ISSN 10637834, Physics of the Solid State, 2011, Vol. 53, No. 7, pp. 1362–1366. © Pleiades Publishing, Ltd., 2011. Original Russian Text © S.F. Dubinin, V.I. Maksimov, V.D. Parkhomenko, V.I. Sokolov, A.N. Baranov, P.S. Sokolov, Yu.A. Dorofeev, 2011, published in Fizika Tverdogo Tela, 2011, Vol. 53, No. 7, pp. 1292–1296.
MAGNETISM
Fine Structure and Magnetism of the Cubic Oxide Compound Ni0.3Zn0.7O S. F. Dubinina, *, V. I. Maksimova, V. D. Parkhomenkoa, V. I. Sokolova, A. N. Baranovb, P. S. Sokolovb, and Yu. A. Dorofeeva a
Institute of Metal Physics, Ural Branch, Russian Academy of Sciences, ul. Sof’i Kovalevskoi 18, Yekaterinburg, 620219 Russia * email:
[email protected] b Moscow State University, Moscow, 119991 Russia Received December 28, 2010
Abstract—The fine structure and spin system of the cubic oxide Ni0.3Zn0.7O 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 Xray 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)2π/a and (1/3 1/3 1/3)2π/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 Ni0.3Zn0.7O compound have been briefly discussed. DOI: 10.1134/S1063783411070092
1. INTRODUCTION In recent years, considerable interest has been expressed in studies of highly doped semiconductor compounds MexZn1 – xO (Me = Ni2+, Co2+, Fe2+, and Mn2+) belonging to a wide class of A2B6 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 A2B6 compounds have either the cubic symmetry (e.g., ZnSe : Me) or the hexagonal symmetry (ZnO : Me). In the sphalerite and wurtzite structures, Me2+ ions sub stituting for Zn2+ have the tetrahedral environment characteristic of mixed ioniccovalent bonds with a predominance of covalence. Earlier, it was established by thermal neutron dif fraction that the DMS system contains, in a wide tem perature range, two types of local Jahn–Teller distor tions of the cubic crystal lattice, namely: (1) nanode formations of the trigonal type in ZnSe : Ni [1], and (2) nanodeformations of the tetragonal type in ZnSe : Cr [2]. Shear deformations are also observed in the hexagonal crystal ZnO : Ni [3].The shear deforma tions revealed from the diffuse neutron scattering cor relate with the change in the propagation velocity of transverse ultrasonic waves [4]. The A2B6 compounds are diamagnetic; thus, the influence of a paramagnetic impurity on local structural deformations can be stud
ied not only by classical diffraction methods, but also using magnetic measurements [2, 3]. The aim of this work is to study the fine structure and magnetism of the metastable cubic oxide com pound Ni0.3Zn0.7O (space group Fm3m) at 300 K, which was produced by quenching from a high tem perature (1470 K) and on applying an external hydro static 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. 2. SAMPLES AND EXPERIMENTAL TECHNIQUE We studied the NixZn1 – xO (x = 0.3) compound. The sample was a 0.05cmthick plate with linear sizes in the flat part of 0.2 × 0.2 cm. In this work, we contin ued the experiments started in [5] on the study of the NixZn1 – xO (x = 0.3) compound with the metastable cubic crystal lattice. The fine crystal structure of Ni0.3Zn0.7O was stud ied using synchrotron and Xray diffraction. The wavelengths of the synchrotron and Xray radiations were λ1 = 0.68805 Å and λ2 = 1.5418 Å, respectively. The magnetization and the magnetic susceptibility of the crystal were measured using a Quantum Design MPMS5XL.
1362
FINE STRUCTURE AND MAGNETISM (200)
(111)
(200)
3
(331)
1
(7/3 1/3 1/3)
(311) (400)
(13/6 1/6 1/6)
(220)
10
(11/6 1/6 1/6)
20
2
(5/3 1/3 1/3) (5/6 7/6 7/6)
30 (111)
(4/3 2/3 2/3)
40 Intensity, arb. units
Intensity, arb. units
50
1363
0 30
40
50
60 70 2θ, deg
80
90
(7/6 5/6 5/6)
100
0 34
38
42 2θ, deg
46
50
Fig. 1. Xray diffraction pattern of the polycrystalline NixZn1 – xO (x = 0.3) compound after thermohydraulic treatment.
Fig. 2. Xray diffraction pattern of the superstructure dif fuse maxima in the cubic NixZn1 – xO (x = 0.3) compound measured at 300 K.
