Dielectric and magnetic properties of mechanically milled nanosized
cupric oxide
Bianchi A.1,2,4, Junciel L. D.2,4, Osorio S.3, Echeverría G.1,2,4, Ayala A.5, Mercader R.
C.2, Torriani I. L.6, Punte G.1,2
1
LANADI e 2IFLP (CCT-La Plata), 3Departamento de Física, FCE and 4Facultad de
Ingeniería UNLP, Argentina.5Departamento de Física, Universidade Federal do
Ceará, Fortaleza, Brasil. 6DFMC-UNICAMP y LNLS, Campinas, SP, Brasil
Cupric oxide (CuO) has been the subject of permanent attention since the advent
of the Cu-based high-Tc superconductors in 1987. Its C2/c monoclinic crystal structure
is unique among the 3d transition-metal monoxides and departs considerably from the
simple NaCl-type structure common to most of them. The crystal structure of CuO can
be viewed as made up of two types of zigzag Cu–O–Cu chains running along the [10¯1]
and [101] directions. The system undergoes two successive magnetic transitions; one to
an incommensurate antiferromagnetic (AF) phase between TN1=230 K and TN2=213 K,
and a second one to a commensurate AF phase below TN2. This magnetic behavior
seems to be ruled by the dominant magnetic interaction along the [10¯1] chain. The angle
involved in this super exchange path, ≈ 146 (intermediate between 90◦ and 180◦),
seems to be responsible for the spiral order arising from the competing AF and
ferromagnetic interactions, while maintaining a high effective J, which in turn induces a
magnetoelectric multiferroic behavior at 230K. In addition, CuO is a charge-transfer
insulator, but micro-traces of Cu3+ turn it into a semiconductor. All the reported studies
in the literature demonstrate the great importance that the microstructure and grain size
have on the magnetic and dielectric properties.
Trying to add to the knowledge of the relation between the magnetic and
dielectric properties of the title system, we have produced nanostructures by controlled
mechanical milling of high purity commercial CuO for different times up to 12 h. The
structural and microstructural properties of all samples were studied by X-ray
diffraction, SAXS and SEM. For increasing milling time, crystal-cell expansion, grainsize diminution, and strain and surface fractality growth have been observed. The
dependence of the magnetic moment with the applied magnetic field (0 ≤ H ≤ 18 kOe)
showed for all the samples the presence at RT of hysteresis loops, whose characteristic
values (saturation magnetization, coercitivity and remanence) raise with milling time.
Impedance spectroscopy at RT (in the 1Hz to 1 MHz range) showed a remarkable
reduction of the real impedance component value when going from commercial CuO
(40 kΩ) to the 12 h milled sample (3.3 kΩ). Besides, the imaginary component of the
impedance, Z”, as function of frequency also shows a noticeable modification when
compared to commercial CuO (maximum at 1.4 x 105 Hz), while in the nanostructured
samples, the Z” maximum values lie at frequencies higher than the measured range. In
this work we discuss the relationship between the magnetic and dielectric properties of
the studied samples with their structural and microstructural properties dependence on
the milling time.
Keywords: CuO, nanostructure, nanomagnetism, impedance spectroscopy
bianchi@fisica.unlp.edu.ar