Non-magnetic Contributions Research Articles

Two-dimensional spatially ordered arrays of cobalt nanowires: polarized SANS study

Structural and magnetic properties of two-dimensional spatially ordered system of ferromagnetic cobalt nanowires embedded into Al2O3 matrix have been studied using polarized small-angle neutron scattering. The small-angle scattering pattern exhibits many diffraction peaks (up to the third reflection order), which corresponds to the scattering from highly correlated hexagonal structure of pores and magnetic nanowires. A comprehensive analysis of contributions to the scattering intensity, including nonmagnetic (nuclear) contribution, magnetic contribution depending on the magnetic field, and nuclear-magnetic interference indicating the correlation between the magnetic and nuclear structures was carried out. A detailed pattern of the remagnetization of an ordered array of the magnetic nanowires has been obtained. It has been illustrated that polarized neutron scattering provides unique information inaccessible by the standard magnetometry techniques.

ChemInform Abstract: The Heat Capacities of the Tantalates MTa2O6, M: Mg, Fe, Co, Ni

The heat capacities at temperatures from 30 K to 300 K of the tantalates MTa 2 O 6 (M = Mg, Fe, Co, Ni) are reported. In this temperature range there are no phase transitions. However, it is known that the Fe, Co, and Ni salts undergo magnetic phase transitions at lower temperatures. The purpose of this work was to determine the non-magnetic contributions to the heat capacities, in order to allow the delineation of the magnetic contributions to the heat capacities at lower temperatures.

Magnetic properties of a two-dimensional spatially ordered array of nickel nanowires

The structural and magnetic characteristics of two-dimensional spatially ordered arrays of magnetic nickel nanowires embedded in the anodized alumina template have been investigated. It has been shown using small-angle polarized-neutron diffraction that, there exists the samples under investigation, in a highly ordered hexagonal structure of pores and magnetic nanowires separated by a characteristic distance d = 106 ± 2 nm. An analysis has been made of different contributions to neutron scattering, such as the nonmagnetic (nuclear) contribution, the magnetic contribution dependent on the magnetic field, and the interference contribution indicating a correlation between the magnetic and nuclear structures. The performed analysis of the results obtained has demonstrated that, when the magnetic field is applied perpendicular to the longitudinal axis of the nanowire in a completely magnetized sample, there arise demagnetizing fields around each nanowire that form a regular hexagonal lattice.

Single Crystal Studies of Ternary Transition-Metal Molybdenum Selenides MMo6Se8 (M = Ti, Fe, Mn, Cr and Co)

Magnetic properties have been measured in single crystals of selenium-based Chevrel-type M xMo6Se8, with M = Ti, Fe, Mn, Cr and Co, in which x(M) is equal to unity, except for Ti (x ~ 0.9) and Co (x ~ 0.5). Presence of impurities, such as the binary Mo3Se4 sub-product, is absolutely excluded and reported results are representative of the intrinsic properties of the reported compounds. For polycrystalline ScMo6Se8 and single crystals of Ti0.88Mo6Se8, the Pauli-like temperature-independent magnetic susceptibility allows having a good estimation of the non-magnetic contribution of the Mo6Se8 sublattice. Crystal field effects were seen for polycrystalline VMo6Se8. Spectacular results were obtained for FeMo6Se8 (canted antiferromagnetic compound with high magnetic anisotropy), CrMo6Se8 (antiferromagnetism, TN = 12 K), MnMo6Se8 (metamagnetic compound) while the properties of Co0.54Mo6Se8 can be interpreted by magnetic exchange interactions JAF ~ 100 K. These results are discussed in connection with structural properties and transport measurements obtained in single crystal specimens.

Optimization of magneto-optical Kerr setup: Analyzing experimental assemblies using Jones matrix formalism

We present a comparative study on an experimental and theoretical optimization of magneto-optical Kerr setups based on photoelastic modulation and phase sensitive detector methodology. The first and second harmonics, I omega,2 omega, of the reflected light intensity are measured for a CoO/Co magnetic reference film. The magnetic field dependence of the optical off-diagonal Fresnel reflection coefficients rps and rsp follows the sample magnetization. Different Kerr setups provide various dependencies of I omega,2 omega on the reflection coefficients and, hence, on the Kerr ellipticity epsilon K and rotation theta K. Jones matrix formalism has been used to analyze the impact of a systematic variation of relative analyzer and polarizer orientations with respect to each other and with respect to the retardation axis of the modulator involved in longitudinal Kerr setups for incoming s-polarized light. We find one particular setup which maximizes I(omega) as well as I2 omega and maximizes the signal-to-noise ratio. Inefficient setups are characterized by I omega,2 omega intensities involving large nonmagnetic contributions of rp and rs.

