Stress-induced large Curie temperature enhancement in ${\text{Fe}}_{64}{\text{Ni}}_{36}$ Invar alloy

We have succeeded in increasing up to 150 K the Curie temperature in the ${\text{Fe}}_{64}{\text{Ni}}_{36}$ invar alloy by means of a severe mechanical treatment followed by a heating up to 1073 K. The invar behavior is still present as revealed by the combination of magnetic measurements with neutron and x-ray techniques under extreme conditions, such as high temperature and high pressure. The proposed explanation is based in a selective induced microstrain around the Fe atoms, which causes a slight increase in the Fe-Fe interatomic distances, thus reinforcing ferromagnetic interactions due to the strong magnetoelastic coupling in these invar compounds.

Figure 1

(Color online) Fitted powder neutron diffraction patterns for standard (upper panel) and MS (lower panel) ${\text{Fe}}_{64}{\text{Ni}}_{36}$ samples. Observed $(+)$ and calculated (solid line) patterns; positions of the Bragg reflections are represented by vertical bars. The observed-calculated difference is displayed at the bottom of each panel. The inset is a magnification of the (111) reflection.

Figure 2

(Color online) Magnetization vs temperature for the standard (red solid circles) and MS (blue solid circles) ${\text{Fe}}_{64}{\text{Ni}}_{36}$ samples measured under an applied magnetic field of 6 kOe. Note the increase in $T_C$ from the standard sample to the MS one. Inset: Temperature dependence of the reciprocal magnetic susceptibility, ${\chi }^{-1}$, showing a Curie-Weiss behavior near $T_C$. ${\theta }_P$ is the paramagnetic Curie temperature with values of 490 and 620 K for the standard and MS samples respectively.

Figure 3

(Color online) Normalized magnetization vs reduced temperature for the standard (red solid circles) and MS (blue open squares) ${\text{Fe}}_{64}{\text{Ni}}_{36}$ samples measured under an applied magnetic field of 300 Oe. The inset shows the M(H) curves measured at $T=5$ and 300 K.

Figure 4

(Color online) Temperature dependence of the fcc cubic lattice parameter, $a$, derived from time-of-flight powder neutron diffraction measurements, for standard (red solid circles) and MS (blue solid circles) ${\text{Fe}}_{64}{\text{Ni}}_{36}$ samples. The inset shows the high temperature dependence of the lattice parameter for both samples.

Figure 5

(Color online) Pressure dependence of the lattice parameter for standard (red solid circles) and MS (blue solid squares) ${\text{Fe}}_{64}{\text{Ni}}_{36}$ samples measured at room temperature. The solid lines correspond to a fit to a first-order Murnaghan equation of state for the mixture of the $\text{HS}+\text{LS}$ states and the pure LS state. The $\text{HS}+\text{LS}$, LS, and NM regions are shaded in light blue, light green and orange colors (see text for more details).

Figure 6

(Color online) Pressure dependence of the peak-to-peak XMCD signal at the Fe K-edge for standard (red solid circles) and MS (blue solid circles) ${\text{Fe}}_{64}{\text{Ni}}_{36}$ samples measured at room temperature. The solid line is a guide for the eyes. The inset shows the pressure dependence of the Curie temperature of both materials (see text for more details). The data of the ${\text{Fe}}_{68.1}{\text{Ni}}_{31.9}$ Invar alloy are taken from the reference.[45]