Ferromagnetic Contribution Research Articles

Magnetic Properties and Structure of TiO2-Mn (0.73%) Nanopowders: the Effects of Electron Irradiation and Vacuum Annealing

Nanopowder TiO2-0.73 % Mn was synthesized by the sol-gel method. Thermal treatment of the samples was carried out in vacuum at a temperature of 500°C. Magnetic properties were studied in the temperature range from 2 to 850 K. The effects of electron irradiation and vacuum annealing on the EPR spectra and magnetic properties of TiO2‑Mn powder are discussed. It was established that a part of manganese ions in the anatase crystal lattice interacts antiferromagnetically, which causes a decrease in magnetization as compared to the result of the calculation for non-interacting ions. Vacuum annealing leads to the formation of oxygen vacancies and, at the same time, to a noticeable increase in the ferromagnetic contribution to magnetization, especially, after preliminary electron irradiation. We assume that the ferromagnetic contribution to the magnetization appears either due to incomplete compensation of antiferromagnetically directed moments of manganese ions, or due to positive exchange interactions of Mn ions via defects in the TiO2 lattice. It is shown that the temperature of magnetic disordering in samples with a spontaneous magnetic moment exceeds 600°C.

On the magnetic contribution to the inverse magnetocaloric effect in Ni-Co-Cu-Mn-Sn metamagnetic shape memory alloys

Abstract Martensitic transformation and magnetic properties in directionally solidified Ni45−xCoxCu5Mn39Sn11 (x = 0, 1, 2) alloys were studied. The martensitic transformation temperature decreases with increasing Co concentration, whereas the Curie temperature of austenite increases, what then alters the magnetic state of austenite at the onset of martensitic phase transformation. Concomitantly the effective magnetic moment of austenite increases by 0.7 μ B / f . u . with Co substitution. Under an applied magnetic field of 5 T the peak values of the magnetic entropy changes reach 11.4 J/kgK (x = 1) and 4.5 J/kgK (x = 2). The variations in magnetic entropy are discussed in relation to a stronger ferromagnetic contribution promoted by Co in the martensite phase.

Physical properties of fluorides barium and calcium nanopowders produced by the pulsed electron beam evaporation method

The mesoporous nanocrystal powders BaF2 and CaF2 with a specific surface up to 34.8 and 88.7 sq.m/g, respectively, are produced by the pulsed electron beam evaporation method in vacuum. The influence of thermal annealing of nanoparticles on air in the range of temperature from 200 to 900°C on the size, morphology of particles and change of their magnetic and luminescent properties investigated have been investigared. The essential stoichiometric impurity (overage of metals) and significant growth in a specific surface of nanopowders (NP) BaF2 and CaF2 after annealing at the temperature of 200°C have been detected. It is established that the synthesized NP BaF2 is a paramagnetic while initial material in the bulk state is diamagnetic. After annealing at 900°C appears the small ferromagnetic contribution at NP BaF2. Produced NP CaF2 showed ferromagnetic behavior. In literature there is no information about the ferromagnetism of CaF2. Appearance of the ferromagnetic response can be explained with formation of structural and radiation defects (F-centers, etc.). The analysis of PCL and magnetization curves of samples BaF2 and CaF2 allows drawing conclusions about their connection.

Structural and Magnetic Properties of Co‒Mn Codoped ZnO Nanoparticles Obtained by Microwave Solvothermal Synthesis

Zinc oxide nanoparticles codoped with Co2+ and Mn2+ ions (Zn(1−x−y)MnxCoyO NPs) were obtained for the first time by microwave solvothermal synthesis. The nominal content of Co2+ and Mn2+ in Zn(1−x−y)MnxCoyO NPs was x = y = 0, 1, 5, 10 and 15 mol % (the amount of both ions was equal). The precursors were obtained by dissolving zinc acetate dihydrate, manganese (II) acetate tetrahydrate and cobalt (II) acetate tetrahydrate in ethylene glycol. The morphology, phase purity, lattice parameters, dopants content, skeleton density, specific surface area, average particle size, average crystallite size, crystallite size distribution and magnetic properties of NPs were determined. The real content of dopants was up to 25.0% for Mn2+ and 80.5% for Co2+ of the nominal content. The colour of the samples changed from white to dark olive green in line with the increasing doping level. Uniform spherical NPs with wurtzite structure were obtained. The average size of NPs decreased from 29 nm to 21 nm in line with the increase in the dopant content. Brillouin type paramagnetism and an antiferromagnetic interaction between the magnetic ions was found for all samples, except for that with 15 mol % doping level, where a small ferromagnetic contribution was found. A review of the preparation methods of Co2+ and Mn2+ codoped ZnO is presented.

