Field Annealing Research Articles

Effects of post-deposition magnetic field annealing on magnetic properties of NiO/Co90Fe10 bilayers

The ferromagnetic (FM)/antiferromagnetic (AF) bilayer structures have drawn intensive attention because of their wide applications in modern spintronic devices. While abundant published works have been reported on the interface effects of the FM/AF bilayers caused by the magnetic field annealing (MFA) process, the volume effects caused by the MFA treatment have been rarely considered. In this work, the microstructural and magnetic properties of the NiO/CoFe bilayers with various CoFe thicknesses were investigated under different annealing temperatures. At high annealing temperature, the interlayer mixing and exchange coupling between NiO and CoFe layers were promoted and consequently the interface effects were facilitated. The interfacial oxides acted as pinning centers and randomly pinned the FM domains, leading to an increase of coercivity and a considerable degradation of uniaxial anisotropy. The increase of coercivity was also contributed by the enhancement of the interfacial exchange coupling between the NiO and CoFe layers after MFA. As the CoFe thickness increased, the volume effects tended to dominate over the interface effects, resulting in the preservation the uniaxially anisotropic features of CoFe. These results indicate that both the coercivity and anisotropic features of the NiO/CoFe bilayers can be directly affected by the MFA process, opening up the possibility of modifying the magnetism in the NiO/CoFe bilayers and offering an effective way to improve the performance of modern spintronic devices.

Microstructure refinement and magnetization improvement in CoFe thin films by high magnetic field annealing

Co-Fe alloy thin films with high magnetization are desirable in numerous applications. Co-Fe thin films prepared by solution methods with refined microstructures have not been achieved up to now. Here, high magnetic field annealing (HMFA) is introduced for the preparation of Co70Fe30 alloy thin films. The effects of HMFA on their microstructures and room temperature magnetic properties are investigated. The thickness of the derived Co70Fe30 thin films is dramatically decreased from a value of 590 nm without HMFA to 320 nm with 7 T magnetic field annealing, and the refinement of microstructures is also observed with HMFA. The derived Co70Fe30 film annealed under 7 T magnetic field exhibits a saturation magnetization of ∼2.43 T, close to the theoretical value of 2.45 T. Overall, HMFA provides a feasible way to fabricate larger-area and denser Co-Fe thin films with excellent magnetic properties.

Structure transformations in cold rolled ferromagnetic alloys upon heating in high DC magnetic field

The processes of recovery and recrystallization in the coldrolled alloys Fe-3%Si, Fe-50%Ni, and Ni-30%Co subjected to annealing at temperatures below and above the Curie point under a high direct current (DC) magnetic field of 10–29Т have been studied. It is established that application of external magnetic field in the course of heat treatment at temperatures below the recrystallization temperature and Curie point retards the processes of material softening. The formation of crystallographic texture initiated by magnetic field starts upon the recovery during the pre-recrystallization magnetic annealing. Then, in the course of subsequent annealing at the recrystallization temperatures without magnetic field, there takes place the development of preferable orientations. The magnetic field annealing favors the formation of such texture components in which the easy-magnetization axis coincides with the direction of the magnetic field applied. This is evidenced by the textures formed in the Fe-50%Ni alloy (easy axis<001>) and Ni-30%Co alloy (easy axis<111>).

Structural, magnetic characterization (dependencies of coercivity and loss with the frequency) of magnetic cores based in Finemet

