Ferromagnetic order and critical behavior at surfaces of ultrathin epitaxial films

The critical behavior, ferromagnetic order and magnetic anisotropies of ultrathin, epitaxial, magnetic films is studied using electron capture spectroscopy (ECS), which is capable of probing the long-ranged and short-ranged electron spin polarization (ESP) at the topmost surface layer of uncoated and coated magnetic structures. For all systems [Ni(100)/Cu(100), Ni(100)/NaCl(100), fcc Fe(111)/Cu(111), Fe(100)/Ag(100), Tb/Fe(100)/Ag(100), Fe(100)/Au(100), hcp Tb(0001)/W(110), Fe(110)/W(110), V(100)/Ag(100), Pd(100)/Ag(100), Pd/W(110)] investigated so far, ferromagnetic order is detected. It is found that the surface Curie temperatureT _Cs depends on film thicknessd. ECS data obtained at the surface of various systems reveal the existence ofT- andd-dependent magnetic anisotropies. Although for V(100)/Ag(100) the measured critical exponent=0.128 agrees very well with=1/8 predicted for the two-dimensional Ising model, for other systems, such as Fe(100)/Au(100), the measured value (0.25) is in disagreement with theoretical predictions. The experimental results are discussed within the framework of presently available experimental and theoretical data.     

Electronic structures and magnetism of Fe nanowires on Cu（001） and Ag（O01）： A first-principles study

The electronic structures and magnetism of Fe nanowires along the [110] direction on Cu（001） and Ag（001） [Fe（nw）/Cu（001） and Fe（nw）/Ag（001）] are investigated by using the all-electron full-potential linearized augmented plane wave method in the generalized gradient approximation. It is found that the magnetic moment of Fe atom for the Fe（nw）/Cu（001） is 2.99#B, which is slightly smaller than that （3.02μB） for the Fe（nw）/Ag（001） but much larger than that （2.22μB） for the bcc iron. The great enhancement of magnetic moment in the Fe nanowires can be explained by the Fe d-band narrowing and enhancement of the spin-splitting due to a reduction in coordination number, From the calculated spin-polarized layer-projected density of states, it is found that the Fe 3d-states are strongly hybridized with the adjacent Cu 3d-states in the Fe（nw）/Cu（001）, and there exists a strong hybridization between the Fe sp-and the adjacent Ag 4d-states in the Fe（nw）/Ag（001）.     

Magnetic imaging at the atomic level

Magnetic contrast at the atomic level has been observed for the first time in scanning tunneling microscopy experiments on a magnetite (Fe₃O₄(001)) surface using in-situ prepared ferromagnetic Fe tips. A periodic corrugation with a 12 Å periodicity is clearly observed along the rows of FeB-sites which corresponds to the repeat period of Fe²⁺ and Fe³⁺ along these rows. This periodicity is not observed by using non-magneticW tips although the rows of FeB-sites can be resolved as well. The magnetic contrast observed with Fe tips is attributed to the different spin configurations of the magnetic ions Fe²⁺ and Fe³⁺ in Fe₃O₄.     

Element-selective spin polarization analysis of surfaces by spin-polarized ion scattering spectroscopy

We discuss the validity of surface spin polarization analysis with element selectivity using spin-polarized ion scattering spectroscopy (SP-ISS). We examined the control of the incident ⁴He⁺ spins and successfully conducted magnetic hysteresis measurement on an Fe(100) surface. The spin polarization of the Fe(100) surface exposed to O₂ atmosphere measured by spin-polarized ion neutralization spectroscopy was consistent with that reported by spin-polarized metastable de-excitation spectroscopy. The element selectivity of SP-ISS is discussed in terms of ion neutralization, re-ionization, and multiple scattering.     

笼状Au$lt;sub$gt;20$lt;/sub$gt;内掺$lt;i$gt;M$lt;/i$gt;$lt;sub$gt;13$lt;/sub$gt;($lt;i$gt;M$lt;/i$gt;=Fe，Ti)团簇磁性的密度泛函计算研究

采用密度泛函理论中的广义梯度近似方法,对M₁₃（M=Fe,Ti）以及M₁₃内掺Au₂₀团簇的几何结构和磁性进行了计算研究.结果表明：M₁₃和M₁₃内掺Au₂₀团簇的几何结构在0.006—0.05 nm误差范围内保持着I_h对称性.Fe₁₃团簇最低能态的总磁矩为44 μ_B,内掺到Au₂₀笼中后形成的Fe₁₃内掺Au₂₀团簇的最低能态总磁矩为38 μ_B,且Au原子与内掺Fe₁₃团簇之间存在着弱铁磁相互作用.Ti₁₃团簇在总磁矩为6 μ_B时能量最低,掺入Au₂₀笼后形成的Ti₁₃内掺Au₂₀团簇最低能态总磁矩是4 μ_B,内表面12个Ti原子与表面Au壳之间是弱铁磁相互作用,而与中心Ti原子之间是弱反铁磁相互作用.由于Au₂₀笼状外壳的影响,Fe₁₃内掺Au₂₀和Ti₁₃内掺Au₂₀团簇中Fe₁₃和Ti₁₃的磁矩比无金壳的Fe₁₃和Ti₁₃团簇的磁矩分别减少了6.81 μ_B和2.88 μ_B. 关键词： 几何结构 磁性 密度泛函理论     

