Isolating the step contribution to the uniaxial magnetic anisotropy in nanostructured Fe/Ag(001) films

We have investigated the possibility of isolating the step-induced in-plane uniaxial magnetic anisotropy in Fe/Ag(001) films on which nanoscale surface ripples were fabricated by the ion sculpting technique. For rippled Fe films deposited on flat Ag(001), the steps created along the ripple sidewalls are shown to be the only source of uniaxial anisotropy. Ion sculpting of ultrathin magnetic films allows one to selectively study the step-induced anisotropy and to investigate the correlation between local atomic environment and magnetic properties.     

Magnetic anisotropy of Fe films on GaAs(113)A substrates

We employ superconducting quantum interference device magnetometry to study the thickness dependence of in-plane and out-of-plane magnetic anisotropic properties of Fe films grown on high-index GaAs(113)A substrates by molecular beam epitaxy. The evolution of the in-plane magnetic anisotropy with film thickness is distinguished into two regions. First, for Fe film thicknesses ≤50 MLs, we observe an in-plane uniaxial magnetic anisotropy with the easy axis along the in-plane 〈332̄〉 axes. Second, for Fe film thicknesses ≥70 MLs, we observe a four-fold magnetic anisotropy with the easy axis along the in-plane 〈031̄〉 axes. The existence of an out-of-plane perpendicular magnetic anisotropy is also detected in ultrathin Fe films. Similar to Fe on GaAs(001), our results provide evidence for the interfacial origin of the in-plane uniaxial and out-of-plane perpendicular magnetic anisotropy. Both the uniaxial and the perpendicular interface anisotropy are found to be independent of the epitaxial orientation and are hence an intrinsic property of the Fe/GaAs interface. PACS 75.70.-i; 75.50.Bb; 81.15.Hi     

Magnetic properties of thin epitaxial films investigated by spin-polarized photoemission

The surface sensitivity of the spin-polarized photoemission experiment was exploited to study two-dimensional magnetism. The magnetization of thin films of Fe, Co, and V in the monolayer (ML) range, grown on Cu(001) and Ag(001) single crystals, was measured as a function of perpendicularly applied field and temperature. Bcc Fe films and fcc Fe and Co films exhibit ferromagnetism down to the single monolayer range, while no evidence for ferromagnetism is found for V on Ag(001). All Co films are magnetized in plane and have a Curie temperature far above room temperature. A thickness dependence of the anisotropy and Curie temperature is observed for the two phases of Fe. Remanent magnetization perpendicular to the surface is found at 30 K for fcc Fe films thicker than 2 ML and for bcc Fe between 3 and 4 ML. The magnetic effects caused by coating and by interdiffusion are discussed in the light of measurements of Cu/Fe/Cu sandwiches and of overlayers obtained by simultaneous evaporation of Fe and Cu. The fcc Fe films are shown to be suitable for thermomagnetic writing.     

Tunable in-plane spin orientation in Fe/Si(557) film by step-induced competing magnetic anisotropies

Although the spin-reorientation transition from out-of-plane to in-plane in Fe/Si film is widely reported, the tuning of in-plane spin orientation is not yet well developed. Here, we report the thickness-, temperature- and Cu-adsorptioninduced in-plane spin-reorientation transition processes in Fe/Si(557) film, which can be attributed to the coexistence of two competing step-induced uniaxial magnetic anisotropies, i.e., surface magnetic anisotropy with magnetization easy axis perpendicular to the step and volume magnetic anisotropy with magnetization easy axis parallel to the step. For Fe film thickness smaller than 32 monolayer(ML), the magnitudes of two effects under various temperatures are extracted from the thickness dependence of uniaxial magnetic anisotropy. For Fe film thickness larger than 32 ML, the deviation of experimental results from fitting results is understood by the strain-relief-induced reduction of volume magnetic anisotropy.Additionally, the surface and volume magnetic anisotropies are both greatly reduced after covering Cu capping layer on Fe/Si(557) film while no significant influence of Na Cl capping layer on step-induced magnetic anisotropies is observed.The experimental results reported here provide various practical methods for manipulating in-plane spin orientation of Fe/Si films and improve the understanding of step-induced magnetic anisotropies.     

