Amorphous Fe-P Research Articles

A critical discussion of some models for the concentration dependence of magnetic moments in amorphous and microcrystalline alloys of Fe, Co or Ni with P

The concept of localized moments as well as the opposite picture of itinerant band-magnetism have been applied up to the present day to explain the magnetism of alloys, leading to two different concepts for the concentration dependence of magnetic moments in 3d-TM-M alloys: The coordination bond model of Corb et al. considering the role of the local atomic coordination around the M-atoms and the band-gap theory of Malozemoff et al. which relates the magnetization of alloys with an averaged magnetic valence. A comparison of the magnetic moments of amorphous and microcrystalline Fe 100− x -, Co 100− x - and Ni 100− x P x (0 ≤ x ≤ 28) shows that the Fe-P system is excellent qualified for a critical discussion of the models. In addition to strong differences in the short range order between the microcrystalline and the amorphous phase the magnetic behaviour continuously changes from the weak ferromagnetism of pure bcc-Fe to the strong ferromagnetism of amorphous Fe-P. This offers the special opportunity to discuss the important influence of changes in the short range order on magnetic moments.

Effect of Yb on Electrodeposited Amorphous Fe-P Alloys

Effect of Yb on Electrodeposited Amorphous Fe-P Alloys

Magnetic and hyperfine interactions in amorphous Fe-P alloys

Amorphous alloys FexP100−x have been fabricated by rf sputtering in the composition range 50≤x≤75. Magnetic ordering temperatures were determined using thermogravimetric analysis (for x=75) and by zero-velocity thermal scanning (for 50≤x≤71). Ordering temperatures are depressed from those of the corresponding crystalline phases and range between 550 K for a-Fe75P25 and 10 K for a-Fe50P50. The Mössbauer spectra at 8 K of alloys with x≤71 all had a central unsplit line and broad, essentially featureless wings, indicating the coexistence of magnetically ordered and nonmagnetically ordered regions. The spectrum of a-Fe75P25 consisted of six broad lines, indicating that this composition is magnetically more homogeneous. Hyperfine-field distributions P(H) and mean isomer shifts (IS), determined using Window’s method, show that mean hyperfine fields have an essentially linear dependence on concentration and, by extrapolation, vanish at x∼46. The IS values at 293 K, however, have only small variations with composition and a weak maximum at x=67, whereas at 8 K the composition dependence is larger and there is a minimum at x=67. This behavior suggests a structural alteration whose nature is, as yet, unclear.

Generalized Slater–Pauling curve and the role of metalloids in Fe-based amorphous alloys

A modification of the generalized Slater–Pauling curve so as to consider the concentration dependence of the number of majority-spin sp electrons per average atom is proposed for amorphous iron-metalloid alloys. In this way an improved matching of the measured magnetic moment dependence on composition is achieved for Fe alloys with B and/or P as metalloids. Comparison of theory with experiment shows that amorphous Fe-P alloys tend to be magnetically rather strong, whereas their Fe-B counterparts are weak itinerant ferromagnets in almost the entire range of compositions.

Structural dependence of the magnetic moment in microcrystalline and amorphous Fe-P alloys

The magnetic moment per metal atom in electrodeposited microcrystalline (0 ≤ x ≤ 15) and amorphous ( x ≥ 15) Fe 100- x P x alloys was calculated from the saturation magnetization measured at 4.2 K. Whereas the moment of amorphous Fe-P shows a linear decrease with x, in microcrystalline Fe-P the moment per Fe-atom is independent of the phosphorus content and equals the value of crystalline α-Fe. In contrast to this finding in Co-P the magnetic moment per metal atom decreases linearly with increasing phosphorus concentration without any visible effect at the transition from the microcrystalline to the amorphous structure. This strange behaviour of the microcrystalline Fe-P is quite well described by the coordination bond model, which attributes the constancy of the moment per Fe-atom in bcc-Fe-metalloid alloys to the special coordination symmetry of the bcc-structure. Moreover, our results show that the band-gap theory is totally unsuitable to describe the dependence of the magnetization of Fe-M alloys.

Electrochemical preparation of amorphous Fe-P alloys

Optimum conditions have been established for the electrodeposition of amorphous fe-P alloys from sulphate electrolytes, containing complex-forming additives (glycine and oxalic acid) and sodium hypophosphite. It was shown that the increase of pH, current density and glycine content in the plating solution leads to a decrease of the amount of phosphorus in the Fe-P alloy. The cathodic current yield decreases with the increase of glycine concentration in the electrolyte. Small amounts of Cu2+ and Mn2+ act as brighteners. Sodium hypophosphite exerts a depolarizing effect on the alloy formation process. This effect is more pronounced at low hypophosphite concentrations and low cathodic current densities (CCD). The measurement of the cathodic potential during the deposition of the investigated alloy provides no evidence for a concentration change of phosphorus in the electrodeposited layers. No qualitative alterations of the surface morphology of the amorphous coatings studied have been established as the composition of the alloy and the plating conditions are changed.

