* Corresponding author. Tel.: #49-821-5983453; fax: #49- 821-5983411. E-mail address: stefan.klimm@physik.uni-augsburg.de (S. Klimm). Journal of Magnetism and Magnetic Materials 226}230 (2001) 216}217 Pressure and doping dependence of Hall e!ect and e!ective mass in V  O  S. Klimm*, M. Herz, R. Horny, G. Obermeier, M. Klemm, S. Horn Universita ( t Augsburg, Lehrstuhl fur Experimentalphysik II Universita ( tsstr. 1, D-86135 Augsburg, Germany Abstract We present results of electrical resistivitity and Hall e!ect measurements on single crystalline V  O  as a function of temperature, pressure and doping y. The pressure dependence of the Hall e!ect suggests an interpretation in terms of spin #uctuations, which is also consistent with the temperature dependence of the electrical resistivity .Ifa ¹ "t is applied to  at low temperatures a saturation rather than a divergence of the corresponding e!ective mass results as the metal}insulator transition is approached.  2001 Elsevier Science B.V. All rights reserved. Keywords: Hall e!ect; High pressure; Spin #uctuations V  O  shows a transition from a paramagnetic metal (PM) to an antiferromagnetic insulator (AFI) below ¹  +170K.Holedoping(V  O  ) shifts this transition to lower temperature, until for y'0.015 a metallic spin density wave (SDW) state with an incommensurate wave vector q along the hexagonal c-axis [1] forms below ¹  +10 K. The PM phase can be stabilized down to the lowest temperatures by hydrostatic pressure. We have performed pressure-dependent transport measurements on single crystal V  O  samples with various vanadium de"ciencies y as a function of the orientation of the direction of current j and magnetic "eld B with respect to the c-axis close to the phase boundaries (AFI-PM and SDW-PM). Samples were grown by the chemical transport technique with TeCl  as transport agent. The vanadium de"ciency y was esti- mated from the measured transition temperatures ¹  or ¹  , respectively, in accordance with Ref. [2]. Pressure was applied using a CuBe piston-cylinder device with a manganin gauge in a quasi-hydrostatic medium (n- pentane-isoamyl). Stoichiometric samples (y"0), which are stabilized in the PM phase by the application of pressure p2GPa, show a residual resistance ratio of RRR"50 and a quadratic temperature dependence of resistivity "  #A ) ¹. The coe$cient A increases on ap- proaching the metal}insulator transition as a function of pressure [3]. The magnetoresistance is positive and scales according to Kohler's rule. The onset of SDW-order at ¹  in samples with y*0.015 is marked by a resistivity minimum for jc and akinkfor jc (Fig. 1). Below ¹  the magnetoresistance is negative and strongly anisotropic and shows metamag- netic behavior [4]. The Hall e!ect depends strongly on temperature and the direction of B. At ¹"300 K the Hall constant is found to be R  , "2.4$0.110 m/C for Bc and R  "0.8$0.110 m/C for Bc and is almost independent of pressure and doping. R  shows a max- imum as a function of temperature at ¹  , which in the doped samples apparently coincides with ¹  and was concomitantly discussed in terms of skew scattering [5]. The pressure dependence of the maximum, however, de- pends on the direction of B (Fig. 1). While the maximum of R  tracks the pressure dependence of the resistivity minimum, R  , is nearly independent of pressure. Also, the maximum in R  disappears for yP0 while R  , shows a maximum around 30 K for paramagnetic stoichiometric samples (under p2GPa). 0304-8853/01/$-see front matter  2001 Elsevier Science B.V. All rights reserved. PII:S0304-8853(00)00907-0