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Origin of mixed anisotropy in crystalline Permalloy and amorphous Cobalt thin films individually deposited on Si substrate

This study investigates the evolution of mixed magnetic anisotropies in rf-sputtered crystalline Permalloy and amorphous Cobalt thin films on Si substrates, revealing how growth conditions and film thickness induce magnetization tilt and defining distinct anisotropy regimes to enhance spintronic device performance.

Original authors: Kirti Kirti, Baisali Ghadai, Abinash Mishra, Rahulkrishnan R, Sucheta Mondal

Published 2026-02-04

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CC BY 4.0

Original authors: Kirti Kirti, Baisali Ghadai, Abinash Mishra, Rahulkrishnan R, Sucheta Mondal

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: Magnetic Thin Films as "Magnetic Maps"

Imagine you are trying to draw a map for a traveler (the magnetic energy) on a very thin sheet of metal. This sheet is so thin it's almost invisible, sitting on top of a silicon computer chip. The goal of this research is to understand which direction the traveler wants to go.

In the world of magnets, this direction is called the "easy axis." Usually, for a flat, thin sheet, the traveler wants to stay flat on the surface (like a skater gliding on ice). However, this paper discovered that for certain materials, the traveler gets confused and starts walking at a weird angle, leaning partly off the ice and partly on it.

The researchers studied two different types of "sheets":

Permalloy (Py): A soft, crystalline magnetic material (like a neatly organized crystal lattice).
Cobalt (Co): A harder, amorphous (glass-like) magnetic material where the atoms are jumbled up randomly.

They deposited these materials on silicon chips at different thicknesses, ranging from very thin (like a single layer of atoms) to quite thick.

The Experiment: The "Oblique Angle" Setup

To make these films, the researchers used a machine called a sputtering system. Think of this like a spray-painting booth.

They shot atoms at the silicon chip to build up the film.
The Twist: They didn't spray straight down. They sprayed at a 45-degree angle (like rain hitting a windshield).
They also rotated the chip slowly while spraying.

This specific angle is crucial. Because the atoms hit at an angle, they tend to pile up in little "shadows," creating tiny, tilted columns or pillars inside the film, rather than a perfectly flat, smooth layer.

The Discovery: Why the Magnetism Gets "Tilted"

The researchers found that the direction the magnetism points depends on a "tug-of-war" between different forces. They identified four main players in this game:

Shape Anisotropy (The "Flat Sheet" Rule):
- The Analogy: Imagine a long, flat pancake. It is much easier to slide a finger across the top of the pancake than to push it through the side.
- The Physics: Because the film is flat and wide, the magnet naturally wants to stay flat (In-Plane). This is the strongest force trying to keep the magnet horizontal.
Growth-Induced Shape Anisotropy (The "Tilted Pillars" Rule):
- The Analogy: Because the spray came in at an angle, the atoms built up like a stack of leaning dominoes or a forest of tilted trees.
- The Physics: The magnet wants to align with the long axis of these tilted columns. This force tries to pull the magnet up (Out-of-Plane).
Stress (The "Rubber Band" Rule):
- The Analogy: Imagine stretching a rubber band or squeezing a sponge. If the film is squeezed or stretched against the silicon chip, it changes how the magnet behaves.
- The Physics: The difference in how the film and the silicon chip expand or contract creates internal stress. Depending on the material, this stress can push the magnet up or keep it flat.
Crystal Structure (The "Internal Compass" Rule):
- The Analogy: In the Permalloy (crystalline) film, the atoms are arranged in a specific grid. This grid has a "preferred direction" built into its DNA.
- The Physics: This internal structure pulls the magnet in a specific direction, regardless of the film's shape.

The Results: Two Different Stories

The researchers found that the outcome of this tug-of-war changed depending on the material and how thick the film was.