We report changes of coercivity, induced magnetic anisotropy, magneto-optical domain structure and frequency dependencies of coercivity and energy loss (up to 10MHz) associated with the structural modifications produced by thermal treatments under applied magnetic field (field annealing) in toroidal wound cores of Fe73.5Cu1Nb3Si15.5B7 amorphous alloy. The thermal treatment (535°C, 1h) leads to the typical nanocrystalline structure of α-Fe(Si) nanograins (60–65% relative volume, 10–20nm average grain size embedded in a residual amorphous matrix, while the magnetic field with the possibility to be applied in two directions to the toroidal core axis, that is in transverse (which is equivalent to the transverse direction of the ribbon) or longitudinal (equivalent to the longitudinal direction of the ribbon), develops a macroscopic uniaxial magnetic anisotropy in the transverse (around 245J/m3) or longitudinal (around 85J/m3) direction of the ribbon, respectively. It is remarkable the quasi-unhysteretic character of the cores with these two kinds of field annealing as comparing with that of the as-quenched one. Magneto-optical study by Kerr-effect of the ribbons provides useful information on the domain structure of the surface in agreement with the direction and intensity of the induced magnetic anisotropy. This induced uniaxial magnetic anisotropy plays a very important role on the Hc(f) and EL(f) curves, (f: frequency), being drastic the presence and direction of the induced magnetic anisotropy. In addition, these frequency dependencies show a significant change at the frequency around 100Hz.

Magnetostriction enhancement by high magnetic field annealing in cast Fe81Ga19 alloy

High magnetic field was introduced to improve the saturation magnetostriction of cast Fe81Ga19 alloy during annealing process. The effects of high magnetic field on the microstructure, magnetic domain structure and magnetostrictive properties were investigated in this paper. Results indicated that high magnetic field could transfer Ga atom from precipitation to parent phase. High magnetic field also enhanced [100] direction orientation of FeGa alloy. The saturation magnetostriction for Fe81Ga19 alloy annealed at 10T increased by about five times compared to one without high magnetic field. The improvement in saturation magnetostriction is attributed to the enhanced [100] orientation and Ga diffusion under high magnetic field during annealing process.

Exchange bias study of sub-100 nm-diameter CoFeB/IrMn antidot and nanodot arrays fabricated by nanosphere lithography

Exchange-coupled bilayers are widely used as pinned layers in nanometric spintronic devices. In this work, sub-100 nm-diameter CoFeB/IrMn antidot and nanodot arrays were patterned by nanosphere lithography. The exchange bias (Hex) and coercivity (Hc) of the nanostructures and continuous films exhibit similar exponential dependence on CoFeB layer thickness. Magnetic field annealing results in changed crystallinity, surface roughness, and magnetic properties. Reduced Hc and enhanced Hex are observed after annealing at low temperatures, while high-temperature annealing results in higher Hc and lower Hex. This work provides physical insights on the magnetization reversal response in nanosized spintronic devices involving CoFeB/IrMn reference layers.

Magnetic and exchange bias properties of YCo thin films and IrMn/YCo bilayers

We report on the structural and magnetic properties of YCo thin films and IrMn/YCo bilayers. X-ray diffraction infer that all the films are amorphous in nature. Magnetization versus magnetic field measurements reveal room temperature soft ferromagnetism in all the YCo films. Thin films which were grown at 100W sputter power with growth rates of 0.677, 0.694 and 0.711Å/sec show better morphology and composition than 50W (0.333, 0.444 and 0.277Å/sec) grown films. Perpendicular exchange bias in as deposited bilayers is evident for IrMn/YCo bilayers. Exchange bias (EB) decreases in case of in plane measurements and enhances for out of plane measurements after perpendicular field annealing. EB is more in case of out of plane direction due to large perpendicular anisotropy in comparison with in plane direction. Above the critical thickness, EB variation is explained on the basis of random field model in the Heisenberg regime, which has been proposed by Malozemoff. Indeed there exists an inverse relationship between EB and IrMn layer thickness. Evidenced vertical shift apart from the horizontal shift for magnetization loops is attributed to frozen magnetic moments in one of the layers at the interface. Present results would prove to be helpful in spintronic device applications.

Magnetic field annealing effect and superparamagnetic contributions in one-dimensional CoPt nanostructures

A low cost versatile electrochemical method has been employed to synthesize highly ordered CoPt nanowires (NWs) in anodic aluminum oxide (AAO) templates with average pore diameter of about 100 nm. The structural properties of as deposited NWs have been studied through XRD analysis showing face centered cubic (fcc) as the dominant phase of CoPt NWs. Magnetic properties at room temperature and lower temperature has been investigated with applied field parallel and perpendicular to NWs axis. The easy magnetization axis is aligned perpendicular to NWs owing to the strong magnetostatic interactions among the NWs due to smaller interwire distance (∼15 nm). Furthermore, the as deposited arrays of NWs have been annealed for 2 h at 300 °C in the presence of 1 T magnetic field applied in the direction perpendicular to NWs axis. Magnetic field annealing gives improved structural and magnetic behavior of these one-dimensional (1D) nanostructures resulting improved crystallinity and increased values of coercivity (Hc) and remnant squareness (SQ). Superparamagnetic contributions at lower temperature due to presence of fine nanoparticles in blocking state play important role and leads to enhanced magnetic behavior of CoPt NWs.

