Revealing Antiferromagnetic Order of the Fe Monolayer on W(001): Spin-Polarized Scanning Tunneling Microscopy and First-Principles Calculations

Abstract

We prove that the magnetic ground state of a single monolayer Fe on W(001) is c2 2 antiferromagnetic, i.e., a checkerboard arrangement of antiparallel magnetic moments. Real space images of this magnetic structure have been obtained with spin-polarized scanning tunneling microscopy. An out-ofplane easy magnetization axis is concluded from measurements in an external magnetic field. The magnetic ground state and anisotropy axis are explained based on first-principles calculations. DOI: 10.1103/PhysRevLett.94.087204 PACS numbers: 75.70.Ak, 68.37.Ef, 71.15.Mb The 3d transition metals bcc Fe, hcp Co, and fcc Ni are known as the prototypical ferromagnets (FM). Much excitement was raised as experiments [1,2] indicated that Fe becomes an antiferromagnet (AFM) when stabilized in the metastable fcc phase. These experiments opened the vista that a proper control of the type of magnetic order is possible, allowing, for example, to turn a ferromagnet into an antiferromagnet or into a spin glass. However, after 40 years of research, all attempts to stabilize antiferromagnetic Fe ended up in Fe phases with fairly complex magnetic structures [3][4][5]. Thus, controlling the magnetic order in solids remains a challenge to solid state physics.Low-dimensional systems offer new possibilities to tune interactions. In this Letter we propose the interface tuning of the exchange interaction as a new route to antiferromagnetism in low-dimensional systems. For one monolayer (ML) Fe on W(001) we provide a clear proof of collinear antiferromagnetic Fe. Although this system has been studied extensively in the past, experiments could show only that the Fe ML is not ferromagnetic above 100 K [6,7], while theoretical predictions are still controversial [8,9]. Employing spin-polarized scanning tunneling microscopy (SP-STM) we obtained atomic resolution images which show that a c2 2 AFM structure with an out-ofplane magnetization direction is the magnetic ground state. This is in clear contrast to the Fe ML on W(110), which is ferromagnetic with an easy axis in the film plane [10], giving evidence that the sign of the exchange interaction can be selected by the choice of the substrate orientation alone. These different experimental results are explained based on first-principles calculations.The lack of remanence of 1 ML Fe=W001 was first discovered by spin-resolved photoemission [6] and Kerr effect measurements [7] and later interpreted on the basis of density-functional theory in the local spin-density approximation (LSDA) as an AFM ground state [8]. In these calculations the AFM state was just 10 meV=Fe atom lower than the nonmagnetic state and surprisingly the FM state did not exist. More recent calculations based on the generalized gradient approximation (GGA), however, found a FM solution, but the AFM state was not considered [9]. Meanwhile, the experimental result, i.e., the absence of remanent magnetization, has been confirmed in numerous experiments [11][12][13][14]. To our knowledge, however, up to now no experimental technique wa...

Spin-correlations and magnetic structure in an Fe monolayer on 5dtransition metal surfaces

“…By manufacturing alloys in different stoichiometries from these two substrate elements, a crossing can be found from the AFM to the FM state of the Fe monolayer [4]. It should be noted that in previous studies [1,4] the Fe layer relaxation on the Ta substrate was assumed to be the same as on W substrate with the reasoning that Ta and W are neighbouring elements in the periodic table. In contrary, we found a considerable difference in the out-of-plane relaxation of the Fe ML on these two substrates, see Table 1.…”

“…Ta(001) and W(001) surfaces have been the focus of a number of theoretical and experimental studies [1,2,3,4]. In case of an Fe monolayer on W(001) surface the magnetic ground state is antiferromagnetic, while for the Fe ML on Ta(001) substrate the ground state is ferromagnetic.…”

“…Whilst a c(2 × 2) antiferromagnetic (AFM) state was discovered for an Fe monolayer (ML) on BCC W(001) surface [1,2], on the BCC Ta x W 1−x (001) surface a crossover was found between AFM (pure W substrate) and ferromagnetic (FM) (pure Ta substrate) states with the same Fe layer relaxation [3,4]. On the other hand, previous theoretical studies demonstrated that the magnetic ground state strongly depends on the relaxation of the magnetic overlayer.…”

Interplay between structural, magnetic, and electronic properties in aFeO∕Pt(111)ultrathin film

“…On the other hand, it has been shown that (homoatomic) magnetic monolayers on transition metal substrates may have magnetic structures which are completely different from the bulk materials. 37,38 This is particularly true for triangular lattices where the AFM interaction is frustrated by the symmetry of the system and can result in complex magnetic structures. 39 In this paper we investigate the relative stability of various FeO/Pt(111) interface structures, taking into account different magnetic configurations.…”

Energy-dependent contrast in atomic-scale spin-polarized scanning tunneling microscopy ofMn3N2(010): Experiment and first-principles theory

“…5 Very recently, Kubetzka et al have applied the same technique to study a single monolayer of Fe on W(001), resolving a longstanding question regarding the magnetic ground state of this system. 11 Atomic-scale SP-STM has therefore been established as a powerful technique for resolving the ultimate limits of magnetic structure on surfaces.…”