Mössbauer study of Co-substituted magnetite

Some aspects of a comprehensive Mössbauer spectroscopic study at variable temperatures and in applied magnetic fields of the spinel system Co _x Fe_3?x O₄ with 0≤x≤0.6 are presented. Emphasis is given to the dipolar contributionB _dip to the magnetic hyperfine field at the octahedral sites. Its dependence on the temperature and on the Co concentration is determined and discussed.     

Experimental evidences of magnetic two-phase state in Eu1−xAxMnO3 (A = Ca,Sr)

For insulating Eu_1−xA_xMnO₃ (A = Ca, Sr; x = 0, 0.3) ceramics the magnetic properties depend on the thermomagnetic history of the sample and the maximum of initial magnetic susceptibility is located at T=T_N, being independent of the AC field frequency. Magnetization isotherms are the superposition of the linear part and spontaneous magnetization. The maximum of electrical resistivity and colossal magnetoresistance are observed in the T_N region. These properties are explained by the magnetic two-phase state.     

The magnetic susceptibility of hydrogen-doped partially crystalline (Zr76Ni24)1−xHx metallic glasses

The magnetizations of Zr₇₆Ni₂₄ metallic glass and hydrogen-doped partially crystalline (Zr₇₆Ni₂₄)_1−xH_x metallic glasses have been measured in the temperature range 10-300 K and magnetic fields up to 2 T for various dopant concentrations (x=0, 0.024, 0.043, 0.054). It is found that the samples are paramagnetic and magnetic susceptibility at room temperature, χ(300 K), shows a nonmonotonic behaviour upon hydrogenation. The values of χ(300 K) of the hydrogen-doped partially crystalline (Zr₇₆Ni₂₄)_1−xH_x metallic glasses are reduced with increase in hydrogen content up to x=0.043, whereas for x=0.054, an enhancement of χ(300 K) has been revealed. The magnetic susceptibility is weakly temperature dependent down to 110 K, below which an increase is observed. A shallow minimum exists between 90 and 120 K. The form and magnitude of the observed temperature dependence of the magnetic susceptibility are well accounted for by the sum of the quantum corrections to the magnetic susceptibility. Hydrogen reduces the electronic diffusion constant and influences strongly the quantum interference at defects, slowing down the spin diffusion and enhancing the magnetic susceptibility in the temperature range from 110 down to 10 K.     

The parallel susceptibility at 0 K of the double magnetic helix

The parallel susceptibility x^∥ at 0 K of a double magnetic helix of pitch angle ? in which two species of magnetic ions are acted on by unequal molecular fields is determined. The conditions under which x^∥ is equal to the 0 K perpendicular susceptibility x^⊥ calculated in the past are determined. Comparison is made with the powder susceptibility measurements performed on the orthorhombic compound La₂Fe_1.5Mn_0.5S₅ which displays a double magnetic helix up to T_N = 85 K.     

Effect of Ti4+ ions on the magnetic and dielectric properties of Mg-ferrite

X-ray diffraction, the real part (ε′), the imaginary part (ε″) of dielectric constant, and the molar magnetic susceptibility (χ_M) for Mg_1+xTi_xFe_2−2xO₄ ferrite (0.1⩽x⩽0.9) were studied. The date of X-ray diffraction showed that the unit cell parameter increases with Ti concentration and ascribed to the predicted variation of the cation distribution, while Mg²⁺ ions are highly diffusible and very sensitive to heat. The effect of dilution by Ti ions is discussed in terms of increasing superparamagnetic and single domain (SP/SD) grains. The measurements of ε′ were performed at different temperatures as a function of frequency, while the magnetic susceptibility was studied at different magnetic field intensities. The variation of the dielectric properties depends mainly on the valence exchange between the different metal ions in the same site or in different sites. All parameters such as ε′, ε″, χ_M showed a decrease in value with increasing Ti and Mg concentration. The dispersion in ε′ with frequency disappeared gradually with increasing Ti concentration.     

