Magnetocaloric properties of DyCo2−xGax alloys

The influence of the substitution of Ga atoms for Co atoms in DyCo₂ compounds on magnetocaloric properties has been investigated. A series of DyCo_2−xGa_x alloys with x=0, 0.03, 0.06, 0.1, 0.15, and 0.2 was prepared by the arc-melting method for this investigation. Experimental results revealed that the Ga substitution for Co in DyCo₂ can form a single phase with the cubic Laves phase structure up to x=0.2. As the Ga content x increases, the lattice parameter and the Curie temperature T_c increases from 143 to 196 K linearly. The maximum magnetic entropy changes in a low field change of 0-1.5 T, increasing from 8.24 to 10.61 J/K kg when the Ga content x increases from 0 to 0.03, but decreasing gradually to 3.51 J/K kg as the Ga content further increases to x=0.2. All the samples show a relatively large magnetic entropy change with very small hysteresis loss.     

Synthesis and structure of aluminum substituted MoSi2

Mo(Si_1−xAl_x)₂ compositions have been prepared by a modified SHS route under pressure. The compositional phase analysis indicates a body centered tetragonal C11_b phase below x<0.05 substitution, above which a biphasic region of C11_b and C40 structure coexists. Occurrence of pure hexagonal C40 phase is observed for x>0.1. The aluminum substitution leads to a continuous increase in the lattice parameters in both these structures. In the biphasic region, redistribution of aluminum concentration takes place with the evolution of aluminum rich C40 phase as indicated by energy dispersive X-ray analysis and supported by micro-hardness measurements.     

High spin polarization in ordered Cr3Co with the DO3 structure: A first-principles study

The electronic structure of the highly ordered alloy Cr₃Co with the DO₃ structure has been studied by FLAPW calculations. It is found that the ferrimagnetic state is stable and that the equilibrium lattice constant of Cr₃Co equals 5.77 Å. A large peak in majority spin density of states (DOS) and an energy gap in minority spin DOS are observed at the Fermi level, which results in a high spin polarization of 90% in the ordered alloy Cr₃Co. The total magnetic moment of Cr₃Co is 3.12μ_B, which is close to the ideal value of 3μ_B derived from the Slater-Pauling curve. An antiparallel alignment between the moments on the Cr (A, C) sites and the Cr (B) sites is observed. Finally, the effect of lattice distortion on the electronic structure and on magnetic properties of Cr₃Co compound is studied. A spin polarization higher than 80% can be obtained between 5.55 and 5.90 Å. With increasing lattice constant, the magnetic moments on the (A, C) sites increase and the moments on the (B, D) sites decrease. They compensate each other and make the total magnetic moment change only slightly.     

Preparation of nano-sized Al-substituted yttrium iron garnets by the mechanochemical method and investigation of their magnetic properties

The polycrystalline Y₃Fe_5−xAl_xO₁₂ compounds with x=0.5, 1.0, 1.5 and 2.0 were prepared by the mechanochemical method. The samples were milled for 40 h in a high-energy planetary mill and then calcined at different temperatures from 1300 to 1100 °C. The minimum calcination temperature to get a single phase garnet decreases by increasing Al concentration. X-ray diffraction patterns reveal that the structures of nano-powders are bcc and the garnet phase has been obtained after calcining. Also, the lattice constant of the samples decreases by increasing Al concentration ,which is discussed based on the substitution of smaller aluminum ions instead of iron ions. The average crystallite sizes are in the range 24-35 nm using Scherrer's formula. The Curie temperature of single phase samples was found to decrease by increasing Al concentration, which can be discussed upon the reduction of magnetic interactions per magnetic ion. When more Al³⁺ is added, the magnetization is reduced because of the reduction of superexchange interactions in crystal lattice.     

