Small thermal expansion (α ∼ 1.19 × 10 −6 /K) of Fe 65 Ni 35 disordered alloy appears within a wide temperature range up to the Curie temperature ( T C ∼ 505 K), which is widely known as the Invar effect. The magnetovolume effect at Fe 65 Ni 35 gives rise to large volume contraction with increasing temperature and reducing magnetization, which compensates the conventional thermal expansion and consequently results in the zero-thermal expansion. On the other hand, Fe-Ni alloys with larger Ni content rapidly recover the thermal expansion of normal metal with a typical value of α ∼ 10 −5 /K and higher T C . Why does the Invar effect appear in a narrow Ni content around 35 at.% of the Fe-Ni alloys?
In this study, the atomic scale origin of the invar effect and the large magnetovolume effect in Fe-Ni alloys is revisited by investigating the Ni-content dependence of bulk modulus B 0 of Fe-Fe, Fe-Ni, and Ni-Ni atomic pairs using reverse Monte Carlo modeling with a dataset of extended x-ray absorption fine structure and powder x-ray diffraction under high pressure. To this end, the results of the Fe 65 Ni 35 invar alloy were compared with those of Ni-rich ferromagnetic Fe 55 Ni 45 and Fe 20 Ni 80 disordered alloys.
The elongation of Fe-Fe pairs compared to the bond lengths of Fe-Ni and Ni-Ni pairs is commonly observed in the ferromagnetic states of all Fe-Ni alloys. The B 0 value of each atomic pair is comparable to the value of bulk B 0 when the ferromagnetic phase is stable under high pressure, while the rapid shrinkage of the elongating Fe-Fe pairs results in a smaller value of B 0 at the destabilized ferromagnetic region. Therefore, the magnetovolume effect in Fe-Ni alloys is interpreted as a result of the elongating Fe-Fe pairs and their population in the alloy. The elongation and subsequent shrinkage of Fe-Fe pairs at the destabilization of the ferromagnetic order plays a crucial role for initiating the invar effect. We consider that the balance between the number of elongating Fe-Fe pairs and Curie temperature as a function of Ni content determines the appearance of the Invar effect.
Physical Review B