Contribution To Magnetic Anisotropy Research Articles

FMR and EPR in Ni@C nanocomposites: Size and concentration effects

One-domain Ni@C nanoparticles encapsulated in carbon coating have been investigated depending on the size and concentration of Ni in carbon. The nanoparticles of nickel were prepared with the average diameters changing in a broad range of 4–45 nm, and the concentration of Ni in C varies in 2–12 wt%. To prepare the Ni@C nanocomposites the solid solutions of nickel phthalocyanine–metal-free phthalocyanine (NiPc) x (H2Pc)1–x , 0 ≤ x ≤ 1 were synthesized and the solidphase pyrolysis of these compounds was performed. In the case of ultradispersive Ni nanoparticles (the interval of quantum dots is 1–10 nm), a considerable shift of the resonance field and broadening of resonance absorption field were revealed in the spectra of FMR at room temperature. The data were interpreted taking into account the essential contribution of the surface magnetic anisotropy, the magnetic field of which far exceeds the magnetic field of volume anisotropy.

Origin of perpendicular magnetic anisotropy in epitaxial Pd/Co/Pd(111) trilayers

Perpendicular magnetic anisotropy in epitaxial $\mathrm{Pd}/\mathrm{Co}/\mathrm{Pd}(111)$ trilayered films grown on Si(111) substrate was investigated. Contributions to perpendicular magnetic anisotropy from the bottom and top Co/Pd interfaces were deduced by replacement of Pd layers by Cu layers and comparative analysis of the magnetic anisotropy in the samples. Perpendicular magnetic anisotropy in Pd/Co/Pd films was induced both by interface electronic effects and by stress caused by lattice mismatch between Pd and Co. Due to asymmetry of the stress in the Co film, the contribution to magnetic anisotropy induced by the bottom Co/Pd interface was stronger than that induced by the top Pd/Co interface. The energy of the perpendicular magnetic anisotropy and asymmetrical contributions from the bottom Co/Pd and top Pd/Co interfaces to anisotropy in Pd/Co/Pd trilayers strongly depend on the thickness of the bottom and top Pd layers and may be precisely controlled. The roughness of the interfaces does not have a large influence on the energy of perpendicular magnetic anisotropy in this system.

Field-angle and DC-bias dependence of spin-torque diode in giant magnetoresistive microstripe

The spin torque diode effect in all metal spintronic devices has been proposed as a microwave detector with a high power limit and resistivity to breakdown. The previous works have revealed the field-angle dependence of the rectified DC voltage (VDC) in the ferromagnetic stripe. The giant magnetoresistive (GMR) microstripe exhibits higher sensitivity compared with the ferromagnetic stripe. However, the influence of the magnetic field direction and bias current in the spin rectification of GMR microstripe is not yet reported. In this work, the angular dependence and bias dependence of resonant frequency (fR) and VDC are investigated. A macrospin model concerning the contribution of magnetic field, shape anisotropy, and unidirectional anisotropy is engaged to interpret the experimental data. fR exhibits a |sin δH| dependence on the in-plane field angle (δH). VDC presents either |sin δH| or |sin2 δH cos δH | relation, depending on the magnitude of Hext. Optimized VDC of 24 μV is achieved under 4 mT magnetic field applied at δH = 170°. Under out-of-plane magnetic field, fR shows a cos 2θH reliance on the polar angle (θH), whereas VDC is sin θH dependent. The Oersted field of the DC bias current (IDC) modifies the effective field, resulting in shifted fR. Enhanced VDC with increasing IDC is attributed to the elevated contribution of spin-transfer torque. Maximum VDC of 35.2 μV is achieved, corresponding to 47% increase compared with the optimized value under zero bias. Higher IDC also results in enlarged damping parameter in the free layer, resulting in increased linewidth in the spin torque diode spectra. This work experimentally and analytically reveals the angular dependence of fR and VDC in the GMR microstripe. The results further demonstrate a highly tunable fR and optimized VDC by bias current without the external magnetic field. GMR microstripe holds promise for application as a high-power, frequency-tunable microwave detector that works under small or zero magnetic field.

