Magnetic Thin Film Structures Research Articles

Influence of the antiphase domain distribution on the magnetic structure of magnetite thin films

A long-range ordered magnetic domain structure was found in magnetitelike (Fe3−δO4, δ≈0.03) thin films prepared by molecular beam epitaxy on MgO (100) substrates. The stripelike magnetic domain structure arising after suitable postprocessing differs significantly from earlier observations. The field-dependent domain structure was investigated by magnetic force microscopy in external magnetic fields. The magnetic domain structure results from a moderate perpendicular anisotropy. The domain structure is pinned, and the pinning centers arise from the magnetite antiphase domain structure.

Carrier-induced ferromagnetism in Ge(Mn,Fe) magnetic semiconductor thin-film structures

We report on the carrier-induced ferromagnetism in Ge(Mn,Fe) magnetic semiconductor insulating-type thin-film structures prepared using sequential deposition at Tg=520K with subsequent annealing at Tg. In the resulting films Mn and Fe are diffused in the Ge matrix without compromising the epitaxial structure. The anomalous Hall effect serves as a manifestation of the carrier-induced magnetism, with p-type conductivity and the Curie temperature TC=209K. The additional doping with Fe stabilizes epitaxial growth and carrier-mediated magnetism at levels of magnetic doping exceeding 10%. We conclude that indirect ferromagnetic exchange is mediated by localized holes with concentration n∼1020cm−3 and mobility μ∼10cm2∕(Vs).

Studies of Magnetic Microstructures With Soft X-Ray Transmission Microscopy

We review recent achievements of magnetic full-field soft X-ray transmission microscopy. This technique allows for an element-specific imaging of magnetic domain structures in thin films and nanopatterned elements. With a lateral resolution down to 15 nm, the ability to record images in external magnetic fields and the application of stroboscopic pump-and-probe schemes detailed insights into fundamental mechanisms of micromagnetism can be obtained which are relevant in the development of high density and ultrafast magnetic storage and sensor technologies.

Quantitative Fresnel Lorentz microscopy and the transport of intensity equation

Imaging of the magnetic structure of thin films by the Fresnel mode of Lorentz microscopy has been re-evaluated in terms of the Ampérian current density within a sample. The conditions for which this imaging can be treated as linear are discussed for quantitative application. Additionally, the consequences for magnetic phase reconstruction using the transport of intensity equation for defocused images are considered. While the range of applicability may initially appear rather limited examples of objects containing different spatial frequency components are used to illustrate the possibilities where relatively large defocus values may be used.

MAGNETISM AND LATTICE DYNAMICS OF NANOSCALE STRUCTURES STUDIED BY NUCLEAR RESONANT SCATTERING OF SYNCHROTRON RADIATION

Nuclear resonant scattering of synchrotron radiation is applied to investigate the magnetic structure and the lattice dynamics of nanoscale systems. The outstanding brilliance of modern synchrotron radiation sources allows for sensitivities to smallest amounts of material. Due to the isotopic sensitivity of the scattering process, ultrathin probe layers of Mössbauer isotopes can be used to map out the magnetic and vibrational structure of thin films with sub-nm spatial resolution. Elastic nuclear resonant scattering is applied to determine the magnetic spin structure of an exchange-coupled bilayer system. Inelastic nuclear resonant scattering was used to determine the vibrational density of states in Fe islands on the W(110) surface.

