Hexagonal Sm Research Articles

Atomic scale understanding of the defects process in concurrent recrystallization and precipitation of Sm-Co-Fe-Cu-Zr alloys

Identifying the defects process in concurrent recrystallization and precipitation during aging a supersaturated solid solution is essential for understanding their interaction mechanisms and for manipulating the microstructure, but was rarely done at the atomic scale. Herein, the concurrent recrystallization and precipitation in the supersaturated hexagonal Sm(Co, Fe, Cu, Zr)7.5 alloys were studied through detailed transmission electron microscopy investigations, where the recrystallization, growth of recrystallized subgrains (cells) and precipitates stem from the gradual formation and dissociation of defects, including basal stacking faults (SFs), vacancies and excess interstitial atoms. The diffusion-controlled glides of <a>-type partial dislocations associated with the SFs not only transform the matrix from the mixture of hexagonal Sm(Co, Fe, Cu, Zr)7 (1:7H) and Sm2(Co, Fe, Cu, Zr)17 (2:17H) to Sm-depleted rhombohedral Sm2(Co, Fe, Cu, Zr)17 (2:17R) cells but also provide continuous diffusion channels to reduce the point defects to form the Sm-enriched hexagonal Sm(Co, Fe, Cu, Zr)5 (1:5H) cell boundary precipitates and Zr-enriched rhombohedral (Sm, Zr)(Co, Fe, Cu)3 (1:3R) platelets. It indicates a diffusion-controlled displacive phase transformation mechanism, characterized by the composition-dependent 2:17R’ intermediate phase due to incomplete basal slip and incomplete solute partitioning. The growth velocities of both recrystallized cells and precipitates are closely related to the defects density, being faster due to the high defects density at early stage, and being slower due to the reduced defects density at later stage. A basal slip model is proposed to explain the formation and dissociation of defects along with the stacking period change and the simultaneous formation of continuous atomic diffusion channels. These new findings may yield a deep understanding of the interaction between recrystallization and precipitation.

Ordered phase formation in Sm-Co1−Cu and Er-Co1−Cu alloy films prepared on Cr(100) single-crystal underlayer

Abstract Sm17(Co1−xCux)83 and Er17(Co1−xCux)83 (at. %, x = 0–1) alloy thin films are prepared on Cr(100) single-crystal underlayers at 500 °C by molecular beam epitaxy. The effect of Cu/Co composition on the ordered phase formation is investigated. Sm17(Co1−xCux)83(11 2 ¯ 0) films with hexagonal Sm(Co,Cu)5 (D2d) ordered phase grow epitaxially on the underlayers for all the compositional range of x = 0–1. On the contrary, the structure of Er17(Co1−xCux)83 film varies depending on Cu composition. Amorphous phase is formed in the Er17Co83 (x = 0) film. Er17(Co1−xCux)83(11 2 ¯ 0) epitaxial films with Er(Co,Cu)5 (D2d) ordered phase are obtained for the compositional range of x = 0.25–0.5. With further increasing the x value, the crystallographic phase varies to a phase other than D2d. Partial substitution of the Co sites in ErCo5 structure with Cu atoms and control of the amount of Cu atom are important to stabilize the Er(Co,Cu)5 phase.

A new ternary arsenide Eu 20Ni 42As 31: Preparation, crystal structure and physical properties

We have investigated the crystal structure, magnetic and electrical transport properties of the new ternary compound Eu 20Ni 42As 31, crystallizing in the hexagonal Sm 20Ni 42P 31 structure type, space group P6 3/ m, a = 21.1496(7), c = 4.0734(2) Å, according to X-ray powder diffraction. From magnetic investigations it is evidenced that in this compound the europium is divalent, Eu 2+. The compound orders antiferromagnetically at a Neèl temperature T N = 5 K. However, in moderate magnetic fields an overall ferromagnetic-like magnetization is observed. The measurements of the electrical resistivity prove this magnetic behaviour as well as the metallic character of the compound Eu 20Ni 42As 31.