3. RESULTS OF XRAY DIFFRACTION ON NixZn1 – xO (x = 0.3)
Actually, the relative intensity of the superstructure reflections of the displacement type is determined by the formula [1]
Figure 1 shows the Xray diffraction pattern at 300 K. As seen from the figure, the pattern of scatter ing includes intense (111), (200), (220), (311), (400), and (331) reflections corresponding to the fcc phase with the lattice parameter ac = 4.25 Å. In addition to the intense Bragg reflections of the fcc phase, the Xray pattern contains a system of low intense diffuse maxima. The diffuse maxima are well seen in the scattering pattern built in a handy scale (Fig. 2). They are thought should be considered super structure 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 q1 = (1/6 1/6 0)2π/ac and q2 (1/3 1/3 0)2π/ac. The positions of the superstructure reflections q1 and q2 on the ( 011 ) 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 Xray scattering amplitudes are very close to one another. It is most likely due to the systematic displacements of Ni2+ 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 (022) and (200) points of the reciprocal lattice. This fact unambiguously dem onstrates that the superstructure is due to the trans verse static displacement of ions in the fcc lattice with respect to the {111} crystallographic directions. PHYSICS OF THE SOLID STATE
Vol. 53
No. 7
2
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 ( 011 ) crystallographic planes, the super structures must to be observed in the direction con necting the (022) and (200) points of the reciprocal lattice. We select two physical parameters in the scattering patterns, namely, the half width Δqobs and the height h of the superstructure diffuse maximum. The observed Δqobs and industrial Δqinst halfwidths allow the deter mination the real halfwidth 2
2
1/2
Δq = ( Δq obs – Δq inst ) ,
(2)
related to the mean size of the structural inhomogene ity by the relationship L = 2π/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 sit uation in the NixZn1 – xO (x = 0.3) compound after a thermal hydraulic treatment. We are interesting, first, in the mean size of the structural inhomogeneity. This
2011
1364
DUBININ et al. (222)
(2/3 4/3 4/3) (5/6 7/6 7/6) (5/6 5/6 5/6) (2/3 2/3 2/3)
(111)
3
Intensity, arb. units
(022)
(111) (7/6 5/6 5/6)
(200)
(4/3 2/3 2/3) 2 (1/3 1/3 1/3) (2/3 2/3 2/3) (5/6 5/6 5/6) 1
(4/3 2/3 2/3) 6
8
10
12 14 2θ, deg
16
18
20
Fig. 4. Synchrotron Xray diffraction pattern of the super structure diffuse maxima in the cubic NixZn1 – xO (x = 0.3) compound measured at 300 K.
(5/3 1/3 1/3) (1/3 1/3 1/3) (11/6 1/6 1/6) (000)
(200) (13/6 1/6 1/6) (7/3 1/3 1/3)
Fig. 3. Positions of the superstructure diffuse maxima on the ( 011 ) plane of the reciprocal cubic lattice.
quantity along the scattering vector found from rela tionships (2) and (3) is L1 ≅ 200 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 Ni0.3Zn0.7O, 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 q1 and q2 (see above). The magnitudes of the q1 and q2 vectors are thought to be the consequence of the chemical composition of the Ni0.3Zn0.7O compound. 4. RESULTS ON SYNCHROTRON DIFFRACTION ON NixZn1 – xO (x = 0.3) Note that we were fortunate to reveal the longitudi nal superstructure maxima of q1 and q2 (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 Xray beam incident on the sample. As noted in Section 2, λ1 is smaller than λ2 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 Xray diffraction pattern. It
is precisely the circumstance that allows us, in our opinion, to observe on a synchrotron diffraction pat tern (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), (2/3, 2/3, 2/3), 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 Ni0.3Zn0.7O compound. The longitudinal corre lation magnitudes estimated by Eqs. (2) and (3) are L2 ≅ 20 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 trans verse correlations of the Ni2+ ion displacements in Ni0.3Zn0.7O because of, first, a relative small λ1 and, second, a high scatter in the instrumental background (compare Figs. 2 and 4). 5. ANALYSIS OF THE MAGNETIC STATE OF NixZn1 – xO (x = 0.3) Let us analyze the magnetic state of the cubic semi conductor compound NixZn1 – xO, 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) tem perature dependence measured in a magnetic field of 10 kOe in the range T = 2–300 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
2011
FINE STRUCTURE AND MAGNETISM
netic and antiferromagnetic correlations in a magnetic compound. The circles in Fig. 5 indicate the data measured during heating of the sample from 2 K to room temperature. The crosses indicate the χ–1(T) data measured during cooling the sample in the range 300–200 K. The data reliably demonstrate that there is no magnetic hysteresis in the vicinity of 225 K. where the χ–1(T) dependence is nonmonotonic. The solid line in Fig. 5 shows a model function of the tempera ture dependence of the reciprocal paramagnetic sus ceptibility. 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 para magnetic values. This fact shows that, in this tempera ture range, there are ferromagnetic spin correlations [4] in the NixZn1 – xO (x = 0.3) compound, which, as seen from Fig. 5, decrease gradually with temperature, and they become zero at 225 K. This effect of suppres sion of the ferromagnetic correlations is thought to be due to appearance of lowtemperature antiferromag netic correlations in the compound [6]. This assump tion is clearly supported by the level of values of χ–1(T) below 200 K. Actually, as seen from Fig. 5, the level of values of χ–1(T) in the lowtemperature 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 –25 K). Note that the Ni2+ ions are the carriers of the mag netic moment in the compound. According to the measurements of χ–1(T) below 300 K, two types of magnetic correlations, namely, ferromagnetic and antiferromagnetic coexist in the cubic lattice of Ni0.3Zn0.7O. 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 dis tortions. In this connection, the ferromagnetic com ponent of the structure is most probably oriented (with allowance for denotations in Fig. 3) along the [ 011 ] direction, and the antiferromagnetic component is directed along the [111] crystallographic direction. As temperature decreases, the relatively more reg ular antiferromagnetic component gradually sup presses the shortrange ferromagnetic order. To obtain more complete information on the magnetic structure of the cubic semiconductor compound, the experi ment on bulk single crystals are necessary. PHYSICS OF THE SOLID STATE
Vol. 53
No. 7
1365
16 χ−1, 107 g Oe/emu
14 12 10 8 6 4 2 0
50
100
150 T, K
200
250
300
Fig. 5. Temperature dependence of the inverse magnetic susceptibility χ–1 in the NixZn1 – xO (x = 0.3) compound measured in the range 2–300 K. Circles show the data measured during heating in the range 2–300 K, and crosses indicate the data measured during cooling from 300 to 200 K.
