Magnetic Anisotropy Measurements Research Articles

Enhanced Stress Stability in Flexible Co/Pt Multilayers with Strong Perpendicular Magnetic Anisotropy.

Due to the magnetoelastic coupling, the magnetic properties of many flexible magnetic films (such as Fe, Co, and Ni) are sensitive to mechanical stress, which deteriorates the performance of flexible magnetoelectronic devices. We show that by stacking Co and Pt alternatively to form multilayers with strong perpendicular magnetic anisotropy (PMA), both magnetic hysteresis and magnetic domain measurements reveal robust PMA against external stress. As the PMA weakens at increased Co thickness, the magnetic anisotropy is vulnerable to external stress. These results were understood based on a micromagnetic model, which suggests that the strength of magnetoelastic anisotropy with respect to initial effective magnetic anisotropy affects the stress-stability of the film. Although the stress coefficient of magnetoelastic anisotropy is enhanced at reduced Co thickness, the concomitant increase of initial effective magnetic anisotropy guarantees a robust PMA against external stress. Our results provide a route to constructing flexible magnetoelectronic devices with enhanced stress stability.

Modelling Crystallographic Texture Evaluation and Non-Destructive Measurement of Magnetic Anisotropy using an Electromagnetic Sensor in Interstitial Free (If) Steels

Interstitial free (IF) steels are popular in the automotive industry for applications requiring good ductility (formability), strength and a superior surface quality. Magnetic anisotropy measurement can be used to gain information about the crystallographic texture, which is an important property controlling strength and especially the formability, for example rvalues. The crystallographic texture is generated during rolling and subsequent recrystallisation during the annealing heat treatment. A set of IF steel specimens, at different states of recrystallisation (cold rolled, partially recrystallised and fully recrystallised) and accordingly different texture components have been used to investigate measurement and prediction of magnetic anisotropy. A finite element microstructure model that considers crystallographic texture based on magnetic domain theory has been used for the evaluation of magnetic anisotropy in polycrystalline steels. In addition, a laboratory based electromagnetic (EM) U shaped sensor has been proposed to determine the magnetic anisotropy in IF samples by measuring at angles varying from 0° to 90°, in steps of 15°, with respect to the rolling direction. Kernel Average Misorientation (KAM) values from Electron Back Scattered Diffraction (EBSD) characterisation of the samples has been used to evaluate the recrystallisation fraction and correlate that to the texture.

Magnetic Damping and Dzyaloshinskii-Moriya Interactions in Pt/Co2FeAl/MgO Systems Grown on Si and MgO Substrates.

Spin-pumping-induced damping and interfacial Dzyaloshinskii-Moriya interaction (iDMI) have been studied in Pt/Co2FeAl/MgO systems grown on Si or MgO substrates as a function of Pt and Co2FeAl (CFA) thicknesses. For this, we combined vibrating sample magnetometry (VSM), microstrip ferromagnetic resonance (MS-FMR), and Brillouin light scattering (BLS). VSM measurements of the magnetic moment at saturation per unit area revealed the absence of a magnetic dead layer in both systems, with a higher magnetization at saturation obtained for CFA grown on MgO. The key parameters governing the spin-dependent transport through the Pt/CFA interface, including the spin mixing conductance and the spin diffusion length, have been determined from the CFA and the Pt thickness dependence of the damping. BLS has been used to measure the spin wave non-reciprocity via the frequency mismatch between the Stokes and anti-Stokes lines. iDMI has been separated from the contribution of the interface perpendicular anisotropy difference between Pt/CFA and CFA/MgO. Our investigation revealed that both iDMI strength and spin pumping efficiency are higher for CFA-based systems grown on MgO due to its epitaxial growth confirmed by MS-FMR measurements of the in-plane magnetic anisotropy. This suggests that CFA grown on MgO could be a promising material candidate as a spin injection source via spin pumping and for other spintronic applications.

