Physica B 259—261 (1999) 969—970
Growth and magnetic properties of (VO) P O -single crystals F. Bu¨llesfeld*, A.V. Prokofiev, W. Assmus, H. Schwenk, D. Wichert, U. Lo¨w, B. Lu¨thi Physikalisches Institut der J.W. Goethe Universita( t, Robert-Mayer-Stra}e 2-4, 60054 Frankfurt a.M., Germany
Abstract We report on crystal growth experiments and discuss the magnetic properties of the low dimensional spin system (VO) P O (VOPO). We were able to grow crystals up to 10;3;3 mm in size. Our magnetic susceptibility data are interpreted numerically using a model with alternating spin chains. Furthermore, we find a spin gap of 67 K from our ESR measurements similar to the gap calculated from susceptibility data. 1999 Elsevier Science B.V. All rights reserved. Keywords: Low-dimensional spin system; Crystal growth; Vanadyl-pyrophosphate
(VO) P O (VOPO) has a layered structure [1,6]. Each layer consists of pairs of vanadyl octahedra, which are separated by phosphate tetrahedra. The layers are stacked with the vanadyl octahedra one above the other so that they form a ladder in the a-direction. Two possible models for the magnetic properties measured on powder samples have been discussed in the literature [2,3]: an alternating chain along the b-direction or a two leg ladder model along the a-direction. Recently, neutron scattering experiments  on an arrangement of many (2 0 0) tiny single crystals (dimensions (1 mm) gave evidence for the alternating chain model. We were able to grow large single crystals . We present magnetic resonance (ESR) and susceptibility data for these single crystals as a function of temperature . We give an interpretation of our results using the alternating chain model [3,6]. To investigate the stability range of the VOPO-phase we performed several heating experiments under different atmospheres followed by a characterization with XRD, SEM and microprobe. It turned out that it is possible to melt VOPO in an atmosphere of argon containing 0.2— 0.7 vol% oxygen without decomposition. The VOPO
* Corresponding author. Tel.: 49-069-798-22621; fax: 49-069798-28520; e-mail: [email protected]
melt however has a tendency to glass formation during cooling due to its high viscosity. VOPO-powder for the growth was prepared by thermal decomposition of the precursor (VO)HPO ) 0.5H O in an argon flow at 700°C. The precursor was synthesised according to Centi et al. . Crystal growth was carried out in a resistance furnace under flow of a gas mixture of argon and oxygen through the silica growth chamber using a Pt-crucible. We used a combination of Czochralski and Kyropoulos methods, i.e. slow cooling of the melt (4—8 K/day) with simultaneous pulling (2—2.5 mm/day) of the grown crystals from the melt. Seed selection was performed by temperature oscillations during the first two days of the growth. After growth the crystals were pulled with a higher pulling speed out of the melt. A cooling of the furnace (40 K/h) with simultaneous decrease of the oxygen content followed. The crystals were examined and oriented using Laue backscattering method. In SEM and microprobe analysis no impurity phase was detected. We have measured magnetic resonances in the frequency range 55—288 GHz. In the three crystallographic directions we find Zeeman split lines with g-factors g "1.937, g "g "1.984. This is in very good agree? @ A ment with the averaged results in the powdered samples . There the g-factors were calculated to be 1.94 and 1.98. In addition we noticed see some small resonances. In Fig. 1 we show the integrated line intensity of the main
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F. Bu( llesfeld et al. / Physica B 259—261 (1999) 969—970
The measured susceptibility is shown in Fig. 1. A small difference for the different crystallographic directions is mainly due to the different g-factors. There is a maximum at 74$2 K for all directions. The defect part because of V> ions is much smaller than in polycrystalline samples . The high temperature region of the susceptibility can be fitted using a Curie law. We get an average valence of 4.0 for the V-ions. If we take an alternating chain model  we can determine J"51 K and d"0.2 with exact diagonalization. An additional frustration shifts the maximum to higher temperatures at constant height (Fig. 1). This work was supported by the SFB 252. Fig. 1. Left: Integrated line intensity. Right: Susceptibility.
References resonance at 134 GHz. The resonance arises from excited triplet excitations. The full line indicates the calculation for a singlet—triplet gap of 67 K"1374 GHz. This energy gap fits to the upper mode of the measured excitation spectra . The temperature dependence of one of the small resonances points to a gap of 40 K comparable with the lower gap in inelastic neutron scattering . Details of these resonances will be discussed elsewhere.
       
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