FeCo Phase Research Articles

The effect of heat treatment on the FeCo phase in Tb-Fe-Co thin films

Investigation of the magnetic behavior of nanostructured Tb-Fe-Co thin films before and after heat treatment below and above Fe-Co phase crystallization temperature has explored by electron microscopy (SEM), X-ray diffraction (XRD), temperature-dependent resistivity, magnetization (MOKE), magneto-resistivity, and Hall resistivity measurements. The thin films are composed of fine crystallites (from 10 to 75 nm) after heat treatment around the FeCo phase crystallization temperature obtained from XRD and SEM measurements. The nanostructured thin films show the unique perpendicular magnetic properties in the out-of-plane direction around the room temperature. The maximum squareness ratio value of as-deposited thin film with 10 nm crystallite size is 0.98, and the maximum coercive field value of 400∘C thin film with 60 nm crystallite size is 3897 Oe. Our findings offer a different perspective for synthesizing the nanostructured Tb-Fe-Co thin films for promising applications in the magnetic sensor technologies.

Engineering perpendicular magnetic anisotropy in Tb-Fe-Co thin films using ion-beam irradiation

This paper presents engineering perpendicular magnetic anisotropy in amorphous Tb-Fe-Co thin films employing irradiation of Ar+ ions at 200 and 250 keV energies with various fluences. The pristine film depicts a rectangular hysteresis loop with high perpendicular magnetic anisotropy, showing elongated, inter-connected maze-like magnetic domains. Irradiation at higher fluences (≥1015 ions cm−2) results in the transformation of magnetization reversal mechanism from rapid domain wall motion to nucleation and propagation of reverse domains due to the pinning of domain walls. The elongated magnetic domains get fragmented with reduced contrast and interestingly inhomogeneous flux-closure patterns are observed at higher fluences, indicating a spin reorientation transition. The modifications in magnetic properties are correlated with the structural investigations which reveal the crystallization of the Fe-Co phase in an amorphous Tb-Fe-Co matrix at higher energy and fluences of the projectile ions. Micromagnetic simulations complement the experimental results of domain modification.

Coercivity enhancement of SmCo/FeCo nanocomposite magnets by doping eutectic Sm-Zn alloy

We discover that the coercivity of SmCo/FeCo magnets can be enhanced by doping eutectic Sm-Zn alloy. The magnets without Sm-Zn alloy, having an average grain size of ~8 nm, are composed of well-dispersed Sm2Co17 phase and FeCo phase, and the coercivity is only 1.5 kOe. In contrast, the magnets with 5 wt% Sm-Zn alloy, having an average grain size of ~12 nm, are composed of SmCo5 dominant phase plus Sm2Co17 minor phase and FeCo phase, and the coercivity is 6.9 kOe. The coercivity enhancement is primarily due to the phase transformation from Sm2Co17 to SmCo5 phase. This work demonstrates a strategy to stabilize the high anisotropy SmCo5 phase and enhance the coercivity in SmCo/FeCo nanocomposite magnets.

Effect of Heat Treatment on the Stability of Nanosized (Co40Fe40B20)34(SiO2)66/ZnO/In2O3 Multilayers

An investigation is performed of the thermal stability and phase transformations of thin-film heterogeneous [(Co40Fe40B20)34(SiO2)66/ZnO/In2O3]85 multilayers obtained via ion beam sputtering. The system contains 85 layers, each consisting of a (Co40Fe40B20)34(SiO2)66 composite layer and ZnO and In2O3 semiconductor spacers. The sample structure in the initial state and after heat treatment is studied by means of X-ray diffraction. It is shown that the samples are stable at temperatures of up to 500°С. Zn2SiO4, InBO3, CoFe, and In2O3 phases form during annealing.

Aging induced segregation and nanoprecipitation in a severely deformed equiatomic high-entropy alloy

To shed light on the impact of severe deformation on the elemental segregation of high-entropy alloys (HEAs) as well as the related changes in mechanical properties, we systematically investigated the equiatomic CrMnFeCoNi HEA cold rolled and aging treated subsequently at 450 °C for varying time periods. Dislocation cells, nano-twins and shear bands of nanostructures form and significantly increase the strength of the as-cold-rolled HEA. Upon aging for 18–96 h, elemental segregation occurs in the dislocation cell and twinning regions. With the aid of higher strain localization, NiMn, FeCo and Cr-rich phases of nanoscale precipitate in the shear band regions. The elemental segregation slightly while the nanoprecipitations significantly increase the local hardness and the bulk strength. The nanoprecipitations also decrease the plasticity of the HEA due to the early cracking along the abundant interphase boundaries. These findings clarify the compositional changes and mechanical properties of the deformed HEAs upon aging, and also provide implications for the design and processing of advanced materials.

