# Suppression of the Shastry-Sutherland phase in Ce$_{2}$Pd$_{2}$Sn at a field induced critical point 

J.G. Sereni $^{1}$, M. G´ omez Berisso $^{1}$, G. Schmerber$^{2}$, A. Braghta2 , 3 and J.P. Kappler 2 

1 Div. Bajas Temperaturas, Centro At´ omico Bariloche (CNEA), 8400 S.C. Bariloche, Argentina 2 

IPCMS, UMR 7504 CNRS-ULP, 23 rue de Loess, B.P. 43 Strasbourg Cedex 2, France 3 

D´epartement de Physique, Universit´e de Guelma, 24000 Guelma, Algeria 

(Dated: November 19, 2018) 

The magnetic phase diagram of Ce 2 Pd 2 Sn is investigated through the field dependence of thermal, transport and magnetic measurements performed at low temperature. The upper transition, T M = 4 . 8 K is practically not affected by magnetic field up to B = 1 T, whereas the lower one T C ( B ) rapidly increases from 2.1 K joining T M in a critical point at T cr = (4 . 2 ± 0 . 3)K for B cr = (0 . 12 ± 0 . 03) T. At that point the intermediate phase, previously described as an unstable Shastry-Sutherland phase, is suppressed. A detailed analysis around the critical point reveals a structure in the maximum of the ∂M/∂B ( B ) derivative which could be related to the formation of a novel phase in that critical region. 

Complex or metastable phases are favored in the proximity of magnetic transitions since, within the Landau theory, the change of sign of the first term of the free energy G ( ψ ) broadens its minimum as a function of the order parameter ψ as T → T$_{C}$ . In these critical conditions the ’roughness’ of G ( ψ ), oc- curring in real systems, may present relative minima which may become relevant in the formation of novel phases [1]. In recent years an increasing number of magnetic systems exhibiting non trivial types of or- der parameters have been reported. Those complex phases may correspond to metastable ground states, such as frustrated or ’spin ice’ [2] systems, Shastry- Sutherland phases [3, 4] or other exotic phases. Those phases can be tuned by non-thermal control parame- ters like pressure (e.g. in non conventional supercon- ductors [5]) or by magnetic field (e.g. in Sr 3 Ru 2 O 7 [6]) and driven to a quantum critical points at T = 0 [7]. Alternatively, some of those phases become unstable under moderate variation of the external parameters and are suppressed in a critical point at finite temper- ature. 

Recently, the formation of an unstable Shastry- Sutherland ShSu phase was reported to form in Ce 2 Pd 2 Sn [8]. That phase is observed within a limited range of temperature, between T$_{M}$ = 4 . 8 > T > T$_{C}$ = 2 . 1 K, having as the upper limit a correlated param- agnetic phase and a ferromagnetic FM one as ground state GS. This exotic phase builds up from FM-dimers formed by Ce nearest-neighbor atoms, and shows the onset of magnetic correlations at T ≤ 20 K. An anti- ferromagnetic AF exchange interaction between those dimers drives the formation of the ShSu phase, real- ized in this compound as a quasi 2D square lattice of effective spin S$_{eff}$ = 1 below T$_{M}$ = 4 . 8 K [8]. 

Stable ShSu phases are predicted for AF-dimers and AF inter-dimer interactions by theoretical cal- culations [4]. This scenario was recently observed in Yb 2 Pt 2 Pb single crystals [9], which undergoes a 

PACS numbers: 75.20.Hr, 71.27.+a, 75.30.Kz, 75.10.-b 

## I. INTRODUCTION 

slight shift of part of Pt atoms that results in two types of Yb-Pt tetrahedral sublattices. On the con- trary, Ce 2 Pd 2 Sn presents a unique crystalline Ce- lattice where the mentioned exotic phase becomes un- stable at T$_{C}$ = 2 . 1 K, undergoing a first order to its FM-GS. Neutron diffraction experiments [10] revealed a modulated character of this phase, with the local moments pointing in the ’c’ crystallographic direction. The incommensurate propagation vector [ qx ] changes from 0.11 (at 4.2 K) to 0.077 (at 2.8 K) where it sud- denly drops to [ qx = 0] once the long range FM order parameter sets on. 

Since the mentioned magnetic phase is unstable and only holds within a short range of temperature, the application of external field is expected to produce significant effects including its eventual suppression in a critical point. To our knowledge there are no sys- tematic investigations of magnetic phase diagrams for Ce 2 T 2 X compounds (with T = Ni, Cu, Pd, Rh, Pt and X = Sn and In) performed yet. This is not a mi- nor point since in the last years a big effort was done searching for critical points at very low temperature and magnetic field may fine tune the critical condi- tions. Furthermore, the scarce examples for FM criti- cal points places the Ce 2 T 2 Sn family of compounds as good candidates for that current topic since it builds up from Ce atoms disposed in triangular prims mimik- ing those of CeT [11] FM compounds. 

Preliminary studies on the effect of the magnetic field [12] on Ce 2 Pd 2 Sn showed that the upper transi- tion is overcome by the lower one by applying moder- ate magnetic filed. In order to investigate the charac- teristics of that critical region, we have performed a detailed investigation of the magnetic field effects on the low temperature thermal, magnetic and transport properties of this compound. Such a study allows to assemble a magnetic phase diagram including the crit- ical point where the intermediate phase is suppressed. 