Ab-initio investigation of electronic properties and magnetism of half-Heusler alloys XCrAl (X=Fe, Co, Ni) and NiCrZ (Z=Al, Ga, In)

The electronic structures and magnetism of the half-Heusler alloys XCrAl (X=Fe, Co, Ni) and NiCrZ (Z=Al, Ga, In) have been investigated to search for new candidate half-metallic materials. Here, we predict that NiCrAl, and NiCrGa and NiCrIn are possible half-metals with an energy gap in the minority spin and a completely spin polarization at the Fermi level. The energy gap can be attributed to the covalent hybridization between the d states of the Ni and Cr atoms, which leads to the formation of bonding and antibonding peaks with a gap in between them. Their total magnetic moments are 1μ_B per unit cell; agree with the Slater-Pauling rule. The partial moment of Cr is largest in NiCrZ alloys and moments of Ni and Al are in antiferromagnetic alignment with Cr. Meanwhile, it is also found that FeCrAl is a normal ferromagnetic metal with a magnetic moment of 0.25μ_B per unit cell and CoCrAl is a semi-metal and non-magnetic.     

Structure and RT ferromagnetism of Fe-doped AlN films

Al_1−xFe_xN_1−δ thin films with 0 ≤ x ≤ 13.6% were deposited by dc magnetron co-sputtering at room temperature (RT). It is found that Fe atom will substitutes the Al atom in the lattice when x ≤ 1.2%, while it will embed into the interstice of the lattice at larger Fe content. RT ferromagnetism was observed in all doped samples. A maximum saturated magnetization 2.81 emu/cm³ of the film is found to be induced by AlFeN ternary alloy when x = 1.2%.     

Effect of Cr on the electronic structure of Co3Al intermetallic compound: A first-principles study

The effect of doping with Cr on the electronic structure and magnetism of Co₃Al has been studied by density functional calculations. It has been found that the Cr atom has a strong site preference for the B-site in Co₃Al. With the substitution of Cr for Co, the total densities of states (DOS) change obviously: A DOS peak appears at E_F in the majority spin states and an energy gap is opened in the minority spin states. The effect of Cr in Co₃Al is mainly to push the antibonding peak of the Co (A,C) atoms high on the energy scale and to form the energy gap around E_F, and also to contribute to the large DOS peak at E_F in the majority spin direction. The calculations indicate a ferromagnetic alignment between the Co and Cr spin moments. The calculated total magnetic moment decreases and becomes closer to the Slater–Pauling curve with increasing Cr content. This is mainly due to the decrease of the Co (A,C) spin moments. At the same time, the moments of Co (B) and Cr (B) only change slightly.     

Magnetic properties of R2Fe17−xGax (R=Y and Gd) compounds

The magnetic properties of Y₂Fe_17−xGa_x for 3≤x≤7 and Gd₂Fe_17−xGa_x for 5≤x≤7 have been investigated using ⁵⁷Fe Mössbauer spectroscopy. These compounds have the rhombohedral Th₂Zn₁₇ structure. X-ray diffraction analysis of aligned powders shows that the easy direction of magnetization is parallel to the c-axis in Y₂Fe₁₀Ga₇ and Gd₂Fe₁₀Ga₇ and is perpendicular to the c-axis in Y₂Fe₁₄Ga₃, Y₂Fe₁₂Ga₅, Gd₂Fe₁₂Ga₅ and Gd₂Fe₁₁Ga₆. Mössbauer studies indicate that those samples are ordered ferromagnetically. The ⁵⁷Fe hyperfine field decreases with increasing Ga content. This decrease results from the decreased magnetic exchange interactions resulting from Ga substitution. The average isomer shift, δ, for R₂Fe_17−xGa_x (R=Y and Gd) at room temperature is positive and the magnitude of δ increases with increasing Ga content.     

Principal energy band gaps of the quaternary alloy AlxGa1−xSbyAs1−y

The correlated function expansion (CFE) interpolation procedure was presented to efficiently estimate principal energy band gaps and lattice constants of the quaternary alloy Al_xGa_1−xSb_yAs_1−y over the entire composition variable space. The lattice matching conditions between x and y for the alloy Al_xGa_1−xSb_yAs_1−y substrated to InAs and GaSb were obtained by optimizing the alloy lattice constant to that of the substrates. The corresponding principal band gaps (E(Γ), E(L), and E(X)) were also calculated along the lattice matching condition on each substrate (InAs and GaSb).     

