Temperature dependence of the hyperfine field distribution in an amorphous ferromagnet

The rapid temperature decrease in magnetization characteristic of amorphous alloys is often attributed to short-range exchange interactions. The temperature dependence of the hyperfine field distribution in an amorphous Fe₈₀B₂₀ (METGLAS 2605) measured by Mössbauer technique disagrees with such an explanation. It is shown that for this alloy each magnetic moment follows the same magnetization curve, i.e. the temperature dependence of the magnetization is determined by long-range correlations.     

Evaluation of the iron hyperfine field distribution in amorphous Fe90Zr10

The separated 2–5 lines of the Mössbauer spectrum of amorphous Fe₉₀Zr₁₀ alloys has been obtained from the linear combination of the spectra of unpolarized and magnetically polarized samples. The hyperfine field distribution has been determined from the original spectra and from the separated 2–5 lines using different evaluation methods (binomial distribution method. Fourier method and Window method).     

Temperature dependence of the hyperfine field of Tc in Fe

Using BaF₂ scintillation counters we made a series of DPAC measurements on⁹⁹TcFe in the temperature range 77 < T < 1020 K. Our results at room temperature are consistent with earlier measurements of Raghavan and Raghavan. Contrary to earlier suppositions, H_hf(T) does not have an anomalous temperature dependence like that of OsFe.     

Temperature dependence of the hyperfine field in amorphous Fe2O3

Mössbauer spectroscopy was used to follow the hyperfine field at ⁵⁷Fe nuclei in amorphous Fe₂O₃. The value of H_hf at T = 0, 470 kG, indicates $\text{J}\text{=}\text{5}\text{2}$ for the Fe ions, while the reduced hyperfine field versus reduced temperature closely follows a $\text{J}\text{=}\text{1}\text{2}$ Brillouin function. This result is at variance with theoretical predictions for a spin glass and is also different from amorphous metals with a high content of magnetic ions as reported in the literature. Paramagnetic Mössbauer spectra of amorphous mixed oxides of Fe₂O₃ with ZnO and CoO show that the ⁵⁷Fe nuclei in all amorphous ferric oxides studied so far are coordinated in a manner similar to the d site in β F₂O₃, and distinctly different from the coordination in their crystalline form.     

Critical behavior of electrical resistivity in amorphous Fe-Zr alloys

Electrical resistivity (ρ) of the amorphous (a-)Fe_100−c Zr _c (c=8.5, 9.5 and 10) alloys has been measured in the temperature range 77 to 300 K, which embraces the second-order magnetic phase transition at the Curie temperature point T _c. Analysis of the resistivity data particularly in the critical region reveals that these systems have a much wider range of critical region compared to other crystalline ferromagnetic materials. The value of T _c and specific heat critical exponent, α has the same values as those determined from our earlier magnetic measurements. The value of α for all the present investigated alloys are in close agreement with the values predicted for three-dimensional (3D) Heisenberg ferromagnet systems, which gives contradiction to the earlier results on similar alloys. It is observed from the analysis that the presence of quenched disorder does not have any influence on critical behavior.     

The hyperfine magnetic field in Fe-Hf amorphous alloys at low temperatures

The time-differential perturbed angular correlation (TDPAC) technique was applied to study the hyperfine magnetic field in amorphous Fe-Hf alloys with different Hf contents at low temperatures. The Hf content was changed from 20 to 40 at% andthe TDPAC measurements were carried out in the temperature range from room temperature down to 18 K. Very strong perturbations were observed for the specimen of low Hf content at low temperatures due to a strong hyperfine magnetic field. The characteristic feature observed by the present study is that the temperature dependence of the perturbation for the Fe-30 at% Hf source is quite different from that for other sources, indicating a maximum around 80 K. Discussions have been given on the explanation of the experimental results in the light of magnetic properties of these alloys.     

Dynamic susceptibility of reentrant Fe-rich inhomogeneous amorphous alloys

Melt-spun amorphous alloys of Fe₉₁Zr₉, Fe₉₁Zr₇B₂, Fe₉₀Zr₇B₃ and Fe₈₈Zr₈B₄ have been characterized by AC susceptibility at frequencies between 30 Hz and 10 KHz. The measurements reflect the existence of reentrant spin glass transitions below 40 K for all cases. The transition shift per frequency decade is large compared to the observed in conventional spin-glass alloys. The shift value increases with the boron content and becomes closer to values in fine-particle systems. We find that the relaxation in Fe₉₁Zr₉ follows a critical slowing down at the reentrant transition, with exponents and . The non-linear susceptibility displays a peak at the transition in this alloy, but wider than in canonical spin glasses. A Vogel-Fulcher (VF) activation process can explain the frequency variation in all the Fe-Zr-B alloys. The reduction of the ideal glass temperature in the VF approach, found in the higher Boron content alloys, is an indication of a superparamagnetic-like behavior. The behavior shown by these alloys is intermediate between a collective freezing and superparamagnetic-like relaxation. We propose that this is arising due to a heterogeneous spin structure. The inclusion of a reduced amount of B, affects the magnetic spin structure.Received: 13 May 2003, Published online: 22 September 2003PACS: 75.50.Lk Spin glasses and other random magnets - 75.50.Kj Amorphous and quasicrystalline magnetic materials - 75.40.Gb Dynamic properties (dynamic susceptibility, spin waves, spin diffusion, dynamic scaling, etc.)J.S. Garitaonandia: Present address: Dept. Física Aplicada, F. Ciencias, Universidad del País Vasco, Bilbao 48080, Spain     

