Twenty years ago, Kirschvink argued that

many paleomagnetic studies were limited by

the sensitivity of the magnetometer systems

then in use [Kirschvink, 1981]. He showed that

sedimentary rocks could preserve detrital

remanent magnetizations at levels of 10^(-14) to

10^(-15) Am^2, about 100-1000 times below the

noise level of today's best superconducting

(SQUID) rock magnetometers. If a more sensitive

magnetometer could be built, it would

dramatically expand the range and variety of

rock types amenable to paleomagnetic analysis.

Just such an instrument is now on the horizon:

the low-temperature superconductivity (LTS)

SQUID Microscope

Baudenbacher, Franz J.

Kirschvink, Joseph L.

Weiss, Benjamin P.

Wikswo, John P.

Caltech Authors - Main

Eos, Vol. 82, No. 44, October 30, 2001 
SECTION NEWS temperature difference between the sample and the coil requires shielding the coils with-in a dewar, heat-sunk radiation baffles, and 
many layers of superinsulation. Because they 
need to encircle both these structures and 
GEOMAGNETISM & 
PALEOMAGNETISM 
Editor: John W Geissman, University of New 
Mexico, Albuquerque, NM 87131 USA; 
Tel: +1-505-277-3433; Fax: +1-505-277-8843 
Section President, William Lowrie; Section 
Secretary, Steven C. Constable 
Magnetic Microscopy 
Promises a Leap 
in Sensitivity 
and Resolution 
PAGES 513,518 
Twenty years ago, Kirschvink argued that 
many paleomagnetic studies were limited by 
the sensitivity of the magnetometer systems 
then in use [ Kirschvink, 1981]. He showed that 
sedimentary rocks could preserve detrital 
remanent magnetizations at levels of 10·14 to 
10·1s Am2,about 100-1000 times below the 
noise level of today's best superconducting 
(SQUID) rock magnetometers. If a more sensi-
tive magnetometer could be built, it would 
dramatically expand the range and variety of 
rock types amenable to paleomagnetic analysis. 
Just such an instrument is now on the horizon: 
the low-temperature superconductivity (LTS) 
SQUID Microscope. 
The LTS SQUID Microscope is the end-product 
of more than a decade of work on the appli-
cation of high-resolution SQUID magnetometers 
for cardiology research by Franz Baudenbacher 
and John Wikswo of Vanderbilt University 
[ Wi"kswo, 1996]. Its current prototype, the Ultra 
High Resolution Scanning SQUID Microscope 
(UHRSSM), maps the vertical component of 
the magnetic field above the surface of a 
sample at room temperature and pressure. It 
achieves this with a spatial resolution of 250 1-1m 
and a moment sensitivity (i .e., minimum 
detectable dipole moment) 10,000 times that 
of the most recent 2G Enterprises® Super-
conducting Rock Magnetometer (2G® SRM). 
The art of LTS SQUID microscopy has now 
progressed to the point that the fields of samples 
placed within the vacuum region of some 
instruments can be measured with spatial 
resolutions of only several micrometers. 
Although magnetic force microscopes (MFMs) 
have better moment sensitivities than SQUID 
microscopes, the current generation of SQUID 
microscopes have field sensitivities 105 times 
better than MFMs and do not suffer from the 
sample-instrument interactions that plague 
MFM-sensing of magnetically soft materials. 
These interactions make MFMs difficult to use 
for quantitative measurements of the magnetic 
field above samples. Although it has -10,000 
times lower spatial resolution than a typical 
MFM, the UHRSSM is able to quantitatively map 
the magnetic fields of rock slices and thin 
sections at spatial resolutions 10-100 times 
better than that of the best existing SQUID 
rock magnetometers. It can therefore provide 
data with a resolution comparable to that of 
other common petrographic techniques such 
as optical and electron microscopy (Figure 1).This 
bridges the roughly six-orders-of-magnitude gap in 
spatial resolution between 2G® SRMs and 
MFMs,such that magnetization can be directly 
correlated with distinct minerals and textures. 
Both the UHRSSM and 2G® SRM use similar 
commercially-available DC-SQUIDs operating 
at or near their thermal noise limits. So how 
can the SQUID microscope achieve such 
superior sensitivity at such high spatial resolu-
tion? Much of the answer lies in the closed 
geometry of dipolar magnetic fields and how 
their detection depends upon instrument size. 
First, consider the 2G® 755 SRM (Figure 2a) . 
A room-temperature sample is inserted into 
the middle of several -8-cm diameter Helmholtz-
style pickup coils held at 4 K.The large 
the sample, the coils need to be rather large 
in size. Although large coils are better for 
detecting uniform magnetic fields, they are 
less sensitive to dipoles than are small coils 
because they encompass more of the sample's 
fringing fields oriented in the opposite sense 
