CC BY
32
www.volsu.ru
ТЕХНИКО-ТЕХНОЛОГИЧЕСКИЕ ИННОВАЦИИ
DOI: https://doi.Org/10.15688/jvolsu10.2016.3.2
УДК 53(075) ББК 22.313
INVESTIGATION OF MAGNETIC PROPERTIES OF RBCin
3 6U
SINGLE CRYSTAL FULLERENE USING TORQUE EXPERIMENTS
Jaba Chigvinadze
Doctor of Physical and Mathematical Sciences, Head of Superconductivity Laboratory, Department of Condenced Medium Physics, Institute of Physics named after E. Andronikashvili jaba@aiphysics.ge
Tamarashvili St., 6, 0177 Tbilisi, Republic of Georgia
о
<N
<D N T3
jS Doctor of Physical and Mathematical Sciences.
Victor Buntar
<D N
S3
q Department of Physics and Astronomy,
McMaster University ^ buntarv@mcmaster.ca
1280 Main Street West, Hamilton, Ontario L8S 4M1, Canada
ä s
? Sabir Ashimov
CO
§ Researcher,
J| Department of Condenced Meduim Physics,
Institute of Physics named after E. Andronikashvili ^ jaba@aiphysics.ge
Tamarashvili St., 6, 0177 Tbilisi, Republic of Georgia
m
<D
л
Teimuraz Machaidze
Researcher,
ö Department of Condenced Meduim Physics,
Institute of Physics named after E. Andronikashvili j jaba@aiphysics.ge
y> Tamarashvili St., 6, 0177 Tbilisi, Republic of Georgia
ТЕХНИКО-ТЕХНОЛОГИЧЕСКИЕ ИННОВАЦИИ Giorgi Donadze
Researcher,
Department of Condenced Meduim Physics, Institute of Physics named after E. Andronikashvili jaba@aiphysics.ge
Tamarashvili St., 6, 0177 Tbilisi, Republic of Georgia
Abstract. For the first time, magnetic properties of fullerides have been studied using a torque technique. A Rb3C60 single crystal has been investigated in these experiments. It was shown that this method is sensitive to the structural phase transition in both superconducting and normal states. In the superconducting state a transition at Tc = 28 K was observed and a pinning force of the magnetic vortices has been estimated. In the normal state, the transition from s.c. to f.c.c. structure has been indicated. A very strong magnetic response has also been observed at temperatures T ~ 200-250 K, which may be explained in terms of the rearrangement on the magnetic system in the material.
Key words: magnetic state, Rb3C60, fullerene, superconductivity, normal state.
Introduction
The appearance of superconductivity in alkali fullerides has led to extensive efforts in attempting to understand their electronic, magnetic, structural, and dynamic properties and to elucidate the origin of their high Tc. In particular, the question of whether or not such a large value of Tc can be caused by coupling to phonons alone is still to be answered and the answer strongly depends on the normal state properties. Despite the apparent simplicity of the structure of A3C60 fullerides (where A is an alkali metal), some important issues are not yet fully resolved.
It is well known that A3C60 fullerides, as well as pure C60, are weak diamagnetics with the appearance of strong diamagnetism at the transition to the superconducting state. At high temperature, solid A3C60 forms a face-centered-cubic (f.c.c) phase. In this phase, C60 molecules freely rotate with a reorientation time scale of the order of 10-11 seconds. With lowering temperature, the transition to s.c. structure occurs, which comes from the fast reorientation of the C60 molecules, and an anisotropic uniaxial rotation of fullerene molecules takes place. This transition has been observed for K3C60 by NMR experiments [7; 8], but the exact transition temperature was hard to evaluate since, in these experiments, the peak related to this transition overlapped with a peak related to appearance of Tc site and is very wide. No special effect on the
magnetic properties at the temperature of the solid state transition from s.c. to f.c.c. structure has been observed.
A3C60 fullerides are extremely air sensitive and usually are sealed in a glass or quartz capsules. Therefore, most of the magnetic measurements on these materials have been done with SQUID magnetometers. Therefore, an alternative method of investigation of the magnetic properties may give new information about the magnetic structure of A3C60 superconductors in the normal state.
Experimental Method and Samples
In these experiments we apply the mechanical torque method in order to study the magnetic properties of alkali doped fullerenes. This method has been extensively used to investigate critical parameters of superconductors such as Tc, critical field Hc, and energy dissipation in the mixed state [3; 4]. A cylindrical (in the ideal case) sample is suspended on a thin elastic thread and torque oscillations of small amplitude of 1 ° or 2° are generated with a short-time impulse. After that the sample performs free axial-torsion oscillations in an external magnetic field, H, which is perpendicular to the axis of the sample. The temperature dependence of the frequency, w (or of the period, t), and of the dissipation of the oscillations, 5, is measured at different magnitudes of the magnetic field.
If there are no fixed (pinned) magnetic moments in the sample, neither the dissipation nor the frequency of the oscillations depend on the external magnetic field. For example, when either i) the external magnetic field does not penetrate the substance, which is the case for a superconductor in an external field smaller than the lower critical field Hc1, or ii) the inner magnetic moments are either zero or disoriented and not fixed.
