Decrease Of Magnetic Moment Research Articles

Magnetic properties of sigma-phase FeCrX (X=Co, Ni) alloys: Experimental and theoretical study

Experimental (vibrating sample magnetometry) and theoretical (electronic structure calculations using self-consistent Korringa-Kohn-Rostoker Green's function method, also including the coherent potential approximation) studies were performed on a series of intermetallic σ-phase FeCrX (X = Ni, Co) compounds in systematic way. In σ-FeCrNi (Ni concentration is limited to ∼ 10 at%) a slight enhancement of magnetic properties when replacing Fe with Ni atoms and a slight attenuation of these properties when replacing Fe with Cr was experimentally observed. In the case of σ-FeCrCo, the reason for disappearance of magnetic properties was explained, since the substitution of Fe with Co leads to systematic attenuation of TC and μ¯. Consequently, σ-CoCr was established to be non-magnetic from accurate measurements of magnetisation for series of σ-FexCrCo samples with x tending to zero. The decrease of average magnetic moment in σ-FeCrCo well corroborates with the results of electronic structure calculations assuming different models for chemical disorder. In order to enlighten reasons for magnetism onset in σ-FeCrX, the KKR-CPA spin-polarised density of states and magnetic moments of Fe impurities diluted on all sublattices were computed in σ-CoCr and hypothetical σ-NiCr. In both cases, the largest Fe magnetic moment was calculated on B (∼2μB) and next C (∼1.3μB and D (∼0.5μB) sites, while almost vanishing values were found on A and E sites.

The effect of magnetic field induced aggregates on ultrasound propagation in aqueous magnetic fluid

Ultrasonic wave propagation in the aqueous magnetic fluid is investigated for different particle concentrations. The sound velocity decreases while acoustic impedance increases with increasing concentrations. The velocity anisotropy is observed upon application of magnetic field. The velocity anisotropy fits with Tarapov’s theory suggests the presence of aggregates in the system. We report that these aggregates are thermodynamically unstable and the length of aggregate changes continuously with increasing concentration and, or magnetic field and resulted in a decrease in effective magnetic moment. The Taketomi's theory fits well with the experimental data suggesting that the particle clusters are aligned in the direction of the magnetic field. The radius of cluster found to increase with increasing concentration, and then decreases whereas the elastic force constant increases and then becomes constant. The increase in cluster radius indicates elongation of aggregate length due to tip-to-tip interaction of aggregates whereas for higher concentration, the lateral alignment is more favorable than tip-to-tip alignment of aggregates which reduces the cluster radius making elastic force constant to raise. Optical images show that the chains are fluctuating and confirming the lateral alignment of chains at higher fields.

Electric field modulated half-metallicity of semichlorinated GaN nanosheets

Abstract Through density-functional theory calculations, we investigated the half-metallic properties of semichlorinated gallium nitride (Cl–GaN) nanosheets (NSs) under an electric field F . The results show that the electric field can modulate Cl–GaN NSs efficiently from ferromagnetic metals to half-metals. More interestingly, under a broad range of electric field intensity (−0.10~−1.30 V/A), Cl–GaN NSs have the excellently half-metallic properties with the band gaps (3.71–0.96 eV) and maximal half-metallic gaps with 0.30 eV in spin-up states and metallic behaviors in spin-down states. Moreover, the total magnetic moment decreases (increases) depending on the negative (positive) F , mainly induced by the unpaired N atoms. Our studies demonstrate that the electronic and magnetic properties of GaN NSs can be delicately tuned by the combined surface modification and electric field, indicating the potential of GaN NSs for developing high-performance spintronic nanodevices.

