Magnetic domain structure of holmium films at low temperatures

In spite of the widespread use of rare-earth metals in magnetism, real space investigations of their magnetic domain structures remain scarce. Here we report on a systematic investigation on the structural and magnetic properties of Ho films by means of low-temperature spin-polarized scanning tunneling microscopy (SP-STM). The films are grown on W(110), resulting in well-ordered surfaces with stacking faults, glide dislocations, and double screw dislocations, all characteristic for epitaxial growth of hcp(0001) films on bcc(110) substrates. At low coverage up to $\ensuremath{\approx}50$ atomic layers (AL), the surface magnetic structure of the Ho(0001) films is found to be dominated by ferromagnetic in-plane domains magnetized along six equivalent high-symmetry axes. Significant pinning of domain walls to crystalline defects is observed. For Ho coverages exceeding 50 AL, additional out-of-plane magnetized stripe domains superimposed to the in-plane magnetization are observed. We relate these stripe domains to the dipolar stray field associated with the known helical magnetic cone structure of Ho which leads to an uncompensated magnetization component along the $c$ axis, i.e., perpendicular to the surface. Domain wall analysis suggests N\'eel-capped Bloch-type walls with widths of ${w}_{60}\ensuremath{\approx}1.2$ nm, ${w}_{120}\ensuremath{\approx}3$ nm, and ${w}_{180}\ensuremath{\approx}4$ nm for ${60}^{\ensuremath{\circ}}, {120}^{\ensuremath{\circ}}$, and ${180}^{\ensuremath{\circ}}$ walls, respectively. Magnetic field-dependent measurements reveal that the stripe domains are annihilated by an out-of-plane field of ${\ensuremath{\mu}}_{0}H=\ifmmode\pm\else\textpm\fi{}300$ mT.