Introduction to the FLAME Project
To be at the high end of international research, the μSR instruments of the SμS need to be kept at the state-of-the-art. This requires a permanent development of new experimental capabilities to be able to address topical research questions. Therefore, LMU took the strategic decision, at the beginning of 2017, to start the planning of a new instrument called FLAME (FLexible and Advanced MuSR Environment).
In view of the facts that: i) the number of muon beamlines at the HIPA complex is limited and not extensible; ii) that LMU is committed to solely provide state-of-the-art instruments with potentially high scientific output and minimum overlap; and iii) that the human resources of LMU is limited; we took the decision that FLAME has to replace the old instrument Low Temperature Facility (LTF) at the secondary muon beamline πM3.3. With an age of 31 years, LTF is the oldest of the existing μSR instruments. It has produced excellent results since it has been the only instrument at PSI which allows for zero field (ZF) and high longitudinal field (LF) measurements at mK temperatures. The data quality of LTF with respect to time resolution, field dependence of the efficiency, and time dependence of the positron background signal does not anymore fulfill up-to-date standards. In addition, due to its detector design, the data contain at least a 30% background. LTF also shows advanced signs of aging and becomes increasingly difficult to operate reliably.
The design objectives for the FLAME instrument have been determined by the needs of the diverse and growing Swiss and international user community of the SμS facility and the compatibility and complementarity with the existing μSR instrumentation. Here we will present a list of key design features of FLAME that will allow addressing a broad range of novel scientific fields that could not be studied up to now.
Feature 1: broad temperature range with magnetic fields of up to 3 T
The sample environment for FLAME will allow performing experiments from 20 mK up to 310 K in the same μSR spectrometer. Please note that after the decommissioning of LTF no other spectrometer will exist at PSI with ZF and LF at DR temperatures. The broad range of temperatures is a great advantage compared to the present situation where the different temperature regions are covered by different spectrometers at different beamlines and systematic errors occur when two datasets have to be combined. Also note that presently at PSI the highest LF for temperatures above 4 K is 0.78 T at GPS. The availability of LF up to 3 T over a broad temperature range up to room temperature greatly enhances the capability of the facility e.g. to study magnetic dynamics. A possible extension to even higher temperatures will be easily possible in the future.
Feature 2: ideal ZF, LF and TF measurements in high spatial homogeneity and temporal stability
Due to the recent upgrade of the muon beamline πM3 with a new spin-rotator, it will be possible to perform both LF as well as TF measurements in a field up to at least 3 T with large signal amplitudes in FLAME. In addition, ideal ZF experiments with residual fields lower than 5 μT will be possible. Since μSR is a highly sensitive magnetic probe, true ZF conditions are essential to determine the real ground state properties of systems with ultra-small electronic or nuclear magnetic moments. The field homogeneity and stability will be far better than presently available at LTF and especially high precision TF measurements e.g. to determine the local susceptibility (Knight shift measurements) will benefit from it.
Feature 3: small samples with practically no background
In FLAME it will be possible to measure small samples with an area of <3x3 mm2 at temperatures from 20 mK up to room temperature with practically no background signal. In comparison to the LTF spectrometer with a sample area of 8x13 mm2 this is an improvement of a factor of 10 in sample to background signal. Please note that a μSR experiment is subject to Poisson statistics. Therefore this improvement corresponds to a factor of 100 in statistical quality of the data. The combination of DR temperatures and small samples size at FLAME will open new possibilities for research for μSR at PSI taking into account that typically high quality, new and topical materials like e.g. the iridates can only be produced as small crystals.
Feature 4: increased time resolution
The newly designed compact detector system will improve the overall time resolution of FLAME by at least a factor of five (to <150 ps) compared to other spectrometers at PSI like Dolly, GPD or LTF. This obviously increases dramatically the spectroscopic accuracy and the observable signal amplitude at high TF and will allow the users of FLAME to investigate higher muon spin relaxation rates and precession frequencies that are typically present in magnetic systems with large magnetic moments like e.g. rare earth based magnets.
