Magnetic properties of amorphous rare-earth—Iron alloys

The magnetic properties of various amorphous alloys of the type R_1-xFe_x (R = Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu) have been determined in the concentration range 0.3 ? x ? 0.5. With the exception of the Gd and Lu alloys pronounced thermomagnetic history effects were observed in the temperature dependence of the magnetization. These effects are due to a strong temperature dependence of the coercive force (H_c) which is found to obey an exponential law of the form H_c ∞ exp(-αT). ⁵⁷Fe Mössbauer spectra were obtained on the alloys of a composition close to 40 at% Fe. From the combined results of the Mössbauer spectroscopy and magnetic measurements it is derived that the Fe moment decreases if one passes through the rare-earth series. It is postulated that this decrease is due to small differences in the compositional short-range order in the amorphous alloys caused by the heat of mixing becoming more negative in the same sense.     

Magnetic studies in amorphous GdxY1−x alloys

We report here our magnetic study on amorphous Gd_xY_1?x alloys with x = 0.17 and 0.70. The alloy with x = 0.17 is paramagnetic down to 4.2 K. For x = 0.70, magnetization shows a peak in low magnetic fields (H < 0.3 T). Magnetic saturation is difficult to obtain even for fields up to 15 T. An extrapolated value gives a moment of $\text{6.2 μ}{}_{\mathrm{B}}\text{Gd at}$. T_c is about 70 K. This led us to the conclusion that ferromagnetic and antiferromagnetic clusters are present in this alloy. The results are discussed by comparing them with crystalline Gd-Y alloys and amorphous Gd-Al, Gd-Au, etc. The effect of Y seems to be specific.     

Low temperature specific heat and magnetic properties of V/FE and V/FE/H alloys

Low temperature specific heats of V_1?xFe_x (0?x?0.34) and V_1?xFe_xH_n (x = 0.02; 0.05; 0.1; 0.15 a 0 ? n ? 0.98) were investigated between 1.5 and 16 K.In V/Fe alloys temperature independent band paramagnetism was observed at iron concentrations below 20at%; at higher iron contents local magnetic moments occur and contribute magnetic terms to the low temperature specific heat. According to Schröder [14] these contributions can be explained by thermal agitation of superparamagnetic clusters of the Fe atoms with local moments. In the temperature range of the specific heat measurements the magnetic terms are dependent on temperature (contrary to Schröder) and can be described by Planck-Einstein functions. A separation of the magnetic contributions from the lattice and electronic term of heat capacity could be obtained in a satisfying way by least squares fitting with three Planck-Einstein functions.In “nonmagnetic” V/Fe alloys with less than 20at% Fe absorption of hydrogen generates local magnetic moments and superparamagnetic contributions. These are, therefore, connected primarily more with the band electron concentration n_e, than with the iron content X_Fe At the highest n_e, values, n_e >0, large negative deviations from the regular course of the heat capacity plot are observed; these can be related to mictomagnetic behavior (freezing temperature of spin-glass state above the range of measurements, T_F > 16 K).After correction for the superparamagnetic contributions there remains in the Fe containing alloys an appreciable enhancement of the electronic heat coefficient γ, up to a factor of about 2 when compared with the iron-free V/H samples. This effect is interpreted as resulting from mass enhancements of the itinerant electrons by electron-magnon (and-paramagnon) interactions.     

Phase transition and extended X-ray absorption fine structure of melt-spun amorphous Fe100−xYx alloys

The phase diagram and local structure of melt-spun amorphous (a-) Fe_100−xY_x (22?x?62) alloys were investigated using AC and DC magnetic and extended X-ray absorption fine structure (EXAFS) measurements. The a-Fe–Y system shows reentrant spin glass (RSG) behavior for 42?x?58 and spin glass (SG) behavior for 60?x. Two SG transition temperatures, T_g and T_f, were obtained in the RSG state. The T_g, T_f and Curie temperature T_C decrease with increasing x, and the T_C and T_g vanish at x=60. A new magnetic phase diagram for the melt-spun a-Fe_100−xY_x alloys was obtained from magnetic measurements for higher Y concentration. The magnetic states of the a-Fe_100−xY_x alloys change remarkably around x=60 and an EXAFS study revealed that the average atomic distance between nearest-neighboring Fe atoms changes at approximately x=60.     

Study of the magnetic and calorimetric properties of (U1−xRx)Cu5Al (R=La and Y)

We have investigated the magnetic and thermodynamic properties of the (U_1−xR_x)Cu₅Al (R=La and Y) series. The results show a quick decrease of T_N for both La and Y substitutions. For the Y series with concentration between 0.3≤x≤0.4, the temperature dependence of the specific heat and magnetic susceptibility below 10 K are consistent with a non-Fermi-liquid regime.     

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.     

