K. E. Kamentsev

Ferrite-Piezoelectric Multilayers for Magnetic Field Sensors

A magnetic field sensor based on magnetoelectric effects in a ferrite-piezoelectric layered sample is proposed. Such sensors are passive, provide direct conversion of magnetic fields into an electrical signal, and allow measurements of both ac and dc magnetic fields. A multilayer sample of nickel zinc ferrite-lead zirconate titanate has been used to characterize the sensor response to ac and dc fields, field orientations, frequency, and temperature. The sample shows a linear response for dc fields up to a maximum of 1750 Oe. The sensor output is temperature independent over 273-337 K, but is dependent on frequency of the ac excitation field. Operating at electromechanical resonance for the element enhances the sensor sensitivity by an order of magnitude. For ac magnetic field sensors, the output varies linearly with amplitude

Preparation and X-ray diffraction, dielectric, and Mössbauer characterization of Co1 − x Ni x Cr2O4 solid solutions

Co1 − xNixCr2O (0 ≤ x ≤ 1) ceramic samples have been characterized by X-ray diffraction and relative dielectric permittivity ɛ(T), loss tangent tan δ(T) (0.1–200 kHz), and thermally stimulated depolarization current (TSDC) measurements in the range 100–350 K. The samples were shown to consist of spinel solid solutions with a cubic (0 < x ≤ 0.98) or tetragonal (0.99 ≤ x < 1) structure. Increasing the Ni content of the samples from 0 to 100 at % increases their tanδ and 1/ρ by three to four orders of magnitude. The ɛ(T), tan δ(T), and TSDC of the samples with 0.2 ≤ x ≤ 0.6 have anomalies at T1 ∼ 220 K and T2 ∼ 240 K, which point to a transition to a polar state below T2. Samples containing 1–4 at % 57Fe as an impurity were characterized by Mössbauer spectroscopy in the range 54–330 K and were shown to be magnetically ordered at 78 K. We conclude that the solid solutions have ferroelectric properties with a Curie temperature T2 and magnetoelectric multiferroic properties.

Polar Order and Frustrated Antiferromagnetism in Perovskite Pb2MnWO6 Single Crystals

Single crystals of the multiferroic double-perovskite Pb2MnWO6 have been synthesized and their structural, thermal, magnetic and dielectric properties studied in detail. Pure perovskite-phase formation and stoichiometric chemical composition of the as-grown crystals are confirmed by X-ray single-crystal and powder diffraction techniques as well as energy-dispersive X-ray and inductively coupled plasma mass spectrometry. Detailed structural analyses reveal that the crystals experience a structural phase transition from the cubic space group (s.g.) Fm3̅m to an orthorhombic structure in s.g. Pn21a at about 460 K. Dielectric data suggest that a ferrielectric phase transition takes place at that same temperature, in contrast to earlier results on polycrystalline samples, which reported a transition to s.g. Pnma and an antiferroelectric low-temperature phase. Magnetic susceptibility measurements indicate that a frustrated antiferromagnetic phase emerges below 8 K. Density functional theory based calculations confirm that the cationic order between Mn and W is favorable. The lowest total energy was found for an antiferromagnetically ordered state. However, analyses of the calculated exchange parameters revealed strongly competing antiferromagnetic interactions. The large distance between the magnetic atoms, together with magnetic frustration, is shown to be the main reason for the low value of the ordering temperature observed experimentally. We discuss the structure-property relationships in Pb2MnWO6 and compare these observations to reported results on related Pb2BWO6 perovskites with different B cations.

Preparation and X-Ray diffraction, dielectric, and Mössbauer characterization of Co1 − x Cu x Cr2O4 ceramics

Co1 − xCuxCr2O4 (0 ≤ x ≤ 1) ceramic samples have been characterized by room-temperature X-ray diffraction and relative dielectric permittivity ɛ(T) and loss tangent tanδ(T) measurements (at frequencies from 0.1 to 200 kHz) in the range 100–350 K. The samples were shown to consist of spinel solid solutions with a cubic (0 < x < 0.53) or tetragonal (0.53 < x < 1) structure. Increasing the Cu content of the samples increases their tanδ and electrical conductivity and produces anomalies in the temperature dependences ɛ(T) and tanδ(T) due to dielectric relaxation processes. Co1−xCuxCr2O4:0.0157Fe2O3 samples were characterized by Mössbauer spectroscopy in the range 77–290 K and shown to be in a magnetically ordered state at 77 K.

Synthesis, X-ray Diffraction Characterization, Mössbauer Spectroscopy, and Dielectric Properties of Solid Solutions in the PbFe2/3W1/3O3–PbSc2/3W1/3O3 System

Ceramic Pb(Fe1–xScx)2/3W1/3O3 samples with 0 ≤ x ≤ 1 have been prepared and characterized by X-ray diffraction, Mössbauer spectroscopy, and dielectric and pyroelectric measurements. The stoichiometry ranges of the perovskite solid solutions in this system have been identified, their structural parameters have been determined, and their dielectric permittivity, dielectric loss tangent, resistivity, and thermally stimulated depolarization current have been measured as functions of temperature. The composition dependences of the dielectric properties for the solid solutions have been obtained. The solid solutions have been shown to exhibit ferroelectric relaxor properties, with a well-defined maximum in their permittivity in the range 180–250 K.

X-ray, Mössbauer, and dielectric studies of the Co1 − xNixCr2O4 ceramic system

The temperature dependences of the dielectric constant, ɛ(T), dielectric loss tangent, tanδ(T), and thermostimulated depolarization currents of (Co1 − xNix)Cr2O4:y57Fe2O3 samples with 0.2 ≤ x ≤ 0.6 and 0.01 ≤ y ≤ 0.04 exhibit anomalies at temperatures of T1 ≈ 220 K and T2 ≈ 240 K that indicate a transition to an ordered ferroelectric state at temperatures below T2. Also observed at the same temperatures are abrupt changes in the isomer shift and quadrupole splitting for two Mössbauer spectral doublets of a sample with x = 0.2.