RE-TM Films Research Articles

Thickness and composition effects on atomic moments and magnetic compensation point in rare-earth transition-metal thin films

Rare-earth (RE) transition-metal (TM) thin films have exhibited great promise for spintronic applications; however, the variation of their magnetic properties with composition and thickness has yet to be adequately quantified. In this paper, we investigate the atomic magnetic moments of few-nanometer-thick GdCo and TbCo via x-ray magnetic circular dichroism and find a significant decrease in both RE and Co average moments with increasing RE concentration, consistent with a local Co environment model description. We observe a shift in compensation composition to higher RE concentrations as the thickness of GdCo and TbCo films decreases $<10$ nm. Based on the dependence of the saturation magnetization $({M}_{s})$ on temperature and thickness, we posit the existence of an intrinsic unalloyed RE dead layer at room temperature that reduces the effective RE concentration in the remaining alloyed region, resulting in significant, nonzero ${M}_{s}$ in thin films at higher RE concentrations than is observed in thick films. We demonstrate a simple model for RE-TM films that accurately describes ${M}_{s}$ as a function of both RE content and film thickness. In addition, the atomic $g$ factors of GdCo and TbCo are found to vary significantly with RE concentration, largely deviating from their elemental values over the bulk of composition space.

Ultrathin (Gd, Tb)-FeCo Films With Perpendicular Magnetic Anisotropy

Perpendicular ultrathin RE-TM films with thickness 3 nm were investigated for potential use as free layers of perpendicular magnetic tunnel junctions. The free layer thickness should be as thin as possible to reduce spin-transfer switching current density while maintaining adequate thermal stability. Using Ti or Al as under- and over-layers, we found that both 3 nm GdFeCo and TbFeCo films had perpendicular magnetic anisotropy (PMA), but DyFeCo had PMA only in cases where thickness exceeded 5 nm. The effective PMA constants of TbFeCo films were of order of 105 erg/cm3, which provided sufficient thermal stability. By annealing at various temperatures, we found that the coercivity of TbFeCo films can be ranged from 120 to 1000 Oe. Therefore, insertions of under/overlayers together with suitable annealing treatment provide a way to fabricate RE-TM ultrathin layer with desired magnetic properties.

Micro-patterning of NdFeB and SmCo magnet films for integration into micro-electro-mechanical-systems

The integration of high-performance RE-TM (NdFeB and SmCo) hard magnetic films into micro-electro-mechanical-systems (MEMS) requires their patterning at the micron scale. In this paper we report on the applicability of standard micro-fabrication steps (film deposition onto topographically patterned substrates, wet etching and planarization) to the patterning of 5–8 μm thick RE-TM films. While NdFeB comprehensively fills micron-scaled trenches in patterned substrates, SmCo deposits are characterized by poor filling of the trench corners, which poses a problem for further processing by planarization. The magnetic hysteresis loops of both the NdFeB and SmCo patterned films are comparable to those of non-patterned films prepared under the same deposition/annealing conditions. A micron-scaled multipole magnetic field pattern is directly produced by the unidirectional magnetization of the patterned films. NdFeB and SmCo show similar behavior when wet etched in an amorphous state: etch rates of approximately 1.25 μm/min and vertical side walls which may be attributed to a large lateral over-etch of typically 20 μm. Chemical–mechanical-planarization (CMP) produced material removal rates of 0.5–3 μm/min for amorphous NdFeB. Ar ion etching of such films followed by the deposition of a Ta layer prior to film crystallization prevented degradation in magnetic properties compared to non-patterned films.

MTJ Elements With MgO Barrier Using RE-TM Amorphous Layers for Perpendicular MRAM

Magnetic properties of an FeCo/TbFeCo bilayer were investigated by MFM and the anomalous Hall effect. In the case of FeCo(x nm)/TbFeCo(50 nm), FeCo as thin as 2 nm behave as perpendicular magnetization film while large thickness of RE-TM films, such as TbFeCo(100nm) causes the increase of the critical thickness up to 4 nm. The prototype of perpendicular MTJ, p-MTJ, consist of TbFeCo/Fe/MgO/Fe/GdFeCo exhibited high squareness ratio of 0.8 having low coercivity of 117 Oe while MgO(100) texture was confirmed in the Fe/MgO/Fe junction prepared on the amorphous GdFeCo layer even though the thickness of MgO is as thin as 3 nm

