Shape Anisotropy Field Research Articles

Vertical giant magnetoresistive read heads for 50-Gb/in/sup 2/ magnetic recording

We have designed and fabricated a vertical giant magnetoresistive (VGMR) head for high-density magnetic recording. In this style of recording head, read current flows through the sensor length, which is oriented perpendicular to the air-bearing surface (ABS), and the transducer is fabricated from GMR sandwich material. In the VGMR style of head design, the correct bias is obtained through a combination of intrinsic anisotropy, shape anisotropy, and longitudinal bias field generated by the sense current. The material at the edge of the sensor is stabilized using an edge-hardening process, which is designed to prevent switching of the magnetization at the edge of the sensor in a reverse applied field. Both top and bottom shields are recessed away from the ABS in order to increase the signal decay length and boost sensor output. We tested read heads incorporating the VGMR reader design under conditions of up to 50 Gb/in/sup 2/. We describe additional sensor concepts, including folded and differential VGMR head designs.

The switching properties of patterned synthetic ferrimagnetic structures

We investigated the switching properties of patterned submicrometer synthetic ferromagnetic (SF) Ni65Fe15Co20(t1nm)∕Ru0.8nm∕Ni65Fe15Co20(t2nm) tri-layers. By changing t1 and t2, the shape anisotropy field, Hksh∝∣t1−t2∣, was changed from 36 to 18Oe, and the effective material anisotropy field, Hkeff,∝α=(t1+t2)∕∣t1−t2∣, was changed from 28 to over 60Oe. We found that a hard axis field, Hhd, is less effective at reducing the easy axis switching field, Hsw, as α is increased, with α=3.7 requiring twice the relative magnitude of Hhd for the same relative reduction in Hsw as a single magnetic layer. In addition, by repeating the basic SF tri-layer structure in circular elements, we demonstrated improved stability against thermal activation by a factor of 2 with no associated increase in Hsw.

Spin waves in an inhomogeneously magnetized stripe

We have observed collective spin-wave modes in inhomogeneously magnetized ${\mathrm{Ni}}_{0.81}{\mathrm{Fe}}_{0.19}$ thin-film stripes. The stripes, 18 nm thick and $2\ensuremath{\mu}\mathrm{m}$ wide, are studied in an in-plane magnetic field oriented along their short axes. When the magnetic field is on the order of the shape anisotropy field, the equilibrium magnetization near the stripe edges rotates 90 deg over a length scale of order $100\mathrm{nm}--1\ensuremath{\mu}\mathrm{m}.$ Time-resolved Kerr microscopy is used to detect a hierarchy of spin-wave modes in these edge regions. Using a combination of semianalytical theory and micromagnetic simulations, we show that these modes span the entire stripe but can only be detected near the edges, where the effective wave vector is small.

Magnetotransport properties of hard magnetic pinned multilayers

Hard magnetic (HM) thin films have been vertically integrated below giant magnetoresistive (GMR) multilayer sensors in order to shift the operating point of an applicable GMR sensor. The shift in maximum magnetoresistance (MR) peaks was found to be dependent on the thickness of the HM layer. As a result of different bias geometries, which have been tested, current shunting effects mainly reduce the maximum MR amplitudes. A strong microstructural influence on the magnetotransport has been found; meander-shaped microstructures with different stripe widths have been microfabricated in order to investigate the influence of shape anisotropy and stray field geometry on the magnetotransport. As a result, the bias strength, as well as the shape of the HM hysteresis, varies with the underlying microstructured pattern geometries and the angle of applied field. Comparing the major MR loops of meanders with 1.5, 5, and 10 μm structure width of HM/GMR systems, different MR behavior has been found. It was determined that the MR loops are shifted by different field values simultaneously with a change in the MR loop shape. The effect depends on the orientation of applied field with respect to the meander structure, as well as on the structure width. A creeping effect, which demagnetizes the hard layer, has not been observed during sine-shaped switching cycles. The MR response signal of biased multilayers can be understood using numerical calculations.

Fabrication and magnetic properties of ultrathin Fe nanowire arrays

Ultrathin Fe nanowire (about 5 nm in diameter) arrays have been fabricated by electrodeposition using anodic porous alumina templates. These ultrathin nanowires exhibited uniaxial anisotropy and a quite large coercivity (4190 Oe) at 5 K. In addition, the field needed to saturate the magnetization, when the field was applied perpendicularly to the easy axis, was much larger than the shape anisotropy field (2πMS). This saturation field increased with decreasing temperature. We believed that this enhanced saturation field was mainly due to the contribution of the surface spins.