The origin and control of the sources of AMR in (Ga,Mn)As devices

We present details of our experimental and theoretical study of the components of the anisotropic magnetoresistance (AMR) in (Ga,Mn)As. We develop experimental methods to yield directly the non-crystalline and crystalline AMR components which are then independently analysed. These methods are used to explore the unusual phenomenology of the AMR in ultra thin (5 nm) (Ga,Mn)As layers and to demonstrate how the components of the AMR can be engineered through lithography induced local lattice relaxations. We expand on our previous [A.W. Rushforth, et al., Phys. Rev. Lett. 99 (2007) 147207] theoretical analysis and numerical calculations to present a simplified analytical model for the origin of the non-crystalline AMR. We find that the sign of the non-crystalline AMR is determined by the form of the spin–orbit coupling in the host band and by the relative strengths of the non-magnetic and magnetic contributions to the impurity potential.

Anisotropic Magnetoresistance Components in (Ga,Mn)As

We explore the basic physical origins of the noncrystalline and crystalline components of the anisotropic magnetoresistance (AMR) in (Ga,Mn)As. The sign of the noncrystalline AMR is found to be determined by the form of spin-orbit coupling in the host band and by the relative strengths of the nonmagnetic and magnetic contributions to the Mn impurity potential. We develop experimental methods yielding directly the noncrystalline and crystalline AMR components which are then analyzed independently. We report the observation of an AMR dominated by a large uniaxial crystalline component and show that AMR can be modified by local strain relaxation. Generic implications of our findings for other dilute moment systems are discussed.

Spatially ordered arrays of magnetic nanowires: Polarized-neutron scattering investigation

The structure and magnetic properties of magnetically ordered arrays of magnetic nanowires formed on the basis of porous aluminum oxide films have been investigated by the small-angle polarized-neutron scattering method. Small-angle scattering intensity patterns exhibit several diffraction maxima (up to the third reflection order), which correspond to scattering from the highly ordered porous structure of the matrix and on the hexagonal packing of magnetic nanowires. The observed reflections are imposed on noticeable diffuse scattering associated with the defects of the structure. A theoretical solution is obtained for describing neutron diffraction in the dynamical limit on the superstructure of magnetic nanowires incorporated into the diamagnetic matrix. Several contributions to scattering that have been analyzed are the nonmagnetic (nuclear) contribution, the magnetic contribution depending on a magnetic field, and the nuclear-magnetic interference indicating the correlation between the magnetic and nuclear structures. A detailed pattern of the remagnetization of an ordered array of the magnetic nanowires has been obtained and it has been shown that polarized neutron scattering provides information inaccessible by the standard magnetometric methods.

Interfacial magnetization in exchange-coupledFe∕Cr∕Festructures investigated by second harmonic generation

We have studied the magnetic field dependences of magnetic optical second harmonic generation (SHG) in MBE-grown $\mathrm{Fe}∕\mathrm{Cr}∕\mathrm{Fe}∕\mathrm{Ag}∕\mathrm{Ga}\mathrm{As}(100)$ heterostructures displaying both bilinear and biquadratic interlayer exchange coupling. The magnetic field $\mathbf{H}$ was applied in the (100) surface plane along both easy ([001]) and hard ([110]) axes of the in-plane fourfold magnetic anisotropy. The SHG has been measured in reflection at near normal incidence for different polarization combinations $(pp,ps,ss,sp)$ of the fundamental and second harmonic light in longitudinal and transversal geometries. The magnetic field variation of the SHG signal clearly reflects the field-induced transformations of the magnetic state at the interfaces in the trilayer. It strongly depends on the configuration of light polarization, experimental geometry (longitudinal or transversal), and orientation of the magnetic field $\mathbf{H}$ relative to the crystal axes. In contrast to linear magneto-optical Kerr effect, which is odd in magnetic field, magnetic SHG is either even in $\mathbf{H}$ or does not display a definite parity at all, depending on the polarization configuration. We interpret the data based on a model accounting for nonmagnetic and magnetic contributions to SHG from the surface and interfaces described by ${C}_{4v}$ point symmetry. Taking into account the changes of the mutual orientation of interfacial magnetizations allows us to describe the general features of the measured field dependences of SHG.

