Perpendicular Magnetic Easy Axis Research Articles

Magnetization reversal of a structurally disordered manganite thin film with perpendicular anisotropy

The field-dependent domain structure of a ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_{3}$ thin film epitaxially grown on a ${\mathrm{LaAlO}}_{3}$ (001) substrate has been investigated at low temperatures by magnetic force microscopy. Due to the lattice mismatch, the ferromagnetic thin film is structurally disordered and exhibits a stress induced perpendicular easy axis of magnetization and stripe domains at zero field. The magnetization reversal is imaged in a movielike manner along the easy axis major hysteresis loop. Individual Barkhausen jumps are distinguished as either nucleation, growth, or annihilation processes, and analyzed with respect to size, contrast, and position. Nucleation and annihilation occur only in a limited field range just below saturation while growth due to wall propagation dominates the domain formation. The role of disorder in the material in comparison to demagnetization effects is discussed in detail, particularly with respect to the size distribution of nucleation and annihilation as well as growth processes, domain wall pinning, and preferred nucleation sites.

Morphology and magnetic properties of ECR ion beam sputtered Co/Pt films

We report the fabrication of magnetic films using electron cyclotron resonance ion beam sputtering. The films consist of a Pt buffer layer, a thin magnetic Co layer and a Pt cap. Varying the thickness of the layers, we can tune the magnetic properties and a uniaxial perpendicular magnetic anisotropy can be obtained. Materials like amorphous silicon oxide, amorphous carbon, glass, mica and potassium bromide have been used as substrates. We have analyzed the structure of Co/Pt films on amorphous carbon by means of transmission electron microscopy. The analysis reveals that the buffer layers are polycrystalline with a (1 1 1)-texture, which increases with Pt thickness. The average grain-size is linearly correlated with the total thickness of the Pt buffer layer. While Co grows predominantly in its HCP stacking with c -axis aligned parallel to the surface normal, FCC grains with a strong (1 1 1) texture are also found. Scanning tunneling microscopy investigations reveal an average surface roughness of the Pt buffer layer of 3.0 Å . For buffer thickness ⩾ 60 Å we find similar magnetic properties for all Co films, not depending on the substrate materials. Co films of 4–8 Å thickness exhibit a perpendicular easy axis of magnetization, while a spin reorientation transition is found in a thickness range of 8–13 Å. Above 13 Å an in-plane easy axis of magnetization is obtained.

Tilted magnetization of a La 0.7Sr 0.3MnO 3/LaAlO 3 (001) thin film

A La 0.7Sr 0.3MnO 3 thin film epitaxially grown on a LaAlO 3 (001) substrate by pulsed laser deposition has been investigated by means of low-temperature magnetic force microscopy under high stability conditions. The ferromagnetic film exhibits a stress-induced perpendicular easy axis of magnetization. The domain structure was imaged at zero field after (i) thermal demagnetization, (ii) saturation in an in-plane field and (iii) saturation in an out-of-plane magnetic field, respectively. Stripe domains were observed in all cases, however, contrast and domain width are increased after out-of-plane saturation while alignment of stripe domains in field direction occurs after in-plane saturation. A quantitative analysis of the image contrast suggests a variable in-plane component of the magnetization, dependent on the magnetic history of the sample. Calculations of domain width and contrast from the relevant energy contributions are compared with experimental results.

Interplay between domain-wall resistance and surface scattering in ferromagnetic thin films

A positive domain-wall resistance at low temperatures has been reported in the literature for cobalt films with a perpendicular magnetic easy axis, in contrast to the behavior observed in microstructures made from cobalt, iron, and permalloy films with an in-plane easy axis and to that seen in perpendicular iron–palladium thin-film compounds. This phenomenon is unexpected if only domain-wall or Fermi-surface properties are considered. It can, however, be understood if an existing domain-wall resistance is considered which is compensated by a magnetoresistance effect arising from surface scattering in a thin film with closure domains. A theory that properly accounts for this interplay between domain-wall resistance and surface scattering is presented and employed to analyze existing experimental data.

Precessional strategies for the ultrafast switching of soft and hard magnetic nanostructures

ABSTRACTWe discuss the precessional, quasi-ballistic switching of magnetization in magnetic nanostructures. In soft spin-valve cells, fast and energy-cost effective magnetization switching can be triggered by a transverse field pulse of moderate amplitude, below the in plane anisotropy field, because of an amplification effect brought by the demagnetizing field at the early stage of the reversal. The same effect is no more possible in hard nanomagnets with perpendicular easy magnetization axis. We propose a new type of nanostructured magnetic device, designed to overcome this limitation. The speed is obtained through the use of a very high effective magnetic field, obtained by incorporating a significant exchange field which stores the energy in the form of a constrained domain wall surrounding a region of high magnetic anisotropy. This stored energy is partially available to accelerate the magnetization reversal in a precessional scenario. We illustrate the concept by studying numerically a model system. The key parameter for the reversal is the ratio of the domain wall width to the structure lateral dimension. Possible routes for device preparation are discussed. Promising application to magnetic storage are anticipated.

