响,得到共振频率随外磁场强度变化关系式．结果发现外应力场强度和方向对系统共振频率的影响在本文中要强于层间耦合强度和交换各向异性场．外应力场方向对光学模共振频率的影响强于声学模,而外应力场强度对声学模共振频率的影响强于光学模．     

Magnetic phase diagram of Fe0.82Ni0.18/V(0 0 1) superlattices

Fe_0.82Ni_0.18/V(0 0 1) superlattices grown by DC magnetron sputtering on MgO(0 0 1) substrates have been investigated using longitudinal MOKE, SQUID magnetometry and magnetoresistance measurements. The varying sign and strength of the interlayer exchange coupling (IEC) were identified in the thin layer region (0.4–2.4 nm) and a magnetic IEC phase diagram was deduced and analyzed in terms of density functional calculations. The maximum giant magnetoresistance effect was determined to be 2.5% at 21 K. The balance between the magnetic anisotropy and IEC was found to be significantly different from that of previously studied Fe/V superlattices, also causing a different dependence of both IEC strength and observed anisotropy on the magnetic layer thickness.     

Coupled perpendicular magnetization in Fe/Cu/Fe trilayers

Ultrathin epitaxial Fe films on Cu(1 0 0) with perpendicular magnetization have been used as templates for the preparation of FCC Fe/Cu/Fe trilayers. The magnetic anisotropy and the coupling of these films have been studied by in-situ magneto optical Kerr effect measurements and Kerr microscopy. The magnetic coupling of both Fe layers is found to be dominated by magnetostatic interaction. Adsorbate-induced spin reorientation in the top layer also causes spin reorientation in the bottom layer. The governing role of the Fe-vacuum interface for the magnetism of the whole trilayer is demonstrated.     

Magnetic and resonance properties of Fe nanowire arrays on oxidised step-bunched silicon templates

Room-temperature magnetic properties of planar nanowire arrays of Fe have been studied using comprehensive analysis of FMR and magnetometry data. It has been shown that the Fe NWs are ferromagnetic at room temperature and their magnetic properties are mainly governed by shape anisotropy. Combining parameters derived from the FMR study with experimental data of magnetometry, simulations of hysteresis loops have been performed based on Stoner–Wohlfarth approach. Calculations show that magnetisation reversal of Fe NWs has a coherent-rotation type of localised character. As NW thickness decreases, localisation of the magnetisation reversal mode is found to enhance due to increased inhomogeneity of thinner NWs.     

双层磁性薄膜系统中铁磁共振性质

利用能量极小原理研究了双层磁性薄膜系统中的铁磁共振特性,给出共振频率与线宽随外加磁场的变化关系.以及考虑应力各向异性、交换各向异性和单轴各向异性后,系统共振频率与线宽的变化情况.数值计算结果表明:外应力场和交换各向异性场对铁磁共振频率、频谱宽度以及磁化行为均有影响.     

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.     

Magnetic properties of arrays of interacting Co nanocrystals

Monodisperse Co FCC nanocrystals with 12 nm diameter were self-assembled into regular quasi-two-dimensional triangular periodic arrays on carbon substrates from a toluene-based colloidal suspension. At 300 K the regular arrays show a collective magnetic behaviour due to dipolar coupling. A remanent magnetization with an easy axis in the film-plane and an uniaxial in-plane anisotropy field of 0.037 T were determined by SQUID magnetometry and angular dependent ferromagnetic resonance.     

Shape-induced anisotropy in epitaxial Fe3O4 nanopillars

Fe₃O₄ nanopillars with remarkably high uniformity and crystallinity are grown on (100)-oriented magnesium aluminate substrates by pulsed laser deposition with a simple anodic-aluminum-oxide template method. Compared with Fe₃O₄ full film, the nanopillars exhibit higher in-plane magnetic saturation field and lower out-of-plane saturation field. The angular-dependent ferromagnetic resonance, investigated by electron spin resonance spectroscopy, confirms the shape-induced anisotropy change in Fe₃O₄ nanopillars. High Gilbert damping parameter of 0.23 is obtained by measuring the resonance field at different microwave frequencies.     

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.     

NiO-thickness dependent magnetic anisotropies in Fe/NiO/Au(001) and Fe/NiO/MgO(001) systems

The in-plane magnetic anisotropy of Fe/NiO bilayers was studied quantitatively as a function of NiO thickness using the magneto-optical Kerr effect with a rotating field. For NiO thicker than the ordering transition thickness, the total in-plane fourfold anisotropy of the Fe layer decreases with NiO thickness in Fe/NiO/Au(001), but increases in Fe/NiO/MgO(001). Our result indicates that the exchange coupling in an Fe/NiO bilayer might induce an additional in-plane fourfold anisotropy, and the opposite thickness dependent behaviors may be attributed to the different Ni²⁺ antiferromagnetic spin orientations for NiO films grown on Au(001) and MgO(001) surfaces.     

