W. B. Cui

Exchange couplings in magnetic films

Recent advances in the study of exchange couplings in magnetic films are introduced. To provide a comprehensive understanding of exchange coupling, we have designed different bilayers, trilayers and multilayers, such as anisotropic hard-/soft-magnetic multilayer films, ferromagnetic/antiferromagnetic/ferromagnetic trilayers, [Pt/Co]/NiFe/NiO heterostructures, Co/NiO and Co/NiO/Fe trilayers on an anodic aluminum oxide (AAO) template. The exchange-coupling interaction between soft- and hard-magnetic phases, interlayer and interfacial exchange couplings and magnetic and magnetotransport properties in these magnetic films have been investigated in detail by adjusting the magnetic anisotropy of ferromagnetic layers and by changing the thickness of the spacer layer, ferromagnetic layer, and antiferromagnetic layer. Some particular physical phenomena have been observed and explained.

Giant room-temperature magnetocaloric effect in Mn1−xCrxAs

A giant magnetocaloric effect was observed at room temperature in Mn1−xCrxAs compounds with x=0.006 and 0.01. The Cr dopant reduces (or even eliminates) the large thermal hysteresis of MnAs, while it lowers the first-order transition temperature from 313K for MnAs to 265K for Mn0.99Cr0.01As. Near the Curie temperature, a magnetic field induces a first-order phase transition from a ferromagnetic hexagonal phase to a paramagnetic orthorhombic phase, leading to a maximum value of ΔSM of 20.2J∕kgK at 267K for a 5T field change for Mn0.99Cr0.01As. The study on the Mn1−xCrxAs system may open an important field in searching proper materials for room-temperature magnetic refrigeration.

Giant magnetocaloric effect in ε-(Mn0.83Fe0.17)3.25Ge antiferromagnet

The magnetic phase transition and magnetocaloric effect are investigated on e-(Mn0.83Fe0.17)3.25Ge compound. A large positive magnetic entropy change ΔSM is observed, which is accompanied with a field-induced metamagnetic transition from a collinear to a triangular antiferromagnetic configuration in this antiferromagnetic compound. The maximum value of ΔSM is 11.6J∕kgK at 93K for a magnetic field change of 7T. The study on systems with antiferromagnetic phases may open an important field in searching new materials for magnetic refrigeration.

The origin of large overestimation of the magnetic entropy changes calculated directly by Maxwell relation

By investigating the nature of the first-order magnetostructural phase transition, we point out the origin of large overestimation of magnetic entropy changes calculated by directly applying the Maxwell relation. Furthermore, if the mass variations in ferromagnetic and paramagnetic phases on temperature are taken into consideration in two-phase region, taking MnAsCx, (Mn, Al)As, and Mn0.994Fe0.004As compounds as example, based on the modified Maxwell relation, the large overestimation and discrepancy of magnetic entropy changes disappear. The magnetic entropy changes are equivalent with the values by Clausius–Claperyon equation, indicating the validity of this modified approach.

Anisotropic behavior of exchange coupling in textured Nd2Fe14B/α-Fe multilayer films

Exchange coupling has been realized in textured Nd2Fe14B/α-Fe multilayer films. A Mo spacer layer has proved to be effective for preventing interdiffusion at the interface region between the hard-magnetic Nd16Fe71B13 and the soft-magnetic α-Fe layer in these multilayer films. Anisotropic behavior of the exchange coupling in the films is observed by means of magnetic measurements. Furthermore, the effective critical correlation length is found to exhibit anisotropic behavior, which is smaller in the parallel than in the perpendicular direction at the same temperature. This anisotropic behavior of the multilayer films can be well explained by taking into account the shape anisotropy of the textured grains.

Large cryogenic magnetocaloric effect of DyCo2 nanoparticles without encapsulation

The structure and formation of nanoparticles without encapsulation of the intermetallic compound DyCo2 were investigated by using x-ray diffraction and high-resolution transmission electron microscopy. The DyCo2 nanoparticles are stable in air without any shell protection. A large magnetic-entropy change of 13.2Jkg−1K−1 was found at 7.5K in an applied-field change from 1to7T, which is ascribed to the large magnetic moment density and the weak interaction energy in the nanoparticles. Such oxidation-resistant rare-earth transition-metal compound nanoparticles with large cryogenic magnetocaloric effect are useful for refrigeration applications at low temperatures.

Exchange bias in antiferromagnetic coupled Fe3O4+Cr2O3 nanocomposites

Exchange bias (EB) and magnetic properties of ferrimagnetic (FI) Fe3O4 and antiferromagnetic (AFM) Cr2O3 nanocomposites prepared by mechanical alloying have been investigated. A large EB field of 2.2 kOe at 10 K is observed in one of the nanocomposites, which may be related to the uncompensated and pinned AFM spins at the interface between FI and AFM phases of the nanocomposites. The EB field varies with the strength of cooling field and the content of the Cr2O3 phase, the phenomena observed are explained in terms of interfacial exchange interaction between the two phases.

Exchange bias and phase transformation in α‐Fe2O3+NiO nanocomposites

[Liu, W.] Chinese Acad Sci, Shenyang Natl Lab Mat Sci, Shenyang 110016, Peoples R China. Chinese Acad Sci, Int Ctr Mat Phys, Shenyang 110016, Peoples R China. Chinese Acad Sci, Inst Met Res, Shenyang 110016, Peoples R China.;Liu, W (reprint author), Chinese Acad Sci, Shenyang Natl Lab Mat Sci, 72 Wenhua Rd, Shenyang 110016, Peoples R China;wliu@imr.ac.cn

Unconventional exchange bias in CoCr2O4/Cr2O3 nanocomposites

Unconventional exchange bias (EB) has been studied in CoCr(2)O(4)/Cr(2)O(3) nanocomposites, in which the Curie temperature of the ferrimagnetic CoCr(2)O(4) is much lower than the Neel temperature of the antiferromagnetic Cr(2)O(3). A negative EB field of about 2.5 kOe at 5 K is achieved upon cooling in a field of 30 kOe. Meanwhile, the coercivity of the CoCr(2)O(4) nanoparticles has been enhanced significantly by coupling with Cr(2)O(3). The effect of the cooling field on the EB field and coercivity at 10 K has also been investigated. The domain-state model is used to interpret the unconventional EB. Cooling field may play a decisive role in the creation of the interfacial spin configuration for the unconventional EB, not only by exchange interaction between the induced magnetization of a polarized paramagnet and interfacial spins of an antiferromagnet but also by Zeeman interaction between the domain-state surplus magnetization and the external field.

Giant reversible magnetocaloric effect in cobalt hydroxide nanoparticles

The magnetocaloric effect associated with magnetic phase transitions in -CoOH 2 nanoparticles has been investigated. A sign change in the magnetocaloric effect is induced by a magnetic field, which is related to a field-induced transition from the antiferromagnetic to the ferromagnetic state below the Neel temperature. The large reversible magnetic-entropy changeS m 20.9 J /kg K at 15 K for a field change of 7 T indicates that -CoOH 2 is a potential candidate for application in magnetic refrigeration in the low-temperature range.