Effects of field annealing on Gilbert damping of polycrystalline CoFe thin films

The effects of annealing and magnetic field annealing on the Gilbert damping constant of polycrystalline CoFe thin films were investigated. Angle dependent ferromagnetic resonance (FMR) demonstrates that an in-plane (IP) anisotropy is developed by field annealing which dramatically reduces the effective Gilbert damping constant. An inverse relation between effective IP anisotropic field and effective damping is shown by FMR measurement in cryogenic environment. This relation is further confirmed in bilayer films of IrMn/CoFe where a strong in-plane unidirectional anisotropy is developed. A large effective IP anisotropic field is possibly associated with a smaller distribution of local fields which reduces the FMR linewidths and subsequently effective damping.

The effects of field annealing on the magnetic properties of FeSiB amorphous powder cores

The effects of transverse magnetic field annealing on the magnetic properties of Fe78Si9B13 amorphous powder cores were investigated. The cores annealed at 400°C under an external transverse magnetic field of 0.5 T show an enhanced and stable effective permeability of 90 in the high frequency range up to 2MHz. Meanwhile, the permeability was improved by about 5% and the core loss was decreased by about 10% compared to the core with zero field annealing, which is attributed to the relief of internal stress and the variation of domain structure. It also shows superior DC bias property with a “percent permeability” of 70% at an external field of 100Oe. Transverse magnetic field annealing is an effective way to improve the soft magnetic properties of the amorphous powder cores. The improvement of soft magnetic properties of the Fe-based amorphous powder cores is encouraging for future applications as functional materials.

Effect of Magnetic Field Annealing on Microstructure and Magnetic Properties of FeCuNbSiB Nanocrystalline Magnetic Core with High Inductance

Effect of Magnetic Field Annealing on Microstructure and Magnetic Properties of FeCuNbSiB Nanocrystalline Magnetic Core with High Inductance

Reduced magnetic coercivity and switching field in NiFeCuMo/Ru/NiFeCuMo synthetic-ferrimagnetic nanodots

The coercivity (Hc) and switching field (Hsw) of free layers increase remarkably with shrinking structural dimensions, reducing the sensitivity of nanosized magnetoresistive sensors. In this work, conetic-alloy (NiFeCuMo) synthetic ferrimagnetic (SyF) trilayers are proposed to reduce Hc and Hsw in magnetic nanostructures. SyF stacks of NiFeCuMo/Ru/NiFeCuMo were patterned into nanodot arrays with diameter of 60nm by nanosphere lithography. The thickness of Ru layer was chosen so that high interlayer coupling energy existed in the continuous film. The linear dependence of Hc and Hsw of SyF nanodot on the amplification factor was revealed. Magnetic field annealing was conducted at various temperatures (Tan) ranging from 373K to 673K. Annealing at low temperature (Tan≤473K) relaxed the structural disorders, resulting in reduced surface roughness and decreased Hc and Hsw. Higher Tan changed the preferred orientations in the crystalline structures, leading to increased roughness and higher Hc and Hsw. This work shows that the Hc and Hsw of nanostructures can be reduced through engaging Conetic alloy in SyF stack. The Conetic-alloy-based SyF structures are a promising candidate as free layers in nanosized spintronic devices.