1. Permalloy (The Crystalline Material)

Thin Films (5–25 nm): The "Tilted Pillars" force and the "Internal Compass" were very strong. They fought the "Flat Sheet" rule so hard that the magnet ended up pointing at a strong angle (about 35 degrees off the flat surface). The researchers call this the "Robustly Tilted" (RT) regime.
- Result: It took the same amount of energy to push the magnet flat as it did to push it up. The magnet was stuck in the middle.
Thicker Films (50–125 nm): As the film got thicker, the "Flat Sheet" rule won the tug-of-war. The magnet mostly stayed flat, but with a slight tilt. The researchers call this the "Subtly Tilted" (ST) regime.

2. Cobalt (The Amorphous Material)

Thin Films (5–90 nm): Cobalt didn't have the "Internal Compass" (because its atoms are jumbled) and the stress forces were different. The "Flat Sheet" rule won easily. The magnet stayed almost perfectly flat. The researchers call this the "Mostly In-Plane" (MIP) regime.
Thicker Films (100–150 nm): As the Cobalt got very thick, the stress forces grew strong enough to fight back. The magnet started to tilt again, entering the "Subtly Tilted" (ST) regime, similar to the thicker Permalloy.

Why Does This Matter?

The paper concludes that by changing the thickness of the film and the angle of deposition, you can control exactly how the magnet tilts.

The Innovation: Usually, magnets are either flat or vertical. This research shows you can create a "tilted" magnet.
The Benefit: The authors mention that having a tilted magnet is a clever way to improve the performance of spintronic devices (the next generation of computer memory and processors). It allows for more efficient switching of magnetic states, which is the basis for storing data.

Summary

Think of the researchers as architects building magnetic skyscrapers. They discovered that by changing the angle of the construction crew (the spray) and the height of the building (the thickness), they could force the building's internal compass to lean. Sometimes it leans a lot (Permalloy, thin), sometimes a little (Permalloy, thick), and sometimes it stays straight (Cobalt, thin). Understanding this "leaning" helps engineers design better, faster, and more energy-efficient magnetic devices.

Technical Summary: Origin of Mixed Anisotropy in Crystalline Permalloy and Amorphous Cobalt Thin Films

Problem Statement
Magnetic anisotropy (MA) is a fundamental determinant of the static and dynamic behavior of magnetic thin films, governing phenomena such as magnetization reversal, domain formation, and spin-wave propagation. While various anisotropy sources exist—including magneto-crystalline (MCA), shape (SA), and magnetostrictive (MsA)—their interplay in sputter-deposited films often leads to complex, mixed anisotropy states. A gap exists in the literature regarding the experimental mapping of inherent anisotropy components in both crystalline and amorphous magnetic films across an intermediate thickness range (ultrathin to thick). Specifically, the emergence and evolution of tilted magnetic anisotropy in seed-free environments for these distinct materials have not been rigorously compared. This study addresses the need to understand the underlying mechanisms of mixed anisotropies in face-centred-cubic Permalloy (fcc-Py) and amorphous Cobalt (a-Co) to inform the design of energy-efficient spintronic devices.

Methodology
The researchers deposited fcc-Py (Ni80Fe20) and amorphous Co (a-Co) thin films on Si (100) substrates using rf magnetron sputtering at room temperature.

Deposition Conditions: Films were deposited with an oblique angle of incidence ( $\alpha \approx 45^\circ$ ) relative to the substrate normal, with the substrate rotating at 2 rpm to partially mitigate shadowing effects. The base pressure was $<7 \times 10^{-7}$ Torr, with a working pressure of 7 mTorr and an Ar flow rate of 40 SCCM.
Thickness Range: Py films ranged from $d = 5$ to $125$ nm; Co films ranged from $t = 5$ to $150$ nm.
Characterization:
- Structural: Grazing-incidence X-ray diffraction (GI-XRD) was used to assess crystallinity and phase. Atomic Force Microscopy (AFM) in tapping mode analyzed surface morphology and roughness.
- Magnetic: Vibrating Sample Magnetometry (VSM) was employed at room temperature to measure in-plane (IP) and out-of-plane (OOP) hysteresis loops.
- Electrical: Resistivity was measured using a four-probe Van der Pauw setup to infer porosity and growth modes.

