Magnetic Dead Layer Research Articles

Synthesis of high saturation magnetic iron oxide nanomaterials via low temperature hydrothermal method

Iron oxide nanoparticles (IONPs) were synthesized through a simple low temperature hydrothermal approach to obtain with high saturation magnetization properties. Two series of iron precursors (sulfates and chlorides) were used in synthesis process by varying the reaction temperature at a constant pH. The X-ray diffraction pattern indicates the inverse spinel structure of the synthesized IONPs. The Field emission scanning electron microscopy and high resolution transmission electron microscopy studies revealed that the particles prepared using iron sulfate were consisting a mixer of spherical (16–40nm) and rod (diameter ~20–25nm, length <100nm) morphologies that synthesized at 130°C, while the IONPs synthesized by iron chlorides are found to be well distributed spherical shapes with size range 5–20nm. On other hand, the IONPs synthesized at reaction temperature of 190°C has spherical (16–46nm) morphology in both series. The band gap values of IONPs were calculated from the obtained optical absorption spectra of the samples. The IONPs synthesized using iron sulfate at temperature of 130°C exhibited high saturation magnetization (MS) of 103.017emu/g and low remanant magnetization (Mr) of 0.22emu/g with coercivity (Hc) of 70.9Oe, which may be attributed to the smaller magnetic domains (dm) and dead magnetic layer thickness (t).

Band alignments in Fe/graphene/Si(001) junctions studied by x-ray photoemission spectroscopy

The control of tunnel contact resistance is of primary importance for semiconductor-based spintronic devices. This control is hardly achieved with conventional oxide-based tunnel barriers due to deposition-induced interface states. Manipulation of single 2D atomic crystals (such as graphene sheets) weakly interacting with their substrate might represent an alternative and efficient way to design new heterostructures for a variety of different purposes including spin injection into semiconductors. In the present paper, we study by x-ray photoemission spectroscopy the band alignments and interface chemistry of iron–graphene-hydrogenated passivated silicon (001) surfaces for a low and a high n-doping concentration. We find that the hydrogen passivation of the Si(001) surface remains efficient even with a graphene sheet on the Si(001) surface. For both doping concentrations, the semiconductor is close to flat-band conditions which indicates that the Fermi level is unpinned on the semiconductor side of the Graphene/Si(001):H interface. When iron is deposited on the graphene/Si(001):H structures, the Schottky barrier height remains mainly unaffected by the metallic overlayer with a very low barrier height for electrons, a sought-after property in semiconductor based spintronic devices. Finally, we demonstrate that the graphene layer intercalated between the metal and semiconductor also serves as a protection against iron-silicide formation even at elevated temperatures preventing from the formation of a Si-based magnetic dead layer.

Brillouin light scattering investigation of the thickness dependence of Dzyaloshinskii-Moriya interaction inCo0.5Fe0.5ultrathin films

$\mathrm{C}{\mathrm{o}}_{0.5}\mathrm{F}{\mathrm{e}}_{0.5}$ (CoFe) ultrathin films of various thicknesses $(0.8\phantom{\rule{0.16em}{0ex}}\mathrm{nm}\ensuremath{\le}{t}_{\mathrm{CoFe}}\ensuremath{\le}1.6\phantom{\rule{0.16em}{0ex}}\mathrm{nm})$ have been grown by sputtering on (001) MgO single crystal or Si/${\mathrm{SiO}}_{2}$ substrates, using Pt as capping or buffer layers, respectively. The x-ray diffraction revealed an in-plane epitaxial (isotropic) growth of Pt on MgO (Si). Their magnetic properties have been studied by vibrating sample magnetometry and Brillouin light scattering (BLS) in the Damon-Eshbach geometry. Vibrating sample magnetometry characterizations show that films grown on MgO are in-plane magnetized, while films deposited on Si are perpendicularly magnetized for CoFe thickness below 1.4 nm. The BLS measurements reveal a pronounced nonreciprocal spin waves propagation, which increases with decreasing CoFe thickness. This nonreciprocity was attributed to an interfacial Dzyaloshinskii-Moriya interaction (DMI) induced by Pt interface with CoFe. Moreover, the DMI sign has been found to depend on the stacks order: it is positive (negative) for CoFe/Pt (Pt/CoFe). The effective thickness dependence of the DMI effective constant shows two regimes due to the degradation of the interfaces as the CoFe thickness decreases. We thus show that the magnetic dead layer should be taken into account to precisely determine the surface DMI constant ${D}_{\mathrm{s}}$. Therefore, for the thickest samples, the surface DMI constants are nearly opposite: $\ensuremath{-}1.27$ and $1.32\phantom{\rule{0.16em}{0ex}}\mathrm{pJ}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}$ for Pt/CoFe and its reversed system, respectively.

