Low field microwave absorption and magnetization process in CoFeNi electroplated wires

Ferromagnetic resonance （FMR）, Ferromagnetic antirresonance （FMAR） and low field magnetoimpedance （MI） are the characteristic features of high frequency losses in applied fields. While some results on FMR and FMAR in CoFeNi electroplated wires were reported earlier, here we present microwave absorption in CuBe wires electroplated by 1 μm FeCoNi magnetic layer at very low fields. These data are comparatively analysed together with longitudinal hysteresis loops in order to reveal the correlation between power absorption and magnetization processes. Microwave studies are made by using the cavity perturbation method at 9.65 GHz for a DC field parallel to the sample axis, and with microwave magnetic field hrf parallel or perpendicular to the wire axis. Two peaks have been observed in all samples, one is due to FMR, and the other is, at very low fields, related to MI. The MI peaks represent minima in power absorption. By comparing with the hysteresis loop we remark the close correspondence between the MI phenomena in the axial mode and the concomitant magnetization process.     

Particle size effect of a.c. superconducting properties in compound

Using a new starting material of Ba₂Cu₃O₅ and a three step heat treatment, single phase Tl₂Ca₁Ba₂Cu₂O₈ high- superconducting samples have been prepared, possessing the onset- and critical temperatures K and K. The morphology dependent value of is 17 Oe, 8 Oe and 5 Oe at 77 K in the case of bulk, crushed and powdered materials, respectively. The a.c. susceptibility, r.f. susceptibility and microwave absorption properties show a significant dependence on the particle size with a sharp change in the interval between 750 μm and 1200 μm. These experiments provide characteristic parameters for intergrain material (treated as 3D Josephson network) as mm, Oe and A/cm² at 77 K. The data are controlled by modulated microwave absorption measurements. The results obtained can be explained well both by the finite size junction model and cavity mode absorption model. The Josephson network is determined unambiguously by metallic S-N-S weak links. Received 10 October 1998     

Application of magnetically modulated microwave absorption to study of giant magnetoresistance effect in the Ni−Fe/Cu multilayer system

Magnetically modulated microwave absorption (MMMA), magnetic hysteresisM(H), giant magnetoresistance (GMR) and ferromagnetic resonance (FMR) have been examined in the antiferro-magnetically coupled Py/Cu (Py=Ni₈₃Fe₁₇, permalloy) multilayer system. The correlation between results obtained by the MMMA technique, the standard GMR measurements, and magnetization reversal studies is shown. Microwave studies of GMR, magnetization reversal, and FMR for different orientations of the magnetic field with respect to the sample surface are presented.     

Giant magnetoimpedance in glass-covered amorphous microwires at microwave frequencies

In this work, results on the giant magnetoimpedance (GMI) of a glass-covered amorphous microwires with nominal compositions Co_70.4Fe_4.6Si₁₅B₁₀ and Fe_77.5Si_7.5B₁₅ are presented. The impedance Z=R+iX has been investigated as function of frequency (up to 1.8 GHz) and magnetic field (up to ±400 Oe), using a HP4396B impedance analyzer and an appropriate coaxial microwave cavity. The effects of the thermal treatments (Joule heating, from 20 mA up to 100 mA for 10 min) for anisotropy induction and, as a consequence, for GMI effect have been investigated.     

An Artificially Garnet Crystal Materials Using In Terahertz Waveguide

A hypothesis is brought forward that the materials with low propagation loss in both optical and microwave band may exhibit good performance in terahertz （THz） band because THz wave band interspaces those two wave bands. For the purpose-of exploring a kind of low-loss material for THz waveguide, Lu2.1Bi0.9Fe5O12（LuBiIG） garnet films are prepared by liquid phase epitaxy （LPE） method on a gadolinium gallium garnet （GGG） substrate from lead-free flux because of the good properties in both optical and microwave bands. In microwave band, the ferromagnetic resonance （FMR） linewidth of the film 2△H = 2.8-5.1Oe; in optical band, the optical absorption coefficient is 600cm^-1 at visible range and about 100-170cm^-1 when the wavelength is longer than 800nm. In THz range, our hypothesis is well confirmed by a THz-TDS measurement which shows that the absorbance of the film for THz wave is 0.05-0.3 cm 1 and the minimum value appears at 2.3 THz. This artificial ferromagnetic material holds a great promise for magnetic field tunable THz devices such as waveguide, modulator or switch.     

