Thick Permalloy Films Research Articles

Different dynamic and static magnetic anisotropy in thin Permalloy™ films

The values of uniaxial anisotropy Hk in thin polycrystalline Permalloy™ films measured by static and dynamic methods differ by as much as a factor of 1.5. The anisotropy obtained with a pulsed inductive microwave magnetometer in 2.5 to 100 nm thick Permalloy films exhibits an additional isotropic component of 120 to 240 A/m not observed in static measurements. The static value of anisotropy was obtained with an inductive magnetic hysteresis loop tracer. The time-resolved precessional response was measured as a function of in-plane applied magnetic bias field and the angle between the easy axis and that of the applied bias field. We interpret the constant-offset field as a transient component of the magnetic susceptibility that affects only dynamical response at time scales below 10 ns.

Search for noncollinear moments at Permalloy/copper interfaces

Recent first-principles electronic structure calculations have shown a tendency for atomic moments near interfaces between Permalloy (e.g., Ni80Fe20) and Cu to become noncollinear with the bulk magnetization direction. One signature of noncollinear moments should be a differential susceptibility at high fields (i.e., above the nominal saturation field) corresponding to a twisting of the noncollinear moments against the exchange fields in the material. We have measured the differential susceptibility for a series of sputtered Permalloy/Cu multilayers and compared it with the bulk (thick Permalloy) film. We have identified a magnetic contribution due to noncollinear moments at the interfaces corresponding to 0.1–0.3 monolayers. Further, we show that the temperature dependence of the differential susceptibility is not consistent with paramagnetic moments, supporting the idea that exchange-driven noncollinearity is present.

Domain structure of circular and ring magnets

We present the results of a study of the domain structure and the magnetization reversal processes in arrays of circular and ring magnets, obtained by magnetic force microscopy and magnetization measurements. The particles, with outer diameter 0.55 and 2.2 μm, were made of Permalloy films of thickness 24 and 66 nm. In the circular elements two domain configurations were observed: a vortex structure with perpendicular magnetization at the core and a ‘bud’ state. In the ring elements the domain structure was found to depend on the ring width: a flux closed state was formed in wide rings and the ‘onion’ state was observed in narrow rings.

Vectorial second-harmonic magneto-optic Kerr effect measurements

A significant modification of an existing experimental technique based on the second-harmonic magneto-optical Kerr effect (SH-MOKE) is introduced. With a p-polarized pumping optical wave incident upon a magnetic film, the transverse component of magnetization causes a change in the second-harmonic generation efficiency of the material and the longitudinal component of the magnetization produces a change in the polarization rotation and/or ellipticity of the second-harmonic signal. This permits simultaneous vectorial measurement of the in-plane magnetization components. Examples of measured hysteresis loops from 50 nm thick permalloy films and procedures for SH-MOKE signal calibration are presented.

High magnetoresistance permalloy films deposited on a thin NiFeCr or NiCr underlayer

Significantly enhanced anisotropic magnetoresistance (MR) in permalloy (Ni0.81Fe0.19) films deposited on a thin (Ni0.81Fe0.19)1−xCrx or Ni1−xCrx underlayer is reported. The maximum ΔR/R enhancement was observed using the underlayer with x≈0.44 at an optimum thickness of ≈30–45 Å, depending on the deposition technique. An enhancement of 75%–150% was observed for 45–430 Å thick permalloy films, compared to the films deposited without this underlayer. The ΔR/R enhancement is attributed to the decrease in the resistivity ρ and the increase in Δρ of the permalloy film due to the formation of large (111) textured crystal grains in the permalloy films deposited on this underlayer, as revealed by the x-ray diffraction results obtained using synchrotron radiation.

Resolution enhancement by applying MFM under UHV conditions

The enhancement in signal-to-noise ratio and lateral resolution in MFM in going from ambient pressure to UHV is demonstrated. The performance of several cantilevers is evaluated using a patterned 50 nm thick permalloy film, with cross-tie as well as /spl sim/90/spl deg/ domain walls, and a 200 nm thick permalloy film with perpendicular magnetization. The increase in the quality factor of the cantilever oscillation in UHV improves the sensitivity, consequently allowing less magnetic material on the tip to achieve the same signal-to-noise ratio. This reduction in magnetic volume sharpens the lateral resolution. We also demonstrate that the magnetic interaction can be so weak that a magnetic contrast is visible only under UHV conditions.

