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Origin of reduced magnetization and domain formation in small magnetite nanoparticles

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ABSTRACT

The structural, chemical, and magnetic properties of magnetite nanoparticles are compared. Aberration corrected scanning transmission electron microscopy reveals the prevalence of antiphase boundaries in nanoparticles that have significantly reduced magnetization, relative to the bulk. Atomistic magnetic modelling of nanoparticles with and without these defects reveals the origin of the reduced moment. Strong antiferromagnetic interactions across antiphase boundaries support multiple magnetic domains even in particles as small as 12–14 nm.

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Spin configurations at 5T.(a) [001] zone axis of a 10 nm nanoparticle without APBs, which has a single magnetic domain. (b) [001] zone axis of a 10 nm nanoparticle with a ¼a0 <110> APB, which leads to a coincident magnetic domain wall. The net magnetisations of the left and right part of the NP are labelled as MR and ML, respectively. (c) Magnified view of (b) near the APB plane showing the canting of the spins at the boundary. (d) 10 nm model with two non-coplanar APBs on <1 1-2> type planes, with net magnetization M> (along the field) and M< (in-plane) for the upper and lower part respectively. The colour coding is in RGB fashion where the function which determines the colour is dependent only on the projection of the spin direction along the applied field. Two limiting cases are pure blue (R = 0; G = 0; B = 1) for spins aligned along the field and pure red colour for spins in opposite direction. Net magnetization direction (determined by the FeB spins) in each of the domains helps identify the FeB sites.
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f4: Spin configurations at 5T.(a) [001] zone axis of a 10 nm nanoparticle without APBs, which has a single magnetic domain. (b) [001] zone axis of a 10 nm nanoparticle with a ¼a0 <110> APB, which leads to a coincident magnetic domain wall. The net magnetisations of the left and right part of the NP are labelled as MR and ML, respectively. (c) Magnified view of (b) near the APB plane showing the canting of the spins at the boundary. (d) 10 nm model with two non-coplanar APBs on <1 1-2> type planes, with net magnetization M> (along the field) and M< (in-plane) for the upper and lower part respectively. The colour coding is in RGB fashion where the function which determines the colour is dependent only on the projection of the spin direction along the applied field. Two limiting cases are pure blue (R = 0; G = 0; B = 1) for spins aligned along the field and pure red colour for spins in opposite direction. Net magnetization direction (determined by the FeB spins) in each of the domains helps identify the FeB sites.

Mentions: Atomistic spin calculations on model particles (with and without APBs) provide insight into the effect of APBs on NP magnetic moment. Using VAMPIRE software34 we calculated the spin configurations at magnetic saturation for various model NPs and compared the results to same size NP without APBs (Fig. 4a). Figure 3c shows the atomic geometry of the simplest model, a spherical nanoparticle 10 nm in diameter, with a ¼a0 <110> APB at the center of the NP. The calculations for this model NP show that there is a reduction in the saturation magnetization by 26% compared to the same size NP without the structural defect (i.e. magnetization value of ~66 emu/g). Furthermore, looking at the spin configuration snapshots (Fig. 4a,b) one can see the formation of two magnetic domains due to the presence of the strong 180°antiferromagnetic bonds across the boundary (Fig. 4c). Within a domain, the FeA and FeB spins are antiparallel, as expected in bulk magnetite. However, across the APB, the FeB spins are canted because of the change in FeB-O-FeB superexchange coupling. In Fig. 4b, where the NP has an APB across its center, the magnetization of the left (ML) and right (MR) sides of the particle are not parallel, even at 5T. The APB reduces the magnetic moment, relative to a defect-free NP.


Origin of reduced magnetization and domain formation in small magnetite nanoparticles
Spin configurations at 5T.(a) [001] zone axis of a 10 nm nanoparticle without APBs, which has a single magnetic domain. (b) [001] zone axis of a 10 nm nanoparticle with a ¼a0 <110> APB, which leads to a coincident magnetic domain wall. The net magnetisations of the left and right part of the NP are labelled as MR and ML, respectively. (c) Magnified view of (b) near the APB plane showing the canting of the spins at the boundary. (d) 10 nm model with two non-coplanar APBs on <1 1-2> type planes, with net magnetization M> (along the field) and M< (in-plane) for the upper and lower part respectively. The colour coding is in RGB fashion where the function which determines the colour is dependent only on the projection of the spin direction along the applied field. Two limiting cases are pure blue (R = 0; G = 0; B = 1) for spins aligned along the field and pure red colour for spins in opposite direction. Net magnetization direction (determined by the FeB spins) in each of the domains helps identify the FeB sites.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5385549&req=5

f4: Spin configurations at 5T.(a) [001] zone axis of a 10 nm nanoparticle without APBs, which has a single magnetic domain. (b) [001] zone axis of a 10 nm nanoparticle with a ¼a0 <110> APB, which leads to a coincident magnetic domain wall. The net magnetisations of the left and right part of the NP are labelled as MR and ML, respectively. (c) Magnified view of (b) near the APB plane showing the canting of the spins at the boundary. (d) 10 nm model with two non-coplanar APBs on <1 1-2> type planes, with net magnetization M> (along the field) and M< (in-plane) for the upper and lower part respectively. The colour coding is in RGB fashion where the function which determines the colour is dependent only on the projection of the spin direction along the applied field. Two limiting cases are pure blue (R = 0; G = 0; B = 1) for spins aligned along the field and pure red colour for spins in opposite direction. Net magnetization direction (determined by the FeB spins) in each of the domains helps identify the FeB sites.
Mentions: Atomistic spin calculations on model particles (with and without APBs) provide insight into the effect of APBs on NP magnetic moment. Using VAMPIRE software34 we calculated the spin configurations at magnetic saturation for various model NPs and compared the results to same size NP without APBs (Fig. 4a). Figure 3c shows the atomic geometry of the simplest model, a spherical nanoparticle 10 nm in diameter, with a ¼a0 <110> APB at the center of the NP. The calculations for this model NP show that there is a reduction in the saturation magnetization by 26% compared to the same size NP without the structural defect (i.e. magnetization value of ~66 emu/g). Furthermore, looking at the spin configuration snapshots (Fig. 4a,b) one can see the formation of two magnetic domains due to the presence of the strong 180°antiferromagnetic bonds across the boundary (Fig. 4c). Within a domain, the FeA and FeB spins are antiparallel, as expected in bulk magnetite. However, across the APB, the FeB spins are canted because of the change in FeB-O-FeB superexchange coupling. In Fig. 4b, where the NP has an APB across its center, the magnetization of the left (ML) and right (MR) sides of the particle are not parallel, even at 5T. The APB reduces the magnetic moment, relative to a defect-free NP.

View Article: PubMed Central - PubMed

ABSTRACT

The structural, chemical, and magnetic properties of magnetite nanoparticles are compared. Aberration corrected scanning transmission electron microscopy reveals the prevalence of antiphase boundaries in nanoparticles that have significantly reduced magnetization, relative to the bulk. Atomistic magnetic modelling of nanoparticles with and without these defects reveals the origin of the reduced moment. Strong antiferromagnetic interactions across antiphase boundaries support multiple magnetic domains even in particles as small as 12&ndash;14&thinsp;nm.

No MeSH data available.


Related in: MedlinePlus