On the magnetoelastic contribution to the magnetic anisotropy of thin epitaxial Permalloy films: an ab initio study

Permalloy films are often used in the magnetic thin film technology. It is expected that in this material the magnetoelastic coupling of the magnetization to the epitaxial film strain does not produce undesired magnetic anisotropy, because the linear magnetoelastic bulk coefficient B₁ of Permalloy is near-zero. It is shown by means of the ab initio density functional electron theory that the nonvanishing nonlinear magnetoelastic couplings also do not lead to a considerable anisotropy. Explicit values for two of the nonlinear magnetoelastic coupling coefficients are given.     

The electrical properties of thin permalloy films

The magnetic properties of thin Permalloy films have been the subject of many investigations, but the work on their electrical properties is very limited [1]. By observing the change in electrical resistance with temperature the structural transformations taking place during the annealing of the condensates can be inferred.The authors of [1] did not undertake a detailed study of the electrical properties of Permalloy. They used Permalloy 79NMA in their investigation, and the dependence of the change in electrical resistance on the temperature of annealing in a magnetic field enabled them to reach conclusions about the nature of the uniaxial anisotropy of thin films.In the present work a detailed study has been made of the electrical resistance of Permalloy films in relation to the temperature of the substrate during evaporation and annealing; the temperature coefficient of resistance (TCR) has also been studied.     

Experimental determination technique for magnetic anisotropy of individual nanowires

We present an experimental technique to determine the magnetic anisotropy of ferromagnetic nanowires. In the technique, the magnetization state is monitored by measuring the anisotropic magnetoresistance with rotating the external magnetic field. The measured magnetoresistance curves exhibit basically the same curves typically appeared in the torque magnetometric measurements, which are then readily analyzed based on the Stoner–Wohlfarth theory with a single fitting parameter – the magnetic anisotropy. By applying the present technique to Permalloy nanowires, it is shown that the shape anisotropy in real nanowires is significantly influenced by the edge roughness.     

Anisotropy characteristics in a Permalloy film induced by a nonuniform magnetic field

Inhomogeneities were observed for the first time in the magnetic structure of a thin Permalloy film, induced by a strongly nonuniform magnetic field applied in the plane of the substrate during fabrication of the samples. The films were obtained by vacuum deposition using a molecular-beam epitaxy system. A nonuniform field was created on the substrate using four samarium-cobalt magnets. The anisotropy of local sections of the samples was measured using a scanning-ferromagnetic-resonance spectrometer. A strong correlation was observed between the distribution of the magnitude and direction of the local magnetic anisotropy of the film and the magnetic-field distribution in the plane of the substrate. Fiz. Tverd. Tela (St. Petersburg) 40, 1291–1293 (July 1998)     

The elastic anisotropy of permalloy sheet

The elastic anisotropy of Permalloy sheet is described by means of Fourier analysis by a free term and only one second harmonic. Analysis has shown that the rolling texture of Permalloy is formed by four mirror-symmetrical positions of the crystallites in which the (135) plane lies in the rolling plane and the direction with the indices [121] lies in the rolling direction.     

Influence of oblique incidence anisotropy on domain structures in GdCo3 sputtered amorphous films

Domain structures in thin obliquely sputtered amorphous GdCo₃ films here been studied by means of the polar Kerr effect and Bitter technique. The influence of induced anisotropy by oblique deposition on domain arrangement is shown and discussed. Discrete stripe domains different from those observed in Permalloy (weak stripes or stripes type I) have been found and a model of this structure proposed.     

Magnetic force microscopy study of magnetic stray-field effects due to mechanical surface modifications of patterned Permalloy thin films

The domain structure of magnetic thin films is strongly influenced by the presence of surface defects. We have used a scanning tunneling microscope for local surface modifications of thin Permalloy films by mechanical interaction between tip and sample. The changes of the magnetic stray-field distribution due to local topographic modifications have been probed by magnetic force microscopy. The relationship between the surface morphology and the micromagnetic structure is studied for different types of surface modifications.     

Unidirectional magnetic interaction in two-layer antiferromagnetic - ferromagnetic films

A study has been made of the directional stability of the unidirectional anisotropy in two-layer films of manganese and Permalloy 82 with respect to temperature and magnetic fields up to 30 kOe. The unidirectional interaction rotates into the direction of the applied magnetic field only after the antiferromagnetic layer is heated above the Neel temperature.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 12, No. 6, pp. 16–20, June, 1969.In conclusion the authors thank A. A. Glazer and R. I. Tagirov for furnishing us the opportunity to become acquainted with the procedure for producing films having such interesting magnetic properties.     