Methods for determination of response times of magnetic head materials

Measurements of magnetization response times in thin-film Permalloy are made by both electrical and optical methods. The electrical method measures the inductive voltage generated in a waveguide by the changing magnetization. The optical technique uses standard pump-probe sampling methods combined with the second-harmonic magneto-optic Kerr effect to directly measure magnetization angle as a function of time. Results of these measurements for a 75 nm thick Permalloy film are in good qualitative agreement.

Structural and magnetic properties of annealed multilayers studied by Mössbauer spectroscopy, x-ray diffraction and magnetization measurements

The influence of annealing on the properties of an ultrathin permalloy/silver multilayer is analysed using Mössbauer effect, x-ray diffraction, magnetization and magnetoresistance experiments. A thick permalloy film is studied for comparison. From Mössbauer experiments, the existence of a `magnetically dead' layer is ruled out. An interfacial zone corresponding to about 1.2 monolayers is observed in the low hyperfine field range in the as-deposited film and disappears after annealing at , when the layer structure is destroyed. The evolution of the hyperfine field is associated with both densification of the NiFe layers and interface improvement. From Mössbauer and magnetization measurements, the individual Ni and Fe magnetic moments have been evaluated. They exhibit different thermal dependences, in agreement with results on multilayers based on pure Fe or Ni layers.

Structure of moving domain walls

The magnetization configuration of moving domain walls is computed for 100 nm thick Permalloy films, and 200 and 1100 nm thick iron films. Changes in the structure, mass and width of the walls as a function of velocity are reported.

Stress effects of water sorption in cure baked photoresist underlayers on the magnetic easy axis of Permalloy film overlayers

Variations in the uniaxial mechanical stress of Permalloy films can change the magnetic easy axis of the Permalloy films through the magnetostrictive effect. Conventional inductive film heads are fabricated with the use of cure baked photoresist (CBP). We show that water absorption in the CBP can produce significant changes in the magnetic properties of Permalloy films similar to those used in recording head yokes. The diffusion coefficient of water in 3 /spl mu/m thick CBP films was measured with a quartz crystal microbalance and found to be 2.2/spl times/10/sup -9/ cm/sup 2//s at 25 degrees C. A large and reversible change in the magnetic easy axis is observed in 1 /spl mu/m thick Permalloy films that overlay a mound of CBP when it is exposed to water vapor. We infer from the observed magnetic easy axis changes that uniaxial stresses of greater than 5/spl times/10/sup 8/ dynes/cm/sup 2/ are produced due to the swelling and contraction of the underlaying CBP. >

Domain-wall dynamics in thick Permalloy films

Micromagnetic numerical simulations are extended to study domain-wall structures and dynamics in micron-thick Permalloy films with an in-plane anisotropy easy axis. The equilibrium domain-wall structure is a flux-closure vortex elongated along the film thickness direction for films thicker than 2 μm. The dynamics of such a vortex wall under an easy-axis field is studied to yield velocity dependence and wall mass. The effect of eddy current damping is simplified by including a larger damping constant in addition to the intrinsic spin damping. With large damping, the steady-state wall structure is more stabilized under applied fields. Under a hard-axis field, wall structure transitions take place for small damping and low-frequency fields; for large damping and higher-frequency fields, these transformations are more suppressed, and the flux conduction is dominated by magnetization rotations instead of wall motions.

