Amorphous Fe-Zr Research Articles

Nonlinear scaling behaviour of magnetization in amorphous ferromagnets

The ‘in-field’ magnetization of amorphous Fe91Zr9, Fe90Zr10 and Fe89Co1Zr10 alloys obeys the scaling equation of state in non-linear variables ε̃ = (T − TC)/T and h̃ = H(TC/T) (TC = Curie point) from T ⋍ 0.4TC to T ⋍ 1.4TC while initial susceptibility follows the generalized Curie-Weiss law from T ⋍ TC to T ⋍ 1.4TC. Non-analytic corrections to the singular behaviour at TC dominate over the analytic ones in the critical region but the reverse is true for T ⪢ TC.

Noncollinear magnetic structures in amorphous iron and iron-based alloys

We present a novel technique for calculating noncollinear spin structures in itinerant magnets. Our approach is based on a tight-binding Hubbard Hamiltonian constructed by a canonical transformation of a self-consistent spin-polarized linear muffin-tin orbital Hamiltonian to a tight-binding form, and on a real-space recursion technique for the self-consistent calculation of the local exchange splitting and local moments. Applications to amorphous Fe at different densities and to Fe-Zr alloys of different compositions are presented. Our results show that with increasing density amorphous Fe shows a continuous transition from a ferromagnetic to an asperomagnetic and finally to a speromagnetic (spin-glass-like) state. This spin-glass state exists over a considerable range of densities before the system becomes paramagnetic. Similar transitions are observed in amorphous Fe-Zr alloys with increasing Fe content.

Spin-Glass Behavior in Amorphous Ti–Fe Alloys Prepared by High-Rate Sputtering

Ti x F 100-x alloy system has been considered to be ferromagnetic. However, it has been confirmed by measurements of the AC magnetic susceptibility and the magnetic cooling effect that the amorphous Ti 50 Fe 50 alloy exhibits the paramagnetic-spin glass transition. The magnetic phase diagram of this system is similar to that of amorphous Zr-Fe alloy system. It is suggested that the triple point consisted of the paramagnetic state, the ferromagnetic state and the spin-glass state is expected to exist in the range from 40 to 45 at.%Ti. The magnetization curves show a relatively easy saturation. The Curie temperature and the magnetic moment per Fe atom decrease with increasing Ti concentration, being similar to the results reported previously.

Temperature dependence of the iron hyperfine field distribution in amorphous Fe-rich Fe-Zr alloys

The temperature dependence of the iron hyperfine field distribution is reported in melt-quenched amorphous Fe-Zr alloys. The most remarkable feature is the compositinal change in the shape of the average hyperfine field versus temperature curves. The unusual increase in the average hyperfine field below about 85 K is a characteristic feature of the RSG systems; however, no anomaly is observed in the width of the hyperfine field distribution as a function of temperature. The results cannot be properly explained in the framework of the existing spin glass models.

Mössbauer study of spin states in amorphous Fe100−x Zr x alloys

Spin states in amorphous Fe100−x Zrx (7≤x≤12) are investigated using hyperfine field distributions (HFDs) as derived from57Fe transmission Mossbauer experiments. The existence of low-, medium- and high-spin states is demonstrated by three-dimensional projections of HFDs. Temperature dependences of the corresponding average values revealed anomalous behaviour in the vicinity ofT f<Tc. An increase in the hyperfine magnetic fields observed towards low temperatures is discussed in terms of a ferromagnet-to-spin-glass transition. Utilizing also theT c values, which were obtained from a thermal scanning Mossbauer technique, we propose a diagram of spin states in amorphous Fe-Zr.

Transverse spin freezing in Fe92.5Hf7.5

Amorphous Fe92.5Hf7.5 has been studied by Mossbauer spectroscopy and magnetization measurements. Transverse spin freezing has been observed at 47 ± 8 K, and the results are compared with those for amorphous iron-rich Fe-Zr alloys in which transverse spin freezing is also observed.

Study of the spin glass transition of amorphous FeZr alloys using small angle neutron scattering

SANS experiments of an amorphous Fe90Zr10 alloy were carried out in the temperature region 20–300 K with and without the application of a magnetic field. The scattering versus temperature for different Q values (zero field cooled magnetization) shows two transitions at Tc=240 K and Tf=40 K. However, both transitions are not observed in magnetic field (2.6 kG) cooling down curves. By switching off the magnetic field and warming up the sample only the transition at Tf=40 K is observed. Application of a magnetic field at different temperatures in a zero field cooled sample reduces the scattering; the scattering returns to its initial value after switching off the field.

