Magnetic nanoparticles formed in glasses co-doped with iron and larger radius elements

Abstract

A new type of nanoparticle-containing glasses based on borate glasses co-doped with low contents of iron and larger radius elements, Dy, Tb, Gd, Ho, Er, Y, and Bi, is studied. Heat treatment of these glasses results in formation of magnetic nanoparticles, radically changing their physical properties. Transmission electron microscopy and synchrotron radiation-based techniques: x-ray diffraction, extended x-ray absorption fine structure, x-ray absorption near-edge structure, and small-angle x-ray scattering, show a broad distribution of nanoparticle sizes with characteristics depending on the treatment regime; a crystalline structure of these nanoparticles is detected in heat treated samples. Magnetic circular dichroism (MCD) studies of samples subjected to heat treatment as well as of maghemite, magnetite, and iron garnet allow to unambiguously assign the nanoparticle structure to maghemite, independently of co-dopant nature and of heat treatment regime used. Different features observed in the MCD spectra are related to different electron transitions in Fe3+ ions gathered in the nanoparticles. The static magnetization in heat treated samples has non-linear dependence on the magnetizing field with hysteresis. Zero-field cooled magnetization curves show that at higher temperatures the nanoparticles occur in superparamagnetic state with blocking temperatures above 100 K. Below ca. 20 K, a considerable contribution to both zero field-cooled and field-cooled magnetizations occurs from diluted paramagnetic ions. Variable-temperature electron magnetic resonance (EMR) studies unambiguously show that in as-prepared glasses paramagnetic ions are in diluted state and confirm the formation of magnetic nanoparticles already at earlier stages of heat treatment. Computer simulations of the EMR spectra corroborate the broad distribution of nanoparticle sizes found by “direct” techniques as well as superparamagnetic nanoparticle behaviour demonstrated in the magnetization studies.

Formation, characterization and magnetic properties of maghemite γ-Fe2O3 nanoparticles in borate glasses

“…Wide FR maximum presented in Fig. 9 (inset) is due to several electron transitions in Fe 3+ ions occupaing both octahedral and tetrahedral positions in accordence with interpretation of MCD spectrum [10]. MCD band centered at 14710 cm -1 was ascribed in [10] to the transition 6 A 1 ( 6 S) -4 T 1 ( 4 G) in the tetrahedral Fe 3+ ions.…”

“…That is why we used RE and some other large-ion-radius elements as co-dopants in order to maximize the magnetooptical activity of potassium-alumina-germanium-borate glasses. Nanoparticles arising in these glasses co-doped with iron and Dy, Tb, Gd, Ho, Er, Y, and Bi were shown to have the γ-Fe 2 O 3 structure independently on co-dopant nature [10]. At the same time, nanoparticle morphology and dimensions, as well as the magnetic and magneto-optical characteristics of these particles containing samples differ strongly for different co-dopants and heat treatment regimes.…”

Magnetic circular dichroism in the canted antiferromagnet α-Fe2O3: Bulk single crystal and nanocrystals

“…In this case, the transition energy is approximately equal to the sum of two single ion Fe 3+ d-d transitions [50]. From this point of view, it seems to be interesting to mention results of papers [50,51], where absorption spectra of the In the absorption and MCD spectra of many iron oxides [46,47], two features in the lower part of the spectrum (1-2 eV) due to the d-d electron transitions 6 A 1g ( 6 S)→ 4 T 1g ( 4 G) and 6 A 1g ( 6 S)→ 4 T 2g ( 4 G) were observed distinctly. As, energies of these transitions depend strongly on the CF value, they are critical for the determining of the CF parameters and identification of the higher energy spectral features with the definite CF transitions.…”

“…Comparison is worth of MCD spectrum with spectra of ferrimagnetic compounds containing only one type of the d-ion, namely, Fe 3+ ions -Y 3 Fe 5 O 12 [46] and γ-Fe 2 O 3 [47] presented in literature. Two strong maxima are observed at energies 2.58 and 2.79 eV for Y 3 Fe 5 O 12 and at 2 3 and 2 6 eV for γ-Fe 2 O 3 .…”

Магнитооптические свойства дисперсий наночастиц на основе Fe-=SUB=-3-=/SUB=-O-=SUB=-4-=/SUB=-, полученных методом импульсной лазерной абляции в жидкости

“…Из всех магнитооптических эффектов магнитный круговой дихроизм (МКД) является наиболее информативным и удобным для измерения методом, поскольку он наблюдается только в области поглощения и при этом, как правило, в нем отсутствует вклад немагнитной компоненты среды [19]. МКД-спектроскопия обладает более высоким спектральным разрешением по сравнению с оптической спектроскопией, она применялась рядом авторов для исследования НЧ оксидов железа [20,21], не только для определения величины магнитооптического отклика, но и в качестве инструмента, иногда ключевого, для идентификации фазового состава НЧ, поскольку спектральная зависимость этого эффекта для разных фаз оксида железа -магнетита (Fe 3 O 4 ), маггемита (γ-Fe 2 O 3 ) и гематита (α-Fe 2 O 3 ) уникальна и служит неоспоримым паспортом этих фаз [22]. Отметим, что получение чистой фазы какого-либо из оксидов железа является весьма сложной технологической задачей, поскольку незначительные термические воздействия могут приводить к структурным преобразованиям в цепочке фаз -магнетит, маггемит, эпсилон фаза и гематит: [23,24].…”