Solvothermally Driven Mn Doping and Clustering of Iron Oxide Nanoparticles for Heat Delivery Applications.

Direct interactions between nanoparticles of Mn-doped magnetite or maghemite (clearly differentiated by Raman spectroscopy) grouped in spherical clusters minimize the effect related to their characteristic magnetic dead layer at the surface. Hence, the clustering process jointly with the manganese doping renders these ferrite nanostructures very attractive as displaying increased saturation magnetization, offering, consequently, outstanding values of the specific absorption rate (SAR) for heat delivery. The whole picture for bio-related applications has been considered, with issues related to magnetic manipulation, colloidal stability, and biocompatibility.

Investigation of extrinsic damping caused by magnetic dead layer in Ta-CoFeB-MgO multilayers with perpendicular anisotropy

We systematically investigated the influence of the dead layer, the oxidation degree of naturally oxidized MgO, the structure of adjacent nonmagnetic metal layers on the damping parameter, and the perpendicular anisotropy of Ta(Ru)/Co20Fe60B20/MgO and MgO/Co20Fe60B20/Ta films using the vector network analyzer ferromagnetic resonance measurement technique. MgO/Co20Fe60B20/Ta film shows almost twice larger extrinsic damping than that for Ta/Co20Fe60B20/MgO film, whereas the perpendicular anisotropy is much smaller. This two-fold enhancement of damping parameter is successfully explained by the extrinsic damping arises from the dead layer at the CoFeB-Ta interface through the Elliott-Yafet mechanism in addition to the conventional spin-pumping model. Furthermore, we found that the oxidation degree of naturally oxidized MgO has no significant impact on the damping parameter, while the perpendicular anisotropy for MgO/Co20Fe60B20/Ta film is enhanced by longer oxidation time.

Annealing-induced alloy formation in Pd/Fe bilayers on Si(1 1 1) for hydrogen sensing

The bilayers of Pd and Fe with different thickness and relative positions were grown on Si(111)-7×7 surface at room temperature. For the investigation of the thermal annealing induced inter-diffusion and the corresponding magnetic behavior, Auger electron spectroscopy (AES) measurement was carried out after various annealing processes, including the variation of annealing duration and temperature. With the annealing temperature of 300–500K, the Pd/Fe bilayers were stable. Slight Si segregated into the thin film at around 700K. Above 700K, more serious Si segregation occurred and most of the Pd/Fe bilayer was mixed with Si, forming a silicide layer. 700–800K annealing also induced change of Pd/Fe AES ratio, indicating the inter-diffusion between Pd and Fe layers. To overcome the unavoidable silicide formation induced magnetic dead layer, a relative thick Fe film of 20 ML capped with 1.5 ML Pd was chosen for the investigation of magnetism. The magnetic coercivity (Hc) increased by 2–3 times with the annealing temperature up to 740K. Obvious hydrogenation effect was observed in 710K-annealed sample; the in-plane Hc increased by more than 10% when the hydrogen pressure was above 200mbar. After further annealing at 740–800K, the hydrogenation effect on Hc became nearly unobservable. The annealing induced Pd-rich magnetic interface is supposed to dominate the hydrogenation effect on magnetism.

