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Magnetic Properties of Perovskite Oxides
Published in Gibin George, Sivasankara Rao Ede, Zhiping Luo, Fundamentals of Perovskite Oxides, 2020
Gibin George, Sivasankara Rao Ede, Zhiping Luo
Spin glasses are a type of magnets in which the magnetic moments are ordered randomly, without a periodicity. Spin glasses are considered as magnetic materials with both ferromagnetic and antiferromagnetic interactions that coexist and compete with each other due to some frozen-in structural disorder, leading to a net magnetic moment of zero, identical to a paramagnetic phase. In spin glasses, the spins are frozen in a spatially random manner irrespective of time, whereas in paramagnets, spatially random spin pattern randomly fluctuates in time. Figure 5.15 compares the magnetic spins of spin glass with ferro- and antiferromagnetic ordering. However, the spin glasses possess a certain periodic long-range order of spins characterized by a certain wave vector (Kawamura and Taniguchi 2015). The term spin glass is originated from its analogy to structural glasses, where the molecules are arranged in random and frozen without any periodicity since glass is a supercooled liquid. Theoretically speaking, the spin-glass ordering is a characteristic of structurally disordered materials.
The chiral anomalous Hall effect at high magnetic fields in Au-Fe alloys
Published in Philosophical Magazine, 2021
F. Wolff-Fabris, P. Pureur, J. Schaf, I. A. Campbell
The strength and behaviour of a contribution due to spin chiralities to the anomalous Hall effect in the spin glass Au-Fe 8 at% and the reentrant ferromagnetic Au-Fe 18 at% alloys were studied under the application of magnetic fields with intensities in the range 1–14 T . The field augmentation progressively leads to a strong attenuation of the chiral term amplitude. This effect, that was not previously observed, occurs because of the partial alignment by the field of the canted spins, which define the scalar chirality. The field-induced reduction of the chiral contribution to AHE is more accentuated in the spin glass ground state of both studied systems. A significant decrease of the chiral term amplitude is also observed in the canted ferromagnetic phase of the reentrant system. Evidence for the occurrence of the chiral Hall term were also detected in the paramagnetic phase of the studied alloys nearly above the ordering temperature. This particular finding reinforces the perception that scattering by spin chiralities may be effective in the transport properties of magnetic materials, irrespective of the system being ferromagnetic, spin glass or paramagnetic, when the time scale for spin fluctuations, quantum or thermal, are long enough as compared to the carrier relaxation time.
Crystal structure and magnetic properties of the Gd8Ag19.5Al45.2 and Ho8Ag21.2Al43.3 compounds
Published in Phase Transitions, 2019
Yuriy Tyvanchuk, Bohdan Stelmakhovych, Tetyana Krachan, Stanisław Baran, Andrzej Szytuła
The explanation of the observed magnetic properties is following: the negative values of the paramagnetic Curie temperatures indicate an antiferromagnetic correlation between the magnetic moments which is in agreement with the antiferromagnetic character of the ordering at low temperatures. The RE–RE interatomic distances (see section “Results and discussion”) indicate that the magnetic interactions are of the RKKY-type. The latter interactions have the long-range character and the J(RKKY) exchange integral is an oscillatory function described by the formula: J(RKKY) ∼ (x cos(x)-sin(x))/x4 where x = 2kFRi, kF is the Fermi wave vector and Ri is the distance between central magnetic atom and the i-th magnetic atom. As a result, the RKKY oscillations of the exchange integral can result in either ferro- or antiferromagnetic coupling. Competing exchange interactions of different character often lead to spin-glass behavior. The data presented for RE = Gd suggest a coexistence of the antiferromagnetic ordering and spin-glass or reentrant spin-glass at low temperatures. In order to decide between them, it is necessary to carry out a neutron diffraction experiment.
Nickel-induced magnetic behaviour of nano-structured α-Fe2O3, synthesised by facile wet chemical route
Published in Philosophical Magazine, 2018
Sharmila Kumari Arodhiya, Astrid Placke, Jaspreet Kocher, Ashok Kumar, Jiri Pechousek, Ondrej Malina, Libor Machala
Figure 8 shows the FC and ZFC magnetisation curve and corresponding differential curve (dM/dT vs. T) of 0 (pure sample), 1%, 2% and 4% Ni containing nanosized α-Fe2O3 in the temperature range of 5–300 K at applied field of 0.1 T. Since coercive field of α-Fe2O3 is nonzero and no doublet peak in Mössbauer spectra has been observed, the separation of ZFC and FC can be ascribed to either spin glass behaviour of the Ni containing α-Fe2O3 or due to bimodal kind of size distribution (one group of particles shows large particles and other very small). However, spin glass behaviour prevails and can be understood as follows. The plateau in FC magnetisation curve below the blocking temperature (TB maximum of ZFC curve) in pristine nanosized α-Fe2O3 points to the strong interparticle interactions [22]. The separating temperature (TS) at which ZFC and FC curve start to separate resembles to the blocking temperature of the largest particle. Difference between TS and TB is known to be a qualitative measure of size distribution. The TB for Ni containing α-Fe2O3 is 255 K and TS is 275–280 K which shows a narrow size distribution of the particles; indicating spin glass behaviour is prevailing over size difference of nanoparticles (this fact is also confirmed by carefully examining transmission electron micrographs). The magnetisation in both curves ZFC and FC clearly exhibit an abrupt change associated to the Morin transition. The inflection point of this variation in the ZFC curve occurs at 257, 245, 247 and 242 K for 0%, 1%, 2% and 4% Ni containing α-Fe2O3, respectively.