Perpendicular Magnetic Films Research Articles

Magnetization reversal dynamics, nucleation, pinning, and domain wall propagation in perpendicularly magnetized ultrathin cobalt films: Influence of the Co deposition rate

We report on the experimental results of magnetization reversal in ultrathin perpendicularly magnetized cobalt films in Au/Co/Au(111) trilayers. We have first studied the static magnetic properties of the samples. Interestingly, the results show the possibility of controlling the coercivity and magnetic anisotropy by changing the Co deposition rate. Second, the magnetization reversal dynamics have been investigated by relaxation measurements and Kerr microscopy. Two magnetization reversal processes are distinguished according to the deposition rate; the dominant magnetization reversal process changes from domain wall motion for cobalt deposited at a rate of 0.2 nm/min to domain nucleation for cobalt deposited at a rate of 0.4 nm/min. The pinning domain wall plays an important role in the observed reversal processes. Analysis shows that in both cases the reversal dynamics is not suitably described by the Fatuzzo–Labrune model.

Improvement of magnetic intergranular isolation and evaluation of read/write characteristics on SmCo 5 perpendicular magnetic thin films

SmCo 5 alloy is a promising candidate for ultra-high-density perpendicular magnetic recording (PMR) media because of its high uniaxial magnetocrystalline anisotropy K u of more than 1.1×10 8 erg/cm 3. Previously, we successfully achieved high K u in a sputter-deposited SmCo 5 thin film by introducing a Cu/Ti dual underlayer. However, in order to apply the SmCo 5 films to practical PMR media, it is necessary to decrease medium noise. A granulated magnetic film comprising of small and magnetically decoupled grains is effective in reducing the medium noise. In this paper, we have proposed a new granular film that is fabricated by partial thermodiffusion of Cu between the Sm–Co continuous layer and the Cu underlayer, which is granulated using compositional segregation caused by the addition of Ta 2O 5. We have analyzed the magnetic properties, magnetic domain size, and magnetization reversal process of the proposed SmCo 5 film. The magnetic domain size decreased and the magnetization reversal process changed from the magnetic-wall-motion mode to a coherent rotation mode to some extent on isolation of magnetic grains. The read/write characteristics of granulated SmCo 5 double-layered media were also evaluated. The medium noise decreased and the signal-to-noise ratio increased for the granulated double-layered (PMR) medium.

Theory and micromagnetics of pinning mechanism at cylindrical defects in perpendicular magnetic films

A theoretical model is developed to describe the pinning energy and the pinning field as a function of pore density, pore diameter and material parameters of a magnetic film. It is shown that the pinning energy and pinning field increases monotonically with increasing pore diameter. A magnetic storage concept is introduced which combines percolated media and graded media which allows to write on magnetic films with ultra high anisotropies.

Magnetization Dynamics in CoFeB Buffered Perpendicularly Magnetized Co/Pd Multilayer

The magnetic properties of CoFeB buffered [Co(0.3 nm)Pd(x nm)] <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> multilayer films have been investigated. It is found that the magnetic properties of the multilayer depended on the Pd thickness and for thickness below 0.5 nm no perperdicular anisotropy is observed. Magnetization dynamics in perpendicularly magnetized CoFeB-[Co/Pd] multilayer films are investigated using time-resolved magneto-optical Kerr effect (TRMOKE). The variation of precession frequency with external magnetic field for different palladium thickness is quantitatively understood using the macrospin approximation of the Landau-Lifshitz-Gilbert equation of motion. The Gilbert damping constant ¿, in the range 0.04-0.1, varying with the palladium thickness is reported. The observed ¿ value is comparable to the damping coefficient of bulk Ni, and much lower than the reported values for perpendicularly magnetized films. The CoFeB buffer layer with in-plane anisotropy appears to significantly affect the precession frequency and thus the damping constant of the films.

Perpendicular FePt-based exchange-coupled composite media

Exchange-coupled composite media were realized by combining perpendicular hard magnetic FePtCu alloy films with perpendicular Co/Pt multilayers which are magnetically softer. We demonstrate that the switching field of the hard layer can be efficiently altered by modifying the material properties of the soft layer by varying the number of Co/Pt bilayers. Moreover, the possibility of effectively tuning the interlayer exchange coupling using rapid thermal annealing was shown. These studies were supported by theoretical modeling revealing the relevant factors to reduce the switching field of the hard layer which are important for future media design.

