Magnetic Properties and Microstructure of FeO x / Fe / FePt and FeO x / FePt Films

The Fe(6 nm)/FePt film with perpendicular magnetization was deposited on the glass substrate. To study the oxygen diffusion effect on the coupling of Fe/FePt bilayer, the plasma oxidation with 0.5∼7% oxygen flow ratio was performed during sputtered part of Fe layer and formed the FeOx(3 nm)/Fe(3 nm)/FePt trilayer. Two-step magnetic hysteresis loops were found in trilayer with oxygen flow ratio above 1%. The magnetization in FeOx and Fe/FePt layers was decoupled. The moments in FeOx layer were first reversed and followed by coupled Fe/FePt bilayer. The trilayer was annealed again at 500C and 800C for 3 minutes. When the FeOx(3 nm)/Fe(3 nm)/FePt trilayer was annealed at 500 C, the layers structure was changed to FeOx(6 nm)/FePt bilayer due to oxygen diffusion. The hard-magnetic FeOx(6 nm)/FePt film was coupled with single switching field. The FeOx/(disordered FePt) layer structure was observed with further annealing at 800C and presented soft-magnetic loop. In summary, the coupling between soft-magnetic Fe, FeOx layer, and hard-magnetic L10 FePt layer can be controlled by the oxygen diffusion behavior, and the oxidation of Fe layer was tuned by the annealing temperature. The ordered L1


Introduction
Equiatomic FePt film with anisotropic face-centered tetragonal (fct) L1 0 ordered structure has high magnetocrystalline anisotropy which is the promised material in energy assisted perpendicular magnetic recording.The L1 0 FePt phase was ordered from disordered FePt phase with face-centered cubic (fcc) structure after high temperature annealing [1][2][3][4][5][6].The disordered and L1 0 FePt phases show soft-and hard-magnetic properties, respectively.The ordering degree can be changed from 0 (disordered) to 1 (ordered) that depends on the process temperature or condition.The granular structure with columnar grains [7,8] and well c-axis alignment normal to the film surface with low switching field distribution are required for FePt perpendicular recording media.For caxis alignment, [001] textured FePt films have been prepared on amorphous glass substrate or MgO underlayer with lower ordering temperatures [9][10][11].To write in perpendicular recording media, measure can be taken from energy assisted writing process called heat assisted magnetic recording (HAMR) [12].To down the writing temperature under fixed writing field, minor adjusting of the intrinsic magnetic anisotropy of FePt film was required.The magnetic anisotropy and the coercivity of FePt film can be tuned by composition and the third element addition [13].The [001] textured FePt film was not easily formed when target composition far deviated from equal atomic ration.In addition, the magnetic anisotropy was usually diluted by nonmagnetic doping.As a result, it is necessary to have high [001] textured FePt granular film with lower coercivity that was accepted by writing temperature and field.Traditionally, exchange spring media and exchange coupled composite (ECC) media with higher and lower magnetic anisotropic layer were introduced to reduce the writing field requirement and maintain the same thermal stability and grater insensitivity to easy axis [14][15][16][17][18][19][20].The motivations of this work are try (1) to know the oxygen diffusion effects on magnetic properties of FePt film and prove the change of magnetic properties by the second annealing process, (2) to modify the morphology of dewetted FePt film by FeO x , and (3) to study the coupling effect via FeO x /Fe/FePt trilayer structure.The structure of FeO x may be cubic FeO, tetragonal -Fe 2 O 3 (maghemite) or spinel Fe 3 O 4 (magnetite), with ferrimagnetism.Plasma oxidation and further annealing were proposed and proven to change the magnetic properties and microstructure of FeO x / Fe/FePt films.The [001] textured FeO x /FePt dewetted film with perpendicular magnetization was also observed.