Diamondoid Structure in a Metal–Organic Framework of Fe4 Single‐Molecule Magnets

A 3D metal-organic framework (MOF) having single-molecule magnet (SMM) linkers was prepared in crystalline form by using a tetrairon(III) complex functionalised with two divergent pyridyl groups, namely [Fe4 (pPy)2 (dpm)6 ] (1; H3 pPy=2-(hydroxymethyl)-2-(pyridin-4-yl)propane-1,3-diol, Hdpm=dipivaloylmethane). Reaction of 1 with silver(I) perchlorate afforded {[Fe4 (pPy)2 (dpm)6 ]2 Ag}ClO4 (2), which crystallises in a cubic face-centred lattice and exhibits two interlocked diamondoid networks. In 2, the SMMs act as linear ditopic synthons, and silver(I) ions as tetrahedral nodes coordinated by four pyridyl nitrogen atoms. The magnetic properties of 1 (S=5 and D≈-0.4 cm(-1) in the ground spin state) are largely preserved in 2, which shows slow magnetic relaxation with an anisotropy barrier of Ueff /kB =11.46(10) K in zero field and 14.25(8) K in an applied field of 1 kOe. However, crystal symmetry triggers highly noncollinear magnetic anisotropy contributions oriented at 109.47° from each other along the threefold axes of AgN4 tetrahedra, a unique scenario fully confirmed by a single-crystal cantilever torque magnetometry investigation. Magnetisation curves down to 0.03 K demonstrated the occurrence of a wide hysteresis loop when the magnetic field was swept along one of the four Ag-N bonds. By symmetry, the crystalline compound can then be persistently magnetised parallel or antiparallel to the four main diagonals of the unit cell, although the crystals have no overall second-order anisotropy.

Statistic regularities of magnetization jumps in K0.4[Cr(CN)6][Mn(R/S)‐pn](R/S)‐pnH0.6 ferrimagnet

A series of two types of stochastic magnetization jumps in K0.4[Cr(CN)6][Mn(R/S)‐pn](R/S)‐pnH0.6 single crystals (pn = 1,2‐diaminopropane) are reported. Large jumps appear on the demagnetization branch of the hysteresis loop at the critical value of the decreasing magnetic field. The correlation of the temperature dependence of jump magnetic field and the temperature dependence of magnetic anisotropy field indicates a contribution of magnetic anisotropy to nonlinear spin‐soliton structure. Small magnetization jumps occur in both magnetization and demagnetization regimes.

Correlations between structural, electronic transport, and magnetic properties ofCo2FeAl0.5Si0.5Heusler alloy epitaxial thin films

The structural and chemical order are the most important parameters governing the physical properties of the Heusler compounds. Here, we give a comprehensive overview of the correlations between structural and chemical order, electronic transport (longitudinal and transverse) and magnetic (static and dynamic) properties of ${\mathrm{Co}}_{2}{\mathrm{FeAl}}_{0.5}{\mathrm{Si}}_{0.5}$ Heusler alloy epitaxial thin films grown on MgO(001) single-crystal substrates. X-ray diffraction measurements indicated that depending on the annealing temperature the films show $B2$ or $L{2}_{1}$ chemical ordering. Longitudinal magnetoresistivity experiments revealed that for the best $L{2}_{1}$ ordered film, at temperatures bellow $125\phantom{\rule{4pt}{0ex}}\text{K}$, the magnon assisted electronic scattering is quenched indicating the appearance of half-metallicity. The presence of quantum correction in resistivity, whose strength is dependent on the structural ordering, was evidenced at low temperatures. Anomalous Hall experiments indicated that the intrinsic band structure contribution has an opposite sign and that is dominant over the extrinsic skew scattering mechanism. The presence of a small uniaxial magnetic anisotropy contribution superimposed on a larger biaxial one was evidenced via ferromagnetic resonance microstrip line measurements. The biaxial term is well correlated with chemical ordering, having a minimum value for the optimum $L{2}_{1}$ ordered film. The damping parameter was evaluated from ferromagnetic resonance linewidth measurements, and a coefficient as low as $1.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ was found for the $L{2}_{1}$ phase.

