Hard Magnetic Films Research Articles

Modeling and closed loop control of a polymer composite-based hard-magnetic micromirror for optical switching applications

In this paper, a mathematical model is first established for an electromagnetic actuated polymer composite-based MEMS hard-magnetic micromirror. The model consists of the analyses of the mechanical parts for torsional scanner, the electro-coils parts and the magnetic actuator parts of the hard-magnetic films. To illustrate the effectiveness of such a model, the static and dynamic performance is further analyzed using both finite element method and it is experimentally validated. The experimental results show that our models are valid. Finally, considering the high performance requirements of optical switching applications proportional-integral-derivative (PID) controller with incomplete differentials and the integral sliding mode surface with PID form controller which is based on the established mathematical model is designed to improve its transient response. And the experimental results of set-point regulation have demonstrated that the 95% setting time is shortened from 45 to 10 ms while improving the overshoot.

Exchange coupled L10 FeCuPt/Fe heterostructures: Magnetic properties and reversal behavior at elevated temperatures

Exchange-coupled heterostructures were prepared by combining hard magnetic L10 FeCuPt alloy films with soft magnetic Fe thin films. For the FeCuPt thin film, the addition of Cu allows adjusting magnetic properties such as perpendicular magnetic anisotropy, coercivity, saturation magnetization, and Curie temperature. The coupling to thin Fe layers, providing high saturation magnetization and high Curie temperature, leads to lowering of the switching field of the hard magnetic FeCuPt layer and to a reduction of thermally induced recording errors. We have investigated the influence of the Fe layer thickness on the switching behavior of the hard layer, revealing a strong reduction for Fe layer thicknesses larger than the exchange length of Fe of about 2nm. Furthermore, superior performance of bilayer structures in comparison to single FeCuPt layers has been demonstrated by recording experiments at temperatures below 150°C, employing a scanning magnetoresistive microscope.

Combinatorial study of Fe-Co-V hard magnetic thin films

Thin film libraries of Fe-Co-V were fabricated by combinatorial sputtering to study magnetic and structural properties over wide ranges of composition and thickness by high-throughput methods: synchrotron X-ray diffraction, magnetometry, composition, and thickness were measured across the Fe-Co-V libraries. In-plane magnetic hysteresis loops were shown to have a coercive field of 23.9 kA m–1 (300 G) and magnetization of 1000 kA m–1. The out-of-plane direction revealed enhanced coercive fields of 207 kA m–1 (2.6 kG) which was attributed to the shape anisotropy of column grains observed with electron microscopy. Angular dependence of the switching field showed that the magnetization reversal mechanism is governed by 180° domain wall pinning. In the thickness-dependent combinatorial study, co-sputtered composition spreads had a thickness ranging from 50 to 500 nm and (Fe70Co30)100-xVx compositions of x = 2–80. Comparison of high-throughput magneto-optical Kerr effect and traditional vibrating sample magnetometer measurements show agreement of trends in coercive fields across large composition and thickness regions.

Exchange-coupled hard magnetic Fe-Co/CoPt nanocomposite films fabricated by electro-infiltration

This paper introduces a potentially scalable electro-infiltration process to produce exchange-coupled hard magnetic nanocomposite thin films. Fe-Co/CoPt nanocomposite films are fabricated by deposition of CoFe2O4 nanoparticles onto Si substrate, followed by electroplating of CoPt. Samples are subsequently annealed under H2 to reduce the CoFe2O4 to magnetically soft Fe-Co and also induce L10 ordering in the CoPt. Resultant films exhibit 0.97 T saturation magnetization, 0.70 T remanent magnetization, 127 kA/m coercivity and 21.8 kJ/m3 maximum energy density. First order reversal curve (FORC) analysis and δM plot are used to prove the exchange coupling between soft and hard magnetic phases.

