Fe Layer Thickness Research Articles

Large perpendicular magnetic anisotropy at Fe/MgO interface

A large perpendicular magnetic anisotropy (PMA) of 1.4 MJ/m3 was observed from ultrathin Fe/MgO(001) bilayers grown on Cr-buffered MgO(001). The PMA strongly depends on the surface state of Fe prior to the MgO deposition. A large PMA energy density of 1.4 MJ/m3 was achieved for a 0.7 nm thick Fe layer having adsorbate-induced surface reconstruction, which is likely to originate from oxygen atoms floating up from the Cr buffer layer. This large magnitude of PMA satisfies the criterion that is required for thermal stability of magnetization in a few tens nanometer-sized magnetic memory elements.

Tunneling magnetoresistance effect in MnGa based perpendicular magnetic tunnel junction with Fe/Co interlayer

In order to enhance the magnetoresistance (MR) of perpendicular magnetic tunnel junctions (pMTJs) based on MnGa alloys, a single ferromagnetic layer such as Fe and Co was previously inserted between MnGa and MgO barrier. In this study, to further enhance the spin-filter effect, we introduced a Fe/Co bilayer as an interlayer in the MnGa/MgO interface. Compared to the single Co interlayer, an apparent MR ratio enhancement was obtained when Fe layer thickness was around 0.3 nm for pMTJs with MnGa compositions of Mn57Ga43, Mn62Ga38, and Mn70Ga30, and the maximum MR ratio reaches 50% at room temperature. In addition, inverted magnetoresistance loops were observed due to the antiparallel alignment of the magnetic moments of Co and MnGa layers separated by the thin Fe layer.

Depth dependent structure and magnetic properties and their correlation with magnetotransport in Fe/Au multilayers

A detailed study on the role of interface structure and magnetism in magnetoresistance (MR) in Fe/Au multilayers has been carried out. Two series of multilayers were deposited on Si substrate by sputtering: one series with varying Fe layer thickness, tFe=30–100Å, and another series with varying Au layer thickness tAu=50–150Å. MR measurements were done using a 4-probe technique. The multilayer interface structure and magnetic properties were studied by specular X-ray reflectometry and polarized neutron reflectometry (PNR). Details of the interface fractal morphology were obtained from diffuse X-ray reflectivity (DXRR). Bulk magnetic measurements were carried out on a superconducting quantum interference device magnetometer. Change in MR can be associated with magnetic and morphological properties of Fe/Au and Au/Fe interfaces in Fe/Au multilayer samples as obtained by reflectometry. Large MR is also associated to interfaces with larger magnetization, large in-plane correlation length and higher Hurst parameter, obtained from PNR and DXRR.

Enhancement of anomalous Hall effect in Si/Fe multilayers

Anomalous Hall effect (AHE) studies were performed at 300 K on Si/Fe multilayers prepared by dc magnetron sputtering. About 60 times enhancement in the saturation Hall resistance and 80 times enhancement in anomalous Hall coefficient were obtained in [Si(50 Å)/Fe(tFe)]20 multilayers when decreasing the Fe layer thickness from 100 to 20 Å. The largest anomalous Hall coefficient (Rs) of 1.4 × 10−7 Ω m T−1 was found for tFe = 20 Å, which is about three orders of magnitude larger than that of pure Fe and Fe/Cr, Al/Fe, Cu/Fe, SiO2/FePt/SiO2 multilayers. The ordinary Hall coefficient R0 was about two orders of magnitude larger than that of pure Fe. The Rs was found to vary with the longitudinal electronic resistivity, ρ as Rsαρ2.2, indicating the role of interfaces for the enhancement of the AHE in the multilayers. An increase of Hall sensitivity from 9 mΩ T−1 to 1.2 Ω T−1 was observed on decreasing tFe from 100 to 10 Å. The high Hall sensitivity obtained was about three orders of magnitude larger than that of Al/Fe and Cu/Fe multilayers, showing it as an emerging candidate for Hall element for potential applications.