Dual mean field search for large scale linear and quadratic knapsack problems

An implementation of mean field annealing to deal with large scale linear and non linear binary optimization problems is given. Mean field annealing is based on the analogy between combinatorial optimization and interacting physical systems at thermal equilibrium. Specifically, a mean field approximation of the Boltzmann distribution given by a Lagrangian that encompass the objective function and the constraints is calculated. The original discrete task is in this way transformed into a continuous variational problem. In our version of mean field annealing, no temperature parameter is used, but a good starting point in the dual space is given by a “thermodynamic limit” argument. The method is tested in linear and quadratic knapsack problems with sizes that are considerably larger than those used in previous studies of mean field annealing. Dual mean field annealing is capable to find high quality solutions in running times that are orders of magnitude shorter than state of the art algorithms. Moreover, as may be expected for a mean field theory, the solutions tend to be more accurate as the number of variables grow.

Field-anneal-induced magnetic anisotropy in highly textured Fe-Al magnetostrictive strips

Highly textured (011)[100] Goss-oriented rolled sheet Fe-Al alloy is a promising magnetostrictive material for use in bending mode sensors and vibrational energy harvesters. In this paper, we performed magnetic field annealing (FA) to induce magnetic anisotropy in strips of highly textured Fe-Al. Prior work suggests FA as a viable alternative to stress annealing (SA), which leads to buckling of Fe-Al rolled sheet samples. The Fe-Al strips studied here exhibited tetragonal magnetostriction values ((3/2)λsat = λ∥ − λ⟂) of ∼136 ppm along their length, which corresponds to ∼78% of the single crystal value along a &amp;lt;100&amp;gt; orientation. The effectiveness of FA on magnetic moment rotation was inferred by comparing post-FA magnetostriction measurements with the maximum possible yield, where λ∥= 0 and λ⟂= (3/2)λsat. Strain gauge data from the middle of the strip indicates that FA achieved ∼27% of the desired built-in uniaxial anisotropy along the parallel direction of the strip length, decreasing λ∥ by 25 ppm of the 95 ppm. Hall effect sensor data was used to assess the potential effect of FA on sensing and energy harvesting performance. FA improved the bending-stress-induced changes in magnetization near the clamped end of the strips by ∼45%. These results suggest that the FA was more effective in the region near the end of the strip than toward the middle of the strip, which we explain may be a result of the use of high temperature permanent magnets at the ends of the strips for the FA protocol.

Enhanced soft magnetic properties of Fe-based amorphous powder cores by longitude magnetic field annealing

Abstract The longitude magnetic field annealing on the soft magnetic properties of Fe-based Fe 78 Si 9 B 13 amorphous powder cores were investigated. The magnetic properties of powder core greatly change when annealed under longitude magnetic field. With the increase of magnetic field intensity, the permeability of annealed powder core further increase. The core shows an enhanced effective magnetic permeability of 66 after longitude magnetic field annealing at an applied field of 30 Oe and temperature of 400 °C, which is attributed to the relief of internal stress and the variation of atomic structure. The permeability was improved by 8% and the frequency stability of FeSiB amorphous powder core is up to 2 MHz. The core loss decreases obviously and DC bias property stays at a same level compared with zero field treatment with the increase of magnetic field intensity. For the powder cores, the core loss ranges from 90 to 180 W/kg at 50 mT and 100 kHz, and DC bias property reaches over 70% at 100 Oe. This work provides a novel approach to improve permeability for Fe 78 Si 9 B 13 powder cores and also validates the application prospect of powder core in the work condition of different ripple currents, different loads and a wide frequency range (10 kHz–2 MHz).

Structure and Magnetic Property Changes of 2–2-Type BaTiO3–CoFe2O4 Composite Thin Films Induced by Magnetic Field Annealing

BaTiO3–CoFe2 O 4 composite thin films with different thicknesses were fabricated via chemical solution deposition. Magnetic annealing and non-field annealing processes were applied in the sintering process, respectively. Variations of the structure and magnetic properties of the composite films induced by the magnetic annealing were investigated. It is found that the mean grain size and grain boundary are increased and decreased, respectively. For the magnetic-annealed films, the internal strain is reduced with the increase of film thickness. Magnetic measurements show that the M s is also increased with the film thickness and the magnetic-annealed films have bigger M s values than the non-field-annealed films. Based on the experimental results, it can be concluded that the effect of magnetic annealing on the structure and properties of the composite films is decreased with the increase of film thickness.