Detection of domain wall propagation in a mesoscopic wire

Extraordinary Hall effect was used to detect the propagation of a domain wall in magnetic devices patterned in sputter grown Pt/Co(1 nm)/Pt sandwiches with perpendicular easy magnetization axis. In such films, domain walls propagate as coherent 1D nano-object in a 2D medium with weak fluctuation energy density. In a patterned device, the competition between global wall energy and Zeeman energy strongly influences the wall propagation.

Irradiation-induced magnetic patterning in magnetic multilayers

Irradiation with light ion (He +) can modify in a precisely controlled way the magnetic properties of thin films, with negligible change of surface roughness and optical indices. In Co/Pt multilayers with perpendicular easy axis of magnetization, we have shown that the magnetic anisotropy decreases with the irradiation fluence. In FePt alloys, partial chemical ordering and high perpendicular anisotropy have been induced by irradiation at moderate processing temperatures. Also, we have shown that perpendicular anisotropy in FePd and CoPt 3 alloys can be decreased depending on the temperature of irradiation. These features allow the realization of a new type of magnetic nanostructures. As the structural modifications are localized in the close vicinity of the ion path in matter, planar magnetic patterning at the sub-50-nm scale can be achieved when the irradiation is performed through a mask. By means of 30 keV He + ion irradiation of Co/Pt multilayers, we have produced a nearly optical contrast-free, planar array of magnetically hard lines embedded in a softer matrix. Such irradiation-fabricated nanostructures exhibit specific magnetization reversal processes: low field nucleation centers and preferred domain wall propagation paths are located at the borders between irradiated and non-irradiated areas. The magnetization reversal dynamic is limited everywhere by domain wall motion, ensuring a relatively weak spread of coercive forces. Striking images of nucleation and domain wall motion in such arrays are displayed and interpreted within a novel theoretical framework of irradiation through a mask.

Structure and magnetism of pulsed-laser-deposited ultrathin films of Fe on Cu(100)

A layer-by-layer growth of ultrathin films right from the beginning would be desirable for establishing a straightforward correlation between magnetism and structure. We give experimental evidence that with the help of pulsed laser deposition ~PLD! we can achieve layer-by-layer growth for Fe on Cu~100! in contrast to deposition by molecular-beam epitaxy. We present the results of a comprehensive study of the structural and magnetic properties of PLD-grown ultrathin Fe films of thicknesses between 2 and 10 monolayers ~ML! deposited at room temperature. We show scanning tunneling microscopy images and low-energy electrondiffraction ~LEED! patterns as well as intensity vs energy ( IV) LEED curves demonstrating that PLD-grown Fe/Cu~100! has an isotropic fcc structure. We characterize the magnetic properties of our films by the magnetooptical Kerr effect. Following the improved growth and morphology, we found strong differences in the magnetic behavior of these films in comparison with Fe thermally deposited onto Cu~100!: PLD-grown ultrathin Fe/Cu~100! shows an in-plane easy axis of magnetization in the thickness range 2‐5 ML and again from about 10 ML, where the film structure is dominated by the bcc-Fe bulk phase, while there is a perpendicular easy axis of magnetization between 7 and 10 ML coverage. These results are discussed in terms of the different growth and structure due to the characteristic features of the PLD technique. @S0163-1829~99!13201-6#

Changes of morphology, structure, and magnetism of Fe on stepped Cu(111)

We have studied the morphology, structure and magnetism of Fe films thermally deposited on stepped Cu(111) by scanning tunnelling microscopy (STM), low energy electron diffraction (LEED) and magneto-optical Kerr effect (MOKE). At room temperature, in the submonolayer range Fe films grow as quasi one-dimensional stripes along the upper step edges due to a strong decoration effect. Between 1.4 and 1.8 monolayer coverage the stripes percolate and become two-dimensional films. Between 2.3 and 2.7 monolayer coverage the films undergo a structural transition from fcc(111) to bcc[110] with Kurdjumov-Sachs orientation. In the fcc range the films have a low net magnetic moment, and a perpendicular easy magnetization axis. The magnetization switches to an in-plane high-moment phase correlated with the fcc to bcc structural transformation.

