Inhomogeneous Magnetic Structures of La 0.7 Ca 0.3 MnO 3 Investigated by ESR and Magnetization Measurements
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Mat. Res. Soc. Symp. Proc. Vol. 602 © 2000 Materials Research Society
ESR study of the single crystal samples of the CMR materials covering a wide temperature range both above and below the magnetic transition temperature Tc. In order to carefully examine the local magnetic properties related with the inhomogeneous nature of the CMR systems, we have made systematic ESR measurements and quantitative analysis for a single crystal sample of a representative CMR material, Lao. 7Cao.3MnO 3 (LCMO), covering a wide temperature range both above and below the magnetic transition temperature (Tc). A careful dc magnetization measurement was also carried out just above Te in order to find a relation between the macroscopic and the microscopic inhomogeneous magnetic properties. As a result of the comprehensive study of this unique system, we observed for the first time that the microscopic magnetic structures are very similar above and below T,, except for the long range ordering. Furthermore, an antiferromagnetic correlation as well as the ferromagnetic clusters were explicitly revealed in this work. EXPERIMENT
The Lao.7Cao.3MnO 3 (LCMO) single crystal sample used in this work was prepared by the floating zone melting method and ground into a spherical shape for the ESR measurements in order to minimize the demagnetization factor. The x-ray diffraction analysis showed a single phase perovskite structure of pseudo-cubic symmetry with a lattice constant a = 3.867 A. The resistivity was measured with the standard four probe method. In order to examine the physical properties associated with the spin ordering transition, the dc magnetization was measured with a superconducting quantum interference device (SQUID) magnetometer as a function of temperature, and as a function of magnetic field at a temperature, 240 K, in the paramagnetic regime. The magnetic ordering temperature is conventionally defined to be the onset of the ferromagnetic magnetization, which gave the Tc of 225 K for the single crystal sample, in good agreement with the conductivity measurements as shown in Fig. 1. The sample dimension of single crystal was greater than the calculated skin depth at room temperature, and the ESR measurements were made at 9.4 GHz employing a Bruker ESP 300E spectrometer using a microwave power of 10 u W and a modulation amplitude of 5 G. 0.030 m_ ._
0.025,
jý)
___--
m -
--
0.020-
E 0.015, 0
N 0.010
o001
cL Mu 0.005
OE.3
0.000
1 1 Temperature (K)
0
50
100
210
'
3
150
200
ie
250
300
350
Temperature (K) Fig. 1. Temperature dependence of the magnetization measured at the external field
of 10 mT. The inset shows the temperature dependence of the resistivity. RESULTS & DISCUSSION
160
A typical differential ESR spectrum for our sample is shown in Fig. 2. As can be seen, the ESR spectra are of an asymmetric lineshape both above and below the Tc. 14 1210-
8-
Linewidth
66-
.0
4
4-
C2-
-4-
, Resonance Field-,
-86
100
300
200
0
500
600
Magnetic Field (mT) Fig. 2. A typical differential ESR spectru
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