Soft Magnetic Permalloy Research Articles

Coercivity enhancement in hematite/permalloy heterostructures across the Morin transition

Interfacial effects between antiferromagnetic (AFM) and ferromagnetic (FM) materials have long been a center of magnetism studies. Aside from the exchange bias occurring at the AFM/FM interface, controlling the coercivity is another significant topic in magnetic recordings. The coercivity of FM materials is often determined through varying grain size, alloy composition, density of defects, etc., which is set during material growth and offers limited room for modification after growth. Hematite (α-Fe2O3) is an AFM material that undergoes a temperature-controlled spin-flip transition, the so-called Morin transition. This transition gives an extra degree of freedom making hematite an intriguing component to study the exchange coupling when interfaced with an FM material. In this work, changes in the magnetic properties of soft magnetic permalloy (Ni81Fe19, or Py) thin films grown on hematite were studied across the Morin transition. Surprisingly, these samples showed a remarkable change in coercivity during the Morin transition. We attribute this effect to the magnetic domain mixture of hematite during the Morin transition. Our findings present a novel method of controlling the coercivity of plain ferromagnetic thin films.

Tailoring of effective biaxial anisotropy in a 2-D array of thin truncated conical nanodisk

In the present study, the magnetization reversal of 2-D rectangular array of soft magnetic permalloy thin truncated conical disks is simulated by using micromagnetic modeling. The base radius (R) of the conical disk is varied between 20–100nm and upper radius (r) varied between 10nm to R−10nm such that the ratio σ=r/R lies between 0 to 1. The analysis of angular dependence (θ=0° to 90°) of hysteresis loops for the case of R=100nm is done in particular and its comparison with Stoner–Wohlfarth (SW) model reveals the presence of a bi-axial anisotropy which is attributed to the dipolar interaction between the nanodisks in the array. When the nanodisks are close to regular cylindrical shape (for σ∼0.9), the orthogonal axes of the facet of the array acts as magnetic easy axes. As the nanodisks are tapered to σ∼0.5 and 0.1, the same set of axes turns as hard axis of the bi-axial anisotropic system. Corresponding bi-axial anisotropy constant is estimated from the coercivity of the hysteresis loop along the easy axis as Keff=13.508kJ/m3, −5.403kJ/m3 and −4.322kJ/m3 respectively for σ=0.9,0.5 and 0.1 with R=100nm. In addition to this, an analytical expression is also formulated to calculate the total demagnetization energy of the 2-D array as a function of the magnetization orientation. This demagnetization energy is fitted with the effective bi-axial anisotropy energy expression considering an angular dispersion of the bi-axial anisotropy. The fitting suggests that the extent of angular dispersion of bi-axial anisotropy reduces as the conical disks are tapered (σ decreased). Irrespective of the magnetic field orientation, the remanent state exhibits a s-state of spatial magnetization profile when σ is large whereas a buckled magnetization state symmetric to body diagonal is exhibited for lower σ.

Additive manufacturing of soft magnetic permalloy from Fe and Ni powders: Control of magnetic anisotropy

Abstract The influence of the process parameters in Laser Beam Melting (LBM) on the element distribution and magnetic properties of permalloy ( Ni 78.5 Fe 21.5 ) is studied. Iron and nickel powders are mixed in the respective proportions to build twenty-five permalloy samples. The process parameters for each sample are varied to achieve different volume energy densities. An increase of the saturation magnetization MS up to 14% of the samples with respect to the initial powder blend is found. For a volume energy density of 428 J mm 3 we detect a stripe-like segregation of iron and nickel in the uppermost layer. In the volume a homogeneous element distribution is found. The segregation at the surface leads to a sizable uniaxial magnetic anisotropy. When using parameter combinations resulting in similar volume energy densities, we observe different surface morphologies depending on scan speed and laser power. The implications for creating tailored magnetic anisotropy directions in Fe-Ni soft magnets are discussed.

