FeCo-based Nanocrystalline Alloys Research Articles

Superior high-temperature magnetic softness for Al-doped FeCo-based nanocrystalline alloys

The temperature dependence of initial permeability (μi-T), the average grain size (D), the Curie temperature of amorphous phase (Tcam) of a series of annealed (Fe0.5Co0.5)73.5Nb3Si13.5Cu1B9−xAlx (x=0, 1, 2, 3) alloys were investigated. The Curie temperature of amorphous phase (Tcam) for the Al-contained alloys decreased compared with the Al-free alloy, but the high-temperature magnetic softness was improved. It was found that the increasing Al content enlarged the interval temperature (ΔTx) between the onset crystallization temperature (Tx1) and the secondary crystallization temperature (Tx2). The μi of 530°C vacuum annealed samples tended to increase both at room temperature (except for x=3 alloy) and elevated temperature when Al replaced B element. The (Fe0.5Co0.5)73.5Nb3Si13.5Cu1B6Al3 alloy exhibited excellent high-temperature magnetic softness, and its higher μi above 1000 can be maintained up to 670°C.

Effect of Mo addition on high-temperature soft magnetic properties for air annealed FeCo-based nanocrystalline alloys

Temperature dependence of initial permeability (μi-T curves) for as-quenched and annealed (Fe0.5Co0.5)73.5Si13.5B9Cu1M3 (M=Nb, Mo) alloys were investigated. The Curie temperature of Mo-alloy is decreased by about 5°C compared with Nb-alloy. Although room-temperature μi of Mo-alloy was lower than that of Nb-alloy when nanocrystallized in air, the more stable value of μi and improved high-temperature soft magnetic properties was observed. The reason for the evolution of μi at elevated temperature was also analyzed.

Excellent high-temperature magnetic softness in a wide temperature for FeCo-based nanocrystalline alloy

Structure and initial permeability (μi) of as-quenched and annealed (Fe1−xCox)72.7Al0.8Si17.5B5Cu1Nb3 (x=0,0.5) alloys were investigated. Substituting 50at.% Co for Fe enhances both the Curie temperature of amorphous alloy, TcA, from 325 to 375°C and the secondary crystallization temperature Tx2, for delaying the formation of Fe–B hard magnetic phase from 720°C to 729°C. The correlation between initial permeability and temperature (μi–T curve) for (Fe0.5Co0.5)72.7Al0.8Si17.5B5Cu1Nb3 alloys heating–cooling cycled at 510–640°C was mainly analyzed. The optimum high temperature magnetic softness was found in 550°C-annealed sample, the μi can maintain above 1400 and shows roughly a horizontal linear correlation with T from room temperature up to 600°C, which make this alloy attractive as an affordable high-temperature soft magnetic material.

Influence of Magnetic Field Annealing Methods on Soft Magnetic Properties for FeCo-Based Nanocrystalline Alloys

Soft magnetic alloys have been widely applied to modern technology devices, such as transformer cores, choke coils and inductive devices. Magnetic field annealing can induce an uniaxial magnetic anisotropy K u [1-4] so as to improve the soft magnetic properties of alloys for the purpose of coping with different application requirements. So far, FeCo-based nanocrystalline alloys have been explored increasingly in order to improve the magnetic softness at higher temperatures[5-6]. Furthermore, the effect of magnetic field annealing on FeCo-based alloys is more pronounced compared with that of single ferromagnetic element composition [7-8]. Therefore, to explore the optimal magnetic field annealing method of FeCo-based alloys can effectively improve their soft magnetic properties to adapt to more strict application requirements at high temperature conditions. Magnetic field annealing of nanocrystalline alloy mainly includes three methods [9-11]. (a) The direct magnetic field annealing above the crystallization temperature of initial crystalline phase T x1 (magnetic field nanocrystallization). (b) Annealing above T x1 for nanocrystallization without magnetic field and then longitudinal magnetic field annealing below the Curie temperature of amorphous phase T c am. (c) After nonmagnetic field annealing above T x1 followed a magnetic field rean-nealing treatment under the same annealing condition. In order to achieve more superior magnetic softness of FeCo-based nanocrystalline alloys, the soft magnetic properties of nanocrystalline (Fe 1−x Co x ) 73.5 Si 13.5 B 9 Nb 3 Cu 1 (x=0.25,0.5,0.75) alloys under different magnetic field annealing were investigated.

Influence of magnetic field annealing on saturation magnetostriction and μi–T curves for FeCo-based nanocrystalline alloy

The influence of magnetic field annealing on the saturation magnetostriction and μi–T curves for the nanocrystalline (Fe0.5Co0.5)73.5Cu1Nb3Si13.5B9 alloy was investigated. In contrast to the nonmagnetic field annealing, the magnetic field annealing reduces λS and remodels the μi–T curves of the samples dramatically. After nonmagnetic annealing at different temperatures, the λS of samples increases with the annealing temperature (Ta), and the value of μi decreases with T increasing at different rates. However, the λS of magnetic annealed samples decreases with increasing Ta because of the ordered rearrangement of the magnetic domain along with the applied field direction. The μi of magnetic annealing samples tends to increase with Ta, but it behaves in a different fashion with increasing T, which is correlated with the lower magnetostriction and the induced uniaxial anisotropy by magnetic annealing.

