BaCuSi$_2$O$_6$ is a quasi-two-dimensional (2D) quantum antiferromagnet containing three different types of stacked, square-lattice bilayer hosting spin-1/2 dimers. Although this compound has been studied extensively over the last two decades, the critical applied magnetic field required to close the dimer spin gap and induce magnetic order, which exceeds 23 T, has to date precluded any kind of neutron scattering investigation. However, the HFM/EXED instrument at the Helmholtz-Zentrum Berlin made this possible at magnetic fields up to 25.9 T. Thus we have used HFM/EXED to investigate the field-induced ordered phase, in particular to look for quasi-2D physics arising from the layered structure and from the different bilayer types. From neutron diffraction data, we determined the global dependence of the magnetic order parameter on both magnetic field and temperature, finding a form consistent with 3D quantum critical scaling; from this we deduce that the quasi-2D interactions and nonuniform layering of BaCuSi$_2$O$_6$ are not anisotropic enough to induce hallmarks of 2D physics. From neutron spectroscopy data, we measured the dispersion of the strongly Zeeman-split magnetic excitations, finding good agreement with the zero-field interaction parameters of BaCuSi$_2$O$_6$. We conclude that HFM/EXED allowed a significant extension in the application of neutron scattering techniques to the field range above 20 T and in particular opened new horizons in the study of field-induced magnetic quantum phase transitions.

Bragg diffracted intensities and q values for crystalline structures with long repeat distances may be obtained by small-angle neutron scattering (SANS) investigations. An account is given of the methods, advantages and disadvantages of obtaining such data by the multichromatic time-of-flight method, compared with the more traditional quasi-monochromatic SANS method. This is illustrated with data obtained from high-magnetic-field measurements on magnetic vortex line lattices in superconductors on the former HFM/EXED instrument at Helmholtz-Zentrum Berlin. The methods have application to other mesoscopic crystalline structures investigated by SANS instruments at pulsed sources.

We report an unexpected magnetic-field-driven magnetic structure in the 5f-electron Shastry-Sutherland system U2Pd2In. This phase develops at low temperatures from a noncollinear antiferromagnetic ground state above the critical field of 25.8 T applied along the a axis. All U moments have a net magnetic moment in the direction of the applied field, described by a ferromagnetic propagation vector q(F) = (0 0 0) and an antiferromagnetic component described by a propagation vector q(AF) = (0 0.30 1/2) due to a modulation in the direction perpendicular to the applied field. We conclude that this surprising noncollinear magnetic structure is due to a competition between the single-ion anisotropy trying to keep moments, similar to the ground state, along the [110]-type directions, Dzyaloshinskii-Moryia interaction forcing them to be perpendicular to each other and application of the external magnetic field attempting to align them along the field direction.

An overview of the high magnetic field facility for neutron scattering at Helmholtz Zentrum Berlin (HZB) is given. The facility enables elastic and inelastic neutron scattering experiments in continuous magnetic fields up to 26.3 T combined with temperatures down to 0.6 K.

