There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Magnetocaloric effect: From materials research to refrigeration devices

H E Stanley Oxford: University Press 1971 pp xx + 308 price ?5 In the past fifteen years or so there has been a lot of experimental and theoretical work on the nature of critical phenomena in the neighbourhood of second order phase transitions. It has not been easy to get a good overall view of this work without digging through the rather complex original literature, although there are some good review articles covering particular aspects of the work.

https://iopscience.iop.org/article/10.1088/0031-9112/23/4/018/pdf

Introduction to Phase Transitions and Critical Phenomena

Extensive calculations based on density functional theory have been carried out to understand the origin of magnetism in undoped ZnO thin films as found in recent experiments. The observed magnetism is confirmed to be due to Zn, instead of O, vacancy. The main source of the magnetic moment, however, arises from the unpaired 2p electrons at O sites surrounding the Zn vacancy with each nearest-neighbor O atom carrying a magnetic moment ranging from 0.490 to 0.740 B. Moreover, the study of vacancy-vacancy interactions indicates that in the ground state, the magnetic moments induced by Zn vacancies prefer to ferromagnetically couple with the antiferromagnetic state lying 44 meV higher in energy. Since this is larger than the thermal energy at room temperature, the ferromagnetic state can be stable against thermal fluctuations. Calculations and discussions are also extended to ZnO nanowires that have larger surface to volume ratio. Here, the Zn vacancies are found to lead to the ferromagnetic state too. The present theoretical study not only demonstrates that ZnO samples can be magnetic even without transition-metal doping but also suggests that introducing Zn vacancy is a natural and an effective way to fabricate magnetic ZnO nanostructures. In addition, vacancy mediated magnetic ZnO nanostructures may have certain advantages over transition-metal doped systems in biomedical applications.

https://absimage.aps.org/image/MAR08/MWS_MAR08-2007-004197.pdf

Zinc Vacancy induced magnetism in ZnO thin films and nanowires

We report here enhancement of ferromagnetism in pure ZnO upon thermal annealing with the ferromagnetic transition temperature Tc above room temperature. We observe a finite coercive field upto 300K and a finite thermoremanent magnetization upto 340K for the annealed sample. We propose that magnetic moments can form at anionic vacancy clusters. Ferromagnetism can occur due to either superexchange between vacancy clusters via isolated F+ centers, or through a limited electron delocalization between vacancy clusters. Isolated vacancy clusters or isolated F+ centers give rise to a strong paramagnetic like behaviour below 10K.

Enhancement of ferromagnetism upon thermal annealing in pure ZnO

We have used the coprecipitation and mechanical-milling methods to fabricate CoFe2O4 nanoparticles with an average crystallite size (d) varying from 81 to ∼12 nm when changing the milling time (tm) up to 180 min. X-ray diffraction and Raman-scattering studies have proved the samples crystalizing in the spinel structure. Both the lattice constant and residual strain tend to increase when tm(d) increases (decreases). The analysis of magnetization data has revealed a change in the coercivity (Hc) towards the hard-magnetic properties. Specifically, the maximum Hc is about 2.2 kOe when tm = 10 min corresponding to d ≈ 29 nm; beyond this tm(d) value, Hc gradually decreases. Meanwhile, the increase of tm always reduces the saturation magnetization (Ms) from ∼69 emu g−1 for tm = 0 to 35 emu g−1 for tm = 180 min. The results collected as analyzing X-ray absorption data have indicated a mixed valence state of Fe2+,3+ and Co2+ ions. We think that the migration and redistribution of these cations between the tetrahedral and octahedral sites together with lattice distortions and defects induced by the milling process have impacted the magnetic properties of the CoFe2O4 nanoparticles.

Towards hard-magnetic behavior of CoFe2O4 nanoparticles: a detailed study of crystalline and electronic structures, and magnetic properties

Giant magnetothermal anomalies and direct measurements of the magnetocaloric effect in Pr0.7Sr0.3−xBaxMnO3 manganites

Electronic Structure, Griffiths Phase, and Magnetocaloric Effect in La0.7Ca0.3Mn1−xCuxO3 (x = 0.1) showing first- and second-order characters

