Physica B 294}295 (2001) 156}159 ESR and CR measurements of Gd monopnictides Keiichi Koyama*, Makoto Yoshida, Takuo Sakon, Hiroyuki Nojiri, Dexin Li, Takashi Suzuki, Mitsuhiro Motokawa Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan Institute for Solid State Physics, University of Tokyo, Kasiwa-shi 277-8581, Japan Abstract Inordertoobtaininformationaboutthemagneticstructureandthecyclotrone!ectivemass(mH  )ofthecarriersinGd monopnictides, electron spin resonance (ESR) and cyclotron resonance (CR) measurements on single crystals have been performed in magnetic "elds up to 25T. The results indicate that GdP and GdAs are easy-plane antiferromagnets with the magnetic moment lying in the (111) plane below the Ne H el temperature. In GdAs, mH  varies from 0.45m  to 0.48m  depending on magnetic "led direction, suggesting that mH  is about two times larger than the masses estimated from the dHvA e!ect.  2001 Elsevier Science B.V. All rights reserved. Keywords: Electron spin resonance; Cyclotron resonance; E!ective mass Rare earth monopnictides RX (X"P,As,Sb,Bi) with NaCl-type structure are strongly correlated systems with low carrier density and show anoma- lous physical properties [1]. Among RX systems, GdX are very simple and good reference com- pounds, because Gd with S"  has no orbital momentum. Magnetic properties of GdX are deter- mined mainly by the indirect exchange and the dipole interactions. Recent results of the magnetic properties were obtained by using high-quality single crystals; these showed that GdN is a ferromagnet with a Curie temperature of 58K, whereas GdP, GdAs, GdSb and GdBi are antifer- romagnets with Ne H el temperatures of 15.9, 18.7, 23.4 and 25.8K, respectively [2,3]. However, the * Corresponding author. Tel.: #81-22-215-2017; fax: #81- 22-215-2016. E-mail address: kkoyama@imr.tohoku.ac.jp (K. Koyama). detailed spin structure has not been reported so far, because no neutron di!raction nor electron spin resonance (ESR) studies for GdX in ordered state have been performed as yet. On the other hand, in order to obtain informa- tion about the electron}electron interaction, it is important to compare the cyclotron e!ective mass (mH  ) determined by cyclotron resonance (CR) measurements with that mH  estimated from the de Haas}van Alphen (dHvA) or the Shubnikov}de Haas (SdH) e!ect measurements. According to Kohn's theorem for electron motion in a Galilean invariant electron gas, mH  is independent of the electron}electron interaction, whereas mH  is addi- tionally in#uenced by the interaction [4]. In this case, mH  is supposed to be smaller than mH  . Experimentally, Singleton et al. showed that mH  is smaller than mH  in organic conductors [5]. However, no comparison between mH  and mH  in strongly correlated electron systems of metals or 0921-4526/01/$-see front matter  2001 Elsevier Science B.V. All rights reserved. PII:S0921-4526(00)00631-1