DOI: 10.1021/cg9010339
Optical Floating-Zone Growth of Large Single Crystal of Spin Half Dimer Sr3Cr2O8
2010, Vol. 10 465–468
A. T. M. Nazmul Islam,*,† D. Quintero-Castro,†,‡ Bella Lake,†,‡ K. Siemensmeyer,† K. Kiefer,† Y. Skourski,§ and T. Herrmannsdorfer§ †
Helmholtz-Zentrum Berlin f€ ur Materialien und Energie, Glienicker Strasse 100, 14109 Berlin, ur Festk€ orperphysik, Technische Universit€ at Berlin, D-10623 Berlin, Germany, and Germany, ‡Institut f€ § Forschungszentrum Rossendorf EV, Hochfeld Magnetlab, D-01314 Dresden, Germany Received August 26, 2009; Revised Manuscript Received November 5, 2009
ABSTRACT: Large single crystals (∼6 mm in diameter and 35-50 mm in length) of spin dimer system Sr3Cr2O8 have been grown in a four-mirror type optical floating-zone furnace under different atmospheres. Single crystals were characterized by polarized optical microscopy, energy dispersive X-ray analysis, X-ray powder diffraction, X-ray, and neutron Laue measurements, etc. Crystals were grown under an optimized growth atmosphere of flowing (6-8 L/min) synthetic air found to be free of any impurities and excellent in quality. Characteristics of the magnetic susceptibility and magnetization along different crystallographic axes are also shown.
Introduction In recent years, extensive research has been performed on quantum magnets because of their interesting and exotic behavior. One class of quantum magnets are the dimerized magnets where a dominant antiferromagnetic exchange interaction couples the spin moments into pairs or dimers; these systems are characterized by a gap in the magnetic excitation spectrum. Such gapped magnets are interesting because they exhibit Bose-Einstein condensation (BEC) where the magnons can be made to condense into the ground state by application of an external magnetic field. In a Bose-Einstein condensate, many Bosons (in this case, magnons) occupy the same ground state. Such systems have attracted much interest because unusual cooperative phenomena can arise from the condensate, for example, superconductivity and superfluidity as in 4He. The first evidence for BEC in a dimerized magnet was found in TlCuCl3,1 and since then, much experimental work has focused on using gapped magnets to test theories of BEC. In this paper, we describe the single-crystal growth of Sr3Cr2O8, which is a new gapped magnet and a candidate compound for BEC; the magnetic chromium ions are in the rare 5þ valence state with a single electron in the 3d levels and a magnetic moment of S = 1/2.2 The room temperature crystal structure of Sr3Cr2O8 was first solved in 1989 by Cuno and M€ uller and found to be hexagonal (space group R3m), as shown in Figure 1.3 Much more recently, Singh et al.2 measured the DC magnetic susceptibility and heat capacity of polycrystalline samples. The susceptibility could be fitted by a model of weakly coupled dimers with an intradimer exchange constant of 5.34 meV at the base temperature, while the specific heat confirmed the absence of long-range magnetic order in this compound. Chapon et al.4 performed powder neutron diffraction down to 10 K and found that Sr3Cr2O8 undergoes a structural phase transition at 275 K, which lowers the symmetry to monoclinic (space group C2/c).
This distortion both lifts the orbital degeneracy of the Jahn-Teller active Cr5þ ion and relieves the frustration while enhancing the dimerization. Very recently, Sr3Cr2O8 was indeed found to exhibit BEC behavior.5 Measurements of heat capacity and the magnetocaloric effect were performed at high magnetic fields to map out the phase diagram as a function of field and temperature. The critical exponent of the ordering temperature as a function of reduced field around the lower critical field was found to be in agreement with the universality class predicted for a three-dimensional BoseEinstein condensate. The single-crystal growth of Sr3Cr2O8 is challenging since it is a thermodynamically unstable material. According to the phase diagram, the Sr3Cr2O8 phase is stable in air only above ∼1065 °C. Above room temperature, Sr3Cr2O8 starts to react strongly with moisture, and oxygen in air decomposes into SrCrO4 and Sr10Cr6O24(OH)2.6 In addition to this, materials with a higher oxidation state of 5þ of the Cr ions are quite rare; hence, information is lacking about the ideal atmosphere, which makes the synthesis of highly phase pure powder and growth of the single crystal unpredictable. Recent work on the growth of single crystals of the isostructural compound Ba3Cr2O8 reported growth in an argon atmosphere.7 However, no report on the growth of single crystals of Sr3Cr2O8 is yet to be found. In this work, we have grown large single crystals of Sr3Cr2O8 by the floating-zone technique under different atmospheric conditions and studied the effect on the as-grown crystals. The quality of the Sr3Cr2O8 single crystals was checked by polarized optical microscopy, X-ray powder diffraction, X-ray and neutron Laue diffraction, and scanning electron microscopy with energy dispersive X-ray analysis (EDX). After the growth conditions were optimized, a large and high-quality single crystal ∼6 mm in diameter and 40 mm in length was obtained using a seed crystal from the previous growth. Experimental Section
*To whom correspondence should be addressed. Tel: þ49(0)30 8062 2188. E-mail:
[email protected].