The appearance of pinned magnetic dipoles produces a nonzero magnetic moment M in the sample. The interaction between M and H makes a torque t = MH sin a, where a is the angle between M and H. This additional moment t affects the oscillating system and makes the dissipation and the frequency of oscillations dependent on the external magnetic field. The sensitivity of this method is very high, 10-17 W [4].
In our experiments we used a crystal of Rb3C60 that was made from a single crystal of C60 by doping it with Rb using the method of vapor phase doping 1. Details of the sample preparation and its characterization can be found in [5; 6] (sample R28 therein). The physical dimensions of the crystal are 3.3 x 2.7 x 1.3 mm3. It consists of a few single crystalline grains of the radius about 350 ^m [1]. The sample is sealed in a quartz capsule to prevent exposure of the material to air.
Results and Discussions
In Fig. 1 we present temperature dependences of both period and dissipation of the oscillations of the sample monitored in the external magnetic field of 100 mT with increasing temperature from the temperature of liquid He. At temperature Tc = 28 K there is a step-like transition to the normal state on both t and 5. This transition temperature is slightly lower than that obtained in [6] for this crystal. This difference is because the thermometer with the electrical cables cannot be fixed on the oscillating sample directly and is placed some distance from it (Fig. 1).
It is clearly seen that the dissipation in the superconducting state is larger than that in the normal state. This is because, in the superconducting state, the material is penetrated by magnetic field in the shape of vortices. These vortices are partly pinned on structural defects and the pinning force depends on several factors like size and number of the defects, and thermal fluctuations. Vortices in the material tend to align along the external magnetic field, which increases the dissipation of the oscillations. When the material undergoes the transition to the normal state, vortices do not exist in the sample anymore and, therefore, do not prevent the oscillation of the sample.
The larger the amplitude of the oscillations, the larger is the force applied on a pinned vortex
8.00
7.95
и
® 7.90
(Л
7.85-
7.80
0.0025
- 0.0020
Ô
- 0.0015
0.0010
0.0005
0 5 10 15 20 25 30 35 40
T(K)
Fig. 1. Temperature dependence of the period (solid squares) and of the dissipation of the oscillations (open circles) of Rb3C60 in the superconducting state and at temperatures close to the transition, Tc,
in the external magnetic field ^0H = 100 mT
by the magnetic field. By measuring the period t and the dissipation 5 at different amplitudes of oscillations one can find the critical amplitude jc at which vortices unpin from structural defects. Using the value of jc one can find the value of the critical moment tc and the strength of the bulk pinning force F as [5]:
F= 3-
p 4 R3 L
I
(1)
where R and L are the radius and the length of the cylindrical sample respectively. The results of the estimation of the bulk pinning force at temperature T =4.2 K are presented in Fig. 2.
With the increasing external magnetic field from zero, vortices penetrate the sample from its surface to the center and the bulk pinning force grows steeply. At the external field Hext = H* (in our experiments H* ~ 50 mT) the sample is completely penetrated by vortices. Further increase of the external magnetic field leads to increasing numbers of vortices (i. e. the volume density of vortices in the sample), and therefore to the decreasing of the bulk pinning force.
Fig. 2. The bulk pinning force as a function of the applied external magnetic field at T = 4.2 K
The temperature dependence of the dissipation of the oscillations in the normal state is presented. At around T1 ~270 K a peak can be observed. This peak may be associated with the first order phase transition from s.c. to f.c.c. structure. This transition is related to the freezing of libration modes and an orientation of C60 molecules. If each C60 molecule is considered as a diamagnetic dipole, then the orientation of
molecules leads to the orientation of magnetic moments in the system and, therefore, strong response on 5(T) and w(T). As expected, the transition temperature in Rb3C60 is higher than that in the pristine C60 at T = 263 K due to the presence of Rb atoms in the interfullerene space.
There is a second peak at T2 ~ 200-250 K. The amplitude of the peak of the dissipation (as shown in Fig. 3) is several orders of magnitude, which is above of the sensitivity of our experimental device. Moreover, this peak is much larger than the one at the s.c. - f.c.c. phase transition. In our experiments, the sample stops oscillating and we cannot generate the oscillations with a short-time impulse any more. We are not aware of any earlier experiments showing such strong magnetic response of fullerides materials.
6
5-1 4 3 2-\ 1
0H
Fig. 3. Temperature dependence of the dissipation 5 of the oscillations at the external magnetic field ^0H = 150 mT
In earlier NMR experiments, some anomalies of the temperature dependence of spin-lattice relaxation times have also been observed in this range of temperatures in K3C60 [8] and Na2CsC60 [6]. However, this anomaly was very weak and, as the authors reported, its amplitude was in the order of experimental error. In our experiments, the magnitude of the effect is huge. Since the effect appears to be so strong in our torque experiments, which are sensitive to the presence of fixed/oriented magnetic moments, it may mean that at this temperature there is a reorganization of the magnetic structure in the material.
