Abstract. The aim of this paper is to present method to investigate the properties of magnetic fluids by means of small angle neutron scattering. Ferrofluids are dispersions of small, single-domain magnetic particles suspended in a fluid carrier. The neutron scattering methods have been largely used the last two decades for the determination of structural properties of magnetic liquids at microscopic level. There can be investigated the structure of the particle, the aggregation phenomena, the magnetic liquid dynamics, particle-surfactant interaction, surfactant liquid-base interaction and structure, magnetic behavior of the samples. Experiments on small angle neutron scattering were carried out on SANS instrument YuMO in function at IBR-2 high-pulsed reactor at the Fran

D Bica

D Hasegan

Gy Torok

L Rosta

L Vekas

M Balasoiu

M Ghenescu

M V Avdeev

V Ghenescu

V L Aksenov

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Romanian Reports in Physics, Vo. 56, No. 4, P. 601-607, 2004 
 
Structural Studies of Ferrofluids by Small-Angle Neutron Scattering 
 
M.Balasoiua,b, M.V.Avdeeva, V.L.Aksenova, V.Ghenescub, M.Ghenescub, Gy.Torokc, 
L.Rostac, D.Bicad, 
L.Vekasd, D.Haseganb 
aFrank Laboratory of Neutron Physics, Joint Institute for Nuclear Research, Dubna, 
Russia 
bInstitute of Space Sciences, Bucharest, Romania 
cResearch Institute for Solid State Physics and Optics, Budapest, Hungary 
dCenter for Fundamental and Advanced Technical Research, Timisoara, Romania 
 
(Received July 22, 2004) 
 
 
Abstract. The aim of this paper is to present method to investigate the properties 
of magnetic fluids by means of small angle neutron scattering. Ferrofluids are dispersions 
of small , single-domain magnetic particles suspended in a fluid carrier.  
The neutron scattering methods have been largely used the last two decades for 
the determination of structural properties of magnetic liquids at microscopic level. There 
can be investigated the structure of the particle, the aggregation phenomena, the magnetic 
liquid dynamics, particle-surfactant interaction, surfactant liquid-base interaction and 
structure, magnetic behavior of the samples.  
Experiments on small angle neutron scattering were carried out on SANS 
instrument YuMO in function at IBR-2 high-pulsed reactor at the Frank Laboratory of 
Neutron Physics, Joint Institute of Nuclear Research, Dubna, Russia. 
Keywords: Neutron, Ferrofluids, SANS, Magnetic particles 
 
INTRODUCTION 
 Colloidal suspensions of single domain magnetic particles of about 100Ö LQ
diameter, stabili zed with a surfactant shell i n a suitable liquid carrier, known as Ferro 
fluids [1], exhibit a remarkably rich variety of behaviors and their study remains very 
active [2]. To understand the fundamental reasons for the interesting macroscopic 
properties of these systems, it is necessary to know the structure of the colloidal particles 
at the microscopic level, and how this structure is influenced by the action of certain 
external parameters. One of the most powerful techniques, which were involved, is small -
angle neutron scattering (SANS). This is particularly well suited to the study of magnetic 
colloids, because of the peculiar features of their magnetic interaction with neutrons, 
which permits the determination of the colloidal dispersion structure under a wide variety 
of experimental conditions. There can be investigated the structure of the particle, the 
aggregation phenomena, the magnetic liquid dynamics, particle-surfactant interaction and 
the surfactant liquid-base interaction, magnetic dimension of the particles [3]. 
M. Balasoiu et al. 602 
We present our investigations on ferrofluids performed for several years at 
different small -angle faciliti es including the YuMO time-of-f light diff ract meter at the 
IBR-2 pulsed reactor at the Frank Laboratory of Neutron Physics, Joint Institute for 
Nuclear Research (Dubna, Russia), SANS instrument at VVR-SM steady-state reactor at 
the Research Institute of Solid State Physics and Optics (Budapest, Hungary).  
 
 
 
 
 
EXPERIMENTAL 
 
 The general theory of the scattering of thermal neutrons (see [4]) shows that the 
differential scattering cross section dΣ/dΩ, can be considered with good approximation, a 
function of momentum transfer, q. 
  dΣ(q)/dΩ = (1/V) Σkl bk bl< eiqrk e-iqrl >                    (1) 
 In the small angle neutron scattering experiment we measure dΣ/dΩ, the 
differential scattering as a function of q, the momentum transfer: 
   q = (4π / λ) sin(θ /2)     (2) 
where λ is the neutron’s wavelength and θ is the scattering angle. 
 From the relation (2) it can be seen that to obtain the dependence  dΣ(q)/dΩ  there 
can be measured or the scattering angle θ, or the wavelength λ, or both parameters. So, 
there can exists two possibiliti es to measure dΣ/dΩ: 
1) with q   variation, by the variation of angle θ for  λ  kept constant; 
2) with q  variation, by the variation  of  wavelength λ (meanning variation of the 
speed of the incident neutrons on the sample, for θ kept constant. 
 In our experiments we have used both these methods to measure dΣ(q)/dΩ, using 
a steady reactor (VVR-SM) and a pulsed source (IBR-2). Besides the high level 
characteristics of the neutron sources, also the used experimental instruments  (Fig.1 and. 
Fig.2) presented performant parameters as shown in Tab.1 and Tab.2. 
 
