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

Magnetic anisotropy (MA) plays a crucial role in deciding both static and dynamic behaviour of magnetic thin films. It controls various phenomena, such as magnetization reversal, domain formation, domain-wall motion, spin-wave generation, and spin-wave propagation etc. We investigate the mixed anisotropies in face-centred-cubic Permalloy (fcc-Py) and amorphous Cobalt (a-Co) thin films deposited via rf magnetron sputtering on Si (100) substrate with thicknesses, d = 5-125 nm and t = 5-150 nm, respectively. X-ray diffraction technique, atomic force microscopy, and vibrating sample magnetometry are employed to study the structural, morphological, and magnetic properties. We adopt a qualitative approach to understand the nature of different anisotropies present in both materials. Mixed anisotropies evolve with film thicknesses for both fcc-Py and a-Co films. The role of growth conditions in the emergence of specific anisotropies is discussed in detail. An alteration of the magnetization easy axis from the conventional in-plane orientation is evidenced due to the collective influence of these mixed anisotropies. Based on the dominance of anisotropy components, their origin, and the direction of magnetization tilt, we categorize our samples as belonging to specific regimes. Introduction of magnetization tilt has been proven to be an extremely innovative way to improve the performance of spintronic devices so far. The one-to-one comparison between a sputter-deposited crystalline and an amorphous magnetic material could be beneficial for building a stronger foundation for that.

💡 Research Summary

This work investigates the evolution of mixed magnetic anisotropies in two fundamentally different thin‑film systems—face‑centred‑cubic Permalloy (Ni₈₀Fe₂₀, fcc‑Py) and amorphous cobalt (a‑Co)—both deposited by RF magnetron sputtering onto Si(100) substrates. The authors prepared a series of films with thicknesses ranging from 5 nm to 125 nm for Py and 5 nm to 150 nm for Co, using identical deposition parameters (base pressure < 7 × 10⁻⁷ Torr, 80 W RF power, 7 mTorr Ar, 40 sccm flow, 45° target‑to‑substrate angle, and a slow 2 rpm substrate rotation). Structural characterization by grazing‑incidence X‑ray diffraction (GI‑XRD) shows that Py develops a clear (111) fcc peak for d ≥ 80 nm, indicating the emergence of crystalline order with an average grain size of ~13 nm; thinner Py films exhibit weak or absent peaks, suggesting poor crystallinity or nanocrystalline domains. In contrast, a‑Co displays no diffraction peaks across the entire thickness range, confirming its amorphous nature. Atomic force microscopy (AFM) reveals smooth surfaces for both materials, with root‑mean‑square roughness values of 0.24–0.63 nm for Py and 0.39–0.95 nm for Co; roughness generally decreases with increasing thickness, especially for Py, reflecting a transition from island‑type nucleation to layer‑by‑layer growth.

Magnetic properties were probed using vibrating sample magnetometry (VSM) to obtain in‑plane (IP) and out‑of‑plane (OOP) hysteresis loops. Very thin films (≤ 25 nm) exhibit nearly identical IP and OOP loops with a coercivity of ~25 Oe, indicating a dominant shape anisotropy that forces the magnetization to lie in the film plane. As thickness increases, OOP loops become progressively more open, coercivity rises, and a clear tilt of the easy axis away from the plane appears. For Py with d = 80–125 nm, the easy axis tilts by roughly 10–20°, while a‑Co shows a similar but weaker trend.

The authors decompose the observed anisotropy into four principal contributions: (1) magnetocrystalline anisotropy (MCA), present only in crystalline Py and aligning the easy axis along the (111) direction; (2) shape anisotropy (SA), which dominates in ultrathin films and confines magnetization to the plane; (3) growth‑induced anisotropy (GI‑SA), arising from the oblique deposition geometry that creates columnar or sheet‑like nanostructures, thereby adding an out‑of‑plane component; and (4) magneto‑elastic (stress‑induced) anisotropy (MsA), generated by intrinsic stresses during sputtering, especially significant for the highly magnetostrictive Co and Py. By comparing the relative strength of these terms as a function of thickness, the authors define three regimes: Regime I (very thin, SA‑dominated, essentially in‑plane magnetization), Regime II (intermediate thickness, competition between GI‑SA and MCA leading to a modest tilt), and Regime III (thick films, MCA and MsA dominate, producing a pronounced out‑of‑plane component).

The practical implication of this mixed‑anisotropy engineering is highlighted for spintronic devices. A controlled tilt of the magnetization reduces the critical current needed for domain‑wall motion, modifies spin‑wave dispersion in a non‑reciprocal manner, and enables low‑power switching without external magnetic fields. Consequently, by simply adjusting film thickness and deposition geometry, one can tailor the balance of anisotropy contributions to achieve desired magnetic states, offering a versatile route toward energy‑efficient, high‑speed magnetic memory and logic elements.

In summary, the paper provides a comprehensive experimental mapping of thickness‑dependent anisotropy in both crystalline and amorphous magnetic thin films, elucidates the underlying mechanisms that generate mixed anisotropy, and demonstrates how these mechanisms can be harnessed to tilt the magnetization easy axis—a strategy that holds promise for next‑generation spintronic technologies.