Although more information about your compound would be needed, the question does not depend on the type of magnetometer. Spin freezing behaviour can be studied by measuring the magnetization vs. temperature M(T) curves under different apply magnetic field values in the following way (the so-called ZFC-FC measurements, zero-field-cooling/field-cooling):

1. After cooling the sample in zero field down to low temperature, then a low magnetic field (less than 100 Oe) is applied and the M(T) curve measured on heating up to the highest temperature.

2. After that the M(T) curve is measured on cooling down under the same applied magnetic field.

The splitting of the M(T) curves after ZFC and FC could be due to spin freezing, but other phenomena such as superparamagnetic (SPM) behaviour also shows this kind of M(T) splitting.

It should be useful to repeat the same measurement with different values of the applied magnetic field in order to show the evolution of the M(T) splitting with the magnetic field.

If the ZFC-FC M(T) curves show splitting and In order to differentiate between SPM, spin-glass-like behaviour, etc, AC magnetic susceptibility measurements under several frequencies is needed, with at least a frequency variation of three decades.

Anyway, if a ferromagnetic contribution is present together with the "spin-freze" it could be difficult to separate both contributions.

Although more information about your compound would be needed, the question does not depend on the type of magnetometer. Spin freezing behaviour can be studied by measuring the magnetization vs. temperature M(T) curves under different apply magnetic field values in the following way (the so-called ZFC-FC measurements, zero-field-cooling/field-cooling):

1. After cooling the sample in zero field down to low temperature, then a low magnetic field (less than 100 Oe) is applied and the M(T) curve measured on heating up to the highest temperature.

2. After that the M(T) curve is measured on cooling down under the same applied magnetic field.

The splitting of the M(T) curves after ZFC and FC could be due to spin freezing, but other phenomena such as superparamagnetic (SPM) behaviour also shows this kind of M(T) splitting.

It should be useful to repeat the same measurement with different values of the applied magnetic field in order to show the evolution of the M(T) splitting with the magnetic field.

If the ZFC-FC M(T) curves show splitting and In order to differentiate between SPM, spin-glass-like behaviour, etc, AC magnetic susceptibility measurements under several frequencies is needed, with at least a frequency variation of three decades.

Anyway, if a ferromagnetic contribution is present together with the "spin-freze" it could be difficult to separate both contributions.

More than bifurcation in M(T) in ZFC and FC mode at various static fields, observation of frequency-dependent peak in temperature dependence of ac susceptibility provides a better test for spin freezing effect in different magnetic systems. The above mentioned bifurcation can be observed in different spin systems including ferromagnets too.!

I am using both methods to define if spin freeze is happenign to Magnetite samples, and the AC susceptibility is prividing a better result.

Is what you guys call "spin freezing" the blocking temperature? Then the effect will depend on the characteristic time constant of your measurement. I.e. VSM or SQUID magnetometry scans will tell a different story from Mößbauer spectroscopy determination of T_B.

Otherwise, i'd be curious to know what you mean by "spin freezing".

I agree with all above mentioned comments. However, as I understand, you study of a semiconductor compound ZnO: Mn. Therefore, I would recommend to apply the approach described in our paper, where we are presented the optical method to study of spin freezing in diluted magnetic semiconductors.(see Journal of Applied Physics, 112, 093715 (2012).

In addition to Pedro Gorria’ comment, I would like to note that the transition from paramagnetic to spin glass (SG) phase is characterized by a sharp cusp in the low-field ac susceptibility at the freezing temperature (Tf). I a also agree that the thermoremanent magnetization studies indicate that there is a very slow dynamics of spins when the temperature is lowered towards Tf. Usually, a spin freezing process can be considered in terms of clustering. Far above Tf (more than 5-7 Tf) SG system is in paramagnetic phase. At the temperature lower about 5Tf SG properties are explained by a collection of independent finite superparamagnetic clusters with local order and fluctuations. According to Myrgovnik and Mydosh’s cluster model, there are local magnetic inhomogeneities in the paramagnetic spin distribution, resulting in superparamagnetic clustering appeared at T < 5 Tf which is larger than it can be explained by random statistics and near-neighbour interactions (small clusters). The clusters are in thermodynamical equilibrium with the phase of free spins at T > Tf. These finite dynamic superparamagnetic clusters (or magnetization fluctuations) vary in magnitude and direction from one magnetic atom site to another, and are a function of time. A typical size of the superparamagnetic clusters for different SG compounds is about 3 nm. For temperatures reaching Tf (Tf < T < 2Tf), the cooperative behavior of spins (or “locked” clusters) becomes more important than behavior of independent clusters. At Tf an infinite cluster appears as a result of the interlocking of different clusters. Besides, at T smaller than Tf there also are part “loose” spins as well as the finite clusters. More detail information on the spin freezing process and its temperature evolution in semiinsulating CdMnTe spin glasses is presented in Journal of Applied Physics 112, 093715 (2012).