Bulk Modulus Anomalies of Fe-Ni and Fe-Pt Invar Alloys

The bulk moduli of Fe 70 Ni 30 , Fe 65 Ni 35 and Fe 72 Pt 28 Invar alloys are obtained by measuring the lattice constants under high pressure up to about 100 kbar using X-ray diffraction technique. Large changes in bulk modulus are observed at ferromagnetic to paramagnetic phase transition. The temperature dependence of bulk modulus shows normal behavior both in the ferromagnetic and in the paramagnetic phases of Fe-Ni Invar alloys but shows anomalous behavior in the ferromagnetic phases of Fe 72 Pt 28 alloy. The different behaviors are discussed in connection with thermodynamical relation and the recent theoretical and experimental results.