to its magnetization (Figure 2a). The result is a 
Catch-22: once the geometry of the system is 
set by the coil diameter and the room-tempera-
ture access, the samples should be made as 
large as possible to achieve the optimum signal-
to-noise ratio. This reduces the effective spatial 
resolution of the system (which is set by the 
diameter of the coils) and means that the sen-
sitivity to small samples is far from optimal. 
Now, consider the UHRSSM (Figure 2b) . The 
major difference is that its pickup coil does 
not encircle the sample, but instead, is brought 
very close, within 100 !Jm.This is not an easy 
task, because the 250-!Jm diameter, 4 K coil is 
only separated from the room-temperature 
environment by a 30-!Jm thick sapphire win-
dow, such that the temperature gradient from 
coi l to sample is several million °C/m! Never-
theless, because the coil is so small, the cryo-
genic surface area that is exposed to room-
temperature thermal radiation is quite small. 
Thus, the thermal loads on the system are 
rather modest. The engineering feat of getting 
a 4 K SQUID to operate within 100 !Jm of a 
room-temperature rock eliminates the require-
ment that the coil encircle the sample, such 
1500 
1000 
500 
0 
-500 
-1000 
I 
-1500 
nT 
Fig. 1. Ultra High Resolution Scanning SQUID Microscope (UHRSSM) image of a 30-j.lm thin 
section of type CR2 carbonaceous chondrite GRA95229, showing the intensity of the out-of-the-
page component of the magnetic field as observed -100 J.lm above the sample. Numerous, ran-
domly oriented dipolar features associated with chrondrules and matrix are visible. Sample 
courtesy of H. Connolly Original color image appears at the back of this volume. 
that much less of the fringing field is encom-
passed (Figure 2b). 
Although the field sensitivity of the coil 
decreases as the coil is made smaller, the 
magnetic field produced by the dipole 
increases as I/r3 for distance r between the 
dipole and coil. If the coil with radius a is at a 
heigh,t a above the sample-which optimizes 
the tradeoff between sensitivity and spatial 
resolution-it is easy to show for a uniformly 
magnetized sample much smaller than a that 
the signal-to-noise ratio scales as roughly 1/a. 
A smaller coil brought closer to the sample 
produces cleaner signals. 
Now, if the coil diameter a is larger than the 
sample-to-coil distance r (as is the case for 
the UHRSSM and 2G® SRM), then the spatial 
resolution of the magnetometer is limited by 
the coil size. Thus, the dramatic increase in 
sensitivity resulting from the small size of the 
coil is accompanied by high spatial resolution. 
This may seem counter-intuitive, because for 
many laboratory instruments-for instance, 
spectrometers or microscopes-spatial resolu-
tion and sensitivity are often negatively 
correlated. But for the imaging of magnetic 
dipoles, the inverse relationship between sen-
sitivity and coil size in SQUID microscopy 
persists as long as the coil (and coil-to-sample 
distance) remains larger than the size of the 
magnetized region that is being targeted. 
The chief limitation of SQUID microscopy 
comes from the problem of inverting the data 
output-a spatial grid of vertical components 
of the magnetic field-into a three-dimensional 
vector magnetization pattern within the sample, 
the quantity usually desired by paleomagnetists. 
This problem is identical to that encountered 
in the inversion of aeromagnetic field data to 
a map of lithospheric magnetization. A similar 
problem is also encountered by those using 
MFMs and 2G® SRMs, which measure the spa-
tial derivative of the magnetic force field and 
net magnetization vector, respectively, of the 
sample. None of these instruments directly 
measures the spatially heterogeneous magne-
tization pattern within the target. All suffer 
from the difficulty that without other constraints, 
there is no unique magnetization pattern that 
can be derived from measurements of the mag-
netic field taken outside the magnetized region. 
In practice, this usually does not mean that 
the inversion is intractable, because many 
magnetization patterns can be-ruled out from 
other constraints. For instance, only certain 
magnetization intensities will be plausible 
given compositional constraints from electron 
microscopy, microprobe, and other techniques. 
Also, magnetizations with high amplitudes 
and spatial frequencies can often be rejected. 
Scanning electron microscopy and other 
textural data can be used to localize the 
boundaries of discrete grains, which are often 
unidirectionally magnetized, to further reduce 
the solution space. Measurements of all three 
components of the net magnetization of the 
sample with a 2G® SRM will further constrain 
the solution. 
Certain more general criteria can also be 