High coercivity Sm-Co thin films from elemental Sm/Co multilayer deposition and their microstructural aspects

Hard magnetic thin films with high coercivity were fabricated by magnetron sputtering on MgO(100) and quartz substrates. The films were grown by depositing sequentially Sm and Co layers at an elevated substrate temperature of 500°C. Subsequent post-annealing was carried out at various temperatures in range of 500–700°C to form Sm-Co hard magnetic thin films. X-ray diffraction studies revealed the formation of randomly oriented SmCo5 crystallites on quartz substrate, whereas, a textured growth of Sm2Co7 with strong (110) crystalline phases was observed on MgO substrate. Microstructural analyses were carried out using Transmission Electron Microscopy (TEM) for samples grown on MgO substrate at 650°C and inferred the presence of high density planar defects along with large grain boundaries. Further microdiffraction studies confirmed the presence of SmCo3 as an impurity phase in the films. Magnetic hysteresis measurements indicate the square hysteresis behaviors with high coercivity value of 3.1T and 2.7T for 650°C annealed samples on both MgO and quartz substrates, respectively. The origin of such high coercivity value was then correlated with pinning type of spin reversal mechanism as confirmed through the analyses of demagnetization curves. The magnetic force microscopy images for films on MgO substrate, annealed at 650°C, revealed the presence of magnetic domains with size higher than 1µm. The formed magnetic domains lacked well defined boundaries indicating an enhanced exchange coupling between the grain clusters.

Gas flow sputtering for manufacture of high quality hard magnetic films

Sputter deposition of magnetic material provides some challenges. In the case of soft magnetic materials, usually thin targets and strong magnetic arrays of the sputtering cathode are used for an effective magnetron sputtering. In the case of hard magnetic material, the situation is different. The magnetic field lines are closed in the target and the magnetron is effectively acting as a diode sputtering system. Hollow cathode processes in contrast require no magnets at all to enhance the ionization. Therefore, high rate deposition of hard magnetic films becomes possible.In this paper results of gas flow sputtering (GFS), a modification of hollow cathode processes, for deposition of hard magnetic CoSm films are presented. Especially for the deposition of thick films (10μm and more) gas flow sputtering is an economic process. Using GFS high dynamic deposition rates of several 10μm per hour are achievable. The influence of the deposition parameters on the magnetic properties coercivity and magnetic remanence are discussed.

A new method for quantifying the nanoscale magnetic domains

Abstract In this work, a signal transformation method to quantitatively analyze fine magnetic structures in nanometer length scales measured by magnetic force microscopy has been developed. Nanosized magnetic domains, magnetic charges with reconstructed polarity as well as quantified magnetic field contours of samples (such as ordered FePt dot arrays, hard magnetic thin films and polycrystalline La0.7Sr0.3 MnO3 (LSMO) films ) were investigated based on the basic principles of deconvolution and micromagnetics. The present technique is crucial for the analysis of fine magnetic structures, and is important for the development of next generation magnetic recording industry.

A model for asymmetric magnetoimpedance effect in multilayered bimagnetic films

The magnetoimpedance in three-layered bimagnetic film structure is studied theoretically. The structure consists of the soft and hard magnetic films separated by highly conductive non-magnetic layer. A model to describe the magnetoimpedance effect in the film structure based on a simultaneous solution of linearized Maxwell equations and Landau–Lifshitz equation is proposed. It is shown that magnetostatic coupling between the magnetic layers results in the asymmetry in the field dependence of the film impedance. The magnetostatic coupling is described in terms of an effective bias field appearing in the soft magnetic layer. The calculated field and frequency dependences of the film impedance are shown to be in a qualitative agreement with previous results of experimental studies of the asymmetric magnetoimpedance in NiFe/Cu/Co film structures. The results obtained may be useful for development of weak magnetic-field sensors.