Decreased oxygen vacancies and improved ferroelectric properties of the BiFeO3 thin films with high magnetic field annealing

High magnetic field annealing (MFA) effects on polycrystalline BiFeO3 (BFO) thin films deposited on ITO buffered glass substrates by the chemical solution deposition (CSD) method have been investigated. Magnetic force induced by magnetic field leads to the increased crystallite size and the decrease in activation energy for boundary motion, especially for the low temperature annealed films. More Fe3+ ions are induced with increasing magnetic field to lower the free energy of the BFO films, which also leads to a decreased oxygen vacancy, resulting in low leakage current density. The increased remanent polarization and the decreased coercive field initially with the increase in magnetic field are attributed to the reduced domain wall pinning resulted from decreased oxygen vacancies and grain boundaries. Generally, the decreased coercivity and the improved ferroelectric polarization suggest high MFA is an effective way to enhance ferroelectric properties of the CSD-derived BiFeO3 thin films.

Fabrication, morphological, structural and magnetic properties of electrodeposited Fe3Pt nanowires and nanotubes

Highly ordered Fe3Pt nanowires (NWs) and nanotubes (NTs) embedded in anodic aluminum oxide (AAO) template have been fabricated by dc electrodeposition method. Response of heat treatment on structural and magnetic properties of the samples has been studied with and without the presence of magnetic field (1T). X-Ray Diffraction analysis shows chemically ordered L12 face centered cubic (FCC) as the dominant phase for Fe3Pt NWs and heat treatment improves crystallinity with retained its phase. Whereas, Fe3Pt NTs show amorphous behavior with and without magnetic field annealing. Furthermore, magnetic properties of the samples have been investigated by vibrating sample magnetometer (VSM). Magnetic parameters of Fe3Pt including magnetic coercivity, saturation magnetization, squareness and shape of MH-loops have been investigated as a result of simple and MF annealing.

Grain alignment due to magnetic-field annealing in MnBi:Bi nanocomposites

High-anisotropy arc-melted and field-annealed Mn100−xBix (x = 50–65) alloys have been investigated. Samples predominantly consist of low-temperature phase (LTP) MnBi and elemental Bi, where the LTP volume fraction and sample magnetization decrease with increasing x. All samples are hard ferromagnetic at room temperature and demonstrate a structural and magnetic phase transition at 630 K. For Mn35Bi65, the highest values for coercivity (4.6 kOe), magnetization remanence ratio (0.97), and energy product (4.9 MGOe) are measured at 540 K, near the melting point of bismuth. We explain the trend in magnetic properties as a consequence of c-axis alignment of MnBi grains facilitated by the molten Bi during magnetic field annealing.

Magnetic properties and magnetoimpedance of FeCoNi/CuBe electroplated tubes with different features of field-annealing induced magnetic anisotropy

The effect of field annealing (in direct (DC) or alternating current (AC) field) on the structure, magnetic properties and giant magnetoimpedance (GMI) of CuBe/ Fe19Co17Ni64 electroplated tubes was studied. The field and frequency dependences of total impedance and its real part were comparatively analyzed together with magnetization processes features. The GMI sensitivity with respect to an applied field was the subject of special attention in view of possible applications of these materials in small magnetic field sensors. The maximum GMI ratio depends strongly on the heat treatments. The DC field annealing leads to the highest total impedance GMI ratio (ΔZ/Z=250%) and real part of the total impedance GMI ratio (ΔR/R=640%), compared to as-cast and AC field annealed samples. The external field response of DC annealed samples presented a single peak GMI response as a consequence of a strong contribution of the longitudinal effective anisotropy. At the same time, the maximum obtained sensitivity of 13.5%/Oe for DC case is much lower compared to the highest sensitivity values obtained for as-prepared (28.6%/Oe) and DC field annealed (22.0%/Oe) tubes for the low frequency of 2.5MHz. The weak dependence of ΔZ/Z ratio in the case of AC field annealed samples in the high frequency range is an important advantage for particular sensor applications.