Ferromagnetic Glass Research Articles

Size effects on charge ordering and magnetic properties in La0.25Ca0.75MnO3

The size effects on the charge ordering (CO) and magnetic properties in La(0.25)Ca(0.75)MnO(3) with mean particle size ranging from 40 to 2000 nm were studied. With decreasing particle size the CO transition temperature shifts to lower temperature and the transition width becomes increasingly wide, indicating the weakening of the CO state. Meanwhile the ferromagnetic (FM) cluster glass state appears and the magnetization at low temperature increases significantly. The behaviour is due to the increasing uncompensated surface spins which weaken the antiferromagnetic interaction and disfavour the formation of the CO state. The suppression of the CO state and appearance of the FM cluster glass state are also found in La(0.25)Ca(0.75)MnO(3) nanowires fabricated by a sol-gel template method. These results indicate that the CO state can be modulated effectively by varying particle size, which has an important implication for nano-device applications of manganites. (c) 2007 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Mössbauer spectroscopy studies of spin reorientations in amorphous and crystalline (Co 0.2Fe 0.8) 72.5Si 12.5B 15 glass coated micro-wires

Thermo-gravimetric, differential scanning calorimetry and comprehensive 57Fe Mössbauer spectroscopy studies of amorphous and crystalline ferromagnetic glass coated (Co 0.2Fe 0.8) 72.5Si 12.5B 15 micro-wires have been recorded. The Curie temperature of the amorphous phase is T C(amorp) ∼730 K. The analysis of the Mössbauer spectra reveals that below 623 K the easy axis of the magnetization is axial-along the wires, and that a tangential or/and radial orientation occurs at higher temperatures. At 770 K, in the first 4 hours the Mössbauer spectrum exhibits a pure paramagnetic doublet. Crystallization and decomposition to predominantly α-Fe(Si) and Fe 2B occurs either by raising the temperature above 835 K or isothermally in time at lower temperatures. Annealing for a day at 770 K, leads to crystallization. In the crystalline material the magnetic moments have a complete random orientation. After cooling back to ambient temperature, both α-Fe(Si) and Fe 2B in the glass coated wire show pure axial magnetic orientation like in the original amorphous state. The observed spin reorientations are associated with changes in the stress induced by the glass coating.

Magnetic properties of Fe-based bulk ferromagnetic glasses

Several Fe-based ferromagnetic bulk amorphous alloys, having compositions Fe 61Co 10Zr 5W 4B 20; Fe 61Co 10Zr 5W 2Nb 2B 20; Fe 61Co 10Zr 2.5Hf 2.5W 2Nb 2B 20, have been prepared by suction casting, in the form of 1 mm diameter rods, and by melt spinning in the form of thick ribbons (thickness ∼200 μm). All these glasses have a large supercooled region T x – T g, ranging from 51 to 70 K, consistent with a large glass forming ability, that is promoted by the substantial differences in the atomic sizes of the constituent elements. We have obtained a saturation polarisation (μ 0M S) of ∼0.8 T for Fe 61Co 10Zr 5W 4B 20 and 0.9 T for Fe 61Co 10Zr 5W 2Nb 2B 20 and Fe 61Co 10Zr 2.5Hf 2.5W 2Nb 2B 20, for both rod and ribbon forms. The coercivities of the samples range from 25 to 50 A/m, for ribbon and from 100 to 300 A/m for rods, when measured at 5 Hz. The coercivity is a parameter that is very sensitive to magnetic inhomogeneity and the data indicate some degree of such inhomogeneity in the samples, especially for the rods. Magnetoelastic resonance experiments were also performed, giving values of Young modulus of about 160 GPa, and magnetostriction coefficients of about 10 −5.

Spin dynamics of ferromagnetic spin glass

The dynamics of ferromagnetic spin glass is studied taking into account the dynamically induced magnetic moment which is not small for disordered magnets. The well defined longitudinal and transverse spin waves have been found. The frustrations do not influence the spectrum of spin waves, however, the transverse spin waves become damped due to dissipative motion of frustrations.

Magnetic materials containing LaSr manganite phase

Ferromagnetic glass–crystalline materials were produced by sintering of glasses from the systems Bi 2O 3B 2O 3MnO and crystalline phase La 0.6Sr 0.4MnO 3. The appropriate preparation procedure was applied as follows: synthesis of the starting components, mixing, milling, pressing and sintering at selected temperatures. The steps of the technological scheme were controlled by X-ray diffraction and infrared spectroscopy. Small crystals with size in the order of 1–5 μm were detected by transmission electron microscopy observations [TEM] after heath treatment. The magnetic properties were studied by a vibrating sample magnetometer [VSM] and the Curie temperature was detected at 353 K. The presence of a narrow hysteresis loop proves the ferromagnetic properties of the materials.

