Seminar Nov. 8: "Neutron Scattering Studies of Magnetic Nanoparticles: from Spin Waves to Self-Assembly" by Mikhail Feygenson

2021-10-22

On November 8, 2021, there will be a seminar by Mikhail Feygenson, ESS, on "Neutron Scattering Studies of Magnetic Nanoparticles: from Spin Waves to Self-Assembly". 

Time: Monday, November 8, 2021, at 15:15-16:00 hrs
Location: Häggsalen, Ångström Laboratory, and on zoom (see link below).

Zoom link: https://uu-se.zoom.us/j/61904577245  

Abstract

With increasing complexity of novel magnetic nanoparticles, understanding of their magnetic properties becomes even more important. The neutron scattering experiments on magnetic nanoparticles have been always a challenge due to small amounts of synthesized nanoparticles and incoherent scattering background from their surfactants. However, construction of new high-flux spallation neutron sources and development of polarized neutron scattering methods at reactor sources have revived the interest in use of neutrons for magnetic nanoparticles studies. In this talk, I will present three examples on how neutron scattering methods were used to gain insight into static and dynamic magnetic properties of nanoparticles. 

My first example is Co/CoO nanoparticles with anomalously large exchange bias effects1,2. In order to understand the establishment of antiferromagnetic order and the lower temperature onset of exchange bias at the blocking temperature, we have studied the magnetic dynamics of a precipitated powder of these core-shell nanoparticles with inelastic neutron scattering. Above the 250K Neel temperature of the CoO shells, the scattering was broad and quasielastic, with little wave vector dependence. The inelastic excitation with a gap of ~3meV and linear dispersion was observed below 200K blocking temperature. This excitation was ascribed to spin-waves in CoO shell, with their dynamic properties ultimately coupled to dynamics of ferromagnetic Co core.

In the following example3, we used polarized neutron diffraction to investigate magnetic phase transition in MnO nanoparticles embedded into porous silica glass matrix. The continuous magnetic phase transition at slightly higher Neel temperature was found for MnO nanoparticles, as compared to their bulk counterpart. The polarization analysis was used to carefully separate a weak magnetic scattering from nuclear and incoherent ones. The analysis of magnetic scattering revealed that about 60% of atoms remain disordered even at low temperatures, presumably due to interactions nanoparticles and glass walls.

Finally, I will show how polarized small-angle neutron scattering (polSANS) can be used to study self-assembly of magnetic nanoparticles in solution4. Iron oxide nanoparticles of 27nm in diameter self-assembled into extended and semi-ordered chains at applied fields as low as 0.004T. This self-assembly was reversible. At the same time, smaller particles of 20nm showed no such self-assembly even at 2.2T. polSANS method allowed us to emphasize the role of magnetic dipolar interactions between the particles by separating the magnetic and structural form factors. The Reverse Monte Carlo simulations were used to model 2D SANS data.

In conclusion, I will outline the new perspective for studying magnetic nanoparticles with neutrons at European Spallation Source5, which is being built in Lund, Sweden.   

References  

  1. M. Feygenson et al., “Controlling the exchange bias field in Co core/CoO shell nanoparticles”, Phys. Rev. B 81, 195445 (2010) 
  2. M. Feygenson et al., “Low-energy magnetic excitations in Co/CoO core/shell nanoparticles”. Phys. Rev. B. 83,174414 (2011) 
  3. M. Feygenson et al., “Magnetism in confined geometry: Magnetic critical scattering of MnO nanoparticles”, Phys. Rev. B 81, 064423 (2010) 
  4. N. Nandakumaran et al., “Unravelling Magnetic Nanochain Formation in Dispersion for In-Vivo Applications”, Adv. Mater. 33, 2008683 (2021) 
  5. K. H. Andersen et al., “The instrument suite of the European Spallation Source”, Nuclear Inst. and Methods in Physics Research, A 957, 163402 (2020). 

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