reaction of fe with c 60 at hpht conditions: formation of fe 3 c and implications for

Reaction of Fe with C60 at HPHT conditions: formation of Fe3C

and implications for “magnetic carbon”

Alexandr Talyzin, (Department of Physics, Umea University, Sweden)A.Dzwilewski (Umea University, Sweden)L.Dubrovinsky (Bayreuth University, Germany)A. Setzer and P. Esquinazi (University of Leipzig, Germany)

A.Talyzin, Umea University, Sweden

1. Introduction: history of “magnetic carbon”

1996-2000- Metallic conductivity in Rh polymer of C60 (Makarova et al. Synth. Met. 121, (2001).)

2001- Pressure polymerized C60 (rhombohedral structure, pure carbon) is ferromagnetic at room temperature (as occasionally discovered) ! Report by Makarova et al., Nature 413, 716-718, (2001)2002-2004 – five experimental papers confirming ferromagnetism of high pressure polymers of C60 (from two groups)2002-2006- about 15 theoretical papers aimed to explain high temperature ferromagnetism of C60 polymers. Poster at this conference (Y.Kitagawa).

August 2005 – Corrigendum to original paper on “magnetic carbon” is published in “Nature”30 March 2006 – The Nature paper is retracted by 7 authors out of 9. Two authors (T.Makarova and P.Scharff have not joined retraction)

Polymeric C60- “magnetic carbon”?

Figure from presentation of T.Makarova, 2005.

Summary of discovery (Makarova et al, Nature,2001), five main arguments:

1. Synthesis temperatures close to the point of C60 collapse.

P=6 GPa, T=1025-1050K

2. Ferromagnetic with Curie T ~500K

3. Ferromagnetism disappears after thermal depolymerisation of samples (700K)

4. Reproducible: 3 sets of samples, one of them “specially synthesized” were reported in original Nature paper.

5. 22 ppm level for all ferromagnetic impurities (30 fold less compared to measured magnetisation)

Magnetic force microscopy on “Nature”sample


Magnetic force gradient image taken in impurity-free parts of samples. (Han et al, Carbon, 2003).

Images were interpreted as magnetic domains and considered as an explicit evidence of ferromagnetism in Rh polymer of C60.

30% of ”impurity-free areas” covered with domains.

More questions and more evidence?


Re-analysis of “Nature samples”by R.Hohne and P.Esquinazi (Adv.Mat., 2002)

-Iron contamination measured by PIXE (30 m depth): 200- 400 g/g,up to about 20 times higher than previously reported(enough to explain ferromagnetism)

-Ferromagnetism do not disappear after prolonged heating at 800 K(on the same sample as shown inNature )

All three samples studied for “Nature” publication (one split on two pieces)(figure from Adv.Mat., 2002)

Ferromagnetism due to polymeric structure?

DSC traces of the R-phase, scanning rate 10 K/min. (A) R-phase; (B) Magnetic carbon. (Korobov et al, Chem.Phys.Lett, 2003)

Nature: magnetisation reversible after heating at 640K for 2 hoursAdv.Mat, 2002: Magnetisation is reversibleAfter heating at 800K for 16 hours

The same sample (E17) depolymerized at 2000C (473K) in 2001 (Makarova et all, Carbon).In Nature (2001) depolymerization was reported below 700K

Depolymerisation of C60 polymers occurs at 500-560K.(including photopolymers)

Ferromagnetism can not be assigned to polymeric structure.

New studies (Han et al, Phys.Rev.B, 2005)


5 10 15 20 25 30 35 40

1193K

1093K

1085K

1073K

1053K

983K

953K

923K

Inte

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Diffraction angle (2)

Over 20 samples synthesized at 3.5 GPa and various temperatures (Bayreuth University, Germany) with all possible precautions against contaminations. Structure and samples and synthesis conditions are very close to those from “Nature” samples.

X-ray diffraction from one set of samples synthesized in 2004 and studied bySQUID in Germany (Prof.Esquinazi group) and by MFM at Umea (Dr.Han).

Below 1085K-polymeric C60 (Rh and T phases). Point of collapse is between 1073-1085K. Perfect agreement with “Nature” data (1075K).

MFM data on new samples.