Magnetic phase diagram of metallic pyrochlore lattice in the double-exchange model

The magnetic phase diagram of metallic kagomeand pyrochlore lattices at the ground state have been studied theoretically in the double-exchange model using a mean-field approximation. The model includes the hopping of itinerant electrons, which is expressed by a single tight-binding band, antiferromagnetic coupling between localized spins, and ferromagnetic exchange interaction between the localized and itinerant electron spins. The phase diagram was found to be rather insensitive to the ferromagnetic exchange interaction but to show a rich variety in magnetic structure with doping. A crossover between ferromagnetic and spin glass phases observed in RE2Mo2O7 pyrochlore lattices, where RE denotes rare-earth ions, is discussed in view of the calculated results.

Structural Characterization of Non-Crystalline Oxides by the Anomalous X-ray Scattering (AXS) Method

The fundamentals of the anomalous X-ray scattering (AXS) method for materials characterization have been described by focusing our attention on the structure of non-crystalline oxide systems. This relatively new method makes it possible to provide the environmental structure around a specific element, which is not obtained from the conventional X-ray diffraction results, in multi-component non-crystalline systems. The validity and usefulness of the AXS method were demonstrated by obtaining the fine structure of GeO 2 glass, ZrO 2 amorphous, the Sr-La-Mn-B-O type ferromagnetic glass and the Cu-I-Mo-O type super-ionic conducting glass.

Low temperature hysteresis in high anisotropy systems

This paper is focused on understanding hysteresis in Dy 3Al 2, which is a member of a class of high anisotropy compounds containing high fractions of rare earth elements with no 3-d transition metals. Analysis of the data suggests that the principal domain wall pinning mechanism does not involve the grain boundaries but instead arises from interactions between either the ferromagnetic glass or the ferromagnetic Dy 3Al 2 phase with the antiferromagnetic DyAl second phase through an exchange bias mechanism.

Magnetic Phase Diagram of Amorphous Fe100?xScx Alloys

An overview will be given on the magnetic properties of amorphous Fe 100-x Sc x alloys. The experimental magnetic phase diagram in an external field of 100 Oe is established on amorphous films that were prepared in ultra high vacuum. At x 41, Sc atoms carry magnetic moments which couple antiferromagnetically to the Fe-atoms. Because of the complexity of the exchange interaction in the present system, antiferromagnetic as well as ferromagnetic and (reentrant) spin glass ordering was observed. Above x > 80, there are indications of superconductivity below 11 K.

Bulk ferromagnetic glasses in the Fe–Ni–P–B System

The ferromagnetic metallic glass Fe 40Ni 40P 14B 6, available only as 30–50 μm thick ribbons, has been extensively studied over the last three decades. We used a flux-melting and water-quenching technique to prepare bulk glassy Fe 40Ni 40P 14B 6 alloys in the form of 2-mm diameter spheres and 1-mm diameter rods. The Curie temperature for the bulk glasses is higher than the average value of Curie temperatures reported for the rapidly quenched ribbons. The glass-transition temperature and the crystallization temperature of the bulk glasses are lower and higher, respectively, than the average values reported for rapidly quenched ribbons, making the supercooled-liquid region as wide as 42 K. The bulk glasses crystallize by a homogeneous nucleation followed by a growth at a constant rate. The nucleation rate in the bulk glasses is four orders of magnitude lower than in the rapidly quenched ribbons, suggesting that the previous thickness limitation was due to impurities in the melt (heterogeneous nucleation).

Ferroelectric and ferromagnetic microcrystalline glass ceramics

Microcrystalline glass ceramics are those materials bearing the major characteristics of both glass and ceramic. In the development of high performance ceramic materials, structural, compositional and chemical purifications are the most important weapons of ceramic scientists and engineers. However, in many cases, even the purist single crystalline materials can no longer satisfy the very complicated and comprehensive requirements on the properties and processes of ceramic materials from modern technology. It is the time for us to look for help from novel multiphase ceramic materials. Sol-gel process is introduced to prepare ferroelectric and ferromagnetic microcrystalline glass powders. Various ferroelectric and ferromagnetic crystallites in the size of nanometers and micrometers can be in-situ precipitated from amorphous glass matrices. Then, conventional ceramic processing is used to sinter such microcrystalline glass powders into ceramics. The processing, structure, property and application of PT, BT and KTP based ferroelectric glass ceramics, spinel Mn-Zn ferrite and magnetoplumbite Ba ferrite ferromagnetic glass ceramics will be discussed.

