First Order Transitions and Multicritical Points in Weak Itinerant Ferromagnets

Abstract

It is shown that the phase transition in low-T c clean itinerant ferromagnets is generically of first order, due to correlation effects that lead to a nonanalytic term in the free energy. A tricritical point separates the line of first order transitions from Heisenberg critical behavior at higher temperatures. Sufficiently strong quenched disorder suppresses the first order transition via the appearance of a critical end point. A semiquantitative discussion is given in terms of recent experiments on MnSi, and predictions for other experiments are made. [S0031-9007(99)09305-9] PACS numbers: 75.20.En, 64.60.Kw, 75.45. + j The thermal paramagnet-to-ferromagnet transition at the Curie temperature T C is usually regarded as a prime example of a second order phase transition. For materials with high T C this is well established both experimentally and theoretically. Recently there has been a considerable interest in the corresponding quantum phase transition of itinerant electrons at zero temperature (T 0), and in the related finite T properties of weak itinerant ferromagnets, i.e., systems with a very low T C . Experimentally, the transition in the weak ferromagnet MnSi has been tuned to different T C by applying hydrostatic pressure [1]. Interestingly, the transition at low T was found to be of first order, while at higher transition temperatures it is of second order [2]. The tricritical temperature that separates the two types of transitions was found to roughly coincide with the location of a maximum in the magnetic susceptibility in the paramagnetic phase. Theoretically, it has been shown [3,4] that in a T 0 itinerant electron system, soft modes that are unrelated to the critical order parameter (OP) or magnetization fluctuations couple to the latter. This leads to an effective long-range interaction between the OP fluctuations. In disordered systems, the additional soft modes are the same "diffusons" that cause the so-called weak-localization effects in paramagnetic metals [5]. In clean systems there are analogous, albeit weaker, effects that manifest themselves as corrections to Fermi liquid theory [6]. A Gaussian theory is sufficient to obtain the exact quantum critical behavior in the most interesting dimension, d 3, for clean as well as for disordered systems (apart from logarithmic corrections in the clean case) [3,4].In this Letter, we show that at sufficiently low temperatures the phase transition in itinerant ferromagnets is generically of first order. This surprising result is shown to be rooted in fundamental and universal many-body physics underlying the transition, viz. long-wavelength correlation effects, and, hence to be independent of the band structure. This suggests that the behavior observed in MnSi is generic, and should also be present in other weak itinerant ferromagnets. We also make detailed predictions about how quenched disorder suppresses the first order transition, which allows for decisive experimental checks of our theory.Let us start by deriving the functional form of the free en...

Quantum critical behavior in the asymptotic limit of high disorder in the medium entropy alloy NiCoCr0.8

“…The extreme chemical disorder in these alloys is an essential component of the physics, suppressing a first order ferromagnetic transition that invariably occurs in many other potential ferromagnetic quantum critical point (FQCP) systems. 9 In carefully prepared single crystals with electropolished surfaces and minimum mechanical damage, we find that the BKV theory 17,18 of metallic ferromagnetic QCPs provides an excellent description of all of our data. The approximate temperature-magnetic field phase diagram that describes the boundary between the quantum critical and the Fermi liquid regions is also determined for the NiCoCr 0.8 composition from magnetization data.…”

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Non-Fermi Liquid Regimes and Superconductivity in the Low Temperature Phase Diagrams of Strongly Correlated d- and f-Electron Materials

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Muon spin relaxation and susceptibility measurements of an itinerant-electron system Sr1−xCaxRuOGat-Malureanu1, Carlo2, Goko3 et al. 2011Phys. Rev. B21090View full textAdd to dashboardCite

“…The SCR theory has so far been applied to second order transitions, and no specific calculations have been performed for free energies of first order transitions based on the SCR theory. Belitz et al 54,55 reported that a first order transition can be expected as generic phenomena due to the logarithmic term in the free energy in ferromagnets with itinerant electrons. The experimental results in MnSi and (Sr,Ca)RuO 3 are consistent with this theory, although they do not necessarily provide definitive proof.…”