Hard Magnetic Films Research Articles

Electrochemical Growth of Hard Magnetic Films: Co-Pt and Fe-Pt

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Micro-patterning of NdFeB and SmCo magnet films for integration into micro-electro-mechanical-systems

The integration of high-performance RE-TM (NdFeB and SmCo) hard magnetic films into micro-electro-mechanical-systems (MEMS) requires their patterning at the micron scale. In this paper we report on the applicability of standard micro-fabrication steps (film deposition onto topographically patterned substrates, wet etching and planarization) to the patterning of 5–8 μm thick RE-TM films. While NdFeB comprehensively fills micron-scaled trenches in patterned substrates, SmCo deposits are characterized by poor filling of the trench corners, which poses a problem for further processing by planarization. The magnetic hysteresis loops of both the NdFeB and SmCo patterned films are comparable to those of non-patterned films prepared under the same deposition/annealing conditions. A micron-scaled multipole magnetic field pattern is directly produced by the unidirectional magnetization of the patterned films. NdFeB and SmCo show similar behavior when wet etched in an amorphous state: etch rates of approximately 1.25 μm/min and vertical side walls which may be attributed to a large lateral over-etch of typically 20 μm. Chemical–mechanical-planarization (CMP) produced material removal rates of 0.5–3 μm/min for amorphous NdFeB. Ar ion etching of such films followed by the deposition of a Ta layer prior to film crystallization prevented degradation in magnetic properties compared to non-patterned films.

Polarized Neutron Reflectometry for the Analysis of Nanomagnetic Systems

In recent years, polarized neutron reflectometry (PNR) has played an essential role for the exploration of magneto- and spintronic structures. Well known systems extensively studied include exchange coupled magnetic superlattices, exchange bias systems between ferromagnetic and antiferromagnetic films, exchange spring valves between soft and hard magnetic films, and more recently magnetic semiconductors and ferromagnetic Heusler alloy films and superlattices. In addition to studies of laterally extended layered systems, neutron scattering has now been applied to the exploration of periodic magnetic arrays, such as stripes and islands on the submicrometer scale. Although the competition with magneto-optics and X-ray resonant magnetic scattering (XRMS) has increased in recent years, there are some advantages PNR offers that are hard to challenge. One of those is the quantitative analysis of the data via fits to theoretical models based on the distorted wave Born approximation (DWBA), which accounts for both specular and off-specular scattering. The second one is spin flip (SF) scattering, which has no counterpart in XRMS. SF scattering probes magnetization fluctuations transverse to the mean magnetization direction and gives access to magnetic roughness and magnetic domain states. Specular and off-specular PNR work is exemplified by most recent work on spintronic materials and on patterned and functionalized magnetic layers.

Remanence enhancement in nanoscaled electrodeposited FePt films

L 1 0 FePt films with a small grain size of around 13nm have been prepared by electrodeposition and postannealing at a low annealing temperature of 400°C. A high remanence of up to 0.9T is achieved due to remanence enhancement by exchange coupling between the nanosized grains. Coercivity increases with longer annealing time as the L10 order becomes more complete. The resulting maximum energy product reaches a high value of 70kJ∕m3 after annealing for 120min, exceeding the maximum energy products so far obtained in electrodeposited hard magnetic films.

Tuning the remanent spin structure of exchange coupled magnetic films

We have investigated the remanent coupling between a soft-magnetic and a hard-magnetic film separated by a nonmagnetic spacer layer. It turned out that the remanent coupling angle of the soft-magnetic layer relative to the hard-magnetic layer can be adjusted by temporarily applied external fields. The underlying mechanism is the competition between the exchange field and the soft layer coercive field. This could be applied in modern magnetic recording techniques like MRAMs to realize multiple or even continuous states per memory cell, thus constituting a re-emergence of analogue recording technology with the potential of much higher information density.

Electrodeposition of Fe-Pt Films and Fe/Pt Multilayers

Electrodeposition from a single bath is used to prepare Fe-Pt films and Fe/Pt multilayers for hard magnetic film applications. Electrodeposition of Fe-Pt is not a simple codeposition of Fe and Pt. Large amounts of oxygen are found in as deposited films. In addition, Fe incorporation takes place at potentials positive to the equilibrium reduction potential of Fe2+. The analysis of the electrode processes and of potentiostatically deposited films indicate that both, iron hydroxide incorporation and underpotential deposition of Fe must be considered. The oxygen content could not be reduced in homogeneous films with an Fe-Pt ratio of 1.0 as the depression of iron hydroxide formation also leads to inhibition of iron incorporation. However, the alternating deposition of Fe-rich and Pt-rich layers, both with low oxygen content, leads to a lower integral oxygen content for an Fe/Pt ratio of 1.0. H2 annealing of the multilayered films reduces remaining iron (hydro)oxides and results in high coercivities and remanences.