Enhancement of Curie temperature and transition temperature range induced by Al doping in Mn1-xAlxCoGe

Abstract Mn 1-x Al x CoGe alloys with a second order transition were produced by arc-melting method. The substitution of Mn by Al increased the Curie temperature ( T C ) from 260.5 K to 300.8 K, the magnetic entropy change (|Δ S M |) decreased from 3.78 J·Kg −1 K −1 to 2.35 J·Kg −1 K −1 under a field change of Δµ 0 H=5 T. In addition, the |Δ S M | well linearly depends on the H 2/3 around T C . Furthermore, the relative cooling power (RCP) can reach 242.3 J·Kg −1 with a large full width at half maximum of |Δ S M | (75.5 K) for x=0.02. The decrease of |Δ S M | is explained by the corresponding monotonical decrease of magnetic moment per formula unit.

The Magnetic Relaxation of MgB2Thin Films Grown by Hybrid Physical–Chemical Vapor Deposition

The magnetic relaxation of MgB 2 thin film deposited on Al 2 O 3 substrate by hybrid physical-chemical vapor deposition method was investigated. The rate of magnetic relaxation (the logarithmic decay rate) S, which was obtained by the time dependence of magnetic moment m(t) at fixed magnetic field H and temperature T, of the MgB 2 thin film was investigated. The decay of magnetic moment m(t) was carried out at temperatures between 5 and 37.5 K in the field ranges of 0.1-1.0 T, using PPMS-VSM (Quantum Design). The decrease of magnetic moment m(t) for 7200 s at T = 5 K was about 2%-3% and 0.2%-0.3% when the field was applied parallel to the c-axis and the ab-plane, respectively. The logarithmic decay rate S in the field parallel to the ab-plane was much smaller than that parallel to the c-axis over all temperature ranges. The time-independent behavior of logarithmic decay rate S at low temperatures below T = 10 K implies the presence of quantum creep effect.

A Copper-Nitroxide Adduct Exhibiting Separate Single Crystal-to-Single Crystal Polymerization-Depolymerization and Spin Crossover Transitions.

A complex cascade of solid-state processes initiated by variation of temperature was found for the heterospin complex [Cu(hfac)2L(Me/Et)] formed in the reaction of copper(II) hexafluoroacetylacetonate [Cu(hfac)2] with stable nitronyl nitroxide 2-(1-methyl-3-ethyl-1H-pyrazol-4-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (L(Me/Et)). The cooling of the compound below 260 K initiated a solid-state chemical reaction, which led to a depolymerization of chains and formation of a pair heterospin complex [Cu(hfac)2L(Me/Et)2][[Cu(hfac)2]3L(Me/Et)2]. Further decrease in temperature below 144 K led to a spin transition accompanied by a drastic decrease in the effective magnetic moment from 2.52 to 2.24 μB. When the compound was heated, the order of effects was reversed: at first, the magnetic moment abruptly increased, and then the molecular fragments of the pair cluster united into polymer chains. Two hysteresis loops correspond to this cascade of temperature-induced structural transformations on the experimental dependence μeff(T): one at high (T↑ = 283 K and T↓ = 260 K) and the other at low (T↑ = 161 K, T↓ = 144 K) temperature. The spin transitions were also recorded for the [[Cu(hfac)2]3L(Bu/Et)2] and [[Cu(hfac)2]5L(Bu/Et)4] molecular complexes, which are models of the trinuclear fragment of the {[Cu(hfac)2]3L(Me/Et)2} pair cluster.

Magnetic and structural properties of Zn‐doped yttrium iron garnet nanoparticles

Zn‐doped YIG was prepared using the sol–gel method with TGA measurements showing the phase formation between 900 and 1000 °C. XRD analysis showed close to 1100 °C the formation of Franklinite phase, coexisting with the cubic YIG. Y3(Fe1–xZnx)5O12 samples with different Zn concentrations (x = 0, 0.01, 0.03, and 0.05) were prepared and analyzed for a magnetic study. A decrease in magnetic moment of the samples was confirmed on increasing the concentration of Zn ions. This decrease is due to the substitution of Fe ions by Zn. This also confirmed the results of XRD showing the linear increase in the lattice parameter. Fittings by Bloch's law shows results compared with those already reported in the literature. The exponent and constant for the Bloch law presented similar values to those reported for YIG doped with other ions. We obtained the parameter α ∼10−6 K−n with n close to 1.9 for all samples.