Control of gigahertz permeability and permittivity dispersion by means of nanocrystallization in FeCo based nanocrystalline alloy

An amorphous (FeCo) based alloy has been prepared by a rapid quench method. Subsequent annealing on the amorphous samples gives rise to the coexistence of two magnetic phases: amorphous matrix and nanocrystalline grains (α′-FeCo) with an average size of 9.8 nm. Permeability dispersion behaviors have been studied by Kittel theory [C. Kittel, J. Phys. Radium 12, 332 (1951)]. The results show that these two magnetic phases contribute to the permeability dispersion. The Cole-Cole dispersion law [K. S. Cole and R. H. Cole, J. Chem. Phys. 9, 341 (1941)] has been employed to explain the permittivity dispersion within microwave region based on the assumption that multiple dielectric relaxation processes existing. Our results indicate the possibility of tuning the high frequency permeability and permittivity values of (FeCo) based alloy by controlling the magnetic microstructure, which suggests an alternative method to develop smart electromagnetic materials.

Observations of ferromagnetic resonance modes on FeCo-based nanocrystalline alloys

Uniform and symmetric resonance modes (known as Aharoni’s exchange resonance modes) are derived from micromagnetic equilibrium condition in the linear approximation. To investigate the uniform and symmetric resonance modes in ferromagnetic nanoscale grains, the microwave permeability of FeCo-based nanocrystalline alloy particles/paraffin composites was measured and calculated in the range 0.5–18 GHz. The measured dynamic permeability curves exhibit a broad resonance band at 4–6 GHz; some curves also exhibit a narrow resonance band at 13 GHz. The former behavior is in qualitative agreement with the uniform mode, and the latter is attributed to the first eigenvalue mode of the symmetric resonance modes excited in nanocrystalline monodomain grains in FeCo-based alloys. The difference value (Δ ω 11) between the uniform resonance frequency and the first frequency eigenvalue of the symmetric resonance modes shows good agreement with experiment.

Improvement of soft magnetic properties in Fe 38Co 38Mo 8B 15Cu amorphous and nanocrystalline alloys by heat treatment in external magnetic field

The influence of both longitudinal and transverse magnetic field applied during heat treatment on the soft magnetic behavior in the amorphous and nanocrystalline Fe 38Co 38Mo 8B 15Cu alloys has been investigated. Sheared loops with good field linearity were achieved after annealing in transverse magnetic field. The value of induced anisotropy constant K u after such annealing increased from 440 J/m 3 for thermally relaxed amorphous samples up to 935 J/m 3 for alloys in advanced stage of primary crystallization. The nanocrystallization under a presence of the longitudinal magnetic field results in squared hysteresis loops that are characterized by reduction of coercivity H c to 3–7 A/m. Such low H c values surpass those previously reported for FeCo-based nanocrystalline alloys.

Mechanism of high-temperature exchange-coupling interaction of FeCo-based nanocrystalline alloy

Temperature dependence of initial permeability is investigated for nanocrystalline Fe38.4Co40Si9B9Nb2.6Cu alloy annealed at 500 and 600℃,and the initial permeability of 600℃-annealed sample is observed not to drop sharply at the Curie temperature of the residual amorphous phase,which is a new magnetic phenomenon in dual-phase nanocrystalline alloys. The origin of the above phenomenon is explored by estimating the Curie temperature of amorphous ribbons which have the same compositions with the residual amorphous phase in annealed nanocrystalline alloys. The results indicate that the Curie temperature of the intergranular amorphous region can be enhanced drastically up to the Curie temperature of the crystalline phase (TAC=TαC) when the exchange-field between adjacent nanograins penetrates through the amorphous interphase thoroughly. Furthermore, the effective exchange penetration length of FeCo-based nanocrystalline alloys (LFeCo) is evaluated to be 0.61 nm much larger than that of Fe-based nanocrystalline alloys, which may be the main reason of the higher permeability of FeCo-based alloys at elevated temperature.

Effect of heat treatment under an external magnetic field on the soft magnetic properties in FeCo-based nanocrystalline alloys

Effect of heat treatment under an external magnetic field on the soft magnetic properties in FeCo-based nanocrystalline alloys

Magnetic domain observations in a FeCo-based nanocrystalline alloy by Lorentz microscopy

Lorentz microscopy was used to investigate the domain structure in an (Fe0.5Co0.5)80Nb4B13Ge2Cu1 alloy annealed at 500, 550, and 610°C, respectively, for 1h. The average grain size d, observed by conventional transmission electron microscopy, is 10nm for the alloy annealed at 500°C, 18nm for 550°C, and 90nm for 610°C. The domain structure of sample annealed at 500°C exhibits few pinning sites and large domains, which correlate with the low coercivity and good soft magnetic properties. More pinning sites and smaller domains are observed in the sample annealed at 550°C. In the sample annealed at 610°C, the domain walls are observed to be localized at the grain boundaries and the domains include several grains. It therefore appears that the grain boundaries act as effective pinning sites in this sample. As expected, domain walls in the 610°C sample are less mobile in small applied fields than in the 500 and 550°C samples. This result is consistent with coercivity measurement and the random anisotropy model commonly used to rationalize the coercivity dependence on average grain size in these alloys.

