Altermagnetic-Like Behavior and Enhanced Coercivity in Ferrimagnets at a Critical Point of an Extended Néel-Diagram

TL;DR

The paper extends Neel's ferrimagnet diagram to identify a critical point where full magnetic compensation persists over a broad temperature range, enabling altermagnetic-like spin-splitting without spin–orbit coupling. It combines a two-sublattice mean-field framework with a DLM-based Gibbs free-energy approach, ab initio exchange constants from DFT (Liechtenstein method with CPA), and Monte Carlo simulations to map composition–temperature behavior in GdCo$_5$-type ferrimagnets. Key findings show that near this critical point one can achieve large coercive fields across a wide temperature span while maintaining vanishing net magnetization and substantial spin splitting, aided by targeted chemical substitution (Ni, Cu, Fe and Ni-Fe co-doping). The work presents practical design principles for engineering ferrimagnets with altermagnetic-like features and robust coercivity for spintronic and magnetic applications.

Abstract

We generalize the classic Néel diagram for ferrimagnets within a mean-field framework and reveal a critical point at which full magnetic compensation is maintained below the Curie temperature and extends past the nominal compensation point. Ferrimagnets tuned to this critical point display altermagnetic-like features and markedly enhanced coercive fields. We show that proximity to this regime requires minimizing the net local moment while balancing exchange interactions with respect to the number of equivalent atoms in each sublattice. The resulting extended Néel diagram provides practical design principles for engineering ferrimagnets near the critical point via targeted chemical substitution that combines atoms with robust and weak local moments, as demonstrated through density functional theory and Monte Carlo simulations for GdCo$_5$-type ferrimagnets.

Figures (8)

• Figure 1: (a) Temperature dependency of the magnetization, normalized to the sum of the two sublattice moment sizes, for P (green), Q (brown) and N-type (pink) typical ferrimagnets with $\mu_a^\text{tot}=2\mu_b^\text{tot}$. An Ň-type, characterized with a low total magnetization, is also shown. (b) Extended Néel diagram for the total net magnetic moment at zero temperature and the ratio of inter and intralattice interaction $\gamma_\text{TM-TM}/\gamma_\text{TM-Gd}$. All phases meet at the critical point. The color bar indicates the maximum value of the total absolute magnetization. DFT data for ternary and quaternary GdCo$_5$-type ferrimagnets are also shown. All data sets originate from $x=0$, corresponding to pristine GdCo$_5$. Data points departing from this common origin correspond to progressively larger values of $x$ in increments of $\Delta x=0.5$.
• Figure 2: Temperature dependence of coercive fields calculated for selected GdCo$_5$-type ferrimagnets. The temperature axis is normalized to the transition temperature to the paramagnetic state, $T_\text{tr}$.
• Figure 3: (a) Composition-temperature magnetic phase diagrams obtained for Gd(Co$_{5-x}$Ni$_{x}$), GdCo($_{5-x}$Cu$_{x}$), Gd(Co$_{5-x}$Fe$_{x}$), and Gd(Co$_{5-x}$Fe$_{0.5x}$Ni$_{0.5x}$) by means of Monte Carlo simulations. The compensation temperature, $T_\mathrm{comp}(x)$, and the transition temperature from the paramagnetic state, $T_\text{tr}(x)$, are indicated with squares and circles, respectively. Available experimental data for Gd(Co$_{5-x}$Ni$_{x}$) amychuang are also shown in black. The black star shows the value of $T_\mathrm{comp}$ provided by DLM-DFT theory for a Gd(Co$_{5-x}$Ni$_x$) material amy. (b) Temperature dependence of the total net magnetization for selected compositions of interest.
• Figure 4: Electronic band structures calculated for GdCo$_5$ (left) and Gd(Co$_2$Ni$_{1.5}$Fe$_{1.5}$) (right) at zero temperature. No spin-orbit coupling is included.
• Figure 5: CaCu$_5$ hexagonal structure of (Gd,TM$_{5}$)-type compounds with a ferrimagnetic ground state. In the unit cell there is one Gd site in purple and two different Wyckoff positions for TM atoms, namely $2c$ in cyan and $3g$ in golden brown. These TM sites can be occupied by one or a solution of different TM atoms (e.g., Co, Ni, Cu, Fe).