Magnetic Anisotropy and Phase Transitions in Co-Doped Yttrium Iron Garnet Films

YIG:Co,Ca,Ge films grown on (001) plane substrate were investigated down to liquid helium temperatures using torque anisometry technique. Successively lowering temperature spin-reorientational transitions were observed between following easy magnetization axes (EMA) orientations: (i) at room temperature four EMA inclined to the film plane from [I 111 directions; (ii) two EMA near the [I101 and [lie] directions, (iii) two other EMA near the [loo] and [OlO] directions; (iv) one EMA near the [OOl] direction at the helium temperature. The garnet YIG:Co,Ca,Ge films have been intensively investigated at room temperature because of their interesting magnetic and magnetooptical properties [1,2]. Knowledge of temperature dependence of magnetic anisotropy in these samples is very important for understanding of recent results of investigation of photomagnetic effects [3,4] or peculiarities of FMR signal [5,6]. Y 3 . ~ a z ~ . x , C ~ G e , 0 1 2 films were grown by liquid phase epitaxy on (001) plane gadolinium gallium garnet GGG substrate. The films were prepared in a form of a disk with diameter of 4mm and thicknesses of 7+10 pm. The measurements were camed out in the temperature region from liquid helium temperature to room temperature by means of an automatic torquemeter. The period of magnetic field rotation was 6 min. Torque curves were measured in three characteristic planes: (OOl), (100) and (110) (examples are shown in Fig.l). The contribution of the paramagnetic GGG substrate was determined by measuring the torque as a function of field amplitude. This contribution was numerically subtracted from the total torque. The analysis of the curves, obtained in this way, displays the orientation of easy magnetization axes. One can distinguish several temperature ranges of different EMA orientation. Four axes EMAlll (inclined from the [Ill]-type directions to the sample plane) were found (Fig.lA) at the room temperature. One of these axes can be described by the angles n n 54.7" <--,I$, ,, = (9, $ are polar and azimuthal angles measured in the x, y, z co-ordinate system connected with 2 4 the standard [loo], [010] and [OOl] crystallographic axis). With decreasing temperature an increase of Qlll was observed. Lowering of the temperature a transition to the configuration with two easy axes EMAllo (near [I101 and [1i0] ) took place see Fig.2. Further lowering of the temperature induced the following EMA configurations: (i) the E m l w oriented near [loo] and [OlO]; (ii) the EMAool near [OOl]. Temperature range of coexistence of the EMAllo, EMAloo and EMAool configurations depends on small difference of sample chemical composition. Only the E f i l configuration was observed at the helium temperature. The EMA reorientation could be discussed in the simple case, taking into consideration %I, Ku first constants describing cubic and uniaxial anisotropy. For different values of these constants, different EMA orientation could be found out, as follow: Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:19971188 C1-462 JOURNAL DE PHYSIQUE IV EMAl, 0 A A A A A A A A A