A.S. Semisalova

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-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 Anisotropy in the (Cr0.5Mn0.5)2GaC MAX Phase

Magnetic MAX phase (Cr0.5Mn0.5)2GaC thin films grown epitaxially on MgO(111) substrates were studied by ferromagnetic resonance at temperatures between 110 and 300 K. The spectroscopic splitting factor g = 2.00 ± 0.01 measured at all temperatures indicates pure spin magnetism in the sample. At all temperatures we find the magnetocrystalline anisotropy energy to be negligible which is in agreement with the identified pure spin magnetism.

Room-temperature ferromagnetism and anomalous Hall effect in Si1−xMnx (x ≈ 0.35) alloys

AdetailedstudyofthemagneticandtransportpropertiesofSi1−xMnx (x ≈ 0.35)filmsispresented.Weobserve the anomalous Hall effect in these films up to room temperature. The results of the magnetic measurements and the anomalous Hall effect data are consistent and demonstrate the existence of long-range ferromagnetic order in the systems under investigation. A correlation of the anomalous Hall effect and the magnetic properties of the samples with their conductivity and substrate type is shown. A theoretical model based on the idea of a two-phase magnetic material, in which molecular clusters with localized magnetic moments are embedded in the matrix of a weak itinerant ferromagnet, is discussed and used to explain experimental results. The long-range ferromagnetic order at high temperatures is mainly due to the Stoner enhancement of the exchange coupling between clusters through thermal spin fluctuations (“paramagnons”) in the matrix. Theoretical predictions do not contradict experimental data when model parameters of a plausible order of magnitude are used.

Ferromagnetism of MnxSi1-x(x ∼ 0.5) films grown in the shadow geometry by pulsed laser deposition method

The results of a comprehensive study of magnetic, magneto-transport and structural properties of nonstoichiometric MnxSi1-x (x ≈ 0.51-0.52) films grown by the Pulsed Laser Deposition (PLD) technique onto Al2O3(0001) single crystal substrates at T = 340°C are present. A highlight of used PLD method is the non-conventional (“shadow”) geometry with Kr as a scattering gas during the sample growth. It is found that the films exhibit high-temperature (HT) ferromagnetism (FM) with the Curie temperature TC ∼ 370 K accompanied by positive sign anomalous Hall effect (AHE); they also reveal the polycrystalline structure with unusual distribution of grains in size and shape. It is established that HT FM order is originated from the bottom interfacial self-organizing nanocrystalline layer. The upper layer adopted columnar structure with the lateral grain size ≥50 nm, possesses low temperature (LT) type of FM order with Tc ≈ 46 K and contributes essentially to the magnetization at T ≤ 50 K. Under these conditions, AHE ...

Properties of Zn1 − xCoxO films produced by pulsed laser deposition with fast particle separation

The study is concerned with the structural, optical, magnetic, and transport properties of Zn1 − xCoxO (x = 0.05–0.45) films produced on Al2O3 (0001) substrates at a temperature of Ts = 500°C by pulsed laser deposition with fast particle separation. The film thickness is d = 60–300 nm. It is found that the Zn1 − xCoxO ternary alloy retains its wurtzite-type crystal structure up to x = 0.35, if the films are produced at low buffer-oxygen pressures (∼10−6 Torr). It is established that, in these conditions, the electron concentration is higher than 1020 cm−3 because of the high density of oxygen donor vacancies. In this case, the films start to exhibit ferromagnetism in the magnetization and the anomalous Hall effect at temperatures above 100 K. The sign of the anomalous Hall effect is found to be positive and opposite to the sign of the normal Hall effect, as occurs in Co metal layers. This is indicative of the cluster nature of ferromagnetism of the Zn1 − xCoxO films. For thin Zn1 − xCoxO layers (d = 60 nm, x = 0.2) in a transverse magnetic field, profound hysteresis of the magnetoresistance is observed, which is indicative of the out-of-plain easy axis of magnetization of the films. The magnetic anisotropy is attributed to the structuring of the layers (elongation of magnetic clusters along the growth axis of the films). The structuring can lead to noticeable strengthening of the layer ferromagnetism.

High-temperature ferromagnetism of Si1 − x Mn x films fabricated by laser deposition using the droplet velocity separation technique

The transport and magnetic properties of Si1 − xMnx films of thickness 55–70 nm with various Mn content (x = 0.44–0.6) are studied in the temperature range of 5–400 K and in magnetic fields up to 2 T. The films are grown by pulsed laser deposition on Al2O3 (0001) substrates at a temperature of 340°C using velocity separation of deposited particles. The films exhibit metal conductivity and the resistivity ρ = (2−8) × 10−4 Ω cm, typical of highly degenerate semiconductors. It is found that the anomalous component of the Hall effect dominates over the normal component at T = 300 K for the Si1 − xMnx alloy with x ≈ 0.5, and that the Curie temperature significantly exceeds room temperature and is estimated as ∼500 K from magnetization measurements (for MnSi silicide the Curie temperature is TC = 30 K). It is shown that the anomalous component of the Hall conductivity at low temperatures is controlled by “side-jump” and (or) “intrinsic” mechanisms independent on the carrier scattering time. The results are explained by features of the formation of defects with localized magnetic moments in the case of Si1 − xMnx films with x ≈ 0.5 and by the significant role of matrix spin fluctuations in the exchange between these defects.

Oriented arrays of iron nanowires: synthesis, structural and magnetic aspects

Iron nanowires with the diameter of ca. 40 nm and a length up to few dozens of microns are fabricated via templated electrodeposition using anodic aluminum oxide (AAO) film as porous matrix. Despite polycrystalline structure of wires the technique allows fabrication of dense deposits with micrometer-sized single crystalline grains within AAO templates and high chemical stability towards oxidation. Nanowire arrays exhibit strong magnetization anisotropy with saturation magnetization of 180 emu/g and coercive field of 815 Oe in direction parallel to the long axis of nanowires and 230 Oe in perpendicular direction. The effective hyperfine fields on iron atoms as extracted from Mossbauer and Nuclear Forward Scattering of sample in demagnetized state indicates slight deviation of magnetization vector (~ 6°) from nanowire long axis appearing probably due to curling of magnetic moments by antisymmetric exchange interactions at the surface of nanowires.Graphical Abstract