Feature 5: in-situ modification of the sample by external stimuli like uniaxial pressure
FLAME and its dilution fridge will possess a large sample space and several high voltage coaxial as well as signal cables entering the sample space allowing for external stimuli of the sample like electric fields or electric currents. Especially we will have a piezoelectrically driven uniaxial strain device. A prototype of this novel device has already been tested in Dolly and will allow in FLAME to in-situ at low temperatures apply tensile and compressive strains up to 1% (forces up to 1800 N) at dilution refrigerator temperatures (and also up to 310K). This will allow e.g. to drive a system though magnetic quantum phase transition (QPT) or to modify the superconducting symmetry in unconventional superconductors and make a multitude of new experiments possible.
Feature 6: multi samples cold finger for fast turnover times and reference samples
FLAME will have the possibility to mount up to 3 samples on a ladder holder on the dilution fridge which can successively be brought into the beam at low temperatures. This will be a worldwide unique feature of FLAME. Since dilution fridge temperatures require considerable cooldown times on the beam, this feature will dramatically reduce the ratio between “down” and “up” periods of the instrument, which is a key factor on a heavily oversubscribed facility as SμS. For example, this system will allow, for the first time, detailed and systematic studies at mK temperatures of e.g. stoichiometry dependent phase diagrams where μSR is traditionally strong. In addition, a sample can be measured together with a reference sample (e.g. Ag) with the same shape and under the same conditions. This is usually not performed due to the lack of time, but is actually essential for high precision Knight shift measurements or when the field dependent drifts of the detector efficiency in large LF need to be corrected for the accurate determination of dynamic relaxation rates.
Feature 7: flexibility for future upgrades and compatibility with equipment of the μSR facility
The magnet and μSR spectrometer are designed to be compatible with other cryogenic environments already used at the μSR facility making FLAME a very versatile instrument and ready for future upgrades. Hence, other sample environments e.g. a flexible temperature range closed cycle refrigerator (4 – 500 K) or even an oven (300 – 1000 K) might be purchased later to further enhance the accessible temperature range for μSR studies in high magnetic fields.
In view of the facts that: i) the number of muon beamlines at the HIPA complex is limited and not extensible; ii) that LMU is committed to solely provide state-of-the-art instruments with potentially high scientific output and minimum overlap; and iii) that the human resources of LMU is limited; we took the decision that FLAME has to replace the old instrument Low Temperature Facility (LTF) at the secondary muon beamline πM3.3. With an age of 31 years, LTF is the oldest of the existing μSR instruments. It has produced excellent results since it has been the only instrument at PSI which allows for zero field (ZF) and high longitudinal field (LF) measurements at mK temperatures. The data quality of LTF with respect to time resolution, field dependence of the efficiency, and time dependence of the positron background signal does not anymore fulfill up-to-date standards. In addition, due to its detector design, the data contain at least a 30% background. LTF also shows advanced signs of aging and becomes increasingly difficult to operate reliably.
The design objectives for the FLAME instrument have been determined by the needs of the diverse and growing Swiss and international user community of the SμS facility and the compatibility and complementarity with the existing μSR instrumentation. Here we will present a list of key design features of FLAME that will allow addressing a broad range of novel scientific fields that could not be studied up to now.
Feature 1: broad temperature range with magnetic fields of up to 3 T
The sample environment for FLAME will allow performing experiments from 20 mK up to 310 K in the same μSR spectrometer. Please note that after the decommissioning of LTF no other spectrometer will exist at PSI with ZF and LF at DR temperatures. The broad range of temperatures is a great advantage compared to the present situation where the different temperature regions are covered by different spectrometers at different beamlines and systematic errors occur when two datasets have to be combined. Also note that presently at PSI the highest LF for temperatures above 4 K is 0.78 T at GPS. The availability of LF up to 3 T over a broad temperature range up to room temperature greatly enhances the capability of the facility e.g. to study magnetic dynamics. A possible extension to even higher temperatures will be easily possible in the future.
Feature 2: ideal ZF, LF and TF measurements in high spatial homogeneity and temporal stability
Due to the recent upgrade of the muon beamline πM3 with a new spin-rotator, it will be possible to perform both LF as well as TF measurements in a field up to at least 3 T with large signal amplitudes in FLAME. In addition, ideal ZF experiments with residual fields lower than 5 μT will be possible. Since μSR is a highly sensitive magnetic probe, true ZF conditions are essential to determine the real ground state properties of systems with ultra-small electronic or nuclear magnetic moments. The field homogeneity and stability will be far better than presently available at LTF and especially high precision TF measurements e.g. to determine the local susceptibility (Knight shift measurements) will benefit from it.
