Studies of the domain structure of anisotropic sintered SmCo5 permanent magnets

In this work, investigations of the magnetic microstructure of anisotropic sintered SmCo₅ permanent magnets with high coercivity have been made using the colloid-scanning electron microscopy (SEM) technique and magnetic force microscopy (MFM). The magnets were produced by powder metallurgy (sintering) process and consisted of oriented grains with an average size of about 20 μm. They were studied in the thermally demagnetized state. Owing to the application of digital image recording, enhancement and analysis, high-quality images of the magnetic microstructure were obtained and analyzed not only qualitatively but also quantitatively. Improvements over previous results were achieved. The grains show the presence of magnetic domains, as expected. At the surface perpendicular to the alignment axis, the coarse domain structure in the form of a maze pattern with surface reverse spikes is observed. The main (maze) domains had typical widths 3–5 μm. The reverse spike domains were imaged as circles typically 1–2 μm in diameter or as elongated regions up to about 6 μm in length. Interestingly, in addition to the coarse maze domains and reverse spikes near the surface, a fine surface domain structure is revealed with MFM. The fine scale domains are found to be magnetized perpendicular to the surface and their occurrence is attributed to further reduction of the magnetostatic energy at the cost of a larger domain wall energy. On the surface parallel to the alignment axis, the main domains within individual grains are imaged as stripe domains with domain walls running approximately parallel to the alignment axis, while reverse spike domains are displayed in the form of triangular domains and occur near some grain boundaries, pores or precipitations. The magnetic alignment of grains was found to be good, but certainly not perfect. In most cases the domain structures within grains were independent of their neighbors, but in some cases (not so rare) observations indicated the existence of significant magnetostatic coupling between neighboring grains. The main and surface domain widths were determined by digital means using the stereologic method of Bodenberger and Hubert. Moreover, the domain wall energy and other intrinsic parameters for the studied magnets were determined.     

Analysis of checkerboard pattern in ultrathin magnetic films

We discuss the magnetostatic energy of checkerboard domain structures in ultrathin magnetic films (of a few monolayer thickness) and in an atomic monolayer using simple magnetostatic considerations where the easy direction of magnetization is perpendicular to the film. The checkerboard domain size, D, the domain-wall width, ω, the ratio f of the uniaxial surface anisotropy, K_s, to the dipolar energy and the binding energy, (BE), have been calculated numerically with the variational parameter δ and the number of atomic layers, n_l, as parameters.     

Simultaneous observation of atomic step and domain wall structure of ultrathin Co films by magnetic force microscopy in ultrahigh vacuum

We have applied magnetic force microscopy in ultrahigh vacuum to study the correlation between the atomic step and magnetic domain wall structure of ultrathin Co films prepared in situ on Au(111) substrates. For the first time we were able to achieve high-resolution images showing simultaneously a clear domain wall contrast and the underlying atomic step structure. Although for in-plane magnetized Co films the domain walls were found to run preferentially in a direction perpendicular to the steps, no such correlation could be observed for out-of-plane magnetized Co films. Received: 3 June 1999 / Accepted: 4 June 1999 / Published online: 29 July 1999     

The domain structure and magnetic anisotropy in Co-based amorphous ribbons after annealing in a helical magnetic field: Part II

The interaction of stray fields and the eddy currents inherent in a sandwich domain structure is studied. This interaction allows us to control the existence of a sandwich domain structure in an amorphous ribbon. The formation of non-uniform magnetic anisotropy in an amorphous ribbon as a result of its annealing in a helical magnetic field is also investigated.     

The influence of size on coercive field of ultra soft magnetic materials

We report here a size dependence of the coercive field in the millimeter–centimeter range length scale of ribbon like samples prepared from ultra soft amorphous and nanocrystalline alloys. A model is proposed where surface pinned domain walls are considered having an effective stiffness constant linearly increasing with the demagnetization factor.     

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.     

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.     

Magnetic Force Microscopy Study of Alternate Sputtered （001） Oriented L1o Phase FePt Films

We present a magnetic force microscopy study of alternate sputtered （001） oriented L1o phase FePt films. It is found that the root-mean-square value of phase shift of magnetic force images, （ △Ф）rms, can be used to characterize the perpendicular anisotropy for a series of specimens. Therefore, the considerable improvement of the perpendicular anisotropy after post-annealing can be characterized. In addition, the magnetic properties, magnetic and crystalline microstructures before and after post-annealing are compared for the typical [Fe5nm Pt5 nm]10 film with substrate temperature T8 = 500℃, single layer thickness d = 5 nm and total layer thickness D=100 nm to confirm the effect of post-annealing on improving the perpendicular anisotropy for Fe-Pt films.     

Magnetic domain wall creep in the presence of an effective interlayer coupling field

We investigate thermally activated domain wall creep in a system consisting of two ultrathin Co layers with perpendicular anisotropy coupled antiferromagnetically through a 4 nm thick Pt spacer layer. The field driven dynamics of domain walls in the softer Co layer have been measured while keeping the harder Co layer negatively saturated. The effect of the interlayer interaction on the soft layer is interpreted in terms of an effective coupling field, _HJ$H_J$, which results in an asymmetry between the domain wall speeds measured under positive and negative driving fields. We show that creep theory remains valid to describe the observed wall motion when the effective coupling field is included in the creep velocity law as a component of the total field acting on the wall. Using the resultant modified creep expression, we determine a value for the effective coupling field which is consistent with that measured from the shift of the soft layer's minor hysteresis loop. The net antiferromagnetic coupling is attributed to a combination of RKKY and orange-peel coupling.     

Extracting domain wall patterns from SEM magnetic contrast images

The magnetic contrast images of a soft magnetic metallic glass Fe₇₉Si₆B₁₄Cu₁ subjected to a periodic magnetic field were recorded with a scanning electron microscope by using a stroboscopic technique. An image processing method for the extraction of domain patterns from these images is presented. By this technique, delicate details of the dynamic magnetization process can be investigated and differences in the local magnetisation and domain wall movement can be analysed.     

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.     

Magnetic field dependence of asymmetry in the magnetization reversal of ultrathin Co films and Co/Pt multilayers with perpendicular anisotropy

We studied the magnetization reversal in ultrathin [Co/Pt]_n films (n=1, 2, and 4) using magneto-optical Kerr microscopy. These materials demonstrate unusual asymmetries in the activity of nucleation centers and domain wall motion. It was found that application of very high holding magnetic field prior to magnetization reversal, exceeding some critical value much larger than the apparent saturation field, suppresses the subsequent ‘asymmetric’ nucleation centers, activity. We revealed that the ‘asymmetric’ nucleation centers become active again after subsequent reversal cycles coming from a smaller holding field and studied how the asymmetry returns with the decrease of applied holding field. It was found that in low-coercivity ultrathin Co films, the asymmetry in domain wall velocity decreased sharply with the applied field increase and disappeared when the reversal field is greater than μ₀H=1.5 mT.     

Domain structures of ultrathin magnetic nanobelts

We use a simple magnetization model to determine domain structures of ultrathin magnetic nanobelts. A train of alternate domains are formed along the length direction. Optimal domain length decreases with belt width. Experimental domain length distributions of Fe bilayer nanobelts can be naturally explained. This approach should be applicable to similar nanomagnets.