1. « Chaînes-Aimants » et «Molécules-Aimants » dans les Oxydes
MEETICC
MCM2
GDR MCM2 & MEETICC Strasbourg, 5 – 6 octobre 2017
« Propriétés magnétiques & électroniques : échelles de temps et d’espace »
Can oxides be complementary to molecular compounds in the investigation of the « 0D & 1D nanomagnets related to blocking effects » ?
Present report Sr4CoMn2O Seikh et al. (2017)
Dy2Ti2O7 Snyder et al. (2003)
Ca3Co2O6 Hardy et al. (2004)
Ca3CoMnO6 Choi et al. (2008)
BaCo2(As3O6)2·2(H2O) David et al. (2013)
Previous reports
in inorganic materials

2. SINGLE-ION AND SINGLE-CHAIN MAGNETISM IN TRIANGULAR SPIN-CHAIN OXIDES
V. Hardy1, M. Seikh2, V. Caignaert1, O. Perez1, and B. Raveau1
1 Laboratoire CRISMAT, UMR 6508, CNRS/ENSICAEN/UNICAEN, Caen , France
2 Department of Chemistry, Visva-Bharati University, Santiniketan, West Bengal, India
CRISMAT
Visva-Bharati
SIM (Single-Ion Magnet) : individual spins
SMM (Single-Molecule Magnet) : clusters
SCM (Single-Chain Magnet) : chains 0D 1D

3. M H
SIM (Single-Ion Magnet)
SMM (Single-Molecule Magnet)
SCM (Single-Chain Magnet)
  Magnet 
Usually : « Magnet »  Long Range Ordering (LRO) below a TC µm
Magnetostatics / Domain boundaries
Here: « Magnet »  Blocking of the spins (without LRO) below a TB(f)
Spin dynamics / Individual reversals nm
log 
1/T
texp
Anisotropy
Barrier 
Blocking :  = 0 exp( / T) > Exp. Time

4. 0D 1D Spin reversal mechanisms 2  2  2JS2 SIM (Single-Ion Magnet)
SMM (Single-Molecule Magnet) 0D 1D
SCM (Single-Chain Magnet) +S
+S-1
-S+1 -S
2  2  2JS2
Thermally Activated  = D S2
(D: anisotropy parameter)
Quantum Tunneling of Magnetization (QTM)
TA-QTM
Thermally Activated  = DS2 + 8JS2

5. DS2 (D+8J)S2 « Mn12 » « Co(hfac) » Single-Molecule Magnet
Single-Chain Magnet
[Mn12O12(CH3CO2)16(H2O)4].2CH3CO2H.4H2O
[Co(hfac)2NITPhOMe]
hfac =hexafluoroacetylacetonate
NITPhOMe=4′-methoxy-phenyl-4,4,5,5,-tetramethylimidazoline-1-oxyl-3-oxide
Mn4+
Mn3+
« Mn12 »
«  Co(hfac) »
Sessoli et al.
Nature 365, 141
(1993)
Caneschi et al.
Angew. Chem. Int. Ed. 40, 1760
(2001)
Co2+
DS2
Easy-axis
(D+8J)S2
Easy-axis+FM-like
M(H) loops
at low T
M(H) loops
at low T
Novak et al., JMMM 294, 133 (2005)
Novak et al., JMMM 294, 133 (2005)

6. ? Sr4CoMn2O9 Spin chains Triangular lattice Mn4+ Co2+ Sr2+
Belongs to the family of the 2D-perovskite related oxides
Spin chains
Triangular lattice
Mn4+
Co2+ TP O
Sr2+ c c
[Trigonal Prism]
[Octahedra]
« 1D » « Geometrical Frustration » ?
JAF JF

7. Sr4CoMn2O9 : magnetic features
Easy-axis anisotropy
Mn4+
Co2+ TP O
Co2+ (3d7) in Trigonal Prism
CEF + SO
GS doublet with g’=8-9
Ising-like anisotropy z
02-
Co2+
Abragam and B. Bleaney (1970).
 F. Lloret et al., Inorg. Chim. Acta. 361, 3432 (2008).
 S. Gómez-Coca et al., Coord. Chem. Rev , 379 (2015).

8. ? Sr4CoMn2O9 : magnetic features
(at least) 4 competing intrachain interactions
Jintra (nn & nnn) expected to be AF
in this family J2 J1 J4 J3
Mn4+
Co2+ TP O
« Geometrical Frustration »
(not only interchain)
but also intrachain  ?

9. Intrachain coupling: trend towards ferrimagnetism
Sr4CoMn2O9 : magnetic features
Intrachain coupling: trend towards ferrimagnetism
Emin (Mn4+ - Mn4+ - Co2+ ) = ()
if (J2-J3) < J4 < (J2+J3)
Mn4+
Co2+ J2 J1 J4 J3
Sr4Mn2NiO9: El Abed et al., JSSC 2002
Ca3CoMnO6 : Zhang et al., PRB 2009
J1  35 K
J2  26 K
J3  27 K
( H = + 2kB J SS )
Emin ()  -1 K < J4 < 53 K
(very likely)

10. Interchain coupling : weak and frustrated
Sr4CoMn2O9 : magnetic features
Interchain coupling : weak and frustrated d*
Sr2+ J*
Small J*
 large d* (5.53 Å)
 weak coupling (Sr2+ )
Strong Geometrical Frustration
 J*is AF
 Ising chains
(AF interchain coupling)

11. Sr4CoMn2O9 1 Ceramic samples [Boulahya et al. (2003)]
Ac susceptibility
Another relaxation mechanism …. 2
No signature of LRO
(Long-Range Ordering)
Heat capacity

