Formation of incommensurate long-range magnetic order in the Dzyaloshinskii-Moriya antiferromagnet Ba2CuGe2O7 studied by neutron diffraction

Abstract

Neutron diffraction on a triple-axis spectrometer and a small-angle neutron scattering instrument is used to study the magnetic phase transition in tetragonal Ba2CuGe2O7 at zero magnetic field. In addition to the incommensurate cycloidal antiferromagnetic (AFM) long-range order, we establish that weak incommensurate ferromagnetism (FM) also arises below the transition temperature TN identified by sharp Bragg peaks close to the Γ point. The intensities of both the incommensurate AFM and FM Bragg peaks vanish abruptly at TN indicative of a weak first-order transition. Above TN , evidence is presented that the magnetic intensity within the tetragonal (a, b) plane is distributed on a ring in momentum space whose radius is determined by the incommensurate wavevector of the cycloidal order. We speculate that the associated soft fluctuations are at the origin of the weak first-order transition in the spirit of a scenario proposed by Brazovskii.

Magnetic Small-Angle Neutron Scattering

“…spin spiral due to DMI below T N ≈ 3.05 K of which a number of neutron scattering studies [see, e.g., and Mühlbauer et al (2012) and references therein] and theoretical work (Bogdanov et al, 2002;Chovan, Papanicolaou, and Komineas, 2002;Chovan, Marder, and Papanicolaou, 2013) have established a quite complete understanding. However, it was predicted that a second sign alternating component of the DMI vector pointing along the c axis should give rise to incommensurate weak FM behavior (Chovan, Papanicolaou, and Komineas, 2002;Chovan, Marder, and Papanicolaou, 2013) with the same k. This prediction was confirmed by SANS (Mühlbauer et al, 2017). A similar situation observed in the ferroelectric AF BiFeO 3 has already been illustrated in Sec.…”

“…Ba 2 CuGe 2 O 7 is also relevant in terms of a Brazovskii scenario (Brazovskii, 1975), however, with reduced 2D symmetry caused by the weak interplane coupling of Ba 2 CuGe 2 O 7 : Because of crystallographic anisotropy, the manifold of fluctuations is reduced to a ring in reciprocal space. SANS and neutron diffraction have been used to shed light on the interplay of Brazovskii and 2D physics (Mühlbauer et al, 2017).…”

Magnetic small-angle neutron scattering

“…therein) and theoretical work (Bogdanov et al, 2002;Chovan et al, 2013Chovan et al, , 2002 have established a quite complete understanding. However, it was predicted that a second, sign alternating component of the DMI vector pointing along the c-axis should give rise to incommensurate weak FM behavior (Chovan et al, 2013(Chovan et al, , 2002 with the same k. This prediction has been confirmed by SANS (Mühlbauer et al, 2017). A silimar situation, observed in the ferroelectric AF BiFeO 3 has already been illustrated in Sec.…”

“…Ba 2 CuGe 2 O 7 is also relevant in terms of a Brazovskii scenario (Brazovskii, 1975), however, with reduced 2D symmetry caused by the weak inter-plane coupling of Ba 2 CuGe 2 O 7 : Due to crystallographic anisotropy, the manifold of fluctuations is reduced to a ring in reciprocal space. SANS and neutron diffraction have been used to shed light on the interplay of Brazovskii and 2D physics (Mühlbauer et al, 2017).…”

The effects of KSEA interaction on the ground-state properties of spin chains in a transverse field

“…Afterwards, Shekhtman, Entin-Wohlman, and Aharony find that the weak ferromagnetism of La 2 CuO 4 can be explained by this nonnegligible symmetric helical interaction [20]. Therefore, the symmetric helical interaction is called KSEA interaction for short [21][22][23][24][25][26][27][28][29][30][31]. By magnetization measurements, neutron diffraction, and inelastic neutron scattering experiments, KSEA interaction is present in Ba 2 CuGe 2 O 7 [22][23][24][25][26], Yb 4 As 3 [27], K 2 V 3 O 8 [28], and La 2 CuO 4 [29].…”