Electron states and the spin density wave phase diagram in Cr(1 1 0) films

Chromium films offer an excellent system to study the impact of dimensional confinement on physical properties associated with the spin-density-wave (SDW) ground state observed in bulk materials. These properties are also of some technological importance since chromium is a common component of thin film magnetic structures. We prepared chromium (1 1 0) films of high crystalline quality on a W(1 1 0) substrate with a wedge-shaped thickness profile so that the impact of confinement can be systematically studied. We have characterized these films using a combination of low-energy electron diffraction and microscopy as well as high-resolution angle-resolved photoemission spectroscopy. We have probed the Fermi surface and the nesting vectors therein that are relevant to the SDW ground state. We find these to predict accurately the observed bulk SDW periodicity. We have also characterized the SDW periodicity in the film directly by measuring the splitting between backfolded bands, and we find that this periodicity deviates markedly from the bulk periodicity for thinner films at higher temperatures. We have systematically mapped the SDW incommensurability and phase diagram as a function of both film thickness and temperature. We find commensurate and incommensurate phases that are separated by nearly continuous transitions. Our results suggest a simple model to explain the delicate interplay between commensurate and incommensurate phases that involves a balance between SDW stabilization energy and surface and interface energetics.

Spin-current effect on ferromagnetic resonance in patterned magnetic thin film structures

We have theoretically investigated the ferromagnetic resonance in the magnetic thin film structures under the influence of spin-transfer torque using a modified Landau–Lifshitz–Gilbert equation in the linearization regime. The study shows that spin currents do not shift the resonance field but rather change both the resonance amplitude and the linewidth. Ferromagnetic resonance under this circumstance can be characterized by an effective damping constant. Depending upon its direction, the spin current can pump energy into or dissipate energy from the magnetic system. In addition, the quality factor of the resonance can be tuned by changing the current intensity. Ferromagnetic resonance excited by ac electrical currents is also theoretically demonstrated and discussed in this article.

Observation of Magnetic Domain Structure of Micro Magnetic Materials and Magnetic Thin Films by Photoemission Electron Microscope (PEEM)

The photoemission electron microscopy using synchrotron radiation (SR-PEEM) combined with the X-ray circular and linear dichroism (XMCD and XMLD) is one of the most powerful techniques to investigate the magnetic domain structure of micro magnetic materials or surfaces and interfaces of magnetic thin films. With the method, we can observe the magnetic domain structure directly to a very high spatial resolution. In this paper, the technique of SR-PEEM for the observation of the magnetic domain structure and its applications to the Ni-micro ring, and the ferromagnetic/antiferromagnetic interface, i.e. the Fe wedge/NiO(001), are introduced.

Near-field second-harmonic imaging of magnetic domains

The principles and mechanisms of scanning near-field microscopy of second-harmonic generation from ferromagnetic films have been investigated. Magnetic domain image formation in near-field second-harmonic microscopy is discussed and the role of the probe tip is considered. It is shown that magnetic contrast can be achieved in the second-harmonic intensity measurements in both the transverse and longitudinal magnetization configurations for both polarizations of the fundamental light. This is related to the electromagnetic near-field properties in the vicinity of the probe tip and the electromagnetic interaction between a tip and a surface. The capabilities of this imaging approach have been illustrated using magnetic domain structures in thin Co and Ni films. The near-field second-harmonic distributions obtained with p-polarized excitation light are strongly correlated to the topography of the films. At the same time, the images obtained with s-polarized excitation light are much less sensitive to the topography.

Surfaces and interfaces characterization by neutron reflectometry

Neutron reflectivity allows to characterize surfaces and interfaces of ultra-thin film layered systems down to a nanometric scale (∼2nm). It is a powerful tool for the study of magnetic and polymer thin film structures. The neutron magnetic interaction is of the same order as the nuclear interaction and makes polarised neutron reflectivity a very sensitive tool for probing magnetic multilayers. It allows the determination of magnetic ordering and coupling in artificial magnetic multilayers (either metallic/semi-conducting or oxides). Examples of studies in the field of giant magneto resistive (GMR) sensors used in hard drive and tape read heads are given. The search for efficient spin-injection materials which could be used in spin electronics is also a growing field of activity. Several material candidates are presently evaluated. Recent studies on such materials are presented: oxide materials (Fe2O3–Fe3O4) epitaxial thin films; multilayer systems involving semiconducting materials ((Fe/Si)n–(GaMnAs/GaAs)). Besides the high sensitivity of neutrons to magnetism, the possibility of isotopic labelling (H/D substitution) offers a way to probe polymer and protein thin film structures with great details: polymer interdiffusion or polymer grafting can be studied at the substrate–polymer or liquid–polymer interfaces. A model study of polymer grafting is presented.