Temperature dependence of ferromagnetic resonance in permalloy/NiO exchange-biased films

The temperature dependencies of the ferromagnetic resonance (FMR) linewidth and the resonance field-shift have been investigated for NiO/NiFe exchange-biased bilayers from 78 K to 450 K. A broad maximum in the linewidth of 500 Oe, solely due to the exchange-bias, is observed at ≈150 K when the magnetic field is applied along the film plane. When the magnetic field is applied perpendicular to the film plane, the maximum in the linewidth is less pronounced and amounts to 100 Oe at the same temperature. Such a behavior of the FMR linewidth is accompanied with a monotonic increase in the negative resonance field-shift with decreasing temperature. Our results are compared with the previous experimental FMR and Brillouin light scattering data for various ferromagnetic/antiferromagnetic (FM/AF) structures, and suggest that spin dynamics (spin-wave damping and anomalous resonance field-shift) in the FM/AF structures can be described in a consistent way by a single mechanism of the so-called slow-relaxation.     

Microwave absorption studies of MgB2 superconductor

Microwave absorption studies have been carried out on MgB₂ superconductor using a standard X-band EPR spectrometer. The modulated low-field microwave absorption signals recorded for polycrystalline (grain size ∼ 10 μm) samples suggested the absence of weak-link character. The field dependent direct microwave absorption has been found to obey a ✓H dependence with two different slopes, which indicated a transition from strongly pinned lattice to flux flow regime.     

Low-field microwave absorption behaviors on single layer magnetic film and exchange coupled multilayer magnetic film

This study examined the magnetization reversal effects on low-field microwave absorption in a Fe_91.6B_2.5N_5.9 single layered film with in-plane uniaxial magnetic anisotropy and a multi-layered film with giant magnetoresistivity using ferromagnetic resonance measurements at 9.84 GHz. Two different kinds of absorption modes were observed at near zero dc field and high dc field. The signals at high-field showed all the features of ferromagnetic resonance due to spin precession. However, the absorption signals at low-field should be associated with the switching field at unsaturated magnetic field region.     

The origin of spin current in YIG/nonmagnetic metal multilayers at ferromagnetic resonance

Spin pumping in yttrium-iron-garnet(YIG)/nonmagnetic-metal(NM) layer systems under ferromagnetic resonance(FMR) conditions is a popular method of generating spin current in the NM layer.A good understanding of the spin current source is essential in extracting spin Hall angle of the NM and in potential spintronics applications.It is widely believed that spin current is pumped from precessing YIG magnetization into NM layer.Here,by combining microwave absorption and DC-voltage measurements on thin YIG/Pt and YIG/NM_1/NM_2(NM_1 =Cu or Al,NM_2 =Pt or Ta),we unambiguously showed that spin current in NM,instead of from the precessing YIG magnetization,came from the magnetized NM surface(in contact with thin YIG),either due to the magnetic proximity effect(MPE) or from the inevitable diffused Fe ions from YIG to NM.This conclusion is reached through analyzing the FMR microwave absorption peaks with the DC-voltage peak from the inverse spin Hall effect(ISHE).The voltage signal is attributed to the magnetized NM surface,hardly observed in the conventional FMR experiments,and was greatly amplified when the electrical detection circuit was switched on.     