Mössbauer study of ion-beam mixing of Fe/Zr multilayers

The Ar-ion-beam mixing of Fe/Zr multilayers is studied in detail by conversion electron Mössbauer spectroscopy (CEMS) and x-ray diffraction (XRD). The dependence of the ion-beam induced amorphization and interfacial mixing on the sublayer thickness and ion dose (1×1013–2×1016 Ar/cm2) is studied systematically for samples with Fe to Zr thickness ratios dFe/dZr=1 and 0.5 and modulation wavelengths Λ=dFe+dZr of 5–80 nm and 7.5–90 nm, respectively. The CEMS results allowed the evaluation of the mixing efficiency from the thickness of the mixed layers. The experimentally determined mixing efficiency was compared with theoretical estimates based on the ballistic collision and thermal spike models, showing good agreement with the predictions of the modified ballistic collision model. For high degrees of amorphization the composition of the amorphous phase formed due to ion-beam mixing is close to the nominal composition of the sample, as revealed by CEMS measurements. These results were compared with those obtained for amorphous Fe-Zr alloys formed by vapor deposition. The XRD results fully agree with CEMS measurements and show that due to ion irradiation the amorphous Fe-Zr phase is formed. The XRD results show that a change of texture occurs from Zr(002) to Zr(100) in the samples with small Λ irradiated with high ion dose. XRD reveals in these samples the formation of the ZrO2 phase.

Magnetic softening of young's modulus of amorphous Fe90Zr10

The Young's modulus and the internal friction of amorphous Fe90Zr10 alloy were measured near the Curie temperature using the vibrating reed technique. The modulus shows softening around Tc(= 227 K) and the internal friction undergoes drastic increase in the same temperature range. It is found that both the Young's modulus and the reciprocal of internal friction are inversely proportional to the magnetic susceptibility in the paramagnetic phase.

Field Induced Magnetic Moments in Amorphous Fe-Zr Spin-Glass-Like Alloys

Field Induced Magnetic Moments in Amorphous Fe-Zr Spin-Glass-Like Alloys

Field induced magnetic moments in amorphous Fe-Zr spin-glass-like alloys.

Rotation of the Fe magnetic moments toward the collinear state in applied magnetic fields up to 7 T was studied by $^{57}\mathrm{Fe}$ M\ossbauer spectroscopy at 4.2 K. The investigated ${\mathrm{Fe}}_{100\ensuremath{-}x}{\mathrm{Zr}}_{x}$ ($7\ensuremath{\le}x\ensuremath{\le}12$) amorphous alloys show spin glass, reentrant spin glass, and ferromagnetic behavior at different concentrations. The collinear state was reached in all the alloys below 7 T. Large high field susceptibilities corresponding to more than 0.1${\mathrm{\ensuremath{\simeq}}}_{\mathrm{B}}$ increase of the average Fe magnetic moments were deduced from the hyperfine fields. The results are in good agreement with bulk magnetic measurements and cannot be explained by antiferromagnetically aligned Fe moments.

Cluster spin-glass model for amorphous Fe-Zr alloys near the critical concentration: a magnetization study

The magnetic properties of melt-quenched amorphous Fe 100− x Zr x ( x = 7-12) alloys were studied in the temperature and magnetic field ranges 15 K < T < 300 K and 0 < H ext < 1.8 T, respectively. Assuming that Fe 93Zr 7 is an assembly of magnetic clusters with a blocking temperature distribution, the magnetization versus magnetic field curves were fitted using Néel's theory for fine particles at several temperatures in order to obtain the evolution of cluster parameters with temperature. The same model is used for Fe 92Zr 8 and Fe 88Zr 12 above the Curie temperature ( T C). When Néels theory is modified to account for the interaction between the cluster moments, the experimental data are properly described by this frame. This model can explain both the low-temperature anomalies of susceptibility observed for x = 7-10 and the high-temperature superparamagnetic behaviour which is shown to be a characteristic feature of the whole a-Fe 100− x Zr x alloy series. The evolution of magnetic cluster behaviour with temperature and concentration is discussed.

Deformation induced crystallization due to instability in amorphous FeZr alloys

Recently, it was shown that mechanical deformation processes can completely crystallize some amorphous transition-metal–metalloid alloys. However, the mechanism is still unclear. We demonstrate here that, for the simpler amorphous Fe90Zr10 alloy, the basic thermodynamic differences between high-energy milling and rapid quenching, associated with a crystallization controlled by elemental diffusion, could be behind this structural transformation.

Effects of exchange bond disorder on transport measurements

Measurements of the low field resistive anisotropy (LFRA) in amorphous Fe91Zr9 with and without hydrogen loading are presented. The temperature dependence of these data is compared with model calculations of the quadrupolar term [〈〈S2Z〉T〉J−S(S+1)/3], from which it is concluded that the model calculations reproduce both the temperature and the exchange bond dependence of this transport coefficient.

Law of approach to magnetic saturation in inhomogeneous ferri- and antiferromagnets and in amorphous iron-rich Fe-Zr alloys

The magnetization curve of a two-phase two-sublattice model is calculated analytically. In one phase a ferromagnetic coupling is assumed which contributes to the total magnetization whereas in the other phase a ferrimagnetic interaction takes place. Within the framework of a molecular field theory the magnetization curve of this ferrimagnetic phase is calculated assuming a certain distribution of the molecular field interactions. The law of approach to magnetic saturation yields for the leading term a H −1 -dependence in contrast to fluctuations of the local magnetic anisotropy which in general yield a H − built1 2 -law. The calculations of the two-phase two-sublattice model agree well with measurements of the high-field magnetization curve of amorphous Fe 1− x Zr x -alloys with 0.076 ≤ x ≤ 0.11 showing that with decreasing Zr-content an increase of the ferrimagnetic phase exists.