Demagnetization dynamics of C-doped FePt film

Magneto-optical information storage has been a hot research subject for several years. FePt exhibits abundant physical properties and has received much attention as a candidate material. Its alloy film with perpendicular anisotropy and small grain size has important applications in magnetic recordings due to the large intrinsic magnetic anisotropy which ensures long-time thermal stability of nanometer sized bits. However, the large coercive field of FePt is a significant factor that hinders its application. As is well known, the magnetic anisotropy in FePt alloy can be precisely modulated by carbon-doping, and as a result, the coercive field of FePt film can be modified effectively with the carbon dopant. On the other hand, the microscopic mechanism of magnetic storage relies on the motion of spin system. Ultrashort femtosecond laser has been demonstrated to be a very effective tool to investigate the dynamical coupling among different degrees of freedom, such as electron, spin and lattice in a ferromagnetic film. The research on spin dynamics has become a new frontier of condensed matter physics, which is crucial for ultrafast magnetic recording materials. In this work, by using the time-resolved magneto-optical Kerr effect spectroscopy, we study the ultrafast spin dynamics of two FePt alloy films with different carbon dopants under the applied magnetic field along the film surface. The FePt alloy films with different carbon dopants are fabricated on silicon substrates by the sputtering method. The main experimental findings in this work are as follows. (i) The transient Kerr signal is linearly proportional to the magnetization with the magnetic field up to 0.8 T, while the transient reflectivity of the film is independent of the applied magnetic field. (ii) For FePt alloy films with different coercive fields, it is found that the demagnetization time of the film with smaller coercive field is significantly faster than that of the larger counterpart: the former shows 0.8 ps demagnetization time, and the latter has a magnitude of 1.2 ps. The demagnetization times for both soft and hard magnetic films are independent of the applied magnetic field. (iii) With ultrafast laser pulse radiation, we observe the propagation of acoustic phonon with a resonance frequency of ～ 49 GHz, and the frequency of the acoustic phonon is independent of the applied magnetic field. From the above, the spin dynamics of the samples shows strong correlation with carbon-doping. Our experimental findings are desired for basic research as well as for the design and development of novel magneto-optical devices.

Some Aspects of Magnetic Force Microscopy of Hard Magnetic Films

A number of aspects of magnetic force microscopy (MFM) specific to the imaging of hard magnetic films have been studied. Firstly, we show that topographic images made in tapping mode with probes characterized by the moderate cantilever stiffness usual for MFM (1--4 N/m), contain artifacts due to strong probe-sample interactions which lead to probe retraction. As a result, stiffer cantilevers (e.g. 40 N/m) are better adapted to characterizing such hard magnetic films. Secondly, imaging with probes coated by a hard magnetic film leads to phase maps which show a twofold symmetry, with paired dark/light contrast on opposite domain edges along the direction of the cantilever. This is due to the tilt of the direction of tip magnetizaiton and direction of oscillation, with respect to the sample normal. Thirdly, due to the long-range nature of the stray field produced by hard magnetic films containing micron-sized domains, MFM phase contrast reflects the stray field itself, as opposed to that of its spatial derivatives, as is generally the case in MFM.

Structural influence of copper substrate on magnetic properties of electrodeposited CoPtP films for MEMS applications

Hard magnetic CoPtP films for MEMS application were electrodeposited on the various structures of copper substrate such as copper foil, copper seed layer and nano porous copper layer. The structural influence of copper substrates provided a reasonable explanation for the correlation of the magnetic properties of electrodeposited CoPtP alloy, because magneto-crystalline anisotropy could account for the enhancement of the perpendicular magnetic properties (Coercivity in highest range is achieved from 9.1 to 14.9 KOe) of the films. Also the influence of deposition current density on the chemical composition that changes the growth morphology, microstructure, and magnetic properties are studied.

Magnetic properties of sputtered anisotropic Pr–Fe–B thin films with different structures and antiferromagnetic materials

Anisotropic Pr–Fe–B films with soft-magnetic layer (Fe) and/or antiferromagnetic layer (Mn, FeMn or MnO) were prepared by direct-current (DC) magnetron sputtering on Si (100) substrates heated at 650 °C. The influence of four types’ different structures on the magnetic properties of Pr–Fe–B films was investigated. The phase and magnetic properties were characterized by means of X-ray diffraction (XRD) and superconducting quantum interference device (SQUID). Addition of antiferromagnetic layer enhances both the coercivity and the remanence ratios of Pr–Fe–B films with suitable structures. The interface number increases and the antiferromagnetic–ferromagnetic exchange interaction is likely to become stronger, which affect the improvement of magnetic properties. To further understand the influence of structures with soft-magnetic Fe layer and/or antiferromagnetic FeMn layer on the magnetic properties of Pr–Fe–B hard-magnetic films, the thickness of Pr–Fe–B layer was designed to decrease from 600 to 50 nm. The improvement of magnetic properties becomes obvious in Mo(50 nm)/Pr–Fe–B(25 nm)Mo(2 nm)FeMn(20 nm)Mo(2 nm)Pr–Fe–B(25 nm)/Mo(50 nm) film.