Anomalous ferromagnetic behavior and size effects in CuO nanowires

In this work we show anomalous ferromagnetic behavior in CuO nanowires grown on top of a CuO/Cu2O bilayer produced via thermal oxidation in air at 400 and 500 °C for various annealing times. Our results show that there are strong size effects as well as temperature dependence governing the ferromagnetic-like behavior of the CuO nanowires. Thinner and longer CuO nanowires resulted in higher saturation magnetization values, which may be related to the presence of well-ordered and aligned surface unpaired electron spins and a high concentration of defects. Our results also show a competition between the anomalous ferromagnetic contribution arising from the CuO nanowires and the diamagnetic contribution arising from the Cu2O layer. Finally, the growth axis of the CuO nanowires was determined to be nearly perpendicular to the easy axis of magnetization of CuO.

Effects of temperature and atomic disorder on the magnetic phase transitions in ZnO nanoparticles obtained by sol–gel method

In this work, we have studied the effects of temperature and atomic disorder on the magnetic phase transitions in ZnO nanoparticles obtained by sol–gel method. The samples were sintered at temperatures of 600 °C, 800 °C 1000 °C and 1200 °C for 2 h. The X-ray diffraction (XRD) analysis have shown a single phase, belonging to the hexagonal structure of ZnO and the Rietveld refinement showed an increase in the lattice parameters a and c with increasing sintering temperature. The crystallite mean size (D) increases from 61 nm to 70 nm, while the lattices strain decreases from 0.181% to 0.169% when the sintering temperature varies from 600 °C to 1200 °C. The magnetic measurements at 300 K, exhibit a ferromagnetic contribution in combination with a paramagnetic component for samples obtained at 600 °C, 800 °C and 1000 °C. A magnetic transition was verified for the sample obtained at 1200 °C, now with diamagnetic behavior. The sol–gel method is presented as an alternative to study the evolution of the different magnetic phases as a function of the degree of atomic disorder in ZnO nanoparticles.

Mictomagnetic State in an EuBaCo2–xO5.5–δ Single Crystal

The magnetic properties of an EuBaCo1.9O5.36 single crystal are studied in the temperature range T = 2–300 K and the magnetic field range H ≤ 90 kOe. This binary layered cobaltite single crystal has vacancies in the cobalt and oxygen sublattices, in contrast to the stoichiometric EuBaCo2O5.5 composition. All cobalt ions in EuBaCo1.9O5.36 are in a trivalent state. The single crystal has an orthorhombic structure with space group Pmmm, and its unit cell parameters are a = 3.883 A, b = 7.833 A, and c = 7.551 A. The field and temperature dependences of the magnetization of the single crystal demonstrate that it is ferrimagnet below TC = 242 K. At T < 300 K, all three spin states of the Co3+ ions are present. The nearest-neighbor interactions give antiferromagnetic (AFM) and ferromagnetic (FM) contributions to the exchange energy. The ratio of the AFM to the FM contributions changes when temperature decreases because of a change in the spin state of the Co3+ ions. The single crystal exhibits signs of mictomagnetism at low temperatures in high magnetic fields. At T = 2 K and H = 90 kOe, the zero-field and nonzero-field magnetizations are strongly different because of a uniaxial magnetic anisotropy, which tends to set magnetization along the magnetic field applied in cooling throughout the crystal volume. As a result, a complex ferrimagnetic structure with a noncollinear direction of Co3+ spins appears. The following phenomena characteristic of mictomagnets are also observed in the EuBaCo1.9O5.36 single crystal: a shift in a magnetization hysteresis loop when temperature decreases, retained hysteretic phenomena and no magnetization saturation in high magnetic fields, and an orientation transition. The mictomagnetic state in EuBaCo1.9O5.36 is shown to be caused by the structural distortions induced by vacancies in the cobalt and oxygen sublattices and by the frustration of AFM and FM exchange interactions.

