Renormalized field theory for the static crossover in dipolar ferromagnets

A field theoretical description for the static crossover in dipolar ferromagnets is presented. New non leading critical exponents for the longitudinal static susceptibility are identified and the existence and magnitude of the dip in the effective critical exponent of the transverse susceptibility found by matching techniques is scrutinized

Critical dynamics of ferromagnets

The crossover in the dynamics from isotropic to dipolar critical behaviour has been a matter of debate over many years. We review a mode coupling theory for dipolar ferromagnets which gives a unified explanation of the seemingly contradictory experimental situation. The shape functions, the scaling functions for the damping coefficients and the precise position of the crossover are computed. Below Tc only the exchange interaction is taken into account

Observation of direct and indirect magnetoelectricity in lead free ferroelectric (Na 0.5Bi 0.5TiO 3)-magnetostrictive (CoFe 2O 4) particulate composite

A particulate composite consisting of 65 mol. % Na 0.5Bi 0.5TiO 3 and 35 mol. % CoFe 2O 4 was synthesized, and it's structure, microstructure, ferroelectric, magnetostrictive, magnetic, and direct/indirect magnetoelectric properties were studied. The composite showed different magnetization behaviour under electrically poled and un-poled conditions. The percentage change in magnetization as a result of poling is approximately -15% at 500 Oe magnetic field. Magnetostriction measurements displayed a value of λ 11 = -57 × 10 -6 and piezomagnetic coefficient δλ 11/δH = 0.022 × 10 -6 kOe -1 at 2.2 kOe for the composite. The maximum magnetoelectric output varied from 1350 mV/cm to 2000 mV/cm with change in the electrical poling condition

３回の対称性を持つ基板上に成膜したエピタキシャルフェライト薄膜の磁気特性

筑波大学 (University of Tsukuba)201

Magnetic and magnetoelastic properties of M-substituted cobalt ferrites (M=Mn, Cr, Ga, Ge)

Magnetic and magnetoelastic properties of a series of M-substituted cobalt ferrites, CoMxFe2-xO4 (M=Mn, Cr, Ga; x=0.0 to 0.8) and Ge-substituted cobalt ferrites Co1+xGexFe 2-2xO4 (x=0.0 to 0.6) have been investigated. The Curie temperature TC and hysteresis properties were found to vary with substitution content x, which indicates that exchange and anisotropy energies changed as a result of substitution of those cations for Fe. The maximum magnitude of magnetostriction decreased monotonically with increase in substitution contents x over the range x=0.0 to 0.8. However, the rate in change of magnetostriction with applied magnetic field (dlambda/dH) showed a maximum value of 5.7 x 10-9 A-1m at x=0.1 Ge sample, which is the highest value among recently reported cobalt ferrite based materials. The slope of magnetostriction with applied field dlambda/dH is one of the most important properties for stress sensor applications because it determines the sensitivity of magnetic induction to stress (dB/dsigma). The results of Ga- and Ge-substituted cobalt ferrite were compared with those of Mn- and Cr-substituted cobalt ferrites, and it was found that the effect of the substituted contents x on magnetic and magnetoelastic properties was dependent on the ionic distribution between two possible interstice sites within the spinel structure: Mn3+ and Cr3+ prefer the octahedral sites, whereas Ga3+ and Ge4+ prefer the tetrahedral sites. Temperature dependence of the absolute magnitude of the magnetic anisotropy constant |K1| of Ga-substituted cobalt ferrites CoGaxFe 2-xO4 (x=0.0 to 0.8) was investigated based on the law of approach to saturation and the results were compared with those of magnetostriction measured at the same temperatures. Based on the results, it was considered that there was a change in sign of K1 around 200 K for Ga-substituted cobalt ferrites. Comparison of the results between Ga- and Ge-substituted cobalt ferrites showed that substitution of Ge4+ ions for Fe made more pronounced effects on magnetic and magnetoelastic properties at room temperature than that of Ga3+ ions. Especially the enhanced value in dlambda/dH by Ge-substitution suggests that adjusting Ge content substituted into cobalt ferrite can be a promising route for controlling critical magnetic properties of the material for practical sensor applications

Crystal field effects on spin pumping

"Spin pumping" is the injection of spin angular momentum by a time-dependent magnetization into an adjacent normal metal proportional to the spin mixing conductance. We study the role of electrostatic interactions in the form of crystal fields on the pumped spin currents generated by insulators with exchange-coupled local moments at the interface to a metal. The crystal field is shown to render the spin currents anisotropic, which implies that the spin mixing conductance of insulator|normal metal bilayers depends on crystal cut and orientation. We interpret the interface "effective field" (imaginary part of the spin mixing conductance) in terms of the coherent motion of the equilibrium spin density induced by proximity in the normal metal.Comment: 8 pages+, 7 figure

Preparation of Spinel and Garnet Ferrites and Identification of Their Magnetic-Energy Losses

