Magnetic impedance of structured film meanders in the presence of magnetic micro- and nanoparticles

The magnetic impedance (MI) of film elements in the form of meanders with a [Fe19Ni81]/Cu]₄/Fe19Ni81/Cu/[Fe19Ni81/Cu]₄/Fe19Ni81 layered structure and variable geometry is studied. For the best meanders (having a maximal MI of up to 125% and an MI maximal sensitivity of about 30%/Oe), the influence of stray magnetic fields is determined. The stray fields are produced by spherical iron particles 500 μm in diameter and ferrofluids containing iron oxide nanoparticles. The feasibility of detecting intricately configured stray fields from a set of ferromagnetic spheres arranged on the surface of an MI element is demonstrated. The sensitivity of film meander MI elements to nonuniform external magnetic fields is simulated. The results of this work may be helpful in developing special-purpose magnetic sensors intended for micropositioning, nondestructive testing, and biomagnetic detection.     

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.     

Photoemission electron microscopy of neodymium-iron-boron (Nd<Subscript>2</Subscript>Fe<Subscript>14</Subscript>B)

The magnetic domain structure of a neodymium-iron-boron single crystal (Nd₂Fe₁₄B) was investigated in a photoemission electron microscope equipped with an aperture for partial restriction of the electron beam. As a result of the influence of magnetic microfields, electron trajectories are deflected in such a way that some of them are stopped by the aperture in the electron optical path. As a result, the contrast caused by the stray fields of the magnetic domains is significantly enhanced. The distribution of the local magnetic fields at the surface is reconstructed from the image by means of the proposed theory on the contrast mechanism. The size of the stray field close to the sample surface under study was 0.5–0.7 T. PACS 68.37.Xy; 75.50.Bb; 75.70-i; 75.70.Kw     

Quantitative Lorentz transmission electron microscopy of structured thin permalloy films

The defocusing of images of ferromagnetic particles in the transmission electron microscope gives rise to magnetic contrast (Lorentz microscopy). We have developed a theory which allows from this contrast to determine quantitatively the distribution of the magnetic fields of the specimen. The measurements were performed on permalloy particles of disc (diameter 50 nm), and rectangular (25×50 nm² and 50×50 nm²) shapes, thickness of 21 nm. These particles had a vortex and Landau–Lifshitz structure, respectively. The determined value of the magnetic induction in the material amounted to 1.1±0.1 T. The stray fields in angular sectors of the rectangular particles reached 0.35±0.05 T. The width of the 90° Néel wall between domains turned out to be equal to 4.5±0.5 nm.     

Two-dimensional spatially ordered system of nickel nanowires probed by polarized SANS

Structural and magnetic properties of two-dimensional spatially ordered system of ferromagnetic nickel nanowires embedded into Al₂O₃ matrix have been studied using polarized small-angle neutron scattering. The small-angle diffraction pattern exhibits many diffraction peaks, which corresponds to the scattering from highly correlated hexagonal structure of pores and magnetic nanowires. Magnetic contribution to the scattering has complex behavior and cannot be explained without taking into account stray fields located between magnetized nanowires.     

On the bubble lattice generation in nonuniform pulse fields

Conclusion The dynamic equilibrium bubble domain lattices created by nonuniform magnetic field pulses were investigated in the region of pulse amplitudes where the domain structure behaved in the most regular way. The lattice formation was found possible in a rather narrow range of the bias field values (withinH _e<H<H _col) and in the area of the sample betweens ₂- ands ₁-boundaries. Detailed parameters on these boundaries (position, field and field gradient) were measured and calculated as functions of the bias field and the pulse field amplitude. The boundarys ₂ was understood as being the position in the sample where the total stray + applied field during the field pulses equalsH _e. A next magnetostatic calculation of the domain structure in the nonuniform field should give an appropriate theoretical formula fors ₁. High-speed photography has been used to record the immediate patterns of the domain structure both during a single field pulse and between the pulses. The obtained pictures made it possible to calculate the magnetic stray fields and stray field gradients when a simplified dipole model of the observed pattern was used. The attraction towards the wire has been compared with the repulsion due to the stray fields and their difference in the case of the equilibrium bubble lattice has been compared with the material coercivity term. The coercivity of the material was found to have very small influence on the translation of the domains during the process of the formation and spreading of the bubble lattice, which was attributed to the repetitive pulse component of the applied field.Dedicated to Jan Kaczér DrSc on the occasion of his 65-th birthday.The authors wish to thank L. Murtinová, P. Novotný and G. Vertésy for their helpful discussions, G. Vertésy for the ion implanted samples and L. Bodis and G. Battistig for taking part at the preliminary experiments.     