6. DISCUSSION OF THE RESULTS In [1, 2], the shear deformations of the transverse type in the 〈110〉 direction were observed only in the cases when impurity centers had the Jahn–Teller effect. In the oxide Ni0.3Zn0.7O compound with the NaCl symmetry, the ground state of the Ni2+ ion (d8 configuration) has no orbital degeneration and, there fore, the Jahn–Teller effect is absent. Likely, the trans verse and longitudinal deformations of the oxide Ni0.3Zn0.7O compound in the 〈111〉 direction have other origin. Recently, the anisotropic 〈111〉 local d–d excita tions sensitive to small symmetry violations were revealed in NiO [7]. Assuming that the ground state of the highly correlated oxide NiO compound and NiO based solid solutions is also sensitive to small symme try violations, it can be additional reason of appear ance of structural distortions revealed in Ni0.3Zn0.7O. 7. CONCLUSIONS In this work, the Xray and synchrotron Xray dif fraction patterns of the cubic oxide NixZn1 – xO (x = 0.3) compound are discussed in detail. Strong argu ments in favor of the assumption that this compound contains a superstructure of atomic displacements with the wave vectors q1 = (1/6 1/6 0)2π/ac and q2 = (1/3 1/3 0)2π/ac (ac = 0.425 nm) that adequately determines the symmetry of local deformations in the metastable cubic compound. The magnetic order in the compound was estab lished to be noncollinear. In this case, the ferromag netic component of the structure is oriented along the [ 011 ] crystallographic direction, and the antiferro
2011
1366
DUBININ et al.
magnetic component is oriented along the [111] direc tion. It was assumed that the structural distortions revealed in NixZn1 – xO (x = 0.3) are due to instability of the ground state of the highly correlated oxide to small deformations.
2.
3.
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. 09T21012 Ural Branch of RAS), State Contract no. 02.518.11.7119, and OUS on Physicote chnical Sciences of the Ural Branch of the Russian Academy of Sciences (grant no. 1M).
4. 5. 6. 7.
REFERENCES
Tela (St. Petersburg) 47 (8), 1494 (2005) [Phys. Solid State 47 (8), 1552 (2005)]. S. F. Dubinin, V. I. Sokolov, A. V. Korolev, S. G. Tep loukhov, Yu. G. Chukalkin, V. D. Parkhomenko, and N. B. Gruzdev, Fiz. Tverd. Tela (St. Petersburg) 50 (6), 1042 (2008) [Phys. Solid State 50 (6), 1087 (2008)]. S. F. Dubinin, V. I. Sokolov, V. D. Parkhomenko, V. I. Maksimov, and N. B. Gruzdev, Fiz. Tverd. Tela (St. Petersburg) 51 (10), 1905 (2009) [Phys. Solid State 51 (10), 2019 (2009)]. V. I. Sokolov, S. F. Dubinin, V. V. Gudkov, and A. T. Lon chakov, Fiz. Tverd. Tela (St. Petersburg) 50 (9), 1697 (2008) [Phys. Solid State 50 (9), 1766 (2008)]. A. N. Baranov, P. S. Sokolov, O. O. Kurakevych, V. A. Tafeenko, D. Trots, and V. L. Solozhenko, High Pressure Res. 28 (4), 515 (2008). S. V. Vonsovskii, Magnetism (Nauka, Moscow, 1971; Wiley, New York, 1974), p. 63. B. C. Larson, Wei Ku, J. Z. Tischler, ChiCheng Lee, O. D. Restrepo, A. G. Eguiluz, P. Zschack, and K. D. Finkelstein, Phys. Rev. Lett. 99, 026401 (2007).
1. V. I. Sokolov, S. F. Dubinin, S. G. Teploukhov, V. D. Parkhomenko, and N. B. Gruzdev, Fiz. Tverd.
PHYSICS OF THE SOLID STATE
Translated by Yu. Ryzhkov
Vol. 53
No. 7
2011
