Tang Shaolong

The magnetic phase transition and the low-field Arrott plots of (GDxDy1−x)Co2 compounds

Abstract A series of (GdxDy1−x)Co2 (0–0.55) compounds have been prepared by arc-melting method. In order to determine the phase transition order in these compounds, we have performed some magnetization measurements. Only from the plots of temperature dependence of magnetization and the low-field magnetization isotherms, it is difficult to judge the phase transition order. The low-field Arrott plot is proved to be a more reliable method to characterize the first-order transition and the second-order transition. No first-order transition is observed in (GdxDy1−x)Co2 compounds (x≠0). The result is discussed in the Inoue-Shimizu like model.

The origin of the large magnetocaloric effect in RCo2 (R=Er, Ho and Dy)

The phase transitions of the RCo2 (R=Er, Ho and Dy) compounds are of first-order type and a metamagnetic transition is observed above the Curie temperature. Owing to their large magnetic entropy change, the RCo2 compounds are competitive candidates as the working substance for magnetic refrigeration. In this paper we discuss the origin of this large magnetic entropy change.

Relation between martensitic transformation temperature range and lattice distortion ratio of NiMnGaCoCu Heusler alloys

In order to study the relation between martensitic transformation temperature range ΔT (where ΔT is the difference between martensitic transformation start and finish temperature) and lattice distortion ratio (c/a) of martensitic transformation, a series of Ni46Mn28−xGa22Co4Cux (x = 2–5) Heusler alloys is prepared by arc melting method. The vibration sample magnetometer (VSM) experiment results show that ΔT increases when x > 4 and decreases when x < 4 with x increasing, and the minimal ΔT (about 1 K) is found at x = 4. Ambient X-ray diffraction (XRD) results show that ΔT is proportional to c/a for non-modulated Ni46Mn28−xGa22Co4Cux (x = 2–5) martensites. The relation between ΔT and c/a is in agreement with the analysis result obtained from crystal lattice mismatch model. About 1000-ppm strain is found for the sample at x = 4 when heating temperature increases from 323 K to 324 K. These properties, which allow a modulation of ΔT and temperature-induced strain during martensitic transformation, suggest Ni46Mn24Ga22Co4Cu4 can be a promising actuator and sensor.

Transformation behaviors, structural and magnetic characteristics of Ni—Mn—Ga films on MgO (001)

Ferromagnetic Ni—Mn—Ga films were fabricated by depositing on MgO (001) substrates at temperatures from 673 K to 923 K. Microstructure, crystal structure, martensitic transformation behavior, and magnetic properties of the films were studied. With increasing deposition temperature, the surface morphology of the films transforms from granular to continuous. The martensitic transformation temperature is not dependent on deposition temperature; while transformation behavior is affected substantially by deposition temperature. X-ray analysis reveals that the film deposited at 873 K has a 7M martensite phase, and its magnetization curve provides a typical step-increase, indicating the occurrence of magnetically induced reorientation (MIR). In situ magnetic domain structure observation on the film deposited at 873 K reflects that the martensitic transformation could be divided into two periods: nucleation and growth, in the form of stripe domains. The MIR occurs at the temperature at which martensitic transformation starts, and the switching field increases with the decrease of temperature due to damped thermal activation. The magnetically induced martensitic transformation is related to the difference of magnetization between martensite and austenite. A shift of martensite temperature of dT/dH = 0.43 K/T is observed, consistent with the theoretical value, 0.41 K/T.

