Ferrimagnetic Component Research Articles

Fluctuating Flame from Suspending Ferrimagnetic Core/Shell Al@Fe3O4 Nanoparticles in a Magnetic Field

AbstractCore–shell fuel@oxidizer nanocomposite can combust in oxygen‐starved environment. A ferrimagnetic reactive particle, being successfully engineered, allows for manipulation using an external magnetic field and facilitates target heat or gas production. This research reports on interesting flame dynamics of newly synthesized core–shell Al@Fe3O4 nanoparticles under the influence of a magnetic field. Serving as an oxidizer and a functional ferrimagnetic component, iron oxide nanoparticles (IONPs) are grown in situ on nano‐sized Al (n‐Al) particles. Electron microscopic images demonstrate nearly monodispersed IONPs ≈7 nm decorating the surface of n‐Al. X‐ray diffractogram and X‐ray photoelectron spectroscopy confirm the formation of Fe3O4. Thermal analysis suggests the as‐prepared core–shell particles predominantly go through a solid‐state reaction mechanism that exhibits 30% lower activation energy compared to physically‐mixed nanocomposite (215.0 vs 310.8 kJ mol−1). The core/shell particles can be ignited and combust under laser irradiation with or without the effect of a magnetic field. When suspended in the middle of the tube by the magnetic field, an interesting combustion process is observed, highlighting a grow‐shrink‐grow flame resulted from the interactions between the combustion products and the external magnetic field, as well as backfiring at the bottom of the sample without major changes in burning rate and ignition delay.

Interplay of lattice-spin-orbital coupling and Jahn–Teller effect in noncollinear spinel Ti x Mn1−x (Fe y Co1−y )2O4: a neutron diffraction study

Local magnetostructural changes and dynamical spin fluctuations in doubly diluted spinel Ti x Mn1‒x (Fe y Co1‒y )2O4 has been reported by means of neutron diffraction and magnetization studies. Two distinct sets of compositions (i) x(Ti) = 0.20 and y(Fe) = 0.18; (ii) x(Ti) = 0.40 and y(Fe) = 0.435 have been considered for this study. The first compound of equivalent stoichiometry Ti0.20Mn0.80Fe0.36Co1.64O4 exhibits enhanced tetragonal distortion across the ferrimagnetic transition temperature T C = 258 K in comparison to the end compound MnCo2O4 (T C ∼ 180 K) with a characteristic ratio c t/√2a t of 0.99795(8) demonstrating robust lattice-spin-orbital coupling. However, in the second case Ti0.40Mn0.60Fe0.87Co1.13O4 with higher B-site compositions, the presence of Jahn–Teller ions with distinct behavior appears to counterbalance the strong tetragonal distortion thereby ceasing the lattice-spin-orbital coupling. Both the investigated systems show the coexistence of noncollinear antiferromagnetic and ferrimagnetic components in cubic and tetragonal settings. On the other hand, the dynamical ac-susceptibility, χ ac(T) reveals a cluster spin-glass state with Gabay–Toulouse (GT) like mixed phases behaviour below T C. Such dispersive behaviour appears to be sensitive to the level of octahedral substitution. Further, the field dependence of χ ac(T) follows the weak anisotropic GT-line behaviour with crossover exponent Φ lies in the range 1.38–1.52 on the H–T plane which is in contrast to the B-site Ti substituted MnCo2O4 spinel that appears to follow irreversible non-mean-field AT-line behaviour (Φ ∼ 3 + δ). Finally, the Arrott plots analysis indicates the presence of a pseudo first-order like transition (T < 20 K) which is in consonance with and zero crossover of the magnetic entropy change within the frozen spin-glass regime.

