Additional Anisotropy Research Articles

Metamagnetoelectric effect in multiferroics A2Cu2Mo3O12 (A=Rb and Cs) quantum spin chain

The magnetoelectric and thermodynamic properties of multiferroic A2Cu2Mo3O12 (A=Rb and Cs) compounds are investigated through the theoretical laboratory of a frustrated one-dimensional spin chain of CuO2 ribbon where inter-chain couplings are ignored. The general framework is built under external magnetic and electric field constraints with additional anisotropies. The field scans of magnetoelectric properties highlight a set of interdependent meta-transition giving birth to the metamagnetoelectric effect. Those meta-transitions are likewise reported by typically measured field dependencies of entropy and further of heat capacity that reveals a breakdown of temperature-energy dependence during meta-transitions. The temperature-energy dependence breakdown vanishing reported for high temperature emphasizes the metamagnetoelectric state death beyond relatively low-temperature values. Furthermore, large temperature-fields dependencies are noticed except for critical values where meta-transitions are occurring in the investigated A2Cu2Mo3O12 compounds.

Unidirectional tilt of domain walls in equilibrium in biaxial stripes with Dzyaloshinskii–Moriya interaction

The orientation of a chiral magnetic domain wall in a racetrack determines its dynamical properties. In equilibrium, magnetic domain walls are expected to be oriented perpendicular to the stripe axis. We demonstrate the appearance of a unidirectional domain wall tilt in out-of-plane magnetized stripes with biaxial anisotropy and Dzyaloshinskii–Moriya interaction (DMI). The tilt is a result of the interplay between the in-plane easy-axis anisotropy and DMI. We show that the additional anisotropy and DMI prefer different domain wall structure: anisotropy links the magnetization azimuthal angle inside the domain wall with the anisotropy direction in contrast to DMI, which prefers the magnetization perpendicular to the domain wall. Their balance with the energy gain due to domain wall extension defines the equilibrium magnetization the domain wall tilting. We demonstrate that the Walker field and the corresponding Walker velocity of the domain wall can be enhanced in the system supporting tilted walls.

Room Temperature Magnetic Memory Effect in Cluster-Glassy Fe-doped NiO Nanoparticles.

The Fe-doped NiO nanoparticles that were synthesized using a co-precipitation method are characterized by enhanced room-temperature ferromagnetic property evident from magnetic measurements. Neutron powder diffraction experiments suggested an increment of the magnetic moment of 3d ions in the nanoparticles as a function of Fe-concentration. The temperature, time, and field-dependent magnetization measurements show that the effect of Fe-doping in NiO has enhanced the intraparticle interactions due to formed defect clusters. The intraparticle interactions are proposed to bring additional magnetic anisotropy energy barriers that affect the overall magnetic moment relaxation process and emerging as room temperature magnetic memory. The outcome of this study is attractive for the future development of the room temperature ferromagnetic oxide system to facilitate the integration of spintronic devices and understanding of their fundamental physics.

Intrinsic anisotropy and pinning anisotropy in nanostructured YBa2Cu3O7−δ from microwave measurements

Anisotropy is an intrinsic factor that dictates the magnetic properties of YBa2Cu3O7−δ and therefore has a great impact on many applications. Artificial pinning centres are often introduced in an attempt to mitigate its effect, resulting in less anisotropic electrical and magnetic properties. However, the nanoengineering of superconductors makes the quantification of the anisotropy itself uncertain: the intrinsic anisotropy due to the layered structure, quantified by the anisotropy factor γ, mixes with the additional anisotropy due to pinning. As a consequence, there is no consensus on the experimental anisotropy factor γ that can result in YBa2Cu3O7−δ when directional (twin planes, nanorods) or isotropic defects are present. We present measurements of the magnetic field and angular dependent surface impedance in very different nanostructured YBa2Cu3O7−δ films, grown by a chemical route and by pulsed laser deposition with different kind of defects (nanorods, twin planes, nanoparticles). We show that the surface impedance measurements are able to disentangle the intrinsic anisotropy from the directional pinning anisotropy, thanks to the possibility of extracting the true anisotropic flux–flow resistivity and by correctly exploiting the angular scaling. We find in all films that the intrinsic anisotropy γ = 5.3 ± 0.7. By contrast, the pinning anisotropy determines a much more complex, feature–rich and nonuniversal, sample–dependent angular landscape.

