Magnetic properties of melt-spun Sm2+Y(Co0.8Fe0.1Mn0.1)17BX alloys

Melt-spun ribbons with composition Sm_2+Y(Co_0.8Fe_0.1Mn_0.1)₁₇B_X (X=0–1.0 and Y=0–0.2) were fabricated with a wheel speed of 50 m/s, followed by annealing in the temperature range of 500–800°C for 2.5–60 min. Our results show that all the ribbons annealed up to 800°C are composed of a TbCu₇-type phase as the main phase. The highest coercivity of 8.7 kOe is obtained in a Sm-rich sample with composition Sm_2.2(Co_0.8Fe_0.1Mn_0.1)₁₇ annealed at 750°C for 5 min. It is found that these magnets show a very promising high-temperature performance – much better than those of typical sintered 2 : 17 magnets.     

The practical limits for enhancing magnetic property combinations for bulk nanocrystalline NdFeB alloys through Pr,Co and Dy substitutions

Pr, Co and Dy additions have been employed to improve the combinations of magnetic properties for nanocrystalline Nd_xFe_94−xB₆ melt spun alloys. The dependences of the magnetic properties on the solute element concentrations have been extensively investigated and the relationships between the measured remanence, maximum energy product (BH)_max and intrinsic coercivity for several compositional series are discussed. The composition ranges for these elemental substitutions which can be used to achieve the highest values of (BH)_max are identified. It is found that, when we employ individual or combined substitutions of Pr and Dy for Nd and Co for Fe in NdFeB alloys with various RE:Fe ratios, the practical limit of (BH)_max lies in the range ∼160–180 kJ/m³, combined with a coercivity in the range ∼400–800 kA/m.     

Nanocrystalline Pr0.5Sm0.5Co5 alloy obtained by mechanical milling

Starting from arc-melted alloys, nanostructured Pr_0.5Sm_0.5Co₅ powders were synthesized by mechanical milling and subsequent vacuum annealing during short times to develop an optimal microstructure with high hard magnetic properties. The best magnetic properties were obtained for the stoichiometric Pr_0.5Sm_0.5Co₅ powders milled for 4 h and annealed at 800 °C for 1 min with a high coercitivity of 13.5 kOe, a high M_r/M_max ratio of 0.72 and a maximum energy product of 11 MGOe. From X-ray diffraction and transmission electron microscopy, CaCu₅-type phase and an average grain size of about 12 nm were determined in the annealed powders, and we calculated that 59% of the volume of the grains/crystallites are exchange-coupled. The observed magnetic hardening is associated to the high magnetic anisotropy field of the PrCo₅ and SmCo₅ phases and also to the microstructure developed by the processing used.     

Effect of hot deformation on texture and magnetic properties of Sm-Co and Pr-Co alloys

Nanocrystalline PrCo₅, SmCo₅ and Sm₂(Co,Fe,Mn)₁₇ alloys were subjected to a high-degree plastic deformation at 950 °C with the height reduction ranging from 70% to 95%. With increasing degree of deformation, the PrCo₅ and SmCo₅ magnets showed improvement of the deformation-induced [0 0 1] texture. The PrCo₅ alloys, known to develop a superior texture at the lower degrees of deformation, showed only modest improvement and their magnetic performance was undermined by a low coercivity. The SmCo₅ alloys had their texture markedly enhanced and, after height reduction by 94.5%, they exhibited a remanence of 8.6 kG, maximum energy product of 18 MGOe and an intrinsic coercivity of 22.8 kOe. No induced texture was found in the alloys based on the Sm₂Co₁₇ structure. The microstructures of the hot-deformed alloys were studied with a transmission electron microscopy, and possible mechanisms of the texture development in the RCo₅ alloys (R=Pr, Sm) are briefly discussed.     

Influence of Al and Ni on the magnetic properties of Finemet alloys

Crystallization behavior and soft magnetic properties of the FeSiBCuNbM (M=Al or Ni) Finemet alloys are investigated by X-ray diffraction, differential scanning calorimetry, hysteresis loop tracer, and vibrating sample magnetometry. The nanocrystalline alloys are prepared by annealing melt-spun amorphous ribbons at different temperatures. Results indicate that the partial substitution of Ni or Al for Nb results in the increase of saturation magnetic induction density (B_s) of the alloys. The alloys with Al or Ni show favorable combination of soft magnetic properties. The partial substitution of Ni for Nb enhances the B_s value, while Al decreases coercivity. The mechanism underlining the magnetic behavior is discussed.     

