Nanostructuring effects in soft magnetic films and film elements with magnetic impedance

The magnetization reversal and magnetic impedance (MI) of films and film elements based on Fe₁₉Ni₈₁ and Fe_72.5Cu_1.1Nb_1.9Mo_1.5Si_14.2B_8.7 alloys with a varied thickness, heat-treatment temperature, and the number of thin Cu interlayers are studied. The dependences of the coercive force and the magnitude of MI on these parameters are found. Layered structuring is shown to be an effective method for improving the functional characteristics of MI elements. In elements containing nanocrystalline Fe₁₉Ni₈₁ layers, this is related to the restructuring of a magnetic structure; in elements containing amorphous Fe_72.5Cu_1.1Nb_1.9Mo_1.5Si_14.2B_8.7 layers, this improvement is likely to be caused by a decrease in the effective electrical resistivity.     

Structure and Magnetic Properties of Powder Permalloy Fe-Ni

The aim of this work was to prepare powder samples of FeNi by long-time milling of microcrystalline ribbon Fe₁₉Ni₈₁ (sample A) and that of pure elements Fe and Ni (sample B) in low-energy vibratory mill for 1000 h and to investigate their magnetic properties. We found that the coercivity of sample A and sample B steeply increases up to 140 h of milling time and then slowly increases for sample A and decreases for sample B. The magnetic moment of both samples monotonously decreases during the milling. The sample A is solid solution Fe₁₉Ni₈₁ and its structure is stable during the milling. New phases with different Curie temperatures are found in sample B during the milling.     

Mössbauer investigation of the Fe/Cu interfaces in BCC Fe/Cu/Fe(0 0 1) structures

A RHEED study shows quasi layer-by-layer growth in BCC Fe/Cu/Fe(0 0 1) structures. The BCC stacking of Cu layers is maintained up to a critical thickness of 11 layers. The different iron sites at the Fe/Cu interfaces can be identified by Mössbauer spectroscopy from the distinct values measured for the magnetic hyperfine fields H_hf and isomer shifts at the ⁵⁷Fe nuclei. This makes it possible to determine the concentration of ⁵⁷Fe atoms in the different iron sites. The roughness of the Cu on Fe interface estimated from the Mössbauer study is more pronounced than that estimated from a RHEED study of the structure. The growth of Fe on Cu produces CuFe alloy layers at the Cu/Fe interface.     

Magnetic anisotropy in multilayers

Magnetic anisotropy between in-plane and out of plane magnetic alignments is studied in a variety of multilayer systems using Mössbauer spectrosopy to observe the (Fe) magnetic orientation. The surface anisotropy in Fe/Au (1 1 1) multilayers is measured as K _s = 0.9 × 10^?3 Jm^?2. In Fe/Ni multilayers the dependence of magnetic orientation on external field applied normal to the layers enables volume and interface anisotropies K _v = (?5 ± 1) × 10⁴ Jm^?3 and K _s = (?0.6 ± 0.4)× 10^?3 Jm^?2 to be evaluated. In similar applied field experiments coherent rotation of the magnetic Fe and NiFe layers in Fe/Cu/NiFe/Cu multilayers was observed for intervening Cu layer thickness x = 5 Å but independent rotation for x = 50 Å. Out of plane magnetic components are observed for DyFe₂, YFe₂ thin films and DyFe₂/YFe₂ multilayers. In fields of up to 0.25 T applied inplane only the moments of the YFe₂ film showed significant rotation.     

Magnetoresistive Fe19Ni81/Tb-Co medium with an internal magnetic bias

The magnetization reversal and magnetoresistance of two-layer exchange-coupled Fe19Ni81/Tb-Co films are studied. Amorphous Tb _x Co_{100 − x} layers with 30 < x < 35 are found to have a uniaxial magnetic anisotropy and a rather high coercive force, which ensures magnetic biasing of the adjacent permalloy layers. In addition, the permalloy layers subjected to selective annealing exhibit a significant anisotropic magnetoresistance and a small magnetic hysteresis. These properties make it possible to consider the developed film structure as an effective magnetoresistive medium. This structure is used to form magnetic sensor samples that have an odd transfer function in the absence of external magnetic biasing.     