Superparamagnetism and ferrimagnetism in the Sr2FeMoO6–δ nanoscale powder

Using combined synthesis modes and optimized conditions for ultrasonic dispersion, we obtained a single-phase nanosized Sr2FeMoO6–δ powder having a high degree of superstructural ordering of Fe and Mo cations (88 %) and an average grain size of 70.8 nm. The results of the Mössbauer spectroscopy and magnetic measurements show that the obtained nanosized strontium ferromolybdate powder is in a magnetically inhomogeneous state, consisting of superparamagnetic and ferrimagnetic phases. In comparison with the superparamagnetic component of magnetization, the ferrimagnetic component has higher values and rises smoothly until reaching saturation as the temperature decreases. Showing that there is no exchange magnetic interaction between the grains in the superparamagnetic phase, allowed us to estimate the critical sizes, of nanoparticles, dspm, in the single-domain state using the Néel-Brown model. The obtained value of dspm is smaller than the size of the single-domain particles, which confirms the absence of the frozen state in some of the superparamagnetic particles. These results are important for understanding the effects of nanosized grains on the magnetic properties and features of magnetization of the Sr2FeMoO6–δ polydisperse powder, a promising material for spintronic device applications.

Influence of functionalization with citric acid on the properties of magnetic nanoparticles Zn-=SUB=-x-=/SUB=-Fe-=SUB=-3-x-=/SUB=-O-=SUB=-4-=/SUB=- (0≤ x≤ 1.0)

The effect of surface functionalization with citric acid on the properties of magnetic nanoparticles (MNPs) ZnxFe3-xO4 (x=0; 0.25; 0.5; 0.75; 1.0) synthesized by the hydrothermal method was studied. To study the properties of MNPs, X-ray diffractometry (XRD) and energy dispersive X-ray spectroscopy (EDS) were used. The magnetic properties of the samples and the phase state of MNPs were studied using a physical property measurement system (PPMS) and Mossbauer spectroscopy (MS). It has been established that the sizes of crystallites and the crystal lattice parameter of ZnxFe3-xO4 MNPs change with increasing Zn2+ concentration. The low values of the coercive force and the presence of a doublet on the MS indicate the presence of both ferrimagnetic and superparamagnetic components. Keywords: ZnxFe3-xO4 ferrite spinel MNPs, hydrothermal synthesis, functionalization of MNPs with citric acid, crystal structure, magnetic properties, magnetic structure.

Influence of Polymorphism on the Magnetic Properties of Co5TeO8 Spinel

This study presents the influence of polymorphism on the magnetic properties of Co5TeO8. This compound with a spinel-like structure [Co2]A[Co3Te]BO8 was synthesized into two polymorphs: one disordered within a cubic Fd3̅m structure, where Co2+ and Te6+ ions are randomly distributed on the octahedral B sites [the disordered polymorph can also be presented as an inverse spinel of the formula Co(Co1.5Te0.5)O4] and the other ordered with a cubic P4332 structure where Co2+ and Te6+ ions are ordered on the B sites. The macroscopic magnetic measurements showed that both polymorphs present a ferrimagnetic ordering, below ∼40 K, and a second transition is also observed at 27 K for the ordered polymorph. Neutron powder diffraction data between room temperature and 1.7 K showed as well the presence of short-range magnetic ordered clusters, which appears for both polymorphs below 200 K. At lower temperature, these short-range orders are transformed into long-range ferrimagnetic orders. Below TC = 40 K, the colinear ferrimagnetic structure of the disordered polymorph is described with the I41/am'd' space group. The ordered polymorph undergoes an incommensurate ferrimagnetic spiral spin ordering below TC1 = 45 K, followed by a second magnetic phase transition at TC2 = 27 K. This last transition is associated with the emergence of an additional ferrimagnetic component and an abrupt change in the magnitude of the magnetic propagation vector k = [0, 0, γ] from γ = 0.086 at T = 30 K to γ ≈ 0.14 in the range between 27 and 1.7 K. The magnetic symmetry of the ordered polymorph is described with the P43(00γ)0 magnetic superspace group. We evidenced that the ordering of Co2+/Te6+ on the B sites changes all of the Co-Co and Co-O distances and thus all JAB, JAA, and JBB exchange interactions, between the A and B sites, which are able to stabilize the incommensurate spin modulation in the ordered polymorph.

