Magnetoresistive telegraph noise in Langmuir-Blodgett films of colloidal magnetite nanocrystals
as seen via scanning tunneling microscopy
Einat Tirosh,
1
Boris Tsukerman,
1
Nurit Taub,
1
Sara A. Majetich,
2
and Gil Markovich
1,
*
1
School of Chemistry, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv 69978, Israel
2
Department of Physics, Carnegie-Mellon University, Pittsburgh, Pennsylvania 15213, USA
Received 6 August 2009; revised manuscript received 12 November 2009; published 31 December 2009
Temperature-dependent fluctuations in the local current passing through close-packed magnetite nanocrystal
NC films were probed by scanning tunneling microscopy. This phenomenon, which peaked near the blocking
temperature T
b
, reflects spin-polarized tunneling fluctuations due to NC magnetization switching events. The
current exhibited telegraph noise patterns, switching between low and high states. Above T
b
both states
occurred with equal probability while below it the high current state dominated, which is consistent with a
superferromagnetic ground state where the NC moments are aligned.
DOI: 10.1103/PhysRevB.80.224427 PACS numbers: 75.75.+a, 68.37.Ef, 73.50.Td, 75.25.+z
I. INTRODUCTION
Assemblies of single domain magnetic nanocrystals
NCs are an important test bed for studies of strongly inter-
acting dipolar systems.
1,2
Their temperature-dependent mag-
netization switching dynamics
3,4
has been related to the mi-
croscopic details of spin-glass transitions,
5–8
and their large
interaction domains have been ascribed to dipolar ferro-
magnetism.
9
The collective magnetic properties are usually
studied on macroscopic scales using magnetometry
3,4,10,11
and other ensemble averaging techniques.
5,7,12,13
Single-
particle measurements have been made on isolated particles
deposited randomly onto a micro-superconducting quantum
interference device micro-SQUID.
14
Spin-polarized scan-
ning tunneling microscopy SP-STM has been applied to
ferromagnetic metal islands deposited in ultrahigh vacuum
on nonmagnetic metals
15,16
and even used to study magneti-
zation switching dynamics in such islands.
17
Typically, such
islands are only weakly interacting and their magnetization is
primarily influenced by the individual magnetic dot aniso-
tropy properties. Collective magnetic behavior has been im-
aged by magnetic force microscopy,
18
electron holography,
19
and x-ray photoemission electron microscopy,
2
which
showed magnetic domains extending over multiple NCs in
the assembly. Here we describe how the local collective dy-
namics of NC assemblies are revealed by the noise in the
tunneling current measured using STM.
Magnetite Fe
3
O
4
is a half metal, with a high degree of
spin polarization at its Fermi level, as confirmed by
magnetotransport experiments on colloidal Fe
3
O
4
NC
assemblies.
20,21
In that work magnetoresistance MR values
up to 10–25 % were obtained around the blocking tempera-
ture 200 K at moderate magnetic fields and were attrib-
uted primarily to interparticle tunneling MR due to the de-
pendence of MR on temperature and bias voltage. The bulk
form of magnetite has been studied by SP-STM in the pio-
neering work of Wiesendanger et al.
22
Interestingly, several
studies of bulk magnetite surfaces reported on the insulating
properties of these surfaces.
23–25
However, the work on our
magnetite NCs, both on scanning tunneling spectroscopy of
single NCs Ref. 26 and on multiple NC arrays
20
consis-
tently showed that the tunneling density of states of these
NCs is high around zero bias the Fermi level and that an
insulating gap opens only below the Verwey transition,
which occurred at 100 K for these NCs.
26
In the present
work we probed the local fluctuations in current passing
through a close-packed Fe
3
O
4
NC film using a nonmagnetic
STM tip.
The basic experimental concept is illustrated in Fig. 1a.
When tunneling current is measured between a metal tip and
a bare gold substrate, the current noise is influenced by shot
noise, junction instabilities, and instrumental noise, but is
relatively low and sets the baseline noise of this experiment
part 1 of Fig. 1a. When current noise is measured over a
magnetic particle in a monolayer thick film over a conduc-
tive nonmagnetic substrate the current noise is expected to be
similar to the baseline STM junction noise part 2 of Fig.
1a. In this case, the nonmagnetic tip emits unpolarized
electrons and the single magnetic particle acts as a spin filter,
but since the original distribution of electron spins is isotro-
pic, to a first approximation, the passing fraction of polarized
electrons would be constant irrespective of the magnetization
orientation of the NC. However, when the tunneling current
passes through at least two Fe
3
O
4
NCs, acting as two indi-
vidual spin filters, it is sensitive to the relative magnetization
orientations of the two or more NCs. The tunneling con-
ductance is roughly proportional to the square of the spin-
polarization level at the Fermi energy of the neighboring
magnetic NCs and to the cosine of the angle between their
magnetic moments.
23
When the temperature is tuned to a
magnetization-switching rate measurable by the STM elec-
tronics, near T
b
, then a modulation in the tunneling current
passing through 2 particles may be detected, as shown in
part 3 of Fig. 1a, providing that the interparticle resistance
is not negligible compared to the tip-sample gap resistance.
The characteristic amplitude and rate of this modulation de-
pend on the temperature, the particle size, the local, time-
varying magnetostatic interaction fields, and the interparticle
resistances relative to particle-tip and particle-substrate re-
sistances.
II. EXPERIMENT
Details of the Fe
3
O
4
NC film preparation method are
given in Ref. 21. In brief, magnetite NCs were synthesized
PHYSICAL REVIEW B 80, 224427 2009
1098-0121/2009/8022/2244275 ©2009 The American Physical Society 224427-1