(a)

(b)

Topographic (left) and magnetic force gradient (right) images for samples of graphite like carbon (“collapsed fullerite”)

All samples diamagnetic (also polymers)

Bulk magnetisation measured by SQUID: maximal magnetisation is only 0.0003 emu/g –three orders of magnitude less! (~0.1 emu/g in Nature)

0.0001 emu/g for sample with “domains”. Less than 1 g/g of Fe.Real contamination: up to 80 g/g of Fe

All magnetisation come from Fe impurities. Samples with “domains” are not polymeric and show absence of bulk ferromagnetism.

Conclusion from the paper: No ferromagnetism at THAT synthesis conditions.

What were conditions of synthesis for ferromagnetic samples?


-Nature (2001)- 6 GPa , T=1025K-1050K (just below collapse point 1075K)

-Recent review (Makarova, Semiconductors, 2004) -“about one hundred degrees below point of C60 collapse..” That will be 975K. Out of range.

-Review by Makarova, (2003)- “….with maximum at 1075K”, exactly the point of collapse.

-MFM paper (Han et al, Carbon, 2003) only one sample studied, the same as in “Nature”: synthesized at ….P=6 GPa and 1125K Above point of C60 collapse!

-The same study published earlier (Nucl. Instr. and Meth. in Phys. Res. B 210 (2003) 531–536) :P=2.5 GPa and 1125K, this sample could not be Rhombohedral at this pressure.

-Hohne and Esquinazi, Adv.Materials, 2002: Three samples: one of them referred as “Nature” sample,second as “different from sample 1”, last sample prepared at “different conditions”. Synthesis conditions not clearly specified for all studied samples.

Verified synthesis conditions for “Nature” samples (Corrigendum, 2005).


Stated in the paper: 6 GPa and 1025-1050K, ferromagnetism only in this “narrow interval of temperatures”, 5 samples ferromagnetic from 3 different sets.

Only three samples were studied by SQUID(Prof. Esquinazi, Germany) prior to paper submission. Only two of them were ferromagnetic:

- “Nature sample” 6 GPa and 975K lowest temperature from all set.

-“MFM sample”- 2.5 GPa and 1125K both P and T out of reported range.

- 6 GPa and 1025K Not ferromagnetic.

Corrigendum published in Nature, 2005. Reports minor mistakes in synthesis conditions. Celsius to Kelvin calculation error,mistake with sample labels.

Implications of “new” synthesis conditions


-Only one ferromagnetic sample was true Rhombohedral polymer, synthesis temperature 975K was lowest from all samples. “Narrow interval” of temperatures do not exist.

-Second sample (2.5 GPa and 1125K) was not polymeric and not fullerene:C60 collapses at these temperatures.

-Magnetic properties of these two samples were identical: ferromagnetism could not be connected to fullerenes!

-over 20 polymeric samples were studied in 2002-2004 in Prof. Esquinazi group: none of them was confirmed ferromagnetic on the level where impurities could not explain it.

-Curie T of 500K was EVER observed only for these two samples, only one of them true Rh polymer.Next set of samples showed Curie T above 800K (Narozhnyj, 2003).

Reproducibility and history of samples.


-Both ferromagnetic samples synthesized in 1998: THREE years prior to “occasional discovery” of ferromagnetism in 2001.

-Samples were split in 1998 by metallic tools and repeatedly touched by unprotected magnet in 2001-2004.

-No precautions against contamination was taken during synthesis, three years of storage and handling of samples.

Would anyone care about metallic dust or tools in high pressure experiments NOT INTENDED for studies of ferromagnetism?

Impurity analysis.


-Citation from Nature paper: “We have paid great attention to chemical analysis of the pristine material as well as of the polymerized phase. The total amount of magnetic (Fe,Ni, Co) impurities is 22 p.p.m. in the pristine phase”.

-Sample cited in Nature was 3 mg (impurity analysis never done on this particlar piece) Impurity analysis mentioned in the paper (atomic absorption spectrometry) required 20 mg.

Contamination introduced during synthesis was not taken into account. Claim of “ferromagntism in pure carbon” was made withoutadequate impurity analysis of samples.

Which impurity could explain Curie T of 500K?


Fe, Nb, W, V are known to induce collapse of C60 with formationof carbides in thin films already at 400K.

