A neutron scattering study of spin precession in ferrimagnetic maghemite nanoparticles

Abstract

We present inelastic neutron scattering studies of 4 nm maghemite nanoparticles. In applied magnetic fields (1-4 T) at 300 K, we observe a clear signal from collective magnetic excitations (spin precession of the whole particle moment). The precession is coherent since it is present at the antiferromagnetic wave vector transfer, 1.31 Å−1 . At high fields, the precession frequency varies linearly with field, corresponding to a Landé factor of g = 2.03(2). The intensity of the precession signal varies with temperature (in the range 6-300 K) and applied magnetic field in quantitative agreement with a simple thermodynamic model. The effective anisotropy field is estimated to 0.3(1) T.

Spin excitations in cubic maghemite nanoparticles studied by time-of-flight neutron spectroscopy

“…The obtained Landé factor is in excellent agreement with g = 2 expected for the spin-only Fe 3+ ions as present in maghemite. The determined effective anisotropy field is slightly larger than reported for spherical maghemite nanoparticles of similar size in the past [B A,eff = 0.3(1) T] [13]. This increased anisotropy field observed here may be due to the anisotropic particle shape and the corresponding increased shape anisotropy.…”

“…4(a)]. For analysis of the field dependence of the energy transfer, we followed the approximation of the average precession frequency in the presence of an external field B ext [13]:…”

“…This approximation is made on the basis of antiparallel orientation of the two ferrimagnetic sublattices with equal Landé factor as is valid for maghemite [13]. The field dependence of the energy transfer is refined using Eq.…”

“…† T.Brueckel@fz-juelich.de the order of 10 −12 -10 −7 s. The collective magnetic excitations in antiferromagnetic and ferromagnetic nanoparticle systems and their dependence on the applied temperature and magnetic field have been intensely studied using inelastic neutron scattering [13][14][15][16][17][18][19]. Whereas most of the previous studies of collective magnetic excitations in magnetic nanoparticles have been carried out using triple-axis neutron spectroscopy in the vicinity of the magnetic Bragg reflections, magnetic excitations in nanoparticles have recently also been studied using time-of-flight neutron spectroscopy [20].…”

Thermoinduced Magnetization in Nanoparticles of Antiferromagnetic Materials

“…In Mössbauer spectroscopy, these magnetic fluctuations result in a reduction of the magnetic hyperfine splitting because they can be considered fast compared to the time scale of the experimental technique [16,17]. In inelastic neutron scattering experiments, transitions between the q 0 precession states can be studied [18,19].…”