Helmholtz-Zentrum Berlin (HZB) operates the medium flux research reactor BER II and the third generation synchrotron source BESSY II.HZB is known for providing an outstanding sample environment, especially high magnetic fields and low temperatures, which is available for both internal and external users.In this contribution we present the latest achievement in this field -High Magnetic Field Facility for Neutron Scattering, which was recently launched at HZB.This facility allows combining neutron scattering with continuous magnetic fields as high as 26 T and temperatures down to 0.65 K (at present) and 0.1 K (in near future).The application of high magnetic fields is a powerful method for revealing the complex behavior in modern materials.In combination with microscopic probe such as neutrons it provides a direct access to static and dynamic correlations in matter.Recently HZB in collaboration with the National High Magnetic Field Laboratory (USA) built a unique horizontal solenoid High Field Magnet (HFM) [1].The magnet utilizes hybrid (resistive insert and superconducting outsert) technology and reaches 26 T at full power of 4 MW.The tapered inner coil allows neutron-scattering to detectors up to +/-15° off the beam axis.Furthermore the magnet can be rotated by an additional 15° to access a larger reciprocal space region.Neutron scattering in high fields is performed using the dedicated Extreme Environment Diffractometer (EXED) [2].EXED uses time-of-flight (TOF) polychromatic technique which compensates very limited angular access available in the HFM.TOF technique combined with 15° magnet rotation provides a gapless coverage of Q-range from 0.1 up to 12 Å-1 for diffraction experiments.The low-Q range can be extended beyond 10-2 Å-1 using a pin-hole TOF Small Angle Scattering mode implemented on the instrument.In addition to the existing elastic capabilities, a direct TOF spectrometer mode was built in this year.The latter will enable inelastic neutron scattering experiments over a limited Q-range < 1.8 Å-1 with an energy resolution of a few percent and incident energies below 25 meV [3].Since the magnet has room temperature bore, several low-temperature inserts have been developed at HZB.They include 4He-cryostat (1.5 K) with a sample rotation stage, 3He cryostat (0.65 K) and a dilution fridge (0.1 K, in fabrication).In this contribution the overview and capabilities of the HFM-EXED facility will be presented together with the selected experimental results.

A series-connected hybrid magnet for neutron scattering experiments has been installed at Helmholtz-Zentrum Berlin. The magnet consists of a superconducting Nb3Sn cable-in-conduit coil and a resistive Bitter magnet with a total maximum field of 26.3 T. The quench detection system monitors the cable-in-conduit coil, the NbTi bus, and the HTS current leads. The total detection system consists of two independent parts, i.e., a main system and an auxiliary system, which have been developed and fabricated by two different suppliers. The main system consists of 16 voltage detection units. Each unit can either monitor a single voltage or a difference of two voltages with a common midpoint. The auxiliary system represents a redundant detection unit for the CICC coil, incorporating the differential signal from a cowound wire. Results of a quench detected with both systems are shown.

Helmholtz-Zentrum Berlin (HZB) operates two large-scale facilities: the research reactor BER 2 and the synchrotron source for soft X-rays BESSY 2. This year HZB's neutron instrument suite around BER 2 has been strengthened by a unique high-magnetic-field facility for neutron scattering. Its main components are the High Field Magnet (HFM), which is the most powerful dc magnet for neutron scattering worldwide, and the Extreme Environment Diffractometer (EXED), which is a dedicated neutron instrument for time-of-flight technique. The hybrid magnet system is projected according to the special geometric constraints of analyzing samples by neutron scattering in a high field magnet. Following our past experience, only steady-state fields are adequate to achieve the goals of the project. In particular, inelastic scattering studies would virtually be excluded when using pulsed magnets. The new series-connected hybrid magnet with a horizontal field orientation was designed and constructed in collaboration with the National High Magnetic Field Laboratory (NHMFL), Tallahassee, FL, USA. With a set consisting of a superconducting cable-in-conduit coil and different resistive coils of conical shape, maximum fields between 26-31 T are possible with cooling power between 4 and 8 MW for the resistive part. A series of commissioning activities of the magnet components and the technical infrastructure systems (20-kA power supply, water cooling, and 4-K Helium refrigerator) was completed at HZB. The maximum field achieved with a 4-MW resistive coil was 26 T.

The final assembly of the Series-Connected Hybrid magnet system for the Helmholtz-Zentrum Berlin for Materials and Energy (HZB) has occurred with the integration of the superconducting cold mass, cryostat, resistive Florida-Bitter coils, and the cryogenic, chilled water, power, and control subsystems. The hybrid magnet consists of a 13-T superconducting Nb3Sn/CICC coil and a set of 12-T resistive, water cooled coils at 4.4 MW. Much of the cryostat and cold mass functional requirements were dictated by the electromagnetic interactions between the superconducting and resistive coils. This includes the radial decentering and axial aligning forces from normal operations and a 1.1 MN fault load. The system assembly was an international achievement with the cold mass being completed at the NHMFL in the USA, cryostat to cold mass interfaces made at Criotec Impianti in Italy, and final assembly at the HZB in Germany.

