Magnetic anisotropy and magnetization reversal of ultrathin iron films with in-plane magnetization on Si（111） substrates

The magnetic anisotropy and magnetization reversal of single crystal Fe films with thickness of 45 monolayer (ML) grown on Si(111) have been investigated by ferromagnetic resonance (FMR) and vibrating sample magnetometer (VSM). Owing to the significant modification of the energy surface in remanent state by slight misorientation from (111) plane and a uniaxial magnetic anisotropy, the azimuthal angular dependence of in-plane resonance field shows a six-fold symmetry with a weak uniaxial contribution, while the remanence of hysteresis loops displays a two-fold one. The competition between the first and second magnetocrystalline anisotropies may result in the switching of in-plane easy axis of the system. Combining the FMR and VSM measurements, the magnetization reversal mechanism has also been determined.     

Effect of dipolar interaction on the magnetization state of chains of rectangular particles located either head-to-tail or side-by-side

Magnetostatic coupling in arrays of closely spaced magnetic elements is becoming an important issue in the path to the fabrication of spintronic devices. Dense chains of rounded-corners rectangular particles (dots) of lateral size 1025 × 450 nm², with interdot spacing variable in the range between 55 and 700 nm, have been patterned by deep UV lithography, followed by the lift-off of two permalloy films of thickness 20 and 40 nm. Magneto-optical Kerr effect (MOKE) and magnetic force microscopy (MFM) experiments, together with micromagnetic simulations, were performed to study the dependence of the magnetization configuration on the dipolar coupling. Both MOKE measurements and MFM images clearly show that, at remanence, the magnetic state of isolated particles of thickness 20 nm takes the form of a distorted single domain (C-state or S-State configurations). Instead, when the particle thickness is double (40 nm), closure states characterized by one, two or three vortices occur at remanence. However, when the 40 nm thick dots are placed in chains along the easy axis (head to tail), as the separation is progressively reduced, the single domain state is stabilized at remanence. On the other hand, when the 40 nm thick particles are placed side by side in chains the effect of dipolar interactions is to favour the nucleation of vortex states. For small inter-element separation, there is only one vortex per particle and it has the same chirality in adjacent particles, due to the dipolar interaction. Different from this, for the 20 nm thick samples and sub-100 nm separation, adjacent particles are single-domain but with antiparallel magnetization in neighbour elements, like in an artificial antiferromagnet.     

The study of exchange coupling in NiFe/Cu/IrMn trilayer structures by MOKE and FMR measurements

The exchange coupling strength of NiFe/Cu/IrMn trilayer films was examined with both a new magneto optical Kerr effect (MOKE) method developed for the exchange coupling field determination and ferromagnetic resonance (FMR) measurements. We found that the value for exchange coupling field obtained by the MOKE technique coincided with FMR result with high accuracy. Other peculiarities of FMR measurements due to interlayer exchange coupling such as angular dependence of resonance field on Cu spacer thickness are also shown in the article.     

High temperature magnetic order in zinc sulfide doped with copper

We report the first observation of magnetic order above room temperature in zinc sulfide doped with Cu²⁺ made by a simple sintering process and detected by ferromagnetic resonance (FMR), SQUID magnetometry and the μSR experiments. FMR study shows nonzero absorption above room temperature indicating a high value of Curie temperature. Field-cooled and zero-field-cooled magnetization data indicate a bifurcation at about 300 K. This temperature is considered to be the blocking temperature at ferromagnetic nanosized regions, which appear to exist in the sample of ZnS doped with copper. The muon spin rotation measurements confirm highly nonuniform magnetic order. It was shown that about 25% of the sample volume fraction exhibits strong magnetism.     

Fe/MnAs bilayers: Magnetic anisotropy and the role of the interface

We have studied different aspects of the magnetic behavior of Fe(5 nm)/MnAs(100 nm) bilayers epitaxially grown on GaAs(1 0 0). Ferromagnetic resonance (FMR) measurements were performed in order to characterize the magnetic anisotropies of the films and the interlayer coupling between them. The chemical composition of the interface was investigated by X-ray photoemission spectroscopy (XPS).     

