Magnetic properties of field-annealed FeCo thin films

Fe₅₀Co₅₀ thin films with thickness of 30 and 4 nm have been produced by rf sputtering on glass substrates, and their surface has been observed with atomic force microscopy (AFM) and magnetic force microscopy (MFM); MFM images reveal a non-null component of the magnetization perpendicular to the film plane. Selected samples have been annealed in vacuum at temperatures of 300 and 350 °C for times between 20 and 120 min, under a static magnetic field of 100 Oe. DC hysteresis loops have been measured with an alternating gradient force magnetometer (AGFM) along the direction of the field applied during annealing and orthogonally to it. Samples with a thickness of 4 nm display lower coercive fields with respect to the 30 nm thick ones. Longer annealing times affect the development of a harder magnetic phase more oriented off the film plane. The field applied during annealing induces a moderate magnetic anisotropy only on 30 nm thick films.     

Structural and magnetoresistive properties of half metallic Co2Mn1−xSi thin films

Polycrystalline Co₂Mn_1−xSi (CMS) thin films with Mn-deficiency can grow on different types of substrates such as MgO (1 0 0) single crystal, α-sapphire (0 0 0 1) and Si coated with SiO₂ either by using V or Ta/Cu as the seed layer. The magnetic property, especially the coercivity of the CMS thin films strongly depends on the crystalline structure and microstructure of the CMS thin film, hence it is affected by the substrate and also the seed layer. Very soft CMS thin film with coercivity of about 20 Oe has been obtained when MgO (1 0 0) is used as the substrate. Magnetic tunnel junctions (with MR ratio of about 9%–18%) by utilizing the CMS as one of ferromagnetic electrodes have been successfully fabricated. The degradation of the magnetoresistive effect of the MTJ after magnetic annealing is attributed to the diffusion of the Mn-atoms into the tunnel barrier during the annealing process.     

Influence of Temperature on Equilibrium Separation Between Vertical Bloch Lines in OHBs in Garnet Bubble Films

The diameters of the ordinary hard bubbles （OHBs） and soft bubbles in epitaxial garnet films are measured under the microscope at various temperatures. It is found that the bubble diameters of OHBs increase with temperature, and it is concluded that the equilibrium separation between two neighbouring vertical Bloch lines （VBLs） Seq is widened with increasing temperature. At the same time, the results can be understood simply as that there are more VBLs in the domain walls of the first dumbbell domains （IDs） than those in walls of OHBs at the same temperature.     

Saturation field direction dependence of domain structures in NiFe elements

The domain structures in NiFe elements were studied by magnetic force microscopy measurement and micromagnetic modeling. The remanent states in the elements were dependent on the direction of the saturation field. The “S” and “U” states were observed at remanence by applying the saturation field at different directions. The “S” and “U” states are metastable: magnetic force microscopy tip field-induced switching from the “S” and “U” states to the flux closure configuration was observed.     

Magnetic domain structure in Fe78.8−xCoxCu0.6Nb2.6Si9B9 nanocrystalline alloys studied by Lorentz microscopy

The magnetic domain structures of Fe_78.8−xCo_xCu_0.6Nb_2.6Si₉B₉ (x=0, 20, 40, 60) alloys are investigated by Lorentz microscopy coupled with the focused ion beam method. The specimen prepared using the FIB method is found to have a considerably more uniform thickness compared to that prepared using the ion-milling method. In Fe_38.8Co₄₀Cu_0.6Nb_2.6Si₉B₉ and Fe_18.8Co₆₀Cu_0.6Nb_2.6Si₉B₉ alloys, 180° domain walls extending in the direction of the induced magnetic anisotropy are observed. Analysis with Lorentz microscopy reveals that the width of the magnetic domains decreases with an increase in the cobalt content or the induced magnetic anisotropy K_u, that is, the domain width d is proportional to the induced magnetic anisotropy (K_u)^−1/4. On the other hand, in the in situ Lorentz microscopy observation as a function of temperature, magnetic ripple structures are found to appear in a localized area due to the fluctuation of magnetization vectors from 423 K. It is observed that the induced magnetic anisotropy caused by the applied magnetic field at 803 K is not suppressed by the magnetic ripple structures observed at 423–443 K.     

Effects of size and interactions on the magnetic behaviour of elliptical (0 0 1)Fe nanoparticles

Arrays of elliptical particles with aspect ratio 1:3 and short axes 50, 100 and 150 nm were prepared by electron-beam lithography and ion-beam milling of epitaxial (0 0 1)Fe films of thicknesses 10 and 20 nm. The domain state of an individual particle imaged by magnetic force microscopy in zero field after demagnetization was observed to change from being bi-domain or multidomain (MD) to stable single domains (SD) as the lateral size and film thickness were decreased. The critical size for SD formation was found to be close to the actual lateral sizes of 100 nm×300 nm and 150 nm×450 nm for the thicknesses of 20 and 10 nm, respectively. Only in the 10 nm thick ellipses of lateral size 100 nm×300 nm, the magnetization reversal may take place through coherent rotation. For all other investigated samples, the experimental switching field is lower than what would be required for this process.     

