A. H. Morrish

Noncollinear magnetic structure of CoFe2O4 small particles

The magnetic structure of small CoFe2O4 particles has been investigated as a function of the particle size. Samples (in the 10–100 nm size range and up) were prepared by chemical precipitation followed by a heat treatment at relatively low temperatures. Mossbauer spectra of the 57Fe nuclei, obtained with a longitudinal magnetic applied field, unambiguously establish that a noncollinear structure exists that is most pronounced for the smallest particles. The analysis indicates that a surface effect of the crystallites that make up a particle is the origin of this phenomenon. A model is proposed in which the CoFe2O4 crystallites that make up a particle consist of a core with the usual spin arrangement and a boundary surface layer with atomic moments inclined to the direction of the net magnetization. The temperature dependency of this structure is also examined.

Surface magnetic properties of fine particles

Abstract Fine particles offer an attractive avenue for the study of the magnetic properties of surfaces. The use of small particles instead of thin films has some advantages. Although several experimental techniques can be employed profitably, Mossbauer spectroscopy is emphasized. The application of large magnetic fields has established that a non-collinear magnetic structure occurs in the surface layers of γ-Fe 2 O 3 , NiFe 2 O 4 , and CrO 2 ; the morphology of the particles appears to be important. Particles doped with Mossbauer isotopes, both throughout the volume and preferentially on the surface, provide additional useful information. Interfacial interactions and surface anisotropy are discussed. Some applications are considered.

MOSSBAUER STUDY OF THE PERMANENT-MAGNET MATERIAL ND2(FE1-XNIX)14B

Samples of Nd2(Fe1−xNix)14B, 0≤x≤0.20, with good phase homogeneity have been made by heating under vacuum for 300 h. 57Fe Mossbauer parameters have been determined for all six different iron crystallographic sites at room temperature. The nickel atoms preferentially substitute for iron atoms on the 16k2 and 8j2 sites. The nickel atoms seem to alter the hyperfine parameters of the 57Fe nuclei via localized interactions.

Structural peculiarities in magnetic small particles

Abstract Nanostructured magnetic materials, consisting of nanometer-sized crystallites, are currently a developing subject. Evidence has been accumulating that they possess properties that can differ substantially from those of bulk materials. This paper illustrates how Mossbauer spectroscopy can yield useful information on the structural peculiarities associated with these small particles. As illustrations, metallic iron and iron-oxide systems are considered in detail. The subjects discussed include: (1) phase stabilities in small particles, (2) deformed or nonsymmetrical atomic arrangements in small particles, and (3) peculiar magnetic structures or non-collinear spin arrangements in small magnetic oxide particles that are correlated with lower specific magnetizations as compared to the bulk values.

Mössbauer study of amorphous and nanocrystalline Fe73.5Cu1Nb3Si13.5B9

The superior soft magnetic material, Fe73.5 Cu 1 Nb 3Si13.5B9, formed by partially crystallizing amorphous ribbons upon annealing, has been investigated using x‐ray diffraction and Mossbauer spectroscopy. The microstructure of the annealed ribbons consists mainly of DO3 α‐FeSi nanocrystallites and an amorphous phase that lies in the grain boundaries. This residual amorphous phase has a Curie temperature of about 600 K; when it becomes a paramagnet the coercivity of the ribbon increases dramatically. It follows that, in addition to the nanocrystallite grain size and the random anisotropy, the coupling between the nanocrystallites by the amorphous grain boundaries is important for the achievement of the excellent soft properties. When Cu is replaced by Ag or Ni, a higher annealing temperature is required to produce nanocrystals.

Mössbauer study of magnetic anisotropy in amorphous Fe40Ni38Mo4B18 (METGLAS® 2628MB)

Abstract The room-temperature anisotropy of amorphous Fe40Ni38Mo4B18 (METGLAS® 2628MB) ribbons after various heat treatments has been studied by Mossbauer spectroscopy. The average magnetization direction becomes significantly more out-of-plane after heating above 650 K but below the crystallization temperature. X-ray diffractograms suggest an atomic rearrangement has occurred.

On the spin collinearity in small iron particles

Abstract In contrast to earlier results for γ-Fe 2 O 3 , the magnetic structure, as determined by Mossbauer spectroscopy, is found to be collinear for small iron particles. Differences in the exchange-coupling mechanism are suggested to be responsible.

Morphology and Physical Properties of Gamma Iron Oxide

Gamma ferric oxide, γ-Fe2O3, is still the most widely used material for magnetic recording devices. Although there are literature references dating in the 19th century, the material was virtually unknown until systematic studies were commenced about 1925. Methods to prepare small, polycrystalline particles of various shapes, both unsupported and supported on iron metal, as well as epitaxial single-crystal films, are indicated. Data on the crystallographic and magnetic structure and other physical properties are summarized. The transformations from magnetite to gamma ferric oxide to hematite are treated. Finally, γ-Fe2O3 with other cation substitutions is considered. Some directions where further research is needed are suggested.

61Ni Mössbauer spectra of fine Ni particles

We report the first measurements of fine Ni particles from61Ni Mössbauer spectroscopy. The average hyperfine field of the 10 nm particles at 4.2K is measured to be 7.7(4) T, compared to the measured value for Ni foil of 7.5(3) T. Application of an external field of 6 T to the fine particles causes a reduction of the hyperfine splitting to 1.5(6) T, a consequence of the negative hyperfine field at Ni nuclei. These results are discussed in terms of fine particle effects.

Crystallization and anisotropy in amorphous Fe81B13.5Si3.5C2

Abstract Magnetic anisotropy in amorphous Fe 81 B 13.5 Si 3.5 C 2 ribbons (Metglas ® 2605SC) that develops after thermal treatments is correlated with the crystallization at the surface layers and in the bulk.