Low temperature magnetic force microscopy (MFM) can be used to study magnetic domains in both thin films and (well polished) bulk materials. MGML scientists have performed MFM studies of an uniaxial ferromagnet UCoGa prepared in single crystalline form. Uranium ferromagnets crystallizing in hexagonal ZrNiAl structure have strong magnetocrystalline anisotropy. High magnetocrystalline anisotropy is expected to lead to thin magnetic domain wall. We chose UCoGa with T_C = 48 K.

All experiments including the single crystal growth (Czochralski method) and magnetic force microscopy measurements at low temperatures were performed in MGML.

Uranium ferromagnets crystallizing in hexagonal ZrNiAl structure have strong magnetocrystalline anisotropy. High magnetocrystalline anisotropy is expected to lead to thin magnetic domain wall. We chose UCoGa with T_C = 48 K [1], from this group to investigate domain structure and domain walls by magnetic force microscopy. In our study [2], we investigated domain structure of UCoGa at several temperatures below the Curie temperature of 48 K. The (001) surface domain structure at 20 K and various magnetic fields is shown in Fig. 1. The domain structure on the surface can be described by domain wall branching. Based on the analysis of sizes of magnetic domains (3.5 µm at 5 K) and considering the crystal structure and the magnetic properties of UCoGa, we conclude that the domain walls are made of only two magnetic atoms with their moments pointing in opposite direction as shown in Fig. 2. There are two main effects supporting this scenario: high magnetic anisotropy and low exchange interaction. Combination of these two properties leads to thin domain walls and the evaluation of domain wall size confirms these findings directly. Our paper was recently highlighted in attocube newsletter.

Fig. 1: MFM images of UCoGa surface at 20 K in different magnetic fields. One can see that at 0.1 and 0.3 T the sample is fully saturated in agreement with the magnetization loop (not shown).

Fig. 2: Schematic of magnetic domain wall in UCoGa. Only magnetic ions are showed.

[1] A. V Andreev, A. V Deryagin, R.Y. Yumaguzhin, Crystal structure and magnetic properties of UGaCo and UGaNi single crystals, Sov. Phys. - JETP. 59 (1984) 1082–1086.
[2] P. Opletal, K. Uhlířová, I. Kalabis, V. Sechovský, J. Prokleška, Extreme narrow magnetic domain walls in U ferromagnets: The UCoGa case, Mater. Today Commun. 24 (2020) 101017.

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