Magnetic Nanoparticles Fundamentals and Protection – explained on the example of Sol-Gel derived Cu-Ni-Fe Nanoparticles – Arne Lüker, 16th of Oct.
Download ReportTranscript Magnetic Nanoparticles Fundamentals and Protection – explained on the example of Sol-Gel derived Cu-Ni-Fe Nanoparticles – Arne Lüker, 16th of Oct.
Slide 1
Magnetic Nanoparticles
Fundamentals and Protection
– explained on the example of Sol-Gel derived Cu-Ni-Fe Nanoparticles –
Arne Lüker, 16th of Oct. 2013
Jožef Stefan Institute, Ljubljana
[email protected]
www.arne-lueker.de
1
Slide 2
Nanoparticles - Classification
Quasi-zero-dimensional (0D) nano-object
all characteristics linear dimensions are in the
same order (not more than 100 nm)
Quasi-one-dimensional (1D) nano-object
nanorods and nanowires: one dimension
exceeds by an order of magnitude the two
others, which are in the nano-range
M. A. Purvis et al.: Relativistic plasma nanophotonics for ultrahigh energy density physics, Nat.
Photonics, online 1. September 2013; DOI: 10.1038/nphoton.2013.21
Quasi-two-dimensional (2D) nano-object
Nanodiscs: two dimensions are an order of
magnitude greater than the third, which is in
the nano-range
Quasi-three-dimensional (3D) nano-object
Complex structures like this toroid in which at
least one dimension is in the nanometer-range
2
Slide 3
Finite Size Effect
single domain limit
d crit 18
A:
Keff:
µ0:
M:
ΔEM:
Edw:
exchange constant
anisotropy constant
vacuum permeability
saturation magnetisation
magnetostatic Energy
domain-wall Energy
A K eff
0 M
2
when
E M E dw
If the sample size is reduced, there is a
critical volume below it costs more
energy to create a domain wall than to
support the external magnetostatic
energy of the single-domain state.
dcrit= 10…20 – 100…200 nm
Fe 20 nm, Co 70 nm, Fe3O4 130 nm, γ-Fe2O3 170 nm
The term »single-domain« does not require a necessary uniform
magnetisation throughout the whole particle bulk but only implies the
absence of domain walls. In addition, a single-domain particle is not
For face-centered cubicnecessarily
cobalt with aa »small«
diameterparticle
of around
nm, about
of the
total number
of
(as1.6
opposed
to a60%
»bulk«
particle)
as regards
(*
spinscharacteristics.
are surface spins!
specific magnetic
*)
X. Batlle and A. Labarta; J. Phys. D 2002, 35, R15
3
Slide 4
Superparamagnetic Limit
Consider: isolated single domain particle
The magnetic anisotropy energy per particle
E ( ) K eff V sin
2
K eff K V
Energy at the saturation magnetisation
ef H S k B T
Keff:
KV:
KS:
V:
θ:
µef:
HS:
effective anisotropy constant
bulk anisotropy constant
surface anisotropy constant
particle volume
magnetisation, easy axis
effective magnetic moment
magnetic field HS
6
d
KS
The energy barrier KeffV separates the
(two) energetically equivalent easy
directions of magnetisation.
With decreasing particle size, the
thermal energy kBT exceeds the energy
barrier KeffV.
The magnetisation is easily flipped
For k B T K eff V : superparamagnet.
approx. when
NS
N
S
V
No hysteresis at
T TC
0 .5
Rule of thumb!
4
Slide 5
Size matters!
We want to have the particle really tiny!
Qualitative dependences of the coercive force HC on the
particle diameter (*
Easy theoretical treatment
Benefits of the »quantum size effect«, e.g.
unusual high magnetisation (per atom), no
hysteresis below Curie or Néel temperatures,
(superparamagnetism) etc. …
But:
Surface atoms are chemically very active
Small particles tend to agglomerate
We need a protective shell around
the nanoparticle (core)!
*)
A. Lüker: http://www.arne-lueker.de/Objects/work/Magnetic/nanoparticles.html
5
Slide 6
Protective Shell
Strategies against oxidation by oxygen, or erosion by acid
or base:
Surface passivation by mild oxidation
Let like be cured by like – Similia similibus curentur –, the (homoeopathic) law
of similars
Surfactant and Polymer Coating (ferrofluids)
The magnetic attraction of nanoparticles is weak enough that the surfactant's
Van der Waals force is sufficient to prevent magnetic clumping or
agglomeration. Problem: not stable at air or/and high temperatures
Precious-Metal Coating
e.g. Au: low reactivity (air stable), can be functionalised with thiol groups, but
direct coating is very difficult
Silica Coating
Controllable (sol-gel) process but unstable under basic conditions, pores in
silica through which oxygen and other species can diffuse
Carbon Coating
High chemical and thermal stability, biocompatibility. Nanoparticles stay in
their metallic state and have a higher magnetic moment. Problem:
agglomeration and formation of clusters.
