Session Y21: High Magnetic Field Measurements, Novel sensors, and Neutron Diffraction

Stand alone experimental setup for measurements of magnetoresistance tensor by dc reversal technique

Several years ago Keithley Instruments, Inc. created a combination of a Current Source and a Nanovoltmeter (Model 6221 and Model 2182A, respectively) for low level transport measurements. That nanovoltmeter/ current source combination was designed for measurements of \textit{one} voltage only. Proposed are the setups assembled from several nanovoltmeter/current source pairs which allow to measure simultaneous \textit{several} voltages associated with the same current. The setups utilize specific wiring and a unique triggering sequence. Several milliseconds delays incorporated into triggering sequence secure stable triggering and proper data flow. The delays might be assured by selection of specific time parameters in the current sources and nanovoltmeters. The setups allow utilizing the built-in functions of the devices. Tested setups consisted of up to four pairs, allowing measurements of up to four voltages.\footnote{A. V. Suslov, Rev. Sci. Instrum. 81, 075111 (2010).} Application of the setups to simultaneous measurements of magnetoresistance tensor components will significantly simplify the experiment, increase precision, and decrease consumption of resources.   

High Magnetic Field Characterization of Cu-Sn Alloys for Distortion-free MRI Probes

For a wide-range of reasons, magnetic resonance imaging (MRI) of brain activity is now exploiting minituraized electrodes and cannulas. However, common construction materials such as stainless steel cause significant distortion of the MRI signals.\footnote{F.M.~Martinez-Santiesteban \emph{et al.}, Phys.~Med.~Biol.~{\bf 52} (2007) 2073.} With the goal of developing brain-susceptibility-matched electrodes and cannula for distortion-free MRI in fields up to 11~T, we have investigated the magnetic properties of a spectrum of Cu-Sn alloys. The results of various characterization studies, including SQUID magnetometry up to 7~T and MRI studies up to 11 T, will be reported and related to the stoichiometric composition of the Cu-Sn solutions. Extensions to device development and other metal alloy combinations will be discussed.   

Addressing signal recovery challenges in pulsed field environment

We review approaches to recovering weak electric signals in the challenging environment of pulsed magnetic fields implemented at the National High Magnetic Field Laboratory Pulsed Field Facility (NHMFL-PFF). Recent technique developments including AC-specific heat measurements at sub-kelvin temperatures, nanosecond-scale resistivity measurements, as well as customized instrumentation and computer-assisted signal detection using Field Programmable Gate Arrays will be discussed.   

The National High magnetic Field Laboratory Pulsed Field Facility. An overview of high field magnet operations and scientific techniques

The National High magnetic Field Laboratory -- Pulsed Field Facility (NHMFL-PFF) is the home to the pulsed field user facility which routinely delivers 85T pulses for user science using a 1.4GW motor generator. The facility also houses a 60T shaped waveform magnet, 65T short pulse and 50T mid pulse capacitor driven magnets. Many techniques are available to users including, Transport, magnetization, calorimeter and cantilever techniques. I will describe the facilities and the measurement techniques available to users.   

Extreme Material Physical Properties and Measurements above 100 tesla

The National High Magnetic Field Laboratory (NHMFL) Pulsed Field Facility (PFF) at Los Alamos National Laboratory (LANL) offers extreme environments of ultra high magnetic fields above 100 tesla by use of the Single Turn method as well as fields approaching 100 tesla with more complex methods. The challenge of metrology in the extreme magnetic field generating devices is complicated by the millions of amperes of current and tens of thousands of volts that are required to deliver the pulsed power needed for field generation. Methods of detecting physical properties of materials are essential parts of the science that seeks to understand and eventually control the fundamental functionality of materials in extreme environments. De-coupling the signal of the sample from the electro-magnetic interference associated with the magnet system is required to make these state-of-the-art magnetic fields useful to scientists studying materials in high magnetic fields. The cutting edge methods that are being used as well as methods in development will be presented with recent results in Graphene and High-Tc superconductors along with the methods and challenges.   