MOCVD growth, structure and magnetic properties of Fe films grown on GaAs (001) substrates

Thin iron films have been grown on (001) GaAs substrates by low pressure metal organic chemical vapor deposition (LP-MOCVD) at different temperatures with the pressure of 150 Torr. X-ray diffraction (XRD) analysis showed that all films have only one strong diffraction peak (110). The surface of Fe film became smooth with increasing the growth temperature. Magnetization measurements showed that the Fe films grown at different temperatures were ferromagnetic with easy axis parallel to the film surface and hard axis perpendicular to the substrates. The field dependence of magnetization along two axes showed a remarkable difference, implying that the samples have strong magnetic anisotropy. Furthermore, when the applied magnetic field is perpendicular to the Fe surface, a sharp jump in the hysteresis loop could be observed, followed by a broad shoulder, which is related to the interface effect, the existence of carbon and the formation of 180^°/90^° magnetic domains.     

NiO-thickness dependent magnetic anisotropies in Fe/NiO/Au(001) and Fe/NiO/MgO(001) systems

The in-plane magnetic anisotropy of Fe/NiO bilayers was studied quantitatively as a function of NiO thickness using the magneto-optical Kerr effect with a rotating field. For NiO thicker than the ordering transition thickness, the total in-plane fourfold anisotropy of the Fe layer decreases with NiO thickness in Fe/NiO/Au(001), but increases in Fe/NiO/MgO(001). Our result indicates that the exchange coupling in an Fe/NiO bilayer might induce an additional in-plane fourfold anisotropy, and the opposite thickness dependent behaviors may be attributed to the different Ni²⁺ antiferromagnetic spin orientations for NiO films grown on Au(001) and MgO(001) surfaces.     

Body-centered-cubic Ni and its magnetic properties

The body-centered-cubic (bcc) phase of Ni, which does not exist in nature, has been achieved as a thin film on GaAs(001) at 170 K via molecular beam epitaxy. The bcc Ni is ferromagnetic with a Curie temperature of 456 K and possesses a magnetic moment of 0.52+/-0.08 micro(B)/atom. The cubic magnetocrystalline anisotropy of bcc Ni is determined to be +4.0x10(5) ergs x cm(-3), as opposed to -5.7x10(4) ergs x cm(-3) for the naturally occurring face-centered-cubic (fcc) Ni. This sharp contrast in the magnetic anisotropy is attributed to the different electronic band structures between bcc Ni and fcc Ni, which are determined using angle-resolved photoemission with synchrotron radiation.     

低相变温度垂直取向的CoPtCu/Ag纳米复合膜

采用磁控溅射(Ag/Cu/CoPt)_n多层膜先驱体结合真空退火的方法制备了一系列CoPtCu/Ag纳米复合薄膜，通过优化薄膜中Ag以及Cu的含量，成功制备出了低相变温度垂直取向的CoPtCu/Ag纳米复合膜，该膜在450℃退火即可发生相变，该温度比目前所报导的CoPtAg纳米复合膜的相变温度降低了150℃. 实验结果表明，薄膜中一定含量的Ag元素能够有效诱导薄膜的(001)取向，Cu元素的加入能有效降低薄膜的有序化温度. 对于特定组分为Co₄₀Pt₃₆Cu₈Ag₁₆的薄膜，经500℃退火后已经显示了明显的(001)取向，垂直于膜面方向上的矫顽力为5.0×10⁵A/m，并且薄膜中晶粒尺寸仅为4—5nm，为将来CoPt-L1₀有序相合金薄膜用于超高密度垂直磁记录介质打下了基础. 关键词： 磁记录材料 CoPt 纳米复合膜     

Growth of Fe films on Rh(001): a photoemission study

The growth mode and electronic structures of Fe thin films, epitaxially grown on Rh(001), were studied by various photoemission measurements for the film thickness of 0–12 monolayers. By comparing the valence band structures obtained from the angle-resolved photoemission spectra and the theoretical band calculations for the bulk Fe, we concluded that the valence band structure of the Fe film resembles that of fcc(001) in the low coverage region and that of bcc(110) in the high coverage region. This transformation of the electronic structure is considered to be related to the thickness dependence of the interlayer spacing of the films.     

