Magnetism and magnetic phase diagram of sigma-phase Fe68V32 compound
Graphical abstract
Magnetic phase diagram of σ-phase Fe68V32 intermetallic compound in the H-T coordinates.
Introduction
The sigma-phase (σ) can occur in alloy systems with, at least, one transition element. Its crystallographic structure (space group D144h-P42/mnm) is constituted by a tetragonal unit cell which accommodates 30 atoms distributed over 5 non-equivalent lattice sites. Characteristic features of σ are high values (12–16) of coordination numbers and lack of stoichiometry [1]. Physical properties of σ depend on the constituent elements, and, for a given choice of elements, on their mutual concentration. Concerning magnetic properties, the subject of the present study, they originally were revealed for σ in the Fe–V [2] and Fe–Cr [3] systems. The magnetic ordering was for a long time regarded as a ferromagnetic one. However, recent studies revealed that it is much more complex as it has a re-entrant character, is weak and itinerant [[4], [5], [6], [7]]. The Fe–V alloy system is exceptional regarding the existence of σ, as its field phase is the largest among the binary alloys. In particular, it can be formed in alloys whose V concentration ranges between ~31 and ~65 at. % and in a wide temperature range [8]. Thanks to the former, its magnetism, and in particular, the Curie temperature, TC, could be tuned from ~10 K for a sample containing 55.1 at.%V to ~316 K for the one whose vanadium concentration was 33.5 at.% [6], what is the record-high value for any σ as found so far. However, it follows from the crystallographic phase diagram of the Fe–V system [8] that the lowest concentration of V at which the σ-phase can be formed equals to ~31 at. %V. Thus the strongest magnetism of σ should be observed in a Fe69V31 alloy. Consequently, the first aim of this study was to prepare such sample and determine its magnetic ordering temperature, TC. Our second aim was to investigate its magnetic properties, and, particularly, to determine its phase diagram in the H-T coordinates. For this purpose magnetization measurements were performed in a wide temperature and applied magnetic field ranges.
Section snippets
Sample preparation
806.2 mg of the 99.999 pure vanadium and 1877.6 mg of the 99.99 pure iron were melted together under protective atmosphere of argon in an arc furnace. The melting process was repeated three times to obtain a more homogenous alloy. The mass of the alloy was unchanged by the melting process, hence the nominal concentration of vanadium i.e. x = 32 at. % was assumed and used in the evaluation of measurements and discussion of results. The conversion of the alloy into the σ-phase was performed by an
Results and discussion
The temperature dependence of ZFC/FC magnetization of the investigated sample (Fig. 1) can be used to determine three characteristic temperatures: (1) the magnetic ordering temperature, Tc; (2) the irreversibility temperature, Tirr; and (3) the crossover temperature, Tcros. Positions of these temperatures are marked with arrows in Fig. 2, which shows the ZFC/FC curves recorded in 500 Oe. The Tc values were obtained independently with two methods, by the minimum of the first derivative of ZFC
Conclusions
The results obtained in this study permit the following conclusions to be drawn:
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The Curie temperature of the studied σ-Fe68V32 intermetallic compound has the value of 335 (2) K which is the highest one ever reported for any σ-phase compound.
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The magnetism of σ-Fe68V32 has a re-entrant character i.e. on lowering temperature a transition from a paramagnetic over a ferromagnetic to a spin-glass state occurs.
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The spin-glass state is heterogeneous and it can be divided into two sub states: the one
Credit roles
S. M. Dubiel: Conceptualization; Sample preparation; Validation; Co-writing - original draft; P. Konieczny: Investigation; Formal analysis; Methodology; Validation; Visualization; Co-writing - original draft.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was financed by the Faculty of Physics and Applied Computer Science AGH UST and Institute of Nuclear Physics, Polish Academy of Sciences statutory tasks within subsidy of Ministry of Science and Higher Education, Warszawa.
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