Fine Structure and Magnetism of the Cubic Oxide Compound Ni 0.3 Zn 0.7 O

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1 ISSN , Physics of the Solid State, 11, Vol. 53, No. 7, pp Pleiades Publishing, Ltd., 11. Original Russian Text S.F. Dubinin, V.I. Maksimov, V.D. Parkhomenko, V.I. Sokolov, A.N. Baranov, P.S. Sokolov, Yu.A. Dorofeev, 11, published in Fizika Tverdogo Tela, 11, Vol. 53, No. 7, pp MAGNETISM Fine Structure and Magnetism of the Cubic Oxide Compound Ni.3 Zn.7 O S. F. Dubinin a, *, V. I. Maksimov a, V. D. Parkhomenko a, V. I. Sokolov a, A. N. Baranov b, P. S. Sokolov b, and Yu. A. Dorofeev a a Institute of Metal Physics, Ural Branch, Russian Academy of Sciences, ul. Sof i Kovalevskoi 18, Yekaterinburg, 619 Russia * dubinin@uraltc.ru b Moscow State University, Moscow, Russia Received December 8, 1 Abstract The fine structure and spin system of the cubic oxide Ni.3 Zn.7 O compound prepared from the initial hexagonal phase by quenching a sample with a high temperature and applying an external hydrostatic pressure to it have been studied using magnetic measurements, synchrotron and X-ray diffraction. It has been revealed that the diffraction patterns of this compound contain a system of weak diffuse maxima with the wave vectors q = (1/6 1/6 1/6)π/a and (1/3 1/3 1/3)π/a, along with strong Bragg peaks of the cubic phase. It has been shown that the origin of the diffuse peaks is due to longitudinal and transverse displacements of ions with respect to symmetric crystallographic directions of the {111} type. The reasons for the ion displacement and specific features of the structure of the spin system of the strongly correlated oxide Ni.3 Zn.7 O compound have been briefly discussed. DOI: /S INTRODUCTION In recent years, considerable interest has been expressed in studies of highly doped semiconductor compounds Me x Zn 1 x O (Me = Ni +, Co +, Fe +, and Mn + ) belonging to a wide class of A B 6 materials. This is mainly due to the idea of designing electronic devices with spin polarization of current (spintronics) based on the aforementioned and related compounds. These materials are usually treated as diluted magnetic semiconductors (DMSs). Note that DMSs based on A B 6 compounds have either the cubic symmetry (e.g., ZnSe : Me) or the hexagonal symmetry (ZnO : Me). In the sphalerite and wurtzite structures, Me + ions substituting for Zn + have the tetrahedral environment characteristic of mixed ionic-covalent bonds with a predominance of covalence. Earlier, it was established by thermal neutron diffraction that the DMS system contains, in a wide temperature range, two types of local Jahn Teller distortions of the cubic crystal lattice, namely: (1) nanodeformations of the trigonal type in ZnSe : Ni [1], and () nanodeformations of the tetragonal type in ZnSe : Cr []. Shear deformations are also observed in the hexagonal crystal ZnO : Ni [3].The shear deformations revealed from the diffuse neutron scattering correlate with the change in the propagation velocity of transverse ultrasonic waves [4]. The A B 6 compounds are diamagnetic; thus, the influence of a paramagnetic impurity on local structural deformations can be studied not only by classical diffraction methods, but also using magnetic measurements [, 3]. The aim of this work is to study the fine structure and magnetism of the metastable cubic oxide compound Ni.3 Zn.7 O (space group Fm3m) at 3 K, which was produced by quenching from a high temperature (147 K) and on applying an external hydrostatic pressure (7.7 GPa) to it [5]. The compounds with the NaCl structure are characterized by the ionic bond with the octahedral environment of the ions. Note that oxide NiO is a highly correlated compound whose energy spectrum and properties have been extensively studied.. SAMPLES AND EXPERIMENTAL TECHNIQUE We studied the Ni x Zn 1 x O (x =.3) compound. The sample was a.5-cm-thick plate with linear sizes in the flat part of.. cm. In this work, we continued the experiments started in [5] on the study of the Ni x Zn 1 x O (x =.3) compound with the metastable cubic crystal lattice. The fine crystal structure of Ni.3 Zn.7 O was studied using synchrotron and X-ray diffraction. The wavelengths of the synchrotron and X-ray radiations were λ 1 =.6885 Å and λ = Å, respectively. The magnetization and the magnetic susceptibility of the crystal were measured using a Quantum Design MPMS-5XL. 136

