Ferromagnetic Bulk Research Articles

Room-temperature ferromagnetism in Co-doped ZnO bulk induced by hydrogenation

The room-temperature ferromagnetism properties of Co-doped zinc oxide are reported in this paper. The Zn 1 − x Co x O ( 0.02 < x < 0.10 ) specimens were synthesized by the solid state reaction method with the mixture of Co 2 O 3 and ZnO powders. The precursor Co 2 O 3 and ZnO powders were mixed and milled for 10 h. The mixed powders were calcined at temperature of 600 ° C and 1000 ° C, respectively for 12 h, and then were cooled at the ambient temperature. Next, the powders were milled, made block and sintered for 12 h. The specimens were then hydrogenated at 600 ° C for 1 h and 3 h, respectively. The room-temperature ferromagnetism (RTFM) was found in the hydrogenated samples, and the hysteresis loops of the hydrogenated Co-doped ZnO samples were measured at 10 K and 300 K by using the superconducting quantum interference device (SQUID) magnetometer (Quantum Design). The coercive field was about 310 Oe for the room-temperature measurement, and the saturation magnetization of the hydrogenated sample was about 0.056 μ B / Co . The samples were further measured by using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HRTEM). The results suggested the observed RTFM in the Co-doped ZnO was partly due to the existence of the cobalt nanoparticles in the hydrogenated samples.

Bulk ferromagnetism in nitronyl nitroxide crystals: a first principles bottom-up comparative study of four bulk nitronyl nitroxide ferromagnets (KAXHAS, YOMYII, LICMIT and YUJNEW)

Searching for general trends that could help to design new bulk (3D) ferromagnets, we have carried out a first-principles bottom-up theoretical study of four bulk ferromagnets of the nitronyl nitroxide (NN) family of crystals: β-p-(nitro)phenyl-NN (KAXHAS), α-o-(hydroxy) phenyl-NN (YOMYII), α-2,5-(dihydroxy)phenyl-NN (LICMIT) and p-(methylthio) phenyl-NN (YUJNEW). This first-principles bottom-up theoretical study connects, in a rigorous form, the microscopic magnetic pair interactions (J AB) with the macroscopic magnetic properties (e.g. magnetic susceptibility or heat capacity curves). The microscopic magnetic pair interactions are computed from the crystal structure using DFT methods. The network of non-negligible J AB magnetic pair interactions between AB radicals provides the magnetic topology of the crystal. Using the room-temperature crystal structure (the only experimentally available) for KAXHAS, YOMYII and LICMIT, we found a 3D magnetic topology. However, although the computed magnetic susceptibility χ( T ) curves reproduce, in all three cases, the ferromagnetic experimental χ( T ) data, not all the J AB pair interactions are ferromagnetic. For YUJNEW, there are three sets of crystallographic coordinates (room temperature, 114 and 10K) and its magnetic topology depends on the temperature: at room temperature, there is a 2D magnetic topology but, using the 114 and 10K crystal structures, the magnetic topology becomes 3D. The computed magnetic susceptibility curve only reproduces the experimental data when using the magnetic topology computed at 10K (at 114K, there is only a qualitative agreement). The dependence of the J AB magnetic pair interactions on the geometry of the A–B radical–radical pair is also analysed. Our results show that small changes in the geometry induce changes in the sign of the interaction, a fact that can be only explained by considering that the nature of the interaction is a cooperative effect that depends on all atoms of the radical.

Formation of ferromagnetic bulk amorphous Fe 40Ni 40P 14B 6 alloys

Ferromagnetic bulk amorphous Fe 40Ni 40P 14B 6 alloy rods with a diameter of ∼ 1.2 mm can be prepared by means of a rapid quenching technique. If a fluxing technique is also used, amorphous rods with a diameter as large as ∼ 2.5 mm can be synthesized. The critical cooling rate R c for the glass formation Fe 40Ni 40P 14B 6 is estimated to be on the order of 10 2 K s − 1 .

