xAlx Compounds Research Articles

The structure, magnetic and magnetocaloric effect of misch-metal (MM)2Fe17−xAlx (x = 0–2) compounds

In this study, the structural, magnetic and magnetocaloric effects (MCE) of misch-metal MM2Fe17−xAlx (0 ≤ x ≤ 2) compounds are investigated. The XRD patterns indicate that all the samples crystallize primarily in a rhombohedral Th2Zn17 type crystal structure coexisting with a small amount of precipitation phase α-Fe. The lattice parameters and the Curie temperature (TC) increase linearly with the increase of Al content. This allows the MM2Fe17−xAlx compounds to achieve magnetic refrigeration over the entire room temperature range. The maximum values of entropy changes (ΔSM) for MM2Fe17−xAlx (x = 0, 0.5, 1 and 1.5) compounds with a magnetic field variation of 0–2 T and 0–5 T are determined to be −2.1 J Kg-1 K−1, −1.3 J Kg-1 K−1, −1.9 J Kg-1 K−1, 1.9 J Kg-1 K−1 and −4.0 J Kg-1 K−1, −2.7 J Kg-1 K−1, −3.5 J Kg-1 K−1, −3.5 J Kg-1 K−1, respectively. The relative cooling power (RCP) for all the studied compounds is larger than 90 and 270 Jkg−1 while the full-width half maxima (δTFWHM) is larger than 60 K and 100 K with the field changes of 0–2 T and 0–5 T, respectively. These results indicate that MM2Fe17−xAlx (0 ≤ x ≤ 2) compounds could have higher magnetocaloric cost-performance comparable to the room-temperature magnetic refrigeration materials R2Fe17 (R = Pr, Nd) and Gd with the second-order phase transition (SOPT).

Synthesis and Study of Influence of Mechanical Activation Processing on the Structure and Magnetic Properties of Sm2Fe17 –xAlxNy Nitride Powders

Sm2Fe17 –xAlx (x = 0.1, 0.2, 0.3 and 0.4) alloys and alloy-based nitrides were synthesized. The chemical and phase composition, microstructure of cast and homogenized alloys, and the hysteresis properties were studied by scanning electron microscopy, local microanalysis, and X-ray diffraction analysis. It was established that nitriding of pseudobinary Sm2Fe17 –xAlx compounds leads to the increase in lattice parameters and simultaneous increase in the specific saturation magnetization. Mechanical activation processing of the nitriding products revealed the different nature of their magnetic behavior depending on aluminum concentration in the alloys. Mechanical activation leads to the increase in coercive force and simultaneous decrease in the values of specific saturation magnetization and the average size of particles at small content of aluminum (18.6 at %). The coercive force of nitride also increases after mechanical activation at high aluminum content (38.3 at %); however, the specific saturation magnetization also increases. Generally, the increase in aluminum content in Sm–Fe–Al–N alloy leads to the decrease in their hysteresis characteristics after mechanical activation processing.

Giant isotropic magnetostriction in NaZn13-type LaFe13−xAlx compounds

The unusual low-temperature magnetostrictive property is of fundamental interest due to significant applications in rapidly developing cryogenic engineering. Here, we report a giant isotropic magnetostriction (λ = 1500 ppm, ω = 4500 ppm) over a wide temperature range (∼210 K), and the saturated volume magnetostriction can be up to 8400 ppm in cubic NaZn13-type LaFe13−xAlx compounds by optimizing the chemical composition. The large magnetostrictive effect often occurs in ferromagnetic materials. However, we discovered that the magnetic-field-induced volume expansion originates from the change of lattice parameters across the first-order transition from the low-volume antiferromagnetic ground state to the high-volume ferromagnetic state. Moreover, the magnetostrictive materials show an excellent zero thermal expansion (ZTE) property, which guarantees their reliability and stability operating at various temperatures. The present study suggests potential applications of La(Fe, Al)13-based compounds as ZTE and magnetostrictive dual-function materials in cryogenic engineering.

ChemInform Abstract: Magnetic Properties and Magnetocaloric Effects in NaZn13‐Type La(Fe,Al)13‐Based Compounds

AbstractReview: 58 refs.

Evolution of magnetostructural transition and magnetocaloric effect with Al doping in MnCoGe1−xAlx compounds

The effect of Al doping in MnCoGe1−xAlx compounds has been investigated. The substitution of Al for Ge enhances Mn–Mn covalent bonding by shortening the distance of nearest Mn atom layers, and thus stabilizes the hexagonal structure. As a result, first-order magnetostructural transition between ferromagnetic martensite and paramagnetic austenite takes place for the optimized compositions (x = 0.01, 0.02). Accompanied with the magnetostructural transition, large magnetocaloric effect (MCE) is observed. More doping of Al(x = 0.03, 0.04) leads to the separation of magnetic and structural transitions and remarkable reduction of MCE.

