Magnetocrystalline Anisotropy, Magnetic Force Microscopy, Thermal Expansion and
Magnetostriction of Gd5(SixGe1-x)4 Materials

Personnel: D.C. Jiles (Senior Physicist, J. Leib, (Graduate Student), C.C.H. Lo (Associate Scientist) and J.E. Snyder (Associate Scientist).

Abstract:
The objective of this research is to conduct systematic experimental studies of the magnetic properties of Gd5(SixGe1-x)4 materials to achieve understanding of the underlying electronic structure and the interactions bringing about extremely strong coupling of the magnetic moments with the lattice. These materials are of specific interest, due to a giant magnetocaloric effect, giant magnetostriction, and giant magnetoresistance. As the magnitude of the magnetic phenomena present in these materials is greatest near a magnetic-martensitic phase transformation which can occur near room temperature, much research has centered on trying to understand the nature of this transition. A secondary objective is the development and validation of a phemenological model of the transformation, which will allow the design of novel material systems exhibiting extremely large responses to small changes of magnetic field, temperature, and pressure.

Recent Results:
Magnetic force microscopy and anisotropy measurements have been carried out using both single crystal and polycrystalline samples, from the region of Gd5(SixGe1-x)4 system where x ~ 0.5. Magnetic susceptibility and magnetization data were obtained to probe the type of magnetic ordering, to establish Curie and/or Neel temperatures, to determine magnetic moments in both paramagnetic and ordered states, and to probe magnetocrystalline anisotropy.

Figure 1. Magnetic force microscopy images of single crystal Gd5(Si,Ge)4 looking along the a, b, and c axes. The domain structures along these directions are seen to be radically different.

High quality single crystals of R5(SixGe1-x)4 were used to conduct anisotropic physical property measurements and neutron scattering studies to determine the magnetic structures and phonon spectra which provide a knowledge base for the complete understanding of the electronic and lattice properties of this novel family of intermetallic compounds. The domain structures of single-crystal surfaces cut perpendicular to the three principal axes were examined by magnetic force microscopy (MFM) utilizing new capabilities of in-situ applied magnetic field and controlled temperature. MFM observations of first order phase transformation from ferromagnetic to paramagnetic states and the reverse were made. Magnetization curves were measured along the principal axes and used for determination of magnetocrystalline anisotropy coefficients of the material. These results suggested that the easy axis is the b-axis and that the strength of the magnetic anisotropy is comparable to that of single-crystal iron.

Figure 2. Magnetization curves along the a, b, and c axes, showing that the b axis is the magnetic easy axis.

Thermal expansion results were used to examine both hysteresis in the order-disorder transformation and to study the change of transformation temperature under the action of a magnetic field. The field-induced shift of transition temperature was found to be a simple linear function of the field strength.

Significance:
Progress has been made toward understanding of the very complex transition - involving significant magnetic and structural transformations, triggered either by thermal or magnetic field energy - in a complex system of materials. Comprehension of the mechanisms involved is critical to understanding quite useful magnetic phenomena that are also present near the transition.

Future Work:
The governing equations for equilibrium between magnetic energy and thermal energies in phase transition temperatures will be determined as thermal expansion and magnetic force microscopy measurements are continue on single crystal Gd5(SixGe1-x)4 samples of different compositions and crystallographic orientations. This data, equation set, numerical results of transition temperature measurements, bulk magnetic measurements, magnetic anisotropy measurements and MFM imagery of surface domain structure will be used to form a phemenological model of the transition.

Interactions:
Cooperation continues with K.A. Gschneidner, Jr., V.K. Pecharsky, V.P. Antropov, B.N. Harmon, L.S. Chumbley, G.J. Miller, and C. Stassis of Ames Laboratory on characterization and modeling efforts on these materials systems. Samples were prepared by T.A. Lograsso and D.L. Schlagel, also of Ames Laboratory.

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