Structure and Magnetic Properties of Co, Ni, Mn, Cr and Cu Substituted Magnetites
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RESULTS AND DISCUSSION Table I shows the values of the average particle diameter and morphology corresponding to the various types of substitutions employed. Both undoped and Ni-substituted magnetites consist of polyhedral particles, whereas Co and Mn-doped magnetites exhibit a spherical/polyhedral morphology, and the Cr and Cu-substituted magnetites have particles of all shapes. The average particle diameter, determined by TEM and confirmed by X-ray diffraction, is in the hundred nanometer range, except for Fe 304:Cu 2÷, which consists of particles in the tens of nanometer range. TABLE I. Average particle size , saturation magnetic moment ms, coercive field H, and particle shape of doped synthetic magnetites. All specimens have the same mass. Composition
Fe 3 0
Concentration x (%)
4
(gim)
ms (emu)
HC (kOe)
Particle shape
0.340
0.156
0.49
polyhedral
2
8.20
0.400
0.083
0.40 spherical/polyhedral
2
8.50
0.680
0.294
0.25
2 Fe................................................................................................................... 10.70 0.540 30 4 :Mn ,
0.089
0.35
spherical/polyhedral
Fe 304:Co + Fe 30 4 :Ni ÷
2
11.06
0.084
0.237
0.99
all shapes
3
12.50
0.580
0.396
0.33
all shapes
+1-0.005 to +/-0.030 +1-0.005
0.01
Fe3O 4 :Cu + Fe 30 4:Cr ÷ Errors:
polyhedral
+/-0.05
Figure 1 shows the hysteresis loop measurements recorded at 4.2 K in an applied field up to 1 T. It can be seen that the saturation magnetic moment, coercive field and hysteresis phenomenon depend strongly on the type of substitution introduced in the magnetite structure. In particular, the coercive field decreases with increasing particle diameter, in agreement with results obtained on ball-milled nanocomposite materials. 1" As expected, the saturation magnetic moment was found to increase with increasing particle size in the case of Fe 3O4 :Ni 2+, which exhibits the same morphology as polyhedral Fe 3 0 4. In all other cases, deviations from the expected proportionality between ms and were obtained and attributed to prevailing shape anisotropy factors. The room-temperature transmission Mossbauer spectra of Fe 3 04, Fe 30 4:Co 2 ÷, Fe3 0 4 :Cr3+, Fe 3O4 :Ni 2+, Fe3O 4:Mn 2' and Fe 30 4 :Cu 2+ are given in Fig. 2. The fitted Mbssbauer parameters obtained from these spectra are listed in Table II. The Mossbauer spectrum of Fe 3 0 4 was analyzed by considering two sextets, corresponding to the tetrahedral (A) and octahedral [B] magnetic sublattices, in a 1:2 areal intensity ratio, typical of stoichiometric magnetite. No matter what charge, the substitutions employed were found to prefer the B sites. The relaxation time for electron diffusion, which allows the Fe[B] charge to redistribute itself, is smaller than the time characteristic to the M6ssbauer experiment, so that the measured hyperfine fields will not be those of the pure Fe 2' and Fe3" states, but time-averaged values for the different local configurations and ratios of the Fe2 ÷ and Fe3" states. The statistical distribution of impurity ions makes i
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