Jung-Wei Liao

Probing the A1 to L10 transformation in FeCuPt using the first order reversal curve method

The A1-L10 phase transformation has been investigated in (001) FeCuPt thin films prepared by atomic-scale multilayer sputtering and rapid thermal annealing (RTA). Traditional x-ray diffraction is not always applicable in generating a true order parameter, due to non-ideal crystallinity of the A1 phase. Using the first-order reversal curve (FORC) method, the A1 and L10 phases are deconvoluted into two distinct features in the FORC distribution, whose relative intensities change with the RTA temperature. The L10 ordering takes place via a nucleation-and-growth mode. A magnetization-based phase fraction is extracted, providing a quantitative measure of the L10 phase homogeneity.

High thermal stability and low Gilbert damping constant of CoFeB/MgO bilayer with perpendicular magnetic anisotropy by Al capping and rapid thermal annealing

We demonstrate that the magnetic anisotropy of the CoFeB/MgO bilayer can be manipulated by adding an aluminum capping layer. After rapid thermal annealing, we can achieve large perpendicular magnetic anisotropy of CoFeB with a high thermal stability factor (Δ = 72) while the Gilbert damping constant can be reduced down to only 0.011 simultaneously. The boron and residual oxygen in the bulk CoFeB layer are properly absorbed by the Al capping layer during annealing, leading to the enhanced exchange stiffness and reduced damping. The interfacial Fe-O bonding can be optimized by tuning annealing temperature and thickness of Al, resulting in enhanced perpendicular anisotropy.

Highly (001)-oriented thin continuous L10 FePt film by introducing an FeOx cap layer

We demonstrate a thin and continuous L10 FePt film with a well-aligned (001) texture directly grown on Si || SiO2 substrates by introducing an FeOx cap layer. The agglomeration of capped FePt films is greatly suppressed by inhibiting the surface diffusion. This, in turn, yields a continuous and smooth film, which significantly promotes the (001) out-of-plane orientation and perpendicular anisotropy. The reduction of Fe oxides occurs during annealing, which not only promotes interdiffusion of Fe and Pt for L10 ordering but also removes the cap layer simultaneously. Therefore, additional etching for the cap layer is not required for further fabricating bit patterned media.

Effects of laminated soft layer on magnetization reversal of exchange coupled composite media

The laminated soft layer (LSL) that comprises the granular [CoPtCr–SiO2/Pt]N multilayers with perpendicular magnetization is designed to reduce the switching field of exchange coupled composite (ECC) media. The magnetic simulation shows that the reduction in the switching field can be optimized by changing the coupling strength between the adjacent CoPtCr–SiO2 layers in LSL. The reversal mechanism of ECC media with LSL depends on the bilayers number N of [CoPtCr–SiO2/Pt]N. Both simulation and experiments reveal that the domain-wall assisting reversal strongly depends on the thickness of LSL. By properly adjusting the coupling strength inside the LSL, the switching and saturation fields can be significantly reduced at a limited thickness of the soft layer due to decreased domain-wall length by the Pt lamination.

001) FePt graded media with PtMn underlayers

(001)-oriented FePt graded media are obtained by using PtMn underlayers. The PtMn underlayer not only behaves as the (001) structural template but provides the diffusion source of Mn. The diffusion of Mn into FePt reduces its anisotropy but, on the other hand, the exchange coupling between antiferromagnetic PtMn and ferromagnetic FePt enhances the anisotropy. Hysteresis loops taken from x-ray magnetic circular dichroism confirm the competition between these two effects, leading to the lowest anisotropy in the middle of FePt.

Magnetic Yoking and Tunable Interactions in FePt-Based Hard/Soft Bilayers

Magnetic interactions in magnetic nanostructures are critical to nanomagnetic and spintronic explorations. Here we demonstrate an extremely sensitive magnetic yoking effect and tunable interactions in FePt based hard/soft bilayers mediated by the soft layer. Below the exchange length, a thin soft layer strongly exchange couples to the perpendicular moments of the hard layer; above the exchange length, just a few nanometers thicker, the soft layer moments turn in-plane and act to yoke the dipolar fields from the adjacent hard layer perpendicular domains. The evolution from exchange to dipolar-dominated interactions is experimentally captured by first-order reversal curves, the ΔM method, and polarized neutron reflectometry, and confirmed by micromagnetic simulations. These findings demonstrate an effective yoking approach to design and control magnetic interactions in wide varieties of magnetic nanostructures and devices.