Key Results and Observations
The study identified distinct regimes of magnetic behavior based on film thickness and material type, driven by the competition between Shape Anisotropy (SA), Growth-Induced Shape Anisotropy (GI-SA), Magneto-crystalline Anisotropy (MCA), and Magnetostrictive Anisotropy (MsA).

Structural Properties:
- fcc-Py: Films with $d < 80$ nm showed no distinct XRD peaks, suggesting poor crystallinity or random orientation. Films with $d \ge 80$ nm exhibited a prominent (111) peak, confirming the fcc phase. Crystallite sizes were calculated between 12.7 and 14.8 nm. A significant lattice mismatch ( $\eta \approx -54\%$ ) with the Si substrate was noted.
- a-Co: No diffraction peaks were observed across the thickness range, confirming an amorphous structure.
- Morphology: AFM revealed that Co films possessed higher surface roughness than Py films, attributed to Co's higher surface energy and island/cluster formation. Roughness in Co decreased slightly with increasing thickness, while Py roughness remained relatively constant.
Magnetic Anisotropy Regimes:
- fcc-Py (Permalloy):
  - Robustly Tilted (RT) Regime ( $5 \le d \le 25$ nm): IP and OOP hysteresis loops coincided, indicating an easy axis tilted $\approx 35^\circ$ from the IP plane. Here, SA (favoring IP) competed with GI-SA (favoring OOP due to columnar growth) and MsA (favoring OOP due to compressive stress and positive magnetostriction). The effective anisotropy ( $K_{eff}$ ) was low due to this cancellation.
  - Subtly Tilted (ST) Regime ( $50 \le d \le 125$ nm): The easy axis shifted closer to the IP plane, though a weak OOP component persisted. SA became the dominant contributor, while GI-SA weakened due to reduced shadowing effects. "Transcritical" loop shapes and "pot-belly" or "wasp-waist" features were observed, indicating the onset of complex domain structures.
- a-Co (Cobalt):
  - Mostly In-Plane (MIP) Regime ( $5 \le t \le 90$ nm): Unlike Py, Co did not exhibit an RT regime. Magnetization remained predominantly IP, driven by SA overpowering other anisotropies.
  - Subtly Tilted (ST) Regime ( $100 \le t \le 150$ nm): A transition occurred where OOP stress-induced anisotropy grew with thickness, competing with SA. This led to a "transcritical" loop shape and a reduction in $K_{eff}$ , signaling a spin-reorientation transition (SRT) and the initiation of stripe domains.

Significance and Claims
The paper claims that the modification of magnetic hysteresis behavior in these films arises directly from the competition between multiple anisotropy components. The authors demonstrate that:

Mixed Anisotropy Origins: The tilt of the magnetization easy axis is not a singular phenomenon but a result of the vector sum of SA, GI-SA, MCA, and MsA.
Material Dependence: Crystalline fcc-Py and amorphous a-Co respond differently to identical growth conditions. The presence of MCA and specific magnetostrictive properties in Py allows for the emergence of the RT regime, which is absent in amorphous Co.
Device Relevance: The study provides a foundational understanding of how growth conditions (specifically oblique deposition) and thickness influence anisotropy. The authors posit that introducing magnetization tilt is an innovative method to improve spintronic device performance. By characterizing these regimes, the work offers a basis for designing devices with tailored magnetic switching properties, potentially facilitating deterministic switching without symmetry-breaking fields, as suggested by prior theoretical proposals.

The authors conclude that their rigorous comparison of crystalline and amorphous materials in a seed-free environment fills a gap in understanding the evolution of tilted anisotropy, providing a concrete framework for future device prototyping.