1 ∕ f noise in linearized low resistance MgO magnetic tunnel junctions

Low RA MgO magnetic tunnel junctions prepared at Singulus (Ta3∕CuN30∕Ta5∕PtMn20∕CoFe2.5∕Ru0.7∕CoFeB3∕MgO1.2∕CoFeB3∕Ta5 (thickness in nanometers) were microfabricated at INESC-MN. The junctions were patterned into micron-sized sensors (5–20μm2) with controlled shape anisotropy (aspect ratio ranging from 2 to 20). A small external longitudinal bias field (15–30 Oe) was further used to improve sensor linearity. The MgO junctions have a resistance-area product of 150Ωμm2 and a maximum tunnel magnetoresistance of 130%. Noise measurements were done in linearized sensors, from dc to 500 kHz. The magnetic and nonmagnetic contributions to the 1∕f noise were determined. From the data fitting, Hooge parameters of ∼2.20×10−9μm2 were obtained for the nonmagnetic 1∕f noise. Analysis of direct experimental data revealed the possibility to detect variations of magnetic fields in the order of 10−10T∕Hz0.5 with these MgO junctions, demonstrating their potential for ultralow-field detection.

Polarization-modulated magnetic soft-x-ray transmission microscopy

An adjustable aperture element has been integrated into the full-field soft-x-ray microscope at the Advanced Light Source to select either the right or left elliptically polarized x rays emitted at an inclined angle from a bending magnet. Magnetic contrast recorded at the Fe L3 edge in a 59-nm-thin Gd25Fe75 layer can be modulated and scales with the degree of circular polarization in agreement with theoretical calculations. Nonmagnetic background contributions can be reduced and magnetic contrast is enhanced by comparing two images taken with opposite circular polarization. The fast modulation speed of this technique allows for lock-in recording schemes in high-resolution magnetic soft-x-ray microscopy.

Femtosecond dynamics of Co thin films on Si support

The ultrafast electron and spin dynamics of Co films was investigated using a femto-second pump-probe technique. The samples were magnetically characterized by means of magnetooptical Kerr effect and non-optical magnetometers, i.e. superconductive quantum interference device and alternating gradient field magnetometer. Time evolution of both, Kerr rotation and ellipticity, was measured at different points of the static hysteresis cycle. This allowed separating magnetic and non-magnetic contributions to the signal. The sample magnetization was found to drop within 200 fs, following the pulse cross-correlation trace, while the magnetization recovery time was found to last few picoseconds.

Ultrasonic absorption in ultra-low carbon steel

The laser-based reverberant technique is used to measure ultrasonic absorption spectra in the 2 to 45 MHz frequency range. This technique, being contactless, allows measurements at high temperature. The absorption spectra of ultra-low carbon steel samples are studied at room temperature in a magnetic field (in order to suppress the magnetoelastic contribution) and in a high temperature furnace (20–1200 °C) without magnetic field. Small steel samples (about 10×10×1 mm3) are used. At room temperature, two main contributions to the ultrasonic absorption are identified: microeddy currents (magnetoelastic contribution) and absorption caused by dislocations (deformation contribution). A typical microeddy current peak is observed and yields a reasonable estimate of the magnetic domain size. Above 10 MHz, the nonmagnetic contribution to the total absorption follows the classical vibrating string model. However, other phenomena also contribute to the absorption spectra. Below 10 MHz, an amplitude-independent damping background is observed. In addition, a small frequency-independent contribution to the absorption is observed at room temperature and is attributed to a thin surface layer. The absorption at high temperature is dominated below the Curie point by the magnetoelastic contribution. Two internal friction peaks are also detected. The first one, at 100 °C, is related to the dislocation kink motion. The second one, measured at 330 °C and 10 MHz, is attributed to the Snoek relaxation of carbon and/or nitrogen in α-iron. The Curie transition as well as the ferrite-austenite transition strongly affect the internal friction spectra.

Antiferromagnetism of almost localized fermions: Evolution from Slater-type to Mott-Hubbard gap

We supplement (and critically overview) the existing extensive analysis of antiferromagnetic solution for the Hubbard model with a detailed discussion of two specific features, namely (i) the evolution of the magnetic (Slater) gap (here renormalized by the electronic correlations) into the Mott-Hubbard or atomic gap, and (ii) a rather weak renormalization of the effective mass by the correlations in the half-filled-band case, which contrasts with that for the paramagnetic case. The mass remains strongly enhanced in the non-half-filled-band case. We also stress the difference between magnetic and non-magnetic contributions to the gap. These results are discussed within the slave boson approach in the saddle-point approximation, in which there appears a non-linear staggered molecular field due to the electronic correlations that leads to the appearance of the magnetic gap. They reproduce correctly the ground-state energy in the limit of strong correlations. A brief comparison with the solution in the limit of infinite dimensions and the corresponding situation in the doubly-degenerate-band case with one electron per atom is also made.