Sub 100 nm ballistic sensors for ultra high spatial resolution magnetic field detection

There is an ongoing drive to develop non-invasive magnetic field sensors with ultra high spatial resolution (UHSR) of 100 nm or less for numerous applications.$^{1,2}$ Conventional field sensors e.g. based on the Hall effect, rely on diffusive transport, where high mobility III-V semiconductors offer the best field sensitivity (T/Hz$^{0.5}$).$^{2}$ For UHSR, the critical dimensions of the device must be reduced below the mean free path where transport is ballistic, and the detection properties are not preserved, e.g. the Hall response can be suppressed and/or nonlinear. We report sub 100 nm sensors utilizing the negative bend resistance of InSb/InAlSb ballistic structures at elevated temperatures.$^{3}$ These devices exhibit an enhanced responsivity that is tunable by geometric design and extremely attractive for the detection of ultra small magnetic fields. Our smallest device studied to date has an active sensor area of 35 x 35 nm$^{2}$, and a sensitivity of 0.87 $\mu$T/Hz$^{0.5}$ at 100 K. The performance and detection properties are reviewed with respect to state-of-the-art technologies.\\ $^{1}$P. Manandhar, Nanotechnol. 20, 355501 (2009). $^{2}$A. Sandhu, Microelectron. Eng. 73, 524 (2004). $^{3}$A. M. Gilbertson, et al., Submitted to Appl. Phys. Letts. (2010).   

dc SQUIDs as displacement detectors for embedded micromechanical resonators

Superconducting quantum interference devices (SQUIDs) can detect tiny amounts of magnetic flux and are also used to study macroscopic quantum effects. We employ a dc SQUID as a linear detector of the displacement of an embedded micromechanical resonator with femtometer sensitivity. We have also measured the backaction of the dc SQUID on the resonator, where the resonance frequency and damping of the resonator can be tuned with bias current and applied magnetic flux. The backaction can tune the resonator from strongly damped to self-sustained oscillation and may be used to cool the resonator.   

A Novel Ambient Operating Force and Acceleration Detector

An investigation to develop a novel accelerometer capable of operating under ambient conditions without any cryogenics is in progress in our laboratory. In this device the proof mass comprises of magnetic or diamagnetic materials. This mass is freely suspended in stable equilibrium under gravity by the combined actions of magnetic attraction and repulsion forces. Stability is achieved along all three Cartesian axes even at zero frequency. For highly dynamical onboard platforms, realtime nulling by active control at high-frequency is desirable. A description of prototypes and measurements will be discussed. Sensitivity in the $\sim $ 0.1ngal regime to both kinematic and gravitational accelerations and $\sim $ pN force resolution is observed. Our initial results including (i) detection of tidal changes in the gravitational background, (ii) seismic tremors, (iii) Fourier analysis of time displacement data and (iv) design considerations for enhanced sensitivity and improved performance will be presented. Several scientific and technological implications will be suggested.   

Driving electronics for a z-positioner in a new SPM design.

We use a modified Pan-type walker as the z coarse approach mechanism in our new SPM design. We developed new electronics for driving and exercising the walker with the main circuit consisting of six 12V relays. Connecting the relays in series produces a timing cascade due to the mechanical delay in each relay. The traditional slow linear ramp has been replaced with the charge and discharge behavior of the RC circuits, where C is the capacitance of the piezolelectric plates. Initial tests with a 6Hz frequency input showed 10 nm step size and a 3 millimeter range. A single 555 timer serves as our frequency generating source. A highly stabilized square wave can be generated in its monostable mode, with the output frequency determined by two external resistors and a capacitor. We also replace the high voltage supply with a voltage quadrupler circuit that is compact and inexpensive, with 64V and 128V DC output in the final configuration.   

A unique 30 Tesla single-solenoid pulsed magnet instrument for x-ray studies

We present a dual-cryostat pulsed-magnet instrument at the Advanced Photon Source (APS) with unique capabilities. The dual-cryostat independently cools the solenoid (Tohoku design) using liquid nitrogen and the sample using a closed-cycle refrigerator, respectively. Liquid nitrogen (LN) cooling allows a repetition rate of seven minutes for peak fields of 30 Tesla. The system is unique in that the LN cryostat incorporates a double-funnel vacuum tube passing through the solenoid's bore preserving the entire angular range allowed by the magnet. This scheme is advantageous in that it allows the applied magnetic field to be parallel to the scattering plane complementing typical split-pair magnets with fields normal to the scattering plane. Performance of the coils along with preliminary x-ray diffraction and spectroscopic studies will be presented.   

