Soft Magnetic Layer Research Articles

High-resolution dichroic imaging of magnetic flux distributions in superconductors with scanning x-ray microscopy

The penetration of magnetic flux into high-temperature superconductors has been observed using a high-resolution technique based on x-ray magnetic circular dichroism. Superconductors coated with thin soft-magnetic layers are observed in a scanning x-ray microscope under the influence of external magnetic fields. Resulting electric currents in the superconductor create an inhomogeneous magnetic field distribution above the superconductor and lead to a local reorientation of the ferromagnetic layer. Measuring the local magnetization of the ferromagnet by x-ray absorption microscopy with circular-polarized radiation allows the analysis of the magnetic flux distribution in the superconductor with a spatial resolution on the nanoscale.

Magnetization dynamics in an exchange-coupled NiFe/CoFe bilayer studied by x-ray detected ferromagnetic resonance

Exchange-coupled hard and soft magnetic layers find extensive use in data storage applications, for which their dynamical response has great importance. With bulk techniques, such as ferromagnetic resonance (FMR), it is difficult to access the behaviour and precise influence of each individual layer. By contrast, the synchrotron radiation-based technique of x-ray detected ferromagnetic resonance (XFMR) allows element-specific and phase-resolved FMR measurements in the frequency range 0.5–11 GHz. Here, we report the study of the magnetization dynamics of an exchange-coupled Ni0.81Fe0.19 (43.5 nm)/Co0.5Fe0.5 (30 nm) bilayer system using magnetometry and vector network analyser FMR, combined with XFMR at the Ni and Co L2 x-ray absorption edges. The epitaxially grown bilayer exhibits two principal resonances denoted as the acoustic and optical modes. FMR experiments show that the Kittel curves of the two layers cannot be taken in isolation, but that their modelling needs to account for an interlayer exchange coupling. The angular dependence of FMR indicates a collective effect for the modes of the magnetically hard CoFe and soft NiFe layer. The XFMR precessional scans show that the acoustic mode is dominated by the Ni signal with the Co and Ni magnetization precessing in phase, whereas the optical mode is dominated by the Co signal with the Co and Ni magnetization precessing in anti-phase. The response of the Co signal at the Ni resonance, and vice versa, show induced changes in both amplitude and phase, which can be ascribed to the interface exchange coupling. An interesting aspect of phase-resolved XFMR is the ability to distinguish between static and dynamic exchange coupling. The element-specific precessional scans of the NiFe/CoFe bilayer clearly have the signature of static exchange coupling, in which the effective field in one layer is aligned along the magnetization direction of the other layer.

Influence of Temperature on Magnetic Properties of Tb26Co74/Co/Fe20Ni80Films with Exchange Bias

Magnetic multilayers with unidirectional magnetic anisotropy, which reveals itself as a shift of the hysteresis loop of the soft magnetic layer along the magnetic eld axis, have been studied intensively for the last two decades [1 6]. Thin lms consisting of exchange coupled Fe20Ni80 and Tb Co layers are of substantial fundamental and applied interest [1 6]. Due to the proper combination of magnetic and magnetoresistive properties such lms have been used in magnetic sensors [5]. Previously hysteresis properties of these layered structures were shown to be determined mostly by the character and e ectiveness of the interlayer coupling, which in turn is de ned by structural and chemical condition of interfaces. Interlayer region can be modi ed arti cially by introduction of the ultrathin nonmagnetic spacer [4] or by selective annealing of the seed permalloy layer [5]. This paper is devoted to the study of in uence of temperature on unidirectional anisotropy in exchange coupled sandwiches based on Tb26Co74 ferrimagnetic layer and Fe20Ni80 soft magnetic layer separated by the ultrathin Co spacer.

Magnetic and Structural Characterization of NiFe/Fe<sub>30</sub>Co<sub>70</sub> Bilayers

Magnetostrictive films are required for a wide range of device applications; by increasing the magnetostriction constant and decreasing the anisotropy field, the devices will become more efficient. This paper has studied Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">30</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">70</sub> films on different thicknesses of the soft magnetic underlayer Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">81</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sub> , to determine how the structural and magnetic properties change. It was found that the anisotropy field of the Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">30</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">70</sub> film could be reduced by 50% to 10 kA/m and the magnetostriction constant increased by a factor 4 to 65 ppm when grown on 30 nm Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">81</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sub> . This was due to the NiFe underlayer inducing a BCC(110) texture within the Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">30</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">70</sub> film and reducing the in-plane stress.

