Investigation of the thermal and magnetic properties of Fe61Co10Zr2 Fe61Co10Zr2.5Hf2.5Me2W2B20 (Me = Y, Nb, W, Ti, Mo, Ni) bulk amorphous alloys obtained by an induction suction method

The microstructure, magnetic properties and thermal stability of Fe₆₁Co₁₀Zr_2.5Hf_2.5Me₂W₂B₂₀ (Me = Y, Nb, W, Ti, Mo, Ni) alloys were investigated. The samples were obtained by an induction suction method as 0.5 mm thickness plates. The microstructure was examined using X-ray diffraction and Mössbauer spectroscopy. It was shown that the investigated samples have amorphous structure throughout the volumes of the samples. The magnetic properties were measured using a Vibrating Sample Magnetometer. The investigated alloys are soft magnetic materials with low coercivity field (from 5.8 A/m to 54 A/m) and high saturation of the magnetization (from 0.87 T to 1.26 T). The studies of thermal stability were performed using a differential scanning calorimeter. It was shown that the addition of respective atoms led to changes of Curie temperature in the range from 497 to 587 K, depending on the composition of the alloys.     

Magnetic entropy changes at early stages of nanocrystallization in amorphous Fe90Zr7B3 ribbons

Microstructure, revealed by transmission electron microscopy and conventional Mössbauer spectroscopy, magnetization versus magnetizing field induction and temperature and isothermal magnetic entropy changes in the as-quenched and subjected to annealing at T_a1 = 723 K for 2 or 3 h and at T_a2 = 743 K for 2.5 h of Fe₉₀Zr₇B₃ amorphous alloy are studied. In the as-quenched state the medium range ordered regions are observed. The annealing at T_a1 leads to early stages of crystallization and nanograins with different diameter embedded in amorphous matrix are formed. At the Curie point of the amorphous phase they are magnetically decoupled and behave like superparamagnetic particles. The Curie point of the residual amorphous phase shifts towards higher temperature as compared to the as-quenched state due to the Invar like effect. The peak of the isothermal magnetic entropy changes appears at the Curie temperature of the main amorphous phase. Their values at the maximum applied field of 0.75 T equals to 0.32 J/kg K⁻¹ in the as-quenched alloy and remain almost unchanged after early stages of nanocrystallization. After the annealing at T_a2 the peak of the entropy changes distinctly decreases. Such behavior is ascribed to the biphasic character of the sample. The main amorphous phase and ordered one, which in some circumstances can be treated as an assembly of superparamagnetic particles, contribute to the total magnetic entropy changes.     

Magnetic properties of the 1/1 approximant Ag42In42Gd16 to the icosahedral quasicrystal Ag-In-Gd

The structural, magnetic, and ¹⁵⁵Gd Mössbauer spectral properties of the 1/1 approximant Ag₄₂In₄₂Gd₁₆ to an icosahedral quasicrystal Ag-In-Gd are reported. Based on dc magnetic susceptibility measurements, it is shown that the studied compound develops no long-range magnetic order in the temperature range 1.8-300 K. The dc zero-field-cooled and field-cooled susceptibility data indicate that the 1/1 approximant Ag₄₂In₄₂Gd₁₆ is a spin glasss with freezing temperature T_f = 3.6(1) K. This is further confirmed by the analysis of the frequency dependence of T_f using the Vogel-Fulcher law and the dynamic scaling behavior near T_f. It is argued that the spin freezing process is a true equilibrium phase transition rather than a nonequilibrium phenomenon. The large frustration parameter of the studied compound indicates that it belongs to a category of strongly geometrically frustrated magnets. The ¹⁵⁵Gd Mössbauer spectra of the 1/1 approximant Ag₄₂In₄₂Gd₁₆ confirm that the Gd spins are frozen at 1.5 K and are fluctuating at 4.6 K. The Debye temperature of the 1/1 approximant Ag₄₂In₄₂Gd₁₆ is 200(1) K.     

Grain size dependent magnetization, electrical resistivity and magnetoresistance in mechanically milled La0.67Sr0.33MnO3

We have studied magnetization, ac susceptibility, resistivity and magnetoresistance in mechanically milled La_0.67Sr_0.33MnO₃. The material with grain size micron to nanometer scale has stabilized in rhombohedral crystal structure with space group R3C. We have found various grain size effects, e.g., decrease of ferromagnetic moment, increase of surface spin disorder, and appearance of insulator/semiconductor type resistivity. In addition to these conventional features, we have identified a magnetic anomaly at 45 K in bulk sample. Ferromagnetic to paramagnetic transition temperature (T_C) is above room temperature for all samples. The samples are typical soft ferromagnet that transformed from multi-domain state to single domain state in nanocrystalline samples. The remarkable observation is that low temperature freezing of ferromagnetic domains/clusters does not follow the conventional spin glass features. Experimental results clearly showed the enhancement of high field magnetoresistance in nanocrystalline samples below 200 K, whereas low field magnetoresistance gradually decreases above 200 K and almost absent at 300 K. We have discussed few more magnetic and electrical changes, highly relevant to the progress of nanomaterial research in ferromagnetic manganites.     