In-plane magnetic anisotropies in Ni/FeMn and Ni90Fe10/FeMn exchange biased bilayers

The in-plane magnetic anisotropy in Ni/FeMn and Ni₉₀Fe₁₀/FeMn exchange-biased bilayers prepared by co-evaporation under molecular beam epitaxy conditions is investigated employing longitudinal magneto-optical Kerr effect (MOKE) and ferromagnetic resonance (FMR). The exchange anisotropy was induced by a magnetic field cooling immediately after the deposition of the bilayers. Besides the induced term, the presence of an additional uniaxial anisotropy in the FM layers was detected both by MOKE and FMR, and the characteristic directions of these two anisotropy terms are not coincident. The interplay between the anisotropy contributions is discussed considering micromagnetic simulations and the in-plane resonance condition for different magnetic field orientation. X-ray diffraction, X-ray photoelectron spectroscopy, and Mössbauer spectroscopy were used to complement the characterization of the samples.     

Optimization of spin-valve parameters for magnetic bead trapping and manipulation

Magneto-optic Kerr effect (MOKE) and magnetoresistance (MR) measurements were used to measure the switching characteristics of spin-valve (SV) arrays currently being developed to trap and release superparamagnetic beads within a fluid medium. The effect of SV size on switching observed by MOKE showed that a 1 μm×8 μm SV element was found to have optimal switching characteristics. MR measurements on a single 1 μm×8 μm SV switched with either an external applied magnetic field or a local magnetic field generated by an integrated write wire (current density ranging from 10⁶ to 10⁷ A/cm²) confirmed the MOKE findings. The 1 μm×8 μm SV low field switching was observed to be +8 and −2 mT with two stable states at zero field; the high field switching was observed to be −18 mT. The low switching fields and the large magnetic moment of the SV trap along with our observation of minimal magnetostatic effects for dense arrays are necessary design characteristics for high-force, “switchable-magnet,” microfluidic bead trap applications.     

Preparation,characterization and magneto-optical investigations of electrodeposited Co/Au films

Au/Co(4–8 ML)/Au single magnetic layers and Au(8 ML)/Co(4 ML)/Au(8 ML)/Co(8 ML)/Au bilayer were sequentially grown by electrodeposition on an Au(1 1 1) buffer layer electrodeposited on Si(1 1 1). The technique used in this work provides full control on the structure and the chemical composition of the different layers (no alloying) as well as on the chemistry at interfaces. scanning tunneling microscopy (STM) and atomic force microscopy (AFM) imaging and X-ray diffraction measurements show that atomically flat continuous Co(0 0 0 1) layers (4–8 ML) can be grown in epitaxy with the Au(1 1 1) substrate and that the 2 nm-thick spacer is also a continuous Au(1 1 1) layer. The Co ultrathin layers (4 and 8 ML) exhibit perpendicular magnetic anisotropy. The lateral magnetic homogeneity and magnetization reversal process have been investigated by scanning magneto-optical Kerr effect (MOKE) magnetometry and global Kerr microscopy. The correlation between magnetization switching behaviour in each layer of the Co-bilayer stack has been evidenced from in-depth sensitive MOKE measurements and microscopy. The strong coupling observed between the two Co layers is attributed to magnetostatic interaction at domain wall boundaries.     

Magnetic phase diagram of Fe0.82Ni0.18/V(0 0 1) superlattices

Fe_0.82Ni_0.18/V(0 0 1) superlattices grown by DC magnetron sputtering on MgO(0 0 1) substrates have been investigated using longitudinal MOKE, SQUID magnetometry and magnetoresistance measurements. The varying sign and strength of the interlayer exchange coupling (IEC) were identified in the thin layer region (0.4–2.4 nm) and a magnetic IEC phase diagram was deduced and analyzed in terms of density functional calculations. The maximum giant magnetoresistance effect was determined to be 2.5% at 21 K. The balance between the magnetic anisotropy and IEC was found to be significantly different from that of previously studied Fe/V superlattices, also causing a different dependence of both IEC strength and observed anisotropy on the magnetic layer thickness.     