Model Analysis of Magnetic Write Head for Shingled Thermally Assisted Magnetic Recording

Magnetic write heads were modeled and analyzed for thermally assisted magnetic recording combined with shingled recording, targeting an areal density of 8 Tbit/in 2 and using the Storage Research Consortium (SRC, Japan) specifications. The head design has an indentation for the near field transducer and has a large recording field at the heat spot. Micromagnetic media recording simulations revealed that an areal density of more than 8 Tbit/in 2 was possible with shingled recording, compared with 4.4 Tbit/in 2 in a nonshingled system. Also, micromagnetic recording head simulations showed that reducing the magnetic head dimensions was effective for high frequency response. Finally, it is shown that a soft magnetic underlayer works quite well at high frequency to enhance the recording field.

Modification of Fe Alloys by Magnetic Field

Electrodeposition is a very promising alternative to the physical vapor deposition techniques (PVD) to produce soft magnetic layers and microstructures. The advantage of electrodeposition in comparison to PVD processes is the fact that it is an inexpensive method.Moreover, it allows depositing high aspect ratio layers with a thickness ranging from a few monolayers up tomore than 1 mm onto a complex geometry substrate.The deposited layer properties could be tailored by a proper selection of the process parameters. A superposition of an external magnetic field during the electrodeposition has gained a considerable attention during the past decade, since it can affect the deposit properties. Mainly the morphology of the deposited layers is infuenced. The electrodeposition process of Fe-Ni films was examined in a presence of homogenous magnetic field(B: up to 0,6 T). Sulphate nickel and sulphate iron salt solution was used with pH 4,3.The changes of surface morphology of Fe films electrodeposited without a magnetic field is observed.The magnetic field superimposing perpendicular to the electric field leads to the hydrogen evolution promotion by MHD convection. This enhances the hydrogen evolution and the changes in Fe ions mass transfer appear near electrode surface. A change of local pH at the electrode is present and formation of different morphology of Fe layer.

Use of Pd–Fe and Ni–Fe–Nb as Soft Magnetic Layers in Ferromagnetic Josephson Junctions for Nonvolatile Cryogenic Memory

Josephson junctions containing ferromagnetic layers are under consideration as the basic elements for cryogenic random access memory. For memory applications, either the amplitude of the critical current or the phase shift across the junction must be controllable by changing the direction of magnetization of one or more of the ferromagnetic layers in the junction. We have measured the critical currents in large-area Josephson junctions containing three ferromagnetic layers. These junctions carry spin-triplet supercurrent. This work addresses the choice of material and optimum thickness for the one soft magnetic layer in such junctions. We have used either a Pd-Fe or Ni-Fe-Nb alloy for the soft layer, and find hysteresis in the low-field “Fraunhofer patterns” due to magnetic switching of the soft layer. The critical current is one order of magnitude smaller in the junctions containing the Ni-Fe-Nb alloy compared to those containing Pd-Fe alloy, which is probably due to strong spin-memory loss in the former. While the large-area junctions studied here are not suitable for memory applications, these experiments lay the groundwork for future studies of submicron junctions where the magnetic state of the junction can be controlled by shape anisotropy.

Noise reduction by magnetostatic coupling in geomagnetic-field sensors

A new magnetoresistive (MR) thin film with a structure of “antiferromagnetic layer/pinned soft magnetic layer/non-magnetic MgO spacer layer/sensitive NiFe layer” was designed. The barber-pole MR elements with a Wheatstone bridge circuit were fabricated using photolithographic methods. The testing results show that, in comparison to the element with a typical structure of Ta/NiFe/Ta, the fabricated MR element shows significant reduction in the Barkhausen noise and the 1/f noise and good magnetic stability while maintaining high magnetic field sensitivity. This element with improved signals can be attributed to the magnetostatic coupling between the pinned soft magnetic layer and the sensitive NiFe layer, which can act as a small stabilizing field, leading to the coherent rotation of magnetic moment in the sensitive NiFe layer.