Influence of shearing process on domain structure and magnetic properties of non-oriented electrical steel

The influence of shearing on the magnetic properties and domain structure of 0.5 mm thick non-oriented electrical steel was studied. In the region from 1 to 1.4 mm from the sheared edge, a striped domain pattern that indicated the existence of elastic strain was observed. From the degradation tendency of flux density with respect to shearing width, the width of the degraded region near the edge increased as the magnetic field decreased. These results suggested that the change in the flux density at high magnetic fields over 300 A/m were mainly dependent on the characteristics of the edge vicinity where the domain pattern was influenced by shearing.     

Designing magnetics of capped perpendicular media with minor-loop analysis

A novel method of analyzing the switching field distribution (SFD) and magnetic correlation length of perpendicular magnetic recording media that uses major and minor magnetization loops is proposed. By applying the analysis to a series of capped perpendicular media, we found that a thick capping layer with a low saturation magnetization effectively reduced SFD without rapidly increasing the magnetic correlation length. Transmission electron microscope observation suggests that the SFD is narrowed by the increased uniformity of intergranular exchange coupling via the thick capping layer. Evaluations of recording characteristics demonstrated a close correlation between narrower SFDs and improved recording performance. Reducing exchange coupling dispersion is a clear solution for improving the performance of recording media.     

Perpendicular magnetic anisotropy of FePt film on Si substrate with SiO2 underlayer and B4C interlayers

FePt multilayer composite films with and without B₄C interlayer have been prepared by magnetron sputtering, respectively, and subsequent annealing in vacuum. It was found that the B₄C layers effectively serve as spacers to separate the FePt layers, enhancing (0 0 1) orientation of FePt alloy. Our results show that highly (0 0 1) oriented FePt/B₄C films have significant potential as perpendicular recording media.     

Microstructure and magnetic properties of FePt:Ag nanocomposite films on SiO2/Si(1 0 0)

FePt:Ag nanocomposite films were prepared by pulsed filtered vacuum arc deposition system and subsequent rapid thermal annealing on SiO₂/Si(1 0 0) substrates. The microstructure and magnetic properties were investigated. A strong dependence of coercivity and ordering of the face-central tetragonal structure on both Ag concentration and annealing temperature was observed. With Ag concentration of 22% in atomic ratio, the coercivity got to 6.0 kOe with a grain size of 6.7 nm when annealing temperature was 400 °C.     

Magnetic domain structures and magnetotransport properties in Co-Ag granular thin films

The magnetic microstructures and magnetotransport properties in granular Co_xAg_1-x films with 17%≤x≤62% were studied. Magnetic force microscopy (MFM) observations showed the presence of magnetic stripe domains in as-deposited samples with x≥45% and the evolution of the magnetic domain patterns to in-plane domains with annealing. A perpendicular magnetic anisotropy as high as about 8×10⁵ ergs/cc for as-deposited Co₆₂Ag₃₈ and about 6×10⁵ ergs/cc for as-deposited Co₄₅Ag₅₅ was observed by magnetization and torque measurements. With increasing annealing temperature, the perpendicular magnetic anisotropy became negative. The origin of the perpendicular magnetic anisotropy may be attributed to a rhombohedral distortion of the cubic cell due to residual substrate-film stresses. The magnetic stripe domains are the consequence of the interplay of the indirect or direct exchange, perpendicular magnetic anisotropy and dipolar interactions. Finally, magnetoresistance (MR) curves displayed training behaviours and different shapes when measured with different configurations (parallel, transverse and perpendicular). It is proposed that the existence and the evolution of the magnetic domain structures strongly affect the magnetotransport properties due to the extra contribution of the electron scattering at the domain walls. Furthermore, an anisotropic MR also contributes to the overall MR curves. Received: 2 March 2000 / Accepted: 28 March 2000 / Published online: 23 May 2001     

Hysteresis of two interacting magnetic pairs with inequivalent perpendicular anisotropy

We consider a model for magnetic memory that consists of strongly coupled dipolar or antiferromagnetic (AF) pairs with inequivalent perpendicular anisotropy _K1$K_1$ and _K2$K_2$. For appropriate parameter values, determined in this work, they have two inequivalent storage states with zero net magnetic moment. Both analytical and numerical calculations are performed, in some cases yielding different results because of relaxation effects (i.e., a dependence on the damping parameter α$\alpha$). Hysteresis loops for a wide variety of parameter values are obtained, both for the AF case and the dipole case. An Appendix gives analytic results for slightly non-collinear spins in an applied field, which were used to test the numerical results.     

