Domain evolution during the spin-reorientation transition in epitaxial NdCo₅thin films

Abstract

The domain structure and its changes with temperature were investigated for an epitaxial NdCo 5 thin film with in-plane texture in which a spin-reorientation transition takes place from the easy c-axis via the easy cone to the easy plane. Scanning electron microscopy with polarization analysis reveals a transition from a two-domain state at temperatures above 318 K via a four-domain state back to a 90 • -rotated two-domain state at temperatures below 252 K. The transition temperatures correspond well to those determined by global magnetization measurements. The magnetization configuration at the three different regimes of magnetic anisotropy and its transition with temperature were analysed in detail. From the local measurements, the spin-reorientation angle and the magnetocrystalline anisotropy constants of first and second order were derived. Gesellschaft phases the individual contributions of the cobalt-and the RE metal sublattices to the magnetocrystalline anisotropy lead to a temperature-driven spin-reorientation transition (SRT). This is due to the fact that the anisotropy of the RE sublattice decreases strongly with increasing temperature owing to the weak RE-RE exchange interaction, J RR , whereas the Co sublattice anisotropy is maintained even at high temperatures (J CoCo J RR ). At higher temperatures, the uniaxial anisotropy of the Co sublattice (parallel to the crystallographic c-axis) will always dominate, while a possible basal plane anisotropy of the RE subsystem may take over at lower temperature. This leads to a temperature-driven change in the easy magnetization direction known as SRT. Upon cooling down, an SRT with an opening of a magnetic easy cone takes place, e.g. in PrCo 5 , where it was investigated in bulk [1][2][3][4] as well as in epitaxial thin film samples [5]. A complete SRT from a magnetic easy c-axis via a magnetic easy cone to a magnetic easy plane in the ab-plane of the hexagonal crystal is observed in NdCo 5 . Additionally, in the easy basal plane the a-axis is energetically preferred over the b-axis [2,[6][7][8]. This thirdorder anisotropy effect is, however, not investigated in the present study. Like in the case of PrCo 5 , for NdCo 5 the SRT was initially observed in bulk samples [2,6,7,[9][10][11][12], where it takes place between 280 and 240 K. We recently succeeded in preparing epitaxial NdCo 5 films on MgO(110) and Al 2 O 3 (0001) substrates with an in-plane and out-of-plane texture, respectively [13,14]. For the in-plane samples the transition temperatures were determined as 310 and 255 K, while for the out-of-plane samples the transitions were qualitatively confirmed, but the exact temperatures were not accessible due to the influence of the shape anisotropy. For the in-plane textured film on MgO(110) investigated here, a sketch of the temperaturedependent easy axis of magnetization and the alignment of the crystal structure with respect to the substrate is shown in figure 1. Due to the shape anisotropy of a thin film, the different possible orientations along the surf...

Micromagnetic investigation of domain and domain wall evolution through the spin-reorientation transition of an epitaxial NdCo5film

“…one pinning centre surrounded by a homogeneous area. However, the results of the simulations reflect characteristic findings from the microscopic imaging, which was performed before on an in-plane textured NdCo 5 thin film by scanning electron microscopy with polarisation analysis (SEMPA) [12]. The SEMPA measurements were conducted in a temperature range between 340 and 240 K, so that the whole SRT from magnetic easy c-axis via easy cone to easy plane was imaged.…”

“…The temperature-dependent SEMPA measurements showed that the domain configuration remained nearly the same after several heating and cooling cycles. This behaviour was also attributed to a dense network of pinning centres, which are present in these thin film samples [12]. Therefore, one focus of the investigations will be on the effect of various types of pinning centres on the domain behaviour.…”

“…The calculations were performed for selected 'temperatures', i.e. the respective magnetocrystalline anisotropy constants of first and second order, which were derived from global vibrating sample magnetometry measurements and local SEMPA measurements [12], were used as parameters in the programme. The temperatures, the values of K 1 and K 2 and the resulting spin-reorientation angle q SR are summarised in table 1.…”

“…For the most part, investigations of the SRT deal with global magnetisation measurements. To analyze the domain structure at the surface of in-plane textured NdCo 5 thin films with sufficient spatial resolution, we recently used scanning electron microscopy with polarisation analysis (SEMPA) [12]. This method [13] allows imaging the local in-plane magnetisation direction.…”

“…Finally, at temperatures below 252K there is again a two domain state now with the magnetisation rotated by  90 and oriented parallel to the easy a-axis. Reference [12] summarises an interpretation of the magnetisation processes which take place during the heating or cooling procedure via the different regimes of magnetocrystalline anisotropy.…”