Fe-Ni Alloy Films Research Articles

Domain-Wall Motion by Néel-Quasi-Néel Wall Pairs in Two-Layer Ni–Fe Alloy Films

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Electron Microscopic Studies on Origins of Magnetization Ripples in Ferromagnetic Thin Films

In order to find the most suitable condition for electron microscopic observation of the magnetization ripples, the images obtained by three methods, that is, the defocus-, the shadow image- and the slit- method, were compared for magnetic and nonmagnetic films quantitatively. The dependence of the ripples on applied magnetic field, on temperature and on a number of parameters of films is discussed in detail. Observation was made in condensed films of Ni, Fe, Co, Ni-Fe alloys, and Ni-Co alloys and also in chemically and electrolytically polished single crystal films of Ni-Fe alloys, Ni and Fe. As the origins of the ripples following conclusions are arrived: (1) The microscopic anisotropies such as the crystalline anisotropy, the stress induced anisotropy, the shape anisotropy and the stray field anisotropy contribute the ripples with equal weights at room temperature. (2) The macroscopic anisotropy due to macroscopic anisotropy due to macroscopic origin only has an effect to decrease the ripples. (3) The shape anisotropy and the stray field anisotropy are more effective at higher temperatures. The ripples near the temperature of domain disappearance. T d c , are due to thermal fluctuation of magnetization, which may appear also in bulk materials.

Temperature Dependence of the Ferromagnetic Resonance Linewidth in Thin Ni–Fe Films

Linewidths for ferromagnetic resonance with the static magnetic field in the film plane have been measured at frequencies from 1–9 Gc/sec and temperatures from 4.2°–300°K for Ni–Fe alloy films (77% Ni) 150 to 1500 Å thick. The linewidth exhibits a maximum in the vicinity of 80°K and the effect is generally larger in thinner films. The amplitude of the linewidth temperature dependence is independent of frequency and the maximum shifts to slightly higher temperatures with increasing frequency. The amplitude can be enhanced by heating the film at 150°C in air (oxidation) and removed by a similar treatment in a hydrogen atmosphere. This enhancement-removal cycle can be repeated reproducibly. The time required for saturation of the enhancement when the oxidation treatment was slowed down by annealing in a vacuum of 10−3 Torr instead of at atmospheric pressure was observed to be independent of film thickness, indicating that a volume-diffusion process is not involved. The apparent importance of surface oxidation suggests two possible mechanisms, valence exchange and exchange anisotropy, which may contribute to the linewidth temperature dependence. The discovery of a temperature-dependent linewidth in thin films which exhibits a maximum in the vicinity of 80°K and the identification of this dependence with a surface oxidation process are the primary results of this investigation.

Relaxation Processes for Ferromagnetic Resonance in Thin Films

Ferromagnetic-resonance linewidth measurements have been made for two hundred Ni–Fe alloy films (77% Ni) 150–3200 Å thick at frequencies from 1–9 Gc/sec with the static field in the film plane. To avoid dispersion effects samples with the smallest linewidth (ΔHmin) were selected for each thickness. For film thickness less than a frequency-dependent critical thickness Dω, ΔHmin is independent of film thickness. For thicker films where D > Dω, ΔHmin increases linearly with film thickness. The observed Dω values (about 1000 Å) are in good agreement with predictions based on magnon scattering involving spin waves degenerate with the uniform mode. Because of the magnetostatic mode modification of the spin-wave dispersion relation for thin films there are no spin-wave states degenerate with the uniform mode for D < Dω and magnon scattering is not allowed. The applicability of two different magnon processes, s-d exchange and two-magnon scattering, is discussed. Neither mechanism provides a completely satisfactory explanation of the data. The present data indicate: (1) For D > Dω, the linewidth is a linear function of film thickness and a magnon process is important. (2) For D < Dω, a magnon process is not important. (3) The anisotropy dispersion makes a contribution to ΔH, which is significant and may mask the above effects if the dispersion is large.