Spin wave-assisted reduction in switching field of highly coercive iron-platinum magnets

Recent rapid progress in spintronic and magnetic storage nanodevices has required nanomagnets to balance competing goals for high coercive field and low switching field. However, a decisive route for highly efficient magnetization switching has not been established yet. Here we propose a novel switching method using a spin wave of magnetic structures twisted in a nanometre scale. We have experimentally demonstrated extremely low field-magnetization switching in a highly coercive FePt by using a spin wave excited in a soft magnetic permalloy (Ni81Fe19), where permalloy is exchange-coupled to FePt through the interface. We can tune the switching field by varying the magnitude and frequency of the radio frequency magnetic field, and a significant decrease in switching field by one order of magnitude is achieved under the optimum conditions. The spin wave-assisted magnetization switching is a promising technique for ultralow-energy magnetization manipulation.

Equilibrium magnetic states in individual hemispherical permalloy caps

The magnetization distributions in individual soft magnetic permalloy caps on non-magnetic spherical particles with sizes ranging from 50 to 800 nm are investigated. We experimentally visualize the magnetic structures at the resolution limit of the x-ray magnetic circular dichroism photoelectron emission microscopy (XMCD-PEEM). By analyzing the so-called tail contrast in XMCD-PEEM, the spatial resolution is significantly enhanced, which allowed us to explore magnetic vortices and their displacement on curved surfaces. Furthermore, cap nanostructures are modeled as extruded hemispheres to determine theoretically the phase diagram of equilibrium magnetic states. The calculated phase diagram agrees well with the experimental observations.

Magnetic vortices on closely packed spherically curved surfaces

We investigate the change of magnetic vortex states driven by curvature. The equilibrium state and magnetization reversal of soft magnetic permalloy (Py, Ni${}_{80}$Fe${}_{20}$) caps on self-assembled spherical particles with diameters of 100, 330, and 800 nm are investigated, revealing the vortex ground state for individual caps and closely packed cap arrays. The magnetic coupling between vortices is substantially reduced due to the shape of the cap as apparent in a much weaker dependence of the magnetization reversal process on the separation distance. Interestingly, the remaining coupling is still sufficiently large to introduce chirality frustrated vortex states in closely packed cap arrays.

Magnetostatic interaction mechanisms in a two-dimensional composite magnet

The magnetization structure and the mechanisms of magnetic interaction in an artificial two-phase magnet are analyzed with magnetic force microscopy and magnetooptics. A model system built up of arrays of hard magnetic CoPt squares (5 μm edge length) embedded in a soft magnetic permalloy matrix is investigated. Special emphasis is put on the characterization of the matrix magnetization and on the interaction between the matrix and the CoPt squares. Different effects on different length scales are observed. At large distances to the hard magnetic dots, the magnetization of the matrix relaxes and is dominated by the characteristic pattern of an antidot array. The stability of this pattern with respect to magnetic fields and its dependence on the magnetic history of the system is analyzed. Nearby the dots, the magnetostatic coupling between the matrix and the CoPt induces a fine scale modulation of the magnetization of the matrix. At inter-dot distances up to 1 μm, the dot matrix interaction is prevalent and the matrix contributes essentially to the dot-dot interaction. We have studied the characteristics of this modulation, particularly considering their decay length and the interaction transfer mediated by the modulation.

Electrostatically driven tunable radio frequency inductor

An electrostatically driven tunable radio frequency (RF) inductor with large inductance tuning range consisting of electroplated soft magnetic permalloy (Ni80Fe20 alloy) plate was proposed in this paper. The fabricated tunable inductor exhibits a large tunable inductance range up to 77.8%, together with improved quality factor (Q-factor) at high frequencies. Such tunability of inductance and Q-factor was due to the magnetoelectric coupling effect between the soft magnetic permalloy plate and the spiral inductor, which lead to the magnetic flux of the inductor and mutual inductance change. The concept of tuning inductance by using electroplated soft magnetic permalloy has large potential for wide range RFIC applications.