First-principles study on the magnetic and half-metallic properties in bulk and (001) surface of Ti2CoSn Heusler alloy

For the bulk and (001) surface of Ti2CoSn Heusler alloy, the electronic and magnetic properties in bulk and the surface effect on the structural, electronic and magnetic properties of the alloy for different terminations of (001) surface have been studied by using first-principles calculations. The spin-gapless semiconductor (SGS) ferromagnetism with the magnetic moment of 3.00μB/f.u. is confirmed in the bulk Ti2CoSn alloy with Hg2CuTi-type structure. For two ideal terminations (TiCo, TiSn) and three modified terminations (CoCo*, TiTi*, SnSn*), the density of states (DOS) indicates that all terminations destroy the SGS character. Furthermore, we find that the atomic magnetic moments (AMM) decrease for the most atoms on the outmost three layers due to structural relaxation of these atoms inward. Both the DOS and AMM of the central layer L9 are similar to the corresponding bulk characters because surface effects fade out at the position of the inner layer, 12Å below the surface.

Structural and magnetic properties of MnPd/Fe grown on MgO(100) substrate: Ab initio studies

Structural and magnetic properties of ultrathin films MnPd/Fe grown on MgO(001) are investigated using a self-consistent pseudopotential plane waves method based on density functional theory in the Perdew–Burke–Ernzerhof generalized gradient approximation. The results obtained reveal the presence of an antiferromagnetic coupling between successive Mn [100] rows, combined with a ripple where Mn outward atoms exhibit a positive magnetic moment, in the case of Mn overlayer on Fe/MgO(001). In the case of MnPd monolayer ordered alloy, the c(2×2) structure formation is more favorable than the p(1×2) one, exhibiting a ferromagnetic coupling between Mn neighbor atoms with a positive induced ferromagnetic moment on Pd atoms. Pd atoms are pushed outward. For 1-ML MnxPd1−x on Fe/MgO, the Mn absolute mean magnetization per atom increases as x coverage increases, whereas the Pd mean induced magnetic moment decreases. For systems alternating Mn and Pd monolayers on Fe/MgO(001), a complex magnetic structure is shown on Mn monolayers: changing from Mn neighboring antiferromagnetic coupling to Mn [010] rows antiferromagnetic behavior.The correlation is made between the electronic structure and the magnetic properties, by comparing filled with partially filled components (Pd, Mn and Fe) d-bands. The magnetization easy-axis changes between the in-plane and the out-of-plane orientations from Fe/MgO to MnPd/Fe/MgO systems.

Modeling and experimental studies of Hf-doped nanocrystalline SmCo7alloys

By combining first-principles calculations and experiments, the effects of Hf doping on the stability and magnetic properties of the metastable SmCo7 alloy were studied. It is found that the single TbCu7-type phase can be stabilized well by Hf doping when the Co-2e sites are occupied in the lattice structure. However, if Hf occupies the Co-3g sites, destabilization of the SmCo7 phase may occur. Moreover, Hf doping changes the partial density of states of Sm atoms in the vicinity of Co-2e sites, which causes a decrease in the net magnetic moments and hence a decrease in the saturation magnetization. The experimental results confirmed the calculation results of the phase stability and magnetic performance of the Hf-doped SmCo7 alloys.

