Seminarul National de Nanostiinta si Nanotehnologie

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Seminarul National de Nanostiinta si
Nanotehnologie
MAGNETIC NANOSTRUCTURES, MULTILAYERS with GIANT
MAGNETO-RESISTANCE (GMR) and TUNNEL
MAGNETO-RESISTANCE (TMR)
Jenica NEAMTU, ICPE-CA, Advanced Research
Institute for Electrical Engineering Bucharest
Jenica NEAMTU, ICPE-CA, Advanced Research
Institute for Electrical Engineering Bucharest
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MAGNETIC NANOSTRUCTURES, MULTILAYERS with GIANT
MAGNETO-RESISTANCE (GMR) and TUNNEL
MAGNETO-RESISTANCE (TMR)
Giant Magneto-Resistance is the subject of a
huge international research effort due to the
numerous technological applications. The largest
is the spin-electronics. Other applications are
the high density data storage, on-chip GMR
sensors for diverse as solid-state compasses,
electrical sensors, automotive sensors. GMR
and TMR effects has been observed in multilayered
nanostructures of the form FM (Å)/NM(Å)/FM(Å),
which FM is a transition-metal ferromagnetic
layer (Fe,Co,Ni or alloys) and NM is a non
ferromagnetic metal (Cr, Cu, Ag etc.) or an
isolating (tunnel) barrier in case of Tunnel
Magneto-Resistance Effect.
The band structure in a ferromagnetic metal
is exchange split, so that the density of states
is not the same for spin up and down electrons at
the Fermi level. Fermi's golden rule states that
scattering rates are proportional to the density
of states, so the scattering rates are different
for electrons of different spin.
Jenica NEAMTU, ICPE-CA, Advanced Research
Institute for Electrical Engineering Bucharest
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MAGNETIC NANOSTRUCTURES, MULTILAYERS with GIANT
MAGNETO-RESISTANCE (GMR) and TUNNEL
MAGNETO-RESISTANCE (TMR)
The two-current model, it is assumed that
there are two independent conduction channels,
corresponding to the spin-up and spin-down
electrons. These two channels are characterized
by different mean free paths and
resistivities. Spin-up electrons (with spin
parallel to magnetization) are scattering very
weakly in comparison with the spin-down
electrons. The first magnetic layer allows
electrons in only one spin state to pass easily -
if the second magnetic layer is aligned then that
spin channel can easily pass through the
structure, and the resistance is low. If the
second magnetic layer is misaligned then neither
spin channel can get through the structure easily
and the electrical resistance is high.
The GMR effectively measures the difference
in angle between the two magnetizations in the
magnetic layers. Small angles (parallel
alignment) gives a low resistance, large angles
(antiparallel alignment) gives a higher
resistance. It is easy to produce the state where
the two magnetic layers are parallel - simply
apply a field large enough to magnetically
saturate both layers.

• Jenica NEAMTU, ICPE-CA, Advanced Research
  Institute for Electrical Engineering Bucharest

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MAGNETIC NANOSTRUCTURES, MULTILAYERS with GIANT
MAGNETO-RESISTANCE (GMR) and TUNNEL
MAGNETO-RESISTANCE (TMR)
Spin valves are dedicated GMR systems, in which
the electrical resistance is high or low,
depending on the direction rather than the
strength of the magnetic field. The change in
resistance is typical in the range of 5 to 10.
Contrary to a GMR multilayer the two
ferromagnetic layers are magnetically decoupled.
This is achieved by increasing the thickness of
the space layer. As a further difference the
magnetization of one of the ferromagnetic layers
is spatially fixed (''pinned) by an
antiferromagnetic layer. Thus it is called the
"pinned layer", the other is called the "free
layer", because it should easily follow the
external magnetic field. There are two
different ways to create a antiferromagnetic
pinning layer. It is an "artificial
antiferromagnet" (AAF) in contrast to "natural
antiferromagnets" such as chromium or manganese.
In AAFs the alignment can be changed in high
fields at any time, which makes the production of
such sensors easy and cheap.

• Jenica NEAMTU, ICPE-CA, Advanced Research
  Institute for Electrical Engineering Bucharest

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MAGNETIC NANOSTRUCTURES, MULTILAYERS with GIANT
MAGNETO-RESISTANCE (GMR) and TUNNEL
MAGNETO-RESISTANCE (TMR)
The objectives of our projects are the
synthesis of GMR, TMR thin films multilayer
structures and studies of the magneto-transport
properties correlated with bulk or interface
electronic scattering process. The GMR of
ferromagnetic/nonmagnetic/ferromagnetic
multilayer are dependent of the thickness of thin
films, the roughness and the nature of thin film
interlayer. The magnetic properties and the
magnetoresistance in correlation with
microstructural properties of permalloy layers
and NiFe(t)/Cu(s)/NiFe(t)n and
NiFe(t)/Mo(s)/NiFe(t) multilayers have been
investigated. In this presentation are
considered the samples 1) Si/SiO2/(Permalloy Ni
80Fe20) monolayer films 2) Si/SiO2/Py (10
nm)/Cu (4 nm)/Py(10 nm) in which Py is Ni80Fe20
3) Si/SiO2/Py(10 nm)/Mo(6 nm)/Py(10 nm) 4)
Si/SiO2/Py(10 nm)/Cu (4 nm)9/Py(10 nm).
The thickness (t) of NiFe layers was ranged from
4 to 12 nm, while the copper and molybdenum
layers was ranged from 3 to 8 nm.

