The investigation of giant magnetic moment in ultrathin Fe3O4 films

Abstract

The magnetic and transport properties of Fe3O4 films with a series of thicknesses are investigated. For the films with thickness below 15 nm, the saturation magnetization (Ms) increases and the coercivity decreases with the decrease in films’ thickness. The Ms of 3 nm Fe3O4 film is dramatically increased to 1017 emu/cm3. As for films’ thickness more than 15 nm, Ms is tending to be close to the Fe3O4 bulk value. Furthermore, the Verwey transition temperature (Tv) is visible for all the films, but suppressed for 3 nm film. We also find that the ρ of 3 nm film is the highest of all the films. The suppressed Tv and high ρ may be related to the islands morphology in 3 nm film. To study the structure, magnetic, and transport properties of the Fe3O4 films, we propose that the giant magnetic moment most likely comes from the spin of Fe ions in the tetrahedron site switching parallel to the Fe ions in the octahedron site at the surface, interface, and grain boundaries. The above results are of great significance and also provide a promising future for either device applications or fundamental research.

Deterministic, Reversible, and Nonvolatile Low-Voltage Writing of Magnetic Domains in Epitaxial BaTiO3/Fe3O4 Heterostructure

“…Local magnetic configuration (Figure b) reveals an antiferromagnetic (AFM) coupling of Fe­( T d )-O-Fe­( O h ) due to superexchange (SE) interaction between T d and O h sites as well as double-exchange (DE) ferromagnetic (FM) coupling within O h sites due to electron exchange between Fe 2+ and Fe 3+ . − When Fe 3 O 4 is fabricated on top of BaTiO 3 with downward polarization in high vacuum, positively charged oxygen vacancies are inevitable, and they will be attracted toward the BaTiO 3 /Fe 3 O 4 interface by the negative polarization charge (Figure c). When the polarization of BaTiO 3 is switched by a negative writing voltage, then the positive polarization charges resulted would repel the oxygen vacancies away from the interface toward the surface (Figure d), which in turn changes the iron–oxygen–iron superexchange interaction according to theoretical predications, switching the spin of Fe ions in the T d and O h sites on the surface from antiparallel (Figure e) to parallel (Figure f). , This is the essence of our concept, reversing the magnetic moment of Fe 3 O 4 by polarization modulated oxygen vacancy distribution controlled by low voltage.…”

Possible magneto-mechanical and magneto-thermal mechanisms of ion channel activation in magnetogenetics

“…It is also instructive to consider the saturation magnetization, M S , of magnetite in my calculation by dividing the particle saturation moment m TOT by the volume V of the 8 nm diameter particle (M S = m TOT /V) which results in M S = 7.8×10 5 (A/m). Commonly reported bulk magnetite magnetization is slightly lower at M S = 4.8×10 5 (A/m) (Coey, 2010), but it should be noted that several reports (Arora et al, 2008; Orna J et al, 2010; Guan et al, 2016) indicate that magnetite magnetization increases significantly in the size range below 10 nm and reaches the value of M S = 1×10 6 (A/m). Therefore, M S = 7.8×10 5 (A/m) value I use for 8 nm diameter ferritin particle of N = 4500 iron spins with 5μ B per iron atom appears reasonable.…”

Direct Evidence of a Graded Magnetic Interface in Bimagnetic Core/Shell Nanoparticles Using Electron Magnetic Circular Dichroism (EMCD)

“…This is somewhat unexpected since in pure Fe 3 O 4 nanoparticles the surface tends to have a lower magnetic moment than the bulk due to surface disorder . Nevertheless, it is important to emphasize that magnetic moments considerably larger than bulk values have been often reported in Fe 3 O 4 (and other ferrites) thin films. − These enhanced moments are typically reported to occur for very thin films or at surfaces and are usually linked to specific defects such as grain boundaries, vacancies, or antiphase boundaries. − Thus, common defects often observed in this type of (and similar) nanoparticles, such as cation inversion, lattice distortions, Fe 2+ vacancies, or grain boundaries could also contribute to the enhanced moment. ,,, The second possible uncommon effect that can be inferred for the magnetic moment is the unusually low moment in the outer Fe 1+ x O core. Namely, due to vacancy clustering Fe 1+ x O is expected to have larger moment than FeO.…”