OBSERVATION OF AN UNUSUAL ESR SIGNAL IN ANTIFERROMAGNETIC EU2CUO4

We report the observation of an unusual electron spin resonanee (ESR) signal in single erystals of Eu 2 Cu0 4• The signal appears to be associated with a resonance mode of the Cu0 2 planes, simÜar to _!he midfield and low-field absorptions we have reported previously (Phys. Rev. B 41, 1934 (1990)]. However, it is only observed when the projection of the :applied de magnetie field in the Cu0 2 plane is within a few degrees of the ( 110) · crysfallog.rapbiC direction. Additionally tbe sample must be field cooled in the Cu0 2 plane, but with a component of the cooling field perpendieular to the ( 110) ESR observation direction. Both the field for resonance and the linewidth exbibit a 1/cos () dependence, where () is the angle of the applied de field between the c axis and the (I 10) .observation direction. Additional constr~ipts for observation of the resonance are that the microwave rf magnetic field must have a component in the Cu0 2 plane, but perpendieular to the de field. The signal disappears above ....,215 K, which we assume is associated with the antiferromagnetic . ordering tel:nperature. We .unusual and di verse properties observed in single erystals of rare-earth R 2

· Z. Fisk and S-W. Cheong Los Alamos Nat (onal Laboratory,Los Alamos,_ New Mexico 87545 We report the observation of an unusual electron spin resonanee (ESR) signal in single erystals of Eu 2 Cu0 4 • The signal appears to be associated with a resonance mode of the Cu0 2 planes, simÜar to _!he midfield and low-field absorptions we have reported previously (Phys. Rev. B 41, 1934(1990]. However, it is only observed when the projection of the :applied de magnetie field in the Cu0 2 plane is within a few degrees of the ( 110) · crysfallog. rapbiC direction. Additionally tbe sample must be field cooled in the Cu0 2 plane, but with a component of the cooling field perpendieular to the ( 110) ESR observation direction. Both the field for r esonance and the linewidth exbibit a 1/cos () dependence, where () is the angle of the applied de field between the c axis and the (I 10) .observation direction. Additional constr~ipts for observation of the resonance are that the microwave rf magnetic field must have a component in the Cu0 2 plane, but perpendieular to the de field. The signal disappears above ....,215 K, which we assume is associated with the antiferromagnetic . ordering tel:nperature.
We have previously reported on the .unusual and diverse magnetic properties observed in single erystals of the rare-earth copper mddes, R 2 Cu0 4 with R = Eu or Gd, be-· low the Cu Neel ordering temperature T N• which occurs at ~270 K. 1 In particular, we have observed two-resonant absorptions, which we termed the low-field absorption (LFA) and midfield absorption (MFA). These absorptions exbibited a· large· out-of"plane anisotropy and were ascribed to resonant niodes of the Cu0 2 planes. They were not considered typ~cal EPR s_i gnals in that tl:ie absorptions could be observed for any orientation of the microwave magnetic field hrr to the applied de magnetic field Hde· The signal was detected with a standard EPR superheterodyne speetroiheter operating at 9.2 GHz in the ·fieid derivative mode. Data was taken at temperatures ranging from 77-300 K. We have. also observed the MAS at a frequeney of 35 GHz a nd in Eu 2 . Cu0 4 ·erystals doped with Gd and/or Ce. These preliminary results, as well as data taken at temperatures below 77 K, ·will be presented in a future publicatio1i". ln this paper we report on a n" ewly discovered mierowave absorption sigiial (MAS) in single . czystals of Eu 2 Cu0 4 which were prepared by standard techniques. 1 In contrast to the LFAand MFA, the MAS is not observable if hrr is parallel to Hdc· The MAS exhibits the same out-ofplane anisotropy found for the low-field and midfield absorptions, but in addition, it exhibits a strong and unusual anisotropy within the Cu0 2 plane. In particular, it is only observed when the de magnetie field is oriented within a few degrees of the ( 110) erystallographic direetionl We find that the MAS is sample dependent. Although we have found it in both pure and Ce doped Eu 2 Cu0 4 , the signal is not observed in all of the Eu 2 Cu0 4 samples which we have investigated. We cannot rule out the possibility that the MAS arises from an impurity within the Eu 2 Cu0 4 compound, but if so, we are unaware of any prior report of an impurity response as unusual as that presented here. In Fig. 1 we present spectra taken for a Eu 2 Cu0 4 single crystaI !lt 77 K with Hdc applied along the Cu0 2 plane at various ~gles near the < ilo) crystallographic direction.
In order to observe the MAS, the sample must be cooled below T N in a large de magnetic field ( discussed below).
When Hdc is applied witbin the Cu0 2 plane, the MAS c " Eu)Cu04 When the sample is field cooled from above T N to 77 K along any ( 110) direction, the MAS is only observed near thc perpendicular ( 110) direction. In Fig. 2 the open circles correspond to cooling in a field HFc along the ( 110) direction, 6. = 4~. In this casc the signal is only observed near the perpendicular (110) direction, !:::. = -45°. If the sample is cooled in a field pointing in an arbitrary direction ( e.g" 6. = -6°) within the Cu0 2 plane, the MAS is thcn observcd in both ( 110) dircctions (solid circles in Fig. 2), with an amplitude whicb is largest for the ( 110) direction most nearly perpendicu1ar to thc direction of Hpc-Thc MAS amplitudc also depends strongly on the strength of the cooling field. This dependence is illustrated in Fig. 3. Tbc samplc was cooled in a field Hpc applied along a (110) direction and tbe maximum MAS amplitude was thcn measured along thc perpendicular (110)  to fully develop the MAS amplitude.
In addition to not observing the MAS ü the Eu2Cu04 is cooled in zero field, the MAS is also not observed if the m.icrowave magnetic field hrr is cithcr perpendicular to thc Cu0 2 plane or parallel to the measuring field

Hdc.
Tbc ficld for resonance Hr and the linewidth IUl of the MAS are botb found to be independent of tbe direction or strength of the coolirig .ficld. H" 6.1/, and the signal intensity are also found to exbibit only a weak temperature dependence between 77 and -150 K. However, as the temperaturc is increased above 150 K, Hr and 6.H both increase, and thc signal intcnsity decreases, until the signal finaJly disappears at T N· Hr and AH are found to exhibit an out-of-plane anisotropy identical to the low-field and midfield absorptions studied previously. 1 Tbey both follow a 1/cos (J dependence wherc 6 is the angle of the applied magnetic field Hdc to the Cu0 2 plane. This out-of-plane anisotropy, and the nonobservability of thc MAS for hrr parallel to Hdc• is suggestive that thc MAS is due to EPR of the Cu-0 system.