“…Here one uses a convergent beam covering a large angular range instead of a collimated beam by using an 4 m long elliptically focusing guide element. This essentially increases the intensity by a factor of 10 in the TOF mode [12]. A resolution of 2 mm was obtained using a position sensitive detector (PSD) positioned about 3 m behind the sample to detect the neutrons.…”
Transition metal-rare earth (TM-RE) Fe/Tb-multilayer systems have been known to show exchange-bias-like shifts in the form of double hysteresis loop (DHL) along and opposite to the field cooling axis. Planar domain walls (DW), with opposite handedness at the interfaces, are held responsible for such DHL. Here, we report on the formation of nanoparticulated Fe layers in the Cu-matrix within a Fe-Cu/Tb multilayer and their eventual low temperature characteristics. AC susceptibility measurements indicate that these diluted magnetic clusters have a superspin-glass type of freezing behavior. Eventually, this Fe-cluster/Tb interlayer interaction, which is conjectured to be mediated by the pinned moments within the individual clusters, has helped in increasing the exchange bias field in the system to a high value of ≈1.3 kOe which gradually vanishes around 50 K. Polarized neutron reflectivity confirms a very strong antiferromagnetic (AF) coupling between the individual layers. The magnitude of the magnetic moment of each of the individual Tb or Fe-Cu layer remains similar but due to the strong AF-coupling at the interfaces, the entire ferrimagnetic Fe-Cu/Tb entity flips its direction at a compensation field of around 3.7 kOe. This study shows that magnetic dilution can be an effective way to manipulate the possible domain walls or the clusters in TM and thereby the exchange bias in TM-RE systems.
“…Here one uses a convergent beam covering a large angular range instead of a collimated beam by using an 4 m long elliptically focusing guide element. This essentially increases the intensity by a factor of 10 in the TOF mode [12]. A resolution of 2 mm was obtained using a position sensitive detector (PSD) positioned about 3 m behind the sample to detect the neutrons.…”
Transition metal-rare earth (TM-RE) Fe/Tb-multilayer systems have been known to show exchange-bias-like shifts in the form of double hysteresis loop (DHL) along and opposite to the field cooling axis. Planar domain walls (DW), with opposite handedness at the interfaces, are held responsible for such DHL. Here, we report on the formation of nanoparticulated Fe layers in the Cu-matrix within a Fe-Cu/Tb multilayer and their eventual low temperature characteristics. AC susceptibility measurements indicate that these diluted magnetic clusters have a superspin-glass type of freezing behavior. Eventually, this Fe-cluster/Tb interlayer interaction, which is conjectured to be mediated by the pinned moments within the individual clusters, has helped in increasing the exchange bias field in the system to a high value of ≈1.3 kOe which gradually vanishes around 50 K. Polarized neutron reflectivity confirms a very strong antiferromagnetic (AF) coupling between the individual layers. The magnitude of the magnetic moment of each of the individual Tb or Fe-Cu layer remains similar but due to the strong AF-coupling at the interfaces, the entire ferrimagnetic Fe-Cu/Tb entity flips its direction at a compensation field of around 3.7 kOe. This study shows that magnetic dilution can be an effective way to manipulate the possible domain walls or the clusters in TM and thereby the exchange bias in TM-RE systems.
“…Figure 7 shows the polarized neutron intensity profiles along Q
z and their fits for S1 (grown on STO (001)) and S2 (grown on STO (110)) after cooling the sample in a saturating field of +5.0 kOe and measuring at +5.0 kOe and at 15 K. The measurements were done using the Selene setup 16 .Figure 7PNR measurements of specimens S1 and S2. Specular neutron reflectivity patterns (solid symbols) along with their best fits (open symbols) as a function of Q
z for the NSF [R − (black) and R + (red)] channels measured at a saturation field H a = +5.0 kOe at 15 K for the sample ( a ) S1 along the [110] direction and the sample ( b ) S2 along the [001] direction.…”
Besides epitaxial mismatch that can be accommodated by lattice distortions and/or octahedral rotations, ferroelectric-ferromagnetic interfaces are affected by symmetry mismatch and subsequent magnetic ordering. Here, we have investigated La0.67 Sr0.33 MnO3 (LSMO) samples with varying underlying unit cells (uc) of BaTiO3 (BTO) layer on (001) and (110) oriented substrates in order to elucidate the role of symmetry mismatch. Lattice mismatch for 3 uc of BTO and symmetry mismatch for 10 uc of BTO, both associated with local MnO6 octahedral distortions of the (001) LSMO within the first few uc, are revealed by scanning transmission electron microscopy. Interestingly, we find exchange bias along the in-plane [110]/[100] directions only for the (001) oriented samples. Polarized neutron reflectivity measurements confirm the existence of a layer with zero net moment only within (001) oriented samples. First principle density functional calculations show that even though the bulk ground state of LSMO is ferromagnetic, a large lattice constant together with an excess of La can stabilize an antiferromagnetic LaMnO3-type phase at the interface region and explain the experimentally observed exchange bias. Atomic scale tuning of MnO6 octahedra can thus be made possible via symmetry mismatch at heteroepitaxial interfaces. This aspect can act as a vital parameter for structure-driven control of physical properties.
