Mechanism of the formation of an uncompensated magnetic moment in bacterial ferrihydrite nanoparticles

Journal of Applied Physics

Krasikov

Dubrovskiy

et al. 2016

Self Cite

We report the results of investigations of the effect of cooling in an external magnetic field starting from the temperature over superparamagnetic blocking temperature TB on the shift of magnetic hysteresis loops in systems of ferrihydrite nanoparticles from ~2.5 to ~5 nm in size with different TB values. In virtue of high anisotropy fields of ferrihydrite nanoparticles and open hysteresis loops in the range of experimentally attainable magnetic fields, the shape of hysteresis loops of such objects in the field-cooling mode is influenced by the minor hysteresis loop effect.A technique is proposed for distinguishing the exchange bias effect among the effects related to the minor hysteresis loops caused by high anisotropy fields of ferrihydrite particles. The exchange bias in ferrihydrite is stably observed for particles no less than 3 nm in size or with TB over 40 K, and its characteristic value increases with the particle size.

“…Typical microphotographs were presented in [32,34]). According to our data, the <d> values are consistent with those obtained from the magnetization curves at T > TB [23,24,32].…”

Section: Samplessupporting

confidence: 89%

“…A technique for preparation nano-ferrihydrite formed during the vital activity of bacteria was described in detail in [29,30,23]. Hereinafter, the initial sample is denoted as b-fh.…”

Section: Samplesmentioning

confidence: 99%

Exchange bias in nano-ferrihydrite

Journal of Applied Physics

Krasikov

Dubrovskiy

et al. 2016

Self Cite

“…Using the data of numerous studies on ferrihydrite and ferritin, it was reliably established that the exponent for nanoparticles of this material is n ≈ ½ [13][14][15][16][17][24][25][26][27][28] and, in this case, the susceptibility of an ideal system of ferrihydrite nanoparticles should not depend on the particle size. However, the increase in the ferrihydrite particle size upon annealing was proven by both the direct (TEM data) and indirect (increasing TB, HC, and P [13]) measurements and with an increase in the particle volume by a factor of about 3, the (300 K) value increase by a factor of almost 4.…”

Section: Discussionmentioning

confidence: 99%

“…The uncompensated magnetic moment of particles starts playing an important role in the magnetic behavior of AFM particles when the number of atoms in a particle is about 10 3 -10 4[20,23]. Numerous investigations of ferrihydrite and ferritin reliably demonstrated that nanoparticles of this material are characterized by an exponent of n ≈ ½[13][14][15][16][17][24][25][26][27][28].We start discussing the experimental data with the Mössbauer spectra.The ideal ferrihydrite structure contains alternating doubled and single layers of octahedral iron positions along the c axis. The doubled layers are characterized by the АВС cubic structure and the single layers, by the hexagonal packing of АВАВ ligands.…”

mentioning

confidence: 99%

Modification of the Structure and Magnetic Properties of Cobalt-Doped Ferrihydrite Nanoparticles Under Heat Treatment

Stolyar

Krasikov

et al. 2017

J Supercond Nov Magn

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Nanoparticles of antiferromagnetic materials acquire the magnetic moment due to the surface effects and structural defects. According to the Neel hypothesis, magnetic moment P of a particle containing N magnetically active atoms with magnetic moment J can be estimated as P ~ J•N n or P ~ V n , where V is the particle volume. Numerous studies of the magnetic properties of ferrihydrite 5Fe2O3•9H2O and ferritin revealed a value of n ≈ ½ for this material, in which Fe atoms have the octahedral surrounding of anions. We investigate the effect of low-temperature annealing of cobalt-doped ferrihydrite nanoparticles on their average size and magnetic properties. Using the Mössbauer spectroscopy study, we demonstrate that doping with Со makes Fe atoms enter the anion tetrahedra, which leads to an increase in the exponent n > ½ in the expression P ~ J•N n .

“…The magnetic moments of iron atoms in ferrihydrite are antiferromagnetically ordered; according to the neutron diffraction data, the Néel temperature of the material is ~350 K [19]. In turn, the imperfection of the ferrihydrite crystal structure leads to the occurrence of an uncompensated magnetic moment in nanoparticles [20][21][22][23][24][25][26]. This fact subsumes ferrihydrite to the class of antiferromagnetic (AFM) nanoparticles [8,27].…”

Section: Introductionmentioning

confidence: 99%

Magnetic anisotropy and core-shell structure origin of the biogenic ferrihydrite nanoparticles

Knyazev

Journal of Alloys and Compounds

Stolyar

et al. 2021

Ferrihydrite is a mineral that exists in a nanoscale form. At the antiferromagnetic ordering of the magnetic moments of iron atoms, it exhibits, due to the uncompensated magnetic moment of its nanoparticles, the magnetic properties very similar to those of ferro-and ferrimagnetic nanoparticles. In this study, we investigate the ferrihydrite nanoparticles that are a co-product of the cultivation of bacteria. The obtained biogenic sample is an aggregate of biogenic nanoparticles with an average size of ~2 nm coated with a polysaccharide shell, which excludes contact magnetic interactions between particles. The features caused by the surface effects and the inhomogeneous structure of ferrihydrite have been examined in the temperature range of 4-300 K using Mössbauer spectroscopy and magnetometry. Based on the data obtained, the superparamagnetic blocking processes have been identified. The dominating contribution of the surface magnetic anisotropy to these processes has been established. This surface effect is caused by the inhomogeneous structure of ferrihydrite nanoparticles, which is confirmed by the Mössbauer spectroscopy data showing the existence of different states of iron nuclei. A core-shell structural model of ferrihydrite particles has been proposed. The particle core is well-crystallized, while the surface layer is loose and contains a greater number of defects. It has been found that the size of the dense core depends on the particle size; the critical value is d ~ 2 nm and the particles with a smaller diameter have no dense core. The results obtained reveal inhomogeneities of the crystal structure in ferrihydrite nanoparticles, which affect the magnetic properties of the investigated materials.