Improving permanent magnetic properties of rapidly solidified nanophase RE-TM-B alloys by compositional modification

Rapid solidification is one of the most important techniques to produce nanocrystalline rare-earth-transition metal-boron (RE-TM-B) hard magnetic materials. To achieve high performance on these NdFeB-based alloys, compositional modification and microstructure optimization have been frequently employed. In this short review, various substitutions and doping elements have been discussed regarding to their behaviors in adjusting the individual or combined hard magnetic properties as well as the microstructure based on our recent results. It has been demonstrated that Pr and Dy enhance coercivity _jH_C, whereas Sm reduces _jH_C due to their effects on intrinsic properties. Co improves the thermal stability as well as the microstructure. Introducing Fe₆₅Co₃₅ is a possible approach to enhance the magnetization and maximum energy product (BH)_max. As a doping element, Ta was found to play an important role to obtain an appropriate combination of magnetic properties for this type of alloys.     

Hard magnetic properties of melt-spun Co82Zr18−xTix alloys

The phases and magnetic properties of Co-Zr-Ti melt-spun ribbons were studied by XRD analysis and magnetic measurements. The optimal magnetic properties of Ms=59.0 emu/g, Mr=4.0 kG, Hc=2.9 kOe, and (BH)_max=3.0 MGOe were obtained in Co₈₂Zr₁₄Ti₄ ribbons produced at a wheel speed of 30 m/s. In this work, we found that Ti was one of the few large atomic radius elements, which could improve hard magnetic properties of Co-Zr alloy.     

High coercivity nanocrystalline YCo5 powders produced by mechanical milling

High coercivity nanostructured YCo₅ powders were successfully prepared by mechanical milling of as-cast alloys and subsequent vacuum annealing. Almost single phase YCo₅ alloys, obtained by arc melting, were processed by high energy mechanical milling using a SPEX 8000 mill. After 4 h of milling, powders become nearly amorphous. DSC scans revealed the existence of an irreversible broad exothermic transition with a maximum at 516 °C associated with the crystallization process. Annealing in high vacuum at 800 °C during 2.5 min led to the formation of YCo₅ nanoparticles with an average particle size of 12 nm. A high intrinsic coercivity of 7.23 kOe together with a σ_r/σ_s ratio of 0.75 were obtained.     

Temperature evolution of magnetic susceptibility during devitrification of Cu-free HITPERM alloy

Development of structural and magnetic properties of the Fe_44.5Co_44.5Zr₇B₄ alloy during its successive devitrification from amorphous to nanocrystalline state was investigated. Temperature independence of the initial susceptibility in a wide temperature range (20-600 °C) was observed. The origin of such susceptibility behavior was described through the compensation of lowering exchange interaction between nanocrystalline grains and their magnetization at increased temperatures. The stabilization of the domain structure, due to atom pair directional ordering, was confirmed by measurements of the amplitude dependence of susceptibility in a wide field range, where the initial susceptibility remains constant (Perminvar effect).     

Influence of structure on coercivity in nanocrystalline (Fe1−xCox)86Hf7B6Cu1 alloys

The relationship between coercivity and structure in nanocrystalline (Fe_1−xCo_x)₈₆Hf₇B₆Cu₁ (x=0–1) alloys was surveyed. It was found that the increase of Co content in the alloys studied was accompanied by the increase of coercivity. However, we suggest that the factors influencing the coercivity change with the concentration of cobalt in these nanocrystalline alloys. In the iron-rich alloys, the average grain size and magnetostriction play predominant roles in the coercivity. On the other hand, in the case of cobalt-rich alloys, the coercivity mostly originates from the FCC-Co phase with large magnetocrystalline anisotropy and the weak exchange coupling between BCC-Fe(Co) and FCC-Co(Fe).     

