High field magnetic structure of the frustrated antiferromagnet TbB₄

H. Nojiri, S. Yoshii and K. Ohoyama (IMR Tohoku University, Japan)
M. Matsuda (JAEA, Tokai, Japan)
S. Michimura and F. Iga (Hiroshima University, Japan)
P. Frings, F. Duc, B. Vignolle and G.L.J.A. Rikken (LNCMI, Toulouse, France)
L.-P. Regnault (INAC-SPSMS-MDN, CEA-Grenoble, France)

We examined the magnetic structure on the multiple magnetisation plateaus in the frustrated antiferromagnet TbB₄ [1]. As shown in the animation (image a), the crystal forms a Shastry-Sutherland lattice, where rectangles and triangles align alternatively in a two dimensional layer.

• ground state: the Tb moments lie within the layer due to the presence of strong planar magnetic anisotropy.
• under magnetic field: several steps appear in the magnetisation process. This contradicts the well-known fact that the magnetisation of a planar antiferromagnet increases linearly with magnetic field intensity when a magnetic field is applied perpendicular to the plane.

Using the pulsed miniature magnet, we measured the time dependence of the [100] magnetic peak intensity in a sinusoidal shaped pulsed magnetic field of 7 milliseconds duration [2] up to 33 T. The data was accumulated over 100-200 shots at the rate of 10 shots/hour. It was then converted into the magnetic field dependence of the intensity as shown in the figure.

We stress here that such a non-trivial structure could be resolved only by neutron diffraction experiments in 30 Tesla fields.
This experiment demonstrates the power of miniature pulsed magnets for neutron diffraction. It also emphasises the importance of developing a high steady magnetic-field installation, which in future may enable inelastic scattering to be combined with very high magnetic fields.

The intensity exhibits multiple steps corresponding to the magnetization steps. By repeating measurements at several Bragg peaks, the magnetic structure model (animation, image b) could be determined for the half-plateau state where the magnetization is half that of the saturation. Surprisingly enough, the structure consists of a stacking of ferromagnetic and planar antiferromagnetic blocks. This shows the large contribution of the biquadratic exchange interaction causing the orthogonal arrangement between the magnetic moments.