Spin freezing properties in LaNi5-xFex

We report magnetic and transport measurements in the metallic pseudobinary compound LaNi_5-xFe_x, which show a Fe spin freezing. The magnetic phase diagram in the x-T plane is derived in the range 0 ⩽ x 1.2, 20 K ⩽ T ⩽ 300 K. Above a critical Fe concentration x_c ≈ 0.3, the system undergoes a superparamagnetic to long range ferromagnetic ordering at a finite temperature T. At lower concentrations, no long range spin ordering is observed, which suggests that x is the percolation threshold. Instead, a transition to a phase characterized by strong irreversibilities is observed at temperature T_f(x). Very strong ferromagnetic coupling interactions between Fe atoms are observed at low concentrations (x < x_c), which contrast with small values of T observed for x ∼ x_c. The results, together with the nature of the spin freezing are discussed in relation to the 2d character of the magnetic lattice.     

Magnetic structure of the random system near the ferro-antiferromagnetic transition

The magnetic structure of the disordered alloy Fe₆₅Ni₂₈Mn₇ was investigated in the temperature 4.2–300 K by the methods: small angle scattering of neutrons, Mössbauer effect, magnetization, magnetic contribution to the thermal coefficient of the thermal expansion, and resistivity. All measurements show that long-range ferromagnetic order appears below T_c ? 160 K. At the same time for T ? 100 K, a dramatic change of magnetic state takes place which is interpreted as the freezing of “spin glass”. An increase of the magnetic contribution to the resistivity with decreasing temperature was also found. This increase was attributed to the existence of poor-bonded magnetic moments of the Kondo-type. A model of the magnetic ground state is proposed which includes the details of magnetic behavior such as long-range ferromagnetic order, spin glass, finite ferro-and antiferromagnetic clusters, and Kondo-type states. A magnetic phase diagram of the system Fe₆₅(Ni_1?xMn_x)₃₅ is also proposed.     

Enhancement of the anomalous Hall effect and spin glass behavior in the bilayered manganite La2−2xSr1+2xMn2O7

The Hall resistivity and magnetization have been investigated in the ferromagnetic state of the bilayered manganite La_2−2xSr_1+2xMn₂O₇ (x=0.36). The Hall resistivity shows an increase in both the ordinary and anomalous Hall coefficients at low temperatures below 50 K, a region in which experimental evidence for the spin glass state has been found in a low magnetic field of 1 mT. The origin of the anomalous behavior of the Hall resistivity relevant to magnetic states may lie in the intrinsic microscopic inhomogeneity in a quasi-two-dimensional electron system.     

Reentrant magnetism - a low field study

We report low field dc magnetization measurements on (Fe_xMn_1?x)₇₅P₁₆B₆Al₃ alloys at 4 ? T ? 300 K. Reentrant magnetic behavior is observed for x = 0.65, 0.7 and 0.8. By comparing field cooled and zero-field cooled states at low T we separate out the reversible and irreversible contributions to the magnetization M and identify the (field-dependent) temperature for the onset of irreversibility. It is shown that the reversible part of the magnetization can be described by the usual scaling laws for critical behavior in magnetic systems not only at the transition from the paramagnetic to the ferromagnetic phase but also when the latter transforms to a spin glass. We identify the irreversible part of M with a spin glass order parameter.     

Magnetic hysteresis properties of metglasses from the system (Fe1−xCrx)85B15 close to the critical concentration for ferromagnetic long range order

We report on measurements of magnetic hysteresis loops for samples from the metglass system (Fe_1−xCr_x)₈₅B₁₅ for concentrations close to the critical concentration for ferromagnetic long range order, where it is assumed that a reentrance of the spin glass phase might occur at low temperatures. For the samples from the “reentrance”-concentration range the coercive force H_c and the initial permeability μ_i depend exponentially on temperature at low temperatures, whereas the remanent polarization is nearly temperature independent. We show that the exponential temperature dependence of H_c and μ_i is probably due to the thermal activation of domain wall movements. There is no indication of a “reentrance”-type of phase transition.     

Magnetic phase transitions and magnetoresistance in the La0.7Ba0.3(Mn1−xMex)O3 (Me=Fe,Al) single crystals and La0.7Sr0.3(Mn1−xMex)O3 (Me=Fe,Cr, Al) ceramics

The single crystals of La_0.7Ba_0.3(Mn_1−xFe_x)O₃ (x⩽0.28) and La_0.7Ba_0.3(Mn_1−xAl_x)O₃ (x⩽0.15) compositions were grown using flux method and characterized by X-ray, electrical and magnetization measurements. The Fe-doping above x=0.2 destroys a long range ferromagnetic order thus leading to a spin glass state. It is found that insulating spin glasses exhibit a large magnetoresistance in the paramagnetic region which is comparable to that for ferromagnetic crystals showing metal–insulator transition close to T_C. The magnetic behavior of La_0.7Ba_0.3(Mn_1−xMe_x)O₃ (Me=Fe, Cr, Al) ceramics is in agreement with superexchange magnetic interactions via oxygen.     