Phase Separation inA-Site-Ordered Perovskite ManganiteLaBaMn2O6Probed byLa139andMn55NMR

$^{139}$La- and $^{55}$Mn-NMR spectra demonstrate that the ground state of the A-site ordered perovskite manganite LaBaMn$_2$O$_6$ is a spatial mixture of the ferromagnetic (FM) and antiferromagnetic (AFI(CE)) regions, which are assigned to the metallic and the insulating charge ordered state, respectively. This exotic coexisting state appears below 200 K via a first-order-like formation of the AFI(CE) state inside the FM one. Mn spin-spin relaxation rate indicates that the FM region coexisting with the AFI(CE) one in LaBaMn$_2$O$_6$ is identical to the bulk FM phase of the disordered form La$_{0.5}$Ba$_{0.5}$MnO$_3$ in spite of the absence of A-site disorder. This suggests mesoscopic rather than nanoscopic nature of FM region in LaBaMn$_2$O$_6$\@.

Room-temperature ferromagnetism in bulk CoxTi1－xO2－δ induced by the phase transformation in the hydrogenation sintering process

Paramagnetic (Co3O4)x/3(TiO2)1- x(03O4 and TiO2 from 700 ℃ to 1000 ℃ in air. X-ray diffraction (XRD) re sults showed that perovskite CoTiO3 existed in all samples. Anatase p hase was also detected in samples sintered at 700 ℃ and when sintering the mixt ure at above 900 ℃, rutile phase formed with anatase phase disappearing. All sa mples exhibited room-temperature ferromagnetism (RTFM) after being hydrogenated in 10％ H2/Ar mixed atmosphere for 1 hour at 500 ℃. X-ray photoelect ron spectroscopy (XPS) showed that Co existed in the +2 formal oxidation state, and no Co clusters were detected. XRD showed that CoTiO3 disappeared and a new phase CoxTi1-xO2-δ formed after hyd rogenation. It is suggested that hydrogenation causes the phase transformation f rom CoTiO3 to CoxTi1-xO2-δ. RTFM of the sample originates from the intrinsic ferromagnetism of CoxTi1-xO2-δ. The exchange interaction between Co2+ occ urring with the phase transformation may be the essential reason for RTFM.

Large Nonlinear Kerr Angle in non-Centrosymmetric Fe/AlGaAs (001) Heterostructure

AbstractA large nonlinear magneto-optical effect is observed in a non-centrosymmetric Fe/AlGaAs (001) heterostructure. This effect is a direct consequence of interference between second-harmonic optical waves of magnetic and crystallographic origin, generated at ferromagnetic Fe interface and bulk AlGaAs, respectively. The longitudinal nonlinear Kerr rotation is measured to be 1.6° along the [1-10] hard axis, about two orders of magnitude stronger than the linear equivalent. The rotational second-harmonic signal shows large magnetic contrast along all the in-plane directions, demonstrating a high sensitivity to the magnetization of an anisotropic interface in the longitudinal geometry.

One-dimensional bulk ferromagnets: NdAl 2 and hcp cobalt

It is shown experimentally that NdAl 2 and hcp cobalt are one-dimensional (1D) bulk ferromagnets. For hcp cobalt this is only under the condition that the sample is magnetically saturated, i.e. that all moments are aligned parallel to the hexagonal c-axis. In 1D magnets the transverse interactions need not to be zero but must be sufficiently weak such that the transverse correlation length does not diverge at the critical temperature. The transverse interactions are then not relevant and the phase transition is driven by the longitudinal interactions. On the other hand, magnetic Bragg scattering relies on finite transverse correlations. For NdAl 2 no conventional magnetic Bragg scattering is observed if all moments are aligned vertical to the scattering plane by a magnetic field. For hcp cobalt the scattering intensity is considerably reduced in this geometry instead of having its maximum. From this observation it can be concluded that the transverse correlation length is practically zero in NdAl 2 but has a finite value in hcp cobalt. The macroscopic magnetization shows normal ferromagnetic saturation.

