Critical behavior of the paramagnetic to antiferromagnetic transition in orthorhombic and hexagonal phases ofRMnO3(R=Sm

Abstract

The critical behavior of the paramagnetic to antiferromagnetic transition in RMnO 3 (R = Sm, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y) has been studied with an ac photopyroelectric calorimeter, which gives simultaneously the thermal diffusivity and specific heat as a function of temperature with high resolution around the Néel temperature. RMnO 3 (R = Sm, Tb, Dy) has an orthorhombic perovskite structure, whereas the other manganites are hexagonal, thus revealing a strong thermal anisotropy. SmMnO 3 with an Type-A antiferromagnetic ordering exhibits a 3D-XY critical behavior, which could be attributed to the peculiar site anisotropy of Sm 3+ and the exchange interaction between Mn 3+ and Sm 3+ ions. The sinusoidal spin ordering in TbMnO 3 and DyMnO 3 can be phenomenologically described by the 3D-Heisenberg model. Finally, the hexagonal manganites RMnO 3 (R = Ho, Er, Tm, Yb, Lu, Y) are close to the 3D-Heisenberg universality class, ruling out the chiral models predicted by the renormalization group theory.

Hexagonal Manganites—(RMnO3): Class (I) Multiferroics with Strong Coupling of Magnetism and Ferroelectricity

“…Furthermore, a large deviation of the lowtemperature ( < ) heat capacity from the phonon contribution seems to indicate the presence of a residual magnetic , possibly due to a magnetic glassy state coexisting with the ordered antiferromagnetism in some RMnO 3 (R = Y, Lu, Sc) [90,144]. In contrast, a recent study of the heat capacity of YMnO 3 and the critical scaling near the Néel transition has argued that the deviation of the low-temperature from the Debye law, previously attributed to an abnormal magnetic contribution, can be accommodated by an additional Einstein contribution and the critical exponents derived are well within the range of a 3D Heisenberg model, thus not supporting 2D or chiral models for the magnetic system [126,158]. It appears that the true nature of the magnetic order and the spin fluctuations below and above has yet to be revealed.…”

“…Near the Néel temperature, ( ) exhibits a -shaped peak characteristic for a second order phase transition. The critical magnetic fluctuations at have been studied recently [126] and the critical exponents were found close to those of the three dimensional Heisenberg universality class. The critical scaling properties prove the second order nature of the transition.…”

“…Based on a systematic study of the heat capacity, Oleaga et al [126] concluded that the critical properties of the hexagonal manganites near fit best to the 3D-Heisenberg universality class. At the spin rotation transition, the thermal expansivities show a narrow peak with opposite sign for the -and -axes.…”

Critical behavior of the ferromagnetic perovskites RTiO3(R=Dy, Ho, Er, Tm, Yb) by magnetocaloric measurements

“…The obtained critical parameters, adjustable fitting parameters (such as A, B, C and E), the fitting ranges and the quality of the fittings (given by the root mean square value) were listed in the Table II. The fitting reduced temperature t ranges is about 10 -1~1 0 -3 in the vicinity of the Curie temperature T C while avoiding the rounding part 22 . The root mean square values R 2 is close to 1, indicating the validity of the fitting.…”

“…The root mean square values R 2 is close to 1, indicating the validity of the fitting. The fitting ranges are in all cases limited by the rounding in the curves near the transition temperature; this rounding is inherent to the quality of the samples and the attribution to the measurement technique 22 . The obtained α (see Table I and Table II) for RTiO 3 single crystal is in excellent agreement with the prediction of the 3D Heisenberg model 13,23 , further supporting the above analysis with the MCE scaling laws.…”