Surface Magnetic Order Research Articles

Ultrafast optical control of surface and bulk magnetism in magnetic topological insulator/antiferromagnet heterostructure

Optical control of the magnetic properties in topological insulator systems is an important step in applying these materials in ultrafast optoelectronic and spintronic schemes. In this work, we report the experimental observation of photo-induced magnetization dynamics in the magnetically doped topological insulator (MTI)/antiferromagnet (AFM) heterostructure composed of Cr-(Bi,Sb)2Te3/CrSb. Through proximity coupling to the AFM layer, the MTI displays a dramatically enhanced magnetism, with robust perpendicular magnetic anisotropy. When subjected to intense laser irradiation, both surface and bulk magnetism of the MTI are weakened by laser-induced heating of the lattice, however, at the surface, the deleterious heat effect is compensated by the strengthening of Dirac-hole-mediated exchange coupling as demonstrated by an unconventional pump-fluence-dependent exchange-bias effect. Through theoretical analyses, the sizes of exchange coupling energies are estimated in the MTI/AFM bilayer structure. The fundamentally different mechanisms supporting the surface and bulk magnetic order in MTIs allow a novel and distinctive photo-induced transient magnetic state with antiparallel spin configuration, which broadens the understanding of the magnetization dynamics of MTIs under ultrashort and intense optical excitation.

DFT study of Pt sub-monolayer adsorption on the positive BiFeO3 (0001) surface

Density functional theory (DFT) calculations have been performed to determine the adsorption mechanism of Pt sub-monolayer (Θ ≤ 1 ML) on the positive BiFeO3 (0001) surface. We predicted that the adsorption behavior is different for Θ ≤ 1/2 ML and Θ > 1/2 ML based on detailed analysis of adsorption configuration, charge transfer effect, and orbital hybridization. For Θ ≤ 1/2 ML, there is only one type of Pt adatom site, which is binding directly with the low-lying O anion and shows electrons depletion/accumulation in the vertical/lateral 5d orbitals. For Θ > 1/2 ML, another type of Pt adatom site can be identified by not only the different adsorption configuration but also the distinct electronic states. Since the nearest neighbor of this type of Pt adatom site is the Bi cation, the charge transfer effect is characterized by significant electrons accumulation of the Pt-5d orbitals at the expense of the electron density of Bi cation. Another interesting feature occurs at Θ = 1 ML, i.e., the surface magnetic order displays ferromagnetic structure instead of antiferromagnetic arrangement. In addition, the existence of two types of Pt sites at Θ > 1/2 ML implies an intriguing possibility to enhance catalytic efficiency in the Pt/BFO (0001) heterocatalysis.

Manipulating surface magnetic order in iron telluride.

Control of emergent magnetic orders in correlated electron materials promises new opportunities for applications in spintronics. For their technological exploitation, it is important to understand the role of surfaces and interfaces to other materials and their impact on the emergent magnetic orders. Here, we demonstrate for iron telluride, the nonsuperconducting parent compound of the iron chalcogenide superconductors, determination and manipulation of the surface magnetic structure by low-temperature spin-polarized scanning tunneling microscopy. Iron telluride exhibits a complex structural and magnetic phase diagram as a function of interstitial iron concentration. Several theories have been put forward to explain the different magnetic orders observed in the phase diagram, which ascribe a dominant role either to interactions mediated by itinerant electrons or to local moment interactions. Through the controlled removal of surface excess iron, we can separate the influence of the excess iron from that of the change in the lattice structure.

Spin-polarized metastable He study of surface magnetic order in Ho thin films

A spin-polarized metastable helium beam operating in magnetic fields of up to 5 T has been used to investigate the spin polarization at the surface of thin films of Ho(0001) grown on W(110) substrates. The extreme surface sensitivity of this technique allows a direct probe of the various magnetic structures that form as a function of film thickness, temperature, and applied magnetic field. Evidence for a transition from a conical ferromagnetic structure to a helical antiferromagnetic (AFM) phase is observed at low temperatures with a further transition to paramagnetism at close to the AFM ordering temperature, TN. In addition, the spin polarization of the 6s CE that mediate the exchange interaction between localized Ho 4f moments (the RKKY interaction) is found to be positive, supporting earlier observations.