Microstructure and magnetic properties of (SmCo+FeCo)/NdFeB multicomponent nanocomposite magnets fabricated by HPTC with change in heating temperature and composition

We report the fabrication of bulk anisotropic (SmCo + FeCo)/NdFeB multicomponent nanocomposite magnets using high-pressure thermal compression (HPTC). The correlations among microstructure, magnetic properties, heating temperature and composition of the HPTC nanocomposite magnets were studied. The HPTC magnet made under variable temperatures (VT), with 19 wt% of FeCo phase, exhibits a maximum energy product of 32 MGOe, which is much higher than that (14 MGOe) for the HPTC magnet made under a constant temperature (CT). When the FeCo content increases to ∼23 wt%, the HPTC magnet made under VT still remains a high energy product of about 25 MGOe. With increasing NdFeB content, the (SmCo + FeCo)/NdFeB multicomponent nanocomposite magnets exhibit an enhanced magnetic anisotropy and coercivity. This work is beneficial to fabricating high-performance and low-cost permanent magnets for practical applications.

Achievement of superior microwave absorption performance and ultra-wide regulation frequency range in Fe-Co-Nd via tuning the phase constitution and crystallinity

To achieve superior microwave absorption and ultra-wide regulation frequency range under a restricted thickness, (FeCo)100−xNdx (0 ≤ x ≤ 15) powders were prepared through a single roller melt spinning technique. The introduction of Nd decreases the crystallinity of FeCo phase and promotes the formation of secondary Nd2(FeCo)17 phase, which results in apparent reduction of electromagnetic (EM) parameters. The correlations between EM parameters and final microwave absorption properties are discussed in detail. It is found that the suppressed EM parameters modifies the coordination between impedance matching and attenuation performance, ultimately leading to superior microwave absorption performance with broad bandwidth of 6.92 + GHz and RLmin of −52.5 dB under thickness of only 1.45 mm. Moreover, due to the dramatic decline of |μr|·|εr| values, the qualified RL of the samples covers almost all the test frequency (2–18 GHz) with thin thickness of 2 mm, showing great practical value in full-spectrum microwave absorption area with a restricted coating thickness.

Precipitation and decomposition in CoCrFeMnNi high entropy alloy at intermediate temperatures under creep conditions

The precipitation and decomposition behaviors of CoCrFeMnNi high entropy alloy (HEA) were studied at intermediate temperatures under creep conditions. One of the most interesting observation is the nanoscale precipitation of bcc Cr particles at dislocations during creep at 515 °C–580 °C. At 580 °C, the formation of intragranular Cr-rich σ phase particles was also observed in addition to nanoscale bcc Cr particles. Typical Kurdjumov–Sachs orientation relationship was exhibited between bcc Cr precipitates and the matrix. Bcc Cr precipitates can be easily nucleated from segregated Cr atoms along < 111 > fcc directions because of the excellent fit between closed packed directions, < 111 > fcc and < 101 > bcc. Strain field contrast at the interface of bcc Cr precipitates suggests that Cr particles is semi-coherent with the matrix. The precipitation of intragranular σ phase precipitates only at a higher temperature (580 °C) is attributed to higher nucleation energy of intragranular σ phase due to the lack of coherency at the interface. The decomposition and precipitation of various phases at grain boundaries were also observed. Interganular σ, NiMn (L1o), and FeCo (B2) phases were observed at 515 °C. As the creep temperature increased, the formation of intergranular phase particles became less significant. The successive and agglomerated formation of three phases could be attributed to the enrichment of expelled atoms (from the transformed phase) and sluggish diffusion in HEA matrix. To improve the predictability of thermodynamic prediction of various phases in high entropy alloys, the local enrichment factor should be taken into consideration in the region of the significant heterogeneous transformation.

