Influence of magnetic field on paramagnetic-ferromagnetic transition in La_(1-x)Ca_(x)MnO₃ (xapprox 0.25) crystal: ultrasonic and transport studies

The ultrasonic properties of La$_{1-x}$Ca$_{x}$MnO$_{3}$ ($x\approx 0.25$) with the Curie temperature $T_C$ about 200 K are studied. Temperature dependences of longitudinal and transverse sound velocities were measured in zero magnetic field and for different constant magnetic fields as well. The ultrasonic study is supported by magnetic, resistive, magnetoresistive, structural and other measurements of the sample that facilitate interpretation of the results obtained. The magnetic field influence on sound properties found in this study presents some new features of the interplay between the elastic and magnetic properties of these compounds. It is shown that the paramagnetic-ferromagnetic transition in the sample studied is first order, but can become second order under the influence of applied magnetic field.