Spin-flop transition in samarium metal investigated by capacitance dilatometry in a steady magnetic field of45T

The longitudinal and transversal forced magnetostrictions of a single crystal of samarium metal have been investigated in a static magnetic field up to $45\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. The data show pronounced features of a spin-flop transition initiating at an applied field of about $30\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. The measured forced magnetostriction is about 1 order of magnitude smaller than the spontaneous magnetostriction. Based on a model calculation of the magnetic phase diagram these features have been interpreted within the exchange striction model. The moments of the quasicubic Sm sites enter a spin-flop phase at about $30\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ and ferromagnetic saturation is predicted at $60\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. Similarly the model predicts a spin-flop phase for the hexagonal Sm sites at $140\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ and ferromagnetic saturation at $390\phantom{\rule{0.3em}{0ex}}\mathrm{T}$.

Study of influence of underlayer on the properties of magnetron sputtered samarium cobalt thin films

This paper presents the results of the investigation on the deposition and characterization of samarium cobalt (SmCo) thin films. To understand the underlayer effects on the magnetic properties, SmCo thin films were deposited onto Cr/SiO 2/Si, Cr/Si and Si at 600 °C and an operating argon pressure of 1.3 Pa using a direct current unbalanced magnetron sputtering technique. These films were characterized for composition, structure, microstructure and magnetic properties using Rutherford Backscattering Spectrometry, X-ray Diffractometer, Atomic Force Microscope and Superconducting Quantum Interference Device, respectively. The films deposited on the three substrate/intermediate layer combinations have the same atomic composition of SmCo 5.51O 0.4. All prepared films are crystalline with predominantly hexagonal Sm 2Co 17 phase having grains of different orientations. The films deposited directly onto a Si substrate show relatively poor crystallinity and have a larger tensile residual stress in the film plane than those deposited with a Cr underlayer. The films prepared with a Cr underlayer are relatively dense with smaller grain sizes than those deposited directly onto a Si substrate. The in-plane hysteresis loops measured at room temperature indicate that the films deposited onto a Cr underlayer have relatively better magnetic properties. The use of a SiO 2 film as a barrier layer between Cr and Si decreases the intrinsic coercivity of the film by about 6.3% when compared with the film without a SiO 2 layer.

Coercivity mechanism of Sm(Co, Cu) 5

The mechanism of the high intrinsic coercivity of the Sm(Co 1− x Cu x ) 5 (0≦ x<1) system was studied by relating the coherency between the lattice constants of hexagonal Sm(Co, Cu) 5 and hcp Co to the coercive force. It was found analytically that the intrinsic coercive force reaches a maximum in the composition range from x=0.6 to 0.8, where the lattice mismatch approaches zero, so that there is a strong correlation between lattice matching and coercive force. When a Sm ion was located within a Sm(Co, Cu) 5 grain and in the outmost edge of the a and c planes of its grain surrounded or not surrounded by the coherent Co phase, the crystal field parameter at each Sm 3+ site was calculated using a point charge model under the assumption that the Co and Cu atoms located in a grain and the hcp Co atoms situated at the interface uniformly have a charge of 3/5−. The results indicated that the Co phase precipitated coherently along the grain boundaries effectively enhances the magnetocrystalline anisotropy of Sm ions located in the outmost edges of the a and c planes of a Sm(Co, Cu) 5 grain.

