Magnetic behaviour of interacting antiferromagnetic nanoparticles

Magnetic properties of interacting La(0.2)Ca(0.8)MnO(3) nanoparticles have been investigated. The field-induced transition from antiferromagnetic (AFM) to ferromagnetic (FM) state in the La(0.2)Ca(0.8)MnO(3) bulk has been observed at exceptionally high magnetic fields. For large particles, the field-induced transition widens while magnetization progressively decreases. In small particles the transition is almost fully suppressed. The thermoremanence and isothermoremanence curves constitute fingerprints of irreversible magnetization originating from nanoparticle shells. We have ascribed the magnetic behaviour of nanoparticles to a core-shell scenario with two main magnetic contributions; one attributed to the formation of a collective state formed by FM clusters in frustrated coordination at the surfaces of interacting AFM nanoparticles and the other associated with inner core behaviour as a two-dimensional diluted antiferromagnet.     

Magnetic relaxations in amorphous soft magnetic alloys

The paper reviews selected results of the extended experimental investigations of magnetic properties, time–temperature stability and workability of the soft magnetic amorphous alloys controlled by structural magnetic relaxation. Complex approach to the magnetic relaxations in multicomponent amorphous alloys is presented. The transition from magnetic after-effect to a new MAE spectrometry is illustrated on ternary amorphous CoSiB alloy.     

Magnetic relaxation in a suspension of antiferromagnetic nanoparticles

A kinetic model is proposed to describe the low-frequency magnetodynamics of antiferromagnetic nanoparticles suspended in a fluid. Because of their small size, apart from an anisotropic magnetic susceptibility typical of antiferromagnets, these particles also have a constant magnetic moment caused by sublattice decompensation. An orientational crossover takes place in such a nanosuspension (colloid) when magnetized by a constant field: the axes of easy particle magnetization that were initially aligned along the field become oriented perpendicularly. This effect changes significantly the characteristics of the system’s magnetic response: the dynamic susceptibility spectrum and the relaxation time in a pulsed field.     

Dipole magnetic anisotropy fields in rhombohedral antiferromagnetic materials

Results of calculations of the contribution of magnetic dipole interactions to the effective uniaxial anisotropy fields of antiferro- and ferromagnetism vectors in rhombohedral antiferromagnetic materials with theS ions are given as functions of the ratio of the hexagonal crystal cell parameters c_H/a_H. There is a strong dependence of the calculated curves on the lattice parameters of real compounds. From the dependences obtained the effective anisotropy fields are calculated for FeF₃, FeBO₃, and MnCO₃. L. V. Kirenskii Institute of Physics. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 82–86, May, 1999.     

Size and anisotropy effects on magnetic properties of antiferromagnetic nanoparticles

Based on the Heisenberg model taking into account single-ion anisotropy and using a Green's function technique we have studied the influence of size and anisotropy effects on magnetization M, Neel temperature T_N, coercive field H_c and spin excitation energy of antiferromagnetic nanoparticles. The properties are compared with those of ferromagnetic nanoparticles. We have shown that the enhanced magnetization M and coercive field H_c of antiferromagnetic nanoparticles is a surface effect, which is due to uncompensated surface spins. Moreover, the shape of the coercive field curve can be significantly influenced by surface magnetic anisotropy.     

Astroid curves of high-moment antiferromagnetic nanoparticles with tunable magnetic properties

We have determined astroids for high-moment antiferromagnetic nanoparticles (AN), which have been recently discovered and used in numerous biomedical applications. The astroid curves for such a system, which is a stack of two isolated disk-shaped ferromagnetic nanoparticles interacting antiferromagnetically, show the regions in the magnetic field plane where different numbers of minima associated with stable or metastable states may exist. We describe the properties of these ANs and estimate their other characteristic parameters such as magnetic saturation field and exchange antiferrtomagnetic coupling. We argue that the finding of these astroids and the properties of ANs is crucial for the use of ANs in numerous applications and for modeling stable information storage devices.     

