Strong magnetoelastic coupling at the transition from harmonic to anharmonic order inNaFe(WO4)2with3d5

Abstract

The crystal structure of the double tungstate NaFe(WO4)2 arises from that of the spin-driven multiferroic MnWO4 by inserting non-magnetic Na layers. NaFe(WO4)2 exhibits a three-dimensional incommensurate spin-spiral structure at low temperature and zero magnetic field, which, however, competes with commensurate order induced by magnetic field. The incommensurate zero-field phase corresponds to the condensation of a single irreducible representation but it does not imply ferroelectric polarization because spirals with opposite chirality coexist. Sizable anharmonic modulations emerge in this incommensurate structure, which are accompanied by large magneto-elastic anomalies, while the onset of the harmonic order is invisible in the thermal expansion coefficient. In magnetic fields applied along the monoclinic axis, we observe a first-order transition to a commensurate structure that again is accompanied by large magneto-elastic effects. The large magnetoelastic coupling, a reduction of the b lattice parameter, is thus associated only with the commensurate order. Upon releasing the field at low temperature, the magnetic order transforms to another commensurate structure that considerably differs from the incommensurate low-temperature phase emerging upon zero-field cooling. The latter phase, which exhibits a reduced ordered moment, seems to be metastable.

Field-induced metastability of the modulation wave vector in a magnetic soliton lattice

“…One example of such metastability is the pressure dependence of the modulation wave vector of the spin density wave ordering in metallic Cr, where the system is locked into the high-pressure state at low temperature after the pressure has been released [ 22 ]. Similar irreversibility features have also been reported in the magnetic-field-induced transitions in NaFe(WO 4 ) 2 [ 23 ] and Ba0.5Sr1.5Zn2(Fe1−xAlx)12O22 [ 24 ], where the high-field magnetic wave vector is maintained as the magnetic field decreases, and in TbMnO 3 [ 25 ], DyMn 2 O 5 [ 26 ], UNiAl [ 27 ], where the propagation vectors of the low-field magnetic structures as the magnetic field sweeps down are different from either the high-field state or the initial low-field phases after zero-field-cooling. A common feature among these irreversible magnetic transitions is that the magnetic history effect can be erased by warming up above the transition temperature, suggesting that thermal fluctuations are essential in the emergence of such metastability.…”

Single-crystal investigations on the multiferroic material LiFe(WO$_4$)$_2$

“…In contrast to NaFe(WO 4 ) 2 [21] no second intrachain spiral with opposite handedness exists as the upper and lower rows of a zigzag chain are alternatingly occupied by magnetic Fe 3+ and nonmagnetic Li 1+ . Furthermore spirals of neighboring chains possess the same handedness, wherefore the effect of inverse DMI is not canceled in LiFe(WO 4 ) 2 .…”

“…4 c) and d) show the temperature dependence of the resulting averaged h and l component of the incommensurate propagation vector for µ 0 H = 0 T and it can be clearly seen that below T N1 the components of k inc vary with temperature but reach a constant value at low temperature. Incommensurate magnetic long-range order persists down to low temperature contrarily to MnWO 4 , which exhibits a first-order transition to a commensurate spin up-up-down-down (uudd) phase at low temperature [21,22].…”

“…al. reported a type-II multiferroic phase in LiFe(WO 4 ) 2 [12], which is beside MnWO 4 [13,14] only the second multiferroic material in the family of tungstates and which exhibits higher transition temperatures compared to related compounds [13][14][15][16][17][18][19][20][21]. In MnWO 4 , the magnetic ions occupy zigzag chains that are lying within the bc plane [22], see Fig.…”

“…Therefore, the a lattice constant as well as the distance between two magnetic zigzag chains along a are doubled lowering the dimensionality of magnetic interaction (see Fig. 1) [21]. Distinctive for LiFe(WO 4 ) 2 is that the zigzag chains are not entirely occupied by magnetic ions but alternatingly by magnetic Fe 3+ and by nonmagnetic Li 1+ ions (see Fig.…”