Gd Mössbauer effect and magnetic properties of GdMn6Ge6

The magnetic properties of GdMn₆Ge₆ have been studied by magnetic measurements and ¹⁵⁵Gd Mössbauer spectroscopy. The Mn sublattice orders ferromagnetically in high magnetic fields and at high temperatures but in low fields and low temperatures there is a tendency to antiferromagnetic ordering. Antiferromagnetic order was also found in YMn₆Ge₆. The electric field gradient derived from the quadrupolar splitting of the ¹⁵⁵Gd Mössbauer spectra is substantially larger than in the isotypic compound GdMn₆Sn₆.     

Gd Mössbauer effect and magnetic properties of GdCo5-χGaχ

We have investigated the effect of Ga substitution on the magnetic properties and the ¹⁵⁵Gd Mössbauer spectra of GdCo₅. A substantial increase was observed in the values of the electric field gradient derived from the quadrupole splitting of the spectra. This was ascribed to a preferred substitution of Ga into the 2c Co site in GdCo₅. This preferred Ga substitution also led to a change in the easy magnetization direction from parallel to the c axis (GdCo₅) to perpendicular to the c axis (GdCo₃Ga₂). Increasing the amount of Ga substitution was found to decrease strongly the magnetic ordering temperature and the Co-induced transferred Gd hyperfine field in the series GdCo_5-χGa_χ.     

Synthesis and properties of CaCd2Sb2 and EuCd2Sb2

High density polycrystalline CaCd₂Sb₂ and EuCd₂Sb₂ intermetallics are synthesized by Spark Plasma Sintering and their thermoelectric properties are investigated. X-ray diffraction measurements reveal both materials have a structure in space group, containing a small amount of CdSb as a second phase. Thermoelectric measurements indicate both are p-type conductive materials. The figure of merit value of CaCd₂Sb₂ is 0.04 at 600 K and that of EuCd₂Sb₂ is 0.60 at 617 K. Theoretical calculations show that CaCd₂Sb₂ is a degenerate semiconductor with a band gap of 0.63 eV, while EuCd₂Sb₂ is metallic with DOS of 13.02 electrons/eV. For deeper understanding of the better thermoelectric properties of EuCd₂Sb₂, its low temperature magnetic, transport and heat capacity properties are investigated. Its Nèel temperature is 7.22 K, convinced by heat capacity anomaly at 7.13 K. Hall effect convinced that it is a p-type conductive material. It has high Hall coefficient, high carrier concentration and high carrier mobility of +1.426 cm³/C, 4.38 × 10¹⁸/cm³ and 182.40 cm²/Vs, respectively. They are all in the magnitude of good thermoelectric materials. The Eu 4f level around Fermi energy and antiferromagnetic order may count for the better thermoelectric properties of EuCd₂Sb₂ than that of CaCd₂Sb₂.     

A Mössbauer effect investigation of site preferences and Fe environments in Ga-substituted Sm2Fe17 compounds

Room temperature ⁵⁷Fe Mössbauer studies of the 2:17 compounds Sm₂Fe_17−xGa_x with x=0, 1, 2, 3, 4 and 5 are reported. These measurements yield values of the Fe hyperfine field and relative concentration of Fe atoms at the four inequivalent transition metal sites, 6c, 9d, 18f and 18h, in the rhombohedral Th₂Zn₁₇ structure. Changes in the Fe hyperfine field as a function of Ga content can be correlated to changes in lattice parameter and Curie temperature and are the result of changes in the transition metal-transition metal exchange coupling. The relative intensity of the various spectra components shows that for small concentrations, Ga preferentially enters into the 18h sites. For x & 1 a substantial quantity of Ga also enters into the 18f sites. It is the Ga which enters into the 18f sites which is responsible for the formation of a uniaxial anisotropy in these materials for higher Ga content.     

Synthesis and magnetic structure of the YbMn2Sb2 compound

A neutron diffraction investigation has been carried out on the trigonal La₂O₃-type (hP5, space group , No. 164; also CaAl₂Si₂-type) YbMn₂Sb₂ intermetallic. A two-step synthesis route has been tried in this work, and successfully utilised to prepare single phase samples of this compound. This study shows that YbMn₂Sb₂ presents antiferromagnetic ordering below 120 K. The magnetic structure of this intermetallic consists of antiferromagnetically coupled magnetic moments of the manganese atoms, in the Mn1 (1/3, 2/3, Z_Mn) and Mn2 (2/3, 1/3, 1 − Z_Mn) sites; the direction of magnetic moments of manganese atoms forming a φ and a θ angle, respectively with the X- and the Z-axis. At 4 K the magnetic moment of the Mn1 atom is μ_Mn = 3.6(1) μ_B, with φ = 0° and θ = 62(4)°, whilst the Mn2 atom has a magnetic moment μ_Mn = 3.6(1) μ_B, with φ = 0° and θ = 242(4)°. On the other hand, in this compound no local moment was detected on the Yb site.     

