Synthesis, structure and magnetic properties of non-crystalline ferrihydrite nanoflakes

Synthesis and characterization of the structural and magnetic properties of a 2-line (2L) ferrihydrite (FHYD) sample based on the composition Fe:Al:Cu=100:25:5 are reported. Typical of 2L-FHYDs, this sample also yields the two broad lines in X-ray diffraction and triplet in the 1400–1700 cm−1 range in IR spectroscopy. However, in transmission electron microscopy, nanoflakes of about 5–20 nm size but without any hint of diffraction fringes characteristic of crystalline order were observed. Temperature dependence (2–380 K) of the magnetization M vs. applied field H (up to ±65 kOe) of this non-crystalline ferrihydrite is used to establish a blocking temperature TB≃20 K, Néel temperature TN≃365 K and a spin-glass ordering of the surface Fe3+ spins at TS≃6 K. These magnitudes of TB and TS are considerably smaller than those of a 5 nm undoped 2L crystalline ferrihydrite with TB=70 K and TS=30 K. The fit of the M vs. H data for several T>TB to a modified Langevin function is shown to collapse onto a universal curve yielding a temperature-independent average magnetic moment μP=70(5)μB per nanoflake. Analysis of these parameters obtained from the fits of M vs. H data above TB is used to show that the effective average volume of the nanoflakes is about 1/3 that of spherical 5 nm crystalline 2L-FHYD. It is argued that these lower magnitudes of μP, TB, and TS for the nanoflakes result from their smaller effective volume determined here.