“…Longer guides improve the homogeneity of the focused beam [2]. Similar concepts can be applied to Montel optics using parabolic or elliptic mirrors yielding the additional benefit of a very homogeneous phase space [6] and allowing even more bulky sample environments. The presented focusing system installed on a mechanical support with provision for a highly reproducible positioning using kinematic mounts allows a very efficient, swift, and cost-effective conditioning of neutron beams at beamlines for diffraction, spectroscopy, imaging, and irradiation.…”
Section: Discussionmentioning
confidence: 99%
“…With the rather dramatic increase of the reflection angles of supermirrors exceeding m = 8 times the critical angle of reflection of Ni [5], neutrons encompassing a wide range of wavelengths and large divergence can be directed towards the sample position. While adjusting the beam size at the sample with Montel mirrors [6] is straightforward using an aperture at the focal point at the entrance of the mirror [6], the selection of the beam size using elliptic or parabolic guides is more complicated and essentially given by the design of the optics [7]. In the following we propose a combination of a focusing parabolic guide with a variable collimation stage in front of the entrance of the guide that allows varying the size of the beam spot at the sample position in the horizontal and…”
Abstract. We have developed a variable focusing system for neutrons that allows varying the vertical and horizontal beam size at the sample position independently. It is based on a focusing parabolic neutron guide coated with supermirror m = 6 times the critical angle of reflection of Ni. The divergence of the incident beam is adjusted by solid-state collimators with horizontally and vertically arranged absorbing blades. The performance of the prototype set-up has been benchmarked at the beamline BOA at SINQ confirming the expected behavior. In particular we show that the beam size at the sample position can be varied between 0.6 and 2 mm without exchanging the focusing guide or by introducing a beam-defining aperture between the exit of the guide and the sample, i.e. the beam size can be adapted more than half a meter away from the sample.
IntroductionThe investigation of novel materials using neutron scattering techniques is challenging because these materials are often only available in small quantities, e.g. encompassing a volume of a few mm³. Moreover, special experimental conditions, e.g. high pressure, high/low temperatures and/or high magnetic fields can only be realized for small volumes. Confining the neutron beam to such small areas using slits reduces the number of neutrons delivered by the rather large neutron beams (e.g. 50-100 cm²) significantly because for the minimization of the background, the penumbra must be suppressed too, to reduce scattering of neutrons from the sample environment. Moreover, it would require an aperture that is placed close to the sample which is usually not compatible with the sample environment. Therefore, it is desirable to focus the neutrons precisely to an area corresponding to the given size of the sample. Ideally, the useful neutrons would be selected already far away from the sample area using neutron optical devices. As already shown in previous works, elliptic mirrors [1]-[4] are well suited to focus neutron and xray beams at the sample. With the rather dramatic increase of the reflection angles of supermirrors exceeding m = 8 times the critical angle of reflection of Ni [5], neutrons encompassing a wide range of wavelengths and large divergence can be directed towards the sample position. While adjusting the beam size at the sample with Montel mirrors [6] is straightforward using an aperture at the focal point at the entrance of the mirror [6], the selection of the beam size using elliptic or parabolic guides is more complicated and essentially given by the design of the optics [7]. In the following we propose a combination of a focusing parabolic guide with a variable collimation stage in front of the entrance of the guide that allows varying the size of the beam spot at the sample position in the horizontal and
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