Applications of superconducting trapped field magnets for x-ray scattering experiments

Two long standing problems in x-ray and neutron scattering studies in applied magnetic fields are, 1) limited optical access and 2) practical impossibility to apply magnetic field parallel to x-ray (neutron) momentum transfer. In order to overcome these obstacles we have developed an application of Type-II superconducting magnets. In this approach, a small and thin plate-like single crystal sample is mounted on the surface of a melt-textured superconductor (SC). The SC is magnetized by cooling it from temperature above its superconducting critical temperature ($T_{c})$ in an applied magnetic field. Below $T_{c}$, magnetic flux gets trapped inside the SC disk after the removal of the external magnetic field. The SC disk acts as a permanent magnet with applied field normal to the flat surface of the disk providing unrestricted optical access to the entire hemisphere allowing a magnetic field parallel to the x-ray momentum transfer.   

Efficient conversion of $^{3}$He($n$,\textit{tp}) and $^{10}$B($n$,\textit{$\alpha $}$^{7}$Li) reaction energies into far-ultraviolet radiation by noble gas excimers

Previous work$^{1,2 }$showed that the $^{3}$He($n$,\textit{tp}) reaction in a cell of $^{3}$He at atmospheric pressure generated tens of far-ultraviolet (FUV) photons per reacted neutron. Here we report amplification of that signal by factors of 1000 when noble gases are added to the cell. Calibrated filter-detector measurements show that this large signal is due to noble-gas excimer emissions, and that the nuclear reaction energy is converted to FUV radiation with efficiencies of up to 30{\%}. Our results have been placed on an absolute scale through calibrations at the NIST SURF III Synchrotron and Center for Neutron Research.$^{3}$ We have also seen large neutron-induced FUV signals when the $^{3}$He gas in our system is replaced with a $^{10}$B film target; an experiment on substituting $^{3}$He with BF$_{3}$ is underway. Our results suggest possibilities for high-efficiency, non-$^{3}$He neutron detectors as an alternative to existing proportional counters. $^{1}$A. K. Thompson, \textit{et al., J. Res. Natl. Inst. Stand. Technol.} \underline {\textbf{113}}\underline {, 69} (2008) $^{2}$M. A. Coplan, A. K. Thompson and C. W. Clark, \underline {U.S. Patent No. 7,791,045} (2010) $^{3}$P.P. Hughes, \textit{et al.,} \underline {arXiv:1009.4707} (\textit{Appl. Phys. Lett. }in press, 2010)   

First Results from the Triple-axis Spectrometer at OPAL

The thermal triple-axis spectrometer TAIPAN is the first instrument for inelastic scattering at Australian research reactor OPAL. TAIPAN started operation in February 2009 and is in full user service since November 2010. The instrument can operate with variable incident or final energies and has a secondary spectrometer with a single detector. Presently the PG (002) double-focusing monochromator and analyzer are in use. The incident energy range on the TAIPAN is from $\sim $ 5 meV up to $\sim $ 100 meV with neutron flux at sample position of $\sim $ 10$^{8 }$n/cm$^{2}$/s [1]. First experiments were performed with superionic conductor Cu$_{2-x}$Se [2]. The measurements reveal a presence of soft mode in addition to the flat optic-like phonon branch. The DFT calculations show that unstable soft mode is related to ordering of Cu atoms followed by $\alpha -\beta $ phase transition at a lower temperature. The evolution of the magnetic structure with temperature in magnetically modulated FePt$_{3}$ thin film was investigated in the diffraction mode of TAIPAN. The results show that the film fabricated by modulation of the chemical order parameter consists of a magnetic FM/AFM superlattice in single-crystalline FePt$_{3}$ [3]. [1] S.A. Danilkin et al., Neutron News, 20 (2009) 37; [2] S.A. Danilkin et al., J. Phys. Soc. Jpn. 79 (2010) Suppl. A, 25; [3] T. Saerbeck et al., Phys. Rev. B 82 (2010) 134409.