N. C. Koon

Giant magnetostriction materials

Abstract One of the significant technical developments in magnetism of the early 1970s was the discovery of a new class of rare earth intermetallic compounds, the RFe 2 Laves phases, which were found to exhibit room temperature magnetostrictive strains approaching 2 × 10 −3 , an order of magnitude larger than any previously known. Since that time both the fundamental and technical properties of these materials have been of intense interest, and they remain the subject of active research even today. The large strains available are useful in such applications as production of high amplitude, low frequency sound waves in water, certain types of strain gages, vibration compensation and compensation for temperature induced strains in large laser mirrors. Because the performance of these materials depends critically on such fundamental properties as the magnetic anisotropy, magnetization and grain orientation of the material, there has been a very strong interplay between fundamental studies and applications. In this article we briefly review the fundamental magnetic and magnetostrictive properties of the RFe 2 Laves phases, focusing especially on the complex behavior of the anisotropy and the success of crystal field theory in explaining it. We also present neutron measurements of magnetic excitation spectra and explain how they provide an understanding of the remarkable success of mean field theory for these systems.

Crystallization of FeB alloys with rare earths to produce hard magnetic materials (invited)

Among the first crystallized melt quenched alloys to exhibit interesting hard magnetic properties was (Fe0.82B0.18)0.9Tb0.05La0.05. The high coercive force of this alloy results from a mixture of R6Fe23 and Fe3B phases with a very fine grained microstructure (∼300 A). The hard magnetic state only exists over a narrow range of annealing temperatures due to phase decomposition and grain growth at higher temperatures. Recent results on alloys with compositions corresponding to varying ratios of R6Fe23 and Fe3B have suggested a new magnetically hard ternary phase near the composition R0.15Fe0.783B0.067. In contrast to earlier alloys, the hard magnetic state of this material remains essentially unchanged over a rather wide range of annealing temperatures up to at least 800 °C. When R is mostly Nd or Pr, isotropic magnets with energy products as high as 13 MGOe can be produced.

Magnetic properties of R2Fe14B single crystals

A whole new class of high‐performance permanent magnet materials is based on the ternary tetragonal structure R2Fe14B, where R is one of the rare‐earth elements. We have successfully grown single crystals of this structure with R=Y, Nd, and Tb. Y is a nonmagnetic rare‐earth substitute, while Nd and Tb couple ferro‐ and ferrimagnetically, respectively, relative to the iron moment. All three of the compounds have [001] easy axes at room temperature, although the Nd compound exhibits a spin reorientation away from the [001] below about 150 K. Nd2Fe14B has a saturation induction at room temperature of 16.2 kG, which places an upper limit of approximately 65.6 MGOe on the energy product obtainable by magnets based on that material. While Tb2Fe14B exhibits a smaller magnetization because of ferrimagnetic coupling of the rare earths and the iron, it also has an extremely large magnetic anisotropy which is nearly temperature independent between 4.2 and 300 K.

Barium ferrite thin-film recording media

Abstract Both longitudinal and perpendicular barium ferrite thin films are being pursued as overcoatless magnetic recording media. In this paper, prior research on thin-film Ba ferrite is reviewed and the most recent results are presented. Self-textured high-coercivity longitudinal Ba ferrite thin films have been achieved using conventional rf diode sputtering. Microstructural studies show that c -axis in-plane oriented grains have a characteristic acicular shape, while c -axis perpendicularly oriented grains have a platelet shape. Extended X-ray absorption fine structure (EXAFS) measurements indicate that the crystal orientations are predetermined by the structural anisotropy in the as-sputtered ‘amorphous’ state. Recording tests on 1500 Oe coercivity longitudinal Ba ferrite disks show performance comparable with that of a 1900 Oe Co alloy disk. To further improve the recording performance, both grain size and aspect ratio need to be reduced. Initial tribological tests indicate high hardness of Ba ferrite thin films. However, surface roughness needs to be reduced. For future ultrahigh-density contact recording, it is believed that perpendicular recording may be used. A thin Pt underlayer has been found to be capable of producing Ba ferrite thin films with excellent c -axis perpendicular orientation.

