Magnetic and Elastic Properties of Magnetostrictive Tb6Fe1-xCoxBi2

The Tb6[6 sub]Fe1-x[1-x sub]Cox[x sub]Bi2[2 sub] (x = 0, 0.125, 0.25, and 0.375) family of compounds has been synthesized and investigated for magnetostrictive behavior using capacitance dilatometry. These measurements have been complemented by measurements of the elastic moduli using Resonant Ultrasound Spectroscopy (RUS), magnetization measurements (with a vibrating sample magnetometer (VSM)), as well as powder X-ray diffraction (XRD) and powder neutron diffraction measurements and heat capacity measurements.

Each of the compounds studied shows little magnetostriction at the Curie temperature but shows pronounced amounts of magnetostriction at temperatures below 100 K. Magnetostriction values of up to ~980 ppm have been discovered at low-temperatures. This increase of magnetostriction coincides with a structural transition at T*~ 50 K of hexagonal to triclinic (for x = 0) or monoclinic (for x = 0.125, 0.25, and 0.375) discovered using RUS and low-temperature XRD.

Our data show that the magnetic model of ferromagnetic terbium moments, with no contribution from iron, put forth in the literature is incomplete. Below the Curie temperature, from approximately 250 K to 50 K, there exists a ferrimagnetic ordering between the terbium and iron moments. Upon transitioning to the lower-symmetry structure below T*, the magnetic moments of each of the three atomic sites cant away from the c-axis by differing amounts.

It is posited here that the large degrees of magnetostriction are due to the softening of the elastic moduli caused by the structural transition. Increasing the cobalt content leads to further elastic softening and magnetostriction although the structural transition remains similar for each Co-doped compound. The difference in elastic softening and magnetostriction is therefore most likely due to the increased disorder caused by the cobalt atoms.

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