# Presentation: 120684

## The Magnetic phase transition in the frustrated antiferromagnet ZnCr2O4 using SPINS

*Group B*
- Ilir Zoto
- Tao Hong
- Yanmei Lan
- Nikolaos Daniilidis
- Sonoko Kanai
- Mitra Yoonesi
- Zhaohui Sun

## Outline

- Principle of triple axis neutron spectrometry
- Sample properties: crystal and magnetic structure
- Sample behavior: macroscopic (magnetic) properties
- Neutron results: structural and dynamic information

## Multiplexing Detection System for TAS

- ai = a + 2i = a - atan(x sina/(L+xcosa))
- kfi = a/2sinai
**Q****i**** = k****i**** - k****f****i**

- Survey (h-Q) space by changing the
- incident energy and scattering angle

## Sample structure (ZnCr2O4)

** ****: Corner-sharing tetrahedra**

**Space group Fd3m**

- Sample structure (ZnCr2O4)

- Edge-sharing
- octahedra

- ?  Multiple energetically equivalent configurations:
-     Geometric frustration

## Magnetic Phase Transition in ZnCr2O4

- Magnetic Phase Transition in ZnCr2O4

- CW = 390 K
- TN = 12.5 K

- Phase transition

## What information do we expect to get from neutron scattering?

- Static information: crystal structure and magnetic ordering, thus perform elastic scattering.
- Dynamic information: what “excitations” do we observe and how they evolve with temperature, thus look for inelastic peaks
- Dynamic and static correlations, thus look at peak linewidths.
- How are the fluctuating spins in the spin liquid phase correlated with each other?
- How do the spin correlations change with the phase transition?

## I. Structural data

- • Perform Q scan at zero
-   energy transfer at several
-   temperatures
- • Estimate Q resolution:
-       ΔQFWHM0.2Å-1
- • Estimate energy resolution:
-       Δ(ћω) FWHM  0.2meV
- • Appearance of several
-   magnetic peaks below the AF
-   TN

## Structural insight gained

- • Position of (1,1,1) nuclear peak doesn’t shift
- • Several half integer indexed peaks appear
- • Comparable peak
-    linewidths: Long
-    range structural
-    order

## II. Dynamical data

- • Scan for energy spectral weight
-   at Q=1.5Å-1
- • Shift in spectral weight from
-   low (quasielastic)to high (inelastic)
-   energy at TN.
- • T>TN: Thermal energy broadening.
- • T<TN: 4.5meV peak
-    FWHM0.5meV   (lifetime8ps).
- • What excitation is it?
- • Why the jump in energy?

## II. More dynamical data

- Q scans at low and high T
- Correlation length at            ħ =1.5meV and T=15K is ~2.5 Å
- Correlation length at            ħ =4.5meV and T=1.5K is ~3.2 Å
- Approximately same range of dynamic spin correlations; comparable to nearest neighbor distance

- ħ=1.5meV, T=15K

- ħ=4.5meV, T=1.5K

## Resolution: The nature of the AF state.

-  Antiferromagnetic spin hexagons form under TCW.
- These can move independently (new degrees of freedom)
-  Still only spin liquid state can be formed (frustration)
-  Dynamical correlations of the formed hexagon span its size only
-  Frustration disappears due to crystal  distortion at TN (lifting of degeneracy).
-   New AF ordered state appears.
* *Why the jump in energy? What is the **Q **dependence? *To be continued...*

## Acknowledgements

- Seung-Hun Lee
- Peter Gehring
- Sungil Park