Evolution of magnetic domain structure of martensite in Ni?Mn?Ga films under the interplay of the temperature and magnetic field

Ferromagnetic shape memory Ni?Mn?Ga films with 7M modulated structure were prepared on MgO (001) substrates by magnetron sputtering. Magnetization process with a typical two-hysteresis loop indicates the occurrence of the reversible magnetic field-induced reorientation. Magnetic domain structure and twin structure of the film were controlled by the interplay of the magnetic and temperature field. With cooling under an out-of-plane magnetic field, the evolution of magnetic domain structure reveals that martensitic transformation could be divided into two periods: nucleation and growth. With an in-plane magnetic field applied to a thermomagnetic-treated film, the evolution of magnetic domain structure gives evidence of a reorientation of twin variants of martensite. A microstructural model is described to define the twin structure and to produce the magnetic domain structure at the beginning of martensitic transformation; based on this model, the relationship between the twin structure and the magnetic domain structure for the treated film under an in-plane field is also described.

Substituting Al for Fe in Pr(AlxFe1–x)1.9 alloys: Effects on magnetic and magnetostrictive properties

The magnetostrictive effects of substituting Al for Fe in Pr(AlxFe1–x)1.9 (x = 0.0, 0.02, 0.05, 0.10) alloys between 5 K and 300 K were investigated. The substitution decreases the Curie temperature and the value of λ111. Fortunately, the substitution slightly increases the magnetostriction in a low magnetic field, which imbues these materials with potential advantages for applications. Rotation of the easy magnetization direction (EMD) from [111] to [100] in the Pr(Al0.02Fe0.98)1.9 alloy as temperature decreases was detected by step scanned XRD reflections.

Magnetic properties and magnetostriction of PrxNd1−xFe1.9 (0 ≤ x ≤ 1.0) alloys at low temperature

The crystal structure, magnetic and magnetostrictive properties of high-pressure synthesized PrxNd1?xFe1.9 (0 ? x ? 1.0) alloys were studied. The alloys exhibit single cubic Laves phase with MgCu2-type structure. The initial magnetization curve reveals that Pr0.2Nd0.8Fe1.9 has a minimum magnetocrystalline anisotropy at 5 K. The magnetostriction curve at 5 K shows that Pr0.2Nd0.8Fe1.9 has a very good low-field magnetostrictive property, and the magnetostriction of the PrxNd1?xFe1.9 alloy in high magnetic field is attributable mainly to Pr. The temperature dependence of the magnetostriction (??) at the field of 5 kOe shows that the substitution of Nd reduces the K1 remarkably, and the values of ?? of Pr0.2Nd0.8Fe1.9 and Pr0.8Nd0.2Fe1.9 alloys are nearly five times larger than that of the PrFe1.9 alloy below 50 K; the ?? of Pr0.8Nd0.2Fe1.9 reaches up to 1082 ppm at 100 K, which makes it a potential candidate for application in this temperature range.

Transport properties, microstructures and domain observation in Ag1-x-(Ni0.8Co0.2)x granular films

Abstract A series of Ag 1-x -(Ni 0.8 Co 0.2 ) x granular film samples were prepared by ion-beam cosputtering technique. Magnetoresistance was measured using conventional four terminal method at room temperature. A transition from giant magnetoresistance (GMR) to anisotropy magnetoresistance(AMR) has been observed in these samples as x rises, and the maximum of magnetoresistance ratio as large as −3% for x = 40 at .%. The VSM results of the samples show gradual changes from superparamagnetism to ferromagnetism as x increases. XRD, TEM and MFM were performed to investigate microstructures of these samples. The lined stripe domains have been observed in x = 75 at .% sample using MFM, but the domains gradually disappear as x decreases. It suggests that the transition from GMR to AMR result from enhanced interparticle interaction in the samples.

Improvement on the magneto-optical Kerr effect of cobalt film with a quadrilayer structure

The magneto-optical Kerr effect of the HfO2/Co/HfO2/Al multilayer structure is investigated in this work, and an obvious cavity enhancement of the Kerr response for the HfO2 semiconductor is found both theoretically and experimentally Surprisingly, a maximum value of about −3° of the polar Kerr rotation for s-polarized incident light is observed in our experiment. We propose that this improvement on the Kerr effect can be attributed to the multiple reflection and optical interference in the cavity, which can also be proved by simulation using the finite element method.