Separating Geometry‐ From Stress‐Induced Remanent Magnetization in Magnetite With Ilmenite Lamellae From the Stardalur Basalts, Iceland

AbstractRealistic geometries of magnetite grains from the Stardalur volcano, Iceland, were obtained by Focused Ion Beam Scanning Electron Microscopy nanotomography. These magnetite grains are subdivided by oxidation‐exsolution lamellae of ilmenite. Magnetic properties of these grains were modeled without internal stress using the three‐dimensional micromagnetic code MERRILL. The influence of grain shape and size was isolated by modeling hysteresis loops of the same grains with and without exsolution microstructures. The resulting coercivities Hc are up to 1.5 times higher, and the Mrs/Ms ratios are twice as high for the grains with exsolution than for those without. Both modeled values are a factor of 10 smaller than the measured bulk data from the same sample. This difference between stress‐free models and measured hysteresis loops suggests that the internal stress due to the formation of the oxidation‐exsolution lamellae is the dominant mechanism of coercivity and remanence enhancement. By comparing the approach‐to‐saturation behavior of modeled and measured hysteresis loops, the internal stress is quantified to about 100 MPa. The formation of lamellae has two effects on magnetic properties. (1) The apparent grain size is geometrically reduced. This effect increases Mrs and Hc by up to a factor of 2. (2) The formation of lamellae produces internal stress fields, which provide additional anisotropy energy that deflect the magnetic spins and apparently increase Mrs and Hc by up to a factor of 10. Accordingly, stress dominates the remanent magnetic properties in the Stardalur basalts and may be the decisive effect explaining its unusual remanent‐dominated ground magnetic anomaly of up to 27,000 nT.

Thermal annealing induced enhancement of room temperature magnetic memory effect in Fe-doped NiO nanoparticles

We report room temperature (RT) ferromagnetism and magnetic memory effect in Ni0.95Fe0.05O nanoparticles (NPs) synthesize by hydrothermal method followed by post-annealing in an ambient atmosphere. The temperature and time-dependent magnetization measurements show that the effect of post-annealing at higher temperatures leads to enhancement in the intraparticle interactions. The enhanced intraparticle interaction has provided additional magnetic anisotropy energy resulting in RT ferromagnetic (FM) properties and enhanced magnetic memory effect. The findings from this study will be useful for the development and understanding of RT FM materials to facilitate the integration of spintronic devices.

Enhanced anisotropy and study of magnetization reversal in Co/C60 bilayer thin film

The interface between the organic semiconductor (OSC)/ferromagnetic (FM) material can exhibit ferromagnetism due to their orbital hybridization. Charge/spin transfer may occur from the FM to OSC layer leading to the formation of “spinterface,” i.e., the interface exhibiting a finite magnetic moment. In this work, the magnetic properties of the Co/C60 bilayer thin film have been studied to probe the interface between the Co and C60 layer. Polarized neutron reflectivity (PNR) measurement indicates that the thickness and moment of the spinterface are ∼2 ± 0.18 nm and 0.8 ± 0.2 μB/cage, respectively. The comparison of the magnetization reversal between the Co/C60 bilayer and the parent single layer Co thin film reveals that spinterface modifies the domain microstructure. Further, the anisotropy of the bilayer system shows a significant enhancement (∼two times) in comparison to its single layer counterpart which is probably due to an additional interfacial anisotropy arising from the orbital hybridization at the Co/C60 interface.

Untwinned YBa2Cu3O7−δ thin films on MgO substrates: A platform to study strain effects on the local orders in cuprates

We have grown untwinned YBa$_2$Cu$_3$O$_{7-\delta}$ (YBCO) films on (110) MgO substrates that were pre-annealed at high temperature in oxygen atmosphere. The annealing results in surface reconstruction with shallow facets, which induce the suppression of the YBCO twinning domains, and the preferential alignment of the CuO chains along one of the in-plane directions of the substrate. Because of the large mismatch between the in-plane lattice parameters of film and substrate, the strain induced by the MgO into the YBCO layer is strong and very peculiar. The YBCO film is compressed, with respect to the bulk, and presents a unidirectional buckling of the atomic planes, along the chains' direction, due to a deformation of the copper-oxygen octahedra. The YBCO films, which can be grown with thicknesses down to few unit cells and oxygen doping levels spanning most of the superconducting dome, are patterned into nanowires with dimensions down to 50 nm. The anisotropies due to the untwinning state are preserved in these structures; moreover, additional anisotropies appear, in ultrathin structures where strain effects become more pronounced. Such untwinned and compressively strained films can therefore be used as a platform to study the interplay between strain and the various local orders in the normal state of YBCO.