Structural and Mössbauer studies on mechanical milled SmCo5/α-Fe nanocomposite magnetic powders

The SmCo₅/α-Fe nanocomposite powders were prepared by high energy ball milling and the inter-diffusion reaction between the SmCo₅ and α-Fe magnetic phases were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), vibrating sample magnetometry (VSM) and ⁵⁷Fe Mössbauer spectroscopy. While structural and magnetic measurements could reveal only the presence of SmCo₅ and α-Fe phases, Mössbauer studies could clearly specify the extent of alloying between Fe and Co atoms in terms of evolution of α-Fe(Co) phase as a function of milling time. It has been found that the fractional volume of α-Fe(Co) solid solution tends to increase at the expense of the initial α-Fe phase upon progressive milling.     

Magnetic, transport and Mössbauer effect study of UFeAl

We present the results of magnetic susceptibility, electrical resistivity and Mössbauer effect measurements performed on two crystallographic structures of UFeAl: high-temperature (HT) and low-temperature (LT) phases, i.e. MgZn₂ and Fe₂P-type structures respectively.

The exchange-enhanced Pauli susceptibility and spin-fluctuation characteristics of the temperature dependence of the electrical resistivity of UFeAl are almost independent of the type of crystal structure.

⁵⁷Fe Mössbauer spectra of both crystallographic phases of UFeAl, recorded in the temperature range 10–295 K, consist of two symmetric quadrupole doublets for each crystal structure associated with the non-equivalent crystallographic positions of Fe. The Debye temperatures for the HT and LT structures of UFeAl were also estimated.     

NMR, magnetism and Mössbauer effect in icosahedral Al63Cu24.5Fe12.5 — a thermodynamically stable quasi-periodic alloy

We report results of ²⁷Al nuclear magnetic resonance (NMR), magnetism and iron-site Mössbauer experiments on the thermodynamically stable and “perfectly” quasicrystalline icosahedral alloy Al₆₃Cu_24.5Fe_12.5. NMR experiments were performed at 11.10, 17.8 and 45.7 MHz and at temperatures as low as 50 K. Magnetization was measured at 295, 100 and 5 K, while iron site Mössbauer was measured at 295 and 4.2 K. We find very small NMR Knight shifts and long relaxation times that we interpret as consistent with a pseudo-gap in the density of states near the Fermi level. NMR line shapes in AlCuFe do not show quadrupolar structure consistent with a single aluminum site. We report results of numerical simulations that effectively reproduce the features and trends in observed line shapes by means of a broad distribution of electric field gradients (EFGs) at aluminum sites. Our magnetization and Mössbauer effect experiments show that there is a very small fraction of the material that is magnetically ordered at low temperatures. This magnetic behavior was only observed well below the lowest temperature of NMR experiments and cannot be responsible for the broad NMR lines. Iron-site Mössbauer lines show significant broadening characteristic of a distribution of EFGs that is qualitatively similar to that indicated for the aluminum site.     

Sn Mössbauer spectroscopy study of the magnetic properties of the HfFe6Ge6-type compounds: SmMn6Sn4Ge2 and GdMn6Sn4Ge2

The HfFe₆Ge₆-type compound SmMn₆Sn₄Ge₂ has been studied by single-crystal magnetisation and ¹¹⁹Sn Mössbauer spectroscopy. The compound orders ferromagnetically at T_c = 420 K and displays an easy-axis anisotropy from T_c to T_SR = 130 K. Below T_SR, both magnetisation and ¹¹⁹Sn Mössbauer spectroscopy measurements indicate a deviation from the [0 0 1] direction and the presence of easy-cone anisotropy. The angle of the moments with respect to the [0 0 1] direction is estimated to 26-31° from Mössbauer spectroscopy results, in good accordance with the magnetisation results. The isotypic compounds GdMn₆Sn₄Ge₂ and GdMn₆Sn₆ studied by ¹¹⁹Sn Mössbauer spectroscopy display easy plane anisotropy in the whole temperature range 300-4.2 K. The anisotropy behaviours of the LMn₆Sn₄Ge₂ compounds are discussed and the coexistence of easy cone anisotropy for both the SmMn₆Sn₄Ge₂ and HoMn₆Sn₄Ge₂ suggests the play of a positive second-order anisotropy constant of the Mn sublattice.     

Gd Mössbauer investigation of GdTSi compounds (T = Co, Fe, Mn)

We have studied the ¹⁵⁵Gd Mössbauer effect in the compounds GdMnSi, GdFeSi and GdCoSi (tetragonal, CeFeSi-type structure). From the quadrupic splitting of the spectra, we determined the electric field gradient at the nuclear site. We also present values for the effective hyperfine field and the isomer shift.     

