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Thermal energy harvesting from natural resources and waste heat is becoming critical due to ever-increasing environmental concerns. However, so far, available thermal energy harvesting technologies have only been able to generate electricity from large temperature gradients. Here, we report a fundamental breakthrough in low-grade thermal energy harvesting and demonstrate a device based on the thermomagnetic effect that uses ambient conditions as the heat sink and operates from a heat source at temperatures as low as 24 degrees C. This concept can convert temperature gradients as low as 2 degrees C into electricity while operating near room temperature. The device is found to exhibit a power density (power per unit volume of active material) of 105 W cm(-3) at a temperature difference of 2 degrees C, which increases to 465 W cm(-3) at a temperature difference of 10 degrees C. The power density increases by 2.5 times in the presence of wind with a speed of 2.0 m s(-1). This advancement in thermal energy harvesting technology will have a transformative effect on renewable energy generation and in reducing global warming.