Volume 4 1 A, number 5 PHYSICS LETTERS 23 October 1972 DEPENDENCE OF THE INSULATOR-METAL TRANSITION IN EuO ON MAGNETIC ORDER* Y. SHAPIRA and S. FONER Francis Bitter NatioruzIh@net Laboratory, MIT, Cambridge, Mass. 02139, USA T.B. REED Lincoln Laboratory, MIT, Lexington, Mass. 02173, USA and H. BIRECKI and H.E. STANLEY Department of PHysics, MIT, Cambridge, Mass. 02139, USA Received 22 August 1972 The Hall coefficient in an EuO sample which exhibited an isulator-metal transition was measured at T = 4Tc in fields uu to 150 kOe. The results indicate that the activation energy does not vary linearly with magnetization, contrary to the conclusion of Penney et al. Recently Penney et al. concluded that the activa- tion energy of insulator-metal transition in EuO varies linearly with the magnetization [l] _ In this letter we present data whichi disagree with this con- clusion. The resistivity p of some EuO samples decreases by many orders of magnitude between the Curie temperature (T,= 69K) and 50K [ 1,2]. This “insu- lator-metal transition” is due primarily to a change in the carrier concentration n, although a change in the mobility p may be present. A model which ex- plains the insulator-metal transition was proposed by Oliver et al. [2] and was modified by the group at IBM [l] . In this model, above - 50K,n is propor- tional to exp (- A/kT), where A is an activation energy which depends on magnetic order. At T 3 T,, A assumes a constant value, Ao, and p - exp(Ao/kT). Recently Penney et al. [ 1) considered two pos- sible dependences of A on magnetic order: 1) A varies linearly with the reduced magnetization (long range order parameter) CJ = W/S, i.e., A = A,( 1 -au) , (1) where a is a constant; 2) A varies linearly with the nearest-neighbor two-spin correlation function (short * Work supported by NSF, by the Department of the Air Force, and by ONR. cl 40 00 120 H CkOe) Fig. 1. H-dependence of the Hall coefficient at 272 K (semi- log scale). The theoretical curves were calculated from eqs. (1) and (2) using the magnetic susceptibility of this sample, and 4 =0.32eV.a= 1.39, b = 1.5, q(H) -q(O) = 1.080’ range order parameter) 77 = W, es,)/@, i.e., A = A,(1 -bp) , (2) where b is a constant. From their analysis of ~(0 at T < T, Penney et al. concluded that A obeys eq. (1) but not eq. (2). However, in this analysis the possi- bility that 1-1 depends on T was not taken into account. In addition, agreement of the resistivity data at 471