Volume 26 • Number 3 • 2015 Neutron News 8 O October 29th and 30th, 2014, the international workshop “Neutron Scattering in Magnetic Fields above 15 Tesla” was held at the Lise Meitner Campus of the Helmholtz Zentrum Berlin für Materialien und Energie (HZB). The high-fi eld hybrid magnet (HFM) at HZB is in the fi nal stages of testing (Figure 1) and, in conjunction with the dedicated Extreme Environment Diffractometer (EXED), will provide neutron diffraction and small angle neutron scattering (SANS) at DC fi elds well beyond the 15 T currently available. The workshop aimed to present HFM to experts in research at high magnetic fi elds and neutron scattering and to discuss possible experiments. The magnet produces a horizontal fi eld and has tapered 30 degrees openings on both sides. It exceeded expectations by achieving 26.2 T, thus opening a new era for neutron sciences in magnetic fi elds. The EXED instrument, where the magnet will be permanently placed from 2015, utilizes the time of fl ight technique and offers diffraction and SANS capabilities. In 2016 EXED will also be able to perform inelastic experiments. Over 50 participants took an active role in this workshop. During the two days, 18 oral contributions and 11 posters covering all major areas of neutron-related research in magnetic fi elds were presented. Talks were divided into eight sessions with the fi rst session presenting the current status of the high-fi eld project. The high-fi eld magnet was presented by P. Smeibidl who is the technical leader of the whole HFM-EXED project. The scattering instrument EXED and its future development was presented by the main instrument responsible O. Prokhnenko. Finally, the sample environment has been described by R. Wahle. The scientifi c part of the workshop started with an invited presentation by J. Wosnitza (HZDR Dresden) who talked about scientifi c opportunities at high magnetic fi elds based on non-neutron measurements performed at HZDR. J. Lynn (NIST, Gaithersburg) presented neutron investigations in a large variety of materials including superconductors, multiferroics and oxides (Figure 2). The next invited talk was delivered by O. Stockert (MPI Dresden) who concentrated on heavy-fermion compounds. The third session commenced with the invited talk of J. Mydosh (Kamerlingh Onnes Laboratory, Leiden) who discussed neutron scattering on Uranium-based compounds that included the famous hiddenorder system URu2Si2. M. Lumsden (ORNL) spoke about recent and future research plans for neutron scattering in high magnetic fi elds at the SNS and HFIR presenting several scientifi c examples. A. Podlesnyak (ORNL) delivered a contributed talk on spin crossover phenomena in transition metal oxides under high magnetic fi elds. The scientifi c program of the fi rst day was closed by a poster session and a guided tour to the magnet, instrument and sample environment sites. A workshop dinner in Potsdam was used to discuss future experiments. The second day started with the invited presentation of G. Boebinger (NHMFL, Tallahassee) who gave an overview of existing high-fi eld projects for neutron sciences and potential studies of quantum matter with neutrons and fi elds. His presentation was followed by an invited talk by T. Kimura (Osaka University) who presented a whole fl avor of physics of multiferroic materials under extreme conditions including also pressure studies. The session was concluded by a contributed presentation given by J. Porras (MPI Stuttgart) on phase diagrams of cuprate superconductors and charge density wave studies in these materials using the neutron technique. The next session was opened by an invited talk given by H. Nojiri (Tohoku University) who concentrated on the complementary technique to DC fi elds of pulse-fi eld technology that is capable to produce even higher magnetic fi elds. Several scientifi c cases were presented in detail. M. Enderle (ILL Grenoble) devoted her talk to nematic spin liquids in high magnetic fi elds and H. Nakotte (New Mexico State University) concentrated on several scientifi c examples comparing neutron and coherent X-ray scattering techniques High-Field Workshop at HZB Berlin