Introducing artificial length scales to tailor magnetic properties

Magnetism is a collective phenomenon. Hence, a local variation on the nanoscale of material properties, which act on the magnetic properties, affects the overall magnetism in an intriguing way. Of particular importance are the length scales on which a material property changes. These might be related to the exchange length, the domain wall width, a typical roughness correlation length, or a length scale introduced by patterning of the material. Here we report on the influence of two artificially created length scales: (i) ion erosion templates that serve as a source of a predefined surface morphology (ripple structure) and hence allow for the investigation of roughness phenomena. It is demonstrated that the ripple wave length can be easily tuned over a wide range (25–175 nm) by varying the primary ion erosion energy. The effect of this ripple morphology on the induced uniaxial magnetic anisotropy in soft magnetic Permalloy films is studied. Only below a ripple wavelength threshold (≈60 nm) is a significant induced magnetic anisotropy found. Above this threshold the corrugated Permalloy film acts as a flat film. This cross-over is discussed in the frame of dipolar interactions giving rise to the induced anisotropies. (ii) Ion implantation through a lithographically defined mask, which is used for a magnetic property patterning on various length scales. The resulting magnetic properties are neither present in non-implanted nor in homogeneously implanted films. Here new insight is gained by the comparison of different stripe patterning widths ranging from 1 to 10 μm. In addition, the appearance of more complicated magnetic domain structures, i.e. spin-flop domain configurations and head-on domain walls, during hard axis magnetization reversal is demonstrated. In both cases the magnetic properties, the magnetization reversal process as well as the magnetic domain configurations depend sensitively on the artificially introduced length scale.

Study of magnetic domains by magnetic soft x-ray transmission microscopy*

X-ray magnetic circular dichroism serves as a strong element-specific magnetic contrast mechanism in full-field transmission soft x-ray microscopy to image micromagnetic domain structures. A lateral resolution down to 25 nm is provided by Fresnel zone plates used as optical elements. Recording the images in varying external magnetic fields and the sensitivity to the direction of the magnetization allows for detailed studies of static magnetization reversal processes in magnetic thin films and nanopatterned elements. Results on highly magnetostrictive Terfenol-D layers are reported. The experimental findings of the switching processes in soft magnetic permalloy rectangular structures are consistent with micromagnetic simulations. The pulsed time structure of polarized synchrotron radiation allows for a stroboscopic imaging of spin dynamics on a sub-nanosecond timescale.

(Fe–Co)–(Mg-fluoride) insulating nanogranular system with enhanced tunnel-type giant magnetoresistance

Insulating nanogranular-type tunnel magnetoresistive thin films made of (Fe or Fe–Co)–(Mg-fluoride) have been investigated. The films were prepared by a tandem deposition method, using Fe, Co, or Fe+Co metal and MgF2 insulator targets. The granular structure was found to consist of Fe or Fe–Co based nanogranules surrounded by thin intergranules of Mg based fluoride with the MgF2 crystal structure. A magnetoresistance value of 13.3% at room temperature and 10 kOe, the largest values ever reported, were obtained at the compositions of 32 vol %(Fe0.51Co0.49)–(Mg–F). To increase the magnetic field sensitivity of the magnetoresistance, a granular-in-gap film consisting of an (Fe–Co)–(Mg–F) granular thin film filling a narrow gap in a soft magnetic Permalloy thin film was prepared. A remarkably high magnetoresistance of 4% or more at 1–2 Oe was obtained.

Magnetic measurements and numerical simulations of interacting nanometer-scale particles

Magnetization processes in arrays of ferromagnetic, nanometer-scale iron particles were investigated experimentally and by numerical simulations. The arrays typically consisted of several hundred, pillar-shaped particles (diameter about 13 nm) in square or triangular arrangement. The particles were grown directly onto a thin soft-magnetic Permalloy film to enhance their interactions. A modified boundary-element method was developed to calculate the magnetization pattern of the particle-Permalloy heterostructures with a proper discretization of the system. The calculated magnetic behavior of the Permalloy film, the particles, as well as the coupled heterostructure for applied fields are compared to the experimental magnetization curves. The complex magnetic behavior observed in these heterostructures (e.g., the formation of domain walls within the Permalloy due to the particle magnetization) was also found numerically. These interaction effects may become relevant for ever smaller and more dense magnetic storage media.

