Rare Earth-transition Metal Thin Films Research Articles

Surface effects in ferrimagnetic TbFe rare-earth–transition-metal thin films and nanoparticles

Surface effects in ferrimagnetic TbFe rare-earth–transition-metal thin films and nanoparticles

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.

Voltage control of ferrimagnetic order and voltage-assisted writing of ferrimagnetic spin textures.

Voltage control of magnetic order is desirable for spintronic device applications, but 180° magnetization switching is not straightforward because electric fields do not break time-reversal symmetry. Ferrimagnets are promising candidates for 180° switching owing to a multi-sublattice configuration with opposing magnetic moments of different magnitudes. In this study we used solid-state hydrogen gating to control the ferrimagnetic order in rare earth-transition metal thin films dynamically. Electric field-induced hydrogen loading/unloading in GdCo can shift the magnetic compensation temperature by more than 100 K, which enables control of the dominant magnetic sublattice. X-ray magnetic circular dichroism measurements and ab initio calculations indicate that the magnetization control originates from the weakening of antiferromagnetic exchange coupling that reduces the magnetization of Gd more than that of Co upon hydrogenation. We observed reversible, gate voltage-induced net magnetization switching and full 180° Néel vector reversal in the absence of external magnetic fields. Furthermore, we generated ferrimagnetic spin textures, such as chiral domain walls and skyrmions, in racetrack devices through hydrogen gating. With gating times as short as 50 μs and endurance of more than 10,000 cycles, our method provides a powerful means to tune ferrimagnetic spin textures and dynamics, with broad applicability in the rapidly emerging field of ferrimagnetic spintronics.

Rare-earth-free ferrimagnetic Mn4N sub-20 nm thin films as potential high-temperature spintronic material

Ferrimagnetic alloy thin films that exhibit perpendicular (out-of-plane) magnetic anisotropy (PMA) with low saturation magnetization, such as GdCo and Mn4N, were predicted to be favorable for hosting small Néel skyrmions for room temperature applications. Due to the exponential decay of interfacial Dzyaloshinskii–Moriya interaction and the limited range of spin–orbit torques, which can be used to drive skyrmion motion, the thickness of the ferrimagnetic layer has to be small, preferably under 20 nm. While there are examples of sub-20 nm, rare earth-transition metal (RE-TM), ferrimagnetic thin films fabricated by sputter deposition, to date, rare-earth-free sub-20 nm Mn4N films with PMA have only been reported to be achieved by molecular beam epitaxy, which is not suitable for massive production. Here, we report the epitaxial growth of sub-20 nm Mn4N films with PMA at 400 °C–450 °C substrate temperatures on MgO substrates by reactive sputtering. The Mn4N films were achieved by reducing the surface roughness of MgO substrate through a high-temperature vacuum annealing process. The optimal films showed low saturation magnetization (Ms = 43 emu/cc), low magnetic anisotropy energy (0.7 Merg/cc), and a remanent magnetization to saturation magnetization ratio (Mr/Ms) near 1 at room temperature. Preliminary ab initio density functional theory calculations have confirmed the ferrimagnetic ground state of Mn4N grown on MgO. The magnetic properties, along with the high thermal stability of Mn4N thin films in comparison with RE-TM thin films, provide the platform for future studies of practical skyrmion-based spintronic materials.

Pervasive artifacts revealed from magnetometry measurements of rare earth-transition metal thin films

A class of artifacts manifesting as soft magnetic components are revealed from magnetometry measurements of rare earth-transition metal (TbFe) thin films prepared by magnetron sputtering. They are not inherent to TbFe, but are a direct result of the manner in which the substrates are mounted prior to sample fabrication, with a material deposited at the substrate sides giving rise to a significant magnetic moment. The authors find the same artifacts to also be present in rare earth-free [Co/Pt] multilayers. Trying to supress the appearance of this type of artifact has an influence on the coercivity and, in some cases, on the shape of the reversal curves. Care needs to be taken during fabrication to ensure reliable and reproducible samples so that sensitive magnetic parameters, such as coercivity and compensation points, can be extracted accurately and that data are not misinterpreted for even more complex systems. This type of artifact is not limited to samples prepared by sputtering but can extend to other conventional thin-film deposition methods.

