[Paper Review] Magnetic Correlations in the Quasi-2D Semiconducting Ferromagnet CrSiTe$_3$

T. J. Williams, A. A. Aczel+5 more|arXiv (Cornell University)|Mar 27, 2015

2D Materials and Applications4 citations

TL;DR

This study uses elastic and inelastic neutron scattering to investigate magnetic correlations in the quasi-2D ferromagnetic semiconductor CrSiTe₃. It reveals strong in-plane static and dynamic magnetic correlations persisting up to at least 300 K, with Heisenberg-like spin dynamics and a small single-ion anisotropy favoring c-axis ordering, challenging prior assumptions of Ising behavior and highlighting the material's 2D character critical for spintronic applications in thin films and monolayers.

ABSTRACT

Intrinsic, two-dimensional ferromagnetic semiconductors are an important class of materials for overcoming the limitations of dilute magnetic semiconductors for spintronics applications. CrSiTe$_3$ is a particularly interesting member of this class, since it can likely be exfoliated down to single layers, where T$_C$ is predicted to increase dramatically. Establishing the nature of the magnetism in the bulk is a necessary precursor to understanding the magnetic behavior in thin film samples and the possible applications of this material. In this work, we use elastic and inelastic neutron scattering to measure the magnetic properties of single crystalline CrSiTe$_3$. We find that there is a very small single ion anisotropy favoring magnetic ordering along the $c$-axis and that the measured spin waves fit well to a model where the moments are only weakly coupled along that direction. Finally, we find that both static and dynamic correlations persist within the $ab$-plane up to at least 300 K, strong evidence of this material's two-dimensional characteristics that are relevant for future studies on thin film and monolayer samples.

Motivation & Objective

• To determine the nature of magnetic order and correlations in bulk CrSiTe₃, a quasi-2D ferromagnetic semiconductor.
• To resolve conflicting prior reports on spin anisotropy and spin wave behavior, particularly the disputed Ising-like character.
• To establish the role of in-plane and out-of-plane exchange interactions in sustaining magnetic order and dynamics.
• To provide a quantitative basis for understanding magnetic behavior in thin films and monolayers of CrSiTe₃, where T_C is predicted to increase significantly.

Proposed method

• Performed elastic neutron scattering to measure magnetic Bragg peaks and critical exponents, confirming T_C = 33.2(1) K and β = 0.151(2), consistent with 2D Ising-like critical behavior.
• Conducted inelastic neutron scattering along (H 0 0) and (0 0 L) directions to map spin wave dispersion and energy, identifying the absence of dispersion along L above T_C.
• Used constant-energy scans at E = 1.75 meV (along H) and E = 1.5 meV (along L) to track temperature evolution of spin waves up to 300 K.
• Performed two-axis measurements to extract in-plane correlation lengths, showing divergence at T_C and persistence above T_C.
• Applied linear spin wave theory to fit spin wave data, yielding J_ab1 = -1.27(23) meV and a small spin gap of 0.075(24) meV.
• Combined X-ray diffraction and DFT calculations to support the presence of weak octahedral distortion and van der Waals interactions along the c-axis.

Experimental results

Research questions

• RQ1 What is the true nature of magnetic anisotropy in CrSiTe₃—Ising-like or Heisenberg-like?
• RQ2 How do static and dynamic magnetic correlations evolve above the Curie temperature T_C?
• RQ3 What are the relative strengths and spatial ranges of in-plane and out-of-plane exchange interactions?
• RQ4 To what extent do two-dimensional magnetic correlations persist in bulk CrSiTe₃, and how do they inform behavior in 2D limit?
• RQ5 How do the observed neutron scattering data reconcile with conflicting prior reports on spin wave branches and spin gap?

Key findings

• The critical exponent β = 0.151(2) extracted from elastic neutron scattering is consistent with 2D ferromagnetic critical behavior.
• Magnetic Bragg peaks exhibit a diffuse component peaking at T_C, indicating strong 2D in-plane correlations persisting above T_C.
• In-plane spin wave dispersion is strongly suppressed above T_C, with no detectable dispersion along the L direction, confirming 2D confinement of dynamics.
• Spin waves are broadened but still observable along (H 0 0) at 40 K, indicating dynamic in-plane correlations persist up to at least 300 K.
• The spin gap is measured as 0.075(24) meV, indicating very weak single-ion anisotropy favoring c-axis alignment, not Ising-like behavior.
• The dominant in-plane exchange interaction J_ab1 = -1.27(23) meV is attributed to superexchange via Te ions, with weaker second- and third-nearest-neighbor interactions likely due to double-superexchange.