Influence of local inhomogeneities on the magnetic properties of thin ferromagnetic films and nanostructures

Soft magnetic permalloy films with uniaxial in-plane anisotropy are widely used as magnetic sensors. The magnetic properties of such films are described by the ripple theory instead of the Stoner–Wohlfarth theory. The ripple theory includes the influence of local anisotropies, which lead to intrinsic demagnetizing fields. In ferromagnetic nanostructures strong demagnetizing fields arise from magnetic divergences at the surface of the structure. These fields dominate the magnetization in the structures and the magnetization reversal in applied fields.

Enhancement of low-field-magnetoresistive response of tunnel-type magnetoresistance in metal–nonmetal granular thin films

A very high response of the tunnel-type magnetoresistance at low magnetic fields for metal and nonmetal granular thin films was achieved by constructing the following hybrid structure: a large TMR Co-based metal–nonmetal film was placed in the lateral gap between a couple of soft magnetic permalloy, Fe 66Ni 34, films. We call our hybrid structure `nano-granular in gap (GIG)' film. This film was prepared by a micro processing technique using ion milling and RF sputtering methods. The best MR values obtained at about 2 Oe were 2–3% for a Fe 66Ni 34–Co 39Y 14O 47–Fe 66Ni 34 GIG film, which was about 250 times larger than the value for a single-layered film of Co 39Y 14O 47. This huge enhancement was attributed to the effect of the strong magnetic flux created at the narrow space between the coupled soft magnetic Fe 66Ni 34 films by small applied magnetic fields.

Reconstruction of induction distributions in thin films from DPC images

Pairs of differential phase contrast (DPC) images, recorded on a scanning transmission electron microscope, can be analysed quantitatively to yield information about the local direction and strength of the induction within a magnetic thin film. The resolution of the images is dependent on the size of the electron probe and can be ≈ 10 nm. The programme used for analysing the magnetic images is part of a versatile suite for digital image processing (DIP) and runs on a PC. Analysis of DPC images to yield induction maps is particularly useful around regions of magnetisation reversal where the spatial variation of magnetisation can be complex. Images of domain walls in soft magnetic permalloy films are presented and their analysis and interpretation discussed.

Galvano-magnetic thin film sensors (abstract)

Magnetoresistance detectors are used to observe the magnetic state of microscopic parts of magnetized samples. The planar Hall effect can be used instead. Both effects can be applied to find out the logic ‘‘1’’ or ‘‘0’’ state of magnetic storage devices. They also can be used for a quantitative and sensitive measurement of magnetic fields as well as a magneto-elastic strain gauge. Soft magnetic permalloy films are appropriate for the field detection while amorphous Fe1−xBx films are suitable for the strain measurements. The sensor films have to show a uniaxial anisotropy. The strength of it determines the range of the detectable fields and strains. The sensitivity of the magnetoresistance emr=dΔU/dH=2( ρ⊥−ρ∥) 1/db JH/HK2(V/Oe) depends linearly on the magnetic field H applied along the hard axis. l, d, b are the film length, thickness and width, respectively. The sensitivity of the planar Hall effect, which is given by epl=dUpl/dH =( ρ∥−ρ⊥)s/db J(1/HK) is independent of the applied field along the hard axis as long as H≪HK. s is the separation of the Hall contacts. Both sensitivities are reciprocal to the uniaxial anisotropy, HK as long as the applied field, H is smaller than HK. The theoretically expected sensitivity usually is decreased by domain splitting and the influence of the magnetization ripple. The smaller HK is (the larger the sensitivity is) the stronger is the influence of the ripple. It gives an upper limit of the interval in which external fields or applied strains can be quantitatively measured. Double layered films can prevent the domain splitting. The sensor theory including the ripple and experimental data will be given.