Thickness dependence of ferrimagnetic compensation in amorphous rare-earth transition-metal thin films.

Magnetic compensation in ferrimagnets plays an important role in spintronic and magnetic recording devices. Experimental results have demonstrated a thickness dependence of the compensation temperature ( ) in amorphous TbFeCo thin films. It was speculated that this thickness dependence originated from a variation in the short-range order. In this work, we have investigated the depth-resolved compositional and magnetization profiles using polarized neutron reflectometry. We find that although the composition is uniform across the film thickness, near the substrate interface, the magnetization exhibits a different temperature dependence from that of the rest of the sample. Monte Carlo simulations show that it is this difference in interfacial magnetization that causes the aforementioned thickness dependence of the compensation. These results demonstrate the critical role of the substrate interface in determining the magnetic properties of amorphous ferrimagnetic thin films for spintronic applications.

Structure and properties of amorphous thin film for optical data storage

In this paper the magnetic and magneto-optical properties of amorphous rare earth-transition metal (RE-TM) alloys as well as the magnetic coupling in the multi-layer thin films for high density optical data storage are presented. Using magnetic effect in scanning tunneling microscopy the clusters structure of amorphous RE-TM thin films has been observed and the perpendicular magnetic anisotropy in amorphous RE-TM thin films has been interpreted. Experimental results of quick phase transformation under short pulse laser irradiation of amorphous semiconductor and metallic alloy thin films for phase change optical recording are reported. A step-by-step phase transformation process through metastable states has been observed. The waveform of crystallization propagation in micro-size spot during laser recording in amorphous semiconductor thin films is characterized and quick recording and erasing mechanism for optical data storage with high performance are discussed. The nonlinear optical effects in amorphous alloy thin films have been studied. By photo-thermal effect or third order optical nonlinearity, the optical self-focusing is observed in amorphous mask thin films. The application of amorphous thin films with super-resolution near field structure for high-density optical data storage is performed.

Influence of spin polarization enhancement layer to rare earth–transition metal thin films for perpendicular tunneling magnetoresistance evaluated by ferromagnetic Hall effect

Magnetic properties of ion beam sputter-deposited TbFeCo and GdFeCo thin films with their compensation point were studied using ferromagnetic Hall effect measurement. Gdx(Fe90Co10)1−x thin films sputter deposited using Kr exhibited high squareness ratio and sharp magnetization reversal compared with those prepared using Ar as sputtering gas. Lower kinetic energy of sputtering ions was effective to attain higher squareness ratio. Ferromagnetic Hall effect clarifies the magnetization reversal process of a FeCo(xnm) layer contacted to a TbFeCo(50nm) layer. FeCo layer with thickness less than 2nm is magnetized together with TbFeCo layer. Ferromagnetic Hall measurement is a powerful tool to determine the magnetization process of very thin films and magnetically coupled layers.

Fluctuation field and time dependence of magnetization in TbFeCo amorphous rare earth-transition metal thin films for perpendicular magnetic recording

In this work we report on the experimental analysis of the time decay of magnetization in the films of amorphous TbFeCo alloys for perpendicular magnetic recording. It is shown that the experimental results can be explained on the basis of the thermally activated incoherent subnetwork magnetization reversal model. Thermally activated reversal of the transition metal subnetwork and associated reversal of the antiferromagnetically coupled rare-earth subnetwork may result in effective stabilization of the net film magnetization. Experimentally obtained activation diameter value Da∼5–6 nm corresponds to the exchange length or the domain wall width in the films under study. The small activation diameter value combined with high stability factor k∼250 mean that the TbFeCo amorphous film should be capable of supporting a thermally stable perpendicular recording at over 100 Gbit/in2 recording density.