The irreversible relaxation of magnetization and curie temperature in amorphous Co-P and Fe-P alloys

Irreversible structural relaxation in amorphous Fe-P and Co-P is accompanied by a small reduction of the saturation magnetization σ 0 of about 1%. Simultaneously the Curie temperature of Fe-P increases by at least 3 K independent of composition. The main contribution to the drop of σ 0 should come from the strengthening of the chemical bonds between metalloid and metal atoms as a consequence of a diffusionless rearrangement, but the influence of chemical decomposition must not be neglected. Decomposition does not cause the changes in T c because the Curie temperature of Fe-P is nearly independent of the phosphorus content. The shift of T c has probably to do with the pronounced irreversible densification of Fe-P upon annealing and a corresponding enhancement of the Fe-Fe correlation. The corresponding results obtained on Co-P fit to this picture though in this case the experimental resolution is insufficient to determine a change of T c smaller than 8 K.

Magnetovolume effect in transition metal-metalloid amorphous alloys

The thermal expansion, spontaneous volume magnetostriction ω s, forced volume magnetostriction ( ∂ω ∂H ) and Young's modulus of amorphous Fe-B, Fe-P, Co-B and (Fe-M) 77Si 10B 13 ( M = Cr, Mn, Co, Ni) alloys have been measured to make clear the magnetovolume effect in transition metal-metalloid amorphous alloys. The thermal expansion coefficient α, ω s and ( ∂ω ∂H ) are dependent on the number of d-electrons per transition metal atom n eff calculated based on the charge transfer model. The n eff vs. α, ω s and ( ∂ω ∂H ) curves are quite similar to the corresponding curves in fcc alloys. The maxima in those curves are, however, found at n eff ≈ 8.2 for the amorphous alloys in contrast with n eff ≈ 8.7 for the fcc Fe-Ni alloys. On the other hand, Young's modulus measured under the saturation of magnetization is governed by the molar volume, irrespective of n eff. The magnetovolume effect in transition metal-metalloid amorphous alloys is discussed in connection with the instability of ferromagnetism of amorphous Fe.

Radial magnetic anisotropy in amorphous electrolytically prepared CoP and FeP cylindrical layers

The presence of radial magnetic anisotropy with the local axis of the easy magnetization perpendicular to the surface of the layers may be identified in the amorphous cylindrical CoP layers by means of the study of the domain structure using the Bitter technique. This anisotropy is maintained also after undercritical isothermal annealing. In amorphous cylindrical FeP layers the radial magnetic anisotropy is formed — in contrast to CoP layers — only after isothermal annealing below the crystallization temperature. However, the amorphous-crystalline transformation leads in both layers to an apparent disturbance of radial anisotropy, which may be connected with the character of the crystallization of layers and with the new distribution of internal stresses.

Magnetoelastic effects and magnetic anisotropy in amorphous Fe-P alloys

The field dependences of elastic moduli were measured in electrodeposited amorphous Fe-P and Co-P tubes by resonance techniques at about 400 Hz and between 0.2 and 8 MHz. On account of magnetic anisotropies established during preparation (intrinsic anisotropy) and by field annealing, the magnetoelastic effects depended sensitively on the stress modes applied and the direction of the magnetizing field within the samples. The observations are consistent with a picture of the domain structure given in a previous paper Additionally it results that field annealing cannot destroy the intrinsic anisotropy.

Elastic properties and structural relaxation of amorphous Co-P and Fe-P alloys

The shear modulus G and Young's modulus E of electrodeposited amorphous Co-P and Fe-P tubes and the corresponding ultrasonic attenuation were measured between 0.1 and 10 MHz by a resonance technique. The accuracy was sufficient to evaluate the bulk modulus K and the Poisson ratio nu from G and E. The effect of structural relaxation on the elastic constants was investigated during continuous heating and several isothermal annealing programs. The quantitative analysis of the results is restrained because of the superposition of reversible and irreversible relaxation processes and because of the nearly unknown pre-annealing effects during the heating periods before isothermal treatment starts. But it is possible to derive some ideas concerning short-range order and structural relaxation from the different behaviour of the elastic moduli and the attenuation as functions of time. Relaxation is described mainly by a homogenisation of the material with respect to excess free volume. There are striking differences between the properties of the two alloy systems, particularly in the effects of structural relaxation on their elastic moduli and on their attenuation of sound, pointing to different short-range orders.

Crystallization of Fe-P amorphous alloys as studied by the EDXD method

The crystallization of amorphous Fe-P alloys, made by electrodeposition, were studied using energy dispersive X-ray diffraction (EDXD), thermomagnetic, and Mossbauer measurements. EDXD proved to be especially useful, because its very short exposure time enabled us to perform isothermal crystallization measurements with practically continuous control by means of diffraction spectra. By measuring the intensity of the appropriate diffraction lines, the crystallization kinetics of each phase could be recorded simultaneously. At 610 K the crystallization of an Fe81.2P18.8 amorphous alloy was found to start within 5 min with the primary crystallization ofα-iron. This step is followed by the eutectic crystallization of the stable phasesα-iron and Fe3P.

Magnetic domains in amorphous Fe-P alloys

Owing to magnetic anisotropy the internal domains within an electrodeposited Fe-P foil are magnetized perpendicular to the surface of the sample. We investigated the closure configuration with the aid of Bitter patterns and the Kerr effect. It consists of domains with nearly triangular cross section which are divided into subdomains.

Time effects in ferromagnetic amorphous Fe-P alloys

Additionally to previous investigations on amorphous Co-P the time variation of the initial permeability μ of amorphous Fe-P alloys was measured. The changes of μ observed in different temperature ranges below crystallisation were attributed to a superposition of several changes in short range order. According to this the measured curves were fitted by a superposition of functions used before to describe the results from Co-P. Moreover some of the physical processes driving the changes of μ could be explained.