Magnetic inter-particle exchange interaction (i.e., coupling between surface atoms of neighbouring particles due to electronic orbital overlap) between nanoparticles of antiferromagnetic materials such as α-Fe₂O₃ (hematite) and α-FeOOH (goethite) is indicative of how the nanoparticles are attached together [1-4]. We see significant variation in the magnetic properties dependent on whether the particles are attached with perfect oriented attachment [2], with low-angle dislocations [3] or with a more random aggregation [4].

For 7 nm α-Fe₂O₃ particles with oriented attachment, the magnetic order, like the crystalline order, continues perfectly across particle interfaces [2]. However, for randomly aggregated 7 nm α-Fe₂O₃ particles, the magnetic spins rotate out of their easy plane because neighbouring particles try to align their magnetizations [4]. In a study of ~50 nm long α-FeOOH rods, we find, in accordance with previous studies [5], that the rods have many defects like low-angle grain boundaries [3]. The magnetic mismatch at these grain boundaries leads to a weakened magnetic coupling and this can explain the rods’ magnetic properties which are neither nano- nor bulk-like [3].

Macroscopic treatments influence particle attachment on the nanoscale. This is also seen from changes in the magnetic properties. For randomly aggregated 7 nm α-Fe₂O₃ particles, intense ultrasonic treatment can break apart aggregates and thereby diminish the magnetic inter-particle interaction (e.g., lead to fast superparamagnetic relaxation) [6]. Subsequent drying of particle suspensions reestablishes the magnetic interaction [6]. This reversible detach-attach mechanism bears some resemblance to observations in Ref. 7. Stronger bonds as found between particles with oriented attachment can be destroyed by gentle grinding of particle powders [2].

References

[1] C. Frandsen & S. Mørup (2003), J. Magn. Magn. Mater. 266, 36.

[2] C. Frandsen et al. (2005), Phys. Rev. B 72, 214406.

[3] D.E. Madsen et al. (2009), J. Phys.: Cond. Matter 21, 016007.

[4] C. Frandsen & S. Mørup (2005), Phys. Rev. Lett. 94, 027202.

[5] D.J. Burleson and R.Lee Penn (2006), Langmuir 22, 402.

[6] C. Frandsen & S. Mørup (2006), J. Phys.: Cond. Matter 18, 7079.

[7] F. Huang et al. (2004), Phys. Rev. Lett. 92, 155501.