Influence of Al substitution on magnetocaloric effect of Pr2Fe17−xAlx

The structural and magnetic properties of the Pr2Fe17−xAlx (x=0, 0.25, 0.5, 0.75, 1) compounds synthesized by arc-melting were investigated. The X-ray diffraction (XRD) coupled with the Rietveld analysis indicate that all samples are single phase with the rhombohedral Th2Zn17 type structure. Moreover, it reveals a 6c preferential occupation for the substituting element Al atoms in R3¯m. The unit cell volume increases slightly with increasing aluminum content while the c/a ratio remains constant. The study of magnetic properties reveals that Curie temperature increases with the Al content. The magnetic entropy change ΔSM was estimated from isothermal magnetization curves and it decreases with the increase of Al content from 5.5Jkg−1K−1 (x=0) to 4.2Jkg−1K−1 (x=1) at μ0H=5T. The saturation magnetization Ms, the magnetocaloric effect (MCE) of the compounds and the relative cooling power (RCP) were determined by measuring their magnetic properties.

Magnetism and large magnetocaloric effect in HoFe2−xAlx

The structural, magnetic and magnetocaloric properties of several HoFe2−xAlx compounds were investigated. The compounds in the Fe-rich region (0.36⩽x⩽0.4) crystallize in the cubic MgCu2 (C15) structure, while for the ones in the intermediate region (0.75⩽x⩽1.125) the hexagonal MgZn2 (C14) structure was observed. Electronic structure calculations were performed, showing a good agreement between theory and experiment. The Curie temperatures were found to decrease with Al content. For the Fe-rich compounds, these are close to room temperature, while for the compounds in the intermediate region, transition temperatures are well below 300K. No magnetic hysteresis was found around the Curie temperature for applied magnetic fields of up to 4T. All of the investigated compounds undergo a second-order magnetic phase transition at the Curie temperature. A maximum magnetic entropy change value of 7.6J/kgK was obtained for the sample with x=1.125, all of the samples displaying rather large RCP values. The possibility of incorporating these materials in magnetic refrigeration devices is discussed.

Anomalous phase composition in the two-phase region of DyFe3−xAlx (x≤1.0)

The structure transitions and phase relationships of DyFe3−xAlx compounds have been investigated by X-ray powder diffraction. Our XRD results show that each of the compounds with x≤0.45 crystallizes in the rhombohedral PuNi3-type structure with space group R3¯m and Z=9; for the 0.8≤x<1.0 compounds, each has a hexagonal structure of the CeNi3 type with space group P63/mmc and Z=6; and each of the samples with 0.45<x<0.8 is a two-phase mixture of the PuNi3- and CeNi3-type structures. The calculated XRD intensities of the DyFe3−xAlx compounds with x=0.2, 0.33, 0.4, and 0.45 indicate that Dy occupies the 3a and 6c sites, Fe and Al distribute randomly on the 18h site, and the 3b and 6c sites are exclusively occupied by Fe, which agrees well with those of our experimental XRD patterns. The XRD intensities of the DyFe3−xAlx compounds with x=0.8 and 1.0 have also been calculated and found to agree with the experimental results with Dy on the 2c and 4f sites, Fe and Al at the 12k site, and Fe at the 2a, 2b, and 2d sites. In the two-phase region with x=0.45–0.8, the values of unit-cell parameters and phase compositions are linearly dependent on the value of x, indicating that the two phases are constituted by the same composition x with different stacking arrangements. This abnormal two-phase equilibrium is further confirmed by the structural analysis of the DyFe2.33Al0.67 (or x=0.67) sample. The samples with x=1.1 and 1.2 were also analyzed, and each found to be a mixture of more than two phases.

Evolution of the optical properties of DyNi5 − x Al x compounds in dependence of aluminum concentration

The refractive (n) and absorption (k) indices of intermetallic DyNi5 − xAlx compounds (x = 0, 0.5, 1, 1.5, 2) have been measured by ellipsometry at room temperature in the spectral range of 0.22–15 μm. It is established that the replacement of nickel by aluminum atoms leads to significant changes in the dispersion relations of the optical conductivity σ(E) in the interband absorption range. With an increase in Al content, the spectrum σ(E) (containing three maxima for DyNi5) is gradually transformed into a single-peak structure. The results obtained are discussed on the basis of the data on the electronic spectrum of these compounds. Concentration dependences of the plasma and relaxation frequencies of conduction electrons are determined.