Structural, magnetic and hyperfine characterization of zinc-substituted magnetites

Nanosized Fe_3???x Zn _x O₄ powders were synthesized by co-precipitation and characterized by total chemical analysis, X-ray diffraction, magnetic susceptibility and Mössbauer spectroscopy. The results showed that, for x?≤?0.15, the as-prepared samples are mostly zinc-substituted magnetites but have maghemite as a minor phase. For x?≥?0.30, only the Fe_3???x Zn _x O₄ solid solution is found. Increasing the zinc content from the end concentration x?=?0, increases the lattice parameter but smaller become the mean crystalline diameter and the magnetic susceptibility. In addition, the magnetic hyperfine fields of the iron sites in the spinel structure, A and B, decrease up to collapse at x?≤?0.90.     

Magnetization and AC susceptibility of TbxY1-xCo2 compounds

Magnetization and ac susceptibility measurements have been performed on Tb_xY_1-xCo₂ compounds. Samples with x > 0.1 order ferromagnetically below room temperature. The cobalt magnetic moments in these compounds are induced by the internal magnetic field exerted by the terbium moments on the cobalt atoms. The ac susceptibility measurements indicate a change from second order to first order in the ferromagnetic transition for samples with x-values equal or smaller than 0.5.     

Quantum bicriticality in Mn1 − x Fe x Si solid solutions: Exchange and percolation effects

The T-x magnetic phase diagram of Mn_{1 ? x} Fe _x Si solid solutions is probed by magnetic susceptibility, magnetization and resistivity measurements. The boundary limiting phase with short-range magnetic order (analogue of the chiral liquid) is defined experimentally and described analytically within simple model accounting both classical and quantum magnetic fluctuations together with effects of disorder. It is shown that Mn_{1 ? x} Fe _x Si system undergoes a sequence of two quantum phase transitions. The first “underlying” quantum critical (QC) point x* ～ 0.11 corresponds to disappearance of the long-range magnetic order. This quantum phase transition is masked by short-range order phase, however, it manifests itself at finite temperatures by crossover between classical and quantum fluctuations, which is predicted and observed in the paramagnetic phase. The second QC point x _c ～ 0.24 may have topological nature and corresponds to percolation threshold in the magnetic subsystem of Mn_{1 ? x} Fe _x Si. Above x _c the short-range ordered phase is suppressed and magnetic subsystem becomes separated into spin clusters resulting in observation of the disorder-driven QC Griffiths-type phase characterized by an anomalously divergent magnetic susceptibility χ ～ 1/T ^ξ with the exponents ξ ～ 0.5–0.6.     

Magnetic behaviour of Fe–Cr nanoparticle systems

The low-temperature magnetic properties of Fe_1−xCr_x nanoparticles with various chromium content (x=2.4−83.0 at%) have been studied. The multiphase particles (α-FeCr, σ-FeCr and Fe/Cr oxides) have a core (metallic)–shell (oxide) structure. The magnetic properties of the Fe–Cr systems depend on the chromium content as well as on the types and relative contributions of the constituent crystalline phases but, in particular, they are determined by long-range interparticle dipolar interactions. The role of the weakly magnetic σ-FeCr phase is also discussed.     

Comparison between the magnetic susceptibilities of Pd1−xAgx and single phase PdHn

The rapid decrease of the isothermal magnetic susceptibility χ of single phase PdH_n with n ( = atomic ratio H/Pd) above the critical temperature T_c = 564 differs from that of the magnetically similar single phase system Pd_1−xAg_x (x: silver mole fraction) at the same temperature and at equal valence electron concentrations (n = x), i.e. χ (PdH_n)−χ(Pd_1−xAg_x) <0. By use of a semiphenomenological susceptibility ansatz related to the nonideal solution behaviour of H (Ag) in Pd the susceptibility difference is interpreted as an electronic excess effect.The analysis of the steep descent of the magnetic susceptibility also applies to the Pd-rich side of PdH_n in the subcritical temperature region (so-called α-phase) and can be supported by ¹⁰⁵Pd Knight shift data at 348 K.     

