Magnetic properties of the RbNd(WO 4 ) 2 single crystal. M.T.Borowiec 1*, E.Zubov 2, T.Zayarnyuk 1, M.Barański 1. Poland.

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1 Magnetc propertes of the RbNd(WO 4 ) sngle crystal M.T.Borowec 1*, E.Zubov, T.Zayarnyuk 1, M.Barańsk 1. 1 Insttute of Physcs, Polsh Academy of Scences, al.lotnków 3/46, -668 Warsaw, Poland. A. A. Galkn Donetsk Physc-Techncal Insttute, Donetsk, Ukrane. Abstract The magnetc nvestgatons as a functon of temperature and magnetc feld for the rubdum neodymum double tungstate RbNd(WO 4 ) sngle crystal have been performed. The magnetzaton was measured n the temperature range from 4. to 1 K and for the magnetc feld up to 1.5 T. The crystal feld and exchange parameters were found. PACS: 75.4.Cx; 75.5.Ee; 75.1.b Keywords: rare earth double tungstate, magnetzaton, ansotropy. * Correspondng author. E-mal: borow@fpan.edu.pl 1. Introducton The rubdum neodymum tungstate RbNd(WO 4 ) (RbNdW) s the representatve of the famly of alkalne (A) and rare earth (Re) double tungstates ARe(WO 4 ) (AReW). RbNdW belongs to the monoclnc system wth space group C/c; whch s sostructural wth - KY(WO 4 ) [1]. Ths crystal s usually grown by hgh temperature soluton method n order to obtan the low temperature monoclnc phase. At present, the AReW tungstates havng the low-symmetrc (e.g., monoclnc) crystallne structure and the atom arrangements n forms of chans or layers are ntensvely studed. Structural, optcal and magnetc nvestgatons of the KDyW, KHoW, KErW and RbDyW compounds were performed earler []. Many of them show complcated structural phase transtons (SPT), caused by the cooperatve Jahn Teller effect (CJTE), and magnetc phase transtons. The rubdum neodymum double tungstate, was also studed, especally ts structural and spectroscopc propertes [1].

2 In ths paper, we show the new results of magnetzaton measurements for the rubdum neodymum double tungstate RbNd(WO 4 ) (RbNdW) sngle crystal.. Magnetc propertes of RbNd(WO 4 ) The temperature, magnetc feld and angular magnetzaton dependences of the RbNd(WO 4 ) sngle crystal were nvestgated usng the vbratng sample magnetometer (PAR Model 45) n a temperature range from 4. to 1 K for magnetc feld up to 1.5 T. The feld was appled both n the ac plane and along the b-axs. The electron confguraton of Nd 3+ s 4f 3. In the crystal feld of monoclnc symmetry the ground multplet 4 I 9/ splts nto fve Kramers doublets. An angular dependence of magnetzaton has allowed to determne the magnetc x and z-axes, whch correspond to the drectons of mnmal and maxmal values of magnetzaton n the ac crystallographc plane, respectvely (fg. 1). The angle between c- and z-axes n clockwse drecton s equal to 86 and the angle between a- and x-axes s equal to 46. The thrd man y magnetc axs s parallel to the second-order axs C and concdes wth the crystallographc b- axs perpendcular to the ac plane c 3 RbNd(WO 4 ) 3 33 Fg. 1. The angular dependence of magnetzaton for RbNd(WO 4 ) sngle crystal. The magnetc feld dependences of magnetzaton M both along all magnetc axes and along the x, y and z crystal drectons do not dsplay the magnetzaton saturaton n temperature

3 nterval from 4. to 1 K and n magnetc feld up to 1.45 T (fg. ), that t s characterstc for paramagnet. 3 1 [emu/mol] -1 - along x axs along y axs along z axs H[ koe] Fg.. The magnetc feld dependences of magnetzaton along three magnetc axs at T=6 K. For a weak magnetc feld H we may use lnear relaton for susceptblty ( T) M / H. The expermental temperature dependences ( T ) were analyzed usng the Cure-Wess law: C (1) T General fttng was used for fndng the parameters : (temperature ndependent part of susceptblty), the Cure constant C = N A ( B g ) J(J+1)/3k B and (paramagnetc temperature) (see table 1). Table 1. The fttg parameters for the Cure-Wess law. χ C g θ H a H=.15 T emu/mol emu K/mol K H b H=.1 T emu/mol 5.55 emu K/mol K H c H=.1 T emu/mol 1.13 emu K/mol K