2 FINE STRUCTURE AND MAGNETISM () () (311) (4) (331) 3 1 (4/3 /3 /3) (5/3 1/3 1/3) (5/6 7/6 7/6) (11/6 1/6 1/6) () (13/6 1/6 1/6) (7/3 1/3 1/3) θ, deg (7/6 5/6 5/6) 38 4 θ, deg 46 Fig. 1. X-ray diffraction pattern of the polycrystalline Ni x Zn 1 x O (x =.3) compound after thermohydraulic treatment. Fig.. X-ray diffraction pattern of the superstructure diffuse maxima in the cubic Ni x Zn 1 x O (x =.3) compound measured at 3 K. 3. RESULTS OF X-RAY DIFFRACTION ON Ni x Zn 1 x O (x =.3) Figure 1 shows the X-ray diffraction pattern at 3 K. As seen from the figure, the pattern of scattering includes intense, (), (), (311), (4), and (331) reflections corresponding to the fcc phase with the lattice parameter a c = 4.5 Å. In addition to the intense Bragg reflections of the fcc phase, the X-ray pattern contains a system of lowintense diffuse maxima. The diffuse maxima are well seen in the scattering pattern built in a handy scale (Fig. ). They are thought should be considered superstructure maxima, since indexing of this diffraction pattern is possible to be performed only in terms of the fcc crystal lattice. In this case, the wave vectors of the superstructure corresponds to quantities q 1 = (1/6 1/6 )π/a c and q (1/3 1/3 )π/a c. The positions of the superstructure reflections q 1 and q on the ( 11 ) reciprocal lattice plane are shown in Fig. 3 by the bright circles with a point in their centers and bright circles, respectively. We first determine the origin of the superstructure. We assume that the superstructure cannot be due to atomic ordering of the nickel and tin ions in the fcc phase, because their X-ray scattering amplitudes are very close to one another. It is most likely due to the systematic displacements of Ni + ions. Arguments in favor of this model are as follows. As seen from Fig. 3, all the superstructure reflections of this series are on the reciprocal lattice plane along the symmetric crystallographic direction of the {111} type arranged between the () and () points of the reciprocal lattice. This fact unambiguously demonstrates that the superstructure is due to the transverse static displacement of ions in the fcc lattice with respect to the {111} crystallographic directions. Actually, the relative intensity of the superstructure reflections of the displacement type is determined by the formula [1] J ( κ u), (1) where κ is the vector of scattering, and u is the vector of atomic displacements. As seen from relationship (1), the superstructure reflections must be observed on the diffraction pattern only in the case, when, in the compound, there is a static component of the atomic displacements along the scattering vector κ. It is precisely in the case of the transverse atomic displacements taking place in the symmetric ( 11 ) crystallographic planes, the superstructures must to be observed in the direction connecting the () and () points of the reciprocal lattice. We select two physical parameters in the scattering patterns, namely, the half width Δq obs and the height h of the superstructure diffuse maximum. The observed Δq obs and industrial Δq inst halfwidths allow the determination the real halfwidth Δq Δq obs Δq inst = ( ) 1/, () related to the mean size of the structural inhomogeneity by the relationship L = π/δq. (3) The diffuse maximum height is proportional to the squared mean amplitude of the ion displacements within one inhomogeneity and to the total number of inhomogeneous formations in the crystal lattice. Let us discuss in more details the experimental situation in the Ni x Zn 1 x O (x =.3) compound after a thermal hydraulic treatment. We are interesting, first, in the mean size of the structural inhomogeneity. This PHYSICS OF THE SOLID STATE Vol. 53 No. 7 11

3 1364 () () DUBININ et al. 3 () (/3 4/3 4/3) (5/6 7/6 7/6) (5/6 5/6 5/6) (7/6 5/6 5/6) (/3 /3 /3) (4/3 /3 /3) (4/3 /3 /3) (1/3 1/3 1/3) (/3 /3 /3) (5/6 5/6 5/6) θ, deg 18 (5/3 1/3 1/3) (1/3 1/3 1/3) (11/6 1/6 1/6) () () (13/6 1/6 1/6) (7/3 1/3 1/3) Fig. 3. Positions of the superstructure diffuse maxima on the ( 11 ) plane of the reciprocal cubic lattice. quantity along the scattering vector found from relationships () and (3) is L 1 Å. As noted in Section 1, divalent nickel ions in weakly doped II VI cubic compounds are chaotically substituted for the zinc ions. Therefore, it is reasonable to assume that, in rapidly cooled highly doped Ni.3 Zn.7 O, the nickel ions will be also disorderly arranged in the zinc sublattice. Of interest is the origin of the numerical values of the wave vectors q 1 and q (see above). The magnitudes of the q 1 and q vectors are thought to be the consequence of the chemical composition of the Ni.3 Zn.7 O compound. 