Unusual magnetic and magnetostriction behavior around the magnetic compensation temperature in Co2TiO4

Some intricate behaviors have been reported at magnetic compensation temperature in ${\mathrm{Co}}_{2}{\mathrm{TiO}}_{4}$ (CTO), but their origins are still unclear. In this paper, we investigate systematically the anomalous properties in CTO crystal by using direct and alternating current magnetic susceptibilities, specific heat, thermal expansion, magnetostriction, and neutron diffraction measurements. CTO exhibits a ferrimagnetic ordering transition at ${T}_{C}\ensuremath{\approx}48\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and a magnetic compensation behavior at ${T}_{\mathrm{comp}}\ensuremath{\approx}31\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Under zero field, there is no obvious anomaly in CTO around ferrimagnetic compensation temperature ${T}_{\mathrm{comp}}$, but a minimum value of M(T), a drop of ${\ensuremath{\chi}}^{\ensuremath{'}}(T)$, a step of $\mathrm{\ensuremath{\Delta}}L/{L}_{80\phantom{\rule{0.16em}{0ex}}\mathrm{K}}$ and a sharp specific heat peak all occur at ${T}_{\mathrm{comp}}$ under the same critical magnetic field of 20 kOe. We propose a phenomenological model to explain these anomalous phenomena, in which the 180 \ifmmode^\circ\else\textdegree\fi{} reversal of the ferrimagnetic component at ${T}_{\mathrm{comp}}$ under $H\ensuremath{\ge}20\phantom{\rule{0.16em}{0ex}}\mathrm{kOe}$, as well as the deflection of the oblique spin moments at B sites under magnetic field, play a key role in how to understand them.

Synthesis and magnetism of MoCo2O4 spinel thin films

Spinel MoCo2O4 was synthesized through epitaxial stabilization. X-ray photoemission spectroscopy revealed that the Mo and Co ions mainly exist as Mo4+ and high-spin (HS) Co2+ in the MoCo2O4 film, respectively. These valence states were consistent with the prediction based on the relation between lattice constants and ionic radii of cations in various spinel cobaltites. The magnetization of the film was composed of paramagnetic and ferrimagnetic components. Density function theory based calculations suggested that the paramagnetic component mainly corresponds to HS Mo4+ 4d2 ions at the octahedral site, while the ferrimagnetic component is derived from the antiferromagnetic interaction between HS Co2+ 3d7 ions at the tetrahedral and octahedral sites in inverse spinel Co[CoMo]O4.

Determination of ferrimagnetic and superparamagnetic components of magnetization and the effect of particle size on structural, magnetic and hyperfine properties of Mg0.5Zn0.5Fe2O4 nanoparticles

The fine-tuning of magnetic parameters and the identification of different magnetic phases are essential for the effective utilization of magnetic nanoparticles. In this work, we aim at controlling the magnetic parameters of Mg0.5Zn0.5Fe2O4 nanoparticles by varying the particle size and to determine magnetic phases using three analytical techniques having different operating time scales: VSM, ESR, and Mössbauer spectroscopy. Single-phase cubic spinel structured Mg0.5Zn0.5Fe2O4 nanoparticles were prepared by the sol-gel auto-combustion route and calcinated at 200, 300, 500, 700, and 900 °C. The cation distribution and structural parameters obtained from the Rietveld analysis had an insignificant variation with the calcination temperature. From the TEM analysis, an increase of the average particle size from 5 to 38 nm and widening of the particle size distribution with the calcination temperature were found. The saturation magnetization increased from 25.6 to 43.7 emu/g with the particle size due to the reduction in the magnetic dead layer thickness. The coercivity decreased from 128 to 31 Oe and the blocking temperature from 76 to 38 K with the particle size and these variations are explained in terms of surface anisotropy and dipolar interactions. Ferrimagnetic, superparamagnetic, and paramagnetic components of magnetization were deconvoluted by the curve fitting of room temperature VSM data. The ferrimagnetic component increased from 56% to 74% and the superparamagnetic component decreased from 37% to 20% respectively with the particle size. The ESR and Mössbauer spectroscopy also identified the ferrimagnetic and superparamagnetic components in the samples and their variations with the particle size were found to be in agreement with that of VSM. Thus, the variation of magnetic parameters in correlation with the particle size offers the possibility of fine-tuning the magnetic parameters of nanoparticles by adjusting the particle size. The deconvolution of magnetic phases using the three analytic methods revealed the coexistence of ferrimagnetic and superparamagnetic components of magnetization in Mg0.5Zn0.5Fe2O4 nanoparticles. Also, this deconvolution unveiled the particle size dependence on the emergence of inhomogeneity in magnetic phases of the samples.

