By characterization of Fe/Si multilayers with a range of layer thickness we show that the diffusion of Si into Fe makes the Si layers 11.4±1.2
Å thinner than expected. We can use this information to ...improve growth of multilayers with high precision of layer thickness. Furthermore, the interdiffusion of Si into Fe creates a magnetically dead layer at the interface of about 5
Å. We have used reactive sputtering of Si with N
2 as a method to reduce interdiffusion. With the ability to control thickness more accurately and to reduce interdiffusion we have deposited polarizing Fe/SiN
x
supermirrors with neutron spin-up reflectivity in excess of 85% at
λ/
θ=0.3°/
λ (i.e., “
m=3”), allowing us to obtain neutron polarization above 95% in reflection geometry and above 90% in transmission geometry, respectively.
Polarized neutron reflectivity (PNR) studies have been performed on granular multilayers Co
80Fe
20(
t
n
)/Al
2O
3 (3
nm)
m
. For a non-percolating superferromagnetic (SFM) sample (
t
n
=1.3
nm) no ...spin-flip (SF) scattering was observed, measured at various points of the hysteresis, confirming that the magnetization reversal is achieved merely by domain nucleation and growth. For a percolated sample (
t
n
=1.6
nm) an oscillating magnetization depth profile from FeCo layer to FeCo layer along the multilayer stack was observed around the coercive field. The competition between long-range dipolar, short-range Néel and external field interactions apparently give rise to such a modulated magnetization depth profile in these magnetic multilayers.
The magnetic and structural properties of epitaxial Fe films grown on Si(1 1 1) are investigated by polarized neutron reflectometry (PNR) at room temperature. The influence of different types of ...interfaces, Fe/Si, Fe/FeSi2 and Au/Fe on the magnetic properties of Fe films deposited by molecular beam epitaxy onto Si(1 1 1) are characterized. We observe a drastic reduction of the magnetic moment in the entire Fe film deposited directly on the silicon substrate essentially due to strong Si interdiffusion throughout the whole Fe layer thickness. The use of a silicide FeSi2 template layer stops the interdiffusion and the value of the magnetic moment of the deposited Fe layer is close to its bulk value. We also evidence the asymmetric nature of the interfaces, Si/Fe and Fe/Si interfaces are magnetically very different. Finally, we show that the use of Au leads to an enhancement of the magnetization at the interface.
Neutron reflectometry (NR) has been employed to study the structure and composition of thin films formed by 4-pentadecyl-pyridine (C15-4Py) at a gold electrode surface. The thickness and the water ...content of films of C15-4Py have been measured as a function of the potential applied to the electrode. At positive potentials, where condensed film is formed, the monolayer contains defects that are filled with water. At very negative potentials, the film is desorbed from the electrode surface. NR has demonstrated that, at these potentials, the amphiphilic molecules remain in close proximity to the gold surface as a thicker and water rich film.
Results of the first neutron test of a new neutron-spin echo (NSE) spectrometer based upon the use of time-gradient magnetic fields are presented: observed performance is in a very good agreement ...with theoretical predictions. The main advantage of this new NSE technique is a practical absence of limits on the shape and size of the spin turners that principally allows for the realization of rather long coils for the high tilt angles required for the phonon-focusing NSE method, cylindrical coils required for the wide angle NSE spectroscopy with a simultaneous data acquisition in a broad
Q-range, etc.
Polarized neutron reflectometry is used to study changes in the magnetic state of the layered heterostructure Pd(2
nm)/V(36.5
nm)/17Fe(1
ML)/V(1
ML)/10Fe(4.7
nm)/V(4.7
nm)/MgO at 1.6, 3, 7 and 293
K. ...The temperature evolution of magnetic depth profiles is investigated. Below the superconducting transition temperature in vanadium, an anomalous decrease of magnetization is observed.
We present a new method for the determination of self-diffusivities in solids and the corresponding activation energy of diffusion using in-situ Neutron Reflectometry. In contrast to the classical ...ex-situ approach based on a sequence of isothermal measurements at different temperatures, the in-situ method allows one to work with a single experiment based on ramping the temperature with a constant rate. Our experiment demonstrates the success of the method for the model system of amorphous germanium. The activation energy of 2.2 eV and the absolute values of diffusivities achieved by the new method are in good agreement with the results of the classical approach, while a significantly lower amount of experimental time and samples are necessary. The presented method allows for an all-in-one type of experiment which can provide clearer and quicker results than similar methods using isothermal annealing procedures.
The horizontal touch and Langmuir–Blodgett techniques have been used to transfer a monolayer of
n-octadecanol from the gas–solution interface of a Langmuir trough onto the metal–solution interface of ...a Au(1
1
1) electrode. Chronocoulometry has been used to determine the charge density at the electrode surface covered by the film of
n-octadecanol. The surface pressure of this film was calculated from the charge density data and was found to be controlled by the electrode potential. We have demonstrated that by dialing the potential applied to the electrode via a potentiostat the monolayer adsorbed on the surface can be compressed or decompressed. Two states of the monolayer were observed. The transition between these states took place at a film pressure ∼12
mN
m
−1. Neutron reflectometry and polarization modulation Fourier transform infrared reflection absorption spectroscopy have been employed to determine the nature of the two states. The results show that octadecanol molecules form a two-dimensional solid film at all film pressures. At film pressures larger than 12
mN
m
−1, the film has low compressibility and
n-octadecanol molecules assume a small tilt angle with respect to the surface normal. At film pressures lower than 12
mN
m
−1 a compressive film is formed in which the tilt angle progressively increases with decreasing surface pressure. We have demonstrated that the properties of a monolayer of
n-octadecanol at the metal–solution interface display many similarities to the properties of that film at the air–solution interface.
Algorithm for the real-structure design of neutron supermirrors Pleshanov, N.K.
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
05/2004, Letnik:
524, Številka:
1
Journal Article
Recenzirano
The effect of structure imperfections of neutron supermirrors on their performance is well known. Nevertheless, supermirrors are designed with the algorithms based on the theories of reflection from ...perfect layered structures. In the present paper an approach is suggested, in which the design of a supermirror is made on the basis of its real-structure model (the RSD algorithm) with the use of exact numerical methods. It allows taking the growth laws and the reflectance of real structures into account. The new algorithm was compared with the Gukasov–Ruban–Bedrizova (GRB) algorithm and with the most frequently used algorithm of Hayter and Mook (HM). Calculations showed that, when the parameters of the algorithms are chosen so that the supermirrors designed for a given angular acceptance
m have the same number of bilayers, (a) for perfect layers the GRB, HM and RSD algorithms generate sequences of practically the same reflectance; (b) for real structures with rough interfaces and interdiffusion the GRB and HM algorithms generate sequences with insufficient number of thinner layers and the RSD algorithm turns out to be more responsive and efficient. The efficiency of the RSD algorithm increases for larger
m. In addition, calculations have been carried out to demonstrate the effect of fabrication errors and absorption on the reflectance of Ni/Ti supermirrors.
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