Experimental
The FePt film was deposited on a glass substrate at room temperature (RT) by magnetron sputtering.The sputtering system was designed for ultrahigh vacuum with base pressure of 5 × 10 −8 Torr, and the load-lock system was used to transfer the substrate via prechamber with base pressure of 5 × 10 −7 Torr.The FePt and Fe targets were used, and a working pressure was fixed at 1.5 × 10 −3 Torr under high purity argon gas.The total thickness of FePt film is 10 nm, and the chemical composition of the FePt layer was Fe 48 Pt 52 measured by an energy dispersive spectrometer (EDS) on a single thicker FePt layer.After deposition, the films were annealed under argon atmosphere by using a rapid thermal annealing process (RTP) at 800 ∘ C for 3 minutes with heating rate of 10 ∘ C/s and formed the L1 0 FePt thin film.The Fe layer with thickness of 6 nm was deposited at RT on FePt film and formed the Fe/FePt bilayer.To study the oxygen diffusion effect on the coupling of Fe(6 nm)/FePt bilayer, the plasma oxidation with varied oxygen flow ration [ =  O 2 /( O 2 +  Ar ) = 0.3, 0.5, 1, 3, 7%] was performed during sputtered part of Fe layer and formed the FeO x (3 nm)/Fe(3 nm)/FePt trilayer.More precisely, the oxygen was introduced during sputtering in half of the Fe layer thickness which is 3 nm.The trilayer was further annealed at 500 ∘ C, 800 ∘ C and finally formed the FeO x /FePt, FeO x /(disordered FePt), respectively.The thickness of each layer or sputtering rate of each material was measured by atomic force microscopy.The crystal structure of the samples was identified using a standard Xray diffraction (XRD) technique (BRUKER, D8 Discover).The film microstructure was observed by scanning electron microscopy (JEOL JSM-6700F) and atomic force microscopy (DI 3100).The surface chemical property was measured by X-ray photoelectron spectroscopy ((XPS) PHI5000Versa-Probe). Magnetic hysteresis loops were measured at room temperature using a vibration sample magnetometer ((VSM) Lakeshore 7400) with a maximum magnetic field of 2T.The magnetic field was applied to be parallel and perpendicular to film surface to obtain in-plane and out-of-plane hysteresis loops, respectively.
Figure 3 shows in-plane and out-of-plane magnetic hysteresis loops of FePt single layer, Fe(6 nm)/FePt bilayer, and FeO x (3 nm)/Fe(3 nm)/FePt trilayer.In Figure 3(a), the FePt single layer shows high perpendicular magnetization, and the out-of-plane coercivity (H c ) is 15.3 kOe.The component in in-plane magnetization is nearly zero and shows the linear loop.In Figure 3(b), the magnetization was increased, and the H c was reduced to 11.8 kOe in soft/hard exchange coupled Fe/FePt bilayer.Figures 3(c)-3(f) show the loops of FeO x /Fe/FePt trilayer with different oxygen flow rationes [ =  O 2 /( O 2 +  Ar ) = 0.5, 1, 3, 7%].In Figure 3(c), there is just a minor shoulder in the loop of FeO x /Fe/FePt film with low oxygen flow rationes ( = 0.5%).When the oxygen flow ration was increased to 1, 3, and 7%, the step or shoulder was found in magnetization curve in Figures 3(d)-3(f).The FeO x layer was decoupled to the Fe/FePt bilayer and reversed the magnetization previously.The magnetization in FeO x layer was decreased as the negative applied field increased, and, up to the critical value, the magnetization was not changed with increased field.The critical value was defined as the H c of FeO x layer.In Figures 3(d)-3(f), the H c of the FeO x layer is 1.37, 1.66, and 1.09 kOe, and the H c values of Fe(3 nm)/FePt bilayer are 13.7, 13.7, and 13.4 kOe, respectively.The H c value of Fe(3 nm)/FePt is between FePt single layer in Figure 3(a) and Fe(6 nm)/FePt in Figure 3  FePt films annealed at 800 ∘ C with oxygen flow ration of 1%, 3%, and 7%, respectively, and the soft-magnetic loops were obtained.The oxygen was diffused into FePt layer and disordered the L1 0 phase, and the layer structure was changed to FeO x /(disordered FePt).In summary, the interlayer coupling and magnetic ansiotropy were tuned by the kinetic diffusion behavior of oxygen driven by temperature.