required of the magnetization solution. For 
Eos, Vol. 82, No. 44, October 30, 2001 
A 
8 em 
B 
4K 
298 K 
Fig. 2. (a) Schematic cross-section of a 2G Enterprises® Superconducting Rock Magnetometer, 
showing pair of Helmholtz coils (dashed) held at 4 K and enclosed by shielding. A room temper-
ature sample with a dipolar magnetization oriented upwards, represented as a current loop, is at 
the center of the coils. The large coils (-8 em diameter) encompass much of the downward, fring-
ing fields. (b) Schematic cross-section of sensing region of the SQUID Microscope, showing pick-
up coils (dashed) held at 4 K, separated from the same sample by a thin (30 f..im) sapphire 
window. The smaller coil (250 f..im diameter) encompasses less of the downward, fringing fields. 
Both diagrams are not to scale. 
instance, one can assume that the solution is 
composed of a grid of many regularly spaced 
fixed-location dipoles whose intensity and 
direction are allowed to vary (an "equivalent 
source" scheme). For samples with a modest 
number of discrete magnetized grains, the use 
of a three-axis SQUID microscope, which is 
currently in development, may also help alle-
viate the inverse problem. 
Initial collaborative work between the 
Cal tech paleomagnetism and Vanderbilt 
SQUID groups on 1-mm-thick slices and 
30-~Jm-thin sections of Martian meteorite 
ALH8400 I [Weiss et a!., 2000] has already 
demonstrated that SQUID microscopy will 
enable a whole new class of paleomagnetic 
analyses. Conglomerate, baked contact, and 
fold tests can be performed on extremely 
small spatial scales, vastly expanding the utility 
of these critical geological field tests of mag-
netic stability. A suite of rock-magnetic and 
paleomagnetic experiments can be done on 
individual grains in standard petrographic 
thin sections at very high rates, allowing the 
observed magnetic components to be matched 
with the minerals that are present. This is only 
the beginning. An improvement in instrumen-
tal sensitivity by only a factor of a few-like 
the magnificent Keck Telescopes, each with a 
collecting area four times that of the Hale 
Telescope-is usually considered a major 
advance that opens new research opportunities. 
A sudden increase in measuring technique by 
four orders of magnitude, accompanied by a 
one to two order-of-magnitude increase in 
spatial resolution, could be a revolution for 
paleomagnetism and rock magnetism. 
We are currently developing a second-gener-
ation LTS SQUID microscope to be located in 
the magnetically shielded clean laboratory at 
Caltech. Funds are being sought to make this 
new instrument open for use by the general 
paleomagnetic community, and within a few 
years these instruments should become com-
mercially available. 
Acknowledgments 
The development of the Vanderbilt LTS 
SQUID microscopes has been funded in part 
by grants from the National Science Founda-
tion and the National Institutes of Health. 
We thank W Goree for valuable discus'!>ions. 
Authors 
Benjamin ?Weiss, Division of Geological 
and Planetary Sciences, California Institute 
of Technology, Pasadena, USA; Franz J Bauden-
bacher and John PWikswo, Department of 
Physics and Astronomy, Vanderbilt University, 
Nashville, Tenn., USA; Joseph L. Kirschvink, 
Division of Geological and Planetary Sciences, 
California Institute of Technology, Pasadena, 
USA 
References 
Kirschvink, J. L., How sensitive should a rock magne-
tometer be for use in paleomagnetism?, in SQUID 
Applications to Geophysics, edited by H.Weinstock 
and W C. Overton, pp. 111-114,The Society of 
Exploration Geophysicists, Tulsa, Okla., 1981. 
Weiss, B. P, et al.,A low temperature transfer of 
ALH84001 from Mars to Earth, Science, 290, 
791-795,2000. 
Wikswo Jr.,J. P, High-resolution magnetic imaging: 
Cellular action currents and other applications, 
in SQUID Sensors: Fundamentals, Fabrication and 
Applications, edited by H. Weinstock, pp. 307-360, 
Kluwer Academic Publishers, The Netherlands, 
1996. 
Eos, Vol. 82, No. 44, October 30, 2001 
E A N 0 c 
S C I E N C E S 
Editor: Keith Alverson, PAGES International Project 
Office, Biirenplatz 2, CH-30 11, Bern, Switzerland; 
Tel: +41-31-312-3133; Fax: +41-31-312-3168; 
Section President, Robert A. Weller; Section 
Secretaries, Alan C Mix, Linda E. Duguay, Paula 
G. Coble, Newell Garfield 
SCOR Announces 
New Activities 
PAGES 514,520 
Roger Revelle had many good ideas during 
his long and productive career. One of them 
came to fruition in 1957 in the form of the Sci-
entific Committee on Oceanic Research (SCOR), 
which the International Council for Science 
created as its first interdisciplinary body, to 
promote international activities in oceanogra-
phy. Revelle served as SCOR's first president 
from 1957 to 1960. SCOR offers opportunities 
