In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

Here, we report the co-substitution of Fe and Co for Ni atoms on metamagnetic behavior, martensitic transformation, and transport and magnetocaloric properties in Ni45(Co1−xFex)5Mn36.6In13.4 (x = 0∼0.05) alloys. It is found that the introduction of Fe atoms stabilizes martensitic phase and shifts martensitic temperature (TM) to higher temperature. Meanwhile, the Curie temperature TC of parent phase notably decreases. Upon Fe doping, the low magnetization of martensitic phase keeps nearly unchanged while the magnetization of parent phase slightly decreases. As a result, the Fe-doped samples maintain strong metamagnetic behavior and show great MR and MCE in an extended temperature range around room temperature. The hysteresis loss is reduced upon Fe-doping, which leads to an enhancement of effective RC by 15%.

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Magnetoresistance and magnetocaloric properties involving strong metamagnetic behavior in Fe-doped Ni45(Co1−xFex)5Mn36.6In13.4 alloys

Our recent progress on magnetic entropy change (ΔS) involving martensitic transition in both conventional and metamagnetic NiMn-based Heusler alloys is reviewed. For the conventional alloys, where both martensite and austenite exhibit ferromagnetic (FM) behavior but show different magnetic anisotropies, a positive ΔS as large as 4.1 Jkg−1 K−1 under a field change of 0–0.9 T was first observed at martensitic transition temperature TM ~ 197 K. Through adjusting the Ni:Mn:Ga ratio to affect valence electron concentration e/a, TM was successfully tuned to room temperature, and a large negative ΔS was observed in a single crystal. The −ΔS attained 18.0 Jkg−1K−1 under a field change of 0–5 T. We also focused on the metamagnetic alloys that show mechanisms different from the conventional ones. It was found that post-annealing in suitable conditions or introducing interstitial H atoms can shift the TM across a wide temperature range while retaining the strong metamagnetic behavior, and hence, retaining large magnetocaloric effect (MCE) and magnetoresistance (MR). The melt-spun technique can disorder atoms and make the ribbons display a B2 structure, but the metamagnetic behavior, as well as the MCE, becomes weak due to the enhanced saturated magnetization of martensites. We also studied the effect of Fe/Co co-doping in Ni45(Co1−xFex)5Mn36.6In13.4 metamagnetic alloys. Introduction of Fe atoms can assist the conversion of the Mn—Mn coupling from antiferromagnetic to ferromagnetic, thus maintaining the strong metamagnetic behavior and large MCE and MR. Furthermore, a small thermal hysteresis but significant magnetic hysteresis was observed around TM in Ni51Mn49−xInx metamagnetic systems, which must be related to different nucleation mechanisms of structural transition under different external perturbations.

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Magnetic entropy change involving martensitic transition in NiMn-based Heusler alloys

Effect of heat treatment on magnetostructural transformations and exchange bias in Heusler Ni48Mn39.5Sn9.5Al3 ribbons

Magnetocaloric and critical behavior in the austenitic phase of Gd-doped Ni50Mn37Sn13 Heusler alloys

We studied the influence of post-annealing on magnetic and transport properties in Ni45Co5Mn36.6In13.4 alloys. Our results demonstrate that post-annealing at low temperatures, ≤300 °C, can lead to a significant change in magnetic properties, martensitic temperature (TM) and magnetoresistance effect through structure relaxations and possible change in atomic order. It was found that TM shifts from 314 to 283 K but the strong metamagnetic behaviour is still retained when the sample is annealed at 300 °C for 3 h. Thereupon, a large magnetoresistance effect over an extended temperature range can be achieved through controlling the heat treatment conditions. Meanwhile, the thermal stability of the novel composition is also disclosed through the investigations on low temperature annealing effect.

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L Q Pan

Papers

Tuning martensitic transformation and magnetoresistance effect by low temperature annealing in Ni45Co5Mn36.6In13.4 alloys