Dynamic formation of columnar lattices in magnetic fluid thin films subjected to oscillating perpendicular magnetic fields

The application of a low-frequency oscillating magnetic field perpendicular to a magnetic fluid thin film leads to the separation of a phase that is concentrated in particles from a dilute phase. The concentrated phase forms cylindrical columns that construct two-dimensional lattices. The ordered structure of magnetic fluid thin films is the basis for potential optical applications. We investigate the dynamical ordering formation of columnar lattices in magnetic fluid thin films subjected to oscillating perpendicular magnetic fields.

Magnetic domain imaging of ferromagnetic GaMnAs films

Scanning Hall probe microscopy has been used to study the magnetic domain structure of GaMnAs thin films after various low temperature anneals to increase the Curie temperature (TC). Samples with in-plane magnetization, which received short low temperature anneals in vacuum directly after growth, exhibit very small (∼2–5 μm) rather random domains. In stark contrast similar samples, which additionally received very long low temperature anneals in air, display large (10–100 μm) domains, which still contain clearly resolvable magnetic disorder. Preliminary scans of air-annealed samples with out-of-plane magnetization also reveal a very irregular, rather fine (1–3 μm) domain structure. In all samples micron-sized regions at domain walls frequently remain ferromagnetic well above the average TC, indicating either the presence of ferromagnetic precipitates (e.g., MnAs) or material there with higher than average Mn concentration.

Magnetic domain structure of colossal magnetoresistance thin films and islands

We synthesized 10–200 nm thick colossal magnetoresistive La0.7Sr0.3MnO3 (LSMO) thin films under compressive strain, resulting in perpendicular anisotropy. Similar magnetic domain structures in LSMO films thicker than 40 nm were observed by magnetic force microscopy and were also seen with photoemission electron microscopy. Recent transport measurements, in conjunction with this result, suggest a dead layer at the interface, not at the surface. When varying the temperature from below the Curie temperature TC to above, the magnetic domains disappeared. These domains nucleated uniformly across the film when the temperature was subsequently lowered, but their positions were unrelated to the initial domains or to surface defects.

Effects of PS–PVP diblock copolymer topography on the magnetic properties of CoCrPt thin films

AbstractThe magnetic properties and growth structure of Co68Cr18Pt14 thin films deposited on self‐assembled PS21400 (styrene)–PVP20700 (4‐vinyl pyridine) diblock copolymer surfaces were investigated using a magneto‐optical microscope magnetometer (MOMM). The perpendicular magnetic anisotropy as well as the coercivity of 200 Å CoCrPt/PS–PVP/Si(100) films in comparison with 200 Å CoCrPt/Si(100) were distinctly enhanced. An atomic force microscope (AFM) revealed that the enhanced perpendicular magnetic anisotropy and coercivity in CoCrPt/PS–PVP/Si(100) films might be ascribed to the reduction of the magnetic dipolar anisotropy of CoCrPt films having a rough surface arising from spherically modulated PS–PVP surface morphology and preferential deposition of CoCrPt on top of the PVP block. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetic structure of FeMn thin films on (001) FCC substrate

Abstract LEED experiments by Offi et al. (Phys. Rev. B 66 (2002) 064419) have recently revealed that the Fe50Mn50 alloy thin films keeps the FCC structure when growing on Cu (0 0 1) and on Co / Cu (0 0 1) . Stability of magnetic configurations of Fe50Mn50 alloy monolayer film is investigated using ab-initio calculations within density functional theory with gradient corrections. Fe induced magnetic moment of 2.85 μ B with Mn magnetic moment of 3.61 μ B aligned parallel with the ferromagnetic moment of the Co layer have been found as groundstate configuration with the Perdew–Wang functional in agreement with X-ray magnetic circular dichroism results.

