Neutron Brillouin Scattering with Pulsed Spallation Neutron Source – Spin-Wave Excitations from Ferromagnetic Powder Samples –

Abstract

Neutron Brillouin scattering (NBS) method was developed using a pulsed spallation neutron source, and the feasibility of NBS was demonstrated by observing ferromagnetic spin waves in La0.8Sr0.2MnO3 and SrRuO3 powders. Gapless spin-wave excitations were observed in La0.8Sr0.2MnO3, which were continuously extended to the lower scattering vector Q from previous results using single crystals. The novel result is a well-defined quadratic Q dependence in the spin-wave dispersion curve with a large energy gap in SrRuO3 indicating robust spin-orbit coupling. PACS numbers: 75.30. Ds, 75.40.Gb, 75.50.Gg In investigations of spin mediated materials physics, it is essential to experimentally determine microscopic interaction energies. For instance, an exchange interaction within the mean field approximation corresponds to the ordering temperature. This value is directly derived by the momentum dependence of spin waves in ordered states. Thus, spectroscopic investigations using thermal neutrons have been established as the most valuable and powerful experimental methods, although actual experiments have been performed mainly with single crystals, which require large volume and high quality samples from advanced spectrometers installed even at the most intense neutron sources.The current research trend in material sciences requires new materials of high performance as well as energy effective functioning for device applications. To support this requirement for magnetic materials as one typical example, ferromagnetic semiconductors are a key material for future spintronics devices, and they have already been proven to be functional at low temperatures. However, now, research directs to the discovery of new materials of high performance at ambient temperatures. Obviously, these materials are required to possess a high ordering temperature (Curie temperature T c for ferromagnets). Therefore, we need to seek magnetic materials with a large spin-wave stiffness constant and a zero energy gap, which show that they are easily magnetized. Most of newly synthesized materials are chemically and structurally complicated such that large single crystals are difficult to be synthesized. Neutron spectroscopy experiments require the use of powder samples, and sometimes, low quality materials with impurity phases.The neutron Brillouin scattering (NBS) method is the most suitable for measuring the momentum dependence of spin-wave excitations from (000) in the forward direction. Here, NBS is defined in comparison with the optical Brillouin Scattering. The principles of NBS are not new, although, until recently, such experiments have been limited to a very small energy range, because intense neutrons have been available in the thermal energy region. Sub-eV neutrons are now used high flux neutron sources. In this work, we demonstrate the first clear result of spinwave excitations from two powder ferromagnets by using a pulsed spallation neutron source: the colossal magnetoresistance manganite La 0.8 Sr 0.2 MnO 3 (T c = 316 K) [1,2] and SrRuO 3 ...

Science from the Initial Operation of HRC

“…As shown in Fig. 1 (a), the spin wave dispersion relation at 7 K was well fitted to E(Q) = E g + DQ 2 with an apparent energy gap E g [3]. We clearly found the energy gap by accessing the low Q by the NBS method.…”

“…Owing to the kinematic constraints of neutron spectroscopy, incident neutron energy (E i ) in the sub-eV region with a high resolution (∆E) is necessary, and the scattered neutrons need to be detected at very low scattering angles, for measuring the scattering near to the forward direction. On the HRC, the NBS experiments became feasible by reducing the background noise at low scattering angles [3,4]. The low background noise was achieved by the T0 chopper and the incident beam collimator.…”

“…The D value was obtained to be 88 ± 2 meVÅ, which agreed well with the previous INS result using a single crystal. In this way, the NBS method became successfully feasible on the HRC [3,4]. Next, spin waves in another polycrystalline sample of a cubic perovskite, SrRuO 3 , which is a metal ferromagnet showing anomalous Hall effect, were measured.…”

Time-of-Flight Elastic and Inelastic Neutron Scattering Studies on the Localized 4d Electron Layered Perovskite La5Mo4O16

“…Refinements of crystal and magnetic structures were carried out by the Rietveld method using the GSAS program. 12) INS measurements were performed at the TOF neutron spectrometers 4SEASONS 13,14) and HRC 15,16) installed at BL01 and BL12, respectively, in MLF, J-PARC. Roughly 11 and 50 g of powder were used for the 4SEASONS and HRC measurements, and the measurements were performed at T = 4, 103, and 224 K. INS data were visualized by software suites Utsusemi.…”

“…6) Since Mo 2 O 10 pillars are nonmagnetic and the distance between the layers is large, the interlayer interaction is expected to be very weak. Susceptibility measurement on polycrystalline La 5 Mo 4 O 16 shows an antiferromagnetic phase transition at T N = 190 K. 6,7) In addition, single-crystal magnetization measurements on La 5 Mo 4 O 16 exhibited a metamagnetic transition from the antiferromagnetic to ferrimagnetic state above 0.5 T. 8) This ferrimagnetic phase is only observed when an external magnetic field is applied along the c axis, suggesting easy-axis magnetic anisotropy. 5,8,9) Recently, intriguing magnetic phenomena in La 5 Mo 4 O 16 were reported.…”