XPS characterization of oxide layers on FeCo-based nanocrystalline alloys

The main reason for the study of FeSi-based (FINEMET) and FeZr-based (NANOPERM) alloys was their magnetic properties. However, one of the limiting factors in commercial uses of these alloys is the oxidation, which occurs at high service temperature such as 800 °C. Due to this high service temperature, a non-magnetic oxide layer is formed on the surface and the soft magnetic properties remarkably diminish. In this work, it was carried out mass gain measurements in order to identify the oxidation resistance of four different FeCo-based nanocrystalline alloys: Fe 38.5Co 38.5Nb 7Cu 1B 15, Fe 36Co 36Nb 7Si 10B 11, Fe 36Co 36Nb 6Cu 1Si 10B 11, and Fe 33.5Co 33.5Nb 7Si 15B 11. XPS analysis was used in order to identify and quantify the oxides formed during the exposure to an oxidant environment at high temperature. SEM also observed these oxide layers. The main conclusion is that the addition of Si improved the oxidation resistance of the nanocrystalline alloys. This is attributed to the increase of SiO 2 amount and decrease of Fe 2O 3 content on the surface layer, enhancing the surface oxide characteristics.

The influence of composition and field annealing on magnetic properties of FeCo-based amorphous and nanocrystalline alloys

We report the influence of composition and very high transverse field annealing on the magnetic properties and structure of four FeCo-based amorphous and nanocrystalline alloys. The compositions (Fe 50Co 50) 89Zr 7B 4 and (Fe 65Co 35) 89Zr 7B 4 were investigated changing the Fe:Co ratio from 50:50 to 65:35. (Fe 50Co 50) 85Zr 2Nb 4B 8.5 was chosen to investigate Nb substitution for Zr in an FeCo-based alloy. This substitution is shown to increase the magnetostrictive constant, λ S , of the nanocrystalline alloy from 36×10 −6 to 54×10 −6. The composition (Fe 65Co 35) 84Cr 5Zr 7B 4 was studied to investigate the influence of Cr on intergranular coupling across the amorphous matrix. Samples of each composition were annealed in the amorphous state at 300 °C and in the nanocrystalline state at 600 °C. Field annealing was performed in 17 T transverse field in an inert atmosphere. Frequency-dependent magnetic properties were measured with an automatic recording hysteresisgraph. Static magnetic properties were measured with a vibrating sample magnetometer. The mass-specific power loss of the alloys decreased with field annealing in both the nanocrystalline and amorphous states for some frequency and induction combinations. Furthermore, the hysteresis loops are sheared after field annealing, indicating a transverse magnetic anisotropy. The nanocrystalline (Fe 50Co 50) 85Zr 2Nb 4B 8.5 composition has a lower relative permeability than the other compositions.

A novel method determining longitudinally induced magnetic anisotropy in amorphous and nanocrystalline soft materials

In ferromagnetic amorphous and nanocrystalline soft magnetic alloys the induced magnetic anisotropy plays a fundamental role in the hysteresis behavior but, due to the elongated shape, it can be measured only if KU is perpendicular to the sample long axis. In order to measure the longitudinal induced anisotropy, an original method derived from known thin layers measurement techniques was used. Hysteresis loops shifted by perpendicular bias field were recorded for this purpose. Direct measurement of the longitudinal induced anisotropy in amorphous and nanocrystalline ribbons or wire without needing sample preparation is reported for the first time. Evidence of self-induced anisotropy is brought in a Fe–Co-based nanocrystalline alloy.

High-temperature magnetic behavior of FeCo-based nanocrystalline alloys

The soft magnetic response of nanocrystalline Fe 7 3 . 5 - x Co x Si 1 3 . 5 B 9 Cu 1 Nb 3 (x=0, 30, and 45) samples are analyzed above room temperature through the temperature evolution of the magnetic permeability and the associated loss factor. Moreover, the actual structure and composition of the crystalline phase is analyzed through neutron-diffraction studies. The results show that the inclusion of Co atoms give rise to an improvement in the soft magnetic behavior at high temperatures with respect to the Fe-based sample as a consequence of the increase in the Curie temperature of the precipitated crystallites. However, the role of the residual amorphous matrix cannot be disregarded and the decrease in its Curie temperature for the Co richest sample gives rise to a deterioration of the high-temperature soft magnetic response. The observed temperature evolution is analyzed within the framework of the random anisotropy model and associated with the temperature dependence of the magnetic coupling between the ferromagnetic crystals.