• Jenica NEAMTU, ICPE-CA, Advanced Research
  Institute for Electrical Engineering Bucharest

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MAGNETIC NANOSTRUCTURES, MULTILAYERS with GIANT
MAGNETO-RESISTANCE (GMR) and TUNNEL
MAGNETO-RESISTANCE (TMR)
The magnetoresistance effect measurements
were performed at room temperature in four-point
contact geometry with the contacts in line, using
a DC current of 10 mA. The magnetoresistance (MR)
is defined as the variation ?R(R0-RH) of the
resistance due to magnetic field normalized by
the resistance R0 at zero magnetic field
MR?R/R0.
Fig. 1. Magnetic properties of GMR thin film
multilayers, Jenica Neamtu, M. Volmer Surface
Science 482-485 (2001) p 1010-1014
Fig. 2. Giant Magneto-Resistance of NiFe(10nm)/
Cu(4nm)/NiFe(10nm) multilayer,deposited in
magnetic field, J.Neamtu, M.Volmer, A.Coraci,
THIN SOLID FILMS 343-344 (1999) p.218-221

• Jenica NEAMTU, ICPE-CA, Advanced Research
  Institute for Electrical Engineering Bucharest

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MAGNETIC NANOSTRUCTURES, MULTILAYERS with GIANT
MAGNETO-RESISTANCE (GMR) and TUNNEL
MAGNETO-RESISTANCE (TMR)
The magnetoresistance effect, MR, (fig.4) is
performed on Py(10 nm)/Mo (6 nm)/Py(10 nm)
multilayer with the magnetic field in the film
plane. The effect is very small because of small
coupling between the magnetic layers with
molybdenum spacer. However one can be observed
the effect of spin-dependent scattering of the
electrons.
Fig. 3. Magnetic properties of Py/Mo/Py thin
film multilayers, Jenica Neamtu, M. Volmer
Surface Science 482-485 (2001) p 1010-1014
Fig. 4. MR effect performed on Py/Mo/Py thin
film multilayers, Jenica Neamtu, M. Volmer
Surface Science 482-485 (2001) p 1010-1014

• Jenica NEAMTU, ICPE-CA, Advanced Research
  Institute for Electrical Engineering Bucharest

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MAGNETIC NANOSTRUCTURES, MULTILAYERS with GIANT
MAGNETO-RESISTANCE (GMR) and TUNNEL
MAGNETO-RESISTANCE (TMR)
The above experimental results for NiFe(10
nm)/Cu(4 nm)9/NiFe(10 nm) multilayers show that
GMR performance is associated with a low
roughness and a sufficiently small average grain
size, directly influencing the amount of grain
boundary scattering.
Fig. 6. 3D AFM image of NiFe(10nm)/Mo(6nm)/
NiFe(10nm) trilayer. The average roughness is
8.5384 nm. Jenica Neamtu, M. Volmer Surface
Science 482-485 (2001) p 1010-1014
Fig. 5. 3D AFM image of NiFe(10nm)/Cu(4 nm)9/
NiFe(10nm) multilayer. Average roughness is
6.4265 nm Jenica Neamtu, M. Volmer Surface
Science 482-485 (2001) p 1010-1014

• Jenica NEAMTU, ICPE-CA, Advanced Research
  Institute for Electrical Engineering Bucharest

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MAGNETIC NANOSTRUCTURES, MULTILAYERS with GIANT
MAGNETO-RESISTANCE (GMR) and TUNNEL
MAGNETO-RESISTANCE (TMR)
For the first time we have performed Hall
effect characterisation of multilayers for
various orientations ?? of thin film regarding
the direction of the applied magnetic field.
In this case Hn is responsible of Hall effect and
Hp is responsible of MR effect.
Field dependencies of the output voltage, U, for
various orientations ?? of the Py (10 nm)
regarding the applied magnetic field. In insert
we present the same dependence for ??0 in low
magnetic field (b) A simulation of magnetic
distributions of remnant state starting from the
the saturated state array of 4x4 single domains
(100x100x10 nm3 each ). ?45º in order to
maximize the AMR effect. Jenica Neamtu, M.
Volmer, MRS Fall Meeting, Symposium R 5.5, Boston
2002, publ. in Journal of Materials Research
vol.746 2003

• Jenica NEAMTU, ICPE-CA, Advanced Research
  Institute for Electrical Engineering Bucharest

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MAGNETIC NANOSTRUCTURES, MULTILAYERS with GIANT
MAGNETO-RESISTANCE (GMR) and TUNNEL
MAGNETO-RESISTANCE (TMR)
From Hall effect and magnetoresistance
measurements on thin films we can make some
assumptions regarding the reversal proceses that
take place in the film. For ??0?, a relatively
large transition region suggests a reversal
process mainly due to domain wall motion. For
??gt10? the magnetisation reversal will only take
place by coherent rotation.

• Jenica NEAMTU, ICPE-CA, Advanced Research
  Institute for Electrical Engineering Bucharest