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.     

Characterization of the martensitic transformation in Ni50−xTi50Cux alloys through pure thermal measurements

The addition of a third element to the Ni-Ti system often changes the product and the path of the martensitic transformation of the alloy, which is a direct B2-B19′ transformation for the NiTi alloy in the fully annealed state. In this study we investigate the martensitic transformation of fully annealed Ni_50−xTi₅₀Cu_x (x = 3-10 at%) shape memory alloy (SMA) samples using differential scanning calorimetry (DSC) and the four-probe electrical resistance (ER) measurements under stress-free conditions. DSC and ER data show that the ternary alloy goes through a direct B2-B19′ transformation for Cu content between 3 and 7 at% and through the two-stage B2-B19-B19′ transformation for Cu content between 8 and 10 at%. We find good agreement between the two techniques as regards the detection of the phase transformation temperatures. B19′ starting and finishing temperatures decreases with the increases of Cu content and show a significant reduction starting from 7 at%; the range of temperatures in which B19 is stable increases with increasing Cu content.     

The effect of the substitution of Cu for Ni on magnetoresistance and magnetocaloric properties of Ni50Mn34In16

The effect of the substitution of Cu for Ni on the magnetoresistance behaviour and the magnetocaloric properties of the Ni₅₀Mn₃₄In₁₆ alloy has been investigated. The (Ni-Cu)₅₀Mn₃₄In₁₆ alloys crystallize in the cubic L2₁ structure in austenite phase. While the M_s temperature is about 160 K for the Ni_47.5Cu_2.6Mn_34.0In_15.9 alloy, the martensitic transition is not observed for Ni_45.5Cu_4.6Mn_33.8In_16.1 alloy. To estimate the magnetic entropy change of the Ni_47.5Cu_2.6Mn_34.0In_15.9 alloy, the magnetization measurements as function of magnetic field are performed by continuous and noncontinuous heating methods. The Ni_47.5Cu_2.6Mn_34.0In_15.9 alloy shows the magnetostructural transition whose irreversible ratio is about 50% at 160 K. The magnetocaloric effect strongly depends on the sample history due to the occurrence of the irreversible magnetostructural transition. For the magnetic field change of 2 T, giant magnetoresistance value is about −68% that is rather big among the similar bulk alloys.     

Effect of ambient air pressure on the oxidation kinetics of Fe-6 at.% Si alloy

Surface oxidation of Fe-6 at.% Si alloy was investigated during annealing in ambient air of various pressures with simultaneous isothermal resistivity registrations. Measurements have been done in the temperature range 500-540 °C. Chemical and phase compositions of the samples were analyzed using X-ray photoelectron spectroscopy, conversion electron Mössbauer spectroscopy (CEMS), transmission Mössbauer spectroscopy (TMS), X-ray diffraction (XRD), and scanning electron microscopy (SEM). Phase analysis showed that during isothermal resistivity measurement in a low pressure air 100 mbar a protective film of hematite α-Fe₂O₃ was formed on the surface of FeSi substrate. By decreasing pressure to 10⁻² mbar the time dependence of the resistivity exhibits an increase due to the transformation of hematite to magnetite Fe₃O₄. The activation energy for this transformation is 115 ± 5 kJ/mol. By regressive increasing the pressure back from 10⁻² to 100 mbar a non-protective oxide scale of hematite + magnetite was formed. The results were interpreted in the light of the iron-oxygen phase diagram.     

MAGNETIC--FIELD--CONTROLLED SHAPE MEMORY EFFECT AND SUPERELASTICITY OF Ni53.2Mn22.6Ga24.2 SINGLE CRYSTAL

证实了Ni_53.2Mn_22.6Ga_24.2单晶发生的两步马氏体相变行为是完全热弹性的. 在磁场作用下, 该材料的马氏体相变和中间马氏体相变展现出相同的应变特征, 且具有磁控双向形状记忆效应. 磁场下应力--应变特性的测量结果表明, 磁场不但对压应力诱发马氏体相变过程中变体重取向所需应力的大小有影响, 而且使原来不可逆的形变成为可逆, 这种磁控超弹性特性预示了该合金用作磁控超弹性元器件材料的可能性.     