Electrodeposited CoNiP Films with Perpendicular Magnetic Anisotropy

CoNiP alloy films have been grown by electrodeposition from a chloride bath under galvanostatic control. Film growth, structure, composition, magnetic properties, and morphology have been found to depend significantly on deposition current density. Hard magnetic films were successfully grown at the current density of with thickness up to , at which a perpendicular coercivity of was achieved. The magnetic hardness and perpendicular anisotropy of these films are the combined result of a microstructure composed of columnar grains and of a hexagonal phase oriented with the axis perpendicular to the film plane. Preliminary investigations of magnetization reversal processes suggest the presence of pinning sites located probably at grain boundaries.

Crystal structure and its correlation to intrinsic and extrinsic magnetic properties of epitaxial hard magnetic Pr-Co films

Pulsed laser deposited epitaxial ${\mathrm{Pr}}_{x}{\mathrm{Co}}_{100\ensuremath{-}x}$ $(x=8.7\char21{}27.6\phantom{\rule{0.3em}{0ex}}\mathrm{at.}\phantom{\rule{0.2em}{0ex}}%)$ thin films were systematically studied as a function of Pr content. Structural and magnetic measurements reveal different phases for specific composition ranges. In some cases, the phases observed are in contrast to their bulk counterparts. Uniaxial anisotropy at room temperature is observed in all the films enabling excellent hard magnetic properties. Polarization decreases monotonically with the increase of Pr content, whereas coercivity exhibits a broad maximum near the highly anisotropic $\mathrm{Pr}{\mathrm{Co}}_{5}$ composition. For the optimum combination of coercivity and polarization, the measured ${(BH)}_{\mathrm{max}}$ reaches values of $310\phantom{\rule{0.3em}{0ex}}\mathrm{kJ}∕{\mathrm{m}}^{3}$, which exceeds the highest-energy product value reported for RE-Co $(\mathrm{RE}=\text{rare}\text{\ensuremath{-}}\text{earth})$ systems. Temperature-dependent ac susceptibility measurements reveal that films with $x=8.7\char21{}20.4$ undergo a spin reorientation from easy axis to easy cone, but films with $x=22.9\char21{}27.6$ maintain their uniaxial anisotropy throughout the temperature range of investigation. From the comparison of the structural investigations and the spin reorientation temperature measurements, it is concluded that the spin reorientation temperature is insensitive to the change in the lattice parameter of the $\mathrm{Pr}{\mathrm{Co}}_{5\ifmmode\pm\else\textpm\fi{}\ensuremath{\delta}}$ phase.

STRUCTURAL ANALYSIS OF LIQUID 3D TRANSITION METALS USING CHARGED HARD SPHERE REFERENCE SYSTEM

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.

Quantitative imaging of stray fields and magnetization distributions in hard magnetic element arrays

In order to determine magnetic stray field and magnetization distributions of thin magnetic patterns and arrays, we developed a new quantitative imaging technique based on magneto-optical indicator films (MOIF) combined with inverse magnetostatic methods. The method is applied to hard magnetic FePt and PrCo5 films which exhibit out-of-plane and in-plane easy magnetization axes, respectively. The films are patterned with standard electron beam lithography into square shaped elements with sizes between 10 μm and 500 nm. The magnetization values obtained from the MOIF method are compared to those of integral magnetometer measurements and show a good agreement, if the sensor properties are taken into account, properly. As an outlook, a concept for combining MOIF imaging with magnetic force microscopy is demonstrated which allows for quantitative magnetization imaging with a resolution down to 10 nm.