Co-SiO2颗粒膜中颗粒相互作用的温度和尺寸依赖关系

我们在真空离子束溅射设备中制备了Co-SiO2颗粒膜，其结构和磁性分别利用透射电子显微术（TEM）和铁磁共振（FMR）来研究。TEM分析表明Co纳米颗粒相互独立地分布在SiO2介质中，且尺寸分布非常窄。室温转角FMR实验显示薄膜具有单轴各向异性，变温FMR实验被用来研究在100－300K温度范围内的颗粒间相互作用强度随温度变化的关系（我们利用一理论模型计算了颗粒集合体中的铁磁部分所占比例和超顺磁部分所占比例）。我们发现颗粒间相互作用强度除了与温度有关之外，还与Co颗粒的尺寸直接相关。     

Ferromagnetism in Mn-doped GaAs layers: Effects of laser annealing

The properties of Mn-doped GaAs layers grown by laser deposition were investigated with measurements of Hall effect and magneto-optical Kerr effect (MOKE). The electrical and magnetic parameters of the layers were defined by growth temperature and quantity of sputtered Mn. It was shown that room-temperature ferromagnetism is revealed by MOKE and, after ruby laser 25 ns pulse annealing, by Hall effect measurements.     

铁磁共振实验中值得注意的几个问题

与顺磁样品相比铁磁样品的磁共振信号不仅是强得多,铁磁样品的许多特点都会在铁磁共振(FMR)谱中反映出来.诸如,起始磁化过程的FMR谱线与有剩余磁化强度Mr的FMR谱是不同的,起始磁化过程中的巴克豪森跳跃也会在FMR谱中反映出来.样品的磁晶各向异性、形状各向异性,铁磁高温顺磁的磁性相变等等都能用FMR方法加以研究.     

Effect of swift heavy ion irradiation in Fe/W multilayer structures

Present study reports effect of swift heavy ion irradiation on structural and magnetic properties of sputtered Fe/W multilayer structures (MLS) having bilayer compositions of [Fe(20 Å)/W(10 Å)]_5BL and [Fe(20 Å)/W(30 Å)]_5BL. These MLS are irradiated by 120 MeV Au⁹⁺ ions up to fluence of 4 × 10¹³ ions/cm². X-ray reflectivity (XRR), wide-angle X-ray diffraction (WAXD), cross-sectional transmission electron microscopy (X-TEM) and magneto optical Kerr effect (MOKE) techniques are used for structural and magnetic characterization of pristine and irradiated MLS. Analysis of XRR data using Parratt's formalism shows a significant increase in W/Fe interface roughness. WAXD and X-TEM studies reveals that intra-layer microstructure of Fe-layers in MLS becomes nano-crystalline on irradiation. MOKE study shows increase in coercivity at higher fluence, which may be due to increase in surface and interface roughness after recrystallization of Fe-layers.     

Magnetooptical properties and FMR in granular nanocomposites (Co84Nb14Ta2) x (SiO2)100? x

For nanodimensional magnetically inhomogeneous amorphous granular films of the system (Co₈₄Nb₁₄Ta₂) _x (SiO₂)_100−x , 30≤x≤60 at.% the concentration dependences of the magnetooptical Kerr effect (MOKE) spectra and FMR have been investigated. The observed changes in the MOKE and FMR spectra are associated with transformations of microstructure and topology of the nanocomposites. For the compositions within the percolation region the transversal Kerr effect increases by an order of magnitude.     

