A. I. Okorokov

Mesoscopic magnetic inhomogeneities in the low-temperature phase and structure of Sm1−xSrxMnO3 (x < 0.5) perovskite

Results are presented of studies of the 154Sm1−xSrxMnO3 system using neutron powder diffraction and small-angle polarized neutron scattering. An analysis of the neutron diffraction spectra showed that at T < 180 K these exhibit typical Jahn-Teller distortions of the manganese-oxygen octahedrons which persist under further cooling and on transition of the sample to a metallic magnetically ordered state. The magnetic contribution to the diffraction is satisfactorily described using the (Ax(Ay)Fz) model and is interpreted as the coexistence of ferromagnetic and antiferromagnetic phases. The exaggerated widths of the diffraction lines indicate an appreciable contribution from microdeformations evidently associated with the inhomogeneity of the system. Small-angle polarized neutron scattering showed that the Sm system for x = 0.4 and 0.25 is magnetically inhomogeneous in the low-temperature phase. Ferromagnetic correlations occur on scales of around 200 Å and having dimensions greater than 1000 Å which, combined with the temperature hysteresis of the magnetic small-angle scattering intensity observed for an x = 0.4 sample in the low-temperature phase, suggests that the transition is of a percolation nature.

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.

Investigation of a magnetic phase transition in fcc iron-nickel alloys

A magnetic phase transition in carbon-doped (0.1 and 0.7 at. %) Fe70Ni30 Invar alloys was investigated by the method of depolarization of a transmitted neutron beam and by small-angle scattering of polarized neutrons. It is shown that for both alloys, two characteristic length scales of magnetic correlations coexist above Tc. Small-angle scattering by critical correlations with radius Rc is described well by the Ornstein-Zernike (OZ) expression. The longer-scale (second) correlations, whose size can be estimated from depolarization data, are not described by the OZ expression, and hypothetically can be modeled by a squared OZ expression, which in coordinate space corresponds to the relation 〈M(r)M(0)〉∝exp(−r/Rd), where Rd is the correlation length of the second scale. The temperature dependence of the correlation radius Rc was obtained: Rc ∝ ((T−Tc)/Tc)−ν, where ν≈2/3 is the critical exponent for ferromagnets, over a wide temperature range up to Tcexp, at which the correlation radius becomes constant and equals its maximum value Rc(Tc)=Rcmax. The maximum correlation radius established (Rcmax=140 Å and 230 Å for the first and second alloys, respectively) characterizes the length-scale of the fluctuation for which the appearance of critical correlations first results in the formation of a ferromagnetic phase, and the phenomenon itself exhibits a “disruption” of the second-order phase transition at T=Tcexp, as a result of which a first-order transition arises. Temperature hysteresis was also detected in the measured polarization of the transmitted beam and intensity of small-angle neutron scattering in the alloy above Tc, confirming the character of this magnetic transition as a first-order transition close to a second-order transition.

Concentrational commensurate-incommensurate magnetic phase transition in Y1 − xTbxMn6Sn6

The magnetic state of Y1 − xTbxMn6Sn6 (with x ≤ 0.25) alloy is studied at different temperatures and magnetic fields by neutron diffraction. The alloy with xc = 0.22 exhibits an incommensurate-to-commensurate structure phase transition. The transition is accompanied by a decrease in the unit-cell volume. The weak dependence of the intensity of satellites on the x concentration and the monotonic displacement of their angular positions with increasing x allow us to conclude that the antiferromagnet → ferrimagnet transition occurs through the uniform magnetic state. The incommensurate phase in YMn6Sn6 exhibits four satellites, which are observed in the low-angle range of X-ray diffraction pattern and demonstrate different behavior with changing temperature. At 293 K, the antiferromagnet → ferromagnet transition in the Y1 −xTbxMn6Sn6 compound can be induced by applied magnetic field, in particular, the field μ0H = 0.3 T induces the transition in Tb0.15Y0.85Mn6Sn6.

