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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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(a) [001] view of a (001) FeB–O lattice plane in bulk magnetite. (b) The presence of a ¼a0 <110> APB (dashed line) leads to shifted right hand side for ¼a0 <110> (in-plane, arrows represent the shift vector), changing the FeB-O-FeB angle from 90° as shown in (a) to 180°. The oxygen sub-lattice is invariant under the shift. (c) 10 nm nanoparticle model with single ¼a0 <110> APB shown along the [11-2] zone axis. Tetrahedral FeA atoms are coloured in red, octahedral FeB in blue and oxygen atoms (not shown in (c)) in green. Drawing of the structural models was generated in VESTA39 software.
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f3: (a) [001] view of a (001) FeB–O lattice plane in bulk magnetite. (b) The presence of a ¼a0 <110> APB (dashed line) leads to shifted right hand side for ¼a0 <110> (in-plane, arrows represent the shift vector), changing the FeB-O-FeB angle from 90° as shown in (a) to 180°. The oxygen sub-lattice is invariant under the shift. (c) 10 nm nanoparticle model with single ¼a0 <110> APB shown along the [11-2] zone axis. Tetrahedral FeA atoms are coloured in red, octahedral FeB in blue and oxygen atoms (not shown in (c)) in green. Drawing of the structural models was generated in VESTA39 software.

Mentions: APBs, extensively studied in magnetite thin films132324252627, are correlated with anomalous properties such as very high magnetic saturation fields23 and negative magnetoresistance1528. However, no direct evidence of APBs in single crystal magnetite NPs has been demonstrated. Recent work on the formation energy of highly stable APB defects29 could explain why APBs are stable even in NPs. A lattice vector shift of ¼ a0 <110> creates an APB (Fig. 3a,b) with a very low formation energy (0.1 J/m2). Figure 3b illustrates how the 90° (Fig. 3a) FeB-O-FeB bulk-like ferromagnetic super-exchange interaction is changed to 180° antiferromagnetic interaction due to the lattice shift caused by the APB. APBs with different lattice shift vectors result in similarly increased AFM interactions. Here we consider only the APBs with ¼ a0 <110> shift for two reasons: a) these APBs are experimentally observed in the studied NPs, and b) the formation energies of other type APBs are an order of magnitude larger, and are therefore much less likely to form in NPs.


Origin of reduced magnetization and domain formation in small magnetite nanoparticles
(a) [001] view of a (001) FeB–O lattice plane in bulk magnetite. (b) The presence of a ¼a0 <110> APB (dashed line) leads to shifted right hand side for ¼a0 <110> (in-plane, arrows represent the shift vector), changing the FeB-O-FeB angle from 90° as shown in (a) to 180°. The oxygen sub-lattice is invariant under the shift. (c) 10 nm nanoparticle model with single ¼a0 <110> APB shown along the [11-2] zone axis. Tetrahedral FeA atoms are coloured in red, octahedral FeB in blue and oxygen atoms (not shown in (c)) in green. Drawing of the structural models was generated in VESTA39 software.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f3: (a) [001] view of a (001) FeB–O lattice plane in bulk magnetite. (b) The presence of a ¼a0 <110> APB (dashed line) leads to shifted right hand side for ¼a0 <110> (in-plane, arrows represent the shift vector), changing the FeB-O-FeB angle from 90° as shown in (a) to 180°. The oxygen sub-lattice is invariant under the shift. (c) 10 nm nanoparticle model with single ¼a0 <110> APB shown along the [11-2] zone axis. Tetrahedral FeA atoms are coloured in red, octahedral FeB in blue and oxygen atoms (not shown in (c)) in green. Drawing of the structural models was generated in VESTA39 software.
Mentions: APBs, extensively studied in magnetite thin films132324252627, are correlated with anomalous properties such as very high magnetic saturation fields23 and negative magnetoresistance1528. However, no direct evidence of APBs in single crystal magnetite NPs has been demonstrated. Recent work on the formation energy of highly stable APB defects29 could explain why APBs are stable even in NPs. A lattice vector shift of ¼ a0 <110> creates an APB (Fig. 3a,b) with a very low formation energy (0.1 J/m2). Figure 3b illustrates how the 90° (Fig. 3a) FeB-O-FeB bulk-like ferromagnetic super-exchange interaction is changed to 180° antiferromagnetic interaction due to the lattice shift caused by the APB. APBs with different lattice shift vectors result in similarly increased AFM interactions. Here we consider only the APBs with ¼ a0 <110> shift for two reasons: a) these APBs are experimentally observed in the studied NPs, and b) the formation energies of other type APBs are an order of magnitude larger, and are therefore much less likely to form in NPs.

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