Elastic interactions of Fe adatoms on Cu(111) at the mesoscale

An extended elastic eigenvector approach had earlier been developed to interpret ab-initio calculations of adatom interactions. It shows oscillating interactions as well as trio- and quarto (multisite) terms within clusters. It is now applied to the interaction of Fe adatoms on Cu(111).The extended approach differs from previous calculations by using a sharp cutoff - generating oscillating interactions - and by taking into account interacting dimers - generating strong anisotropies and multisite terms. Additional weak anisotropy stems from the substrate and from the surface Brillouin zone shape.This approach has 3 free parameters which have been fitted to first principles interactions of Fe adatoms on Cu(111). Elastic adatom pair interactions and dimer-dimer interactions at mesoscale separations show a reasonable good fit to the first principles interactions. At smaller separations elastic interaction values remain questionable. To enable a comparison with future first principle calculations also dimer-monomer interactions are shown.Some conclusions on initial adatom formation and diffusion are proposed and open questions are formulated.

Marked Enhancement of Roll-Off Frequency in FeCoN Synthetic Antiferromagnetic Films Deposited by Oblique Incidence.

A series of FeCoN films were successfully deposited on glass substrates in a magnetron sputtering system. Using oblique incidence method and FeCoN/Ru/FeCoN synthetic antiferromagnetic (SAF) structure, two additional anisotropies energy were introduced: oblique incidence anisotropy and exchange anisotropy energy, which marked enhancement of the effective magnetic anisotropy (Hk). The increment of Hk results in a significant improvement in the roll-off frequency of these films. The roll-off frequency of FeCoN/Ru/FeCoN films with SAF structure can reach up to 8.6 GHz. A feasible approach to conveniently controlling Hk of soft magnetic thin films by using oblique deposition and SAF structure can further improve their properties for the potential applications in the high frequency region.

Flower-like Mn-Doped Magnetic Nanoparticles Functionalized with αvβ3-Integrin-Ligand to Efficiently Induce Intracellular Heat after Alternating Magnetic Field Exposition, Triggering Glioma Cell Death.

Despite the potential of magnetic nanoparticles (NPs) to mediate intracellular hyperthermia when exposed to an alternating magnetic field (AMF), several studies indicate that the intracellular heating capacity of magnetic NPs depends on factors such as cytoplasm viscosity, nanoparticle aggregation within subcellular compartments, and dipolar interactions. In this work, we report the design and synthesis of monodispersed flowerlike superparamagnetic manganese iron oxide NPs with maximized SAR (specific absorption rate) and evaluate their efficacy as intracellular heaters in the human tumor-derived glioblastoma cell line U87MG. Three main strategies to tune the particle anisotropy of the core and the surface to reach the maximum heating efficiency were adopted: (1) varying the crystalline anisotropy by inserting a low amount of Mn2+ in the inverse spinel structure, (2) varying the NP shape to add an additional anisotropy source while keeping the superparamagnetic behavior, and (3) maximizing NP-cell affinity through conjugation with a biological targeting molecule to reach the NP concentration required to increase the temperature within the cell. We investigate possible effects produced by these improved NPs under the AMF (f = 96 kHz, H = 47 kA/m) exposure in the glioblastoma cell line U87MG by monitoring the expression of hsp70 gene and reactive oxygen species (ROS) production, as both effects have been described to be induced by increasing the intracellular temperature. The induced cell responses include cellular membrane permeabilization and rupture with concomitant high ROS appearance and hsp70 expression, followed by cell death. The responses were largely limited to cells that contained the NPs exposed to the AMF. Our results indicate that the developed strategies to optimize particle anisotropy in this work are a promising guidance to improve the heating efficiency of magnetic NPs in the human glioma cell line.

Reflection and transmission responses for layered transversely isotropic media with vertical and horizontal symmetry axes

Reflection and transmission (R/T) responses characterize the energy distributions for incident and generated waves across the subsurface interface. The R/T coefficients are considerably influenced by the local anisotropy, and this implies the significance of the R/T responses analysis for anisotropic media. We have considered the plane interface bounded by two transversely isotropic media with, respectively, vertical and horizontal symmetry axes, and R/T coefficients normalized by the vertical energy flux are obtained in the phase domain. We define two simple anisotropic layered models characterized by fewer independent model parameters. Under the assumption of weak contrast model parameters across the interface, the R/T coefficient approximations are obtained as the perturbations from the simple models’ counterparts. The isotropic background medium is also used to obtain the approximations under an additional weak anisotropy assumption. Compared with approximations degenerated from more general cases, our approximations rely on fewer independent parameters. Numerical tests are implemented to evaluate our approximations.