The effect of Fe doping on structural, magnetic and electrical transport properties of CaMn1−xFexO3 (x=0-0.35)

A comprehensive study of the effect of Fe doping on CaMnO₃ is carried out by means of experiments on the structural, transport conduction, and magnetic properties of CaMn_1−xFe_xO₃ (0≤x≤0.35). With a sol-gel process for sample preparation, Fe is substituted for Mn up to x=0.35. This substitution substantially brings out the lattice expansion and gradually suppresses the antiferromagnetism. For x=0.08 and 0.10 in particular, the magnetization curves with a field-cooled mode under the field of 1 kOe behave as those of a ferrimagnetic-like system and present low-temperature negative magnetization. For x≥0.15, the negative magnetization phenomenon disappears, and a ferromagnetic component coexists with an antiferromagnetic one, but the antiferromagnetic interaction still dominates in these compounds. Electrical transport measurements show insulating behavior for all compositions. Fe doping, even at a level as low as x=0.02, can cause a marked resistivity increase in the temperature range studied. Further increasing the Fe content causes the resistivity to gradually decrease due to the increasing carrier presence.     

Ab initio study of 5d-shells Ir substitution for Fe-based SmOFe1−xIrxAs

The lattice and electronic properties for 5d-shells Ir substituted Fe-based superconductor SmOFe_1−xIr_xAs (x=0,0.2,0.25,0.3) are investigated based on the density functional theory (DFT) with a spin generalized gradient approximation SGGA+U method. The electronic density of states (DOS) of SmOFe_1−xIr_xAs is studied and well compared with the results of experimental X-ray photoemission spectroscopy (XPS). The calculation indicated that iridium substitution at the Fe site induced a modification of the FeAs₄ tetrahedron and suppressed the magnetic ordering corresponding to the Fe-3d, which may be the main cause of inducing superconductivity in Ir-doped SmOFeAs system.     

Effect of preparation and metal content on the introduction of Fe in BEA zeolite, studied by DR UV-vis, EPR and Mössbauer spectroscopy

Fe-containing SiBEA zeolites were prepared by a two-step postsynthesis method: creation of vacant T-sites by dealumination of tetraethylammonium BEA zeolite with nitric acid and then impregnation of the resulting SiBEA zeolite with an aqueous solution of Fe(NO₃)₃. X-ray diffraction shows that iron is incorporated in SiBEA at lattice sites. The presence of Fe in its oxidation state +3 and at isolated tetrahedral sites for low metal content, was demonstrated by diffuse reflectance UV-vis, EPR and Mössbauer spectroscopy. For high iron content, diffuse reflectance UV-vis and Mössbauer spectra revealed the additional presence of extra-lattice FeO_x oligomers and superparamagnetic Fe-oxyhydroxide. Mössbauer spectroscopy identified superparamagnetic Fe-oxyhydroxide as the main phase when basic conditions are used for the preparation.     

Magnetic properties of the Ce2Fe17−xMnx helical magnets up to high magnetic fields

Magnetic properties of the Ce₂Fe_17−xMn_x, x=0–2, alloys in magnetic fields up to 40 T are reported. The compounds with x=0.5–1 are helical antiferromagnets and those with 1<x?2 are helical ferromagnets or helical antiferromagnets at low and high T, respectively. Mn ions in the system carry average magnetic moment of 3.0±0.2 μ_B that couple antiparallelly to the Fe moments. Easy-plane magnetic anisotropy in the Ce₂Fe_17−xMn_x compounds weakens upon substitution of Mn for Fe. The absolute value of the first anisotropy constant in the Ce₂Fe_17−xMn_x helical ferromagnets decreases slower with increasing temperature than that calculated from the third power of the spontaneous magnetization. Noticeable magnetic hysteresis in the Ce₂Fe_17−xMn_x, x=0.5–2, helical magnets over the whole range of magnetic fields reflects mainly irreversible deformation of the helical magnetic structure during the magnetization of the compounds. A contribution from short-range order (SRO) magnetic clusters to the magnetic hysteresis of the helical magnets has been also estimated.     