Influence of quadratic contributions in magnetization‐induced second harmonic generation studies of magnetization reversal

AbstractMagnetization‐induced optical Second Harmonic Generation (MSHG) from an exchange‐biased CoO/Fe multilayer produces an asymmetrical hysteresis loop that indicates different magnetization reversal behaviour between the interface and the bulk ferromagnet. A more careful analysis of the data demonstrates that this asymmetry is in fact due to a quadratic dependence on the magnetization of the MSHG intensity. (© 2005 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Formation and magnetic properties of Co–Fe-based bulk metallic glasses with supercooled liquid region

A new Co–Fe-based ferromagnetic bulk metallic glass (BMG) was synthesized by copper mould casting method. The thermal stability and crystallization processes were investigated by differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The soft magnetic behavior was studied by DC magnetic measurements. The high glass formation ability was interpreted in terms of the effective suppression of nucleation and growth of the intermetallic compounds which appear in the multicomponent system during solidification. The high thermal stability indicates that the new Co–Fe-based BMG could be used as high-temperature magnetic material. The low coercivity which was as low as 8 A/m for the as-cast sample was found in the Co–Fe-based metallic glass cylinder with a diameter of 1.5 mm.

23Na NMR investigations of the itinerant ferromagnets NaFe 4Sb 12 and Na 0.5Ca 0.5Fe 4Sb 12

A 23Na ( I = 3 2 ) NMR study is presented on the itinerant ferromagnets NaFe 4Sb 12 and the isostructural Ca-substituted compound Na 0.5Ca 0.5Fe 4Sb 12 performed in 7.05 and 11.74 T in the temperature range from 4 to 300 K. Both compounds show a bulk ferromagnetism below 85 K. Static (Knight-shift) and dynamic (spin-lattice relaxation) measurements are described in the framework of the SCR theory for itinerant d band metals.

Review of add-on process modules for high-frequency silicon technology

Add-on process modules as enhancements for standard high-frequency silicon integration processes are discussed. Such modules can cost-effectively be added without any interference with the core process before (pre-process modules), during (mid-process modules), or after (post-process modules) the circuit integration. In addition, layout options providing a most cost-effective means of enhancement are discussed. High-resistivity silicon substrates, ferromagnetic thin-film integration, bulk micromachining, saddle-add-on metallization, spacer-substrate integration, and metal layer shunting are presented as examples in those categories.

Temperature-dependent spectral generalized magneto-optical ellipsometry for ferromagnetic compounds

Spectral generalized magneto-optical ellipsometry is presented as an optical tool for the simultaneous measurement of the complex index of refraction ñ=n+ik, the complex magneto-optical coupling parameter Q=Qr+iQi (i.e., the Voigt-parameter), and the orientation of the saturation magnetization Ms of isotropic ferromagnetic bulk materials. For wavelengths between 220nm and 790nm and at temperatures between 4.2K and 800K measurements on iron and permalloy demonstrate the comfortable application of this technique in order to resolve the spectral response of spin-polarized carriers and bands, which can provide valuable insight about the formation of the ferromagnetic state.