Development of magnetic anisotropies in ultrathin epitaxial films of Fe(001) and Ni(001)

Ultrathin films, bcc Fe(001) on Ag(001), fcc Fe(001) on Cu(001) and Fe/Ni(001) bilayers on Ag, were grown by molecular beam epitaxy. A wide range of surface science tools were employed to establish the quality of epitaxial growth. Ferromagnetic resonance and Brillouin light scattering were used to extract the magnetic properties. Emphasis was placed on the study of magnetic anisotropies. Large uniaxial anisotropies with easy axis perpendicular to the film surface were observed in all ultrathin structures studied. These anisotropies were particularly strong in fcc Fe and bcc Fe films. In sufficiently thin samples the saturation magnetization was oriented perpendicularly to the film surface in the absence of an applied field. It has been demonstrated that in bcc Fe films the uniaxial perpendicular anisotropy originates at the film interfaces. In situ measurements indentified the strength of the uniaxial perpendicular anisotropy constant at the Fe/vacuum, Fe/Ag and Fe/Au interfaces asK _us = 0.96, 0.63, and 0.3 ergs/cm² respectively. The surface anisotropies deduced for [bulk Fe/noble metal] interfaces are in good agreement with the values obtained from ultrathin films. Hence the perpendicular surface ansiotropies originate in the broken symmetry at abrupt interfaces. An observed decrease in the cubic anisotropy in bcc Fe ultrathin films has been explained by the presence of a weak 4th order in-plane surface anisotropy,K _1S=0.012 ergs/cm². Fe/Ni bilayers were also investigated. Ni grew in the pure bcc structure for the first 3–6 ML and then transformed to a new structure which exhibited unique magnetic properties. Transformed ultrathin bilayers possessed large inplane 4th order anisotropies far surpassing those observed in bulk Fe and Ni. The large 4th order anisotropies originate in crystallographic defects formed during the Ni lattice transformation.     

Electric manipulation of spin relaxation using the spin Hall effect

Using the spin Hall effect, magnetization relaxation in a Ni_{81}Fe_{19}/Pt film is manipulated electrically. An electric current applied to the Pt layer exerts spin torque on the entire magnetization of the Ni81Fe19 layer via the macroscopic spin transfer induced by the spin Hall effect and modulates the magnetization relaxation in the Ni81Fe19 layer. This method allows us to tune the magnetization dynamics regardless of the film size without applying electric currents directly to the magnetic layer.     

Magnetooptical visualization of stray magnetic fields of sample rocks using magnetouniaxial garnet ferrite film

The stray magnetic fields of rock samples with inclusions of the magnetic grains and magnetic crystals were observed using epitaxial (Bi,Lu)₃(Fe,Ga)₅O₁₂ films with uniaxial magnetic anisotropy. Basalt rocks with inclusions of titano-magnetite and native elsen but also crystals of titano-magnetite and pyrrhotin were investigated. Stray magnetic fields were visualized with the Faraday Effect.     

The influence of different doping elements on microstructure, piezoelectric coefficient and resistivity of sputtered ZnO film

ZnO film is attractive for high frequency surface acoustic wave device application when it is coupled with diamond. In order to get good performance and reduce insertion loss of the device, it demands the ZnO film possessing high electrical resistivity and piezoelectric coefficient d₃₃. Doping ZnO film with some elements may be a desirable method. In this paper, the ZnO films undoped and doped with Cu, Ni, Co and Fe, respectively (doping concentration is 2.0 at.%) are prepared by magnetron sputtering. The effect of different dopants on the microstructure, piezoelectric coefficient d₃₃, and electrical resistivity of the film are investigated. The results indicate that Cu dopant can enhance the c-axis orientation and piezoelectric coefficient d₃₃, the Cu and Ni dopant can increase electrical resistivity of the ZnO film up to 10⁹ Ω cm. It is promising to fabricate the ZnO films doped with Cu for SAW device applications.     

Peculiarities of the Giant Magnetoimpedance in Permalloy-Based Film Structures in the Important Temperature Range for Practical Applications

Multilayer structures based on Fe19Ni81 films have been obtained by ion-plasma sputtering and investigated on an impedance spectroscopy apparatus equipped with a temperature unit. An increase in the magnetoimpedance ratios for the total impedance and its real part has been found for the multilayer structure (Cu/FeNi)₅/Cu/(Cu/FeNi)₅ upon heating from 25 to 50°C. The maximum of the giant magnetoimpedance ratio of the total impedance ΔZ/Z = 56% has been observed at a frequency of 80 MHz with a sensitivity of 18%/Oe, while the maximum of the real part ΔR/R = 170% was observed at the frequency of 10 MHz with the sensitivity of 46%/Oe. Magnetization and resistivity at the direct current have been found to depend insignificantly on the temperature and, hence, the relaxation mechanism due to the magnetoelastic anisotropy was proposed as the most probable mechanism of increasing the value and sensitivity of the magnetoimpedance effect.     

