The dipole moment of the spin density as a local indicator for phase transitions

Abstract

The intra-atomic magnetic dipole moment - frequently called 〈Tz〉 term - plays an important role in the determination of spin magnetic moments by x-ray absorption spectroscopy for systems with nonspherical spin density distributions. In this work, we present the dipole moment as a sensitive monitor to changes in the electronic structure in the vicinity of a phase transiton. In particular, we studied the dipole moment at the Fe2+ and Fe3+ sites of magnetite as an indicator for the Verwey transition by a combination of x-ray magnetic circular dichroism and density functional theory. Our experimental results prove that there exists a local change in the electronic structure at temperatures above the Verwey transition correlated to the known spin reorientation. Furthermore, it is shown that measurement of the dipole moment is a powerful tool to observe this transition in small magnetite nanoparticles for which it is usually screened by blocking effects in classical magnetometry.

Suppression of the Verwey Transition by Charge Trapping

“…Having a look at the temperature dependence of XANES and XMCD, it is obvious that there is a change of the intensities between 50 K and 100 K for the bare magnetite nanoparticles (Figure , upper panel). These changes clearly indicate the presence of the Verwey transition as discussed in the literature . The transition is not visible in the spectra of the silica capped nanoparticles (Figure , lower panel).…”

“…In ensembles of nanoparticles, the Verwey transition is difficult to be observed by diffraction methods, magnetometry or resistivity measurements as already discussed elsewhere . While tunnel spectroscopy and scanning tunneling microspectroscopy have been used to investigate the Verwey transition in single nanoparticles, analysis of the X‐ray absorption near‐edge structure (XANES) and its associated magnetic circular dichroism (XMCD) has turned out to be a powerful tool to monitor the transition in ensembles of magnetite nanoparticles . Hence it is used in this work to investigate the influence of a 3.5 nm thick amorphous silica (SiO 2 ) shell on the properties of magnetite nanoparticles with a mean diameter of (6.3 ± 0.9) nm.…”

Towards sub-nanometer real-space observation of spin and orbital magnetism at the Fe/MgO interface

“…The latter contribution denotes the expectation value of the spin dipole moment operator ( T z ), which is negligible in cubic materials27, but can be appreciable in e.g. oxides28. The results shown in Fig.…”

The UE46 PGM-1 beamline at BESSY II

“…• Charge order of high-Tc Superconductors (Blanco-Canosa et al, 2013;Comin et al, 2014;da Silva Neto et al, 2014;Fink et al, 2009;Ghiringhelli et al, 2012) • Coupling of electronic / lattice degrees of freedom in multiferroic materials (Glavic et al, 2013;Partzsch et al, 2011;Schierle et al, 2010;Schmitz-Antoniak et al, 2013;Skaugen et al, 2015) • Microcrystals of novel materials (Leininger et al, 2011;Matsuda et al, 2015) • Interfacial electronic properties in heterostructures (Frano et al, 2013;Wadati et al, 2009) • Element-speci c magnetic hysteresis loops (Radu et al, 2012) • Single molecular magnets (Bernien et al, 2015;Hermanns et al, 2013) • Electronic depth pro les • Electronic ground states and phase transitions in correlated materials (Schmitz et al, 2014;Strigari et al, 2013;Willers et al, 2012Willers et al, , 2011 • Magnetic clusters in carbon nanotubes (Shiozawa et al, 2015) • Nanoparticles for medical applications (Graf et al, 2015) • Magnetic semiconductors (Khalid et al, 2014) …”