Manipulating superconductivity in perpendicularly magnetized FSF triple layers

We investigate the superconducting transition temperature T_c of epitaxial ferromagnet/superconductor/ferromagnet (FSF) triple layers with perpendicular magnetic anisotropy. Due to the different coercive fields of the top and bottom F layers (F=[Co/Pt] multilayer) different magnetized states can be achieved: a fully magnetized state where the F layer magnetizations are parallel oriented, a state DM where one layer is demagnetized, and a state DD where both layers are demagnetized. T_c is maximum in the fully magnetized state and decreases consecutively from the DM to the DD state due to the different contributions from magnetic stray fields originating from the domain walls present in the demagnetized layers. The role of the proximity effect and the effect of the stray fields on the superconductivity in the S layer can be distinguished by analyzing the temperature dependence of the upper critical field and by comparison with data taken on an FISIF multilayer where I is an insulating SiO₂ barrier. Hence, we demonstrate that T_c can be manipulated by the intentional creation of different stray-field configurations in the F layers. PACS 68.55.JK; 74.45.+c; 74.78.Db; 74.78.Fk; 75.47.-m     

Mössbauer studies of magnetite nanoparticles

Magnetite Fe₃O₄ samples with different levels of dispersion (“bulk” and nanoscale particles with an average diameter of 24, 14 and 7 nm) are studied using Mössbauer spectroscopy. It is shown that this method of analysis makes it possible to obtain important information about the features of magnetic states of iron-containing nanoparticles and, more specifically, about the distinction in effective magnetic fields for iron nuclei located in different regions within them. The obtained results are fully consistent with known theoretical data and allow estimation of the thickness of the surface region of the nanoparticles.     

The structural instabilities in ferronematic based on liquid crystal with negative diamagnetic susceptibility anisotropy

The studied ferronematic is a nematic liquid crystal (ZLI1695) of low negative anisotropy of the diamagnetic susceptibility (χ_a<0) doped with the magnetic particles Fe₃O₄. Structural instabilities are interpreted within Burylov and Raikher's theory. The high magnetic fields were oriented perpendicular (Freedericksz transition) or parallel to the initial director. Using capacitance measurements the Freedericksz threshold magnetic field of the ferronematic B_FN, and the critical magnetic field B_max, at which the initial parallel orientation between the director and the magnetic moment of magnetic particles breaks down, have been determined. The values of these quantities have been used to estimate the surface density of the anchoring energy W of liquid crystal molecules on the surface of the magnetic particles. The obtained values indicate a soft anchoring of the liquid crystal on the magnetic particles with a preferred parallel orientation of the magnetic moment of magnetic particles and the director.     

Oscillation galvanomagnetic effect in double-layer metal films

A semiclassical approximation has been used to analyze theoretically the dependence of the conductivity σ (resistivity ρ) of a double-layer film consisting of plane-parallel single-crystal metal layers of different thickness d_j ≠ d_n and grade l_j ≠ l_n on the layer thickness ratio d_2,1 = d₂/d₁ and on the magnitude of the magnetic field normal to the layer interface. A general expression (for any d_2,1) and asymptotic expressions (for thick and thin (compared to the electron free path l_j) metal layers in a weak and a strong magnetic field) for the conductivity of a double-layer specimen have been obtained. A nonmonotonic relation between the conductivity of a double-layer film and the layer thickness ratio at weak magnetic fields has been predicted which is sensitive to the character of the interaction of charge carriers with the conductor boundaries. At strong magnetic fields, ρ becomes an oscillating function of magnetic field (layer thickness). A detailed numerical analysis of the resistivity of a double-layer film on the layer thickness ratio and on the magnetic field strength has been performed for arbitrary values of the parameters that describe the character of the interaction of charge carriers with the specimen boundaries. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 26–33, July, 2007.     

