Ferromagnetic-to-antiferromagnetic transition in (Hf1−xTix)Fe2 intermetallic compounds induced by geometrical frustration of the Fe(2a) sites

The ferromagnetic-to-antiferromagnetic transition in the hexagonal (Hf_1−xTi_x)Fe₂ (0?x?1) intermetallic compounds has been investigated by ⁵⁷Fe Mössbauer spectroscopy. At 10 K, the transition occurs within rather narrow concentration limits, around x=0.55–0.65. We found that the key factor governing the unexpected quick change of the magnetic structure is the magnetic frustration of the Fe(2a) sites. The magnetic frustration is caused by the noncollinearity of the Fe(6h) magnetic sublattice. The noncollinearity arises from the rotation of the magnetic moments due to the competition between the ferromagnetic exchange interactions and the antiferromagnetic Fe(6h)–Ti–Fe(6h) interaction. In the compounds with x=0.4–0.6, the temperature transitions to the antiferromagnetic state are observed. As an example, the Hf_0.4Ti_0.6Fe₂ compound is completely antiferromagnetic above 200 K.     

Selected properties of EuCo2(Si1−xGex)2 alloys

EuCo₂(Si_1−xGe_x)₂, x=0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1 samples were synthesised by induction melting followed by annealing at 900 °C and rapid quenching. X-ray powder diffraction and Auger electron spectroscopy studies revealed that solid solutions are formed only for x?0.2 and x?0.7. Magnetic susceptibility investigations for the solid solutions revealed a dominant divalent europium valence state in the germanium-rich samples and a dominant trivalent europium component in the silicon-rich samples. In the germanium-rich samples, a long-range antiferromagnetic ordering was observed. In all samples studied, additional magnetic transitions at various temperatures were detected, which could be attributed to small clusters containing different europium chemical surrounding from that in the predominant phase.     

Magnetocaloric effects in (Gd1−xTbx)Co2

The structures and magnetocaloric effects of (Gd_1−xTb_x)Co₂ (x=0, 0.25, 0.4, 0.5, 0.6, 0.7, 0.8, and 1) pseudobinary compounds were investigated by X-ray powder diffraction and magnetic properties measurement. The results show that the T_c of the alloy is near room temperature when X=0.6. The magnetic entropy changes of the compounds increase from 1.7 to 3.6 J/kg K with increasing the content of Tb under an applied field up to 2 T. All the compounds exhibit second order magnetic change. As a result, the values of their ΔS_M are lower than that of some large magnetocaloric effect materials.     

Magnetic properties and specific heat of Dy1−xLaxNi2 compounds

Magnetic and specific heat measurements have been carried out on polycrystalline series of single-phase Dy_1−xLa_xNi₂ (0?x?1) solid solutions. The compounds have a Laves-phase superstructure (space group F4¯3m) with the lattice parameter gradually increasing with decreasing Dy content. The samples with x?0.8 are ferromagnetic with the Curie temperature below 22 K. At high temperatures, all solid solutions are Curie-Weiss paramagnets. The Debye temperature, phonon and conduction electron contributions as well as a magnetic contribution to the heat capacity have been determined from specific heat measurements. The magnetocaloric effect was estimated from specific heat measurements performed in a magnetic field of 0.42 and 4.2 T.     

Ferromagnetic and spin-wave resonance in Ni0.8Fe0.2/Dy1−x Cox bilayer films

The standing spin wave spectra of Ni_0.8Fe_0.2(1000–3000 Å)/(Dy_1?x Co_x(700 Å) bilayer exchange-biased films with two different (precompensation Dy_0.2Co_0.8 and postcompensation Dy_0.3Co_0.7) compositions of the hard magnetic layer are analyzed. Measurements are performed at room temperature. It is found that the effective magnetic layer thickness (d _eff=d ₀±Δd), which determines the wave vectors of the first modes in the spectrum, differs from the d ₀ value specified in film technology. The sign of |Δd| ～ 500 Å is governed by the composition of the DyCo hard magnetic layer.     

Nuclear magnetic resonance of intermetallic compounds Gd2Ni17−xAlx

²⁷Al Knight shifts vs temperature and magnetic susceptibility for the intermetallic compounds Gd₂Ni_17?xAl_x (x = 17; 16.2; 16; 15) are presented. The results are discussed in terms of the uniform polarization model fo the conduction electrons by the 4f and 3d spins localized on the Gd and Ni ions. The phenomenological exchange constants J_sf and J_sd range between ?1.80×10^?3 and 1.19×10^?3 eV and ?0.63×10^?3 and ?0.52×10^?3 eV, respectively.     