Tailoring the substitution on the surface of Fe13/Pt42 core–shell nanoparticles by Au atoms

Fe13/Pt42 and Fe13/Pt42−nAun (n=0–12) core–shell nanoparticles were investigated using density functional theory calculation. The most stable structure was found by excess energy. Vertex is a preferential site for Au atoms to substitute. In addition, Au atoms prefer to substitute the closer vertical Pt. Edged Pt atoms are substituted by Au atoms only being found in Fe13/Pt39Au3 and Fe13/Pt37Au5 NPs. With the substitutions, higher stabilities are achieved in Fe13/Pt41Au1 and Fe13/Pt40Au2 NPs. However, an increasing of the Au atoms is not causing an enhancement in stability. Electrons are transferring from Pt and Au atoms to Fe atoms. The substitution brings about the decrease of Bader charge and total magnetic moment. Furthermore, substitutions result in the shift of the d-orbitals of Fe, Pt and Au atoms. The average bond length of Fe–Fe, Bader charge of center Fe, spin splitting of Fe d-band center and local magnetic moment of center Fe reveal a similar trend with the increasing of Au atom, demonstrating the electronic and magnetic properties strongly depend on the atomic ratios and average bond lengths.

Collapse of the magnetic moment under pressure of AFe2 (A=Y, Zr, Lu and Hf) in the cubic Laves phase

The electronic structures of four Laves phase iron compounds (e.g. YFe2, ZrFe2, LuFe2 and HfFe2) have been calculated with a state-of-the-art full potential electronic structure code. Our theoretical work predicted that the magnetic moments collapse under hydrostatic pressure. This feature is found to be universal in these materials. Its electronic origin is provided by the sharp peaks in the density of states near the Fermi level. It is shown that a first order quantum phase transition can be expected under pressure in Y(Zr, or Lu)Fe2, while a second order one in HfFe2. The bonding characteristics are discussed to elucidate the equilibrium lattice constant variation. The large spontaneous volume magnetostriction gives one of the most important characteristics of these compounds. Invar anomalies in these compounds can be partly explained by the current work when the fast continuous magnetic moment decrease with the decrease of the lattice constant was properly considered. This work may be as a first insight into the rich world of quantum phase transition and Invar mechanism in these Laves phase compounds.

Thickness-dependent magnetic properties and strain-induced orbital magnetic moment inSrRuO3thin films

Thin films of the ferromagnetic metal SrRuO3 (SRO) show a varying easy magnetization axis depending on the epitaxial strain and undergo a metal-to-insulator transition with decreasing film thickness. We have investigated the magnetic properties of SRO thin films with varying thicknesses fabricated on SrTiO3(001) substrates by soft x-ray magnetic circular dichroism (XMCD) at the Ru M2,3 edge. Results have shown that, with decreasing film thickness, the film changes from ferromagnetic to non-magnetic around 3monolayer thickness, consistent with previous magnetization and magneto-optical Kerr effect measurements. The orbital magnetic moment perpendicular to the film was found to be ~ 0.1{\mu}B/Ru atom, and remained nearly unchanged with decreasing film thickness while the spin magnetic moment decreases. Mechanism for the formation of the orbital magnetic moment is discussed based on the electronic structure of the compressively strained SRO film.

Ab-initio study on the electronic, optical and ferroelectric properties of LiOsO3

A comprehensive investigation has been made on the ground state, magnetic, ferroelectric and optical properties of the recently synthesized ferroelectric-like LiOsO3 by first-principle calculations. It is found that the ground state of perfect LiOsO3 is G type antiferromagnetic (G-AFM) order by both LSDA+U with Ueff>0eV and HSE. For the G-AFM order, when Ueff>1.75eV the charge gap opens for LiOsO3 while the gap is about 0.60eV by HSE. So the perfect LiOsO3 should be a G-AFM semiconductor. As for the Li-defected Li0.94OsO3, the Fermi level shifts to the valence bands. On the other hand the translational periodicity is destroyed by various defects, leading to the disappearance of magnetic order which is responsible for the ferroelectric metal state observed in experiment. This is proved by the decrease of the Os average magnetic moments of Li-defected LiOsO3. The calculated spontaneous polarization of perfect LiOsO3 is about 22.23μC/cm2 and the largest component of second order nonlinear optical coefficient is 80.795pm/V, which indicate that it should be a potential ferroelectric and nonlinear optical material.

