Magnetic force microscopy characterization of a first-order transition:: Magnetic-martensitic phase transformation in Gd5(SixGe1-x)4

The magnetic-martensitic phase transformation of Gd-5(SixGe1-x)(4) (xapproximate to0.5), which occurs close to room temperature, has been observed for the first time using a magnetic force microscope (MFM) equipped with a heating-cooling stage. MFM images obtained from a polycrystalline Gd-5(Si2.09Ge1.91) sample and single crystal Gd-5(Si1.95Ge2.05) and Gd-5(Si2Ge2) samples showed transition to a domain structure at low temperatures indicative of a ferromagnetic phase. Some samples exhibited complex domain structures, suggesting that Gd-5(SixGe1-x)(4) (xapproximate to0.5) has a strong magnetic anisotropy. As the sample temperature increased the domain structure diminished, reflecting the transformation from ferromagnetic to paramagnetic state. On cooling the sample the domain structure reappeared, but at a lower transformation temperature than on heating. This magnetic phase transformation is highly unusual because it is an "order-disorder" phase transition, which is normally second order, but in this case the "order-disorder" (ferromagnetic to paramagnetic) transition exhibits hysteresis in temperature, indicating that it is first order. Such thermal hysteresis in phase transformation was also observed in thermal expansion experiments. The transformation temperatures obtained in the MFM study are in good agreement with those determined from thermal expansion data. (C) 2002 American Institute of Physics.