Surface energies of stoichiometric FePt and CoPt alloys and their implications for nanoparticle morphologies

We have calculated surface energies and surface magnetic order of various low-indexed surfaces of monoatomic Fe, Co, and Pt, and binary, ordered FePt, CoPt, and MnPt using density-functional theory. Our results for the binary systems indicate that elemental, Pt-covered surfaces are preferred over Fe and Co covered and mixed surfaces of the same orientation. The lowest energy orientation for mixed surfaces is the highly coordinated (111) surface. We find Pt-covered (111) surfaces, which can be realized in the $\text{L}{1}_{1}$ structure only, to be lower in energy by about 400 meV/atom compared to the mixed $\text{L}{1}_{0}$ (111) surface. We conclude that in small nanoparticles this low surface energy can stabilize the $\text{L}{1}_{1}$ structure, which is suppressed in bulk alloys. From the interplay of surface and bulk energies, equilibrium shapes of single-crystalline ordered nanoparticles and crossover sizes between the different orderings can be estimated.

Surface Phase Transition in Anisotropic Heisenberg Models

The behavior of an anisotropic Heisenberg model on a semi-infinite cubic lattice is studied. We employ the Green's function formalism and the random-phase approximation in our analysis. We consider different possible configurations for the anisotropy parameters of the surface and bulk. We determine the profile of the magnetization and the renormalized magnon energy spectrum as a function of temperature and of the anisotropy parameters of the surface and bulk. We also find the complete phase diagram for the multicritical points which is in agreement with the real-space renormalization group calculations. The onset of surface ordering takes place when the surface magnons become more energetic than the bulk modes. In the limit where bulk and surface are both described by a pure Heisenberg model, there is no long-range surface magnetic order over the paramagnetic bulk, whatever the values of the exchange couplings are. We relate this result with the Mermin-Wagner theorem. We show that the slope of the surface magnetization is discontinuous at the bulk critical temperature and that the discontinuity increases when the model on the surface varies from Ising to Heisenberg. In the particular case when the exchange couplings become antiferromagnetic, we have found the canted-paramagnetic critical field at the surface which is larger than that of the bulk. At very low temperatures, the critical field at the surface does not follow a T3/2 Bloch law, but instead decreases linearly with temperature.

X-ray dichroism and scattering studies of magnetic systems

Recent experimental and theoretical work at ESRF, on circular magnetic X-ray dichroism and surface resonant X-ray scattering for the study of transition metal systems is briefly reviewed. The applicability and accuracy of dichroism sum rules is critically assessed by discussing the extent to which real systems satisfy the necessary assumptions. Special emphasis is given to results for the L 2,3 edges of the 3d ferromagnetic metals. The recent demonstration of the feasibility and potential of X-ray scattering for the investigation of surface magnetic order is also discussed.

Magnetic structures of bct manganese in the bulk and at the (001) surface.

We have studied the magnetic structure of bct Mn in bulk and at the (001) surface using a tight-binding $d$-band model with site-dependent Stoner-like exchange splitting. In the bulk we have studied the relative stability of a ferromagnetic (F) state and antiferromagnetic (AF) states in four different configurations as a function of tetragonal distortion, $\frac{c}{a}$, for atomic volumes from 12 to 16 ${\mathrm{\AA{}}}^{3\ensuremath{'}}$. Between $\frac{c}{a}=1$ (bcc) and $\frac{c}{a}=\sqrt{2}$ (fcc) we find three AF phases with different spin alignments. For two particular bct lattices, as observed in epitaxial Mn films on Fe(001) and on Pd(001), we have studied the magnetic structure at the (001) surface. Starting from the corresponding ground-state magnetic structure in the bulk we have compared several magnetic configurations in the surface layer. The in-plane AF [$c(2\ifmmode\times\else\texttimes\fi{}2)$ symmetry] configuration is ground state for both lattices. The relative stability of the different configurations has been interpreted in terms of competing AF exchange couplings between neighboring surface and subsurface sites. This has been done by constructing an effective Ising model for the surface magnetic order from the energies obtained from the electronic structure calculation.