Strain engineering the magnetic states of vacancy-doped monolayer MoSe2

The effect of vacancy defect and strain on the magnetic property of monolayer MoSe2 was investigated by the first-principles calculation. Firstly, the most stable structure of monolayer MoSe2 and four kinds of vacancy, VSe, VSe2, VMoSe3 and VMoSe6 geometries were systematically studied. Our calculations show that 1H-MoSe2, as well as VSe, VSe2 and VMoSe3 vacancy doped MoSe2 monolayer are nonmagnetic, while VMoSe6 vacancy geometry shows magnetism and the magnetic moment mainly comes from the six Mo atoms around the vacancy although it is most difficult to form. Meanwhile, our calculations indicate that MoSe2 monolayer under tensile strain ranging from 0% to 10% does not show any magnetism. However, it is found that the combined tensile strain and VSe, or VSe2 and VMoSe3 vacancy indeed induce magnetism in MoSe2 monolayer by breaking the Mo–Mo metallic bonds. As for VMoSe6 vacancy geometry, the vacancy induced magnetic moment decrease under the imposed tensile strain.

Low-temperature co-fired Ni–Ti co-substituted barium ferrites

Divalent Ni2+ ion and tetravalent Ti4+ ion equiatomic co-substituted two trivalent Fe3+ ions of M-type barium ferrite in the form of Ba(NiTi)xFe12-2xO19 with x = 1.0, 1.1, 1.2, 1.3. The samples were prepared using solid state reaction route and added 2.5 wt% Bi2O3 to lower sintering temperature. The samples were sintered at 925℃ and 950℃ for 6 h. As the Ni–Ti molar content changed from 1.0 to 1.3, all XRD patterns showed the single phase of the M-type barium ferrite without other intermediate phase. The microstructures were analyzed with scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). The saturation magnetization Ms (49.74–29.58 emu/g) and the coercivity Hc (426.44–329.63 Oe) decreased with increasing Ni–Ti composition x and indicated that a decrease of magnetic moment and anisotropy was achieved by substitution of Ni–Ti. These properties attribute to Ni and Ti occupancies at different sites in the barium ferrite crystalline structure.

Magnetic analysis of a melt-spun Fe-dilute Cu60Ag35Fe5 alloy

The magnetic properties of a melt-spun Fe-dilute Cu60Ag35Fe5 alloy are examined by X-ray diffraction, magnetic measurements, and transmission electron microscopy (TEM). The X-ray diffraction patterns show that the as-spun and annealed (773K×36ks) samples contain Cu and Ag phases and no Fe phases; thus, most Fe atoms are dispersed as clusters. Magnetic measurements indicate that the as-spun and annealed samples exhibit superparamagnetic behavior at 300K, whereas ferromagnetic and superparamagnetic behaviors coexist at 4.2K. The magnetic moments of small clusters at 300K are determined by the nonlinear least squares method as 5148 and 4671μB for as-spun and annealed samples, respectively, whereas those at 300K are experimentally determined as 3500 and 3200μB. This decrease in magnetic moments may imply the formation of anti-ferromagnetic coupling by annealing. TEM observation of the melt-spun sample suggests that there are three regions with different compositions: Cu-rich, Ag-rich, and Fe-rich with no precipitation in the matrix. In addition, these regions have obscure interfaces. The magnetic clusters are attributed to the Fe-rich regions.

