Low Temperature Specific Heat of Dy₂Ti₂O₇in the Kagome Ice State

Abstract

We report the specific heat of single crystals of the spin ice compound Dy 2 Ti 2 O 7 at temperatures down to 100 mK in the so-called Kagome ice state. In our previous paper, we showed the anisotropic release of residual entropy in different magnetic field directions and reported new residual entropy associated with spin frustration in the Kagome slab for field in the [111] direction. In this paper, we confirm the first-order phase transition line in the field-temperature phase diagram and the presence of a critical point at (0.98 T, 400 mK), previously reported from the magnetization and specific-heat data. We newly found another peak in the specific heat at 1.25 T below 0.3 K. One possible explanation for the state between 1 T and 1.25 T is the coexistence of states with different spin configurations including the 2-in 2-out one (Kagome ice state), the 1-in 3-out state (ordered state) and paramagnetic one (free-spin state).

Finite-Temperature Transitions in Dipolar Spin Ice in a Large Magnetic Field

“…A similar proposal had been made earlier for the case of garnet systems [9]. Experimental evidence [10] suggests that the 0.5 K feature in powder samples arises due to a multicritical point at a temperature of 0.5 K and field of 1 T along the [111] crystallographic direction, which is related to the interesting phenomenology of kagomé ice [11]. However, to date, there is no concensus as to whether or not the specific heat features at 0.35 K and 1.2 K are caused by the magnetic field directed on crystallites of particular orientation [12,13,14].…”

Spin ice in a field: Quasi-phases and pseudo-transitions

“…Meanwhile a wealth of more or less sharp anomalies in the applied magnetic fields H of different directions is observed in their thermodynamic parameters [5][6][7][8][9][10][11][12][13][14]. Some of these anomalies are interpreted as the field-induced transitions while others are thought to indicate the crossover between the regions with different types of collective spin fluctuations.…”

“…While there is no true phase transition between paramagnet and spin-liquid state still the temperature dividing these "quasi-phases" can be pointed out -it is the temperature T m of specific heat maximum in its temperature dependence C(T) [4]. Indeed, this maximum indicates the more or less sharp drop of entropy due to the confinement of spin fluctuations at low T. One may hope that such definition of T m can justify the notion of "pseudotransition" between the "quasi-phases" with different types of spin fluctuations and may help to quantify in the framework of rigorous theory the regions where various spin-liquid states exist.Implicitly the notions of "quasi-phases" and "pseudo-transition" are widely used to interpret heuristically the observed field-induced anomalies of C(T) in spin ices and to identify the regions belonging to the different spin-liquid states on the H-T planes [9][10][11][12][13]. Yet it is important to discriminate the "pseudo-transitions" and the ordinary ones as the microscopic models describing first of them would not have any singular point but only the crossover regions.…”

Observation of Magnetic Monopoles in Spin Ice

“…20) Consequently they strongly suggest existence of magnetic monopoles interacting via the magnetic Coulomb force. Further investigations of critical phenomena, 15) screening of the Coulomb interaction, and effects of the anisotropic motion of the monopoles within the kagomé lattices are of interest.…”