Tuning the magnetocaloric properties of La0.7Ca0.3MnO3 manganites through Ni-doping

The effect of Ni²⁺ doping on the magnetic and magnetocaloric properties of La_0.7Ca_0.3MnO₃ manganites synthesized via the auto-combustion method is reported. The aim of studying Ni²⁺-substituted La_0.7Ca_0.3Mn_1 ? xNi_xO₃ ($x=0,0.02,0.07$, and 0.1) manganites was to explore the possibility of increasing the operating temperature range for the magnetocaloric effect through tuning of the magnetic transition temperature. X-ray diffraction analysis confirmed the phase purity of the synthesized samples. The substitution of Mn³⁺ ions by Ni²⁺ ions in the La_0.7Ca_0.3MnO₃ lattice was also corroborated through this technique. The dependence of the magnetization on the temperature reveals that all the compositions exhibit a well-defined ferromagnetic to paramagnetic transition near the Curie temperature. A systematic decrease in the values of the Curie temperature is clearly observed upon Ni²⁺ doping. Probably the replacement of Mn³⁺ by Ni²⁺ ions in the La_0.7Ca_0.3MnO₃ lattice weakens the Mn³⁺–O–Mn⁴⁺ double exchange interaction, which leads to a decrease in the transition temperature and the magnetic moment in the samples. By using Arrott plots, it was found that the phase transition from ferromagnetic to paramagnetic is second order. The maximum magnetic entropy changes observed for the $x=0,0.02,0.07$, and 0.1 composites was 0.85, 0.77, 0.63, and 0.59 J/kg?K, respectively, under a magnetic field of 1.5 T. In general, it was verified that the magnetic entropy change achieved for La_0.7Ca_0.3Mn_1 ? xNi_xO₃ manganites synthesized via the auto-combustion method is higher than those reported for other manganites with comparable Ni²⁺-doping levels synthesized via standard solid state reaction. The addition of Ni²⁺ increases the value of the relative cooling power as compared to that of the parent compound. The highest value of this parameter (～60 J/kg) is found for a Ni-doping level of 2% around 230 K in a field of 1.5 T.     

Chemical order and magnetic properties of the Ni2−xMnSb system

The results of extensive magnetisation, X-ray and neutron powder diffraction measurements on the intermetallic compound series Ni_2-xMnSb, for 0?x?1, are reported. For x ?0.4, a high degree in the C1_b structure is observed, but for x?0.3, some disorder is evident. The series is ferromagnetic, with Curie temperatures rising from 368 to 732 K with decreasing nickel concentration. The magnetic moments all lie in the range (4.0±0.3)μ_B, with a maximum at a composition near x=0.45. The magnetic moments are largely associated with the ordered manganese sites, but it is possible that a small negative moment ?0.25μ_B may be associated with the “nickel sites” for x?0.3. This latter moment may be accounted for by disordered Mn atoms antiferromagnetically aligned.     

Magnetic phase diagrams of Ho1−xDyxNi2B2C

We performed the magnetization measurement on Ho_1−xDy_xNi₂B₂C single crystals (x=0.1, 0.2, 0.3, 0.4, and 0.6) with magnetic field applied perpendicular and parallel to the c-axis. But only for the magnetic field perpendicular to the c-axis, the increase of Dy³⁺ concentration affects the magnetically ordered states of HoNi₂B₂C compound and makes the phase diagram more complicated. The antiferromagnetic ordering state attributed to Dy³⁺ sublattice starts to appear from a case of x=0.2 and finally the magnetic phase diagram becomes analogous to that of DyNi₂B₂C as x is increased which is consistent with the neutron scattering result.     

NMR spectral densities and two dimensional diffusion in TiS2(NH3)1.0 and TaS2(NH3)x

NMR measurements of proton spin-lattice relaxation times T₁ and T_1? in the layered intercalation compounds TiS₂(NH₃)_1.0 and TaS₂(NH₃)_x (x = 0.8, 0.9, 1.0) are reported as functions of frequency and temperature (100 K – 300 K). These observations probe the spectral density of magnetic fluctuations due to motions of the intercalated molecules at frequencies accessible to the T₁ (4–90 MHz) and T_1? (1–100 kHz) measurements. Since the average molecular hopping time (τ) can be changed by varying temperature, different regions of the spectral density can be examined. For T > 200 K, both T^?1₁ and T^?1_1? vary logarithmically with frequency, reflecting the two dimensional character of the molecular diffusion. The temperature dependence of T₁ suggests that a more accurate picture of the short time dynamics is required. No dependence of relaxation rate on vacancy concentration is found.     

