Chapter 1: Introduction
1.6 Magnetization Reversal
The magnetization reversal (MR) or negative magnetization (NM) is defined as the change in the magnetization from a positive value to a negative value under a fixed positive applied magnetic field, either by varying the magnitude of applied magnetic field or by varying the temperature. The temperature at which the net magnetization within the material becomes zero (M = 0) is known as compensation temperature (Tcomp) below which the magnetization becomes negative. It was Neel who first predicted the magnetization reversal in ferrimagnetic material in 1948 and later it was experimentally observed in spinel ferrites in 1950 [40]. Beside ferrimagnetic material, magnetization reversal has also been reported in various magnetically ordered materials such as canted antiferromagnets, intermetallic alloys, ferromagnets, molecular magnets and multilayers [7,41]. The materials exhibiting magnetization reversal find potential technological application in thermomagnetic switches,
magnetic memory, spin valves, thermally assisted magnetoresistive random access memory (TAMRAM) [7,19,42]
Magnetization reversal is an intrinsic phenomenon that appears in a material when two or more specifically arranged magnetic moments coupled with a magnetic anisotropy.
Thus, the presence of finite magnetic anisotropy is very much essential for the occurrence of MR. In the absence of any magnetic anisotropy, a compensation behavior is observed but without any reversal of magnetization because below the compensation temperature the magnetization of the dominant component (opposite to the applied field) rotates and gets aligned in the direction of the applied magnetic field. Such a behavior is observed in some of the garnets [7,43,44]. Magnetization reversal has been reported in a variety of magnetic materials and the mechanism of its origin can be explained differently in different classes of materials which are briefly discussed below.
1.6.1 Magnetization Reversal in Ferrimagnetic materials
Magnetization reversal was first predicted by Neel in ferrimagnetic materials and its origin was explained by considering the different temperature dependence of the antiferromagnetically coupled sublattice magnetization corresponding to different crystallographic sites [40]. At T = Tcomp the magnetization of the two antiparallel sublattices becomes equal resulting zero magnetization in the sample, whereas for T < Tcomp the magnetization becomes negative.
The temperature dependence of FC magnetization of polycrystalline Co2VO4
manifesting magnetization reversal is shown in Fig. 1.8. Negative magnetization is observed below Tcomp = 70 K which is explained by considering the antiparallel moments of Co2+
located at octahedral site and Co2+/V4+ at tetrahedral site and their different temperature dependent behavior in accordance with the Neel’s hypothesis [45]. Fe2MoO4 [46], Co(Cr1- xFex)2O4 [47], Ni(Cr1-xFex)2O4 [48] and FeCr2-xAlxS4 [49] are few more examples of spinel compounds where the observed magnetization reversal can be explained satisfactorily by considering the above framework. Beside spinel compounds, magnetization reversal has also been observed in rare earth garnets [50] and Prussian blue analogues [51,52] where its origin can be explained with the same framework of Neel’s mean field theory.
Figure 1.8: Reversal of magnetization observed in the temperature dependent field cooled magnetization of Co2VO4 for H = 700 Oe. Adapted from [45].
1.6.2 Magnetization Reversal in Intermetallic compounds
In these compounds, the magnetization reversal is observed due to the competition between the antiparallely arranged spin and orbital moments of an atom or ion occupying the same crystallographic site and their different temperature dependences. Ferromagnetic alloys of (Sm1-xGdx)Al2 [41,53]shows the MR behavior and its origin is explained on the basis of different temperature dependences of spin and orbital moments of Sm3+ and Gd3+ ions and the moments of polarized conduction of electrons [54]. Magnetic compensation along with the negative magnetization observed in bulk Nd0.75Ho0.25Al2 [55], Sm2Al and Sm1.988Gd0.012Al [56] can be explained with similar mechanism.
1.6.3 Magnetization Reversal in Ferromagnetic and Antiferromagnetic Heterostructures
In multilayer compounds with FM and AFM interfaces, the magnetization reversal behavior is observed due to different temperature dependence of these layers. In Ni-FeF2
multilayer system, the negative magnetization is observed as a result of the strong interfacial AFM coupling between the FM Ni layer and the AFM FeF2 layer [57]. The negative magnetization reported in Gd-Fe, Gd-Co, and Gd-CoNi multilayers is attributed to the
antiparallel alignment of the Gd moments at low temperature with respect to that of transition elements [58,59].
1.6.4 Magnetization Reversal in Canted Antiferromagnetic Materials
In some materials, the presence of spin canted AFM sublattices occupying different crystallographic sites and their interaction leads to MR. For example, rare earth orthovandates such as YVO3 [60], LaVO3 [61,62], SmVO3 [63], and NdVO3 [64] fall in this category of materials. In the case of LaVO3, the observed magnetization reversal is explained in terms of the response of vanadium orbital moments to the first order structural transition at To = 138 K which can create a canted spin component opposite to the applied field below To [62,65].In YVO3 single crystals,the magnetization reversal is ascribed to the competition between the canted spin moments of V3+ ions produced by DM interaction and single ion magnetic anisotropy [60]. However, in NdVO3 and SmVO3, it is ascribed to the different quenching rates of orbital moments of V3+ ions [63,64].
The temperature dependence of FC magnetization of BiFe0.5Mn0.5O3, which crystallizes in an orthorhombic structure, shows the magnetization reversal behavior with Tcomp = 208 K. Here the origin of magnetization reversal is explained by considering the competition between the single-ion magnetocrystalline anisotropy and the antisymmetric DM interaction [66].
In some materials the presence of a paramagnetic sublattice along with the FM/canted AFM sublattice also leads to the magnetization reversal. The competition between paramagnetic moment and the FM/canted AFM sublattices residing at different crystallographic sites bring about the MR in such systems. The moments of the paramagnetic atom or ion get partially aligned opposite to the moments of the FM/canted AFM sublattices under the influence of an effective molecular field arising from the ordered FM/AFM sublattices. The net magnetization of such compounds can be expressed as [12]
/ 3
(557 * 86)9
(1.9) where, MCr is the weak FM moment of canted Cr3+ ions, and C is the Curie constant. H and HI represent the external magnetic field and internal field respectively.
In the single crystal of La1-xGdxMnO3 [67], the Gd spins behave like a paramagnet and are partially aligned antiparallel with respect to the FM component of Mn spins. When the net Gd moment exceeds the FM component of Mn moments, reversal in the sign of magnetization or negative magnetization is observed. The above mechanism of also holds good for the observed negative magnetization in SmMnO3 [68], ErCo0.5Mn0.5O3, and GdNi0.3Mn0.7O3 [69] compounds. The MR observed in some rare earth orthochromites RCrO3 (R = Gd, Ce, Tm, Yb, Sm) [12,15,70–72] and in polycrystalline NdCr1-xMxO3 (M = Fe, Mn) [73,74] can be satisfactorily explained by the above phenomenological model of competition between the paramagnetic moment of the rare earth ion and the net FM moment of canted transition metal ion.