Feature 3: small samples with practically no background
In FLAME it will be possible to measure small samples with an area of <3x3 mm2 at temperatures from 20 mK up to room temperature with practically no background signal. In comparison to the LTF spectrometer with a sample area of 8x13 mm2 this is an improvement of a factor of 10 in sample to background signal. Please note that a μSR experiment is subject to Poisson statistics. Therefore this improvement corresponds to a factor of 100 in statistical quality of the data. The combination of DR temperatures and small samples size at FLAME will open new possibilities for research for μSR at PSI taking into account that typically high quality, new and topical materials like e.g. the iridates can only be produced as small crystals.
Feature 4: increased time resolution
The newly designed compact detector system will improve the overall time resolution of FLAME by at least a factor of five (to <150 ps) compared to other spectrometers at PSI like Dolly, GPD or LTF. This obviously increases dramatically the spectroscopic accuracy and the observable signal amplitude at high TF and will allow the users of FLAME to investigate higher muon spin relaxation rates and precession frequencies that are typically present in magnetic systems with large magnetic moments like e.g. rare earth based magnets.
Feature 5: in-situ modification of the sample by external stimuli like uniaxial pressure
FLAME and its dilution fridge will possess a large sample space and several high voltage coaxial as well as signal cables entering the sample space allowing for external stimuli of the sample like electric fields or electric currents. Especially we will have a piezoelectrically driven uniaxial strain device. A prototype of this novel device has already been tested in Dolly and will allow in FLAME to in-situ at low temperatures apply tensile and compressive strains up to 1% (forces up to 1800 N) at dilution refrigerator temperatures (and also up to 310K). This will allow e.g. to drive a system though magnetic quantum phase transition (QPT) or to modify the superconducting symmetry in unconventional superconductors and make a multitude of new experiments possible.
Feature 6: multi samples cold finger for fast turnover times and reference samples
FLAME will have the possibility to mount up to 3 samples on a ladder holder on the dilution fridge which can successively be brought into the beam at low temperatures. This will be a worldwide unique feature of FLAME. Since dilution fridge temperatures require considerable cooldown times on the beam, this feature will dramatically reduce the ratio between “down” and “up” periods of the instrument, which is a key factor on a heavily oversubscribed facility as SμS. For example, this system will allow, for the first time, detailed and systematic studies at mK temperatures of e.g. stoichiometry dependent phase diagrams where μSR is traditionally strong. In addition, a sample can be measured together with a reference sample (e.g. Ag) with the same shape and under the same conditions. This is usually not performed due to the lack of time, but is actually essential for high precision Knight shift measurements or when the field dependent drifts of the detector efficiency in large LF need to be corrected for the accurate determination of dynamic relaxation rates.
Feature 7: flexibility for future upgrades and compatibility with equipment of the μSR facility
The magnet and μSR spectrometer are designed to be compatible with other cryogenic environments already used at the μSR facility making FLAME a very versatile instrument and ready for future upgrades. Hence, other sample environments e.g. a flexible temperature range closed cycle refrigerator (4 – 500 K) or even an oven (300 – 1000 K) might be purchased later to further enhance the accessible temperature range for μSR studies in high magnetic fields.
FLAME Milestones
- 2017/01 NUM department supports FLAME
- 2017/05 PSI research committee supports FLAME
- 2017/05 PSI directorate supports FLAME
- 2017/12 SNF R'Equip funding application accepted
- 2018/01 Funding secured from DIR, NUM, and SNF
- 2018/07 WTO tender for purchasing the main magnet
- 2018/12 Purchase order for main magnet at Cryogenics Ltd.
- 2018/12 Dismantling of LTF finished
- 2019/01 Painting of the piM3.3 area
- 2019/02 Local area crane (250 kg) installed
- 2019/05 New measurement cabin installed
- 2019/09 Purchase order of dilution refrigerator (KelvinoxJT) at Oxford Instruments
- 2020/03 Original delivery date for main magnet - delayed
- 2020/07 Magnet dummy tube installed due to delay of magnet delivery
- 2020/08 Area platform installed
- 2020/09 Spectrometer test with beam successful (first muSR spectrum at FLAME)
- 2020/12 4He Orange cryostat delivered and initial tests successful
- 2020/12 Dilution refrigerator (KelvinoxJT) delivered and tested (SAT) successfully
- 2022/02 Delivery of the superconducting magnet and installation in the PiM3.3 area