J.E.L. Bishop

Magnetic force microscopy of nanocrystalline NdFeB ribbons: A study of tip-sample interaction using a well-characterised sample

Abstract The magnetic domain structure of near stoichiometric nanocrystalline Nd 11.8 Fe 82.5 B 6 alloy ribbon has been examined using magnetic force microscopy (MFM). Tip-sample interactions have been compared for Si tips coated with remanent CoCr and Fe/SiO 2 . The former behaves as a permanent magnet dipole, the latter as a super-paramagnet. They provide consistent and complementary information. The observed magnetic force gradient contrast was interpreted in terms of interaction domains (a group of grains with their spins closely aligned by exchange coupling). The average length scale of these domains for the ribbon in the as-cast state was 350 ± 30 nm, increasing to an average size of 620 ± 30 nm in the remanent state. Reproducible detail on a scale of ∼20 nm was detectable.

Computer simulation of the effect of an ultra soft phase on the magnetic properties of nanocrystalline permanent magnets

Abstract Micromagnetic models of assemblies of randomly oriented, exchange coupled, nanocrystals consisting of a magnetically hard uniaxial phase (typified by Nd 2 Fe 14 B) together with an ultra-soft phase (typified by α -iron) have been studied numerically. The hard crystallites were located on a three dimensional cubic lattice, and cubic crystallites of the soft phase were inserted at the junctions of every group of eight hard crystallites. Demagnetizing curves were obtained as a sequence of static equilibrium states in an incrementally changing applied field. Values of the coercivity, remanence and energy product are reported as functions of Δ GH , the grain size of the hard phase, and of ϕ S , the fractional volume of the soft phase. The dependence on Δ GH falls quite rapidly with increasing ϕ S . The low anisotropy and high saturation magnetization of the α -iron make approximately equal contributions to remanence enhancement in the model. Its remanence values are in quite good agreement with experimental values for Nd 2 Fe 14 B containing α -iron. However, its values for the coercivity and energy product, though decidedly lower than those obtained computationally by some other authors, significantly exceed those obtained experimentally, but show a similar variation with Δ GH and ϕ S .

The effect of tip type and scan height on magnetic domain images obtained by MFM

In the last few years there has been a considerable expansion in the application of Magnetic Force Microscopy (MFM). MFM images the stray field from the sample, offers excellent lateral resolution (/spl sim/10 nm) and requires minimal sample preparation. The interaction between the stray field from the sample and from the sensing tip depends on the shape of the tip, the composition and thickness of its coating and the scan height above the sample. The effects of tip type and scan height on the MFM image are reported for a Nd-Fe-B specimen composed of a few large crystallites of similar orientation. The observed structures of order /spl ges/1 /spl mu/m are discussed in the context of established models for the domain structure of strongly uniaxial materials. Unexpected fine magnetic structure /spl ap/25 nm is reported.

Simulation of domain wall bowing in

The bowing of domain walls spanning (110) [001] laminations has been simulated for sinusoidal flux variations using a model wall subdivided into 16 active segment pairs. Dynamic equilibrium of drive field, eddy field, and wall surface forces is assumed, using Graham and Neuraths expression for the orientation dependence of the specific wall energy γ in material with first-order positive cubic anisotropy. Hybrid wall surfaces are assumed where they have minimum energy. Bowing thresholds occur near Wβ = 3.5 for

Domain wall bowing interpretation of eddy current loss measurements in a (110) [001] Si-Fe monocrystal

The measurements of cyclic magnetization loss in a 3% Si-Fe monocrystal with known domain structures reported by Swift et al. (1974) are compared with the predictions of the Pry and Bean model, a skew planar wall model, and a model of domain wall bowing which neglects the dependence of domain wall energy per unit area on the crystallographic orientation of the wall. Detailed agreement is not obtained, but the trend of the bowing model behaviour is encouraging. Consideration of the consequences of the dependence of wall energy on orientation indicates that it should assist the initiation of bowing at lower frequencies and restrain severe bowing at high frequencies. This is expected to lead to a reduction in the losses, especially at low frequencies, and to a general straightening of the loss/frequency characteristic in accord with the trend of the observations. The discrepancy between the measured losses and the Pry and Bean model has previously been attributed to edge effects according to the calculations of Young et al. (1975) It is shown in the present paper, however, that the Pry and Bean model requires negligible correction for edge effects provided the domain walls are well separated on the scale of the sheet thickness.

