Influence of continuous laser annealing on magnetic properties of amorphous ribbons: Connections with structural modifications and crystallization occurrence,as revealed by Mössbauer spectroscopy

In this paper the influence of continuous laser annealing on the magnetic properties and amorphous structure of Fe₄₀Ni₄₀P₁₄B₆ ribbons is studied; an interpretation is given relating the measured magnetic behaviour with the structural aspects revealed by Mössbauer spectroscopy. The saturation induction, the magnetostriction, the initial permeability, the constant of magnetic anisotropy and the residual stress behaviour versus the annealing temperature are reported. The experimental results show that the continuous laser irradiation can be used to improve magnetic properties under appropriate conditions. In particular the average temperature of heating must not exceed 450 K. On the other hand, we show that the effects of continuous laser annealing are pre-eminently due to both the quenching stress relaxation and the formation of crystalline phases.     

Dynamics of first- and second-order phase transitions in amorphous magnetooptic TbFeCo films

The dynamics of phase transformations in thin amorphous TbFeCo films under the action of ～ 1 ps laser pulses is investigated. The films are heated to the Curie temperature in the amorphous state (T _C1), to the crystallization temperature (T _ac), and to the Curie temperature in the crystalline phase (T _C2). The change in magnetization is detected by Faraday magnetooptic effect during and after the action of the heating pulse. A static external magnetic fieldH～1?12 kOe, whose flux lines are directed perpendicular to the plane of the film, is used in the experiments. Amorphous TbFeCo films possess a perpendicular magnetic anisotropy, which on crystallization becomes reoriented in the plane of the film. It is observed that crystallization and magnetization reorientation occur during the heating pulse (within ～ 1 ps). The spin subsystem is heated to the Curie temperature several picoseconds after the end of the laser pulse. The characteristic spin relaxation time is ～ 10 ps. A model of the dynamics of the electronic, spin, and phonon subsystems that makes it possible to explain the experimental results is proposed on the basis of the data obtained.     

Studies of thin FeAlN films prepared by different techniques

The microstructure, morphology, and magnetic properties of FeAlN films deposited by reactive rf magnetron sputtering with subsequent treatment by three techniques, namely, in situ, ex situ (with the sputtering and annealing processes separated), and thermal crystallization of amorphous alloys, have been studied. FeAlN films prepared by the ex situ technique exhibit the best soft magnetic characteristics. Thermal crystallization of amorphous alloys produced films with properties having the highest thermal stability. Films 800-to 1000-nm thick were found to have the best soft magnetic properties. The dependences of the properties of FeAlN films on nitrogen content and annealing temperature were established. The conditions favoring the preparation of thin nanostructured FeAlN films featuring the best soft magnetic characteristics (saturation induction B _S = 1.8 T, coercivity H _C = 1.2 Oe, magnetic susceptibility μ₁ (1 MHz) = 3400) were determined.     

Medium range ordering and some magnetic properties of amorphous Fe90Zr7B3 alloy

High-resolution electron microscopy (HREM) reveals in the as-quenched Fe₉₀Zr₇B₃ alloy the existence of medium range ordered (MRO) regions 1-2 nm in size. Transmission Mössbauer spectroscopy confirms that these regions are α-Fe MRO ones. Above the Curie point of the amorphous phase (T_C=(257±2)K) they behave like non-interacting superparamagnetic particles with the magnetization decreasing linearly with the temperature. For these particles the average magnetic moment of 390μ_B and the average size of 1.7 nm, in excellent agreement with HREM observations, were estimated. The maximum of the isothermal magnetic entropy change at the maximum magnetizing field induction of 2 T occurs at the Curie temperature of the amorphous phase and equals to 1.05 Jkg⁻¹ K⁻¹. The magnetic entropy changes exhibit the linear dependence on the maximum magnetizing field induction in the range 0.5-2 T below, near and above T_C. Such correlations are attributed to superparamagnetic behavior of α-Fe MRO regions.     

Paramagnetic centers in nonmetallic amorphous polyphthalocyanines

Electron paramagnetic resonance (EPR) spectra of nonmetallic amorphous polyphthalocyanines are investigated in the temperature range 295–500 K. The EPR spectrum of nonmetallic amorphous polyphth-alocyanine samples at room temperature prior to heating is a narrow singlet of approximately Lorentzian shape with a linewidth ΔH_pp ≈ 1.7 Oe, a splitting factor g=2.00, and an intensity I_EPR ≈ 10¹⁷ spins/g. It is found that the intensity and linewidth of the EPR spectrum increase with increasing temperature. Beginning with a characteristic temperature T₁, both parameters, ΔH_pp and I_EPR, become dependent on time (under isothermal conditions). Computer calculations of the spectra demonstrate that the EPR spectrum can be represented as a superposition of two lines with substantially differing parameters whose dependences on the temperature and micro-wave power also differ significantly. The possible reasons for the existence of electron paramagnetic resonance centers of two types with different degrees of delocalization of a charge carrier with a magnetic moment in nonmetallic amorphous polyphthalocyanines are discussed.     