Amorphous magnetic alloys near critical concentration: Low field reversible dc magnetization

Measurements of the reversible magnetization at low dc fields have been used to investigate the magnetic response near the multicritical point (x_c) of two sets of amorphous alloys. In both cases, the ferromagnetic (FM) to spin glass (SG) transition line is found to be non-monotonic. The collapse of the magnetization as x→x⁺_c and the rapid increase in the susceptibility as x→x^-_c are suggestive of a percolation transition in the magnetic network at x=x_c. From a study of the non-linear susceptibility in the most concentrated spin glass alloy in each system, we obtain scaling exponents in agreement with previous reports providing further support for a thermodynamic phase transition at the spin glass temperature. For the first time we find a divergence in the linear susceptibility in these samples similar to that expected for the non-linear susceptibility and attribute it to their proximity to the FM phase. Dramatic changes in the transition temperatures and a perceptible shift in x_c are observed when normal boron is replaced by enriched boron (≈100%¹¹B) in one series of alloys.     

Magnetic phase diagram of diluted spinel Zn1−xCuxCr2Se4 system

By using mean field theory, we have evaluated the nearest-neighbour and the next-neighbour super-exchange J₁(x) and J₂(x), respectively, for Zn_1−xCu_xCr₂Se₄ in the range 0?x?1. The intraplanar and the interplanar interactions are deduced. High-temperature series expansions are derived for the magnetic susceptibility and two-spin correlation functions for a Heisenberg ferromagnetic model on the B-spinel lattice. The calculations are developed in the framework of the random phase approximation. The magnetic phase diagram is deduced. A spin glass phase is predicted for intermediate range of concentration. The results are comparable with those obtained by magnetic measurements. The critical exponents associated with the magnetic susceptibility (γ) and the correlation lengths (ν) have been deduced. The values are comparable to those of the 3D Heisenberg model, and are insensitive to the dilution x.     

A semimetal model of the normal state magnetic susceptibility and transport properties of Ba(Fe1−xCox)2As2

A simple two-band 3D model of a semimetal is constructed to determine which normal state features of the Ba(Fe_1?xCo_x)₂As₂ superconductors can be qualitatively understood within this framework. The model is able to account in a semiquantitative fashion for the measured magnetic susceptibility, Hall, and Seebeck data, and the low temperature Sommerfeld coefficient for 0 < x < 0.3 with only three parameters for all x. The purpose of the model is not to fit the data but to provide a simple starting point for thinking about the physics of these interesting materials. Although many of the static magnetic properties, such as the increase of the magnetic susceptibility with temperature, are reproduced by the model, none of the spin-fluctuation dynamics are addressed. A general conclusion from the model is that the magnetic susceptibility of most semimetals should increase with temperatures.     

Probing local magnetic cluster development in (CuZr)93−xAl7Gdx bulk metallic glasses by Al NMR

We report magnetization properties of (CuZr)_93−xAl₇Gd_x bulk metallic glasses from temperature dependent ²⁷Al nuclear magnetic resonance spectroscopy and magnetic susceptibility measurements. Significant non-linear line broadening of ²⁷Al spectra commencing at high temperatures is attributed to the development of a local magnetic susceptibility distribution that prevails over a finite temperature range. Magnetization measurements confirm the linewidth enhancement due to strong frustrated magnetic short-range order. This study provides insight into the nature of magnetic development and frustration in paramagnetic systems.     

Superconducting and magnetic properties of the oxide superconductors La1.85Sr0.15Cu1−xFexO4

Mössbauer and magnetic measurements have been made in the perovskite superconductors La_1.85Sr_0.15Cu_1−x ⁵⁷Fe_xO₄ for x=0.0025, 0.005, 0.01, 0.05 and 0.10. The superconducting transition temperature is rapidly depressed as x increases, reaching T_c=0 for x≈0.03. Mössbauer data for x=0.05 show behavior below ≈15 K which indicates magnetic ordering. Magnetic susceptibility data give an effective moment of 3–4 μ_B/Fe atom, and a hand-like susceptibility which increases with x.     

