PHYSICS OF MAGNETIC PHENOMENA

SYNTHESIS AND PHYSICAL PROPERTIES OF TI-Ca-Ba-Cu-O

HIGH-TEMPERATURE SUPERCONDUCTORS

G. A. Petrakovskii, V. E. Volkov, G. S. Patrin, T. A. Bidman, N. I. Kiselev, and V. N. Vasil'ev

UDC 537.312;538.27

We present results of an investigation of Ti-Ca-Ba-Cu-O, high-temperature super- conductors obtained under various technological conditions. Data from elec-~ trical, magnetic, and ultra-high frequency (UHF) measurements are given. We discuss the possibilities of applying magnetic and UHF methods to analyze the quality of HTSC ceramics.

Among the presently known high-temperature superconductors (HTSC), compounds of the TI-Ca-Ba-Cu-O system have the highest temperature T c of transition to the superconducting state. Depending on the ratio of the elements in the chemical formula TlnCamBas or,

in short, (nms T c takes on various values. Thus, for compounds with compositions (2122), (1223), and (2223), resistance to electrical current is zero at the temperatures 105, ii0, and 125 K, respectively [i]. In the utilization of ceramic technology of such compounds, their properties depend very strongly both on the composition of initial components and on the preparation conditions [2].

In this work we present results of studies of TI-Ca-Ba-Cu-O HTSC ceramics obtained under various conditions. Chemically reactive Ca2CuO3, Ba(NO3) 2 and CuO were taken as the initial compounds, in their respective proportions. The composition was mixed and annealed at a temperature T = 800-810~ This process was repeated three times. Then everything was ground up again, thallium oxide T1203 was added, and everything was thoroughly mixed in alcohol and dried out. After this, tablets were pressed. Final annealing was carried out at T = 820 ~ for 3-5 min. Depending on whether or not the oxidizer was added before pressing the tablets, various results were obtained. In what follows, those specimens obtained by using the oxidizer will be denoted by a I, and those without the oxidizer, by a 2.

Measurements of the electrical resistance were made using the 4-probe method. The mag- netic data were obtained on a pendulum magnetometer; the measurement precision was 0.02 emu/g. The UHF response was studied on a standard EPR spectrometer by the usual method at a frequency f = 10.7 GHz. X-ray diffraction spectra were taken on a DRON-3 apparatus (the copper Ks-line).

Figure 1 shows the temperature path of the electroresistance of specimen 1 and 2. It is evident that specimen 1 has a higher temperature and a narrower width for its superconducting transition. Here, far from the transition, in the normal phase the specific resistance of specimen I is app=oximately five times less than that of specimen 2.

Figure 2 shows the temperature dependences of the magnetization of these specimens. For comparison, the curve taken on the ceramic Ho (123) (curve 3) is shown. It is obvious that the magnetization for specimen 2 (curve 2) passes through zero in the neighborhood of i00 K, which practically coincides with T c obtained from the electroresistance. For specimen 1 (curve i), the dependence a(T) has two slope changes. The asymptote of the low-temperature part fits to a temperature T ~ ii0 K, which correlates with electroresistance data, and the value of o(T) = 0 occurs at a temperature near T = 120 K. The size of the magnetization in a magnetic field H = 750 Oe at T = 78 K for specimen 1 is about three times larger than for specimen 2. The field dependences for these specimens, cooled in zero magnetic field, also have different forms (Fig. 3). For different specimens the magnitudes of the fields cor- responding to Jamaxl differ extremely strongly, with lOmax[ for specimen i larger than the corresponding quantity for specimen 2 by approximately an order of magnitude.

L. V. Kirenskii Institute of Physics, Siberian Branch, Academy of Sciences of the USSR. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika,'No. 9,~pp. 18-22, Septem- ber, 1990. Original article submitted March 2, 1989.