The fragile magnetic structures of Fe/CeH2−δ multilayers

Fe/CeH_2−δ multilayers exhibit at room temperature evidence of interlayer exchange coupling. Subsequent Fe layers are either parallel or antiparallel to each other, depending on the Fe and CeH_2−δ layer thickness. However, when both layers have thickness larger than 15 Å, the antiferromagnetic structure becomes fragmented into domains laterally limited to a few microns, and the magnetic structures become very fragile. Small magnetic fields of a few Oersteds acting on the samples during growth induce helimagnetic configurations which coexist with antiferromagnetic coupling. The magnetic structures can be permanently destroyed by applying magnetic fields larger than 150 Oe.     

Experimental determination of ultra-sharp stray field distribution from a magnetic vortex core structure

The fine magnetic stray field from a vortex structure of micron-sized permalloy (Ni₈₀Fe₂₀) elements has been studied by high-resolution magnetic force microscopy. By systematically studying the width of the stray field gradient distribution at different tip-to-sample distances, we show that the half-width at half-maximum (HWHM) of the signal from vortex core can be as narrow as ∼21 nm at a closest tip-to-sample distance of 23 nm, even including the convolution effect of the finite size of the magnetic tip. a weak circular reverse component is found around the center of the magnetic vortex in the measured magnetic force microscope (MFM) signals, which can be attributed to the reverse magnetization around the vortex core. Successive micromagnetic and MFM imaging simulations show good agreements with our experimental results on the width of the stray field distribution.     

Modes of natural vibration for magnetic domains

We analyze wall-vibration modes for the case of plane parallel stripe domains in a uniaxial film whose easy axis is normal to the film plane, using Landau-Lifshitz equations carried to the limit of vanishing wall thickness. We take into account long-range dipole interactions and wall-moment twist due to stray fields from magnetic charges on the film surfaces. The small-amplitude wall displacement q(k, z) depends on the position coordinate z normal to the film plane, and on a two dimensional wave vector k parallel to the film plane. Numerically computed natural frequencies v_n(k) depend on the number of nodes n(=0, 1, 2 …) in the dependence of q on z. Surface and bulk modes are distinguished by the z-dependence of computed eigenmodes q_n(k, z). The spectrum of computed natural frequencies compares favorably with available experimental data.     

Magnetische streufelder über den bereichen sehr genau orientierter Ni-einkristalloberflächen zwischen 5.5–500 K

The previously described electron optical measurements of the magnetic stray fields of Ni single crystals above the domain structure of plane ( 10)-surfaces oriented within 0.01° at room temperature are now extended to the temperature range 5.5–500 K. Their dependence on the angle ₀, 0° ₀ 5.41°, between exact 10 direction and the normal to the surface is investigated along the crystal axis 110. The results are discussed and compared with calculations of the μ^*-correction.     

Oriented O(3P2), Ne(3P2), and He(3S1) atoms emerging from a bent magnetic guide

We describe our observation of strongly oriented total electronic angular momentum J in O(³P₂), Ne(³P₂), and He(³S₁) atoms emerging from a bent magnetic multipole guide, as measured by resonant multiphoton ionisation. This was contrary to our expectation because no additional (uniform) magnetic fields were applied to orient the atoms behind the exit of the guide. Two- and three-photon ionisation techniques were employed to determine the degree of J polarisation, from which we infer that atoms become oriented as a result of a combination of weak fringe fields, possible stray fields, and the fact that molecular beam packets do not oscillate around the geometric center of the bent multipole guide. We conclude that similar effects may exist in other, related experiments and that a detailed characterisation of the degree of orientation is required prior to any study of chemical dynamics or spectroscopy. This paper should serve as a warning for anybody using similar devices not to assume isotropic angular momentum distributions of atoms and molecules emerging from a magnetic guide or a decelerator, particularly when it is bent; whenever possible, the possibility for a J anisotropy should be experimentally checked.     

Temperature dependence of the critical magnetic field of the structural transition in MBBA-based ferronematics

We studied the structural transitions in ferronematics based on the thermotropic nematic liquid crystal MBBA (4^′ -methoxybenzylidene-4-n-butylaniline) having a nematic-to-isotropic transition temperature T _N–I?=?48.0^○C and in MBBA-based ferronematics doped with a magnetic suspension consisting of Fe₃O₄ particles (10?nm in diameter) coated with oleic acid as a surfactant. The ferronematic samples were prepared with different volume concentrations of magnetic particles φ?=,1× 10^?4, 2× 10^?4 and 5×10^?4. The temperature dependences of the critical magnetic fields in a bias electric field under strong applied magnetic fields are presented. We calculated the surface density of anchoring energy W at the nematic–magnetic particle boundary. Scaling of the structural transition in the MBBA and MBBA-based ferronematics with the temperature of the nematic-to-isotropic transition was observed.     

