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Register a new userMagnetic phase diagrams in the H-T plane of the magnetically strongest sigma-phase Fe-V compounds
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Magnetization measurements were performed on two sigma-phase samples of Fe(100-x)V(x) (x=35.5, 34.1) vs. temperature, T, and in DC magnetic field, of various amplitudes. Using three characteristic temperatures, magnetic phase diagrams in the H-T plane have been designed testifying to a re-entrant character of magnetism. The ground magnetic state, a spin glass (SG), was evidenced to be composed of two sub phases: one with a weak irreversibility and the other with a strong irreversibility. Two critical lines were reconstructed within the SG state. Both of them show a crossover from the Gabay-Toulouse behavior (low field) to a linear and/or quasi-Almeida-Touless behavior. A strong difference in the effect of the applied magnetic field on the SG phase in the two samples was revealed.
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Magnetization measurements were carried out in the in field-cooled (FC) and in zero-field-cooled (ZFC) conditions versus temperature, T, and external magnetic field, H, on a sigma-phase Fe47Mo53 compound. Analysis of the measured M_FC and M_ZFC curves yielded values of characteristic temperatures: magnetic ordering (Curie) temperature, T_C, irreversibility temperature, T_ir, temperature of the maximum in M_ZFC, T_m, identified as the N.eel (T_N) temperature, and cross-over temperature, T_co. Based on these temperatures a magnetic phase diagram in the H-T plane was outlined. The field dependences of the characteristic temperatures viz. of the irreversibility and of the cross-over temperatures were described in terms of a power law with the exponent 0.5(1). In the whole range of H i.e. up to 800 Oe, except the one H>50 Oe, a rare double re-entrant transition viz. PM-FM-AF-SG takes place. For small fields i.e. H<50 Oe a single re-entrant transition viz. PM-FM-SG is revealed.
In-field DC and AC magnetization measurements were carried out on a sigma-phase Fe55Re45 intermetallic compound aimed at determination of the magnetic phase diagram in the H-T plane. Field cooled, M_FC, and zero-field cooled, M_ZFC, DC magnetization curves were measured in the magnetic field, H, up to 1200 Oe. AC magnetic susceptibility measurements were carried out at a constant frequency of 1465 Hz under DC fields up to H=500 Oe. The obtained results provide evidences for re-entrant magnetism in the investigated sample. The magnetic phase diagrams in the H-T plane have been outlined based on characteristic temperatures determined from the DC and AC measurements. The phase diagrams are similar yet not identical. The main difference is that in the DC diagram constructed there are two cross-over transitions within the strong-irreversibility spin-glass state, whereas in the AC susceptibility based diagram only one transition is observed. The border lines (irreversibility, cross-over) can be described in terms of the power laws.
Systematic experimental (vibrating sample magnetometry) and theoretical (electronic structure calculations using charge and spin self-consistent Korringa-Kohn-Rostoker Green function method) studies were performed on a series of intermetallic sigma-phase Fe(100-x)Re(x) (x = 43-53) compounds. Clear evidence was found that all investigated samples exhibit magnetism with an ordering temperature ranging between 65 K for x = 43 and 23 K for x = 53. The magnetism was revealed to be itinerant and identified as a spin-glass (SG) possibly having a re-entrant character. The SG was found to be heterogeneous viz. two regimes could be distinguished as far as irreversibility in temperature dependence of magnetization is concerned: (1) of a weak irreversibility and (2) of a strong one. According to the theoretical calculations the main contribution to the magnetism comes from Fe atoms occupying all five sub lattices. Re atoms have rather small moments. However, the calculated average magnetic moments are highly (ferromagnetic ordering model) or moderately (antiparallel ordering model) overestimated relative to the experimental data.
Formation energy of the sigma-phase in the Fe-V alloy system, Delta E, was computed in the full compositional range of its occurrence (34 < x < 60) using the electronic band structure calculations by means of the KKR method. Delta E-values were found to strongly depend on the Fe concentration, also its variation with different site occupancies was characteristic of a given lattice site. Calculated magnetic and configuration entropy contributions were used to determine sublattice occupancies for various compositions and temperatures. The results agree well with those obtained from neutron diffraction measurements.
A series of sigma-phase Fe_{100-x}V_x samples with 34.4 < x < 59.0 were investigated by neutron and X-ray diffraction and Mossbauer spectroscopy (MS) techniques. The first two methods were used for verification of the transformation from alpha to sigma phase and they also permitted to determine lattice parameters of the unit cell. With MS the Debye temperature, T_D, was evaluated from the temperature dependence of the centre shift, <CS>, assuming its entire temperature dependence originates from the second-order Doppler shift. To our best knowledge, it is the first ever-reported study on T_D in sigma-FeV alloys. Both attice parameters i.e. a and c were revealed to linearly increase with x. T_D shows, however, a non-monotonic behaviour as a function of composition with its extreme values between 425K for x=40 and 600K for x=59. A local maximum of 525K was found to exist at x=43.
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