Effects of high pressure, magnetic fields, and substitutions - ICMR ...

Strong spin-spin and spin-lattice coupling in RMnO 3
In-plane Mn-Mn superexchange interactions are very strong
Model description:
AFM long range order is suppressed due to frustration and weak inter-plane exchange
Sizable spin-spin correlations well above mT N
growing estronger for decreasing me T→T N
H = H + H +
(evidence from magnetization and neutron scattering, Lonkai et al. J. Appl. Phys. 93, 8191, 2003)
Magnetostriction (spin-lattice coupling) leads to a shortening of in-plane distances with
m
// z z ⊥ x x y y
// z z ⊥ x x y y
z
decreasing H T = and, 2 ( Jvia s lattice s + J elasticity, ( s s + s s to ) −the 2αincrease J s s of + Jthe ( sc-axis s + s length s ) − h s
∑
i,
j
i
j
i
j
i
j
N
These effects are dramatically enhanced at T N
(peaks of α a
and α c
with opposite signs)
15.6
0.9995
2
Magnetic subsystem
Through the e spin-lattice ⎛ P interaction ⎞
i
a 2 b 4 the spin fluctuations couple with the FE
H =
displacement ∑coordinate
⎜ − u +
⎟ −
i
ui
∑Uui
u
j
−∑Eui
i ⎝ 2m
2 4
15.4
⎠ i,
j
i
Ferroelectricity
H
me
=
z z x x y y
( g s s + g ( s s s s
)
2
∑ uk
1
+
i j 2 i j
k,
i,
j
i
j
H
∑(
15.8
) ∑
i l i l i l
i,
l
ε
16.2
16.0
T SR
T T
2
T N
0 50 100
T (K)
Spin-lattice coupling
ε ∼ 1/ 2 scales with 1/c 2
C. Zhong and J. Fang Solid State Comm. 128, pp449 (2003)
i
i
1.0005
1.0000
[c(T)/c(293 K)] -2