4. RESULTS ON SYNCHROTRON DIFFRACTION ON Ni x Zn 1 x O (x =.3) Note that we were fortunate to reveal the longitudinal superstructure maxima of q 1 and q (Section 3) only in the experiments performed on a synchrotron. We discuss this fact in more detail. The angular extent of the diffraction maximum is substantially dependent of the wavelength of the X-ray beam incident on the sample. As noted in Section, λ 1 is smaller than λ by a factor of more than two. It means that the angular extent of the diffraction effects is substantially smaller than that on the classical X-ray diffraction pattern. It Fig. 4. Synchrotron X-ray diffraction pattern of the superstructure diffuse maxima in the cubic Ni x Zn 1 x O (x =.3) compound measured at 3 K. is precisely the circumstance that allows us, in our opinion, to observe on a synchrotron diffraction pattern (Fig. 4) low wide diffuse maxima in the angular range in which there are no highly intense structural Bragg reflections. The indices of these extent diffuse maxima are indicated in Fig. 4. Note that the diffuse maxima (1/3, 1/3, 1/3), (/3, /3, /3), and (5/6, 5/6, 5/6) are arranged along the [111] crystallographic direction. Their positions in the reciprocal lattice are shown by square symbols in Fig. 3. It implies that, in the compound under study, there are weak longitudinal correlations of ions in the cubic Ni.3 Zn.7 O compound. The longitudinal correlation magnitudes estimated by Eqs. () and (3) are L Å, i.e., they are practically an order lower than the transverse correlations. We briefly note, in the end of this Section, that the method of synchrotron diffraction is ineffective for observing the scattering effects provided by the transverse correlations of the Ni + ion displacements in Ni.3 Zn.7 O because of, first, a relative small λ 1 and, second, a high scatter in the instrumental background (compare Figs. and 4). 5. ANALYSIS OF THE MAGNETIC STATE OF Ni x Zn 1 x O (x =.3) Let us analyze the magnetic state of the cubic semiconductor compound Ni x Zn 1 x O, which is thought to be in a compete agreement with its atomic structure. From this standpoint, the temperature dependence of the reciprocal magnetic susceptibility χ 1 (T) is most informative function. Figure 5 depicts the χ 1 (T) temperature dependence measured in a magnetic field of 1 koe in the range T = 3 K. Note that the χ 1 (T) function is very sensitive to the existence of ferromag- PHYSICS OF THE SOLID STATE Vol. 53 No. 7 11

4 FINE STRUCTURE AND MAGNETISM 1365 netic and antiferromagnetic correlations in a magnetic compound. The circles in Fig. 5 indicate the data measured during heating of the sample from K to room temperature. The crosses indicate the χ 1 (T) data measured during cooling the sample in the range 3 K. The data reliably demonstrate that there is no magnetic hysteresis in the vicinity of 5 K. where the χ 1 (T) dependence is nonmonotonic. The solid line in Fig. 5 shows a model function of the temperature dependence of the reciprocal paramagnetic susceptibility. It is convenient to identify the types of the magnetic correlations appeared in a magnetically active compound with respect to this line at relatively low temperatures. Actually, as seen from Fig. 5, the values of χ 1 (T) of our semiconductor compound deviate from a linear paramagnetic dependence of the reciprocal magnetic susceptibility near room temperature. In this case, the experimental values of χ 1 (T) are lower than the paramagnetic values. This fact shows that, in this temperature range, there are ferromagnetic spin correlations [4] in the Ni x Zn 1 x O (x =.3) compound, which, as seen from Fig. 5, decrease gradually with temperature, and they become zero at 5 K. This effect of suppression of the ferromagnetic correlations is thought to be due to appearance of low-temperature antiferromagnetic correlations in the compound [6]. This assumption is clearly supported by the level of values of χ 1 (T) below K. Actually, as seen from Fig. 5, the level of values of χ 1 (T) in the low-temperature range, is, first, higher then the paramagnetic level and, second, the paramagnetic Curie point obtained by extrapolation of the experimental dependence to the temperature axis is negative (about 5 K). Note that the Ni + ions are the carriers of the magnetic moment in the compound. According to the measurements