Effect of thermal instability on the magnetic properties of Cu<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Zn<Subscript>x</Subscript>Cr<Subscript>2</Subscript>Se<Subscript>4</Subscript> solid solutions

Annealing in vacuum is found to affect magnetic order in polycrystalline Cu_1?xZn_xCr₂Se₄ samples (x=0.88, 0.90). Samples subjected to heat treatment exhibit a temperature dependence of dynamic magnetic susceptibility characteristic of a non-single-phase magnetic state. The annealing-induced magnetic order is assigned to the zinc off-stoichiometry formed in the process.     

The magnetic transition in Fe-substituted hexagonal β-Al9SiMn3 phase

Recently we have studies β-Al₉Si(Mn_1?x Fe _x )₃ using Mössbauer effect spectroscopy withx=0.28 and discovered a new transition at about 80 K. Neutron diffraction studies down to low temperatures reveal no structural changes, so that this transition at 80 K is indeed of magnetic origin. The magnetic susceptibility shows a small maximum near this temperature superimposed on a large Curie-like background. In addition to this transition, the Curie-like background shows a cusp near 4 K. We thus infer a magnetic double transition in this disordered alloy. These magnetic transitions have been followed to lower Fe concentrations (x=0.10 and 0.20) by both Mössbauer spectroscopy (T _c ) and by magnetic susceptibility (T _g). It is found thatT _C decreases rapidly with decreasing Fe concentration, whileT _g remains relatively constant. The saturation hyperfine fieldB _hf(0) decreases rapidly with decreasingx, indicating that the Fe atoms loose their magnetic moments in the limit of infinite dilution, and the important role played by Fe?Fe nearest neighbors in determining the magnetic properties.     

Competition between spin-glass and antiferromagnetic order in EuxSr1−xAs3

Measurements of the magnetic a.c. susceptibility χ of Eu_xSr_1?xAs₃ single crystals (0.10 ? x ? 0.78) are reported. For the lower concentrations, spin-glass like maxima with a.c. frequency dependent positions and heights are observed. Samples with x = 0.54 and 0.65 exhibit a complex magnetic behavior (with directional dependence of χ observed for x = 0.54) which gives way to antiferromagnetic ordering when x is further increased towards EuAs₃.     

Dipolar interactions in rare earth orthoferrites—I.: YFeO3 and HoFeO3

The part of dipolar interactions in the magnetic properties of YFeO₃ and HoFeO₃ is examined in detail.In YFeO₃, one finds that the contribution of these interactions to the anisotropy in the x0z plane (easy magnetization plane for the iron moments) has the same order of magnitude as the crystalline anisotropy.In the case of HoFeO₃, the molecular field formalism is used in order to interpret the existence of a single, ferromagnetic ordered structure for temperatures below a rearrangement temperature, T_R. The physical parameters introduced within the framework of this formalism are fitted by comparison with the available experimental results.     

Magnetic properties,phase transitions and microstructural effects in mixed GdLa-based glasses

Magnetic properties of amorphous Gd_xLa_65-xCo₂₅B₁₀ are investigated from 4.2 to 300 K. AC susceptibility and magnetization measurements to 80 kOe are used to determine a phase diagram including paramagnetic, ferrimagnetic and spin-glass regions. For x ≲ 40 diffraction and susceptibility data indicate the increasing development of chemical short-range order and phase separation. An analysis of the magnetic structure and local-moment magnitudes is presented. Results of a scaling analysis for one paramagnetic-ferrimagnetic transition are discussed and some conclusions are drawn concerning the relationship between phase transitions and chemical short-range-order in ternary or higher-order magnetic glasses.