Applied field Mössbauer studies of the iron storage proteins ferritin and haemosiderin

Applied field Mössbauer spectra obtained from ferritin and haemosiderin have been fitted using a spin-Hamiltonian model, to determine the effect of the applied field on the direction of the iron magnetic moments. The results of these fits indicate that the ordering of magnetic moments within the ferrihydrite core of these proteins is antiferromagnetic. The values of the anisotropy field obtained from these fits correspond to much higher values of the superparamagnetic blocking temperatures than those actually observed. This anomaly has implications for our understanding of the superparamagnetic relaxation processes in these materials.     

Size-dependent magnetism of deposited small iron clusters studied by x-ray magnetic circular dichroism

The size-dependent magnetic properties of small iron clusters deposited on ultrathin nickel films have been studied with circularly polarized synchrotron radiation. With the use of sum rules, orbital and spin magnetic moments have been extracted from x-ray magnetic circular dichroism spectra. The ratio of orbital to spin magnetic moments varies considerably with cluster size, reflecting the dependence of magnetic properties on cluster size and geometry. These variations can be explained in terms of enhanced orbital moments in small clusters.     

Modeling of magnetic Barkhausen noise in single and dual easy axis systems in steel

Angular dependent magnetic Barkhausen noise (MBN) signals measured on plate steel, and on the inside and outside surfaces of sections of seam welded and spiral welded 2% Mn steel pipe are modeled by considering a system of dipole moments. The relative orientation of dipole moments is fixed within the material, but their magnitude grows in the presence of an applied field. Growth of the moments is proportional to the magnetic field projected along a particular moment axis. A single easy axis material consists of an isotropically aligned population of moments, giving the background, upon which is superimposed a population of moments with relative orientations that result in a net moment within the sample. A dual easy axis system is proposed to consist of: (i) a second population of moments with orientations resulting in a net moment with orientation different from that of the first and (ii) interactions, possibly quadrupolar in nature, that occur between the individual moments of each population. The model is used to explain differences in the angular-dependent MBN signal between the seam welded pipe, with a single easy axis, and the spiral welded pipe, with a dual easy axis. The source of the dual easy axis system in the spiral welded pipe, which is different on the two pipe surfaces, is considered in terms of the asymmetric manufacturing processes, relative to the pipe axis, applied during its production.     

Atomic-layer resolved magnetic and electronic structure analysis of ni thin film on a Cu(001) surface by diffraction spectroscopy

Up until now there has been no direct method for detecting the electronic and magnetic structure of each atomic layer at the surface, which is an essential analysis technique for nanotechnology. For this purpose, we have developed a new method, diffraction spectroscopy, based on the photon energy dependence of the angular distribution of Auger electron emission. We have applied this method to analyze the magnetic structure of a Ni ultrathin film on a Cu(001) surface around the spin reorientation transition. Atomic-layer resolved x-ray absorption and magnetic circular dichroism spectra were obtained. Surface and interior core-level shifts and magnetic moments are determined for each atomic layer individually.     

The behaviour of superparamagnetic small particles in applied magnetic fields: A Mössbauer spectroscopic study of ferritin and haemosiderin

Small magnetically ordered particles display characteristic temperature dependent superparamagnetic behaviour in their Mössbauer spectra, as a result of the interplay between the thermal energy and the magnetic anisotropy energy of each particle. In the presence of an applied magnetic field there is an additional energy term arising from the net magnetic moment of the particles. The resulting Mössbauer spectra depend on the size of the anisotropy energy relative to the energy of the magnetic moment of the particles in the applied field. The present applied field Mössbauer measurements on ferritin and haemosiderin indicate that in these materials the anisotropy energy dominates, although the spectra can still be analysed to give information on the magnetic moment of the particles.     

Huge exchange-coupling field observed in FeMn-pinned spin valve with ultra-thin pinned NiFe layer

A conventional Ta/NiFe/Cu/NiFe/FeMn/Ta spin valve multilayer was prepared to investigate the exchange bias variations of the pinned NiFe layer. An exchange bias field of 560 Oe has been found in a valve multilayer with ultra-thin pinned NiFe layers (1 nm), in which a large constant magnetic field of 700 Oe was applied during film deposition procession. The observed results are attributed to the large applied magnetic field, which produced more net spins of the antiferromagnet at the interface. These interfacial uncompensated spins provide the net spin moments required for exchange coupling and bias.     

