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2008/2/28

Magnetic atoms of gold, silver and copper have been obtained

An international team led by Physics and Chemistry teams from the Faculty of Science and Technology at the University of the Basque Country (UPV/EHU) and directed by Professor Jose Javier Saiz Garitaonandia, has achieved, by means of a controlled chemical process, that atoms of gold, silver and copper - intrinsically non-magnetic (not attracted to a magnet) - become magnetic. The article has been published in the February issue of the prestigious international magazine in nanotechnology, Nanoletters (Vol.8, No. 2, 661-667 (2008)).

According to the research, in which researchers from the UPV/EHU as well as teams from Australia and Japan have taken part, the magnetism appears reduce the dimensions of the material to nanometric dimensions and surround it with previously selected organic molecules. The magnetism of these nanoparticles is a permanent one (like iron) which, even at ambient temperature, is quite significant. This amazing behaviour has been obtained not just with gold (a phenomenon which had already been put forward as experimentally possible) but, in this research, nanoparticles of silver and copper (the atoms of which are intrinsically non-magnetic) with a size of 2 nm (0.000002 mm) have also been shown to be magnetic at ambient temperature.

The contribution of this work, part of the PhD of Ms Eider Goikolea Núñez and led by Professors Mr Jose Javier Saiz Garitaonandia and Ms Maite Insausti Peña, is not limited to obtaining these amazing magnetic nanoparticles. In fact, by means of complex techniques, using experimental systems based on particle accelerators and nuclear techniques, both in Japan and in Australia, have clearly shown for the first time that magnetism exists in atoms of gold, silver and copper, metals which, in any other condition, are intrinsically non-magnetic (a magnet does not attract them).

This discovery goes beyond the mere fact of converting non-magnetic elements to magnetic ones. These properties appear in smaller-sized particles that have never been seen in classical magnetic elements. In fact, they can be considered as the smallest magnets ever obtained. Moreover, such properties do not occur only at low temperatures but they are conserved, apparently without any degradation, at temperatures well above the ambient ones.

This work poses new questions as regards what have been the accepted up to now as the physical mechanisms associated with magnetism and opens the doors to interesting applications yet to be discovered, some of which are related to the use of magnetic nanoparticles for the diagnosis/treatment of illnesses. Likewise, this article is destined to be a point of no return for research into fundamental questions about magnetism.

Kimika, Fisika, Nanoteknologia Go to top of page
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2008/2/28 | Terry

Previously done

Some years ago, I was involved in outsourcing some electrical products to Chinese manufacturers. All of the contacts made out of electrical copper we received from that country could be picked up with a magnet.

2008/3/6 | Jose Javier Saiz Garitaonandia

Reply to "previously done"

Thank you, Terry, for your comment.
It is clear that, when an announcement of scientific result is published for non-specialist readers, most of the deep or specific details are omitted in the report. Only the most outstanding conclusions are commented. So, the general public can understand the importance of the advance without all the scientific details.

Background
Less than five years ago, a Canadian research group published a work titled “X-Ray Studies of the Structure and Electronic Behavior of Alkanethiolate-Capped Gold Nanoparticles: The Interplay of Size and Surface Effects” (Peng Zhang and T. K. Sham, Physical Review Letters 24 (2003) 245502). This is the first time (as far as I know) somebody probes and comments that the internal electronic distribution (including the 5d electrons) in the Au atoms can be radically changed by an interaction with an external capping. The authors do not say, but suggest, that that redistribution could imply a change in the intrinsic magnetic properties of the Au atoms of the fcc Au particle. In the work, authors use an element selective experiment as XPS or EXAFS which allows observing selectively the physic of a concrete element. One year latter, other Spanish group managed to synthesize the first Au nanoparticles with a permanent magnetic behaviour (Physical Review Letters 8 (2004) 087204) and, at the same time a Japanese one synthesized by similar procedure superparamagnetic Au nanoparticles (Physical Review Letters 11 (2004) 116801). Both papers represented great advances in the research of the physics of the magnetic nanoparticles. But there was a great difference among them. In the first one, the apparition of permanent magnetism in nanoparticles of only 100 atoms symbolized a breaking-off with the some basic concepts of the theory of magnetism accepted up to now. However, the characterization of the samples was completely macroscopic and the major criticism was, effectively, that the magnetic signal could come from other magnetic impurities. On the other hand, the superparamagnetism observed in the samples synthesized by the Japanese group was analyzed by XMCD technique, an element selective technique which permits to check the intrinsic magnetism of a selected atom. So, in this second work, even if the samples contained magnetic impurities, the intrinsic magnetism of the Au atoms was completely probed. On the other hand, this magnetism remained or could be explained by the frame of the accepted magnetic concepts.

