Investigations of the Co-Pt alloy phase diagram with neutron diffuse scattering, inverse cluster variation method, and Monte Carlo simulations

Abstract

The short-range order in a CoPt alloy was determined at 1203 K and 1423 K using neutron diffuse scattering measurements. The effective pair interactions provided by data analysis reproduce well the experimental order-disorder transition temperature in Monte Carlo simulations. They complete previous results reported for the Co-Pt system and are compared to those obtained within tightbinding and ab initio formalisms. Our results show that the important dependence of the nearestneighbor pair interactions with composition is not related to the sample magnetic state at the measured temperatures. Interactions measured in the paramagnetic domain for the CoPt alloy behave like those in the ferromagnetic domain for the Co 3 Pt and Co 0.65 Pt 0.35 alloys. The effective pair interactions related to the tight-binding Ising model provide a relatively good description of the CoPt alloy thermodynamics close to the ordering temperature (short-range order and temperature of phase transformation), even if they strongly differ from those measured in this study. The average magnetic moment of Co atoms at high temperatures was determined from the analysis of the intensity contribution that is not dependent on the scattering vector. The obtained value is very close to the moment measured at room temperature or determined from ab initio calculations. This confirms the Curie-Weiss behavior of the CoPt alloy. Finally, transmission electron microscope observations carried out on samples annealed for about 30 days confirmed that the order-disorder transition takes place in the 830 K-843 K temperature interval at the Co 3 Pt composition.

Synthesis of highly ordered L10 MPt alloys (M = Fe, Co, Ni) from crystalline salts: an in situ study of the pre-ordered precursor reduction strategy

“…Similar to the MPt 1 : 1 case, the order–disorder transition temperature in correspondence of the 1 : 3 composition is higher for the FePt 3 system ( T O–D -FePt 3 ∼ 1350 °C; T O–D -CoPt 3 ∼ 850 °C; T O–D -NiPt 3 ∼ 500 °C). 33 This corresponds to a higher driving force and, consequently, to a higher stability of the intermediate FePt 3 compound that forms as soon as the process starts and gradually evolves, by further reducing the compound, towards a highly ordered 1 : 1 FePt alloy. The lower driving force for CoPt 3 formation, together with the higher reduction potential of Co ++ with respect to the Fe ++ , implies a shorter temperature range in which different intermediate phases can coexist and this is further confirmed in the case of the Ni-based compound, where the formation of the chemically ordered NiPt 3 phase is complicated by unfavorable thermodynamic constraints, thus explaining the single-step behavior observed in such a system.…”

“…, Mn–Pt and Mn–Ni) for spintronic devices. 31 According to the bulk equilibrium phase diagram, 32–35 around the equiatomic composition, the chemically ordered L1 0 phase, consisting of planes of pure atoms alternating along the c -axis of the face-centered tetragonal (fct) unit cell (ESI,† Fig. S1), is stable below the order/disorder transition temperature T O–D , which is characteristic of each system.…”

“…25 Paramount examples of chemically ordered compounds include: (i) Fe(Co)-Pt and Fe(Co)-Pd ferromagnetic alloys for data storage/processing and catalysis, which feature a huge uniaxial magnetic anisotropy (K = 0.5-1 Â 10 7 J m À3 ) 26 that can be intrinsically obtained without resorting to complex multilayered structures; [27][28][29] (ii) high-anisotropy Mn-Al and Fe-Ni alloys (K = 1-2 Â 10 6 J m À3 ) 30 that are cost-effective alternatives to materials containing critical elements for the development of sustainable permanent magnets and electronics; and (iii) antiferromagnetic alloys (e.g., Mn-Pt and Mn-Ni) for spintronic devices. 31 According to the bulk equilibrium phase diagram, [32][33][34][35] around the equiatomic composition, the chemically ordered L1 0 phase, consisting of planes of pure atoms alternating along the c-axis of the face-centered tetragonal (fct) unit cell (ESI, † Fig. S1), is stable below the order/ disorder transition temperature T O-D , which is characteristic of each system.…”

Effective site energy and cluster expansion approaches for the study of phase diagrams

“…We have determined the ESEs of a Co c Pt 1−c linear chain to define our energetic model using interatomic potentials already considered in the literature for their abilities to reproduce bulk and surfaces properties [51,52]. CoPt is a well-known alloy, which has often been studied both in bulk and in two-dimensions (2D) [26,[53][54][55]; this paper is an extension to 1D. Note that the purpose of this paper is not the alloy itself.…”

“…CE is a powerful tool for the analysis of alloy thermodynamics and is widely used to study bimetallic alloys [19][20][21][22][23][24][25][26], high entropy alloys [27], semiconductors, and oxides [28][29][30][31][32][33]. However, its application to real systems is often questionable and remains a topical issue even before considering the difficulties associated with complex structures such as inhomogeneous systems.…”