Low temperature heat capacity and magnetic excitation of HoAl2 in a magnetic field

The specific heat of single crystalline HoAl₂ in magnetic fields up to 7.5 T has been measured for the temperature range 1.5–16 K. In addition the energy of a magnetic excitation in a magnetic field of 5 T at 4.2 K has been determined by inelastic neutron scattering. The results have been interpreted with a cubic crystalline electric field and an exchange interaction using the same parameter set B₄=-0.85×10^-4 meV, B₆=+0.71× 10^-6 meV and T_C=31.5 K previously obtained by magnetization measurements.     

Magnetic properties of R2Re2Si2C (R=Ho and Er)

We have investigated magnetic properties of R₂Re₂Si₂C (R=Ho and Er) using magnetic susceptibility, magnetization and heat capacity measurements. Both the materials order antiferromagnetically. The ordering temperatures (T_N) for Ho₂Re₂Si₂C and Er₂Re₂Si₂C are, ∼8.8 and ∼7.6 K, respectively. Our measurements indicate crystal field effects in the bulk properties of both these compounds. The experimental results have been analyzed by taking into account the effect of crystalline electric field and magnetic exchange interaction.     

Bulk magnetization study of a DyNi5 single crystal

Magnetization and susceptibility measurements were performed on a single crystal of DyNi₅ along the three main symmetry axes of the ortho-hexagonal cell. Below its ordering temperature (T_c = 11.6 K), b and c are respectively the easy and hard magnetization axes. The strong anisotropy originates from the crystalline electric field acting on the 4f electrons of the Dy³⁺ ions. A small magnetization is induced on nickel atoms by the applied field and the exchange interactions with the dysprosium atoms. The crystal field parameters, the molecular field coefficients and the susceptibility of nickel atoms are determined from the experimental data.     

Low temperature heat capacity of some heavy REA12 compounds (RE = Tb,Dy, Er and Tm) in a magnetic field

The molar heat capacity of single crystalline TbAl₂, DyAL₂ and ErAl₂ and of polycrystalline TmAl₂ has been measured for the temperature range from 1.5 to about 20 K in external magnetic fields up to 7.5 T. The magnetic contribution to the heat capacity has been interpreted by means of a simple theoretical approach containing a cubic crystalline electric field interaction and an exchange interaction in mean field approximation. The three model parameters have been determined from the interpretation of the experimental results for the four compounds. A comparison of the reduced crystal field parameters for the heavy rare earth-Al₂ compounds shows systematic behaviour and supports the physical relevance of the applied model.     

Specific heat and crystalline electric field parameters in TmxY1−xAl2

Specific heat measurements on Tm_xY_1?xAl₂ have been performed to determine the crystalline electric field parameters in Tm_xY_1?xAl₂. We find for the ground state the Γ⁽¹⁾₅ triplet and for the Lea, Leask and Wolf parameters X = (0.47 ± 0.03) and W = (0.034 ± 0.013) meV.     

Exchange and hyperfine fields acting on dilute Eu In SmCo5, Sm2(Co1-xFex)17 and Sm2CoxMy( rm M = Fe, Cu, Zn, Zr)

Mössbauer source experiments of dilute ¹⁵³Eu and ⁵⁷Fe in SmCo₅ and ¹⁵³Eu in Sm₂(Co_1-xFe_x)₁₇ and Sm₂ Co_x M_y at 4.1 K were performed. After the ${}^{153}\text{Sm}\text{→β?}{}^{153}$ decay the Eu ion is trivalent and exhibits extremely large hyperfine interactions due to strong exchange fields. Since the exchange interactions are comparable to the Eu³⁺ spin-orbit coupling, we calculated the expectation values of the Eu³⁺ spin, magnetic hyperfine field and electric field gradient as a function of exchange field and second order crystalline field, by diagonalization of the full Hamiltonian of spin orbit, exchange and crystalline field. For SmCo₅ and Sm₂(Co_1-xFe_x)₁₇ the exchange and crystalline fields are known and thus allow us to analyze our experimental results and obtain the polarized conduction electron contributions to the magnetic hyperfine field. The contribution due to magnetic neighbour polarization of conduction electrons is found to be linear in exchange field. The experimental results together with the theoretical analysis yield the Eu electric field gradient 4f Sternheimer shielding factor R_Q to be 0.26±04. It is shown that Mössbauer studies of two probes (¹⁵⁵Gd and Eu³⁺) in magnetic systems yield directly the second order crystalline field, the exchange field and the various contributions to the hyperfine field acting on the Eu nucleus. From the experimentally measured magnetic hyperfine fields alone, approximate values for the exchange fields in the mixed systems Sm₂Co_xM_y were determined.     