Non-destructive evaluation of magnetic anisotropy associated with crystallographic texture of interstitial free steels

Interstitial free (IF) steels, as third generation of deep drawing steel, are used in the automotive industry for applications requiring good formability, strength and a superior surface quality. The formability is achieved by having a strong gamma fibre (γ-fibre) crystallographic texture and is quantified by measuring the r-value from tensile tests. Whereas the crystallographic texture is typically measured using EBSD or XRD on samples after careful surface preparation, it is desirable to be able to evaluate it non-destructively on production material. To do this aim, a magnetic anisotropy measurement is an attractive approach as the magnetic behaviour of steel is known to be affected by its crystallographic texture. A set of interstitial free steel samples, at different stages of recrystallisation process (i.e., commercially cold rolled and annealed to achieve fully and partially recrystallised microstructures) and consequently different texture components have been employed to study the measurement and possibility of model-wise predicting magnetic anisotropy in IF steels. In order to compare the predicted magnetic anisotropy to the measured values, a finite element (FE) microstructure model that takes into account crystallographic texture was applied.The findings indicate that the non-destructive technique deployed in this study - a U-shaped electromagnetic (EM) sensor that can be placed onto a sheet specimen - is promising for a rapid assessment of the magnetic anisotropy in IF steels.

Probing the manipulation of antiferromagnetic order in CuMnAs films using neutron diffraction

We describe measurements of the uniaxial magnetic anisotropy and spin-flop rotation of the Néel vector in antiferromagnetic CuMnAs thin films using neutron diffraction. The suppression of the magnetic (100) peak under magnetic fields is observed for films as thin as 20 nm, indicating that they undergo a spin-flop transition. Good agreement is found between neutron diffraction and electrical transport measurements of the spin-flop rotation in the same layer with similar shape and hysteresis of the obtained curves, while the neutron measurements provide a quantitative determination of the spin flop extending throughout the antiferromagnet layer.

Advanced Characterization of FeNi-Based Films for the Development of Magnetic Field Sensors with Tailored Functional Parameters.

Magnetometry and ferromagnetic resonance are used to quantitatively study magnetic anisotropy with an easy axis both in the film plane and perpendicular to it. In the study of single-layer and multilayer permalloy films, it is demonstrated that these methods make it possible not only to investigate the average field of perpendicular and in-plane anisotropy, but also to characterize their inhomogeneity. It is shown that the quantitative data from direct integral and local measurements of magnetic anisotropy are consistent with the direct and indirect estimates based on processing of the magnetization curves. The possibility of estimating the perpendicular magnetic anisotropy constant from the width of stripe domains in a film in the transcritical state is demonstrated. The average in-plane magnetic anisotropy field of permalloy films prepared by magnetron sputtering onto a Corning glass is almost unchanged with the thickness of a single-layer film. The inhomogeneity of the perpendicular anisotropy field for a 500 nm film is greater than that for a 100 nm film, and for a multilayer film with a total permalloy thickness of 500 nm, it is greater than that for a homogeneous film of the same thickness.

Effects of synthetic antiferromagnetic coupling on back-hopping of spin-transfer torque devices

A synthetic antiferromagnetic (SAF) layer is a key component in spin-transfer torque magneto-resistive random-access memory devices. This study reveals that slight fluctuations in SAF coupling at the margin of the reference layer and hard layer (i.e., concurrent reversal) can lead to write errors in the form of back-hopping (BH). It appears that variable BH behavior can be attributed to competition between antiparallel (AP) → parallel (P) and P → AP transitions associated with SAF coupling. Our conclusions are supported by careful analysis of switching phase diagrams and measurements of self-heating and voltage-controlled magnetic anisotropy. We also observed that one form of coupling provided higher perpendicular magnetic anisotropic energy and thermal stability, which is likely due to the Dzyaloshinskii–Moriya interaction (DMI) effect. Thus, minimizing variations in DMI by optimizing SAF coupling is crucial for minimizing write error rates.

Magnetic Pore Fabrics: The Role of Shape and Distribution Anisotropy in Defining the Magnetic Anisotropy of Ferrofluid‐Impregnated Samples

