Yaocen Wang

Magnetization reversal in a preferred oriented (111) L10 FePt grown on a soft magnetic metallic glass for tilted magnetic recording

L1(0) FePt is an important material for the fabrication of high density perpendicular recording media, but the ultrahigh coercivity of L1(0) FePt restricts its use. Tilting of the magnetic easy axis and the introduction of a soft magnetic underlayer can solve this problem. However, high temperature processing and the requirement of epitaxial growth conditions for obtaining an L1(0) FePt phase are the main hurdles to be overcome. Here, we introduce a bilayered magnetic structure ((111) L1(0) FePt/glassy Fe(71)Nb(4)Hf(3)Y(2)B(20)/SiO(2)/Si) in which the magnetic easy axis of L1(0) FePt is tilted by ~36° from the film plane and epitaxial growth conditions are not required. The soft magnetic underlayer not only promotes the growth of L1(0) FePt with the preferred orientation but also provides an easy cost-effective micro/nanopatterning of recording bits. A detailed magnetic characterization of the bilayered structure in which the thickness of (111) L1(0) FePt with the soft magnetic Fe(71)Nb(4)Hf(3)Y(2)B(20) glassy underlayer varied from 5 to 60 nm is carried out in an effort to understand the magnetization switching mechanism. The magnetization switching behavior is almost the same for bilayered structures in which FePt layer thickness is >10 nm (greater than the domain wall thickness of FePt). For FePt film ~10 nm thick, magnetization reversal takes place in a very narrow field range. Magnetization reversal first takes place in the soft magnetic underlayer. On further increase in the reverse magnetic field, the domain wall in the soft magnetic layer compresses at the interface of the hard and soft layers. Once the domain wall energy becomes sufficiently large to overcome the nucleation energy of the domain wall in L1(0) FePt, the magnetization of the whole bilayer is reversed. This process takes place quickly because the domain walls in the hard layer do not need to move, and the formation of a narrower domain wall may not be favorable energetically. Our results showed that the present bilayered structure is very promising for the fabrication of tilted bit-patterned magnetic recording media.

Nano-crystallization and magnetic mechanisms of Fe85Si2B8P4Cu1 amorphous alloy by ab initio molecular dynamics simulation

Iron-based amorphous and nano-crystalline alloys have attracted a growing interest due to their potential in the application of magnetic coil production. However, fundamental understanding of the nano-crystallization mechanisms and magnetic features in the amorphous structure are still lack of knowledge. In the present work, we performed ab initio molecular dynamics simulation to clarify the ionic and electronic structure in atomic scale, and to derive the origin of the good magnetic property of Fe85Si2B8P4Cu1 amorphous alloy. The simulation gave a direct evidence of the Cu-P bonding preference in the amorphous alloy, which may promote nucleation in nano-crystallization process. On the other hand, the electron transfer and the band/orbital features in the amorphous alloy suggests that alloying elements with large electronegativity and the potential to expand Fe disordered matrix are preferred for enhancing the magnetization.

Investigation on the crystallization mechanism difference between FINEMET® and NANOMET® type Fe-based soft magnetic amorphous alloys

In this article, the atomic behaviors of Nb and P in Fe-based amorphous alloys during nano-crystallization process were studied by the combination of ab initio molecular dynamics simulations and experimental measurements. The inclusion of Nb is found to be tightly bonded with B, resulting in the formation of diffusion barrier that could prevent the over-growth of α-(Fe, Si) grains and the promotion of larger amount of α-(Fe, Si) participation. The P inclusion could delay the diffusion of the metalloids that have to be expelled from the α-(Fe, Si) crystallization region so that the grain growth could be reduced with fast but practically achievable heating rates. The combined addition of P and Nb in high Fe content amorphous alloys failed in exhibiting the potential of good magnetic softness with slow heating (10 K/min) annealing at various temperatures. The sample with optimum crystallization process with confined grain size was annealed at 653 K, with the grain size of 31 nm and a coercivity of ∼120 A/m, ...

Magnetic Influence of Alloying Elements in Fe-Rich Amorphous Alloys Studied by Ab Initio Molecular Dynamics Simulations

The magnetic influence of silicon, boron, phosphorous, niobium, as well as Cu in Fe-rich amorphous was studied through ab initio molecular dynamics simulations. The small concentration of metalloids with large electronegativity is beneficial to the saturation magnetization of Fe-rich amorphous alloys, but may reduce the magnetization by p-d orbital hybridization with a large amount of inclusion. On the other hand, owing to their low electronegativity, early transition metals may bring excess electrons to the alloys system and reduce the effect of Fe electron absorption by boron or phosphorous; therefore, the inclusion of them will significantly reduce the magnetization of the alloy. The minor inclusion of Cu slightly exhibits negatively charged in Fe-rich amorphous alloys, which indicates that the transition metals with low electron losing tendency are acceptable in the alloy to maintain excellent soft magnetic properties with high magnetization.

