الملخص الإنجليزي
Three series of nanocrystalline compounds, Gd1-xRxCrO3 (where 0 ≤ x ≤ 1 and R: Pr,
Sm and Y), and the fourth series of GdMn0.5-xFexCr0.5O3 (where 0 ≤ x ≤ 0.5) were
synthesized by the auto-combustion route followed by annealing at 700 °C. XRD revealed
that all samples in all series are perovskites with a distorted orthorhombic structure and
crystallized into the Pbnm space group. TEM and XRD showed that the average particle
size for the samples annealed at 700 °C for 4 hours was in the range of (29 - 97) nm. FT IR spectra for all samples under investigation are recorded and its results for the change
in bonds lengths agree with the change in the unit cells volume obtained from the XRD
structure results. It is noted in the Gd1-xRxCrO3 series that with the increase in the
concentration of R, the magnetization decreases due to the reduction of Gd concentration.
On the other hand, by increasing the Fe content in the GdMn0.5-xFexCr0.5O3 series, the
magnetization increases at 2 K. The compounds in the different series exhibit the
magnetization reversal (MR) phenomenon at ZFC/FC modes. The Néel transition
temperature (TN) is found to increase with increasing Pr, Sm, and Fe concentrations in
Gd1-xRxCrO3 and GdMn0.5-xFexCr0.5O3 seriesrespectively while it decreases linearly as the
Y concentration increases in Gd1-xYxCrO3 series. Moreover, the compensation
temperature (Tcomp2) increases as x increases in the Gd1-xPrxCrO3 series although it
decreases as the Y and the Sm concentrations increases in the Gd1-xYxCrO3 and the Gd1-
xSmxCrO3 series respectively. It is found that the spin flipping temperatures (TSF) for the
compounds in Pr- series and Y-series decrease as the dopant concentration increases. All
samples under investigation in the Gd1-xRxCrO3 series and some samples in GdMn0.5-
xFexCr0.5O3 (under the same applied magnetic field) showed magnetic switching, which
makes them useful for magnetic switching applications. The samples were tested for
switching by changing the value of the applied magnetic field to switch the magnetization
of the compounds and different ΔH was needed to change the sign and magnitude of the
magnetization of the compound at a certain temperature which is less than the
compensation temperature of the compound. The negative magnetization ranges were
determined based on the change in the compensation temperatures for all the compounds
and the best compounds for the application in the different series were found. The
compounds of x = 0.1 and x= 0.3 are the best for switching for GdMn0.5-xFexCr0.5O3 series
and Sm-series respectively while for Pr-series and Y-series, the compounds of x =1 show
best magnetization reversal. At different temperatures, the magnetization as a function of
the magnetic field was measured. Weak ferromagnetism was observed just below (TN),
and notable magnetic features at lower temperatures. Above (TN), the compounds are
paramagnetic. A giant change in the exchange bias field (HEB) value as x changes from 0
to 1 was found in Pr-series and Sm-series at low temperatures. However, in Y-series the
compound of x = 0.7 has the largest value of HEB. That means these compounds can be
used in the applications of exchange bias (EB)-based magnetic devices at low
temperatures. It is observed that the coercivity is enhanced as the dopant concentration
increases in Pr-series and Y-series whereas, in Sm-series, it increases to reach a maximum
at x = 0.5 then it decreases. The exchange bias (EB) phenomenon was studied for all the
samples in all the series without applying a cooling, magnetic field (ZFC-EB) and under
different cooling, magnetic fields (FC-EB) at (1 T, 2 T, and 3 T) and found that applying,
magnetic fields (FC-EB) causes further increase in the percentage of change of the
exchange bias (EB) field as x changes in Pr-series and Y-series compared to the
corresponding percentage of ΔHEB under (ZFC-EB). Room temperature Mössbauer
spectra for the GdMn0.5-xFexCrO3 samples (x = 0.1- 0.4) exhibit paramagnetic central
doublets and for x = 0.5 it showed an onset of magnetic order. At 20 K, each spectrum is
composed of a magnetic six-line pattern superimposed on a central doublet. The fitted
doublets of the Mössbauer spectra at 300 K and the sextets of the spectra at 20 K show a
slight decrease in the value of the quadrupole splitting with increasing the Fe
concentration in the samples and this is due to the reduction in lattice distortion with
increasing x and a more symmetric crystal structure agreeing with XRD analysis. The
ranges of isomer shift for the doublets are (0.27 - 0.30) mms-1
at 300 K and (0.18 - 0.22)
mms-1
at 20 K and they indicate that the iron exists in the Fe3+ valence state in all samples
which agree with the XPS results.
