Magnetodielectric and magnetoviscosity response of a ferromagnetic liquid crystal at
low magnetic fields
Rasmita Sahoo, M. V. Rasna, D. Lisjak, A. Mertelj, and Surajit Dhara
Citation: Applied Physics Letters 106, 161905 (2015); doi: 10.1063/1.4918995
View online: http://dx.doi.org/10.1063/1.4918995
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APPLIED PHYSICS LETTERS 106, 161905 (2015)

Magnetodielectric and magnetoviscosity response of a ferromagnetic liquid
crystal at low magnetic fields
Rasmita Sahoo,1 M. V. Rasna,1 D. Lisjak,2 A. Mertelj,2 and Surajit Dhara1,a)1

School of Physics, University of Hyderabad, Hyderabad 500046, India
J. Stefan Institute, Jamova cesta 39, Ljubljana SI-1001, Slovenia

2(Received 21 January 2015; accepted 13 April 2015; published online 23 April 2015)
We report on experimental studies of the viscoelastic, magnetodielectric, and magnetoviscosity
properties of ferromagnetic liquid crystals (LCs) prepared by dispersing ferromagnetic
nanoparticles in a thermotropic LC. Both the splay elastic constant and rotational viscosity of theferronematic LCs are found to be considerably lower than that of the pure LC and advantageous to
the device applications. The ferromagnetic LCs show unique magnetodielectric andmagnetoviscosity response at very low magnetic fields that are useful for smart fluid applications.
C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4918995]
V

Ferrofluids are liquids prepared by suspending nanometersized ferromagnetic particles in isotropic carrier fluids. The
magnetic attraction of nanoparticles is weak enough that theentropic forces of the particles and steric and/or electrostatic
repulsion due to the surfactant prevents magnetic clumping or
agglomeration. The magnetic moments of the nanomagnets arealigned along the direction of the external magnetic field
resulting in chain formation due to the dipole-dipole interaction. As a result, the viscosity of the ferrofluids increases
significantly and these materials are useful for various magnetorheological applications.1–3 Long ago, Brochard and deGennes theoretically predicted various interesting properties ofanisotropic ferrofluids that can be prepared by dispersing
different types of ferromagnetic nanoparticles in nematic liquid
crystals (NLCs). They provided a continuum theory of magnetic suspensions in NLCs and so contrived the term as ferronematics.4 In spite of several attempts in the past, only very
recently the successful experimental realization of a ferromagnetic liquid crystalline material was reported by Mertelj et al.5
They prepared colloidal suspension of platelet type ferromagnetic nanoparticles in pentyl cyanobiphenyl (5CB) liquid
crystal at room temperature. Subsequently, they reported onmagnetooptic and converse magnetoelectric effects in thesematerials and described the system by a simple macroscopic
theory.6
Nematic liquid crystals exhibit anisotropic physicalproperties. The colloidal dispersion of nanoparticles (withlow concentration) in liquid crystals does not affect the overall molecular orientation. Nevertheless, the small particles
affect significantly the anisotropic physical properties that
depend on the size, shape, concentration, and properties of
the nanoparticles.7–18 The colloidal dispersion of nanoparticles in liquid crystals combines the physical properties ofnanoparticles and the orientational order of liquid crystals.
There are a few reports on the physical measurements ofliquid crystal nanocomposites based on ferromagnetic nanoparticles. The experimental results showed that the nanocomposites are paramagnetic.18,19 The recent discovery ofunambiguous ferromagnetic liquid crystals by Mertelj et al.5a)Electronic mail: sdsp@uohyd.ernet.in