Structure and magnetism of epitaxial rare-earth–transition-metal films

Epitaxial rare-earth–transition-metal thin films have been grown by magnetron sputtering onto buffered single-crystal MgO substrates. Results are presented for SmFe12(001) and magnetically hard Sm2Co7 and Sm2Co17 (110) films using W (100) buffer layers. The methodology of utilizing epitaxial W buffer layers combined with cosputtering from elemental sources provides a general approach to tailor the orientation and phase of the films.

HIGH COERCIVITY AND VOIDS IN RE-TM THIN FILMS

The majority of rare earth/transition metal (RE-TM) magneto-optical (MO) materials are made by a sputtering process. The coercive force Hc depends very much not only on the composition but calculations have shown that it will depend also upon the size and number of voids. In this paper, we report on the effects of argon pressure on the value of Hc, on the numbers and sizes of voids, and on the values of the gap densities in the coatings with columnar structures. The numbers and sizes of the voids were determined by using small angle x-ray scattering (SAXS) measurements. The gap densities were measured by a new method using TEM. We have also determined the position of these voids using TEM. According to measurements from a vibrating sample magnetometer (VSM), SAXS, and TEM, it was found that there is a significant contribution to the increase of Hc with coating pressure by the increasing volume fraction of these voids located mainly in the gaps between the columns of the MO films.

Kerr Ellipticity and Optical Response of Amorphous Rare-Earth Transition-Metal Thin Films at Shorter Wavelength

The Kerr ellipticity angle γ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</sub> of TbFeCo film and its influence on the magnetooptic response of magnetooptic media were investigated. The γ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</sub> was derived throughout the wavelength range from 400 to 830 nm by a simple method which adjusts the calculated value of the Kerr rotation angle to match the measured value by varying the value of γ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</sub> . The Kerr ellipticity angle γ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</sub> of TbFeCo film increased with shortening wavelength, although the Kerr rotation angle θ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</sub> decreased. The contributions to the Kerr rotation angle of θ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</sub> and θ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</sub> for films with an enhancing structure could be separated, and were found to be additive; the latter was more dominant at shorter wave-lengths. It was also shown that the maximum value of the figure of merit of four-layer media is enhanced by the contribution of γ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</sub> .

Pseudocrystalline model of the magnetic anisotropy in amorphous rare-earth-transition-metal thin films.

A pseudocrystalline model is proposed to explain the occurrence of perpendicular anisotropy in amorphous rare-earth--transition metal (R-T) thin films. It is based on the central hypothesis that during layer-by-layer growth small planar hexagonal units are formed defining on average a preferential axis perpendicular to the film plane. The units are similar in structure to relaxed crystalline ones and are estimated to typically comprise six rare-earth atoms. They are regarded as an idealized model of the short-range order and are consistent with the known nearest-neighbor R-T and T-T coordination numbers in the amorphous state. This model is able to explain the known experimental results concerning the influence of composition, substrate temperature, annealing, and bombardment effects during sputter deposition on the magnetic anisotropy of thin amorphous rare-earth--transition-metal films of the system (Nd, Tb, Dy) (Fe, Co), as well as the destruction of this anisotropy by additives.

Thermal conductivity of amorphous rare earth—transition metal thin films for magneto-optic recording

The thermal conductivity of a magneto-optic recording layer of sputtered amorphous TbFeCoZr is measured, and the results correlated with the thin film microstructure. Thermal conductivity K⊥ perpendicular to the plane of the film is measured using a thermal comparator method, while thermal conductivity K ‖ parallel to the plane of the film is calculated from electrical conductivity using the Wiedemann-Franz conversion. K⊥ ∼ 7 W mK −1 for non-columnar dense films, while K ‖ ∼ 4 W mK −. For columnar, less dense films sputtered at higher pressures, K⊥ ∼ 4 W mK −1, while K ‖ ∼ 0.3 W mK −1. The lowering of K⊥ is probably due to the decrease in density, while the strong anisotropy in the thermal conductivity of the columnar material is due to the introduction of intercolumnar contacts perpendicular to the film plane. Magnetic coercivity also drops with increasing deposition pressure, owing in part to the introduction of voids and defects into the film.

