Speaker
Description
Applying a magnetic field to an antiferromagnet can cause abrupt changes to its magnetic state. A metamagnetic transition to a paramagnetic state is accomplished in sufficiently high magnetic fields. This transition is usually of a second-order type. However, a first-order transition is observed at low temperatures well below the Néel temperature in some cases. The antiferromagnet - paramagnet transition line in the field-temperature magnetic phase diagram having the first-order and second-order type segments is expected in Ising antiferromagnets with competing antiferromagnetic and ferromagnetic interactions. D. P. Landau has interpreted the point at which the two segments meet as a tricritical point [1]. Detailed experimental studies of tricriticality in antiferromagnets are rarely reported in the literature, although understanding these phenomena has fundamental importance. The field-induced state achieved by a first-order metamagnetic transition is often mistakenly considered ferromagnetic. However, it is a polarized paramagnetic state.
We have observed and studied in detail these intriguing phenomena in several diverse antiferromagnetic materials, such as a van der Waals insulator VBr$_3$ and intermetallic compounds UNiAl, UIrSi$_3$, CePtGe$_2$, UIrGe. We will discuss the results of the measurements of magnetization, specific heat, electrical, and Hall resistivity as functions of temperature and magnetic field characteristics for the first- and second-order magnetic phase transitions in these materials. Uniaxial crystal structures characterize the first three compounds, while the last two are orthorhombic. Nevertheless, all exhibit extreme magnetocrystalline anisotropy with one preferential magnetization axis, implying the Ising-like character of magnetism and spin-flip type metamagnetic transition. The source of considerable magnetocrystalline in VBr$_3$ is a significant orbital moment of the V$^{3+}$ ion. Theoretical calculations have confirmed the metamagnetic transition in this compound.
Unlike the first four colinear antiferromagnets, UIrGe exhibits a noncollinear magnetic structure but still displays characteristics of magnetic tricriticality. This aspect corroborates the idea that the appearance of the first-order type spin-flip metamagnetic transition at low temperatures results from the enormous magnetocrystalline anisotropy, which doesn’t allow continuous rotation of magnetic moments.
Acknowledgment
This work was supported by the Czech Grant Agency, project no. 25-15448S.
References
[1] D. P. Landau, “Magnetic Tricritical Points in Ising Antiferromagnets,” Physical Review Letters, vol. 28, no. 7. American Physical Society (APS), pp. 449–452, Feb. 14, 1972. https://doi.org/10.1103/physrevlett.28.449
