Spin-lattice coupling in the frustrated antiferromagnet ZnCr2Se4probed by ultrasound

Abstract

Ultrasound and magnetization studies of the frustrated spinel ZnCr 2 Se 4 are performed as a function of temperature and magnetic field up to 14 T. In zero field, the sound velocity and attenuation reveal significant anomalies at the antiferromagnetic transition at T N ≈ 21 K indicating strong spin-lattice coupling. External magnetic fields shift these anomalies to lower temperatures concomitantly with the reduction of the Néel temperature. At 2 K, the sound velocity as a function of magnetic field manifests three pronounced anomalies: a deep minimum at 5.4 T related to an inflection point of the magnetization followed by two plateaus with distinct stiffness at fields above 7 and 10 T. The first plateau is ascribed to a transformation from a tetragonal to a cubic phase, while the second one corresponds to a state with fully polarized magnetization. The evolution of magnetic and structural states is discussed within a H -T phase diagram and compared with related frustrated magnetic spinels with strong spin-lattice coupling.

Magnetic phase diagram of the helimagnetic spinel compound ZnCr2Se4revisited by small-angle neutron scattering

“…The possibility of a spin-nematic state arising in a helimagnetic system is a long-standing prediction [11], and such a scenario has been identified in LiCuVO 4 [12]. The field dependence of the magnetisation in the proposed spin-nematic state of ZnCr 2 Se 4 [7] is analogous to that of LiCuVO 4 [12], where a shallow plateaux preceding the ferromagnetic state is …”

“…Crystal preparation was described previously in [7]. The experiment was performed with the magnetic field applied horizontally, i.e.…”

“…At low temperature, the spin-spiral state persists until 6 T, although in applied field the magnetic structure is transformed, at H C1 , into a longitudinal conical spiral, as the opening angle is reduced with increasing field [2,4,7]. At 6 T, the lattice is nearly ferromagnetically polarised, and above this field the system gives way to a new high-field phase which is proposed to be a spin-nematic state [7].…”

“…A variety of spin-screw structures are observed in helimagnetic compounds, and a comprehensive discussion of these spin arrangements in various materials was the subject of several reviews [5,6]. Our sample magnetically orders at T N ≈ 21 K [7], and at low temperature in zero field the incommensurate screwlike magnetic structure has a reported helical pitch of 22.4 Å, which is equivalent to 6.1 Cr-Cr distances and corresponds to an angle of 42…”

“…It is seen that the spin-spiral structure propagates along the crystal axis which experiences the maximum distortion at the structural transition [1,8], which is the c axis in the previous definition. There are three possible domains, owing to the cubic crystal symmetry at high temperatures, and in a small magnetic field, domain selection occurs [7,9]. Measurements on a single crystal show that there is usually one dominant domain, although there may be a minor fraction of the other two possible domains present in the crystal, and the relative population of these domains exhibits a hysteresis across the magnetic transition line [1,8].…”