Magnetic Materials Processing Research Articles

Critical BEOL Aspects of the Fabrication of a Thermally-Assisted MRAM Device

Recent developments in new switching methods, such as spin transfer torque [1] and temperature-assisted switching (TAS) [2], have greatly increased interest in MRAM. MRAM fabrication is challenging, however, due to the lack of magnetic materials processing experience by semiconductor lines. In this talk, we will discuss key back-end-of-line (BEOL) fabrication aspects for a CMOS-integrated thermally-assisted (TAS) MRAM on 200 mm wafers with, e.g., opens/shorts yields of 100% and 99% for 1Mb and 16 Mb arrays, resp. The cell sizes were 33F^2 for the 16M cell, and 73F^2 for the 1M cell, where F = 90 nm. This was achieved in our Microelectronics Research Laboratory (MRL) facility through careful process optimization and adherence to good BEOL processing practice. Factors influencing opens and shorts yields will be discussed. In the TAS-MRAM magnetic tunnel junction (MTJ) device, data retention is enhanced through synthetic antiferromagnetic exchange coupling; during writing; coupling is overcome through passage of a heating current through the MTJ, and a lower switching field suffices to switch the heated device [2]. Figure 1 contains TEMs of a TAS MRAM structure with two dual-Damascene Cu levels and an Al level after the MTJ. The 90 nm base FEOL/BEOL CMOS technology wafers were fabricated by a third party. Fabrication at IBM commenced at V3, and trilayer via and trench RIE masking schemes were used below the final Al level. The field line (M4) had ferromagnetic (FM) cladding of either PVD Ni-Fe alloy or Co. Cladding proved challenging for narrow field lines (≤ 0.5 µm); e.g., in the initial stages of Cu plating, imperfect corner seedlayer coverage led to solution attack of the Co. A Damascene TaN layer (ca. 50 nm thick) served as the pedestal for the MTJ. The MTJ resist mask patterns were trimmed using RIE to afford, after MTJ methanol-based RIE etching, final device sizes of about 110 nm. After MTJ encapsulation with SiN and dielectric deposition, V4 vias were RIE etched, stopping on SiN. During M5 dielectric RIE, the MTJ pillar tops were exposed, and the V4 vias’ SiN cleared. The final V5/M6 Cu level was followed by an Al level. Besides the FM liner, other processing challenges included clearing isolated vias, and developing a robust trench RIE process to contact the MTJs. [1] J. C. Slonczewski, J. Magn. Magn. Mater.,159, L1 (1996). [2] For example, I. L. Prejbeanu et al., J. Phys: Condens. Matter, 19 (16), 165218 (2007). Acknowledgements: Back-end-of-line fabrication was carried out in the Microelectronics Research Laboratory (MRL) at the T. J. Watson Research Center. The MRAM stacks were deposited at Crocus Technology in Grenoble, France. Figure 1

Finite element modeling of a double-diffusive mixed convection flow of a chemically-reacting magneto-micropolar fluid with convective boundary condition

In this paper, the steady, two-dimensional, heat and mass transfer flow of a chemically reacting mixed convection magneto-micropolar fluid over a wedge with a convective surface boundary condition is investigated. Using similarity transformations found by Falkner and Skan, the governing transport equations are reduced to a system of non-linear ordinary differential equations which are solved by employing the extensively-validated, finite element method. The results are depicted graphically to illustrate the influence of the various pertinent parameters on velocity, micro-rotation, temperature and concentration functions. Additionally, skin friction coefficient, local Nusselt number and local Sherwood number are also computed and presented graphically for the flow regime. The numerical solutions are compared with earlier studies for special cases of the wedge angle parameter and found to be in excellent agreement, thereby validating the accuracy of the present numerical code. The study finds applications in chemical reaction engineering processes, magnetic materials processing, solar collector energy systems, etc.

Electron Holography of the Magnetic Phase Shift of a Current-Carrying Wire

The controlled generation of magnetic fields to study switching processes in magnetic materials in the transmission electron microscope (TEM) is highly challenging [1-3]. Here, we use electron holography to study the magnetic field around a carefully fabricated current-carrying wire. As a result of the fact that a current flowing parallel to the plane of an untilted TEM specimen produces no net magnetic phase shift, we examine a short nano-fabricated segment of wire that is oriented parallel to the electron beam direction. Different regions of such a wire could in principle be used to apply either in-plane or out-of-plane magnetic fields to closely-adjacent nanomagnets.

