hall probe magnetometers mpms sample mounting and...

Novel Materials and Ground States

Magnetic leftovers

590B F08

Sergey L. Bud’ko

Hall probe magnetometers

MPMS sample mounting and measurements considerations

Arrott

plots


Hall gradiometer

Novel Materials and Ground StatesHysteresis loops of ~ 600 Fe nanoparticles

Hall gradiometer -

applications

Quantum tunneling in Mn12

-

acetate (50x50x300 μm3

crystal)


Submicron Hall probes

Domain wall movement

Novel Materials and Ground StatesDomain wall movement



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.



(*) Magnetic field relaxation

History is important

Approach is important

Note the time scale!Superconducting solenoid



(**) 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



(**) 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?



(***) Field variation

~ 1 kOe

at max field!

MPMS is factory calibrated for 6 cm scans

Need standard !



(****) Temperature relaxation



3 cmScan length (6 cm)

Uniform sample holder for AT LEAST ±

scan length from the sample (transparent plastic drinking straw is a reasonable solution)



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.



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.



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



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…



Glued sample

Quartz rodNecked quartz tube

Straight, low signal, low thermal expansion mount

Fragile, signal from glue



Oxygen contamination

250.1 mg




Complex shape/size of the transition(s). Critical in powders.

Check O-rings and sliding assembly. Purge!



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


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



Arrott

plotsHow to determine Tc

of ferromagnets

from magnetization measurements?

Often does not work


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


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


Reading: Quantum Design web-site

S. Chikazumi, Physics of ferromagnetism.

A. Aharoni, Introduction to the theory of ferromagnetism

hall probe magnetometers mpms sample mounting and...

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