Hard magnetic CoCr layer in ferromagnetic tunnel junctions

In magnetic tunnel junctions a highly spin-polarizing layer is usually exchange biased by an antiferromagnetic layer, an artificial antiferromagnetic layer system or a combination of both, while the magnetically soft layer is free to rotate. The use of a single layer of a hard magnetic material is rarely investigated up to now. In this paper, we present the electric and magnetic properties of tunnel junctions with a hard magnetic Co₈₃Cr₁₇ layer. The soft magnetic electrode consists of either a single Co layer or a Co/Ni₈₀Fe₂₀ bilayer. The magnetic anisotropy and coercive field H_C of the CoCr layer depend on its thickness and the kind of the bottom layer (Cu or Ta) and can vary from H_C=50–700 Oe. It is found that a thin Co cap layer also influences the hysteretic behavior. Furthermore, only small changes after annealing up to 450°C promise a high thermal stability for the application in magnetic tunnel junctions. Measurements of the tunnel magnetoresistance on large area junctions, however, show a strong magnetic coupling of the hard and soft electrodes.     

Nano-sized disorders in hard magnetic grains and their influence on magnetization reversal at artificial Nd/Nd2Fe14B interfaces

We have investigated the effect of lattice disorders near the surface of hard magnetic Nd₂Fe₁₄B grains on coercivity using artificial interfaces created by sputter depositing Nd on polished surface of Nd-Fe-B sintered magnets. The interfacial structure was manipulated by annealing the coated samples at 550 °C in vacuum with and without Ta cap. Nano-beam electron diffraction revealed a few nm thick disordered layer within the Nd₂Fe₁₄B phase at the Nd/Nd₂Fe₁₄B interface of a low coercivity sample, while a high coercivity sample showed a well-defined crystal structure of Nd₂Fe₁₄B near the NdO_x/Nd₂Fe₁₄B interface.     

Exchange-coupled nanoscale SmCo/NdFeB hybrid magnets

Nanoscale hybrid magnets containing SmCo₅ and Nd₂Fe₁₄B hard magnetic phases have been produced via a novel “in-one-pot” processing route. The grain size of the processed bulk composite materials is controlled below 20 nm. The refinement of the nanoscale morphology leads to effective inter-phase exchange coupling that results in single-phase like magnetic properties. Energy product of 14 MGOe was obtained in the isotropic nanocomposite magnets at room temperature. At elevated temperatures, the hybrid magnets have greatly improved thermal stability compared to the Nd₂Fe₁₄B single-phase counterpart and have substantially increased magnetization and energy products compared to the single-phase SmCo₅ counterpart.     

The effect of magnetic field on the electrodeposition of CoFe alloys

The superimposition of a homogenous magnetic field during an electrodeposition of CoFe alloy was investigated. A magnetic field superimposed parallel to the electrode surface increases the limiting current density and the deposition rate due to the magnetohydrodynamic (MHD) effect. The deposits obtained in this field configuration appear smoother and more homogenous than the ones obtained without the magnetic field. On the contrary, a magnetic field superimposed perpendicular to the electrode surface does not influence significantly the electrochemical reaction but the morphology of the deposited layers is strongly affected. The roughness is strongly increased in this field configuration and grains grow as separated columns aligned perpendicular to the electrode surface, in the field direction. A magnetic field applied during the deposition affects the magnetic properties of the deposited layers as well. These changes are discussed with respect to the surface roughness and the internal stress state of the layer.     

Strain rate sensitivity of ultra-fine and nanocrystaline metals and alloys

The mechanical behavior of metals and alloys is strongly related to grain size. In particular, the grain refining leads to the increase in yield strength in the ultra-fine grain (<1 μm) and nanocrystalline (<100 nm) regimes.Instrumented nanoindentation measurements allow a rapid evaluation of mechanical properties of materials, and the possibility to perform tests in a very wide range of loads. The strain rate sensitivity of ultra-fine and nanocrystalline metals can be derived by changing loading rates. The present paper presents the results on the strain rate sensitivity of ultra-fine grain metals produced by equa-channel angular pressing and nanocrystalline materials produced via electrodeposition. The results were obtained by systematic experiments performed at different loading rates (3, 30 and 300 mN/s) showing broad ranges of variations for the investigated metals. Also, the strain rate sensitivity of the studied materials was derived from the load vs. depth curves.     

Nanophase hard magnets

The dramatic developments that occurred in nanophase hard magnetic materials over the last two decades are reviewed. Much of the research was done after the discovery of Nd₂Fe₁₄B-based magnets in an attempt to develop more economical Fe-based magnets with better properties. This led to the discovery of 1 : 12-based magnets, 2 : 17 nitrides and carbides, and more recently, to nanocomposite magnets consisting of a fine mixture of exchange coupled soft and hard phases. Currently, much emphasis is devoted to nanocomposite films consisting of nanoparticles of a high anisotropy material embedded in a non-magnetic matrix because they have a great potential for applications in high density recording media.     