of χ 1 (T) below 3 K, two types of magnetic correlations, namely, ferromagnetic and antiferromagnetic coexist in the cubic lattice of Ni.3 Zn.7 O. In other words, the spin structure of the compound is noncollinear, and it is in many respects determined by the character of its local structure distortions. In this connection, the ferromagnetic component of the structure is most probably oriented (with allowance for denotations in Fig. 3) along the [ 11 ] direction, and the antiferromagnetic component is directed along the [111] crystallographic direction. As temperature decreases, the relatively more regular antiferromagnetic component gradually suppresses the short-range ferromagnetic order. To obtain more complete information on the magnetic structure of the cubic semiconductor compound, the experiment on bulk single crystals are necessary. χ 1, 1 7 g Oe/emu T, K 3 Fig. 5. Temperature dependence of the inverse magnetic susceptibility χ 1 in the Ni x Zn 1 x O (x =.3) compound measured in the range 3 K. Circles show the data measured during heating in the range 3 K, and crosses indicate the data measured during cooling from 3 to K. 6. DISCUSSION OF THE RESULTS In [1, ], the shear deformations of the transverse type in the 11 direction were observed only in the cases when impurity centers had the Jahn Teller effect. In the oxide Ni.3 Zn.7 O compound with the NaCl symmetry, the ground state of the Ni + ion (d 8 configuration) has no orbital degeneration and, therefore, the Jahn Teller effect is absent. Likely, the transverse and longitudinal deformations of the oxide Ni.3 Zn.7 O compound in the 111 direction have other origin. Recently, the anisotropic 111 local d d excitations sensitive to small symmetry violations were revealed in NiO [7]. Assuming that the ground state of the highly correlated oxide NiO compound and NiObased solid solutions is also sensitive to small symmetry violations, it can be additional reason of appearance of structural distortions revealed in Ni.3 Zn.7 O. 7. CONCLUSIONS In this work, the X-ray and synchrotron X-ray diffraction patterns of the cubic oxide Ni x Zn 1 x O (x =.3) compound are discussed in detail. Strong arguments in favor of the assumption that this compound contains a superstructure of atomic displacements with the wave vectors q 1 = (1/6 1/6 )π/a c and q = (1/3 1/3 )π/a c (a c =.45 nm) that adequately determines the symmetry of local deformations in the metastable cubic compound. The magnetic order in the compound was established to be noncollinear. In this case, the ferromagnetic component of the structure is oriented along the [ ] crystallographic direction, and the antiferro- 11 PHYSICS OF THE SOLID STATE Vol. 53 No. 7 11

5 1366 DUBININ et al. magnetic component is oriented along the [111] direction. It was assumed that the structural distortions revealed in Ni x Zn 1 x O (x =.3) are due to instability of the ground state of the highly correlated oxide to small deformations. ACKNOWLEDGMENTS This study was supported by the Branch of Physical Sciences of the Russian Academy of Sciences (research program Neutron Studies of the Material Structure and the Fundamental Properties of Matter, project no. 9-T--11 Ural Branch of RAS), State Contract no , and OUS on Physicotechnical Sciences of the Ural Branch of the Russian Academy of Sciences (grant no. 1-M). REFERENCES 1. V. I. Sokolov, S. F. Dubinin, S. G. Teploukhov, V. D. Parkhomenko, and N. B. Gruzdev, Fiz. Tverd. Tela (St. Petersburg) 47 (8), 1494 (5) [Phys. Solid State 47 (8), 155 (5)].. S. F. Dubinin, V. I. Sokolov, A. V. Korolev, S. G. Teploukhov, Yu. G. Chukalkin, V. D. Parkhomenko, and N. B. Gruzdev, Fiz. Tverd. Tela (St. Petersburg) (6), 14 (8) [Phys. Solid State (6), 187 (8)]. 3. S. F. Dubinin, V. I. Sokolov, V. D. Parkhomenko, V. I. Maksimov, and N. B. Gruzdev, Fiz. Tverd. Tela (St. Petersburg) 51 (1), 195 (9) [Phys. Solid State 51 (1), 19 (9)]. 4. V. I. Sokolov, S. F. Dubinin, V. V. Gudkov, and A. T. Lonchakov, Fiz. Tverd. Tela (St. Petersburg) (9), 1697 (8) [Phys. Solid State (9), 1766 (8)]. 5. A. N. Baranov, P. S. Sokolov, O. O. Kurakevych, V. A. Tafeenko, D. Trots, and V. L. Solozhenko, High Pressure Res. 8 (4), 515 (8). 6. S. V. Vonsovskii, Magnetism (Nauka, Moscow, 1971; Wiley, New York, 1974), p B. C. Larson, Wei Ku, J. Z. Tischler, Chi-Cheng Lee, O. D. Restrepo, A. G. Eguiluz, P. Zschack, and K. D. Finkelstein, Phys. Rev. Lett. 99, 641 (7). Translated by Yu. Ryzhkov PHYSICS OF THE SOLID STATE Vol. 53 No. 7 11