Magnetic properties of Cr-Fe-Mn alloys

The magnetic behaviour of a Cr_80−xFe₂₀Mn_x alloy system with x=2, 7, 10, 13 and 22 has been investigated in the temperature range 2-400 K through measurements of magnetization, electrical resistivity, magnetoresistivity, specific heat and thermal expansion. The temperature vs. Mn concentration magnetic phase diagram of the system is rich in magnetic behaviour with ferromagnetic (FM), antiferromagnetic (AFM) and paramagnetic phase regions and a spin-glass (SG) region at the lowest temperatures. Phase transition temperatures amongst these different magnetic phases could be identified from well-defined anomalies of magnetic origin that are displayed by graphs of the above-mentioned physical properties as a function of temperature. The time relaxation of the thermoremanent, isothermal remanent and field-cooled magnetizations below and above the SG freezing temperature show unusual aspects. These relaxations do not follow the usual superposition principle that is expected for typical SG materials. Negative giant magetoresistance (GMR) is observed in the alloys at 4 K. The GMR initially increases sharply on increasing the Mn content in the alloy system, followed by a tendency towards a saturation negative value for concentrations of more than about 10 at% Mn. Low-temperature plots of C_p/T vs. T², where C_p is the specific heat, present anomalous behaviour for alloys with x=2, 10 and 22. For x=2 the plot shows an upturn at the lowest temperatures that changes over to a prominent downturn for x=10 and 22. This behaviour is attributed to Fe concentration fluctuations in the alloys, confirming the theoretical model of Matthews.     

Specific heat of Na1−x V2O5 single crystals

Temperature dependences of the specific heat C and the magnetic susceptibility χ of Na_1?x V₂O₅ single crystals (x=0, 0.01, 0.02, 0.03, and 0.04) are studied. In NaV₂O₅, the transition to the spin-gap state (T _c =34 K) is accompanied by a sharp decrease in χ, while C exhibits a λ-shaped anomaly. At low temperatures, the specific heat of NaV₂O₅ is approximated by the sum of phonon ～T ³ and magnon ～exp(?Δ/T) contributions, which makes it possible to estimate the Debye temperature Θ_D=336 K and the gap in the magnetic excitation spectrum Δ=112 K. With the departure from stoichiometry, the anomalies observed in the behavior of χ and C are spread and shifted to lower temperatures. The low-temperature specific heat of nonstoichiometric samples is determined by the sum of phonon and magnon components and the contribution due to the presence of defects. The values of magnetic entropy characterizing the phase transitions in Na_1?x V₂O₅ are calculated.     

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.     

Structural and magnetic studies of the alloy system GdInxAg1-x

The structural and magnetic studies below room temperature of the alloy system GdIn_xAg_1-x(0?x?1) are reported. It has been found that alloys with x?0.5 crystallize in CsCl type cubic structure, but the distortion of the unit cell starts for alloys with x#62;0.5 and for these alloys the unit cell becomes tetragonal. No phase change is evident from the low temperature (295-8 K) structural studies. However, a break appears in the X^-1_m vs. T linear plot of each alloy of this system at a specific temperature (designated as break temperature T_B). The variation of T_B with x is similar to the variation of phase transition temperature with x reported for LaIn_xAg_1-x. Close agreement has been found in the values of effective magneton number (p), magnetic ordering [Néel (T_N) or Curie (T_C)] and paramagnetic Curie (θ_p) temperature for materials studied by us and earlier workers. The variation of magnetization with applied field strength (2.5-65)×10⁵ Am^-1) at 4.2 K has also been reported for ferromagnetic of this system. It has been concluded that alloys with 0.4?x?0.6 are simple ferromagnets with parallel alignment of magnetic moment in the ground state. The angular arrangement of the magnetic moment starts appearing in the ground state for alloys with x?0.4 for x#62;0.6 and continues till x becomes closure to 0.17 or 0.84. The alloys with x=0.17 or x=0.84 have θ_p and T_C equal to zero and appear paramagnetic. Angular arrangement in spins again appear for alloys with x?0.17 or x#62;0.84, however all materials with 0?x?0.17 or 0.84?x?1 remain antiferromagnetic.     

Superconductivity in zirconium-nickel glasses

The superconducting transition temperatures (T_c) and magnetic susceptibilities of amorphous Zr_100?xNi_x alloys have been measured. T_c decreases linearly with increasing x. The results are compared to those for amorphous ZrPd and ZrCu alloys and discussed in terms of changes in the electron to atom ratio on alloying.     

Vibrational spectra of crystalline Bi1−xSbx alloys

Second order Raman scattering is reported at 300 K in crystalline Bi_1?x Sb_x alloys. The spectra, which are interpreted in terms of dominant overtone processes, indicate variations in the phonon density of states for the optical bands with composition. The form of the spectra, which include contributions from Bi-Bi, Bi-Sb and Sb-Sb bonds, implies microscopic statistical clustering effects analogous to those in Ge_1?xSi_x alloys. A comparison of the spectra with inelastic tunneling results in superconducting amorphous Bi_1?xSb_x indicates covalent bonding contributions in the latter.     