Structural and magnetic phase transitions of Fe on stepped Cu(111)

The magnetism and its correlation with morphology and structure of ultrathin Fe/Cu(111) films have been studied. At room temperature, the films grow in a quasi-one-dimensional form (stripes) in the submonolayer range. Between 1.4 and 1.8 ML the stripes percolate and become two-dimensional films. The remanent magnetization of the percolated films was observed to be significantly more stable with respect to time than that of the stripes. At low thickness (2.3 ML) the films adopt the fcc structure from the substrate and later transform to bcc(110) structure with Kurdjumov-Sachs orientation. Experimental evidence suggests that the fcc films have a low-spin ferromagnetic or ferrimagnetic phase, and a perpendicular easy magnetization axis. The magnetization switches to an in-plane high-spin phase after the fcc to bcc structural transformation has been accomplished.

X-ray magnetic circular dichroism at the Gd L edges in Gd-Ni-Co amorphous systems

X-ray magnetic circular dichroism (XMCD) experiments have been performed at the ${\mathrm{L}}_{2,3}$ edges of gadolinium in the amorphous Gd-Ni-Co and Gd-Ni alloys in order to observe the effect of the transition metal on the polarization of the rare-earth 5d band. The magnetic properties of the Gd-Ni-Co alloys, weak anisotropy or perpendicular easy axis of magnetization, are very well suited to XMCD experiments and very large XMCD signals are observed at the Gd ${\mathrm{L}}_{2,3}$ edges. The signals corrected from the circular polarization rate (${\mathrm{P}}_{\mathrm{c}}$ =0.65) can reach up to -22% at the ${\mathrm{L}}_{2}$ and +11% at the ${\mathrm{L}}_{3}$ for some samples. The V-like behavior of the XMCD signal at the Gd L edges near the compensation temperature is understood in terms of distribution of the Co concentration in the Gd-Ni-Co sputtered films. Within the spin-dependent radial matrix model, the evolution of the XMCD intensity in the Gd-Ni-Co films as a function of the Co concentration qualitatively accounts for the variation of the spin-dependent final-state density \ensuremath{\Delta}\ensuremath{\rho}/\ensuremath{\rho} monitored by the Co concentration.

Magnetic anisotropy of Ni/Al multilayers

The magnetic anisotropy of Ni/Al multilayers has been investigated by vibrating sample magnetometer (VSM) and ferromagnetic resonance (FMR) techniques. The FMR spectra are obtained as a function of the orientation of the applied magnetic field from in-plane to out-of-plane, and are fitted theoretically to determine the magnetic anisotropy. From VSM and FMR, a positive value for Ni/Al interface anisotropy, favoring a perpendicular easy axis, is obtained. Furthermore, for the ultrathin magnetic layer below a critical Ni thickness of 20 Å, a perpendicular easy axis of magnetization can be extrapolated.

Kerr rotation and perpendicular magnetic anisotropy oscillations versus Cu(111) coverage thickness in Cu(111)/Co/Au trilayers

We report here the observation of polar magneto-optical Kerr rotation and coercive force oscillations versus Cu overlayer thickness on Cu/Co/Au(111) UHV deposited trilayers with different Co and Cu thicknesses and perpendicular easy magnetization axis. The coercive force oscillation is attributed to an oscillatory behavior of the perpendicular magnetic anisotropy due to a spin-dependent electron confinement effect in the Cu(111) overlayer.

Study of large area high density magnetic dot arrays fabricated using synchrotron radiation based x-ray lithography

Large area arrays of dots have been patterned in Au/Co/Au(111) sandwiches with ultrathin Co layers (0.6 to 2 nm) and a perpendicular easy magnetization axis. Dot dimensions down to 0.2 μm have been achieved using x-ray lithography, with either positive resist and direct ion beam etching or a lift-off process with aluminum mask. Both processes have been tested against the damages they induce to the fragile structure of the samples. The magneto-optical effects and magnetization reversal processes in the arrays have been characterized versus Co thickness, dot dimension, and lattice aspect ratio. For high quality samples, the domain walls propagation mechanism that drives magnetization reversal in as-grown films is drastically modified in dot arrays, leading to a large increase of the coercive field with dot diameter reduction, together with changes in the shape of the hysteresis loops.

Correlation between the spin magnetic dipole term and the magnetic anisotropy in Co/Pd multilayers

Using the sum rules for magnetic X-ray dichroism we obtain trends for the spin—orbit interaction and the z-components of the orbital magnetic moments of Co in Co/Pd multilayers with perpendicular easy axis of magnetization. We found a correlation between the spin magnetic dipole term T z , which is related to the spin—orbit interaction, and the crystal-field. This is the first experimental evidence that the origin of the perpendicular easy axis of the magnetization in magnetic layers is determined by a fine interplay between the crystal-field and spin—orbit coupling.

