Thermal expansion and pressure effect in

Abstract

M nW O4 has attracted attention because of its ferroelectric property induced by frustrated helical spin order. Strong spin-lattice interaction is necessary to explain ferroelectricity associated with this type of magnetic order. We have conducted thermal expansion measurements along the a, b, c axes revealing the existence of strong anisotropic lattice anomalies at T1=7.8 K, the temperature of the magnetic lock-in transition into a commensurate low-temperature (reentrant paraelectric) phase. The effect of hydrostatic pressure up to 1.8 GPa on the FE phase is investigated by measuring the dielectric constant and the FE polarization. The lowtemperature commensurate and paraelectric phase is stabilized and the stability range of the ferroelectric phase is diminished under pressure. Multiferroic magnetoelectric compounds exhibit the coexistence of ferroelectric (FE) and magnetic orders in some temperature range. The mutual correlation between these orders is of fundamental physical interest and it bears the potential for future applications utilizing the magnetoelectric effect in which the magnetization (FE polarization) is controlled by internal or external electric (magnetic) fields [1,2]. Recently, this property has been observed in M nW O 4 in a phase with an incommensurate (IC) helical spin density wave [3]. M nW O 4 crystalizes in the wolframite structure (monoclinic space group P2/c). Below 15 K competing magnetic exchange interactions result in a high level of magnetic frustration with several magnetically ordered states quasi-degenerated in energy. As a consequence, M nW O 4 undergoes three successive magnetic transitions, antiferromagnetic (AFM) order of the Mn-spins with an IC sinusoidal spin modulation appears at T N =13.5 K (AF3 phase) followed by an elliptical IC magnetic order below T 2 =12.6 K (AF2 phase) and a commensurate collinear magnetic phase below T 1 =7.8K (AF1 phase) [4]. Ferroelectricity was observed in the AF2 phase only and it can qualitatively be explained by the loss of inversion symmetry due to the helical magnetic order and a strong spin-lattice coupling [5]. * Corresponding author. Tel: (713) 743-8314 fax: (713) 743-8201Email address: rajit.chaudhury@uh.edu (R. P. Chaudhury). The magnetic phase transitions are also visible in anomalies of the specific heat, the dielectric constant, and the magnetic susceptibility [6].The coupling between AFM and FE orders observed in M nW O 4 must be mediated by strong spin-lattice interactions. The existence of such spin-lattice coupling can be experimentally proven by detecting the strain of the lattice by high-resolution thermal expansion measurements [7,8]. The macroscopic lattice strain along the principal crystallographic orientations, a, b, and c, is measured employing a high-resolution capacitance dilatometer. The results shown

Room-temperature vibrational properties of multiferroic MnWO4 under quasi-hydrostatic compression up to 39 GPa

“…Despite the technological and fundamental science interest that MnWO 4 generates, the study of its structural and optical properties under pressure in quasi-hydrostatic conditions has been limited to pressures below 10 GPa. 6,11,12 In this work, we report a Raman spectroscopy study under quasi-hydrostatic conditions of the vibrational properties of MnWO 4 up to 39.3 GPa that shows an increase of 8 GPa in the structural stability of the wolframite phase with respect to previous works.…”

“…However, most of those properties have been studied in the low-temperature range where the magnetic behavior shows up. One of those physical interesting properties is the thermal expansion that has been recently measured under pressure up to 1.4 GPa below 20 K. 12 However, the value of the thermal expansion coefficient has not been reported so far in a higher temperature range and only a predicted value can be found in the bibliography at ambient conditions. 19 A way to obtain an estimation of its value near the room temperature range can be done if the gr€ uneisen parameters of all the Raman active vibrational modes are known.…”

Infrared-active phonon modes in monoclinic multiferroicMnWO4

“…High-resolution thermal expansion data show subtle but clear anomalies at both T N1 and T N3 . 35 The ferroelectric polarization is of the order of 50 µC/m 2 , 8,9 more than an order of magnitude smaller than in, e.g., TbMnO 3 .…”

Pressure-induced decoupling of rare-earth moments and Mn spins in multiferroicGdMn2O5

“…The driving force for the commensurate order is the exchange interaction between moments of the correlated system of Mn 3+ , Mn 4+ spins, and Gd moments, which requires more free energy to order the complete magnetic system, External pressure can play a crucial role in tuning the dielectric and ferroelectric properties of magnetic multiferroics. 22,27,[31][32][33][34][35][36][37] For example, theoretical calculation predicted the huge increase in polarization as well as T N of binary magnetic multiferroic CuO to room temperature under high pressure. 38 Recently, Aoyama et al .…”