High Rate Deposition of Magnetic Material by Gas Flow Sputtering

For different applications like thin film sensors or microactuators soft and hard magnetic films are required to create and guide the magnetic flux. In many cases, the required thickness of the coatings is well above 10 µm. For the preparation of coatings various techniques are used. In this paper we will present gas flow sputtering as a high potential method for high rate preparation of soft and hard magnetic coatings. The preparation conditions were varied and the influence of gas flow, working pressure, deposition temperature and substrate bias were investigated. In the field of soft magnetic films NiFe (81/19) was investigated. After optimization of the deposition parameters, growth rates exceeding 50 µm · h−1 were realized. The permeability µr was also tailored to higher values. Actual results showed µr values higher than 240 for a 10 µm thick coating. For films sputtered at 300 °C the coercitivity Hc of was below 340 A·m−1. As a hard magnetic material CoxSmy was investigated. Since the stoichiometry of the target was Co:Sm = 80:20 at.-%, CoSm coatings with mainly Co5Sm were prepared. The angular dependence of the coercivity Hc and remanence Br were investigated. The resulting coercitivity of the films was up to 1 220 kA·m−1 even with a high content of residual oxygen in the films. Deposition rates of 85 µm·h−1 could be realized.

Influence of Bath Composition on Magnetic Properties of Electrodeposited Co-Pt-W Thin Films

Effect of bath composition ([Co2+]/[PtIV] and [WO42, [cit−]) and pH on the magnetic properties of electrodeposited Co-Pt-W thin films has been investigated. Electrodeposited Co-Pt-W thin films exhibited strong perpendicular magnetic anisotropy when the ratio of [Co2+] to [PtIV] was 10; cathode current efficiency and perpendicular magnetic anisotropy showed little variations when [WO42] was lower than 0. 1 mol/L, but perpendicular magnetic anisotropy had strengthened when [WO42−] was over 0. 1 mol/L, which could be explained by the fact that the hydrogen evolution could produce pores as magnetic domain pinnings; citrate as complexing reagent can promote the polarization of [Co2+] and [PtIV]. As a result, the equilibrium electrode potentials of cobalt and platinum moved to negative direction, which led to the co-deposition of Co, Pt, and W. It was also found out that the as-deposited Co-Pt-W hard magnetic thin films were very homogeneous, smooth, and had the maximum coercivity for the bath pH 8. 5 and the concentration of citrate 0. 26 mol/L.

High performance hard magnetic NdFeB thick films for integration into micro-electro-mechanical systems

5 μ m thick NdFeB films have been sputtered onto 100mm Si substrates using high rate sputtering (18μm∕h). Films were deposited at ⩽500°C and then annealed at 750°C for 10min. While films deposited at temperatures up to 450°C have equiaxed grains, the size of which decreases with increasing deposition temperature, the films deposited at 500°C have columnar grains. The out-of-plane remanent magnetization increases with deposition temperature, reaching a maximum value of 1.4T, while the coercivity remains constant at about 1.6T. The maximum energy product achieved (400kJ∕m3) is comparable to that of high-quality NdFeB sintered magnets.

Size and Shape Control of Monodisperse FePt Nanoparticles

Morphological control of FePt nanoparticles has been systematically studied. By varying synthetic parameters including precursors, solvents, amount of surfactants, and heating rate of the solution, the particle size from 2 to 9 nm can be tuned with 1 nm accuracy. While most particles are spherical in shape, cubic particles can be obtained when particles are greater than 7 nm. Rod-shape nanoparticles have also been obtained. The as-synthesized nanoparticles are found to be superparamagnetic at room temperature and their blocking temperature is size dependent that increases with particle size. After annealing in a reducing atmosphere, the nanoparticles form hard magnetic films with ordered fct structure and high coercivity up to 2.7 T.

Crystallization and magnetic hardening of SmCo thin films

Hard magnetic thin films preparation is a key issue on the development of new micro-electro-mechanical systems. Vapour phase preparation techniques are desired because they are more easily incorporated into the typical microfabrication techniques. In this work we present the deposition of SmCo thin films by both magnetron sputtering (MS) and pulsed laser deposition (PLD). Thin films prepared by MS at temperatures below 400°C are amorphous and present soft magnetic properties, whereas those deposited at 450°C are nanocrystalline and present hard magnetic properties with a well defined in plane easy axis. The PLD prepared samples are nanocrystalline, but present soft magnetic properties. In both cases, post deposition annealing treatments have been successfully carried out, giving rise to a maximum coercivity of 2.7T.