Electrical transport properties of perpendicular magnetized Mn-Ga epitaxial films

We report electrical transport properties of perpendicular magnetized Mn-Ga epitaxial films with various thicknesses. The maximum extraordinary Hall resistivity and Hall angle is 11.5 μΩ cm and 5.7%, respectively, which is comparable to the highest value reported in amorphous Fe0.79Gd0.21 alloy. In the low temperature region, resistivity was proportional to T2.9 owing to the unconventional one-magnon scattering processes, indicating high spin polarization of this material.

Ultrafast heating effect on transient magnetic properties of L1 0-FePt thin films with perpendicular anisotropy

Laser-induced ultrafast magnetization dynamics and transient coercivity behavior in perpendicular magnetized L1 0-FePt films are investigated using the time-resolved polar magneto-optical Kerr technique. The magnetization after photo-excitation shows a dramatic reduction on the picosecond time scale. In contrast, the coercivity shows a weak decrease, accompanied by a skewed Kerr loop shape for low and intermediate fluences. The results can be interpreted by the laser-induced non-uniform demagnetization due to the weakened coupling between FePt grains of different size and/or components. The remaining coercivity vanishes when the film is fully demagnetized at higher fluence. We claim that the remaining coercivity can be manipulated by employing appropriate laser energy and film thickness, which may be helpful for application in heat-assisted magnetic recording.

First order reversal curve diagrams of perpendicular magnetic anisotropy films

The exchange coupling and magnetic properties distributions in Co/Pd multilayer perpendicular magnetic anisotropy films with different magnetic properties are investigated using the experimental first order reversal curve (FORC) diagram with assistance of Landau–Lifshitz–Gilbert simulation. The simulated FORC diagrams of perpendicular magnetic anisotropy films with different exchange couplings and magnetic property distributions are quite different, which make FORC diagrams very powerful for characterizing perpendicular magnetic recording media.

Non-adiabatic spin-torques in narrow magnetic domain walls

There is considerable debate over the size and direction of the non-adiabatic component of the spin-torque generated when a current flows across a domain wall in a ferromagnet. Measurements of this property in a wall just 1–10 nm wide suggest its value is small, arising from purely magnetic dissipation mechanisms. Torques appear between charge carrier spins and local moments in regions of ferromagnetic media where spatial magnetization gradients occur, such as a domain wall, owing to an exchange interaction. This phenomenon has been predicted by different theories1,2,3,4,5,6,7 and confirmed in a number of experiments on metallic and semiconductor ferromagnets8,9,10,11,12,13,14,15,16,17,18,19. Understanding the magnitude and orientation of such spin-torques is an important problem for spin-dependent transport and current-driven magnetization dynamics, as domain-wall motion underlies a number of emerging spintronic technologies20,21. One outstanding issue concerns the non-adiabatic spin-torque component β, which has an important role in wall dynamics, but no clear consensus has yet emerged over its origin or magnitude. Here, we report an experimental measurement of β in perpendicularly magnetized films with narrow domain walls (1–10 nm). By studying thermally activated wall depinning, we deduce β from the variation of the Arrhenius transition rate with applied currents. Surprisingly, we find β to be small and relatively insensitive to the wall width, which stands in contrast to predictions from transport theories2,5,6,7. In addition, we find β to be close to the Gilbert damping constant α, which, in light of similar results on planar anisotropy systems15, suggests a universal origin for the non-adiabatic torque.

Orbital contribution to perpendicular magnetic anisotropy in Co80Pt20 thin films

We have measured x-ray magnetic circular dichroism spectra at the Co and Pt L2,3-edges in Co80Pt20 perpendicular magnetization films, wherein the uniaxial magnetic anisotropy constant, Ku, significantly increases with decreasing film thickness, δ, and reaches 1.7×107 erg/cm3 for δ=3 nm. The anisotropy of the orbital magnetic moment (AOM) was estimated as a function of δ, and the relationship between AOM and Ku is discussed. When the thickness was reduced, the AOM for Pt 5d was nearly constant, whereas a weak increase was observed for Co 3d. Both AOMs are small in contrast to the large Ku, and any proportionality between AOM and Ku was hardly recognized. This result indicates that a large AOM is not always a necessary and sufficient condition for the appearance of perpendicular magnetization.

