Abstract:
The origin of the spin glass (SG) phase in the well-known multiferroic Pb(Fe1/2Nb1/2)O3 compound remains controversial due to the complications introduced by the coexistence of SG and long-range ordered (LRO) antiferromagnetic (AFM) phases. We have addressed this controversy through a comprehensive study on a Pb-free system Ca(Fe1/2Nb1/2)O3 (CFN) which does not exhibit LRO AFM transition. The SG transition in CFN is confirmed by the appearance of a cusp in the temperature dependence of dc magnetization M(T) with a SG freezing temperature Tf∼25K, and bifurcation of the zero-field-cooled and field-cooled magnetization M(T) below the irreversibility temperature Tirr∼27K. Using ac susceptibility [χ(ω,T)] measurements, we show that the spin dynamics follows power/Vogel-Fulcher law-type critical dynamics which diverges at TSG∼24K with an attempt time τo∼10-6s suggesting cluster spin glass (CSG) behavior. The field dependence of Tf(H) and Tirr(H) is shown to follow the de Almeida-Thouless line which separates the ergodic and nonergodic phases in the H-T plane and gives Tf(H=0)∼25K, which is in close agreement with TSG obtained from χ(ω,T). The existence of the glassy phase below TSG is further confirmed by the observation of slow nonexponential decay of thermoremanent magnetization with time, memory and rejuvenation effects, and unidirectional exchange-bias effect in the M-H hysteresis loop of field-cooled samples. The neutron powder-diffraction patterns reveal the absence of any magnetic peak due to LRO AFM phase but show a broad diffuse peak due to the presence of ∼2-nm-size AFM spin clusters which are responsible for the CSG freezing in CFN. ©2019 American Physical Society.