Evaluation of interlayer exchange coupling in α-Fe(100)/Nd2Fe14B(001) films

We fabricated α-Fe(100)/Nd2Fe14B(001) bilayer films with different thicknesses values of the α-Fe layer on MgO(100) substrates by using an ultra-high-vacuum (UHV) magnetron sputtering system. The exchange coupling constant at the heterointerfaces of α-Fe(100)/Nd2Fe14B(001) was evaluated by analyzing the magnetization curves and the ferromagnetic resonance (FMR) spectra. We observed a significant lower shift of the α-Fe resonance field compared to that of the α-Fe single layer irrespective of the Fe-layer thickness, which suggests the existence of an internal field owing to a positive exchange coupling between the α-Fe(100) and the Nd2Fe14B(001) layers.

Enhanced perpendicular magnetic anisotropy in Fe/Mn bilayers by incorporating ultrathin ferromagnetic underlayer through magnetic proximity effect

The perpendicular magnetic anisotropy(PMA) was shown to be established in ferromagnetic (FM)/fcc-Mn bilayers through the FM-antiferromagnetic (AFM) exchange coupling. We demonstrate here that such PMA can be further enhanced by incorporating an ultrathin Fe film as an underlayer. In a series of Fe/Mn bilayers, hysteresis loop measurement shows that the thickness of top Fe layer with PMA can be extended to a thicker range while an ultrathin Fe underlayer is inserted. Such enhancement of PMA is attributed to an increase of AFM ordering on the Mn film originated from the magnetic proximity effect with the Fe underlayer.

Structural and magnetic properties of L10-FePt/Fe exchange coupled nano-composite thin films with high energy product

Nanoparticle-FePt/Fe bilayer thin films with different Fe layer thicknesses and [nanoparticle-FePt/Fe]n multilayer thin films with various periods were fabricated by DC magnetron sputtering. Their structures and magnetic properties (energy product and exchange coupling) were investigated by X-ray diffraction technique and vibrating sample magnetometer, respectively. The results show that coercivities of FePt/Fe bilayer films are dropped from 10.7kOe to 0.53kOe with increasing Fe soft magnetic layer thickness, due to the interconnecting of FePt nano-particles by Fe-rich soft magnetic phase and decreasing of the ordering degree of L10-FePt phase. The critical Fe layer thickness changing the annealed film from granular to continuous microstructure is about 4nm. For multilayer [FePt/Fe]n films, both ordering degree of L10-FePt phase and magnetic properties are improved with the number of layer periods. Moreover, a maximum energy products as high as 21.65MGOe is obtained in the [FePt/Fe]5 multilayer film. The enhancement of saturation magnetization and energy products can be ascribed to the high ordering degree of FePt phase as well as the strong exchange coupling among L10-FePt hard magnetic phase and Fe-rich soft magnetic phase.

Tunable resonant properties of perpendicular anisotropy [Co/Pd]/Fe/[Co/Pd] multilayers

We describe the static and dynamic magnetic behaviors of Fe films (thicknesses 2, 4, and 6 nm) sandwiched between Co/Pd multilayers with strong perpendicular magnetic anisotropy. Out-of-plane measurements of both magnetization and ferromagnetic resonance confirm well-defined Fe layer response modified by large perpendicular exchange field arising from the coupling with the Co/Pd. The field/frequency dispersion is linear for all samples with field intercepts increasing with Fe layer thickness. Analysis in terms of shape anisotropy and interfacial exchange model yields a large out-of-plane interfacial coupling of ∼3.0–3.7 erg/cm2 that is mediated by the coupling across thin Pd layers. The value of this interface exchange is also shown to be tunable with interfacial Pd thickness.