The origin and interpretation of fine scale magnetic contrast in magnetic force microscopy: A study using single-crystal NdFeB and a range of magnetic force microscopy tips

We have used an oriented single crystal of Nd2Fe14B1 as a test sample to explore contrast and resolution as a function of magnetic force microscopy tip coating, tip scan height, and sample history. We find that resolution is independent of tip type; that contrast is greater at low scan heights and for higher moment tips; and that the fine length scale structure of order 25 nm can be resolved. We further show how the surface structure is a function of state (demagnetized or remanent), demonstrating the ability of well-characterized studies to give high-resolution information on carefully prepared samples.

3-D simulation of Bloch lines in 180° domain walls in thin iron films

Abstract The micromagnetic structure of Bloch lines separating 180° domain-wall sections with differing symmetry in 60 nm {{1 0 0}} iron films has been calculated. The flux of transverse magnetization in the wall sections was ‘S’-shaped, normal to the film plane in the interior and parallel to it near the surfaces. Stable structures were obtained for (i) two ‘S’ sections with opposite normal flux, (ii) ‘S’ and ‘mirror-S’ sections with parallel flux. The energy of the latter is slightly lower than that of the former.

Simulation Of 3-D Micromagnetic Structures In A Thin Iron Platelet

The three dimensional classical Landau-Lifshitz domain structure in a 30 nm thick (100) iron rectangular platelet was simulated by a 3-D extension of the LaBonte method. Two stable micromagnetic structures with different symmetries and energies were obtained. The four 90/spl deg/ walls of both the structures are of the simple symmetric Neel type. However, the single 180/spl deg/ domain walls in the centres of the two structures have different complicated 3-D cross-tie periodic structures containing circular swirls and cross swirls which we believe correspond to the complicated zip-like patterns that have been observed by magnetic force microscopy. The period of the spacing of the swirls is comparable to that of these zip-like patterns. In addition to the 3-D simulation, 2-D simulations were also employed to study the variation in periodicity of the circular and cross swirls with the length of the platelet.

Micromagnetic and MFM studies of a domain wall in thick {110} FeSi

A 180° domain wall on a {1 1 0} surface of a 0.3 mm thick crystal of FeSi (3.25% Si), and containing a Bloch line, has been studied by magnetic force microscopy (MFM) with particular attention to disturbance by the probe. A technique for distinguishing the contributions to the MFM signal arising from the undisturbed structure and from the disturbance to that structure, by repeating the observations with probe moment reversed, is discussed. Valid to first order, and applicable only to mild reversible perturbations, it is used to compare the observed wall structure with that of a micromagnetic model. Asymmetry in the MFM signal is consistent with the presence of a Neel cap that closes the flux to one side of the wall at the surface. Scanning by the probe sometimes created (or destroyed) kinks in attractive wall sections. These kinks are identified as junctions between lengths of wall having a common Bloch core but opposite Neel cap orientations. Comparison is made with a micromagnetic model of such a kink. The kinks and the Bloch line could be moved by scanning, depending on the polarisation and flying height of the tip.

Eddy current dominated magnetization processes in grain oriented silicon iron

The magnetization reversal process in grain oriented silicon-iron with good texture is considered as a function of magnetization rate. Barkhausen jumps in the quasistatic and low-field regimes are briefly discussed. The interaction of eddy currents with the orientation dependent domain wall surface energy is discussed quantitatively and in some detail. The use of two dimensional flexible domain wall models is described and justified at fields well above the coercivity threshold. Examples of such model motion are discussed with some mention of the difficulties presented by severe wall bowing, merging and nucleation at high magnetization rates. Comparison is made with experimental data and the origin of the downward curvature of the loss versus frequency characteristic is considered.