Magnetic domain wall pinning and coercivity in FePt/MgO and FePt/Pt thin films

FePt thin layers have been epitaxied either on Pt(0 0 1) or on MgO(0 0 1) substrates, and magnetically characterized using extraordinary Hall effect magnetometry and magnetic force microscopy. The coercivity originates in both cases from the pinning of domain walls on structural defects. Whereas the coercivity increases with the FePt layer thickness in FePt/Pt samples, it decreases in FePt/MgO samples. This discrepancy is explained on the basis of structural observations, and of atomistic simulations of magnetic domain wall pinning.     

Self-assembled L10 FePt thin films for high-density magnetic recording

A new technique, which utilizes the interlayer diffusion, for preparation of self-assembled nanodot magnetic structures has been proposed. L1₀-phase Pt/FeCu and Pt/FeAg films have been successfully synthesized by this technique. Both the coercivity of Pt/FeCu and Pt/FeAg films exhibited, respectively 4.1 and 8.0 kOe in perpendicular direction. Pt/Fe and Pt/FeAg films show positive values, while Pt/FeCu shows negative value in δm plot. The results indicate that the exchange coupling between the grains has been decoupled in the self-assembled nanodot structure in Pt/FeCu film.     

Comparative evaluation of performance of percolated perpendicular media using FePt–MgO/Pt/Cr trilayer and FePt–MgO/MgO bilayer

(Fe₄₈Pt₅₂)_100−x–(MgO)_x films were used to examine the performance of a perpendicular percolated medium. Two underlayers, Pt(0 0 1)/Cr(0 0 2) and MgO(0 0 2), were used for comparison. The (Fe₄₈Pt₅₂)_100−x–(MgO)_x film with the MgO underlayer exhibits a strong preference to segregate at FePt grain boundaries. The microstructure with small closely packed MgO particles (2–4 nm) dispersed uniformly in the L1₀ FePt matrix was achieved in the Pt/Cr underlayered sample. Structural data reveal that the precipitate is crystallographically coherent with the surrounding L1₀ FePt phase and preserves good lattice alignment. Magnetic results indicate significant pinning behavior for those introduced non-magnetic columns with an enhanced coercivity of about 70%—much greater than that of the MgO underlayered samples. Percolated perpendicular medium can be realized in the FePt system and a Pt(0 0 1)/Cr(0 0 2) underlayer promotes the formation of pinning sites within the FePt grains.     

Investigation of hard magnetic properties in the Fe-Pt system by combinatorial deposition of thin film multilayer libraries

The magnetic properties of annealed Fe-Pt multilayer thin films with a broad composition range were investigated in order to identify the effects of composition and annealing temperature on the achievable coercive field, and to identify its maximum at low processing temperatures. Two types of multilayer systems were deposited as materials libraries to vary the composition from Fe₂₀Pt₈₀ to Fe₇₅Pt₂₅. The first type of multilayer was comprised of alternating opposing wedges, whereas the second type consisted of repeated uniform Fe and Pt layers interspersed periodically with Fe wedge layers. It was found that coercive fields μ₀H_C > 0.7 T can be achieved at an annealing temperature of about 300 °C (60 min) for both types of multilayers as long as the composition is close to 50:50. Higher annealing temperatures are needed for films, which deviate from this composition. Increasing the annealing temperature up to 700 °C leads to increased coercivity values. Multilayers with additional Fe layers showed increased remanence but reduced coercive fields.     

Investigation of ferromagnetic coupling between Co layers in a Co/Pt multilayer with perpendicular anisotropy

The coercivity of a Co/Pt multilayer with out-of-plane anisotropy can be lowered greatly if it is grown onto an ultrathin NiO underlayer . By making use of this characteristic, a series of samples glass/NiO(10 Å)/[Co(4 Å)/Pt(5 Å)]₃/Pt(x Å)/[Co(4 Å)/Pt(5 Å)]₃ with different Pt spacer thickness have been prepared to determine the ferromagnetic (FM) coupling between Co layers across the Pt layer. The measurements of major and minor hysteresis loops have shown that the FM coupling between the top and bottom Co/Pt multilayers decreases monotonically with the Pt layer thickness and disappears above the Pt layer thickness of 40 Å. This thickness of 40 Å is much larger than that in the literature. In addition to the FM coupling between the top and bottom Co/Pt multilayers across the Pt spacer, there exists a weak biquadratic coupling, which induces the broad transition of the bottom Co/Pt multilayer.     

