[Paper Review] Crystal facet orientated Altermagnets for detecting ferromagnetic and antiferromagnetic states by giant tunneling magnetoresistance effect

Boyuan Chi, Leina Jiang+5 more|arXiv (Cornell University)|Sep 18, 2023

Magnetic properties of thin filmsPhysics and Astronomy38 references3 citations

TL;DR

This paper proposes an altermagnetic tunnel junction (ATMTJ) using RuO2/TiO2/CrO2 heterostructures, where crystal facet orientation enables giant tunneling magnetoresistance (TMR) up to 6100% when current flows along the [110] direction due to momentum-resolved spin splitting in RuO2's band structure. The ATMTJ acts as a robust, field-resistant magnetic sensor capable of detecting Néel vector states via TMR contrast, offering a pathway to high-TMR spintronic devices with altermagnetic electrodes.

ABSTRACT

Emerging altermagnetic materials with vanishing net magnetizations and unique band structures have been envisioned as an ideal electrode to design antiferromagnetic tunnel junctions. Their momentum-resolved spin splitting in band structures defines a spin-polarized Fermi surface, which allows altermagnetic materials to polarize current as a ferromagnet, when the current flows along specific directions relevant to their altermagnetism. Here, we design an Altermagnet/Insulator barrier/Ferromagnet junction, renamed as altermagnetic tunnel junction (ATMTJ), using RuO$_2$/TiO$_2$/CrO$_2$ as a prototype. Through first-principles calculations, we investigate the tunneling properties of the ATMTJ along the [001] and [110] directions, which shows that the tunneling magnetoresistance (TMR) is almost zero when the current flows along the [001] direction, while it can reach as high as 6100\% with current flows along the [110] direction. The spin-resolved conduction channels of the altermagnetic RuO$_2$ electrode are found responsible for this momentum-dependent (or transport-direction-dependent) TMR effect. Furthermore, this ATMTJ can also be used to readout the Néel vector of the altermagnetic electrode RuO$_2$. Our work promotes the understanding toward the altermagnetic materials and provides an alternative way to design magnetic tunnel junctions with ultrahigh TMR ratios and robustness of the altermagnetic electrode against external disturbance, which broadens the application avenue for antiferromagnetic spintronic devices.

Motivation & Objective

• To design a novel magnetic tunnel junction using altermagnetic materials that achieve giant TMR ratios.
• To overcome the challenge of detecting Néel vector states in antiferromagnetic materials by leveraging spin-polarized conduction in altermagnets.
• To demonstrate that altermagnetic electrodes can function as effective spin-polarizing barriers in tunnel junctions, similar to ferromagnets, when current flows along specific crystallographic directions.
• To provide a practical, field-resistant alternative to all-antiferromagnetic MTJs by using a ferromagnetic reference layer while retaining the robustness of the altermagnetic electrode.
• To establish a design principle for high-TMR spintronic devices based on altermagnetic materials with tunable transport-direction-dependent TMR.

Proposed method

• First-principles density functional theory (DFT) calculations are used to model the electronic band structure and Fermi surface of RuO2, identifying spin-splitting along high-symmetry lines.
• The tunneling magnetoresistance (TMR) is calculated for RuO2(110)/TiO2/CrO2 heterostructures along [001] and [110] crystallographic directions to assess transport anisotropy.
• Spin-resolved conduction channels are analyzed to correlate TMR behavior with the shape and symmetry of spin-up and spin-down Fermi surface sheets.
• The magnetic space group of RuO2 is examined to confirm the breaking of TPτ and Uτ symmetries, which enables non-relativistic spin splitting in momentum space.
• The feasibility of extending the design to other altermagnets, such as CrSb, is evaluated using similar band structure and conduction channel analysis.
• Theoretical TMR in a potential all-altermagnetic RuO2/TiO2/RuO2 junction is also computed for comparison, yielding 685.1% TMR.

Experimental results

Research questions

• RQ1 Can altermagnetic materials with momentum-resolved spin splitting generate giant TMR in a tunnel junction when used as an electrode?
• RQ2 How does the crystallographic orientation of the altermagnetic electrode influence the TMR ratio in a magnetic tunnel junction?
• RQ3 Can the Néel vector state of an altermagnetic electrode be read out via TMR in a hybrid altermagnet/insulator/ferromagnet junction?
• RQ4 What is the role of spin-polarized Fermi surface topology in enabling direction-dependent TMR in altermagnetic materials?
• RQ5 Can altermagnetic tunnel junctions outperform conventional antiferromagnetic or ferromagnetic tunnel junctions in terms of TMR magnitude and robustness against external fields?

Key findings

• The ATMTJ exhibits a TMR ratio of nearly zero when current flows along the [001] direction due to symmetric spin conduction channels.
• When current flows along the [110] direction, the TMR ratio reaches 6100%, driven by asymmetric spin-up and spin-down conduction channels in RuO2(110).
• The giant TMR effect arises from momentum-space spin splitting in RuO2, which creates a spin-polarized Fermi surface that behaves like a ferromagnet along specific transport directions.
• The altermagnetic RuO2 electrode can be used to detect the Néel vector state via TMR contrast, enabling a readout mechanism for antiferromagnetic order.
• The design is robust against external magnetic fields because the altermagnetic reference layer is intrinsically field-resistant, making it suitable for magnetic sensor applications.
• The method is generalizable: CrSb, another altermagnet with high Néel temperature, also supports giant TMR when current flows along the [10̄11] direction, confirming the broader applicability of the approach.