Single crystal growth and physical properties of pyroxene CoGeO$_3$

We report on the synthesis and physical properties of cm-sized CoGeO$_3$ single crystals grown in a high pressure mirror furnace at pressures of 80~bar. Direction dependent magnetic susceptibility measurements on our single crystals reveal highly anisotropic magnetic properties that we attribute to the impact of strong single ion anisotropy appearing in this system with T$_N$~$\sim$~33.5~K. Furthermore, we observe effective magnetic moments that are exceeding the spin only values of the Co ions which reveals the presence of sizable orbital moments in CoGeO$_3$.


I. INTRODUCTION
Pyroxenes are one of the main rockforming minerals in the Earth's crust [1][2][3][4] and have the general formula AM X 2 O 6 (A = mono-or divalent metal, M = di-or trivalent metal, X = Si 4+ , Ge 4+ or V 5+ ). This class of materials gained considerable interest due to their large amount of diverse properties 5,6 including the observation of multiferroicity and magnetoelectric effects 7 . The quasi 1D system CoGeO 3 having two Co sites belongs to the family of pyroxene minerals 8 and crystallizes in two polymorphs 8,9 -the monoclinic polymorph is stable above 1351 • C and the orthorhombic one below this temperature 9 . As reported in literature, single crystals of the monoclinic phase with space group C2/c (a = 9.64Å, b = 8.99Å, c = 5.15Å and β = 101 • 10') have been grown from the melt with crystal sizes of the order of 1 × 0.25 × 0.1 mm 39 . The corresponding pyroxene structure of monoclinic CoGeO 3 10 consist of Co1 ions that are forming CoO 6 octahedral zigzag chains running in c-direction, with adjacent Co2 octahedra, compare Fig. 1. The so formed Co ladders (or double-zigzag chains) are separated by GeO 4 tetrahedra from each other. Throughout this article we refer to the monoclinic form of CoGeO 3 which orders antiferromagnetically below T N ∼ 36 K 8 .

II. RESULTS AND DISCUSSION
A photo of our several cm 3 -sized, as-grown single crystal of CoGeO 3 is shown in Fig. 2(a). Powder XRD measurements performed on crushed and powderized parts of the single crystal indicate an impurity-free monoclinic phase, compare Fig. 3. The lattice parameters obtained from a Rietveld-refinement can be found in the crystal structure table (Tab. 1). X-ray Laue and single crystal X-ray diffraction measurements indicate the single crystalline nature of our as-grown crystal. As can be seen in Fig. 2(b) the single crystals are twined with the underlying twin matrix (-1 0 -0.732, 0 1 0, 0 0 1). A precise structural analysis by means of single crystal X-ray  diffraction has been performed which confirms that we have synthesized the monoclinic pyroxene CoGeO 3 -see Tabs. 1-2. The resulting positional parameters (x, y, z) for the six different atoms in the asymmetric unit are in agreement with literature data 10 , but within the high precision of our measurement we were additionally able to determine the anisotropic displacement parameterssee Tabs. 1-2. From the obtained structural parameters also the accurate bond distances could be determined, see Tab. 3. According to the bond valence sum (BVS) formalism these results indicate Co oxidation states which are very close to 2+, see Tab. 3.
The oxidation state of the Co ions in CoGeO 3 was further investigated by soft X-ray absorption spectroscopy (XAS) measurements at the Co L 2,3 edge in the total electron yield mode using a CoO single crystal as a Co 2+ reference. The similarity of the Co-L 2,3 XAS spectra of CoGeO 3 and CoO, see Fig. 4, reveals a Co 2+ high spin state in octahedral coordination [11][12][13] in CoGeO 3 . These observations corroborate the results of the BVS for the Co ions and further confirm the stoichiometry of our single crystals.
The magnetic susceptibility χ of CoGeO 3 shows a drop at T N ∼ 33.5 K, see

III. MATERIALS AND METHODS
The floating zone growth of monoclinic CoGeO 3 was carried out in a high pressure optical mirror furnace (HKZ, SciDre GmbH ). Initially, Co 3 O 4 and GeO 2 with an excess of 3% GeO 2 were mixed together and sintered at 1200 • C for 72 hours with intermediate grindings. From these powders, polycrystalline rods were made using a hydrostatic press and subsequently sintered at 1300 • C for 24 h.
During the floating zone growth pressures of 80 bar of an Argon/O 2 mixture (with a ratio of 98 : 2) were used  I. Crystallographic & structural refinement data of a single crystal X-ray diffraction measurement. The crystallographic software Jana was used for the structural refinement 15 . The lattice parameters were obtained from a powder X-ray diffraction measurement using Cu Kα1 radiation (Rietveld refinement with Fullprof 16 ; χ 2 = 2.24). [*: also unobserved reflections were used.] and a growth rate of 3.6 mm per hour was successful for growing large (twined) CoGeO 3 single crystals (heating power ∼2700 W).
Powder and single crystal X-ray diffraction (XRD) measurements have been performed on a Bruker D8 Discover A25 (Cu K α1 radiation) and on a Bruker D8 VEN-TURE diffractometer (Mo K α radiation) respectively. Soft X-ray absorption spectroscopy measurements have been performed at the BL11A Beamline of National Synchrotron Radiation Research Centre (NSRRC) in Taiwan. The oxidation state of Co ions in CoGeO 3 was determined by soft X-ray absorption spectroscopy (XAS) measurements at the Co L 2,3 edge in the total electron  yield mode using a CoO single crystal as a Co 2+ reference.
Direction dependent magnetic properties of single crystals of CoGeO 3 were initially studied using a SQUID magnetometer (MPMS-5XL, Quantum Design Inc.).

IV. CONCLUSIONS
We have grown sizable single crystals of CoGeO 3 in a high pressure floating zone furnace that were characterized by XRD and XAS measurements. Our direction dependent magnetic susceptibility measurements on our single crystals reveal highly anisotropic magnetic properties with large effective moments of ∼5 µ B per Co ion which are indicative for (i) the significance of single ion anisotropy and (ii) the occurrence of large orbital moments in this system.