Training effects in Gd5Ge4: role of microstructure

Abstract

Detailed optical metallographic studies at room temperature on polycrystallineGd5Ge4 are presented for two cases: (a) a sample in the as-cast condition and (b) the samesample subjected to a known number of temperature and magnetic field cycles. Aherringbone- (criss-cross-) like pattern whose characteristic feature size extends to≈20 µm is observed in the as-cast sample. The herringbone pattern at room temperature isinterpreted as arising due to a long-lived (kinetically arrested) metastable phase. Thisherringbone pattern can be trained to form a pattern having a different morphology bycycling the sample through the low-temperature field induced magneto-structuraltransition. Training effects are also observed in magneto-transport and magnetizationmeasurements as the sample is subjected to temperature and field cycles. Based onheuristic arguments, a model is proposed which self-consistently explains anomalies intransport and magnetization properties in terms of changes occurring in the microstructureof the sample. These results highlight the need for analysing the interesting properties ofGd5Ge4 and its family of alloys with microstructure as an important component.