Microstructure and magnetic properties of FePt:Ag nanocomposite films on SiO2/Si(1 0 0)

FePt:Ag nanocomposite films were prepared by pulsed filtered vacuum arc deposition system and subsequent rapid thermal annealing on SiO₂/Si(1 0 0) substrates. The microstructure and magnetic properties were investigated. A strong dependence of coercivity and ordering of the face-central tetragonal structure on both Ag concentration and annealing temperature was observed. With Ag concentration of 22% in atomic ratio, the coercivity got to 6.0 kOe with a grain size of 6.7 nm when annealing temperature was 400 °C.     

Enhancement of L10 ordered FePt by Pt buffer layer

L₁₀-ordered FePt thin films prepared by molecular-beam epitaxy on MgO (0 0 1) substrate at 320 °C with different thickness of Pt buffer layer have been investigated. The out-of-plane coercivity increases with increasing thickness of Pt buffer. The maximum values of the long-range order parameter and uniaxial magnetic anisotropy energy are 0.72 and 1.78×10⁷ erg/cm³, respectively, for films with 12 nm thick Pt buffer layer, where the c/a ratio (0.976) shows the minimum value. The reason for the enhancement in ordering is due to the proper lattice strains Pt buffer bestows on FePt layer, these strains are equal to the contraction in lattice parameter c and the expansion in a. Studies of angular-dependent coercivity revealed that the magnetization reversal behaviour shifts from a domain-wall motion dominated case towards a near rotational mode with increasing thickness of Pt buffer layer.     

Thermal annealing of FePt thin films by millisecond plasma arc pulses

A series of 20 and 100 nm Fe₅₃Pt₄₇ thin films sputter-deposited onto Si substrates have been thermally annealed using a pulsed thermal plasma arc lamp. A series of one, three or five pulses were applied to the thin films with widths of either 50 or 100 ms. The microstructure and magnetic properties of these annealed Fe₅₃Pt₄₇ films are discussed according to the various annealing conditions and A1 to L1₀ phase transformation. Upon pulse annealing, the average in-plane grain size of 15 nm (nearly equivalent for both film thicknesses) was observed to increase to values near 20 nm. In general, increasing the pulse width or number of pulses increased the L1₀ order parameter, tetragonality of the c/a ratio and coercivity of the specimen. The exception to this trend was for five pulses at 100 ms for both film thicknesses, which indicated a reduction of the order parameter and coercivity. This reduction is believed to be a result of the interdiffusion of Fe and Pt into the Si substrate and the formation of iron oxide clusters in the grain boundaries characterized by atom probe tomography.     

Structure of PtFe/Fe double-period multilayers investigated by X-ray diffraction, reflectivity, diffuse scattering and TEM

Double-period [(Pt 1.7 nm/Fe 0.9 nm)₅Fe(t_Fe2)]₈ and [(Pt 1.8 nm/Fe 0.6 nm)₅Fe(t_Fe2)]₈ multilayers with different thickness t_Fe2 (between 0.23 and 4.32 nm) of the additional Fe layers, prepared by combinatorial sputter deposition, show differences in the mosaic spread and the vertical interfacial roughness when deposited on native or thermally oxidised Si wafers. Simulations of the wide-angle X-ray scattering intensities revealed the presence of interdiffusion in the (Pt/Fe)₅ bilayers and systematic variations of the grain sizes, perpendicular to the film surface, as well as the rms variations of the two superlattice periods with the total film thickness. A comparison of ω-rocking scans shows an increase of the correlated vertical roughness of the (Pt/Fe)₅ multilayers with the total multilayer thickness.     

Control of crystallographic orientation in L10 FePt films by using Cr and CrW underlayers

The microstructure and magnetic properties of FePt films grown on Cr and CrW underlayers were investigated. The FePt films that deposited on Cr underlayer show (2 0 0) orientation and low coercivity because of the diffusion between FePt and Cr underlayer. The misfit between FePt magnetic layer and underlayer increases by small addition of W element in Cr underlayer or using a thin Mo intermediate layer, which is favorable for the formation of (0 0 1) orientation and the transformation of FePt from fcc to fct phase. A good FePt (0 0 1) texture was obtained in the films with Cr₈₅W₁₅ underlayer with substrate temperature of 400 °C. The FePt films deposited on Mo/Cr underlayer exhibit larger coercivity than that of the films grown on Pt/Cr₈₅W₁₅ because 5 nm Mo intermediate layer depressed the diffusion of Cr into magnetic layer.     