SANS study of new magnetic nanocomposites embedded into the mesoporous silica

Abstract Magnetic nanocomposites embedded into mesoporous silica have been studied by Small Angle Polarized Neutron Scattering. It is well known that mesoporous silica represents a highly regular hexagonal structure of nanotubes. For the samples under study a diffraction peak in SANS at q c ≈1.7 nm −1 is observed which corresponds to a hexagonal structure with periodicity a≈3.5 nm . We found that the intercalation of iron into the silica matrix leads to some changes of the matrix itself. Additionally, the nuclear–magnetic interference in scattering of polarized neutrons was investigated. The interference reveals the absence of a periodical magnetic structure at room temperature which would be consistent with the hexagonal structure of nanotubes. The correlation between magnetic nanoparticles and pores of the matrix is revealed in the interference scattering at q⪡qc.

Spin correlations in YBa2(Cu1−xFx)3O7+y ceramic

To detect scattering by magnetic correlations and to estimate their characteristic space scale, YBa2(Cu1−xFex)3O7+y ceramic with x=0.13 and y=0.4 is investigated by the small-angle scattering of polarized neutrons. The measurements are carried out in the range of temperatures 15 K⩽T⩽315 K and magnetic fields 0<H⩽4500 Oe. Anomalies in the temperature curves of the intensity I(T,q) (where q is the momentum transfer) and the polarization P(T,q) are observed in the temperature range T<40 K. Interference between nuclear and magnetic scattering is also observed in this temperature range. The observed phenomena are interpreted as scattering by magnetic correlations having a scale 70 Å<R<370 Å. Irreversible effects and the type of magnetic ordering are discussed.

Peculiarities of the construction of broad-band adiabatic flipper of neutrons

Abstract An available model for calculation of broad-band adiabatic flipper of polarized neutrons is suggested. This model of the sine-cosine modulation of an effective field enables one to optimize and to simplify calculation for constructing the flipper. Functional relation between spin-flip probability and main parameters of the device such as length of flipper, neutron wavelength and both form and size of magnetic fields have been found. The test of the flippers constructed on the base of the model has been successfully carried out.

The observation of the magnetic correlations in YBa2(Cu1−xFex)3O7+y ceramics by small-angle polarized neutron scattering

Abstract The small-angle polarized neutron scattering (SAPNS) technique has been used to observe the magnetic ordering in the system of YBa2Cu2.61Fe0.39O7+y ceramics. The anomalies of the temperature dependences of the scattering intensity I(T, q), the polarization P(T, q) and the difference Δ(T, q) in SAPNS intensities of neutrons polarized along with or opposite to the applied magnetic field were found in the temperature region T

Investigation of the chiral structure of the Y/Dy multilayer system by the method of the small-angle scattering of polarized neutrons

The magnetic spiral structure of a Y/Dy sample has been investigated for temperatures from 30 to 190 K by the method of the small-angle scattering of polarized neutrons. The sample is a sequence of layers Y50nm[Dy4.3nm/Y2.8nm]350/Y234nm/Nd200nmAl2O3 (substrate) that is grown as a single crystal with the [001] axis of the hexagonal lattice, which is perpendicular to the layer plane. The experiments demonstrate the appearance of the magnetic peak below TN = 165.4 K, which is associated with the helicoidal phase, and the helicoid coherence length is larger than the layer thickness of the Y/Dy layer. The use of polarized neutrons allows the separation of the polarization-independent and polarization-independent components of magnetic scattering. The polarization-independent component of the magnetic neutron cross section is proportional to magnetization squared 〈SZ〉2, whereas the polarization-dependent component is proportional to the average chirality of the system 〈C〉 = 〈[·S1 × S2]〉. The critical exponents βC = 1.02(1) and β = 0.39(1) have been determined for the average chirality and magnetization, respectively. The magnetization critical exponent β for Dy/Y coincides with the exponent obtained for Dy bulk samples. The difference βC − 2β = 0.24(2) shows that the chirality can be a component of the order parameter that is independent of magnetization. This experiment corroborates the results that were obtained for the critical chirality in Ho and were reported in Phys. Rev. B 64, 100402(R) (2001).

The spin correlations in YBa2(Cu1−xFex)3Oy ceramics at T=15–500 K investigated by the small-angle scattering of polarized neutrons

Abstract The ceramic YBa2(Cu1−xFex)3Oy system was studied by means of the small-angle scattering of polarized neutrons in a wide range of iron concentration 0⩽x⩽0.2 in both oxygen-rich y⩾7 and oxygen-reduced y⩽6.7 states. The measurements were carried out in the range of temperatures 15≤T⩽550 K and magnetic fields 0