The starting material for crystal growth was prepared from highpurity powder of SrCO3 (99.994%, Alfa Aesar) and Cr2O3 (99.97%
r 2009 American Chemical Society
Published on Web 12/01/2009
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Islam et al.
Figure 2. As-grown single crystals of Sr3Cr2O8 grown under different atmospheres: (a) Air flow (6 L/min), (b) argon flow (2 L/min), and (c) synthetic air flow (8 L/min).
Figure 1. Unit cell of Sr3Cr2O8 showing a network of Cr5þ ions having a bilayer structure. Alfa Aesar) mixed thoroughly in a 3:1 molar ratio. After the powders were mixed, the powder of stoichiometric composition was calcined in an alumina crucible in air at 900 °C for 24 h. The powder was then pulverized and packed into a cylindrical rubber tube and pressed hydrostatically up to 3000 bar in a cold isostatic pressure machine. A cylindrical rod with a diameter of about 6 mm and a length of 7-8 cm prepared in this process was then sintered in air at 1200 °C for 12 h. Because Sr3Cr2O8 reacts with atmospheric oxygen and moisture below 800 °C and disintegrates into SrCrO4 and Sr10Cr6O24(OH)2, calcinations of powder and sintering of feed rod were both followed by rapid quenching to room temperature in argon. A dense and crack-free feed could be obtained in this process. Growth was carried out in an optical image furnace (Crystal Systems Corp., Japan) equipped with four 300 W tungsten halide lamps focused by four ellipsoidal mirrors. The feed rod of stoichiometric composition was suspended from the upper shaft using nickel wire, while another small feed rod was fixed to the lower shaft to support the melt. Crystal growths were performed under different atmospheres such as flowing air (6-8 L/min), argon (2 L/min), a mixture of argon and oxygen flow (0-0.4 L/min of O2 and 2 L/min of Ar), synthetic air (20.5% oxygen in N2) (6-8 L/min), etc. The reason for a strong flow of gas from below is to quench the crystal after growth and avoid any reactions. The typical growth rate was about 6-8 mm/h. A seed crystal from a previous growth was used for subsequent crystal growths to avoid random nucleation and to obtain one large single crystal. After growth, a piece of the crystal was ground and checked with X-ray powder diffraction (Bruker D8) for phase purity. A piece of single crystal was also polished and checked with EDX integrated to a scanning electron microscope (SEM) for any residual phase, grain boundaries, or inclusions. Single crystals of Sr3Cr2O8 were also checked with both X-ray and neutron Laue diffraction. The temperature dependence of susceptibility measurements was done in a SQUID magnetometer, while the field dependence of magnetization was measured in a high field magnet up to 65 T on a Sr3Cr2O8 single crystal.
Results and Discussion Figure 2 shows as-grown single crystals grown under different atmospheres. We observe that the crystal grown in flowing air from the compressor attached to the floating-zone machine has a rough surface, while crystals grown in argon and synthetic air have smooth surfaces with a metallic luster. Figure 3 shows X-ray powder diffraction patterns taken with powder obtained by grinding a piece of the crystal. We find that crystals grown in synthetic air and argon are phase pure, while crystal (a) grown in atmospheric air contains a significant amount of the Sr10Cr6O24(OH)2 phase due to reaction
Figure 3. X-ray powder diffraction patterns of crushed powder of different crystals.
with moisture; this is the reason for roughness of the crystal surface. Crystal (b) grown in argon breaks into small pieces after a few weeks. We believe that since the Cr5þ ions are in a higher oxidation state in this system, crystals grown under a lower oxygen partial pressure, for example, as in argon, creates oxygen deficiency in the crystal, which is responsible for the eventual breaking of the crystal. Another difference to note is that the crystals grown in air had strong faces on either side of the crystal, while crystals grown in argon had no facets and are round in shape. A slice of each crystal was taken perpendicular to the growth direction, polished mechanically, and checked with polarized optical microscope and scanning electron microscope with an EDX. The crystals were found to be free of any grain boundaries or inclusion of secondary phase. The composition measured at different points across the cross-section of the single crystal (c) was found to be homogeneous within in the limit of the detector. The single crystallinity of crystal (c) was checked with the automatic neutron Laue diffractometer known as Orient Express in the ILL.8 The Laue photograph taken in back refection geometry is given in Figure 4 (left). The incident neutrons were parallel to the c-axis, and the picture shows 3-fold symmetry from the hexagonal plane as expected. An indexed pattern superimposed on an X-ray Laue photograph taken from the same plane is shown in Figure 4 (right). We have measured the temperature dependence of magnetic susceptibilities on single-crystal samples with field applied along the a- and c-axes as shown in Figure 5. The susceptibilities above 100 K follow a Curie-Weiss behavior and are independent of growth atmosphere or orientation. Below this temperature, χ exhibits a broad peak at 38 K and a sharp drop with decreasing temperature toward zero, which is a characteristic of a dimerized system and is in agreement with
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Figure 4. Neutron Laue backscattering diffraction photograph confirms that the whole as-grown boule (L = 35 mm, D = φ 6 mm) is one single-crystal (left). X-ray Laue diffraction (right) with well-defined round spots gives the orientation of the crystal.
