Search

A technique based on torque magnetometry is proposed for precise determination of the uniaxial magnetic anisotropy constant, K u, of perpendicularly magnetized thin films, in particular for K u larger than 106 J/m3, by electrical detection of the anomalous Hall effect and exact determination of the magnetic field direction. This technique can measure the K u value of several-nanometer-thick polycrystalline films. Further, the K u value of FePt–C granular films, which are expected to be useful for high-density magnetic recording media, is evaluated. The intrinsic K u value of 4-nm-thick FePt–C thin films fabricated at 450 °C is approximately 3 MJ/m3 for carbon contents up to 34 vol % C.

First-principles calculation predicts a large negative to positive change in the intrinsic perpendicular magnetic anisotropy (PMA) on the order of 107 erg/cm3 of a W/Fe(001) multilayer upon reducing the in-plane lattice constant. This PMA arises at the W/Fe interface, and the second interfacial W site plays an important role. To experimentally verify this theoretical prediction, we have grown W/Fe/W(001) epitaxial trilayers on Cr(001) underlayers. By varying the W layer thickness, the in-plane lattice constant of W can be widely controlled, and the large change in PMA from negative to positive is successfully demonstrated.

Microwave-assisted magnetization switching (MAS) in a granular magnetic film is examined by computer simulation. Contrary to the macrospin calculation and the experiments on single magnetic dots reported so far, in which the switching field linearly decreases with increasing rf frequency and then sharply increases at the critical frequency, the granular film exhibits considerably broad MAS behavior against the rf frequency. This broad MAS behavior is mainly caused by the dispersion of magnetic properties and thermal agitation. On the other hand, intergrain dipolar and exchange interactions enhance the MAS effect in the granular film and suppress the MAS broadening.

The addition of Ru to L10 (fct)-FePt thin films was investigated to lower the Curie temperature (T c) of the films, which is beneficial for heat-assisted magnetic recording. The FePtRu films exhibited L10-ordering and perpendicular anisotropy for Ru contents up to at least 18 at. %, and T c was reduced by ∼100 °C/6 at. % Ru independent of the Fe/Pt ratio between 0.8 and 1.1. The T c versus uniaxial magnetic anisotropy diagrams exhibited a linear trend similar to that of FePtCu films but with a larger slope. Systematic analysis clarified that the Ru additive caused a slight decrease of the anisotropic field but greatly decreased the saturation magnetization, which was related to the observed reduction in T c.

So far, microwave assistance effects have been investigated for several perpendicular magnetic anisotropy materials with the sample dimension of at least above submicron. In this work, we study the microwave assisted magnetization switching in nanostructured Co/Pt dots as small as 50 nm in diameter. Significant reduction of the switching field and the increase of the critical frequency for the microwave assistance effect are clearly observed with the enhancement of the radio frequency magnetic field. This switching behavior is similar to that predicted by the single macrospin calculation and is well explained using the Stoner–Wohlfarth model in the rotating frame.

Microwave assisted magnetization switching (MAS) is a key technology for future ultra high-density magnetic recording. To explore the feasibility of MAS, we have studied how the magnetization switching behavior of individual Co/Pt multilayer nanodots with perpendicular magnetic anisotropy is modified by the application of rf fields. It is found that the behavior depends on the rf field amplitude. For a large rf field but much smaller than the effective perpendicular anisotropy field, the switching field can be reduced remarkably. Moreover, the broad dispersion of the switching field in the absence the of rf field also becomes quite narrow.

Microwave assisted magnetization switching is implemented for Co/Pt multilayer nanodots. Application of rf fields significantly reduces the switching field of Co/Pt nanodots, and the reduction behavior is similar to that predicted by macrospin simulation. However, some dots are inert to rf fields. These very different switching behaviors may be attributable to a slight difference in the nucleation process of a reversed domain. It is also found that the switching field reduction remains almost constant regardless of the pulse width and interval of rf fields, indicating that the switching event is sufficiently fast and reliable in the ns regime.

We have examined the bistability condition of a circular magnetic dot by extending Thiele's bubble domain theory for an infinite continuous film to a finite one, and have found that the dot is always bistable at least when its diameter is D≤4 l, where l is the characteristic length given by l =σw/4πMs2 (σw: wall energy per unit area, Ms: saturation magnetization). This domain theory naturally explains the dot size dependence of the nucleation field without any assumptions.