Mǒsbauer study of the field induced uniaxial anisotropy in electro-deposited FeCo alloy films

Thin ferromagnetic films with in-plane magnetic anisotropy are promising materials for obtaining high microwave permeability.The paper reports a Mo¨ssbauer study of the field induced in-plane uniaxial anisotropy in electro-deposited FeCo alloy films.The FeCo alloy films were prepared by the electro-deposition method with and without an external magnetic field applied parallel to the film plane during deposition.Vibrating sample magnetometry and Mo¨ssbauer spectroscopy measurements at room temperature indicate that the film deposited in external field shows an in-plane uniaxial anisotropy with an easy direction coinciding with the external field direction and a hard direction perpendicular to the field direction,whereas the film deposited without external field does not show any in-plane anisotropy.Mo¨ssbauer spectra taken in three geometric arrangements show that the magnetic moments are almost constrained in the film plane for the film deposited with applied magnetic field.Also,the magnetic moments tend to align in the direction of the applied external magnetic field during deposition,indicating that the observed anisotropy should be attributed to directional ordering of atomic pairs.     

73Ge NMR spectra in germanium single crystals with different isotopic composition

We have studied the influence of isotopic disorder on the local deformations in Ge single crystals from both experimental and calculation points of view. The nuclear magnetic resonance (NMR) spectra of⁷³Ge nuclei (the nuclear spin equals 9/2) in perfect single crystals of germanium with different isotopic content were measured at temperatures 80, 300 and 450 K. Abnormal broadening of the spectrum was found to occur when the magnetic field was aligned along the [111] axis of a crystal. The observed specific angular dependence of the quadrupole broadening was attributed to isotopic disorder among atoms of germanium sited around the⁷³Ge NMR probe. Local lattice deformations in germanium crystal lattice due to isotopic impurity atoms were calculated in the framework of the adiabatic bond charge model. The results obtained were applied to study random noncubic crystal field interactions with the nuclear quadrupole moments and corresponding effects in NMR spectra. Simulated second and fourth moments of resonance frequency distributions caused by the magnetic dipole-dipole and electric quadrupole interactions are used to analyze the lineshapes, theoretical predictions agree qualitatively with the experimental data.     

Some magnetic properties of the cores of various ferritins

⁵⁷Fe Mössbauer spectra of the mineral cores of various ferritins show both paramagnetic and superparamagnetic relaxation effects and in some cases a combination of both. Correlations have been observed between data on the crystallinity, phosphate content and percentage iron in the cores and their magnetic ordering and mean blocking temperatures derived from the Mössbauer spectra.     

Mössbauer study of small-particle maghemite

Two small-particle maghemite (λ-Fe₂O₃) samples have been investigated with the Mössbauer effect. From the results of the model-independent hyperfine-field distribution fits some characteristic temperature-dependent parameters have been obtained. The spectra at the lowest temperatures could be fitted with two strongly overlapping hyperfine-field distributions with different isomer shifts. Spectra in applied magnetic fields ranging from 40 to 60 kOe, and at 4.2 K showed non vanishing Δm ₁=0 absorption lines. The hyperfine-field distribution and canting-angle distribution method for fitting these spectra did not yield reasonable results. Therefore, a bi-dimensional hyperfine-field-canting-angle distribution has been applied, and was found to reproduce the experimental line shapes with remarkable adequacy. The resulting distribution profiles revealed a linear correlation between H_hf and the angle between the magnetic moments and the external field.     

Hadron magnetic moment in the spinor strong interaction model

Hadron magnetic moments are considered in the framework of the spinor strong interaction theory for hadron spectra proposed by the author. Expressions of magnetic moments of ground-state hadrons are derived. These differ from the conventional ones in that they are no longer phenomenological and are basically relativistic. Pseudoscalar mesons have no magnetic moment. Charged vector meson magnetic moment values are given. The magnetic moment operators operate in the internal space, so that the ground-state octet baryons have the same spin-space symmetry, including the A. A formula for the ground-state octet baryon magnetic moment is derived from the basic spinor strong interaction baryon equations previously given, essentially without approximation and in a way analogous to the way in which the electron magnetic moment is derived.     

Local magnetic ordering of Fe impurities in Pd2MnSn

Mössbauer effect of Fe⁵⁷ embedded as very dilute substitutional impurities in Pd₂MnSn was studied. The impurities are seen to replace the three elements in the alloy. Although the Curie temperature of the alloy is 189K, well below the room temperature, the Mössbauer spectrum recorded at room temperature consisted of two distinct 6-finger magnetic hyperfine spectra and a single unsplit line. One of the 6-finger patterns which corresponds to an internal magnetic field ofH _int=?375 kOe is inferred to arise due to local magnetic coupling of the localized magnetic moments of Fe impurities at the Pd sites with those of the 4 Mn first nearest neighbours of the Fe impurities. The other 6-finger pattern which corresponds to an internal magnetic field ofH _int=?335 kOe is inferred to arise due to the local magnetic coupling of the localized magnetic moments of the Fe impurities at the Sn sites with those of the 6 Mn second nearest neighboours of the Fe impurities. The difference in the internal magnetic fields observed at the Pd and Sn sites in the alloy could be understood qualitatively, on the basis of RKKY theory, as arising due to the different conduction electron polarization contributions to the net internal magnetic field at the Fe impurity sites. The results of the measurements suggest that the localized magnetic moments of Fe⁵⁷ impurities at Pd and Sn sites are antiferromagnetically coupled with the moments of their neighbouring Mn atoms.     