Competing magnetic phases in mixed-valent manganese oxide perovskites

Measurement of specific heat ${C}_{p}(T)$ below $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ of melt-grown samples of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ $(0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.3)$ and ${R}_{0.7}{A}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ $(R=\text{rare}\text{\ensuremath{-}}\text{earth},A=\text{alkaline}\text{\ensuremath{-}}\text{earth})$ with room-temperature tolerance factor $0.950\ensuremath{\leqslant}t\ensuremath{\leqslant}0.996$ have been supplemented by transport and magnetic measurements. Comparison of the phase diagram of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ with that of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ and the evolution with $t$ of the ${R}_{0.7}{A}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ family illustrate the sensitivity to $t$ of the crossover from localized to itinerant behavior of the $\ensuremath{\sigma}$-bonding electrons and support the model of two magnetic phases in the crossover compositional range that has been used to account for the colossal magnetoresistance (CMR) phenomenon found in these oxides. A vanishing of the specific-heat anomaly at the Curie temperature ${T}_{c}$ and the magnetic data at crossover are typical of a spin glass, and a broad hump in ${C}_{p}(T)$ below a ${T}_{h}&gt;{T}_{c}$, where there is no anomaly at the ${T}_{c}$ signal ferromagnetic ordering within isolated pockets of a hole-rich, conductive $\mathrm{O}*$ minority phase at ${T}_{h}$. On cooling through ${T}_{N}$ of the antiferromagnetic matrix, the spins freeze at a spin-glass temperature ${T}_{g}$ in zero magnetic field $H$ if the ferromagnetic phase does not percolate; the ferromagnetic phase grows in an applied $H$, and a modest $H$ converts the spin glass to a bulk ferromagnet with a Curie temperature ${T}_{c}\ensuremath{\approx}{T}_{g}$, where the ferromagnetic phase grows to beyond percolation. As $x$ increases in ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$, a ferromagnetic-insulator ${\mathrm{O}}^{\ensuremath{''}}$ phase having a charge ordering and a different orbital ordering than the parent ${\mathrm{O}}^{\ensuremath{'}}$ phase percolates below a ${T}_{g}\ensuremath{\approx}{T}_{c}$, and the minority ${\mathrm{O}}^{\ensuremath{'}}$ phase remains paramagnetic until it becomes antiferromagnetic below a ${T}_{M}&lt;{T}_{c}\ensuremath{\approx}{T}_{g}$. In the interval $0.15&lt;x&lt;0.25$, an orbitally disordered conductive, ferromagnetic vibronic phase appears in a narrow temperature interval ${T}_{\mathrm{oo}}&lt;T&lt;{T}_{c}$, the majority phase transforming to the charge and orbitally ordered ${\mathrm{O}}^{\ensuremath{''}}$ phase below a ${T}_{\mathrm{oo}}$. At $x\ensuremath{\geqslant}0.25$, the system transforms from a polaronic paramagnetic to a ferromagnetic metal at ${T}_{c}$.

Ferromagnetism above room temperature in bulk sintered gallium phosphide doped with manganese

Evidence for ferromagnetism in bulk sintered gallium phosphide (GaP) doped with 3% manganese, having a Curie temperature of 600 K considerably higher than previous observations, is obtained using ferromagnetic resonance (FMR) and AC magnetization measurements. The field position and line width of the resonance showed a strong temperature dependence characteristic of FMR spectra. A non-resonant derivative signal centered at zero field was also observed starting at 600 K further confirming high temperature ferromagnetism. AC magnetization measurements also show the existence of ferromagnetism at high temperature.

The Mechanism of Magnetic Interactions in the Bulk Ferromagnet para‐(Methylthio)Phenyl Nitronyl Nitroxide (YUJNEW): A First Principles, Bottom‐Up, Theoretical Study

The mechanism of magnetic interactions in the bulk ferromagnet para-(methylthio)phenyl nitronyl nitroxide crystal (YUJNEW) has been theoretically reinvestigated, using only data from ab initio calculations and avoiding any a priori assumptions. We first calculate the microscopic magnetic interactions (JAB exchange couplings) between all unique radical pairs in the crystal, and then generate the macroscopic magnetic properties from the energy levels of the corresponding Heisenberg Hamiltonian. We thus propose a first principles, bottom-up (i.e. micro-to-macro) approach that brings theory and experiment together. We have applied this strategy to study the magnetism of YUJNEW using data from the previously reported 298 and 114 K crystal structures, and also data from a 10 K neutron diffraction structure fully reported in this work. The magnetic topology at 298 K is two-dimensional: noninteracting planes, with three different in-plane JAB pair interactions (+0.24, +0.09, and -0.11 cm(-1)) and one numerically negligible (+0.02 cm(-1)) inter-plane JAB interaction. In contrast, the magnetic topology at 114 and 10 K is three-dimensional, with two non-negligible in-plane JAB constants (+0.11 and +0.07 cm(-1) at 114 K; +0.22 and +0.07 cm(-1) at 10 K) and one inter-plane pair interaction (+0.07 cm(-1) at 114 K; +0.08 cm(-1) at 10 K). Although this three-dimensional magnetic topology is consistent with YUJNEW being a bulk ferromagnet, there is only a qualitative agreement between computed and experimental magnetic susceptibility chiT(T) data at 114 K. However, the experimental chiT(T) curve is quantitatively reproduced at 10 K. The heat capacity curve presents a peak at around 0.12 K, close to the estimated experimental peak (0.20 K).