Ordering formation of columnar lattices in magnetic fluid thin films subjected to oscillating perpendicular magnetic fields

Applying an oscillating magnetic field perpendicularly on the high-quality magnetic fluid thin film, the phase separation of particles in the liquid matrix will occur. The concentrated phase makes up the cylindrical columns that can form two-dimensional lattices. The ordered structure of magnetic fluid thin films is the basis for the potential optical application. We explore the dynamical ordering formation of columnar lattices in magnetic fluid thin films subjected to oscillating perpendicular magnetic fields in this study.

ELECTRONIC STRUCTURE AND MAGNETISM OF NICKEL THIN FILMS

We present calculations of the electronic and magnetic structure of thin films of Ni, using the recursion technique based on the tight-binding linearized muffin-tin orbitals basis. We find that the surface layers of free standing Ni thin films do carry enhanced magnetic moment.

Imaging Amperian currents by Lorentz microscopy

The magnetic structure of thin films may be imaged in a transmission electron microscope by the techniques of Lorentz microscopy. In this paper, we re-interpret the underlying principles behind this imaging mode to show that it may be considered, in appropriate circumstances, as Amperian current microscopy. One of the benefits of this interpretation is that results from micromagnetic simulations may now be more easily formulated into the corresponding images expected from Lorentz microscopy. Examples are given in this paper which demonstrate the utility of this approach and also provide excellent agreement by direct comparison between experimental and simulated images of magnetic thin film structures.

Optical ferromagnetic resonance studies of thin film magnetic structures

We have used a magneto-optical pump-probe technique to stimulate and characterize small amplitude precession of the magnetization in a variety of thin film magnetic structures. The sample is stimulated by an optically triggered magnetic field pulse and its response probed by means of a stroboscopic measurement of the magneto-optical Kerr effect. We demonstrate that the experimental technique is sensitive to small amplitude deflections of the magnetization and present formulae that describe the frequency of precession in continuous films containing either one or two magnetic layers. We show how the dependence of the precession frequency upon the orientation and magnitude of the static field may be used to characterize a 100 Å Co thin film, a Ni81Fe19(50 Å)/Cu(20 Å)/Co(50 Å)/IrMn(100 Å) spin-valve structure, and a square Ni81Fe19 element of 10 µm side and 150 nm thickness. Information may be obtained about the g factor, demagnetizing field, exchange bias effect and magnetic anisotropy within an individual ferromagnetic layer, and about the coupling between layers. We deduce the presence of a four-fold anisotropy in the square element that is associated with non-uniformity of the magnetization. Time resolved images show that the dynamic magnetization is generally non-uniform. A stripe pattern is observed when the static field is significantly larger than the anisotropy field. Measurements at low field show that the dynamic magnetization initially has a regular pattern associated with the underlying domain structure, but then evolves into a highly disordered state. Finally we discuss the outlook for further development and application of the experimental technique.

X-ray photoemission electron microscopy investigation of magnetic thin film antidot arrays

Periodic square arrays of antidots in 10 nm-thick cobalt films have been investigated with antidot periods, p, ranging from 2 μm down to 200 nm and various ratios of antidot size to antidot separation, w/d. For p=2 μm, the extent of modification of the thin film magnetic domain structure increases with increasing w/d, forming domains pinned diagonally between antidots for w/d⩾0.2 and resulting in a two-dimensional periodic checked domain contrast commensurate with the antidot lattice for w/d⩾0.9. As p is decreased while maintaining d≈w, we observe a dramatic change in the magnetic domain configuration at p=400 nm resulting in chains of magnetic domains running parallel to the intrinsic hard axis and with lengths corresponding to a multiple of the antidot period.