Temperature dependence of magnetic stripe domain period in ultrathin films

In ultrathin films, due to the thermal activation and temperature dependencies of the magnetic parameters, magnetization reversal processes are strongly affected by thermal effects. We analyze changes of domain periods of ultrathin cobalt and L₁₀$\mathrm{L}{1}_0$ films in a wide temperature range. With regard to the temperature dependencies of the film magnetic parameters we calculate the equilibrium stripe period as a function of temperature. It is shown that on film heating the equilibrium domain structure (DS) period decreases and at the reorientation phase transition (RPT) approaches its minimal value corresponding to the temperature independent period of the sinusoidal domain structure. Just below the RPT temperature (or thickness) the stripe domain period was found to exponentially decrease with temperature. Irreversible temperature changes of the domain period affected by coercivity are also discussed.     

Instantons and solitons in a monolayer film of ferromagnetic grains with biaxial anisotropy

We investigate domain wall and domain structure in a monolyer film consisting of ferromagnetic grains with biaxial anisotropy, which are seen to be the static versions of instanton and soliton, respectively. The equation of motion of the magnetization vector is reduced to the (1 + 2)-dimensional sine-Gordon field equation in strong anisotropy limit and the instanton and soliton configurations are obtained analytically. Various new domain structures in the ferromagnetic film are found.     

Magnetic microstructure of candidates for epitaxial dual Heusler magnetic tunnel junctions

Heusler alloys are considered as interesting ferromagnetic electrode materials for magnetic tunnel junctions, because of their high spin polarization. We, therefore, investigated the micromagnetic properties in a prototypical thin film system comprising two different Heusler phases Co₂MnSi (CMS) and Co₂FeSi (CFS) separated by a MgO barrier. The magnetic microstructure was investigated by X-ray photoemission electron microscopy (XPEEM). We find a strong influence of the Heusler phase formation process on the magnetic domain patterns. SiO₂/V/CMS/MgO/CFS and SiO₂/V/CFS/MgO/CMS trilayer structures exhibit a strikingly different magnetic behavior, which is due to pinhole coupling through the MgO barrier and a strong thickness dependence of the magnetic ordering in Co₂MnSi.     

Magnetic properties of FeSiB thin films displaying stripe domains

FeSiB amorphous thin films with thicknesses from 25 to 600 nm have been produced by rf sputtering on Si₃N₄ substrates. A spin reorientation transition has been observed on the as-prepared samples, as a function of thickness and temperature. Spin reorientation transition is shown to depend on the thermal treatments to which the as-prepared samples have been submitted. Static hysteresis loops obtained as a function of temperature, and magnetic force microscopy images taken at room temperature at the remanence and as a function of an applied magnetic field, have been employed to study the magnetic domain configuration of all the samples, and to see how it is affected by sample thickness, measurement temperature and annealing conditions.     

Direct observation of room temperature magnetism in (In,Mn)As thin films by magnetic force microscopy

Variable-temperature magnetic force microscopy (MFM) has been performed over the temperature range of 298-348 K on ferromagnetic (In,Mn)As thin films deposited by metal-organic vapor phase epitaxy (MOVPE). Ferromagnetic domains were observed with submicron resolution in both single and two phase (In,Mn)As films, persisting up to 328 K. Isolated cylindrical domains ranging from 100 to 350 nm in diameter with densities of 2-5 × 10⁸ cm⁻² were observed in phase pure films. Longer range magnetic order, in the form of ribbon-like domains up to 1 μm in length, are present in the regions between the cylindrical domains. Two phase (In,Mn)As films produced a well-resolved complex domain structure consisting of 180° parallel and antiparallel domains. Excellent agreement between the temperature dependence of the relative magnetization obtained by MFM and superconducting quantum interference device measurements was observed.     

Microstructure and magnetic properties of nanocomposite FePt/MgO and FePt/Ag films

An in-plane magnetic anisotropy of FePt film is obtained in the MgO 5 nm/FePt t nm/MgO 5 nm films (where t=5, 10 and 20 nm). Both the in-plane coercivity (H_c∥) and the perpendicular magnetic anisotropy of FePt films are increased when introducing an Ag-capped layer instead of MgO-capped layer. An in-plane coercivity is 3154 Oe for the MgO 5 nm/FePt 10 nm/MgO 5 nm film, and it can be increased to 4846 Oe as a 5 nm Ag-capped layer instead of MgO-capped layer. The transmission electron microscopy (TEM)-energy disperse spectrum (EDS) analysis shows that the Ag mainly distributed at the grain boundary of FePt, that leads the increase of the grain boundary energy, which will enhance coercivity and perpendicular magnetic anisotropy of FePt film.