Magnetic and microwave absorption properties of FeCo/CoFe2O4 composite powders

FeCo/CoFe2O4 composite powders were synthesized by conducting solution combustion method in a closed system and using microwave heating. Phase evolution studies by X-ray diffraction method showed the appearance of FeCo phase at the cobalt to ferric molar ratio of 2, while the impure CoO phase was formed at the higher molar ratio. The synthesized powders had spongy structure with spherical nanoparticles (∼30 nm), as observed by scanning and transmission electron microscopes. The highest saturation magnetization of 99 emu/g and lowest coercivity of 450 Oe was attained by the CoFe2O4/17 wt % FeCo composite powders. The paraffin-FeCo/CoFe2O4 composite sample exhibited a reflection loss intensity value of −20 dB in C band (4–8 GHz) which showed that the metal/ferrite composite powders may be a promising candidate for microwave absorption applications.

Exchange Bias in Chemically Reduced FeCo Alloy Nanostructures

The equiatomic FeCo alloy with average particle sizes of 140 and 30 nm is obtained using an instant chemical reduction process. The mean hyperfine field of 36.0(5) T estimated from 300 K 57Fe Mössbauer spectra is consistent with a disordered FeCo phase with a relative fraction of 90(2)% for both the samples, along with γ‐Fe2O3. A CoO shell with an average thickness of 2.7 nm is inferred from transmission electron microscopy in the size‐reduced FeCo. The room temperature coercivity of the size‐reduced FeCo is higher than the expected theoretical values. An exchange bias field (HE of 296 Oe), vertical remanence shift at 15 K, and an exchange bias blocking temperature of 150 K are exhibited by the 30 nm FeCo particles. The observed HE and its temperature dependence are fairly explained based on a random field model.

FACILE SYNTHESIS OF FECO NANOPARTICLES BY ONE-POT POLYOL PROCESS

Ferromagnetic FeCo nanoparticles were prepared by a simple one-pot polyol process and followed by simple annealing treatment. The prepared ferromagnetic FeCo nanoparticles have spherical shape and the size was controlled by the annealing temperature. Importantly, single FeCo phase was obtained at 400°C and these samples have spherical shape and size about 50 nm. While at higher temperature (at 600°C) the nanoparticles have very lower aggregation and have higher coercivity. The prepared FeCo nanoparticle at low temperature with good magnetic properties are to be considered as potential candidate for many applications.

Nano-imprinting potential of magnetic FeCo-based metallic glass

Fabrication of magnetic nanostructures at low cost is strongly desired for applications such as sensors, actuators, magnetic memory, etc. In conventional nano-patterning techniques, the magnetic field of a magnetic material interferes with the patterning process, making nano-patterning challenging. Here, we report on the low cost patterning potential of FeCo-based magnetic metallic glass using a nano-imprinting technique. We show that out of a large number of magnetic metallic glasses, Fe40Co35P10C10B5 glassy alloy exhibits high saturation magnetic flux density (Bs ∼ 1.24 T), a large super-cooled liquid temperature range (ΔTx ∼ 49 °C), and a relatively low glass transition temperature (Tg ∼ 430 °C) with good thermal stability. The quasi-static viscosity (∼108 Pa.s at a heating rate of ∼40 °C min−1) in ΔTx, which is one of the most important parameters for nano-imprinting, is lowest among the reported magnetic metallic glasses. The deformability of this magnetic alloy is similar to the well-known non-magnetic metallic glasses, which can be patterned to a few tens of nanometers. Crystallization of Fe40Co35P10C10B5 glassy alloy leads to the precipitation of a high Bs FeCo phase that may exhibit high magnetocrystalline anisotropy. Based on detailed investigations of structural, thermal, and magnetic behavior, along with imprinting experiments, we show that the Fe40Co35P10C10B5 glassy alloy is the most desirable material for making various nano-patterns with tailorable magnetic properties.

Microstructures and magnetic properties of cast alnico 8 permanent magnets under various heat treatment conditions