Effects of catalyst thickness on the fabrication and performance of carbon nanotube-templated thin layer chromatography plates

The effects of iron catalyst thickness on the fabrication and performance of microfabricated, binder-free, carbon nanotube (CNT)-templated, thin layer chromatography (TLC) plates are demonstrated. The iron catalyst was deposited at thicknesses ranging from 4 to 18 nm in increments of 2 nm. Its thickness plays a key role in governing the integrity and separation capabilities of microfabricated TLC plates, as determined using a test dye mixture. Atomic force microscopy and scanning electron microscopy show that smaller and more numerous catalyst nanoparticles are formed from thinner Fe layers, which in turn govern the diameters and densities of the CNTs. The average diameter of the Fe nanoparticles, Dp, is approximately six times the initial Fe film thickness, tFe: Dp ≈ 6tFe. After deposition of relatively thick silicon layers on CNTs made with different Fe thicknesses, followed by oxidation, all of the resulting CNT-templated SiO2 wires had nearly the same diameter. Consequently, their surface areas were very similar, although their areal densities on the TLC plates were not because thinner catalyst layers produce denser CNT forests. For tFe = 6 nm, nanotube growth appears to be base growth, not tip growth. Best TLC separations of a test dye mixture were obtained with plates prepared with 6 or 4 nm of catalyst. Calculations suggest a loss of surface area for TLC plates made with thicker Fe layers as a result of fewer, thicker CNTs, where the density of silica nanotubes (device surface area) goes approximately as 1/tFe2. While the focus of this paper is toward a greater understanding of the processing conditions that lead to the best TLC plates, a baseline separation of three analgesics (caffeine, phenacetine, and propyphenazone) is shown on a normal phase TLC plate grown with 6 nm of iron.

Magnetic tunnel junctions of perpendicularly magnetized L10-MnGa/Fe/MgO/CoFe structures: Fe-layer-thickness dependences of magnetoresistance effect and tunnelling conductance spectra

The tunnel magnetoresistance (TMR) effect and the bias voltage dependence of tunnelling conductance spectra were measured in L10 Mn-Ga/Fe/MgO/CoFe magnetic tunnel junctions (MTJs). The TMR ratio and bias-voltage dependences on annealing conditions and Fe-layer thickness were investigated. The TMR ratio showed an increase subsequent to Fe-layer deposition under the optimum annealing condition, and a maximum value of 24% was achieved in an MTJ with a perpendicularly magnetized Fe layer of thickness of 1.1 nm at room temperature; this corresponded to a 57% TMR ratio in the case of completely antiparallel magnetization configuration. In the tunnelling conductance spectra, an anomalous dip, the so-called zero-bias anomaly, was observed for all the samples. The zero-bias anomaly is speculated to have appeared because of an increase in magnon excitation at the magnetic layer/barrier interfaces according to the model by Zhang et al (1997 Phys. Rev. Lett. 79 3744). In our experiments magnitude of the zero-bias anomaly depended on both the annealing condition and the Fe-layer thickness. We discuss our observation of the variation in the Curie temperature of the magnetic layer at the barrier layer interface depending on the the preparation conditions of the Fe insertion layer, which caused the change in the magnitude of the zero-bias anomaly.

Effect of Fe layer thickness and Fe/Co intermixing on the magnetic properties of Sm–Co/Fe bilayer exchange-spring magnets

Sm–Co(20 nm)/Fe(tFe) bilayers are fabricated with different Fe layer thicknesses (tFe = 0, 2, 4, 6 and 8 nm) in order to systematically investigate the intermixing effect between Fe and Sm–Co layers and its influence on the magnetic properties. X-ray diffraction analysis indicates that the deposition of soft layer (Fe) promotes crystallization of hard layer (Sm–Co) at a low annealing temperature (400 °C). The annealed films consist of hexagonal Sm2Co7, Sm2Co17 and SmCo5 phases in the hard layer and bcc-structured Fe(Co) in the soft layer. Rutherford backscattering is employed to investigate the atomic composition of the individual layers and thereby the extent of diffusion of Co atoms into the Fe layer. The estimated Co content is higher for lower tFe. The magnetic properties of the bilayers very close to the interface are analysed by the magneto-optical Kerr effect. The calculated Kerr intensities (which represent the magnetization process) are significantly affected by the extent of diffused Co in the Fe layer. Superconducting quantum interference device magnetic measurements demonstrate smooth and single-phase magnetic behaviour for tFe up to 6 nm and a good combination of high coercivity (6.5 kOe) and high magnetic remanence (834 emu cm−3) is obtained for tFe = 4 nm.