FRONT PROPAGATION IN UNIAXIAL FERROMAGNETS

This work is a theoretical study of fronts propagating into an uniaxial ferromagnetic medium with velocity v, when the magnetization, M, is driven by a DC applied magnetic field, H0, from the demagnetized to the magnetized state. It is assumed that the dynamics of M is govern by the Landau–Lifshitz–Gilbert equation (LLGE). We also consider an effective field that includes in-plane uniaxial, HU, and shape anisotropy fields, HD. We show that, in the particular case of uniformly translating profiles, this equation reduces to a damped-forced Duffing's equation, with a family of solutions that describe periodic oscillating (PO), damped oscillating (DO) or exponential front profiles (EF). Of particular interest is the existence of a critical speed, v0, below which there are no stable states for the magnetization. When the velocity of the profile exactly equals v osc , the only stable states are the PO profiles. Above v osc , there is an asymptotic speed, v*, that separates the DO profiles from EF profiles. This velocity (v*) is connected with the existence of a non-linear marginal stability point for front propagation. Both v0 and v* depend on the applied field and on the anisotropy constants of the material. A stability diagram for front propagation and magnetization curves are also calculated.

Materials Issues in the Application of Ferromagnetic Films in Tunable High Q VHF/UHF Devices

AbstractThin ferromagnetic films have narrow ferromagnetic resonance bandwidths, and large RF permeabilities that are attractive for tunable high Q device applications. They can also be processed at low temperatures and integrated with most RF and microwave materials. At VHF/UHF frequencies, where discrete devices can be realized, magnetic loss tangents (Q−1) less than 10−3 can be obtained, if the effects of spin resonance and eddy current losses are reduced. The material figure of merit requires a tradeoff of the permeability, and Q. The highest FOM is obtained by maximizing the magnetization, anisotropy, resistivity, and minimizing the magnetic loss. This paper discusses the interplay of these properties, and the fill factor and magnetic bias required for tunable inductors with Q>1000 in the ∼50-500 MHz range. The most promising candidates for these applications are shown to be reactively sputtered nanocrystalline films. The large resistivity, magnetization, and thickness of these films can be used to attain large film permeances, and increased shape anisotropy fields from patterned film objects. The latter eliminates the need for using a large bias magnet to achieve Q's ∼1000 at VHF/UHF operating frequencies.

Magnetic anisotropy and switching field of spindle-shaped metal particles for particulate recording media

The difference of magnetic properties between spindle- and needle-shaped metal particles, which have almost the same thickness of about 17–27 nm, is discussed through a magnetic torque analysis for particulate films made of these metal particles. The effective magnetic anisotropy field of particles H ptc k(eff) are 6.3 and 6.5 kOe (averaged values), respectively, for spindle- and needle-shaped metal particles, showing no significant difference. Because of the interparticulate magnetostatic coupling, these H ptc k(eff) values are estimated to be about 1 kOe smaller than intrinsic values of shape anisotropy field. Saturation magnetization M s in both kinds of metal particles is found to show almost the same value of about 1300 emu/cm 3. However, the switching field H EA r normalized by H ptc k(eff) of the spindle-shaped particles is about 10–15% larger than that of the needle-shaped particles. Furthermore, the value of SFD of the spindle-shaped particles is about 20% smaller than that of the needle-shaped particles. The values of H EA r/ H ptc k(eff) and rotational hysteresis loss R h in both kinds of metal particles are strongly related to the thickness of particles, however, significant differences in these relations between spindle- and needle-shaped metal particles were not observed.

Magnetic switching and uniaxial anisotropy in lithographically defined anti-dot Permalloy arrays

The magnetic switching of a continuous Permalloy film is artificially modified by lithographically structuring an anti-dot array. The anti-dots introduce a spatially variant shape anisotropy field which competes with the intrinsic uniaxial anisotropy of the continuous film. This results in two different spin reversal mechanisms, depending on the relative orientation of the applied magnetic field and the intrinsic anisotropy axes and which have been observed by scanning Kerr microscopy. When the field is applied along the intrinsic easy axis, reversal proceeds by the sweeping of two 180° domain walls (DWs) through the sample. The anti-dots become the dominant DW pinning sites which leads to a slight increase in the coercivity. This could be used in the future as a way of artificially controlling coercivity. When the field is applied along the intrinsic hard axis, a well defined periodic domain structure occurs at remanence. This could be significant for high density data storage. A micromagnetic calculation explains the two reversal mechanisms and indicates optimum anisotropy conditions for data storage applications.