Reversible electric-field-driven magnetization in a columnar nanocomposite film

Self-assembled heteroepitaxial bismuth ferrite - cobalt ferrite (BFO-CFO) nanocomposite films were grown on (001) oriented piezoelectric Pb(Mg1/3Nb2/3)0.72Ti0.28O3(001) (PMN-PT0.28) substrates by pulsed laser deposition in which BFO and CFO targets were alternately ablated. Completely reversible electric field induced biaxial strain in this substrate has been exploited to produce reversible electric field control of magnetization in the BFO-CFO nanocomposite film. The surface of the CFO-BFO films shows rectangular CFO pillars protruding out of a flat BFO matrix. The pillars are aligned with their edges along the pseudocubic {110} directions and are homogeneously distributed in the matrix. X-ray diffraction revealed an out-of-plane compression of the CFO unit cell by ~ 0.4%. Magnetization hysteresis (M-H) loops show a moderate perpendicular anisotropy of the magnetostrictive CFO pillars, which is related to their vertical compression. The application of an electric field to the electromechanical PMN-PT0.28 substrate produced significant and reversible changes in the magnetization due to an additional strain-induced magnetic anisotropy. This demonstrates completely reversible, room-temperature electric-field-assisted control of magnetization in self-assembled vertical nanocomposites of CFO and BFO.

Dynamics of Spatially Inhomogeneous Spin Polarization of Nonequilibrium Conduction Electrons in Magnetic Transitions

The equation of the dynamics of the magnetic moment motion that has been averaged over the ensemble of nonequilibrium spin-injected electrons in a ferromagnetic transition in the presence of exchange interaction, interaction with an external electromagnetic field, and a thermostat has been obtained taking into account the spatial inhomogeneity of the charge carrier distribution. It has been shown that taking into account the spatial inhomogeneity of the charge carrier distribution in the equation of the dynamics of the magnetic moment motion allows for describing the various processes of electron transfer in the magnetic transition. The probability of quantum transitions of electrons with opposite spin directions, which determine spin relaxation in interaction with a thermostat, has been estimated. It has been shown that the anisotropy of the radiation medium is determined not only by the anisotropy of the sd-exchange tensor, but also by the additional anisotropy that is caused by the electron impulse derivatives of this tensor. The considered phenomena have a great potential for the detection of new effects and development of new devices based on them, including compact tunable radiation sources in the terahertz frequency range, which is obviously difficult to generate.

Magnetic order in ErFeO3 single crystals studied by mean-field theory

Recent experiments showed magnetic moment reversal and pronounced exchange-bias (EB) effect in the vicinity of the compensation temperature ${T}_{\mathrm{comp}}$ of orthoferrites $R\mathrm{Fe}{\mathrm{O}}_{3}$ ($R=\mathrm{Nd}$, Sm, Er). Although different orthoferrites exhibit diverse $R$-Fe interactions, ${T}_{\mathrm{comp}}$ values, and spin-reorientation temperatures, the EB field in the like manner emerges and diverges upon approaching ${T}_{\mathrm{comp}}$ and changes its sign with crossing ${T}_{\mathrm{comp}}$. In order to explain these observations, a mean-field theory approach for the representative $\mathrm{ErFe}{\mathrm{O}}_{3}$ orthoferrite is proposed. The general case of two sublattices, antiferromagnet with exchange anisotropy and rare-earth--iron interactions, is considered. A small applied magnetic field appears to be a source of additional anisotropy, resulting from canting of sublattice moments. This anisotropy leads to an imbalance of free energy for two different types of magnetic domains, causing a spin jump near the ${T}_{\mathrm{comp}}$. The suggested approach allows reproduction of magnetization reversal and main features of the coercive and exchange-bias fields observed in $\mathrm{ErFe}{\mathrm{O}}_{3}$.

Reflection and transmission responses in a layered transversely isotropic medium with horizontal symmetry axis

Seismic wave reflection and transmission (R/T) responses characterize the subsurface local property, and the widely spread anisotropy has considerable influences even at small incident angles. We have considered layered transversely isotropic media with horizontal symmetry axes (HTI), and the symmetry axes were not restricted to be aligned. With the assumption of weak contrast across the interface, linear approximations for R/T coefficients normalized by vertical energy flux are derived based on a simple layered HTI model. We also obtain the approximation with the isotropic background medium under an additional weak anisotropy assumption. Numerical tests illustrate the good accuracy of the approximations compared with the exact results.

