Observation of antiferromagnetic ordering in Bi2Sr2YCu2Oy above room temperature by the μSR method

PhysicaC 156 (1988) 625-628 North-Holland,Amsterdam

OBSERVATION OF ANTIFERROMAGNETIC ORDERING IN Bi2Sr2YCu2Oy ABOVE ROOM TEMPERATURE BY THE ltSR M E T H O D N. NISHIDA 1, H. MIYATAKE 1, S. OKUMA l T. TAMEGAI K. NISHIYAMA 4 and K. NAGAMINE 4

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R. YOSHIZAKI 3

Department of Physics, Tokyo lnstitute of Technology, Meguro-ku, Tokyo15 2, Japan 2 Institute for Solid State Physic, University of Tokyo, Minato-ku, Tokyo 106, Japan 3 Institute of Applied Physics, University ofTsukuba, Tsukuba, lbaraki 305, Japan 4 Meson Science Laboratory, University of Tokyo, Bunkyo-ku, Tokyo 103, Japan

Received 8 September 1988

We have observeda long-rangemagnetic ordering (probablyan antiferromagneticone) with a transition temperature above 300 K in Bi2Sr2YCu2Oyby the ~t+SRmethod. This is the first observationof the antiferromagneticordering with a high N6el temperature in the Bi-Sr-Ca-Cu-O system. The Bi2Sr2YCu20r may be treated as an analogue of the high-Tosuperconductorrelated antiferromagnetssuch as YBa2Cu306or La2CuO4_#.

Since the discovery of hi gla-Tc superconductors, LnBa2Cu3OT_~ ( L n = Y or rare earth atoms) and La2_xAxCuO4_,~(A=alkaline earth atoms), a great deal of effort has been devoted to understand the mechanism of superconductivity both experimentally and theoretically. Various theoretical models proposed the magnetic coupling as an origin of the superconductivity [1-3]. In high-To superconductor-related insulators, La2CuO4_~ and YBa2Cu306, antiferromagnetic orderings with high N~el temperatures have been observed by ~t+SR [4-6], NMR [7,8] and neutron experiments [9-11 ]. When holes are doped into these magnetic insulators, the superconducting phase sets in. Recently, Maeda et al. found new superconductors in the Bi-Sr-Ca-Cu-O system [ 12] with Tc values of 80 K and 110 K. The low Tc phase has been identified as Bi2Sr2CaCu20~ which has been characterized by double CuO2 layers separated by BiO double layers. It is interesting and important to study the magnetism of CuO2 layers of an insulator phase of the BiESrECaCu2Ox system. Recently, Bi2Sr2YCu2Oy,where Ca atoms are substituted by Y atoms in Bi2SrECaCu2Ox, has been proposed [ 13 ] as the insulator phase of this system. The electrical resistivity and magnetic susceptibility measurements of BiESrEYl_~CaxCUEOy have been

performed [ 13-15 ]. However, since a single-phase sample is not available at the present, the physical properties have not been well understood. Here we report the observation of an antiferromagnetic ordering of Bi2Sr2YCu20 r above room temperature by the positive muon spin rotation or relaxation method (Ix+SR). The sample of Bi2Sr2YCu2Oy was prepared by a solid-state reaction of Bi203, SrCO3, Y203 and CuO as starting materials. The details are described in a previous paper [ 13 ]. The X-ray diffraction pattern is very similar to that of superconducting Bi2Sr2CaCu20r Our Bi2Sr2YCu2Oy crystal is orthorhombic with the lattice parameters a=5.465 A, b = 5.428 A and c = 30.175 ~,. The electrical resistivity shows a semiconductor-like temperature dependence. The magnetic susceptibility shows a cusp at 13 K and obeys a Curie-Weiss law at higher temperatures. This cusp peak stays at the same temperature when we change x in Bi2Sr2Yt_xCaxCu2Oy. Therefore, the antiferromagnetism with TN= 13 K is speculated to be extrinsic and due to the impurity phase Y2Cu205 which has a N6el temperature of 13 K. Thus, it is very difficult to study the magnetism only from bulk susceptibility measurements of the present Bi2Sr2YCu2Oy sample. From the known