SPETTROSCOPIA MUONICA: UNA SONDA MICROSCOPICA PER IL MAGNETISMO

The muon spectroscopy is an experimental technique which employs muon beams to study the properties of condensed matter. In particular, it allows us to study static and dynamic processes in magnetic materials (e.g. iron magnets/antiferromagnets, spin glasses, frustrated, etc) and superconductors, as well as chemical reactions and diffusion phenomena of charged and neutral particles. Here we will describe at the elementary level the physical principles of this technique, highlighting especially the use in the microscopic study of the magnetic properties of materials.

Element free Galerkin modeling of radiative hydromagnetic micropolar flow saturated Darcy medium with heat transfer over a stretching sheet with Joule heating

ABSTRACTIn this paper, the effect of thermal radiation on heat transfer in the steady, incompressible hydromagnetic flow of an electrically conducting, micropolar fluid through a Darcian porous medium adjacent to a stretching sheet under a uniform transverse magnetic field has been examined. The partial differential conservation equations governing the flow regime have been nondimensionalized using appropriate similarity transformations and the resulting ordinary differential equations have been solved numerically by using the element‐free Galerkin method. The influence of dimensional less flow parameters on the velocity, microrotation, and temperature distribution have been studied graphically. Excellent correlation of selected results has been achieved with the finite element method. Dimensionless temperatures have been shown to be considerably increased with higher thermal radiation contribution. The flow is also shown to be decelerated in the boundary layer with increasing magnetic field, whereas microrotation of the micro‐elements is amplified with increasing magnetic field and temperatures are also elevated. Increasing porosity of the regime accelerates the flow and also boosts microrotation on the surface of sheet but depresses temperatures in the regime. The mathematical model has important applications in polymeric sheet processing at high temperatures, magnetic materials processing, hydromagnetic control of conducting polymeric sheets, and so on. © 2013 Curtin University of Technology and John Wiley & Sons, Ltd.

High Steady Magnetic Field Processing of Functional Magnetic Materials

The materials science community has been enriched for some decades now by the “magneto-science” approach, which consists of applying a magnetic field during material processing. The development of anisotropic properties by applying a steady magnetic field is now a well-established effect in the material processing of magnetic substances, which benefits from the unidirectional and static nature of the field delivered by superconducting magnets. Among other effects, magnetic anisotropy in functional magnetic materials, which arises from the alignment of magnetic moments under external field, can be developed at various structural scales. Magnetic ordering, magnetic patterning, and texturation are at the origin of this anisotropy development. Texture is developed in materials from magnetic orientation due to magnetic forces and torques or from stored energy. In metals and alloys, for instance, this effect can occur either in their liquid state or during solid-state thermomagnetic treatments and can thus impact significantly the material functional magnetic properties. Today’s improved superconducting magnet technology allows higher field intensities to be delivered more easily (1 T up to several tens of Teslas) and enables researchers to gather evidence on magnetic field effects that were formerly thought to be negligible. The magneto-thermodynamic effect is one of them and involves the magnetization energy as an additional parameter to tailor microstructures. Control of functional properties can thus result from magnetic monitoring of the phase transformation, and kinetics can be impacted by the magnetic energy contribution.

Soret-Dufour Effects on the MHD Flow and Heat Transfer of Microrotation Fluid over a Nonlinear Stretching Plate in the Presence of Suction

In this work, the Micropolar fluid flow and heat and mass transfer past a horizontal nonlinear stretching sheet through porous medium is studied including the Soret-Dufour effect in the presence of suction. A uniform magnetic field is applied transversely to the direction of the flow. The governing differential equations of the problem have been transformed into a system of non-dimensional differential equations which are solved numerically by Nachtsheim-Swigert iteration technique along with the sixth order Runge-Kutta integration scheme. The velocity, microrotation, temperature and concentration profiles are presented for different parameters. The present problem finds significant applications in hydromagnetic control of conducting polymeric sheets, magnetic materials processing, etc.