It is unclear now what kind of magnetic structure is formed in A3C60 These magnetic moments cannot be the moments of C
60
molecules since most of the molecules are already
5
oriented at T1 = 280 K and, moreover, the magnetic effect is much stronger than that at the phase transition. One may speculate that some persistent currents may appear, or we may assume that the magnetic moments involved in the
crossover are due to distortions of the C„
60
molecules or the A3C60 lattice. In general, magnetism is mainly related to properties based on electron spins. The solid lattice, though important for some of effects, usually does not provide an important contribution to magnetism. However, there is a specific type of materials for which the Jahn-Teller effect plays a very important role determining both structural and magnetic properties.
NOTE
1 Sample preparations have been done in the Institute for Material Physics, Vienna University, Vienna, Austria.
REFERENCES
1. Buntar V., Haluska M., Kuzmany H., Sauerzopf F.M., Weber H.W. Preparation and Characterization of Rb3C60 Bulk Single Crystals.
Supercond. Sci. Technol., 2003, no. 16, p. 907.
2. Buntar V., Zauerzopf F.M., Weber H.W., Halushka M., Kuzmany H. Systematic Treatment of Displacements, Strains, and Electric Fields in Density-functional Perturbation Theory. Phys. Rev. B, 2005, no. 72, p. 024521.
3. Chigvinadze J. Effect of Surface and Volume Defects. JETP, 1972, no. 63, p. 2144.
4. Chigvinadze J. Effect of Surface and Volume Defects on the Dissipative Processes in Type-II Superconductors. JETP, 1973, no. 65, p. 1923.
5. Fuhrmans M., Heiden C. The Impact of Cryogenics Was Wide and Varied. Cryogenics, 1976, no. 8, p. 451.
6. Matus P., Alloul H., Kriza G., Brouet V., Singer P.M., Garaj S., Forro L. Influence of Local Fullerene Orientation on the Electronic Properties of Na2AC60 (A = Cs, Rb, K) Compounds. Phys. Rev. B, 2006 , no. 74, p. 214509.
7. Nemes N.M., Fischer J.E., Baumgartner G., Forró L., Fehér T., Oszlányi G., Simon F., Jánossy A. Conduction-electron Spin Resonance in the Superconductor K3C60. Phys. Rev. B, 2000, vol. 61, no. 10, p. 7118-7121.
8. Yoshinari Y., Alloul H., Brouet V., Kriza G., Holczer K., Forro L. Molecular Motion and Phase Transition in K3C60 and Rb3C60 by Nuclear Magnetic Resonance. Phys. Rev. B, 1996, no. 54, p. 6155.
9. Yoshinari Y., Alloul H., Kriza G., Holczer K. Liquid Hydrogen Targets. 93Yo06 Molecular Dynamics in K3C60: A 13C NMR study. Phys. Rev. Lett., 1993, no. 71, J0. 2413.
ИССЛЕДОВАНИЕ МАГНИТНЫХ СВОЙСТВ КРИСТАЛЛОВ ФУЛЛЕРИДОВ RB3Cб0 С ПОМОЩЬЮ МЕТОДА КРУТЯЩЕГО МОМЕНТА
Джаба Чигвинадзе
Доктор физико-математических наук, заведующий лабораторией сверхпроводимости, отдел физики конденсированной среды, Институт физики им. Е. Андроникашвили jaba@aiphysics.ge
ул. Тамарашвили, 6, 0177 г. Тбилиси, Республика Грузия
Виктор Бунтар
Доктор физико-математических наук, отдел физики и астрономии, Университет Макмастера, buntarv@mcmaster.ca
ул. Мэйн Стрит Уест, 1280, L8S 4М1 г Гамильтон (Онтарио), Канада
ТЕХНИКО-ТЕХНОЛОГИЧЕСКИЕ ИННОВАЦИИ Сабир Ашимов
Научный сотрудник,
отдел физики конденсированной среды,
Институт физики им. Е. Андроникашвили
jaba@aiphysics.ge
ул. Тамарашвили, 6, 0177 г. Тбилиси, Республика Грузия
Теймураз Мачаидзе
Научный сотрудник,
отдел физики конденсированной среды,
Институт физики им. Е. Андроникашвили
jaba@aiphysics.ge
ул. Тамарашвили, 6, 0177 г. Тбилиси, Республика Грузия
Гиорги Донадзе
Научный сотрудник,
отдел физики конденсированной среды,
Институт физики им. Е. Андроникашвили
jaba@aiphysics.ge
ул. Тамарашвили, 6, 0177 г. Тбилиси, Республика Грузия
Аннотация. Впервые магнитные свойства фуллеридов были изучены с использованием метода крутящего момента. В этих экспериментах был исследован монокристалл Rb3C60. Было показано, что этот метод чувствителен к структурному фазовому переходу и в сверхпроводящих и в нормальных состояниях.
Ключевые слова: магнитное состояние, Rb3C60, фуллерен, сверхпроводимость, нормальное состояние.