Magnetic properties of ErxY1−xF3 solid solutions

The Er_xY_1−xF₃ (x=0.1, 0.2, 0.7, 0.9, 1) solid solutions were synthesized and characterized by X-ray powder diffraction and magnetic measurements. The crystal structure refinements done by the Rietveld profile method show that no significant change of the structure parameters with the erbium concentration occurs. On the basis of DC susceptibility measurements in the 2-300 K range the lowest four crystal field levels have been determined, giving the ground level magnetic moment value of 6.7 μ_B. Results of M(H) studies point to the presence of complex exchange interactions between erbium ions.     

Magnetoelastic coupling and spin reorientation in RECo5 uniaxial magnets (RE = Pr,Dy, Ho and Y). II

Magnetostriction measurements, between ≈ 5 and 300 K, of powder aligned samples of the uniaxial compounds RECo₅ (RE = Pr, Ho, Dy and Y) for strong applied magnetic fields up to 15 T are reported. The strains have been measured, parallel, perpendicular and at 45° form the c-axis, with the field applied also along these directions. These sets of measurements allow us to determine the six irreducible magnetoelastic coupling constants: λ^α₁₁, λ^α₂₁, related to the strain dependence of isotrop ic exchange, and λ^α₁₂, λ^α₂₂, λ^γ and λ^ϵ, describing the strain dependence of the magnetocrystalline anisotropy ene rgy. Anomalies on the strains, associated with the spin reorientation (SR) regime are observed. The thermal variation of the strains is well explained by the standard magnetostriction model, including as ingredients exchange striction and the single-ion anisotropic one, as well as a dependence on the angle of reorientation under field. The point charge model of the crystalline magnetoelastic field is far from agreement with the present results.     

Metamagnetism in ErGa2 studied on a single crystal

Magnetization measurements and neutron difrraction experiments were performed on a single crystal of ErGa₂. Because of the strong crystal field effects a huge uniaxial anisotropy imposes the Er moments to be parallel to the c-axis of the hexagonal structure. We have shown that the transition from the antiferromagnetic structure to the ferromagnetic one, when the field is applied along c, occurs through an intermediate phase. Such a behaviour is very well accounted for using a simple Ising-like model which takes into account first and second nearest neighbours exchange interactions in the hexagonal layers. A quantitative determination of the crystalline electric field effects is also presented.     

Ferromagnetism modulation by phase change in Mn-doped GeTe chalcogenide magnetic materials

In this work, an effective method to modulate the ferromagnetic properties of Mn-doped GeTe chalcogenide-based phase change materials is presented. The microstructure of the phase change magnetic material Ge_1?x Mn _x Te thin films was studied. The X-ray diffraction results demonstrate that the as-deposited films are amorphous, and the crystalline films are formed after annealing at 350 °C for 10 min. Crystallographic structure investigation shows the existence of some secondary magnetic phases. The lattice parameters of Ge_1?x Mn _x Te (x = 0.04, 0.12 and 0.15) thin films are found to be slightly different with changes of Mn compositions. The structural analysis clearly indicates that all the films have a stable rhombohedral face-centered cubic polycrystalline structure. The magnetic properties of the amorphous and crystalline Ge_0.96Mn_0.04Te were investigated. The measurements of magnetization (M) as a function of the magnetic field (H) show that both amorphous and crystalline phases of Ge_0.96Mn_0.04Te thin film are ferromagnetic and there is drastic variation between amorphous and crystalline states. The temperature (T) dependence of magnetizations at zero field cooling (ZFC) and field cooling (FC) conditions of the crystalline Ge_0.96Mn_0.04Te thin film under different applied magnetic fields were performed. The measured data at 100 and 300 Oe applied magnetic fields show large bifurcations in the ZFC and FC curves while on the 5,000 Oe magnetic field there is no deviation.     

