J. K. Mei

Effect of initial stress/strain state on formation of (001) preferred orientation in L10 FePt thin films

The stress state of FePt thin films deposited at room temperature was controlled within the range from 1.01 GPa compressive to 0.18 GPa tensile before taking rapid thermal annealing (RTA). After the order–disorder transformation triggered by RTA at temperatures (Ta) from 650 to 800 °C for 5 min, the tensile-stressed films exhibit significant preferred orientation of (001) of L10 structure. However, the compressive-strained ones show isotropic texture. Strong (001) texture with high Lotgering orientation factor of 0.9 is obtained at Ta = 800 °C, resulting in enhanced perpendicular magnetic anisotropy. The results provide direct evidence of stress-induced (001) texture, which could be significant for future applications.

Effect of thickness of MgO, Co-Fe-B, and Ta layers on perpendicular magnetic anisotropy of [Ta/Co60Fe20B20/MgO]5 multilayered films

This study elucidates the magnetic properties of [Ta/Co60Fe20B20/MgO]5 as functions of thickness of each layer. Its perpendicular magnetic anisotropy (PMA) is found to depend strongly on the thickness of the MgO (tMgO) and Co-Fe-B (tCoFeB) layers. The Ta/CoFeB interface is critical to inducing PMA. A maximum room-temperature (RT) anisotropic energy (KuRT) of about 1 × 106 erg/cm3 and an anisotropic field (HkRT) of 4.7 kOe are obtained in the RT-prepared multilayered sample with tMgO = 1.0, tCoFeB = 1.3, and thickness of layer tTa = 10 nm. These values are comparable to the published values for Co60Fe20B20 after optimized field annealing. In this investigation, post-annealing has a more complicated effect on PMA in a multilayered structure than in a single or a double Co-Fe-B layer, both of which structure have been examined elsewhere. This result may be explained by the competing effects of the thermal process, which is an improvement of the crystallinity of Co-Fe-B and roughening of the interface.

Low temperature growth of FePt and CoPt films on MgO(111) substrate

Equiatomic CoPt and FePt thin films grown on MgO(111) substrates at temperatures (Ta) from room temperature (RT) to 400 °C were studied. Distinct phase evolution was observed. In CoPt films, a metastable phase of L11 appears at Ta = 250 °C prior to the formation of thermodynamic equilibrium L10 phase; whereas no intermediate structure is found in the FePt films. Good epitaxial growth of CoPt films can be obtained at low Ta before ordering. However, in FePt films, high quality epitaxy appears after the occurrence of L10 ordering. Either L11 or L10 ordering induces magnetic hardening. Perpendicular magnetic anisotropy was obtained in the L11 films, yet the L10 sample exhibited isotropic magnetism. Corresponding evolution in magnetic domain structure was also reported.

Structure and magnetic properties of FePt(001) graded films deposited on glass substrates

This study examines L10 FePt (001) hard magnetic films that were coated with a layer having graded perpendicular magnetic constant (Ku) and with a disordered FePt soft layer on glass substrates. The 5 nm-thick hard layer exhibits a clear (001) texture, large perpendicular magnetic anisotropy (PMA), and an island-like morphology with particle size of 15–25 nm. The deposition of the graded or soft layer with a thickness of 5–15 nm does not change the particle size. The graded and hard/soft samples both exhibit reduced out-of-plane coercivity (Hc⊥) but their reversal behaviors differ. Strong exchange coupling appears between the hard and graded layers, maintaining high PMA. However, hard/soft films exhibit weak coupling such that the film becomes in-plane anisotropic when the thickness of the soft layer exceeds 5 nm. The investigations of magnetic domain structures further reveal different magnetic coupling configurations between different types of films. The results herein are consistent with theoretical pr...

Critical Thickness of (001) Texture Induction in FePt Thin Films on Glass Substrates

Development of (001) texture in FePt thin films deposited on glass substrates with different thickness (t) treated by rapid thermal annealing (RTA) is studied. A critical thickness of 30 nm is characterized: below which the (001) preferred orientation of the films develops with increasing t ; when t >; 30 , the films become isotropic. Remarkable perpendicular anisotropy in magnetic properties is achieved in the 30 nm thick sample with the best (001) texture. Discontinuous changes are also observed in surface morphology, microstructure, magnetic domain structure, and internal stress. Direct evidences are presented relating the formation of (001) texture to abnormal grain growth. The internal stress/strain analysis indicates that the residual tensile (σ) stress is proportional to the degree of (001) preferred orientation. A large value of σ of about 8.9 GPa is obtained in the film with t = 30, suggesting the driving force forming the texture.