The role of boron in nanocrystalline FeZrB soft magnetic alloys

The microstructure and soft magnetic properties of crystallization-induced alloys in FeZr and FeZrB systems were investigated. Melt-spun amorphous Fe 92Zr 8 and Fe 91Zr 7B 2 alloys were found to crystallize through two distinct stages; the first and second stage are due to primary crystallization of a nanocrystalline b.c.c. phase and crystallization of a residual solute-enriched amorphous phase into Fe 3Zr and Fe 2Zr respectively. The addition of B into amorphous FeZr alloys causes the refinement of the primary b.c.c. crystals, resulting in a significant improvement of the soft magnetic properties of the annealing-induced alloys. Grain coarsening of the nanocrystalline b.c.c. phase as well as the formation of compounds is suppressed by the addition of B. This is presumably of a significant increase in the crystallization temperature of the residual amorphous phase resulting from the combined redistribution of Zr and B elements between the primary and parent amorphous phases.

Ferromagnetism and spin glass in amorphous FeZr alloys

Magnetic properties of the amorphous FeZr alloys have been investigated on the basis of a finite temperature theory of magnetism for amorphous alloys with strong local environment effects. The calculated magnetic phase diagram and magnetization vs. concentration curve explain well those obtained from experimental data, exhibiting a spin-glass ferromagnet transition. It is demonstrated that the spin-glass behaviour near the amorphous pure Fe is mainly caused by the competition between the short-range ferromagnetic interactions and long-range antiferromagnetic interactions via structural disorder, while the formation of ferromagnetism with increasing Zr concentration originates in the band narrowing at the Fe site, which is caused by a reduction in average coordination number of Fe atom due to the difference between Fe and Zr atomic sizes. The re-entrant spin-glass behaviour in Fe-rich amorphous alloys is shown to be realized by the thermal spin fluctuations of amplitude of local magnetic moment.

Magnetic properties and core losses of nanocrystalline FeMB (M  Zr, Hf or Nb) alloys

A mostly single b.c.c. phase with nanoscale grain sizes of 10–20 nm was found to form by annealing amorphous FeZrB, FeHfB and FeNbB alloys prepared by the melt-spinning method for 3.6 ks in the temperature range 773–923 K. The highest μ c and B s values reach 22 000 and 1.63 T for Fe 90Zr 7B 3, 32 000 and 1.59 T for Fe 89Hf 7B 4 and 22 000 and 1.49 T for Fe 84Nb 7B 9 alloys. The low core loss of 0.066 W kg −1 at 1 T and 50 Hz was confirmed for the b.c.c. Fe 89Hf 7B 4 alloy and is considerably smaller than that of a commercial Fe-based amorphous alloy being used in practical uses as core materials in transformer. The magnetostriction λ s of FeMB alloys shows a tendency to decrease with increasing annealing temperature and is as small as −1 × 10 −6 for the b.c.c. Fe 89Hf 7B 4 alloy. The small λ s and the small grain size are concluded to be the reasons for the good soft magnetic properties. The residual existence of the amorphous phase with a large amount of solute elements in the grain boundaries of the b.c.c. phase was observed by high resolution transmission electron microscopy observations on the Fe 84Nb 7B 9 alloy annealed for 3.6 ks at 923 K and is presumed to suppress the grain growth of the nanocrystalline b.c.c. phase.

Influence of boron on the magnetic and transport properties of FeZr amorphous and nanocrystalline alloys

The magnetic properties and electrical resistivity of amorphous and nanocrystalline FeZr and FeZrB(Cu) alloys are compared in a wide range of temperatures (4 to 1000 K). The addition of boron increases the Curie temperature of the alloys and induces a broad minimum in the resistivity vs temperatures. A first step of crystallization occurs around 700 K in all the alloys, giving rise to /spl alpha/-Fe crystallites of very small size. Small amounts of boron greatly influence the exchange interactions, enhancing the ferromagnetic character of these compounds.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Finite-temperature theory of amorphous magnetic alloys.

A finite-temperature theory of magnetism in amorphous magnetic alloys with strong environment effects is developed on the basis of the functional integral method for thermal spin fluctuations and the distribution-function method for random distribution of local magnetic moments. The theory drastically simplifies numerical calculations by means of the geometrical-mean model for amorphous structure as well as electronic structure, and allows us to investigate the magnetism in amorphous transition-metal alloys with large difference in atomic size via average coordination number which depends on the type of central atom. Numerical example is given of the amorphous Fe-Zr alloys. It is demonstrated that the theory reproduces quantitatively the local densities of states obtained from the first-principles calculations, and describes the magnetic phase diagram, in particular, the itinerant-electron spin glass. It is also shown that the atomic-size effects play an important role in the formation of ferromagnetism in concentrated Fe-Zr amorphous alloys. The reentrant spin-glass behavior around 90 at. % Fe is shown to be due to the thermal spin fluctuations of amplitudes of local magnetic moments.