Effect of Al on the structural and magnetic properties ofEr2Co17−xAlx compounds

Powder x-ray diffraction and magnetization measurements were employedto investigate the effect of Al on the structural and magnetic properties ofEr2Co17−xAlx compoundswith 0≤x≤4. Al substitution leads to a monotonic increase in the lattice parameters and anapproximately linear decrease in the saturation magnetization. The Curie temperature isalso found to decrease with Al concentration. Large thermomagnetic irreversibility seen inthese compounds has been attributed to the domain wall pinning effect. The temperaturevariation of magnetization shows compensation points in the compounds withx = 3 and 4. The field-cooled magnetizations of these two compounds in low fields are found to benegative at temperature below the compensation temperatures, which has been explainedon the basis of the temperature variations of the exchange coupling strengths of Er and Cosublattices. The easy magnetization direction at room temperature is found to be along thec-axis in all the compounds. The first order anisotropy constant at room temperature isfound to increase with Al concentration, which has been attributed to the increase in theaxial anisotropy of both the Er and Co sublattices as a result of Al substitution.

Influence of aluminum substitution on the magnetic couplings in R-T alloys Ho6Fe23−xAlx

Magnetic properties of Ho6Fe23−xAlx compounds (x=0, 2.5, and 5) are reported in the temperature range of 5–550K under a magnetic field of up to 90kOe. A large decrease of the Curie temperature is obtained (−20K per Al atom) as in Y6Fe23−xAlx. The compensation temperature varies from 185K (x=0) to 280K (x=5); it is associated with a strong increase of low-temperature spontaneous magnetization (+2.3μB per Al atom). In the vicinity of Tcomp field-induced transitions are observed for x=0 and x=2.5. The different results are analyzed within the frame of the Néel ferrimagnetic model: the Ho–Fe and Fe–Fe magnetic interactions and the canting angle are then determined. The presence of the rare-earth Ho “stabilizes” the ferrimagnetism.

Magnetic and electronic properties of DyNi5−xAlx compounds

The DyNi5−xAlx compounds crystallize in CaCu5-type structure for an aluminum content x⩽1.5 and in HoNi2.6Ga2.4 type for x⩾2. The saturation moments per formula unit extrapolated at 0 K, in composition range x<2 are ∼9.2 μB in agreement with x-ray photoelectron spectroscopy studies which show negligible crystal field effects. For x⩾2 a mictomagnetic type contribution to magnetic ordering develops which becomes dominant only for the DyNi2Al3 compound. Band structure calculations on DyNi5−xAlx show that the Dy 5d band is polarized parallel to the 4f moment. For DyNi5 the nickel moments at 2c and 3g sites are antiparallel oriented to that of Dy. The nickel contributions to magnetization decrease when increasing aluminum content and are near nil for x⩾2. Above the Curie points the reciprocal susceptibilities follow linear dependencies. The effective nickel moments decrease from 2.11 μB/atom (x=0) to 0.87 μB/atom (x=2.5). The magnetic behavior of nickel is analyzed in models which take into account the electron correlation effects in d bands.

Magnetocrystalline anisotropy of Ce2Co17−xAlx compounds with x=0–3

An investigation of the structure and magnetocrystalline anisotropy of Ce2Co17−xAlx (x=0, 1, 2, and 3) compounds has been performed by means of x-ray diffraction and magnetization measurements. It is observed that the unit-cell volume, Curie temperature, and saturation magnetization of Ce2Co17−xAlx compounds are anomalously low in comparison with the values obtained from a simple extrapolation by the other R2Co17−xAlx compounds, such as R=Pr and Nd. These anomalies may be an evidence of a mixed-valence behavior of the Ce ion. X-ray diffraction measurements on magnetically aligned Ce2Co17−xAlx powders with x=0–3 exhibit an easy-axis type of magnetic anisotropy at room temperature. Substitution of Al for Co leads to a change of the magnetocrystalline anisotropy of the Co sublattice from the basal plane to c axis and strongly increases the uniaxial anisotropy of Ce2Co17−xAlx. The magnetocrystalline anisotropy field HA is found first to increase and then decrease with increasing Al concentration, attaining a maximum value of 18 kOe at x=2. The magnetocrystalline anisotropy constant K1 shows a small decrease with increasing temperature and K2 is almost a constant.