This paper presents a detailed study on the Co-doping influence on the electrical and magnetotransport properties of ${\rm La}_{0.7}{\rm Ca}_{0.3}{\rm Mn}_{1-x}{\rm Co}_{x}{\rm O}_{3}(x={0.09}{-}0.17)$ prepared by solid-state reaction. Magnetic measurements versus temperature revealed a gradual decrease of the magnetization $(M)$ and Curie temperature $(T_{C})$ with increasing Co concentration $(x)$ . The $T_{C}$ values vary from 194 to 159 K as changing $x$ from 0.09 to 0.17, respectively. $H/M$ versus $M^{2}$ performances around $T_{C}$ prove the $x={0.09}$ sample undergoing a first-order magnetic phase transition (FOMT) while the samples with $x\geq 0.11$ undergo a second-order magnetic phase transition (SOMT). The other with $x={0.10}$ is considered as a threshold concentration of the FOMT–SOMT transformation. Considering temperature dependences of resistivity, $\rho(T)$ , in the presence and absence of the magnetic field, the samples (excepting for $x={\rm 0.17}$ ) exhibit a metal–insulator transition at $T_{P}=60{-}160~{\rm K}$ , which shifts toward lower temperatures with increasing $x$ . In the metallic-ferromagnetic region, the $\rho(T)$ data are well fitted to a power function $\rho(T)=\rho_{0}+\rho_{2}T^{2}+\rho_{4.5}T^{4.5}$ . This indicates electron–electron and electron–magnon scattering processes are dominant at temperatures $T . In addition, the conduction data at temperatures $T>\theta_{D}/2$ ( $\theta_{D}$ is the Debye temperature) and $T_{P} obey the small-polaron and variable-range hopping models, respectively. The values of activation energy $E_{p}$ , and density of states at the Fermi level $N(E_{F})$ were accordingly determined. Here, $N(E_{F})$ increases while $E_{p}$ decreases when an external magnetic field is applied. We also have found that $N(E_{F})$ increases when materials transfer from the FOMT to the SOMT, and $N(E_{F})$ value becomes smallest for the sample having the coexistence of the FOMT and SOMT (i.e., $x={\rm 0.10}$ ).

Electrical and Magnetotransport Properties of ${\rm La}_{{0.7}}{\rm Ca}_{{0.3}}{\rm Mn}_{{1-x}}{\rm Co}_{{x}}{\rm O}_{3}$

Việc điều khiển tính dị hướng và trường trao đổi dịch (HEB) trong các màng mỏng sắt từ/phản sắt (FM/AFM) từ được coi là chìa khóa trong sự phát triển các thiết bị spintronics thế hệ mới. Trong bài báo này, chúng tôi nghiên cứu khả năng điều biến dị hướng từ và HEB dựa trên các thông số cấu trúc trong các màng nano xốp đa lớp FM/AFM. Các màngmỏng đa lớp [Co/Pd]-IrMn được phún xạ trên đế phẳng SiO2 và trên các đế xốp nano Al2O3 và TiO2có kích thước lỗ xốp và độ nhám bề mặt khác nhau. Ảnh hưởng của hình thái bề mặt, kích thước lỗ xốp lên các đặc trưng cấu trúc và tính chất từ của các màng xốp đa lớp [Co/Pd] và [Co/Pd]-IrMn được khảo sát một cách kỹ lưỡng. Tất cả các màng (phẳng và xốp) đều thể hiện dị hướng từ mạnh và HEBlớn theo phương vuông góc ở nhiệt độ phòng, đặc biệt trong màng xốp TiO2-[Co/Pd]-IrMn. Thêm nữa, sự tăng cường đáng kể lực kháng từ và sự giảm độ từ dư trong các màng xốp đã chế tạo, phụ thuộc vào kích thước lỗ xốp và hình thái bề mặt, cũng quan sát thấy.

Tran Dang Thanh

Papers

Towards hard-magnetic behavior of CoFe2O4 nanoparticles: a detailed study of crystalline and electronic structures, and magnetic properties

Giant magnetothermal anomalies and direct measurements of the magnetocaloric effect in Pr0.7Sr0.3−xBaxMnO3 manganites

Electronic Structure, Griffiths Phase, and Magnetocaloric Effect in La0.7Ca0.3Mn1−xCuxO3 (x = 0.1) showing first- and second-order characters

Electrical and Magnetotransport Properties of ${\rm La}_{{0.7}}{\rm Ca}_{{0.3}}{\rm Mn}_{{1-x}}{\rm Co}_{{x}}{\rm O}_{3}$

ĐIỀU BIẾN DỊ HƯỚNG TỪ VÀ TRƯỜNG TRAO ĐỔI DỊCH THEO PHƯƠNG VUÔNG GÓC TRONG CÁC MÀNG ĐA LỚP [Co/Pd]-IrMn CÓ CẤU TRÚC NANO DẠNG ANTIDOT ARRAYS