Magnetic properties and the mechanism of formation of the uncompensated magnetic moment of antiferromagnetic ferrihydrite nanoparticles of a bacterial origin

The magnetic properties of the superparamagnetic ferrihydrite nanoparticles that form as a result of the vital activity of Klebsiella oxytoca bacteria are studied. Both an initial powder with an average number of iron atoms N _Fe ～ 2000–2500 in a particle and this powder after annealing at 140°C for 3 h in air are investigated. The following substantial modifications of the magnetic properties of the ferrihydrite nanoparticles are detected after annealing: the superparamagnetic blocking temperature increases from 23 to 49.5 K, and the average magnetic moment of a particle increases (as follows from the results of processing of magnetization curves). The particles have antiferromagnetic ordering, and the magnetic moment resulting in the superparamagnetism of the system appears due to random spin decompensation inside the particle. For this mechanism, the number of uncompensated spins is proportional to the number of magnetically active atoms raised to the one-half power, and this relation holds true for the samples under study at a good accuracy. The possible causes of the detected shift of magnetic hysteresis loops at low temperatures upon field cooling are discussed.     

Magnetic properties and heating effect in bacterial magnetic nanoparticles

A suspension of bacterial magnetosomes was investigated with respect to structural and magnetic properties and hyperthermic measurements. The mean particle diameter of about 35 nm was confirmed by transmission electron microscopy (TEM), X-ray and magnetic analysis. The X-ray powder diffraction peaks of magnetosomes fit very well with standard Fe₃O₄ reflections. The found value for specific absorption rate (SAR) of 171 W/g at 5 kA/m and 750 kHz means that magnetosomes may be considered as good materials for the biomedical applications in hyperthermia treatments. Moreover, they have biocompatible phospholipid membrane.     

Magnetic nanostructures as amplifiers of transverse fields in magnetic resonance

We introduce the concept of amplifying the transverse magnetic fields produced and/or detected with inductive coils in magnetic resonance settings by using the reversible transverse susceptibility properties of magnetic nanostructures. First, we describe the theoretical formalism of magnetic flux amplification through the coil in the presence of a large perpendicular DC magnetic field (typical of magnetic resonance systems) achieved through the singularity in the reversible transverse susceptibility in anisotropic single domain magnetic nanoparticles. We experimentally demonstrate the concept of transverse magnetic flux amplification in an inductive coil system using oriented nanoparticles with uni-axial magnetic anisotropy. We also propose a composite ferromagnetic/anti-ferromagnetic core/shell nanostructure system with uni-directional magnetic anisotropy that, in principle, provides maximal transverse magnetic flux amplification.     

Magnetic hyperfine splitting in superparamagnetic particles in external magnetic fields

The magnetic hyperfine splitting in Mössbauer spectra of superparamagnetic particles, induced by an external magnetic field, has been calculated. Numerical results have been obtained both for isolated particles with a finite value of the magnetic anisotropy energy constant and for strongly interacting particles. Moreover, analytical approximations are derived. The theoretical results are compared with results of experimental studies of supported α-Fe particles and magnetic particles in ferrofluids.     

Thermoinduced magnetization in nanoparticles of antiferromagnetic materials

We show that there is a thermoinduced contribution to the magnetic moment of nanoparticles of antiferromagnetic materials. It arises from thermal excitations of the uniform spin-precession mode, and it has the unusual property that its magnitude increases with increasing temperature. This has the consequence that antiferromagnetism is nonexistent in nanoparticles at finite temperatures and it explains magnetic anomalies, which recently have been reported in a number of studies of nanoparticles of antiferromagnetic materials.     

Enhancement of surface magnetization in antiferromagnetic nanoparticles

We report enhancement of magnetization below the antiferromagnetic ordering temperature T_N in nanoparticles of two antiferromagnets, viz CoRh₂O₄ and Cr₂O₃. The enhancement of magnetization below T_N is systematic, being larger for sample with smaller particle size. Scaling analysis showed that such enhancement of magnetization in CoRh₂O₄ nanoparticles is due to the superparamagnetic type contribution of surface (shell) spins. The present work shows that similar analysis can also be applied in Cr₂O₃ nanoparticles.     

Peculiarities of stochastic motion in antiferromagnetic nanoparticles

Antiferromagnetic (AFM) materials are widely used in spintronic devices as passive elements (for stabilization of ferromagnetic layers) and as active elements (for information coding). In both cases the switching between different AFM states, to a great extent depends on the environmental noise. In the present paper we derive stochastic Langevian equations for an AFM vector and a corresponding Fokker-Plank equation for a distribution function in the phase space of generalised coordinate and momentum. Thermal noise is modelled by a random delta-correlated magnetic field that interacts with the dynamic magnetisation of AFM particle. We scrupulously analyse a particular case of a collinear compensated AFM in the presence of spin-polarised current. The energy distribution function is found for normal modes in the vicinity of two equilibrium states (static and stationary) in sub- and super-critical regimes. It is shown that the noise-induced dynamics of AFM vector has some pecuilarities compared to the dynamics of magnetisation vector in ferromagnets.     

