Introduction

Although MgAl₂O₄ (MALO) spinel belongs to highest (m3m) symmetry group, and, therefore, not much effort is required to cut properly oriented seeds for pulling these crystals by Czochralski method, by no means this is easy to be grown due to its high melting point (2105^oC [1] to 2130^oC [2]). Therefore, only a limited number of papers exist on growing these crystals. Basically, some details on Czochralski growth were only provided in a few earlier works (e.g. [3]). However, many researchers has paid considerable attention to MALO (also grown by other techniques (e.g. micro-pulling down [4]), since this is an interesting material to be used in many applications, including saturable absorbers in laser heads. However, the researchers usually buy this crystal from different manufacturers, and they usually neither pay much attention to growth conditions, nor to real parameters of the crystals themselves, which as a standard procedure are taken from the manufacturers’ catalogues.

We grew undoped and Co-doped MALO by classical Czochralski method from iridium crucible. Optical quality of crystals was verified by conoscopic and polariscopic methods. Although we made a couple of other experiments, our major efforts were concentrated upon investigation of thermal properties of this crystal, because they are very important in laser technique, and to our knowledge they haven’t been yet well verified by other investigators.

State of the art, crystal growth and investigations

Although there has been nothing odd about growing MALO by the Czochralski method, one has to be careful about the process itself, since in temperatures exceeding 2100^oC a certain deformation as well as a certain evaporation of the crucible material (iridium) can be observed. Especially then a suitable thermal isolation of the crucible is needed. Another problem is doping of MALO with Co⁺² ions, which makes the crystal to work as a saturable absorber in laser heads. Apart of the mentioned work by Volk et al. [1],as said only a few reports have been presented on making single crystals. More detailed studies of alumina-rich MALO were conducted by Tang et al. [5-7]. There have been also a couple of investigations on powdered MALO [e.g. 8-9]. A very nice example of using MALO as saturable absorber at eye-safe wavelength of 1.5 µm was given by Mlynczak et al. [10].

Since we expect several kW of absorbed pumping powers, as well as ab. 2-5 KW of the output powers from miniature laser structures (it’s expected that MALO will be thermally bonded to the host material (e.g. Er,Yb glass)) it is important to know basic thermal parameters of the absorber that can be later compared to this of the host. Since we expected that these parameters may depend on Co concentration, we grew crystals doped with Co between 0.06 and 0.21 at. %.

Since at the beginning we had no seeds, we grew two undoped MALO using iridium wire. By X-ray method it was found that the both crystals were pulled in <111> direction. Next we grew Co-doped MALO using regular seeds that were cut out from the undoped crystals. All of the crystals were checked by XRD (powder) technique for eventual admixtures of different crystallographic phases. After cutting off the conical and tail parts, the end faces of remaining boules were polished for optical investigations.

By conoscopic, polarimetric, plane- and circular-polariscopic methods it was found that the majority of crystals were grown without core in their central parts, which, on the contrary, is a typical case in e.g. <111> YAG crystals which also grow, like MALO, with convex front of crystallization (to the melt), as well as they also belong to m3m symmetry group. Although it’s rather a well know fact that the core “consumes” practically all of so-called residual stresses (the core is avoided when cutting the samples for practical use), a minor residual stresses in these crystals were, generally, a good news from the point of view of MALO’s further applications. After measuring refractive indices and their optical dispersion in all crystals, we also cut a certain number of wafers and other elements for spectral and thermal investigations. So, we examined undoped and Co-doped crystals for linear expansion coefficient, specific heat, thermal diffusivity and thermal conductivity. Although from general theories (e.g. [11]) it follows that the thermal properties are (or should be) isotropic in cubic crystals, we also cut a certain number of <100> oriented samples for these investigations. In the future we are planning researches of laser parameters of these crystals as saturable absorbers. There is no room for futher discussion of this problem here, but, however, there is a need for checking whether a couple of these parameters are either orientation dependent, or they depend on the plane of polarization of the incident radiation.

References

[1] Y. V. Volk, A. M. Malyarevich, K. V. Yumashev, V. N. Matrosov, T. A. Matrosova and M. I. Kupchenko, “Anisotropy of nonlinear absorption in Co²⁺:MgAl₂O₄ crystal”, Appl. Phys. B88, 443-447 (2007).

[2] Products catalogue of MTI Corp. www.mtixtl.com

[3] E. Kasper, P. Korczak and H. Henkel, “X-ray topographic analysis of dislocations in Czochralski-grown stoichiometric MgAl₂O₄ spinel single crystals”, J.Mater. Sci. 9, 1699-1700 (1974).

[4] A. Jouni, A. Yoshikawa, T. Fukuda and G. Boulon, “Growth and characterization of Mn²⁺-Activated magnesium aluminate spinel single crystals”, J. Cryst. Growth 293, 517-521 (2006).

[5] H. Tang, J. Xu, Y. Dong and F. Wu, “Growth and annealing properties of Mg_0.4Al_2.4O₄ crystal”, J. Alloys Comp. 470, L29-L32 (2009).

[6] H. Tang, J. Xu, H. Li, Y. Dong, F. Wu and M. Chen, “Structure, thermal expansion and optical property of alumina-rich spinel substrate”, J. Alloys Comp. 479, L26-L29 (2009).

[7] H. Tang, J. Xu, H. Li, Y. Dong, F. Wu and X. Yang, “Thermal and mechanical properties of novel substrate crystal Mg_0.4Al_2.4O₄”, Mater. Lett. 63, 1800-1802 (2009).

[8] I. Ganesh, R. Johnson, G.V. N. Rao, Y. R. Mahajan, S. S. Madavendra and B. M. Reddy, “Microwave-assisted combustion synthesis of nanocrystalline MgAl₂O₄ spinel powder”, Ceramics Int. 33, 67-74 (2005).

[9] V. Singh, M. Haque and D. K. Kim, “Investigation of a new red-emitting, Eu³⁺-activated MgAl₂O₄ phosphor”, Bull. Korean Chem. Soc. 28, 2477-2480 (2007).

[10] J. Mlynczak, K. Kopczynki, Z. Mierczyk, M. Malinowska and P. Osiwianski, „Pulse generation at 1.5 µm wavelength In New EAT14glasses moped with Er³⁺ and Yb³⁺ ions”, Opto-Electron. Rev. 20, 14-17 (2012).

[11] J. F. Nye, “Physical properties of crystals”, Clarendon, Oxford 1957.

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