Ultrafast magneto-optical response of iron thin films

Optical pump probe techniques are utilized to measure the time-resolved magneto-optical Kerr effect (TRMOKE) of a ferromagnetic iron film at room temperature in air. We focus on the interpretation of the TRMOKE signal, and find the experimental data to be consistent with a simple phenomenological model. Nonmagnetic contributions are present in the TRMOKE signal up to 100 ps after excitation. Without making further assumptions, this prevents a determination of the true magnetization dynamics. The initial magnetization does not change its direction, and within the first picoseconds there is no detectable dependence of the magnetization dynamics on the external magnetic field.

Growth of nanosized MnAs/Si(111) magnetoelectronic heterostructures and their magnetooptical study

Thin (6–12 nm) epitaxial MnAs films were MBE-grown on Si(111) substrates under different technological conditions. The films feature essentially different surface morphology. This manifests itself in the formation, on the silicon surface, of hexagonal-shaped crystallites, whose dimensions vary depending on the growth conditions. The volume and surface magnetic properties of the films were studied using the magnetooptical Kerr effect and optical second harmonic generation. The Kerr effect was found to scale linearly with the effective thickness of the magnetic layer. The thickness of the magnetically disordered transition layer formed near the interface with the substrate was estimated. The surface and volume hysteresis properties of the films were found to be different. A contribution to the second-harmonic intensity was observed which is an odd function of magnetization. This effect originates from the interference of the magnetic and nonmagnetic contributions to the nonlinear polarization.

Separation of magnetic from non-magnetic information in the Bitter pattern method

The paper deals with the problem of separating magnetic and non-magnetic contributions to the image contrast in the Bitter pattern method. With the help of the digital image difference procedure, it is demonstrated for the first time for the Bitter method that the separation is easy to achieve for relatively soft magnetic specimens, when an external field can be applied to simply produce the non-magnetic reference image of the specimen area under study. It is also shown that obtaining satisfactory results is principally impossible when removing the colloid from the specimen surface is used for the purpose of recording the non-magnetic image.

Growth and magnetic ordering of Ni and Co films on Rh(0 0 1) investigated with second-harmonic generation

Second-harmonic generation (SHG) was used to study the magnetic properties of cobalt and nickel films on Rh(0 0 1) during growth. We observed strong stress-induced magnetostrictive effects due to the large lattice mismatch of 7.3% for Co and 7.9% for Ni. For Co films in-plane magnetic ordering starts at 2 ML at room temperature and remains in-plane up to about 23 ML. A shift of the relative phase between magnetic and nonmagnetic SH contributions leads to sign changes of the magnetic contrast for both, p → P and s → P polarization combinations. In case of Ni, no in-plane magnetization was observed at room temperature but we could verify the onset of perpendicular magnetization between 5 and 6 ML.

非線形磁気光学効果による［Ｆｅ（ｘＭＬ）／Ａｕ（ｘＭＬ）］人工格子界面の評価

The magnetic second-harmonic generation (MSHG), one of nonlinear magneto-optical effects, was measured by using a Ti-sapphire laser (λ = 810 nm) combined with filters, a polarizer, an analyzer, a detector and a photon-counting apparatus. The polar plot of the azimuthal dependence of the MSHG from a [Fe(3.75ML)/Au(3.75ML)] superlattice showed a four fold symmetry pattern. A reversal of the magnetic field caused reflection of the azimuthal patterns with respect to a certain axis. Nonlinear magneto-optical Kerr rotation as large as 31.1° was observed in [Fe(1.75ML)/Au(1.75ML) for the S-polarization incidence. The magnetic contribution to the azimuthal pattern was explained by symmetry analysis, taking into account the surface non-magnetic (electric dipole), bulk non-magnetic (electric quadrupole) and surface magnetic (electric dipole) contributions.

Magnetic properties of Gd-containing Langmuir–Blodgett films studied by magneto-induced optical second harmonic generation

Nonlinear magneto-optical and structural properties of organo-metallic Langmuir–Blodgett (LB) films containing Gd monolayers are studied by second harmonic generation (SHG). The X-ray diffraction data show that the Gd–LB films consist of periodic layered structures with the Gd monolayers spaced at about 5 nm. The diffuseness and depolarization of the SHG radiation indicate an in-plane island-like inhomogeneity of 2D Gd-layers. Giant nonlinear magneto-optical Kerr effect (NOMOKE) is observed for these films and is interpreted in terms of the interference between the dipole quadratic nonmagnetic contribution to the SH field from electronic system of 2D Gd islands, and magnetization induced contribution from their spin system.