Magnetic characterization of surface-coated magnetic nanoparticles for biomedical application

The influence of the oleic acid surface coating on Fe₃O₄ and NiFe₂O₄ nanoparticles on their magnetic and calorimetric characterization was investigated. Fe₃O₄ nanoparticles (particle sizes of 15-20 and 20-30 nm) and NiFe₂O₄ nanoparticles (particle sizes of 20-30 nm) were dispersed in oleic acid. The surface coating resulted in a decrease in the dipole-dipole interaction between the particles, which in turn affected the coercivity and heat dissipation of the nanoparticles. The coercivity of the oleic-acid-coated nanoparticles was found to be lower than that of the uncoated nanoparticles. The temperature rise in the oleic-acid-coated nanoparticles was greater than that of the uncoated nanoparticles; this temperature rise was associated with the relaxation losses. The viscosity dependence on the self-heating temperature of Fe₃O₄ nanoparticles (15-20 and 20-30 nm) under an ac magnetic field was measured. The temperature rise for both the Fe₃O₄ nanoparticles (15-20 and 20-30 nm) exhibited a strong dependence on viscosity at each magnetic field frequency, and the contribution of Brownian relaxation loss to the temperature rise was revealed. Moreover, an in vitro cytotoxicity test of Fe₃O₄ and NiFe₂O₄ was performed using human cervical carcinoma cells (HeLa), and the cytotoxicity of NiFe₂O₄ nanoparticles was compared to that of Fe₃O₄ nanoparticles.     

Quasioptical study of antiferromagnetic resonance in YFeO3 at submillimeter wavelength under high pulsed magnetic fields

Transmission spectra, T(H), of linearly polarized electromagnetic waves through YFeO(3), weak ferromagnet, measured at frequencies nu=96-1000 GHz in long-pulsed magnetic fields (H||k||c-axis, Faraday geometry) exhibit strong rotation of the polarization plane near the quasiferromagnetic AFMR as well as low frequency impurity modes. New ascending impurity branch including five lines was observed at high magnetic field (10-30 T) at 96 GHz and 140 GHz in addition to the known low-field descending impurity branch. Behavior of all the impurity modes assigned to transitions in (6)S(5/2) multiplet of Fe(3+) "impurity" ions in c-sites was described self-consistently by one spin-Hamiltonian. A theoretical calculation of dynamical magnetic susceptibility at AFMR and impurity modes and further simulation of transmission spectra made it possible to describe the main features of the observed spectra T(H). It was found that the T(H) behavior is determined at resonances not only by non-diagonal components of the magnetic susceptibility but also by the anisotropy of the dielectric permittivity (epsilon(xx)(') not equal epsilon(yy)(')), i.e. birefringence.     

Memory and aging effects in antiferromagnetic nanoparticles

We investigate slow dynamics of collection of a few noninteracting antiferromagnetic NiO nanoparticles. Our purpose is to enquire the role of size-dependent magnetization fluctuations in temperature and time dependent properties of antiferromagnetic nanoparticles. The zero-field cooled magnetization exhibits size dependent fluctuations. We find memory effects in field cooled magnetization, as well as aging effects in thermoremenant magnetization of antiferromagnetic nanoparticles. The antiferromagnetic nanoparticles show a stronger memory effect than the corresponding effect in the ferromagnetic particles, when the distribution of particles include very small sizes. The situation reverses for bigger sizes. The relaxation of the magnetization after a sudden cooling, heating and removal of fields reiterate the memory effects. We also see a weak signature of size-dependent magnetization fluctuations in aging effect of antiferromagnetic nanoparticles. We find a two-step relaxation of thermoremenant magnetization in antiferromagnetic case, which differs qualitatively from relaxation of ferromagnetic nanoparticles.     

Magnetic frustration in the antiferromagnetic Kondo lattice YbPtAl: Anomalous magnetic behavior at high pressures

We have investigated the magnetic and electronic properties of the antiferromagnetic Kondo lattice YbPtAl using the ¹⁷⁰Yb Mössbauer effect at ambient pressure (1.8<T<10 K), electrical resistance (1.8<T<300 K) and X-ray diffraction (T=300 K) techniques at high pressures up to 26 GPa. We find a complex magnetic state in YbPtAl at ambient pressure and an unusual volume-induced change of T_N. It is suggested, that the anomalous volume dependence of T_N is due to the interplay between frustrated anisotropic exchange interactions and magnetocrystalline anisotropy. The magnetic frustration originates from the topology of the crystal lattice.