Low temperature behavior of magnetic domains observed
using a magnetic force microscope
S. H. Chung
a)
Department of Physics, University of Maryland, College Park, Maryland 20742
and Laboratory for Physical Sciences, College Park, Maryland 20740
S. R. Shinde, S. B. Ogale, T. Venkatesan, and R. L. Greene
Department of Physics, University of Maryland, College Park, Maryland 20742
M. Dreyer and R. D. Gomez
Department of Electrical and Computer Engineering, University of Maryland, College Park, Maryland 20742
and Laboratory for Physical Sciences, College Park, Maryland 20740
A commercial atomic force microscope/magnetic force microscope ͑MFM͒ was modified to cool
magnetic samples down to around 100 K under a high vacuum while maintaining its routine imaging
functionality. MFM images of a 120 nm thick La
0.7
Ca
0.3
MnO
3
film on a LaAlO
3
substrate at low
temperature show the paramagnetic-to-ferromagnetic phase transition. Evolution of magnetic
domains and magnetic ripples with decreasing temperature are also observed near the edge of a 20
nm thick patterned Co film on a Si substrate. © 2001 American Institute of Physics.
͓DOI: 10.1063/1.1360262͔
INTRODUCTION
Low temperature magnetic force microscopy ͑MFM͒ has
been a recent interest for the study of low temperature mag-
netism such as the ferromagnetic phase transition of colossal
magnetoresistive ͑CMR͒ materials
1
and vortex structures of
high T
c
superconductors.
2–4
Operating at low temperatures is
very useful because it increases sample stability and de-
creases piezo creep. Most of the samples become more rigid
and less affected by the lateral motion of the MFM tip at low
temperatures. The signal-to-noise ratio also improves as ther-
mal fluctuation and phonon scattering is reduced. We can
thus investigate the physical properties which may be
masked by thermal excitation such as magnetic quantum tun-
neling or the magnetic anisotropy of small particles that are
superparamagnetic at room temperature. There is also a great
need to develop an instrument that is highly adaptable and
whose operation can be performed routinely and has less
restriction on the sample preparation.
There are some intrinsic problems in the instrumentation
of low temperature atomic force microscopy ͑AFM͒.Itis
very important to keep the sample free from contaminants
since materials with high freezing points would be frozen to
screen the real features of the sample surface. Therefore a
high vacuum or a water-free environment is critical for the
low temperature MFM experiment. Additionally, isolating
the vibration from a mechanical noise source such as boiling
liquid nitrogen or liquid helium and preventing the thermal
drift of piezo are also important.
In this work we report the conversion of a commercial
AFM/MFM for use in low temperature experiments. We
demonstrate its capability by imaging magnetic structures of
a perovskite manganite and a patterned thin Co film on a Si
substrate at low temperatures.
INSTRUMENTATION AND EXPERIMENT
The commercial MFM used in this work is a Nanoscope
III from Digital Instruments,
5
in standard ͑Interleave™͒
mode. The MFM measures the topographic and magnetic
information in two successive scans. Both commercial and
homemade MFM tips were used in our experiments.
Figure 1 shows the schematic diagram of our low tem-
perature AFM/MFM system. The commercial MFM assem-
bly fits in a vacuum chamber 30.5 cm high and 20.3 cm in
diameter. The system consists of a pumping–purging part, a
vacuum measurement part, cold finger feedthrough, and elec-
tric feedthrough. The tube chamber can be pumped down to
6ϫ10
Ϫ7
Torr using a turbomolecular pump and a copper
cold trap. The vacuum chamber is sealed with Viton O-rings.
The temperature of the samples can be set from 100 up
to 350 K by a temperature controller. The sample holder is
attached on top of the tube piezo of the MFM by a small
magnet embedded inside the piezo. Magnetic samples are
attached on top of the sample holder, which is connected to
fine copper braids as a heat flow conduit. The other end of
a͒
Electronic mail: chungsh@glue.umd.edu FIG. 1. Schematic diagram of the low temperature AFM/MFM system.
JOURNAL OF APPLIED PHYSICS VOLUME 89, NUMBER 11 1 JUNE 2001
67840021-8979/2001/89(11)/6784/3/$18.00 © 2001 American Institute of Physics