Role of magnetic chirality in polarization flip upon a commensurate-incommensurate magnetic phase transition in YMn2O5

Abstract

We have performed simultaneous measurements of magnetic chirality by using polarized neutrons and electric polarization along the b-axis of single crystals of YMn 4+ (Mn1−xGax) 3+ O5 with x = 0.047 and 0.12, in which nonmagnetic Ga-ions dilute Mn 3+ spins. The x = 0.047 sample exhibits high-temperature incommensurate (HT-ICM), commensurate (CM), and low-temperature incommensurate (LT-ICM) magnetic phases in order of decreasing temperature, whereas the x = 0.12 sample exhibits only HT-ICM and LT-ICM phases. Here, the CM and LT-ICM phases are ferroelectric and weak-ferroelectric, respectively. Measurements conducted under zero field heating after various field-cooling conditions evidence that the microscopic mechanisms of the spin-driven ferroelectricity in the CM and LT-ICM phases are different: the magnetic chirality of Mn 4+ cycloidal spins plays a dominant role in the LT-ICM phase, whereas the magnetic exchange striction by the Mn 4+ -Mn 3+ chain plays a dominant role in the CM phase. The polarization of YMn2O5 flips upon CM to LT-ICM phase transition because the ferroelectricity driven by the magnetic chirality and the exchange striction provides opposite directions of polarization.Many of the multiferroic compounds that have attracted much attention in recent years exhibit a giant magnetoelectric effect owing to the strong coupling between magnetism and ferroelectricity. The presence of this effect makes these multiferroic compounds potential candidates for industrial device applications. In addition, the coupling mechanism underlying the giant magnetoelectric effect is one of the central issues in this research field.Multiferroic material RMn 2 O 5 (R: rare earth, Bi, and Y)1 provides a unique opportunity for studying the coupling mechanism between magnetism and ferroelectricity. RMn 2 O 5 , which has an orthorhombic structure with the P bam space group, consists of edge-shared Mn 4+ O 6 octahedral chain along the c-axis and pairs of Mn 3+ O 5 pyramids connecting the Mn 4+ O 6 octahedral chains.2 Frustrated spin exchange interactions result in noncollinear magnetic structures and complex magnetic phases.3 RMn 2 O 5 generally exhibits a magnetic phase transition from a paramagnetic to an incommensurate magnetic (HT-ICM) phase at T N 1 , which is typically ∼ 45 K, followed by a successive magnetic phase transition into a commensurate (CM) phase at T CM , typically a few kelvins below T N 1 . Then, at lower temperature, the magnetic structure becomes incommensurate again (a phase referred to as LT-ICM) at T N 2 .4,5 These three magnetic phases exhibit different dielectric properties. The HT-ICM phase is paraelectric, whereas the CM phase exhibits spontaneous polarization concomitantly, and therefore, the CM phase is ferroelectric (FE). The LT-ICM phase is still ferroelectric, however the polarization is suppressed below the CM to LT-ICM phase transition temperature. Thus, the LT-ICM phase is recognized as weak ferroelectric (WFE). Remarkably, a rich variety of magnetoelectric effects have been identified in th...

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

“…45 The reduction of the polarization and the sign reversal in YMn 2 O 5 was suggested to originate from a competition of two contributions to the ferroelectric polarization, one arising from the exchange striction mechanism which involves the Mn 4+ -Mn 3+ chains (dominating in the high-temperature commensurate phase) and a second contribution due to the cycloidal spin modulation of Mn 4+ moments along the c-axis. 46,47 The question whether or not the same mechanism could explain the pressure-induced FE2 phase in GdMn 2 O 5 needs to be discussed in more detail.…”

Understanding the multiferroicity in TmMn2O5 by a magnetically induced ferrielectric model

“…Second, it is noted that all these data were obtained in single crystal samples grown by flux method. These single crystals are claimed to have strong self-polarized effect which would self-polarize the FE domains3233, leading to the strong path-relevance of the measured data. This consequence was often mentioned in literature on RMn 2 O 5 3335 to account for those anomalous data on polarization below T N2 .…”

“…The two cases, taking R = Tm and R = Y, are plotted in Fig. 1(b,c) for a guide of eyes3133. It has been suggested that component P C is related to the symmetric exchange striction, i.e.…”

Multiferroic Materials: Physics and Properties

“…Wakimoto et al 107 selectively replaced Mn 3 þ in the host lattice by Ga 3 þ , disrupting the AFM zig-zag chain and destroying the CM order for 12% substitution. However, they did observe a polarization below 20 K, where the cycloidal spin phase appears.…”

“…Figure 6. 107 The improper ferroelectricity in the collinear (CM) phase can be explained in terms of symmetric exchange striction effects. 102 A magnetically frustrated spin chain runs along the b axis and consists of pairs of almost parallel and antiparallel Mn 3 þ and Mn 4 þ spins with ↑↑↓↑ order, as shown schematically in the central panel of Figure 6.…”