Coercivity Analysis in Sm(CoFeCuZr)z Magnets with Abnormal Temperature Behavior

A comprehensive characterization of sintered magnets with composition Sm(CobalFe0.06CuxZr0.03)z, where 0.088 ≤ x ≤ 0.128 and 5.8 ≤ z ≤ 7.2, was undertaken with the objective of clarifying the coercivity mechanisms. For the range of compositions evaluated in this study Cu has a small effect on microstructure, while z has a more pronounced effect. The abnormal coercivity behavior as function of temperature takes place preferentially in samples with low Cu and low z. The microstructure of the samples is composed by cells of rombohedral Sm2(Co,Fe)17 phase, a cell boundary hexagonal Sm(Co,Cu)5 phase and lamellae of the crystallographically coherent Zr rich platelet phase. Magnetic viscosity studies show that the activation length La (7–20 nm) has a dimension very similar to the width of the cell boundary phase (∼10 nm) and is also compatible with the cell size (10–90 nm). Through the Barbier plot, it is found a law of the type SV ∼ Hc0.71, for Sm(CoFeCuZr)z samples with coercivity between 0.09 and 4.2 T. Initial magnetization curves and the angular dependence of the coercivity are used to discuss coercivity mechanisms.

Structural chemistry, magnetic and electrical properties of ternary rare-earth nickel phosphides, R 20Ni 42P 30 (R=Ce, Sm)

We have investigated the magnetic and electrical transport properties of two ternary compounds R 20Ni 42P 30 (R=Ce, Sm), which crystallize in the hexagonal Sm 20Ni 41.6P 30 structure type, space group P6 3/ m. Both samples undergo magnetic transitions below 50 K. The cerium atom adopts an intermediate valence state ( v∼3.8). The temperature dependence of the resistivity, ρ, of Sm 20Ni 42P 30 resembles the shape of a simple non-magnetic metal, whereas ρ( T) of Ce 20Ni 42P 30 reveals the typical feature of a Kondo lattice.

Magnetic properties of Sm 2(Fe,Ti) 17 compounds and their nitrides with Th 2Zn 17 and Th 2Ni 17 structures

In Sm 2(Fe 1− x Ti x ) 17 compounds ( x=0.02, 0.03, 0.04) the hexagonal Th 2Ni 17 structure has been stabilized at high temperatures. The type of crystal structure affects the magnetic properties of Sm 2(Fe,Ti) 17. For the Th 2Ni 17 structure, higher values of the Curie temperature and constants of planar magnetocrystalline anisotropy than those for the Th 2Zn 17 structure are observed. For both structures, the substitution of Ti for Fe leads to growing importance of the anisotropy constants of orders higher than second. No change in the type of structure is found upon nitrogenation. The hexagonal Sm 2(Fe,Ti) 17 nitrides as well as the rhombohedral nitrides show uniaxial anisotropy at room temperature.

A model relating remanence and microstructure of SmCo 5 magnets

The question of remanence in SmCo 5 sintered magnets is reviewed, giving special attention to the influence of microstructure. In SmCo 5 magnets, the microstructural constituents are the matrix phase SmCo 5, another ferromagnetic phase (Sm 2Co 7), oxides (Sm 2O 3), a recently reported carbide SmCoC 2 and pores. A method to estimate alignment via the ratio J r/ J s from remanence measurements and microstructural analysis is proposed. The results agree very well with the degree of alignment calculated from Schulz pole figures. During sintering it was observed that the nucleation of SmCo 5 on the Sm 2Co 7 phase is epitaxial. Hexagonal Sm 2Co 7 is present in our sintered magnets but rhombohedral Sm 2Co 7 may also be present. The effect of chemical composition (Sm content and oxygen content) on remanence and on the ratio J r/ J s was evaluated. In the sintering step, the densification occurs more slowly for compositions with higher samarium content (or higher Sm 2Co 7 volume fraction). A model able to calculate the best chemical composition to maximize coercivity and remanence is presented and discussed.

Structural effect of Zr substitution in the Sm 2Co 17 phase

In the SmCoZr system, a hexagonal Sm 2Co 17 structure is observed in the isothermal section at 1150°C. Recent investigations have shown that only rhombohedral Sm 2Co 17 is stable in the SmCo binary system. X-ray patterns of samples annealed up to 1325°C were indexed with the 2 17 R structure. The hexagonal structure is not a disordered form of the rhomboheedral Sm 2Co 17 phase. It is obtained by deformation of the SmCo 5 structure through the disordered SmCo 7 structure. In this work, the crystal structure of this (Sm+Zr) 2Co 17 compound has been determined and its stabilisation is discussed.