Spin-resolved photoelectron spectroscopy of the MgO/Fe(110) system

The surface structure and electronic properties of ultrathin MgO layers grown on epitaxial Fe(110) films were investigated at room temperature by means of electron diffraction, Auger electron spectroscopy, scanning tunneling microscopy, and spin-resolved photoelectron spectroscopy. The spin polarization at the Fermi level (E_F) of the Fe(110) film decreases sharply with increasing thickness of the MgO layer. This behavior arises from the formation of a thin FeO layer at the MgO(111)/Fe(110) interface, as revealed by structural and spectroscopic investigations. The strong attenuation of the intrinsic spin polarization is qualitatively attributed to the scattering of spin-polarized electrons at the unoccupied d-orbitals of Fe²⁺. PACS 68.35.-p; 68.55.-a; 73.20.r; 75.70.Cn; 79.60.-I     

Cluster calculations of the electronic structure of Fe4C

The electronic structure and magnetic properties of Fe₄C are investigated with the first-principles discrete variational method (DV) in spin-polarized case. It is used for the local spin density approximation of the density functional theory. Clusters of 21 and 27 atoms were adopted to represent the two non-equivalent iron sites in this compound. The calculated magnetic moments for the corner and face centered iron sites are 2.63\mu_B and 1.77\mu_B, respectively, which are in very good agreement with the experimental results. A collapse of the magnetic moment for both inequivalent iron sites was verified above 12% of contraction of the lattice spacing. The C atom acts as an acceptor for electrons in this host. This revised version was published online in August 2006 with corrections to the Cover Date.     

Theory of localized magnetic moments in metals I finite cluster approach

The origin of localized magnetic moments formation in metals is investigated theoretically using a self-consistent local spin density molecular cluster approach. Clusters with up to 55 atoms are employed to describe isolated impurity local moment behavior in the cases of Fe$\text{Ag}$ and Fe$\text{Pd}$. Densities of states and spin magnetic moments were determined and compared with results of spectroscopic (notably photoemission) and magnetization measurements, respectively. In the case of a noble metal host, the spin magnetization density is found to be highly localized around the Fe site; the iron moment is ≈ 3.9μ_B and the polarization of the host Ag atoms is small. In the case of a transition metal host, the iron moment is ≈ 3.2 μ_B but here the strong hybridization of the Fe-3d and Pd-4d states results in a large induced magnetic moment in the host PD metal — in essential agreement with experiment for this giant moment system.     

Short range order at the surfaces of annealed amorphous Fe40Ni40P14B6 ribbons

Conversion electron Mössbauer spectroscopy (CEMS) and gamma transmission Mössbauer spectroscopy were used to measure the effects of annealing at 583 K in vacuum into about 200 nm thick layer below the two surfaces and on the bulk of theFe ₄₀ Ni ₄₀ P ₁₄ B ₆ amorphous ribbons prepared by means of melt-spinning technique. The results show a large distribution of hyperfine magnetic fields on the bulk and in the surfaces of the samples. By means of selective analysis of hyperfine magnetic field distribution, we have evalueded the correlation between the different degree of short range orders at the surfaces and in the bulk of the samples, and the phosphorus segregation associated with mechanical cubrittlement induced at low annealing temperature.     

Application of M?ssbauer spectroscopy in magnetism

An overview is provided on our recent work that applies ⁵⁷Fe M?ssbauer spectroscopy to specific problems in nanomagnetism. ⁵⁷Fe conversion electron M?ssbauer spectroscopy (CEMS) in conjunction with the ⁵⁷Fe probe layer technique as well as ⁵⁷Fe nuclear resonant scattering (NRS) were employed for the study of various nanoscale layered systems: (i) metastable fct-Fe; a strongly enhanced hyperfine magnetic field B_hf of ～39?T at 25?K was observed in ultrahigh vacuum (UHV) on uncoated three-monolayers thick epitaxial face-centered tetragonal (fct) ⁵⁷Fe(110) ultrathin films grown by molecular-beam epitaxy (MBE) on vicinal Pd(110) substrates; this indicates the presence of enhanced Fe local moments, μ_Fe, as predicted theoretically; (ii) Fe spin structure; by applying magnetic fields, the Fe spin structure during magnetization reversal in layered (Sm–Co)/Fe exchange spring magnets and in exchange-biased Fe/MnF₂ bilayers was proven to be non-collinear and depth-dependent; (iii) ferromagnet/semiconductor interfaces for electrical spin injection; CEMS was used as a diagnostic tool for the investigation of magnetism at the buried interface of Fe electrical contacts on the clean surface of GaAs(001) and GaAs(001)-based spin light-emitting diodes (spin LED) with in-plane or out-of-plane Fe spin orientation; the measured rather large average hyperfine field of ～27?T at 295?K and the distribution of hyperfine magnetic fields, P(B_hf), provide evidence for the absence of magnetically “dead” layers and the existence of relatively large Fe moments (μ_Fe ～ 1.8?μ_B) at the ferromagnet/semiconductor interface. - Finally, a short outlook is given for potential applications of M?ssbauer spectroscopy on topical subjects of nanomagnetism/spintronics.     

