Magnetic properties of ultra-small goethite nanoparticles

TY - JOUR

T1 - Magnetic properties of ultra-small goethite nanoparticles

AU - Brok, Erik

AU - Frandsen, C.

AU - Madsen, Daniel Esmarch

AU - Jacobsen, Henrik

AU - Birk, Jonas Okkels

AU - Lefmann, Kim

AU - Bendix, Jesper

AU - Pedersen, Kasper Steen

AU - Boothroyd, C.B.

AU - Berhe, A.A.

AU - Simeoni, G.G.

AU - Mørup, Steen

PY - 2014/9/10

Y1 - 2014/9/10

N2 - Goethite (α-FeOOH) is a common nanocrystalline antiferromagnetic mineral. However, it is typically difficult to study the properties of isolated single-crystalline goethite nanoparticles, because goethite has a strong tendency to form particles of aggregated nanograins often with low-angle grain boundaries. This nanocrystallinity leads to complex magnetic properties that are dominated by magnetic fluctuations in interacting grains. Here we present a study of the magnetic properties of 5.7nm particles of goethite by use of magnetization measurements, inelastic neutron scattering and Mössbauer spectroscopy. The 'ultra-small' size of these particles (i.e. that the particles consist of one or only a few grains) allows for more direct elucidation of the particles' intrinsic magnetic properties. We find from ac and dc magnetization measurements a significant upturn of the magnetization at very low temperatures most likely due to freezing of spins in canted spin structures. From hysteresis curves we estimate the saturation magnetization from uncompensated magnetic moments to be σs=0.044Am2kg-1 at room temperature. Inelastic neutron scattering measurements show a strong signal from excitations of the uniform mode (q=0 spin waves) at temperatures of 100-250K and Mössbauer spectroscopy studies show that the magnetic fluctuations are dominated by 'classical' superparamagnetic relaxation at temperatures above ∼170K. From the temperature dependence of the hyperfine fields and the excitation energy of the uniform mode we estimate a magnetic anisotropy constant of around 1.0×105Jm-3.

AB - Goethite (α-FeOOH) is a common nanocrystalline antiferromagnetic mineral. However, it is typically difficult to study the properties of isolated single-crystalline goethite nanoparticles, because goethite has a strong tendency to form particles of aggregated nanograins often with low-angle grain boundaries. This nanocrystallinity leads to complex magnetic properties that are dominated by magnetic fluctuations in interacting grains. Here we present a study of the magnetic properties of 5.7nm particles of goethite by use of magnetization measurements, inelastic neutron scattering and Mössbauer spectroscopy. The 'ultra-small' size of these particles (i.e. that the particles consist of one or only a few grains) allows for more direct elucidation of the particles' intrinsic magnetic properties. We find from ac and dc magnetization measurements a significant upturn of the magnetization at very low temperatures most likely due to freezing of spins in canted spin structures. From hysteresis curves we estimate the saturation magnetization from uncompensated magnetic moments to be σs=0.044Am2kg-1 at room temperature. Inelastic neutron scattering measurements show a strong signal from excitations of the uniform mode (q=0 spin waves) at temperatures of 100-250K and Mössbauer spectroscopy studies show that the magnetic fluctuations are dominated by 'classical' superparamagnetic relaxation at temperatures above ∼170K. From the temperature dependence of the hyperfine fields and the excitation energy of the uniform mode we estimate a magnetic anisotropy constant of around 1.0×105Jm-3.

U2 - 10.1088/0022-3727/47/36/365003

DO - 10.1088/0022-3727/47/36/365003

M3 - Journal article

SN - 0022-3727

VL - 47

JO - Journal Physics D: Applied Physics

JF - Journal Physics D: Applied Physics

IS - 36

M1 - 365003

ER -