Ferromagnetic resonance and magnetization in permalloy films with nanostructured antidot arrays of variable size

We present the results of structural and magnetic characterization of Permalloy (Fe₂₀Ni₈₀) films deposited by sputtering on self-organized nanoporous alumina previously treated with phosphoric acid to vary the pore diameters. SEM and AFM images of the top film surfaces show a triangular array of pores with diameters similar to the untreated porous alumina. However, the underlying pore enlargement is evidenced by the magnetic study. Indeed magnetization measurements reveal a decrease in the easy-plane anisotropy energy with elapsing time of chemical etching. Consistent with these results, ferromagnetic resonance measurements perpendicular to the film indicate a systematic reduction of the resonance field which can be directly related to an increase in the pore diameter. The effect of lateral confinement (due to the pores) in the spin-wave resonance is evidenced in multiple absorption lines when the applied field is in the film plane and perpendicular to it. This contrasts with the results for the continuous test film.     

Sudden critical current drops induced in S/F structures

In the search for new physical properties of S/F structures, we have found that the superconductor critical current can be controlled by the domain state of the neighboring ferromagnet. The superconductor is a thin wire of thickness d_s ≈2ξ_S. Nb/Co and Nb/Py (Permalloy Ni₈₀Fe₂₀) bilayer structures were grown with a significant magnetic anisotropy. Critical current measurements of Nb/Co structures with ferromagnet thickness d_F > 30 nm show sudden drops in two very defined steps when the measurements are made along the hard axes direction (i.e. current track parallel to hard anisotropy axes direction). These drops disappear when they are made along the easy axis direction or when the ferromagnet thickness is below 30 nm. The drops are accompanied by vortex flux flow. In addition magnetorestistance measurements close to T_C show a sharp increase near saturation fields of the ferromagnet. Similar results are reproduced in Nb/Py bilayer structure with the ferromagnet thickness d_F ~ 50 nm along the easy anisotropy axes. These results are explained as being due to spontaneous vortex formation and flow induced by Bloch domain walls of the ferromagnet underneath. We argue these Bloch domain walls produce a 2D vortex-antivortex lattice structure.     

Thermodynamically stable fractal-like domain structures in magnetic films

A fractal-like structure of the domain boundaries was revealed in “overcritical” uniaxial Permalloy magnetic films. The fractal dimension of domain boundaries at the film surfaces was determined as a function of the film thickness. It is shown that the phase transition between the two possible types of fractal-like structures is accompanied by a jump in fractal dimension.

     

A quantitative analysis of magnetic vortices in Permalloy nanoparticles characterized by electron holography

The present work investigates experimentally curling magnetic configurations locally observed in almost dispersed Permalloy nanoparticles in the remanent state. Magnetic analysis is performed in a field emission TEM using off-axis electron holography. Particularly, electron holography is used to characterize the magnetic microstructure of Fe₃₀Ni₇₀ nanoparticles, whose average diameter (50 nm) is expected to be close to the critical size for a curling magnetic structure (vortex) formation. The vortex core diameter D_core and the bulk magnetic profile of the vortex are measured and compared with a “rigid vortex” micromagnetic model. The connection between vortex structure and the characteristic micromagnetic length of the system deduced from magnetization curve measurements is discussed.     

具有条纹磁畴结构的NiFe薄膜的制备与磁各向异性研究

具有条纹磁畴结构的磁性薄膜表现出面内转动磁各向异性,对于解决高频电子器件的方向性问题起着至关重要的作用.本文采用射频磁控溅射的方法,研究了NiFe薄膜的厚度、溅射功率密度、溅射气压等制备工艺参数对条纹磁畴结构、面内静态磁各向异性、面内转动磁各向异性、垂直磁各向异性的影响规律.研究发现,在功率密度15.6 W/cm~2与溅射气压2 mTorr(1 Torr=1.33322×102Pa)下生长的NiFe薄膜,表现出条纹磁畴的临界厚度在250 nm到300 nm之间.厚度为300 nm的薄膜比250 nm薄膜的垂直磁各向异性场增大近一倍,从而磁矩偏离膜面形成条纹磁畴结构,并表现出面内转动磁各向异性.高溅射功率密度可以降低薄膜出现条纹磁畴的临界厚度.在相同功率密度15.6 W/cm~2下生长300 nm的NiFe薄膜,随着溅射气压由2 mTorr增大到9 mTorr,NiFe薄膜的垂直磁各向异性场逐渐由1247.8 Oe(1 Oe=79.5775 A/m)增大到3248.0 Oe,面内转动磁各向异性场由72.5 Oe增大到141.9 Oe,条纹磁畴周期从0.53μm单调减小到0.24μm.NiFe薄膜的断面结构表明柱状晶的形成是表现出条纹磁畴结构的本质原因,高功率密度下低溅射气压有利于柱状晶结构的形成,表现出规整的条纹磁畴结构,高溅射气压会导致柱状晶纤细化,面内转动磁各向异性与面外垂直磁各向异性增强,条纹磁畴结构变得混乱.     