Electrical spin transport in a GaAs (110) channel

A homoepitaxial GaAs (110) channel gives a great interest in the field of semiconductor spintronics due to the longer spin diffusion. By utilizing optimal temperature process and V/III flux ratio control, the GaAs layer is grown without a serious defect. In a ferromagnet/semiconductor hybrid device, Tb₂₀Fe₆₂Co₁₈/Ru/Co₄₀Fe₄₀B₂₀ films are deposited on the GaAs (110) channel as a spin source to investigate the spin transport in (110)-oriented channel. To measure the Hanle signal, an in-plane magnetic field is applied to the perpendicularly polarized spins which are injected from the Tb₂₀Fe₆₂Co₁₈ layer. From the experimental results, the spin diffusion length in a GaAs (110) is longer than that in a GaAs (100) by up to 25%. The proper selection of crystalline growth direction for the spin transport channel is a viable solution for an efficient spin transport.     

Local spin density total energy study of surface magnetism: V (100)

Results of self-consistent all-electron local (spin) density functional studies of the electronic and magnetic properties of vanadium (100) 1-, 3-, 5- and 7-layers films are reported using our full-potential linearized augmented plane wave (FLAPW) method. The calculated work function, 4.2 eV, agrees very well with the experimental value of 4.12 eV. From both Stoner factor analyses and spin-polarized total energy calculations, it is concluded that V(100) undergoes a ferromagnetic phase transition only for the monolayer system. The magnetic moment is found to be 3.09μ_B per atom of this monolayer film and to have a total energy 57 mRy below that of the paramagnetic structure. For multilayer V(001) systems, the sharp surface density-of-states peak which is characteristic of the occurrence of surface magnetism in the 3d transition metals is located 0.3 eV above the Fermi level. As a result, the paramagnetic state is stable. In addition, no enhancement of the exchange-correlation integral is found for the surface atoms compared with the bulk value. The lower energy of the paramagnetic structure is further supported by total energy investigations of the multilayer relaxation of V(100) — the calculated interlayer spacings for the paramagnetic surface with a 9% contraction of the topmost interlayer spacing and a 1% expansion of the second interlayer spacing with respect to its bulk value are in good agreement with LEED measurements. It is suggested that the surface magnetism of V(100) may be associated with surface oxygen or caused by impurity induced surface reconstructions.     

Quantum transformation of spin structure of a Fe8 nanocluster in ultrastrong magnetic fields

The transformation of the spin structure of a high-spin Fe₈ cluster in a strong magnetic field has been investigated. The magnetization and magnetic susceptibility of the material are calculated at different external magnetic fields and temperatures. It is shown that the magnetic field induces transformation of the spin structure of a Fe₈ cluster from the quasi-ferrimagnetic structure with an average magnetic moment of 20 μ_B per molecule to the quasi-ferromagnetic structure with a magnetic moment of 40 μ_B. Unlike a similar transformation of a Néel ferrimagnet, which is continuous and occurs through an intermediate angular phase, this process in Fe₈ at low temperatures manifests itself as a cascade of discrete quantum jumps, each being the transition accompanied by an increase in the spin number of the complex. At high temperatures, the behavior of the magnetic cluster approaches the cluster behavior described by the classical theory. The nature of quantum jumps is discussed in terms of the magnetic-field-induced energy level crossing in the ground state of a magnetic cluster. __________ Translated from Fizika Tverdogo Tela, Vol. 42, No. 6, 2000, pp. 1068–1072. Original Russian Text Copyright ? 2000 by Zvezdin, Plis, Popov.     

Low temperature magnetic structure of erbium iron garnet

The magnetic and crystallographic structure of erbium iron garnet {Er₃}[Fe₂](Fe₃)O₁₂ has been refined simultaneously from powder and single crystal neutron diffraction data. At 5 K the ferrimagnetic structure has a spin direction parallel to [100]. Magnetic moments of iron and erbium and canting angles for the erbium sublattices were derived in space groupI4₁/acd for 5 K<T<65 K. The magnetic moments of the two erbium sites are 6.9 _B and 4.3 _B at 5 K. An indication of further symmetry reduction of the magnetic structure from tetragonal to orthorhombic below 5 K is discussed.     

