Unconventional Magnetostructural Transition inCoCr2O4at High Magnetic Fields

Abstract

The magnetic-field and temperature dependencies of ultrasound propagation and magnetization of single-crystalline CoCr2O4 have been studied in static and pulsed magnetic fields up to 14 T and 62 T, respectively. Distinct anomalies with significant changes in the sound velocity and attenuation are found in this spinel compound at the onset of long-range incommensurate spiral-spin order at Ts = 27 K and at the transition from the incommensurate to the commensurate state at T l = 14 K, evidencing strong spin-lattice coupling. While the magnetization evolves gradually with field, steplike increments in the ultrasound clearly signal a transition into a new magneto-structural state between 6.2 and 16.5 K and at high magnetic fields. We argue that this is a high-symmetry phase with only the longitudinal component of the magnetization being ordered, while the transverse helical component remains disordered. This phase is metastable in an extended H-T phase space. Multiferroic materials, which exhibit concomitant magnetic and ferroelectric order, are of great current interest, both from a fundamental as well as applicationoriented view. They challenge our understanding of ordering phenomena, but in addition provide new functionalities in spintronics, since these dielectric and magnetic polarizations can be tuned either by external magnetic or electric fields [1][2][3][4]. Among multiferroics magnetic AB 2 X 4 compounds with spinel structure attracted considerable interest revealing colossal magnetocapacitance and spontaneous dielectric polarization in the magnetically ordered state [5][6][7][8][9]. The appearance of dielectric polarization is associated either with a non-collinear arrangement of spins, with charge order, or with magnetic ions moving off-center from their symmetric site positions in the lattice due to strong magneto-elastic effects.Significant spin-lattice coupling and magnetic frustration are important features of spinels, specifically, of chromium oxides and chalcogenides. Despite quenched orbital moments these compounds reveal structural instabilities which are governed explicitly by ordering of spins, e.g., giant magnetostriction, negative thermal expansion [10], and spin Jahn-Teller instabilities [11][12][13][14][15]. In Cr spinels with only one magnetic sublattice, where the Cr 3+ ions are located solely on the pyrochlore lattice of the B sites, it is well known that the oxides with dominating antiferromagnetic (AFM) exchange reveal strong geometrical frustration, while in the sulfides and selenides ferromagnetic (FM) exchange becomes important. ZnCr 2 S 4 and ZnCr 2 Se 4 are strongly frustrated due to competing AFM and FM interactions, with the ground state still being antiferromagnetic [16]. At low temperatures, the geometrically frustrated oxide spinels exhibit magnetization plateaus as function of an external magnetic field, with a 3-up 1-down spin configuration [17][18][19]. Theoretically, it has been suggested that these plateaus are stabilized by lattice distortions [20]. Indeed, in high-fiel...

Spin-lattice effects in selected antiferromagnetic materials (Review Article)

“…12. Our data evidence that in the high-field (high-symmetry) phase only the longitudinal component of the magnetization is ordered, while the long-range transverse spiral order is suppressed [37]. Concomitantly, effects of the inverse DM interaction are canceled and the structure is released into presumably a cubic symmetry.…”

“…Astonishingly, upon reducing the applied field, the ground state is not recovered at the same magnetic-field values t H , but the high-field phase remains metastable down to a lower transition field and seems to persist even to zero field at 6.7 K. The situation changes at higher temperatures. At 16.5 K, the metastable state exists between approximately 40 and 60 T, whereas at 30 K no metastable state is seen up to 63 T. It is worth noting that the magnetization does not demonstrate any anomalies in this temperature and magnetic-field range [37].…”

“…We now concentrate on high-field ultrasound results obtained on CoCr 2 O 4 , ZnCr 2 S 4 , and CdCr 2 O 4 [35][36][37]. Figure 10 shows / ∆v v of CoCr 2 O 4 as function of applied pulsed magnetic field at different temperatures.…”

“…Here [35][36][37][38][39]. At room temperature these materials possess a cubic structure with space group 3 Fd m .…”

First-principles investigations into the thermodynamics of cation disorder and its impact on electronic structure and magnetic properties of spinel Co(Cr1–xMnx)2O4

“…Among these the oxide spinel CoCr 2 O 4 is found to be a promising multiferroic material as it exhibits switchable electric polarisation under reversal of magnetic field 21 . The richness of it's magnetic phase diagram comprising of various long range and short-range, collinear and non-collinear magnetic structures [24][25][26][27] prompted researchers to examine the effects of substitution of one of the magnetic atom with a new one on it's properties. Subsequently, new and interesting phenomena like tunable Exchange Bias, temperature dependent magnetic compensation and magnetostrictions were observed upon controlled substitution of Cr with Fe atoms [28][29][30][31] .…”