Magnetic domain formation in lithographically defined antidot Permalloy arrays

The magnetic switching of a continuous Permalloy film is artificially modified by lithographically structuring an antidot array. The antidots introduce a spatially variant shape anisotropy field that competes with the intrinsic uniaxial anisotropy of the continuous film. This results in a well-defined periodic domain structure at remanence as observed by scanning Kerr microscopy, and which could be significant for high-density data storage. A micromagnetic calculation explains the structure and indicates optimum anisotropy conditions for data storage applications.

Ferromagnetic resonance and magnetic disaccommodation of Ti-doped single crystal lithium ferrites

Disaccommodation and ferromagnetic resonance measurements for single crystal titanium-doped lithium ferrites are presented. Magnetic disaccommodation is performed at 1 kHz from 77 to 400 K while ferromagnetic resonance is carried out at 11 GHz from 77 to 300 K. Ferromagnetic resonance line shape asymmetry and anisotropy field data are also shown. A relationship between the asymmetry and the disaccommodation spectra is suggested.

Effective magnetic anisotropy field of Pd/Ni multilayers

The effective magnetic anisotropy field of Pd/Ni multilayer structures has been investigated. The strain between the substrate and magnetic thin film may contribute significantly to the effective internal field via magnetostriction since Ni is a strong magnetostrictive ferromagnet. The multilayer structures with Ni thicknesses larger than the exchange length (3.2 nm) were studied by means of FMR at the X-band. Polycrystalline samples were deposited (rf-sputtered) on Corning glass and mica. FMR measurements yield an effective anisotropy field including various anisotropy contributions; in our case we have the magnetic shape anisotropy field H u, the stress-induced anisotropy field H σ and the magnetic interaction field H int· H σ was found to be dependent on the Ni thickness, but was not altered by the thicknesses t 1 and t 2 of Pd for Pd( t 1)/Ni and Pd( t 1)/Ni/Pd( t 2) structures. The magnetic interaction field H int in the Pd( t 1)/Ni/Pd( t 2)/Ni/Pd( t 3) structure versus the thickness t 2 of the Pd interlayer (the middle layer) is also altered by stresses. For each value of t 2 (from 1.0 to 5.0 nm) H int of a more strained film (Corning glass as the substrate) is smaller than that for a less strained one (mica as the substrate). The surface mode excitations in the spin wave resonance (SWR) experiment were used to derive the surface anisotropy energy constant K s. For the Pd( t 1)/Ni/Pd( t 2)/Ni/Pd( t 3) structure, K s is altered by stresses but does not depend on the thickness t 2 of the Pd interlayer.

Magnetoresistance and magnetization in submicron ferromagnetic gratings

A technique for engineering micron and submicron scale structures from magnetic films of transition metals has been developed using a combination of electron- and ion-beam lithography enabling high-quality arrays of submicron magnetic Fe wires to be fabricated. This process can be used to fabricate novel devices from a variety of metal combinations which would not be possible by the usual liftoff metallization method. The structure and magnetic properties are reported of an epitaxial 25 nm Fe(001)/GaAs(001) film and the wire gratings which are fabricated from it. The width of the wires in the grating is 0.5 μm for all structures studied, but the separation of each wire is varied in the range 0.5 to 16 μm. An artificially induced shape anisotropy field of around 1 kG, consistent with a magnetostatic calculation, was observed for all separations studied. The field dependence of the magneto-optic Kerr effect and magnetoresistance (MR) data is consistent with a twisted magnetization configuration across the width of the sample beneath saturation for transverse applied fields. In this case, the detailed form of the field dependence of the MR is strikingly modified from that observed in the continuous film and is consistent with coherent rotation of the magnetization.

A Discussion about Soft Magnetic Properties of Highly Oriented Fe-Base Films

I-H loops of ≪111≫, ≪100≫ and ≪100≫ oriented single-crystal films of Fe-base alloys were obtained by calculations in which the single-domain model was used, with the total energy including the shape anisotropy, magnetocrystalline anisotropy and magnetic field energy. These calculations show that the coercive force of ≪111≫ oriented film is as small as 50 A/m, less than one one-hundredth of that for ≪110≫ and ≪100≫ oriented film. In addition, the K <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</inf> (magnetic anisotropy constant) dependence and shape dependence of the coercive force for ≪111≫ oriented film were obtained.

