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Novel Materials and Ground States
Magnetic leftovers
590B F08
Sergey L. Bud’ko
Hall probe magnetometers
MPMS sample mounting and measurements considerations
Arrott
plots
Novel Materials and Ground StatesHysteresis loops of ~ 600 Fe nanoparticles
Hall gradiometer -
applications
Quantum tunneling in Mn12
-
acetate (50x50x300 μm3
crystal)
Novel Materials and Ground States
MPMS sample mounting and measurements considerations
Two relevant quotes:price
“It's not the size that matters, it's how you use it”“Even the most beautiful girl cannot give you more than she has”
Two main misconceptions about MPMS magnetometer (and many other instruments):
• I’ve paid ¼
million $$ for it, so all the data is right and I am the best.
• Prof. ZZZ has 100 times better resolution when she measures susceptibility of Y at temperature X, so MPMS is an expensive piece of junk.
Novel Materials and Ground States
MPMS sample mounting and measurements considerations
(*) Magnetic field relaxation
History is important
Approach is important
Note the time scale!Superconducting solenoid
Novel Materials and Ground States
MPMS sample mounting and measurements considerations
(**) Remnant field
5.5 T magnet:
2-6 G –
Oscillate mode
20-40 G –
No Overshoot mode AND changing for several hours
Factor of 10 difference
Novel Materials and Ground States
MPMS sample mounting and measurements considerations
(**) Remnant field –
cures
-Oscillating Mode (many of us have DEMAG sequence)
-Use of shorter scans (but potentially lower quality of the fit)
-Use Magnet Reset Option (quench from high fields) –
LHe
loss, magnet integrity?
Novel Materials and Ground States
MPMS sample mounting and measurements considerations
(***) Field variation
~ 1 kOe
at max field!
MPMS is factory calibrated for 6 cm scans
Need standard !
Novel Materials and Ground States
MPMS sample mounting and measurements considerations
(****) Temperature relaxation
Novel Materials and Ground States
MPMS sample mounting and measurements considerations
3 cmScan length (6 cm)
Uniform sample holder for AT LEAST ±
scan length from the sample (transparent plastic drinking straw is a reasonable solution)
Novel Materials and Ground States
MPMS sample mounting and measurements considerations
Sample (crystal or “solid”
polycrystal)
> 6 cm
Two straws (inner one with cut along it) Mount using friction and elasticity of plastic. No signal from glue, threads, holders.
Teflon tape Few holes in the straws poked with a needle
Sample slightly off center. Sample can move (in principle). Torque may be a problem. Small stress on the sample.
Novel Materials and Ground States
MPMS sample mounting and measurements considerations
Variation for “H plate” sample
cross-section Very small addenda (gap between two pieces of straw).
Torque may be a problem. “Medium”
accuracy of the orientation. Easy to loose sample. May need to use paper disks or plastic film (“Glad Press N’
Seal”…) if sample is small.
Novel Materials and Ground States
MPMS sample mounting and measurements considerations
Gelatine
capsules (χ
~ 10-7
emu/g, m ~ 40 mg)
Good for powders/small pieces
Need to prevent sample movement. Need to be sure that will not pop-up (positive pressure) –
hole, glue, …
Hygroscopic
Also have fun in local pharmacy when buying large quantities
Novel Materials and Ground States
MPMS sample mounting and measurements considerations
Quartz tube with quartz rods. Sealed on both sides.
Used for air sensitive samples (ask John Corbett’s students).
Non-negligible contribution of the gap (but easy to calibrate). Powder size –
not too small.
Hygiene…
Novel Materials and Ground States
MPMS sample mounting and measurements considerations
Glued sample
Quartz rodNecked quartz tube
Straight, low signal, low thermal expansion mount
Fragile, signal from glue
Novel Materials and Ground States
MPMS sample mounting and measurements considerations
Oxygen contamination
250.1 mg
Novel Materials and Ground States
MPMS sample mounting and measurements considerations
Oxygen contamination
Complex shape/size of the transition(s). Critical in powders.
Check O-rings and sliding assembly. Purge!
Novel Materials and Ground States
MPMS sample mounting and measurements considerations
Horizontal sample rotator
Signal from the pad –
180o
periodicity
Signal from glue
Torque –
upper limit for sample size (thread and pulley)
Signal from the solder in the bottom
Novel Materials and Ground States
MPMS sample mounting and measurements considerations: SUMMARY
Things to remember:
Magnet relaxation/history
Magnet inhomogeneity
Temperature lag
Sample holder/glue signal
Sample length
Torque
Sample displacement
Oxygen contamination
Novel Materials and Ground States
Arrott
plotsHow to determine Tc
of ferromagnets
from magnetization measurements?
Often does not work
Novel Materials and Ground States
Arrott
plots
Expansion of Brillouin
function
Use mean field
You getM2
H/M
It works!
What are “low fields”?
-
Should be in a single-
domain state
Novel Materials and Ground States
Arrott
plots -
generalization
Equation of state (critical exponents)
Nickel
Things to remember:
Domains
Tc
distribution
Demagnetization factor
Crystal Electric Field
After all, often one can use resistivity or Cp to determine Tc