Mobility and Loss Mechanisms for Domain Wall Motion in Thin Ferromagnetic Films

Domain-wall mobility for Ni–Fe alloy films has been measured as a function of film thickness from 300 to 1650 Å. Between 300 and 800 Å, the mobility decreases with increasing film thickness, ranging from 8×103 cm/sec·Oe at 300 Å to 3×103 cm/sec·Oe at 800 Å. Between 900 and 1000 Å, the mobility increases rapidly with increasing film thickness to about 7×103 cm/sec·Oe. Above 1000 Å, the mobility increases slowly with film thickness. These mobility data have been compared with predictions based on intrinsic and eddy-current loss mechanisms utilizing available static domain wall-shape information. Predictions based on a 180° Bloch or Néel wall model in which the magnetization rotates linearly through the wall are in disagreement with the present data over the entire range of film thickness. For the 300–800 Å thickness range, predictions based on Lorentz microscopy wall-shape measurements are in good agreement with the measured mobility values. The crosstie structure associated with Néel walls in this thickness range does not appear to affect the mobility. Bitter patterns indicate that the sharp increase in mobility between 900 and 1000 Å may be associated with a wall structure transition in this region. For thicker films, mobility predictions based on static Bloch wall-shape calculations are an order of magnitude lower than the observed mobilities. Lorentz microscopy wall-shape measurements for 1200-Å films yield wall widths of about 3000 Å significantly wider than calculated Bloch wall widths. Predictions based on these measured widths are in good agreement with the data. Although the calculation includes both intrinsic and eddy-current losses, intrinsic losses dominate for crosstie or Néel wall motion in the 300–800-Å thickness range and for Bloch wall motion in the 1000–1600 Å thickness range.

Eddy-Current-Limited Domain-Wall Motion in Thin Ferromagnetic Films

The effect of eddy current losses on domain-wall motion in thin ferromagnetic films has been investigated from a theoretical point of view. Earlier calculations neglecting wall shapes and stray demagnetizing fields do not adequately explain existing experimental data. A general expression has been developed for the current distribution about a moving Bloch wall of arbitrary shape in a thin film of infinite extent and arbitrary thickness. Stray demagnetizing field effects have been included in an approximate manner for a simple 180° wall in which the magnetization rotates linearly through a wall whose width is calculated from static energy considerations. The current distribution was evaluated numerically and the eddy-current-limited wall mobility was obtained as a function of film thickness from 400 to 4000 Å, using material parameters typical for Ni–Fe alloy films with 80% Ni. Although the current distributions exhibited a significant dependence on wall width, the mobilities were not significantly different from previous predictions. In order to explain existing mobility data on the basis of eddy current losses, wall widths much larger than calculated static Bloch wall widths must be assumed. Otherwise, some other loss mechanism must dominate.

Unidirectional Anisotropy in Multilayer Films of Cr and NiFe

Layered films of Cr and NiFe alloy have been made which exhibit biased hysteresis loops in the easy direction and asymmetric hysteresis loops in the hard direction. The asymmetry in the hard direction is reversed by rotating the film a few degrees in the loop tester. The magnitude of the bias is 0.5 Oe for films consisting of a 500-Å layer of 76% NiFe on top of a 500-Å layer of Cr on a soft glass substrate. No loop bias was observed for films of 82% NiFe, for films on hard glass substrates, or for films with layer thicknesses departing from 500 Å by a factor of two. The films exhibit high sensitivity to externally applied stress. Loop bias can be eliminated by raising the temperature above the Néel point of Cr. The unidirectional axis is a function of the direction of a magnetic field applied during cooling of a film through the Néel point. The observed phenomena are believed due to exchange anisotropy between antiferromagnetic (strain sensitive) Cr and ferromagnetic NiFe. Crystal anisotropy, shape anisotropy, magnetostriction, etc. are not unidirectional. The only unidirectional effects reported in the literature have resulted from exchange anisotropy.