for scientists from different countries to coop-
erate in planning and executing international 
programs in ocean sciences. Over its 44 years 
in existence,SCOR has sponsored 120 work-
ing groups and has actively participated in 
many of the major international oceanographic 
projects. Thirty-six nations presently participate 
as SCOR members. 
SCOR Working Groups 
National committees and other organizations 
propose working group activities to SCOR to 
focus the best expertise worldwide on ocean 
science issues. Working group meetings are 
often supplemented with additional workshops 
and special sessions at meetings of scientific 
societies to increase involvement of the inter-
national ocean science community. Such activ-
ities usually result in articles in peer-reviewed 
journals [ e.g.,Millero, 2000], special issues of jour-
nals [e.g.,Hollingworth,2000],or books [Turner 
and Hunter,2001]. Information about current 
SCOR working groups and other SCOR activi-
ties is accessible through the SCOR Web site 
(http://www.jhu.edu/-scor). Several new 
working groups have been formed in 2001: 
Standards for the Survey and Analysis of 
Plankton, Quantitative Ecosystem Indicators 
for Fisheries Management, and Marine Phyto-
plankton and Global Climate Regulation: 
The Phaeocystis spp. Cluster as a Model. 
Large-scale Oceanographic Programs 
SCOR plays a leading role in planning 
longer-term, large-scale research programs 
designed to study the role of the ocean in 
global change and the effects of global change 
on the ocean. For example, SCOR was instru-
mental in developing and sustaining the inter-
national Joint Global Ocean Flux Study 
(JGOFS) and the Global Ocean Ecosystem 
Dynamics (GLOBEC) project. Now both are 
co-sponsored by SCOR and the International 
Geosphere-Biosphere Programme (IGBP), 
while GLOBEC is also co-sponsored by the 
Intergovernmental Oceanographic Commission 
(IOC). Likewise, SCOR is cooperating with the 
IOC in the Global Ecology and Oceanography 
of Harmful Algal Blooms program, which 
recently published a science plan for global 
research on harmful algal blooms. IGBP's 
Phase II will be implemented on January 1, 2003, 
and will feature integrated projects of terrestri-
al, oceanic, and atmospheric research, as well as 
interface projects on land-atmosphere, ocean-
atmosphere, and land-ocean interactions. 
Two of these new projects are being planned 
and implemented jointly by SCOR and IGBP: 
• Surface Ocean-Lower Atmosphere Study 
(SOLAS)-Processes at and beyond the air-
sea interface govern the transfer of chemical 
species, momentum, and energy between the 
ocean and atmosphere. More accurate and 
precise knowledge of the magnitude and tem-
poral variability of such transfers is needed to 
develop a predictive understanding of global 
change, including climate change. SO LAS will 
focus on understanding biogeochemical and 
physical interactions of the uppermost layer 
of the ocean (0-200 m) and the portion of 
the atmosphere above the ocean surface (to 
about 1 km). SOLAS will serve as IGBP II's 
ocean-atmosphere interface project. Professor 
Peter Liss of the University of East Anglia is 
chairing the international SOLAS Scientific 
Steering Committee. The World Climate 
Research Programme and Commission on 
Atmospheric Chemistry and Global Pollution 
(CACGP) of the International Association for 
Meteorology and Atmospheric Sciences are 
also involved in SOLAS. More information 
about SOLAS can be found at www.ifm.uni-
kiel.de/ch/solas/main.html. 
• The Future of Ocean Research in Earth 
System Science-SCOR and IGBP are cooper-
ating to develop a new framework for future 
ocean research in Earth system science. The 
new framework will build on the results of 
JGOFS and other programs, interface with 
ongoing projects (that is, GLOBEC, SO LAS, 
and the Land-Ocean Interactions in the 
Coastal Zone project), and address new 
research questions. This activity will serve as 
the integrated marine project of IGBP II and 
may result in one or more new projects or 
augmentations of existing projects. Professor 
Peter Burkill of the Plymouth Marine Labora-
tory in the United Kingdom is chairing the 
first phase of this activity. More information 
about it can be found on the SCOR Web site. 
SCOR has gone through a major transition 
in the past year, with the retirement of Elizabeth 
Gross from the SCOR Executive Director 
position and appointment of a new SCOR 
president, Robert Duce, and Secretary, Julie 
Hall. Gross was succeeded by Ed Urban, who 


Magnetic Microscopy

Promises a Leap

in Sensitivity

and Resolution

https://authors.library.caltech.edu/36542/1/Kirschvink_2001p513.pdf

Magnetic Microscopy Promises a Leap in Sensitivity and Resolution

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