Dielectric Studies of Ferrimagnetic-Piezoelectric Composites

Magneto-electric composites of two distinct phases, ferrimagnetic-piezoelectric system with general compositional formula (x) ferromagnetic + (1-x) piezoelectric were synthesized using a hybrid technique, mechano-chemical method by sintering the mixtures of piezo-electric BaTiO3 (BTO) and ferri-magnetic Mg0.2Cu0.5Zn0.3Fe2O4 (MCZF). Here, ferri-magnetic phase component MCZF (Mg0.2Cu0.5Zn0.3Fe2O4) was prepared using auto-combustion method, whereas piezo-electric BTO was procured commercially from Sigma-Aldrich. Here, the general composition of composites is given by (x) Mg0.2Cu0.5Zn0.3Fe2O4+(1-x) BaTiO3(x=15%, 30% and 45%). Presences of two phases in these magneto-electric composites were probed using X-ray diffraction (XRD) studies. Peaks observed in the XRD spectrum indicated spinel cubic structure for MCZF ferrite and tetragonal perovskite structure for BTO and, both spinel and pervoskite structures for synthesized composites. Micro-structure of the samples has been investigated using Field Emission Scanning Electron Microscope (FESEM). Frequency dependent dielectric properties of synthesized composites were measured from 100 Hz to 1 MHz at room temperature using a precision HIOKI make LCR HI-TESTER. Dielectric dispersion was observed at lower frequencies for the synthesized composites.

Changes in the Magnetic Properties of Soddy-Podzolic Soils Depending on the Conditions of Soil Formation

This paper presents the results of a study of soil cover in the Mari Chodra national park in the southeastern part of the Republic of Mari El. Magnetic properties of soddy-medium-podzolic sandy loam soils and soddy-podzolic loam soils are studied. Samples are taken along the shoreline of the karst Yalchik Lake and up the slope from the lake. Based on the results of magnetic susceptibility measurements, it is shown that the studied soil profiles are characterized by both the accumulative and eluvial–illuvial distribution of mineral components, depending on the conditions of soil formation and recreational activity of the given territory. It is shown that the increase in magnetic susceptibility in the organogenic horizons of soddy-medium-podzolic and soddy-podzolic soils is due to the contribution of a ferrimagnetic component. Differential thermomagnetic analysis and decomposition of coercive spectra into components revealed that the main carriers of the magnetization of the soils are fine-grained magnetite and hematite that have a pedogenic origin.

Neutron diffraction evidence for local spin canting, weak Jahn–Teller distortion, and magnetic compensation in Ti1−xMnxCo2O4 spinel