Multilayer Permalloy films grown by molecular-beam epitaxy

Single and multilayer [111]-textured films of Permalloy and Cu were grown by molecular-beam epitaxy (MBE) on (111) Si substrates. The magnetic properties of the films were measured by ferromagnetic resonance and M-H loop tracer. The microstructure was observed by transmission electron microscopy, reflection high-energy electron diffraction, and x-ray diffraction. Even the single-layer films had lower easy axis coercivity Hce (≊0.6 Oe) and a lower in-plane anisotropy field (≊1.1 Oe) than permalloy films of similar thickness (≊80 nm) deposited by sputtering. In these single-layer films, the grain size and Hce both increased with improved pre-MBE cleaning of the Si substrate. The multilayers consisted of five identical thickness Permalloy layers separated by Cu interlayers. Multilayers with total magnetic thickness greater than 100 nm exhibited lower Hce than equivalent single-layer films. 4πMs, measured by a combination of ferromagnetic resonance and M-H loop tracer in very thin (<5 nm) permalloy layers was found to drop off relative to the bulk value of 9.5 kG; a value of 5.0 kG was measured for a multilayer with 1-nm-thick Permalloy layers.

Fabrication and Magnetic Properties of Thin Films Sputtered on the Substrate Excited by SAW

Permalloy films of thickness 300 A were deposited by d.c. magnetron sputtering onto LiNbO 3 single-crystal substrates excited by SAW (surface acoustic waves) at 45 MHz. Film structures, such as grain shapes and sizes were found to change drastically as functions of the excitation amplitude. Particularly when deposited at an excitation amplitude of 24 A o-p , films consisted of very fine homogeneous grains and showed soft magnetic properties, namely a minimum coercive force. A uniaxial anisotropy was induced in the film plane in all films examined; however, at the critical amplitude, the in-plane easy axis was found to change significantly. It was concluded that deposition of film onto substrates excited by SAW is a highly useful method for controlling the film microstructure and magnetic properties.

弾性表面波で励振された基板上への薄膜の作製とその磁性

Permalloy films of 300A thickness were prepared by d.c. magnetron sputtering onto LiNbO3 single crystal substrate excited by SAW at 45 MHz. Film structure such as shapes and size of grains was found to be drastically changed as a function of excited amplitude. Especially in the case of 24Ao-p for excited amplitude, films consisted of very fine homogeneous grains and showed magnetic softness, namely minimum coercive force. Uniaxial anisotropy was induced in plane for all the films examined; however, in the critical excited amplitude mentioned above, a drastic change of the easy axis in plane was found. As a conclusion, film preparation onto the substrate excited by SAW is considered to be a highly useful method to control the microscopical film structure and also the magnetic properties.

Use of the longitudinal magneto-optical Kerr effect to study nonmagnetic/magnetic bilayers

We report the results of longitudinal magneto-optical Kerr effect (LMKE) measurements on bilayers consisting of a thick permalloy film covered by varying thicknesses of Nb. The LMKE pertains to situations in which light with a finite angle of incidence is reflected from a plane surface, and M in the sample is parallel both to the plane of incidence and the surface. Either s- or p-linearly polarized 6328-Å light is incident on the sample at 60° to the normal. The two components of the complex Kerr rotation are measured with a modulation technique utilizing a λ/4 plate and a Faraday rotator driven by dc and 2000-Hz signals. With cycling of an in-plane magnetic field, square magneto-optical hysteresis loops are observed in both rotation and ellipticity. There is a change in sign, as well as a decrease in the magnitude of the switching discontinuity in these quantities with increasing overlayer thickness. These results are very similar to those of the corresponding idealized boundary value problem. The technique gives experimental information on the magnetic properties of films buried below a nonmagnetic material or of thin films bounded on both sides by a nonmagnetic material. Further, it gives a probe of the phase and intensity of optical frequency electric fields below a reflecting surface.

Exchange forces between domain wall and electric current in permalloy films of variable thickness

Wall displacements are induced by large current pulses crossing a wall, in Ni81Fe19 films. In films of thickness w<35 nm containing Néel walls, the sense of wall motion is found to be independent of the magnetization sense in the two domains adjacent to the wall, and is identical to the sense of motion of the electronlike charge carriers. This shows that the wall motion is not caused by stray magnetic fields, but rather by s-d exchange forces generated by conduction electrons. The value ≂5×107 A cm−2 of the needed current density agrees with predictions of a theory based on s-d exchange. In the case of cross-tie walls in films with 35 nm <w<86 nm, the sense of wall motion does depend on the sense of the domain magnetizations. The force of the circumferential magnetic field of the current is probably important, here.

