Spin correlations and magnetic order in Co-Ga alloys: A comprehensive study

• Published in: Journal of alloys and compounds Vol. 649; pp. 1011 - 1018
• Main Authors: Yasin, Sk Mohammad, Saha, Ritwik, Srinivas, V., Kasiviswanathan, S., Nigam, A.K.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15.11.2015
• Subjects: Alloys; Cluster glass; Clusters; Cobalt base alloys; Concentration (composition); Ferromagnetism; Magnetization; Phase transformations; Phase transition; Re-entrant magnetism; Spin glass; Ferromagnetism; Cluster glass; Re-entrant magnetism; Phase transition
• ISSN: 0925-8388; 1873-4669
• DOI: 10.1016/j.jallcom.2015.07.072

Low temperature magnetic properties of binary CoxGa100−x alloy with Co concentration in the range 54 ≤ x ≤ 61.5 at% have been investigated. From the temperature and magnetic field dependent magnetization measurements magnetic phase diagram has been identified. Cluster spin glass like features are noticed in x = 54, 55 compositions, while the compositions x > 57 exhibit double magnetic transition i.e., at higher temperatures paramagnetic (PM) – ferromagnetic (FM) and at lower temperatures FM-SG like transition. The critical concentration is identified to be near x = 57 composition where discernible spontaneous magnetization emerges and the long range ferromagnetic order develops above this composition in addition to the spin glass transition (or mixed magnetic phase). Analysis of temperature dependence magnetization data in the different temperature ranges for the compositions x = 60 and 61.5 indicate that the mean field models are not suitable to understand the phase transition. Magnetic isotherms in the critical region were analyzed using non-mean-field approach and the critical exponents (γ = 1.31 and β = 0.337) found to be close to 3D Heisenberg model suggesting the importance of short range magnetic order. The data satisfies magnetic equation of state characteristic of a second order phase transition. The results obtained from the present study corroborate well with the phenomenological interacting spin cluster model. Low temperature magnetic properties of binary CoxGa1−x alloy with Co concentration in the range 54 ≤ x ≤ 61.5 at% have been investigated. From the temperature and magnetic field dependent magnetization measurements magnetic phase diagram has been identified. Cluster spin glass (SG) like features are noticed in x = 54, 55.5 compositions, while the compositions x > 57 exhibit double magnetic transition i.e., at higher temperatures paramagnetic (PM) – ferromagnetic (FM) and at lower temperatures FM-SG like transition. The critical concentration is identified to be near x = 57 composition where discernible spontaneous magnetization emerges and the long range ferromagnetic order develops above this composition in addition to the spin glass transition (or mixed magnetic phase). Analysis of temperature dependence magnetization data in the different temperature ranges for the compositions x = 60 and 61.5 indicate that the mean field models are not suitable to understand the phase transition. Magnetic isotherms in the critical region were analyzed using non-mean-field approach (see figures below) and the critical exponents (γ = 1.31 and β = 0.337) found to be close to 3D Heisenberg model suggesting the importance of short range magnetic order. The Coercivity and irreversibility (FC-ZFC) of magnetization data at low temperatures supports the presence of isolated magnetic clusters in the intermediate concentration range. Such granular states can provide large values of magnetoresistance. The results obtained from the present study corroborate well with the phenomenological interacting spin cluster model. A overall picture with concentration can be viewed as non-magnetic x = 50, CoxGa1−x, develops into a cluster glass (x = 54–55.5) and at critical concentration (x = 57) an interaction between clusters result in development of ferromagnetic interactions. Coercive field and δM as a function of Co concentration. [Display omitted] •Through systematic magnetic study CoxGa1−x, (x = 54–61.5) phase diagram is established.•M−H data near critical composition/temperature suggest presence of spin clusters.•Present analyses disagree with earlier reports of presence of SPM state.•Through critical behavior analysis second order magnetic transition is established.•Established weakly-interacting ferromagnetic spin clusters at critical concentration.