Fundamentals and Protection

– explained on the example of Sol-Gel derived Cu-Ni-Fe Nanoparticles –

Magnetic Nanoparticles

Arne Lüker, 16th of Oct. 2013

Jožef Stefan Institute, Ljubljana

arnelueker@aim.com

www.arne-lueker.de

1

Nanoparticles - Classification

2

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-zero-dimensional (0D) nano-objectall characteristics linear dimensions are in the

same order (not more than 100 nm)

Quasi-one-dimensional (1D) nano-objectnanorods and nanowires: one dimension

exceeds by an order of magnitude the two others, which are in the nano-range

Quasi-two-dimensional (2D) nano-objectNanodiscs: two dimensions are an order of magnitude greater than the third, which is in

the nano-range

Quasi-three-dimensional (3D) nano-objectComplex structures like this toroid in which at least one dimension is in the nanometer-range

Finite Size Effect

3

For face-centered cubic cobalt with a diameter of around 1.6 nm, about 60% of the total number of spins are surface spins! (*

single domain limit

20

18M

KAd

eff

crit

when dwM EE

A: exchange constantKeff: anisotropy constantµ0: vacuum permeabilityM: saturation magnetisationΔEM: magnetostatic EnergyEdw: domain-wall Energy

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 necessarily a »small« particle (as opposed to a »bulk« particle) as regards specific magnetic characteristics.

*) X. Batlle and A. Labarta; J. Phys. D 2002, 35, R15

Superparamagnetic Limit

4

2sin)( VKE eff SVeff Kd

KK6

Keff: effective anisotropy constantKV: bulk anisotropy constantKS: surface anisotropy constantV: particle volumeθ: magnetisation, easy axisµef: effective magnetic momentHS: magnetic field HS

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.

Consider: isolated single domain particle

The magnetic anisotropy energy per particle

The magnetisation is easily flipped

TkH BSef Energy at the saturation magnetisation

5.0 VS

NNS

Rule of thumb!

approx. when

For : superparamagnet.VKTk effB No hysteresis at

CTT

Size matters!

5

We want to have the particle really tiny!

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)!

Qualitative dependences of the coercive force HC on the particle diameter (*

*) A. Lüker: http://www.arne-lueker.de/Objects/work/Magnetic/nanoparticles.html

Protective Shell

6

Surface passivation by mild oxidationLet like be cured by like – Similia similibus curentur –, the (homoeopathic) law of similars

Strategies against oxidation by oxygen, or erosion by acid or base:

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 Coatinge.g. Au: low reactivity (air stable), can be functionalised with thiol groups, but direct coating is very difficult

Silica CoatingControllable (sol-gel) process but unstable under basic conditions, pores in silica through which oxygen and other species can diffuse

Carbon CoatingHigh 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 nanoparticlesEasy way to avoid agglomeration if isolated particles are not mandatory

Magnetic CoatingsExample: Fe2O3/Fe3O4 core with a Ni/Cu shell

The soft magnetic core provides a high saturation magnetisation and the relative hard magnetic shell ensures a high coercive force.

7

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 synthesis of Cu–Ni–Fe ferromagnetic nanocomposites by modified Sol–Gel(*

8

Aqueous solution ofNi(NO3)2∙6H2O

Aqueous solution ofCu(NO3)2∙3H2O

Stirring and pH-control (7.5-8.5) with 25% aqueous solution of ammonia

Stirring and heating at 70-80°Ctransparent green „Gel“

Mixing with 70% aqueoussolution of glycolic acid

Evaporation of Volatiles 70-80°C for 5 h

„Sol“

Aqueous solution ofFe(NO3)2∙9H2O

Calcination at 600°Cnanocomposite „Powder“

Cold pressing into form,Sintering at 800-900°C for 30 min., Final Bake at 1200°C for 4 h in air.

*) A. Lüker, Sol–gel route for ferromagnetic Cu-Ni-Fe nanocomposites, Research Notes 4159, 2009

Particle size: 20 … 150 nm

Dependence of the coercive force HC on the particle diameter of magnetic nanoparticles

9

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.

The state of the nanoparticle (superparamagnetic or blocked) depends on the measurement time now.

HC = 0. Magnetisation can randomly flip direction under the influence of temperature.

The Magnetisation of (Nano)-Ni-Cu-Fe

If  , 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 , 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.

flipmeas

flipmeas

Transition to single-domain particles entails in an increase of thermal fluctuations. HC decreases for d<dcr

τ flip : »flipping time« or time of thermal fluctuations

τ 0: time constant; 10−9…10−13 secτ meas: measurement time

Tk

VK

flipB

eff

e0

In the »Blocking Region«

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.

A factor of two in particle size can change the flipping time from 100 years to 100 nanoseconds!

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

T

dT

Tk

E

T

dTd

B

25

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.

0ln

measB

effb

k

VKTTk

VK

measflipB

eff

e0

10

For τ meas = 75 and τ 0 = 10-9 sec we find:

B

effb k

VKT

25

Rule of thumb!

: superparamagnet. (page 4)VKTk effB

11

Thanks!

More information on www.arne-lueker.de Cu-Ni-Fe Nanoparticle, SEM-based Digital Illustration