0003-6951/2015/106(16)/161905/4/$30.00

has created immense interest in the scientific community. A
suspension of ferromagnetic nanoplatelets in a NLC can be~ and n^, where M~
macroscopically described by coupled M
describes the density of magnetic moments. The coupling is
a result of the interaction of the NLC with the surface of the~ directly responds to magnetic and n^ to elecnanoplatelets. M~ also indirectly
tric fields. However, due to their coupling, M
responds to electric field and inversely n^ to small magnetic
fields. In this letter, we report on the measurements of splay
elastic constant, rotational viscosity, magnetodielectric, andthe magnetoviscosity response of ferromagnetic LCs. We
show that the viscoelastic properties are considerably lower
than the pure sample and unique magnetodielectric and magnetoviscosity response at very low magnetic fields.
We used 4-cyano-40 -octylbiphenyl (8CB) liquid crystal
obtained from Sigma-Aldrich. It exhibits the following phasetransitions: I 38.8  C N 32.5  C SmA 21.5  C Cr. We prepared isotropic suspension of scandium-doped barium
hexaferrite single-crystal (BaFe11.5Sc0.5O19) magnetic nanoplatelets in isopropanol. The thickness of the platelets isabout 5 nm, the distribution of the platelet diameter isapproximately 70 nm, and the standard deviation is 38 nm.
The details of the preparation of the magnetic nanoplatelets
and preliminary characterization are reported in Ref. 20. The
preparation of ferronematic sample and the experimental
techniques are briefly mentioned in the supplementarymaterial.21
Figure 1(a) shows the TEM image of the nanoplatelets.
Figure 1(b) shows the physical appearance of the suspensions in the nematic phase. The sample appears reddish withincreasing concentration of the nanoparticles. Figures 1(c)and 1(d) show the photomicrographs of a planar cellobtained from polarising optical microscope. The director(the average alignment direction of the LC molecules) isaligned uniformly along the rubbing direction with no evidence of large agglomeration of the platelets. A schematicorientation of the magnetic nanoplatelets together with the
LC director and cross-section of disclination lines is also
shown in Fig. 1(e). The direction of magnetization is perpendicular to the plane of the nanoplatelets. The LC director isanchored perpendicular to the nanoparticle’s surface and106, 161905-1

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Appl. Phys. Lett. 106, 161905 (2015)

FIG. 1. (a) TEM image of the nanoplatelets. (b) Magnetic nanoplatelets suspended in 8CB liquid crystal at threedifferent concentrations: 0 wt. %,0.3 wt. %, and 0.6 wt. % at 35  C. (c)Photomicrographs of aligned cell of aferromagnetic suspension (0.6 wt. %magnetic nanoplatelets) under opticalpolarized microscope, polariser (P) andanalyser (A) are crossed and the rubbing direction (R) parallel to P (d) the
rubbing direction is at 45 with respect
to P and A. (e) A schematic presentation of the director, n^ (black) and themagnetic field (green) around theplatelets (red and vertical short-lines).
Black dots represent cross-section ofdisinclination lines. Photomicrographs(c) and (d) are taken in the absence ofmagnetic field.

forms a stable suspension with a macroscopic magnetization
along the director.5
First, we present and discuss some physical properties
measured in the absence of magnetic field. Figure 2(a) shows
the temperature dependence of parallel (jj) and perpendicular (?) components of dielectric constant. It is observed thatthe dielectric anisotropy D (¼ jj – ?) remains unaffectedby the incorporation of nanoparticles. For example, in all the

FIG. 2. Temperature variation of (a) parallel (jj) and perpendicular (?)components of dielectric constant, (b) splay elastic constant (K11), and (c)rotational viscosity (c1) in the nematic phase. Vertically downward arrows
indicate the decrease of respective properties with increasing concentrationof magnetic nanoparticles. Cell thickness 13.1 lm.

