Linear and nonlinear elastic properties of antiferromagnetic γ-manganese-nickel alloys

Abstract

The linear and nonlinear elastic properties of single crystals of f.c.c. γ-manganese-nickel alloys with compositions Mn₇₃Ni₂₇, Mn_80.5Ni_19.5 and Mn₈₄Ni₁₆ have been studied using ultrasonic pulse-echo techniques. The elastic stiffness tensor components of Mn₇₃Ni₂₇ have been determined as a function of temperature and pressure on both sides of the Nee1 temperature. The longitudinal moduli c ₁₁, and c _L(= (c ₁₁ + c ₂₂ + 2c ₄₄)/2) and the bulk modulus B ^S _O are larger in the antiferromagnetic than in the paramagnetic state; the reverse is true for the shear stiffness c ₄₄ and c′(= (c ₁₁ - c ₁₂)/2). The hydrostatic pressure derivatives (∂c _IJ/∂P) _{P= 0}, of the elastic stiffness tensor components have been measured in both the antiferromagnetic and paramagnetic states of Mn₇₃Ni₂₇. In the antiferromagnetic state (∂c ₁₁/∂P) _P=0, and (∂c′/∂P) _P=0, increase with temperature, while (∂c ₄₄/∂P) _P=0 decreases; in the paramagnetic state (∂c ₁₁/∂P) _{P = 0} and in consequence (∂B ^S _O/∂P) _P=0 are much smaller. Large differences are found between the mode Grüneisen gammas of the longitudinal acoustic modes in the antiferromagnetic and paramagnetic states; the shear modes are less affected. From hydrostatic and uniaxial pressure effects on the velocities of ultrasonic modes, the six third-order elastic stiffness tensor components of Mn_80.5Ni_19.5 have been obtained at room temperature; c ₁₁₁ is the largest, indicating the predominance of contributions from the nearest-neighbour repulsive forces to the higher-order elastic effects.