Fabrication of MgO-based magnetic tunnel junctions with CoCrPt perpendicularly magnetized electrodes

The applicability of perpendicularly magnetized CoCrPt films to the MgO-based magnetic tunnel junctions (MTJs) was investigated. For this study, CoCrPt films deposited on the Ru buffer exhibited hcp(0002)-oriented growth by sputtering method using the low substrate temperature of 250 °C, low saturation magnetization of around 360 emu/cm3, and high magnetic anisotropy field of 6 kOe, which is sufficient to retain the thermal stability of the magnetization direction. The MgO-based MTJs with a synthetic ferrimagnetlike structure were fabricated: CoFe was coupled magnetically with CoCrPt through the thin Ru layer. Transport properties with a magnetic field applied perpendicular to the film plane revealed a tunnel magnetoresistance ratio of about 6% at room temperature.

Photoemission Magnetic Circular Dichroism using Laser

We report magnetic circular dichroism (MCD) on ultrathin films using lasers as excitation sources. By using threshold photoemission, we can obtain high MCD asymmetry. On the other hand, when the laser photon energy is far away from the threshold, the MCD asymmetry is small. The MCD asymmetry depends also on the magnetization, and MCD is larger for the perpendicularly magnetized films. The reason for the enhanced MCD asymmetry near the threshold is spontaneous momentum and energy selection in the photoelectron excitation. The MCD dependence on the magnetization direction is explained by the polarization change of reflected light, which is negligible for x-ray MCD. Also shown is the photoemission electron microscope observation of magnetic domains using lasers.

Microstructural evolution and magnetization reversal mechanism of CoPt films with perpendicular magnetic anisotropy

Microstructural evolution and magnetization reversal mechanisms of perpendicular magnetic anisotropic Co72Pt28 films were investigated. Results showed that in the initial stage of film growth, the Co72Pt28 film was continuous and followed a dome-shaped structure with increasing film thickness. When the film thickness further increased above a certain critical value, the film growth acquired an inverted frustum shape structure and increased the intergranular magnetic interaction. The magnetization reversal mechanism showed a strong dependence on microstructures. The magnetization reversal followed the domain wall motion behaviour when the film was continuous and deviated towards the Stoner–Wohlfarth (S–W) model in the dome-shaped regime. A further deviation away from the S–W model was observed, when the film acquired an inverted frustum shape structure in order to minimize the surface energy.

Drift of a twisted domain wall (TDW) in perpendicularly magnetized films of uniaxial ferromagnetic materials

Drift motion of a twisted domain wall (TDW) in a strong magnetic field circularly polarized in the plane of a perpendicularly magnetized film is considered. The twisted structure reduces the threshold for the regime of stationary motion of a TDW (on both the velocity of motion and frequency of the rotating field) in comparison with a one-dimensional DW of the Bloch or Neel type. The dependence of the boundary of optimum (from the experimental viewpoint) observation of drift of a TDW in the regime of stationary motion on the ratio of the width of a Bloch line to the thickness of the film has been determined.

Photoinduced magnetic softening of perpendicularly magnetized L1-FePt granular films

Ultrafast spin dynamics has for the first time been studied in perpendicular magnetized granular films. For FePt continuous films and FePt–MgO and FePt–Ag granular films with femtosecond laser excitations, the coercivity HC and the saturation Kerr rotation θKS are dramatically reduced, accompanied by a sharp increase in the reflectivity R. Afterward, these physical quantities are slowly recovered. The changes in HC, θKS, and R are all different among FePt, FePt–MgO, and FePt–Ag films. The difference is caused by different film thicknesses and in particular by the surface plasmon resonance of metallic nanoparticles.