Matrix-Dispersed magnetic nanoparticles
Easy way to avoid agglomeration if isolated particles are not mandatory
6
Slide 7
Magnetic Coatings
Example: Fe2O3/Fe3O4 core with a Ni/Cu shell
The rhombohedral α-Fe2O3 (hematite) is antiferromagnetic (soft magnetic)
at temperatures below 950 K, while above the Morin point (260 K) it exhibits
so-called ›weak‹ ferromagnetism. All Fe3+ ions have an octahedral
coordination.
The
cubic spinel Fe3O4 (magnetite) is ferrimagnetic (soft magnetic) at
temperatures below 858 K.
Ferromagnetic
(hard magnetic) Ni–Cu forms a face-centred-cubic (fcc)
structure with giant magnetoresistance and magnetic properties over the entire
composition range. It has a variety of properties including high strength,
corrosion resistance and good wear resistance, which make it a perfect
protective coating.
The exchange coupling across the
antiferromagnetic/ferrimagnetic – ferromagnetic interface
provides an extra source of anisotropy leading to
magnetisation stabilisation.
The soft magnetic core provides a high saturation magnetisation and
the relative hard magnetic shell ensures a high coercive force.
7
Slide 8
The synthesis of Cu–Ni–Fe ferromagnetic nanocomposites by
modified Sol–Gel(*
Aqueous solution of
Aqueous solution of
Aqueous solution of
Ni(NO3)2∙6H2O
Cu(NO3)2∙3H2O
Fe(NO3)2∙9H2O
Mixing with 70% aqueous
solution of glycolic acid
Stirring and pH-control (7.5-8.5) with
25% aqueous solution of ammonia
Evaporation of Volatiles
70-80°C for 5 h
„Sol“
Stirring and heating at 70-80°C
transparent green „Gel“
Calcination at 600°C
nanocomposite „Powder“
Cold pressing into form,
Sintering at 800-900°C for 30 min.,
Final Bake at 1200°C for 4 h in air.
Particle size: 20 … 150 nm
*) A. Lüker, Sol–gel route for ferromagnetic Cu-Ni-Fe nanocomposites, Research Notes 4159, 2009
8
Slide 9
The Magnetisation of (Nano)-Ni-Cu-Fe
As the particle decreases, the number of domains decreases,
and the role of interdomain boundaries in magnetisation reversal
becomes less pronounced, The coercive force HC increases with
a decrease in d.
Transition to single-domain particles entails in an increase of
thermal fluctuations. HC decreases for d
HC =
0. Magnetisation can randomly flip direction under the
influence of temperature.
Dependence of the coercive force HC on the
particle diameter of magnetic nanoparticles
K eff V
τ flip :
τ 0:
τ meas:
flip
0e
k BT
»flipping time« or time of
thermal fluctuations
time constant; 10−9…10−13 sec
measurement time
The state of the nanoparticle (superparamagnetic or
blocked) depends on the measurement time now.
If meas flip , the nanoparticle magnetisation will flip several times
during the measurement, then the measured magnetisation will
average to zero and the nanoparticle will appear to be in the
superparamagnetic state.
If meas flip , the magnetisation will not flip during the measurement,
so the measured magnetisation will be what the instantaneous
magnetisation was at the beginning of the measurement. The
nanoparticle will appear to be “blocked” in its initial state.
9
Slide 10
In the »Blocking Region«
K eff V
flip
meas 0 e
k BT
Tb
K eff V
k B ln meas
0
For τ meas = 75 and τ 0 = 10-9 sec we find:
Tb
In experimental studies in the »Blocking Region«,
sharp changes in magnetisation are never observed,
because a size spread (and, generally, other types of
spread) always exists for the particles. Small
particles pass into the superparamagnetic state
earlier than large particles and the magnetisation
jump is blurred.
25 k B
Rule of thumb!
Tb, the average temperature of particle transition into the
superparamagnetic state, corresponds to the maximum
distribution of all particles over the volume V.
Even for absolutely identical particles, the flipping time
increases smoothly rather than by a jump, although
rapidly. Under the same conditions as before, we find
for the relation
k B T K eff V : superparamagnet. (page 4)
dT E
T k BT
d
A factor of two in particle size can change the
flipping time from 100 years to 100
nanoseconds!
K eff V
dT
25
T
i.e., in the »Blocking Region« the relative
change in the flipping time is 25 times as fast
as the relative change in the temperature.
