Understanding the spin-glass state through the magnetic & electronic properties of Mn-doped ZnTe

Presenter Information

A. AlcantaraFollow
S. BarrettFollow
D. Matev
I. Miotkowski
A. K. Ramdas
T. Pekarak
J. T. Haraldsen

Faculty Sponsor

Dr. Jason T. Haraldsen, Dr. Tom Pekarak

Faculty Sponsor College

College of Arts and Sciences

Faculty Sponsor Department

Physics

Location

SOARS Virtual Conference

Presentation Website

https://unfsoars.domains.unf.edu/2021/posters/understanding-the-spin-glass-state-through-the-magnetic-electronic-properties-of-mn-doped-znte/

Keywords

SOARS (Conference) (2021 : University of North Florida) – Archives; SOARS (Conference) (2021 : University of North Florida) – Posters; University of North Florida -- Students -- Research – Posters; University of North Florida. Office of Undergraduate Research; University of North Florida. Graduate School; College students – Research -- Florida – Jacksonville – Posters; University of North Florida – Undergraduates -- Research – Posters; University of North Florida. Department of Physics -- Research – Posters; Project of Merit Winner

Abstract

Project of Merit Winner

International Research Symposium Exhibitor and Honorable Mention

To gain insight into the spin-glass state of diluted magnetic semiconductors, we have examined the magnetic and electronic properties of Mn-doped ZnTe using density functional theory. Using a generalized gradient approximation, we investigate the electronic and magnetic properties for x=0, 0.25, and 0.50 doping levels using the magnetic moment of Mn2+ as guide for the dependence of the Hubbard onsite potential on the electronic structure as well as a geometry optimization to assure an anti-ferromagnetic (AFM) ground state which is consistent with a zero magnetic moment spin glass state. An onsite potential of up to 8 eV on the Mn 3d-orbitals is needed to harden the magnetic moment toward S = 5/2. From our analysis of the electronic structure evolution with doping and onsite potential, we confirm the semiconducting state of the Mn-doped ZnTe as well as show that the presence of Mn incorporated into the ZnTe matrix at the Zn lattice site produces magnetic interactions through the Te ions with a distinct Te-Mn pd-orbital hybridization. Furthermore, we show that this hybridization is activated with the Mn doping above 25%, which corresponds to the doping level in which the spin-glass transition begins to rise. Therefore, it is likely that the coupling of pd-orbital hybridization of the Mn and Te p-orbitals is a precursor to the spin-glass state.

Comments

Audio Presentation Transcript:

Hello everyone, my name is Alexandria Alcantara, and I am an undergraduate at the University of North Florida. I am a physics major, and my research mentor is Dr. Jason Haraldsen. This abstract is on understanding the spin-glass state through the electronic and magnetic properties of manganese-doped zinc telluride. We also have work from Dr. Pekarak from the physics department.

First, let me explain what a spin-glass is; it’s similar to a paramagnet, similar to what our fridge magnets do – the spins of the electrons face the fridge all in the same direction. However, as soon as you take off that magnet, it goes back to a disordered orientation. Now, for a spin-glass, what happens is, it exhibits that paramagnetic behavior, but when it is induced by a certain temperature, it locks onto that spin state, making it into this glassy formation, and it has a measurement of remnant magnetization.

And so to gain insight into the spin-glass state, specifically for diluted magnetic semiconductors (DMS), we examine a known DMS, zinc-manganese-telluride, in its density of states to see how the electrons prefer to interact in the system to gain any type of understanding of how the spin-glass transition occurs. And this is mostly due to the fact that, we do know what materials can exhibit spin-glass behavior, however, we don’t know how they transition into that state. And so, what we did is, we took two structures, using Density Functional Theory (DFT), using QuantumATK for DFT, and this is because DFT examines how electrons move through a material, and because there are so many electrons, it can’t be calculated manually.