Anomalous temperature dependence of the upper critical fields in magnetic-superconducting Y1−xGdxRh4B4

We report that the dominant mechanism governing the upper critical field H_c2 in Y_1?xGd_xRh₄B₄ with x ≥ 0.1 is the magnetic exchange field of Gd spins. The H_c2 vs. temperature curve determines the magnetic structure to be of spin glass for x = 0.1 and ferromagnetic for x ≥ 0.2.     

Magnetostriction of amorphous GdxAg1−x alloys

Magnetostriction measurements above 4.2 K and up to 2.2 T have been performed in the amorphous alloys Gd_xAg_1−x (0.30 ⩽ x ⩽ 0.40). Magnetostriction is purely of volume character. It shows with composition a maximum at x = 0.35, and this behaviour can be explained if forced volume magnetostriction derives form the strain dependence of the two-ion longitudinal spin correlation. The isotropic spin exchange correlation seems irrelevant in these series, as also put forward by thermal expansion measurements.     

Magnetic resonance in amorphous FexNi80-xP14B6: I. Ferromagnetic and reentrant alloys

We present a study of ferromagnetic resonance in amorphous Fe_xNi_80-xP₁₄B₆ for Fe concentrations down to that required for ferromagnetism. The resonance was studied at microwave frequencies between 10 and 35 GHz and at temperatures between 2 and 300 K. We find i) in accord with previous data on amorphous ferromagnets, each alloy investigated is magnetically inhomogeneous even in its ferromagnetic state, ii) the intrinsic relaxation parameter λ / Mγ for each alloy falls between the value for pure Fe and the value for pure Ni, iii) a low temperature linewidth rise which is frequency independent and follows an empirical form suggested earlier, iv) frequency dependent linewidth maxima at low T which do not correspond to low field spin freezing temperatures, and v) anisotropy fields intruding at low temperatures. The corresponding anisotropy energy is similar to that proposed for spin glasses as are the temperature and frequency dependences of the anisotropy constant. With part II of this paper, this represents the most complete resonance study to date of the evolution of spin glass behavior in Fe based alloys.     

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.     

Mössbauer studies in reentrant spin glass (Pd x Pt1−x )3Fe

Mössbauer and Magnetization studies in the reentrant spin glass regime of the series (Pd_x Pt_1–x)₃ Fe clarifies some aspects of the multiple magnetic phase transitions. The data presented refute the presence of a long range ferromagnetic order at temperatures between the spin glass and the paramagnetic states. The ferromagnetic-like response can arise from the presence of magnetic clusters.     

A study on magnetoelastic properties of Tb3 (Fe28−xCox) V1.0 (x=0, 3, 6) compounds

In this work, The magnetoelastic properties of polycrystalline samples of Tb₃ (Fe_28−xCo_x) V_1.0 (x=0, 3, 6) intermetallic compounds are investigated by means of linear thermal expansion and magnetostriction measurements in the temperature range of 77–515 K under applied magnetic fields up to 1.5 T. The linear thermal expansion increases with the Co content. The well-defined anomalies observed in the linear thermal expansion coefficients for Tb₃ (Fe_28−xCo_x) V_1.0 (x=0, 3, 6) compounds are associated with the magnetic ordering temperature for x=0 and spin reorientation temperatures for x=3, 6. Below transition temperatures, the value of the longitudinal magnetostriction (λ_Pa) at 1.6 T increases with Co content.     

A Comparative Study on the Magnetic Properties of Arc-Melted and Ball-Milled Fe0.9−x Mn0.1Al x Alloys

Samples of nominal composition Fe_0.9?x Mn_0.1Al _x (0.1 ≤x≤0.5) were prepared both by mechanical alloying and arc-melting. In order to elucidate the effect of the synthesis method upon the magnetic properties of this system, we have carried out a comparative study involving the use of different experimental techniques (Mössbauer, X-ray diffraction, vibrating sample magnetometry and magnetic susceptibility). Results revealed that independently of the employed method and milling time, the samples exhibit ferromagnetism below ～34 at.% Al. Above this concentration, the preparation method became a determinant factor upon the magnetic properties of the system. The differences are attributed, in the case of the mechanically alloyed samples, to Fe contamination arising from jars material. The results of our study are summarized in a magnetic phase diagram including ferromagnetic, paramagnetic, pure spin glass and reentrant spin glass regions.     

