Strong coupling between the structure and magnetism of manganese perovskites

K.V. Kamenev, A.J. Campbell, D. McK Paul, M. R. Lees, G. Balakrishnan (Univ. Warwick), G.J. McIntyre (ILL).

The system La_1-xSr_xMnO₃ belongs to the family of doped manganese perovskites which exhibit colossal magnetoresistance near the ferromagnetic spin-ordering temperature. Samples with x = 0.165 undergo a first-order phase-transition from the high-temperature rhombohedral (R-3c) to a low-temperature orthorhombic (Pbnm) phase at T_S = 295 K, and a second-order phase transition from the para- to the ferromagnetic state at T_C = 261 K. Doping by Sr also results in a change in the ordering; with increasing x, T _C increases from 238 K for x = 0.15 to 283 K for x = 0.175, whereas T _S decreases from 380 K to 190 K. The magnetic and structural phase-transitions intersect on the x-T-phase diagram for a critical doping level x _C ~ 0.165 at about 270 K. Application of a magnetic field to samples with x _C ~ 0.165 induces the structural transition from the orthorhombic to rhombohedral state at room temperature. The same phenomenon also occurs on application of pressure. A recent strain-gauge dilatometric study showed that T_S = T _C at a pressure of approximately 3 kbar in La_0.835Sr_0.165MnO₃. However, only isobaric temperature scans were possible in this study, and the type of transition, structural or magnetic, is only inferred indirectly. Here we report a more comprehensive investigation of the P-T phase diagram of La_0.835Sr_0.165MnO₃ by means of neutron diffraction.

Neutron-diffraction measurements were carried out on a sample of La_0.835Sr_0.165MnO₃ on the single-crystal diffractometer D10. A high-pressure cell using gaseous helium as pressure-transmitting medium allowed continuous variation of the temperature and pressure in situ over a temperature range of 1.2-310 K and pressure range of 0-5 kbar. The orthorhombic (2 0 0) and (4 0 5) reflections were chosen to monitor the magnetic and structural phase-transitions, respectively. The results of this investigation are summarised in the pressure-temperature phase diagram in Fig. 1.

The P-T phase diagram of La_0.835Sr_0.165MnO₃ is defined by the lines of the structural and magnetic phase-transitions. The first-order structural phase-transition, T _S, is shown by the two "Z"-shaped curves which enclose a metastable region where, depending on the history of the sample, either the rhombohedral or the orthorhombic phase can occur. The single line which starts at T _C = 261 K at ambient pressure and ends at 282 K at a pressure of 5 kbar corresponds to the (normally) second-order para- to ferromagnetic phase-transition. The hatched area indicates the intersection of T_S and T _C, and separates the four phases given by all possible different combinations of the two structural and two magnetic phases. These phases are marked on the diagram as Pr, Po, Fo or Fr, where "P" or "F" specifies the magnetic state as paramagnetic or ferromagnetic and "r" or "o" denotes the structure as rhombohedral or orthorhombic.

One of the key features of the phase diagram is that, due to the peculiar "Z"-shape of the structural phase-boundaries near the intersection, some of the lines of the phase-transitions cannot be seen by pure temperature variations. The phase boundaries which can be found during temperature variations at a constant pressure are delineated by solid lines in Fig. 1; the phase boundaries shown by broken lines represent the transitions which can only be seen on isothermal pressure changes.

By observing the temperatures and pressures of the phase transitions found during pressure variations we could connect the phase boundaries which were invisible in heating-cooling runs, and had gone undetected in the dilatometric study. Isothermal pressure variations allowed us to establish the "Z"-shape. It is also clear that the magnetic phase-transition mirrors the hysteresis of the structural phase-transition in the crossing region which leads us to conclude that the structural change is a primary effect with respect to the magnetic one, and that the type of the ferro- to paramagnetic phase-transition switches from second-order to first-order, a most unusual phenomenon. Pressure scans at 270 K (Fig. 2) illustrate clearly the locking together of the structural and magnetic transitions in the region of intersection of the phase boundaries.

Due to the hysteretic character of the first-order structural and, locked to it, the magnetic phase-transition, the intersection occupies the hatched area on the P-T-phase diagram. In this area the sample can be in any one of the Pr, Po, Fo, or Fr phases depending on the pressure-temperature path by which it was brought to the area.

In sharp contrast to the pressure-independent T _C seen in the orthorhombic phase below 3 kbar, the Curie temperature in the rhombohedral phase increases with applied pressure with dT_C/dP = 3.5°C/kbar. Given that the cell volumes are almost identical in the two structural phases, the change in the pressure dependence is particularly intriguing. To understand this we note that T _C is sensitive to the Mn-O-Mn bond angle. When the crystal is compressed in the orthorhombic phase, the Mn-O-Mn angle is 180° by symmetry, and pressure cannot affect it, changing instead the inter-planar distances. In the rhombohedral phase the "easy" direction of compression is the three-fold axis, which is at an angle to the Mn-O-Mn bonds, so that the Mn-O-Mn bond angle is more easily affected by applied pressure.

Finally, comparison of the x-T, H-T, and P-T-phase diagrams reveals a common trend in the behaviour of La_1-xSr_xMnO₃ (x ~ 0.160), i.e. increase in Sr-content, magnetic field or pressure leads to intersection of T_S and T _C. Even though it is impossible to give a definition of the Curie temperature in a magnetic field, we can suppose that the abrupt decrease of T _S near H = 2 T is related to the intersection with the magnetic phase-transition in the plane of magnetic field and temperature (Fig. 3).

A comprehensive study of the intersection region can only be done if both the temperature and another parameter, one of x, H or P, is varied. Since isothermal variations of Sr-content (x) are obviously impossible and because of the difficulty with the definition of T_C in magnetic field, use of pressure as a second variable is the only practical means to probe this complicated phase diagram.