The study reveals the dependence of the loss of spin memory in various interfaces

Courtesy: Gupta et al.

Researchers from Twente University and Beijing Normal University recently conducted a study examining a parameter known as spin memory loss (SML) for many different interfaces using a combination of theoretical and computational methods. Their article published in Physical Review Lettersoffers valuable new insights that can help develop better interfaces.

“The Holy Grail in our research area is a new concept for storing magnetic memory, which will be 100% electronic; that is, potentially faster, denser and more reliable than today hard disks (HDD), which form the backbone of the Internet (for example, data farms) and which are based on a mechanically rotating magnetic disk, accessed by a read / write head floating all in nanometers above a rapidly rotating hard disk, “Paul Kelly, One of the researchers doing the research, told “The new concept is based on the so-called spin-Hall effect (SHE), which was theoretically predicted 50 years ago, but was first observed in semiconductors in 2004 and two years. later in metal. "

In addition to the presence of a charge, electrons have a spin, which means that they can act as "rotating vertices." A magnetic moment is associated with this spin. SHE is a direct result of the relativistic effect called spin-orbit coupling (SOC), which “binds” the way electrons rotate (clockwise or counterclockwise) with how they move around atoms.

As a result of this effect, when the charge current passes through a plate of a heavy metal such as platinum, it excites the spin current at right angles to the charge current. If platinum is in contact with a magnetic material such as iron, nickel or permalloy, an FeNi alloy, a “spin current” is supplied to this adjacent magnetic material.

The study reveals the dependence of the loss of spin memory in various interfaces

Figure explaining the effect of the Spin Hall. Courtesy: Gupta et al.

“Under the right circumstances, this spin current can reorient the direction in which magnetic moment points: up – "1", down – "0"; and we have the foundation of a new type of magnetic memory, "Kelly explained." That's where we get in. "

As Kelly goes on to explain, spin current usually worsens as it passes from Pt's wire magnetic materialthat often happens on interfaces between two different materials. This current degradation, known as “spin memory loss” (SML), has been at the center of many studies, including by the Kelly team, and very little is known about this at present.

“What is still known about SML was obtained from low-temperature experiments, while 99% of the interest in what happens at room temperature is important for many applications,” Kelly said. “Our study was aimed at studying properties such as this.”

The main goal of the study by Kelly and his colleagues was to study the LSM and its behavior on various interfaces and at finite temperatures (where temperature-induced atomic vibrations and fluctuations in magnetic moments are unavoidable). Researchers have focused on four material combinations that are commonly used in the development of a fully magnetic storage device.

The study reveals the dependence of the loss of spin memory in various interfaces

A fully polarized spin current is introduced into a two-layer Au / Pt with a clear boundary (vertical black line), two layers of the Au50Pt50 interface (yellow shaded area) and four layers of the Au50Pt50 interface (green shaded area) between them. The calculated spin currents for three cases are shown as gray circles, yellow rhombs, and green squares, respectively. A solid blue line indicates compliance with the VF equation in Au. The solid, dashed, and dashed red lines indicate compliance with the VF equations in Pt for Au / Pt, Au / Au50Pt50 (2) Pt and Au / Au50Pt50 (4) jPt, respectively. (Insert) δ versus ARI for N ¼ 0, 2, and 4 interface layers of mixed Au50Pt50. Courtesy: Gupta et al.

Over the past 20 years, Kelly and his colleagues have developed computer codes that can be used to study the transport of electrons and spins (that is, spin transport) in complex materials. These codes are based on solving the "Schrödinger equation" of quantum mechanics in a form called the "scattering theory", which means that the behavior of electrons is in terms of the waves of matter.

“Two important steps in developing these codes were to include relativistic effects, namely SOC and temperature in the form of a temperature-induced lattice and spin disorder"Kelly said." As the temperature of the material increases, the atoms that make up the material vibrate more and more; this is called lattice disorder. If the material is ferromagnetic, then the magnetic moments on the atoms rotate from their original, uniform orientation. "

As a final step in developing code for studying spin transport through interfaces, Kelly and his colleagues used the results of their quantum-mechanical scattering calculations for calculate charge and spin currents observed by experimenters. This the process eventually allowed them to study IT on interfacesas well as degradation of spin currents during their transition from one material to another (i.e., SML).

“The key difference between our research and research conducted by other research groups is that we have long identified interfaces as a key goal and focused our code development on the ability to study interfaces between materials that have very different sizes (i.e. constant lattices) . "Kelly said." This included the widespread use of "sparse matrix methods" to be able to process huge numerical arrays that are the result of a realistic description of the interfaces. "

The study reveals the dependence of the loss of spin memory in various interfaces

Unfilled circles: spin current jS (z) through the three-layer Pt Py Pt calculated for T ¼ 300 K. The solid blue (orange) curve corresponds to the VF equations in the volume of Pt (Py). These adjustments are extrapolated to the zI interface to obtain the values ​​of js, Pt (ZI) and Js, Py (ZI), which are shown in detail in the right box. (Left inset) Spin current with (red) and without (blue) moments of proximity in Pt. Courtesy: Gupta et al.

Kelly and his colleagues were the first to study spin transport as a function of temperature through realistic interfaces. In addition to introducing numerical values ​​for the parameters describing this transport, they gathered valuable information on how these parameters vary on different interfaces, as well as their dependence on the types of disorder they affect.

In particular, the researchers found that non-magnetic interfaces have a minimum temperature dependence, while interfaces containing ferromagnets are highly temperature dependent. They also found that SML was greater for certain interfaces, especially when the transition between different materials is more abrupt (for example, Co / Pt interfaces).

Finally, Kelly and colleagues found that SML could be significantly improved due to lattice mismatches and interface alloying. In the future, the observations and ideas they have gathered will help develop more efficient interfaces for various possible applications.

“As a next step, we want to directly study the process by which the spin current generated by SHE in a heavy metal is injected into various other materials, both magnetic and magnetic, for closer contact with magnetic memory and related nanodevices.” Said Kelly. “We will also study the properties of new two-dimensional van der Waals ferromagnetic materials, which may have different charge and spin transfer properties and whose“ interfaces ”should play a key role in determining their magnetic properties.”

Spin current generation and control for modern electronic devices

Additional Information:
Criti Gupta et al. Violation of the dependence of the memory spin on interface loss, Physical Review Letters (2020). DOI: 10.1103 / PhysRevLett.124.087702

Yi Liu et al. Improving the interface of Gilbert Dumping from the First Principles, Physical Review Letters (2014). DOI: 10.1103 / PhysRevLett.113.207202

Yi Liu et al. A direct method for calculating temperature-dependent transport properties, Physical Review B (2015). DOI: 10.1103 / PhysRevB.91.220405

R.J.H. Wesselink et al. Calculation of spin-transport properties from first principles: spin currents, Physical Review B (2019). DOI: 10.1103 / PhysRevB.99.144409

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The study reveals the dependence of spin memory loss in various interfaces (2020, March 24)
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