Magnetic leftovers. Sergey L. Bud ko

Transcription

1 Magnetic leftovers 590B F08 Sergey L. Bud ko Hall probe magnetometers MPMS sample mounting and measurements Arrott plots

2 Hall gradiometer

3 Hall gradiometer - applications Quantum tunneling in Mn 12 - acetate (50x50x300 μm 3 crystal) Hysteresis loops of ~ 600 Fe nanoparticles

4 Submicron Hall probes Domain wall movement

5 Domain wall movement

6 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. 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

7 (*) Magnetic field relaxation History is important Approach is important Superconducting solenoid Note the time scale!

8 (**) Remnant field 5.5 T magnet: 2-6 G Oscillate mode G No Overshoot mode AND changing for several hours Factor of 10 difference

9 (**) 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?

10 (***) Field variation MPMS is factory calibrated for 6 cm scans Need standard! ~ 1 koe at max field!

11 (****) Temperature relaxation

12 3 cm Scan length (6 cm) Uniform sample holder for AT LEAST ± scan length from the sample (transparent plastic drinking straw is a reasonable solution)

13 > 6 cm Teflon tape Sample (crystal or solid polycrystal) Few holes in the straws poked with a needle Two straws (inner one with cut along it) Mount using friction and elasticity of plastic. No signal from glue, threads, holders. Sample slightly off center. Sample can move (in principle). Torque may be a problem. Small stress on the sample.

14 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.

15 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

16 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

17 Glued sample Quartz rod Necked quartz tube Straight, low signal, low thermal expansion mount Fragile, signal from glue

18 Oxygen contamination mg

19 Oxygen contamination Complex shape/size of the transition(s). Critical in powders. Check O-rings and sliding assembly. Purge!

20 Horizontal sample rotator Signal from the pad 180 o periodicity Signal from glue Torque upper limit for sample size (thread and pulley) Signal from the solder in the bottom

21 : SUMMARY Things to remember: Magnet relaxation/history Magnet inhomogeneity Temperature lag Sample holder/glue signal Sample length Torque Sample displacement Oxygen contamination

22 Arrott plots How to determine Tc of ferromagnets from magnetization measurements? Often does not work

23 Arrott plots Expansion of Brillouin function Use mean field You get M 2 It works! What are low fields? - Should be in a singledomain state H/M

24 Arrott plots - generalization Equation of state (critical exponents) Nickel Things to remember: Domains T c distribution Demagnetization factor Crystal Electric Field After all, often one can use resistivity or Cp to determine T c

25 Reading: Quantum Design web-site S. Chikazumi, Physics of ferromagnetism. A. Aharoni, Introduction to the theory of ferromagnetism