Interesting thermomagnetic history effects in the antiferromagnetic state of SmMn₂Ge₂

Abstract

We present results of magnetization measurements showing that the magnetic response of the antiferromagnetic state of SmMn 2 Ge 2 depends on the path used in the field(H)-temperature(T) phase space to reach this state. Distinct signature of metastablity is observed in this antiferromagnetic state when obtained via field-cooling/field-warming paths. The isothermal M-H loops show lack of end-point memory, reminiscent of that seen in metastable vortex states near the field-induced first order phase transition in various type-II superconductors. Typeset using REVT E X 1The intermetallic compound SmMn 2 Ge 2 with its interesting magnetic properties has been a subject of intensive study during last two decades [1][2][3][4][5][6][7][8][9][10]. In low applied magnetic fields it shows at least three magnetic transitions as a function of temperature [1][2][3]6,7].First it undergoes a paramagnetic (PM) to ferromagnetic (FM1) transition at around 350K, followed by an FM1 to antiferromagnetic (AFM) transition at around 160 K (T N 1 ). On reduction of the temperature further this AFM state transforms again into another ferromagnetic (FM2) state around 100 K(T N 2 ). There is a large spread in the reported values of the transition temperatures from FM1 to AFM and AFM to FM2 states. Quality of the samples may be a possible source for the reported differences in the transition temperatures, especially when it is known that the microscopic magnetic properties of RE(rareearth)1-2-2 compomuds are quite sensitive to their underlying crystal lattice structure. On the other hand, there exist now enough evidences from various studies that both of these transitions are probably first order in nature [2,4,11,12]. The first order nature of these magnetic phase transitions can also provide a natural explanation to the reported spread in the transition temperatures. Supercooling(superheating) can take place down(up) to a temperature T * (T * * ) while cooling (heating) across a first order transition point (T N ) [13]. The extent of supercooled/superheated phases will depend on the path followed in the field(H)-temperature(T) phase space [14]. In addition in the samples with defect structures the lower(higher) temperature phase will start nucleating around these defect structures once the sample is cooled(heated) across T N . This nucleation of the lower(higher) T phase will be completed at T * (T * * ), and in the temperature regime T N -T * (T * * -T N ) there will be co-existence of two phases. All these properties will give rise to thermal hysteresis, and such thermal hysteresis is actually observed across the FM2-AFM and AFM-FM1 phase transitions in SmMn 2 Ge 2 [2,4,7].Confined between two FM phase at low and high temperature and reached via first order phase transitions, the AFM phase in SmMn 2 Ge 2 is something special. In this paper, based on our careful dc magnetization studies we shall show that the magnetic response of this AFM state actually depends on the path used in the (H,T) phase space to reach 2 this state. Distinc...

First order magneto-structural phase transition and associated multi-functional properties in magnetic solids

“…The coexistence of FM and AFM phases across this FOPT was studied in detail using the minor hysteresis loop (MHL) technique [44]. This MHL technique was originally developed to study a first order vortex matter phase transition in type-II superconductors [45,46] and has been used subsequently to study phase transitions in various magnetic materials [47][48][49][50][51].…”

“…This region contains mixtures of AFM and FM clusters, and is metastable in nature. A new concept, namely the 'lack of end point memory effect' [47], has been used to study the metastable nature of this phase-coexistence regime in detail [52,58]. In this metastable phase-coexistence region (obtained via an FCC path) any field cycling introduces energy fluctuations, which would drive the clusters of the metastable FM state to the equilibrium AFM state.…”

First order magneto-structural transition in functional magnetic materials: phase-coexistence and metastability

“…A new concept viz. 'lack of end point memory effect' (Roy et al 2002) has been used to study, in detail, the metastable nature of this phase-coexistence regime (Chattopadhyay et al 2003;Sokhey et al 2004). Any field cycling in this metastable phase-coexistence region (obtained via FCC path) introduces energy fluctuations, which drive the clusters of metastable FM state to the stable AFM state.…”