The objective of this work was to explain the magnetic-energy loss mechanisms of some magnetic materials. The study was divided into three parts., The first part involved fabrication of NiZn-based and YIG ferrites in toroidal and pellet form, employing ceramic processing technique of the starting oxides. Characterisation of chemical, microstructural, magnetic, electrical, mechanical and thermal properties were carried out. In the second part, sol-gel method was employed to obtain high quality and fine-grained microstucture. The Y3FeS012 and NiFe204 samples were fabricated using this technique. The third part dealt with some preliminary studies on the magneto-optical Kerr effect, which were carried out on the NiFe204 and YSFeS012 samples. The characterisation of samples in the first part was divided mainly into two parts: the extrinsic-microstructure properties and the intrinsic-composition properties. The results showed that the initial permeability, relative loss factor, impedance, power loss, quality factor, saturation induction, core loss, coercive force, curie temperature and temperature coefficient of the sintered samples depended chiefly on both the microstructure and the composition of the samples. Adopting ZnO, which acted as a modifier, in the NiZn ferrite series (first premise) had greatly influenced the magnetic properties of the samples, as occurrence of Zn loss was a major factor that affected the grain growth kinetics. Adopting an iron-deficit composition (second series) was fruitful when high density and wide operating frequencies were required in the NiZn ferrite composition. Samples with excess Fe203 (third series) were deleterious in terms of losses due to the formation of Fe2+. There was no significant contribution of the zero magnetostriction affecting the magnetic and electrical properties that was concluded from this premise. CoO was seen to affect the growth anisotropy in the rich NiO content (fourth premise) and thus affected the microstructure of the samples. Interesting, however, was sample with composition Nio.8Zno.2Fe204 that gave very homogeneous and moderate grain size (:::::10.9 flm) exhibited large -KJ , played a dominant role in the frequency extension. Evidence by the reduced permeability, it was believed that the damping of domain wall was restricted by the anisotropy effects. Simultaneously, the relative loss factor was significantly reduced at higher frequencies. In the fifth premise where both C0 2 + and Fe 2 + were adopted in the excess non NiZn based composition, the C02+ content was believed to stabilise the domain wall movement at high frequencies. When a small concentration of cobalt with the formula Nio.70COO.0191sZno.27S8SFe2.00S04.00S was adopted, a vast decrease of power loss was seen to occur. It was speculated that C02+ ions diffused or moved through the vacancies and hence caused them to reside in the vacancies created by the slight iron excess. This reduced the stress and strain created by them and as a result, power loss was reduced significantly. In the second part of this work, high quality and fine grained single-phase ferrite (-0.9 Ilm) was obtained by using the sol-gel technique. Finally, Kerr rotation (Nl deg) was observed for both the NiFe204 and Y3Fes012 samples. Kerr rotation was accompanied by optical energy reflection . This was actually a measure of energy reflected when ferromagnetic order exists. This shed new light in the area of magnetooptic

Effect of pulsed magnetic field pre-treatment of AISI 52100 steel on the coefficient of sliding friction and wear in pin-on-disk tests

Disc specimens manufactured from commercial bearing rollers (AISI 52100 steel, 62–63 HRC) in initial state and after pre-treatment by pulsed magnetic field (PMF) with a magnetic field strength of 1–7 MA/m were tested with sunflower oil using pin-on-disk apparatus. According to the obtained results the treatment causes a reduction in the coefficient of friction and wear. To explain the results, nano- and microhardness tests as well as optical and atomic force microscopy were used. Reasons of the effect of PMF on the friction and wear were discussed.Peer reviewe

Hardening of cobalt ferrite nanoparticles by local crystal strain release: implications for rare earth free magnets

In this work, we demonstrate that the reduction of the local internal stress by a low-temperature solvent-mediated thermal treatment is an effective post-treatment tool for magnetic hardening of chemically synthesized nanoparticles. As a case study, we used nonstoichiometric cobalt ferrite particles of an average size of 32(8) nm synthesized by thermal decomposition, which were further subjected to solvent-mediated annealing at variable temperatures between 150 and 320 °C in an inert atmosphere. The postsynthesis treatment produces a 50% increase of the coercive field, without affecting neither the remanence ratio nor the spontaneous magnetization. As a consequence, the energy product and the magnetic energy storage capability, key features for applications as permanent magnets and magnetic hyperthermia, can be increased by ca. 70%. A deep structural, morphological, chemical, and magnetic characterization reveals that the mechanism governing the coercive field improvement is the reduction of the concomitant internal stresses induced by the low-temperature annealing postsynthesis treatment. Furthermore, we show that the medium where the mild annealing process occurs is essential to control the final properties of the nanoparticles because the classical annealing procedure (T > 350 °C) performed on a dried powder does not allow the release of the lattice stress, leading to the reduction of the initial coercive field. The strategy here proposed, therefore, constitutes a method to improve the magnetic properties of nanoparticles, which can be particularly appealing for those materials, as is the case of cobalt ferrite, currently investigated as building blocks for the development of rare-earth free permanent magnets.This work was supported by EU-H2020 AMPHIBIAN Project (Grant no. 720853). A.L.O. acknowledges support from the Universidad Pública de Navarra (Grant no. PJUPNA2020). Open access funding provided by Universidad Pública de Navarra