national high magnetic field laboratory
PULSED FIELD FACILITY
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TORQUE & FORCED MAGNETOMETRY IN PULSED FIELDS
CRYOGENIC ROTATION DOWN TO 350mk, FIELDS UP TO 85T
RESOLVING ANISOTROPY CHANGES AT MAGNETIZATION STEPS
COMPARISON OF TORQUE & INTEGRATED SUSCEPTIBILITY MEASUREMENTS
FERMIOLOGY OF OXYPNICTIDE SUPERCONDUCTORS
RECENT PNICTIDE-FERMIOLOGY PUBLICATIONS USING CANTILEVERS IN PULSED DC FIELDS
torque & forced magnetometry in pulsed fields
Piezoresistve AFM lever (Seiko) 120μm x 50 μm x 4 μm
Unloaded resonant frequency 250‐300 kHz
Sample loading can lower the response to ~10 kHz (for big ~1 μg samples)
→Use small samples to keep response fast (c.f. 65 T – 8 ms rise time)
Spring constant = 40 N/m → very small angular deflecton
Balanced bridge to detect small ΔR/R
Ground shielding to prevent inductve/capactve cross talk
Noise floor of balanced bridge resistance measurements < 5mΩ/500Ω = 10-⁻5 (~150 μA @ 297.5kHz, 10kHz bandwidth, during 65T, 8ms rise time pulse)
For a Seiko PRC120 torque = 4.7x10-‐8ΔR/R
→Torque noise floor = ½ pNm
Converting to a magnet moment via τ ∝ M × μ0H gives 10-⁻14 Am2 (or 109μB) @ 50T
specialization and methods
Technique successfully used in 65T short pulse, 50T mid pulse, 60T long pulse and 85T magnets.
Second Bdot coil to measure projection of field.
Rotate the sample with respect to the applied field: τ ≈ M × H
cryogenic rotation down to 350mk, fields up to 85T
resolving anisotropy changes at magnetization steps in a quantum magnet system
Torque signal is small near symmetry directions.
Change the balance of force and torque by moving in field gradient.
Change of sign of the magnetic torque @ the steps in magnetization requires nontrivial anisotropy, c.f. Ds ² is the same sign for all S .
comparison of torque and integrated susceptibility measurements
The angle dependence of
quantum oscillations map out the Fermi surface topology and allow the Spin Density Wave nesting vector (Q) of the As- parent phase to be identified.
fermiology of oxypnictide superconductors
some recent pnictide-fermiology publications using the cantilevers in pulsed and DC fields
• Quantum oscilla)ons in the parent pnic)de BaFe2As2 : I)nerant electrons
in the reconstructed state' (J.G. Analyis et al Phys. Rev. B 80, 06450 (2009))
• 'Fermi Surface of SrFe2P2 Determined by the de Haas–van Alphen Effect
(J.G. Analy)s et al, Phys. Rev. Lel. 103, 076401 (2009))
• Topological Change of the Fermi Surface in Ternary Iron Pnic)des with
Reduced c/a Ra)o: A de Haas–van Alphen Study of CaFe_2P_2 ' (A.I. Coldea
et al Phys. Rev. Lel. 103, 026404 (2009))