Epitaxial Growth of Co&lt;sub&gt;0.75&lt;/sub&gt;Fe&lt;sub&gt;2.25&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt;/NiO Bilayer on MgO(001) Substrate

Co x Fe 3−x O 4 (CFO) films epitaxially grown on MgO(001) substrates show perpendicular magnetic anisotropy as high as 10 Merg/cm3[1]. However, magnetization is dependent on thickness. The magnetization per unit area linearly increases with increasing thickness of the films. Although the slope corresponds to the bulk magnetization, the slope intersects the horizontal (thickness) axis, suggesting that a magnetic dead layer exists at the interface/surface of the films. There are several possible reasons for the decrease in magnetization at the interface such as diffusion of Mg from the substrates, existence of anti-phase boundaries (APBs), and so on. In order to prevent the formation of the dead layer and improve the magnetic properties at the interface, we investigated the effect of a NiO insertion layer on the magnetization of CFO films. It has been reported that the insertion layer of NiO prevents Mg diffusion from MgO substrates in Fe 3 O 4 /MgO(001)[2]. Since NiO is isostructural with MgO, we can expect that APBs in CFO films are formed on the NiO insertion layers as well as on the MgO surfaces.

Atomic-Scale Interfacial Magnetism in Fe/Graphene Heterojunction.

Successful spin injection into graphene makes it a competitive contender in the race to become a key material for quantum computation, or the spin-operation-based data processing and sensing. Engineering ferromagnetic metal (FM)/graphene heterojunctions is one of the most promising avenues to realise it, however, their interface magnetism remains an open question up to this day. In any proposed FM/graphene spintronic devices, the best opportunity for spin transport could only be achieved where no magnetic dead layer exists at the FM/graphene interface. Here we present a comprehensive study of the epitaxial Fe/graphene interface by means of X-ray magnetic circular dichroism (XMCD) and density functional theory (DFT) calculations. The experiment has been performed using a specially designed FM1/FM2/graphene structure that to a large extent restores the realistic case of the proposed graphene-based transistors. We have quantitatively observed a reduced but still sizable magnetic moments of the epitaxial Fe ML on graphene, which is well resembled by simulations and can be attributed to the strong hybridization between the Fe 3dz2 and the C 2pz orbitals and the sp-orbital-like behavior of the Fe 3d electrons due to the presence of graphene.

Buffer influence on magnetic dead layer, critical current, and thermal stability in magnetic tunnel junctions with perpendicular magnetic anisotropy

We present a detailed study of Ta/Ru-based buffers and their influence on features crucial from the point of view of applications of Magnetic Tunnel Junctions (MTJs) such as critical switching current and thermal stability. We study buffer/FeCoB/MgO/Ta/Ru and buffer/MgO/FeCoB/Ta/Ru layers, investigating the crystallographic texture, the roughness of the buffers, the magnetic domain pattern, the magnetic dead layer thickness, and the perpendicular magnetic anisotropy fields for each sample. Additionally, we examine the effect of the current induced magnetization switching for complete nanopillar MTJs with lateral dimensions of 270 × 180 nm. Buffer Ta 5/Ru 10/Ta 3 (thicknesses in nm), which has the thickest dead layer, exhibits a much larger thermal stability factor (63 compared to 32.5) while featuring a slightly lower critical current density value (1.25 MA/cm2 compared to 1.5 MA/cm2) than the buffer with the thinnest dead layer Ta 5/Ru 20/Ta 5. We can account for these results by considering the difference in damping which compensates for the difference in the switching barrier heights.