10
Slide 11
Thanks!
More
informationSEM-based
on www.arne-lueker.de
Cu-Ni-Fe
Nanoparticle,
Digital Illustration
11
Magnetic Nanoparticles
Fundamentals and Protection
– explained on the example of Sol-Gel derived Cu-Ni-Fe Nanoparticles –
Arne Lüker, 16th of Oct. 2013
Jožef Stefan Institute, Ljubljana
[email protected]
www.arne-lueker.de
1
Slide 2
Nanoparticles - Classification
Quasi-zero-dimensional (0D) nano-object
all characteristics linear dimensions are in the
same order (not more than 100 nm)
Quasi-one-dimensional (1D) nano-object
nanorods and nanowires: one dimension
exceeds by an order of magnitude the two
others, which are in the nano-range
M. A. Purvis et al.: Relativistic plasma nanophotonics for ultrahigh energy density physics, Nat.
Photonics, online 1. September 2013; DOI: 10.1038/nphoton.2013.21
Quasi-two-dimensional (2D) nano-object
Nanodiscs: two dimensions are an order of
magnitude greater than the third, which is in
the nano-range
Quasi-three-dimensional (3D) nano-object
Complex structures like this toroid in which at
least one dimension is in the nanometer-range
2
Slide 3
Finite Size Effect
single domain limit
d crit 18
A:
Keff:
µ0:
M:
ΔEM:
Edw:
exchange constant
anisotropy constant
vacuum permeability
saturation magnetisation
magnetostatic Energy
domain-wall Energy
A K eff
0 M
2
when
E M E dw
If the sample size is reduced, there is a
critical volume below it costs more
energy to create a domain wall than to
support the external magnetostatic
energy of the single-domain state.
dcrit= 10…20 – 100…200 nm
Fe 20 nm, Co 70 nm, Fe3O4 130 nm, γ-Fe2O3 170 nm
The term »single-domain« does not require a necessary uniform
magnetisation throughout the whole particle bulk but only implies the
absence of domain walls. In addition, a single-domain particle is not
For face-centered cubicnecessarily
cobalt with aa »small«
diameterparticle
of around
nm, about
of the
total number
of
(as1.6
opposed
to a60%
»bulk«
particle)
as regards
(*
spinscharacteristics.
are surface spins!
specific magnetic
*)
X. Batlle and A. Labarta; J. Phys. D 2002, 35, R15
3
Slide 4
Superparamagnetic Limit
Consider: isolated single domain particle
The magnetic anisotropy energy per particle
E ( ) K eff V sin
2
K eff K V
Energy at the saturation magnetisation
ef H S k B T
Keff:
KV:
KS:
V:
θ:
µef:
HS:
effective anisotropy constant
bulk anisotropy constant
surface anisotropy constant
particle volume
magnetisation, easy axis
effective magnetic moment
magnetic field HS
6
d
KS
The energy barrier KeffV separates the
(two) energetically equivalent easy
directions of magnetisation.
With decreasing particle size, the
thermal energy kBT exceeds the energy
barrier KeffV.
The magnetisation is easily flipped
For k B T K eff V : superparamagnet.
approx. when
NS
N
S
V
No hysteresis at
T TC
0 .5
Rule of thumb!
4
Slide 5
Size matters!
We want to have the particle really tiny!
Qualitative dependences of the coercive force HC on the
particle diameter (*
Easy theoretical treatment
Benefits of the »quantum size effect«, e.g.
unusual high magnetisation (per atom), no
hysteresis below Curie or Néel temperatures,
(superparamagnetism) etc. …
But:
Surface atoms are chemically very active
Small particles tend to agglomerate
We need a protective shell around
the nanoparticle (core)!
*)
A. Lüker: http://www.arne-lueker.de/Objects/work/Magnetic/nanoparticles.html
5
Slide 6
Protective Shell
Strategies against oxidation by oxygen, or erosion by acid
or base:
Surface passivation by mild oxidation
Let like be cured by like – Similia similibus curentur –, the (homoeopathic) law
of similars
Surfactant and Polymer Coating (ferrofluids)
The magnetic attraction of nanoparticles is weak enough that the surfactant's
Van der Waals force is sufficient to prevent magnetic clumping or
agglomeration. Problem: not stable at air or/and high temperatures
Precious-Metal Coating
e.g. Au: low reactivity (air stable), can be functionalised with thiol groups, but
direct coating is very difficult
Silica Coating
Controllable (sol-gel) process but unstable under basic conditions, pores in
silica through which oxygen and other species can diffuse
Carbon Coating
High chemical and thermal stability, biocompatibility. Nanoparticles stay in
their metallic state and have a higher magnetic moment. Problem:
agglomeration and formation of clusters.
