Magnetocaloric effect (MCE) in TbCo 2-x Fe x has been studied by dc magnetization measurements.On substituting Fe in TbCo 2 , not only the magnetic transition temperature is tuned to room temperature, but also the operating temperature range for MCE is increased from 50 K for TbCo 2 to 95 K for TbCo 1.9 Fe 0.1 . The maximum magnetic entropy change (-ΔS M ) for TbCo 1.9 Fe 0.1 is found to be 3.7 J kg -1 K -1 for a 5 T field change, making it a promising candidate for magnetic refrigeration near room temperature. The temperature dependent neutron diffraction study shows a structural phase transition (from cubic to rhombohedral phase with lowering of temperature) which is associated with the magnetic phase transition and these transitions broaden on Fe substitution.To investigate the nature of the paramagnetic to ferrimagnetic phase transition, we performed a critical exponent study. From the derived values of critical exponents, we conclude that TbCo 2 belongs to the 3D Heisenberg class with short-range interaction, while on Fe substitution it tends towards mean-field with long-range interaction. The derived values of critical exponents represent the phenomenological universal curve for the field dependence of ΔS M , indicating that TbCo 2 and TbCo 1.9 Fe 0.1 belong to two different universality classes.PACS number(s): 75.30.Sg, 61.05.fm, 75.40.Cx * Electronic mail: smyusuf@barc.gov.in FAX: +91 22 25505151

The magnetic properties and transition from an antiferromagnetic (AFM) to a ferromagnetic (FM) state in semi Heusler alloys Cu 1-x Ni x MnSb, with x < 0.3 have been investigated in details by dc magnetization, neutron diffraction, and neutron depolarization.We observe that for x < 0.05, the system Cu 1-x Ni x MnSb is mainly in the AFM state. In the region 0.05 ≤ x ≤ 0.2, with decrease in temperature, there is a transition from a paramagnetic to a FM state and below ~50 K both AFM and FM phases coexist. With an increase in Ni substitution, the FM phase grows at the expense of the AFM phase and for x > 0.2, the system fully transforms to the FM phase. Based on the results obtained, we have performed a quantitative analysis of both magnetic phases and propose a magnetic phase diagram for the Cu 1-x Ni x MnSb series in the region x < 0.3. Our study gives a microscopic understanding of the observed crossover from the AFM to FM ordering in the studied semi

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Magnetocaloric effect in Mn 4 FeGe 3-x Si x compounds has been studied by dc magnetization measurements. For the parent compound Mn 4 FeGe 3 , the paramagnetic to ferromagnetic transition temperature T C is above room temperature (320 K), which initially remains constant for small Si substitution at the Ge site and then decreases marginally with an increase in Si concentration. A large change in magnetic entropy at the T C, under a magnetic field variation of 50 kOe, with typical values of 5.9, 6.5, 5.9 and 4.4 J kg -1 K -1 for x = 0, 0.2, 0.6, and 1 samples, respectively, along with a broad operating temperature range and a negligible hysteresis make Mn 4 FeGe 3-x Si x series a promising candidate for magnetic refrigerant material around room temperature. Mn 4 FeGe 3-x Si x series is found to undergo a second-order magnetic phase transition. The field dependence of the magnetic entropy change has been brought out and implemented it to deduce the critical exponents. The critical behavior study shows that the magnetic interactions for x = 0 and 0.2 samples have two different behaviors below and above T C . Below T C , it follows the mean field theory with long-range magnetic interaction and above T C it follows the Heisenberg three-dimensional model with short-range or local magnetic interaction. The magnetic exchange interactions for the x = 0.6 and 1 samples follow the mean-field theory. PACS number(s): 75.20.En, 75.30.Sg, 75.40.Cx a) Electronic mail: smyusuf@barc.gov.in FAX: +91 22 25505151 15

We report magnetic properties and magnetocaloric effect in intermetallic compounds NdMn2−xCoxSi2 (x = 0.2, 0.4, 0.6, 0.8, and 1). dc magnetization study shows that these compounds undergo a ferromagnetic-like transition at around 45 K. Neutron diffraction study at 5 K for the x = 0.2 sample shows a canted-ferromagnetic state at lower temperature (5 K), and a collinear antiferromagnetic state at higher temperature (above ∼50 K). At 5 K, the Nd moments are aligned along the crystallographic c-axis and the Mn moments are canted to the c-axis. At higher temperatures (50, 100, 200, and 300 K), Nd sublattice does not order but Mn sublattice orders antiferromagnetically. A magnetocaloric effect (MCE) is found with a magnetic entropy change of 14.4 and 12.4 J kg−1 K−1 for the x = 0.2 and 0.4 samples, respectively, at 47.5 K under a field variation of 50 kOe. Various interesting phenomena such as metamagnetic transitions and domain wall pinning have been observed, and their role in obtaining a large MCE and an inverse MCE, respectively, has been brought out. The hysteresis (in magnetic field dependent dc magnetization study) reduces significantly at temperatures near and above the magnetic transition temperature (TC), which makes these materials important for their practical applications in magnetic refrigeration around TC.