Effects of interfacial Fe electronic structures on magnetic and electronic transport properties in oxide/NiFe/oxide heterostructures

We report that the magnetic and electronic transport properties in oxide/NiFe(2nm)/oxide film (oxide=SiO2, MgO or HfO2) are strongly influenced by the electronic structure of NiFe/oxide interface. Magnetic measurements show that there exist magnetic dead layers in the SiO2 sandwiched film and MgO sandwiched film, whereas there is no magnetic dead layer in the HfO2 sandwiched film. Furthermore, in the ultrathin SiO2 sandwiched film no magnetoresistance (MR) is detected, while in the ultrathin MgO sandwiched film and HfO2 sandwiched film the MR ratios reach 0.35% and 0.88%, respectively. The investigation by X-ray photoelectron spectroscopy reveals that the distinct interfacial redox reactions, which are dependent on the oxide layers, lead to the variation of magnetic and transport properties in different oxide/NiFe/oxide heterostructures.

Capping layer-tailored interface magnetic anisotropy in ultrathin Co2FeAl films

Co2FeAl (CFA) thin films of various thicknesses (2 nm ≤ d ≤ 50 nm) have been grown on (001) MgO single crystal substrates and then capped with Cr, V, and Ta. Their magnetic and structural properties have been studied by x-ray diffraction (XRD), vibrating sample magnetometry, and broadband microstrip ferromagnetic resonance (MS-FMR). The XRD revealed that the films are epitaxial with the cubic [001] CFA axis normal to the substrate plane and that the chemical order varies from the B2 phase to the A2 phase when decreasing the thickness. The deduced lattice parameters showed that the Cr-capped films exhibit a larger tetragonal distortion, as compared with the films capped with V or Ta. The presence of magnetic dead layers has been observed in CFA samples capped with V and Ta but not in the case of the Cr-capped ones. The effective magnetization, deduced from the fit of MS-FMR measurements, increases (decreases) linearly with the CFA inverse thickness (1/d) for the Cr-capped (Ta-capped) films while it is constant for the V-capped ones. This allows quantifying the perpendicular surface anisotropy coefficients of −0.46 erg/cm2 and 0.74 erg/cm2 for Cr and Ta-capped films, respectively. Moreover, the fourfold and the uniaxial anisotropy fields, measured in these films, showed different trends with a respect to the CFA inverse thickness. This allows inferring that a non-negligible part of the fourfold magnetocrystalline term is of interfacial origin.

Enhanced Local Magnetization by Interface Engineering in Perovskite‐Type Correlated Oxide Heterostructures

Elimination of a magnetic dead-layer at the LSMO/STO interface is achieved by interface engineering through incorporation of a single La0.33Sr0.67O layer. Controlled growth of interfacial atomic stacking in combination with local magnetic probing by polarized neutron reflectometry enables controlled interface engineering and results in optimal interfacial magnetization in perovskite-type correlated oxide heterostructures.

Influence of Mo capping layer on magnetic anisotropy of MgO/CoFeB/Mo

In this paper, the influence of Mo capping layer on magnetic anisotropy of MgO/CoFeB/Mo with varying thickness is studied. It is found that Mo capping layer shows more saturated magnetic moments than Ta capping layer. The direction of the external magnetic field has a great influence on the magnetic anisotropy. The MgO/CoFeB/Mo sample prepared in an applied magnetic field parallel to the plane shows in-plane magnetic anisotropy (IMA). IMA becomes weak as the CoFeB thickness decreases, and it still exists when the thickness decreases to 1.1 nm. At the same time, the saturation field vertical to the plane decreases. When the thickness of CoFeB layer decreases to 0.9 nm or less, the IMA disappears. In our study, the saturated magnetization and magnetic dead layer are 1600 emu/cm3 and 0.26 nm at the annealing temperature 200 ℃, and the interface anisotropy is 0.91 erg/cm2, which is smaller than previous research results. Increasing the annealing temperature helps the sample keep the saturated state under a small magnetic field vertical to the plane, and makes IMA weak and transform into PMA. The variation of the Mo capping layer thickness affects the saturation magnetic moment of the sample. The magnetic moment shows a sharp downtrend when the Mo layer is between 1.2 and 1.6 nm, then it turns stabler with Mo capping layer thickening. Meanwhile, when the Mo capping layer is 1.6 nm, the external vertical saturation field becomes smallest. However under the parallel magnetic field, changing the thickness or annealing temperature, or changing both leads to no PMA occurring. When the magnetic field direction changes from parallel to vertical direction, some of the samples show PMA after the annealing process. The magnetic anisotropy of MgO/CoFeB/Mo varies with the thickness of Mo capping layer. IMA is present when the Mo capping layer is 1 nm or less while PMA is present when the Mo capping layer is between 1.2 and 5 nm. The sample coercive force in the vertical direction varies with thickness, and its magnetic hysteresis loss is much larger when the thickness of Mo capping layer is 1.4 nm.