Figure 5. Temperature dependence of the susceptibility of the Sr3Cr2O8 single crystal with a field applied parallel to different crystallographic axes.
previous powder susceptibility measurements.2 The dimers have spin gaps from a singlet ground state to a triplet excited state, while interdimer interactions make the triplet dispersive, giving it a finite bandwidth. For Sr3Cr2O8, the size of the dimer exchange interaction was found to be 5.5 meV from fitting the susceptibility. A small mismatch observed around the maxima of the susceptibilities along c- and a-axes indicates a weakly anisotropic g tensor in the system. The normalized susceptibility below 15 K shows an upturn upon decreasing temperature, which is believed to be the Curie tail from any paramagnetic impurities in the crystals. We observe that crystal (c) has a significantly smaller amount of impurities as compared to crystal (a). Figure 6 shows magnetization of the Sr3Cr2O8 single crystal for a field up to 65 T applied along the c-axis. We observe the appearance of two plateaus at zero magnetization Mo and at saturation magnetization Ms, respectively, with increasing field. The minimum magnetic field Hc required to excite the spin singlet state to the spin triplet state can be obtained from the field derivative of magnetization and was around 30.78 T, which corresponds to a spin-gap energy of 3.562 meV. Note that the data collected on the 65 T magnet (green line) have not
Figure 6. Field dependence of the magnetization of a Sr3Cr2O8 single crystal with pulsed fields up to 65 T along the crystallographic c-axis.
been background corrected; therefore, the first plateau has a small magnetization value. The saturation magnetization was reached at around 62 T. Detailed results of our inelastic neutron scattering measurements on this crystal can be found in a separate paper.9 In this paper, the magnetic excitation spectrum is shown and is found to be in agreement with the interacting dimer model for Sr3Cr2O8. Conclusion We have grown large single crystals of spin 1/2 dimer system Sr3Cr2O8 in a four-mirror type optical floating-zone furnace under different atmospheres and found that single crystals grown in flowing synthetic air are highest in quality as characterized by polarized optical microscopy, EDX, X-ray powder diffraction, and single-crystal neutron Laue diffraction measurement, etc. Our temperature-dependent susceptibility and high field magnetization measurements show the characteristics of a dimerized system with a dimer interaction of 5.5 meV and a spin gap of about 3.5 meV. Acknowledgment. We thank D. Argyriou for help with the lab facilities and Y. Singh for suggestions about powder
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preparations. We also thank M.-H. Lemee-Cailleau and B. Ouladdiaf for help with the Neutron Laue instrument, Orient Express at ILL, and H. Kropf for the SEM and EDX measurements.
References (1) Nikuni, T.; Oshikawa, M.; Oosawa, A. Phys. Rev. Lett. 2000, 84, 5868–5871. (2) Singh, Y.; Johnston, D. C. Phys. Rev. B 2007, 76, 012407. (3) Cuno, E.; Mullerbuschbaum, H. Z. Anorg. Allg. Chem. 1989, 572, 95. (4) Chapon, L. H.; Stock, C.; Radaelli, P. G.; Martin, C. arXiv: 0807.0877.
Islam et al. (5) Aczel, A. A.; Kohama, Y.; Marcenat, C.; Weickert, F.; Jaime, M.; McDonald, R. D.; Selesnic, S. D.; Dabkowska, H. A.; Luke, G. M.; arXiv:0908.3049 [cond-mat.mtrl-sci], 2009. (6) Kisil, Y. K.; Sharova, N. G.; Slobodin, B. V. Izv. Akad. Nauk SSSR, Neorg. Mater. 1989, 25, 1755. Kisil, Y. K.; Sharova, N. G.; Slobodin, B. V. Inorg. Mater. (Engl. Transl.) 1989, 25, 1490. (7) Aczel, A. A.; Dabkowsha, H. A.; Provencher, P. R.; Luke, G. M. J. Cryst. Growth 2008, 310, 870–873. (8) Ouladdiaf, B.; Archer, J.; McIntyre, G. J.; Hewat, A. W.; Braub, D.; York, S. Phys. B 2006, 385-386, 1052–1054. (9) Quintero-Castro, D.; Lake, B.; Wheeler, E. M.; Islam, A. T. M. N.; Rule, K.; Guidi, T.; Izaola, Z.; Kiefer, K.; Russina, M.; Skourski, Y., Herrmannsdoerfer, T., 2009, arXiv:physics/0909.3941 [cond-mat.mtrlsci], arXiv.org e-print archive. http://arxiv.org/PS_cache/arxiv/pdf/ 0909/0909.3941v2.pdf.
