L. Folks

The use of MFM for investigating domain structures in modern permanent magnet materials

Abstract Magnetic force microscopy occupies a special niche in the array of techniques which are currently available for imaging magnetic structures of high energy permanent magnet materials, yielding high resolution data under ambient conditions on bulk samples. The high anisotropies of permanent magnet samples mean that to a good approximation the tip stray field does not modify the magnetic state of the sample. However strong stray fields, of the order of 1xa0Tesla, may be experienced by a tip in close proximity to the sample. These stray fields are known to perturb the magnetic state of the tip considerably. As a result, contrast may be generated between neighboring domains, or at domain boundary walls or a combination of both, depending on the degree of perturbation of the tip. Examples demonstrating this variation with a series of batch-fabricated tips with various magnetic coatings ranging in coercivity from ∼1400 Oe to Oe are presented. The origin of the different types of observed contrast is discussed, and it is shown that very low coercivity tips coated with Fe nanoparticles embedded in a SiO 2 matrix provide the most readily understood contrast, and have an additional benefit in that they are `self-focusing. That is, the active magnetic volume scales with the characteristic dimension of the sample domain structure. Image interpretation is also discussed in the context of specific MFM images from samples of neodymium iron boron (sintered, melt-quenched and nanocomposite) and barium ferrite.

Magnetic force microscopy images of high-coercivity permanent magnets

Abstract The stray magnetic field distributions at the surfaces of isotropic and anisotropic NdFeB-type magnets have been imaged by magnetic force microscopy. Polished surfaces of bulk samples produced by melt-quenching, hot-rolling, and sintering were examined using two techniques which provided complementary information on the structure of the domains at the surface. For the first technique a high-coercivity magnetic tip was used, and for the second, a soft magnetic tip was used. The structures which have been observed are compared with those reported by other workers using techniques of Lorentz electron microscopy and optical Kerr microscopy and generally appear to be in good agreement. Preliminary analysis of the images collected from the hot-rolled material suggests the presence of circular ‘bubble’ domains, alongside the more regularly observed stripe domains. These appear to be reverse domains, involving only a small amount of material, which have arisen at the exposed surface to minimize the magnetostatic energy at the surface. Imaging of the die-upset material proved to be somewhat more difficult than for the other materials due to strong stray fields at the surface that affected the tips magnetic moment during imaging. This problem was largely overcome by the use of special high-coercivity tips. The results nonetheless are in accordance with recently published Kerr micrographs for the same material.

Investigation of interaction mechanisms in melt‐quenched NdFeB

Investigation of the magnetization processes in melt‐quenched Nd2Fe14B materials has been made by analysis of measurements of the time dependence of the magnetizing curves (after thermal demagnetization) and demagnetizing curves at room temperature. The quantities S (the magnetic viscosity parameter) and χirri (the intrinsic irreversible susceptibility) have been determined for the commercially available materials MQI (isotropic, resin bonded), MQII (slightly anisotropic, hot pressed), and MQIII (anisotropic, hot formed) produced by Delco Remy, U.S. In addition δM(H), which compares the fraction of irreversible processes occurring at a particular field in the magnetizing mode with those occurring at the same field in the demagnetizing mode, has been determined as a function of internal field, and the results are interpreted in terms of interactions between grains. From these results it is shown that although the magnetization and demagnetization behavior of these materials differ substantially, this may b...

A magnetic equation of state

Abstract In general, the intensity of magnetization, M, is dependent on both magnetic field, H, and on time and may be described in terms of a constitutive equation, or equation of state as dH = (1/χ)dM + Λd In M, where 1/χ = ϖH/ϖM‖Ṁ and Λ = ϖH/ϖ In Ṁ‖M. Measurements have been made on Pr15Fe77B8 from which values of Λ and χ as functions of M are derived using the constitutive equation. It is shown that χ is a function of M but is independent of Ṁ. At room temperature Λ = (86.0±1.4) Oe over the magnetization range + 300 to -500 G.