A systematic study using neutron diffraction and magnetic susceptibility is reported on Mn substituted ferrimagnetic inverse spinel Ti1−xMnxCo2O4 in the temperature interval 1.6 K T 300 K. Our neutron diffraction study reveals cooperative distortions of the TO6 octahedra in the Ti1−xMnxCo2O4 system for all the Jahn–Teller active ions T = Mn3+ , Ti3+ and Co3+ , having the electronic configurations 3d1, 3d4 and 3d6, respectively which are confirmed by the x-ray photoelectron spectroscopy. Two specific compositions (x = 0.2 and 0.4) have been chosen in this study because these two systems show unique features such as; (i) noncollinear Yafet–Kittel type magnetic ordering, and (ii) weak tetragonal distortion with c/a < 1, in which the apical bond length dc(TB-O) is longer than the equatorial bond length dab(TB-O) due to the splitting of the eg level of Mn3+ ions into and . For the composition x = 0.4, the distortion in the TBO6 octahedra is stronger as compared to x = 0.2 because of the higher content of trivalent Mn. Ferrimagnetic ordering in Ti0.6Mn0.4Co2O4 and Ti0.8Mn0.2Co2O4 sets in at 110.3 and 78.2 K, respectively due to the presence of unequal magnetic moments of cations, where Ti3+ , Mn3+ , and Co3+ occupy the octahedral, whereas, Co2+ sits in the tetrahedral site. For both compounds an additional weak antiferromagnetic component could be observed lying perpendicular to the ferrimagnetic component. The analysis of static and dynamic magnetic susceptibilities combined with the heat-capacity data reveals a magnetic compensation phenomenon (MCP) at TCOMP = 25.4 K in Ti0.8Mn0.2Co2O4 and a reentrant spin-glass behaviour in Ti0.6Mn0.4Co2O4 with a freezing temperature of ∼110.1 K. The MCP in this compound is characterized by sign reversal of magnetization and bipolar exchange bias effect below TCOMP with its magnitude depending on the direction of external magnetic field and the cooling protocol.

Spin glass and exchange bias behavior in magnetically frustrated Ni1−xMgxCr2O4 (x = 0.0–0.50)

Single-phase polycrystalline samples of Ni1−xMgxCr2O4 (x = 0.0–0.50) were prepared by sol gel route and their structural and magnetic properties were studied. Structural transformation from tetragonal (I41/amd) to cubic (Fd3-m) phase is observed at room temperature due to Mg substitution. Mg substitution gives rise to reduction in ferrimagnetic and antiferromagnetic transition temperatures along with a signature of spin glass like phase in samples of intermediate compositions, i.e., for x = 0.10, 0.20 and 0.30. The observed stretched exponential type relaxation of thermoremanent magnetization and the highly frustrated magnetic behavior confirm the glassy magnetic phase. Significant increase in exchange bias field under field cooled condition for the Mg substituted samples is observed. The origin of exchange bias in theses samples is explained by considering exchange anisotropy between the ferrimagnetic and antiferromagnetic components of canted spin. The training effect of exchange bias field is also observed.

Periodization of Holocene Climatic Cycles Based on Synchronous Variations in the Magnetic and Geochemical Parameters of the Sediments of Lake Bolshoe Yarovoe (Southwestern Siberia)

Abstract —Variations in the magnetic and geochemical properties of the sediments of Lake Bolshoe Yarovoe (Altai Territory) were studied. The data were derived from five core columns (up to 4.5 m long) covering a time interval of more than 8000 years. In addition, coercive spectra were obtained for 792 samples taken every 2 cm. Coercive spectra were then used to identify soft magnetic (10–15 mT) and hard magnetic (35–50 mT) components. The soft magnetic component is detrital, and the hard magnetic component is biogenic, which is confirmed by microscopic studies. Moreover, the samples contain micrometeorite particles. Variations in geochemical properties allow reconstruction of the environmental history of the lake. Sediments in the lower part of the section vary both in the content of ferrimagnetic components and in geochemical properties, which indicates a sharp environmental change between ~6100 and ~7600 years ago. Quasiperiodic alternation of warm (dry) and cool (humid) periods is observed higher in the section, between ~4100 and ~6100 years ago. The variations in the magnetic components are consistent with the environmental changes and can be used for the historical reconstruction.

Insights into magnetoelectric coupling mechanism of the room-temperature multiferroic Sr3Co2Fe24O41 from domain observation