Chemically assisted sputter etching of permalloy using CO or CL2

Redeposition-free patterns have been etched in 3000 Å thick permalloy films using the technique of chemically assisted sputter etching. This technique involves the simultaneous adsorption of a reactant gas from a molecular beam source and normally incident argon ion bombardment. Experimental conditions with Cl2 adsorption can be maintained to etch the permalloy at 80 Å/min and to etch positive photoresist at 110 Å/min. These etch rates are not significantly different from rates obtained with argon ion milling, even though the permalloy surface is known to incorporate Cl2. Redeposited material observed with Cl2 chemically assisted sputtering has been shown to be water soluble, and can be completely removed from feature sidewalls prior to stripping the resist. Linewidth measurements show that no feature broadening occurs on 2 μ line and space patterns, within the precision of the measurement (±0.2 μ). Chemically assisted sputter etching with the adsorption of CO resulted in reduced etch rates and increased redeposition. Both of these effects are attributed to the formation of carbides on the alloy surface during the process.

Spin-wave spectra of insulating films: Comparison of exact calculations and a single-wave-vector model.

Spin-wave spectra are usually calculated within the circular-precession approximation which neglects ellipticity of the microwave magnetization. This approximation yields rigorously correct results only for perpendicular resonance and becomes worse for parallel resonance. Even if the ellipticity is accounted for, only single-wave-vector modes are commonly postulated for nonperpendicular resonance. Such modes cannot satisfy the boundary conditions except for zero surface anisotropy energy when the model is exact for any configuration of applied external field. Rigorous results correspond to normal modes made up of waves with two different wave vectors. Calculations with a set of parameters typical for a 1000-A\r{}-thick Permalloy film indicate that the exact results may significantly differ from the single-wave-vector model both in the position of the normal modes and their intensities. The predicted critical angle is also different from the \ensuremath{\pi}/4 observed. It is concluded that the exact procedure is, in principle, required.

Thickness Dependence of Induced Magnetic Anisotropy in Evaporated Permalloy Films

The uniaxial magnetic anisotropy (K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">u</sub> ) of evaporated Permalloy films of different thicknesses was measured. For thinner films, measurements relied on the magnetoresistive (ΔR) effect. Although there was considerable scattering in the results, K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">u</sub> tended to decrease as films were made thinner than 500 Å, and disappeared at around 100 to 300 Å. This decrease is thought to be related to the island structure of extremely thin films, in which case the anisotropy finally disappears in fine crystal grains of size arotund 100 Å.

Magnetic, structural and magnetoresistive properties of magnetron-sputtered thin NiFe films

The resistivity, magnetoresistance ratio, magnetic hysteresis loop and structure of magnetron-sputtered permalloy (Ni 81Fe 19) films of thickness 200–1600 Å were studied and compared with the corresponding properties of permalloy films prepared by electron beam evaporation. The r.f.-sputtered films were prepared in an argon plasma from a sputter gun with an aligning magnetic field of 400 Oe applied in the plane of the substrates. No external heating or substrate bias voltage was used during the deposition. The resistivities of sputtered permalloy films show a similar thickness dependence to that for evaporated films, although with higher values. The residual resistances of the sputtered films are also higher; this is attributed to the greater amount of incorporated gases and possibly less atomic ordering. From the thickness dependence of the resistivity as measured in the range 200–1600 Å, the value of the boundary reflection coefficient was estimated and was found to be almost the same as that reported earlier for permalloy films deposited in an ultrahigh vacuum. X-ray diffraction techniques were employed to measure the intrinsic isotropic stress σ which was found to be about 6×10 9 dyn cm −2. The magnetoresistance ratio of the sputtered films is less than 2%, which might be explained by their higher resistivity values. The coercivity of the films is in the range 1.5–3 Oe, which is higher than that of evaporated permalloy films. The same was true of the anisotropy field, which is between 5 and 6.5 Oe.