Thickness-Driven Magnetic Behavior in Ni–Cr Nanocrystalline Thin Films: Implications for Spintronics and Magnetic Cooling

Abstract

Alloying antiferromagnetic Cr with ferromagnetic Ni forms an excellent starting point to design materials for various room temperature applications, such as spintronics, the magnetocaloric effect, and hyperthermia, thanks to its tunable Curie temperature, T C , and competing magnetic interactions. In this report, we present a comprehensive analysis of the relationships among film thickness, Cr surface segregation, and magnetic properties in nanocrystalline Ni 100−x Cr x (x = 5−15 at. %) thin films. We assess the impact of Cr segregation evolution on various magnetic transitions as the thickness and Cr doping level increase. These films exhibited ferromagnetic to paramagnetic transitions at higher temperatures and ferromagnetic to spin-glass transitions at low temperatures, which resulted in a phase diagram that differed from the bulk equilibrium phase diagram. Further, magnetometry and magnetic force microscopy were performed to investigate the magnetic anisotropy and magnetic domain structure as a function of film thickness. It is found that the perpendicular component of magnetic anisotropyled to a stripe domain structure in films with critical thicknesses above 20 nm. Substantial changes in the magnetization behavior of nanocrystalline Ni 100−x Cr x thin films within a few Cr at. % doping provide a means for adjusting both soft and hard magnetic component applications.

Coexistence of normal and inverse magnetocaloric effect in inhomogeneous Ni95Cr5 layers

“…The Curie temperature, T C of bulk Ni 95 Cr 5 alloy value is indicated in the same figure (black vertical dashed lines) for comparative study. These layers exhibit higher T C values compared to their bulk value, which suggests Cr-surface segregations as has been explained earlier [18,19]. However, the T C values of the 10 nm-thick Ni 95 Cr 5 layer approach their bulk values, and making them composite with Ag further reduces T C values.…”

“…Interestingly, the transcritical nature of the M-H loops of the annealed Ni 95 Cr 5 layer appears to have vanished, as shown in figure 6(f). This is because layer's microstructure, texturing, residual stresses, exchange coefficient and domain structures are expected to significantly affect after annealing [18]. Furthermore, in the lesser thick 20 nm annealed Ni 95 Cr 5 layer, typical FM type behavior was observed even for lower fields M-T curves (Supporting Information figure S3).…”

“…% Cr, with the 5 at. % Cr being notable for its Tc around room temperature [15,18,19], holding promise for MCE applications. Our recent study demonstrates that the thickness variation of these Ni 100−x Cr x layers is a second lever of action that enables external tuning competence of the soft magnetic properties (T C , saturation magnetization, M S , and coercive field H C ) by inducing Cr-surface segregation [18], which can eventually be used to create various magnetic ground states for MCE and IMCE functionalities.…”

“…% Cr being notable for its Tc around room temperature [15,18,19], holding promise for MCE applications. Our recent study demonstrates that the thickness variation of these Ni 100−x Cr x layers is a second lever of action that enables external tuning competence of the soft magnetic properties (T C , saturation magnetization, M S , and coercive field H C ) by inducing Cr-surface segregation [18], which can eventually be used to create various magnetic ground states for MCE and IMCE functionalities. In this study, we have therefore chosen Ni 95 Cr 5 layer materials as model substances for calculational approaches to the problem of magnetic cooling near elevated magnetic ordering temperatures, T C , and spin glassy transition temperature, T G far below the T C that drives the change in the sign of S M D as a function of thickness.…”