Controllable magnetic anisotropy of ferromagnet/antiferromagnet bilayers coupled with piezoelectric strain

Coupling of magnetoelastic and piezoelectric effects is achieved with the ferromagnetic (FM) Co60Fe20B20/antiferromagnetic (AFM) Co0.7Ni0.3O bilayers deposited on the surface of piezoelectric single crystalline substrate. Piezoelectric strain of the substrate is successfully transferred to the magnetic layers, and controllable variation of magnetic anisotropy in response to the elastic deformation is demonstrated in the M-H loops measured at various azimuthal angles with respect to the direction of AFM spin alignment. Magnetic behaviors of the exchange coupled bilayers can be understood by considering the competition of many magnetic anisotropy energies within the magnetic film based on the modified Meiklejohn and Bean (MB) model. From the comparison of experimentally measured results with the modified MB model, it is found that the strain in the film from the substrate effectively introduces additional uniaxial anisotropy in the FM layer with its anisotropy direction and magnitude determined by the applied strain.

Analysis of a rotordynamic system with anisotropy and nonlinearity using the Floquet theory and the method of normal forms

The system under analysis is composed of an asymmetric rotor, two asymmetric bearings and a nonlinear spring-damper system representing an annular gas seal. The anisotropy in the rotor stiffness introduces time-periodic coefficients in the equations of motion which lead to parametric excitation with the typical resonance effects. As the additional anisotropy in the bearings may increase the area of unsafe operation conditions, the impact of these two different sources of anisotropy on the stability of the trivial solution is investigated in detail by a numerical approach based on the Floquet theory. Further insight into the stability behavior in case of an unstable trivial solution is obtained by accounting for stiffness and damping nonlinearities due to the gas seal. In this nonlinear case, the stability of the periodic solutions is analyzed by the semi-analytical method of normal forms revealing the influence of individual system parameters. The results of the semi-analytical approximation are verified by numerical calculations showing good agreement.

Observation of surface step bunch induced perpendicular magnetic anisotropy using spin-polarized low energy electron microscopy

Using spin-polarized low energy electron microscopy (SPLEEM), we observed surface step bunch induced perpendicular magnetic anisotropy in Fe/Ni bilayers grown on Cu(001) single crystal as well as in Ni/Co/Pd trilayers grown on W(110) crystal. On Cu(100) the formation of step bunches can be stimulated or suppressed by high- or low-temperature annealing cycles, respectively. SPLEEM images resolving the three dimensional magnetization vector in the Fe/Ni films grown on step bunched Cu(100) reveal an additional perpendicular magnetic anisotropy in regions near step bunches. In contrast, no extra perpendicular magnetic anisotropy is observed on low-temperature annealed Cu(100) featuring single-atom height step arrays. Additional investigation of Ni/Co/Pd trilayers on W(110) reveals the influence of step bunch orientation on magnetic anisotropy. Our observations may lead to opportunities for tailoring or patterning anisotropy in magnetic thin-films by controlling film morphology.

A family of high-temperature ferromagnetic monolayers with locked spin-dichroism-mobility anisotropy: MnNX and CrCX (X = Cl, Br, I; C = S, Se, Te)

Two-dimensional magnets have received increasing attention since Cr2Ge2Te6 and CrI3 were experimentally exfoliated and measured in 2017. Although layered ferromagnetic metals were demonstrated at room temperature, a layered ferromagnetic semiconductor with high Curie temperature (Tc) is yet to be unveiled. Here, we theoretically predicted a family of high Tc ferromagnetic monolayers, namely MnNX and CrCX (X = Cl, Br and I; C = S, Se and Te). Their Tc values were predicted from over 100 K to near 500 K with Monte Carlo simulations using an anisotropic Heisenberg model. Eight members among them show semiconducting bandgaps varying from roughly 0.23 to 1.85 eV. These semiconducting monolayers also show extremely large anisotropy, i.e. ∼101 for effective masses and ∼102 for carrier mobilities, along the two in-plane lattice directions of these layers. Additional orbital anisotropy leads to a spin-locked linear dichroism, in different from previously known circular and linear dichroisms in layered materials. Together with the mobility anisotropy, it offers a spin-, dichroism- and mobility-anisotropy locking. These results manifest the potential of this 2D family for both fundamental research and high performance spin-dependent electronic and optoelectronic devices.