A review of the hybrid techniques for the fabrication of hard magnetic microactuators based on bonded magnetic powders

Polymer composites based on permanent magnetic bonded powders exhibit immense potential for applications in microactuators and sensors with magnetic performances comparable to their fully dense counterparts. While fabrication and integration of magnetic devices based on bonded magnetic powders is challenging via conventional deposition and electrochemical growth techniques, hybrid fabrication offers a promising alternative. This paper presents the evolution of permanent magnetic materials into bonded magnetic powders, the magnetic performance figures of merit of permanent magnetic materials significant for the design and manufacture of polymer based sensors and actuators. A review of the hybrid fabrication techniques such as replica molding, squeegee coating, spin casting etc are reported. Critical factors affecting the fabrication of polymer magnetic composites such as filler particle size and effect of magnetic field during fabrication are discussed. Prior art based on polymer magnetic composites for the fabrication of hard magnetic films and hard magnetic actuators are presented.

A Simple Process to Fabricate Micro Flux Sources with High Magnetic Field Gradient

In this paper, we present a fabrication process to produce the micro-sized magnetic structures based on the hard magnetic powders. Under the magnetic field originated from a micro patterned hard magnetic film, these magnetic powders are magnetically aligned to form arrays of the micro magnets on a polydimethysiloxane (PDMS) substrate. The high magnetic field gradient and stable magnetic flux can be obtained at certain micro-sized area on the surface of the micro magnets. The fabricated structures have been used for trapping iron oxide particles. Generally this fabrication process is simple, low cost and the micro magnets can be used for further applications in biology, medicine and beyond.

Micro-magnet arrays for specific single bacterial cell positioning

In various contexts such as pathogen detection or analysis of microbial diversity where cellular heterogeneity must be taken into account, there is a growing need for tools and methods that enable microbiologists to analyze bacterial cells individually. One of the main challenges in the development of new platforms for single cell studies is to perform precise cell positioning, but the ability to specifically target cells is also important in many applications. In this work, we report the development of new strategies to selectively trap single bacterial cells upon large arrays, based on the use of micro-magnets. Escherichia coli bacteria were used to demonstrate magnetically driven bacterial cell organization. In order to provide a flexible approach adaptable to several applications in the field of microbiology, cells were magnetically and specifically labeled using two different strategies, namely immunomagnetic labeling and magnetic in situ hybridization. Results show that centimeter-sized arrays of targeted, isolated bacteria can be successfully created upon the surface of a flat magnetically patterned hard magnetic film. Efforts are now being directed towards the integration of a detection tool to provide a complete micro-system device for a variety of microbiological applications.

Micro-magnetic imprinting of high field gradient magnetic flux sources

We report here on the fabrication of hard magnetic powder based micro-flux sources using micro-patterned hard magnetic films as templates or master structures. The micro-magnetic imprinting (μMI) process is simple and the constituent materials of the final structures, commercial hard magnetic powders and polymer, are inexpensive. The structures may be transparent, and either flexible or rigid, depending on the choice of polymer matrix used. The peak-to-peak intensity of the z-component of the stray magnetic field measured above a test μMI structure made with spherical NdFeB particles of average particle size 16 μm is in good agreement with simulated field values (150 mT at 5 μm). Simulations indicate magnetic field gradients of up to 5 × 105 T/m at the surface of such μMI structures. The trapping of cells functionalised with superparamagnetic beads by these structures has been demonstrated. The μMI fabrication technique has much potential for the development of high field gradient magnetic flux sources for applications in biology and beyond.