Magnetic property and electronic structure of BaFe4-xTi2+xO11

In this paper, polycrystalline BaFe4-xTi2+xO11 (x=0, 0.25, 0.5, 0.75, 1) samples have been synthesized by the conventional solid-state reaction method. X-ray diffraction (XRD) patterns of all the samples show that the diffraction peaks correspond to that of an R-type hexagonal ferrite structure, and no trace of second phase is detected. Measurement of X-ray photoelectron spectroscopy (XPS) reveals that most of the Fe ions in BaFe4Ti2O11 are trivalent and the fitting of two peaks in Fe 2p spectrum corresponding to different Fe ion sites, while the amount of Fe2+ ions increases with the increase of Ti ions in BaFe4-xTi2+xO11. The spectroscopy of Ti ions confirms that the valence of Ti in BaFe4-xTi2+xO11 are tetravalent. Magnetic susceptibility of BaFe4-xTi2+xO11 (x= 0, 0.25, 0.5, 0.75, 1) reveals two magnetic transitions at T1～250 K and T2～83 K, which indicate a complex magnetic order driven by competing interactions on a frustrated lattice with a noncentrosymmetric structure. For all the samples, the magnetic susceptibility obeys Curie-Weiss law above T1, and M-H curves exhibit a linear variation with magnetic field in this temperature range, which is consistent with the paramagnetic behavior. A decrease of the effective magnetic moment is due to the increase of Fe2+ ions with the increase of Ti content in BaFe4-xTi2+xO11. Below T1, the magnetization curve as a function of temperature (M-T) and the magnetization versus magnetic field (M-H) at different temperatures imply its characteristic of a typical canted antiferromagnetic or ferrimagnetic state. Meanwhile, the transition temperature T2 drops gradually with the increase in Ti content, which might be related to the change of occupying of Fe ions in the kagome layers.

First-principles study on the effect of defects on the electronic and magnetic properties of the Ti2NiAl inverse Heusler alloy

The effect of atomic antisite and swap defects on the electronic and magnetic properties of Ti2NiAl inverse Heusler alloy is investigated by the first-principles calculations within density functional theory. In the ordered Ti2NiAl alloy, there are eight antisites and five swaps which are established by the replacement of an atom by another and the exchange in positions of atoms, respectively. The NiTi(A) antisite is found to be the most probable defect due to the lowest formation energy, whereas the least probable defects are the AlTi(A)/Ti(B) and NiAl/Ti(B) antisites as well as Ni-Ti(A) and Al-Ti(B) swaps due to the higher formation energies compared with NiTi(A) antisite. The TiNi/Al and AlNi antisites as well as Al-Ti(A)/Ni and Ni-Ti(B) swaps are highly unlikely to be formed due to the positive values of formation energy. Moreover, we deduce from the relative binding energy of the swap with respect to their antisites that the Ni prefers atomic antisite to site swap, while Al prefers site swap to atomic antisite. The spin polarization is markedly reduced in Ni/AlTi(B) antisite as well as Ni-Ti(A) and Al-Ti(B) swaps due to the occurrence of defect states at the Fermi level, while a very high spin polarization is obtained for Ni/AlTi(A) antisites and only the NiAl antisite retains the half-metallicity with a perfect spin polarization. The magnetic moments of all the likely defected structures decrease in comparison to the ordered Ti2NiAl mainly due to the decrease of local magnetic moments of the defect atom and its near neighbors.

Corrosion behavior and ΔS-T relation of LaFe13−−Co Si C compounds near room temperature

Corrosion behavior and ΔS-Tc relation of LaFe13−x−yCoxSiyCz compounds near room temperature have been investigated. The ΔS-Tc relation shows that element doping of Si, Co and C shifts Tc to high temperature, but decreases the ΔS value. Si doping has the largest effect on ΔS value decrease. The rapid decrease in ΔS value is associated with a significant decrease of saturation magnetic moment. While taking high Si content, Co doping decreases ΔS value more slowly. On the other hand, introducing interstitial C atoms only decreases ΔS slightly with increasing Tc to high temperature. Both replacing Fe with Co and adding C can improve the corrosion resistance, but adding C has more prominent effect. Therefore introducing interstitial C atoms makes it possible to adjust Tc to high temperature, while still keeping ΔS at a large value and increasing the corrosion resistance.