MULTI WAVELENGTHS READ-OUT FOR MULTI-VALUED MO RECORDING

We propose a new multi wavelengths read-out method which solves the fatal problem of decreasing the signal to noise ratio (SNR) in multi-valued magneto-optical (MO) recording. Theory and method to find the optimal condition are discussed and the two examples for the disks of RE-TM films and the combination of RE-TM and magnetic garnet films are investigated. This method provides theoretically 12.5 dB higher SNR than conventional one, and is suitable to realize 3-dimensional MO recording with the combination of the multi-valued MO recording proposed by Ohta and Shimazaki et al.[1]

Microstructure of Re-Tm films and anisotropy of domain wall motion

The structure of thin amorphous rare earth-transition metal (Re-Tm) alloy films and domain wall propagation in these films were studied with the help of scanning tunneling microscopy and magnetooptical techniques. The anisotropic displacement of the domain walls arising from the anisotropic film structure was found in the films investigated. >

Domain wall motion in RE-TM films with different thickness

A thermally activated domain wall motion is studied in amorphous TbFe films with different thicknesses. The activation energy of the domain wall displacement process is determined for the films with various composition and thicknesses. It is about 2.5 eV. The activation volume of this process nonlinearly increases from 2*10/sup -18/ cm/sup 3/ to 7*10/sup -18/ cm/sup 3/ as the film thickness increases from 25 nm to 400 nm. The change of the domain shape with the film thickness growth and domain wall coercive force dependence on film thickness are discussed in terms of thermoactivated domain wall motion.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

DyFeCo magnetooptical memory with reflective film structure

The magnetic properties and magnetooptic effects of amorphous DyFeCo thin films with various compositions and thicknesses were investigated. The product of magnetization times coercive force of 15-30-nm-thick DyFeCo film gave a high value of 2.5-3 erg/cm/sup 3/, comparable to that of Tb-enriched rare earth (RE) transition metal (TM) films. The dynamic characteristics of a quadrilayered magnetooptical disk having a reflective layer, i.e., carrier-to-noise ratio (C/N), optimum recording power and jitter were also evaluated. A high reproducing 60-dB C/N performance, comparable to that of Tb-enriched RE-TM films, was obtained by the 15-30-nm-thick DyFeCo films having a room temperature compensation composition. It is concluded that the use of this film is as efficient as the use of Tb films, while at the same time being much more inexpensive.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A Boundary Oxidation Model for Amorphous Rare Earth-Transition Metal Films

Transmission electron microscopy and electron diffraction analysis of amorphous rare earth-transition metal alloy (RE-TM) films have been carried out to clarify the origin of experimentally obtained thickness dependence of their magnetic properties. It has been clarified that there is no islandlike structure even in a 5-nm-thick sample. It has also been clarified that there are an RE-oxide phase and a TM phase in samples with a thickness of below 10 nm. A boundary oxidation model for RE-TM films is proposed. The thickness of the boundary oxidation region (dB) and magnetization excluding the influence of the boundary oxidation region (Ms0) have been calculated based on this model. For films with an RE content of 27-28 at.%, the dB values were 10 to 20 nm, and the Ms0 values were 272 to 300 emu/cm3, which were 17% to 35% higher than the measured Ms values of 100-nm-thick films.

Kerr effect enhancement and corrosion resistance improvement by AlN and AlSiN films (abstract)

RE-TM amorphous thin films with perpendicular magnetic anisotropy are promising for use in erasable optical recording media. In order to improve the drawback of easy oxidation and lower C/N of RE-TM films, some protective layers such as SiO, SiO2, ZnS, AlN, and Si3N4 films were studied.1,2 We have studied the Kerr effect enhancement and corrosion resistance improvement by AlN and AlSiN films. AlN and AlSiN films were prepared on glass, PC, and PMMA substrates by a rf magnetron sputtering system with three targets using low sputtering power. The films have a high refractive index (2–2.15), high optical transparency (over 90%), and high stability. The relation between optical properties and rf reactive sputtering conditions (Ar: N2 ratio, total pressure, sputtering power, sputtering time), composition, spectral transmittance, and uniformity of sputtering AlN and AlSiN films were studied. The Kerr rotation angle was up to 1.5° in AlN/TbFeCo/glass and AlSiN/TbFeCo/glass multilayer structures (laser is incident from air). We also studied AlN/TbFeCo/AlN/glass, AlN/TbFeCo/AlN/Al/glass, AlSiN/TbFeCo/AlSiN/glass and multilayer structure films. The results show that AlN and AlSiN films provide sufficient Kerr effect enhancement and superior corrosion resistance improvement to the RE-TM films. The microstructure of those films were also studied by JEM, XRD, and XPS.