Strain-mediated 180° perpendicular magnetization switching of a single domain multiferroic structure

Analytical results for a voltage induced 180° perpendicular magnetization switching in a single magnetic nanoelement are presented. This 180° switching is achieved by combining perpendicular magnetic anisotropy due to surface effects with a temporally short voltage induced strain pulse. The angular momentum of the magnetic moment results in its precession in response to an in-plane effective magnetic field induced by strain, causing it to overshoot the 90° reorientation to an in-plane equilibrium position. Removal of the strain at the peak overshoot results in a 180° magnetization switch. This process is simulated using a micromagnetic dynamic approach implemented in a finite element framework that includes shape anisotropy and perpendicular magnetic anisotropy contributions to the energy functional in addition to the exchange and magnetoelastic energy terms. Control of the energy landscape to enable the 180° switching was accomplished by controlling the film thickness, and the strain pulse amplitude and duration. A film thickness near the critical thickness for in-plane to out-of-plane equilibrium magnetization direction was used to reduce the energy barrier to switching and thus reduce the necessary strain to levels that can be achieved in ferroelectric thin films clamped by a substrate.

Rotatable magnetic anisotropy in Si/SiO2/(Co2Fe)xGe1−x Heusler alloy films

Polycrystalline (Co2Fe)xGe1−x Heusler alloy films are fabricated by sputtering on amorphous substrates and shown to possess three types of magnetic anisotropy. The nearly stoichiometric composition of x = 50 m.f.% shows a rectangular hysteresis loop and isotropic coercive and ferromagnetic resonance fields when the film is field-magnetized along any in-plane direction, thus predominantly possessing rotatable in-plane magnetic anisotropy. Higher-x compositions show evidence of two- and fourfold in-plane anisotropy superposed on the rotatable one. A qualitative model of the observed anisotropic magnetic properties is proposed. The model explains the rotatable anisotropy by taking into account dry friction for the in-plane rotation of the magnetization direction in a fine-grained polycrystalline film with the magnetic grain size smaller than the correlation length of the inter-grain exchange interaction. The observed two- and fourfold magnetic anisotropy contributions are attributed to partial texturing of the fine-grained films, even though the films are grown on amorphous SiO2 substrates. These results should be valuable for understanding and controlling the magnetic behaviour of highly spin-polarized Heusler alloy films used in various magnetic nanodevices.

Thermal stability of high frequency properties of gradient-composition-sputtered FeCoHf films with and without stripe domains

A systematic study on the thickness dependence of dynamic magnetic properties and their thermal stability behaviors of FeCoHf thin films grown by gradient-composition sputtering technique was carried out. The critical thickness for the formation of stripe domain structures at room temperature was found to be around 120 nm based on the appearance of the multiple resonance peaks and the shape of the magnetization curves. For the films with thicknesses less than 90 nm, the ferromagnetic resonance frequency and in-plane magnetic anisotropy are increased with the increasing of temperature; however, for the films with thicknesses in the range from 90 nm to 120 nm, the ferromagnetic resonance frequency is reduced with temperature and the secondary peak appears as the temperature is raised suggesting an evolution of stripe domains. For the films with stripe domains at room temperature, the ferromagnetic resonance frequency is also decreased with increasing temperature, and in some cases, there is a splitting up of the resonance peaks indicating that there is a change in the stripe domain states when the sample is heated up. All the results are discussed considering the contribution of stress-induced magnetic anisotropy and the increasing of a weak perpendicular magnetic anisotropy.

Determination of magnetic anisotropy constants in Fe ultrathin film on vicinal Si(111) by anisotropic magnetoresistance.

The epitaxial growth of ultrathin Fe film on Si(111) surface provides an excellent opportunity to investigate the contribution of magnetic anisotropy to magnetic behavior. Here, we present the anisotropic magnetoresistance (AMR) effect of Fe single crystal film on vicinal Si(111) substrate with atomically flat ultrathin p(2 × 2) iron silicide as buffer layer. Owing to the tiny misorientation from Fe(111) plane, the symmetry of magnetocrystalline anisotropy energy changes from the six-fold to a superposition of six-fold, four-fold and a weakly uniaxial contribution. Furthermore, the magnitudes of various magnetic anisotropy constants were derived from torque curves on the basis of AMR results. Our work suggests that AMR measurements can be employed to figure out precisely the contributions of various magnetic anisotropy constants.