AbstractPore fabrics define physical properties of a rock, such as permeability and elasticity, both of which are important to many geological, hydrological, and environmental applications. Minerals and hence pores are often preferentially aligned, leading to anisotropy of physical properties and preferred flow directions. Preferred flow paths are defined by the shape and arrangement of pores, and a characterization of this pore fabric forms the basis for prediction of fluid flow directions. Magnetic pore fabrics (MPFs), that is, magnetic anisotropy measurements on ferrofluid‐impregnated samples, are a promising and fast way to characterize the pore fabric of connected pores in 3‐D, while analyzing a large number of pores with sizes down to 10 nm, without the need for any a priori knowledge about fabric orientation. Empirical relationships suggest that the MPF is related to the pore shape and orientation and approximates permeability anisotropy. This study uses models including shape and distribution anisotropy to better understand and quantify MPFs, using simple pore shapes and pore assemblies measured in previous studies. The results obtained in this study show that (1) shape anisotropy reliably predicts the MPF of single pores, (2) both shape and distribution anisotropy are needed to predict MPFs of pore assemblies, and (3) the anisotropy parameters P, L, and F are affected by the intrinsic susceptibility of the ferrofluid in addition to pore geometry. These findings can help explain some of the variability in empirical relationships and are an important step toward a quantitative understanding and application of MPFs in geological and environmental studies.

Tuning the magnetic anisotropy of Ga-rich FeGa thin films deposited on rigid substrates

We have investigated the possibility of growing on outward bent rigid substrates to create an in-plane magnetic anisotropy in Ga-rich FeGa layers grown in the diffusive flow that in principle are expected to be magnetically isotropic in the sample plane when using non-bent rigid substrates. In this work, samples were grown by sputtering in the diffusive regime in rigid outward bent substrates, i.e. substrates under tensile stress. After the growth of the layers, the release of the substrate promotes a compression in the sample plane confirmed by x-ray diffractometry. This compression strain induces a uniaxial magnetic anisotropy axis in the sample plane perpendicular to the mechanical stress as expected for a material with a positive magnetostriction. Moreover, a value of 73 ppm for the magnetostriction constant has been inferred from the magnetic anisotropy measurements in the as-grown layer. A thermal treatment at a moderate temperature of 300 °C partially release the strain, but it does not modify the magnetic anisotropy either in magnitude or direction.

Role of growth parameters on structural and magnetic properties of Fe4N thin films grown by reactive magnetron sputtering

We studied the effect of growth parameters on the phase formation of Fe4N in a reactive sputtering process. The Fe4N phase formation was found to be extremely sensitive to N2 gas flow and sputter rate but not so much on the substrate temperature. But post-deposition annealing (PDA) time was found to be a critical parameter to get single phase Fe4N. As PDA time increases, the anti-bonding states characteristic of Fe4N start to diminish due to N out-diffusion. Signatures of such N diffusion process can also be seen from the Fe and N depth profiles. We found that the residual resistivity ratio was maximum for the optimized Fe4N films. Magnetization measurements were performed using bulk magnetization as well as polarized neutron reflectivity. We found that the magnetic moment of optimized Fe4N film was about 2.3 μB/atom at 15 K. Magnetic anisotropy measurements carried out using magneto optical-Kerr effect exhibit a rotatable anisotropy.

Operational Efficiency Evaluation of a Setup for Removing Stresses in the Steam Header of PGV-1000 Steam Generator

The effectiveness of a setup for removing stresses in the steam header of a PGV-1000 steam generator is evaluated. The evaluation is based on an investigation of the stress state by means of magnetic anisotropy measurements performed in the zone of the No. 111 weld seam of the hot header of steam generators operating in one of the power-generating units with a V-320 series reactor arrangement. Inspections made it possible to obtain cartograms of the distribution of the difference of the principal mechanical stresses along the perimeter of the weld connection of a PGV-1000 header equipped with a cooling mantle and headers without such a mantle. By means of statistical analysis of the data, a diagram of the distribution of the principal mechanical stresses was obtained. In addition, the distribution of the standard deviation of the principal mechanical stresses as a measure of the dispersion of a random quantity relative to its mathematical expectation was determined.

The Study of Exchange Bias Effect in Pt x Co1−x /CoO Bilayers

We investigated the exchange bias effect in PtxCo1−x/CoO bilayers as a function of Pt concentration. Chemical analysis on the PtxCo1−x layer confirmed that x varies from 0.6 to 0.9 by 0.1 step. Hereby, we present magnetic anisotropy measurements as well as magnetic hysteresis measurements at low temperatures. It was found that exchange bias field and blocking temperature values of PtxCo1−x/CoO bilayers strongly depend on Pt concentration. Moreover, we showed that it is possible to obtain a higher exchange bias effect with a less Co content in PtCo alloy layers. Both blocking temperature and exchange bias effect results of the bilayer system were discussed with two different scenarios.