Structural and magnetic properties of nanostructured Fe50(Co50)-6.5 wt% Si powder prepared by high energy ball milling

This work investigates the effects of 6.5 wt% Si addition and milling times on the structural and magnetic properties of Fe₅₀Co₅₀ powders. For this purpose, at first the elemental Fe and Co powders were milled for 10 h to produce Fe₅₀Co₅₀ alloy and then Si was added and the new product was milled again for different times. The microstructural and magnetic properties were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and vibrating sample magnetometer (VSM). The results show that the minimum crystallite size of the as-milled powders (∼12 nm) has been achieved after introducing Si and milled for 8 h (total milling time of 18 h). Also an amount of 188 emu/g has been achieved for M_s. This amount of M_s is higher than most of those which have been already reported for M_s of different Fe-Si systems.     

High temperature transformations of the Au7Cu5Al4 shape-memory alloy

The β-phase of Au₇Cu₅Al₄ undergoes a reversible shape-memory phase transformation, however there has been some uncertainty regarding the crystal structure or structures of the parent phase. Here we show that, under equilibrium conditions, the parent phase possesses the L2₁ structure between its A_p (about 79 °C) and ∼630 °C, and the B2 primitive cubic structure between ∼630 °C and its melting point. It melts directly from B2 into the liquid state and hence never achieves the random bcc A2 structure that has been previously mooted. Splat-cast samples of the alloy are martensitic, proving that development of equilibrium order and defect concentration are not pre-requisites for the A → M transformation to occur.     

Temperature and field induced strain in polycrystalline Ni50Mn35In15−xSix magnetic shape memory Heusler alloys

Magnetic shape memory properties of polycrystalline Ni₅₀Mn₃₅In_15−xSi_x were investigated. A reversible strain of more than 0.4% was observed for x = 0 at a magnetic field H = 5 T that was found to be associated with a field induced reverse martensitic transformation. The strains were found to increase with the substitution of In by Si and strains larger than 1% were observed for x = 2 at H = 5 T. Both the positive and negative strain changes were observed in the vicinity of martensitic transition temperatures. The strain in Ni₅₀Mn₃₅In_15−xSi_x was found to depend on silicon concentration, and on samples texture.     

Thermal cycling behaviors of the intermartensitic transformation in a polycrystalline Ni52.5Mn23.7Ga23.8 alloy

Thermally induced intermartensitic transformation in polycrystalline Ni_52.5Mn_23.7Ga_23.8 has been investigated by differential scanning calorimetry (DSC) and X-ray diffraction (XRD). It is found that after annealing at 500 °C for 4 h an intermartensitic transformation, seven-layered orthorhombic martensite (7M) → five-layered tetragonal martensite (5M), appears in polycrystalline Ni_52.5Mn_23.7Ga_23.8 alloy quenched from 800 °C, where the sequence of phase transformations is austenite phase (A) → 7M → 5M during cooling and 5M → 7M → A during heating. The intermartensitic transformation is an independent phase transformation, but the critical transition temperatures and the transformation temperature ranges of 7M → 5M are strongly affected by the martensitic transformation.     

Synthesis, structure, and high temperature Mössbauer and Raman spectroscopy studies of Ba1.6Sr1.4Fe2WO9 double perovskite

Ba_1.6Sr_1.4Fe₂WO₉ has been prepared in polycrystalline form by solid-state reaction method in air, and has been studied by X-ray powder diffraction method (XRPD), and high temperature Mössbauer and Raman spectroscopies. The crystal structure was resolved at room temperature by the Rietveld refinement method, and revealed that Ba_1.6Sr_1.4Fe₂WO₉ crystallizes in a tetragonal system, space group I4/m, with a = b = 5.6489(10)Å, c = 7.9833(2)Å and adopts a double perovskite-type A₃B′₂B″O₉ (A = Ba, Sr; B′ = Fe/W, and B″ = Fe/W) structure described by the crystallographic formula (Ba_1.07Sr_0.93)_4d(Fe_0.744W_0.256)_2a(Fe_0.585W_0.415)_2bO₆. The structure contains alternating [(Fe/W)_2aO₆] and [(Fe/W)_2bO₆] octahedra. Mössbauer studies reveal the presence of iron in the 3+ oxidation state. The high temperature Mössbauer measurements showed a magnetic to paramagnetic transition around 405 ± 10 K. The transition is gradual over the temperature interval. The decrease in isomer shift is in line with the general temperature dependence. While the isomer shift is rather linear over the whole temperature range, the quadratic dipolar ΔE temperature dependence shows an abrupt change at 405 K. The latter results allow concluding that a temperature-induced phase transition had occurred. The high temperature Raman study confirms the Mössbauer results on the magnetic to paramagnetic transition.     