Giant forced volume magnetostriction of Fe-rich iron-cobalt-zirconium amorphous alloys

The forced volume magnetostriction has been measured as a function of composition and temperature in amorphous (Fe_1?xCo_x)₉₀Zr₁₀ alloys. The maximum value of $\text{δω}\text{δH}$ (T = 0 K), equal to 285 × 10^-10 Oe^-1, has been observed for Fe₉₀Zr₁₀ alloys. The effect of pressure on the magnetization σ₀ at T = 0 K was calculated from $\text{δω}\text{δH}$ data. The pressure dependence of the Curie temperature T_c and σ₀ behaves in a similar way as those observed for crystalline NiPd and NiRh alloys.     

Temperature dependence of the magnetic hyperfine field at cerium impurity in Co

Perturbed gamma-gamma angular correlation (PAC) technique was used to measure the magnetic hyperfine field (B _hf ) at Ce impurity in Co using ¹⁴⁰La→ ¹⁴⁰Ce probe. The radioactive ¹⁴⁰La produced by neutron irradiation of lanthanum metal with thermal neutrons was introduced in Co by arc melting in argon atmosphere. The present measurements cover the temperature range from 4.2– 1300 K. Two pure magnetic interactions were observed at impurity sites, corresponding to a ferromagnetic ordering of Co moments in hcp and fcc phases. The temperature dependence of B _hf for both phases, however, shows a sharp deviation from an expected standard Brillouin-like behavior for the host magnetization. The results are discussed in terms of a simple molecular-field model where the localized moment at impurity ions as well as the conduction electron contributions to the hyperfine field are taken into account.     

Mössbauer study of hyperfine field relaxations in soft magnetic amorphous alloys

Superimposed asymptotic exponential relaxation between 125 and 200°C of average hyperfine inductions and their orientations were detected by the⁵⁷Fe Mössbauer spectroscopy in the ferromagnetic Fe₄₀Ni₄₀B₂₀ and Fe₇₀Co₁₀B₂₀ amorphous alloys. From their Arrhenius type temperature dependence, average activation enthalpies ΔH were derived in agreement with the resistometric and coercivity data. The interpretation is proposed in terms of individual irreversible processes: stress relief, free volume shrinking and short-range ordering.     

Temperature dependence of57Fe hyperfine field in Gd: YIG

The NMR spectra of⁵⁷Fe in Gd: YTG polyeryslallaline samples were measured between liquid-helium and room temperatures. In spectra of⁵⁷Fe on tetrahedral sites, beside the main line, weaker and broader peak at higher frequencies appears. It can be decomposed in to the three satellites corresponding to Fe³⁺ ion with one Gd³⁺ ion in the nearest (two satellites) and next nearest dodecahedral site. The temperature dependence of the stellite resonance frequency indicates, that the part of the transferred hyperfine field on⁵⁷Fe with the Gd³⁺ in the nearest c-site depends on the magnetic moment of Gd³⁺ ion. The resonance frequency of the satellite corresponding to the⁵⁷Fe with Gd³⁺ in the next nearest c-site follows the temperature dependence of the dipolar field, the change of the transferred field is small.     

Mixed hyperfine interaction in amorphous Fe-Zr sputtered films in external magnetic field — A57Fe Mössbauer study

Conventional⁵⁷Fe-Mössbauer spectroscopy provides only information about the magnitude of the splitting QS in the case of electric quadrupole hyperfine interaction, but not on the sign of the main component of the electric field gradient (EFG), V_zz, or the asymmetry parameter, η, which are sensitive to the local environment of the⁵⁷Fe nuclei. This kind of information is obtained by measurements in external magnetic fields. In the case of amorphous Fe-Zr sputtered films mixed hyperfine interaction leads to a clear change in the behaviour of the Zr-rich and the Fe-rich alloys, indicating the existence of magnetic clusters in the Fe-rich samples.     

Saturation and forced volume magnetostriction of Fe-rich FeZr amorphous alloys

AC susceptibility, saturation and forced volume magnetostriction were studied on iron-rich zirconium amorphous alloys a-Fe_100-xZr_x (8 at% ≤ x ≤ 12 at%). The experimental results are discussed in relation to the re-entrant spin glass state. It is found that the transverse magnetostriction accompanies a remarkable magnetic aftereffect in the spin glass phase, and that the forced volume magnetostriction shows an apparent peak at the ferromagnet to spin glass transition where the ac susceptibility has a cusp.