Thickness-dependent electronic structure modulation of ferromagnetic films on shape memory alloy substrates based on a pure strain effect

Pure strain-induced electronic structure modulation in ferromagnetic films is critical for developing reliable strain-assisted spintronic devices with low power consumption. For the conventional electricity-controlled strain engineering, it is difficult to reveal the pure strain effect on electronic structure tunability due to the inseparability of pure strain effect and surface charge effect. Here, a non-electrically controlled NiTi shape memory alloy was utilized as a strain output substrate to induce a pure strain on attached Fe films through a thermally controlled shape memory effect. The pure strain induced electronic structure evolution was revealed by in-situ X-ray photoelectron spectroscopy and correlated with first-principles calculations and magnetic anisotropy measurements. A compressive strain enhances the shielding effect for core electrons and significantly tunes their binding energy. Meanwhile, the strain modifies the partial density of states of outer d orbits, which may affect spin-orbit coupling strength and related magnetic anisotropy. This work helps for clarifying the physical nature of the pure strain effect and developing the pure-strain-assisted spintronic devices.

Acquisition of detrital magnetization in four turbidites

AbstractTurbiditic events are mostly avoided in paleomagnetic studies and therefore their remanence and magnetic properties are poorly described. Turbidites are exempt of bioturbation and potentially provide pertinent information about depositional remanence. We studied four quaternary turbidites of different origins in marine sediment cores. Upward fining of both magnetic and sedimentary fractions indicates that coarser grains reached the bottom first. We observe a progressive shallowing of the magnetic inclinations between the upper and bottom layers of the turbidites that increases with the size of the events and obeys a simple linear scaling law. Measurements of magnetic anisotropy suggest that hydrodynamic conditions prevailing during deposition seem to be dominant for the alignment of the magnetic grains. We suggest that small spherical grains are randomly oriented with zero resultant magnetization in presence of strong turbulent conditions, while the alignment of elongated grains is constrained by the competition between gravity and magnetic forces. A possible scenario is that under turbulent conditions they tend to rest at the bottom with their long axes parallel to the sediment surface and therefore with shallow inclinations, whereas weakly turbulent conditions like during the smallest (26 cm thick) event do not disturb the magnetic alignment and therefore do not generate inclination shallowing.

Measurement of magnetic anisotropy of multiwalled carbon nanotubes in nematic host

The magnetic anisotropy of multiwalled carbon nanotubes (MWCNT-s) is measured using their dispersion in nematic liquid crystal (NLC). Due to their ability to align themselves with inserted nano-particles, NLC are very useful for the study of the physical properties of MWCNT as well as for other micro or nano-particles. Thus an organized system is obtained from the beginning and the influence of initial random orientation is considerably reduced. The average magnetic anisotropy of MWCNT dispersed in NLC was calculated from the system relaxation time and the obtained value (6.61×10−5) was in good agreement with other reported results.

Magnetic anisotropy, damping, and interfacial spin transport in Pt/LSMO bilayers

We report ferromagnetic resonance measurements of magnetic anisotropy and damping in epitaxial La0.7Sr0.3MnO3 (LSMO) and Pt capped LSMO thin films on SrTiO3 (001) substrates. The measurements reveal large negative perpendicular magnetic anisotropy and a weaker uniaxial in-plane anisotropy that are unaffected by the Pt cap. The Gilbert damping of the bare LSMO films is found to be low α = 1.9(1) × 10−3, and two-magnon scattering is determined to be significant and strongly anisotropic. The Pt cap increases the damping by 50% due to spin pumping, which is also directly detected via inverse spin Hall effect in Pt. Our work demonstrates efficient spin transport across the Pt/LSMO interface.