Spin-glass behavior of RNi1−xCuxAl compounds

The NdNi_1−xCu_xAl compounds were studied by means of heat-capacity, magnetization and AC-susceptibility measurements. The previously observed loss of long-range magnetic order in other RNi_1−xCu_xAl series with R=Tb, Dy, Er around x=0.6-0.8 that is accompanied by enhanced tendency to spin-glass character of behavior, is spread to wider concentration region of x=0.4-0.8 in the case of the newly studied Nd-based series, a candidate with rare-earth element coming from the light rare-earth group.     

Phase relationships and magnetic phase diagram in the system Mn1−xCuxCr2S4

Ternary spinel compounds have been found in the system Mn_1?xCu_xCr₂S₄ for x < 0.4 and x ? 0.8. The unit cell parameter does not follow a Vegard law. Magnetization measurements up to 150 kOe for Mn_0.95Cu_0.05Cr₂S₄ show three magnetic structures: Néel ferrimagnet, Yafet-Kittel and tridimensional. The magnetic phase diagram has been calculated within the molecular field approximation. There is strong evidence for the non-existence of Mn³⁺. Our findings are in conformity with a model proposed by Goodenough.     

The origin of magnetism in UNi2 — a simple band approach

The magnetization and susceptibility investigation of pseudo-binary U(Ni_1?xFe_x)₂ and U(Ni_1?xCu_x)₂ for chosen concentrations of x ? 0.1, x ? 0.8, and x ? 0.06, respectively, are presented. The most significant result is that the substitution of Ni (in UNi₂) by Fe reduces both the magnetic moment and the ordering temperature rapidly although it appears to be established that the magnetic moment of UFe₂ is predominantly residing on the Fe sites. The small concentration of Cu substituted into UNi₂, on the other hand, increases the magnetic moment. The obtained results are discussed together with those of U(Ni_1?xCo_x)₂ and seem to support the recently proposed explanation on the origin of magnetism in UNi₂.     

Spectroscopic and laser properties of (Pr,La)P5O14 and (Pr,Y)P5O14 crystals

Systematic emission and absorption spectroscopic studies of Pr_xLa_1?xP₅O₁₄ (for x=1, 0.9, 0.8, 0.7, 0.5, 0.1) and Pr_xY_1?xP₅O₁₄ (for x=0.75 and 0.5) were performed at 300 and 77 K. The structure of Pr³⁺ energetic levels in pentaphosphates was determined for each concentration considered. Numerical values of the most important spectroscopic parameters — the emission cross section, fluorescence quantum yield and branching ratio — determining the laser properties of the compounds investigated, were evaluated. Also, the radiative lifetime and normalized quenching rate values were obtained. The results of the measurements complete the information about the laser properties of praseodymium pentaphosphates, which were reported in previous papers.     

Mössbauer study of the re-entrant spin-glass behaviour in the mixed spinel ferrites Mg(0.9+x)Fe2(1−x)Ni0.1TixO4 (x=0.5 and 0.6)

Samples of the mixed spinel ferrite series Mg_(0.9+x)Fe_2(1−x)Ni_0.1Ti_xO₄ with x=0.5 and 0.6, prepared by solid state reaction of the appropriate oxides, have been investigated with ⁵⁷Fe Mössbauer spectroscopy. The as-prepared samples are found to be mainly superparamagnetic due to magnetic cluster formation. Samples after at least three times reheated exhibit spectra, which can be rather interpreted by a transversal relaxation of the spin above and spin-glass behaviour below the respective freezing temperatures T_f. External-field spectra reveal the canting to occur only on the octahedral sites. From the derived transition temperatures and thresholds together with data from earlier investigated sample with x=0.7 a compositional magnetic phase diagram for this spinel series is obtained.     