 
 
 
 
 
 
 
 
 
Fig.1 - YuMO SANS TOF diffractometer in function at IBR-2 reactor [5, 6]: 1) movable 
reflector; 2) moderator/cold moderator; 3)  
 
 
Studies of ferrofluids by neutron scattering 603 
chopper (adapted to cold moderator); 4)  first  collimator; 5) vacuum tube; 6)  second  
collimator; 7)  liquid bath thermostat; 8) V, graphite, H2O standards;  10 ) second circular 
detector (PSD) of thermal neutrons; 11) detector of direct beam;       12) first circular 
detector of thermal neutrons; 13) third circular detector of thermal neutrons. 
Studies of ferrofluids by neutron scattering                                          603 
 
The parameters characterizing the diffractometer are collected in the Table 1 
Table 1 
Monochromatization:  Time of f light method 
Used wavelength  0.5A to 8A   # 
Q-resolution Low, 5-20% 
Size of beam on the sample 8-22 mm2    ∑ 
Transferred momentum range:  7x10-3 - 0.5 A-1 
Flux on the sample (thermal 
neutrons) 
107-4x107 n/(s cm2) [1] 
Detectors He3 -fulfiled, home made preparation, 8 independent 
wires  
Detector (direct beam) 6Li-convertor (home made preparation)  
Sample environment Thermostatted sample changer, electromagnet, 
pressure camera 
 
 
 
 
 
 
 
 
 
 
 
 
Fig.2 - SANS diffractometer in function at VVR-SM steady reactor [7]. 
 
The parameters characterizing the diffractometer are collected in the Table 2: 
 
Table 2 
Monochromator:  Mechanical velocity selector   
Incident wavelength:  3 - 24 Å  
Wavelength spread:  usually 12 – 30%  
Maximum beam size at sample:  16 mm diameter  
Transferred momentum range:  0.004 - 0.5 Å-1 
 
 
 
Studies of ferrofluids by neutron scattering 605 
The studied ferrofluid was produced at the Laboratory of Magnetic Fluids, 
CFATR, Timisoara, Romania [8,9]. 
 
 
RESULTS AND DISCUSSIONS 
 
The contrast variation method, applied in a small angle scattering (SANS) 
experiment, was used to simultaneously obtain information about molecular and magnetic 
structure of diluted magnetite/C6D6 ferrofluids. There it was shown that the separate 
determination of the contribution of nuclear and magnetic interactions to scattering 
patterns may be accomplished even with unpolarized neutrons, i.e. in the absence of any 
perturbation produced by an external magnetic field, otherwise involved in the usual 
methods with polarized beams [10, 11]. 
The effect of the colloidal particle concentration on the structure of the 
magnetite/C6D6 ferrofluid stabili zed by oleic acid was investigated by SANS (Fig.3). The 
thickness of surfactant shell i s the key parameter, which determines the stabili ty of the 
fluids. It was shown that in the case of the magnetite/oleic acid/benzene ferrofluid the 
thickness of the surfactant layer (monolayer of oleic acid around magnetite particles) 
changes significantly when changing the magnetite concentration.  A significant decrease 
in the thickness of the surfactant layer with increase in the magnetite concentration was 
observed when the concentration of particles increases within an interval of 0.1-19 vol. 
%.  The magnetic scattering in the system was not taken into account, but the model 
curves fitted well to experimental ones, which points to a small effect of the magnetic 
scattering in the studied systems[12-15]. 
The situation is more complicated in ferrofluids based on polar solvents [13, 16-
18], in particular, water. 
 
 
 
1
0,01
0,1
1
10
a)
1.1%
3.8%
6.3%
9.5%
19%
I(
q
),
 c
m
-1
q, nm-1
 
 
Fig.3 - Experimental (points) and model (lines) scattering curves at different magnetite 
concentrations indicated to the right of the graph. Magnetic scattering was not taken into 
account when fitting. 
 
In the case of the magnetite/dodecylbenzenesulphonic acid/water ferrofluid the 
large interpenetration between sublayers of the double surfactant layer 
(dodecylbenzenesulphonic acid around magnetite particles) was detected which does not 
change much with the concentration [15]. 
M. Balasoiu et al. 606 
Further, it was also studied in this systems the magnetic scattering by the analysis of 
the anisotropic SANS patterns from ferrofluids in a magnetic field. The isotropic A(q) and 
anisotropic B(q) contributions to two-dimensional scattering patterns were separated 
according to the expression: 
I(q, ϕ) = A(q) + B(q) sin2ϕ, (3) 
where ϕ is the angle between the direction of the applied magnetic field and scattering 
vector[15,19].The specific anisotropy appears in the scattering and in the case of 
magnetization saturation the isotropic and anisotropic parts of the scattering correspond to 
nuclear and magnetic contributions, respectively. 
 The comparison of the nuclear and magnetic scattering from the benzene-based 
ferrofuid with the magnetite concentration cm=5 vol. % is presented in Fig.4. It was 
surprising that the B(q) function  is not fitted by the model of simple polydisperse spheres 
corresponded to the magnetic core of the colloidal particles in ferrofluids. The reason for 
this is unclear for us at the moment, which requires the further analysis and experiments. 
 
 
 
0,1 1
0,1
1
10
 A(q), H = 1.2 T
 B(q), H = 1.2 T 
I(
q
),
 c
m
-1
q, nm
-1
 
Fig.4 - A(q) and B(q) functions at saturation magnetization for the the benzene-based 
ferrofuid with the magnetite concentration cm=5 vol. %. In this case A(q) and B(q) 
correspond to nuclear and magnetic scattering, respectively. 
The analysis of the effect of interparticle interaction, as well as of the magnetic 
scattering, on the accuracy of the structural parameters require additional experiments and 
special preparation of samples. 
 
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Structural Studies of Ferrofluids by Small-Angle Neutron Scattering

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Structural Studies of Ferrofluids by Small-Angle Neutron Scattering

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