I synthesized NiCoFe2O4 using microemulsion route. I obtained two different morphologies which one of them is needle-like and the other is spherical. VSM was used to study magnetic property of them. The results show the perfect superparamagnetic behavior for both of them, with ZERO Hc !!!! So can we say that all nanoferrites are anisotropic materials?? Does it depend on the morphology or no?? And how can I discuss about effect of anisotropy on magnetic property?? (It is not possible to use another characterization)
Both of them (NiCoFe2O4 nanoferrites with needle-like and spherical shape ) have spinel structure. Can I use XRD for investigation of their anisotropy??
Thanks for your help
You have to ask yourself first what does anisotropy means? It describes the direction dependence of a property. Since crystals are characterized by a periodic arrangement of atoms and BONDS it means that ALL crystals are anisotropic, anyway which habit they reflect. However, you can generate additional "anisotropies", i.e. direction dependencies, e.g. by axial loading, coating, cooling etc. One expression is crystal growth. If you have a cubic phase and you grow long needles, the phase ist still cubic but effective mechanical properties changes to ether tetragonal or a Neumann symmetry called infinite/mmm.
The size-anisotropy can be also measured by XRD if the anisotropy correlates to the crystal symmetry. In case of the cubic material the problem is that the small planes perpendicular to the long axis are at the same 2theta angles as equivalent but big planes which are in the zone of the long axis. Therefore, I would not assume that you can see any asymmetry for cubic phases. In contrast, for tetragonal if should work (if there is no pseudosymmetry and peak positions of "small" and "big" planes do not overlay that much!). If you presuppose some model, you can of course also try to derive the anisotropy in isometry using the anomaly in peak broadening. For spheres you should not find this effect, for needle shaped you should clearly see it. But it is more challenging since the same peak describes than both which is certainly more complicated. The question I cannot answer is, which software is able to do this. BGMN (Rietveld refinement) certainly does it but needs a lot of time to become familiar with. Maybe, other programs do this job as well.
CoFe2O4 has an easy axis along the [100] with magnetocrystalline anisotropy constant K1 ∼ 2 × 106 ergs/cm3, greater than other spinel ferrites, resulting in a high coercivity (Hc). But when the particle size reaches a certain value as in the attached picture (red line), the effect of anisotropy become undistinguishable even with shape anisotropy particles.
"So can we say that all nanoferrites are anisotropic materials??" Answered by Dr. Gert
"Does it depend on the morphology or no??" Answered by Dr. Gert
"how can I discuss about effect of anisotropy on magnetic property??" Go for single crystal or grow epitaxialy (thin film). You may try to compact your needle-like shape powder to a bulk form but you have to apply magnetic field during compacting.
First you have to measure magnetic hysteresis loops of your sample in two mutually perpendicular directions. If two loops overlap, sample is isotropic. If two loops are different, then sample is anisotropic. If the difference between two loops is higher, then the anisotropy is also higher. The anisotropic properties mainly depend on the sample preparation conditions.
Further details can be found in following research article
P. Samarasekara and F.J. Cadieu, “Magnetic and Structural Properties of RF Sputtered Polycrystalline Lithium Mixed Ferrimagnetic Films” Chinese J. Phys. 39(6), 635-640 (2001)
This paper can be downloaded freely. Figure 3 in this paper shows hysteresis loops of isotropic sample. Figure 4 in this paper shows hysteresis loops of anisotropic sample.
The magnetic anisotropy is obtained by minimizing the total energy. The total energy of a magnetic material has several contributions, mainly Zeeman, shape and magnetoelastic. In your case, shape and magnetoelastic contributions could be calculated if you know the size (length and diameter) of needles and strain in unitcell (calculated from XRD peak-position shifts).
In your case shape anisotropy should be prominent if all the “needles” are aligned. For a needle of aspect ratio ~30 (300 nm length and 10 nm of diameter), anisotropy field is approximately 2.5 kOe for CoFe2O4. But, I guess you have randomly oriented “needles” and this could be a reason for “zero Hc”. The other reason for zero Hc could be that you have more "nanospeheres than needles" and nanospheres are in the paramagnetic/superparamagnetic limit.
You did not mention the ratio between Co and Ni in the compound (NixCo1-xFe2O4). Also, you did not mention the crystallite size of the prepared samples. NiFe2O4 is superparamagnetic while CoFe2o4 is ferromagnetic. If the crystallite size is so small, the superparamagnetic behavior is highly expected, even for CoFe2O4.
Dear Mohsin do you have reference on this: "In your case, shape and magnetoelastic contributions could be calculated if you know the size (length and diameter) of needles and strain in unitcell (calculated from XRD peak shift."
How do you make sure that the crystal shape depend on the magnetic anisotropy? As Dr. Gert pointed out; it's cubic. e.g. hexagonal BaFe12O19 has an easy axis along [0001], the shape anisotropy cud be platy or needle depend on synthesize route not depend on magnetic anisotropy, more over by partial substitution the magnetic ion one cud tune the it magnetic anisotropy direction. What i want to say is it's very difficult task to deal with particles. You have to make sure that single particle is single domain, non interacting (> exchange interaction) and align properly with applied magnetic field.
"calculate the size-strain from peak shift" ???
Dear Maykel Manawen,
Please go through this book:
Modern Magnetic Materials: Principles and Applications
Book by R. C. O'Handley
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