A method for measurement of "Creep" in thin magnetic films

The phenomenon in thin magnetic films is measured using a field consisting of a static field parallel to the film's easy axis, and a high-frequency sinusoidal field along the transverse axis. A special field-gradient coil is used to establish a two-domain magnetization configuration in the film plane, and the Kerr magneto-optic effect is employed to measure the position of the disturbed domain wall. Measurements on Ni-Fe-Co and NiFe alloy films show the typically sharp threshold field below which there is no wall creep; nonuniform creep gives evidence for wall pinning phenomena. With the method described for field calibration and the simple form of fields employed, this procedure should be valuable in establishing comparative creep sensitivity data for films formed from different alloys or by different technologies.

Magnetic Domain Pattern and Magnetocrystalline Anisotropy in Epitaxially Grown Thin Films of Fe, Ni, and Ni-Fe Alloys

Crystal grains and domain patterns of Ni-Fe alloy films which were condensed on cleaved surfaces of heated NaCl in a vacuum of 5×10 -5 torr were studied by transmission electron microscopy and electron diffraction. Magnetic anisotropy was also measured by a torque magnetometer for thin films. The substrate temperature was varied between room temperature and 600°C. The films obtained range from polycrystal to nearly perfect single crystals. The degree of perfection of the crystal is classified into several types. The magnetic domain patterns and crystal anisotropies of the films depend on the composition and also on the degree of perfection of the films. Polycrystalline films are, in general, found to consist of domains of irregular shapes with curved walls and ripple structure within the domains. In good epitaxial films, however, the domains have a regular cross-hatched shape and when the films are magnetized parallel to one of the crystal anisotropy axes, the films are composed of linear domain walls alm...

Exchange Coupling of Uniaxial Magnetic Thin Films

The properties of two uniaxial magnetic films in intimate contact have been investigated. It is shown that when the uniaxial easy axes of the two films are orthogonal, and the magnitude of the uniaxial anisotropy the same in each film, uniaxial anisotropy in the composite film vanishes, to be replaced by a field-dependent biaxial anisotropy. The theoretical analysis relates this biaxiality to exchange coupling between the two halves of the composite film. Experimental measurements on several films are compared with the theory and imply for A, the exchange coupling constant, an average value of 0.4×10−6 erg/cm in a 66–34 Ni-Fe alloy film.

Rotating-Field Technique for Galvanomagnetic Measurements

The isothermal galvanomagnetic behavior of an isotropic conductor in a magnetic field can be described by the vector equation E=ρ⊥j+n(j·n) (ρ∥−ρ⊥)+ρH(n×j),relating the electric field E to the current density j. The resistivities ρ∥ and ρ⊥ are those which result when the magnetic induction is parallel to j, and normal to j, respectively; ρH is the Hall resistivity. The vector n is a unit vector which has the direction of the induction. When j is directed along the x axis, and n is made to rotate in the x-y plane with angular frequency ω, one obtains an electric field with ac components Ẽx=12jx(ρ∥−ρ⊥) cos2ωt,Ẽy=12jx(ρ∥−ρ⊥) sin2ωt.These equations describe the magnetoresistive effects in a rotating magnetic field. On the other hand, if n is allowed to rotate in the x-z plane, the y component of the electric field becomes Ẽy=ρHjxsinωt. This equation describes the Hall effect in a rotating field. By measuring the ac components of E having frequencies ω and 2ω when jx is known, ρ∥−ρ⊥ and ρH can be determined. This rotating-field technique is being used to study the galvanomagnetic properties of ferromagnetic films of rectangular geometry. The films are placed in the field of a magnet rotating at a frequency of 20 cps. When the magnetic field is in the plane of the film, the magnetoresistive equations apply, as has been verified by measuring Ẽx and Ẽy using a wave analyzer. No components having a frequency other than 2ω (40 cps) were detected. The absence of a component with frequency ω shows that no ``Umkehreffekt'' is present in the magnetoresistance. If the wave analyzer is replaced by a high-gain amplifier having a narrow band-pass at 40 cps, it is possible to detect a magnetoresistive change (ρ∥−ρ⊥)/ρ∥=10−6 in a typical film of 10-Ω resistance. A particular advantage of this method is that it is not susceptible to small variations in ρ∥ or ρ⊥ due to fluctuations in temperature. Thus, measurements as a function of temperature are easily and rapidly obtainable. The utility of the technique is demonstrated by measurements of ρ∥−ρ⊥ as a function of composition in Ni-Fe alloy films having fcc and bcc structures near 30% Ni. The effect of allotropic transformations (fcc→bcc) on magnetoresistance in these films is also presented. It has been found that this transformation occurs with greater difficulty in films on glass substrates than in the bulk alloys.