Fe3C is ferromagnet with Curie Temperature 480-500K

Model experiments: Fe+C60 at HPHT conditions

Two samples synthesized: one with 10% (mass) of Fe and 3% of Fe.Samples subjected to the same P-T treatment as C60 used forpreparation of “magnetic carbon. Uniform size of Fe particles: 2-3 m

Reaction of Fe with C60 at 2.5 GPa and 1040K

35 40 45 50 55

35 42 49

Si

10% Fe

3% Fe

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Diffraction angle 2


20 30 40 50

Fe (110)

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Pristine C60

/ 10% Fe powder

The same sample after HPHT treatment

Iron peaks disappear after high pressure high temperature treatment. Excess of C60 transforms into polymeric phase.

More deteail XRD: iron transformed into Cementite, Fe3C.Curie T (Fe3C)= 480-500K

Fe2O3+C Fe3O4+C Fe+C Fe3C(“Magnetism of cigarette ashes”, Jordanova, 2006)

Ferromagnetism in C60 polymer/Fe3C mixture (Prof. P.Esquinazi group, University of Leipzig)


Pristine C60/10 % Fe mixture

HPHT treated with 10% FeHPHT treated with 3% Fe

Saturation magnetization of Fe3C at (RT) Ms=128 emu/g.

Expected magnetic moments at saturation due to Fe3C were calculated from known Fe concentrations and masses of samples.Values calculated for both samples are in agreement with the measured data within experimental error.Iron do not induce ferromagnetism in in carbon

Rh polymer,“Nature”

Comparing C60/Fe3C to ”magnetic carbon”.


Pristine C60/10% Fe mixture

Right axis: two samples of ”magnetic carbon”: E17 (Nature, 2001) and E16 (Adv.Mat., 2002)

Left axis: C60/Fe3C samples obtained at HPHT.

800 g/g of Fe in form of Fe3Ccalculated for ”Nature” curves.

Conclusion: ”magnetic carbon” and C60/Fe3C show nearly identical magnetic properties. Submitted to Phys.Rev.B Available on line at: http://arxiv.org/abs/cond-mat/0602306

Amounts and source of iron in ”magnetic carbon”


Figure from Han et al, Carbon 2003Figure from Spemann et al, (2003),conference proceedings

Particles of tens micrometer size.

ImageJ software shows 2-3% of surface are covered by subsurface particles (counting only sharp ones). (~30 m depth of method).

These particles were likely Fe3C(if iron was introduced before or during synthesis)

22 ppm of Fe??

Averaging from PIXE:2-3 m spots, 5 points.

1.370, 100, 200,16.000 and 100 (average 482 g/g)

Hundreds of points required!

Independent confirmations.1) 2.5 GPa- Makarova et al (Synth.metals, 2003). No impurity analysis was done, no magnetic data shown, Ms= 0.01 emu/g (~50 g/g Fe)

2) 6 GPa- Narozhnyj et al (2003), proceedings, 22 ppm in pristine C60 (with reference to Nature paper).Curie T above 800K, not determined.

3) 9 GPa- Wood et al, (2002).Impurity analysis not presented at all.Only one sample shown to be ferromagnetic.

4) 9 GPa- Chan et al,2005 (the same group as 3) Hydrogen amount determined but notFe, Ni,Co.

Confirmations are not convincing.

Summary and Retraction (29 March 2006, Nature)


“..T.L.Makarova and P.Scharff decline to sign this retraction because they do not believe that the earlier results, supported in subsequent studies, are totally invalidated by these findings…”.

Retraction submitted by: B. Sundqvist, R. Hohne, P. Esquinazi, Y. Kopelevich, V. Davydov, L. S. Kashevarova & A. V. Rakhmanina

All existing data on ferromagnetism of high pressure polymers of C60 can be explained by impurities without suggesting “magnetic carbon”.

Theoretical papers about ferromagnetism in Rh polymer are not supported by experimental evidence…

Metallic conductivity in high pressure polymers of C60:result of mistakes in Celsius to Kelvin calculations

Makarova, T. L. et al. Anisotropic metallic properties of highly-oriented C60 polymer. Synth. Met. 121, (2001).Makarova, T. L. et al. Electrical properties of two-dimensional fullerene matrices. Carbon 39, (2001).Okotrub et al, J.Chem.Phys. (2001) –model proposed to explain metallic Rh C60.