Electronic structure and thermoelectric properties of the thioantimonate FePb4Sb6S14

The title compound was synthesized and its thermoelectric properties investigated. Electronic structure calculations and electrical conductivity measurements show semiconducting behavior. The results of thermopower measurements are presented. The high thermopower motivated us to investigate the effects arising from chemical doping. Cobalt and tin doped variants were synthesized and their physical property measurements show improved electrical conductivity.     

Thermoelectric properties of semi-conducting compound Zn4Sb3

Hot-pressed samples of the semi-conducting compound Zn₄Sb₃ with the stoichiometric composition were prepared and characterized by X-ray and microprobe analysis. Thermoelectric characterization was done through measurements of the electrical and thermal conductivities as well as the Seebeck coefficient between room temperature and 650 K. All samples had p-type conductivity. High thermoelectric figures of merit (ZT) were obtained between 450 and 650 K and a maximum of about 1.3 were obtained at a temperature of 650 K.     

Mn substitution effect on magnetic and magnetostrictive properties of HoFe2

The cubic Laves phase compounds Ho(Fe_1-χMn_χ)₂ (χ = 0, 0.1, 0.2, 0.3, 0.4) are investigated by magnetization and magnetostriction measurements. With increasing χ value, the saturation magnetization increases and the Curie temperature drops. It was found that the easy direction magnetostriction λ₁₀₀, which is estimated from polycrystalline data, increases with increasing Mn content, while the hard direction magnetostriction λ₁₁₁ becomes low. This can be related to the fact that the first-order anisotropy constant K₁ for the compounds, obtained by the “law of approach to saturation”, decreases with increasing Mn content.     

Crystallographic and magnetic properties of HfFe6Ge6-type REFe6Sn4Ge2 compounds (RE = Y, Gd–Er)

The REFe₆Sn₄Ge₂ (RE = Y, Gd–Er) compounds have been synthesized and studied by powder X-ray diffraction and magnetisation measurements. These compounds crystallize in the hexagonal HfFe₆Ge₆ structure although the parent ternary compounds REFe₆X₆ (X = Ge, Sn) display more complicated orthorhombic crystal structure. This evolution is discussed and interpreted on the basis of the relaxation of some RE–X contacts in the quaternary compounds. The iron sublattice order antiferromagnetically above room temperature (554 ≤ T_N ≤ 560 K) while the paramagnetic RE compounds display a second transition at low temperature (7.3 ≤ T_t ≤ 42.7 K). The magnetisation versus field curves display a metamagnetic behaviour at 4.2 K. The corresponding value of the magnetisation suggests a non-collinear ordering of the RE sublattice.     

Structural and electrical properties of Gd(Ni1/2Zr1/2)O3

The Gd(Ni_1/2Zr_1/2)O₃ (GNZ) ceramic is synthesized by the solid-state reaction technique. The X-ray diffraction pattern of the sample shows monoclinic phase at room temperature. The dielectric dispersion of the material is investigated in the temperature range from 303 K to 673 K and in the frequency range from 100 Hz to 1 MHz. The relaxation peak is observed in the frequency dependence of the loss tangent. The relaxation time at different temperatures is found to obey Arrhenius law having activation energy of 1.1 eV which indicates the hopping of ions at the lattice site and may be responsible for the dielectric relaxation of GNZ. The scaling behaviour of loss tangent suggests that the relaxation mechanism is temperature independent. The frequency dependent conductivity spectra follow the power law. In the impedance formalism, the Cole-Cole model is used to study the relaxation mechanism of GNZ.     

New Zr6CoAs2-type compounds: Y6CoBi2, Ho6CoBi2 and Tm6CoBi2

Results of a powder X-ray diffraction investigation of new ternary compounds are reported. The compounds Y₆CoBi₂ [a=0.8312(1) nm, c=0.4144(1) nm], Ho₆CoBi₂ [a=0.8246(2) nm, c=0.4095(1) nm], and Tm₆CoBi₂ [a=0.8155(2) nm, c=0.4066(1) nm] crystallize in the hexagonal Zr₆CoAs₂-type structure (space group P6b2m No. 189). The Zr₆CoAs₂-type structure is a superstructure of the Fe₂P-type structure.     

Structural, magnetic and Mössbauer spectral studies of nanocrystalline Ni0.5Zn0.5Fe2O4 ferrite powders

Nanocrystalline Ni_0.5Zn_0.5Fe₂O₄ powders, synthesized by a combustion method are investigated by X-ray diffraction, vibrating sample magnetometry and Mössbauer spectroscopic techniques. We adopt a strategy to systematically control the particle sizes between 4 and 45 nm simply by changing the elemental stoichiometric coefficient, Φ_e, of the combustion mixture. Curie temperature of the superparamagnetic particles of size 4 nm is higher than that of the bulk particles. Interestingly, bigger particles (45 nm) show a comparable room temperature saturation magnetization and exceptionally very high Curie temperature of 833 K, when compared to that of the bulk Ni_0.5Zn_0.5Fe₂O₄ material (563 K).