Frozen low-spin interface in ultrathin Fe films on Cu(111)

In ultrathin film systems, it is a major challenge to understand how a thickness-driven phase transition proceeds along the cross-sectional direction of the films. We use ultrathin Fe films on Cu(111) as a prototype system to demonstrate how to obtain such information using an in situ scanning tunneling microscope and the surface magneto-optical Kerr effect. The magnetization depth profile of a thickness-driven low-spin to high-spin magnetic phase transition is deduced from the experimental data, which leads us to conclude that a low-spin Fe layer at the Fe/Cu interface stays live upon the phase transition. The magnetically live low-spin phase is believed to be induced by a frozen fcc Fe layer that survives a thickness-driven fcc-->bcc structural transition.     

Preparation and characterization of epitaxial Fe(001) thin films on GaAs(001)-based LED for spin injection

We report on the successful fabrication and characterization of ferromagnetic Fe/Tb multilayer contacts with perpendicular magnetic anisotropy (PMA) on the clean As-terminated surface of a GaAs(001)-based light-emitting diode (LED) structure. PMA was inferred from magnetometry and ⁵⁷Fe conversion electron Mössbauer spectroscopy. Such contacts are a potential candidate for optical detection of electron spin injection from the remanent magnetic state into the semiconductor.     

Uniaxial magnetocrystalline anisotropy of metal/semiconductor interfaces: Fe/ZnSe(001)

A theoretical study of the magnetic moments and the in-plane magnetic anisotropy of an interface between a cubic ferromagnet and a cubic semiconductor, Fe/ZnSe(001), is presented. Theory confirms the observed, much debated, uniaxial anisotropy of the iron film. This result is important since the calculations are for perfect interfaces with squarelike environments, proving that the fourfolded symmetry of the interface Fe atoms is broken beyond the nearest neighboring semiconducting layer, effects that are usually assumed small. It is demonstrated how the uniaxial anisotropy is produced by the directional covalent bonds at the interface, even without atomic relaxations.     

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.     

Magnetism of CrO overlayers on Fe(001)bcc surface: first principles calculations

Riva et al. [Surf. Sci. 621, 55 (2014)] as well as Calloni et al. [J. Phys.: Condens. Matter 26, 445001 (2014)] have studied the oxydation of Cr films deposited on Fe(001)bcc through low-energy electron diffraction, Auger electron spectroscopy and scanning tunneling microscopy. In the present work we perform a density functional approach within Quantum Expresso code in order to study structural and magnetic properties of CrO overlayers on Fe(001)bcc. The calculations are performed using DFT+U. The investigated systems include O/Cr/Fe(001)bcc, Cr/O/Fe(001)bcc, Cr_0.25O_0.75/Fe(001)bcc, as well as the O coverage O _x /Cr/Fe(001)bcc (x = 0.25; 0.50). We have found that the ordered CrO overlayer presents an antiferromagnetic coupling between Cr and Fe atoms. The O atoms are located closer to the Fe atoms of the surface than the Cr atoms. The ground state of the systems O/Cr/Fe(001)bcc and Cr/O/Fe(001)bcc corresponds to the O/Cr/Fe(001)bcc system with a magnetic coupling c(2 × 2). The effect of the O monolayer on Cr/Fe(001)bcc changes the ground state from p(1 × 1) ↓ to c(2 × 2) and produces an enhancement of the magnetic moments. The O _x overlayer on Cr/Fe(001)bcc produces an enhancement of the Cr magnetic moments.     

A comparison of ferromagnetic resonance and magneto optical Kerr effect on thin Fe films on InAs(001)

A combination of STM, SQUID magnetometry, FMR and MOKE is used to study the structural and magnetic properties of thin iron films grown on InAs(001) (4×2)/c(8×2). The different magnetic characterization methods of this paper allow measurements of the magnetic anisotropies in the saturated and non-saturated state. Here we show results of a SQUID/FMR investigation on a 12 monolayer thick Fe film. As expected, FMR measurements find a four-fold symmetry of the magneto crystalline anisotropy, but with an additional uniaxial contribution. The dependence of the remanence on the magnetization angle computed from the magnetic parameters obtained in the saturated state is compared to experimental remanence data measured using MOKE. Good agreement is found. The InAs-substrate quality prior to growth, the nucleation behavior and the thickness-dependent granular structure of the Fe-layer are studied with STM. The origin of the magnetic anisotropies is discussed in terms of these structural data.     