Systematics of enhancement in magneto-optical thin films

The results of a previous analytical study [R. Gamble, P. H. Lissberger, and M. P. Parker, IEEE Trans. Magn. MAG-21, 1651 (1985)] of shot-noise-limited signal-to-noise ratio (SNR) in thin multilayers containing a magnetic storage layer are compared with those obtained using practical numerically designed quadrilayers and bilayers. Both studies clearly demonstrate that multilayer enhancement of the Kerr magneto-optical effect is at its least favorable for currently fashionable RETM films such as GdTb/FeCo alloys. By contrast, low-absorption film media such as garnets can derive comparatively much greater benefits from appropriate multilayer design. In the present analysis the general characteristics of the resonant dispersion of Kerr rotation in bilayer systems are also examined.

Properties and structure of AlSiN and AlSiON films

Magneto-optical disks employ rare earth-transition metal amorphous alloy films (RE-TM films) as recording films, therefore improvement of the corrosion resistance of such disks is an important problem which must be resolved to enable practical application. Development of protective layers for magneto-optical disks is important to improving corrosion resistance. Such protective layers must offer excellent protective performance, and must also have a large refractive index in order that a large Kerr effect enhancement be obtained. AlSiN and AlSiON films are being developed for use as protective films meeting these requirements. However, as yet not much is known about the structure, bonding states, and relation between composition and refractive index of such films. This article reports on the refractive indices, structures, and bonding states of AlSiN and AlSiON sputtered films.

A NEW METHOD TO MEASURE THE PERPENDICULAR ANISOTROPY OF RE-TM AMORPHOUS FILMS USING SPONTANEOUS HALL EFFECT

A new method to measure the effective magnetic anisotropy field (H:) of amorphous RETM films with perpendicular magnetic anisotropy is described. In this method, H; is determined by the measurement of spontaneous Hall effect. The experimental data for TbFe sputtered films agreed with the theoretical prediction. RETM amorphous films with perpendicular magnetic anisotropy, such as GdCo, GdFe, TbFe, GdTbFeCo, etc., have now attracted special interest because of their promissing possibility as magnetooptical recording material, and intensive studies have been concentrating on these materials. However, it has not yet been clearly explained why these amorphous films have the perpendicular anisotropy. By the way, it is not so easy to investigate the magnetic anisotropy of such RETM films by conventional torque method. The reasons are: 1) they are ferrimagnetic thin films and have compensation temperature, T,,,,, in the vicinity of room temperature, so that the value of saturation magnetization M, is very small, on the other hand, 2) the magnetic anisotropy field Hk is very large. Okarnoto, the first author of this paper, has recently developed a new method to measure the effective mag/ netic anisotropy field Hk = Hk-4rM, using the spontaneous Hall effect, which is very large in RETM films independent of their M, values [I]. According to his method, if the film has a rectangular Hall hysteresis loop (VH H loop) with magnetic remanence Hall voltage VH= this means the film has a rectangular M H / loop with remanence Mr then Hk can be determined from the VHr HII curve. Where, HII is the magnetic field applied in the direction of film plane. And if the film has a linear type of VH H loop as shown in figure 1, in which VH,? 0 due to the linear magnetization characteristics of the film, H; is determined from the Hall loop for the obliquely applied magnetic field in the direction a = 45' as illustrated in figure 2. In figure 1, a predicted Hall loop for a = 45' is also shown. In this paper, we deal with the latter case more detailed. Let us consider the case of figure 2, where the film has become a single domain state due to the sufficiently large applied field H. We define the effective uniaxial magnetic anisotropy energy K as 1 K = K, 2 a ~ : = -M,Hk. 2 (1) a= 4 5 O V H S / D --4 ---

Anisotropy and coercivity of amorphous RE-TM films

The origins of magnetic anisotropy and coercivity in sputter-deposited amorphous rare earth-transition metal thin films are discussed, focussing on microscopic mechanisms. We point out that two distinct mechanisms, directional compositional short range order and bond-orientational anisotropy, have to be considered, and discuss relevant experimental results.