There is a close relationship between the magnetic properties of alnico permanent magnetic alloys and the nano-scaled spinodally decomposed structure which results from various heat treatment cycles. In this paper, different heat treatment cycles are employed to cast alnico 8 alloys with composition 32.3Fe-37.5Co-13.7Ni-6.2Al-5.8Ti-3.4Cu-0.2Zr-0.1S-0.8Nb. It is observed that magnetic field treatment at high temperature for specific time is the most effective method to obtain better microstructure and magnetic properties. The mosaic structures consisting of Fe-Co phase and Al-Ni rich phase are best separated during thermomagnetic treatment for 4–5 min and refined during low temperature treatments. The bias growth of the ferromagnetic phase does not develop in absence of magnetic field, and hence the phases are not refined and separated completely. This produces isotropic alnico alloys with low magnetic properties. However, continuous cooling of the alloys in magnetic field followed by isothermal treatment without magnetic field provides moderate magnetic properties. Treatment at high temperatures in field and without field for longer time leads to the coarsening of spinodal phases and poor magnetic properties. The optimum magnetic properties of Hcj = 1.7 kOe, Br = 8.0 kGs and (BH)max = 5.02 MGOe are attributed to the refined microstructure of the thermomagnetically treated alloys.

Improvement of Magnetic Properties of SmCo5/α-Fe Nanocomposite Magnets by Magnetic Field Annealing

In this paper, magnetic field heat treatment was carried out for the SmCoSUB5/SUB/α-Fe amorphous powders prepared by high-energy ball milling. The effects of annealing temperature and magnetic field on the crystallization, microstructure, and magnetic properties of the SmCoSUB5/SUB/α-Fe nanocomposite permanent magnets were studied. The results show that the magnetic field can benefit the crystallization of SmCoSUB5/SUB phase and the degree of crystallinity of SmCoSUB5/SUB/α-Fe alloy. Thus, the remanence and coercivity of the SmCoSUB5/SUB/α-Fe magnets annealed with magnetic field are significant higher than those of the samples annealed without magnetic field. With the increase of annealing temperature in magnetic field heat treatment, the Sm(Co, Fe)5 phase and FeCo phase are gradually crystallized and grew up, while a small amount of Sm₂CoSUB17/SUB phase appears. The best magnetic properties are MSUBr/SUB = 73.29 emu/g, MSUBr/SUB/MSUBs/SUB = 0.59, and HSUBci/SUB = 6.20 kOe, when the annealing temperature is 700 ℃.

Influence of substrate temperature driven magnetic anisotropy on the magnetostrictive behavior of Tb[sbnd]Fe[sbnd]Co thin films

TbFeCo thin films were grown at different substrate temperatures on Si 〈100〉 substrates to engineer their structure, microstructure, magnetic anisotropy and magnetostriction. Structural studies showed an amorphous structure for the as-deposited film. However, films deposited at higher substrate temperatures exhibited formation of Tb2(Fe,Co)17 and FeCo phases. Magnetization studies displayed presence of in-plane magnetic anisotropy for all the films. In addition to this, strong out-of-plane magnetic anisotropy co-existing with in-plane component has been observed for the film grown at 400 °C. These observations were further confirmed by the magnetic microscopy studies which showed strong magnetic phase contrast for the film processed at 400 °C. Consequently, the magnetostriction calculated from tip deflection measurements was found to be very small for this film. A maximum magnetostriction of about 300 ppm has been realized for the film grown at 550 °C owing to the presence of in-plane magnetic anisotropy, enhancement in crystallization and relaxation of stresses.

On spinodal decomposition in alnico - A transmission electron microscopy and atom probe tomography study

Alnico is a prime example of a finely tuned nanostructure whose magnetic properties are intimately connected to magnetic annealing (MA) during spinodal transformation and subsequent lower temperature annealing (draw) cycles. Using a combination of transmission electron microscopy and atom probe tomography, we show how these critical processing steps affect the local composition and nanostructure evolution with impact on magnetic properties. The nearly 2-fold increase of intrinsic coercivity (Hci) during the draw cycle is not adequately explained by chemical refinement of the spinodal phases. Instead, increased Fe-Co phase (α1) isolation, development of Cu-rich spheres/rods/blades and additional α1 rod precipitation that occurs during the MA and draw, likely play a key role in Hci enhancement. Chemical ordering of the Al-Ni-phase (α2) and formation of Ni-rich (α3) may also contribute. Unraveling of the subtle effect of these nano-scaled features is crucial to understanding on how to improve shape anisotropy in alnico magnets.

