Store programmer
Faktaark
www.forskningsradet.no/nanomat
Rock Hard Memory Systems
Prosjekt: The nature and origin of natural magnetic nanoscale minerals
This project resulted in a proof of the original lamellar magnetism hypothesis.
This proof developed through the study of exchange bias in a natural magnetic
mineral intergrowth of ilmenite-exsolution lamellae in a hematite host. The
studied samples from Modum, Norway, show the largest exchange bias (1300
mT) ever measured in any material, man-made or natural.
Bakgrunn:
Naturally occurring nanoscale magnets are all around us. Before they can be manipulated,
we must develop a basic knowledge of how they are produced in nature, and why they are
stable. We have been studying natural mixed hematite-ilmenite (Fe2O3-FeTiO3) minerals
from Norway, because they exhibit an unexpectedly strong and stable remanent
magnetization. Through transmission electron microscopy (TEM) studies, we discovered that
these minerals contain exsolution lamellae, which are thinner than a single unit cell < 1.4
nm. By mapping of chemical elements, we found that sizes and patterns of subsequently
exsolved nanoscale lamellae (Fig. 1) are controlled by stranded metastable diffusion profiles
developed at moderate temperatures.
Figure 1. TEM images showing multiple generations of hematite exsolution lamellae,
from 1µm to 1-2nm in thickness, in an ilmenite host. Scale bar is 100nm.
A surprising result has been that the natural nanoscale lamellae have retained their magnetic
memory for far longer than any hard drive that will ever be needed, nearly 1 billion years.
The magnetic field needed to remove half of their memory is at least 75-120 mT (milliTesla), far above the Earth's magnetic field of 0.05 mT. Variability of this extreme magnetic
stability depends on the state of stress of the intergrowth, and on whether hematite or
ilmenite is the host mineral. Many of our natural samples stay magnetized to very high
temperatures, ~ 600oC or greater, depending on composition. Thus, such materials can have
extremely stable magnetic memories, while resisting very high temperatures, making these
attractive as magnetic data-storage media for space and security applications. Our project
combined magnetic-property measurements, nanoscale chemistry, atomic simulations, phase
relations and thermodynamics of the mineral system, to understand why these nanophases
are so stable, and to study the processes that formed them.
Mål:
This research was designed to gain understanding of the natural magnetic system provided
by nanoscale-exsolution features in the hematite-ilmenite solid solution series, termed
"lamellar magnetism". Gaining understanding is a crucial step before investigating
technological applications and included: 1) Determining lamellar size/magnetic domain size
that yields the predominant magnetic moment and highest coercivity; 2) Thermal stability;
3) Favorable chemical substitutions; 4) Direct imaging and/or proof of the nature of "contact
layers"; 5) Testing magnetic reorientation of natural nanomagnetic material; 6) Determining
properties of lamellar magnetism vs. crystallography in single crystals and possibilities for a
six-way memory system.
Resultater:
This project resulted in a proof of the original lamellar magnetism hypothesis presented by
Robinson et al., (Nature 2002) and (Robinson et al., 2004, 2006). This proof developed
through the study of exchange bias in a natural magnetic mineral intergrowth of ilmeniteexsolution lamellae in a hematite host. The studied samples from Modum, Norway, show the
largest exchange bias (1300 mT) ever measured in any material, man-made or natural.
(Nature Nanotechnology, McEnroe et al. 2007; Physics Reviews B., Harrison et al., 2007;
and Earth and Planetary Science Letters, Fabian et al., in press). Prior to this work, the
record for exchange bias was 950 mT, discovered in the late 1950s. Exchange bias in
synthetic thin-film materials is used in giant magnetoresistive heads, and is central in
modern data-storage technology. The thermal stability in the hematite-ilmenite system is
high, and our study of chemical substitutions to increase the temperature range for the
magnetic properties resulted in interesting findings which we continue to work with today.
