Magnetocrystalline Easy Axis Research Articles

Crystallographic texture and angular dependence of coercivity of ordered CoPt thin film

Effect of crystallographic texture on the angular dependence of coercivity of ordered CoPt thin film was studied by successfully controlling the texture of the ordered CoPt thin film. We have developed, based on the Stoner–Wohlfarth interacting particles model, a micromagnetic simulation technique which can simulate the effect of distribution of magnetocrystalline easy axis on the magnetization behavior of thin film. The good agreement between the experiments and simulation suggested that the magnetization reversal in ordered CoPt thin film occurs not by the domain wall motion but by the Stoner–Wohlfarth interacting particles model.

Mapping orientations of easy axes in nanodot arrays by magnetic force microscopy

An experimental method has been developed for mapping the three-dimensional (3D) orientations of easy axes in nanodot arrays by means of magnetic force microscopy. This method was applied to a CoCrPt-patterned medium to measure 3D easy-axes orientations of individual cylindrical dots of an average of 36 nm in diameter and 40 nm in height. A main easy-axes orientation was found according to the planar number distribution of dots being in the same range of easy-axes orientations. The dot-number dependence on inclining angles of magnetocrystalline easy axes from the perpendicular direction agreed well with the x-ray diffraction result. The magnetization reversal mechanism in the medium was also discussed.

Stepwise and continuous low‐temperature demagnetization

Magnetites with sizes from 1 μm to 135 μm were cooled in zero field and their magnetizations M(T) measured continuously. M(T) changed reversibly in cooling from T0 = 300 K to 200 K, and in subsidiary warming‐cooling cycles Ti → T0 → Ti for any Ti. Changes in M(T) in cooling from 200 K to 130 K were largely irreversible due to decreasing magnetocrystalline anisotropy which promotes wall unpinning and domain nucleation. Low‐temperature demagnetization (LTD) is almost complete by 130 K in 20–135 μm magnetites but in 1–14 μm magnetites further LTD occurs on cooling to 120 K as magnetocrystalline easy axes change and domains reorganize at the Verwey transition. The observed irreversible changes are the basis of stepwise LTD as a method of paleomagnetic “cleaning.” Decrements ΔM in remanence due to cooling are most accurately measured at T0, requiring a set of warming‐cooling cycles Ti → T0 → Ti. A less accurate method, continuous LTD, measures decrements M(Ti) − M(Ti−1) from the main cooling curve below 200 K, without intermediate warming‐cooling cycles; this requires remanence measurements at Ti < T0. Stepwise or continuous LTD curves M(Ti) discriminate among remanence types and grain sizes. The signal of finer (PSD) grains is enhanced compared to coarser (MD) grains. Analogous to the Lowrie and Fuller [1971] test, the inverse thermoremanence (ITRM) of 1–14 μm grains is harder to stepwise LTD than saturation remanence (SIRM), while anhysteretic remanence (ARM) is harder than either; for 20–135 μm multidomain grains, ITRM is softer than SIRM.

Magnetic behavior of atomically engineered NiO–Co–Cu-based giant magnetoresistance spin valves using Pb as a surface modifier

The present study illustrates the efficacy of using surface modifiers, or simply surfactants, to alter, modify, or control the nature of interfaces and film morphology in order to control physical properties in magnetic multilayers. In particular, the magnetic properties of giant magnetoresistive NiO–Co–Cu-based symmetric spin valves are discussed in relation to the modification of interfaces and nanostructure using surface modifier Pb. Results show that a ML of Pb deposited on the first Co/Cu bilayer of the symmetric spin valve leads to reduced in-plane magnetic anisotropy and coupling. In the presence of Pb, the coherent growth mode of Co–Cu layers is disrupted, leading to a fine grain size (≈1–5 nm) in the metal layers. Although this grain size corresponds to the critical wavelength for maximum coupling due to Néel’s so-called “orange-peel” effect, the absence of topographical correlation between various interfaces prevent the occurrence of this form of coupling. At the same time, averaging of exchange interactions over such a fine grain size, which is much lower than the characteristic exchange length for Co (≈25–45 nm), precludes the display of local magnetocrystalline easy axes, thereby leading to low switching fields. The nature of magnetization reversal in the “free” Co layer of the Pb-free spin valve is highly local in nature and occurs by nucleation, growth and coalescence together of irregular micron or sub-micron sized domains.

Micromagnetism and magnetization reversal of micron-scale (110) Fe thin-film magnetic elements

Magnetic force microscope ~MFM! imaging in conjunction with longitudinal Kerr hysteresis loop measurements have been used to investigate the micromagnetic behavior of micron scale epitaxial ~110! bcc Fe thin-film elements~50-nm thick! with rectangular, triangular, and needle-shaped ends and competing magnetic anisotropies. Thin-film elements of 2-mm width and 6-mm length and greater have been fabricated with their long axis oriented either parallel or perpendicular to the @001# in-plane magnetocrystalline easy axis. For elements with their long axis perpendicular to the @001# direction, the end shape is critical in determining domain nucleation, domain configurations, and magnetic hysteresis. The magnetization reversal mechanisms are revealed by direct field dependent MFM imaging. Magnetic vortex configurations within elements during reversal are seen to be affected by small changes in element corner shape. Similarly, small trapped domains and domain walls are observed to applied fields significantly larger than the coercive field and apparent magnetic saturation field, as determined by hysteresis loop measurements of arrays of elements. These are shown to have a dramatic effect on the character of the low-field magnetic hysteresis. Particles with long axis parallel to the @001# direction have large remanence and switching fields which also depend sensitively on end shape. The angular dependence of the switching field observed in these elements is contrasted to that of magnetization reversal by coherent rotation.

Ferromagnetic resonance of particulate magnetic recording tapes

The room-temperature ferromagnetic resonance (FMR) spectra of γ-Fe2O3, CrO2, and barium ferrite particulate magnetic recording tapes have been measured at microwave frequencies of 9.35 and 35 GHz for various orientations of the static and high-frequency magnetic fields with respect to the tape. For CrO2 tapes, the influence of the width of the angular distribution of the particle orientations on the FMR spectra has been studied from the nearly isotropic case up to the highly oriented case. Hysteretic behavior for a CrO2 tape as well as the effect of tape calendering for a γ-Fe2O3 tape has been observed by FMR. Experimental results are found to be in reasonable agreement with results of theoretical calculations based on a model of an ellipsoidal single-domain particle with both shape and magnetocrystalline anisotropy. Magnetostatic interaction inside the magnetic film has been introduced by expressing the total magnetostatic energy as a combination of a part dependent on particle shape and a part dependent on the shape of the tape. As a result of a comparison of experimental data with calculated data from the model, the magnetocrystalline easy axis of the CrO2 particles is found to be parallel with the particle axis.