Magnetic patterning: local manipulation of the intergranular exchange coupling via grain boundary engineering

Magnetic patterning, with designed spatial profile of the desired magnetic properties, has been a rising challenge for developing magnetic devices at nanoscale. Most existing methods rely on locally modifying magnetic anisotropy energy or saturation magnetization, and thus post stringent constraints on the adaptability in diverse applications. We propose an alternative route for magnetic patterning: by manipulating the local intergranular exchange coupling to tune lateral magnetic properties. As demonstration, the grain boundary structure of Co/Pt multilayers is engineered by thermal treatment, where the stress state of the multilayers and thus the intergranular exchange coupling can be modified. With Ag passivation layers on top of the Co/Pt multilayers, we can hinder the stress relaxation and grain boundary modification. Combining the pre-patterned Ag passivation layer with thermal treatment, we can design spatial variations of the magnetic properties by tuning the intergranular exchange coupling, which diversifies the magnetic patterning process and extends its feasibility for varieties of new devices.

Direct probing magnetization reversal of exchange-coupled-composite media by x-ray magnetic circular dichroism

X-ray magnetic circular dichroism (XMCD) was used to directly probe the depth-dependent magnetization reversal of CoPtCr-SiO2-based exchange-coupled-composite media with laminated soft layers. A thin Fe-marker layer in the soft layer was used as the indicator of local magnetization. Element-specific XMCD loops of Fe-marker layers confirmed the transition of the magnetization reversal from rigid magnets to exchange-spring magnets with increasing thickness of the soft layer. The micromagnetic simulations revealed the importance of the reduced exchange constant (Asoft) by laminating the soft layer for domain-wall assisting reversal. By comparing XMCD loops with simulations, we can deduce the effective Asoft.

Fabrication of FePt networks by porous anodic aluminum oxide

It is demonstrated that the large-area FePt network nanostructures with strong perpendicular anisotropy can be obtained by growing the mask of porous anodic aluminum oxide (AAO) directly on the L10-FePt films and subsequent plasma etching. The aspect ratio of the AAO mask is critical to achieve well-organized FePt networks. The out-of-plane coercivity of FePt networks is enhanced by 20% compared to that of the FePt film, due to the domain wall pinning effects imposed by the presence of pores.

Thermal dewetting with a chemically heterogeneous nano-template for self-assembled L10 FePt nanoparticle arrays

A design for the fabrication of metallic nanoparticles is presented by thermal dewetting with a chemically heterogeneous nano-template. For the template, we fabricate a nanostructured polystyrene-b-polydimethylsiloxane (PS-b-PDMS) film on a Si|SiO2 substrate, followed by a thermal annealing and reactive ion etching (RIE) process. This gives a template composed of an ordered hexagonal array of SiOC hemispheres emerging in the polystyrene matrix. After the deposition of a FePt film on this template, we utilize the rapid thermal annealing (RTA) process, which provides in-plane stress, to achieve thermal dewetting and structural ordering of FePt simultaneously. Since the template is composed of different composition surfaces with periodically varied morphologies, it offers more tuning knobs to manipulate the nanostructures. We show that both the decrease in the area of the PS matrix and the increase in the strain energy relaxation transfer the dewetted pattern from the randomly distributed nanoparticles into a hexagonal periodic array of L10 FePt nanoparticles. Transmission electron microscopy with the in situ heating stage reveals the evolution of the dewetting process, and confirms that the positions of nanoparticles are aligned with those of the SiOC hemispheres. The nanoparticles formed by this template-dewetting show an average diameter and center-to-center distance of 19.30 ± 2.09 nm and 39.85 ± 4.80 nm, respectively. The hexagonal array of FePt nanoparticles reveals a large coercivity of 1.5 T, much larger than the nanoparticles fabricated by top-down approaches. This approach offers an efficient pathway toward self-assembled nanostructures in a wide range of material systems.