Neutron diffraction evidence for local spin canting, weak Jahn–Teller distortion, and magnetic compensation in Ti 1− x Mn x Co 2 O 4 spinel

Abstract

A systematic study using neutron diffraction and magnetic susceptibility is reported on Mn substituted ferrimagnetic inverse spinel Ti

1−
x

Mn

x

Co
2
O
4
in the temperature interval 1.6 K

T

300 K. Our neutron diffraction study reveals cooperative distortions of the
T
O
6
octahedra in the Ti

1−
x

Mn

x

Co
2
O
4
system for all the Jahn–Teller active ions
T
  =  Mn
3+
 , Ti
3+
and Co
3+
 , having the electronic configurations 3
d
1
, 3
d
4
and 3
d
6
, respectively which are confirmed by the x-ray photoelectron spectroscopy. Two specific compositions (
x
  =  0.2 and 0.4) have been chosen in this study because these two systems show unique features such as; (
i
) noncollinear Yafet–Kittel type magnetic ordering, and (
ii
) weak tetragonal distortion with
c/a
  <  1, in which the apical bond length
d

c

(
T

B

-O) is longer than the equatorial bond length
d

ab

(
T

B

-O) due to the splitting of the
e

g

level of Mn
3+
ions into

and

. For the composition
x
  =  0.4, the distortion in the
T

B

O
6
octahedra is stronger as compared to
x
  =  0.2 because of the higher content of trivalent Mn. Ferrimagnetic ordering in Ti
0.6
Mn
0.4
Co
2
O
4
and Ti
0.8
Mn
0.2
Co
2
O
4
sets in at 110.3 and 78.2 K, respectively due to the presence of unequal magnetic moments of cations, where Ti
3+
 , Mn
3+
 , and Co
3+
occupy the octahedral, whereas, Co
2+
sits in the tetrahedral site. For both compounds an additional weak antiferromagnetic component could be observed lying perpendicular to the ferrimagnetic component. The analysis of static and dynamic magnetic susceptibilities combined with the heat-capacity data reveals a magnetic compensation phenomenon (MCP) at
T
COMP
  =  25.4 K in Ti
0.8
Mn
0.2
Co
2
O
4
and a reentrant spin-glass behaviour in Ti
0.6
Mn
0.4
Co
2
O
4
with a freezing temperature of  ∼110.1 K. The MCP in this compound is characterized by sign reversal of magnetization and bipolar exchange bias effect below
T
COMP
with its magnitude depending on the direction of external magnetic field and the cooling protocol.