Makarova (JMMM), 2004

1075K

1025K

975K

Makarova, (2002)

1025 K

1050 K

1075K

Makarova, (Mol.Mat) 2000

1048K

1073K

1123K

Point of C60 collapse: 1075K (Nature)

Magnetic carbon in meteorite? (Coey et al, Nature, 2002)


Magnetisation measured from graphite nodule reported to be 30% higher compared to magnetisation calculated from mineral composition. Amount of magnetic minerals estimated by XRD, Mossbauer spectroscopy, chemical analysis, (EDAX). Curie Temperature of “magnetic graphite” : ~500KInterpretation: iron induces ferromagnetism ingraphite. Fe3C is actually named

as mineral composing the meteorite.

Cohenite (typical mineral of metheorites): Fe (Ni,Co)3C- antiferromagnetic or ferromagnetic, not counted in the calculations.Cementite: (also known in meteorites) Fe3C- ferromagnetic stronger than magnetite.

Cigarette ashes and magnetic meteorites


Thermal demagnetisation of ashes from Bulgarian cigarettes ”Shipka” (N.Jordanova, Journal of Magnetism and Magnetic Materials, 2006). “Camel” and “Marlboro” also studied…Fe2O3 - Fe3O4 - Fe3C reactions occur at temperatures higher than 1020K in processof cigarette burning

Fe3C

Fe3O4

Thermal demagnetisation of ”Magnetic meteorite”, J.M.D. Coey et al., Nature (2002)

Fe must have reacted with carbonat temperatures of meteorite fallingand impuct.

More carbon magnets?


-Talapatra et al, Phys.Rev.Lett., 2005: ferromagnetism in nitrogen irradiated Nanodiamonds. Commercial detonation nanodiamonds are always contaminated with metallic alloy catalyst. Impurity analysis not presented

-Proton irradiated graphite (Prof. Esquinazi group). Impurity analysis presented. Something to study…

-Carbon nanofoam (A.V.Rode, Phys.Rev.B, 2004 ). Impurity analysis presented. Ferromagnetism below 90K

-Chemically etched graphite (A.V.Mombru, PRB, 2005). Impurity analysis presented. Results not confirmed (J.M.D.Coey group), large Fe particles found in their samples in Umea (K. Han)

”Magnetic rubber” byA.Talyzin and A.Dzwilewski

Questions ?

“Ferrocarbon”: EU project started in 2005 to study “magnetic carbons”.

Not to be confused with:“Ferrocarbon”- a form of steel with a very high concentration of carbon (Wiktionary).

Increased stability of magnetic carbon to depolymerization?


Depolymerisation of ALL C60 polymers occurs below 600K(including photopolymers).

T.Makarova, Semiconductors, 2004

XRD shown in figure were taken from two different samples, both are not from Rh polymer!

Heated at 800K(2.5 GPa, 1125K)

Pristine polymer(2.5 GPa, 1050K)

Makarova (Semiconductors, 2004):“One of the samples lost only 2% of magnetization, and the X-ray diffraction patterns remained unchanged after annealing and corresponded as before to the rhombohedral phase of polymerized C60.” This sample (E17) was destroyed during heating due to oxygen leak (Prof. Esquinazi)

The same sample (E17) depolymerized at 2000C in 2001 (Makarova et all, Carbon).

In Nature (2001) depolymerization was reported below 700K

Source of iron? Synthesis details...


Toroid apparatus (Prof. Davydov group,Troitsk, Russia)

Parts of high pressure assemblage:Nb, Ta capsule, graphitic heater,BN cage, pressure medium-Catlenite(mostly CaCO3).

Unpublished impurity analysis ( Prof. Esquinazi, g/g)): E16-N (0.4%), Nb (329), Ca (571)

H7 (6 GPa, 1100K): Ta-3.24%, Ca (1%)Sample was reported once as ”intrinsically superconductive”

Capsules most likely to break at point ofC60 collapse (10% volume decrease)

Material of capsuleNb,Ta also carbide forming metals!

Extra-care against impurities?


Samples were split in 1998 by metallic tools and handled by metallic Instruments and even by unprotected magnet.

“The main impurities were two elements: calcium that was transferred from the hands of the experimentalist and iron that was introduced from the forceps and tools used in splitting the samples.” citation from T.Makarova (Semiconductors, 2004)

Supplementary materials on Nature web page show how samples were operated (until 2004!). The same works with rubber pieces(see www.magneticcarbon.com)

reaction of fe with c 60 at hpht conditions: formation of fe 3 c and implications for

Documents

dzwilewski umea university

rh polymer of c60 makarova

nature publication

original nature paper

nature samplesby r

magnetic domains

rphase b magnetic carbon

point of c60 collapse