Mechanism for Unconventional Superconductivity in the Hole-Doped Rashba-Hubbard Model.

Motivated by the recent resurgence of interest in topological superconductivity, we study superconducting pairing instabilities of the hole-doped Rashba-Hubbard model on the square lattice with first- and second-neighbor hopping. Within the random phase approximation, we compute the spin-fluctuation-mediated pairing interactions as a function of filling. Rashba spin-orbit coupling splits the spin degeneracies of the bands, which leads to two van Hove singularities at two different fillings. We find that, for a doping region in between these two van Hove fillings, the spin fluctuations exhibit a strong ferromagnetic contribution. Because of these ferromagnetic fluctuations, there is a strong tendency towards spin-triplet f-wave pairing within this filling region, resulting in a topologically nontrivial phase.

Origin of magnetism probed through electron spin resonance studies in Zn1-xMnxO synthesized by chemical route

Nanocrystalline Zn1-xMnxO (0.005 ≤ x ≤ 0.02) shows room temperature ferromagnetic behaviour. The electron spin resonance spectra of Zn1-xMnxO (0.005≤ x ≤ 0.02) consist of a broad resonance line stemming from interacting spins and a set of hyperfine structure lines originating in the presence of the paramagnetic Mn2+ ions. The broad resonance lines and the observation of resonance at low field indicate the magnetic inhomogeneity of the sample. The ferromagnetic contribution coming from the cationic interaction like Zn2+-Zn2+, Zn2+-Mn2+ independently or through oxygen vacancies dominate over the paramagnetic behaviour of Mn2+-Mn2+ interaction and gives rise to ferromagnetic behaviour in Mn doped ZnO system.

Separation of Diamagnetic and Paramagnetic Fabrics Reveals Strain Directions in Carbonate Rocks

AbstractWe present a new procedure for separating magnetic fabrics in coccolith‐bearing chalk samples, demonstrated in the case studies of three sites located within the Dead Sea Fault (DSF) plate boundary. The separation is achieved by combining measurements of room temperature and low‐temperature anisotropy of magnetic susceptibility (RT‐AMS and LT‐AMS, respectively) with anisotropy of anhysteretic remanence magnetization (AARM). The LT‐AMS, measured at ~77 K, enhances the fabric of paramagnetic clay minerals. The AARM represents the fabric of ferromagnetic Fe oxides. By subtracting the paramagnetic and ferromagnetic fabrics from the RT‐AMS, the diamagnetic fabric is separated. In the studied samples, we found that the ferromagnetic contribution to the bulk magnetic fabric is negligible and could be excluded from the subtraction procedure. Our analysis indicates that in chalks with a negligible ferromagnetic contribution, diamagnetic fabric predominates the rock bulk magnetic fabric, if the mean susceptibility is &lt;−6 × 10−6 SI, whereas with a mean susceptibility &gt;11 × 10−6 SI, paramagnetic fabric predominates. In the studied rocks, the paramagnetic clay minerals preserve the original depositional fabric, whereas the diamagnetic minerals show a tectonic fabric. We propose a mechanism by which coccolith rotation under tectonic strain contributes to the development of the diamagnetic fabric parallel to the shortening direction. We infer that the diamagnetic fabrics of the studied rocks indicate strain regime of approximately N‐S horizontal shortening near strands of the DSF system. This suggests a deflection of the regional principal strain axes near the DSF. The diamagnetic fabric is more sensitive to tectonic strain than paramagnetic fabric in chalks and provides a valuable strain indicator near major faults.

Appearance of itinerant electrons detected by IR spectroscopy and its correlation with surface magnetism in Co-doped TiO2 nanopowders

IR absorption and magnetic data for the titanium dioxide nanopowders are studied. Additional absorption in IR range is revealed in nanocrystalline TiO2 and Co-doped TiO2 after reducing annealing and explained by appearance of the oxygen vacancies and delocalised charge carriers. Increasing of the absorption in IR range after annealing of Co-doped TiO2 in vacuum (or decreasing of this contribution after annealing in air) correlates with arising (or vanishing) of ferromagnetic contribution.