Atomic packing and diffusion in Fe85Si2B9P4 amorphous alloy analyzed by ab initio molecular dynamics simulation

In the work reported in this paper, ab initio molecular dynamics simulation was performed on Fe85Si2B9P4 amorphous alloy. Preferred atomic environment of the elements was analyzed with Voronoi polyhedrons. It showed that B and P atoms prefer less neighbors compared with Fe and Si, making them structurally incompatible with Fe rich structure and repulsive to the formation of α-Fe. However, due to the low bonding energy of B and P caused by low coordination number, the diffusion rates of them were considerably large, resulting in the requirement of fast annealing for achieving optimum nano-crystallization for its soft magnetic property. The simulation work also indicates that diffusion rate in amorphous alloy is largely determined by bonding energy rather than atomic size.

First-principle simulation on the crystallization tendency and enhanced magnetization of Fe76B19P5 amorphous alloy

Iron-based amorphous alloys have attracted a growing interest due to their potential in the application of magnetic coil production. However, the magnetization of this kind of material is usually low due to the lack of long range ordering and high alloying element content. In this paper, an Fe76B19P5 amorphous alloy was simulated with ab initio molecular dynamics based on a previous simulation work on an Fe76Si9B10P5 amorphous alloy exhibiting that electron absorbers such as B and P can help enhance the magnetization of nearby Fe atoms. The present simulation results show that replacing Si with B can destabilize the amorphous structure, making it easier to crystallize, but no separate α-Fe participation can be observed in experiments during annealing due to its high B/P content. The results also show an increase in saturation magnetization by 8% can be expected due to the intensified electron transfer from Fe to B/P, and the glass forming ability decreases correspondingly. The idea of enhancing electron transfer can be applied to the development of other Fe-based amorphous alloys for the purpose of larger saturation magnetization.

Structural and magnetic properties on the Fe-B-P-Cu-W nano-crystalline alloy system

In the present article, the structural and soft magnetic properties of Fe-B-P-Cu alloy system with W addition have been studied as well as the annealing configurations required for magnetic softness. It is found that the substitution of B by W deteriorates the soft magnetic properties after annealing. The reason of such impact with W addition may lie in the insufficient bonding strength between W and B so that the addition of W is not effective enough to suppress grain growth against the high concentration and high crystallization tendency of Fe during annealing. The addition of 4 at.% W is also found to reduce the saturation magnetization of the nano-crystalline alloy by 14%. It is also found that the addition of P in the Fe-based alloys could help reduce the coercivity upon annealing with high heating rate. The existence of P could also help slightly increase the overall saturation magnetization by enhancing the electron transfer away from Fe in the residual amorphous structure.

NANOMET ® and FINEMET ® : Investigation on the crystallization mechanism between different kinds of Fe-based soft magnetic nanocomposite alloys

The potential electrical applications for soft magnetic materials have spurred the research of Fe-based nano-crystalline alloys.

Effects of metalloids in Fe-rich soft magnetic amorphous alloys on magnetization

A large amount of research efforts have been focused on the development of Fe-based amorphous alloys, a kind of soft magnetic materials that is promising in the potential application of motors, transformers and choke coils due to their excellent soft magnetic property. The extraordinarily low coercivity is caused by the disordered structure and the lack of micro-scale anisotropy. Generally, the inclusion of the minor alloying elements necessary for the formation of the amorphous structure can interfere with the Fe-Fe ferromagnetic exchange and reduce the maximum magnetization [1], which is a disadvantage for the efficiency and minimization of the produced devices. On the other hand, it is reported that some common alloying metalloids, such as B and P, can promote the Fe atoms in amorphous alloys into high spin state with larger magnetic moment [2]. The present study is to clarify and optimize the magnetic effect of the alloying elements in Fe-rich amorphous alloys. In this research work, ab initio molecular dynamics simulations were performed for Fe85Si2B8P-4Cu1, Fe76Si9B10P5 and Fe73.5Si13.5B9Nb3Cu1 amorphous alloys. Considering the electric charge transfer, electron structure as well as the cluster formation, it is clarified that minor inclusion of B and P can effectively absorb electrons from Fe atoms, making the radii of 3d orbitals of Fe decrease towards optimum ferromagnetic exchange between Fe-Fe atoms. However, with increasing B/P content, the replacement of Fe-Fe bonds by Fe-metalloids bonds makes severe magnetically inert p-d hybridization which reduces the spin polarization of 3d electrons as well as the magnetic moments [3]. Therefore, B and P have complicated magnetic effect in Fe-based amorphous alloys, which appears to promote magnetization with low concentration, but reduces it at larger concentration. Besides, it was found that Si shows no beneficial effect on increasing the magnetization of the amorphous alloys due to the hybridization between Si 3p and Fe 3d orbitals, although experimental data indicate that Si is good for amorphous formation or crystallization controllability [4].

Structural and Magnetic Study of

Fe76Si9B10P5 metallic glass is an excellent material for its good glass-forming ability and soft magnetic property. However, a large gap still exists toward a better theoretical understanding, which explains their excellent properties that have been experimentally observed. In this paper, we perform molecular dynamics simulation to study the atomic scale structure and clarify the origin for the good magnetic property. The results suggest that the glassy alloy consists of B/P centered dense spaces and sparsely spread Si-rich regions. Fe atoms in the dense area are positively charged and possess larger magnetic moment up to 2.6 μB, which is larger than that in pure α-Fe. The negatively charged B and P atoms were possible reasons for the charge transfer and enhancement of magnetic moment.