734 0038-5697/90/3309-0734512.50 �9 1991 Plenum Publishing Corporation

"cm

o J I

270

Fig. i

G.cm 3/2 l

I4/',, | ] t t , I ( I I

70 ~'~'0 fSO 300 7,

Fig. 2

G.cm3/2

1o

o,5~

o

-..f

6

Fig. 3

f2 H kOe

The difference in the values of T c from the data of the electroresistance and from the temperature dependence of the magnetization is due to the fact that diamagnetism, when tem- peratures are lowered, arises earlier'in isolated clusters, while its occurence throughout the entire system takes place later.

From these data of the magnetic and electrical measurements, one can draw the conclusion that specimens obtained under different conditions not only have differing contents of the superconducting phase, but also differ in phase composition.

This is supported by the x-ray diffraction data, as shown in Fig. 4. For specimen i (a), the appearance of peaks, missing in the spectrum of specimen 2 (b), is characteristic. Except for some details, the spectrum of specimen 1 resembles the spectrum identified in [3], as for the (2223) system. The corresponding difference is related to traces of other phases.

We also conducted a study on specimen 1 of the reaction of HTSC ceramics with UHF radia- tion. Figure 5 shows the change in form of the absolute value of the derivative of the UHF response in zero magnetic fields as a function of temperature and the magnitude of the starting field H s. We note that, in comparison with the ceramic system Ho (123), in the present case the "spectrum" is much wider, and larger hysteresis effects are observed. How- ever, the intensity of the signal of the ceramic Ho (123) is greater than for the TI-Ca-Ba- Cu-~3 prepared according to the described technology. The amplitude of the signal of the derivative of specimen 1 increases with increasing temperature, reaching a maximum in the region T = 75-80 K, and then sharply decreases with further increase of the temperature to Tc = 120 K. The temperature dependences of AH/2 and Hm, corresponding to the "right" half- width of the UHF-response line and to the value the magnetic field where the derivative is zero, respectively, are given in Fig. 6. The magnitude of AH/2 decreases monotonically with temperature up to T m 82 K, where the slope changes, and a sharper decrease is observed; with an increase in the temperature, H m at first increases, reaching its maximum near T = 50-55 K, and then it decreases, while, just as for AH/2, at T ~ 82 K there is a sharp drop. The values of both H m and AH/2, reach zero at a temperature Tc = 120 K.

735

i I I I I

I I I I I

Fig. 4

b

I

I

a

I

500 H, Oe

. ~,Oe

3OO !T;5 0 500 H, Oe 0

Fig. 5

80 /20 T,t:

Fig. 6

.- m.Oe

180

90

If we consider the Josephson nature of the UHF absorption, and we view the integral effect from the totality of weak links connecting the superconducting granules, then the behavior of the UHF response can be explained within the framework of a model of a super- conducting glass [4]. Comparison of the data of UHF measurements with the data of magnetic measurements (see Fig. 2, curve i) points to a correlation of peculiarities in the temper- ature dependences of the magnetization and the absorption-line parameter. This forms a basis for using the behavior of'parameters of the UHF-response line for analyzing the quality of HTSC ceramics. In this case the amplitude of the derivative of the response line can serve as a measure of the amount of superconducting phase, as one can attempt to relate the spec- trum width (for a fixed starting magnetic field) to the distribution of current contours by sizes, since the magnetic moments of the latter determine the scatter of local magnetic fields in the individual transitions, and, perhaps, the linewidth.

For the ceramic specimens the true diamagnetic susceptibility appears only in the weak- field limit, since only then are both the bulk part of the granules and all the weak bonds at work [5]. When the magnetic field is increased significantly, the diamagnetism is to some degree suppressed, since the links begin to uncouple as the magnetic field is increased. During UHF investigations of the HTSC ceramics, for the most part the weak-bond state is probed. Therefore, it is the combined use of the methods of the radiation of a static mag- netic susceptibility and of methods of UHF spectroscopy that allows one to obtain sufficiently full information on the properties and processes taking place in the superconducting phase of HTSC ceramics.