4 The results of fttng are also presented n Fg. 3(a,b,c). The paramagnetc temperature has a large value along the x and y drectons experment along axs y general fttng a [mol/emu] 8 4 experment along axs x general fttng b [mol/emu] T [K] T [K] 7 experment along axs z general fttng 5 c [mol/emu] T [K] Fg. 3 abc. The temperature dependences of nverse susceptblty along three magnetc axes: ponts (experment) and lne (theory). The obtaned values of paramagnetc Cure temperature were used to fnd the crystal feld parameters. For the C symmetry of Nd 3+ ste H ˆ cr nvolves 15 non-zero crystal feld parameters because for the z-axs parallel to the C axs the crystal feld parameters wth the odd q are equal to zero. Followng to Ref. [3] we shall restrct consderaton only to second order crystal feld parameters. Our system has the symmetry axs of the second order that excludes n crystal feld Hamltonan the Stevens operators wth odd powers. Acordng to that the Hamltonan has the followng form wth the frst nonvanshng terms: Hˆ B 3 J J( J 1) B J J () cr. z x y

5 We attempted to estmate crystal feld parameters B and B. At hgh temperatures, the relatons between the crystal feld parameters and crystal feld contrbuton n paramagnetc temperatures cr n the drecton have a form 1 B 1 B B cr. x cr. y cr. z B B, (3) where 1 J 1 ( J 3) [3]. For J = 9/ = 96/5. In our case, the x, y and z axes 5 concde wth the c, a and b axes, respectvely. Then the paramagnetc temperature can be expressed as [3], (4) cr. exch. where the exchange contrbuton n paramagnetc temperature s equal to exch. 33 J( J 1) J() J(). The J()=z*J, where z -number of the nearest neghbours of 3 the rare-earth on. J s the parameter of the par exchange nteracton. Snce the contrbuton of crystal feld n sum of paramagnetc temperatures along the 3 rd drecton s equal to zero, only the 3 rd contrbuton from exchange reman. 99 From relaton x y z J () we obtan J() = K, and the crystal feld contrbuton n paramagnetc temperatures s equal to cr. cr. cr. x 68.3K, y K and z -3.16K. Then we obtan the followng parameters of crystal feld Hamltonan B 1.68K, B 5.44K. It gves fve Kramers doublets wth energes of, 85.4, 149.4, and 13.9 K. In summary, we have found that the magnetzaton shows a strong ansotropy n temperature range studed. In the temperature range up to 1 K, the expermental curves of susceptblty follow to the Cure-Wess law. By fttng the calculated susceptblty to the expermental data, the crystal feld and exchange parameters and g-factors along man crystal drectons were calculated. The energes of Nd 3+ spectra n a low symmetrc crystal feld were estmated. Acknowledgements

6 Ths work was supported by EU project DT-CRYS, NMP3-CT , of the Polsh State Commttee for Scentfc Research (KBN) (decson of project No. 7/E-67/SPB/6. PR/DIE 43/4-6), and by the European Regonal Development Fund through the Innovatve Economy grants POIG /8 and POIG /9. Reference [1] M.T. Borowec, A.D. Prokhorov, I.M. Krygn, V.P. Dyakonov, K. Woznak, L. Dobrzyck, T. Zayarnyuk, M. Baransk, W. Domukhowsk, H. Szymczak, Physca B 371, 5 (6) M.T.Borowec, A.A.Prokhorov, A.D.Prokhorov, V.P.Dyakonov, H.Szymczak J. Phys., Condens. Matter 15, 5113 (3) G. Lenec, T. Skbnsk, S.M. Kaczmarek, P. Iwanowsk, M. Berkowsk Cent.Eur.J.Phys. 1, 5 (1) []. M.T. Borowec, A. Watterch, T. Zayarnyuk, V.P. Dyakonov, A. Majchrowsk, J. Zmja, M. Baransk, H. Szymczak, J. Appl. Spectrosc. 71, 888 (4) M.T. Borowec, I. Krynetsk, V.P. Dyakonov, A. Nabałek, T. Zayarnyuk, H. Szymczak, New J. Phys.8, 14 (6) M.T. Borowec, V.P. Dyakonov, K.Woznak, L. Dobrzyck, M. Berkowsk, E.E. Zubov, E. Mchalsk, A. Szewczyk, M.U. Gutowska, T. Zayarnyuk, H. Szymczak, J. Phys., Condens. Matter 19, 566 (7) M.T. Borowec, Proc. SPIE 441, 196 (1) M.T. Borowec, A.D. Prokhorov, I.M. Krygn, V.P. Dyakonov, K. Woznak, L. Dobrzyck, T. Zayarnyuk, M. Baransk, W. Domukhowsk, H. Szymczak, Physca B 371, 5 (6) [3] A.K.Zvezdn, V.M.Matveev, A.A.Mukhn, A.I.Popov, Rare earth ons n magnetcally ordered crystals, Moscow, Nauka, 1985, p.1.