配向性Ｆｅ系薄膜の軟磁気特性に関する考察

I-H loops of , or oriented single crystal films of Fe base alloys were obtained by calculation in which the single domain model is used with the total energy including shape anisotropy, magnetocrystal anisotropy and magnetic field energy. This calculation shows that the coercive force of oriented film is as small as 50 [A/m], which is less than a hundredth of those for or oriented film. And K1 (magnetic anisotropy constant) dependence and shape dependence of coercive force for oriented film were obtained.

Magnetic properties and anisotropy of CoCr thin films for perpendicular recording

CoCr thin films, 50 Å to 1 μm thick, have been rf sputtered onto a number of different substrates. These films have been studied for their nucleation, microstructure, magnetic properties, and anisotropy. It has been observed that at lower thicknesses, the grains are extremely small (200 Å at 0.1 μm) but cover the entire substrate surface quite uniformly. Below the thickness of 100 Å, the coercivity Hc approaches zero very rapidly and the film approaches superparamagnetic state. As the film thickness is increased, the grains get larger (800 Å at 1 μm) and the coercivity of the film increases. The Δθ50 value for the (0002) x-ray peak reached 2°. The grain size clearly dominates the coercivity of these films. The effective anisotropy constants increased from 1.31 to 3.16×105 ergs/cm3 for the above thickness range. Estimates of the magnetocrystalline anisotropy field are determined from a uniform thin-film model and a model that accounts for the columnar grains. When the circular magnetic columns are in contact, the shape anisotropy field is 5% lower than the uniform thin-film approximation. When the magnetic radius is 80% of the grain radius, then the shape anisotropy field is 34% lower.

Magnetic properties and anisotropy of CoCr thin films for perpendicular recording (abstract)

The magnetic properties, microstructure, and anisotropy of CoCr thin films have been studied using VSM, x-ray, torque magnetometer, and electron microscopy techniques. These films were grown on a number of different substrates including sapphire, glass, mica, Al, and Ni-P, by rf sputtering, in the thickness range 300 Å–1 μm. The microstructures dominate the magnetic properties of these films and the coercivity Hc continues to increase even up to the grain diameters of 1000 Å. The Δθ50 value for the (0002) x-ray peak reached 2°. For the sapphire the anisotropy field Hk is positive down to a film thickness of 500 Å. Effective anisotropy constants as measured by torque magnetometer increase from 1.31 to 3.16×10+5 erg/cm3 for film thicknesses ranging from 0.1 to 1 μm. We have estimated the magnetocrystalline anisotropy from the measured anisotropy values from the torque magnetometer. In calculating the shape anisotropy, this model accounts for magnetostatically interacting cylindrical columns as opposed to the simple uniform thin-film model. Since there is experimental evidence that Cr segregates towards the grain-boundary walls, a variety of models were used where the diameter of the grains were kept constant while the diameter of the magnetic parts of the grains were gradually reduced. When the magnetic columns are in contact, the shape anisotropy field Hs is 5% lower than the uniform thin film approximation. When the magnetic radius is 80% of the grain radius, then the Hs is 34% lower, but the magnetocrystalline anisotropy constant Ku is 7% higher.

Some effects of comprehenssive stress on an amorphous ribbon

A simple apparatus has been built and tested for measuring the dc magnetostriction of amorphous ribbon material under compressive stress. The magnetostriction was found to increase linearly from 6.6 to 9.3 μϵ as the stress was increased from zero to 7 MPa. The initial value of the observed magnetostriction and the increase with stress have been explained in terms of the effect that the shape anisotropy field and the stress have in rotating the magnetization vector away from the axis of the ribbon. This effect is strongly dependent upon the shape of the test sample.

Measurement of saturation magnetostriction of a thin amorphous ribbon by means of small-angle magnetization rotation

A new method was developed to measure the saturation magnetostriction of a thin amorphous ribbon. It is based on the use of small-angle magnetization rotation to measure the change in anisotropy field caused by the tensile stress. Measurements have been performed on Metglas 2826, 2605, and Co-Si-B amorphous ribbons. The maximum experimental error of the measurement was estimated to be about ±5 percent. It is shown that the sensitivity of the method depends on the shape anisotropy field. The estimated sensitivity was about 2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-7</sup> for Metglas 2826 ribbon with 1.8 mm × 40 μm × 12 cm in dimensions.

Experimental method of measuring the anisotropy field in small samples

The method uses a vibrating reed magnetometer and measures the variation of mechanical resonance frequency when the static applied field is increased. The experimental results obtained when measuring shape anisotropy fields of thin Ni and Fe wires show that this method is very sensitive.