F. T. Yuan

Effect of initial stress/strain state on order-disorder transformation of FePt thin films

Initial stress (σi) of a room-temperature deposited FePt films was manipulated to study the order-disorder transformation. We observed that, while σi was increased from −1.01 (compressive) to 0.18 GPa (tensile), the phase transformation activation energy decreased from 0.387 to 0.23 eV/atom. This causes a reduction in ordering temperature of about 100 °C. We also found that densification induces an increase in tensile stress of about 1 GPa before ordering. In the films with small σi, strong tension facilitates the nucleation of L10 FePt; however, if highly compressive σi cancels the densification tension, ordering is retarded causing higher ordering temperature.

Effects of Pt and Fe underlayers on the microstructure and magnetization reversal of epitaxial FePt films for high areal density magnetic recording

The effects of underlayer on the microstructure and magnetic characterizations of epitaxial FePt films were studied. Enhanced out-of-plane coercivity and ordering were obtained in the Pt-underlayered FePt samples, which may be due to the proper lattice strain from the Pt underlayer. The larger lattice parameter of Pt may enlarge the a axis and reduce the c axis of the L10 FePt phase and result in low-temperature ordering. However, such an effect does not appear in Fe-underlayered samples because of the smaller lattice parameter and the considerable interdiffusion. The angular dependence coercivity revealed that the dominant reversal mechanism of the FePt films changed from domain-wall motion to rotation, which is caused by the formation of L10 island structures induced by the Pt underlayer. On the other hand, the Fe-underlayered FePt films showed a broad peak roughly at 45°, implying the tilt of the easy axis about 45° away from the film normal.

Magnetization reversal and microstructure of FePt–Ag (001) particulate thin films for perpendicular magnetic recording media

FePt thin films with the addition of Ag layers have been fabricated in order to study the microstructure and magnetic properties via a molecular beam epitaxy technique on MgO (001) single-crystal substrates at 350°C. The enhancement of the coercivity is due to the film structures changed from continuous state to an isolated particulate character, not an ordering enhancement of FePt films with Ag addition. Studies of angular dependent coercivity show a tendency of a domain-wall motion shift toward rotation of reverse-domain type with the addition of Ag layers into the FePt films. The intergrain interaction was confirmed from the Kelly-Henkel plot that indicated FePt films with Ag addition can lead to the reduction of intergrain exchange coupling.

Fabrication of L 1 1 Phase CoPt Film on Glass Substrate With [Co/Pt] Multilayer Structure

L1₁ CoPt hlms were fabricated on glass substrate with Pt underlayer by alternate deposition of Co and Pt hlms. The effect of individual Co and Pt thickness, and the total CoPt hlm thickness on structural and magnetic properties were investigated. In this paper, the experiments were divided into two parts. In the hrst part, [(x)Co₁/(x)Pt_0.75]_n hlms with varying Co and Pt hlms were deposited and overall film thickness was kept at 7 nm. In the second part, [(x)Co₁/(x)Pt_0.75]_n hlms with different total thickness were deposited. In the first case, the coercivity (H_c⊥), squareness (S_⊥), and perpendicular magnetic anisotropy are found to deteriorate with increasing Co and Pt layer thickness due to longer diffusion length. In the second case, the higher H_c⊥ with value of 110 kA/m is obtained in [(x)Co₁/(x)Pt_0.75]_n film, and increasing total thickness of film results in lower H_c⊥. Therefore, it is demonstrated that the CoPt film sputtered by multilayer type with lower thicknesses of Co and Pt layer promotes the formation of L1₁ structure with excellent perpendicular magnetic properties.

Sputtering perpendicular magnetic anisotropy CoPt thin film on glass substrate at room temperature

[Co1.0/Pt0.75]n multilayers consisting of 1.0-nm-thick Co and 0.75-nm-thick Pt layers with n = 2–6 were grown on glass substrates with and without a Pt underlayer at room temperature (RT). In this study, the effects of total thickness and Pt underlayer on magnetic properties and microstructures are investigated. Without a Pt underlayer, the [Co1.0/Pt0.75]n film exhibits soft magnetic behaviors due to poor crystallization. After the inclusion of a Pt underlayer between magnetic layer and glass substrate, the [Co1.0/Pt0.75]n films show significant perpendicular magnetic anisotropy (PMA), out-of-plane coercivity, and squareness as the total thickness of [Co1.0/Pt0.75]n is in the range of 5.25–10.5 nm. The induced PMA is attributed to the epitaxial growth of CoPt grains along Pt(111) underlayer. The RT-prepared CoPt film with PMA shows high potential in future applications of spintronics.

Preannealing effect on ordering transformation and magnetic properties of CoPt thin films

Single layer CoPt thin films with thicknesses from 30to100nm were sputtered on glass substrates. A preannealing at 250°C for 80min was applied to the samples, before ordering treatment at 700°C for 10min. This preannealing process resulted in a 33%–65% decrease in coercivity and 23%–55% decrease in volume fraction of order phase f0. Results of morphology investigation showed that the preannealing increases the surface roughness, which mainly results from the elimination of defects. Structural results further suggest that the eliminated defects are mostly low dimensional, such as dislocations or vacancies. It can thus be concluded that those low dimensional imperfections in the CoPt films play an enhancing role in ordering transformation process.

Large energy density enhancement in FePt films by microstructure refining

The effect of microstructure refinement on magnetic properties in single-phase Ll0 Fe58Pt42 films was studied. Samples with thickness t of 100, 200, and 300nm were sputtering deposited on glass substrate with elevated temperatures Ta of 400 and 800°C. In the t=300 series of films, the order parameter of films with Ta=400 and 800°C were identical, about 0.663, but the average grain size is different by approximately eight times in diameter (from ∼75to∼610nm). Comparing to the film of Ta=800°C with large grains, significant enhancement in remanence of about 32% was obtained in the 400°C annealed film (∼880emu∕cm3) and the energy density was achieved an increment of about 72% (19.6MGOe), which exceeds the theoretical preditions. The reasonable explanations are summarized as the dimension confinement of interactions and the degradation of anisotropy.

Perpendicular magnetic anisotropic Pr-Fe-B thin films on glass substrates

The correlation between microstructures and magnetic properties of Pr-Fe-B thin films with thickness in the range of 25–150 nm sputtered on Ta underlayer buffered glass substrates was investigated. The films with thickness of 50–125 nm showed Pr2Fe14B phase with (004) preferred orientation. Perpendicular magnetic properties, including perpendicular magnetic anisotropy, coercivity ((Hc⊥)2T), remanence ratio ((Mr/M2T)⊥), and energy product [(BH)max⊥]2T were improved with increment of magnetic layer thickness to 150 nm. Higher (Hc⊥)2T of 10.3 kOe and energy product [(BH)max⊥]2T of 19.8 MGOe were attained in the film with the thicker thickness, in turn resulted from fine microstructure and impeded domain wall motion due to nonmagnetic Pr-rich grain boundary phase as a pinning site.