The relationship between the microstructure and magnetic properties of sputtered Co/Pt multilayer films (abstract)

Co/Pd multilayer films (MLFs) are of interest because of their potential application as high-density magneto-optical recording media. Co/Pd MLFs with varying Co and Pd layer thicknesses were grown by sputter-deposition onto (100) Si wafers. X-ray diffraction and high resolution electron microscopy were used to study the microstructure of the films, and Lorentz microscopy was used to analyze their magnetic domain structure. The films show an fcc crystal structure with a compromised lattice parameter and a strong (111) crystallographic texture in the growth direction. The compromised interplanar spacing parallel to the surface increased with decreasing thickness ratio (tCo/tPd), and the columnar grain size decreased with increasing Pd layer thickness. Films with tCo=0.35 nm and tPd=2.8 nm (columnar grain diameter 20 nm) showed promising magnetic properties, namely a high perpendicular magnetic anisotropy (1.85×105 J m−3), with a perpendicular coercivity of 98.7 kA m−1, a perpendicular remanence ratio of 99%, and a perpendicular coercivity ratio of 88%. The magnetic domains were uniform and of a narrow stripe type, confirming the perpendicular easy axis of magnetization. The Curie temperature was found to be about 430 °C. Films of pure Co and Pd, grown for comparison, also showed columnar grain structure with grain-sizes of the same order as those seen in the MLFs. In addition the Pd films showed a (111) textured fcc structure.

Short period oscillations in the Kerr effect of 4d- and 5d-transition metal wedges on Co films (invited) (abstract)

We report a novel type of short period oscillations in electron-beam evaporated Co films covered with wedge-shaped films of Pt, Au, and Pd. Oscillatory behavior, with periods in the range 2–3 Å, is observed both in the saturation polar Kerr angle and in the high field susceptibility, as a function of the wedge position (see Fig. 1). The structures of the type buffer/Co/X wedge/Y cap were made by high vacuum evaporation using fused silica as substrates. They consist of a 200 Å Pt buffer layer, a thin Co layer revealing a perpendicular easy magnetization axis, a transition metal wedge (X) with thickness in the range 0–25 Å, and a thin Cu or Au cap (Y) layer. The presence of oscillations depends critically on the choice of the cap-layer Y. They are distinctly different from long and short range period oscillations observed in magnetic sandwich or multilayer structures, since there is only one magnetic film involved. The present results will be discussed in the framework of spin polarized quantum well states.

Transition to the perpendicular easy axis of magnetization in Ni ultrathin films found by x-ray magnetic circular dichroism.

Using soft-x-ray magnetic circular dichroism we have observed a change in the easy axis of magnetization in Ni ultrathin films on Cu(001) from parallel to the surface for 5--9 monolayers (ML) to perpendicular for 10--75 ML. This is an unusual effect which is counter to the predictions of magnetic surface anisotropy arguments. We conclude that the effect is due to a shape anisotropy, introduced either by a change in the Ni lattice structure or by three-dimensional morphological changes in the film.

57Fe Mössbauer spectroscopy from Tb/Fe and Er/Fe multilayers

Mössbauer spectroscopy was carried out on Tb/Fe and Er/Fe multilayers. The dependence of the extension of the interface on the substrate temperature during the deposition process and its magnetic behaviour are shown. Easy plane of magnetization and perpendicular easy axis of magnetization have been observed in Er/Fe and Tb/Fe multilayers respectively. The effects of the ion anisotropy and of the thickness of the layers are discussed.

Preparation and structural characterization of epitaxial Co/Pd(111) superlattices

High-quality epitaxial fcc Co/Pd(111) superlattices have been prepared on single-crystal GaAs(110) substrates using molecular-beam epitaxy. An ultrathin (6 Å) buffer layer of bcc Co(110) was used to promote epitaxial growth on the GaAs substrate. A thick (≊ 500 Å) buffer layer of fcc Pd(111) was then deposited to provide a smooth surface for superlattice growth. In addition, the Pd buffer layer isolates the superlattice from chemical interaction with the GaAs substrate. Co/Pd superlattices with ultrathin (<20 Å) Co layers are found to have a perpendicular easy axis of magnetization. The source of the perpendicular magnetization in these superlattices is attributed to an interface anisotropy, which depends on the interface quality and crystal structure. The structure of the superlattices has been investigated using a wide range of techniques, including reflection high- energy electron diffraction, low-energy electron diffraction, scanning tunneling microscopy, Rutherford backscattering spectroscopy, and a variety of x-ray-diffraction techniques. Details of the structure, including grain size, coherence length, and interfacial roughness are discussed. The magnetic properties of these superlattices are presented elsewhere in these proceedings.