Martensitic transformation and magnetic anisotropy in Ni-Mn-Ga/NaCl(001) thin films probed by ferromagnetic resonance

By the ferromagnetic resonance and magnetometry measurements in a broad temperature range, it is found that the 220-fiber textured Ni51.4Mn28.3Ga20.3 (at. %) film of 100 nm thickness deposited onto NaCl(001) exhibits in the martensitic state an easy-plane magnetic anisotropy. This is in a drastic difference with the identical film of the same texture deposited onto Si(001) wafer showing out-of-plane oblique type of anisotropy [Golub et al., J. Appl. Phys. 105, 07A942 (2009)]. The results are discussed in terms of the different martensitic variant selections and stress-induced magnetic anisotropy contributions having opposite signs.

Surface Structural and Magnetic Properties of Cobalt Bilayer Deposited on Pt(111) with Ni Buffer Layer

A cobalt bilayer film was deposited on a monolayer (ML) Ni/Pt(111) surface at room temperature. The dependencies of the structural and magnetic properties of the surface on the annealing temperature were investigated. During the annealing, it was found that the formation of Co–Ni alloy occurs before the formation of Co–Ni–Pt alloy. After high-temperature annealing, the surface consisted of Co–Ni–Pt alloys with a Pt-rich phase. We found that the perpendicular magnetic anisotropy (PMA) contribution of the Co–Ni–Pt alloy at the surface was greater than that of the Co–Pt alloy below. The ultraviolet photoelectron spectrum (UPS) of the 2 ML Co/1 ML Ni/Pt(111) system showed a redistribution of electron density on the surface after annealing. The UPS peak height near the Fermi edge was mostly dependent on the surface composition. However, the change in the work function was not dependent on the PMA after high-temperature annealing. The strength of the PMA of this system can be tuned to a desired level by annealing without affecting the work function.

Electrical Detection of Individual Magnetic Nanoparticles Encapsulated in Carbon Nanotubes

We report on low-temperature electrical transport measurements of single-walled carbon nanotubes (SWNTs) filled in their inner core with one-dimensional cobalt nanoparticles. The electrical transport properties of the hybrid devices are strongly sensitive to the magnetization reversal of isolated magnetic nanoparticles, resulting in strong hysteretic variations of the magnetoconductance. The magnetic anisotropy of a one-dimensional encapsulated cobalt nanoparticle is investigated, establishing an unusually strong dominating contribution of magnetic surface anisotropy.

Analysis of dynamic magnetics and high effective permeability of exchange-biased multilayers fabricated onto ultrathin flexible substrates

A detailed investigation of the influence of substrate thickness on the static and dynamic properties of exchange-biased FeCo/MnIr multilayers fabricated onto flexible substrates by sputtering technique was performed together with the analyses based on Landau–Lifshitz–Gilbert equation. Reducing the substrate thickness to lower than 3 μm significantly enhances the exchange bias field which is attributed to the deformation of the substrates creating more pinned antiferromagnetic spins at the interface. The effect of substrate thickness on effective magnetization, effective damping coefficient, frequency linewidth, ferromagnetic resonance frequency, and static permeability is also discussed. The observed difference between static and dynamic magnetic anisotropies is attributed to the contribution of rotatable magnetic anisotropy presumably arising from the changes in the antiferromagnetic domain which also results in the enhancement of coercivity. It is evidently suggested that by using ultrathin flexible substrates one can obtain high effective permeability which is useful for microwave applications.

ChemInform Abstract: Probing the Magnetizability Distribution of Ferrocene as Determined via Anisotropic Contributions to the NMR Shielding and Its Application to Several Substituted Ferrocenes.

Molecular magnetic anisotropies of ferrocene derivatives are derived from solution-state measurements of the Cotton–Mouton and Kerr constants. A calculation is presented of chemical shifts induced at the hydrogen and carbon nuclei, arising from the experimental magnetic anisotropy. Ferrocene, ruthenocene and analogously substituted benzenes were also examined. Comparisons of the chemical shifts were used to probe the effects of metal–carbon bonding in the metallocenes. The experimental chemical shifts for pentamethylferrocene have been interpreted, allowing for magnetic anisotropy contributions, to give carbon atom charges that are in close agreement with predictions from the measured electric dipole moment.