Al versus Si competition in FeSiAl alloys

In FeSiAl alloys, when Si substitutes for Al, important changes take place in the magnetism as well as in the structural properties. Alloys in the two composition series Fe₇₅Al_25−xSi_x (x=0, 7.5, 12.5, 17.5, 25) and Fe₇₀Al_30−xSi_x (x=0, 9, 15, 21, 30) were prepared by induction melting; afterwards they were crushed and then annealed in order to recover the DO₃ stable phase. The deformed FeAl samples show larger lattice parameters than the ordered ones; however, this difference (Δa) decreases when Si substitutes for Al until it becomes zero (i.e. until the ordered samples and the deformed ones have the same lattice parameters). This trend is the same for both sample series and does not depend on the Fe content of the alloy. However, the magnetization has a different behaviour depending on the Fe content. For deformed Fe₇₅Al_25−xSi_x alloys the saturation magnetization decreases with increasing Si content while for Fe₇₀Al_30−xSi_x deformed alloys the saturation magnetization has a plateau in which the saturation magnetization values do not vary.     

Luminescent properties of stabled hexagonal phase Sr1−xBaxAl2O4:Eu (x=0.37-0.70)

Stabled hexagonal phase Sr_1−xBa_xAl₂O₄:Eu²⁺ (x=0.37-0.70) was prepared by solid-state method. Result revealed that the structure behavior of the SrAl₂O₄:Eu²⁺ calcined at 1350 °C in a reducing atmosphere for 5 h strongly depended on the Ba²⁺ concentration. With increasing Ba²⁺ concentration, a characteristic hexagonal phase can be observed. When 37-70% of the strontium is replaced by barium, the structure of the prepared sample is pure hexagonal. Photoluminescence and excitation spectra of the samples with different x and doped with 2% Eu²⁺ were investigated. Changes in the emission spectra were observed in the two different phases. The green emission at 505 nm from Eu²⁺ was found to be quite strong in the hexagonal phase. The intensity and peak position of the green luminescence from Eu²⁺ changed with increasing content of Ba²⁺. The strongest green emission was obtained from Sr_0.61Ba_0.37Al₂O₄:Eu²⁺. The decay characteristics of Sr_1−xBa_xAl₂O₄:Eu²⁺ (x=0.37-0.70) showed that the life times also varied with the value of x. Furthermore, the emission colors and decay times varying with x could be ascribed to the variation of crystal lattice.     

First-principles investigations of Cr doping effects on electronic structure and magnetic properties in Sr2FeReO6

The electronic structures and magnetic properties of double perovskites Sr₂Fe_1−xCr_xReO₆ (x=0.0, 0.25, 0.5, 0.75, 1.0) have been studied within the local spin density approximation (LSDA) and LSDA+U schemes. The calculated results reveal that with increasing Cr content the cell volume shrinks 2.61%; the Fe/Cr site magnetic moment decreases while the Re-site moment increases. The total spin magnetic moment linearly decreases with the Cr doping from 3.00μB for x=0.00 down to 1.00μB for x=1.00 per formula unit. The magnetic coupling constants increase with increasing x. The electronic structure calculations indicate that the electronic concentration in the Re spin-down subband slightly increases resulting from the increase of bonding-antibonding interaction between the localised and the delocalised states in spin-down band; the coupling of O-2p_↑ and transition-metal-3d_↑ is substantially enhanced with the Cr doping. We discuss the origin of the anomalously high TC of Cr-doped Sr₂FeReO₆ compounds in terms of band hybridization effects.     

Magnetocaloric effect of GdCo2−xAlx compounds

The structure, magnetic property and magnetocaloric effect of GdCo_2−xAl_x (x=0, 0.06, 0.12, 0.18, 0.24, 0.4) compounds have been investigated by X-ray diffraction (XRD) and magnetic measurement techniques. The experimental results show that the GdCo_2−xAl_x (x≤0.4) compounds are single phase with a Laves-phase MgCu₂-type structure. The Curie temperature T_c initially increases, and then decreases with increasing Al content. The maximum value of T_c, 418 K, is reached for the compound with x=0.06. The magnetic entropy change, which is determined from the temperature and field dependence of the magnetization by the Maxwell relation, decreases almost linearly with increasing Al content.     