So, the Zinc is actually replaced by 25 and 50% manganese and we use a geometry optimization on these to relax the figures, and to assure the ground state is anti-ferromagnetic. We use a spin-polarized-generalized-gradient approximation in the Perdew, Burke, and Ernzenhof functional using a fairly large k-point sampling to properly model the system. We found that a Hubbard-onsite potential was necessary to achieve the proper magnetic moment for the manganese, which should be a high-spin state of five-halves. And so, a Hubbard-U was applied to the system to achieve this. We show the bandstructures and density of states in the evolution of the doping in its electronic structure.

And so, if you take a look at the blue middle area on the poster, the first image on the left is a graph of the transition temperatures as a function of their doping levels. We have here metals, insulators, and diluted magnetic semiconductors (DMS). The reason why we didn’t choose metals for this study is because of their high conduction; there’s too much going on in the electronic system to truly understand what is happening. And then, in contrast, the insulators, they are very stable in their configuration, thus, not much is happening in the electronic system to study. However, the DMS seem to be that sweet spot for the spin-glass transition as it shows this quite linear slope, but you can see that after the 20% doping there’s a shift in that slope. And so, this is why we chose the DMS for this study, to see what’s happening after that 20% doping level. Maybe this can tell us why this spin-glass transition happens.

And so, if you look at the, in the blue area, the top right image, this is the magnetic moment as a function of the Hubbard-U that I talked about, and this is for the proper manganese magnetic moment, and it starts off without a Hubbard-U, it starts off at a spin-state of two, and that is extremely low for the manganese. And so, as we apply the Hubbard-U, it grows into that high-spin state of five halves for both the 25% and 50%. Below that magnetic moment there is the energy gap and this is because when we inserted the manganese, there was impurity bands that showed, but they were very flat, and there was not much electron mobility available in the system. So, Hubbard-U was also necessary for the energy gap, and also, the 50% doping should be higher in the energy gap than the 25%, and this is actually from experimental procedure that I say this. And so, you can see that as Hubbard-U is applied, it does raise those bands, and it exhibits that flip in the energy gap to where now the 50% is higher, and this gives us the proper standards to analyze the system.

And so, if you look at the right side, the white, at the top, you have the structures that were developed; they are 25% and 50%. This is a zinc-blende structure. Below that, is the bandstructure and the + density of states for the zinc-telluride, which was used as the mother figure. If you look at the color coding, you can see that the orbitals are exhibiting a mostly p-orbital interaction, mostly because in the local density of states the s and d orbitals are not interacting at all. And then if you look below that, the next three bandstructures and density of states is the 25% doped, and you can see without the Hubbard-U, which is the first set of plots, those impurity bands are quite low, as I stated. And then if we look below that, down to the Hubbard-U value of eight, which is the third one, we can see that the manganese inserted is starting to provide interaction for the tellarium, exhibited through the d-orbitals. And then below that, the next three, these are the 50% doped structures. You can see at the Hubbard-U value of eight, which is the very last bandstructure and density of states, you can see that the p and d-orbitals are now occurring together in the total density of states. And this can actually only happen through the tellarium and the manganese due to the d-orbitals. And you can see that the zinc is essentially, completely ignored from the system at this point.

And if we look back to the middle portion, the blue area, below the transition vs. doping levels plot on the left side, this figure was made to exhibit a depiction of what’s really happening in the system. And so, the manganese are the red, the tellarium is the orange, and the zinc is the purplish-blue. And you can see that around the zinc, it’s just mostly a grey area; the system is ignoring the zinc, and the tellarium in orange, you can see, is now preferring its nearest manganese neighbors through this multi-lobe interaction, where the darks are the spin-downs, and the light areas are the spin-ups.

And so overall, what is shown here through this analysis, is that the spin-glass transition after that 20% doping level can proceed through the p and d-orbital, possible, hybridization, which is conveyed in that depiction of the multi-lobe interactions.”

Streaming Media 1

Rights Statement

http://rightsstatements.org/vocab/InC/1.0/