Thermo-Diffusion and Diffusion-Thermo Effects on MHD Free Convective Heat and Mass Transfer from a Sphere Embedded in a Non-Darcian Porous Medium

The problem of combined heat and mass transfer by natural convection over a sphere in a homogenous non-Darcian porous medium subjected to uniform magnetic field is numerically studied, taking Soret/Dufour effects into account. The coupled, steady, and laminar partial differential conservation equations of mass, momentum, energy, and species diffusion are normalized with appropriate transformations. The resulting well-posed two-point boundary value problem is solved using the well-tested, extensively validated Keller-Box implicit finite difference method, with physically realistic boundary conditions. A parametric study of the influence of Soret number (Sr), Dufour number (Du), Forchheimer parameter (Λ), Darcy parameter (Da), buoyancy ratio parameter (), Prandtl number (Pr), Schmidt number (Sc), magnetohydrodynamic body force parameter (), wall transpiration () is the blowing/suction parameter, and streamwise variable (ξ) on velocity, temperature, and concentration function evolution in the boundary layer regime is presented. Shear stress, Nusselt number, and Sherwood number distributions are also computed. Applications of the study arise in hydromagnetic flow control of conducting transport in packed beds, magnetic materials processing, geophysical energy systems, and magnetohydrodynamic chromatography technology.

First order reversal curve studies of permanent magnets

First order reversal curve (FORC) diagrams are a useful tool to analyze the magnetization processes in magnetic materials. FORC diagrams are computed from measured first order reversal curves on sintered Nd2Fe14B magnets. It is shown that the FORC diagram simplifies if the first order reversal curves a desheared using the macroscopic demagnetizing field given by the geometry of the sample. Furthermore the resulting FORC diagram is almost identical to the FORC diagram measured for a thin platelet of the same material. This opens the possibility to compare experimental FORC diagrams with FORC diagrams computed by micromagnetic simulations.

Magnetization Dynamics of Amorphous Ribbons and Wires Studied by Inductance Spectroscopy.

Inductance spectroscopy is a particular formulation variant of the well known complex impedance formalism typically used for the electric characterization of dielectric, ferroelectric, and piezoelectric materials. It has been successfully exploited as a versatile tool for characterization of the magnetization dynamics in amorphous ribbons and wires by means of simple experiments involving coils for sample holding and impedance analyzer equipment. This technique affords the resolution of the magnetization processes in soft magnetic materials, in terms of reversible deformation of pinned domain walls, domain wall displacements and spin rotation, for which characteristic parameters such as the alloy initial permeability and the relaxation frequencies, indicating the dispersion of each process, can be defined. Additionally, these parameters can be correlated with chemical composition variation, size effects and induced anisotropies, leading to a more physical insight for the understanding of the frequency dependent magnetic response of amorphous alloys, which is of prime interest for the development of novel applications in the field of telecommunication and sensing technologies. In this work, a brief overview, together with recent progress on the magnetization dynamics of amorphous ribbons, wires, microwires and biphase wires, is presented and discussed for the intermediate frequency interval between 10 Hz and 13 MHz.

MATHEMATICAL MODELING OF OSCILLATORY MHD COUETTE FLOW IN A ROTATING HIGHLY PERMEABLE MEDIUM PERMEATED BY AN OBLIQUE MAGNETIC FIELD

We study theoretically the incompressible, viscous, oscillatory hydromagnetic Couette flow in a horizontal fluid-saturated highly permeable porous medium parallel-plate channel rotating about an axis perpendicular to the plane of the plates under the action of a uniform magnetic field, B0, inclined at an angle θ to the axis of rotation. The flow is generated by the non-torsional oscillation of the lower plate of the channel. The reduced unsteady momentum equations are nondimensionalized with appropriate variables. Exact solutions under specified boundary conditions are obtained using the Laplace transform method (LTM). The flow regime is found to be controlled by a rotational parameter (K2), which is the reciprocal of the Ekman number (Ek), the square of the Hartmann magnetohydrodynamic number (M2), a porous medium permeability parameter (Kp), which is the inverse of the Darcy number (Da), oscillation frequency (ω), dimensionless time (T), and magnetic field inclination (θ). The influence of these parameters on the primary (u1) and secondary (v1) velocity field is presented graphically and studied in detail. Asymptotic behavior of the solutions is also examined for several cases of the square of the Hartmann number, rotation parameter, and oscillation angular frequency. The existence of modified Hartmann boundary layers is also identified. The present study has important applications in MHD (magnetohydrodynamic) energy generator flows, chemical engineering magnetic materials processing, conducting blood flows, and process fluid dynamics.

