Surface magnetism of Gd(0001): Evidence of ferromagnetic coupling to bulk

Abstract

Previous polarized electron experiments and recent ab initio calculations suggest that the surface layer magnetic moments of Gd(0001) are antiferromagnetieally coupled to the bulk magnetic moments.Spin-polarized photoemission data are presented which show that the spin polarization of the magnetic surface state and the surface 4f states of Gd(OOOI ) are coupled ferromagnetically to the bulk magnetic moment.PACS numbers: 75. 10.Lp, 75.50.Cc, 79.60.Cn Gadolinium has been considered an important prototype material for testing local-spin-density approximation (LSDA) calculations on rare-earth metals where localized 4f electrons, exchange, and relativistic (spin-orbit) eA'ects play important roles [1][2][3][4][5][6]. In the rare earths, the LSDA appears to overestimate the itineracy of 4f levels leading to predictions of structural, electronic, and magnetic properties often in relatively poor agreement with experiment, a few exceptions being La and Lu which have either an empty or filled 4f shell. Calculations for bulk Gd typically yield lattice constants which vary by a factor of 2 beyond limits considered appropriate for systems where the LSDA is judged accurate. LSDA calculations of band structure and magnetic properties are often in reasonable agreement with experiment. Recent self-consistent calculations for bulk Gd have yielded a conduction-band magnetic moment per atom (-0. 57pti) in good agreement with measurements (-0.63), and an average 5d-band exchange splitting (-0.6 eV) in reasonable agreement with recent angle-resolved photoemission measurements [7] of the A2 band dispersion and exchange splitting. However, the binding energies and dispersion of the h2 bands obtained from existing LSDA calculations do not agree with experimental results (-50% discrepancies), suggesting either inaccuracies in the calculations, inliuence of surface eA'ects due to short electron mean free paths [8], or strong many-body eITects [9] in the photoemission similar to those apparent in Ni. Surface properties of Gd(0001) are of intrinsic interest and also provide an additional means of testing LSDA calculations. Previous surface-sensitive probes of magnetism applied to Gd(0001) have established novel behavior. Spin-polarized low-energy electron-diA'raction (SPLEED) and magneto-optic Kerr eAect (MOKE) measurements [10] of single-crystal Gd(0001) surfaces [11] prepared on W(110) confirmed the existence of enhanced surface ferromagnetic order at temperatures up to 310 K ( -20 K above the bulk Curie temperature Tcb =293 K). These results, along with spin-polarized photoemission experiments [10] that probed bulk and surface 4f excitations, were interpreted as indicating antiferromagnetic coupling between the surface 4f spins and those of the underlying ferromagnetic bulk. Recent ab initio calculations [12] have examined the structural, electronic, and magnetic properties of Gd (0001), taking into account the total system energy.Several important results were obtained that can be directly compared with experimental results: First, th...

Oxygen Induced Suppression of the Surface Magnetization of Gd(0001)

“…These results are generally consistent with the majority of the previous measurements of the (0001) surface of Gd [4,8,10]. In addition, the deconvolutions of the majority and minority components indicate that the surface is ferromagnetically coupled to the bulk, in agreement with earlier studies [10,12].…”

Spin-resolved photoemission study of photohole lifetimes in ferromagnetic gadolinium

“…As the 4f-derived moments disorder, the exchange splitting reflecting the local mean field decreases and the net polarization in the surface state decreases. [14,15] and calculations [16] indicate that the surface state should be 100% majority spin, due t o parallel alignment of the surface and bulk moments. Our observation of a minority spin component could be taken as evidence of either incomplete saturation of the magnetization or the presence of minority domains.…”

Spin‐polarized Photoelectron Spectroscopy as a Probe of Magnetic Systems

“…The state, which is spin polarized through an exchange interaction with the localized 4f orbitals has an important history and indeed it was spin-resolved photoemission studies of the surface state that finally confirmed that the surface moments were ferromagnetically aligned with the bulk of the material. (Mulhollan et al, 1992) The results of a more recent photoemission study of this same state held at 20K with both high energy and spin resolution are illustrated in figure 13. (Fedorov et al, 2002) As noted earlier, both experiment (Mulhollan et al, 1992, Li et al, 1993) and theoretical calculations indicate that the surface state should be 100% majority spin, reflecting parallel alignment of the surface and bulk moments.…”

“…(Mulhollan et al, 1992) The results of a more recent photoemission study of this same state held at 20K with both high energy and spin resolution are illustrated in figure 13. (Fedorov et al, 2002) As noted earlier, both experiment (Mulhollan et al, 1992, Li et al, 1993) and theoretical calculations indicate that the surface state should be 100% majority spin, reflecting parallel alignment of the surface and bulk moments. The coexistence of both spin components at the same energy in figure 13 is therefore an intrinsic property of the surface state arising from a combination of spin-orbit and spin exchange processes.…”