Microstructure and magnetic properties of L10 FePt–MgO/Fe–MgO thin films

The L10 FePt–MgO/Fe–MgO bilayers were fabricated on thermal oxide silicon substrate by using magnetron sputtering. The TEM image shows that the FePt–MgO single layer has small and uniform granular grain. In the FePt–MgO/Fe–MgO bilayer system, with increasing Fe layer thickness, the grain size become larger and the grain boundary become obscure, while the magnetization process transforms from a coherent magnetization to an exchange spring behavior, and finally a two-phase magnetization reversible process.

Effects of the elemental layer thickness on the properties of Fe/Co grown at 200 °C

Thin Fe/Co multilayers were grown at 200°C onto glass and naturally oxidized Si substrates, changing the elemental layer thickness. Onto glass substrates, the multilayers show a large in-plane uniaxial magnetocrystalline anisotropy, which strengthens by increasing the Fe layer thickness. Onto naturally oxidized Si substrates, an appreciable out-of-plane contribution to the magnetization vector is present. This can be due to the absence in the multilayer stack of a pure-Co layer as a consequence of a large intermixing occurring at the Fe/Co interfaces, that gives rise to a structure only constituted by intercalated Fe and FeCo layers. However, by increasing the Co and Fe layer thickness, the intermixing lowers because of a change in the sample morphology and microstructure, which determines the disappearance of the out-of-plane tilting of the magnetization vector while promoting the establishing of an in-plane anisotropy.

Effect of multilayer structure on cyclic performance of Si/Fe anode electrode in Lithium-ion secondary batteries

A buffer-strengthened Si/Fe multilayer film, consisting of amorphous silicon layers and polycrystalline Fe layers, is investigated as the anode for Li-ion batteries. This film can achieve a stable cycle-life performance with a high capacity. Decreasing the thickness of the Fe layer can lead to a higher capacity, which is related to the fast transport of the Li ion, but the cyclic performance deteriorates with repeated cycling. In contrast, increasing the thickness of the Fe buffer layers and the number of deposit stacks improves the cycle life with high reversibility. Because of the strain in the Si layers suppressed by the primary multilayer structure, the long-term strength is preserved and the substantial fracture toughness is enhanced by the increasing numbers of effective grain boundaries and interfacial layers. In addition, we demonstrate that the Ti underlayer promotes the electrochemical properties in the Si/Fe multilayer for various Fe layer thicknesses because of the enhanced adhesion of the interfacial electrode and current collector. The mechanically optimized Si/Fe multilayer films can have superior cycle-life performances and higher capacities. Notably, the 16-bilayer deposited electrode exhibits an excellent capacity retention of ~95% with ~204 mAh g(-1) over 300 cycles at a 1 C rate.

Monte Carlo Study of Magnetism of the Fe/Gd Multilayers: Dependence on the Layers Thickness and Interface Morphology

We present the results of the Monte Carlo simulations of Fe/Gd multilayers and a comparison of the models studied with experimental results. The Heisenberg model interactions are considered for both ferromagnetic and antiferromagnetic cases. At thermal equilibrium magnetization, the Curie temperature is investigated for different thicknesses of Fe layers and interface morphology in Fe/Gd multilayers. It turns out that the magnetic properties of the Fe/Gd multilayers strongly depend on magnetization amplitude with interface composition, the spatial arrangement of magnetic atoms, and the thickness of Fe layers.

The Influence of Quantum Size Effects on Magnetic Properties of Thin-Film Systems

Results on the investigation of magnetic and magneto-optical properties of Fe/NML/Fe (NML: Mo, Ta) thin-film systems are presented. The examined samples were prepared by DC magnetron sputtering technique. The hysteresis loops and spectral dependencies of the transverse Kerr effect (TKE) were measured employing the magneto-optical magnetometer and the magneto-optical spectrometer, respectively. The saturation field of the examined trilayers was revealed to oscillate as a function of the NML layer thickness, and the period of these oscillations to depend on the Fe layer thickness. The TKE spectra of the studied systems were found to be identical but the TKE values depend on the thickness of both Fe and Mo, Ta layers.