Oxygen induced suppression of the surface magnetization of Gd(0001).

The effects of oxygen adsorption on the surface magnetization of Gd(0001) have been determined by measuring the polarization of the Gd 4f core levels and the oxygen induced states with spin-resolved photoemission. Calculations of the partial density of states of O/Gd(0001) using the LMTO-ASA method support the photoemission measurements, which indicate that oxidation occurs at low coverages. The loss of surface magnetic order with oxygen exposure is explained in terms of decreasing indirect coupling.

An effective transfer matrix approach for semi‐infinite ferromagnetic systems

AbstractAn effective transfer matrix approach for the Ising model is proposed. Taking a semi‐infinite ferromagnetic system as an example, its surface magnetic order is calculated. The results also show that the discontinuity of the surface magnetization will not appear in our treatment.

Magnetization behavior of thick epitaxial Gd(0001) films on W(110).

The temperature-dependent magnetic properties of thick Gd(0001)/W(110) films were investigated with respect to the annealing temperature as well as the film thickness (d=10--100 nm). After annealing to sufficiently high temperatures, thick films (d\ensuremath{\ge}40 nm) show evidence for a canted magnetization, accompanied by a strong anomaly of the coercive field just below the Curie temperature ${\mathit{T}}_{\mathit{C}}$. These effects can be attributed to the temperature-dependent magnetic anisotropy of bulk Gd. For thinner films, the magnetization behavior implies the existence of residual strain, which cannot be fully annealed by thermal treatment. The implications of our experimental results for surface magnetic order are discussed.

Anisotropic Heisenberg ferromagnet on a semi-infinite lattice

The behavior of an anisotropic Heisenberg ferromagnet on a semi-infinite cubic lattice is studied within the context of the Green function formalism. The bulk is described by a pure Heisenberg model while on the surface an exchange anisotropy parameter permits to consider a sequence of models from pure Heisenberg to Ising. We show that in the limit where bulk and surface are both described by a pure Heisenberg model, there is no long range surface magnetic order over a paramagnetic bulk, whatever the values of the exchange couplings are. We relate our results with the Mermin-Wagner theorem.

The Surface Atomic and Magnetic Order in Crystals

The Surface Atomic and Magnetic Order in Crystals

A Study for Critical Behaviour of Ferromagnetic Thin Films

AbstractThe critical behavior of ferromagnetic thin film with two surfaces is investigated by the use of effective field theory with correlations. Surface magnetic order phase is found under certain conditions. The effects of couplings at surface, couplings between surface and nearest neighbouring layer, film thickness, and concentration of magnetic atoms at surface on surface magnetism are discussed.

Magnetic order at Tb surfaces determined by electron capture spectroscopy (ECS)

The magnetic order at surfaces of the 4f-rare earth metal terbium is investigated using electron capture spectroscopy (ECS). ECS probes the magnetic order of the topmost surface layer. In ECS, the capture of spin-polarized electrons during grazing-angle ion-surface reflection is used to determine the electron spin polarization (ESP) due to long-ranged and short-ranged surface magnetic order. It is found that long-ranged ferromagnetic order exists far above the bulk Curie temperature T cb of Tb. using magneto-optical Kerr effect and ferromagnetic induction, T cb is found to be at 220 K. At 140 K, the long-ranged ESP amounts to 24%. With increasing temperature, the ESP decreases monotonically up to the bulk Néel temperature T Nb = 228 K, then exhibits a pronounced maximum at T = 238 K and vanishes at the surface Curie temperature T Cs found to be located at 248 K. These novel and striking results on enhanced magnetic order at Tb surfaces suggest the presence of very large surface anisotropies.

