Handbook of Modern Coating Technologies
Experimental results
In this chapter we summarise experimental results obtained with neutron reflectometry with a focus on magnetic and polymer systems for which neutron scattering methods have the particular advantage of direct sensitivity to the mangetic induction and a large scattering cross section of light elememts, with the additional possibility of isotope contrast variation.
- Data storage
Due to its magnetic sensitivity down to magnetic moments on the atomic level (« qB) and its spatial resolution in the nanometer (NR and GISANS) and micrometer (OSS) ranges NR is very well suited to study thin-film magnetism [28,29]. As an example NR played a crucial role in the development of spintronics [30], which play an important role for data storage (e.g., the discovery of Giant magnetoresistance) [31] or magnetic memories (MRAM) [32]. Ferromagnetic/oxide interfaces play an important role in magnetic tunnel junctions, which are a central component of modern spintronics. Ta/CoFeB/MgO layer structures have recently gained increased attention since they can provide a high tunnel magnetoresistance and large perpendicular magnetic anisotropy (PMA) [32]. Due to the high differences in neutron cross sections between Co, Fe, B, and Ta, PNR is a highly suitable technique to study these interfaces. Important insight into the elemental diffusion processes were inferred using PNR by ex situ annealing [33,34]. Moreover the PMA of the CoFeB layer was revealed by NR as well [35].
Another phenomenon widely studied with PNR is the one of exchange bias (EB), which is of high technological importance providing a magnetic stability exploited for example in read heads of magnetic storage devices or random access memory units. EB manifests as a shift in the magnetic hysteresis loop after field cooling of a ferromagnet in contact with an antiferromagnet due to the interface exchange coupling resulting in an unidirectional magnetic anisotropy [36]. In addition, depending on material and structure, an increase in the coercivity and a training effect can be observed.
Different models have been proposed to explains the EB effect from an theoretical point of view [37,38] and the magnetization reversal for the ascending and descending part of the hysteresis loop are being investigated [39] experimentally. A CoO/Co [40] structure was studied [41] by NR to understand the asymmetric behavior of the magnetization reversal.
First reflectivity curves with polarization analysis have been conducted to locate interesting Q values to later run hysteresis loops measurements. Fig. 4—12 depicts a typical hysteresis loop as extracted with polarized neutrons at a specific q value. From the analysis of the spin flip data together with the diffuse scattering the remagnetization process is extracted. It turns out that the remagnetization process changes after reaching saturation for the first time. The first magnetization reversal is dominated by nucleation and domain wall motion. Later reversals are found to be dominated by coherent rotation of the magnetization.
Heusler alloy systems are possible candidates offering 100% spin polarization at the Fermi level. This makes them extremely interesting as spintronic materials [42—44]. Bergmann et al. [45] have studied the domain structures in a Heusler multilayer sample by specular and off-specular neutron reflectivity measurements. The samples have an antiferromagnetic coupling between the layers. This is seen directly from the appearance of a half order Bragg reflection in the specular reflectivity. This peak is purely magnetic and allows an
| FIGURE 4-12 Hysteresis loop for the exchange bias sample during the first magnetization reversal (upper panel). The lower panels depict the nonspin flip and spin flip scattering extracted from neutron reflectivity measurements. At Hc1 the remagnetization is connected to domain wall motion, whereas at Hc2 it takes place via coherent rotation, visible from the large amount of spin flip scattering. Adapted from F. Radu, M. Etzkorn, R. Siebracht,
T. Schmitte, K. Westerhold, H. Zabel, Interfacial domain formation during magnetization reversal in exchange- biased coo/co bilayers. Phys. Rev. B 67 (2003) 134—409. |
easy investigation of the correlations in the magnetic structure. From the specular reflectivity the direction and the amount of the magnetization in the layers could be extracted as described in Section 4.1.1.4. From the detailed analysis of the off-specular scattering [46,47] the lateral domain size, the out-of-plane antiferromagnetic correlation length, and deviations of domain magnetization vectors from the mean magnetization direction could be extracted. From the scattering pattern it is concluded that the layers show a Landau type of pattern [48] with four possible types of domains with perfect antiferromagnetic coupling between layers (Fig. 4—13).
