N. S. Perov

Motion of ferroparticles inside the polymeric matrix in magnetoactive elastomers

Ferroelastic composites are smart materials with unique properties including large magnetodeformational effects, strong field enhancement of the elastic modulus and magnetic shape memory. On the basis of mechanical tests, direct microscopy observations and magnetic measurements we conclude that all these effects are caused by reversible motion of the magnetic particles inside the polymeric matrix in response to an applied field. The basic points of a model accounting for particle structuring in a magnetoactive elastomer under an external field are presented.

Weak ferromagnetism in hexagonal orthoferrites RFeO3 (R = Lu, Er-Tb)

Hexagonal orthoferrites of rare earths RFeO3 (R = Lu, Er-Tb) were grown epitaxially on (111)ZrO2(Y2O3) substrates using metal-organic chemical vapour deposition. Temperature and field dependences of magnetization were measured and analyzed for all samples and revealed weak ferromagnetic behavior below T = 120–140 K. The difference in electronic structure along with a distinct similarity in the crystal structure of hexagonal manganites RMnO3 and hexagonal orthoferrites RFeO3 are brought into focus in order to explain the results. Hexagonal orthoferrites are regarded as a promising family of multiferroics.

Magnetic and viscoelastic response of elastomers with hard magnetic filler

Magnetic elastomers (MEs) based on a silicone matrix and magnetically hard NdFeB particles have been synthesized and their magnetic and viscoelastic properties have been studied depending on the size and concentration of magnetic particles and the magnetizing field. It has been shown that magnetic particles can rotate in soft polymer matrix under applied magnetic field, this fact leading to some features in both magnetic and viscoelastic properties. In the maximum magnetic field used magnetization of MEs with smaller particles is larger while the coercivity is smaller due to higher mobility of the particles within the polymer matrix. Viscoelastic behavior is characterized by long relaxation times due to restructuring of the magnetic filler under the influence of an applied mechanical force and magnetic interactions. The storage and loss moduli of magnetically hard elastomers grow significantly with magnetizing field. The magnetic response of the magnetized samples depends on the mutual orientation of the external magnetic field and the internal sample magnetization. Due to the particle rotation within the polymer matrix, the loss factor increases abruptly when the magnetic field is turned on in the opposite direction to the sample magnetization, further decreasing with time. Moduli versus field dependences have minimum at non-zero field and are characterized by a high asymmetry with respect to the field direction.

Strong magnetodielectric effects in magnetorheological elastomers

The effect of a uniform magnetic field on the permittivity of magnetorheological elastomers (MREs) is studied. MREs were synthesized on the basis of silicone rubber and magnetic fillers of various chemical nature (Fe, NdFeB and Fe3O4) and particle sizes. The value of permittivity was obtained from the measurements of the capacity of a plane capacitor with MRE samples. A strong increase of the permittivity (magnetodielectric effect) was observed when the applied field was perpendicular to the capacitor plates. The value of the magnetodielectric effect was found to be strongly dependent on the type of magnetic filler as well as on the size and concentration of magnetic particles within MRE composites. The highest magnetic response reaching 150% was observed for the MRE based on a magnetically hard NdFeB filler. A simple model explaining physical reasons for the magnetodielectric effect in a MRE is proposed. The developed MRE with a strong magnetodielectric effect is very promising for a wide range of applications, in particular, as magnetic field sensors and actuators.

High-frequency giant magneto-impedance in multilayered magnetic films

We present the results of experimental and theoretical investigations of giant magneto-impedance (GMI) in the thin-film multilayer structure of F/SiO2/Ti/Cu/Ti/SiO2/F, where F stands for the ferromagnetic alloy having a composition of Fe73.5Cu1Nb3Si16.5B6. The layers were deposited under a transversemagnetic field Hp = 200 Oe applied in the plane of the sample perpendicular to its long side and were annealed at 280°C under Hp = 420 Oe for 2h. The GMI effect was measured at frequencies ranging from 0.5 to 1250 MHz using a waveguide methhod. An external magnetic field with a strength of up to 130 Oe was applied along the direction of the alternating current. The GMI is shown to exist in the multilayer structure even if the skin depth in the ferromagnetic layers is of the order of their thickness. Both real and imaginary parts of the impedance vary significantly under external magnetic field. A maximum GMI sensitivity of 10–35%/Oe was achieved at high frequencies.