Improvement of magnetic properties and read/write characteristics in SmCo 5 perpendicular thin films

An SmCo 5 alloy is a promising candidate for ultra-high density magnetic recording media because of its strong uniaxial magnetocrystalline anisotropy, whose constant, K u, is more than 1.1×10 8 erg/cm 3. Recently, we successfully obtained high perpendicular magnetic anisotropy for a sputter-deposited SmCo 5 thin film by introducing a Cu/Ti dual underlayer. However, it is necessary to improve magnetic properties and read/write (R/W) characteristics for applying SmCo 5 thin films to perpendicular magnetic recording media. In this study, we focused on reduction of magnetic domain size and change of a magnetization reversal process of SmCo 5 perpendicular magnetic thin films by introducing carbon (C) atoms into the constituent Cu underlayer. The magnetic domain size became small and the ratio of coercivity ( H c) against magnetic anisotropy ( H k) which is an index of the magnetization reversal process was increased by adding C atoms. We also evaluated the R/W characteristics of SmCo 5 double-layered media including C atoms. The medium noise was decreased and signal-to-noise ratio increased by introducing the C. The addition of C into the Cu underlayer is effective for changing the magnetization reversal process, reducing medium noise and increasing SNR.

Fabrication of cobalt nanowires from mixture of 1-ethyl-3-methylimidazolium chloride ionic liquid and ethylene glycol using porous anodic alumina template

Porous anodic alumina template is synthesized by electrochemical anodization of aluminum and used to grow cobalt nanowires. The cobalt nanowires produced by direct current electrodeposition are characterized by field emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction and physical property measurement system. Test results indicate that the average diameter of cobalt nanowires is about 45 nm, which is generally the same as the pore diameter of porous anodic alumina template, and the cobalt nanowires electrodeposited from mixture of 1-ethyl-3-methylimidazolium chloride ionic liquid and ethylene glycol have a smoother surface and better magnetic properties than cobalt nanowires electrodeposited from aqueous solution, and they show a better squareness. Therefore it can be concluded that the cobalt nanowires electrodeposited from mixture of 1-ethyl-3-methylimidazolium chloride ionic liquid and ethylene glycol using porous anodic alumina template can be used as a perpendicular magnetic recording film.

Planar self‐biased magnetic resonance microscopy “lenses”

AbstractWe present novel planar permanent magnet designs that create points above the plane where the magnitude of the static magnetic field is a local nonzero magnetic field point minimum. The designs represent potential magnetic resonance microscopy “lenses” where only a point region of the sample located in the “focus” spot of the structures is resonant. The high magnetic field curvatures of the “lenses” ensure that the regions outside of the “focus” spot are inactive or filtered out in the resonance detection process. In contrast to the original magnetic resonance “lens” design that required an applied uniform external bias field, the new designs are self‐biased. Each planar permanent magnet design is accompanied by the equivalent planar single noncrossing conductor design. We also introduce the concept of Amperean current doubling in planar perpendicularly magnetized thin films that can be used to improve the performance of each permanent magnet design we present. We experimentally constructed macroscale representative permanent magnet and wire structures and measured their magnetic field properties that confirm our numerical predictions. Single conductor current‐carrying designs are suitable for single layer lithographic fabrication, as we also experimentally demonstrate. Finally, we present the case that nanometer scale recording of perpendicular anisotropy thin magnetic films using presently available data storage technology can potentially provide the ultimate miniaturization of the presented designs. © 2008 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 33B: 21–31, 2008

Preparation of Perpendicular GdFeCo Magnetic Thin Films with Pulse Electrodeposition Technique Utilizing Molten Salt as Electrolyte

We have utilized ZnCl2-dimethylsulfone (DMSO2) as the electrolyte with added GdCl3, FeCl2, and CoCl2, for electrodepositing a perpendicular GdFeCo magnetic thin film. The reaction at the electrode surface and the electrical conductivity of the ionic substance at different ionic concentrations were studied by cyclic voltammetry and a computerized direct current method. Moreover, the electrodeposition of the GdFeCo thin film was determined by a pulse potential method. Relation between the composition of the deposited thin film and control parameters including applied potentials was determined by EDS analysis. An amorphous structure and the thickness of the thin film were obtained by TEM analysis. Its roughness and uniformity were determined by AFM analysis. Meanwhile, a perpendicular magnetic property and pinning magnetic domain of the thin film were analyzed from results of AGM and MFM.

Nonlinear precession dynamics of magnetization in (111) garnet ferrite films

The nonlinear dynamics of magnetization precession in perpendicular-magnetized (111) garnet ferrite films is studied by numerically solving the equations of motion of magnetization. Bifurcational changes in the magnetization precession and dynamic-bistability states are detected. The conditions are found under which both regular and stochastic large-amplitude dynamic regimes arise.