In our case, as can be seen, specimen 1 is two-phase, where each phase has a Tc equal to Ii0 and 120 K, which corresponds to compositions (1223) and (2223), respectively; specimen 2 has a composition of the type (1223). Also, specimen 1 has a much greater total content of superconducting phase. We explain this result in general. It is known that, during heating of thallium oxide, thallium reduces easily and becomes volatile, and thus the com- position of the charge changes. However, when there is an oxidizer in the original mixture the volatile component is retained, which allows a composition close to optimal to be pre- served. This conclusion agrees with the result of [6], where the yield of (2223) phase in- creased when the tablet was wrapped in a platinum foil before the final annealing. It is dif- ficult for us to make any kind of quantitative comparison due to the absence in the literature of data on the absolute content of the various phases in the specimens.

736

In conclusion, we note that the process used here for creating technological conditions close to optimum is simple and extremely promising for systems of thallium-containing high- temperature superconducters.

LITERATURE CITED

I. R. Beyers, S. S. Parkin, V. Y. Lee, et al., Appl. Phys. Lett., 53 432-434 (1988). 2. R. M. Hazen, L. W. Figner, R. J. Angel, et al., Phys. Rev. Lett., 60, 1657-1660 (1988). 3. M. Kikuchi, N. Kabayashi, H. lwasaki, et al., Jpn. J. Appl. Phys., 27, LI050-LI053

(1988). 4. G. A. Petrakovskii, G. S. Patrin, Yu. N. Ustyuzhanin, et al., Preprint No. 512F, Krasno-

yarsk (1988). 5. A. K. Grover, C. Radhkrishnamutry, P. Chaddah, et al., Pramana J. Phys., 30, 569-595

(1988). 6. A. K. Ganguli, K. S. Nanjunda Swamy, G. N. Subbana, et al., Solid State Commun., 67, 39-42

(1988).

SURFACE MAGNETIZATION OF A MODEL ANTIFERROMAGNET

S. Yu. Davydov and V. I. Margolin UDC 538.22.001

A semi-infinite Hubbard lattice of Anderson "impurities" with antiferromagnetic ordering of the inner layers is examined. The magnetization is analyzed for three models of magnetic ordering in the near-surface layers. The cases of zero and finite temperatures are examined. The possibility is discussed of a surface mag- netic phase transition of the first kind.

Investigation of the surface magnetization of magnets is an urgent problem [i]. Within the framework of a model approach (a Hubbard lattice of Anderson impurities) the magnetiza- tion of a ferromagnet (FM) surface was considered in [2]. The present paper is devoted to application of this approach to the simplest models of a semi-infinite antiferromagnet (AFM).

Let a lattice of mutually noninteracting single-level atoms, "impurities", be placed in an electron gas. Interaction between such an "impurity" and the electron gas can be described by an Anderson Hamiltonian [3], to which a Green's function (GF) of the form

G71 = w -- so + iF (1)

will correspond in a medium field approximation, where go is the location of the quasi-level of an "impurity" with spin projection o and half-width r. We shall later consider the elec- tron gas of an infinitely wide s-zone shape while the parameters go and r will determine the seeding d-zone of the metal (s-~ model modification).

Let us now allow the electrons to tunnel between adjacent "impurities" located in planes parallel to the surface. Then, as is shown in [4, 5], the GF for the FM and AFM layers will have the respective forms

D~,FM = 6~ [1 - - t'~ (x) O,]-~;

U ~ -- G [I + t7 (x) G_o] [1 - t272 (x) G, O_ ,1- , ,

(2)

where t is the integral of electron transition between adjacent "impurities", 7(x) =~eiX~ , !~I

is the distance between nearest neighbors (NN) and m is a wave vector parallel to the surface.

V. I. Ul'yanov (Lenin) Leningrad Electrotechnical Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 22-26, September, 1990. Original article submitted March 20, 1989.

0038-5697/90/3309-0737512.50 �9 1991 Plenum Publishing Corporation 737