Stability and microstructural phenomena in RE-TM films for thermo-magneto-optic recording

TbFe films have been subjected to aging treatments over a range of temperatures and relative humidities. Magnetic, optical and microstructural measurements show that the properties of unprotected films deteriorate rapidly with time, particularly in high relative humidities. Films coated with protective aluminium or aluminium nitride layers are generally far more stable but cannot tolerate high humidities. The process of corrosion is associated with the preferential oxidation of the rare earth leading to local diffusion and segregation of iron. Large scale corrosion is particularly serious at pinholes or other obvious defects in the deposited films. These variations in composition and structural inhomogeneities correlate well with magnetic measurements.

Magnetic Properties of Y-RE-Fe Sputtered Films

The authors substituted Y for the rare-earth (RE) element in RE-TM films and studied the changes in the anisotropy of the RE atoms and the magnetic characteristics of the films. Gd substituted for Y tended to increase M s . All of the films studied had amorphous macroscopic structures as determined by X-ray analysis.

Improvement of corrosion resistance of GdTbFe by metal coatings

For an application of rare earth (RE) transition metal (TM) alloys for erasable optical recording the corrosion sensitivity has to be limited in order to obtain long-term stability. The use of thin RE-TM films as a storage medium needs a top coating to avoid strong surface-induced change of intrinsic material properties due to oxidation. This paper presents annealing experiments of GdTbFe films in air within the temperature range of 65 to 160 °c. The GdTbFe films of 40 and 80 nm thickness were coated with Al,Cr,CrNi and Cu coatings of 50 and 200 nm thickness, respectively. The values of coercivity and the angular dependence of torque were measured in the initial state right after the preparation of samples. These values were compared with the results of the annealed films. The angular dependence of torque and the corresponding oxygen depth profiles (SIMS) show different results for the coated and non-coated samples. Coatings of Cr or Al having 20 or 50 nm thickness both are suitable being an excellent barrier against the diffusion of oxygen into the magnetic layer.

Polarity Reversal of Hall Voltage at Compensation Temperature in Amorphous RE-Ni (RE:Gd, Tb, Dy, Ho) Films with Perpendicular Magnetic Anisotropy

Various amorphous rare earth - Ni (ferrimagnetic) films were prepared, and the temperature dependences of their Hall loops studied, to elucidate the mechanism by which the Hall polarity is suddenly reversed at the compensation temperature T comp . The polarity of V H was found to change from positive to negative at T comp in all cases, in contrast with RE-Co(Fe) films, which change from negative to positive. At temperatures below T comp , where the Gd spin dominates, ? H ≫ 0, lending support to the hypothesis that the transition metal spin plays a dominant role in the Hall effect mechanism in RE-TM films.

Properties of Amorphous Rare Earth-Transition Metals thin films for magnetooptical recording

Amorphous RE-TM alloys thin film can be used as high density storage media for magnetooptieal (M.O.) recording. The poor reading efficiency of these alloys can be enhanced if they are deposited in a multilayered structure computed to combine M.O. and interference effects. The perfect rectangularity in the inductive and magnetooptic hysteresis cycles is attributed to the large value of the quality factor Q = K u /2πM s 2which can be much larger than that of other perpendicular recording materials such as CrCo. The magnetization process of RE-Fe film occurs by domain nucleation and domain wall motion which are assumed to be thermally activated. Aging phenomena are observed on GdCo, GdFe and TbFe which are due to the oxidization of RE atoms. In the case of SiO 2 - protected GdFe films the aging phenomena at room temperature are attributed to the segregation of RE atoms which build up a nonpermeable layer when they oxidize. RE atoms can diffuse in the RE-TM amorphous layer as can be seen by the change in the magnetic properties which occurs when RE atoms are deposited below and above the magnetic film. The consequences of these properties on the use of RE-TM films as M.O. recording media are discussed.

Noise Characteristics of Amorphous Rare Earth-Transition Metal Thin Films

The origin of media noise affecting the polarization of the incident beam (and with intensity proportional to the reproduced signal) in magneto-optical reproduction was studied. RE-TM films were RF sputtered on glass disks, and the C/N ratio measured. The C/N was low at peripheral disk regions, probably due to the sputtering geometry, which would also cause the magnetization to be diagonal rather than perpendicular. Magnetic field annealing resulted in lower noise levels, also consistent with the theory that deviation of the magnetization from the perpendicular may affect the medium noise.