Matrix-Dispersed magnetic nanoparticles
Easy way to avoid agglomeration if isolated particles are not mandatory
6
Slide 7
Magnetic Coatings
Example: Fe2O3/Fe3O4 core with a Ni/Cu shell
The rhombohedral α-Fe2O3 (hematite) is antiferromagnetic (soft magnetic)
at temperatures below 950 K, while above the Morin point (260 K) it exhibits
so-called ›weak‹ ferromagnetism. All Fe3+ ions have an octahedral
coordination.
The
cubic spinel Fe3O4 (magnetite) is ferrimagnetic (soft magnetic) at
temperatures below 858 K.
Ferromagnetic
(hard magnetic) Ni–Cu forms a face-centred-cubic (fcc)
structure with giant magnetoresistance and magnetic properties over the entire
composition range. It has a variety of properties including high strength,
corrosion resistance and good wear resistance, which make it a perfect
protective coating.
The exchange coupling across the
antiferromagnetic/ferrimagnetic – ferromagnetic interface
provides an extra source of anisotropy leading to
magnetisation stabilisation.
The soft magnetic core provides a high saturation magnetisation and
the relative hard magnetic shell ensures a high coercive force.
7
Slide 8
The synthesis of Cu–Ni–Fe ferromagnetic nanocomposites by
modified Sol–Gel(*
Aqueous solution of
Aqueous solution of
Aqueous solution of
Ni(NO3)2∙6H2O
Cu(NO3)2∙3H2O
Fe(NO3)2∙9H2O
Mixing with 70% aqueous
solution of glycolic acid
Stirring and pH-control (7.5-8.5) with
25% aqueous solution of ammonia
Evaporation of Volatiles
70-80°C for 5 h
„Sol“
Stirring and heating at 70-80°C
transparent green „Gel“
Calcination at 600°C
nanocomposite „Powder“
Cold pressing into form,
Sintering at 800-900°C for 30 min.,
Final Bake at 1200°C for 4 h in air.
Particle size: 20 … 150 nm
*) A. Lüker, Sol–gel route for ferromagnetic Cu-Ni-Fe nanocomposites, Research Notes 4159, 2009
8
Slide 9
The Magnetisation of (Nano)-Ni-Cu-Fe
As the particle decreases, the number of domains decreases,
and the role of interdomain boundaries in magnetisation reversal
becomes less pronounced, The coercive force HC increases with
a decrease in d.
Transition to single-domain particles entails in an increase of
thermal fluctuations. HC decreases for d
HC =
0. Magnetisation can randomly flip direction under the
influence of temperature.
Dependence of the coercive force HC on the
particle diameter of magnetic nanoparticles
K eff V
τ flip :
τ 0:
τ meas:
flip
0e
k BT
»flipping time« or time of
thermal fluctuations
time constant; 10−9…10−13 sec
measurement time
The state of the nanoparticle (superparamagnetic or
blocked) depends on the measurement time now.
If meas flip , the nanoparticle magnetisation will flip several times
during the measurement, then the measured magnetisation will
average to zero and the nanoparticle will appear to be in the
superparamagnetic state.
If meas flip , the magnetisation will not flip during the measurement,
so the measured magnetisation will be what the instantaneous
magnetisation was at the beginning of the measurement. The
nanoparticle will appear to be “blocked” in its initial state.
9
Slide 10
In the »Blocking Region«
K eff V
flip
meas 0 e
k BT
Tb
K eff V
k B ln meas
0
For τ meas = 75 and τ 0 = 10-9 sec we find:
Tb
In experimental studies in the »Blocking Region«,
sharp changes in magnetisation are never observed,
because a size spread (and, generally, other types of
spread) always exists for the particles. Small
particles pass into the superparamagnetic state
earlier than large particles and the magnetisation
jump is blurred.
25 k B
Rule of thumb!
Tb, the average temperature of particle transition into the
superparamagnetic state, corresponds to the maximum
distribution of all particles over the volume V.
Even for absolutely identical particles, the flipping time
increases smoothly rather than by a jump, although
rapidly. Under the same conditions as before, we find
for the relation
k B T K eff V : superparamagnet. (page 4)
dT E
T k BT
d
A factor of two in particle size can change the
flipping time from 100 years to 100
nanoseconds!
K eff V
dT
25
T
i.e., in the »Blocking Region« the relative
change in the flipping time is 25 times as fast
as the relative change in the temperature.
10
Slide 11
Thanks!
More
informationSEM-based
on www.arne-lueker.de
Cu-Ni-Fe
Nanoparticle,
Digital Illustration
11