Potential of Metallic Glass Thin Films as a Soft Magnetic Underlayer for &lt;inline-formula&gt; &lt;tex-math notation="TeX"&gt;${\rm L}1_{0}$ &lt;/tex-math&gt;&lt;/inline-formula&gt; FePt-Based Recording Media

We report on the use of metallic glass (FeHfNbYB) thin film as a soft magnetic underlayer for the fabrication of L1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> FePt-based bit patterned recording media (BPM). Preferred oriented phase of L1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> (111) FePt is obtained either by depositing it in situ on a heated soft magnetic metallic glass/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Si or at room temperature (FePt/metallic glass/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Si) followed by annealing at 400 °C-450 °C for 20 min. Fabrication of recoding bits of size ~2 μm to 20 nm is demonstrated using electron beam lithography. Magnetic measurements on ~500 nm bit patterns show a narrow switching field (SF) distribution with SF of ~3 kOe. The ability to read/write this BPM is demonstrated with a commercial head in perpendicular geometry in a static tester. Results strongly suggest that the soft magnetic metallic glass thin films can solve the problem of writability in high anisotropy L1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> FePt-based recording media.

Peculiarities of magnetization reversal in exchange-coupled ferro/ferromagnet NiFe/CoP film structures

The effect of layer thicknesses on the magnetic properties and mechanism of magnetization reversal in exchange-coupled NiFe/CoP film structures has been studied. The process of magnetization reversal was studied by analysis of the magnetic-induction and magneto-optical hysteresis loops. It is established that, as the thicknesses of layers in the NiFe/CoP film structure are increased, the system exhibits a transition from homogeneous magnetization reversal in the structure to exchange spring formation in the soft magnetic layer.

Manipulation by exchange coupling in layered magnetic structures

Exchange coupling in magnetic heterostructures can be modified via introduction of additional magnetic spacer layers at the interfaces. The magnetic characteristics and the spacer layer thickness determine the functional properties of the whole system. We show that the hysteresis loop area of trilayer spring magnets with two different soft magnetic layers (s1, s2) and one hard magnetic layer (h) with the sequence s1/s2/h can be increased as compared to both bilayer structures s1/h and s2/h with the same total thickness of the soft layers and for definite thickness ratios of the soft layers and their sequences. For ferrimagnetic spin valves, the perpendicular exchange bias effect can be tuned via the thickness of non-magnetic spacer layers at the interface, which determine the exchange coupling between ferrimagnets. A simple quasi one-dimensional phenomenological model is able to describe the magnetic hysteresis of even complex layered structures and to predict optimal geometrical and magnetic parameters of such heterostructures.

Influence of soft magnetic underlayers on the magnetic properties of Co90Fe10 films

The structure and magnetic properties of thin Co90Fe10 films have been studied to determine how different soft magnetic underlayers (Fe81B13.5Si3.5C2 (Metglas) and Ni81Fe19) influence these properties with the aim to achieve soft fcc Co90Fe10 films with large magnetostriction constants. The thickness of the Co90Fe10 films was 25nm with the thickness of the magnetic underlayer ranging from 15nm to 35nm. The effect of applying a magnetic field during the growth of both layers was also investigated. From X-ray diffraction, it was found that the Co90Fe10 films grown on NiFe had lower in-plane stresses compared to those grown on silicon and Metglas. While the coercive fields of all the Co90Fe10 films were smaller than the monolith Co90Fe10 film, the magnetostriction constants were strongly dependent on the underlayer they were grown on. Thus it is possible to tune the magnetostriction constant of the Co90Fe10 film to be positive or negative by selecting the correct soft magnetic underlayer.