Revisiting magnetization processes in granular hard magnetic materials

We show that the generally accepted demagnetizing field corrections are not applicable to the broad class of modern granular magnetic materials, where magnetization reversal occurs via discrete switching of the moments of individual grains, and thus the self-demagnetizing field of a given grain is proportional to the spontaneous magnetization Ms, not to the sample magnetization M. This leads to the fact that the self-demagnetizing field must be considered as a contribution to the coercive field and that the slope of the magnetization variation is higher than the classical demagnetizing field slope. This description of demagnetization processes is confirmed by numerical modeling.

Network numerical analysis of magneto-micropolar convection through a vertical circular non-Darcian porous medium conduit

Abstract The fully developed, mixed convection heat transfer of a magneto-micropolar fluid in a Darcy–Forchheimer porous medium containing heat sources contained in a vertical circular conduit is investigated in this article. The conservation equations for mass, linear momentum, micro-inertia, angular momentum (micro-rotation) and energy are presented in a cylindrical coordinate system (r, θ, z) with appropriate boundary conditions. A Darcy–Forchheimer drag force model is employed to simulate the effects of bulk linear porous impedance and second order porous resistance. The governing partial differential equations are non-dimensionalized into a set of ordinary differential equations in a single independent variable (η) and solved using the Network Simulation Method. Benchmark solutions are compared with earlier computations using the finite element method, showing excellent agreement. The influence of Darcy number, Forchheimer number, Grashof number, Hartmann number, geometric scale ratio (conduit radius to length ratio), Eringen parameter (ratio of vortex viscosity to Newtonian viscosity) and heat source/sink parameter on the linear velocity, angular velocity (micro-rotation) and temperature functions are studied in detail. Flow i.e. linear (translational) velocity, f, is seen to be inhibited with increasing magnetic field (Hartmann number), Forchheimer number and Eringen parameter, but accelerated with increasing Darcy number. Micro-rotation (g) is decreased with increasing Forchheimer number and Hartmann number, but increased with a rise in Grashof number, Darcy number, geometric scale ratio and Eringen parameter. Both velocity (f) and micro-rotation (g) are increased in the presence of a heat source but decreased with a heat sink. Several special cases of the flow regime are also documented. Applications of the problem include the cooling of porous combustion chambers, geophysical transport in electrically-conducting zones, exhaust nozzles of porous walled flow reactors, hydromagnetic control processes in nuclear engineering and magnetic materials processing (ceramic foams).

Network simulation solutions for laminar radiating dissipative magneto-gas dynamic heat transfer over a wedge in non-Darcian porous regime

We study the steady-state, magnetohydrodynamic, optically thick, dissipative gas boundary layer flow and heat transfer past a non-isothermal porous wedge embedded in a scattering, homogenous, isotropic Darcy–Forchheimer porous medium, with significant thermal radiation effects in the presence of heat sink/sources and surface transpiration, in an ( x , y ) coordinate system. The Rosseland diffusion approximation is employed for which radiative flux can propagate only small distances prior to scattering or absorption. Joule electric dissipation, viscous heating and also stress work are incorporated in the boundary layer equations and a temperature-dependent heat source/sink term utilized. Following a transformation to a ( ξ , η ) coordinate system, the transformed coupled, nonlinear pseudosimilar equations for momentum and energy are solved using a powerful computational method based on thermoelectric analogy, viz the Network Simulation Method. The effects of magnetism (Hartmann number), Darcy number, Forchheimer number, Rosseland radiation–conduction parameter, pressure gradient parameter, Eckert number, transpiration (wall mass transfer) parameter, and heat source/sink parameter on velocity and temperature profiles are depicted graphically. Numerical solutions are compared where possible with earlier non-porous and non-dissipative studies and found to be in excellent agreement. The current study has potential applications in simulating laminar radiative-magnetohydrodynamic heat transfer over astronautical bodies in debris-laden (porous) regimes and also in geothermal physics and magnetic materials processing.