FMR studies in compositionally modulated Co-Nb and Co films

Compositionally modulated (CM) films of Co-Nb have been prepared by sequential evaporation onto Si substrates using two E-guns fitted in an UHV system. The thickness of the Co sublayer was varied in the range 50 to 105 Å and that of Nb in the range 18 to 90 Å. A single layer Co film of about 850 Å was also prepared and studied. Magnetization was measured using a VSM at 290 K. FMR was observed at about 9.8 GHz in the range 3 to 290 K. For Co a layer thickness of 50 Å the magnetization in CM film is about 12% lower at 4.2 K, as compared to the bulk value. The FMR spectrum of single layer Co film shows a single absorption in the perpendicular configuration with a narrow line width of 40 Oe. Other properties of this film agree with those published in the literature. In CM films, multiple absorption modes are observed. A small perpendicular anisoropy is also measured. The magnetization in all the samples varies at T^3/2. The Curie temperature of CM films is lower than that of bulk Co. The resonance line width in CM films at lower temperatures, increases much faster than in Co film.     

Synthesis and magnetic properties of Cu0.5Ni0.5Fe2O4 nanoparticles produced by glycothermal and hydrothermal processes

Cu_0.5Ni_0.5Fe₂O₄ nanoparticles have been synthesized in ethylene glycol solution and in deionised water. The glycothermal reaction was carried at 200°C under gauge pressure of 100 psi. The hydrothermal treatment was done at 100°C under zero pressure. Complete single-phase cubic spinel structure in the samples made by glycothermal (sample G) and hydrothermal (sample H) processes formed after annealing at 600°C and 900°C respectively. The coercive field of sample H increases from 72 Oe to 133 Oe after sintering at 700°C and then decrease to 11 Oe on sintering at 1000°C. This variation is attributed to surface effects and crossover from single to multidomain behavior due to increasing particle size.     

Optical and magnetic properties of nitrogen ion implanted MgO single crystal

The microstructure,optical property and magnetism of nitrogen ion implanted single MgO crystals are studied. A parallel investigation is also performed in an iron ion implanted single MgO sample as a reference. Large structural,optical and magnetic differences are obtained between the nitrogen and iron implanted samples. Room temperature ferromagnetism with a fairly large coercivity field of 300 Oe (1Oe=79.5775 A/m),a remanence of 38% and a slightly changed optical absorption is obtained in the sample implanted using nitrogen with a dose of 1×1018 ions/cm2 . Tran- sition metal contamination and defects induced magnetism can be excluded when compared with those of the iron ion implanted sample,and the nitrogen doping is considered to be the main origin of ferromagnetism.     

Magnetoacoustic resonance in ferrite-ferroelectric nanopillars

This work is concerned with the nature of ferromagnetic resonance (FMR) under the influence of acoustic oscillations with the same frequency as FMR. Here we provide the theoretical model for ME coupling at FMR in the nanopillars of ferrite in a piezoelectric matrix. Our calculations show that magnification of ME coefficient is obtained at the magnetoacoustic resonance (MAR) region where FMR and acoustic oscillations at electromechanical resonance (EMR) overlap. The clamping effect of the substrate for nanopillars is considered in determining the ME voltage coefficient. In addition, nanostructures based on single crystal ferrites take on special significance as magnetic resonance line width of such materials may be narrow enough to enable the observation of effects connected with magnetoelastic interaction. As an example, ME coefficient is estimated for the nanopillars of yttrium iron garnet in lead zirconate titanate matrix. The phenomenon is of importance for the realization of multifunctional ME nanosensors/transducers operating at microwave frequencies.     

FMR determination of the period of an incommensurate magnetic structure in chiral organometallic crystals

A sequence of maxima of microwave absorption has been found in the ferromagnetic resonance (FMR) spectra of the chiral molecular ferrimagnet [Mn{(R/S)-pn}]₂[Mn{(R/S)-pn}₂(H₂O)][Cr(CN)₆]₂, which, as is shown, corresponds to the spin-soliton resonance. It has been established that this sequence corresponds to an incommensurate magnetic structure induced by the competition between the symmetric and antisymmetric exchange interactions. On the basis of the FMR spectra and their dependence on the temperature, the parameters of the modulated magnetic structure have been estimated.     

