Preparation and photoemission investigation of bulk-like a-Mn films on W(110)

We report the successful stabilization of a thick bulk-like, distorted $\alpha$-Mn film with (110) orientation on a W(110) substrate. The observed $(3\times3)$ overstructure for the Mn film with respect to the original W(110) low-energy electron diffraction pattern is consistent with the presented structure model. The possibility to stabilize such a pseudomorphic Mn film is supported by density functional total energy calculations. Angle-resolved photoemission spectra of the stabilized $\alpha$-Mn(110) film show weak dispersions of the valence band electronic states in accordance with the large unit cell.

Manganese (Mn) can be considered as the most complex of all metallic elements from the crystallographic point of view. Assuming regular structural trends as in the series of the 4d and 5d transition metals, one would expect crystallization of Mn in a hexagonal close-packed (hcp) structure. In the row of the 3d elements, the occurence of magnetism disturbs this regular structural sequence: Fe and Co crystallize in body-centered (bcc) and hcp structures, respectively, whereas hcp and face-centered cubic (fcc) structures would be expected from the group sequences [1]. Being a member of the 3d-row, Mn behaves in a completely different way. Depending on temperature and pressure, it exists in five allotropic forms [2,3,4]. α-Mn, the stable phase below 1000 K, has an exotic bcc crystal structure containing 58 atoms in the conventional cubic unit cell [ Fig. 1(a)]. β-Mn is stable between 1000 K and 1368 K and has a simple cubic structure with 20 atoms per unit cell. An fcc γ-Mn phase exists between 1368 K and 1406 K and a bcc δ-phase from 1406 K up to the melting point. An hcp ǫ-phase of Mn exists above a pressure of 165 GPa [5,6].
Mn was grown in the form of ultrathin films on a number of fcc (Cu [7], Ni [8], Ir [9], Au [10,11]) and bcc (Fe [12,13], V [14], W [15,16]) metal surfaces. In case of lowtemperature growth of Mn on noble and transition metal surfaces a tetragonally expanded fct Mn layer (with antiferromagnetic ordering) is formed which can be considered as distorted γ-Mn. Deposition of Mn at higher temperatures leads to interdiffusion and to the formation of surface alloys with c(2 × 2) structure, where the Mn atoms are coupled ferromagnetically in the alloy layer.
A misfit of only 2.5% between the lattice constants of W (3.16Å) and of the hightemperature bcc phase δ-Mn (3.08Å at 1133 • C) opens a chance prepare relatively thick distorted δ-Mn(110) films on W(110) [see Fig. 1(b), lower part]. Recently, the possibility to stabilize thin distorted δ-Mn films up to several monolayers (ML) on W(110) was confirmed by low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) [15,16]. The epitaxial relationship is mantained up to a coverage of about 3 ML, but in contrast to Fe/W(110), already the first Mn layer exhibits a modest growth anisotropy in the 001 direction [15].
Concerning the study of electronic properties, all 3d metals except Mn were routinely investigated by means of photoelectron spectroscopy with angle-as well as spinresolution [17,18]. This lack of photoemission (PE) data for Mn seems surprising, but might be due the complicated crystal structure of α-Mn, although epitaxial growth of Mn overlayers would be a possible way to obtain crystalline bulk-like samples.
Here we report crystallographic and electronic structure investigations of stabilized bulklike distorted α-Mn films on W(110). The formation of α-Mn was achieved by annealing of freshly in situ evaporated Mn films at T ≈ 200 • C. LEED images show a clear (3 × 3) overstructure with respect to the W(110) surface that we assigned to the formation of a highly strained α-Mn thick film with (110) orientation [see Fig. 1 The experiments were performed in a photoelectron spectroscopy setup consisting of two chambers described in detail elsewhere [19,20].  The electronic structure of the every-time freshly prepared Mn overlayers was studied by means of angle-resolved PE after applying the two described different annealing schemes.  [22] (for example, at ∼ 1.2 eV BE in the around normal-emission spectra) are easily detectable on top of broad Mn-derived features. These data confirm our previous conclusion that in this case high Mn-islands are formed. Fig. 3(b) shows PE spectra measured in the same conditions for the Mn/W(110) system but annealed at a lower temperature of In order to get more confidence in the interpretation of the observations, electronic structure calculations of non-magnetic bcc α-Mn and other competing phases were performed.
Magnetism was disregarded, since the temperature during experiment was 300 K, much larger than the magnetic ordering temperature T N = 95 K. The program package CRYS-TAL 06 [23] was used. The gradient corrected functional of Perdew et al. [24] was employed.
A k-point net with 12 × 12 × 12 points was used. The chemically inactive [Ne] core of Mn was simulated by an energy-consistent full-relativistic pseudopotential: Mn 15+ -PP [25]. The basis set consists of contracted Gaussian-type orbitals [26].
In Fig. 4 the calculated total energies as functions of volume per Mn-atom are presented for the four Mn structural modifications (α-, β-, γ-, and δ-Mn). Experimental values for the internal parameters were used. In accordance with previous calculations [3,4], the αphase is for all volumes the most stable Mn phase in the considered non-magnetic state. We attribute the deviations of the optimized lattice constants from the related experimentally values (for example, for α-Mn: a opt. = 8.56Å and a expt. = 8.91Å [21] to the neglect of spontaneous magnetostriction. In order to describe the experimental situation of a strained bulk-like α-Mn film, we also performed calculations for orthorhombic Mn (space group F mm2). The in-plane lattice parameters were chosen to match the experimental strain, i.e., c orth. = a opt. α−Mn × 1.033 = 8.84Å and b orth. = a opt. α−Mn × √ 2 × 1.043 = 12.63Å. The out-of-plane parameter a orth. was varied in order to mimick the relaxation toward zero out-of-plane stress and the internal parameters were fixed at their experimental values. The related energy vs. volume behavior is included in Fig. 4. While this strained bulk-like film has as total energy somewhat higher than the ground state phase, its minimum energy is still below the energy of the other competing phases. It is thus very reasonable to assume that such a strained thick film can be (meta)stable under suitable conditions. In conclusion, we successfully stabilized bulk-like α-Mn films with (110)