59Co NMR analysis of CoFeB-MgO based magnetic tunnel junctions

59Co Nuclear Magnetic Resonance (NMR) and X-Ray Diffraction (XRD) has been used to investigate annealing effects on crystallographic structure and magnetic properties of ferromagnetic CoFeB layer in pseudo spin valves (P-SV): Ta/CoFeB/MgO/CoFeB and in an exchange bias spin valves (EB-SV): PtMn/CoFe/Ru/CoFeB/MgO/CoFeB magnetic tunnel junctions. It was found, that the as-deposited CoFeB layers in the P-SV and EB-SV are amorphous. The annealing treatment at 340 °C and 360 °C induces in all studied systems progressive phase segregation in CoFeB layer to areas of bcc CoFe regions displaying different degree of B2 order and to areas of an amorphous magnetic reminder with a higher B content. In the CoFeB layers grown on Ru (EB-SV) and Ta (P-SV) and covered by MgO in the upper interface, after annealing at 500 °C a state of substantially full phase segregation to B2 ordered bcc CoFe phase has been observed. The annealing at 500 °C of the CoFeB layers grown on MgO and covered by Ru in P-SV and EB-SV triggers extensive thermal diffusion and induces mixing at the CoFeB/Ru interface which transforms the CoFeB into a non-magnetic layer. Moreover, it has been found that CoFeB crystallizes better into the bcc CoFe (100) texture when deposited on Ta than on a Ru layer. XRD profiles show that while CoFeB layer is growing on Ru it crystallizes into bcc CoFe (110) oriented texture, and the one grown on Ta crystallizes into bcc CoFe (200) when annealed at 500 °C. 59Co NMR restoring field analysis (Hrest - magnetic stiffness) showed that the insertion of the antiferromagnetic PtMn layer into the EB-SV system increases the magnetic hardness of the CoFeB layer by an order of the magnitude. However, adding CoFeB/MgO layers between PtMn/CoFe/Ru and CoFeB layers in the EB-SV decouples the CoFeB from the hard-magnetic system making it magnetically soft.

High-coercivity SmCo5/α-Fe nanocomposite magnets

Bulk nanocomposite SmCo5/α-Fe magnets were produced by the spark plasma sintering method from SmCo5/α-Fe nanocomposite powders, which were prepared by ball milling of SmCo5 and Fe powders. Although the SmCo5/α-Fe nanocomposite powders did not show high coercivity, the bulk magnets produced from them by the spark plasma sintering method exhibited high coercivity. Synchrotron radiation X-ray diffraction studies revealed that the SmCo5 phase was transformed into the Sm2Co17 phase after sintering at 973 K. The bulk nanocomposite magnet produced at 873 K consisted of the Fe and SmCo5 phases, while that produced at 973 K consisted of the FeCo and Sm2Co17 phases. It was found that the bulk nanocomposite SmCo5/α-Fe magnet produced at 873 K exhibited a high remanence of 83.1 Am2/kg with a high coercivity of 0.98 MA/m.

Depth selective crystallization study of CoFeB film on MgO

Depth resolved study of crystallization of CoFeB film on MgO film has been done using GIXRD measurements. The quality and thermal stability of MgO film deposited using EB evaporation, was ensured using GIXRD measurements. Depth selectivity for studying the crystallization of CoFeB film has been achieved by performing the GIXRD measurements as a function of the angle of incidence on as-deposited as well as annealed samples. GIXRD measurements on partially crystallized sample annealed at 300 °C reveal that the crystallization of CoFeB film occurs preferentially at CoFeB/MgO interface with the thickness of crystallized layer being 7 nm. This is in conformity with suggestions made in some earlier studies. Anomalous XRD measurements near the Fe absorption edge on completely crystallized sample achieved after annealing at 450 °C shows the presence of (1 0 0) superlattice peak. This evidences the formation of ordered bcc CoFe phase. This result is important as it has been suggested that an ordered CoFe bcc phase may improve the symmetry filtering effect and hence the Tunnel Magnetoresistance value.

The microstructure and magnetic properties of FeCo@SiO2 core-shell nanoparticles synthesized by using a solution method

FeCo@SiO2 nanoparticles were prepared by a solution method and annealed in gas mixtures. The influence of annealing temperature and SiO2 shell thickness on the morphology, microstructure and magnetic properties has been checked. After annealing at 450 °C, FeCo@SiO2/0.2 ml nanoparticles with pure FeCo phase show large saturation magnetization of 138.5 emu/g. With the increasing of SiO2 shell thickness, the pure FeCo phase of FeCo@SiO2 nanoparticles was kept, the magnetization decreased and the coercivity increased respectively. These FeCo nanoparticles with large saturation magnetization have great potential in some industry fields.