Direct evidence for the source of reported magnetic behavior in “CoTe”

In order to unambiguously identify the source of magnetism reported in recent studies of the Co-Te system, two sets of high-quality, epitaxial CoTex films (thickness ≈ 300 nm) were prepared by pulse laser deposition (PLD). X-ray diffraction (XRD) shows that all of the films are epitaxial along the [001] direction and have the hexagonal NiAs structure. There is no indication of any second phase metallic Co peaks (either fcc or hcp) in the XRD patterns. The two sets of CoTex films were grown on various substrates with PLD targets having Co:Te in the atomic ratio of 50:50 and 35:65. From the measured lattice parameters c = 5.396 Å for the former and c = 5.402 Å for the latter, the compositions CoTe1.71 (63.1% Te) and CoTe1.76 (63.8% Te), respectively, are assigned to the principal phase. Although XRD shows no trace of metallic Co second phase, the magnetic measurements do show a ferromagnetic contribution for both sets of films with the saturation magnetization values for the CoTe1.71 films being approximately four times the values for the CoTe1.76 films. 59Co spin-echo nuclear magnetic resonance (NMR) clearly shows the existence of metallic Co inclusions in the films. The source of weak ferromagnetism reported in several recent studies is due to the presence of metallic Co, since the stoichiometric composition “CoTe” does not exist.

Phase diagram of Sm and Mn co-doped bismuth ferrite based on crystal structure and magnetic properties

Sm and Mn co-substituted BiFeO3 (BFO) ceramic has been synthesized by the tartaric acid modified sol-gel method to understand the effect of structural modification on the magnetic properties. The secondary phases (i.e., Bi2Fe4O9, Bi25FeO39 etc.) present in the parent compound BiFeO3 have been significantly reduced with increase in the substitution percentage of Sm and Mn at Bi and Fe sites of BFO respectively. The Rietveld refinement of the X-ray diffraction (XRD) patterns reveals a structural transition from rhombohedral (R3c, space group) to orthorhombic (Pbnm, space group) crystal symmetry with the concentration of Sm and Mn in the sample. The chemical state and elemental ratio of the samples have been studied by analyzing the X-ray photoelectron spectroscopy (XPS) data. Crystallite size obtained from the transmission electron microscopy analysis is in well comparable with the XRD analysis. Fourier transform infrared spectroscopy (FTIR) analysis supports the XRD results. A change in the phonon frequency and line widths in Raman spectra confirms the lattice distortion in the co-substituted samples which are in agreement with the XRD analysis. Ferromagnetic signature has been observed in Sm and Mn co-substitution. The magnetic hysteresis loop at room temperature has been analyzed by considering both ferromagnetic and antiferromagnetic contribution. The saturation magnetization for ferromagnetic phase increases up to 10% substitution of Sm and Mn in BFO and, decrease with the further increase of percentage of substituents. A correlation between crystal structure and magnetic behavior has been observed for Sm and Mn co-substituted sample. Phase diagram of crystal symmetries percentage, saturation magnetization (MS), remnant magnetization (Mr) and coercivity (HC) of Bi1-xSmxFe1-xMnxO3 for x = 0.000, 0.025, 0.050, 0.075, 0.100 and 0.150 ceramics.

Properties of granular (CoFeB)x(Al2O3)100-x and (CoFeB)x(LiNbO3)100-x nanocomposites: Manifestation of superferromagnetic ordering effects