12. Cole-Cole Coexistence of 2 relaxation processes    (0.4-0.6)
Generalization of the Debye model
with a distribution of relaxation times around c characterized by a parameter (0 <  < 1)
0 : isothermal susceptibility
 : adiabatic susceptibility
 : width of the distribution 1
Coexistence of
2 relaxation processes
   ( ) 2

13. Spin relaxation time c(T)  Peak in  ’’(T, f)
c(Tpeak)=1/(2f) 1
c(T)=1/(2fpeak) 1 2 2
log 
1/T
log 
1/T
Max ’’(T)  2f c  1
Max ’’(f)  2f c = 1

14. Sr4CoMn2O9 1 1 2 2 1 1
SCM ?
(Single-Chain Magnet)

15. ① ② ③ The basic ingredients for SCM : ① Easy-axis anisotropy
② Chains with a net magnetization
③ Free (uncoupled) chains ① ② ③
Mn4+
Co2+ z

16. Sr4CoMn2O9 ? SIM of Co2+ ? in TP SCM Co2+ Mn4+ 1 2 2 Low T behavior ↓
Quantum Tunneling ?
SMM / SIM ?
JACS (2015)
SIM of Co2+ ?
in TP

17. SCM SIM Two relaxation mechanisms … Co2+ Mn4+ Mn4+ Single-Chain Magnet
Single-Ion Magnet

18. SIM : -1 = [0 exp(/T)]-1 + [r/Tn]-1 + [QTM]-1
Sr4CoMn2O9
Mn4+
Co2+
SCM
(Single-Chain Magnet)
(Single-Ion Magnet)
SIM
Dynamical responses in zero field well compatible with SCM & SIM
SCM : = 0 exp(/T)
0 10-11 sec &   167 K
SIM : -1 = [0 exp(/T)]-1 + [r/Tn]-1 + [QTM]-1
0 10-7 sec &   40 K
… but what about the M(H) ?

19. M(H) loop while no (detected) LRO
Sr4CoMn2O9 : dc magnetization
Mrem
Hcoer
M(H) loop while no (detected) LRO

20. Sr4CoMn2O9 : dc magnetization
« Saturation » magnetization as expected
Mrem
Hcoer
Remanent magnetization is « robust »

21. Sr4CoMn2O9 : dc magnetization
Mrem : 0 → 9T → 0
ZFC
FCC

22. Sr4-xCaxCoMn2O9 x=2  Sr2Ca2CoMn2O9 (Sr,Ca) IR (Sr2+)  1.26 Å
IR (Ca2+)  1.12 Å
Chemical pressure effects
x=2  Sr2Ca2CoMn2O9

23. x=0 : Sr4CoMn2O9 & x=2 : Sr2Ca2CoMn2O9
101 – 104 Hz
NPD
x=2
300 K 5K

24. Observed magnetic LRO « consistent with » PDA
x=2 : Sr2Ca2CoMn2O9
NPD
x=2
300 K 5K I + -
Partially Disordered Antiferromagnet (PDA)
M. Mekata, J. Phys. Soc. Jpn. 42, 76 (1977)
Incoherent (zero net magnetization)
Ferrimagnetic UP
Ferrimagnetic DOWN
Ising chains
Triangular lattice
Observed magnetic LRO « consistent with » PDA

25. x=0 : Sr4CoMn2O9 & x=2 : Sr2Ca2CoMn2O9
Sr4CoMn2O9  Sr2Ca2CoMn2O9
SCM → LRO
SIM → SIM

26. « Moléculaires » Sr4-xCaxCoMn2O9 Crossover vs. x : Sr4-xCaxCoMn2O9
Maxima in ’’(T)
Filled symbols : 102 Hz
Empty symbols : 104 Hz
SIM
SMM
SCM
LRO
SIM (Single-Ion Magnet)
SMM (Single-Molecule Magnet)
SCM (Single-Chain Magnet)
LRO (Long -Range Ordering)
« Moléculaires »
Sr4-xCaxCoMn2O9

27. Cp : Sr4-xCaxCoMn2O9 x = 0 x = 2 x = 0 x = 2 H = 0 T H = 9 T H = 0 T
OK with SCM ?
OK with SIM ?

28. Signature of blocking on the Cp of SMM/SCM ?
Fominaya et al. , PRB 59, 519 (1999)
Cbi
Cuni
Cbi
Tblocking T
Cuni
► SCM … ?
1D Ising
Insensitive to « blocking » ?

29. SMM TB TB TB ? Mn12Ac 0.3T 0T 0T & 9T 0T 1D Ising chains made
Gomes et al., PRB 57, 5021 (1998)
Mettes et al., PRB 64, (2001) TB
Mn12Ac
0.3T TB 0T
1D Ising chains made
of S=9 Mn4 clusters
Bhattacharjee et al., Polyhedron 20, 1607 (2001)
0T & 9T
[Mn4(hmp)6R2](ClO4)2
A : R=OAc
B : R=Cl 0T
TB ?

30. ? Cp : Sr4-xCaxCoMn2O9 x = 0 x = 2 x = 0 x = 2 H = 0 T H = 0 T
ZFC 0.1 T
ZFC 0.1 T

31. SINGLE-ION AND SINGLE-CHAIN MAGNETISM IN TRIANGULAR SPIN-CHAIN OXIDES
V. Hardy, V. Caignaert, O. Perez, B. Raveau CRISMAT, Caen
M. Seikh Visva-Bharati University, Santiniketan
To be done :
►Planned tasks : Single crystals , NPD, INS, High-fields , Other 3d combinations , etc
► Collaborations with people of « Molecular Magnetism » !
We hope that oxides might be complementary to molecular compounds in the investigation of the « 0D & 1D nanomagnets related to blocking effects »
Thank you
GDR MCM2 & MEETICC Strasbourg, 5 – 6 octobre 2017