High magnetic fields can create exotic states which challenge our basic understanding of matter. This requires a deep and precise knowledge of the spatial ordering of atoms and associated magnetic moments. Particularly interesting would be to disclose magnetic field dependency of various fluctuations and collective modes. Such information can be obtained from neutron scattering experiments. The Helmholtz Zentrum Berlin (HZB) is known for its sample environment that is available for both internal and external users. Presently, a project that combines dedicated scattering instrument (EXED) with a horizontal hybrid solenoid magnet with tapered 300cones is being finalized at HZB. To achieve an optimal performance, a 13 T superconducting Nb3Sn cable-in-conduit coil is combined with resistive insert coils of 12 T to 18 T (see figure), depending on electric power (between 4.4 and 8.0 MW), to give a maximum of 25 to 31 T. The magnet that has been developed in collaboration with the National High Magnetic Field Laboratory of Florida State University, Tallahassee, FL, USA [1] provides a 50 mm diameter room temperature bore. For sample cooling a 3He cryostat with a pulse tube precooling stage is being developed. The magnet will be permanently mounted at the dedicated time-of-flight instrument EXED at the end of a multispectral neutron guide NL4a, about 76 m away from the neutron source. The EXED instrument is optimized for diffraction under restricted geometrical conditions and is being upgraded to include inelastic option. This unique experimental setup is supposed to play a major role in high-field neutron studies and should be ready for use in early 2015. The contribution describes not only the most important design features of the system, the outline of the building for the technical infrastructure and the status of the installation and commissioning but also the scientific possibilities and limitations of the setup.

A new series connected 25 T hybrid magnet system is being set up by the Helmholtz Zentrum Berlin (HZB) for neutron scattering experiments. CRPP has designed and manufactured a pair of 20 kA current leads for the powering of the outer superconducting coils of the hybrid magnet system. In connection with the test of joints for JT60SA, the current leads were tested at ENEA at low voltage up to a current of 18 kA. The mass flow rates required to cool the current leads at different currents measured in the test are in line with the design calculations. For the sum of the resistances of the warm and cold end copper contacts of the HTS module values of 13 (Lead A) and 11 n Omega (Lead B) were measured. In addition, the helium flow through the heat exchanger part was stopped at 10 and 12 kA to study the behaviour of the current leads in case of a loss of flow. The time elapsed between stopping of the helium mass flow and the initiation of a quench was found to be 117 s (Lead A) and 125 s (Lead B) compared to a calculated value of 86 s. The lower value obtained by the calculation can be attributed to the lower initial temperatures in the experiment.

We have investigated nonmagnetic impurity effect on the H-parallel to c-T magnetic phase diagram of an isosceles triangular lattice Ising antiferromagnet CoNb2O6, by means of neutron diffraction measurements using single crystals of Co1-x-MgxNb2O6 with x = 0, 0.004, and 0.008. We have found that the commensurate antiferromagnetic (AF) ground state disappears by substituting only 0.8% of nonmagnetic Mg2+ ions for the magnetic Co2+ ions. On the other hand, the phase boundaries between the other phases, namely the field-induced ferrimagnetic phase, thermally-induced incommensurate (IC) magnetic phase and the paramagnetic phase, are hardly affected by the small amount of nonmagnetic substitution. We have also performed Monte Carlo simulations for the isosceles triangular lattice Ising model to understand the extremely high sensitivity to the nonmagnetic substitution. Consequently, we have revealed that the disappearance of the AF phase is not because the small amount of nonmagnetic impurities destabilize the AF phase, but because the phase transition from the IC phase to the AF phase is strongly suppressed by a pinning effect due to the impurities.

The Helmholtz Centre Berlin (HZB) is a user facility for the study of structure and dynamics with neutrons and synchrotron radiation with special emphasis on experiments under extreme conditions. Neutron scattering is uniquely suited to study magnetic properties on a microscopic length scale, because neutrons have comparable wavelengths and, due to their magnetic moment, they interact with the atomic magnetic moments. At HZB a dedicated instrument for neutron scattering at extreme magnetic fields and low temperatures is under construction, the Extreme Environment Diffractometer ExED. It is projected according to the time-of-flight principle for elastic and inelastic neutron scattering and for the special geometric constraints of analysing samples in a high field magnet. The new hybrid magnet will not only allow for novel experiments, it will be at the forefront of development in magnet technology itself. With a set of superconducting and resistive coils a maximum field above 30 T will be possible. To compromise between the needs of the magnet design for highest fields and the concept of the neutron instrument, the magnetic field will be generated by means of a coned, resistive inner solenoid and a superconducting outer solenoid with horizontal field orientation. To allow for experiments down to Millikelvin Temperatures the installation of a 3He or a dilution cryostat with a closed cycle precooling stage is foreseen.