Magnetostriction of the NdCo5 uniaxial permanent magnet

Magnetostriction measurements, between ≈5 and 300 K, of powder magnetically aligned samples of the hexagonal compound NdCo₅, in strong pulsed magnetic fields up to 15 T are reported. The measurement have been performed parallel, perpendicular and at 45° to the alignment c-axis, for the field also applied in those directions. This set of measurements allows us to determine the six irreducible magnetoelastic modes: λ^α₁₁, λ^α₂₁, related to the strain dependence of the isotropic exchange, and λ^α₁₂, λ^α₂₂, λ^γ, λ^ϵ, related to the strain dependence of the crystalline field anisotropy energy. Anomalies on the strains, associated with the spin reorientation (SR) regime are observed, in particular at the temperatures of beginning, T_SR1, and end, T_SR2, of the SR. The thermal variation of the strains is explained by the standard magnetostriction model, including an ingredients: exchange striction (varying as m²_Nd), single-ion anisotropy (varying as m³_Nd), (where m_Nd is the reduc ed Nd sublattice magnetization), as well as an angular dependence of the form {sin²θ(H, T)−sin²θ(0, T)}, where θ(H, T) and θ(0, T), respectively, are the SR angles under field and spontaneous. The irreducible magnetoelastic coupling constants at 0 K have been estimated, although the point charge model is far from agreement with those results.     

Obtaining the Optical and EPR Parameters of Vanadyl-Doped Sodium Dihydrogen Phosphate Dihydrate Powders by Experimental and Theoretical Methods

In this study, the spin-Hamiltonian parameters (g and A), molecular orbital coefficients, and other spectroscopic properties of vanadyl-doped sodium dihydrogen phosphate dihydrate (NaH₂PO₄·2H₂O) powders have been investigated by experimental and theoretical methods, including electron paramagnetic resonance (EPR) and optical absorption spectroscopies. The results show axially symmetric crystalline field around VO²⁺ ion. The optical absorption spectra exhibit three characteristic bands of VO²⁺ ions in tetragonal symmetry. EPR and optical data were used in a complementary way to calculate spin-Hamiltonian parameters and molecular orbital coefficients. The octahedral and the tetragonal field parameters were theoretically calculated on the basis of crystal field theory. These parameters were used to determine various bonding parameters which characterize the nature of bonding in the complex. The theoretical results are supported by experimental results.     

EPR conduction-electron relaxation by cerium and gadolinium impurities in LaAl2

The conduction-electron relaxation by cerium impurities is computed by use of the effective exchange Hamiltonian which takes into account both combined spin and orbit exchange scattering and crystalline field effects, which finally can account for recent EPR measurements of Gd in LaAl₂ with cerium impurities.     

EPR and optical study of Mn doped ammonium oxalate monohydrate

Studies on fine and hyperfine structures of paramagnetic resonance spectra in single crystals of Mn²⁺: ammonium oxalate monohydrate are reported. As sufficient numbers of lines were not obtained at room temperature, measurements have been done at liquid nitrogen temperature and at the frequency of X-band. The Mn²⁺ spin Hamiltonian parameters have been evaluated employing a large number of resonant line positions observed for various orientations of the external magnetic field and the surrounding crystalline field has been discussed. The values of the zero field parameters that give good fit to the observed EPR spectra have been evaluated. The values obtained for g, A, B, D, E and a are 2.0002±0.0002, (100±2)×10⁻⁴, (79.5±2)×10⁻⁴, (257±2)×10⁻⁴, (85±2)×10⁻⁴ and (−18±1)×10⁻⁴ cm⁻¹, respectively. The percentage of covalency of the metal-ligand bond is also determined. The optical absorption study has been done at room temperature. The observed bands are assigned as transitions from the ⁶A_1g(S) ground state to various excited quartet levels of Mn²⁺ ion in a cubic crystalline field. The electron repulsion parameters (B and C), the crystal field splitting parameter(Dq) and the Trees correction (α) providing good fit to the observed optical spectra have been estimated and the values obtained for the parameters are B=897, C=2144, Dq=910 and α=76 cm⁻¹.     