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N. Nishida et al. / Antiferromagnetism of Bi 2Sr2YCu eO~

magnetic susceptibility of Y 2 C u 2 O s , about 10 percent of the sample is estimated to be this impurity phase. After the contribution from Y2Cu205 has been subtracted, the magnetic susceptibility of Bi2Sr2YCu2Oy is very small (of the order of 10 - 7 emu/gram) and slightly decreases with temperature. It is very similar to the case of the YBa2Cu3Ox system [ 16 ]. In the YBa2Cu3Ox system magnetic susceptibility measurements have failed in finding 3D magnetic ordering, thus far. The IX+SR method was powerful enough to detect the static magnetic ordering in the YBa2Cu3Ox system [4], whether it might be regularly or randomly ordered. Therefore, we applied the IX+SR method to magnetism studies of Bi2Sr2YCu2Oy. The experiments were performed at surface positive muon channel ~1 at the BOOM facility, Meson Science Laboratory, University of Tokyo, located at KEK. Polarized pulsed (50 ns width and 20 Hz) Ix+'s were implanted into the sample. The numbers of decay positrons were counted time-differentially at time t after Ix+implantation as NF(t) and Na(t) by forward counter (situated parallel to initial ix+ spin) and backward counter (situated anti-parallel to initial ix+ spin), respectively. As the asymmetry ratio, [NF(t) --Na(t) ] / [NF(t) +NB(t) ], is proportional to the polarization of ix+ spin at time t, we are able to obtain the ix+ spin relaxation function Gz(t) even in zero external field. When we deduced Gz(t), we paid special attention to the full asymmetry ratio and the positive muons stopped other than in the target. In fig. 1, the relaxation functions Gz(t) in zero external field are plotted at temperatures 50, 150, 250 and 300 K. The ix+ spin precessions are observed up to 300 K; the frequencies gradually decrease with increasing temperature as 0.45 + 0.01 MHz at 14 K and 0.20 + 0.02 MHz at 300 K. This means that some part of positive muons implanted into Bi2Sr2YCu2Oy feel a unique local magnetic field of about 30 G in Bi2Sr2YCu2Oy and that Bi2Sr2YCu2Oy is magnetically long-range ordered [ 17 ] even at 300 K. The ix+ local fields corresponding to the precession frequencies are plotted against temperatures in fig. 2. The dephasing time of the precessions (T2 or T~) becomes very short above 250 K. At 340 K, the/t ÷ spin precession was not seen, but the ~÷ spin relaxed in a Gaussian-like shape, with a time constant about 3 Ixs. It will be noticed that all the G~(t) do not start

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Fig. 1. It + SR relaxation functions Gz (t) under zero applied field in Bi2Sr2YCu2Oy are shown for 50, 150, 250 and 300 K. The it + spin precession signals are seen up to 300 K. The precession frequency becomes lower as the temperature increases. This means that Bi2Sr2YCu2Oy is magnetically ordered at 300 K. LF-it+SR results at various longitudinal fields: ( O ) 0 G, ( • ) 30 G, ( • ) 100 G, and (-k) 500 G are shown. A different decoupling scheme of It + spins is seen at 300 K.

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N. Nishida et aL / Antiferromagnetisrn of Bi2SreYCu20~

from 1.0 at t=0. This will be discussed later in more detail. The behaviour of the Gz(t) below 250 K is summarized as composed of the following five components: ( 1 ) a precessing component of about 0.4 MHz (about 13%), (2) a relaxing component with a time constant of about 1 Ixs (about 20%), (3) a missing asymmetry component (when Gz(t) does not start from 1.0 at t=0, we call the reduction a missing asymmetry component) (about 20%), (4) a slowlyrelaxing component with a time constant of 10 Ixs (about 13%) and (5) a non-relaxing component (about 33%). The percentage of each signal is given in parentheses. In order to understand the meaning of the observed Gz(t) more in detail, we performed transverse field IX+SR (TF-IX+SR) and longitudinal field IX+SR (LF-IX+SR) measurements, applying external magnetic field perpendicular and parallel to the initial ix+ spin, respectively. The transverse field IX+SR experiments under 30 G have been performed to measure what portion of the implanted Ix+'s precess with the frequency corresponding to the applied transverse magnetic field. We were able to determine the fraction of paramagnetic component in the sample: IX+ spins implanted into the magnetically ordered part relax with time constants corresponding to the local fields (in the present Bi2Sr2YCu2Oy case the relaxation time is shorter than 1 Ixs) and the precession frequency is different from the one corresponding to an applied magnetic field. The paramagnetic part which was thus determined by the above-mentioned transverse field ~t+ SR was about 13% of the sample and almost temperature-independent. In paramagnetic phase g+ spin relaxes slowly due to nuclear magnetic moments in the crystal. Therefore, the slowly-relaxing component (4) in the zero-field relaxation function Gz(t) is assigned to the signal from a paramagnetic part in the sample. This component might be from Y2Cu2Os. An experiment below 13 K will verify this speculation. In fig. 1, the results of the longitudinal field IX+SR experiments at 30, 100 and 500 G are also shown for 150 K and 300 K. Below 250 K, the experimental results are similar to those at 150 K. At 150 K, the longitudinal external field of 100 G was enough to decouple IX+ spins which recess at the frequency of