Optical magnetometer with submicron spatial resolution based on Rb vapors

It is shown experimentally that use of fluorescence and transmission spectra obtained from nanocells with the thickness of column of rubidium atomic vapor L = λ/2 and L = λ, respectively (λ = 794 nm is the wavelength of laser radiation close to resonance with D ₁-line transition of Rb atoms), by means of a narrowband diode laser allows spectral separation and study of variations of probabilities of atomic transitions between ground and excited states of hfs of D ₁ lines of ⁸⁵Rb and ⁸⁷Rb atoms in the range of magnetic fields from 10 to 5000 G. Small thickness of atomic vapor column (∼390 nm and ∼794 nm) allows applying permanent magnets simplifying essentially creation of strong magnetic fields. Advantages of this technique are discussed as compared with the technique of saturated absorption. The obtained results show that a nanocell with submicrom thickness of vapor column may serve as a basis for designing a magnetometer with submicron local spatial resolution which is important in case of measuring strongly inhomogeneous magnetic fields. Experimental data are in good agreement with the theoretical results.     

Study of the magnetic state of a highly dispersed BaO · 6Fe2O3 system with near-critical particle size

An investigation was made of the magnetic state of a system of highly anisotropic BaO · 6Fe₂O₃ nanocrystals several lattice parameters thick and having a near-critical volume (∼10⁻¹⁸ cm³), obtained using cryochemical technology. It is observed that the particles are transformed to the superparamagnetic state and it is shown that the external magnetic field plays a role in its formation. An H-T diagram was obtained for the temperature range 300 K-T _c, which shows various regions uncharacteristic of the macro-object, which are specifically attributed to the distribution over the anisotropy fields in the system and the impaired magnetic structure in the surface zone of the particles. Relatively large regions of magnetic fields and temperatures were observed where reversible rotation of the magnetization vector of particles with near-critical volume plays an important role. Fiz. Tverd. Tela (St. Petersburg) 40, 1294–1297 (July 1998)     

On the Mössbauer spectrum of γ-Fe2O3

High quality Mössbauer spectra of γ-Fe₂O₃ particles have been analyzed in detail to determine the effects of overlapping patterns, quadrupole splitting, and applied magnetic fields. The results are discussed with regard to the use of Mössbauer spectroscopy to measure particle and magnetic moment orientation. Data were collected from two samples at 4.2–315K and with 6.1T longitudinal and 1.65T transverse magnetic fields applied. It is shown that electric field gradient effects alone are insufficient to account for the presence of Δm_I=0 lines when a large longitudinal field is applied.     

Impact of silica environment on hyperfine interactions in &#x01d716;-Fe2O3 nanoparticles

Magnetic nanoparticles have found broad applications in medicine, especially for cell targeting and transport, and as contrast agents in MRI. Our samples of ??-Fe₂O₃ nanoparticles were prepared by annealing in silica matrix, which was leached off and the bare particles were then coated with amorphous silica layers of various thicknesses. The distribution of particle sizes was determined from the TEM pictures giving the average size ～20 nm and the thickness of silica coating ～5; 8; 12; 19 nm. The particles were further characterized by the XRPD and DC magnetic measurements. The nanoparticles consisted mainly of ??-Fe₂O₃ with admixtures of ～1 % of the α phase and less than 1 % of the γ phase. The hysteresis loops displayed coercivities of ～2 T at room temperature. The parameters of hyperfine interactions were derived from transmission Mössbauer spectra. Observed differences of hyperfine fields for nanoparticles in the matrix and the bare ones are ascribed to strains produced during cooling of the composite. This interpretation is supported by slight changes of their lattice parameters and increase of the elementary cell volume deduced from XRD. The temperature dependence of the magnetization indicated a two-step magnetic transition of the ??-Fe₂O₃ nanoparticles spread between ～85 K and ～150 K, which is slightly modified by remanent tensile stresses in the case of nanoparticles in the matrix. The subsequent coating of the bare particles by silica produced no further change in hyperfine parameters, which indicates that this procedure does not modify magnetic properties of nanoparticles.