Spin liquid in a single crystal of the frustrated diamond lattice antiferromagnet CoAl2O4

Abstract

We study the evidence for spin liquid in the frustrated diamond lattice antiferromagnet CoAl2O4 by means of single crystal neutron scattering in zero and applied magnetic field. The magnetically ordered phase appearing below TN =8 K remains nonconventional down to 1.5 K. The magnetic Bragg peaks at the q=0 positions are broad and their lineshapes have strong Lorentzian contributions. Additionally, the peaks are connected by weak diffuse streaks oriented along the <111> directions. The observed short-range magnetic correlations are explained within the spiral spinliquid model. The specific shape of the energy landscape of the system with an extremely flat energy minimum around q=0 and many low lying excited spiral states with q= <111> results in thermal population of this manifold at finite temperatures. The agreement between the experimental results and the spiral spin-liquid model is only qualitative indicating that microstructure effects might be important to achieve quantitative agreement. Application of a magnetic field significantly perturbs the spiral spin-liquid correlations. The magnetic peaks remain broad but acquire more Gaussian lineshapes and increase in intensity. The 1.5 K static magnetic moment increases from 1.58 µB/Co at zero field to 2.08 µB/Co at 10 T. The magnetic excitations appear rather conventional at zero field. Analysis using classical spin wave theory yields values of the nearest and next-nearest neighbor exchange parameters J1=0.92(1) meV and J2=0.101(2) meV and an additional anisotropy term D=-0.0089(2) meV for CoAl2O4. In the presence of a magnetic field, the spin excitations broaden considerably and become nearly featureless at the zone center.

Low-temperature nuclear magnetic resonance investigation of systems frustrated by competing exchange interactions

“…µ of Co 2+ is estimated to be 2.41(9) µ B /Co using a relation H int = |A hf | µ . This value is smaller than the expected value of 3 µ B /Co for S = 3/2 of Co 2+ ions with g = 2 and is larger than the value of 1.58 µ B /Co reported from the neutron diffraction study done previously by Zaharko et al [204] The shoulderlike structures of the spectrum are due to a distribution of Co 2+ ordered moments in the AFM state. …”

Spiral spin-liquid and the emergence of a vortex-like state in MnSc2S4

“…Among the other compounds, CoAl 2 O 4 and MnSc 2 S 4 manifest the strongest frustration. For CoAl 2 O 4 , the ratio of |J 2 /J 1 | has been identified as 0.109 19 , which is near, but still lower than, the 0.125 threshold for the spiral spin-liquid state. Many experimental studies of MnSc 2 S 4 21,23-27 suggest its relevance to the spiral spin-liquid, but the spiral surface has not been observed.…”

“…Until now, several series of A-site spinels, in which the magnetic A ions form a diamond lattice, have been investigated, including: the cobaltates Co 3 O 4 and CoRh 2 O 4 16 ; the aluminates M Al 2 O 4 with M = Fe, Co, Mn [17][18][19][20] ; and the scandium thiospinels M Sc 2 S 4 with M = Fe, Mn 21 . For the spinels with Fe 2+ at the A-site, the e g orbital angular momentum of Fe 2+ is active, making the pure spin J 1 -J 2 model inadequate 22 .…”

Revisiting the ground state of CoAl2O4 : Comparison to the conventional antiferromagnet MnAl2O

“…The higher level of cation inversion in the MnAl2O4 sample indicates that this novel behavior is primarily an effect of greater next-nearest-neighbor exchange. The A-site spinels, AB 2 X 4 with A magnetic, have seen a surge of interest in the past decade, due to a series of interesting experimental observations [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] and theoretical predictions of novel spin-liquid ground states [16][17][18][19][20] . Magnetic cations in these materials comprise a bi-partite diamond lattice, and novel behavior is argued to be the result of a competition between nearest (J 1 ) and nextnearest (J 2 ) neighbor superexchange interactions 2,21 .…”