627

0.4 MHz (component ( 1 ) ) and which relax with a time constant of I Ixs (component (2)) from the IX+ local fields in Bi2SrEYCuEOr A further increase of the external field brings about a recovery of a missing asymmetry (component (3) ); at 500 G, most of the missing asymmetry is recovered as seen in fig. l and at l kG IX+ spin is decoupled completely from the IX+ internal magnetic field in Bi2SrEYCuEOr These recovery schemes mean that all the IX+ internal fields in BiESr2YCu2Oy are static below 250 K and that positive muons of the previously mentioned missing asymmetry component feel local magnetic field of several hundred gauss, because they can be almost decoupled by 500 G. It is not known whether these magnetic fields are unique ones that cause IX+ spin precession or static random ones that bring about fast IX+ spin relaxation. As the time resolution of the muon beam at KEK-BOOM is 50 ns and it is hard to observe Gz(t) in the early time range, we cannot observe the contribution in Gz(t) in the present experiments if the relaxation time is short. In IX+SR experiments on antiferromagnetic YBa2Cu3Ox [4], the IX+ spin precession of about 4 MHz has been accompanied by the fast relaxation of IX+ spin with a time constant of 0. l Ixs. They were from the s a m e magnetic origin. Similarly in Bi2Sr2YCuEOy, the five components of Gz(t) in zero external field are interpreted as follows: component ( l ) is from Ix+'s which are located in the magnetically ordered part and feel a unique local field of about 30 G; component (2) is from Ix+'s that stay in the magnetically ordered part, but feel distributed magnetic fields of the order of 30 G. Component (3) is from Ix+'s that were implanted into the magnetically ordered part where the magnitude of local fields is of the order of a few hundred gauss, though it is not known whether they are definite ones or random ones. As all the IX+ local fields are static, l / 3 of the implanted Ix+'s feel local magnetic fields effectively parallel to the initial IX+ spin direction. Those IX+ spins do not relax and are called the 1/3 component. Component (5) is the l / 3 component of ( 1 ), (2) and (3). We wil consider that the components ( l ), (2) and (3) have the same magnetic origin. This speculation was valid in the case of YBa~Cu3Ox. Then, more than 80% of the Bi2Sr2YCu2Oy (the sum of components ( 1 ), (2), (3) and ( 5 ) ) is considered to be magnetically ordered.

628

N. Nishida et aL ~Antiferromagnetism o f Bi :,Sr2YCu 204

At 300 K, where we still observe the ix+ spin precession, but T2 is short, the results of the longitudinal field ix+SR experiments are different from those below 250 K. As seen in fig. 1, in the longitudinal magnetic field of 30 G or 100 G, ix+ spins relax exponentially and are not decoupled. Furthermore, the 1/3 components are observed to relax. This means that Ix÷'s feel some dynamically fluctuating local magnetic fields. Two candidates are considered as the origin of the dynamic fluctuation: (a) a critical fluctuation near TN, and (b) a kind of Ix÷ motion in Bi2Sr2YCu2Oy which is magnetically ordered. Though we need further studies to distinguish them, we tentatively ascribe the origin of fluctuation of IX+ local fields to ix+ motion. In fact, in YBa2fu307, Nishida et al. [4] reported that ix÷ starts to move above 250 K. This might be the reason why the T2 becomes shorter above 250 K. In conclusion, we have observed a long-range magnetic ordering in the Bi2Sr2YCu2Oy system up to 300 K. The magnetic order is probably antiferromagnetic, as the magnetic susceptibility is small. The temperature dependence of ix+ local magnetic fields has been measured below 300 K. Bi2Sr2YCu2Oy will be treated as the analogue of La2CuO4_x and YBa2Cu3Ox(x=6.0) in the context of antiferromagnets closely related with high-Tc superconductors. Further experiments on the antiferromagnetism o f BiESr2Yt _xCaxCu2Oy a r e n o w i n p r o g r e s s .

Acknowledgements We acknowledge Professor T. Yamazaki for stimulating discussions and for reading the manuscript. The present work is supported by a Grant-in-Aid for Special Project Research on Meson Science and a Grant-in-Aid for Scientific Research on Priority Areas "Mechanism Of Superconductivity" of the Ministry of Education, Science and Culture of Japan. One of the authors (N.N) thanks the Kyo-Cera Corporation for the scholarship to study on high-Tc superconductors.

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