Hafnium based ferromagnetic half metals for spintronics and thermoelectric applications – Materials Computation

Climate change is one of the most pressing issues facing the globe today because of the significant carbon footprint produced by the combustion of coal and gasoline engines used to produce traditional power. Furthermore, thermodynamic constraints result in the waste of half of the world’s total heat energy production [1]. Waste heat delivered from different sources contain thermal gradients and effluents which are harmful to the green society. The major drawbacks of these thermal gradients is such that they increase global warming which is threatening to the entire world. The advanced technology which converts this thermal heat energy into an re usable electricity is known as thermoelectricity works under the principle of Seebeck effect [2]. The best thermoelectric materials are parametrized by a factor know as figure of merit which directly proportional to power factor as well as Temperature and inversely proportional to thermal conductivity [3]. The merit values greater than unity led the thermoelectric materials as the potential candidates for low, mid and high temperature applications [4].

In the field of IT sectors and technological parks, there is a necessary of large mass storage processors for data storage applications [5]. Commonly, data storage devices use large number of registers and counters made up of conventional flips, diodes and transistors etc [6]. The main disadvantage of these conventional electric devices are they will heat up very quickly and its heating rate is up to several million electron volts [7]. Furthermore, these devices have very less long lasting and storage capability [8]. But neglecting the principle of conventional electronics which mainly corresponds to charge of an electron, it is possible to spin of an electron [9]. The development in the spin of an electrons led to the new advancement in the technology known as spintronics which is nothing but spin-based electronics [10]. The spintronic principle is used for making spin-based registers, computer, diodes, transistors etc., But unfortunately, properties of best spintronic devices have high spin polarization, integer magnetic moment and high Curie temperature [11]. Semiconductors does not show high spin polarization it is only possible with the help of metals [12]. And moreover, ferromagnetic materials have high Curie temperature and integer magnetic moment [13]. So, it is a need to find new class of materials which shows semiconducting as well as metallic behavior [14]. In the case, there is group of metals, they have the name half metals with both semiconducting and metallic behavior. Because of its properties such as band gap, magnetic behavior, integer magnetic moment etc., they are suitable for both thermoelectric and spintronic applications [15].

Due to their unique and particular traits, Heusler alloys which belong to one of the interesting classes of materials have attracted an immense amount of interest during the preceding three decades [15]. Both thermoelectricity and spintronics make significant use of these materials. These materials were initially created in 1903 when Friedrich Heusler, a German mining engineer, discovered that Mn-Cu could be alloyed with metalloids such as Sn, Al, As, Sb, Bi, or B to create ferromagnetic alloys, despite the fact that none of the constituent elements are ferromagnetic in and of themselves [16], [17]. Since then, these Heusler alloys have garnered a lot of interest because of their many physical characteristics, including semiconductor thermoelectric activity, spin polarization effects, superconductivity, ferromagnetic, and half-metallic [18], [19], [20]. These solids include the ternary combinations X2YZ and XYZ, in which Z is an element belonging to group III, IV, or V of the periodic table and X and Y are transition metals [21]. Sometimes an alkaline earth metal or a rare earth element is used in place of Y [22]. Half-Heusler compounds are a promising new family of functional materials for a variety of spintronic technology applications, including memory devices, tunneling magneto-resistance (TMR), and magnetic tunnel junctions [23], [24], [25]. Additionally, they are great options for low-cost and eco-friendly thermoelectric applications, which can have two functions: producing energy from heat sources or cooling by applying electricity. A significant class of half-Heusler compounds has recently drawn attention because of its intriguing characteristics. This class of materials works well in spintronics and a variety of thermoelectric applications [26], [27].

The half metals VPtSi, VPtGe and VPtSn are ferromagnetic with positive magnetic moment 1 [28]. The materials shows high Curie temperature for VPtSi, for VPtGe and for VPtSn. Similarly, alkali based half metals LiCrGe, LiCrSn and LiCrPb shows a phase transition from ferromagnetic to anti-ferromagnetic phase exhibits a negative magnetic moment of −3 [29]. They also show high Curie temperature for LiCrGe, LiCrSn and LiCrPb respectively. Muhammad Atif Sattar et al. studied the thermoelectric and spintronics properties of VCrTe, VMnTe, VFeTe and VCoTe [30]. Among the 4 compounds investigated, VCrTe and VFeTe are stable in ferrimagnetic phase, while VCoTe is ferromagnetic and VMnTe is non-magnetic. The nonmagnetic semiconductor VMnTe shows high power factor of 249.4 and high figure of merit of 1.2 at 1200 K. The half Heusler alloys, FeCrSb, RuCrSb and OsCrSb shows high Seebeck coefficient of 299 at 300 K [31]. The high ZT values of 3.27 and 1.43 is obtained for FeCrSb and RuCrSb suggest the stability of Heusler alloys for TE applications. S.Majumdar et al. concluded that there is paramagnetic to ferromagnetic phase transition in due to their electron transport properties [32]. When an external magnetic field is applied, band gap of the material decreases, and negative magneto resistance increases near Curie temperature.

In this article, we investigate the Heusler alloys () for its half metallic property, spintronic and thermoelectric applications. We organized the paper as follows: in Section 2, we have given the computational details employed for DFT in this investigation. The crystal structure and its stability is discussed in Section 3.1. Following that, we have investigated the mechanical property of the alloys in Section 3.2. The electronic and magnetic study results are given in the Sections 3.3 Electronic properties of, 3.4 Magnetic properties of respectively. The transport properties of the alloys () are discussed in Section 3.5. Finally, we concluded and presented our results in Section 4.