The magnetic hyperfine field at Cu in Fe,Co and Ni and the magnetic moment of the 41 keV, 2+ state of62Cu

The magnetic hyperfine interaction of Cu in Fe, Co and Ni was studied by means of the γ-γ perturbed angular correlation method using⁶²Zn(⁶²Cu) as a probe. With the publishedg-factor (g=+0.661(12)) of the 41 keV, 2⁺ state hyperfine fields ofB _HF=16.95(51) T,B _HF=13.15(41) T andB _HF=4.05(30) T atT=0 K for Cu in Fe, Co and Ni are derived, respectively. The systematic discrepancy of these values with several independent measurements of these hyperfine fields is removed by deriving a new value ofg=0.55(5) for the 41 keV, 2⁺ state of⁶²Cu.     

Giant magnetic impedance of film nanostructures adapted for biodetection

A comparative study of the magnetic properties and giant magnetic impedance of Fe₁₉Ni₈₁/Su/Fe₁₉Ni₈₁ film structures fabricated by the method of ion-plasma evaporation is performed versus the geometry of an impedance element. The geometrical parameters of the Fe₁₉Ni₈₁/Сu/Fe₁₉Ni₈₁ structures are estimated from the viewpoint of obtaining strong magnetic impedance effect and possible application of these structures for detection of bioelements with magnetic markers. Widening of the structures necessary for increasing their active surface area weakens the magnetic impedance effect, and the length increase is limited by the optimal working biodetector frequencies and electromagnetic wave propagation processes.     

Layer-resolved microscopy of magnetic domains in multi-layered systems

Photoelectron emission microscopy in connection with magnetic circular dichroism in soft X-ray absorption can be used for the microscopic imaging of magnetic domains in layered thin film structures consisting of several magnetic layers. Due to the element-selectivity of the method, the different magnetic layers in such a structure can be imaged separately, provided that they contain different elements. This has been applied for the investigation of Co/Cu/Ni trilayers, epitaxially grown on Cu (001). The magnetic coupling between the Co and Ni layers can be directly visualized from comparing layer-resolved magnetic domain images of both layers. As a consequence of the competition between the anisotropy energies of the two magnetic layers and the magnetic coupling energy, spin-reorientation transitions between collinear and non-collinear magnetic configurations are observed. Apart from this globally observable magnetic interlayer coupling a micromagnetic coupling mechanism is also evident from the layer-resolved domain images. It is caused by magnetostatic interaction of local stray fields from domain walls. Received: 22 August 2002 / Accepted: 2 October 2002 / Published online: 5 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-345/5511-223, E-mail: kuch@mpi-halle.de     

Soft magnetic properties of NiFe compacted powder alloys

It is known that Ni–Fe based alloys (permalloys) are important soft magnetic materials, which have been widely applied in the field of electronic devices and industry. The most suitable permalloys for application exhibit low value of coercivity and magnetostriction (for about 80 at% Ni), high saturation magnetic induction (for about 50 at% Ni), higher electrical resistivity (for about 35 at% Ni). The aim of this work was to investigate the structure and magnetic properties of _Ni81Fe19${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ (wt%) compacted powder material in the form of small cylinders.     

Untersuchung gestörter Winkelverteilungen an F19 in ferromagnetischen Gittern

The perturbation of the 197 keV transition angular distribution in F¹⁹ was investigated by time-dependent spin rotation measurements following excitation with a pulsed beam. The recoil implantation technique was used to determine the internal magnetic fields for F¹⁹ in Fe, Co and Ni lattices. The results are:H _HF(F¹⁹ in Fe)=+(95700±500) Oe,H _HF(F¹⁹ in Co)=+(59500±1500) Oe,H _HF(F¹⁹ in Ni)=?(21830±350)Oe. The temperature and field dependence of the effective fields was studied. Strong satellite fields due to lattice perturbations were detected. The half life and the gyromagnetic ratio of the 197 keV 5/2⁺ state in F¹⁹ were redetermined asT _1/2=(80.2±0.5) nsec andg=+1.436±0.007.     