Ion beam induced chemical vapor deposition (IBICVD) of cobalt particles

A novel method aiming at fabricating submicron-scale particles utilizing dicobalt octacarbonyl as a precursor for cobalt, formed upon localized ion beam induced decomposition, is presented. Patterns of deposited particles are fabricated through vector scan rastering. Measurements of cumulative magnetic properties (arrays of 2 μm–size dots) show coercivity of about 100 Oe and the saturation magnetization of approximately 1000 emu/cm³.     

Effect of the domain structure on ferromagnetic resonance in easy-axis ferromagnets

The results of experimental and theoretical investigations of ferromagnetic resonance (FMR) in ferrite single crystals having hexagonal structure and magnetocrystalline anisotropy of the easy-axis type are reported. Experiments are carried out on disk-shaped samples of materials differing in anisotropy strength. The values of the effective anisotropy fields and the gyromagnetic ratios (g-factors) of the materials are determined from measured frequency dependences of the FMR field along the easy and hard magnetization directions for homogeneously magnetized samples. It is shown that if the FMR spectra are also measured for the same samples in the presence of a domain structure, then one can experimentally determine not only the above-mentioned parameters but also the saturation magnetization of uniaxial magnetic materials. It is shown that the theory of the FMR frequency spectrum of a partially magnetized sample with a simple domain structure in the form of a system of plane-parallel layers is in good agreement with the experiment.     

Periodic microwave absorption in superconducting whiskers

The microwave absorption and structural studies of BiSrCaCuO (2212) whiskers have been performed. The low-angle boundaries in the (a, b) and (a, c) planes have been observed. The periodic microwave absorption signal indicated the flux quantization in a system of weak links and superconducting loops formed on the boundaries. Regular character of this signal suggests possible applications in SQUID magnetometry. Lower Josephson critical fieldH _clJ=1.6 Oe and the critical temperatureT _c=89 K have been determined. The Josephson penetration depth was estimated from the Ginzgurg-Landau, and Clem models.     

Magnetic anisotropy-induced spin-reorientation in spinel FeCr2S4

In this paper, the low-field magnetic behavior of polycrystalline FeCr₂S₄ was investigated by thermal circling the sample from 5 to 200 K at a constant magnetic field. We show that, upon cooling the sample at 50 Oe to a temperature below the Curie temperature then warming back, the magnetization displays irreversibility between cooling and warming sequence. The irreversible behavior was suppressed gradually with increasing magnetic field. By considering spin-reorientation due to the increase of magnetic anisotropy upon cooling, the irreversible behavior has been explained qualitatively.     

NiFe/Pt薄膜中角度相关的逆自旋霍尔效应

NiFe/Pt双层薄膜样品在铁磁共振时, NiFe磁矩进动所产生的自旋流注入到Pt层中, 由于逆自旋霍尔效应产生直流电压V_ISHE, 此电压会叠加到NiFe薄膜由于自旋整流效应而产生的电压V_SRE 上, 实验测量所得电压为V_ISHE和V_SRE的叠加. 为了区分这两种不同机理对电压的贡献, 本文采取旋转外加静磁场的方法, 通过分析所测电压随磁场角度的变化从而分离出V_ISHE 的大小. 研究结果表明, 相比于单层NiFe(20 nm)薄膜样品, NiFe(20 nm)/Pt(10 nm)双层膜样品中由于NiFe自旋注入到Pt 中导致铁磁共振线宽增加. 与逆自旋霍尔效应产生的电压相比, 自旋整流效应的贡献较小, 但不可忽略. 本文工作有助于认清铁磁/非磁性金属材料中的自旋相关效应, 并提供了一种准确的分析逆自旋霍尔效应的方法.