The mechanism of a magnetoelectric coupling in a room-temperature multiferroic, $\mathrm{S}{\mathrm{r}}_{3}\mathrm{C}{\mathrm{o}}_{2}\mathrm{F}{\mathrm{e}}_{24}{\mathrm{O}}_{41}$, with the Z-type hexaferrite structure, is examined by three approaches: observations of domain structures and their field responses, measurements of magnetic-field effect on electric polarization, i.e., magnetoelectric effect, and phenomenological discussions on the interplay among coexisting order parameters. With use of a resonant soft x-ray microdiffraction technique, we visualized magnetic-field responses of two types of magnetic domains ascribed to ferrimagnetic and spiral components inherent in a transverse conical magnetic structure of the hexaferrite. A simultaneous inversion of these magnetic domains by a magnetic-field reversal was observed, meaning that the process of a magnetization reversal corresponds to a 180\ifmmode^\circ\else\textdegree\fi{} rotation of the cone axis. The reversal process of the magnetic structure, together with experimental results of the magnetoelectric effect, leads us to the conclusion that the magnetoelectricity in the Z-type hexaferrite originates mainly from the spin-dependent metal-ligand orbital hybridization, with minor contribution from the asymmetric spin-exchange interaction. Furthermore, such a mechanism is discussed by the symmetry analysis based on the Landau theory and is well described in terms of couplings among the coexisting order parameters included in the free energy. Thus, observations on field responses of multiple domains in multiferroics provide insights into underlying microscopic magnetoelectric coupling mechanisms.

High-pressure magnetism of the double perovskite Sr2FeOsO6 studied by synchrotron Fe57 Mössbauer spectroscopy

The pressure dependence of the magnetism of the double perovskite ${\mathrm{Sr}}_{2}{\mathrm{FeOsO}}_{6}$ was investigated by temperature- and magnetic-field-dependent synchrotron $^{57}\mathrm{Fe}$ M\ossbauer spectroscopy in the energy domain up to $\ensuremath{\sim}50$ GPa. ${\mathrm{Sr}}_{2}{\mathrm{FeOsO}}_{6}$ is known to feature antiferromagnetic ordering below ${T}_{\mathrm{N}}\ensuremath{\sim}140$ K and a change in spin structure from AF1 to AF2 near 70 K at ambient pressure. Previous Os ${L}_{2,3}$ x-ray magnetic circular dichroism (XMCD) spectra [Veiga et al., Phys. Rev. B 91, 235135 (2015)] indicated a pressure-driven change from the antiferromagnetic to the ferrimagnetic state. Raman spectra of ${\mathrm{Sr}}_{2}{\mathrm{FeOsO}}_{6}$ collected at room temperature and up to 34 GPa are in agreement with previous x-ray-diffraction studies which verified that the tetragonal ambient pressure crystal structure is retained at high pressure. The M\ossbauer investigations show that the Fe ions remain in the +3 high-spin $({{t}_{2g}}^{3}{e}_{g}^{2})$ state over the whole pressure range. The magnetic ordering is strongly stabilized by high pressure and for $pg30$ GPa the ordering temperature is increased to above room temperature. Broad magnetic hyperfine patterns as well as coexistence of magnetic and paramagnetic signals at high pressures indicate the formation of inhomogeneous magnetic states. The shapes of applied-field M\ossbauer spectra at low temperature are similar at 2 and 33 GPa and do not allow resolution of distinct antiferromagnetic and ferrimagnetic components. Pressure-induced spin canting in a basically antiferromagnetic spin structure or magnetic phase separation involving a pressure-induced minority ferrimagnetic phase may be the origin for the ferromagnetic XMCD signal.

Magnetic field regulated, controlled hyperthermia with Li Fe3-O4 (0.06 ≤ x ≤ 0.3) nanoparticles

Abstract The single phase LixFe3-xO4 (0.06 ≤ x ≤ 0.3) nanoparticles were synthesized using microwave refluxing technique and found to be spherical with the size in the range of ∼ 3–22 nm. The structure and the size were confirmed by X-ray diffraction patterns and transmission electron microscopy. X-ray photoelectron spectroscopy data of Li0.3Fe2.7O4 sample indicate the oxidation states for Li-ions (+1) and Fe-ions (+2 and + 3). The sample Li0.06Fe2.94O4 had superior magnetization value (∼54.1 Am2/kg) and it diminished with increasing Li content. The low coercivity values and the presence of a doublet in Mossbauer spectra advise the presence of both ferrimagnetic and superparamagnetic component. The ferrofluids of these samples show magnetic field dependent stabilization of temperature near 42 °C at the time of magnetic hyperthermia experiments. The specific absorption rate value (up to 100 W/g) found to be comparable with the literature values.

