Spin Lattice order in frustrated Nickel- Zinc-Chromite (Ni x Zn 1-x Cr 2 O 4 ) System
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Abstract
ZnCr
2
O
4
compound is well known to show the frustration of the spin structure. At 12 K,
ZnCr
2
O
4
distorts to break symmetry of the degenerated frustrated spin states by the spin-
Peierls-like phase transition, accompanying with the anti-ferromagnetic ordering. On the
other hand, NiCr
2
O
4
undergoes a Jhan-Teller [JT] Phase transition at a temperature of
310K, differing from the low temperature ferromagnetic transition temperature T
c
of
about 60K. It is also reported that NiCr
2
O
4
shows another magnetic phase transition at
about 30K. These two phase transitions accompanying with the lattice change can be
understood by the magneto-elastic interactions. Two interactions, the Jhan-Teller
interaction and the spin-Peierls-like interaction co-exist in Ni
x
Zn
1-x
Cr
2
O
4
system. In this
report the Nickel-Zinc-Chromite [Ni
x
Zn
1-x
Cr
2
O
4
] compounds with x = 0.8, 0.6 and 1 are
investigated by the X-ray diffraction measurements. From these measurements the
Crystal structures are determined. The full width at half maximum [FWHM] and
integrated intensity [I.I.] give the fruitful information for magnetic elastic
interactions.The Phase transition of Nickel-Chromite [NiCr
2
O
4
] at 30 K is not yet well
understood. To investigate this phase transition, the magnetic moment of NiCr
2
O
4
and
also the magnetic moment of other two components Ni
0.8
Zn
0.2
Cr
2
O
4
and Ni
0.6
Zn
0.4
Cr
2
O
4
were performed by Superconducting Quantum Mechanical Interference Device [SQUID].
From this measurement estimated effective Bohr magneton for NiCr
2
O
4
, Ni
0.8
Zn
0.2
Cr
2
O
4
and Ni
0.6
Zn
0.4
Cr
2
O
4
are 5.77, 6.34 and 7.84 respectively. The asymptotic Curie
temperatures (θ
CW
) are obtained as - 470 K, -565 K and -870 K. The corresponding
frustration strengths are -7.2, -18.8 and -29.00. From the previous measurement when x =
0.5 and x = 0, the asymptotic Curie temperatures (θ
CW
) were -498K and -380K and the
corresponding frustration strengths were -30.18 and -31.00 respectively.