To understand the oxidation state of Fe and FeO x (oxygen flow ration of 3%) layer in FeO x (3 nm)/Fe(3 nm)/FePt film, XPS were performed on samples with different depth profiles.Figures 5(a)-5(c) show the Fe-2p X-ray photoelectron spectra of FeO x (3 nm)/Fe(3 nm)/FePt films at the depth position below 3 nm from film surface.The curve in Figure 5(a) shows the Fe 0 spectrum that is obtained in FeO x (3 nm)/Fe(3 nm)/FePt film without further annealing.The metallic Fe-2p 3/2 shows the peak at the binding energy of 706.5 eV [22].In Figure 5(b), the FeO x (3 nm)/Fe(3 nm)/FePt films were further annealed at 500 ∘ C, and the binding energy shifts the Fe-2p 3/2 core level to 709∼711 eV.This shift may prove the formation of iron in the Fe 2+ oxidation state that was FeO, and the binding energy of the Fe-2p 3/2 core level was shifted around to 709.6 eV [23].In Figure 5(c), the FeO x (3 nm)/Fe(3 nm)/FePt films were further annealed at 800 ∘ C, and the peak was smeared out in the iron-oxide area.Figures 5(d)-5(f) show the Fe-2p X-ray photoelectron spectra of FeO x (3 nm)/Fe(3 nm)/FePt films at the depth within 3 nm from film surface.In Figures 5(d)-5(e), the FeO x layer before and after further annealing at 500 ∘ C was in the Fe 2+ oxidation state that was FeO, and the binding energy of the Fe-2p 3/2 core level was shifted around to 709.3 eV.In Figure 5(f), the peak was also smeared out in the iron-oxide area when annealed at 800 ∘ C. In summary, first, further annealed at 500 ∘ C, the Fe (3 nm) may oxidize or mix with FePt film and form FeO x /FePt layer structure.Second, further annealed at 800 ∘ C, the oxygen was diffused into the FePt lattice, deteriorated the ordering degree, and formed the FeO x /(disorder FePt) film.
Figure 6 shows the scanning electron microscopy (SEM) image of Fe(6 nm)/FePt and FeO x (3 nm)/Fe(3 nm)/FePt films with oxygen flow ration of 7%.In Figure 6(a), the annealed Fe(6 nm)/FePt film was dewetted in the network structure, and the dewetted area was 44%.When the Fe layer was partially plasma oxidized and formed the FeO x (3 nm)/Fe(3 nm)/FePt layer structure, the dewetted area was reduced to 30% as shown in Figure 6(b).The FeO x has lower surface energy than FePt and was easy to wet on the glass substrate.As a result, the dewetted area was reduced around 14%.In Figures 6(c) and 6(d), the FeO x (3 nm)/Fe(3 nm)/FePt film was furthered annealed at 500 ∘ C, 800 ∘ C, and the dewetted area was 42%, 55%, respectively.Due to large difference of surface energy between FePt and glass substrate, the dewetted area was increased again with annealing temperature.
Figure 7 shows the surface roughness of Fe/FePt and FeO x /Fe/FePt film with oxygen flow ration of 7% measured by atomic force microscopy (AFM).In Figure 7(a), the average surface roughness of Fe/FePt film was 7.1 nm.In Figures 7(b) and 7(c), when the FeO x layer was capped on Fe/FePt film at RT without annealing or with further annealing at 500 ∘ C, the average surface roughness became 4.9 nm and 3.2 nm, respectively.In Figure 7(d), the surface roughness was up to 13 nm when the annealed temperature was high to 800 ∘ C. In summary, the dewetted area in Fe/FePt film was partially covered by FeO x layer deposited at RT but aggregated into small area again with further annealing at high temperature.The surface roughness shows the same tendency to the dewetted area.The FePt film with thickness of 10 nm was dewetted due to strain release when the ordering (rapid phase transformation) was complete at 800 ∘ C. The dewetted area and surface roughness were increased with annealing temperature, and the critical dewetted temperature is 650 ∘ C in this study.After annealing, the sample was cooled down naturally in the argon atmosphere in RTP system.Cooling rate was not changed in this experiment and will be check in the future.