Luminescent and magnetic properties of YVO4:Ln@Fe3O4 (Ln = Eu or Dy) nanocomposites

A series of core-shell bifunctional magnetic-optical YVO₄:Ln³⁺@Fe₃O₄ (Ln³⁺ = Eu³⁺ or Dy³⁺) nanocomposites have been successfully synthesized via two-step method. The nanocomposites have the advantage of high magnetic responsive and unique luminescence properties. The structure, luminescent and magnetic properties of the nanocomposites were investigated by XRD, TEM, PL and VSM. The maximum emission peaks of the nanocomposites are at 618 nm (doping Eu³⁺), 574 nm (doping Dy³⁺). The special saturation magnetization of the nanocomposites is 54 emu/g. The diameter of the nanocomposites is 400-900 nm.     

Electric field-modulated Hall resistivity and magnetization in magnetoelectric Ni-Mn-Co-Sn/PMN-PT laminate

The effects of electric field on the magnetization and Hall resistivity were investigated in a laminated composite consisting of Ni₄₃Mn₄₁Co₅Sn₁₁ alloy and Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ ferroelectric single crystal. Upon applying an electric field (3 kV/cm) on the single crystal, the change of Hall resistivity in the alloy is up to 45%. The co-action of magnetization change and the different carrier concentration between the martensitic and austenite phases of alloy, which result from the stress-induced martensitic transformation, are responsible for the electric field-modulated Hall resistivity.     

Enhanced magnetoelectric properties in Bi0.95Ho0.05FeO3 polycrystalline ceramics

Polycrystalline samples of Ho doped BiFeO₃ were prepared by solid state reaction method and effect of partial substitution of Ho on dielectric, magnetic and ferroelectric properties was studied. High temperature dielectric results show two dielectric anomalies both in ? and tan δ, out of which, anomaly at higher temperature (∼400 °C) could be ascribed to antiferromagnetic Néel temperature which, is a signature of magnetoelectric coupling. The magnetic moment is greatly improved and the maximum magnetization was found to be 0.736 emu/g. Saturated ferroelectric hysteresis loops were observed for Bi_0.95Ho_0.05FeO₃ with remnant polarization (P_r) = 1.59 μC/cm², maximum polarization (P_max) = 2.56 μC/cm² and coercivity (E_c) = 5.45 kV/cm. We have conducted comprehensive magnetoelectric and magnetodielectric properties at room temperature. Magnetic field induced ferroelectric hysteresis loop observed in Bi_0.95Ho_0.05FeO₃ is of prime importance.     

Ultrasonic-assisted in situ synthesis and characterization of superparamagnetic Fe3O4 nanoparticles

Superparamagnetic Fe₃O₄ nanoparticles were synthesized via a modified coprecipitation method, and were characterized with X-ray diffraction (XRD), vibrating sample magnetometer (VSM), Zeta potential and FT-IR, respectively. The influences of different kinds of surfactants (sodium dodecyl benzene sulfonate, polyethyleneglycol, oleic acid and dextran), temperatures and pH values on the grain size and properties were also investigated. In this method, Fe³⁺ was used as the only Fe source and partially reduced to Fe²⁺ by the reducing agent with precise content. The following reaction between Fe³⁺, Fe²⁺ and hydroxide radical brought pure Fe₃O₄ nanoparticles. The tiny fresh nanoparticles were coated in situ with surfactant under the action of sonication. Comparing with uncoated sample, the mean grain size and saturation magnetization of coated Fe₃O₄ nanoparticles decrease from 18.4 nm to 5.9-9.0 nm, and from 63.89 emu g⁻¹ to 52-58 emu g⁻¹ respectively. When oleic was used as the surfactant, the mean grain size of Fe₃O₄ nanoparticles firstly decreases with the increase of reaction temperature, but when the temperature is exceed to 80 °C, the continuous increase of temperature resulted in larger nanoparticles. the grain size decreases gradually with the increasing of pH values, and it remains unchanged when the PH value is up to 11. The saturation magnetization of as-prepared Fe₃O₄ nanoparticles always decreases with the fall of grain size.     

Effect of high temperature sintering on the structural and the magnetic properties of La1.4Ca1.6Mn2O7

We investigate the effect of sintering temperature on the structural and magnetic properties of La_1.4Ca_1.6Mn₂O₇ double perovskite manganite compound prepared by the sol-gel route. Three sintering temperatures (1273, 1473 and 1673 K) are utilized for the heat treatment. Regardless of the sintering temperature, the tetragonal crystal structure with the I4/mmm space group is stable. However, a reduction in a and an increase in c lattice parameters along with a significant increase in crystallite-size occur on increasing the sintering temperature. This is also accompanied by the growth of grains. Grain-growth and changing crystallite-sizes account for the changes in magnetization, Curie temperature and magnetic entropy-change values. As the sintering temperature increases from 1273 to 1673 K the Curie temperature increases from 225 to 268 K and the magnetic entropy-change increases from 0.58 to 3.1 J kg⁻¹ K⁻¹ respectively.