Interplay between the magnetic anisotropy contributions of cobalt nanowires

We report on the magnetic properties and the crystallographic structure of the cobalt nanowire arrays as a function of their nanoscale dimensions. X-ray diffraction measurements show the appearance of an in-plane hcp-Co phase for nanowires with 50 nm diameter, suggesting a partial reorientation of the magnetocrystalline anisotropy axis along the membrane plane with increasing pore diameter. No significant changes in the magnetic behavior of the nanowire system are observed with decreasing temperature, indicating that the effective magnetoelastic anisotropy does not play a dominant role in the remagnetization processes of individual nanowires. An enhancement of the total magnetic anisotropy is found at room temperature with a decreasing nanowire diameter-to-length ratio $(d/L)$, a result that is quantitatively analyzed on the basis of a simplified shape anisotropy model.

Effect of martensitic transformation and magnetic field on transport properties of Ni-Mn-Ga and Ni-Fe-Ga Heusler alloys

We study the magnetic-field influence on the martensitic transformation temperatures and the accompanying anomaly in the resistivity by extensive measurements of the temperature dependencies of resistivity of several Ni-Mn-Ga and Ni-Fe-Ga alloys. A low-field minimum of the martensitic transformation temperature versus magnetic-field curves is observed for single and polycrystalline Ni-Mn-Ga alloys. This minimum is confirmed by magnetization loop measurements showing a magnetic anisotropy contribution to the ordinary Clausius-Clapeyron relationship which describes the martensitic transformation. This minimum did not appear for Ni-Fe-Ga polycrystalline alloys because in this case the difference in the magnetic anisotropy of relevant phases is small. We did not find any systematic behavior of the magnetoresistance related to the phase in which it is measured. In the vicinity of the martensitic transformation, the magnetoresistance exhibits a peaklike behavior which is explained in terms of the combined influence of the zero-field resistivity anomaly at the transition and the magnetic-field-induced shift of the transformation temperature.

Magnetic field effect on premartensitic transition in Ni–Mn–Ga alloys

Extensive measurements of the temperature dependencies of resistivity under constant magnetic fields made it possible to clarify the premartensitic transition sensitivity to applied magnetic field up to 14 T in polycrystalline and monocrystalline Ni–Mn–Ga alloys with compositions close to the stoichiometric one. A low-field minimum on the transition temperature versus magnetic field curves is found in both alloys. This minimum is attributed to the magnetic anisotropy contribution to the Clausius–Clapeyron relationship, which describes the premartensitic transition. This contribution is confirmed by magnetization loops measurements.

Magnetic properties of [formula omitted] compounds

The magnetic properties of GdCo 4− x Fe x B have been investigated using 57Fe Mössbauer spectroscopy at room temperature. Those compounds have the hexagonal CeCo 4B type of structure and are ordered ferrimagnetically. In GdCo 1.4Fe 2.6B, the 57Fe hyperfine fields are 224 and 180 kOe at the 2c and 6i sites, respectively, whose corresponding Fe moments are 1.50 and 1.20 μ B, respectively. The increase in the Fe magnetic moment for 1 ≤ x ≤ 2.6 in GdCo 4− x Fe x B are 0.033 and 0.079 μ B at the 2c and 6i sites, respectively. The weighted average values of the isomer shift in GdCo 4− x Fe x B increase for 1 ≤ x ≤ 2.6 with increasing Fe content. The result of quadrupole splitting shows the difference of the contribution of magnetic anisotropy of the Fe atoms at the 2c and 6i sites in GdCo 4− x Fe x B.

Spin reorientation transition and magnetic domain structure of Co ultrathin films grown on a faceted Au(455) surface

The spin reorientation transition (SRT) and magnetic domain structure of Co ultrathin films on faceted Au(455) surfaces have been investigated as a function of thickness by using magneto-optic Kerr effect and x-ray photoemission electron microscopy in combination with x-ray magnetic circular dichroism. The magnetization easy axis of the Co films is found to rotate from an out-of-plane to an in-plane orientation perpendicular to the atomic steps of the substrate due to the dominant contribution of the step-induced magnetic anisotropy. Moreover, magnetic stripe domains are observed to replicate the underlying periodic facets of the Au surface. Depending on the facet, the SRT occurs at a well-defined critical thickness and proceeds via a different mechanism: a state of uniform canted magnetization is formed for (233) facets, while the SRT involves continuous magnetization rotation followed by the nucleation of small in-plane magnetic domains for (677) facets.