Epitaxial Fe films on ZnSe(001): effect of the substrate surface reconstruction on the magnetic anisotropy

It is well known that Fe films deposited on a c(2 × 2)-reconstructed ZnSe(001) surface show a strong in-plane uniaxial magnetic anisotropy. Here, the effect of the substrate reconstruction on the magnetic anisotropy of Fe has been studied by in situ Brillouin light scattering. We found that the in-plane uniaxial anisotropy is strongly reduced for Fe films grown on a (1 × 1)-unreconstructed ZnSe substrate while the in-plane biaxial one is nearly unaffected by the substrate reconstruction. Calculations of magnetic anisotropy energies within the framework of ab initio density functional theory reveal that the strong suppression of anisotropy at the (1 × 1) interface occurs due to complex atomic relaxations as well as the competing effects originating from magnetocrystalline anisotropy and dipole-dipole interactions. For both sharp and intermixed c(2 × 2) interfaces, the magnetic anisotropy is enhanced compared to the (1 × 1) case due to the further lowering of symmetry. The theoretical results are in agreement with the experimental findings.     

Local magnetic properties of ultrathin ferromagnetic fcc-Fe-films on Cu(001)

Conversion electron Mössbayer spectroscopy (CEMS) on three monolayers (ML) thick metastable fcc-Fe(001) films grown epitaxially on a Cu(001) substrate under different conditions shows that these films are characterized by a distributionP(B _hf) of magnetic hyperfine fieldsB _hf. The vast majority of⁵⁷Fe nuclei experience relatively large hyperfine fields at low temperature. The temperature dependence of the most probable fieldB _peak was found to follow aT ^3/2 spin-wave law below 300 K. It is shown from the relative line intensities that preferential Fe spin orientation perpendicular to the film plane exists in films grown at 120 K, while preferential in-plane spin orientation is found for a growth temperature of 300 K. Coating a low-temperature grown Fe film by 2 ML of Cu(001) drastically reduces the hyperfine field, in contrast to the case of room-temperature grown Cu-coated films.Dedicated to Professor Ulrich Gonser on the occasion of his 70th birthday     

Magnetic surfaces

The magnetic ground state, magnetic anisotropies and spin excitations of surfaces, interfaces and ultra-thin films of ferromagnetic 3d-metals are discussed. Enhanced magnetic ground state moments and altered hyperfine fields as predicted by ab initio band calculations have not been conclusively verified by experiments up to now. Future calculations should take into account dipolar fields and the role of interface roughness. Very large magnetic anisotropies are observed at magnetic surfaces and interfaces. In Ni/Cu multilayered films, the superposition of surface and stress-induced anisotropies was used to switch the easy axis of magnetization from the film plane to a perpendicular orientation by a proper choice of the Ni layer thickness. This could be an attractive possibility to develop new magnetic materials for technical applications. The temperature dependence of the spontaneous magnetization at surfaces and in ultra-thin films deviates from the behaviour of bulk material. Size effects as well as surface effects of spin wave excitations are discussed, comparing theoretical and experimental results. The need for more complete theories including surface exchange, surface anisotropy and realistic surface structures is emphasized.     

Structure and magnetic properties of CoPtCu:Ag nanocomposite films with (001) texture

Nanocomposite (001) textured CoPtCu:Ag films consisting of well separated L1₀ structure CoPtCu nanoparticles have been prepared using a CoPt/Cu/Ag multilayer precursor on SiO₂/Si(100) substrate by magnetron sputtering and subsequent vacuum thermal annealing. With a Cu concentration of 6–10% and Ag of 10–20% in atomic ratio, the films start ordering at an annealing temperature of 450 °C, which is roughly lower by 150 °C than that needed for most CoPt-based films especially with (001) texture. The perpendicular coercivities for the film are in the range from 5 kOe to 7 kOe after annealing at 500 °C in vacuum. The (001) texture for the film is partially due to the formation of an Ag underlayer after annealing, the decreasing of the ordering temperature is most-likely related to the formation of CoPtCu alloy nanoparticles in the film. PACS 75.30.Gw; 75.50.Kj; 75.70.Ak     

Interplay between anisotropic strain relaxation and uniaxial interface magnetic anisotropy in epitaxial Fe films on (001) GaAs

Grazing incidence x-ray diffraction study of Fe epitaxial ultrathin films (1.5-13 nm) on GaAs (001) reveals an anisotropy of both domain shape and strain, with [110] and [1-10] as the principal directions. It is shown that the observed thickness-dependent strain anisotropy, together with a uniaxial interface term, can provide an unambiguous explanation to the usual in-plane magnetic anisotropy and its thickness dependence observed in this magnetic thin-film system.