In situ surface magneto-optical Kerr effect (s-MOKE) study of ultrathin soft magnetic FeCuNbSiB alloy films

Herein we report on an in situ surface magneto-optical Kerr effect (s-MOKE) study of ion-beam-sputtered ultra-thin films of an amorphous Fe73.9Cu0.9Nb3.1Si13.2B8.9 (FINEMET) alloy with film growth that ranges from a fraction of a nm to a few tens of nms. Extrapolating the linear variation of the Kerr signal with film thickness suggests the absence of a magnetic dead layer at the substrate/FINEMET film interface, and hence the absence of any intermixing. The presence of a thin SiO2 film at the surface of the Si substrate may be responsible for preventing possible intermixing of Fe with Si to form nonmagnetic silicide. Close to the onset of ferromagnetic ordering, a steep increase in the coercive field with film thickness can be explained in terms of the Volmer–Weber growth of the film. Furthermore, the temperature dependence of the hysteresis loops of a 41 nm-thick FINEMET film has been studied. The Curie temperature of the amorphous film is found to be lower than that of a ribbon of the same composition. The origin of a uniaxial magnetic anisotropy in the as-prepared stage is attributed to the generation of some long-range stresses in the film, which are relieved close to the onset temperature for nanocrystallization.

Large enhancement of magnetoresistance in NiFe film with MgO layers sandwiched after annealing

A large enhancement of magnetoresistance (MR) up to 6.0% has been observed in NiFe sandwiched by MgO layers, which is 50% larger than the highest MR (4%) in bulk materials. The great improvement of MR derives from the slight increase in corresponding resistivity change Δρ and the great decrease in resistivity ρ. The enhancement of Δρ is attributed to the strengthened spin-dependent scattering of the interfacial conductive electrons but the contribution will be slight with the increase in NiFe thickness. The main contribution is from the significant decrease in ρ, originating from confinement effect on electrons from well-formed oxide/metal interfaces after annealing. Surprisingly, this effect still exists when the NiFe thickness reaches 100nm. Meanwhile, the oxide (MgO) layers inserted prevents the atomic interdiffusion of Ta and NiFe at the interfaces, which decreases the thickness of magnetic dead layer.

Perpendicular Magnetic Anisotropy in MgO/CoFeB/Nb and a Comparison of the Cap Layer Effect

High perpendicular magnetic anisotropy (PMA) has been recently revealed in CoFeB/MgO/CoFeB tunnel junctions if the thickness of CoFeB is <;1.5 nm. However, PMA has been observed in MgO/CoFeB/Ta but not in MgO/CoFeB/Ru, indicating a metallic cap layer effect. In this paper, we extend the study to MgO/CoFeB(0.8-1.8 nm)/Nb(1.3 nm) by sputtering and find that the magnetic behavior of MgO/CoFeB/Nb resembles the magnetic behavior of MgO/CoFeB/Ta. The interface anisotropy constant of both MgO/CoFeB/Ta and MgO/CoFeB/Nb is ~2.2 erg/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> while MgO/CoFeB/Ru has a much weaker interface anisotropy constant ~0.68 erg/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . In addition, the magnetic dead layer (MDL) of MgO/CoFeB/cap (cap = Ta and Nb) is ~0.1 nm while MgO/CoFeB/Ru has a much larger MDL ~0.5 nm. The metallic layer effect on the PMA in CoFeB/MgO-based perpendicular structure is attributed to the interdiffusion during the post-annealing process. The possible correlation between PMA and MDL provides a possible mechanism for the metal layer effect in the CoFeB/MgO-based structures.