Tailored dielectric and magnetic properties of composite electroceramics with ferroelectric and ferrimagnetic components

High-frequency coaxial waveguide measurements are used to evaluate composition- and frequency-dependent complex permittivities and permeabilities of polycrystalline ferroelectric and ferrimagnetic ceramics. Material dielectric and magnetic property measurements were performed on ferroelectric barium titanate (BaTiO3) and ferrimagnetic yttrium iron garnet (YIG), with the upper measurement frequency bound below gyromagnetic resonance of YIG. These polycrystalline materials were chosen because of their widespread use in voltage- and magnetic field-tunable devices. Wideband frequency material measurements were used to predict dielectric and magnetic properties of various two-phase composite media incorporating BaTiO3 and YIG. Two effective medium theories were used: an asymmetric, modified Maxwell-Garnett type and a symmetric Bruggeman model. The Bruggeman effective medium model is extended to include three-phases. The three-phase Bruggeman predictive model is employed when controlled porosity is introduced into the composite and when two component phases are combined with polytetrafluoroethylene.

Influence of substrate effects in magnetic and transport properties of magnesium ferrite thin films

In order to study the substrate influence in Mg ferrite properties, thin films were simultaneously grown by DC magnetron sputtering on different substrates (MgO (100) and SrTiO3 (100)). In both cases samples show high values of saturation magnetization and Curie temperatures above room temperature. While in the case of the sample grown on MgO the hysteresis loops indicate the existence of more than one ferrimagnetic component and lower value of remanence to saturation ratio, the deposited one on SrTiO3 is mono-component and has a rectangular-shape magnetic loop at all temperatures below 300 K. The difference is attributed to different microstructures due to the misfit strain caused by the different lattice constants between substrates and ferrite. The electric transport and photoconductivity properties have been investigated on both samples. Thin film growth on STO presents an increase around 40% of the photoconductance in the near UV range. The photocurrent shows two clear onsets that coincide with the indirect and direct band gaps of STO indicating the important role of this substrate in the generation of photo carriers.

Surfaces and their effect on the magnetic properties of polycrystalline hollow γ-Mn2O3 and MnO nanoparticles

Manganese oxide nanoparticles were prepared in an inert gas condensation system. X-ray Diffraction (XRD) studies revealed presence of multiple manganese oxide phases while high resolution transmission electron microscopy (HRTEM) showed polycrystalline hollow nanoparticle morphology. The additional inner surface of the hollow nanoparticle directly affect the magnetic properties of these particles. Combined physical structure, electronic structure and magnetic susceptibility analyses led to the conclusion that the prepared nanoparticles are polycrystalline and composed of γ-Mn2O3 and MnO crystallites. Magnetic study found a sharp peak around 38K with no frequency dependence in the AC susceptibility measurement. Large coercivity (HC) and exchange bias (HEB) fields, up to 11kOe and 7kOe, respectively, were observed below the order temperatures. HC and HEB were found to increase and decrease, respectively, as a function of cooling field. Both HC and HEB were found to decrease monotonically as the temperature approached paramagnetic phase transition of the ferrimagnetic component.

Exchange bias and bistable magneto-resistance states in amorphous TbFeCo thin films

Amorphous TbFeCo thin films sputter deposited at room temperature on thermally oxidized Si substrate are found to exhibit strong perpendicular magnetic anisotropy. Atom probe tomography, scanning transmission electron microscopy, and energy dispersive X-ray spectroscopy mapping have revealed two nanoscale amorphous phases with different Tb atomic percentages distributed within the amorphous film. Exchange bias accompanied by bistable magneto-resistance states has been uncovered near room temperature by magnetization and magneto-transport measurements. The exchange anisotropy originates from the exchange interaction between the ferrimagnetic and ferromagnetic components corresponding to the two amorphous phases. This study provides a platform for exchange bias and magneto-resistance switching using single-layer amorphous ferrimagnetic thin films that require no epitaxial growth.