We present a comparative study of (CoFeB)x(Al2O3)100-x and (CoFeB)x(LiNbO3)100-x film nanocomposites (NCs) with various structure deposited onto a glass–ceramic substrate by the ion-beam sputtering (CoFeB = Co40Fe40B20). NCs consist of the CoFe nanogranules embedded into the nonstoichiometric oxide matrix. For (CoFeB)x(Al2O3)100-x the granules are rounded (2–4 nm in size), whereas for (CoFeB)x(LiNbO3)100-x the granules are strongly elongated along the NC growth axis up to15 nm with the same in-plane size 2–4 nm. In both cases, the anomalous Hall effect takes place and its behavior is similar to the one observed in films with easy-plane magnetic anisotropy. Moreover, the analysis of magnetization showed that for (CoFeB)x(LiNbO3)100-x film there is no any structurally induced magnetic anisotropy, i.e. the magnetic anisotropy of samples is practically entirely determined by their shape. For both NCs along with the ferromagnetic (FM) contribution, additional paramagnetic (PM) component in the total magnetization is observed at T < 20 K; in case of (CoFeB)x(LiNbO3)100-x PM component even exceeds the FM one in three times. We have shown that PM component is caused by Fe2+ and Co2+ ions dispersed between granules with the concentration reaching Ni ≈ 3·1022 cm−3 and explain our results by a crucial role of these ions in the FM exchange between granules forming percolation network as well as in the total magnetization of the system.

Oxygen vacancy induced electrical conduction and room temperature ferromagnetism in system BaSn1−xNixO3 (0 ⩽ x ⩽ 0.20)

Powder and ceramics of few compositions of BaSn1−xNixO3 (with x = 0, 0.05, 0.10 and 0.20) have been prepared bysolid-state reaction route. Synthesized samples have been characterized using XRD, FTIR, SEM and XPS techniques. XPS studies have shown that all samples contain a significant number of oxygen vacancies (VO), Ba is present in the Ba2+ state, Sn is present in both Sn2+ and Sn4+ states and Ni exists mainly in the Ni2+ state. The AC conductivity spectra of all compositions followed universal Johnscher’s power law and the time-temperature superposition principle (TTSP). Above 300 °C conduction in these samples is governed by long-range diffusion of doubly ionized oxygen vacancies (). M-H loop at room temperature shows that undoped BaSnO3 is ferromagnetic with saturation magnetization (Ms) ~ 5.46 × 10−3 emu g−1 and coercive field (Hc) ~ 540 Oe, due to spins of electrons trapped in oxygen vacancies (F-center) and direct interaction between F-centers. Ni-doped samples exhibit both ferromagnetic and paramagnetic behavior and paramagnetic contribution increases with doping concentration. The room temperature magnetic parameters have been extracted by fitting the experimental data to the bound magnetic polaron (BMP) model. The saturation magnetization increases whereas coercive field decrease with increasing concentration of dopant (Ni).

Magnetic properties of NiO nano particles: Contributions of the antiferromagnetic and ferromagnetic subsystems in different magnetic field ranges up to 250 kOe

The magnetic properties of antiferromagnetic NiO nanoparticles prepared by thermal decomposition of nickel hydroxocarbonate are investigated. According to the data of magnetization measurements in fields of up to 250 kOe, the magnetic moment linearly grows in strong fields, which is caused by the contribution of the antiferromagnetically ordered nanoparticle core, and the antiferromagnetic susceptibility corresponds to that of bulk polycrystalline NiO. This allowed the antiferromagnetic and ferromagnetic contributions to the total magnetic response of a sample to be quantitatively determined. The latter occurs due to the incomplete spin compensation in an antiferromagnetic nanoparticle caused by defects on its surface. It is demonstrated that to correctly determine the superparamagnetic blocking temperature, it is necessary to take into account the antiferromagnetic susceptibility of the particle core.

Microwave Interferometry for High Sensitivity VNA-FMR Measurements

Two-port vector network analyzer ferromagnetic resonance (VNA-FMR) measurements using coplanar waveguides (CPWs) are a fast and powerful tool for the analysis of ferromagnetic film parameters, including damping and effective field contributions like anisotropies, exchange coupling, and demagnetization fields. Thereby, VNA-FMR appears to be a good choice for the analysis of patterned magnetic nanostructures showing a rich spectrum of different anisotropy terms. However, due to their small volume, the absorption of nanostructures in FMR is low and the measurement signal is superimposed by the excitation signal. To eliminate the background signal, we propose an interferometric approach with the following two key features: 1) the reference path has the same geometry as the measurement path, including an identical but empty CPW parallel to the measurement CPW to remove spurious field dependent contributions to the signal and 2) additionally, we extend the reference path by 0.5 m of an RF coaxial cable. This path length difference leads to a frequency-dependent phase difference of the two paths, assuming a value of 180 ° in ≈ 500 MHz intervals. Here, the excitation RF signals from both paths add to zero. In broadband measurements, this simple approach allows to obtain a sufficient number of grid points for dispersion analysis without the need for a phase shifter. Our experiments reveal an improvement of the sensitivity by up to two orders in magnitude under optimum conditions. The amplitude and the shape of the resonance peaks will be discussed.