Study of the microstructure of a new hexagonal Sm 2Fe 17 phase with the Th 2Ni 17-type structure

A study of the microstructure of the rapidly solidified Sm 2Fe 17N 3− x , alloy was carried out using transmission electron microscopy. A new hexagonal Sm 2Fe 17 phase, with space group P6 3/mmc and lattice parameters a = 0.854 and c = 0.828 nm, was found. This hexagonal phase is metastable and can only be formed during rapid solidification. The high resolution image also identified many 2:17H domains inside the 2:17R phase. Large-angle trigrain boundaries with a 120° rotation relationship between any two c axes were often formed in the alloy.

Nitrogenation of Sm 2Fe 17 prepared by mechanical alloying and subsequent heat treatment

SmFe alloy was prepared by mechanical alloying of the elemental metals, and the Sm 2Fe 17 phase was obtained through properly crystallizing the SmFe alloy. Subsequently, the nitrogenation behaviour of the Sm 2Fe 17 phase was investigated. The results show that nitrogenation consists of the following processes: incubation period, coexistence of Sm 2Fe 17 and nitride phase, and transformation of the residual Sm 2Fe 17 into nitride phase. During nitrogenation, small amounts of very fine α-Fe particles segregate. After nitrogenation, the metastable phase formed during crystallization disappears; this is attributed to the nitrogen-induced transformation of the hexagonal Sm 2Fe 17 phase into the rhombohedral one.

Magnetic properties of rapidly quenched R 2(Co, Fe) 7B 3 alloys

The magnetic properties of melt-spun R 2Co 7- x Fe x B 3 with R = Sm, Nd and Pr 2Fe 7B 3 alloys habe been investigated. The as-spun samples are magnetically soft but they develop a high coercivity in the range of 10–30 kOe after a heat-treatment at around 700°C. The coercivity of Fe-rich samples is higher indicating a larger anisotropy of the R 2(Co, Fe) 7B 3 phase. The Curie temperature of this phase is in the range of 300–430°C. X-ray diffraction studies on crystallized samples showed a multiphase structure with the majority phase having the hexagonal Sm 2Co 7B 3 -type structure.

Temperature and pressure dependence of magnetic and electric hyperfine interactions in metallic Samarium

The magnetic and electric hyperfine interaction at111Cd impurities in Samarium has been investigated by TDPAC measurements. The quadrupole frequency is νQ=20.0(2) MHz at 290 K and has a linear temperature dependence with the same slope (dlnνQ/dT)290K=−7.3(2) 10−4 K−1 in the rhombohedral and the hcp phase. The pressure dependence up to 7 kbar is (dlnνQ/dT)=+8.7(1.4) 10−3 kbar−1. The magnetic hyperfine field of111Cd in Sm is Hhf=242(6) kG at 4.2 K. Its temperature dependence confirms the existence of 2 different magnetic phases in Sm. The crystal field parameters B20 and B40 have been estimated from a comparison of Hhf(T) with molecular field models. The TDPAC spectra in the magnetic phases suggest that the impurities preferentially occupy the hexagonal Sm sites.

Magnetic characteristics of hexagonal Sm 2− xR xCo 16.4Zr 0.6

Magnetic properties (Curie temperatures, saturation magnetizations and anisotropy fields) of alloys of the composition Sm 2− x R x Co 16.4Zr 0.6 with R = La, Ce and Y are presented. The ternary system Sm 2Co 16.4Zr 0.6 is hexagonal and hence contains Sm in the 2b and 2d positions. Partial substitution of Sm by the other rare earth ions was carried out in an attempt to increase the anisotropy fields. This will occur only if the substituent preferential occupies the 2d site. Anisotropy fields are weakened by the special alloying employed and by about as much as would occur were the site occupancy random.