Detecting and Correcting for Paleomagnetic Inclination Shallowing of Sedimentary Rocks: A Review

Magnetic anisotropy and the elongation/inclination (E-I) approaches have been increasingly employed as two important means for detecting and correcting the paleomagnetic inclination shallowing in sedimentary rocks that was first recognized sixty years ago. Both approaches are based on certain assumptions, and thus have advantages and intrinsic limitations in investigating shallow inclinations in sedimentary rocks. The E-I approach is relatively easy to use, but it needs a large dataset to adequately sample paleomagnetic directions due to paleosecular variation of the geomagnetic field. Also, slow sediment accumulation rates and local tectonics could lead to under- or over-corrections using the E-I approach. For the magnetic anisotropy technique, labor-intensive, sophisticated laboratory rock magnetic experiments are required in order to accurately determine both bulk magnetic anisotropy of remanence-carrying grains and magnetic anisotropy of an individual particle, i.e., “a” factor, of samples. Our review shows that, despite the intensive laboratory work necessary for applying anisotropy-based inclination corrections, it is worth investing the effort. In addition, the joint use of magnetic susceptibility and remanence anisotropy measurements as well as detailed rock magnetic measurements for determining the particle anisotropy “a” factor have the advantage of retrieving direct evidence of inclination shallowing and correcting for it with high confidence. We caution against use of either of the two approaches without full appreciation of the underlying assumptions and intrinsic limitations of each technique. The use and comparison of both techniques could provide the most robust inclination shallowing correction for sedimentary rocks.

Direct and converse magneto-electric coupling in ferromagnetic shape memory alloys based thin film multiferroic heterostructures

The ferromagnetic shape memory driven alterations in strain mediated direct and converse magnetoelectric coupling (DME & CME) was realized in sputtered deposited PZT/Ni-Mn-In multiferroic hetero-junctions. The ferroelectric (P-E loops), dielectric (ε vs frequency, ε-E), and voltage modulated magnetic anisotropy measurements (M-E curves) were executed in the plane (hard axis) and out of the plane (easy axis) of the functional magnetic material based PZT/Ni-Mn-In bilayer structure. A gain of ∼16 μC/cm2 in maximum polarization (Pmax) and ∼12% in tunability (nr) were observed along an easy magnetic axis of Ni-Mn-In. The butterfly shaped normalized magnetization (M/Ms) vs electric field (applied across the heterostructure) [M-E] plots evident the strain character of CME coupling. The co-action of (i) dissimilar carrier concentration between high symmetric austenitic cubic phase and low symmetry martensite phase of ferromagnetic shape memory alloys and (ii) martensitic transformation induced magnetization change in Ni-Mn-In cause an electrical field modulated hall resistivity; a change of ∼42% in hall resistivity was observed at 60 kV/cm electric field and 0.2 T magnetic field at 270 K. The reversible manipulation of remnant magnetization (Mr) with applied electric field was demonstrated as on/off switch using a square pulse of 60 kV/cm amplitude.

Correlated oscillations of the magnetic anisotropy energy and orbital moment anisotropy in thin films: The role of quantum well states

We report the first-principles study of the correlated behavior of the magnetic anisotropy energy (MAE) and orbital moment anisotropy (OMA) as the functions of the thickness $N$ of the Fe film. The work is motivated by recent experimental studies combining photoemission, x-ray magnetic circular dichroism, and magnetic anisotropy measurements. In agreement with experiment, the correlated oscillations of $\mathrm{MAE}(N)$ and $\mathrm{OMA}(N)$ are obtained that have their origin in the formation of the 3d quantum well states (QWS) confined in the films. The main contribution to the oscillation amplitude comes from the surface layer. This is an interesting feature of the phenomenon consisting in the peculiar dependence of the physical quantities on the thickness of the film. We demonstrate that the band structure of the bulk Fe does not reflect adequately the properties of the 3d QWS in thin films and, therefore, does not provide the basis for understanding the oscillations of $\mathrm{MAE}(N)$ and $\mathrm{OMA}(N)$. A detailed point-by-point analysis in the two-dimensional (2D) Brillouin zone (BZ) of the film shows that the contribution of the $\mathrm{\ensuremath{\Gamma}}$ point, contrary to a rather common expectation, does not play an important role in the formation of the oscillations. Instead, the most important contributions come from a broad region of the 2D BZ distant from the center of the BZ. Combining symmetry arguments and direct calculations we show that orbital moments of the electronic states possess nonzero transverse components orthogonal to the direction of the spin magnetization. The account for this feature is crucial in the point-by-point analysis of the OMA. On the basis of the calculations for noncollinear spin configurations we suggest interpretations of two interesting experimental findings: fast temperature decay of the oscillation amplitude in $\mathrm{MAE}(N)$ and unexpectedly strong spin mixing of the initial states of the photoemission process.

ChemInform Abstract: Broken Symmetries in URu2Si2

AbstractReview: 52 refs.