New experiments on origin of anisotropy in Permalloy films

A series of new experiments has been performed to determine the origin of the uniaxial anisotropy in Permalloy films. By means of a double magnetic shield enclosing the vacuum system, it was possible to deposit films in extremely small ambient fields (10^–6 to 10^–3 oersted). It was found that for deposition field strengths higher than about 10^–1 oersted, the anisotropy constant and the squareness of the hysteresis loop are essentially invariant with respect to the value of the deposition field. However, forH _d <10^–1 oersted, both the anisotropy and squareness of the loop decreases dramatically. From these experimental results, it appears that both the anisotropic Fe-pair orientation and anisotropic imperfection-alignment are deposition-field induced.     

Coherent inelastic light scattering from a microwave-excited array of magnetic particles

Inelastic light scattering from an array of Permalloy particles driven by a microwave magnetic field is shown to be a coherent phenomenon in which the scattered radiation is observed only at diffraction angles corresponding to the reciprocal lattice of the array. The results are explained in terms of the phase coherence of the inelastically scattered light by each of the particles.     

High-frequency magnetic modulation of recoilless gamma radiation of57Fe

Radio-frequency (rf) magnetic modulation has been used to generate sidebands in⁵⁷Fe Mössbauer spectra of Fe_0.18Ni_0.82 Permalloy foils which have the smallest constant of magnetostriction among Fe?Ni alloys. Sidebands in Mössbauer spectra were observed at 30 MHz and 55 MHz. In addition to the generation of sidebands, the external rf magnetic field was found to alter the line positions of the original six line spectrum. An attempt was made to study acoustic vibrations in the foil by means of X-ray diffraction. The rf magnetic field caused changes in diffraction peak intensities and positions. It was found that X-ray diffraction can be used to study the amplitude of acoustic vibrations in Permalloy foils.     

Micromagnetic dissipation, dispersion, and mode conversion in thin permalloy platelets

Micron-sized ferromagnetic Permalloy disks exhibiting an in-plane ferromagnetic vortex structure are excited by a fast rise time perpendicular magnetic field pulse and their modal structure is analyzed. We find azimuthal and axial modes. By a Fourier filtering technique we can separate and analyze the time dependence of individual modes. Analysis of the experimental data demonstrates that the azimuthal modes damp more quickly than the axial modes. We interpret these results as mode conversion from low-frequency azimuthal modes to the fundamental mode which is higher in frequency, i.e., mode-mode coupling in a system with a single Landau-Lifshitz-Gilbert phenomenological damping constant alpha.     

Control of magnetic fluctuations by spin current

We use microfocus Brillouin light scattering spectroscopy to study the interaction of spin current with magnetic fluctuations in a Permalloy microdisk located on top of a Pt strip carrying an electric current. We show that the fluctuations can be efficiently suppressed or enhanced by different directions of the electric current. Additionally, we find that the effect of spin current on magnetic fluctuations is strongly influenced by nonlinear magnon-magnon interactions. The observed phenomena can be used for controllable reduction of thermal noise in spintronic nanodevices.     

Magnetic microstructure of the spin reorientation transition: a computer experiment

The scenario of the spin reorientation in two-dimensional films within first-order anisotropy approximation is theoretically studied by means of Monte Carlo simulations. The magnetic microstructure is investigated as a function of the ratio of the perpendicular anisotropy energy to the dipolar one. If the anisotropy dominates, out-of-plane domains will be found while in-plane vortices appear for a vanishing anisotropy. In the range of comparable anisotropy and dipolar energies a complex domain pattern evolves yielding a continuous transition between the two structures. The structure with equally distributed magnetic moment orientations is stable at the point where anisotropy and dipolar energies cancel each other.