A new series connected 25 T hybrid magnet system is being developed by the Helmholtz Zentrum Berlin (HZB) for neutron scattering experiments. In collaboration with CRPP, high temperature superconducting (HTS) current leads have been developed for the powering of the outer superconducting coil. These HTS current leads, with a nominal current rating of 20 kA, have been designed and are being manufactured by CRPP, based on the design of the 18 kA EDIPO leads. Each of the two current leads consists of an HTS module cooled only by heat conduction from the cold end and a copper part actively cooled by helium gas of 44 K inlet temperature. To reach a temperature of 53.7 K at the warm end of the HTS a helium mass flow rate of 1.37 g/s per lead is required at a current of 20 kA. The estimated heat leak at the 4.5 K level caused only by heat conduction is as low as 1.4 W. The evolution of the temperatures in the case of a loss of flow has been calculated. In addition to the design, the main fabrication steps are described.

The outsert coils of the Series-Connected Hybrid magnets for the National High Magnetic Field Laboratory and Helmholtz Zentrum Berlin each contain approximately 4000 kg of Nb3Sn/Cu cable-in-conduit conductor (CICC). There are three different sizes of CICC that grades the amount of superconductor. Significant progress has been made in all aspects of the CICC fabrication. The Nb3Sn strand, consisting of high J(C) RRP Nb3Sn and conduit, composed of 316 LN with critical chemistry modifications, have been manufactured and quality control measurements made. The additional service to create the ten lengths of superconducting and three lengths of prototype, multi-stage twisted Cu cable is also complete. Jacketing of the cables has successfully been carried out at a new facility (subset of ICAS) for the insertion and compaction of fusion technology CICC using a weld and pull method. Fabrication processes and quality controls have been developed through a collaborative effort between the NHMFL, HZB, Criotec Impianti, and ENEA Superconductivity Division.

The outsert coils of the Series-Connected Hybrid magnets for the National High Magnetic Field Laboratory and Helmholtz Zentrum Berlin each contain approximately 4000 kg of Nb 3 Sn/Cu cable-in-conduit conductor (CICC). There are three different sizes of CICC that grades the amount of super-conductor. Significant progress has been made in all aspects of the CICC fabrication. The strand, consisting of high RRP Nb 3 Sn, and conduit, composed of 316 LN with critical chemistry modifications, have been manufactured and quality control measurements made. The additional service to create the ten lengths of superconducting and three lengths of prototype, multi-stage twisted Cu cable is also complete. Jacketing of the cables has successfully been carried out at a new facility (subset of ICAS) for the insertion and compaction of fusion technology CICC using a weld and pull method. Fabrication processes and quality controls have been developed through a collaborative effort between the NHMFL, HZB, Criotec Impianti, and ENEA Superconductivity Division.

The outsert coils of the Series-Connected Hybrid magnets for the National High Magnetic Field Laboratory and Helmholtz Zentrum Berlin each contain approximately 4000 kg of Nb3Sn/Cu cable-in-conduit conductor (CICC). There are three different sizes of CICC that grades the amount of super-conductor. Significant progress has been made in all aspects of the CICC fabrication. The strand, consisting of high RRP Nb3Sn, and conduit, composed of 316 LN with critical chemistry modifications, have been manufactured and quality control measurements made. The additional service to create the ten lengths of superconducting and three lengths of prototype, multi-stage twisted Cu cable is also complete. Jacketing of the cables has successfully been carried out at a new facility (subset of ICAS) for the insertion and compaction of fusion technology CICC using a weld and pull method. Fabrication processes and quality controls have been developed through a collaborative effort between the NHMFL, HZB, Criotec Impianti, and ENEA Superconductivity Division.