Publisher Summary This chapter provides an overview of the magnetoelastic properties of the highly magnetostrictive rare earth-Fe2 alloys. The chapter describes a general treatment of magnetostriction for the cases of hexagonal and cubic symmetry, which is applicable to the rare earth elements and the rare earth-iron compounds. The chapter presents the magnetostriction of binary rare earth-iron alloys and the magnetostriction of single crystal and polycrystal RFe2 compounds are compared to other magnetostrictive materials at room temperature. The chapter discusses a possible source of startling magnetostriction anisotropy, measurements of magnetization, sublattice magnetization, and magnetic anisotropy, and the role of intrinsic as well as extrinsic effects. It reports the effects of the strong magnetoelastic coupling on sound velocities and elastic moduli and observes extraordinarily large ∆E effects and changes in sound velocity in single crystals, polycrystals, and amorphous rare earth-Fe2 alloys. The chapter concludes with a discussion of the recent measurements of linear and volume magnetostriction on the amorphous form of the RFe2 alloys.

Chapter 7 Magnetostrictive rare earth-Fe2 compounds

By the early 1960’s, it was widely recognized that the rare earths possessed many extraordinary magnetic properties. Neutron diffraction measurements, for example, showed that the spin structures were much more complex than those of any of the classical ferromagnets or antiferromagnets. More importantly, in the heavy rare earth metals, the parallel coupling of large orbital and large spin angular momenta yielded huge magnetic moments of 9μ B and 10μ B, dwarfing the conventional values of 0.6 for Ni and 2.2 for Fe. Enormous magnetic anisotropies were also encountered in the heavy rare earth elements. In 1963, a breakthrough in magnetostrictive materials occurred with the measurement of the basal plane magnetostrictions of Tb and Dy at low temperatures (Legvold et al. 1963, Clark et al. 1963, 1965, Rhyne and Legvold 1965). These basal plane strains are 100 to 10000 times typical magnetostrictions and still remain today the largest known (~1%). Over wide temperature ranges, thermal expansions are dominated by the temperature dependences of the magnetostrains. Elastic moduli were found to be strongly influenced by the unprecedented magnetoelastic interactions. However, because of the low ordering temperatures of the rare earths the application of these magnetostrictive properties to devices operating at room temperature could not be achieved with the elements. Only Gd, which is essentially non-magnetostrictive, possesses a Curie point as high as room temperature.

Magnetostrictive Rare Earth-Fe2 Compounds

The interesting physics and potential memory technologies resulting from topologically protected spin textures such as skyrmions have prompted efforts to discover new material systems that can host these kinds of magnetic structures. Here, we use the highly tunable magnetic properties of amorphous Fe/Gd multilayer films to explore the magnetic properties that lead to dipole-stabilized skyrmions and skyrmion lattices that form from the competition of dipolar field and exchange energy. Using both real space imaging and reciprocal space scattering techniques, we determined the range of material properties and magnetic fields where skyrmions form. Micromagnetic modeling closely matches our observation of small skyrmion features (\ensuremath{\sim}50 to 70 nm) and suggests that these classes of skyrmions have a rich domain structure that is Bloch-like in the center of the film and more N\'eel-like towards each surface. Our results provide a pathway to engineer the formation and controllability of dipole skyrmion phases in a thin film geometry at different temperatures and magnetic fields.

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Tailoring magnetic energies to form dipole skyrmions and skyrmion lattices

A new concept is proposed for the microactuation based upon magnetostriction. Magnetostrictive bimorph cantilever actuators and a traveling machine, composed of the magnetostrictive amorphous Tb‐Fe and Sm‐Fe thin films on a polyimide substrate, were fabricated. These actuators moved without power supply cables. The 3‐mm‐long cantilever actuator exhibited the large deflection above 100 μm in as low a magnetic field as 300 Oe and above 500 μm at resonant frequency in an alternating magnetic field of 300 Oe. Such unique characteristics suggest that magnetostriction is useful as the driving force of the microactuator.

Fabrication of magnetostrictive actuators using rare‐earth (Tb,Sm)‐Fe thin films (invited)

Structural, magnetic and dielectric properties of Co-Zr substituted M-type calcium hexagonal ferrite nanoparticles in the presence of α-Fe2O3 phase

A systematic investigation was made of magnetostriction and magnetization in amorphous thin films (∼1 micron) of TbxFe1−x with x=0 to 0.5 prepared by electron‐beam co‐evaporation. The amorphous or crystalline character of the films was determined using x‐ray and Mossbauer techniques. Net magnetization, coercivity, and the initial in‐plane and out‐of‐plane distribution of magnetization was determined using a vibrating sample magnetometer. Both the sign and magnitude of the magnetostriction was measured as a function of magnetic field using a capacitance method. Over most of the concentration range the amorphous samples have a positive magnetostriction with a maximum of Δl/l=285×10−6 at x=0.4 and going to zero at the compensation point (x=0.22) and near x=0.5 where the amorphous alloys are no longer magnetically ordered at room temperature. In the region x=0.1 to 0.2 the magnetostriction is more complex, being negative for fields up to 5 kOe and positive at higher fields. All the films studied with x?0.1 we...

Magnetostriction of amorphous TbxFe1−x thin films

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...

Amorphous YFe2—A concentrated spin glass

Magnetic properties of substituted M-type hexagonal ferrites

Microwave magnetic properties of gadolinium-iron films

A novel film synthesis procedure is used to explore the amounts of hydrogen absorbed by amorphous a−R1−xFex alloys with 0≤x≤0.9 and R=Gd, Dy, Tb, Ho, Y. Magnetization and Mossbauer measurements on a‐Gd1−xFex: Hy are illustrative of the dramatic magnetic changes produced by the hydrogen. For a‐Gd1−xFex: H3 the compensation temperatures are reduced by about 200 K and the Curie temperatures by 50 K. Detailed analysis of magnetization M(T) data for a‐GdFe2: Hy with y=0, 1.25, and 3.0 are presented to demonstrate the changes of magnetic moments and exchange parameters. JFe‐Fe and JFe‐Gd exhibit small percentage decreases with added hydrogen whereas JGd‐Gd has a large change, going from ferromagnetic (y=0) to antiferromagnetic coupling (y=3). The iron moment increases by about 10%. The magnetic changes are interpreted in terms of charge transfer as mediated by anionic behavior of the hydrogen.

J. H. Schelleng

Papers

Magnetostriction of amorphous TbxFe1−x thin films

Amorphous YFe2—A concentrated spin glass

Magnetic properties of substituted M-type hexagonal ferrites

Microwave magnetic properties of gadolinium-iron films

Hydrogenation and magnetic properties of amorphous rare-earth-iron (R-Fe) alloys