Seminars



Department of Magnetic Reseach Seminar

13:30, 19-12-11
sala nr 6 (bud. II)

Peculiar magnetic susceptibility and phase transitions in LiNiPO4

prof. dr hab. Andrzej Szewczyk

Instytut Fizyki PAN w Warszawie

Among orthorhombic olivines, considered for cathodes of Li-ion batteries, LiNiPO4 shows a unique set of physical properties, i.e., (i) two-step formation of a quasi-2D antiferromagnetic structure (at first, the 2nd order transition to an incommensurate phase at 21.8 K and then the 1st order transition to the commensurate antiferromagnetic phase at 20.9 K appear), (ii) complex exchange interactions, and (iii) strong linear magnetoelectric effect.

Detailed studies of specific heat, magnetization and magnetic torque of a LiNiPO4 single crystal, aimed at (i) seeking for a transition to a multiferroic state, (ii) constructing the phase diagram in the magnetic field, B, up to 9 T, (iii) verifying whether the com­plex exchange interactions, strong anisotropy, and the quasi 2D magnetic structure lead to any uncommon macroscopic magnetic properties - will be presented.

The specific heat studies, supplemented by the "slope analysis method", al­lowed to find a splitting of the specific heat anomaly associated with the 1st order transition, suggesting that these are two coupled transitions, one of which can be the ferroelectric one. Measurements of the specific heat, CP, of a single crystal as a function of temperature, T , for nonzero B allowed to determine analytical equa­tions describing the phase transition lines in the T-B plane and to model evolution of a shape of the specific heat anomalies accompanying the phase transitions in a powder sample under influence of B.

By measurements of angular dependences of magnetic torque and magnetiza­tion for B rotating within the a-c and b-c crystalline planes, for numerous fixed T and B values, we found a new effect, that we called "off-diagonal nonlinear magnetic susceptibility", i.e., we found that for each of the main crystalline axis (a, b, and c), an additional component of magnetic susceptibility, proportional to the square of the perpendicular to this axis component of B exists. A phenomenological model of this effect, describing the experimental results very well, was proposed.

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