Study of Martensitic Transformation in Evaporated Iron-Nickel Alloy Films by Electron Microscopy and Diffraction

Seven kinds of Fe-Ni alloy films (14.35∼27.1 at% Ni) 500, 1000 and 1500 Å thick were prepared by vacuum evaporation. The films were austenitized by heating and subsequently cooled in liquid nitrogen to induce the martensitic transformation. Electron microscope and diffraction studies showed that repeated twins parallel to (112)b.c.c. were produced. The mean twin plate thickness was 60 Å. The martensite-start temperature is much lower than that for the bulk material, and it falls not only with increasing nickel content but also with decreasing film thickness. The small crystal size of austenite in the evaporated films, which causes a strong interaction between the transformation dislocations and the crystal surfaces, in considered to be responsible for the suppression of the transformation.

Magnetoresistive Measurements on Domain Rotation and Wall Development in Ni-Fe Alloy Films

In low saturating magnetic field, the magnetoresistive effect in a polycrystalline ferromagnet is described by the relation Δρ/Δρ0 = cos2α where Δρ is the resistivity change which occurs when the saturation magnetization Ms is oriented first normal then at an angle α to the electric current i. The maximum change Δρ0 is characteristic of the material. The uniform rotation model of a uniaxially anisotropic film in a transverse magnetic field H⊥ leads to a dependence of Δρ on H⊥ of the form Δρ/Δρ0 = h⊥2 when i is normal to the anisotropy axis (K axis) and of the form Δρ/Δρ0 = 1−h⊥2 when i is parallel to the K axis. In these expressions h⊥ is the ratio of H⊥ to HK, where HK = 2K/Ms, and K is the anisotropy energy constant. The present paper treats the transverse magnetization process in Ni-Fe alloy films on the basis of a simple model in which the K axes in local regions of the film are dispersed in direction about an average 〈K〉 axis. For a single numerical value of the dispersion angle β the transverse magnetoresistive behavior is described by Δρ/Δρ0 = (h⊥+β)2 when i is normal to 〈K〉 and Δρ/Δρ0 = 1−(h⊥+β)2 when i is parallel to 〈K〉. The model also requires that the ratio of transverse remanent magnetization to Ms be sinβ. These predictions are experimentally verified for films which gave values of β from 0.1 to 0.3 (6° to 18°), and show the applicability of the Stoner-Wohlfarth theory to domain rotation.

Magnetic Fields of Square-Loop Thin Films of Oblate Spheroidal Geometry

Thin films of Ni-Fe alloy may be prepared to be anisotropic and exhibit square-loop M-H characteristics. In films that are single-domained with flux changes involving only rotation of intrinsic magnetization controlled by cross-magnetization fields, very fast switching action can be obtained for storage and logic functions. Problems of coupling to the flux changes and interaction in an array of such films require study of the magnetic-field distribution. In the treatment given, a circular, single-domain, thin film is represented by a very flat oblate spheroid. The field distribution outside the spheroid is found by assuming that the magnetic properties are characterized by an intrinsic magnetization M constant in magnitude, but varies in direction depending on field and energy considerations. Calculation of the field distribution is given for a typical film with diameter to thickness ratio of 105. From the regions over which field changes are most significant, conclusions are drawn as to the proper size of sensing loops and spacing to avoid interaction during switching in film arrays.