Size-dependent magnetic properties of FeGaB/Al2O3 multilayer micro-islands

Recently, micrometer-size patterned magnetic materials have been widely used in MEMS devices. However, the self-demagnetizing action is significantly influencing the performance of the magnetic materials in many MEMS devices. Here, we report an experimental study on the magnetic properties of the patterned micro-scale FeGaB/Al₂O₃ multilayers. Ferromagnetic hysteresis loop, ferromagnetic resonance (FMR), permeability and domain behavior have been demonstrated by complementary techniques. Magnetic annealing was used to enhance the performance of magnetic multilayers. The comparisons among micro-islands with different sizes in the range of $200\mathrm{\mu }\text{m}～500\mathrm{\mu }\text{m}$ as well as full film show a marked influence of size-effect, the exchange coupling effect, and the different domain structures inside the islands.     

The magnetic properties across the martensitic transition in the Co38Ni34Al28 alloy

The magnetic properties of the Co₃₈Ni₃₄Al₂₈ alloy have been studied. The alloy exhibits a first order austenite-martensite phase transition in the temperature region between 155 and 247 K. A strain of 0.07% is produced across this phase transition. The Arrott plots obtained from the isothermal magnetic field dependence of magnetization indicate the presence of spontaneous magnetization both in the austenite and martensite phases, confirming the ferromagnetic character of the alloy up to room temperature. The temperature dependence of the high field magnetization indicates the presence of spin wave excitations, spin wave excitation gap and spin wave-spin wave interactions in the martensite phase. The magnetic anisotropy energy constant for the Co₃₈Ni₃₄Al₂₈ alloy is estimated both with the help of the standard law of approach to saturation of magnetization, and also from the field dependence of magnetization using the field for technical saturation of magnetization. The temperature dependences of these energy terms are compared. The estimated values of the magnetic anisotropy constant seem to be in agreement with the magnitude of the spin wave excitation gap estimated from the temperature dependence of high field magnetization.     

Determination of intrinsic FMR line broadening in ferromagnetic (Fe44Co56)77Hf12N11 nanocomposite films

Ferromagnetic nanocomposite (Fe₄₄Co₅₆)₇₇Hf₁₂N₁₁ films were deposited to investigate their intrinsic damping mechanisms due to scattering of itinerant electrons, which carry the magnetic moment of the ferromagnetic transition elements. The films were produced by reactive r.f. magnetron sputtering using a 6 in. Fe₃₇Co₄₆Hf₁₇ target. They were annealed at 400 °C in a static magnetic field, in order to induce in-plane uniaxial anisotropy. Subsequently, the films can be considered as uniformly magnetised. A ferromagnetic resonance frequency (FMR) of around 2.3 GHz could be attained, which was determined by measuring the real and imaginary parts of the frequency dependent permeability up to 5 GHz. The imaginary part, which represents a typical resonance curve, was utilised to obtain its full-width at half-maximum Δf_eff (FWHM) for the total damping behaviour characterisation. Thereby, it is possible to extract the intrinsic Gilbert damping parameter α_int, which in turn can be decomposed into two additional damping terms α_sf and α_os allocated to “spin-flip” and “ordinary scattering”, respectively. This result is correlated and discussed in terms of a verified theoretic model, to identify whether damping due to spin-flip scattering and/or ordinary scattering is dominant.     

Magnetic Compton profiles of Fe thin film and anisotropy of Co/Pd multilayer

The magnetic Compton profiles (MCPs) of Fe thin film with 1 μm thickness have been successfully measured, using polyethylene terephtalate (PET) substrates with 4 μm thickness to reduce scattering photons from substrate.We have succeeded for the first time to observe the anisotropy of MCPs in the Co/Pd multilayer. The magnetic out-of-plane anisotropy of the current Co (0.8 nm)/Pd (1.6 nm) multilayer sample can be explained by the model of a large number of the |m|=1 states of 3d-bands.     

Oscillatory ferromagnetic interlayer coupling in [Pt(5 Å)/Co(4 Å)]3/Cr(x Å)/[Co(4 Å)/Pt(12 Å)]3 multilayers with perpendicular anisotropy

Investigation has been performed on the interlayer coupling between two Co/Pt multilayers with perpendicular anisotropy separated by Cr spacers. As a function of the Cr spacer thickness, only ferromagnetic interlayer coupling has been observed between the two Co/Pt multilayers in contrast to the oscillatory interlayer coupling between ferromagnetic and antiferromagnetic observed in ferromagnetic layers with in-plane anisotropy separated by Cr spacers. It is the strength of the ferromagnetic interlayer coupling that has been observed to be oscillatory as a function of the Cr spacer thickness with a period of about 7 Å.     

Finite size effects with variable range exchange coupling in thin-film Pd/Fe/Pd trilayers

The magnetic properties of thin-film Pd/Fe/Pd trilayers in which an embedded - thick ultrathin layer of Fe induces ferromagnetism in the surrounding Pd have been investigated. The thickness of the ferromagnetic trilayer is controlled by varying the thickness of the top Pd layer over a range from 8 to . As the thickness of the top Pd layer decreases, or equivalently as the embedded Fe layer moves closer to the top surface, the saturated magnetization normalized to area and the Curie temperature decreases, whereas the coercivity increases. These thickness-dependent observations for proximity-polarized thin-film Pd are qualitatively consistent with finite size effects that are well known for regular thin-film ferromagnets. The critical exponent β of the order parameter (magnetization) is found to approach the mean field value of 0.5 as the thickness of the top Pd layer increases. The functional forms for the thickness dependences, which are strongly modified by the nonuniform exchange interaction in the polarized Pd, provide important new insights to understand nanomagnetism in two-dimensions.