On the magnetic properties of mechanosynthesized Co-Fe-Ni ternary alloys

X-ray diffraction, Mössbauer spectroscopy and magnetization measurements were used as complementary methods to obtain structural data and to determine magnetic properties of the mechanically synthesized and subsequently thermally treated Co-Fe-Ni alloys. New, however approximate, phase diagrams were established on the basis of X-ray diffraction investigations. Mössbauer spectroscopy and magnetization measurements allowed to reveal practically linear correlation between the average values of the hyperfine magnetic field induction, 〈B_hf〉, and the effective magnetic moments, μ_eff, of the alloys. The decrease in 〈B_hf〉 with the number of electrons per atom, e/a, was observed. Moreover, the dependence of μ_eff on the valence 3d and 4s electrons per atom follows the Slater-Pauling curve. Thermal treatment of mechanosynthesized Co-Fe-Ni alloys led to some changes in the phase diagrams, increase in the grain size and decrease of the level of internal strains in alloys. Dependencies of lattice constants, average hyperfine magnetic fields, effective magnetic moments and Curie temperatures on the number of electrons per atom have the same trends for mechanically synthesized as well as for thermally treated alloys.     

Phase formation and change of magnetic properties in mechanical alloyed Ni0.5Co0.5Fe2O4 by annealing

The effects of milling time and annealing temperature on phase formation, microstructure and magnetic properties of nickel-cobalt ferrite synthesized from oxide precursors by mechanical alloying were studied. The study of milling time effects on phase formation of milled materials showed that if milling continues up to 55 h, single phase nano-sized nickel-cobalt ferrite is obtained. Also, magnetic properties of powders versus milling time and annealing at different temperatures extensively changed, so that annealing at 1200 °C increased the magnetization saturation of the as-milled powder from 15.1 to 53.6 emu/g. X-ray powder diffraction technique (XRD) with Cu-Ka radiation was employed for phase identification. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were also used to determine the morphology and size of the particles. The magnetic properties were measured by a vibration sample magnetometer (VSM).     

Magnetic properties of Sm–Co–B-based nanocomposite magnets

The magnetic properties of nanocomposite melt-spun magnets with composition Sm_16−xCo_68+xB₁₆ (x=0–10, 2 at% interval) and Sm₈Co_92−yB_y (y=10–18, 2 at% interval) have been studied systematically. Several ribbons were fabricated with a wheel speed of 50 m/s, followed by annealing in the temperature range of 700–800°C for 2.5–40 min. XRD results and magnetization versus temperature curves showed that almost all of the samples were composed of the tetragonal Sm₂Co₁₄B and rhombohedral SmCo₁₂B₆ phases which are not magnetically hard at room temperature. However, a relatively high coercivity in the range of 3.5–5.5 kOe has been obtained in these samples. The highest coercivity of 5.5 kOe and a very promising β value of −0.28%/°C were obtained in Sm₈Co₇₄B₁₈ ribbons annealed at 750°C for 5 min. The high coercivities are attributed to the small grain size of the 2 : 14 : 1 phase, in which the large surface areas enhance its effective anisotropy, and make it uniaxial type.     

Annealing temperature effect on microstructure,magnetic and microwave properties of Fe-based amorphous alloy powders

Fe₇₄Ni₃Si₁₃Cr₆W₄ amorphous alloy powders were annealed at different temperature (T) for 1.5 h to fabricate the corresponding amorphous and nanocrystalline powders. The influences of T on the crystalline structure, morphology, magnetic and microwave electromagnetic properties of the resultant samples were investigated via X-ray diffraction, scanning electron microscopy, vibrating sample magnetometer and vector network analyzer. The results show that the powder samples obtained at T of 650 °C or more are composed of lots of ultra-fine α-Fe(Si) grains embedded in an amorphous matrix. When T increases from 350 to 750 °C, the saturated magnetization and coercivity of the as-annealed powder samples both increase monotonously whereas the relative real permittivity shows a minimal value and the relative real permeability shows a maximal value at T of 650 °C. Thus the powder samples annealed at 650 °C show optimal reflection loss under −10 dB in the whole C-band. These results here suggest that the annealing heat treatment of Fe-based amorphous alloy is an effective approach to fabricate high performance microwave absorber with reasonable permittivity and large permeability simultaneously via adjusting T.