Magnetic properties of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> surface

The magnetic properties of the magnetite Fe₃O₄(110) surface have been studied by spin resolved Auger electron spectroscopy (SRAES). Experimental spin resolved Auger spectra are presented. The results of calculation of Auger lines polarization carried out on the basis of electronic state density are presented. Problems related to magnetic moments of bivalent (Fe²⁺) and trivalent (Fe³⁺) ions on the Fe₃O₄(110) surface are discussed. It is established that the deposition of a thin bismuth film on the surface results in significant growth of polarization of iron Auger peaks, which is due to additional spin-orbit scattering of electrons by bismuth atoms.     

Determining pore sizes using an internal magnetic field

A concept is proposed to measure the pore size length scale by the internal magnetic field (B_i) in porous materials. The spatial distribution of the magnetic field inhomogeneity, a result of the magnetic susceptibility contrast between the porous material and the fluid, reflects the underlying pore geometry. Diffusion in B_i causes the initial decay of magnetization. At long times, the effect of B_i saturates when the diffusion length reaches a characteristic pore size. This method is independent of surface spin relaxation in determining pore sizes. Nuclear magnetic resonance experiments on packed glass beads and sedimentary rock samples will be presented.     

Hydrostatic pressure driven spin,volume and band gap collapses in SmFeO3: a GGA + U study

The crystal structure, magnetic and electronic properties of SmFeO₃ under hydrostatic pressure have been studied by first-principles calculations within the generalized gradient approximation plus Hubbard U (GGA + U). The iso-structural phase transition with spin, volume and band gap collapses can be induced by a large enough hydrostatic pressure. The high-spin (HS) state of Fe³⁺, with the magnetic moment of ~4 μ_B, is retained at low pressure. The spin crossover occurs at a transition pressure (~68 GPa) with the magnetic moment of Fe³⁺ decreasing to ~1 μ_B in low-spin (LS) state. Meanwhile, the reductions of cell volume (by ~?5.43%) and band gap (from >2 eV to ~1.6 eV) of SmFeO₃ are obtained when the HS–LS transition happens. Finally, the critical pressure of HS–LS transition, magnetic and electronic properties are found to be Hubbard U dependent.     

Effect of magnetic pulse treatment on the magnetic characteristics of yttrium–iron garnets

The effect weak (10–100 kA m^–1) low-frequency (10–20 Hz) pulsed magnetic fields have on the surface structure and magnetic characteristics of yttrium–iron garnet Y₃Fe₅O₁₂ is studied by means of electron and Mössbauer spectroscopy. A mechanism is proposed for the variation of saturation magnetization in Y₃Fe₅O₁₂ after magnetic pulse treatment. The mechanism is associated with the change in the spin state of iron ions localized in the tetrahedral sublattice.     

Magnetization studies of TbFeO3

The spontaneous magnetization and principal magnetic susceptibilities of TbFeO₃ were measured from 4.2 to 300 K. The weak ferromagnetic moment is along the c crystallographic axis in the entire temperature range. The field dependence of the magnetization at 4.2 K was also studied. The magnetic behavior is interpreted in terms of an interaction between the ordered Fe³⁺ spin system and the electrons occupying the lowest lying “accidental” doublet of the Tb³⁺ ions. The FeTb interaction and the Tb³⁺ Van Vl eck susceptibility along the c axis play significant roles in determining the magnetic configuration of the Fe³⁺ spin system. No indication was found that the TbTb interaction plays a significant role in the magnetic behavior of TbFeO₃ at temperature above 4.2 K.     

Half-metallicity in Fe-based Heusler alloys Fe2TiZ (Z = Ga,Ge, As,In, Sn and Sb)

The electronic structure and magnetic properties of new Fe-based Heusler alloys Fe₂TiZ (Z = Ga, Ge, As, In, Sn and Sb) have been studied by first-principles calculations. In these alloys, the 24-electron Fe₂TiGe, Fe₂TiSn are nonmagnetic semiconductors and other compounds are all ferrimagnetic metals. Fe₂TiAs and Fe₂TiSb are predicted to be half-metals with 100% spin polarization. The spin polarization ratio in Fe₂TiGa and Fe₂TiIn is also quite high. The calculated total moment for Fe₂TiAs and Fe₂TiSb is 1 μ_B, which is mainly determined by the Fe partial moment. The half-metallicity of Fe₂TiSb is stable under lattice distortion. The spin polarization of Fe₂TiSb is found to be 100% for the lattice variation in a range of 5.6–6.1 Å, which is attractive in practical applications.