Ferromagnetic order at Tb surfaces above the bulk Curie temperature

The magnetic order at surfaces of the 4f rare-earth metal terbium is investigated using electron capture spectroscopy (ECS), which probes the electron spin polarization (ESP) of the topmost surface layer. In ECS, the capture of spin-polarized electrons during grazing-angle ion-surface interaction is used to determine the ESP due to long- and short-ranged surface magnetic order. It is found that long-ranged ferromagnetic order exists far above the bulk Curie temperature, measured to be TCb =220 K. At 140 K, the long-ranged ESP amounts to 24%. With increasing temperature, the ESP first decreases montonically up to the bulk Néel temperature TNb=228 K, then exhibits a pronounced maximum at T=238 K, and ultimately vanishes at the surface Curie temperature TCs =248 K. These striking results on enhanced magnetic order at Tb surfaces suggest the presence of very large surface anisotropies.

Summary Abstract: Spin-polarized photoelectron diffraction: A new probe of surface magnetic order

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Strong temperature dependence of ferromagnetism at Ni and Tb surface determined with electron capture spectroscopy ECS

The magnetic structure of the topmost atomic layer of Ni and Tb surface has been investigated using electron capture spectroscopy (ECS). Contrary to bulk behavior, at Ni(111) surfaces, the long-range magnetic order, characterized by the long-range electron spin polarization (ESP), decreases from −43% at T= 293 K almost linearly to zero at the bulk Curie temperature T cb = 631 K. The “local” surface magnetic order (SMO) at Ni(111) surfaces is temperature independent and is detected far above T cb. At Tb surfaces, however, the “local” SMO is strongly temperature dependent and disappears above T cb. ft]+|Walter-Meissner-Institute f. Low-Temp.-Res., Bav. Acad. Sci., D8046 Garching, F.R.G.

Temperature dependence of ferromagnetism at Ni(111) surfaces determined with electron capture spectroscopy (ECS)

Electron capture spectroscopy has been used to probe the electron spin polarization (ESP) at the topmost atomic layer at Ni(111) surfaces. Contrary to bulk behavior, the ESP-signal measuring the long-range surface magnetic order decreases from -43% at T=293 K almost linearly to zero at the bulk Curie temperature T cb=631 K. “Local” surface magnetic order is temperature independent and is detected far above T cb. Such a behavior is only found for the 3d transition metal Ni.

Criticality of the semi-infinite Potts ferromagnet: a renormalisation group approach

Within a real space renormalisation group framework which uses rather sophisticated clusters, the authors discuss the phase diagram and the universality classes of a semi-infinite cubic-lattice q-state Potts ferromagnet. In particular, they study the influence, on the surface magnetism, of q and Delta identical to JS/JB-1 (where JS and JB are respectively the surface and bulk coupling constants). The exact d=2 critical temperature Tc2D is recovered for all values of q. The q-dependence of the value Delta c above which surface magnetic order can exist even if the bulk is disordered, is calculated and, through a convenient extrapolation, reliable results are obtained (for the Ising particular case, i.e. q=2, they obtain the extrapolated value Delta c=0.569, which compares satisfactorily with the series result 0.6+or-0.1 and the Monte Carlo one, 0.50+or-0.03). At the surface-bulk (SB) multicritical point they calculate the q-dependences of the critical amplitude A and the crossover exponent phi (defined in the Delta to Delta c+0 limit through (TcS( Delta )/Tc3D-1) approximately A( Delta / Delta c-1)1 phi /, where TcS( Delta ) and Tc3D identical to TcS( Delta c) respectively are the surface and the bulk critical temperatures), as well as the critical exponent yt1SB. For the Ising particular case they obtain phi equivalent to 0.641 (which compares satisfactorily with the epsilon -expansion (0.68) and the Monte Carlo (0.56+or-0.04) results), A equivalent to 0.4 and 1/yt1SB equivalent to 1.623. (As far as we know, no series or Monte Carlo results are available in the literature for A or 1/yt1SB.).