The features of GMI effect in amorphous wires at microwaves

A new waveguide method is used to investigate high frequency GMI and magnetic permeability in Co-rich amorphous wire. The GMI spectra at microwaves are quite different from those measured at relatively low frequency. The amplitude of magnetic permeability decreases with increasing frequency, however, its relative change under external magnetic field is still large which enables a high sensitivity of GMI sensors of up to 1000 MHz.

Resonance magnetoelectric interactions in an asymmetric ferromagnetic-ferroelectric layered structure

Strain mediated magnetoelectric (ME) interactions have been investigated in a sample consisting of oppositely poled lead zirconate titanate (PZT) and asymmetric magnetostrictive layers. A thin layer of Ni with negative magnetostriction and amorphous ferromagnetic Metglas with positive magnetostriction are bonded to the PZT layers. It is shown that the magnetic layers facilitate effective excitation of bending oscillations in the structure, whereas the use of oppositely poled PZT layers results in an increase in the ME voltage at the bending resonance frequency, suppression of the voltage at the longitudinal electromechanical resonance frequency, and cancellation of thermal fluctuation in the voltage. The ME voltage coefficient at resonance is 18 V/(cm Oe); that is an order of magnitude higher than the value measured for a Ni–PZT bilayer of similar dimensions. Theoretical estimates of the ME voltage and resonance frequency are in good agreement with the data.

Experimental Setup for Investigating Topochemical Transformations of Ferromagnetic Nanoparticles

An experimental setup controlling the topochemical transformations in synthesis of ferromagnetic metal nanoparticles is described. The setup is based on a vibrating magnetometer. The range of operating temperatures in the reaction zone is 300–870 K. The required sensitivity is maintained by a magnetic field of up to 0.6 T. Gases (or gas mixtures) may be blown through the reactor at a flow rate as high as 150 cm3/min. The experimental results illustrating the capabilities of the setup are presented.

High-temperature ferromagnetism in Si1 − xMnx (x ≈ 0.5) nonstoichiometric alloys

It has been found that the Curie temperature (TC ≈ 300 K) in nonstoichiometric Si1 − xMnx alloys slightly enriched in Mn (x ≈ 0.52–0.55) in comparison to the stoichiometric manganese monosilicide MnSi becomes about an order of magnitude higher than that in MnSi (TC ∼ 30 K). Deviations from stoichiometry lead to a drastic decrease in the density of charge carries (holes), whereas their mobility at about 100 K becomes an order of magnitude higher than the value characteristic of MnSi. The high-temperature ferromagnetism is ascribed to the formation of defects with the localized magnetic moments and by their indirect exchange interaction mediated by the paramagnetic fluctuations of the hole spin density. The existence of defects with the localized magnetic moments in Si1 − xMnx alloys with x ≈ 0.52–0.55 is supported by the results of numerical calculations performed within the framework of the local-density-functional approximation. The increase in the hole mobility in the nonstoichiometric material is attributed to the decay of the Kondo (or spin-polaron) resonances presumably existing in MnSi.

Magnetic properties of short amorphous microwires

Abstract Magnetic properties of amorphous Fe- and Co-rich wires with different magnetostriction are studied in dependence on their length (8 mm L L * is estimated. It is shown that for L L * the influence of demagnetizing fields on the wire magnetization is substantial and the wire remanent magnetization is proportional to the square of the wire length. The oscillations of the magnetic moment component parallel to the wire axis during the cross remagnetization processes are found.