Critical properties of the quasi-two-dimensional metallic ferromagnet Fe2.85GeTe2

We have investigated the critical behavior of Fe_2.85GeTe₂ single crystals near the paramagnetic-ferromagnetic transition by bulk dc magnetization measurements. The critical exponents β, γ and δ, obtained from modified Arrott plot, Kouvel-Fisher method, and critical isothermal magnetization analysis, could fulfill the Widom scaling law. The self-consistency and reliability of these exponents are further verified by the magnetic state equations below and above the Curie temperature at high magnetic field. In addition, the exchange distance deduced from the susceptibility exponent is shown to decay as [Formula: see text]. Based on the observations, we suggest that the competition between direct magnetic exchange and Coulombic and/or RKKY interactions should be responsible for the intrinsic magnetism in this system.

From tricritical ferromagnetism to metamagnetism in quasi-two dimensional Tl(Co1-x Ni x )2S2 (x = 0, 0.05)

We have investigated the critical behavior near the ferromagnetic transition of TlCo₂S₂ single crystals and the magnetic properties of Tl(Co_0.95Ni_0.05)₂S₂ by means of magnetization measurements. The obtained critical exponents β, γ and δ of TlCo₂S₂ could basically satisfy the scaling equations and are found very close to the prediction of the tricritical mean-field theory. 5[Formula: see text] Ni doping drives the system to an antiferromagnetic ground state which is unstable to magnetic field, yielding metamagnetic transition. The possible existence of a tricritical point (TCP) in Tl(Co_0.95Ni_0.05)₂S₂ is also discussed.