Structure, transport and magnetic properties in La2xSr2−2xCo2xRu2−2xO6

The perovskite solid solutions of the type La_2xSr_2−2xCo_2xRu_2−2xO₆ with 0.25≤x≤0.75 have been investigated for their structural, magnetic and transport properties. All the compounds crystallize in double perovskite structure. The magnetization measurements indicate a complex magnetic ground state with strong competition between ferromagnetic and antiferromagnetic interactions. Resistivity of the compounds is in confirmation with hopping conduction behaviour though differences are noted especially for x=0.4 and 0.6. Most importantly, low field (50 Oe) magnetization measurements display negative magnetization during the zero field cooled cycle. X-ray photoelectron spectroscopy measurements indicate the presence of Co²⁺/Co³⁺ and Ru⁴⁺/Ru⁵⁺ redox couples in all compositions except x=0.5. Presence of magnetic ions like Ru⁴⁺ and Co³⁺ gives rise to additional ferromagnetic (Ru-rich) and antiferromagnetic sublattices and also explains the observed negative magnetization.     

The influence of substitution of Co for Mn on magnetism of the HoMn6−xCoxSn6 compounds (0⩽x⩽0.25)

We have studied the effects of Co substitution for Mn on the structure and magnetic properties of the HoMn_6−xCo_xSn₆ compounds (0?x?0.25) with HfFe₆Ge₆-type structure (space group P6/mmm) by X-ray powder diffraction and magnetization measurements. A monotonic decrease of the lattice parameters a and c is observed with increasing Co content. While the compounds with x=0 and 0.05 exhibit ferrimagnetism in the whole temperature range, the compounds with 0.1?x?0.15 show ferrimagnetism, helimagnetism and re-entrant ferrimagnetism with decreasing temperature. For the compounds with x=0 and 0.05, the spin reorientation temperature is observed. A metamagnetic transition from helimagnetic magnetic ordering to ferrimagnetism is observed for the compounds with x=0.1 and 0.2. The results are summarized in the HoMn_6−xCo_xSn₆ magnetic phase diagram.     

The structural and magnetic properties of Ni2Mn1−xBxGa Heusler alloys

The influence of the substitution of manganese by boron on the crystal structure and magnetic properties of Ni₂Mn_1−xB_xGa Heusler alloys with 0?x?0.5 has been investigated using X-ray diffraction, thermal expansion, resistivity, and magnetization measurements. The samples with concentrations x<0.25 were found to be of single phase and belonged to the cubic L2₁ crystal structure at room temperature. Crystal cell parameters of the alloys decreased from 5.830 to 5.825 Å with increasing boron concentration (x) from 0 to 0.25. The alloys were ferromagnetically ordered at 5 K and the saturation magnetization decreased with increasing boron concentration. The ferromagnetic ordering and structural transition temperatures for 0?x?0.3 have been observed and the phase (x−T) diagram of the Ni₂Mn_1−xB_xGa system was constructed. The phase (x−T) diagram indicates that the ground state of Ni₂Mn_1−xB_xGa alloys belongs to ferromagnetic martensitic, premartensitic, and austenitic phases in x?0.12, 0.12<x?0.18, and 0.18<x?0.3, respectively. The relative influence of cell parameters and electron concentrations on the phase diagram is discussed.     

Magnetic characteristics of the intermetallic system Gd2Ni17−xAlx

The magnetic behaviour of the intermetallic compounds Gd₂Ni_17?xAl_x, (0?x?2) has been investigated both in the ferrimagnetic and paramagnetic regions. The unusual temperature dependence of the intensity of magnetization may be interpreted as a second order phase transition of order-order type, i.e. from colinear to non-colinear ferrimagnetic structure.     

EPR and magnetic susceptibility studies of the interaction between Cu2+ and Mn2+ ions in x(CuO·MnO)(1−x)[2B2O3·K2O] glasses

EPR and magnetic susceptibility experiments have been performed on x(CuO·MnO)(1?x)[2B₂O₃·K₂O] glasses with x varying in the range 0?x?50 mol.%. For x?3 mol.% both Cu²⁺ and Mn²⁺ ions are present mostly as the isolated species. The increase of the g-tensor values and bonding parameters (α², β², δ²) for Cu²⁺ ions together with the increase of TM ions concentration in the 0.2–1 mol.% range was noticed. In the case of 5 ? x ? 30 mol.% the dipole-dipole and superexchange interactions occur between transition metal ions, the first type of interactions prevailing in this range of concentration. For x30 mol.% the superexchange interaction prevail. The strong interaction between Cu²⁺ and Mn²⁺ gives rise to the exchange coupled Cu²⁺Mn²⁺ pairs in the studied glasses with x 3 mol.%.