samples, D ’ 8. The dielectric anisotropy is proportional to
the scalar order parameter S of a nematic LC.22 This result
suggests that the magnetic nanoparticles do not disturb nematic order significantly. Figure 2(b) shows the temperaturedependence of splay elastic constant (K11). K11 was meas22
ured directly
frompﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
ﬃ the Frederiks threshold voltage, i.e.,Vth ¼ K11 =o D, o being the permittivity of the free space.
It is observed that K11 decreases (Fig. 2(b)) with increasing
concentration of magnetic nanoparticles. For example, at aparticular temperature (35  C), K11 in the suspension (with
0.6 wt. % magnetic nanoparticles) decreases by 22% compared to the pure LC. The rotational viscosity (c1) was
measured from the measurement of relaxation time (so) in aplanar cell using a phase-decay-time measurement technique23,24 (see supplementary material). The temperature
variation of c1 is shown in Fig. 2(c). It is observed that c1
decreases with increasing concentration of magnetic nanoparticles. For example, at 35  C, c1 (0.6 wt. % nanoparticles)
is reduced by 11% compared to the pure nematic LC. In themean field theory,22,25 both K11 / S2 and c1 / S2. The
decrease of S in the suspensions (0.6 wt. % magnetic nanoparticles), according to dielectric anisotropy measurement isless than 4%. Hence, the decrease of K11 and c1 cannot be
due to the decrease of S alone as both decrease more thanthat expected from the decrease of S.
To study the magnetodielectric effect, the sample wasfilled in a planar cell and the magnetic field was applied per~ n . Figure 3(a) shows
pendicular to the director (^n ), i.e., B?^
the variation of effective dielectric constant (eff) with magnetic field in pure 8CB. eff increases rapidly beyond theFrederiks threshold magnetic field (’128 mT) (Fig. 3(a)) andtend to saturate at higher field as expected. The magnetic
field dependence of eff in the suspensions is significantly
different. The cells were filled in the absence of external
magnetic field, so initially they were magnetically polydomain state. During the first measurement (Fig. 3(b)), we
observed that eff increases slightly with magnetic field up to
4 mT, and beyond this field it increases steeply. Finally, it
saturates above the magnetic field of 10 mT. The saturation

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FIG. 3. (a) Variation of effective dielectric constant (eff) with magnetic field(B) of pure 8CB at 36  C. (b) Variation of eff with magnetic field of a few
samples with different concentrations: 0 wt. % (blue spheres), 0.3 wt. % (redsquares), and 0.6 wt. % (green hexagons). Upward and downward arrows
indicate the data collected during increasing and decreasing of the magnetic
field. (Inset) Magnetodielectric response from the same sample in the second
measurement. Cell thickness of 58 lm. Applied voltage of 0.6 V and frequency of 3.111 kHz.

in eff is a signature of a magnetically monodomain state. In
the decreasing field, it shows hysteresis in the dielectric constant. It is due to the transformation from a polydomain to a
monodomain state. However, due to the domain wall pinningat the surface imperfections, this transformation is not complete; as a result, we also observe the hysteresis in the successive measurements. The second measurement was
performed 20 min after the first one. During this time, thevalue of eff (at 1 mT) is reduced from 10 to 8, which showsthat the additional magnetic relaxation of the sample
occurred (e.g., slow motion of domain walls). In the second
measurement, eff steeply increases with the external field asexpected in a monodomain sample (inset of Fig. 3(b)). In thedecreasing field, the curve is almost identical to the first measurement. The time lapse between two successive measuringAppl. Phys. Lett. 106, 161905 (2015)

points in both measurements was 30 s. The first measurement
thus shows the growth of magnetic domains, which is governed by the motion of domain walls and during this measurement the sample is almost transformed from polydomain
to a monodomain state. In pure 8CB, n^ is directly coupled to~ whereas in the suspensions the coupling between n^ and B~
B,~ directly induces
is indirect. The external magnetic field B~ and since M~ is coupled toreorientation of magnetization M~ indirectly causes a reorientation of n^.
the orientation of n^; B
The latter results in the increase of eff. As explained in Ref.
6, the reorientation of n^ is smaller than the reorientation of~ and so the saturated value of eff in the suspensions isMsmaller (Fig. 3(b)) than in the pure 8CB (where orientationof n^ is directly coupled with the external field) (Fig. 3(a)).
We observe that the saturated value of eff in the suspensions
with higher concentration of platelets is larger (Fig. 3(b)).
This is expected, since the saturated value depends on the~ and n^, which is larger for higher density
coupling between Mof the nanoplatelets.
Magnetoviscosity was measured by using a rheometer(Anton Paar MCR-501) with parallel plates having diameterof 20 mm (MRD-170) and plate-gap of 0.2 mm. The directionof the magnetic field is perpendicular to the plane of theplates. A schematic experimental setup of the magnetoviscosity measurement is shown in Fig. 4. Figure 5 shows the variation of shear viscosity with magnetic field. In pure 8CB, theshear viscosity (geff) at zero magnetic field is 25 mPa s and it
is comparable to the Miesowicz viscosity g2 of the nematic
phase of the pure 8CB.26 It suggests that the untreated platesof the rheometer induce almost planar alignment of the director along the shear direction (Fig. 4(b)). As the magnetic field
is increased, the viscosity increases beyond 120 mT andreaches to about 40 mPa s around 1 T. The measured viscosity
in the decreasing field is lower than that measured during
increasing magnetic field. This is because in the freshly loaded
samples usually there are many defects (disclination lines)
and the sample is less homogeneous. In the ferromagnetic LC,
the viscosity exhibits three distinct regimes. In the very low
field region (below 10 mT), geff (’ g2) is almost constant andincreases rapidly in the intermediate field range (10 mT to
20 mT). At relatively higher field, geff saturates and exhibit
hysteresis, while the field is reduced to zero (Fig. 5(b)). In thesuspension with 0.3 wt. % magnetic nanoparticles, at zero