Our group’s work
During the last three years we have been working on two physical concepts. First, when the number of atoms is reduced certain electronic levels can stay closer to the Fermi level. This would be the consequence of an electronic redistribution when the energy is minimized to stabilize the nanoparticle. Second, if a effective ligand is used, a charge transfer from even below the Fermi level could happen, changing, at the same time, the electronic structure of the elements or, what it is the same, their properties. At the end we got the optimal chemical-physical procedure to obtain permanent magnetic Au nanoparticles with magnetic moments of al least one order of magnitude higher that those previously published for any magnetic behaviour in any Au magnetic nanoparticle. One of our main contributions was to check by two different and independent element selective techniques the origin of the magnetism at this size scale. The first one was 197Au Mossbauer spectroscopy, a nuclear resonant technique, the second one XMCD at the L edges of the Au at the Spring-8 synchrotron in Japan. The combination of the results located unquestionably the magnetism on the Au atoms of the surface of the nanoparticles. This procedure was extended also to 4d10 and 3d10 electronically structured atoms, that is Ag and Cu. The d levels are completed or, what is the same, they are diamagnetic. In our new Ag and Cu nanoparticles permanent magnetism is also presented and, besides, with a collective magnetic behaviour. This is our second outstanding contribution.

Electrical copper
It is very known that electrical copper is formed by an alloy of several metals with different quantities depending of the quality of the material. Among these metals you can easily find Fe, Ni and Co which are intrinsically magnetic. It is also very known that Cu and, for example, Fe or Co are non-mixable, that is, it is not possible to get naturally a diluted sold solution of Fe or Co atoms in the fcc Cu structure. These atoms appear in small conglomerations which are essential for certain properties of this electrical Cu. The magnetism of this electrical Cu can be provided by these conglomerations. It is also known that a fcc CuFe solid solution can be obtained after a long high energy ball milling of the alloy. The alloy, in this case, is also magnetic with a fcc crystallographic structure but the magnetism continues residing in the Fe atoms. The Fe atoms are magnetically exchanged among the Cu atoms. A magnetic polarization in these Cu atoms can be possible but this would not be intrinsically resident in the Cu atoms. It is also very known that certain Cu oxide is antiferrimagnetic and, so, could be also macroscopically picked up by a magnet.

Criticism to the comment
First. I guess that the commentator is not a specialist in the subject. Pure bulk Cu is not magnetic at all, with a completed 3d electronic level. That fact can be checked in any well documented chemistry or physic book with a good periodic table.
Second. When the number of magnetic atoms is reduced down to certain quantity characterized to each magnetic phase, the material loses the ferromagnetic behaviour becoming superparamagnetic. The exact size or the exact quantity of atoms depends basically on the magnetic anisotropy of the material. The limit for the magnetic material with the highest anisotropy known would be around 4-5 nm at room temperature for independent or disperse nanoparticles.
Third. A effective rapprochement of the 3d electronic level to the Fermi level in pure Cu can be only achieved by decreasing the number of atoms in the nanoparticle which, at the same time, reduces considerable the consecution of a ferromagnetic behaviour according to the second point.
Fourth. In any case the commentator mentions the uses of element selective technique to identify the origin of the magnetism of his electrical Cu. He only mentions the effect of a magnet on bulk electrical cupper. So, the magnetism can come from any other element but Cu.
Fifth. If, effectively, magnetic Cu has been previously obtained, I earnestly recommend to the commentator the submission of the result in a scientific specialist publication where the details can be analyzed by independent and anonymous researchers previous the publication

I understand that comment has not been ill-intentioned. I also understand that the lack of deep knowledge on the subject of the commentator induces errors in the objective valuation of a research work. However, I sadly can not avoid that three years of intensive and high quality work can be obscured by a simple and unfortunate couple of sentences.

Dr. J.S. Garitaonandia

2008/3/7 | Terry

apology

I most humbly appologize for my comment. I did not consider that the language translation might cause any ill will between us; my comment was very much tongue-in-cheek, only refering to the fact that Chinese exported parts commonly contain a very high level of impurities. In this case, electrical copper for contacts, the presence of iron is absolutely unacceptable.

Again, I am sorry if I upset the author of this fine piece, and I meant no disrespect to the researchers for what is a real breakthrough.

2008/5/22 | Laura

Elementary School Teacher

Where can I find out about nanomagnetism, explained in a very simple, elementary way? Help. Congratulations on your breakthrough, I wish I could understand it better.

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