Направления / Наноструктуры / Металлические многослойные системы |
||
Металлические многослойные системы | ||
Field induced chirality in the helix structure of Y/Dy multilayer superstructure | ||
Yu.O. Chetverikov, S.V. Grigoriev (Petersburg Nuclear Physics Institute, St-Petersburg, Russia) |
||
Скачать постеры: постер Ю. Четверикова. Скачать публикации по теме: [1] | ||
Motivation |
|||||||||
|
|||||||||
The polarized neutron scattering in helimagnets [6,7] |
|||||||||
![]() |
|||||||||
s(q) – neutron cross section |
|||||||||
Qh- the helix scattering vector |
|||||||||
![]() |
|||||||||
![]() |
|||||||||
![]() |
|||||||||
![]() |
|||||||||
Sample |
|||||||||
Experimental detales |
|||||||||
The experiments were carried out at the NeRo reflectometer of FRG-1 research reactor in Geesthacht (Germany). The scattered neutrons were detected by the position sensitive detector (PSD) with (256x256) pixels. Bragg condition 2dsinQB/2=l with q = 2/d was satisfied for several peaks associated with the interaction of neutrons with the spin helix and nuclear superstructure. The polarization followed the direction of the guide magnetic field of order of 0.01T, which was applied perpendicularly to the incident beam and perpendicularly to multilayer surface (along c-axis). |
|||||||||
|
|||||||||
Rocking scan | |||||||||
After ZFC to T=150K the intensity integrated over the Bragg peak is shown as a function of the rocking angle w for two incident polarizations I(±P0). The rocking scans are well fitted by the Lorentzian centered at wc and with a relatively large width showing a large dispersion in the helix orientation. |
![]() |
||||||||
q scan |
|||||||||
|
|||||||||
Interpretation of polarization-dependent scattering |
|||||||||
For our experiment (P0 || ĉ) the scattering intensity with a certain polarization is attributed to one of two types of the helix domains: left- or right- handed ones. The non-zero difference between two intensities I(+P0) and I(-P0) demonstrates existence of the non zero average chirality in the sample i.e. there is a non equal population of the left- and right- handed helixes. This difference is significant near the maximum of the peak intensity of order of 10% in Fig.1 (a and b). The difference is also noticeable in a value of a width of these curves. The more populated domains have smaller width (for 2-3 %) both in q- and w-dependence. This means that more populated domains better ordered than other ones. For comparison of two contributions to scattering coming from left- and right- handed helix domains we introduce the ratio |
|||||||||
![]() |
|||||||||
where I(+P0) and I(-P0) are q-integrated intensities at w = wC. |
|||||||||
Dependence of temperature/field prehistory |
|||||||||
![]() |
|||||||||
For Zero Field Cooling (ZFC – red symbols) RD is negative and decreases from zero at T above TN to (–0.1) at low temperature. Fitting of RD to the scaling function of t=(TN – T)/TN gives the value bC-2b=0.22 in a good agreement with the previous experiment on SANS2 [5]. It should be noticed that thermocycling (cooling up and down through the critical temperature range in zero field) has no effect on the value of RD. The sample, being cooled to a certain T in the field H=0.9T applied in the sample plane (H ^ĉ) and measured in a small guide field (H || P0 || ĉ), gives RD value of the opposite sign than in the ZFC experiment. Now RD is zero above TN and positive (+0.2) below TN (open symbols). Even more intriguing behavior of RD is observed for measurements after FC (H=0.9T) to 150 K with the subsequent warming up in zero field (blue symbols). RD is constant in the range from 150 to 160 K, and then increases as temperature approaches TN. It has maximum at TN, and then decreases to zero in the paramagnetic range. |
|||||||||
Magnetic control of chirality |
|||||||||
![]() |
|||||||||
The value RD depends on the strength of the magnetic field applied in the sample plane in the FC procedure but it does not depend on the direction (±) of the field. It is clear that the magnetic field applied upon cooling is able to change RD and repopulate the left- and right-handed domain structure. |
|||||||||
Conclusions |
|||||||||
We have observed the spin chirality, or, the non-equal helix domain population numbers in the Y/Dy multiplayer system upon zero field cooling below TN. Furthermore the field-induced chirality have found after cooling in the magnetic field applied in the (ab) plane of the sample. This break the chiral degeneracy is mostly forming in the critical temperature range near TN. |
|||||||||
References |
|||||||||
[1] I.E. Dzyaloshinskii, Zh. Exp. Teor. Fiz. 46, 1420 (1964)[Sov.Phys. JETP 19, 960 (1964)]. |
|||||||||