Mössbauer and magnetic resonance experiments on amorphous iron-silicon films

Mössbauer measurements at room temperature and 4.2 K, and room temperature magnetic resonance measurements on a series of amorphous Fe_xSi_1-x thin films (0.23?x?0.81) are presented. The concentration dependence of the isomer shifts and quadrupole splittings provides information on the nature of the local coordination in these amorphous materials. Analysis of the distributions of magnetic hyperfine splitting combined with the presence of multiple resonances in the magnetic resonance data indicates that magnetoanisotropy plays a dominant role in determining the magnetic behaviour of these films.     

Rare earth-transition metal compound-based MOSLM for the visible spectral range

We have demonstrated a magneto-optical spatial light modulator in which functionality is realized by (i) heating up to Curie temperature (T_c) magneto-optical elements (pixels) with a semiconductor laser and (ii) application of a switching magnetic field. The pixels were made of films of amorphous rare earth-transition metal compounds (TbFe films with T_c=403 K and DyFe films with T_c=343 K) having good magneto-optical responses for wavelengths from the visible spectral range. We have found that the magnetization direction of pixels can be modulated with a laser radiation density of 5 mJ/cm² and in a switching magnetic field of 15 Oe.     

Microstructure and some magnetic properties of bulk amorphous (Fe0.61Co0.10Zr0.025Hf0.025Ti0.02W0.02B0.20)100−xYx (x=0, 2, 3 or 4) alloys

Microstructure, revealed by X-ray diffraction, transmission electron microscopy and Mössbauer spectroscopy, and magnetic properties such as magnetic susceptibility, its disaccommodation, core losses and approach to magnetic saturation in bulk amorphous (Fe_0.61Co_0.10Zr_0.025Hf_0.025Ti_0.02W_0.02B_0.20)_100−xY_x (x=0, 2, 3 or 4) alloys in the as-cast state and after the annealing in vacuum at 720 K for 15 min. are studied. The investigated alloys are ferromagnetic at room temperature. The average hyperfine field induction decreases with Y concentration. Due to annealing out of free volumes its value increases after the heat treatment of the samples. The magnetic susceptibility and core losses point out that the best thermal stability by the amorphous (Fe_0.61Co_0.10Zr_0.025Hf_0.025Ti_0.02W_0.02B_0.20)₉₇Y₃ alloy is exhibited. Moreover, from Mössbauer spectroscopy investigations it is shown that the mentioned above alloy is the most homogeneous. The atom packing density increases with Y concentration, which is proved by the magnetic susceptibility disaccommodation and approach to magnetic saturation studies.     

Formation and magnetic properties of the silicon-cobalt interface

Formation of the Si/Co interface and its magnetic properties have been studied by high-resolution photoelectron spectroscopy with synchrotron radiation. The experiments have been performed in situ in superhigh vacuum (5 × 10^?10 Torr) with coating thicknesses up to 2 nm. It has been found that, in the initial stage of silicon deposition on the surface of polycrystalline cobalt maintained at room temperature, ultrathin layers of the Co₃Si, Co₂Si, CoSi, and CoSi₂ silicides are formed. The three last phases are nonmagnetic, and their formation gives rise to fast decay of magnetic linear dichroism in photoemission of Co 3p electrons. At deposition doses in excess of ～0.4 nm Si, a film of amorphous silicon grows on the sample surface. It has been established that the Si/Co interphase boundary is stable at temperatures up to ～250°C and that further heating of the sample brings about escape of amorphous silicon from the sample surface and initiates processes involving silicide formation.     