Itenerant magnetic properties of amorphous metallic systems: Numerical studies of the Fe-B alloys

A self-consistent method to compute local magnetic moments and spin polarized electron densities of states and to test the stability of the ferromagnetic vs. the paramagnetic state is developed within the framework of the itinerant electron theory. The tight-binding Hamiltonian, with Coulomb correlation among d electrons, is parametrized according to the Slater-Koster interpolation scheme. The method goes beyond the rigid band splitting approximation. We apply it to the Fe_1−xB_x amorphous system for 0 ⩽x⩽0.6 and obtain the magnetization for T=0 as a function of x in agreement with experimental data in the experimentally-accessible concentration region.     

Magnetic and magneto-optical studies on Zn1−xCrxTe (x=0.05) films grown on glass substrate

Zn_1−xCr_xTe (x=0.05) films were prepared by thermal evaporation onto glass substrates. X-ray diffraction (XRD) was used to determine the crystalline quality of the ZnTe:Cr film. Magnetic force microscopy (MFM) investigation has shown a non-uniform distribution of magnetic domains with an average size of 4 nm at room temperature. SQUID measurements have further shown that the non-uniform distribution of domains does not affect the room temperature ferromagnetism of this material. Electron spin resonance spectroscopy (ESR) was done to determine the Cr valence state in the ZnTe lattice. Magnetic circular dichroism (MCD) analysis was used to confirm the ZnCrTe phase contributing to the ferromagnetic behavior.     

Anomalous thermal expansion of high-temperature superconductors Bi2Sr2−x LaxCuO6 at low temperatures

The low-temperature thermal expansion of Bi₂Sr_2?x La_xCuO₆ single crystals with different doping levels is measured. The effect of a magnetic field on the thermal expansion of Bi₂Sr_2?x La_xCuO₆ is studied. Anomalous (negative) thermal expansion whose characteristics depend on the doping level of the sample is revealed at T ≤ 10–20 K. It is found that these characteristics are significantly affected by moderately strong magnetic fields (2–4 T). The experimental results can be interpreted within a model relating the anomalous thermal expansion in high-temperature superconductors at low temperatures and the strong effect of the magnetic field on this expansion to the change in the stabilizing role of the charge-density waves in the oxygen sublattice.     

Magnetic and transport property studies of nanocrystalline ZnxFe3−xO4

The magnetic and transport properties of nanocrystalline Zn_xFe_3−xO₄ with x=0.0, 0.2, 0.4, 0.5, 0.6, 0.8 and 1.0, respectively, fabricated by the sol-gel method have been investigated. Large magnetoresistance (MR) was observed and found to be originated both from the tunneling of the spin-polarized electrons across the adjacent ferromagnetic grains and the scattering by the canted spins at the grain surface near the grain boundaries. It has been revealed that the MR for the Zn_xFe_3−xO₄ samples (x=0, 0.5 and 1.0) increases with the temperature decreasing from room temperature until a maximum is reached at around 55 K. Then a sharp drop occurs with the further decrease in temperature, regarded as a spin (cluster) glass transition. For the samples studied, a biggest low field (0.5 T) MR value of about 20% for x=0 at 55 K has been obtained. The mechanism of the MR behavior of the materials was discussed.     

Magnetic dilution and electronic nature in La0.7Pb0.3Mn1−xZnxO3

We have studied the magnetic dilution and electronic nature of Zn doping on the Mn site in the colossal magnetoresistant material La_0.7Pb_0.3MnO₃ (x≤0.3). Small non-magnetic Zn²⁺ doping tends to separate the system into ferromagnetic clusters to weaken the long-range ferromagnetic order and to reduce the Curie temperature. The spin polarizability of the x=0–0.3 samples is estimated to be 0.97–1.00, indicating that the x=0–0.3 samples are the spin polarized materials in which the conductivity is dominated by single-spin charge carriers. Small doping (x≥0.1) induces the metal–insulator transition and destroys the metallic state with long-range ferromagnetic order.     

Magnetic behavior of Fe0.2Mg0.8TiO3 and Fe0.3Mg0.7TiO3 probed by Mössbauer timescale

Magnetization and Mössbauer measurements have been made on a diluted antiferromagnets Fe_xMg_1?xTiO₃ with x=0.2, 0.3 and 0.7. It has been demonstrated that this system exhibits a spin glass or a reentrant spin glass like behavior in the samples with x around the percolation concenrration x_c≈0.25. It has been shown that various sizes of magnetic clusters are formed well above the transition temperatures determined by the magnetization measurements in the samples with x near x_c. The behavior of Fe_0.2Mg_0.8TiO₃ is discussed as a typical cluster glass.