Anomalous magnetism of the nanocrystalline oxide TiO2 surface

The magnetic properties of an oxygen-deficient nanocrystalline undoped titanium dioxide synthesized by the gas-phase, electric-explosion, and chemical method have been studied. The defect state was controlled using reduction treatments in vacuum or in a hydrogen atmosphere. It is shown that the defect state of the surface of nanocrystalline oxides (for example, the existence of vacancies in the anion sublattice and other defects) has a dominant influence on the formation of the magnetic properties of the samples under study. The main contributions to the magnetism of TiO2 nanoparticles after the reduction treatments are the paramagnetic contribution of the matrix, the paramagnetic Curie–Weiss contribution, and the contribution of the spontaneous magnetic moment provided by the existence of regions with different spin ordering. A heterogeneous magnetic state is found to exist in the TiO2 nanopowders; for example, at low temperatures, shifted hysteresis loops are observed as a result of a possible set of magnetic states with different spin orders. It is shown that a soft compaction or grinding of nanopowders in an agate mortar lead to substantial increase in the magnetization, sometimes, by a factor of more than two, regardless of the nanopowder synthesis method and the initial phase state of TiO2 (anatase or rutile structures). This experimental fact proves the key role of the surface defects and the magnetic moment carriers with different spin configurations localized mainly on the nanoparticle surface. The compaction changes the magnetization only in the case when the initial magnetic state has a nonlinear “quasi-superparamagnetic” character of the magnetization curve. As a result of predominant exchange interaction between the nanoparticles with a frustrated character of spin ordering on the nanoparticles surface, the ferromagnetic contribution increases as nanoparticles contact.

Magnetic structure of martensite in Ni–Mn–Ga–Co alloys

Ni[Formula: see text]CoxMn[Formula: see text]Ga[Formula: see text] ([Formula: see text]) alloys undergo martensitic transformation between ferromagnetic (FM) austenite and weak-magnetic martensite. However, the type of magnetism of martensite remains unclear. In this work, the results of a detailed study of the magnetic state of martensite in a Ni[Formula: see text]Co7Mn[Formula: see text]Ga[Formula: see text] are shown. The evolution with post-quench ageing indicates that improvement of atomic order increases the FM contributions to the weak-magnetic martensite, major changes occurring as the austenite switches from paramagnetic (PM) to FM. The results are consistent with the presence of FM clusters inside a PM matrix.

A facile approach to the elucidation of magnetic parameters of CuFe2O4 nanoparticles synthesized by hydrothermal route

Pure pseudo cubic shaped copper ferrite nanoparticles with narrow size distribution in the range 6–17 nanometer are prepared by hydrothermal method under various synthesis conditions namely, hydrothermal temperature, heating time, and pH. The structural and morphological studies are carried out in detail using XRD and TEM analysis. The crystallite size and particle size are calculated from different characterization techniques. The distribution of cations among the tetrahedral and octahedral sites is determined from the XRD intensity calculation. Compositional features are determined from EDS analysis. Magnetic studies are carried out using VSM at room temperature and the important magnetic parameters are extracted from it. Contributions due to various types of magnetization to the total magnetization are determined from the theoretical fitting of the magnetization curve. Excellent fits are obtained for all samples prepared under various conditions. The ferromagnetic, superparamagnetic and paramagnetic contributions to the magnetization are determined from the analysis of fitted M-H curve. It is observed that the hydrothermal reaction time and temperature has little effect on the structural and magnetic parameters of the material. However, pH plays a crucial role in the physical properties of nanoparticles. Optimized synthesis conditions are identified for changing the soft ferrimagnetic nature of copper ferrite nanoparticles to superparamagnetic nature.