Development of SuperHRPD, a TOF neutron powder diffractomter at J-PARC

SuperHRPD is one of six time-of-flight neutron powder diffractometers in the Materials and Life Science Experimental Facility (MLF) of the Japan Proton Accelerator Research Complex (J-PARC). SuperHRPD is looking at a newly developed high resolution moderator which gives narrow & symmetrical neutron pulse with less tails. With using this moderator and lower repetition rate of 25Hz as well as the flight path shorter than 100 m, a high resolution and wide dynamical range is attainable with limited loss of neutrons. The designed highest resolution of SuperHRPD is as high as Δd/d = 0.035 % in the backward bank. Although unplanned shutdown for two years due to the earthquake and the Hadron radiation accident, SuperHRPD has been upgraded repeatedly by the scattering chamber replacement, the increase of detector solid angle, and the improvement of the detector systems, and improvement of resolution. Sample environments cover 4 K – 1000 K, 10GPa and 14 T with up to d = 40 Å. It is emphasized the magnet was designed to detect tiny structural changes precisely as well as magnetic reflections up to 14 T. After three years of operation, we confirmed higher resolution can reduce systematic errors in structural analyses. The current status of SuperHRPD will be reported.

A study of perpendicular magnetic recording media using polarized SANS

The study of thin film magnetic systems that are structured on the nanoscale is an area of intense interest. Small-angle neutron scattering is an extremely powerful probe of nanomagnetism in the bulk, but in thin-film systems the experiments are challenging due both to the small scattering volume available and also to scattering from other sources such as the substrate and sample environment. We have demonstrated that such experiments are however possible in magnetic films as thin as 10 nm. A good example to illustrate this is the case of perpendicular magnetic recording media. These materials are found in all modern magnetic hard drives, the data storage technology that continues to be of tremendous commercial and technological importance. These media are advanced functional multilayered materials, containing an active recording layer of only around 10 nm in thickness. This recording layer is compositionally segregated into 8 nm-sized grains of a magnetic CoCrPt alloy separated by a thin oxide shell, typically SiO2. These media have their magnetic moments oriented perpendicular to the plane of the film. Determining the local magnetic structure and reversal behavior is key to understanding the performance of perpendicular media in recording devices. Polarised SANS has proved to be a very effective tool to measure these materials at a sub-10nm length scales. The signal of interest must however also be distinguished from the scattering from other layers in the structure, some of which are also magnetic. We will present a summary of some recent results on recording media, including measurements of the grain-sized dependent switching with and without the presence of an exchange spring. We will also briefly mention experiments that demonstrate the viability of extending this approach to measurement for lithographically defined structures similar to those for application in bit-patterned media, including 2d artificial spin-ice and structurally glassy arrays.

Large in-plane deformation of RuO₆ octahedron and ferromagnetism of bulk SrRuO₃

Glazer tilting system with tilting and rotation of oxygen octahedron, can describe ABO3 perovskite structure effectively. In highest symmetry, Pm-3m(No. 221) crystal structure is a0a0a0 without tilting and rotation. If temperature is lower, the different atomic radius of A and B causes tilting and rotation of BO6 octahedron. Glazer tiling notation of Pbnm(No. 62, cab lattice) orthorhombic structure is a-a-c+ with antiphase tilting along [110]cubic and in-phase rotation along [001]cubic for neighboring octahedron. SrRuO3 is rare example of itinerant ferromagnetic among 4d oxides. It shows zero thermal expansion, so called Invar effect below ferromagnetic transition(Tc=165 K). Otherwise, paramagnetic CaRuO3 has same Pbnm crystal structure without magnetic transition. To understand Invar effect and ferromagnetism of SrRuO3, We carried out high resolution Time-of-flight powder neutron diffraction using SuperHRPD beamline in J-PARC, with the best resolution Δd/d=0.03% of backscattering bank. Itinerant ferromagnetic SrRuO3 shows 50 fetometer increase of <Ru-O> mean bond below ferromagnetic transition while paramagnetic CaRuO3 shows decrease of <Ru-O> and follows well by the usual thermal expansion. For SrRuO3, Glazer tilting with deformation of RuO6 octahedron explains Invar effect and why lattice a is larger than lattice b in Pbnm structure. The increased <Ru-O> mean bond is considered as coupled order parameter with ferromagnetic transition. The band width of CaRuO3 is almost constant in the whole temperature range whereas ones of SrRuO3 decrease at low temperature. Then more localized Ru 4d orbitals probably contribute ferromagnetic transition.