Imprinting of fine-scale magnetic patterns in electroplated hard magnetic films using magnetic foil masks

This paper presents a batch-fabrication approach to imprint complex magnetic pole patterns (perpendicular north/south poles) into hard magnetic films using fine-scale, laser-machined iron foil “masks.” In this approach, pulsed magnetic fields varying between 0.1 and 1.0 T are used in conjunction with the field-concentrating effect of the iron foil masks to selectively reverse the magnetization of selected regions of 10-μm-thick Co80Pt20 films (Br = 1 T, Hci = 340 kA/m). Simple stripe array patterns are systematically studied both experimentally and via simulation, achieving patterned feature sizes (poles) down to 30 μm. Double exposures are also explored to create checkerboard patterns from simple stripe-like masks. Finally, masks with more complex patterns are used to demonstrate the viability of the approach to imprint arbitrary features.

Influence of temperature on the magnetic properties of electroplated Co-rich Co–Pt thick films

In this paper, the magnetic properties of 10 µm thick Co-rich Co–Pt films (∼80:20 atomic ratio) hard magnetic films electroplated on silicon substrates are characterized at temperatures from 300 to 1000 K. With an increase in temperature up to 500 K, the coercivity and remanence of the films both decrease rapidly, dropping to only 10% of their respective as-deposited values. Above 500 K, the coercivity and remanence continue declining but at a slower rate, reaching nearly zero at 1000 K. Conversely, the saturation magnetization reduces by only 15% at 800 K and 40% at 1000 K from the as-deposited value. In addition to these measurements at elevated temperature measurements, thermal cycling tests are performed to examine the influence of various thermal exposures. Cycles up to 500 K are shown to have little impact on the film morphology but notably reduce the layer adhesion. Most importantly, a thermal cycle to just 400 K is shown to essentially destroy the hard magnetic properties. Because of these temperature sensitivities, Co-rich Co–Pt films may be significantly limited for end applications.

Microfluidic immunomagnetic cell separation using integrated permanent micromagnets

In this paper, we demonstrate the possibility to trap and sort labeled cells under flow conditions using a microfluidic device with an integrated flat micro-patterned hard magnetic film. The proposed technique is illustrated using a cell suspension containing a mixture of Jurkat cells and HEK (Human Embryonic Kidney) 293 cells. Prior to sorting experiments, the Jurkat cells were specifically labeled with immunomagnetic nanoparticles, while the HEK 293 cells were unlabeled. Droplet-based experiments demonstrated that the Jurkat cells were attracted to regions of maximum stray field flux density while the HEK 293 cells settled in random positions. When the mixture was passed through a polydimethylsiloxane (PDMS) microfluidic channel containing integrated micromagnets, the labeled Jurkat cells were selectively trapped under fluid flow, while the HEK cells were eluted towards the device outlet. Increasing the flow rate produced a second eluate much enriched in Jurkat cells, as revealed by flow cytometry. The separation efficiency of this biocompatible, compact micro-fluidic separation chamber was compared with that obtained using two commercial magnetic cell separation kits.

Effects of Intermediate Layers on Chemical Ordering and Hard Magnetic Properties of L1<sub>0</sub> FePt Films

The FePt (30 nm)/NiO (2 nm) bilayer films with or without an intermediate layer were deposited on Si substrate at ambient temperature by magnetron sputtering. The films were then post-annealed at 700 °C for 3 min by Rapid Thermal Annealing (RTA) with a high heating ramp rate of 100 °C/sec. The perpendicular coercivity (Hc), in-plane coercivity (Hc//), perpendicular squareness (S) and in-plane squareness (S//) of the film with no intermediate layer are 8.9 kOe, 9.0 kOe, 0.48 and 0.60, respectively. Its chemical ordering is around 0.82. This degrades to 0.62 and 0.48 respectively when Pt and Pd intermediate layers of 5 nm thick are inserted between the FePt and NiO layers. However, when an Ag intermediate layer with a thickness of 5 nm is introduced, the chemical ordering of the film is further increased to 0.91. Its Hc, Hc//, S and S// also increases to 10.2 kOe, 10.6 kOe, 0.62 and 0.68, respectively. The reason for the enhancement in chemical ordering and hard magnetic properties of the FePt film is mainly due to the fact that Ag atoms are immiscible with both Fe and Pt atoms. Instead, Ag tends to segregate at the grain boundary of FePt ; this increases the grain boundary energy, promoting the transformation of soft magnetic fcc FePt into hard magnetic fct L10 FePt films.