Universal conductivity variation in glassy Zr-M alloys

Temperature dependence (3–300 K) of the electrical conductivity in a number of amorphous Zr_1?xM_x alloys (M = Cu, Ni, Co and Fe, 0.19 < x < 0.71) has been analysed in some detail. Like in some other alloys with a high electrical resistivity, the conductivity varies as T at lower temperatures (T < 80 K) and √T at higher temperatures. A new feature observed is that the ratio of the coefficients of a low temperature T and a high temperature √T conductivity variation is practically constant for allalloys. Therefore a universal conductivity-temperature curve can be constructed for all amorphous Zr_1?xM_x alloys with the resistivity higher than 150 μohms cm. These results are consistent with the effects of incipient localisation and indicate that the electron-phonon coupling determines the conductivity variation.     

BCS-like behaviour in the superconducting state of itinerant antiferromagnetic CrRe alloys

Measurements of the heat capacity and the magnetic susceptibility have revealed BCS-like behaviour in the superconducting state of itinerant antiferromagnetic Cr_1?xRe_x alloys for x = 0.30) and 0.26. The thermodynamic quantities, such as electronic heat capacity and thermodynamic critical field have been reproduced with the BCS theory with the energy gap Δ = (1.76 ± 0.05)k_BT_s, where T_S is the superconducting transition temperature for the corresponding system: T_S = 3.61 K (2.35 K) for x = 0.30 (0.26).     

Unexpected magnetic behaviour in a singlet crystal field ground state induced by Tb 4f-instability

The magnetic susceptibility () of Th_1-x Tb _x alloys (with 5<x<55a/o Tb) in the range of 1 K<T<350K and the specific heat (forx=1.5a/o Tb) for 0.5<T<10 K were measured. A maximum in (T) was observed at a temperature which depends on concentration. The crystal field level scheme that fits the specific heat measurements shows two non-magnetic singlets as lowest levels. Such ambiguity is discussed within the framework of the 4f-Tb instability. The results are in agreement with the matrix superconductiveT _c decrease. The contribution of the interconfigurational mixing on and the specific heat is also discussed.This work was partially supported by the KfA-CNEA cooperation program and by the Deutsche Farschungsgemeinschaft through SFB125     

Magnetic properties and hydrostatic pressure effect on the curie temperature of (Co,Mn)2B amorphous alloys

The temperature dependence of magnetization and magnetic susceptibility and hydrostatic pressure effect on the Surie temperature (dT_c/dP) are measured for (Co_1-xMn_x)₂B (0?x?0.4) amorphous alloys and the results are compared with those of crystalline compounds with the same composition. The Curie temperature decreases linearly with an increasing Mn content but magnetization shows a maximum around x=0.15. The reciprocal magnetic susceptibility of all the prepared alloys obeys the Curie-Weiss law above T_c. The magnitude of the negative value of dT_c/dP decreases linearly with increasing x from about 1.1 K/kbar (x=0) to zero (x=0.4), the composition dependence of which is opposite to that of the crystalline compound. The composition dependence of the average magnetic moment per transition metal atom and the Curie temperature and dT_c/dP are analysed on the basis of the local environment and the pair order interaction mode, respectively.     

Effect of compositional variation on the soft magnetic properties of Fe(87−x−y)CoxTi7 Zr6By amorphous ribbons

The effect of the replacement of Fe by Co or B on the thermal stability and soft magnetic properties of the Fe-based amorphous metallic ribbons with Fe_(87−x−y)Co_xTi₇Zr₆B_y (x = 10, 20% and y = 8, 10, 12%) produced by melt-spinning technique was investigated. For the melt-spun amorphous ribbons, the values of saturation magnetization and coercivity were observed to range from 107.00 to 152.38 emu/g and from 0.012 to 0.446 Oe, respectively. The thermal properties such as T_g, T_x, and ΔT_x were in the range of 796.7–809.6 K, 840.2–853.5 K, and 35.8–54.5 K, respectively. In the Fe–Co–Ti–Zr–B alloys, the Co substitution for Fe improved the soft magnetic properties but decreased the thermal stability. For magnetic properties, the coercivity (H_c) decreased and saturation magnetization (M_s) increased by the addition of Co. However, the supercooled liquid region (ΔT_x) decreased by the addition of Co. Meanwhile, the B substitution for Fe had no meaningful change on the thermal stability and soft magnetic properties. The amorphous ribbon of Fe₅₉Co₂₀Ti₇Zr₆B₈ exhibited the best soft magnetic properties such as the low coercivity of 0.025 Oe and the high saturation magnetization of 152.38 emu/g.     

Re-entrant magnetic behaviour in amorphous Fe-rich Fe100−x Zr x alloys

We report results of⁵⁷Fe Mössbauer measurements on the magnetically concentrated re-entrant amorphous Fe₁₀₀?xZr _x alloys (8≤x≤12) which give convincing evidence for a microscopic origin of the low-temperature magnetization anomaly atT _f<T _c. This is shown by an anomalous increase in the magnetic hyperfine field and in the relative intensities of the (?m=0)-Mössbauer lines belowT _f upon cooling. No evidence was found for an antiferromagnetic phase. The shape of the hyperfine-field distribution atT=4.2 K in zero external field and in a field of 3 T is unchanged, indicating homogeneous behaviour of all Fe moments near saturation. Ferromagnetic order is not long-ranged, but determined by exchange-coupled magnetic clusters. The cluster moment nearT _c is found to decrease strongly with increasing Fe content and extrapolates to zero atx≈96.