Preparation and characterization of hydrophobic magnetite microspheres by a simple solvothermal method

Superparamagnetic magnetite microspheres with a hydrophobic surface were successfully prepared through a simple solvothermal method based on hydrolysis of iron-oleate complex in diphenyl ether in the presence of oleic acid as the ligands. The microspheres size and size distribution were analyzed by a laser diffraction particle size analyzing method using ZETASIZER. The morphology and crystalline structure of the products were characterized using transmitting electron microscopy (TEM), scanning electron microscopy (SEM), X-Ray diffraction (XRD), and the magnetic property was studied by a Quantum Design MPMS SQUID. TEM and SEM images showed that as-prepared spherical nanostructures are of about 140 nm in sizes, which self-assembled by many 10 nm primary magnetic nanoparticles. The XRD analysis revealed that the magnetic microspheres are composed of magnetite. The magnetic measurements demonstrated that the spherical nanostructures are superparamagnetic at room temperature with no magnetic remanence and coercive force. In addition, the microspheres can be well dispersed in various non-polar solvents due to their surfaces capped of hydrophobic surfactants in situ.     

Magnetic shape anisotropy calculations of Fe nanostructures at the Si/Fe(1 1 1) interface

The influence of Si capping layers on the magnetic properties of thin Fe films grown on Si(1 1 1) has been studied by means of shape anisotropy calculations. Fe surface morphology simulations are realized using experimental STM data. Surface modifications induced by the interaction between the Si overlayer and the Fe surface are performed in agreement with the model proposed in a previous work by Stephan et al. [J. Magn. Mater. 293 (2005) 746]. Calculations of the uniaxial anisotropy energy constant K_u are then performed on the modified Fe surface morphology for different Si deposition geometries as proposed in the model. The relevant data deduced by this method such as anisotropy constants and their related easy axis direction, are directly compared to the experimental ones obtained by ex situ magneto-optical Kerr effect (MOKE) measurements at room temperature using the transverse bias initial inverse susceptibility and torque (TBIIST) method. We show that a very good agreement between those results leads to a confirmation of the proposed model.     

FMR Studies of Ultra-Thin Epitaxial Pd<Subscript>0.92</Subscript>Fe<Subscript>0.08</Subscript> Film

Magnetic anisotropies of 20 nm epitaxial film of palladium–iron alloy Pd_0.92Fe_0.08 grown on the (001) MgO substrate were studied. Ferromagnetic resonance (FMR) spectroscopy and vibrating sample magnetometry (VSM) were exploited to determine magnetic parameters of the film. It was found that the synthesized film reveals cubic anisotropy with tetragonal distortion. The simulated magnetic hysteresis loops, obtained utilizing the magnetic anisotropy constants taken from the FMR spectra analysis, agree well with those measured by VSM.     

Two-jump magnetic switching in ultrathin Co(0 0 1) films probed by giant magnetoresistance measurements

Ultrathin epitaxial FCC-Co films, which form part of a spin-valve structure, were found to undergo one- or two-jump magnetic switching, in GMR and MOKE measurements depending upon the field orientation. The transitions are mediated by the propagation of 180° or 90° domain walls. The Co two-jump spin switching in the spin-valve structure has contributed to the formation of three stable GMR states: parallel, antiparallel and a new intermediate state.     

Magnetic properties of self-assembled nanostructure films on the base of amorphous Ni granules

We report on structural and magnetic properties of granular films consisting of 2.5 nm Ni nanoparticles. The films are fabricated by the original laser electrodispersion technique, which allows producing nearly monodisperse and amorphous particles. Atomic force microscopy (AFM) study shows that in 8 nm thickness films the particles are self-assembled in clusters with the lateral size 100-150 nm and the height of about 8 nm. Performed by SQUID, the films magnetization measurements reveal superparamagnetic behaviour, characteristic for an ensemble of non-interacting single domain magnetic particulates. It is found that the magnetic moment of the particulate is equal to that of about 3000 individual Ni nanoparticles and the blocking temperature is close to room temperature. Defined from magnetic measurements, the size of single domain particulates correlates well with the size of the clusters determined from AFM images. We propose that exchange interaction plays an important role in the formation of the particulates by aligning the magnetic moments of the individual Ni nanoparticles inside the clusters. Presence of magnetic clusters with high blocking temperature makes the fabricated films potentially useful for high-density magnetic data storage applications.