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Investigations on compensated ferrimagnetism in the Mn2Co0.5V0.5Al Heusler alloy

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Highlights

  • Compensated ferrimagnetism in Mn2Co0.5V0.5Al Heusler alloys by tuning composition.

  • N/P type of thermomagnetic behavior in Mn2+zCo0.5V0.5-zAl.

  • P type of thermomagnetic behavior in Mn2+δCo1-δV0.5Al.

  • The Curie temperatures are highly compositional sensitive.

  • Mn1.96Co0.5V0.54Al alloy close to the fully compensated ferrimagnetic behavior.

Abstract

We present theoretical and experimental investigations on the electronic and magnetic properties of the Mn2+δCo0.5-δV0.5Al (δ = 0 ÷ 0.08) and Mn2+zCo0.5V0.5-zAl (z = −0.14 ÷ 0.08) Heusler alloys. The electronic band structure calculations by the Korringa-Kohn-Rostoker (KKR) Green's function method show the dependence of the total spin moment on the preferential site occupation for different crystal sites and predict compensated ferrimagnetic behavior for the compounds obtained from Mn2Co0.5V0.5Al by slight variation of composition. Experimental investigations by tuning the Mn content in Mn2Co0.5V0.5Al alloys have been performed in order to achieve the fully compensated ferrimagnetic behavior which is of particular importance for spintronics. The thermo-magnetic measurements show different types of ferrimagnetic behavior with the threshold influenced by Mn content: N-type of ferrimagnetic behavior for Mn2+δCo0.5-δV0.5Al, while for the Mn2+zCo0.5V0.5-zAl Heusler alloys a combined N/P-type of behavior, depending on z, is observed: N-type for Mn excess/P-type for Mn deficiency. The Curie temperatures of the investigated alloys show high sensitivity on the Mn content. The results of our study may give an insight on the influence of the composition on the magnetic properties of the Mn2Co0.5V0.5Al alloys in order to adjust their properties and make them suitable for spintronic applications.

Introduction

Heusler compounds are an important class of materials already widely used for decades as monochromators or polarizers for neutron scattering i.e: Cu2MnAl [1]. Also, some Heusler compounds are exhibiting excellent thermoelectric properties [2]. These compounds have attracted much interest recently due to their huge potential for spintronics or future energy applications [3]. Several Mn2-based full Heusler compounds exhibit half-metallic character with a spin gap within minority spin channel which leads theoretically to 100% spin polarization of the conduction electrons at the Fermi level. This kind of materials are required for the new generation of devices using spin polarized currents but the improvement of their magnetic properties has a particular importance.

The Mn2-based Heusler compounds have major advantages as high tunability of their magnetic structure, with the magnetic moments of Mn atoms ferro- or ferrimagnetically coupled depending on the distance between them and on their magnetic environment [[4], [5], [6], [7]]. Recently, several half-metallic ferrimagnetic (HMFi) fully compensated Mn2-based Heusler compounds have been theoretically predicted [[8], [9], [10]]. For few of them the almost fully compensated ferrimagnetic behavior has been experimentally proven [[9], [10], [11], [12], [13]]. The advantage of using these compounds for spintronics arises from the possibility to obtain materials with lower average magnetic moment which creates lower magnetic strains. As a consequence, the HMFi containing devices would have lower energy losses and will be less affected by the external magnetic fields. One of the major drawbacks of these materials could appear due to low values of the Curie temperatures or low individual magnetic moments [14].

According to Slater-Pauling rule [3], the Mn2Co0.5V0.5Al Heusler alloy should have zero average spin moment and would be a fully compensated HMFi. In our previous investigation, we showed that only near fully compensated HMFi can be found through the Mn2Co0.5V0.5Al Heusler alloy, a small value of saturation magnetization (0.29 μB/f.u.) being measured. On the other side, the half-metallic character deduced from density of states (DOS) calculations is not affected by slight variation on alloys composition [10].

We carried out further theoretical and experimental studies on the magnetic behavior in Mn2Co0.5V0.5Al Heusler alloys by slight adjustment of the Mn/Co and Mn/V ratios. The aim of this study is to identify the composition for which the fully ferrimagnetic compensation is achieved. Theoretical band structure calculations have been performed in order to determine the behavior of sublattices magnetic moments in Mn2+δCo0.5-δV0.5Al (δ = 0 ÷ 0.08) and Mn2+zCo0.5V0.5-zAl (z = −0.14 ÷ 0.08) Heusler alloys. We show the magnetic measurements for the as-cast samples and discuss the experimental magnetizations and Curie temperatures in relationship with the Mn occupation on the 4a and 4b crystal sites.

Section snippets

Experimental and computational details

The as-cast samples were prepared by arc melting from the high purity elemental constituents, Mn (99.95 wt%), Co (99.99 wt%), V (99.99 wt%) and Al (99.999 wt%) as starting materials. The homogeneity was insured by turning and melting repeatedly the samples. The as-cast samples have been obtained by water-cooling using copper crucibles. The loss of weight of the final materials was under 1%. The investigations of the crystal structure have been performed by XRD measurements with a Brüker D8

Synthesis and characterization of the samples

The XRD patterns for the Mn2+δCo0.5-δV0.5Al (δ = 0, 0.04, 0.06 and 0.08) and Mn2+zCo0.5V0.5-zAl (z = −0.14, −0.06, 0.0 and 0.08) as-cast samples are plotted in Fig. 1, Fig. 2. The intensity scale for 2θ interval of 25–35 deg., the region of (111) and (200) superlattice peaks, is shown four times enlarged. The Hg2CuTi type of structure has been considered for the refinement of all XRD patterns, due to lower total energy obtained in concurrence with L21 type of structure [10]. The XRD data

Acknowledgements

Financial support through the UEFISCDI grants PN-III-P3-3.1-PM-RO-FR-2016-0057 (Brancuşi), PN-III-P1-PCCDI-2017-0871 and the MCI Nucleu-Program, PN 19 35 01 01 is gratefully acknowledged.

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