Bulk ferromagnetic glasses prepared by flux melting and water quenching

Several ferromagnetic bulk amorphous alloys of the type Fe–(Co, Cr, Mo, Ga, Sb)–P–B–C, containing between 62 and 71 at. % Fe, have been prepared in the form of 4-mm-diam rods. The glass synthesis consists of mechanically alloying the constituents, purifying the melts in B2O3 flux inside fused silica tubes, and quenching the melts at cooling rates on the order of 100 K/s. All these glasses have a large supercooled region Tx−Tg, ranging from 35 to 61 K, within which the glass can be shaped under a relatively small applied load. It is shown that the value of Tx−Tg depends strongly on the metalloid composition. These bulk metallic glasses have very low coercivity and low hysteresis losses.

Ferromagnetic Glasses with Stable Supercooled Liquid in Gd-Al-(Cu,Ni,Co) Alloys

Metallic glasses with ferromagnetism and stable supercooled liquid formed in melt-quenched Gd 55 Al 25 TM 20 (TM = Cu, Ni, and Co) alloys have been studied by means of vibrating-sample magnetometer (VSM) and differential scanning calorimeter (DSC). These glasses show a large temperature interval between glass transition temperature (T g ) and crystallization temperature (T x ) of about 60K which is comparable with typical metallic glasses. The glasses are predominantly ferromagnetic with curie temperature (T o ) from 90 K to 120 K and saturation magnetization (σ s ) between 160 emu/g and 200 emu/g.

Magnetic phase diagram of the Fe x Mn 0.7-x Al 0.3 alloys series obtained by Mössbauer spectroscopy

By Mossbauer spectroscopy the magnetic phase diagram of the FexMn0.7-xAl0.3 spin glass alloy has been obtained. X-ray diffraction (XRD) has shown that all the alloys are in the BCC phase. The broad spectral lines observed in the Mossbauer spectra are indicative of the disordered character of these alloys. Depending on the composition and the temperature the alloys behave as paramagnetic (P), ferromagnetic (F) and reentrant spin glass (RSG). For the alloys rich in iron the only detected magnetic transition is from F to P. For medium iron content two transitions were observed, namely from RSG to F and from F to P. The RSG phase is obtained as a consequence of the disordered character of the alloys and the competitive exchanges due to iron and manganese atoms.

Spin dynamics in the reentrant spin glass (Fe 0:65 Ni 0:35 ) 1 x Mn x

The spin dynamics in the reentrant spin glass ( Fe0.65 Ni0.35)1-x Mn_x has been studied by zero, longitudinal and transverse field μSR. In the ferromagnetic reentrant and pure spin glass regimes (x\leqslant 0.175), zero field experiments reveal a stretched exponential muon relaxation with a universal behaviour of the dynamic exponent \beta above the spin glass transition. There are no qualitative differences between the ferromagnetic and paramagnetic phases. In transversal field μSR experiments the divergence of the relaxation rate close to the spin glass transition is suppressed for manganese doping up to x=0.113 but enhanced for slightly higher doping (x\geqslant 0.12). We understand this behaviour as a crossover from an itinerant to a more localized state of the 3d electron system. This is also supported by the fact that in the highly doped regime with dominant antiferromagnetic interactions the muon relaxation rate diverges above the antiferromagnetic transition temperature.

Pressure and Isotope Effects in the Manganese-Oxide Perovskites

ABSTRACTMeasurements of the temperature dependence of the resistivity ρ(T) and thermoelectric power α(T) under several hydrostatic pressures on l8O/l6O isotope-exchanged (La1-xNdxCa0.3MnO3 polycrystalline samples spanning the structural O'-O orthorhombic transition have demonstrated that the perovskite tolerance factor t increases with pressure, signaling an unusually compressible Mn-0 bond. They have also indicated a change at the O'-O transition from static to dynamic cooperative Jahn-Teller deformations, from a second-order to a first-order magnetic transition, from Mn(IV) to two-Mn polarons in the paramagnetic region, from Mn(IV) polarons to a vibronic state below Tc, and a phase segregation in the O phase above Tc that traps out mobile polarons into ferromagnetic Mn(IV)-rich clusters within a Mn(IV)-poor matrix. Specific heat data show a transfer of spin entropy to configurational entropy on cooling through Tcin the O phase, and a single-crystal study of La1-xSrxMnO3, x = 0.12 and 0.15, has demonstrated a transition from polaronic to itinerant e electrons below Tc within the O phase. Magnetic-susceptibility measurements in low fields confirm the phase segregation above Tc and the existence of a ferromagnetic glass below Tc in the O phase. The intrinsic “colossal” magnetoresistance (CMR) is attributed to the growth to their percolation threshold of the ferromagnetic clusters existing above Tc, and the dramatic rise in Tc with increasing tolerance factor t in the O-orthorhombic phase to an increase in the density, and also the mobility, of the untrapped polarons above Tc.