Buffer effects of Ag layers on magneto-optical Co/Ge(1 0 0) ultrathin films

Magnetic properties of the Co/Ag/Ge(1 0 0) films grown at room temperature and 200 K were studied by the surface magneto-optical Kerr effect (SMOKE). More than 1.5 monolayer Ag buffer layers not only effectively block the interdiffusion between the capped Co layers and the Ge(1 0 0) substrate but also stabilize the magnetic phase. The temperature and thickness dependence on coercivity measurements show that interactions upon the interfaces are strongly correlated to the microstructures.     

Coercivity and squareness enhancement in ball-milled FeCo–MnO nanocomposites

FeCo–MnO nanocomposites were prepared by ball milling mixtures of Fe, Co and Mn, where MnO was obtained from the oxidation of Mn after or during the milling process. The coercivity and squareness of FeCo–MnO nanocomposites increase with increasing milling time. After annealing treatment, the coercivity and squareness increase and exhibit a maximum at 120 h milling time. The temperature dependence of the magnetic properties of FeCo–MnO was also studied and there is a distinct boundary at 120 K. The exchange-bias field and the coercivity decrease quickly with increasing temperature from 30 to 120 K. However, there are no shifted hysteresis loops and the coercivity decreases slightly with increasing temperature from 120 K to room temperature. The enhancement of the coercivity and squareness is mainly attributed to the exchange-bias effects and the reduced magnetic interactions between the FeCo particles by the efficient isolation in MnO matrix.     

Microstructure and magnetic properties of nanocomposite FePt/MgO and FePt/Ag films

An in-plane magnetic anisotropy of FePt film is obtained in the MgO 5 nm/FePt t nm/MgO 5 nm films (where t=5, 10 and 20 nm). Both the in-plane coercivity (H_c∥) and the perpendicular magnetic anisotropy of FePt films are increased when introducing an Ag-capped layer instead of MgO-capped layer. An in-plane coercivity is 3154 Oe for the MgO 5 nm/FePt 10 nm/MgO 5 nm film, and it can be increased to 4846 Oe as a 5 nm Ag-capped layer instead of MgO-capped layer. The transmission electron microscopy (TEM)-energy disperse spectrum (EDS) analysis shows that the Ag mainly distributed at the grain boundary of FePt, that leads the increase of the grain boundary energy, which will enhance coercivity and perpendicular magnetic anisotropy of FePt film.     

Fabrication and magnetic properties of ordered 20 nm Co-Pb nanowire arrays

Ordered Co-Pb nanowire arrays embedded in anodic alumina template were successfully fabricated by electrodeposition. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) observations revealed that the Co-Pb nanowires were polycrystalline with uniform diameters around 20 nm and lengths up to several micrometers. Magnetic measurements showed that the coercivity and remanence of the as-deposited Co-Pb nanowires decreased with the increase of the Pb content. After annealing the Co-Pb nanowires present higher coercivities (2.4-2.5 kOe) than that of pure Co nanowires (2.1 kOe) and the dependence of coercivity and remanence on the Pb content is inconspicuous. A phase separation of Co and Pb occurred after annealing. The familiar pinning model was employed to explain the above experimental results.     

Magnetic properties of CoPt–SiNx/Ag nanocomposite films

When the thickness of Ag under layer is 25 nm, the CoPt/Ag film has maximum out-of-plane squareness (S_⊥), minimum in-plane squareness (S_∥), and the largest out-of-plane coercivity (H_c⊥), they are 0.95, 0.35, and 15 kOe, respectively. Different volume percent of SiN_x ceramic materials were co-sputtered with Co₅₀Pt₅₀ films on the Ag under layer to reduce the grain size of the CoPt film. Comparing the X-ray diffraction pattern of CoPt-SiN_x/Ag films without annealing with that of the films which annealed at 600 and 700 °C, it is found that the intensities of CoPt (0 0 1) and CoPt (0 0 2) superlattice lines were reduced after annealing. As the SiN_x content is raised to 50 vol%, the particle size of CoPt is reduced to be about 9 nm.     