Structure and magnetic properties of Ce2Co17−xAlx compounds

The structural and magnetic properties of Ce2Co17−xAlx samples for Al concentrations up to x=5 were studied by means of X-ray diffraction and magnetic measurements. The 2:17 phase was found to exist only up to x=3.5. The easy magnetisation direction is parallel to the c-axis in all of the latter compounds. The Co moments and the Curie temperatures decrease strongly with increasing Si concentration. From magnetic measurements made on magnetically aligned powders, we have determined the concentration dependence of the anisotropy constant K1. The values of K1 increase with increasing x. This behaviour is discussed in terms of preferential Al substitution.

Magnetic properties of Pr2Co17−xAlx and Y2Co17−xAlx

The magnetic properties of Pr2Co17−xAlx and Y2Co17−xAlx compounds with 0≤sx≤4 were studied by means of magnetic measurements. These measurements have shown that increasing Al concentration leads to a sign reversal of the magnetocrystalline Co-sublattice anistropy from easy-plane anisotropy for low Al concentration to easy-axis anisotropy for higher Al concentration. In the Pr2Co17−xAlx compounds, the competition between the Pr- and Co-sublattice anisotropies leads to spin-reorientation transitions at temperatures that decrease strongly with increasing Al concentration. There is an almost linear decrease of the Curie temperature with Al concentration. Also the saturation moments at 5 K decrease strongly with Al concentration although this decrease is less strong in Pr2Co17−xAlx than in Y2Co17−xAlx.

Influence of Al substitution on the structure and Co-sublattice magnetocrystalline anisotropy of Gd2Co17 compounds

The structure and magnetic properties of Gd2Co17−xAlx compounds were investigated. All samples have a rhombohedral Th2Zn17-type structure and the replacement of Co by Al results in an approximately linear increase in the unit cell volumes at a rate of 8.2 Å3/Al. The Curie temperature decreases monotically with the increase of Al concentration. On the basis of magnetization curves at the compensation temperature, the intersublattice-molecular-field coefficient, nRT, and the RT exchange-coupling constant JRT have been determined. It is noteworthy that the substitution of Al has a significant effect on the magnetocrystalline anisotropy of the Co sublattice, and changes the easy magnetization direction of Gd2Co17−xAlx compounds from the basal plane to the c axis.

Metamagnetic transition in Nd6Fe14−xAlx compounds

Ferrimagnetic Nd6Fe14−xAlx compounds with x=3, 3.5, and 4 have been studied by magnetization measurements in fields up to 9 T. Néel temperatures were determined from Mössbauer spectra to be 308, 286, and 239 K, respectively. The net magnetization at T=4.2 K is 3.6 μB/f.u.(x=3), 2.2 μB/f.u.(x=3.5), and zero for x=4. Metamagnetic transitions were observed for all samples, at decreasing critical fields with increasing x. Both the metamagnetic transition hysteresis and the net magnetization decrease with increasing temperature and become negligible near 40 K. The initial susceptibility of Nd6Fe10Al4 shows an anomalous temperature dependence near T=25 K, which might be associated with a spin reorientation. The electrical resistance of the latter compound, measured as a function of applied field, exhibits an abrupt 3% increase at the metamagnetic transition.

Magnetic properties of Pr2Co17−xAl x compounds

The magnetic properties of Pr2Co17−xAlx compounds with 0⩽x⩽4 were studied by means of magnetic measurements. From these measurements it is derived that there is an almost linear decrease of the Curie temperature with Al concentration accompanied by a somewhat less strong decrease of the saturation moment at 5 K. For the compounds with 2⩽x⩽4, we have observed spin-reorientation transitions at temperatures that decrease strongly with increasing Al concentration. These spin-reorientations are interpreted as originating from a competition between the Pr- and Co-sublattice anisotropies.

Crystal Structure and Magnetic Properties of LaCo13—xAlx Compounds

The crystal structure and magnetic properties of LaCoIii 13—xAlx compounds have been investigated by X-ray powder diffraction and magnetization measurement. X-ray powder diffraction shows that the compounds crystallize in cubic NaZn13-type structure with lattice parameter a = 11.345(2) to 11.473(4) Å in the range of x ≤ 2.7, and in the tetragonal Ce2Ni17Si9 structure at x = 3. Magnetization measurements reveal that both cubic and tetragonal LaCo13—xAlx are ferromagnetic with Curie temperature higher than 840 K, and the magnetic moment per Co atom in LaCo13—xAlx compounds decreases with Al content.