Low temperature magnetic excitations in amorphous light rare-earth—transition-metal thin films

The temperature dependence of magnetization has been studied on amorphous RT (R = Ce, Pr, Nd: T = Fe, Co, Ni) thin films. It was found that both spin wave excitations and band excitations exist for most RFe samples in temperature range T 0–200K, and a marked deviation from Bloch T 3 2 is noted at T< T 0 in all RT samples. T 0 is the temperature below which the T 3 2 law is unsuitable to describe the temperature dependence of the magnetization. T 0 ranges from 10 to 100 K. The deviation is accounted for the magnetic structure transition and can be explained by Kaneyoshi's theory.

Measurements of domain-wall energy density for amorphous rare earth–transition metal thin films

The value of domain-wall energy density (σw) has been most often assumed to be proportional to (AxKu)1/2, where exchange stiffness (Ax) is usually calculated from the mean-field theory. We report a simple method to derive σw from the difference between the onset of expanding and collapsing fields (ΔH) for circular domains. A micro-Kerr hysteresis-loop tracer with submicrometer resolution is used to measure the destabilizing field of domains written thermomagnetically in TbFeCo and GdTbFeCo films.

Modeling studies of rare earth-transition metal optical recording media in relation to experiment

Ballistic aggregation models were utilized to explore and explain the possible origins of thickness, compositional and magnetic variations encountered in the sputter deposition of rare earth-transition metal thin films. They were also used to examine the influence of sputter gas pressure on details of microstructural evolution including columnar growth, defect propagation and surface roughness. The simulations, in agreement with experimental observations, pointed to the advantages of low pressure (<10 mTorr) deposition conditions which resulted in dense, featureless films and minimized the adverse effects of substrate irregularies and defects. The observed increase in corrosion resistance of these films was attributed to this densification.

Magnetooptical thin film optical constants measurement using surface plasmon resonances

Rare-earth-transition-metal thin films are extensively studied as materials for erasable magnetooptical (MO) data storage media. MO Kerr effects can be enhanced by interference effects using multilayer designs. The optical constants n and k are required for the design of multilayer structures, but little data exist for MO materials. We have developed a method to measure the constants using the surface plasmon resonance phenomenon. We measured the constants of TbFe films deposited by rf sputtering. The accuracy estimation was about +/-0.005 and could be as good as +/-0.001 for optimum conditions.

Recording experiments on rare-earth transition-metal thin films are studied with Lorentz microscopy

Domains written thermomagnetically under recording conditions in rare-earth transition-metal thin films are studied with Lorentz microscopy. A novel technique in which the magnetooptical layer is deposited on specially prepared silicon wafer disks provided with Si/sub 3/N/sub 4/ windows is described. This technique allows direct observation of the written domain patterns in the electron microscope. It is demonstrated that the nucleation process plays a crucial role when writing in TbFeCo. Different compositions having different temperature dependences of the magnetic properties lead to markedly different domain-formation behavior. With the detailed insight into the shape and structure of domains obtained by Lorentz microscopy, carrier and noise levels obtained from recording experiments can be understood. This is demonstrated with data on GdTbFe. It is shown that magnetic-field modulation, apart from the advantage of direct overwrite, offers the possibility of very-high-density recording.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The Design and Fabrication of Magneto-Optic Recording Materials

A review of the relationships between film composition, deposition process and properties of sputtered amorphous rare earth-transition metal thin films for magneto-optical recording applications is given. It is shown that to maximize Kerr rotation, anisotropy, corrosion resistance and reflectivity, the sputtering conditions should be selected to produce Zone T featureless microstructure. Although equivalent properties may be achieved with either RF diode or DC magnetron sputtering, DC magnetron sputtering is preferred for manufacturing because of higher deposition rates and lower substrate temperatures. To maximize magnetooptic effects, alloys of Fe and Co should be used as the transition metal. It is shown that perpendicular anisotropy, coercivity, compensation temperature and Curie temperature may be adjusted over a wide range through careful selection of rare earth constituents and Fe/Co ratio. Systematic data on the dependences of the magnetic properties on film composition are given.