Absence of room temperature ferromagnetism in bulk Mn-doped ZnO

Structural and magnetic properties have been studied for polycrystalline Zn1−xMnxO (x=0.02,0.03,0.05). Low-temperature (∼500 °C) synthesis leaves unreacted starting ZnO and manganese oxides. Contrary to a recent report, no bulk ferromagnetism was observed for single-phase materials synthesized in air at temperatures above 900 °C. Single-phase samples show paramagnetic Curie–Weiss behavior.

Synthesis of ferromagnetic Fe-based bulk glassy alloys in the Fe–Nb–B–Y system

The effect of substitution of Y for Fe on the glass forming ability (GFA), magnetic and mechanical property has been studied in Fe 77 − x Nb 6B 17Y x ( x = 0–4) alloys. Replacement of 1–4 at.% Fe by Y increased the thermal stability of the glass and liquid phase in the Fe–Nb–B–Y alloys. It was found that the Fe 74Nb 6B 17Y 3 alloy exhibited the largest T rg (0.60) and γ (= T x /[ T g + T L]) (0.396) values enabling the fabrication of bulk metallic glass (BMG) with a diameter of up to 2 mm. The BMG alloy (≥74 at.% Fe) exhibited high compressive fracture strength and hardness of about 3 GPa and 1060 Hv, respectively. The partial replacement of 3 at.% Fe by Y in Fe–Nb–Y–B improved the alloy magnetization saturation by about 10%.

Formation and magnetic properties of the RE-based compounds of type RE 6Fe 13−xAl 1+x (RE = Pr, Sm, Gd)

Selected rare-earth (RE)-based δ-phase compounds of the composition RE 6Fe 13− x Al 1+ x (1 ≤ x ≤ 4) have been prepared and studied to ascertain their role in conveying coercivity to the so-called ferromagnetic bulk metallic glass (BMG) alloys (RE) 60Fe 30Al 10. It was found that δ-phase does not exist for RE = Sm or Gd. However, nearly single δ-phase Pr-based compounds of the composition Pr 6Fe 13− x Al 1+ x in a range of Al concentration 1.5 ≤ x ≤ 3.5 were successfully synthesized. The antiferromagnetic behavior of the Pr-based δ-phase compounds is clearly seen from temperature-dependent magnetization measurements and the Néel temperature shows a quasi-linear decrease from 320 K to 115 K as x in Pr 6Fe 13− x Al 1+ x is increased from 1 to 4. The observed Al-concentration dependence of the Pr 6Fe 13− x Al 1+ x Néel temperatures is expected to be reflected in the magnetic behavior of the BMG alloys of composition Pr 60Fe 30Al 10.

Interactions between demagnetizing field and minor-loop development in bulk ferromagnets

A phenomenon of mutual influence of the demagnetizing field and a gradual increase of width of minor loops in bulk ferromagnets during magnetization processes is scrutinized by computational modeling. Classical static micromagnetics is here enriched by a suitable activated dissipation mechanism allowing for rate-independent hysteresis. The activation threshold is allowed to depend, again by a rate-independent manner, on local history of a magnetization process, which reflects (only on a mesoscopical level) gradual development of a domain structure. Influence of the shape of the specimen is thus automatically involved via nonlocal interactions through the demagnetizing field.

Classical Description of Spin Wave Excitation by Currents in Bulk Ferromagnets

A modification of the Landau-Lifshitz equation accounts for the effect of the current on the magnetization in the slow passage limit, according to which the spins precess about the exchange field quickly in comparison to the rate of change of the orientation of the magnetization as seen in the reference frame of the conduction electrons. This model implies that the spin polarization of the current provokes an instability of the magnetization in the bulk of a ferromagnet when the current density reaches a threshold.