Texture and magnetic properties of hard magnetic RE–Fe–B thin films deposited on a Nb buffer layer

RE–Fe–B thin films with a strong c-axis texture were prepared by dc magnetron sputtering on to a Nb (1 1 0) buffer layer on thermally oxidized Si substrates. The microstructure and magnetic properties of these RE–Fe–B films were studied as a function of the deposition temperature and Fe:RE ratio. The degree of c-axis orientation improved with decreasing deposition temperature. Good perpendicular hard magnetic properties could be obtained over a wide substrate temperature and concentration range. Perpendicular coercivities up to 1.67 MA/m (2.1 T) were obtained for RE-rich films, while perpendicular maximum energy products up to 200 kJ/m 3 were achieved for high Fe content films. Our results show that Nb, as for Ta, is a suitable buffer layer for deposition of highly textured hard magnetic RE 2Fe 14B films.

Thermodynamics and kinetics during pulsed laser annealing and patterning of FePt films

Laser annealing using 25ns pulses of a KrF excimer laser was applied to electrochemical and pulsed laser deposited FePt films of 700nm and 40 or 80nm thicknesses, respectively. The dependence of phase formation and magnetization behavior on the film thickness and the as-deposited state are studied. Models of the film heating are compared to the experimentally observed results. Furthermore a time temperature transformation (TTT) diagram for the ordering of FePt is proposed, summarizing the phase formation in dependence on annealing time and temperature. In agreement with the TTT diagram, laser annealing leads to a disordering of L10-ordered films. By only partially disordering the L10 phase, this approach provides control over the coercivity of hard magnetic FePt films. Local disordering is applied for magnetic patterning of the FePt films with dot patterns in the micrometer size range.

Effect of organic additives on the magnetic properties of electrodeposited CoNiP hard magnetic films

The properties of hard magnetic CoNiP films electrodeposited in presence of organic additive in various concentrations were studied with respect to thickness of the films. Films were electrodeposited in various current densities and for different time in order to get different thickness and uniform deposits. Elemental composition of the films was obtained using atomic absorption spectrometry. The phosphorous content was found to be less than 1%. Vibrating sample magnetometric studies indicate that organic additive has favourable impact on the magnetic properties of these films. Surface structural analysis was carried out using X-ray diffractometry and scanning electron microscopy. Reasons for variation in magnetic properties and structural characteristics are discussed. Hardness and adhesion of the films were also studied.

Anisotropic polymer bonded hard-magnetic films for microelectromechanical system applications

Thick hard-magnetic films have been prepared from NdFeB microparticles dispersed in polymethylmethacrylate solutions. The preparation process included the application of different magnetic fields during the drying of the hard particles dispersion, which resulted on different degrees of out-of-plane macroscopic anisotropy. From the analysis of the temperature dependence of the coercivity of the films it was concluded that the induced macroscopic anisotropy was linked to the occurrence of columnar particle aggregates.

Polymer Bonded Anisotropic Thick Hard Films for Micromotors/Microgenerators

Hard magnetic films are very promising elements for the production of MEMS, due to the optimum scaling properties of magnetic interactions. These elements are specially well suited for the development of micromotors and microgenerators. A magnetization multipolar structure is required on the material to be used in such applications. In order to generate this multipolar structure, a well defined magnetic anisotropy is required, this anisotropy being directed in many cases in the direction perpendicular to the film plane. The maximum torque obtainable from a micromotor depends mainly on the mass of magnetic material (i.e. the film thickness). Anisotropic hard magnetic films with thicknesses on the order of tenths of millimetres can not be prepared by any of the common thin film deposition techniques available nowadays. Though some preparation techniques have been developed that can be used to prepare hard magnetic films with the desired thicknesses, they are not able to produce materials with a well defined anisotropy along the direction perpendicular to the film plane. In this work, we describe a method to prepare anisotropic hard magnetic films with thicknesses of tenths of millimetres by dispersing NdFeB anisotropic microparticles on a polymethyl methacrylate (PMMA) solution at room temperature under low intensity magnetic fields. Large macrospic anisotropics have been reached for magnetic fields as low as 150 Oe applied during the deposition, with room temperature coercivities in excess of 1.2 T and maximum energy products up to 9 MGOe. The analysis of the temperature dependence of the magnetic properties indicates that a columnar arrangement of the particles on the direction parallel to the field applied during the preparation is the reason of the observed macroscopic anisotropy. The films can be easily deposited onto different substrates and mechanised in order to obtain a smooth surface with the desired shape. Their use in micromotors/microgenerators has been tested.