Evolution of magnetic microstructure in high-coercivity permanent magnets imaged with magnetic force microscopy

Magnetic force microscopy (MFM) has been shown to give high-resolution imaging of magnetic domain structures in a variety of high-coercivity permanent magnets [Folks et al., J. Magn. Magn. Mater. (in press)]. We show that this technique can be extended by the application of external fields during imaging, thus allowing direct observation of submicron microstructural evolution as a function of field. Electromagnets mounted on the MFM supplied fields up to 7 kOe laterally and 3 kOe vertically. In sintered materials, submicron processes such as depinning of domain walls at grain boundaries, domain fragmentation, and hysteresis were observed. MFM tips having very low coercivity highlighted domain walls, whereas higher-coercivity tips suffered unpredictable rotation of their magnetic moment due to both the sample and applied fields, leading to images which are difficult to interpret. For imaging of the finer-grained melt-quenched magnets, however, relatively high-coercivity tips were superior. These results sh...

Magnetic properties of novel resin-bonded exchange coupled rare-earth magnets

Abstract The magnetic properties of compaction-molded resin-bonded magnets which exhibit exchange spring behaviour have been measured. The starting compositions of the alloys were Nd4.5Fe73B18.5Co3Ga1 and Nd3.5Dy1Fe73B18.5Co3Ga1 and the preparation method consisted of melt-quenching followed by annealing for a short period. The magnets have remanent magnetisation approximately 70% of saturation magnetisation. The variation of the magnetic properties with temperature in the range 250–320 K has been investigated and magnetic viscosity measurements have been made. These data are compared with those from a conventional resin-bonded NdFeB-type permanent magnet. It has been found that the exchange coupled materials have smaller variations in magnetic properties over the temperature range investigated, and they exhibit less time dependence of magnetisation, compared with the conventional material.

Interpretation of low-coercivity tip response in MFM imaging

Abstract Response of a ‘zero-coercivity’ point dipole to the stray field over a model domain structure is computed. Comparison is made with images of a bulk sample of BaFe acquired with a recently developed ultra-low coercivity sputter-coated tip and standard tips of moderate coercivity. The results of this work are discussed in relation to the interpretation of images in which both low- and high-coercivity tip characteristics are exhibited.

MFM study of NdFeB and NdFeB/Fe/NdFeB thin films

Abstract Domain structures of NdFeB thin films, ranging in thickness between 1500 and 29 nm, have been studied qualitatively by magnetic force microscopy (MFM). Samples were prepared using a range of sputtering conditions resulting in differences in properties such as texture, coercivity and magnetic saturation. MFM images of all the films showed extensive interaction domain structures, similar to those observed in nanocrystalline bulk NdFeB. An exchange-coupled NdFeB/Fe/NdFeB trilayer with layer thicknesses 18 nm/15 nm/18 nm, respectively, was also examined using MFM.

Magnetocaloric dependence of magnetic viscosity measurements in NdFeB

Measurements of the time dependence of magnetization may be analyzed by means of a constitutive equation to give information regarding the fundamental nature of magnetization reversal processes. The quantity Λ (≡ the fluctuation field Hf) may be measured and then related to the activation volumes involved in the magnetization reversal processes. These measurements are commonly made at or near room temperature for hard permanent magnet materials, since this is the usual operating temperature, and the system is assumed to be isothermal. However, the experimental work described shows for these materials the magnetocaloric effect (MCE) cannot be neglected at these temperatures. Magnetic viscosity measurements have been made in two ways which lead to different thermal behavior in the sample. Thermometric measurements were made concurrently with the magnetic viscosity measurements in a sample of sintered NdFeB. The measurements show that the calculated value of Λ can be changed by as much as 10% when isothermal...

Magnetization surfaces in (M, H, Ṁ) space

Abstract Thermal activation of magnetization processes leads to time dependence of intensity of magnetization, M. The description of the equation of state of a magnetized material thus requires the specification of the time rate of change of magnetization, Ṁ, in addition to M and the magnetic field, H. The magnetic state can be derived from a constitutive equation, the solutions of which are represented by surfaces in three-dimensional (M, H, Ṁ) space. Magnetization behaviour, subject to such conditions as constant H, constant Ḣ and constant Ṁ, is given by trajectories on the (M, H, Ṁ) surfaces. A computer generated (M, H, Ṁ) surface derived from experimental data on sintered Pr15Fe77B8 has been calculated. The variations of M vs. H under conditions of constant Ṁ and of constant Ḣ are derived from trajectories on this surface and they are compared with direct experimental data.