Site preference and elastic properties of ternary alloying additions in B2 YAg alloys by first-principles calculations

First-principles calculations were preformed to study the site preference behavior and elastic properties of 3d (Ti–Cu) transition-metal elements in B2 ductility YAg alloy. In YAg, Ti is found to occupy the Y sublattice whereas V, Cr, Co, Fe, Ni and Cu tend to substitute for Ag sublattice. Due to the addition of 3d transition metals, the lattice parameters of YAg is decreased in the order: V<Cu<Cr<Ni<Co<Fe<Ti. The calculated elastic constants show that Cr, Fe, Co and Cu can improve the ductility of YAg alloy, and Fe is the most effective element to improve the ductility of YAg, while Ti, Ni and V alloying elements can reduce the ductility of YAg alloy, especially, V transforms ductile into brittle for YAg alloy. In addition, both V and Ni alloying elements can increase the hardness of YAg alloy, and Y₈Ag₇V is harder than Y₈Ag₇Ni.     

Magnetic stripe domains in coupled magnetic sandwiches

Magnetic stripe domains in the spin reorientation transition region are investigated in (Fe/Ni)/Cu(001) and Co/Cu/(Fe/Ni)/Cu(001) using photoemission electron microscopy. For (Fe/Ni)/Cu(001), the stripe domain width decreases exponentially as the Fe/Ni film approaches the spin reorientation transition point. For Co/Cu/(Fe/Ni)/Cu(001), the Fe/Ni stripe orientation is aligned with the Co in-plane magnetization, and the stripe domain width decreases exponentially with increasing the interlayer coupling between the Fe/Ni and Co films. By considering magnetic stripes within an in-plane magnetic field, we reveal a universal dependence of the stripe domain width on the magnetic anisotropy and on the interlayer coupling.     

Mössbauer spectroscopy in highT c superconductors

Mossbauer spectroscopy has recently been applied to study the new high T_c superconducting compounds RBa₂Cu₃O_z, using isotopes of rare earths mainly¹⁵⁵Gd and¹⁵¹Eu, and⁵⁷Fe, with different amounts of Fe ions replacing Cu. It was shown that magnetic moments on the rare earth site do not interfere with superconductivity. Fe at low concentrations (<1%) was found to replace Cu mainly in the Cu(1) site, and the Mossbauer spectra reveal different quadrupole doublets-fol lowing the different oxygen coordination around the Fe ion. The change of the relative intensities of the different doublets with z can easily be followed. For higher iron concentrations, it seems that increasing amounts of iron replace Cu in the Cu(2) site. For z<6.5, the iron reflects the magnetic ordering of Cu in this site, and the ordering temperature as function of z can be obtained. The agreement between neutron diffraction and Mossbauer measurements prove that Fe is a good probe for the magnetic behaviour of the Cu(2) ions. At low temperatures, Fe Mossbauer spectra of Fe in the Cu(1) site are also magnetically broadened, for all z. Superconducting-magnetic phase diagrams are also obtained in Y_1−x Pr_x Ba₂ Cu₃O_z as function of x and z. For z=7.1, T_N changes sharply with x. T_N=300, 230 and 35 K for x=0.8, 0.6 and 0.4 respectively, whereas for z=6.1 T_N changes very little with x. Mossbauer measurements performed on 5 at %⁵⁷Fe doped in CalaBaCu₃O_z show that most of the la occupy the Ba site. For z=7 about half the iron in the Cu(2) sites are magnetically ordered, with H_eff=520 kOe and T_N=400 K, even though the sample is superconducting with T_c=35 K. The possibility of coexistence between superconductivity and magnetic order in these systems will be discussed.     

Synthesis and magnetic properties of Cu-coated Fe composite nanoparticles

The Fe/Cu nanocomposites with iron as core and copper as shell have been successfully synthesized by a two-step reduction method. A spherical nanoparticle of γ-Fe was first fabricated by the reduction of ferrous chloride, and then the Fe particle was coated by nanocrystalline Cu through the reduction of copper sulfate. The thickness of copper shell has been tuned by varying the initial concentration of copper sulfate. The morphology, crystalline structure, chemical composition and magnetic properties of the products were investigated by using transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS) and vibrating sample magnetometer (VSM). It was found that the saturation magnetization (M_s) values of the Fe/Cu core–shell particles are varied owing to the different thickness of copper layer. Though the M_s value of the Fe/Cu nanocomposite is lower than that of pure iron nanoparticles, the higher M_s value (22.411 emu/g) of the Fe/Cu composites is also investigated. The result of the thermogravimetric analysis (TGA) showed the enhanced antioxidation capacity of the Fe/Cu nanocomposites. This kind of nanocomposites combined the excellent magnetism of iron and the electronic, thermal conductivity of copper, suggesting potential application as a novel electromagnetic material.