Deformation-induced texture in nanocrystalline 2:17, 1:5 and 2:7 Sm-Co magnets

The effect of crystal structures on texture induced by hot plastic deformation was studied for Sm-Co, Sm-Zr-Co, Sm-Zr-Co-Fe and Sm-Co-Fe-Mn nanocrystalline alloys with 9-22 at% Sm. Nanocrystalline precursors were obtained via high-energy ball milling and subsequent hot consolidation; deformation was carried out at 800-1150 °C. The analysis of X-ray diffraction and magnetic measurements showed that the degree of the axial [0 0 1] texture after deformation was negligible for the ordered 2:17 structure, but became increasingly noticeable for the disordered 2:17 (“1:7”), 1:5 and 2:7 structures. Because of interplay of several factors including the [0 0 1] texture, saturation magnetization and magnetocrystalline anisotropy, there was no universal trend in the hard magnetic properties with the Sm content. Optimum compositions for the maximum energy product varied from Sm₁₁(Co, Fe, Mn)₈₉ in the Sm-Co-Fe-Mn series to Sm₁₁Zr₂(Co, Fe)₈₇ in the Sm-Zr-Co-Fe series to Sm₁₇(Co, Fe)₈₃ in the Sm-Co-(Fe) series. Iron substitution for cobalt strongly suppresses the 1:5 structure, whereas the Fe-free magnets based on the SmCo₅ compound showed by far the highest room-temperature coercivity.     

Exchange coupling in CoO–Co bilayer

In this work, exchange bias and coercivity enhancement in ferromagnet (FM)–antiferromagnet (AFM) bilayer have been investigated. CoO film (50 nm) was deposited by sputtering with a relatively high oxygen partial pressure. The deposited films were subsequently annealed at varied temperature up to 973 K in the air atmosphere. The CoO film shows a disordered structure in the as-deposited state and an increase of crystallinity after annealing characterized by XRD and Raman spectra. A 40-nm Co film was deposited on the as-deposited CoO and annealed films. The Co–CoO bilayer shows a large exchange bias up to 1600 Oe and relatively high coercivity up to 3200 Oe (H_C−) at 5 K, which is much larger than that of crystalline Co–CoO bilayer films without any treatment. The spin glass behavior combined with increasing crystallinity, surface roughness of CoO after annealing may be attributed to the large exchange bias and high coercivity.     

Abnormal reduction of coercivity on magnetic cobalt–platinum alloy films

Ultrathin Co–Pt alloy films as substrate were studied by the surface magneto-optical Kerr effect. As the growth of Ni, the films show uniquely high polar Kerr responses without any in-plane signals. The coercivity decreased until the thickness of Ni film was higher than 5 ML. A new surface structure was discovered at 7–10 ML Ni/Co–Pt films by the low-energy electron diffraction. Interestingly, polar Kerr signal and coercivity of the 10 ML Ni/Co–Pt(1 1 1) template film reduced rapidly as Co films were further deposited onto only about 1–2 ML. Then the films show a canted magnetization with a rollback hysteresis in the polar configuration during the growth of Co. Coercivity of the 7 ML Co/Ni/Co–Pt film was found unusually down to almost 100 Oe.The corresponding magic number at around 7 ML of Co in the abnormal reduction of coercivity may be attributed to the cluster formations of Co.     

Microstructures and coercivities of SmCox and Sm(Co,Cu)5 films prepared by magnetron sputtering

We have investigated the influence of composition and annealing conditions on the magnetic properties and microstructural features of SmCo_x films that were prepared by sputtering and subsequent annealing. A huge in-plane coercivity of 5.6 T was obtained from an optimally annealed Sm–Co film, which was attributed to the nanometer sized polycrystalline microstructure of the highly anisotropic SmCo₅ phase. Although a high density of planar defects were observed in the films that were annealed at high temperatures, they did not act as strong pinning sites for domain wall motion. The effect of Cu on [SmCo_4.5(9 nm)/Cu(xnm)]₁₀ multilayer thin films was also studied. An appropriate Cu content increased the coercivity.     

As-grown preparation of FePt-ordered alloy thin films from Pt/Fe bi-layered structure

A novel method to fabricate FePt-ordered alloy thin films with (0 0 1) preferential orientation from Pt/Fe bi-layered films has been developed. It is able to omit post-annealing with a furnace, and the whole process can be conducted in a chamber without exposing samples to air. In this method, the inter-diffusion and the hetero-epitaxial relation between Fe and Pt are key factors to attain an FePt-ordered alloy thin film with (0 0 1) preferential orientation. An FePt-ordered alloy thin film with (0 0 1) preferential orientation fabricated from Pt[3 nm]/Fe[3 nm] bi-layer employing this method exhibited a relatively large coercivity of 5.6 kOe and high squareness ratio of 0.94 on perpendicular magnetization property. This novel method has a potential to fabricate FePt-based perpendicular magnetic recording media more easily than several conventional methods.