1. Phase diagram of a mixed spin-1 and spin-3/2 Ising ferrimagnet
Milan Žukovič
and
Andrej Bobák
P.J. Šafárik University in Košice

2. Outline Motivation Model Monte Carlo simulations » Parameters Results
» Evaluated quantities
» Finite-size scaling
Results
» Previous approximations
» Current MC results
Summary and conclusions

3. Motivation
Synthesis of new molecular magnets with interesting technological applications
Mixed spin-SA and spin-SB Ising systems - simple models of bimetallic molecule-based magnets
Difficulties to apply nonperturbative approaches to models with higher spins and complex lattices
Mixed SA=1 and SB=3/2 Ising model on square lattice » mean-field theory (MFT)
» effective-field theory (EFT)
» cluster variation theory (CVT)
Qualitative disagreements!!!!
Goal: help to resolve discrepancies by MC simulation

4. Model
Hamiltonian
where ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●

5. Monte Carlo simulations Parameters
» Monte Carlo technique: standard and histogram
» Dynamics: Metropolis
» Lattice sizes LxL with: L=20,…,200
» MC sweeps: 105 ~ 107
» Boundary conditions: periodic

6. Monte Carlo simulations Evaluated quantities
Sublattice magnetizations:
Total magnetization:
Staggered magnetization:

7. Monte Carlo simulations Evaluated quantities
Int. energy and specific heat per site:
Direct and staggered susceptibilities:
and

8. Monte Carlo simulations Finite-size scaling
Scaling of maxima of the quantities
where
- correlation length critical exponent
- susceptibility critical exponent

9. Results Phase diagram by previous approximative approaches
(a) Effective field theory (EFT) (b) Cluster variation theory (CVT)
A. Bobák, Physica A 258 (1998) J.W. Tucker, J. Magn. Magn. Mater. 237 (2001) 215.

10. Phase diagram by current MC simulations
Results
Phase diagram by current MC simulations
kBTc/|J|=0.1

11. Results Finite-size scaling at kBTc/|J|=0.1
2D Ising values Second-order behavior

12. Multicompensation behavior
Results
Multicompensation behavior

13. Critical and compensation behavior
Results
Critical and compensation behavior

14. Summary and conclusions
Only order-disorder phase boundary in (kBTc/|T|,D/|J|) plane (MFT,EFT,CVT)
Order-disorder transition temperature is a single-valued function of D/|J| (MFT,EFT,CVT)
Order-disorder transition is of second-order for entire range of D/|J| (MFT,EFT,CVT)
Multicompensation behavior with up to two compensation points around D/|J|≈-1.96 (MFT,EFT,CVT)
Approximative approaches: caution drawing conclusions about phenomena in a narrow parameter space!