Magnetic Properties of FePt based Nanocomposite Thin Films Grown on Low Cost Substrates

We report a systematic investigation of temperature dependent magnetic properties of FePt single and FePt(30)/M/Fe(5) nanocomposite thin films prepared by sputtering technique on low cost substrates at ambient temperature and post annealed at different temperatures. With increasing annealing temperature, L10 ordering, hard magnetic properties and thermal stability of FePt films are improved. The formation of interlayer exchange coupling between hard and soft magnetic layers in FePt/M(Al,Cu,C)/Fe films depends strongly on interlayer materials and interface morphology. A strong interlayer exchange coupling was achieved when the C interlayer thickness was about 0.5nm, which enhances saturation magnetization largely. Also, the magnetization reversal process changes from incoherent to coherent switching process, which results a single hysteresis loop. High temperature magnetic studies revealed that the effective reduction in the coercivity decreases from 34Oe/K to 13Oe/K by the introduction of a thin C(0.5nm) layer in FePt/C/Fe film. This reveals a promising approach to improve the stability of hard magnetic properties at high temperatures, suitable for high temperature magnetic applications.

Spin configuration of magnetic multi-layers: effect of exchange, dipolar and Dzyalozhinski–Moriya interactions

We investigate the effect of coupling (intensity and nature), applied field, and anisotropy on the spin dynamics of a multi-layer system composed of a hard magnetic layer coupled to a soft magnetic layer through a nonmagnetic spacer. The soft layer is modeled as a stack of several atomic planes while the hard layer, of a different material, is either considered as a pinned macroscopic magnetic moment or again as a stack of atomic planes. We compute the magnetization profile and hysteresis loop of the whole multi-layer system by solving the Landau–Lifshitz equations for the net magnetic moment of each (atomic) plane. We study the competition between the intra-layer anisotropy and exchange interaction, applied magnetic field, and the interface exchange, dipolar or Dzyalozhinski–Moriya interaction. Compared with the exchange coupling, the latter two couplings present peculiar features in the magnetization profile and hysteresis loop that may help identify the nature of the interface coupling in multi-layer magnetic systems.

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.

Investigations on the removal mechanisms of diverse alumina based polishing slurries for chemical mechanical polishing of electro-plated NiFe 45/55

Magnetic micro electro–mechanical systems (MEMS), such as magnetic sensors or actuators, are used in various areas of application, including bio-medical, wireless communication or power supplies. Many functional elements of these systems are made of soft magnetic materials. The increasing demands on miniaturization and higher efficiency of these functional elements require the application of thin-film structuring (photolithographic patterning) and electroplated magnetic materials. As a result, micro patterns filled with relatively thick magnetic materials (∼10–40 μm) are desired. However, the deposition of thicker soft magnetic layers (NiFe, CoFe) is restricted by a strong increase in roughness (edge overflow) as well as the formation of internal stress resulting in warping or delamination of electroplated structures. An effective technique that reduces surface roughness and thus allows the manufacture of multi-layered, multi-material systems is the planarization of each layer by chemical mechanical polishing (CMP). CMP of common semiconductor materials, such as silicon and its oxides, copper and tungsten is well-investigated, but there is almost no published research on CMP of soft magnetic materials, especially regarding iron based alloys (NiFe, CoFe). Therefore, the development of a reliable and efficient CMP process for these materials can be a crucial drive for improving functionality, decreasing dimensions and extending the application field of magnetic MEMS. In this paper, three commercial alumina based polishing slurries with different chemical compositions are tested. Electroplated NiFe 45/55 thin-film structures on silicon are polished under constant experimental conditions. During the experiments, surface roughness and material removal rate (MRR) are determined. In order to expose the underlying effects of material removal for each of the slurries, randomly selected sections of the NiFe surface are analyzed in detail using atomic force microscopy (AFM) before and after the experiments. The impact of the chemical components of the slurries on the mechanical properties of exposed surfaces is investigated using nanoindentation techniques. The results permit to identify the impact of the composition of the used slurry on the determinant removal mechanism in CMP of NiFe 45/55. These results can be applied as well to most technically used NiFe alloys.

