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Register a new userSpin glass like ground state and observation of exchange bias in Mn_{0.8}Fe_{0.2}NiGe alloy
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The ground-state magnetic properties of hexagonal equiatomic alloy of nominal composition Mn_{0.8}Fe_{0.2}NiGe were investigated through dc magnetization and heat capacity measurements. The alloy undergoes first order martensitic transition below 140 K with simultaneous development of long range ferromagnetic ordering from the high temperature paramagnetic phase. The undoped compound MnNiGe has an antiferromagnetic ground state and it shows martensitic like structural instability well above room temperature. Fe doping at the Mn site not only brings down the martensitic transition temperature, it also induces ferromagnetism in the sample. Our study brings out two important aspects regarding the sample, namley (i) the observation of exchange bias at low temperature, and (ii) spin glass like ground state which prevails below the martensitic and magnetic transition points. In addition to the observed usual relaxation behavior the spin glass state is confirmed by zero field cooled memory experiment, thereby indicating cooperative freezing of spin and/or spin clusters rather than uncorrelated dynamics of superparamagnetic like spin clusters. We believe that doping disorder can give rise to some islands of antiferromagnetic clusters in the otherwise ferromagnetic background which can produce interfacial frustration and exchange pinning responsible for spin glass and exchange bias effect. A comparison is made with doped rare-earth manganites where similar phase separation can lead to glassy ground state.
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Magnetic and magneto-functional behavior of a Fe-doped MnNiGe alloy with nominal composition Mn$_{0.85}$Fe$_{0.15}$NiGe have been investigated in ambient as well as in high pressure condition. The alloy undergoes first order martensitic phase transition (MPT) around 200 K and also shows large conventional magnetocaloric effect (MCE) ($Delta S$ $sim$ -21 J/kg-K for magnetic field ($H$) changing from 0-50 kOe) around the transition in ambient condition. Application of external hydrostatic pressure ($P$) results a shift in MPT towards the lower temperature and a clear decrease in the saturation moment of the alloy at 5 K. The peak value of MCE is also found to decrease with increasing external $P$ ($sim$ 18 J/kg-K decrease in $Delta S$ has been observed for $P$ = 12.5 kbar). The most interesting observation is the occurance of exchange bias effect (EBE) on application of external $P$. The competing ferromagnetic and antiferromagnetic interaction in presence of external $P$ plays the pivotal role towards the observation of $P$ induced EBE.
In conventional exchange-bias system comprising of a bilayer film of ferromagnet (FM) and antiferromagnet (AFM), investigating the role of spin-disorder and spin-frustration inside the AFM and at the interface has been crucial in understanding the fundamental mechanism controlling the exchange-bias -- an effect that leads to a horizontal shift in the magnetization hysteresis response of the FM. Similarly, in the recently reported monolayer molecular exchange-bias effect requiring no AFM layer, probing magnetic-disorder at the FM/molecule interface or inside the FM layer can provide new insights into the origin of molecular exchange-bias and the associated physics. In this article, by cooling the Fe/metal-phthalocyanine devices in oscillating magnetic field, we demonstrate a characteristic temperature dependent response of exchange-bias shift and ferromagnet coercivity that is supportive of a spin-glass behavior. Here, the origin of spin-glass is attributed to the spin frustration created in the magnetic structure of the Fe layer, which was absent in our reference-Fe studies. These results highlight the strong influence of FM/molecule interface pi-d hybridization on the magnetic exchange interactions extending deeper into the FM layer.
The ground state properties of the ferromagnetic shape memory alloy of nominal composition Ni2Mn1.36Sn0.64 have been studied by dc magnetization and ac susceptibility measurements. Like few other Ni-Mn based alloys, this sample exhibits exchange bias phenomenon. The observed exchange bias pinning was found to originate right from the temperature where a step-like anomaly is present in the zero-field-cooled magnetization data. The ac susceptibility study indicates the onset of spin glass freezing near this step-like anomaly with clear frequency shift. The sample can be identified as a reentrant spin glass with both ferromagnetic and glassy phases coexisting together at low temperature at least in the field-cooled state. The result provides us an comprehensive view to identify the magnetic character of various Ni-Mn-based shape memory alloys with competing magnetic interactions.
We report on comprehensive results identifying the ground state of a triangular-lattice structured YbZnGaO$_4$ to be spin glass, including no long-range magnetic order, prominent broad excitation continua, and absence of magnetic thermal conductivity. More crucially, from the ultralow-temperature a.c. susceptibility measurements, we unambiguously observe frequency-dependent peaks around 0.1 K, indicating the spin-glass ground state. We suggest this conclusion to hold also for its sister compound YbMgGaO$_4$, which is confirmed by the observation of spin freezing at low temperatures. We consider disorder and frustration to be the main driving force for the spin-glass phase.
Using ab initio calculations and special quasirandom structures, we have characterized the distribution of defect formation energy and migration barrier in Ni-based solid-solution alloys: Ni_{0.5}Co_{0.5}, Ni_{0.5}Fe_{0.5}, Ni_{0.8}Fe_{0.2} and Ni_{0.8}Cr_{0.2}. As defect formation energies depend sensitively on elemental chemical potential, we have developed a computationally efficient method for determining it which takes into account the global composition and local short-range order. We find that Fe has the biggest alloy effects for Ni among these four elements. Our results show that the distribution of migration energies for vacancies and interstitial have a region of overlap, which will facilitate the recombination between them.
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