Observation of collisionless plasma heating by strong shock waves

The heating of a plasma by collisionless shock waves is investigated by measuring the variation of magnetic field (with magnetic probes), density and electron temperature (from Thomson scattering of laser light) in the shock waves. The compression waves are produced in a tube of 14 cm diameter by the fast rising magnetic field (12 kG in 0.5Μsec) of a theta pinch. For shocks with Mach numbers between 2 and 3 propagating into a hydrogen or deuterium plasma with a localΒ of about 1 (Β=ratio of particle pressure to magnetic pressure) the measured jump in density and magnetic field across the front is 2 to 4, and the electron temperature increases in the front from 3 to 50 eV with a further rise to between 100 and 250 eV in the piston region. Only about 20% of the measured electron heating can be explained by adiabatic heating and resistive heating based on binary collisions, indicating a high turbulent plasma resistance. Both the observed electron heating and the width of the shock front, which is about 0.6 ·c/Ω _p, can be accounted for using an effective collision frequency close to the ion plasma frequencyΩ _p. The ion heating in the almost stationary shock fronts can be inferred indirectly from the steady state conservation relations. For shock waves with Mach numbersM<M _crit it seems to be consistent with an adiabatic heating process, whereas forM>M _crit the calculated ion temperatures exceed those one would except for a merely adiabatic heating.     

Magnetic properties of BaFe11.6–2xCoxTixO19 particles produced by sol–gel and spray-drying

Ba-ferrite powders of composition BaFe_11.6−2xCo_xTi_xO₁₉ (with x varying from 0 to 1.1) were prepared by the sol–gel and spray-drying techniques. Green powders showed an amorphous structure. They were heat-treated from 850°C to 1000°C to promote crystallisation and to study the dependence of the magnetic properties on the annealing temperature. Unlike the evaporation-dried powders, the spray-dried samples showed a highly homogeneous structure, which consisted of spherical particles with sizes ranging from 0.1 to ∼0.3 μm. As for the magnetic properties, the saturation magnetisation and remanence remained practically constant up to x=0.8 for all heating temperatures, whereas the coercivity fell uniformly as x increased from ∼5.5 to ∼0.1 kOe.     

Switching field dependence on heating pulse duration in thermally assisted magnetic random access memories

The minimum applied field H_SW required to reverse the magnetic moment of the ferromagnetic/antiferromagnetic storage layer of a thermally assisted magnetic random access memory (TA-MRAM) device during the application of a heating electric pulse is investigated as a function of pulse power P_HP and duration δ. For the same power of the heating pulse P_HP (or, equivalently, for the same temperature of the storage layer), H_SW increases with decreasing heating time δ. This behavior is consistently interpreted by a thermally activated propagating domain-wall switching model, corroborated by a real-time study of switching. The increase of H_SW with decreasing pulse width introduces a constraint for the minimum power consumption of a TA-MRAM where writing combines heating and magnetic field application.     

Advances in amorphous and nanocrystalline materials

A new amorphous alloy has been recently introduced which shows a saturation magnetic induction B_s of 1.64 T which is compared with B_s=1.57 T for a currently available Fe-based amorphous alloy and decreased magnetic losses. Such a combination is rare but can be explained in terms of induced magnetic anisotropy being reduced by the alloy's chemistry and its heat treatment. It has been found that the region of magnetization rotation in the new alloy is considerably narrowed, resulting in reduced exciting power in the magnetic devices utilizing the material. Efforts to increase B_s also have been made for nanocrystalline alloys. For example, a nanocrystalline alloy having a composition of Fe_80.5Cu_1.5Si₄B₁₄ shows B_s exceeding 1.8 T. The iron loss at 50 Hz and at 1.6 T induction in a toroidal core of this material is 0.46 W/kg which is 2/3 that of a grain-oriented silicon steel. At 20 kHz/0.2 T excitation, the iron loss is about 60% of that in an Fe-based amorphous alloy which is widely used in power electronics. Another example is a Fe₈₅Si₂B₈P₄Cu₁ nanocrystalline alloy with a B_s of 1.8 T, which is reported to exhibit a magnetic core loss of about 0.2 W/kg at 50 Hz and at 1.5 T induction. This article is a review of these new developments and their impacts on energy efficient magnetic devices.     

Ferromagnetism modulation by phase change in Mn-doped GeTe chalcogenide magnetic materials

In this work, an effective method to modulate the ferromagnetic properties of Mn-doped GeTe chalcogenide-based phase change materials is presented. The microstructure of the phase change magnetic material Ge_1?x Mn _x Te thin films was studied. The X-ray diffraction results demonstrate that the as-deposited films are amorphous, and the crystalline films are formed after annealing at 350 °C for 10 min. Crystallographic structure investigation shows the existence of some secondary magnetic phases. The lattice parameters of Ge_1?x Mn _x Te (x = 0.04, 0.12 and 0.15) thin films are found to be slightly different with changes of Mn compositions. The structural analysis clearly indicates that all the films have a stable rhombohedral face-centered cubic polycrystalline structure. The magnetic properties of the amorphous and crystalline Ge_0.96Mn_0.04Te were investigated. The measurements of magnetization (M) as a function of the magnetic field (H) show that both amorphous and crystalline phases of Ge_0.96Mn_0.04Te thin film are ferromagnetic and there is drastic variation between amorphous and crystalline states. The temperature (T) dependence of magnetizations at zero field cooling (ZFC) and field cooling (FC) conditions of the crystalline Ge_0.96Mn_0.04Te thin film under different applied magnetic fields were performed. The measured data at 100 and 300 Oe applied magnetic fields show large bifurcations in the ZFC and FC curves while on the 5,000 Oe magnetic field there is no deviation.     