RF noise suppressor using FeCoNiB soft magnetic thin films on the microstrip line

A radio-frequency noise suppressor based on a microstrip line using FeCoNiB magnetic thin film is presented, whose noise suppression effects have been investigated. The FeCoNiB thin films with excellent soft magnetic properties and high-frequency characteristics were synthesized by the RF magnetron sputtering. It is found that the intensity and frequency of RF transmission signal attenuation can be adjusted by controlling the geometric dimension of the soft magnetic FeCoNiB thin film and SiO2 dielectric layer, owing to the L–C resonance, eddy current loss and FMR loss. Appropriate nickel content in FeCoNiB magnetic thin film is propitious to RF noise suppression characteristics. The maximum power loss of RF noise suppressor achieves 0.87 with the nickel content is 0.09. It indicates that the mentioned FeCoNiB thin film RF noise suppressors have great potential for the application of electromagnetic interference in the gigahertz range.

Programmable Input for Nanomagnetic Logic Devices

A programmable magnetic input, based on the magnetic interaction of a soft and hard magnetic layer is presented for the first time. Therefore, a single-domain Co/Pt nanomagnet is placed on top of one end of a permalloy bar, separated by a thin dielectric layer. The permalloy bar of the introduced input structure is magnetized by weak easy-axis in-plane fields. Acting like a 'magnetic amplifier', the generated fringing fields of the permalloy pole are strong enough to control the magnetization of the superimposed Co/Pt nanomagnets, which have high crystalline perpendicular magnetic anisotropy. This magnetostatic interaction results in a shift of the hysteresis curve of the Co/Pt nanomagnet, measured by magneto-optical Kerr microscopy. The Co/Pt nanomagnet is fixed by the fringing field of the permalloy and thereby not a!ected by the magnetic power clock of the Nanomagnetic Logic system. MFM measurements verify the functionality of the programmable magnetic input structure. The fringing fields are extracted from micromagnetic simulations and are in good agreement with experimental results. The introduced input structure enables switching the logic functionality of the majority gate from NAND to NOR during runtime, o!ering programmable Nanomagnetic Logic.

Magnetic dispersion in a soft amorphous layer with a helical anisotropy profile

A new analysis method of the magnetization dispersion in a thin magnetic film is presented. It is based on the angular measurement of the permeability spectra and on the evaluation of the integral relation. It provides the average orientation of the magnetization in the layer and a dispersion parameter which quantifies the magnetic dispersion. The method is successfully applied on a soft CoNbZr 800nm magnetic layer which possesses a helical anisotropy profile. This helical profile is obtained by rotating continuously the sample during the sputtering deposition on a scale from R = 0 to 16 turns. The study reveals that, for about 1/2 turn, a maximal dispersion is achieved and, for more elevated rotation speed, the magnetization no longer follows the anisotropy profile but lines up along an easiest axis direction. The experimental data are well described by a one-dimensional micromagnetic model which takes both exchange coupling and a helical anisotropy into account. The analytical cases with an exchange constant null and infinite are also considered in order to gain more insight onto the observed magnetic behaviour in the soft magnetic thin film.

Magnetic Properties and Microstructure of FeOx/Fe/FePt and FeOx/FePt Films

The Fe(6 nm)/FePt film with perpendicular magnetization was deposited on the glass substrate. To study the oxygen diffusion effect on the coupling of Fe/FePt bilayer, the plasma oxidation with 0.5~7% oxygen flow ratio was performed during sputtered part of Fe layer and formed the FeOx(3 nm)/Fe(3 nm)/FePt trilayer. Two‐step magnetic hysteresis loops were found in trilayer with oxygen flow ratio above 1%. The magnetization in FeOx and Fe/FePt layers was decoupled. The moments in FeOx layer were first reversed and followed by coupled Fe/FePt bilayer. The trilayer was annealed again at 500°C and 800°C for 3 minutes. When the FeOx(3 nm)/Fe(3 nm)/FePt trilayer was annealed at 500°C, the layers structure was changed to FeOx(6 nm)/FePt bilayer due to oxygen diffusion. The hard‐magnetic FeOx(6 nm)/FePt film was coupled with single switching field. The FeOx/(disordered FePt) layer structure was observed with further annealing at 800°C and presented soft‐magnetic loop. In summary, the coupling between soft‐magnetic Fe, FeOx layer, and hard‐magnetic L10 FePt layer can be controlled by the oxygen diffusion behavior, and the oxidation of Fe layer was tuned by the annealing temperature. The ordered L10 FePt layer was deteriorated by oxygen and became disordered FePt when the annealed temperature was up to 800°C.