Effect of diamagnetic substitutions in BaFe12O19 on the magnetic properties

The exchange parameters of BaFe₁₂O₁₉ have been calculated from the temperature dependence of the saturation magnetization using the molecular field theory. Under the assumption that the exchange parameters do not change for diamagnetic substitutions of the Fe³⁺ ions, it is shown that the temperature coefficient of the magnetization at room temperature cannot be decreased without decreasing the magnetization. Diamagnetic substitution in the octahedralf ₂ site would solely increase the saturation magnetization.     

Austausch und Spinresonanz in den Gadolinium-Pniktiden GdP,GdAs und GdSb

Powder samples of mixed crystals Gd _x Y_1?x P, Gd _x Y_1?x As and Gd _x Y_1?x Sb have been studied by electron-spin-resonance for Gd-concentrations 0.01≦x≦1 between the ordering temperatures and room temperature. All measurements show a single exchange-narrowed and asymmetric absorption line of Lorentzian shape in the centre region. Theg-factors are 1.995±0.008 in agreement with the expected value for a⁸ S _7/2-state of the half-filled 4f-shell. The linewidths are explained by dipolar and exchange interactions. It is shown that the effect of exchange narrowing is proportional to the sum of the absolute values of the exchange parameters. Therefore a new method for studying the magnetic coupling far above the ordering temperature is proposed. The dependence of exchange on changes of the lattice parameters has been evaluated. The observed asymmetry of the resonance line gives a measure for the electric conductivity. In particular the beginning of critical scattering near the ordering temperature is observed.     

Magnetic properties of armchair graphene nanoribbons: A Monte Carlo study

The magnetic properties of armchair graphene nanoribbons have been studied using a Monte Carlo study. The ground state phase diagrams have been determined for mixed spins S={±2; ±1; 0} and σ={±5/2; ±3/2; ±1/2}. The topologies of the phase diagrams depend on the values of the parameters in the Hamiltonian such as exchange interactions, crystal field and external magnetic field. The some diagrams show some coexistence between regions. The Néel temperature t_N is obtained for different values of layer (n). The effect of reduced exchange interactions between the mixed spins σ and S and reduced crystal field on total magnetization has been given. Magnetic hysteresis cycles are given for different values of n, reduced exchange interactions and reduced temperatures. The double hysteresis loop and superparamagnetism were observed.     

Electron paramagnetic resonance of some lanthanide ions in yttrium and lutetium vanadate

The electron paramagnetic resonance (EPR) parameters have been determined for the trivalent ions neodymium, erbium and ytterbium, at low abundance in lutetium vanadate. Crystal field splittings produce ground states which are Kramers doublets, and measurements were made of theg-values both parallel and perpendicular to the tetragonal axis; they are compared with previous measurements in yttrium vanadate. Two of the crystal field splitting parameters of the trivalent gadolinium ion (S = 7/2) in LuVO₄ show noticeable difference from those in YVO₄.     

Crystal field and magnetic properties of the tetragonal TbNi2Si2 compound

The magnetic properties of a TbNi₂Si₂ single crystal have been investigated experimentally by specific heat, anisotropy of the susceptibility and magnetization, and inelastic neutron scattering, leading to the determination of its fundamental coupling parameters. The lowest crystalline electric field states are two singlets close to each other, that explains the different magnetic structures evidenced in TbNi₂Si₂: an amplitude modulated structure, close to T _N , a simple antiferromagnetic one at low temperature and an antiphase one when an external magnetic field is applied along the c direction.     

Magnetic properties of A DyNi2 single crystal

Magnetic measurements have been performed on a single crystal of DyNi₂ in applied fields up to 135 kOe. In the ferromagnetic range (T_c = 25 K), the easy magnetization direction is [100] and the hardest one is [111]. Crystal field parameters have been determined from the field and temperature dependence of the magnetization measured along the three principal axes. A two-dimensional model has been used to take into account the rotation of magnetization towards the field. The deduced parameters are W = -0.8 K and x = 0.49. The corresponding anisotropy is very large: especially even a field of 135 kOe applied along a difficult magnetization axis cannot rotate the magnetization along this direction.