TEM imaging of the lamellae, hosts, and associated contact layers was made on numerous
samples (McEnroe et al., 2004, 2005, 2007 a,b; Kasama et al., 2004; Robinson et al., 2006
a, b). By collaborations with crystallographers, mineralogists and material scientists we
greatly enhanced and developed our research. Experiments made at high temperature (953
K) and a pressure of 10 kbars directly lead to an understanding of stabilization of the
lamellae (McEnroe et al., 2004), and are important to understand the nature of magnetic
anomalies in the Earth’s crust. We also studied the low-temperature (200 K to 5 K) phase
diagram of the hematite-ilmenite system (Burton et al., in press). It reveals essential
fundamental properties, and has direct applications in space science. The hematite-rich end
of this diagram is of interest with our finding of extremely high magnetizations (> 200 A/m)
in large and magnetically extremely stable hematite grains (Schmidt et al., 2007). These
samples proved to a Ti-free hematite with a fine intergrowth of a cubic Fe-oxide, which had
its high magnetization pinned to the weaker host so that the large magnetization and
coercivity could be retained up to nearly 670oC. This work was highlighted by ESF, because
these samples are analogous to source rocks of the yet unexplained high magnetic anomalies
on Mars.
Nytteverdi og anvendelse:
We have studied the physical, chemical and magnetic properties of natural oxides showing
nanoscale exsolution of either hematite or ilmenite lamellae in a host of ilmenite or hematite.
These closely interlocked exsolutions have properties lacking in either of the two endmember minerals. The unusual magnetic properties measured: extreme stability, resistance
to high- and low-temperature demagnetization, and extremely large exchange bias, are all
very advantageous qualities for nanomagnets. Can we produce synthetic materials with
these special and highly desirable properties? Our next goal is to use our newly gained
understanding of chemical and magnetic properties of these materials to synthesize artificial
nanomagnets having similar properties to those found by this project in naturally occurring
rocks in Norway. The accomplished research has been published in Nature Nanotechnology,
Physics Reviews B, Earth and Planetary Science Letters, Journal of Geophysical Research,
Geophysical Research Letters, and other international journals.
Annen relevant informasjon:
This project has created a network between UK, Germany, Switzerland, Denmark, Australia,
USA, Russia and Norway, involving material scientists, geophysicists, physicists,
computational theoreticians and rock magnetists to unravel the magnetic properties of these
unusual nanomagnets with stable high temperature memories of nearly a billion of more
years. Web-related stories can be found at:
http://www.forskning.no/Artikler/2007/september/1190796254.98
http://www.geo365.no/nyheter/mars_magn/
http://www.esf.org/activities/eurocores/news/ext-news-singleview/article/what-makesmars-magnetic-298/news-browse/1.html
Sammendrag på norsk:
Flere års studier av naturlig nanomagnetisme i mineraler førte til teorien om at
magnetismen er bundet til grenseflaten mellom ilmenitt-lameller og hematitt. Gjennom
studien er hypotesen blitt bevist og akseptert. Den største ”exchange bias” - en
bestemt magnetisk egenskap - som noen gang er funnet i et naturlig eller syntetisk
materiale, ble også oppdaget i disse undersøkelsene. Oppgaven fremover er å
produsere denne robuste nanomagnetismen syntetisk ved å ta i bruk de helt spesielle
egenskapene som er funnet naturlig i mineralene.
163556/S10
Ansvarlig: Norges geologiske undersøkelser
01.06.2004 30.06.2007
Prosjektleder: Suzanne McEnroe
Kontaktperson: Suzanne McEnroe, Norges geologiske undersøkelser Adresse: 7491 Trondheim
Telefon: 73 90 44 05 E-mail: [email protected]
Lenker: www.ngu.no
http://www.forskning.no/Artikler/2007/september/1190796254.98
http://www.geo365.no/nyheter/mars_magn/ http://www.esf.org/activities/eurocores/news/ext-newssingleview/article/what-makes-mars-magnetic-298/news-browse/1.html
Samarbeidende institusjoner og bedrifter: National Institute of Standards and Technology (NIST),
Institute for Rock Magnetism ved universitetet I Minnesota, britiske Department of Earth Sciences
ved universitetet i Cambrigde, tyske Bayerisches Geoinstitut ved universitetet i Bayreuth og Max
Planck Institute for Metals Research, Russian Academy of Sciences og CSIRO, Australia