Applied physics Lab, Physic department, CBNU Supervised by Professor, Seong-Cho Yu Student, The-Long Phan Department of Physics, Chungbuk National University, 361-763, Cheongju, Korea  The colossal magnetoresistance (CMR) effect in the doped manganites R1-xA′ xMnO3 (R=La, Nd; A′ =Ca, Sr, Ba, etc.) with perovskite structure has recently become a subject of intense interest  The CMR effect in manganites, occurring near TC, is explained by the Mn3+-Mn4+ double-exchange process, Mn3+-Mn3+ superexchange and Jahn-Teller distortion processes, caused by substituting divalent ions (Ca, Sr, Ba, ) into the A-site of perovskite structure  Electron paramagnetic resonance (EPR) is an useful probe to study spin dynamics of the Mn3+ and Mn4+ ions, Jahn-Teller distortion in the CMR materials  Measure EPR spectra from: - Polycrystallines: La0.7A′ 0.3MnO3 (A′=Sr,Ba,Cd), and (Nd1-xYx)0.7Sr0.3MnO3 (0≤x≤0.28) - Single crystal: La0.7Sr0.2Ca0.1MnO3  Theoretical calculations apply for analyzing experimental EPR data  Find out influences of the effective radius of the A-site cation , the Mn3+/Mn4+ ratio and the skin effect on the EPR parameters X-band 9.21 GHz (Frequency) Magnetic field (H) Sample Perovskite Temperature Controller ESR signal: dP/dH Working conditions: - Temperature range: ~77 to 473 K - External magnetic field: to T - The fixed frequency: f = 9.2 GHz, in the microwave range I La0.7A′ 0.3MnO3 (A′=Sr,Ba,Cd) compounds  EPR signals for the samples FM FM PM PM TC = 365 K TC = 322 K FM PM TC = 239 K  The linewidth and intensity of EPR spectra From above EPR signals, we determined the temperature dependences of the EPR linewidth and intensity Temperature dependence of the EPR linewidth for LSM, LBM, and LCM Temperature dependence of the EPR intensity for LSM, LBM, and LCM  Analyze the EPR linewidth data - In the T≥Tmin regime, if we base on the relationship between ∆ Hpp(T) and the electrical conductivity, the linewidth is given by ∆ Hpp(T) = ∆ H0+A/T[exp(-Ea/kBT)] (1) The linewidth data of the samples are fitted to Eq (1) The fitting parameters are shown in Table - In the case of using the model of the isothermal regime, where Lande’s factor is constant (as seen in the figure), temperature dependence of ∆ Hpp is linear ∆ Hpp(T) = a + bT Lande’s factor in the isothermal regime (2) Eq (2) gives the best fits to the ∆ Hpp(T) data at temperatures T>Tmin, a and b are in Table - If we supplement into Eq (2) a term, causing from critical fluctuations near phase transition Tmin, Eq (2) becomes ∆ Hpp(T) = a + bT + c/|T-Tmin|p (3) This equation is in good agreement with the ∆ Hpp(T) data at temperatures T≥Tmin, as shown in Figure - Therefore, the temperature dependence of ∆ Hpp is due to spinspin interaction and the critical fluctuation  Analyze the EPR intensity - At high temperatures, the relation between ∆ Hpp(T) and I(T) is via a expression ∆ Hpp(T) ~ C/[T*I(T)] (4) Using Eq (1) for this case, we have I(T) = I0exp(Ea/kBT) The EPR intensity data at T>Tmin fit to Eq (5), Ea is shown in Table (5) The table summarizes the experimental parameters for La0.7A′ 0.3MnO3 (A′=Sr,Ba,Cd) compounds II (Nd1-xYx) 0.7 Sr0.3MnO3 (0≤x≤0.28) compounds EPR signals: In this system, we have carried out as the above analyses, the results are shown in the flowing figures: The linewidth data are fitted to ∆ Hpp(T) = ∆ H0+A/T[exp(-Ea/kBT)] The linewidth data are fitted to ∆ Hpp(T) = a + bT The intensity data are fitted to I(T) = I0exp(Ea/kBT) The table summarizes the experimental parameters for (Nd1-xYx) 0.7 Sr0.3MnO3 (0≤x≤0.28) compounds III Single crystal sample of La0.7Sr0.2Ca0.1MnO3 - The single crystal in the bulk form EPR signals at different temperatures, for the c⊥H case EPR spectra are fitted to a Dysonian function, α is the dispersion-to-absorption ratio ∆H + α( H + H res )  dP d  ∆H + α( H − H res ) =A +   dH dH  4( H − H res )2 + ∆H 4( H + H res )2 + ∆H    - The single crystal in the powder form EPR signals at different temperatures EPR spectra are well fitted to a Lorentzian function when α is small, EPR becomes symmetrical The difference between EPR shapes of the single crystal in bulk and powders is due to the skin effect The shift of the resonance position is due to the difference of anisotropic fields existing in the samples We carried out the works the following:  Measured in detail the EPR spectra for compounds La0.7A′ 0.3MnO3 (A′=Sr,Ba,Cd) and (Nd1-xYx)0.7Sr0.3MnO3 (0≤x≤0.28) (polycrystallines), and La0.7Sr0.2Ca0.1MnO3 (single crystal)  Analyzed the EPR parameters by means of the critical fluctuation, isothermal, and adiabatic models in the temperature regime T≥Tmin  Fitted EPR signals of the single crystal sample to Dysonian and Lozentzian functions  Studied the influences of the effective radius of the A-site cation , the Mn3+/Mn4+ ratio, and the skin-depth effect on the shape and parameters of the X-band EPR spectra