Calculation of the relaxation time from the frequency spectra of ferrites

A method for determining the relaxation times of the magnetic moments of ferrites from inverse Fourier transforms is suggested. The effect of the alternating external magnetic field strength on the relaxation times was studied. It was found that the dispersion and absorption ranges in the magnetic spectra are associated with changes in the relaxation times of magnetic moments of ferromagnets.     

Hyperfine fields and magnetic moments in intermetallic ScFe2 with cubic and hexagonal structures

The Sc, Fe and Al NMR spectra have been measured. The spectra and hyperfine fields are dependent on the crystal structures. The magnetic moments of Fe sites were estimated. The magnetic moments of Sc sites μ(Sc) in ScFe₂ (MgCu₂) were evaluated from the magnetic moment per formula μ_∫ as well. μ_∫ in ScFe₂ with hexagonal structures are sharply dependent on the technological conditions and the impurities in the samples.     

X-ray circular magnetic dichroism in the presence of strong spin fluctuations

The use of X-ray magnetic circular dichroism (XMCD) spectra for determininghe t magnitude of atomic magnetic moments in compounds of rare-earth and transition elements is discussed. The standard sum rule approach often yields a magnitude of moments that is often smaller than values obtained from magnetic measurements. We attribute this to strong spin fluctuations in the surface layers in which XMCD signals form. A way of determining the values of local magnetic moments in the presence of strong fluctuations is proposed and tested.     

Diluted graphene antiferromagnet

We study RKKY interactions between local magnetic moments for both doped and undoped graphene. In the former case interactions for moments located on definite sublattices fall off as 1/R2, whereas for those placed at interstitial sites they decay as 1/R3. The interactions are primarily (anti)ferromagnetic for moments on (opposite) equivalent sublattices, suggesting that at low temperature dilute magnetic moments embedded in graphene can order into a state analogous to that of a dilute antiferromagnet. In the undoped case we find no net magnetic moment in the ground state, and demonstrate numerically this effect for ribbons, suggesting the possibility of an unusual spin-transfer device.     

Role of the chemical ordering on the magnetic properties of Fe–Ni cluster alloys

The spin and orbital moments of fcc Fe-Ni cluster alloys are determined within the framework of a d-band Hamiltonian including the spin-orbit coupling non perturbatively. Different sizes (up to 321 atoms), compositions, and chemical configurations (random alloys as well as core-shell arrays of iron and nickel atoms) are considered in order to reveal the crucial role played by local order and stoichiometry on the magnetic moments of the clusters. Interestingly, we have found considerably reduced average magnetizations for Fe-Ni clusters with Fe cores compared to that of the bulk alloy with the same composition. Indeed, in these configurations not only antiparallel arrangements between the local moments of some Fe atoms within the iron core are found, but also the total magnetization of the surface Ni atoms is significantly quenched. On the opposite, the disordered and Ni-core cluster alloys are characterized by high magnetizations resulting from saturated-like contributions from both Ni and Fe atoms, in agreement with recent ab-initio calculations. In general, the local orbital magnetic moments are strongly enhanced with respect to their bulk values. Finally, the variation of the orbital-to-spin moment ratio with the chemical order is discussed.     

Ferromagnetism in epitaxial orthorhombic YMnO3 thin films

Epitaxial orthorhombic YMnO₃ thin films, (0 0 1) oriented, have been grown by pulsed laser deposition on (0 0 1)SrTiO₃ substrates. Their crystal structure and magnetic response have been studied in detail. Although bulk o-YMnO₃ is antiferromagnetic, our magnetic measurements reveal intriguing thermal hysteresis between the zero-field-cooled and field-cooled curves below the onset of the antiferromagnetic ordering temperature, thus signaling a more complex magnetic structure with net ferromagnetic moments. We discuss on the possible origin of this net magnetization and we have found a correlation of the magnetic response with the strain state of the films. We propose that substrate-induced strain modifies the subtle competition of magnetic interactions and leads to a non-collinear magnetic state that can thus be tuned by strain engineering.     

Magnetic-field enhancement of superconductivity in ultranarrow wires

We study the effect of an applied magnetic field on sub-10-nm wide MoGe and Nb superconducting wires. We find that magnetic fields can enhance the critical supercurrent at low temperatures, and do so more strongly for narrower wires. We conjecture that magnetic moments are present, but their pair-breaking effect, active at lower magnetic fields, is suppressed by higher fields. The corresponding microscopic theory, which we have developed, quantitatively explains all experimental observations, and suggests that magnetic moments have formed on the wire surfaces.