Cation distribution in NiZn-ferrite films via extended x-ray absorption fine structure

We have applied extended x‐ray absorption fine structure (EXAFS) spectroscopy to study the cation distribution in a series of spin‐sprayed NiZn‐ferrite films. A least‐squares fitting of experimental EXAFS data with theoretical, multiple‐scattering, EXAFS data allowed the quantitative determination of site distributions for all transition metal cations.

Direct measurement of octahedral and tetrahedral site environments in NiZn-ferrites

The local environment of metal ions in a series of spin-sprayed Ni/sub 0.15/Zn/sub y/Fe/sub 2.85-y/O/sub 4/ (y=0.16, 0.23, 0.40, 0.60) films have been studied using extended X-ray absorption fine structure (EXAFS). Fourier transforms of the EXAFS data show peaks which are identified with specific local atom correlations within the spinel ferrite unit cell. From the data the relative occupancy of the cations on the octahedral and tetrahedral sites are determined. We find that Ni ions occupy the octahedral sites, while the Zn ions occupy the tetrahedral sites. Fe cations occupy both sites in a ratio determined by the Zn/Fe ratio. The valence distribution of Fe ions on these sites is consistent with that of an inverted spinel. The saturation magnetization varies with increasing Zn/Fe ratio, first increasing due to the imbalance on the Fe spin lattice and later decreasing due to the disruption of the A-B superexchange.

Phase-separated Fe and Co particles in a BN matrix

Using co‐sputtering, 10–300 A particles of Fe and Co have been prepared in an insulating BN matrix. The Fe particles have the α‐Fe structure. The saturation magnetic moment of the iron particles per at. % Fe was found to be approximately independent of the Fe concentration and equal to the value of α‐Fe. The system undergoes a metal–nonmetal transition at approximately 40 vol % Fe. For concentrations of Fe particles above this threshold the temperature dependence of the resistivity is metallic and the room‐temperature coercivity is large (50–100 Oe). For the Co particles, the room‐temperature coercivity is about twice as large as Fe above the metal–nonmetal threshold. Below the metal–nonmetal threshold the particles behave as superparamagnets and the coercivity is approximately zero.

Hyperfine fields and spin orientations in (Fe57/Ag) superlattices from conversion electron Mössbauer studies (invited)

Conversion electron Mossbauer studies have been carried out as a function of temperature on a series of single‐crystal [(100)57Fe/(100)Ag] superlattices grown by MBE on GaAs substrates. The 57Fe layer thickness was varied from approximately 1 to 5.5 ML. From the relative intensity of the Mossbauer lines we show conclusively that the orientation of the magnetic moment for films 2.4 ML and thinner in zero applied field is perpendicular to the film plane at low temperatures. For a 5.5‐ML film the spin alignment at room temperature is in the film plane and partially out of the plane at low temperatures. The low‐temperature magnetic hyperfine fields are enhanced relative to bulk Fe.

Amorphous YFe2—A concentrated spin glass

Magnetic susceptibility, Mossbauer, and neutron scattering studies of the same alloy sample all indicate that amorphous YFe2 (a‐YFe2) exhibits spin‐glass behavior with a truly thermodynamic spin‐glass transition at TS.G.=58K. These studies show that spin‐glass behavior can persist to quite high magnetic concentrations in an amorphous alloy system which is dominated by competing ferromagnetic and antiferromagnetic exchange interactions. At a lower temperature of T?20K, the susceptibility versus T curve shows a break and the coercive field is anomalous. We interpret this phenomenon near T=20K as arising from magnetic ’’blocking’’ of individual spins or regions of correlated spins which persist below TS.G.. This produces a cluster‐freezing‐type ’’pseudo‐transition’’ which is merely a manifestation of the slowing down of spin fluctuations to a time interval comparable with the experimental susceptibility measurement time. The temperature dependence of the spin‐glass order parameter is obtained from the suscep...

Mössbauer evidence of spin reorientation in single crystal Nd2Fe14B

Abstract Using 57 Fe Mossbauer spectroscopy, we have measured the canting angle of the iron spins in Nd 2 Fe 14 B. Our measurements are made on a single crystal sample cut with its c -axis normal to the sample face. The iron spins rotate away from the c -axis by about 27° at 4.2 K.