Numerical study of magnetohydrodynamic viscous plasma flow in rotating porous media with Hall currents and inclined magnetic field influence

A numerical solution is developed for the viscous, incompressible, magnetohydrodynamic flow in a rotating channel comprising two infinite parallel plates and containing a Darcian porous medium, the plates lying in the x–z plane, under constant pressure gradient. The system is subjected to a strong, inclined magnetic field orientated to the positive direction of the y-axis (rotational axis, normal to the x–z plane). The Navier–Stokes flow equations for a general rotating hydromagnetic flow are reduced to a pair of linear, viscous partial differential equations neglecting convective acceleration terms, for primary velocity (u′) and secondary velocity (v′) where these velocities are directed along the x and y axes. Only viscous terms are retained in the momenta equations. The model is non-dimensionalized and shown to be controlled by a number of dimensionless parameters. The resulting dimensionless ordinary differential equations are solved using a robust numerical method, Network Simulation Methodology. Full details of the numerics are provided. The present solutions are also benchmarked against the analytical solutions presented recently by Ghosh and Pop [Ghosh SK, Pop I. An analytical approach to MHD plasma behaviour of a rotating environment in the presence of an inclined magnetic field as compared to excitation frequency. Int J Appl Mech Eng 2006;11(4):845–856] for the case of a purely fluid medium (infinite permeability). We study graphically the influence of Hartmann number (Ha, magnetic field parameter), Ekman number (Ek, rotation parameter), Hall current parameter (Nh), Darcy number (Da, permeability parameter), pressure gradient (Np) and also magnetic field inclination (θ) on primary and secondary velocity fields. Additionally we investigate the effects of these multiphysical parameters on the dimensionless shear stresses at the plates. Both primary and secondary velocity are seen to be increased with a rise in Darcy number, owing to a simultaneous reduction in Darcian drag force. Primary velocity is seen to decrease with an increase in Hall current parameter (Nh) but there is a decrease in secondary velocity. The study finds important applications in magnetic materials processing, hydromagnetic plasma energy generators, magneto-geophysics and planetary astrophysics.

Numerical study of free convection magnetohydrodynamic heat and mass transfer from a stretching surface to a saturated porous medium with Soret and Dufour effects

We examine the combined effects of Soret and Dufour diffusion and porous impedance on laminar magnetohydrodynamic mixed convection heat and mass transfer of an electrically-conducting, Newtonian, Boussinesq fluid from a vertical stretching surface in a Darcian porous medium under uniform transverse magnetic field. By applying two equal and opposing forces along the x-axis, the sheet is stretched with a speed proportional to the distance from the fixed origin x = 0. The steady-state boundary layer equations are non-dimensionalized into non-similar form and then solved numerically by the local non-similarity method with shooting quadrature. The effects of the Darcian linear porous drag parameter (Da), Soret number (Sr), Dufour number (Du), Prandtl number (Pr), Schmidt number (Sc), magnetohydrodynamic parameter (M) and concentration-to-thermal-buoyancy ratio parameter (N), on the fluid velocity, temperature, concentration, local skin friction, Nusselt number function and Sherwood number function distributions in the regime are depicted graphically and analyzed in detail. Applications of the study include magnetic materials processing and chemical engineering systems.

Nano-processing with gas cluster ion beams

The gas cluster ion beam process has become a candidate technique for advanced nano-fabrication, where both throughput and precise functionality are required. It is demonstrated that both extreme high-speed and precise nano-processing with low damage can be realized using reactive cluster ion beams. To date, this technique has been successfully applied to photonic, magnetic, electronic, and biological materials processing. The energy and size of cluster have become well controllable and many kinds of gaseous materials become available as cluster source gas. The fundamentals and applications of gas cluster ion beam process are reviewed in this paper.

Processing and Modeling of Novel Nanocrystalline Soft Magnetic Materials

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Rate dependence of first-order reversal curves by using a dynamic Preisach model of hysteresis

First-order reversal curves (FORCs) are used to analyze the kinetic effects described by dynamic Preisach models of hysteresis. It is shown that dynamic processes in magnetic materials strongly affect the symmetry of FORC diagrams. The regions that are most strongly affected when the applied magnetic field varies at high speed are located in the proximity of the diagonal of the FORC diagrams. In these regions, the differential magnetic susceptibility can be negative and result into negative values of the FORC diagrams. An identification technique based on the FORC diagrams is presented for the kinetic parameters of the dynamic Preisach model.

Processing, properties and some novel applications of magnetic nanoparticles

Magnetic nanoparticles have been prepared by various soft chemical methods including self-assembly. The bare or surface-modified particles find applications in areas such as hyperthermia treatment of cancer and magnetic field-assisted radioactive chemical separation. We present here some of the salient features of processing of nanostructured magnetic materials of different sizes and shapes, their properties and some possible applications. The materials studied included metals, metal-ceramic composites, and ferrites.