Coverage-dependent magnetic properties of Ni ultrathin films on Pd(001) investigated using X-ray magnetic circular dichroism

Coverage-dependent magnetic properties of Ni ultrathin films on Pd(001) was investigated using X-ray magnetic circular dichroism (XMCD). Magnetization curves and Ni L2,3 XMCD spectra were measured as a function of Ni coverage. These results revealed a spin reorientation transition from in-plane to out-of-plane as the Ni coverage increased. Ni 3d spin and orbital magnetic moments were evaluated using the magneto-optical sum rules. The presence of a magnetic dead layer inside the Ni film explains the Ni coverage dependence of spin magnetic moment.

Impact of fluorine based reactive chemistry on structure and properties of high moment magnetic material

The impact of the fluorine-based reactive ion etch (RIE) process on the structural, electrical, and magnetic properties of NiFe and CoNiFe-plated materials was investigated. Several techniques, including X-ray fluorescence, 4-point-probe, BH looper, transmission electron microscopy (TEM), and electron energy loss spectroscopy (EELS), were utilized to characterize both bulk film properties such as thickness, average composition, Rs, ρ, Bs, Ms, and surface magnetic “dead” layers' properties such as thickness and element concentration. Experimental data showed that the majority of Rs and Bs changes of these bulk films were due to thickness reduction during exposure to the RIE process. ρ and Ms change after taking thickness reduction into account were negligible. The composition of the bulk films, which were not sensitive to surface magnetic dead layers with nano-meter scale, showed minimum change as well. It was found by TEM and EELS analysis that although both before and after RIE there were magnetic dead layers on the top surface of these materials, the thickness and element concentration of the layers were quite different. Prior to RIE, dead layer was actually native oxidation layers (about 2 nm thick), while after RIE dead layer consisted of two sub-layers that were about 6 nm thick in total. Sub-layer on the top was native oxidation layer, while the bottom layer was RIE “damaged” layer with very high fluorine concentration. Two in-situ RIE approaches were also proposed and tested to remove such damaged sub-layers.

Annealing temperature and thickness dependence of magnetic properties in epitaxial L1-Mn1.4Ga films

Mn1.4Ga films with high perpendicular magnetic anisotropy and high crystalline quality were grown on MgO substrates with Cr buffer layer using molecular beam epitaxy. The crystalline structure and the surface morphology of the films have been systematically investigated as functions of in-situ annealing temperature (Ta) and film thickness. It is found that the magnetic properties can be largely tuned by adjusting Ta. As Ta increases, both saturation magnetization (Ms) and uniaxial perpendicular magnetic anisotropy constant (Ku) increase to the maximum values of 612 emu/cc and 18 Merg/cc at 300 °C, respectively, and then decrease. The morphology also changes with Ta, showing a minimum roughness of 2.2 Å at Ta = 450 °C. On the other hand, as the thickness increases, Ms and Ku increase while coercivity decreases, which indicates there is a magnetic dead layer with a thickness of about 1.5 nm at the interfaces. The detailed examination on the surface morphology of the films with various thicknesses shows a complicated film growth process, which can be understood from the relaxation mechanism of the interfacial strain.

Exchange bias induced by the fully strained La2/3Ca1/3MnO3 dead layers

A pure compressively strained La2/3Ca1/3MnO3 (LCMO) dead layer grown on (001)-oriented LaAlO3 substrate can show all the rich phenomenon of large bias field shift, coercive field enhancement, and high blocking temperature. The obtained exchange bias field (∼350 Oe) and the enhanced coercivity of about 1160 Oe at 5 K under 500 Oe cooling field are superior to that have been reported in LCMO-based ferromagnetic/antiferromagnetic superlattices or nanoscale systems. Our results clearly demonstrate that the inhomogeneous magnetic dead layer of LCMO can induce a strong exchange bias effect, which may be exploited as a very simple structure for spin-valve device application.