Thermal conductivity of La0.67(CaxSr1−x)0.33MnO3 (x=0, 0.5, 1) and La0.6Y0.07Ca0.33MnO3 pellets between 10 and 300 K

Thermal conductivity (λ) of nanocrystalline La_0.67(Ca_xSr_1−x)_0.33MnO₃ (x=0, 0.5, 1) and La_0.6Y_0.07Ca_0.33MnO₃ pellets prepared by a novel ‘pyrophoric’ method have been studied between the temperature range 10 and 300 K. Our data show that the magnitude of thermal conductivity is strongly influenced by the ion substitutions at La-site. The analysis of the thermal conductivity data indicates that the thermal transport is governed largely by phonons scattering in these systems and the electronic contribution is as small as 0.2-1% of total thermal conductivity (λ_total). At low temperatures (<90 K) 2D like lattice defects contribute to the phonon scattering dominantly and its strength increases with increasing Sr content and also with partial substitution of La by Y. Depending upon the composition of the samples, the magnon thermal conductivity contributes 2-15% of λ_total close to T_C. In the paramagnetic regime the unusual increase in λ_total keeps signature of large dynamic lattice distortion.     

Structural and some magnetic properties of manganese-substituted lithium ferrites

The structural and magnetic properties of Mn-substituted lithium ferrites having the general formula Li_0.5−0.5xMn_xFe_2.5−0.5xO₄ (where x=0.0–1.0) prepared by the standard ceramic technique have been studied. Single phase cubic structure is confirmed by X-ray diffractometer. This result demonstrates that the samples are homogeneous, and the sharp peaks reveal that the samples are in crystalline form. The lattice parameter ‘a’ and average grain diameter ‘D’ increase with increasing Mn²⁺ ion substitution. The saturation magnetization and the experimental magnetic moment are found to increase with manganese up to x=0.5 and then tends to decrease for x>0.5. The increase in magnetic moment with manganese is attributed to Neel's two sublattice model according to which the magnetic moment is the vector sum of lattice magnetic moment. The decrease in magnetization for x>0.5 obeys the Yafet–Kittel (Y–K) model. The increase in Y–K angles for x≥0.5 indicates the increased favor for triangle spin arrangements on B-sites. This suggests the existence of canted spin structure in the ferrite system with higher content of Mn. Hystersis loops were measured. The initial permeability μ_i was measured as a function of temperature.     

The effect of Fe substitution on the electrical and thermal conductivity and thermopower of Ca3(FexCo1−x)4O9 synthesised by a sol–gel process

The effect of Fe substitution for Co on direct current (DC) electrical and thermal conductivity and thermopower of Ca₃(Co_1−xFe_x)₄O₉ (x = 0, 0.05, 0.08), prepared by a sol–gel process, was investigated in the temperature range from 380 down to 5K. The results indicate that the substitution of Fe for Co results in an increase in thermopower and DC electrical resistivity and substantial (14.9–20.4% at 300K) decrease in lattice thermal conductivity. Experiments also indicated that the temperature dependence of electrical resistivity ρ for heavily substituted compounds Ca₃(Co_1−xFe_x)₄O₉ (x = 0.08) obeyed the relation lnρ ∝ T^−1/3 at low temperatures, T < ~55K, in agreement with Mott’s two-dimensional (2D) variable range hopping model. The enhancement of thermopower and electrical resistivity was mainly ascribed to a decrease in hole carrier concentration caused by Fe substitution, while the decrease of thermal conductivity can be explained as phonon scattering caused by the impurity. The thermoelectric performance of Ca₃Co₄O₉ was not improved in the temperature range investigated by Fe substitution largely due to great increase in electrical resistivity after Fe substitution.