Microwave absorption in a thin La 0.7 Sr 0.3 MnO 3 film: Manifestation of colossal magnetoresistance D. L. Lyfar, 1 S. M. Ryabchenko, 1 V. N. Krivoruchko, 2 S. I. Khartsev, 3 and A. M. Grishin 3 1 Institute of Physics NAS of Ukraine, 46, prospect Nauki strasse, Kiev 03028, Ukraine 2 Donetsk Physics & Technology Institute NAS of Ukraine, 72, R. Luxemburg strasse, Donetsk 83114, Ukraine 3 Division of Condensed Matter Physics, Department of Microelectronics and Information Technology, Royal Institute of Technology, Electrum 229, S-164 40 Kista, Stockholm, Sweden ~Received 22 December 2003; published 19 March 2004! Microwave ~MW! absorption by a thin La 0.7 Sr 0.3 MnO 3 film on a SrTiO 3 substrate is investigated at a 9.1 GHz as a function of a dc magnetic field. Features of this absorption, namely, the jump in the absorption derivative, have been detected as the applied field passes through its zero value. Hysteretic behavior of the jumps is also observed. The results are discussed based on the model in which MW losses, additional to the ferromagnetic resonance, arise due to attenuation of MW currents induced in the sample by both variable magnetic induction and MW electrical field near the substrate surface with high dielectric permittivity. We show that zero-field anomalies in MW absorption are directly coupled with manganite magnetoresistive prop- erties. DOI: 10.1103/PhysRevB.69.100409 PACS number~s!: 75.70.2i, 75.47.Gk, 76.50.1g Electron magnetic resonance ~EMR!, namely, ferromag- netic resonance ~FMR! at temperatures ~T! below the Curie point T C , and electron paramagnetic resonance ~EPR! at T .T C have been extensively used for the study of doped manganite oxides. For example, the surface impedance in the range of the metal-insulator transition was studied in the works of Lofland et al. 1–4 EMR measurements were em- ployed for studying the spin structure and magnetic ~in!ho- mogeneities in bulk and thin-film samples ~see, e.g., Refs. 5–8!, too. Even though the data on EMR in conductive mag- netic materials can be influenced by eddy current losses of a magnetic component of the microwave ~MW! field, 9,10 investigations 1–8 do not show any manifestation of the colos- sal magnetoresistivity ~CMR! of manganites in the MW re- gion. The MW absorption of thin La 1 2x Ba x MnO 3 2z films with a linear dependence on the modulus of the external magnetic ~H! field was observed in Ref. 11. The authors interpreted the data as a manifestation of the MW CMR of the studied films; however, the physics behind the phenom- ena was not considered. In this paper, the losses at frequency ;10 10 s 21 in the thin La 0.7 Sr 0.3 MnO 3 film on the SrTiO 3 substrate are inves- tigated as a function of the applied H field. New anomalies in the MW absorption of the manganite film at the low mag- netic field, which to a certain extent are equivalent to those observed in Ref. 11 but reveal new aspects of it, have been observed. The anomalies, namely, the jumps in the absorp- tion derivative on the magnetic field at the field changing the sample magnetization sign are directly connected with the CMR of manganites that is manifested at this frequency. The hysteretic behavior is a representative feature of the jump as well. The results can be explained taking into account the Joule losses related both to FMR in a conducting ferromag- net and the MW electrical ~e! field on the substrate surface with high permittivity «. The condition when the anomalies can be observed is also discussed. Experiments were performed on the 2.534.0 mm 2 film with ;0.3 mm thickness grown on the 0.3 mm ~001!- oriented SrTiO 3 substrate. The sample was placed at the MW magnetic field loop of TE102 cavity of the X-band EPR spec- trometer Radiopan SE/X-2544. The temperature was in the range 1004400 ~60.5! K. The accuracy of the sample to the H-field direction orientation was about 62°. The investigated film resistivity achieves its maximum at ’320 K and drops down with decreasing temperature, i.e., it shows a tempera- ture dependence typical for such materials. The magnetore- sistive effect, @ R ( H 50) 2R ( H 51 T ) # / R ( H 50) 525% at 310 K, is a representative one, too. At T >360 K a rather narrow isotropic EPR signal with a g factor g 51.9760.01 was observed. The peak-to-peak line- width m 0 D H pp was about 20 mT ( m 0 is the permeability of free space!. At lower temperatures we observed a depen- dence of the resonance line position on the angle u between the film’s normal and H-field direction n, which further transforms at T ,T C ’316 K into a typical dependence for the FMR line of a film with the ‘‘easy plane’’ type anisot- ropy. Figure 1 shows the FMR field temperature behavior for parallel and perpendicular film plane geometries. They are described well by the usual Kittel’s equations. 12 A compari- son of Fig. 1 data with that known from the literature ~see, e.g., Refs. 3 and 13! shows that the film’s anisotropy is mainly determined by the sample shape ~the demagnetization factor anisotropy!. In our experiment, at least one of the sub- strate linear size, d i , fulfills the relation d i A «>n l /2, where l is the MW wavelength in air and n is an integer. Due to the temperature dependence of « some dielectric resonator modes can be equal to the measured frequencies at appropri- ate narrow temperature areas. Its influence on the registered spectra can also be seen ~see the features at ’145 and 195 K, and others in Fig. 1!. The representative records of the FMR spectra for the derivative of the MW absorption on a field, dP ( H )/ dH , ob- tained at T 5298 K and different angles u are shown in Fig. 2 for the MW magnetic field of the cavity, h MW , in the film plane. The jumps on the spectra at the zero-field region are clearly seen. Except for the main FMR line, one can also RAPID COMMUNICATIONS PHYSICAL REVIEW B 69, 100409~R!~2004! 0163-1829/2004/69~10!/100409~4!/$22.50 ©2004 The American Physical Society 69 100409-1