Curie temperatures and crystallization behavior of amorphous Fe81−xNixZr7B12 (x=10, 20, 30, 40, 50, 60) alloys

Curie temperature, crystal structure and crystallization behavior of amorphous alloys with the stoichiometry Fe_81−xNi_xZr₇B₁₂ (x=10–60) have been studied by X-ray diffractometry (XRD), differential scanning calorimetry (DSC) and AC-magnetization (TMAG) measurements as functions of temperature. The thermal stability of long-range magnetic order, T_C vs. Ni content in as-quenched amorphous alloys exhibits maximum at 352 °C for x=40. The primary crystallization has been detected during annealing at the first crystallization stage of all ribbons investigated.     

A case of magnetic field-induced change in final product

An interesting case of magnetic field-induced change in the final product of chemical reaction was observed. XRD patterns indicate that both of the products prepared at room temperature are amorphous regardless of the external magnetic field applied or not. However, after vacuum-annealed at 873 K, the product prepared at room temperature with 0.25 T magnetic field applied was orthorhombic Fe₃BO₅, but the product prepared without magnetic field applied changed to orthorhombic Fe₃O₄ or tetragonal Fe₂B. It indicates that the magnetic field induction is a dominating factor to the final product of chemical reaction. It suggests us an important method to fabricate new materials under mild magnetic field induction.     

Invar-type new ferromagnetic amorphous Fe-B alloys

Measurements of magnetization, electrical resistivity, thermal expansion and differential thermal change were made on amorphous Fe_100-xB_x (9 ≦ X ≦ 21) alloys prepared by rapid quenching from the liquid state.With decreasing boron content in the alloys, the Curie temperature falls remarkably, while the magnetic moment increases sluggishly. The thermal expansion curves exhibit the invar characteristics below the Curie temperature due to a large positive spontaneous volume magnetostriction, and the reduced magnetization curves decrease much more rapidly with increasing temperature than those of other ferromagnetic amorphous alloys.     

Structure and magnetic properties of Nb-doped FeZrB soft magnetic alloys

Structure and magnetic properties of Nb-doped (FeZrB)_100−xNb_x alloy are investigated by X-ray diffraction (XRD), differential scanning calorimetry and vibrating sample magnetometer. The fully amorphous structure of the as-quenched ribbons is confirmed by the XRD pattern. With increasing Nb, the glass transition temperature and the onset crystallization temperature are increased, indicating increased stability of the amorphous structure. For x=1, the saturation magnetization of the ribbons is 125.7 emu/g and the optimized annealing temperature increases from 550 to 630 °C. The morphology of the crystallized phases is observed by scanning electron microscopy. The results show that nanocrystalline α-Fe grains are dispersed in the amorphous matrix.     

Mössbauer studies of the magnetic behavior of amorphous Mn x Sn1−x alloys

We present systematic low temperature in situ¹¹⁹Sn Mössbauer effect (ME) studies in vapor quenched amorphous Mn _x Sn_1?x (0.09<x<0.95) alloys between 150 and 4.2 K. It is shown that the magnetic behavior of the system is correctly displayed by the transferred magnetic hyperfine (hf) interactions detected at the¹¹⁹Sn site. Combining the results of the concentration dependence of the transferred magnetic hf field and the ordering temperature with recent ac-magnetic susceptibility data reported on this system, a complete magnetic phase diagram is proposed. The effect of an external magnetic field (up to about 3 T) on the spin correlations in the spin-glass state is also discussed.     

A mean-field study on the thermo-magnetic properties of GdxCo1−x amorphous alloys (0.16⩽x⩽0.25)

We present a mean-field study on the thermo-magnetic properties of Gd_xCo_1−x amorphous alloys in the 0.16⩽x⩽0.25 composition range. A single set of exchange integrals and fixed values of the angular momenta of Gd and Co fairly describe the temperature dependence of magnetization. The magnetic specific heat and magnetic entropy show field and composition dependence. Both the specific heat anomaly and the saturated entropy, at the temperature of the magnetic phase transition, increase with increasing Co concentration. The two magnetic subnetworks and their cross-interactions contribute differently to the specific heat.