First-principles calculation of x-ray absorption spectra and x-ray magnetic circular dichroism of ultrathin Fe films on BaTiO3(001)

First-principles calculations of x-ray absorption spectra (XAS) and the related x-ray magnetic circular dichroism (XMCD) are presented for ultrathin Fe layers on a BaTiO${}_{3}$ (BTO) single-crystal surface within the framework of density functional theory. We have investigated the dependence of XAS and XMCD of Fe ${L}_{2,3}$ edges as a function of Fe layer thickness (1--3 ML) and on polarization direction of BTO. The calculations give a detailed insight concerning the relation between structural, electronic, and magnetic properties at the multiferroic interface Fe/BTO. In dependence on the Fe layer thickness we find characteristic features in the related XMCD, which depend strongly on the interface structure. The critical temperature of the ultrathin Fe layer is calculated to be between 170 to 230 K.

Magnetic Properties of Anisotropic Pr–Fe–B/Fe/Pr–Fe–B Films

Pr-Fe-B/Fe/Pr-Fe-B films with different Fe layer thicknesses were deposited by magnetron sputtering on Si (100) substrate heated at 650 degrees C. Structural and magnetic properties of the Pr-Fe-B/Fe/Pr-Fe-B films were investigated. X-ray diffraction and magnetic measurement results reveal that the Pr-Fe-B/Fe/Pr-Fe-B films are anisotropic when the thickness of Fe layer is smaller than 50 nm. The enhancement of the saturation magnetization in nanocomposite films is attributed to the exchange coupling between the soft and hard phases. The highest coercivity of about 13.9 kOe is achieved in the Mo(50 nm)/Pr-Fe-B(50 nm)/Mo(2 nm)/Fe(2 nm)/Mo(2 nm)/Pr-Fe-B(50 nm)/Mo(50 nm) film and increasing the thickness of soft-magnetic layer results in a continuous decreasing of the coercivity.

GROWTH AND DEPTH-DEPENDENCE OF SATURATION MAGNETIZATION OF IRON NITRIDE THIN FILMS ON MgO SUBSTRATE

We used polarized neutron reflectometry (PNR) to investigate the depth-dependent saturation magnetization (4πMs) of bi-layer structured Fe–N/Fe grown on MgO substrates, which were prepared by a facing target sputtering method followed by a subsequent annealing process to purposely study the N inter-diffusive effect. It is observed that by tuning the Fe layer thickness from 2 nm to 5 nm, the magnetic properties of the resulting product varies substantially. According to X-ray diffraction, an additional peak, indexed to Fe16N2(004) , was developed in the sample with thinner (2 nm) underlayer. Its corresponding PNR study shows a 4πMs of up to 2.82 T towards the substrate interface, which is 14% higher than the known limit ( Fe 65 Co 35 ~ 4πMs = 2.45 T ). We attribute this giant magnetization to the presence of chemically ordered Fe1N2 . We have seen evidence that the high Ms is favorably stabilized due to the lattice misfit. The thickness range is consistent with the strained region of the films. This depth-dependent saturation magnetization analysis can shed a light to help understand the previous magnetization reports by classical magnetometry methods for the samples prepared using the same underlayer.

Thickness-dependent evolution of magnetization reversal in micron-scale polycrystalline Fe rings

The evolution of magnetic switching mechanism is investigated for micron-scale polycrystalline Fe ring arrays with Fe layer thickness, tFe, varying between 10 nm and 50 nm. Single-step and double-step switching are observed for the 10 nm and 50 nm rings, with the 30 nm sample showing a transient behavior. As thickness increases, the first-step switching field, Hc1, increases, while the second-step switching field, Hc2, and remanent magnetization, Mr decreases. Magnetic force microscopy imaging and micromagnetic simulations reveal that in the reversal process, Hc1 and Hc2 correspond to the switching fields of two distinct halves of the ring. The relative separation between these two fields decides the switching behavior of the ring.