Surface magnetism, Morin transition, and magnetic dynamics in antiferromagnetic α-Fe2O3 (hematite) nanograins

Abstract

The grain size of α-Fe2O3 decreases to ∼20 nm by 64 h mechanical milling of the bulk sample. X-ray diffraction pattern suggested identical crystal structure in bulk and mechanical milled samples. Magnetic study (at temperatures of 100–900 K and fields of 0–±15 kOe) showed many interesting features during the decrease in grain size in antiferromagnetic α-Fe2O3, e.g., suppression of Morin transition, enhancement in low temperature magnetization, magnetic blocking at high temperature, exchange bias effect, and unusual relaxation of magnetic spin moment. We understand the results in terms of core-shell spin structure of nanograins, where the core part essentially retained the magnetic structure of the bulk sample and the magnetic structure of the shell part is modified due to grain size reduction and surface modification during mechanical milling. Core-shell structure also plays an important role in exhibiting the increasing soft ferromagnetic character in the present hematite samples. The in field magnetic relaxation at room temperature revealed some interesting properties of the magnetic spin ordering in hematite system.

Synthesizing Iron Oxide Nanostructures: The Polyethylenenemine (PEI) Role

“…Here, the combination of a shallow increase and a brisk settlement could be attributed to the Morin transition, which indicates the passage of hematite from a low-temperature antiferromagnetic to a high-temperature weakly ferromagnetic state. In fact, the low initial value of 250 K and the large spread up to 300 K is consistent with the formation of small-sized (<50 nm) hematite domains, which act as a secondary phase to the main one of magnetite [30]. Conversely, no such a trend can be observed in the FeO_nopei sample, where the disappearance of the Morin transition suggests the formation of hematite domains smaller than 20 nm [31].…”

Doping of Ga in antiferromagnetic semiconductor α-Cr2O3 and its effects on magnetic and electronic properties

“…In the first process, a fraction of spin attempts to restore the original AFM order of the core through magnetic exchange coupling at the interfaces of core and shell. Such spin dynamics in AFM system dominates at low magnetic fields [28][29], showing the increase of freezing temperature The magnetic field dependence of magnetization (M(H)) curves in Fig. 6 (a-c) [2,26,[30][31].…”

“…At higher magnetic field, T m2 of the samples showed a usual decrease with the increase of magnetic field. It seems that two spin relaxation processes control magnetic dynamics in Ga doped samples depending on the competition between magnetic anisotropy and interactions in the AFM system [4,[28][29]. In the first process, a fraction of spin attempts to restore the original AFM order of the core through magnetic exchange coupling at the interfaces of core and shell.…”

Magnetic, dielectric and photo-absorption study of a ferromagnetic semiconductor α-Fe1.4Ga0.6O3

“…6 Similar lattice expansion with decreasing grain size was observed in mechanical milled α-Fe 2 O 3 . 35 The increase of cell parameters in milled samples with milling time up to 100 hours is primarily due to the effect of better alloying of two binary oxides (α-Ga 2 O 3 and α-Fe 2 O 3 ) to attain the structure of α-Fe 2 O 3 phase with higher cell parameters (a = b= 5.0386 Å, c = 13.7498 Å, V = 302.3 Å 3 ). Similar structural modification was also seen in the formation of mechanical milled Cr 1.4 Fe 0.6 O 3 alloy.…”