Rapid Thermal Annealing Induced Metastable Phase in FePt Thin Films

Room-temperature-deposited FePt thin films with thickness (t) ranged from 5 to 100 nm treated by rapid-thermal annealing (RTA) were studied. With annealing condition of 900°C for 60 seconds at heating rate of 80° C/sec., a metastable phase of FePt was observed in the films with t ≥ 40 nm. The phase is chemically ordered with a face-centered-cubic (fcc) structure. The lattice parameter of it is found identical to the planar spacing of L10 (100). The metastable structure is dominant in the film with t = 40 nm and gradually replaced by L10 phase with increasing t. The fcc phase is soft magnetic with saturation magnetization similar to that of disordered FePt. Drastic changes were also observed in surface morphology. The results infer the connection between the metastable transformation and internal strain. The formation of the fcc structure is sensitive to processing parameters except t . Slower heating rate, lower annealing temperature, and longer annealing time, tend to suppress the formation of it. The interesting findings provide additional knowledge for FePt thin films.

Microstructure and Magnetic Performance of Perpendicularly Magnetic Anisotropic Fe

L1₀-FePt(001) films with compositional gradient was proposed to realize the graded film with magnetic perpendicular anisotropy in this study. A hard magnetic layer of 5 nm-thick stoichiometry L1₀ FePt was initially deposited onto a glass substrate with a thin MgO underlayer. The hard layer exhibits strong (001) texture, island-type surface morphology, high perpendicular magnetic anisotropy (PMA), and large out-of-plane coercivity (H_c⊥) of about 20 kOe at room temperature (RT). A 5-nm-thick compositionally graded Fe-Pt layer was then deposited on the L1₀ FePt hard layer at deposition temperatures (T_d) from 300 to 450°C. Good (001)-texture is retained in the graded layer and the island-type morphology is preserved when T_d ≤ 400°C. A significant reduction in H_c⊥ from 20 to around 7 kOe was achieved. Besides, the graded films exhibit magnetically single-phase reversal behavior with a high anisotropy field (H₀) of ~90 kOe. When T_d > 400°C, PMA of the films was reduced due to the extensive diffusion between the Fe-rich graded and hard layers. Our proposed scheme demonstrate the feasibility of magnetic gradation induced by binary composition variation in FePt films, providing useful information for designing future ultrahigh magnetic recording media.

_{3}

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.

Pt/Fe

Structure and magnetic properties of the sputter-prepared FePt (160 nm) and FePt(160 nm)/Zr(10 nm) are studied. The films were deposited at room temperature and followed by a post annealing for magnetic hardening by L10-ordering. Distinct ordering processes are observed. In FePt films, extensive ordering occurs after annealing at Ta=700 °C and forms strong (111) texture. Hc increases to 8.5 kOe with a decrease in Mr of about 10%. In contrast, FePt/Zr samples orders at Ta=500 °C and results in drastic increase in Hc reaching 12.5 kOe with only 2.7% decrease in Mr. The difference in surface morphology of the two samples suggests that the low temperature ordering is a result of the nucleation promotion by Zr underlayer. The refined grain size strengthens the intergrain coupling resulting in greatly enhanced magnetic properties. The value of (BH)max for the film with Ta=500, 600, and 700 °C are 16.8, 20.0, and 19.6 MGOe, respectively. The maximum value is more than 110% higher than that of the FePt film annealed at Ta=700 °C (9.5 MGOe).

_{2}

Fe₄₉Pt₅₁ thin films with a thickness of 20 nm were deposited by conventional dc co-sputtering at substrate temperature of 400 and 500^°C before undergoing glow-discharge-induced ion bombardment at room temperature. The energy of the incident Ar ions was adjusted by applying a radio-frequency bias (<i>V</i>_b) in the range of 50 to 600 V. Significant abilities of ion bombardment in modifying both chemical ordering and surface morphology are demonstrated. With <i>V</i>_b in the range of 50 to 150 V, the bombardment smoothes the surface and enhances ordering by promoting surface diffusion. Bombarding at high <i>V</i>_b greater than 200 V evolves the surface morphology towards discontinuous island-like structure or rough pinhole topography and causes disordering by progressive etching. The magnetic properties can be tuned with the structural transformations by ion bombardment. The coercivity (<i>H</i>_c) can be increased from 0.57 to 0.75 MA/m by improving ordering for the films deposited at 400^°C. On the other hand, the disordering and mass loss may produce the magnetic softening and drastically reduced magnetization.