FIG. 4. (a) Schematic setup for themeasurement of magnetoviscosity. The
green arrow shows the direction ofshear. The applied magnetic field (redarrow) is perpendicular to the shear
direction. Orientation of director (^n)~ at (b) zero andand magnetization (M)(c) finite field. The horizontal dottedlines represent the direction and mag~).
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Appl. Phys. Lett. 106, 161905 (2015)

and the magnetic field induces perpendicular orientation of
the same. On the other hand, in the rheological experiment,
parallel orientation is induced by flow and this competes
with the magnetic alignment (Figs. 4(b) and 4(c)).
In conclusion, we showed that the splay elastic constantand rotational viscosity are reduced in the ferronematic LC.
The low field magnetodielectric and magnetoviscosity
responses remarkably differ in the suspension from the pure
LC due to the orientational coupling between the director
field and the magnetization. The low magnetic field effects
in these ferronematic materials make them useful for multipurpose applications because these are useful for both display and non-display applications such as a smart fluid. The
small amount of magnetic nanoparticles (e.g., 0.3 wt. %) do
not change the Miesowicz viscosities but facilitate the measurement at a significantly low magnetic field.
We gratefully acknowledge the support from the
UPE-II, DST (SR/NM/NS-134/2010), CSIR (03(1207)/12/
EMR-II), and DST-PURSE. M.V.R. and R.S. acknowledge
UGC-BSR for fellowship. A.M. and D.L. acknowledge thesupport from Slovenian Research Agency (A.M., Grant No.
P1-0192; D. L., Grant No. P2-0089-4).
1

FIG. 5. (a) Variation of shear viscosity (geff) with magnetic field of pure 8CB
LC and (b) suspensions with different concentrations: 0 wt. % (blue spheres),
0.3 wt. % (red squares), and 0.6 wt. % (green hexagons) at shear-rate of
10 s1. Upward and downward arrows indicate the data collected during
increasing and decreasing of the magnetic field. (Inset) Magnetoviscosity
response from the same sample (0.6 wt. %) in the second measurement. The
measurements in the suspensions are restricted up to the field of 30 mT.

magnetic field, geff ¼ 25 mPa s and it increases up to 62 mPa s
in the saturation region and finally reduced to 27 mPa s atzero field. The viscosity at the zero field is equal to that of thepure 8CB sample and hence comparable to Miesowicz viscosity g2. The effective viscosity in the saturated region is almost
comparable to the Miesowicz viscosity g1 of the pure 8CB atthe same temperature.26 This suggests that the director is almost
parallel to the magnetic field (perpendicular to the shear direction) as schematically shown in Fig. 4(c). This technique thus
helps us to measure two Miesowicz viscosities using a verysmall magnetic field. In the suspension with 0.6 wt. % magnetic
nanoparticles, the overall behavior is almost similar except thatthe values are larger and this is expected as the viscosity is linearly proportional to the phase volume of the nanoparticles.
The magnetic field dependence of viscosity at the firstmeasurement is significantly different from the second measurement and somewhat similar to that is observed in themagnetodielectric measurements. We attribute this behavior
to the growth of magnetic domains, resulting in the magnetically monodomain sample as mentioned previously. The second measurement thus shows the behavior of a monodomain
sample. In the second measurement at the smallest field(1 mT), the viscosity is larger than that is measured at thefirst measurement, further it steeply increases and saturates
above ’15 mT (inset of Fig. 5(b)). In the magnetodielectricmeasurements, there is a competing orienting mechanism.
The cell surfaces induce parallel orientation of the director,

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