Magnetic Critical-Point Behavior of CrO2

Abstract

The magnetization σ of the ferromagnetic compound CrO₂ was measured as a function of field H and temperature T near the Curie point T_c. From isotherms of σ² vs H/σ, the initial susceptibility χ₀ above T_c was obtained, which when tested against the relationship χ₀⁻¹ ∝ (T−T_c)^γ gives a constant γ of 1.63±0.02 from just above T_c (386.5°K) up to about 1.15 T_c. This γ value contrasts with the values near$\frac{4}{3}$ recently computed for the Heisenberg model and later found experimentally in various ferromagnetic metals and compounds. At higher temperatures the effective γ decreases rapidly towards unity. Up to the highest field used (25 kOe), the critical isotherm obeys the relationship σ∝H^1/δ with δ=5.75±0.05, which differs markedly from the theoretical δ values of 3 (molecular field) and 5.2 (3‐dimensional Ising) and from various experimental values. Gradual departure from this relationship below 1.5 kOe is attributed to the magnetocrystalline anisotropy that persists at T_c. Furthermore, we find that all the σ(H, T) data for CrO₂ just above T_c can be represented by a universal function of the form, σ/σ′=f(H/H′), in which σ′ ∝ (T−T_c)^λ and H′ ∝ (T−T_c)^λ+γ, where λ=0.34. This ``corresponding states'' representation is the exact magnetic analog of an equation of state recently proposed by Widom for a fluid near its critical point.