Ferromagnetic Interaction in C60 Charge Transfer Complexes

Abstract C60[TDAE] was the first and so far the only C60 CT complex which ex-hibits molecular ferromagnetism with a Curie temperature of about 16 K [1]. Despite extensively investigated a convincing access to the origin of the low temperature phase of this C60 CT complex is still missing. The 3 fold degeneracy of the C60 HOMO is lifted by a change of the C60 conformation, and we find a transition from a static low temperature Jahn-Teller distortion to a dynamic one at about 140 K. The excess electron is concentrated in a belt around the equator of the C60-. The distorted C60 molecules have a spin density distribution with an axial symmetry and are aligned along the crystallographic axes. This orientational ordering of the C60- molecules is the necessary precursor to a ferromagnetic spin glass type ordering in C60[TDAE] which was interpreted in terms of spin polarisation between C60- and TDAE+*by Yamaguchi et a1 [2].

Synthesis of Magnetite Particles by Pulsed Alexandrite Laser Processing of Metallic Glass Precursors

ABSTRACTSamples of Fe78B13Si9 and Fe81B13.5Si3.5C2 metallic glasses were irradiated with a pulsed alexandrite laser (λ=750 nm, τ=60 μs) using different laser fluences. Kinetics of laser-induced phase transformations and fluence dependence of magnetic properties were studied by scanning electron microscopy (SEM) and Mössbauer spectroscopy. Low laser fluences were found to induce changes in magnetic texture and onset of crystallization. High laser fluences, however, correlated with additional oxidation effects and the formation of stoichiometric Fe3O4 particles in the irradiated alloy system. An activation energy of 11.9 eV was estimated for the laser-driven synthesis of magnetite nanoparticles. Pulsed alexandrite laser processing is an intriguing alternative technique for the controlled synthesis of iron oxide phases from ferromagnetic glass precursors.

Magnetoelastic waves application to investigate laser annealing effects in ferromagnetic glass

The dependence of magnetoelastic wave amplitude on structure-sensitive parameters, in addition to the possibility of producing and detecting these waves in different parts of a sample, has been used to study the effects induced by laser heating in ferromagnetic materials, during and after heat treatment. Here we report experimental results and theoretical considerations that explain the correlations between the magnetoelastic wave amplitude and the magnetic permeability, the stress sensitivity and the elastic coefficient. In particular, it is shown that structural changes can be seen, and a simple model of thermomagnetic coupling is developed.

Spin-resolved and high-energy-resolution XPS studies of cobalt metal and a cobalt magnetic glass.

The role of electron spin in photoelectric emission from the 2s, 2${\mathit{p}}_{1/2}$, 2${\mathit{p}}_{3/2}$, and 3p core levels of cobalt metal is investigated through a combination of spin-resolved x-ray-photoelectron spectroscopy (SRXPS) and high-energy-resolution XPS (HRXPS) studies. The sensitivity of the Co core-level spectra to a variation in the valence spin polarization is revealed by comparing the Co-metal data with SRXPS and HRXPS spectra from the amorphous cobalt-based ferromagnetic glass ${\mathrm{Co}}_{66}$${\mathrm{Fe}}_{4}$${\mathrm{Ni}}_{1}$${\mathrm{B}}_{14}$${\mathrm{Si}}_{15}$. The results demonstrate explicitly the role of intra-atomic exchange in deep-core-level photoemission from Co, and reveal core-hole spin dependencies to core-hole-induced electron-hole-pair excitation, to the ${\mathit{L}}_{1}$${\mathit{L}}_{2,3}$V Coster-Kronig transition rate, and to the rates of Auger transitions involving the valence band. The data also suggest that the reduction of the cobalt magnetic moment in the ${\mathrm{Co}}_{66}$${\mathrm{Fe}}_{4}$${\mathrm{Ni}}_{1}$${\mathrm{B}}_{14}$${\mathrm{Si}}_{15}$ glass results from the transfer of majority-spin 3d electron density into the minority-spin 3d band caused by a hybridization-induced weakening of the 3d electron-electron interaction.